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Recent Advances in Carbon Dioxide Hydrogenation to Methanol via
Heterogeneous Catalysis
Xiao Jiang,@
Xiaowa Nie,*,@
Xinwen Guo,* Chunshan Song,* and Jingguang G. Chen*
Cite This: https://dx.doi.org/10.1021/acs.chemrev.9b00723 Read Online
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ABSTRACT: The utilization of fossil fuels has enabled an unprecedented era of prosperity
and advancement of well-being for human society. However, the associated increase in
anthropogenic carbon dioxide (CO2) emissions can negatively affect global temperatures and
ocean acidity. Moreover, fossil fuels are a limited resource and their depletion will ultimately
force one to seek alternative carbon sources to maintain a sustainable economy. Converting
CO2 into value-added chemicals and fuels, using renewable energy, is one of the promising
approaches in this regard. Major advances in energy-efficient CO2 conversion can potentially
alleviate CO2 emissions, reduce the dependence on nonrenewable resources, and minimize the
environmental impacts from the portions of fossil fuels displaced. Methanol (CH3OH) is an
important chemical feedstock and can be used as a fuel for internal combustion engines and
fuel cells, as well as a platform molecule for the production of chemicals and fuels. As one of
the promising approaches, thermocatalytic CO2 hydrogenation to CH3OH via heterogeneous
catalysis has attracted great attention in the past decades. Major progress has been made in the
development of various catalysts including metals, metal oxides, and intermetallic compounds.
In addition, efforts are also put forth to define catalyst structures in nanoscale by taking advantage of nanostructured materials, which
enables the tuning of the catalyst composition and modulation of surface structures and potentially endows more promising catalytic
performance in comparison to the bulk materials prepared by traditional methods. Despite these achievements, significant challenges
still exist in developing robust catalysts with good catalytic performance and long-term stability. In this review, we will provide a
comprehensive overview of the recent advances in this area, especially focusing on structure−activity relationship, as well as the
importance of combining catalytic measurements, in situ characterization, and theoretical studies in understanding reaction
mechanisms and identifying key descriptors for designing improved catalysts.
CONTENTS
1. Introduction B
2. Transition Metal and Oxide Catalysts C
2.1. Innovation of Classic Cu-Based Catalysts C
2.1.1. Identification of Active Sites C
2.1.2. Quantification of Cu Surface Areas F
2.1.3. Catalytic Structure−Activity Relation-
ship F
2.1.4. Reactor Design and Optimization R
2.1.5. Magnetic Field-Assisted Reactor S
2.2. Precious Metal-Based Catalysts (Pd and Pt) S
2.2.1. Monometallic Pd and Pt Catalysts T
2.2.2. Pd/Pt-Based Alloy Catalysts T
2.3. ZnO-Based Binary Solid Solution Catalysts V
3. Main Group Metal and Oxide Catalysts V
3.1. In2O3 Catalysts W
3.1.1. Selective CH3OH Synthesis on In2O3
Catalysts W
3.1.2. Transition Metal-Doped In2O3 Catalysts W
3.2. Ga-Based Intermetallic Compounds Y
3.2.1. Identification of Metallic Combinations
and Optimal Atomic Ratios Y
3.2.2. Evaluation of Support Materials Z
4. MOF/ZIF-Derived Nanostructured Catalysts Z
4.1. Bottom-Up Method AA
4.2. Top-Down Method AC
5. Mechanistic and Kinetic Studies AD
5.1. Transition Metal and Oxide Catalysts AD
5.1.1. Intermediates and Plausible Reaction
Pathways: HCOO* vs COOH* AD
5.1.2. H2O Effect AH
5.1.3. Kinetic Models AI
5.2. Theoretical Studies on In2O3-Based Catalysts AK
5.2.1. In2O3 Catalysts: Uniqueness of High
Methanol Selectivity AK
Received: November 5, 2019
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5.2.2. Transition Metal and Oxide Modified
In2O3 Catalysts AL
5.3. Mechanistic Studies on MOF-Derived Cata-
lysts AM
6. Conclusions and Future Research Opportunities AN
Author Information AP
Corresponding Authors AP
Author AP
Author Contributions AP
Notes AP
Biographies AP
Acknowledgments AP
References AP
1. INTRODUCTION
Carbon dioxide (CO2), a main component of greenhouse gases,
can bring both advantages and disadvantages. For example, the
existence of CO2, together with other greenhouse gases, enables
the creation of a warm environment for living creatures on earth.
However, the excess burning of fossil fuels causes a continuous
rise of CO2 concentration in the atmosphere, leading to
substantial and probably irreversible changes of the world’s
climate. The growth rate in the past 60 years has become more
rapid. In 2018, about 33890.8 million tons of CO2 was released
to the atmosphere.1
As of September, 2019, the global CO2
concentration in the atmosphere reached 407.65 ppm, an
increase of ca. 20% in the past 40 years.2
Undoubtedly, the
mitigation of CO2 emission has become an urgent issue, the
breakthrough of which can potentially offer innovative solutions
pertaining to global “3E” issues, namely energy-environment-
economy challenges.3
In 2014, the Intergovernmental Panel on
Climate Change (IPCC) has concluded that the costs of climate
change mitigation (costs between 2015 and 2100 relative to
default technology consumptions) could be increased by 138%
without considering the deployment and development of
Carbon Dioxide Capture, Utilization, and Storage (CCUS).4
The control of CO2 emission has been the subject of extensive
research efforts. For the summary of recent progress on CO2
capture and storage, one can refer to several comprehensive
reviews.5−7
As an important part of CCUS, the utilization of
CO2 as an untraditional and renewable carbon source has
attracted great attention worldwide because a major advance in
energy-efficient catalytic CO2 conversion using renewable
energy can potentially alleviate CO2 emissions and reduce the
dependence on fossil resources such as petroleum.8
The CO2
molecule is thermodynamically and chemically stable. It would
be energy-demanding if CO2 is used as a single reactant.5
However, it becomes thermodynamically easier if another
substance with higher Gibbs free energy is introduced as a
coreactant, such as H2.5
Therefore, CO2 hydrogenation to value-
added products is one of the promising approaches for utilizing
the abundant carbon source in CO2, leading to the production of
oxygenates (alcohols and dimethyl ether) and hydrocarbons
(olefins, liquid hydrocarbons, and aromatics).9−18
Methanol
(CH3OH) is an important chemical feedstock and can be used
as a fuel for internal combustion engines and fuel cells. With the
depletion of nonrenewable energy source, methanol is also an
alternative building block to produce chemicals and even
gasoline. As discussed by Olah et al. regarding the “methanol
economy” concept,19
methanol could play an indispensable role
in the near future, and one of the promising and regenerative
routes of methanol production is through CO2 hydrogena-
tion.20,21
Nowadays, with the successful development of active
catalysts (zeolite-based catalysts), methanol-to-olefin, MTO,
and methanol-to-propylene, MTP, have attracted great
attention. The market demand of methanol-derived fuels has
been significantly increased from 6% in 2011 to 22% in 2016.22
Therefore, CO2 conversion to methanol is one of the attractive
and potentially profitable routes in CCUS and should play an
important role in mitigating CO2 emissions and creating a new
carbon cycling process.
Hydrogen is primarily produced based on fossil fuels through
steam reforming, partial oxidation of methane, and coal
gasification, generating CO2 as a byproduct.23
However, the
depletion of fossil fuels and recovery of high purity H2 can
become a technical and economic obstacle to the implementa-
tion of large-scale processes for CO2 hydrogenation to
methanol. In this context, the usage of renewable energy (i.e.,
solar/wind power, photovoltaic cells, and geothermal power,
etc.) to produce H2 via water electrolysis is indispensable and
should be integrated concurrently with the progress of CO2
utilization. In fact, the integration of water electrolysis as
hydrogen supply is already taken into consideration in both
laboratory-scale research24−26
and practical implementation.27
In October 2009, a CO2-to-Renewable Methanol Plant was
established by Carbon Recycling International (CRI) at the
Svartsengi geothermal power station.28
One of the notable
features of the facility is that H2 is supplied from water
electrolysis. As reported, in 2015, CRI expanded the plant from a
capacity of 1.3 million liters per year to more than 4 million liters
per year, and 5.5 thousand tonnes of CO2 is recycled a year.28
Most notably, the use of the renewable methanol from the plant
releases 90% less CO2 in comparison to the use of a comparable
amount of energy from fossil fuels.28
Recently, Tackett et al. have analyzed the energy balances for
the thermocatalytic (TC) and electrocatalytic (EC) conversion
of CO2 to methanol and the associated net-reduction in CO2
emissions.8
As shown in Figure 1, the net-reduction is correlated
with the extent of CO2 emission per unit of electricity. At the
current value of 0.48 kg CO2 emitted per kWh electricity, both
TC and EC processes, as well as two hybrid (HB) processes
involving TC and EC, lead to a net-production of CO2. This
analysis highlights the importance of using renewable energy for
CO2 conversion to methanol. For example, with utilization of
CO2-free renewable energy (inset of Figure 1), the HB2 process
(involving the thermocatalytic hydrogenation of CO2 using H2
from water electrolysis) should lead to a net-reduction of CO2
emission in methanol synthesis.
Table 1 lists reactions involved in CO2 hydrogenation to
CH3OH, and changes of equilibrium CO2 conversion and
CH3OH yield as functions of temperature, pressure, and H2/
CO2 ratio are illustrated in Figure 2. The hydrogenation
reactions of CO2 and CO are exothermic, while the reverse
water−gas shift (RWGS) is endothermic. The thermodynamic
properties determine that lower reaction temperatures and
higher pressures are more favorable for CH3OH synthesis
(Figure 2a,b). Higher H2/CO2 ratios also favor CH3OH
synthesis (Figure 2c).29
At 523 K and 4 MPa, equilibrium
CO2 conversion and methanol yield are ca. 23% and 14%,
respectively.
Due to the similarity between syngas-to-methanol conversion
and CO2-based methanol synthesis, Cu-based catalysts have
been extensively investigated. The catalyst structure−activity is
well established, although debates continue in understanding
the mechanism and identifying the active sites. However, these
Chemical Reviews pubs.acs.org/CR Review
B

catalysts suffer from short life cycles and poor activity at lower
temperatures where methanol synthesis is more thermodynami-
cally favorable.30−32
To seek solution, precious metal-based
catalysts have been explored. However, these catalysts suffer
from a low selectivity. Bimetallic oxides such as ZnO/ZrO2 are
emerging as active, cost-effective catalysts for efficient methanol
synthesis.33
More importantly, these catalysts show good
stability even in the presence of sulfur species H2S and SO2.
Recently, there is growing attention to the development of main
group metal-based catalysts for methanol synthesis. As a
representative, In2O3 catalysts display promising activity,
selectivity, and stability.34
Intermetallic compounds offer
alternative to the bimetallic catalyst system, especially in terms
of controlling the formation of favorable active sites with
uniform bimetallic ratios and structure stability.35
Apart from
the advances in developing conventional catalysts, there is a
continuous interest in defining the catalyst structure in
nanoscale or atomic level because this can allow one to
manipulate the activity performance by creating more active
sites with desired catalytic features. Taking advantage of the
rapid advances in nanoscience and nanotechnology offers
opportunities in this regard, such as metal−organic frameworks
(MOF)- and zeolitic imidazolate frameworks (ZIF)-derived
catalysts.36
Similar advances have been made in using methanol as a
platform for CO2 hydrogenation to fuels and chemicals (olefin,
liquid hydrocarbons, and aromatics) via the well-known MTO
and MTA processes. There have been several excellent reviews
regarding catalytic CO2 utilization, especially on catalytic
conversion.10,37−43
However, these reviews primarily focus on
general aspects of CO2 conversion, and the discussion is focused
on Cu-based catalysts for CO2 hydrogenation to methanol. In
this review, we emphasize on the progress in developing
transition metal-based and main group metal-based catalysts for
CO2-to-methanol conversion, focusing on the activity and
structure−activity relationship. We also include the comparison
of transition metal catalysts such as precious metals and
transition metal oxides. A comprehensive summary of metal
oxides (In2O3) from main groups, intermetallic compounds
(Ni−Ga), and novel nanostructured catalysts (MOF/ZIF-
derived catalysts) is covered, which are emerging as alternative
catalysts. The review also underlines the importance of catalytic
measurements, in situ characterization, density functional
theory (DFT), and experimental kinetic-model fitting in
understanding reaction mechanisms and identifying key
descriptors for designing improved and selective catalysts. For
other promising approaches, such as homogeneous catalysis,
photocatalysis, and electrocatalysis, one can refer to review
articles that have been published recently.44−47
2. TRANSITION METAL AND OXIDE CATALYSTS
2.1. Innovation of Classic Cu-Based Catalysts
In industry, methanol is produced from syngas with added CO2
(CO2 + CO)/H2 over Cu/ZnO/Al2O3 catalysts, such as the new
Lurgi MegaMethanol process for plants with 5000 tonnes of
methanol production per day.48
A two-step process, RWGS and
subsequent CO hydrogenation to form methanol, is proposed
(∼75 Mt year−1
) to produce methanol from CO2 and H2 in
tandem reactions, in which ZnAl2O4 and Cu/ZnO/ZrO2/
Ga2O3 catalyze the RWGS and CO-to-methanol reactions,
respectively.37,49
Cu-based catalysts have been extensively studied for CO2-to-
methanol conversion via thermocatalysis, and the innovation
continues. Major efforts are devoted to (i) identifying the active
sites, (ii) developing the catalytic structure−activity relation-
ship, and (iii) improving the understanding of reaction
mechanisms. In addition, reactor design and optimization are
also explored to alleviate H2O-induced catalyst sintering,
increase methanol selectivity, and reduce energy consumption.
The following sections will cover major progresses in these areas.
2.1.1. Identification of Active Sites. Extensive studies
have been carried out to reveal the structure sensitivity by
examining the activity of single crystals such as Cu(100),50
Cu(110),51
and Cu(111).52
Results indicate that CO2 hydro-
genation to CH3OH is structure-sensitive to the Cu facet and
surface structure. As for the Cu/ZnO catalysts, the interface is
crucial for CH3OH synthesis. Generally, two possible active sites
are proposed at the interface, though there are intense debates
regarding the exact nature of the interfacial sites. One possibility
is the synergy between Cu and ZnO at the interface;53
Cu−Zn
surface alloy sites are the other possibility,54
the formation of
which might promote the partial reduction of ZnO particles to
the Znδ+
state or modification of Cu surfaces with metallic
Zn.55−59
The roles of ZnO include the stabilization of the Cu+
species by the ZnO moieties on the Cu surface,60
hydrogen
Figure 1. Net CO2 emission for the four CO2-to-methanol conversion
cases as a function of CO2 emission per unit of electricity. Values above
zero on the y-axis are net-CO2 emitting, and values below zero are net-
CO2 consuming. The basis for calculations is the production of 1000
tons of methanol per day. TC: thermocatalytic route; EC: electro-
catalytic route; HB1 (hybrid): electrocatalytic CO2-to-syngas + TC;
HB2 (hybrid): electrolysis of H2O + TC. Reproduced with permission
from ref 8. Copyright 2019 Springer Nature.
Table 1. Reactions Involved in CO2 Hydrogenation to CH3OH
Reaction I, CO2 HYD CO2 + 3H2 = CH3OH + H2O ΔH298 K = −49.4 kJ mol−1
Reaction II, CO HYD CO + 2H2 = CH3OH ΔH298 K = −90.4 kJ mol−1
Reaction III, RWGS CO2 + H2 = CO + H2O ΔH298 K = +41.0 kJ mol−1
C

reservoir,60
stabilization of key intermediates such as for-
mate,61,62
and promotion of the hydrogenation of formate
species.63
Progress for the first possibility has been made by Kattel et al.
recently in which XPS, DFT, and kinetic Monte Carlo (KMC)
simulations are integrated to identify the active sites of the Cu/
ZnO catalysts, especially the sites on the top layer of the catalyst
surface.64
ZnCu(111) and ZnO/Cu(111) model surfaces are
synthesized for comparison. Both experimental and theoretical
results indicate that ZnCu undergoes surface oxidation under
the reaction conditions, resulting in the transformation of Zn
into ZnO and then reaching a comparable activity as ZnO/Cu
(Figure 3). Therefore, the synergy between Cu and ZnO at the
interface should account for the methanol synthesis activity.
As a representative of the second possibility, Behrens et al.
have reported that the Zn atoms-decorated Cu steps are the
active sites and the stabilization of which relies on the
coexistence of well-defined bulk defects (disordered lays of Cu
NPs in Figure 4) and surface species.65
Karelovic et al. have also
studied the role of Cu particle size on the catalytic performance
of CO2 hydrogenation to CH3OH and also suggested the crucial
roles of steps and kinks in the case of small particles.66
A partial reduction occurs to adjust the Zn oxidation state to
Znδ+
, which originates from the strong metal−support
interaction (SMSI) and adsorbate-induced oxidation.65
The
SMSI is supported by the XPS results, which reveal an uneven
Zn distribution from the surface to the bulk: the surface is Zn-
rich (Zn/Cu = 70/30), whereas the ratio tends to gradually
approach the value in calcined form (Zn/Cu = 30/70).65
Such
SMSI has also been observed by temperature-programmed
desorption using CO as a probe molecule67
and FT-IR analysis
for CO2-containing syngas-to-methanol conversion.68
A recent
complementary study, led by Lunkenbein et al., has provided
visual evidence of the SMSI-induced formation of metastable
ZnOx overlayer on Cu nanoparticles via detailed chemical
transmission electron microscopy study over reduced industrial
Cu-ZnO-Al2O3 catalysts.59
As presented in Figure 5(a), Cu
nanoparticles (NPs) are covered with a layer of ZnO, indicating
the morphological SMSI effect for the reduced catalyst. HR-
TEM micrographs reveal details of overgrowth in atomic scale,
in which a broad distribution of interlayer distance is evidenced
(Figure 5(b)), demonstrating the presence of a metastable,
distorted ZnOx overlayer (“graphitic ZnO”). This overlayer is
also detected by electron energy loss spectroscopy (EELS) and
energy-filtered TEM (EFTEM). By being exposed to electron
beam, the ZnOx overgrowth transforms from metastable
structure to thermodynamically stable wurtzite ZnO (Figure
5(c)). DFT calculations reveal that the Zn decoration can
increase the adsorption strength of some intermediates such as
HCO*, H2CO*, and H3CO*.65
Such oxophilicity of Zn results
in a formal oxidation of Zn and adjusted Zn oxidation state in the
form of partially oxidized Znδ+
.
As summarized by Behrens et al., two factors can render high
activity in the design of Cu/ZnO catalysts: (i) the presence of
steps at the Cu surface is indispensable, the stabilization of which
can be achieved by bulk defects such as stacking faults or twin
boundaries terminating at the surface (Figure 4) and (ii) the
Figure 2. (a) Equilibrium CO2/CO conversion and (b,c) product yield as a function of temperature, pressure (CO2/H2 = 1:3), and H2/CO2 ratio.
Calculated by HSC 6.0 software.
Figure 3. (a) Rate for the conversion of CO2 to methanol on ZnCu(111) as a function of reaction time. The Cu substrate was precovered with 0.2 ML
of metallic Zn. Reaction conditions: 525 or 550 K, PH2 = 0.45 MPa, PCO2 = 0.05 MPa. (b) Zn 2p3/2 XPS binding energies measured after performing the
hydrogenation of CO2 on the Zn/Cu(111) catalyst. Reproduced with permission from ref 64. Copyright 2017 Science.
D

Figure 4. (a−d) Aberration-corrected HRTEM images of Cu particles in the conventionally prepared, most-active Cu/ZnO/Al2O3 catalyst. Panel (d)
is a close-up of the marked area in (c). Reproduced with permission from ref 65. Copyright 2012 Science.
Figure 5. (a) HAADF-STEM image of reduced Cu/ZnO/Al2O3 catalyst. (b) HRTEM micrograph of Cu/ZnO/Al2O3 with the inset denoting the
corresponding line scans taken from the assigned regions of interest (ROI). (c−e) HRTEM images of Cu/ZnO/Al2O3 after diﬀerent times of electron
beam exposure, demonstrating the transformation from graphitic-like ZnOx to the wurtzite structure. The color indicates the diﬀerent state during
phase transformation. The red-colored sites correspond to Cu particles; yellow indicates graphitic-like ZnOx; green highlights the rock salt ZnO; and
blue regions correspond to the wurtzite ZnO structure. Reproduced with permission from ref 59. Copyright 2015 John Wiley and Sons.
E

presence of Znδ+
at the defective Cu surface, resulting from a
dynamic SMSI effect.65
In a recent paper by Gogate, the nature of active sites of the
Cu/ZnO methanol synthesis catalyst is evaluated by a newly
developed microstructural analysis method based on HRTEM/
EDX/X-ray.69
Both high CO2 conversion and methanol
selectivity can be achieved through the unique nanostructured
features of the active sites in a Cu−Zn nanoensemble. The
characterization results identify the presence of a large amount
of under-coordinated Cu atoms in individual microcrystalline
domains, including those at steps and edges, discontinuities,
planar boundaries, and other local inward/outward curvatures.
The SMSI still plays a key role in the active sites, and partially
reduced ZnOx overlayers are formed on the surface of Cu
microdomains in different structural forms. These results
illustrate the importance of controlling the nanoscale features
of methanol synthesis catalysts.
2.1.2. Quantification of Cu Surface Areas. The normal-
ization of reactivity based on the Cu surface area has been widely
applied for Cu-based catalysts, as it allows a direct comparison of
activity in the form of turnover frequency (TOF) among
different catalysts. The mostly used approach to quantify the
surface Cu sites is N2O titration. However, overoxidation of Cu0
to Cu+
may occur, leading to inaccuracy of quantification. Other
factors that might affect the accuracy include temperature70,71
and Cu-support interaction.72
In early studies, the amount of N2 formed via N2O
decomposition has been determined by measuring the heat of
adsorption in a calorimeter in a static flow system.73,74
Osinga et
al. have performed N2O titration on supported Cu catalysts and
concluded that a suitable range of decomposition temperature
should not be higher than 373 K.74
This temperature range has
been further optimized by Scholten et al. in a vacuum
recirculation system.75
They have found that, due to the active
nature of N2O adsorptive decomposition, the heat of reaction
during adsorption may cause a subtle increase in temperature,
leading to an overestimated Cu surface area. Such fluctuation in
coverage further causes poor reducibility when the decom-
position occurs at lower temperatures such as 293−323 K.
Therefore, 363−373 K is suggested for N2O decomposition.
Moreover, this temperature range enables a full surface overage,
which sets a relatively higher kinetic barrier (ca. 20 kcal/mol) to
be overcome for bulk oxidation.75
Chinchen et al. have introduced the reactive frontal
chromatography (N2O RFC) to determine the specific Cu
surface area in an isothermal flow experiment, which has been
widely used since then.76
This method is characteristic of a flow
switching technique that enables the flow switch from He to a
mixture of N2O/He; the evolved N2 before the breakthrough of
N2O in He exhibits an equivalent amount as the number of
oxygen atoms chemisorbed on the Cu surface.
Luys et al. have confirmed that the subsurface oxidation is
accompanied by the surface oxidation for supported Cu catalysts
(Cu/SiO2 and Cu supported on magnesium silicate) during
N2O adsorptive decomposition.70
This oxidation of subsurface
layers is temperature-dependent, but the rate is slower than the
surface oxidation. A correction method has been proposed, in
which a back-extrapolation of the line representing subsurface
oxidation is applied in kinetic measurements. The intercept
should represent the extent of monolayer. Recently, Jensen et al.
have presented an improved approach, in which the extent of
surface and bulk oxidation can be differentiated.71
This is
achieved by a continuous measurement of evolved N2 from
adsorptive decomposition. Specifically, a continuous flow
experiment has be carried out: (1) He and 2%N2O/He are
used; (2) the unreacted N2O is trapped with liquid nitrogen, but
evolved N2 is not and measured continuously by thermal
conductivity detector (TCD). For all tested Cu/ZnO/Al2O3
catalysts with different surface areas, N2 evolution profiles
feature a significant peak, followed by a long tail, representing
the formation of N2 due to a fast surface oxidation and a
diffusion-limited, slow bulk oxidation, respectively. The total
amount of oxygen consumed by metallic Cu thus includes the
initial uptake from the surface reaction and the subsurface
diffusion.71
Pulse injection of N2O has also been explored using the
chromatographic technique for supported Cu catalysts and
proven effective in controlling the overoxidation caused by the
Cu-support interaction.72
Giamello et al. have found that the
heat of interaction of N2O with Cu is independent of coverage or
Cu loading level.77
Therefore, the microcalorimetry, which
evaluates the heat released during adsorptive decomposition, is
introduced to enable a more precise estimate of the amount of
reacted N2O.77
Although being widely used, N2O titration may result in
significant, irreversible changes in the catalyst structure,
negatively impacting the determination of Cu surface areas.78
In this case, H2-TPD provides an alternative.79
Muhler et al.
have measured the Cu surface sites over the commercial Cu/
ZnO/Al2O3 catalyst by means of H2-TPD and recommended
two key factors for an accurate and reproducible determination:
(1) lowering the temperature to 300 K before H2 exposure is
important; (2) maintaining the sample at ca. 250 K for 1−2 h
before cooling the temperature to liquid N2 temperature.78
Hinrichsen et al. have conducted a comparative study by
evaluating the Cu surface areas measured by different methods
such as chemisorption/RFC (continuous flow and pulse) and
H2-TPD.80
A mild measuring temperature is imperative to
achieve comparable, meaningful Cu surface areas measured for
different supported Cu catalysts.
2.1.3. Catalytic Structure−Activity Relationship. CO2
hydrogenation to methanol over Cu catalysts is generally known
as a structure-sensitive reaction,9
in which the catalytic
properties are closely associated with (i) metal dispersion and
surface Cu metallic area, (ii) dimension, composition, and
electronic properties of the Cu−ZnO interface, and (iii)
capability of adsorption of reagents and mass transfer. Generally,
these factors are tunable by means of promoter effect, support
effect, preparation methods, and the incorporation of core-shell
structure and hydrotalcite-like compounds. Significant efforts
have been devoted to developing an understanding of the
catalyst composition−structure−activity relationship, as sum-
marized in this section.
2.1.3.1. Promoter/Modifier-Mediated Heterogeneous Cu
Catalysts. The type of promoters/modifiers is diverse, and the
commonly used metals are alkali and alkaline-earth metals,81
rare-earth metals,82
transition metals,83
and main group
metals.82,84
Their functionalities for CH3OH synthesis include
(i) improving Cu dispersion and surface area, (ii) adjusting the
adsorption properties and the surface H/C ratios, (iii) tuning
interaction between Cu and metal oxide for H2 spillover, and
(iv) tailoring the support materials with desired single-metal
sites at the periphery of Cu NPs. Some nonmetal materials such
as graphene oxide and C3N4 are also promising as modifiers.85
Metal-Modified Cu Catalysts. Alkali and alkaline earth
metals are widely used as promoters due to their basicity.
F

Table 2. Summary of Reaction Conditions with Conversion, Space-Time-Yield (STY), and Selectivity to CH3OH for Selected
Catalysts
flow CH3OH formation
catalyst CO2/H2 ratio GHSV/h−1
W/F/g-cat h mol−1
press/
MPa temp/K
CO2
conv/%
STY/
mol kg-cat−1
h−1
selec/
C-mol % ref
Cu/Al2O3 1:3.8 4000 − 10.0 473 2.4 1.05 46.2 81
Cu-Ba/Al2O3 1:3.8 4000 − 10.0 473 3.6 0.14 4.2 81
Cu-K/Al2O3 1:3.8 4000 − 10.0 473 2.8 1.62 62.2 81
CuZnZr 1:3 − 10 3.0 503 19.6 2.3 44.4 82
CuZnZrLa 1:3 − 10 3.0 503 20.5 2.7 49.8 82
CuZnZrCe 1:3 − 10 3.0 503 22.8 3.2 53.0 82
CuZnZrNd 1:3 − 10 3.0 503 19.0 2.0 40.5 82
CuZnZrPr 1:3 − 10 3.0 503 19.3 2.2 42.0 82
Cu/AlCeO 1:3 − 1.56 3.0 533 ca. 17 11.9 ca. 45 86
CuNi2/CeO2-NT 1:3 6000 − 3.0 533 17.8 18.1 78.8 87
Cu/Ti@SiO2 1:3 − − 2.5 503 <10 64800a
85.0 88
Cu/Zr@SiO2 1:3 − − 2.5 503 <10 38880a
73.0 88
Cu/SiO2 1:3 − − 2.5 503 <10 12960a
49.0 88
Cu1La0.2/SBA-15 1:3 − 1.87 3.0 513 5.7 5.96 81.2 89
CuZn 1:3 − 9.3 3.0 513 16.1 1.39 36.5 90
CuZnTi 1:3 − 9.3 3.0 513 16.4 1.51 38.8 90
CuZnZr 1:3 − 9.3 3.0 513 17.0 1.65 41.5 90
CuZnTi-Zr 1:3 − 9.3 3.0 513 17.4 1.79 43.8 90
CuO-ZnO-ZrO2 1:3 − − 3.0 513 18.2 1.91 41.6 91
CuO-ZnO-ZrO2-Cr2O3 1:3 − 9.33 3.0 513 18.1 1.82 40.0 91
CuO-ZnO-ZrO2-MoO3 1:3 − 9.33 3.0 513 19.0 2.23 46.7 91
CuO-ZnO-ZrO2-WO3 1:3 − 9.33 3.0 513 19.4 2.33 47.8 91
Cu/ZrO2 1:3 − 2.68 1.0 503 ca. 4.2 ca. 1.24 39.0 92
(5 wt %)Ag/Cu/ZrO2 1:3 − 2.68 1.0 503 ca. 1.4 ca. 0.69 65.0 92
Cu/5 wt % g-C3N4-ZnO/
Al2O3
1:3 − 3.3 1.2 523 − ca. 1.53 38.8 85
CuZnZr 1:3 − 2.3 5.0 513 22.4 14.13 64.0 93
CuZnZr 1:3 − 0.4 5.0 513 9.7 37.05 62.0 93
CuZnCe/TNTs 1:3 − 2.99 3.0 533 23.3 9.33 59.8 94
Cu/ZrO2 (III) 1:3 3600 − 8.0 533 15.0 ca. 6.45 86.0 95
Cu/ZrO2 (IV) 1:3 3600 − 8.0 533 8.6 ca. 4.81 92.0 95
Cu/HAl 1:3 − 3.11 3.0 513 5.6 ca. 1.88 41.7 96
Cu/UAl 1:3 − 3.11 3.0 513 6.4 ca. 1.84 35.7 96
CuZnMn/SBA-15 1:3 − 0.19 4.0 453 5.7 69.8 >99 97
CuZnMn/MCF 1:3 − 0.19 4.0 453 3.9 47.9 >99 97
CuZnMn/KIT-6 1:3 − 0.19 4.0 453 8.2 105.3 >99 97
CuZnSBA-15 1:3 6600 0.51 3.0 523 8.9 4.94 27.7 98
CuZnZrSBA-15 1:3 6600 0.51 3.0 523 19.2 11.75 30.6 98
Cu/ZrO2/CNT-3 1:3 3600 − 3.0 533 16.3 2.62 43.5 99
CuZn/NrGOae-U 1:3 2444 1.40 1.5 523 24.16 12.67 − 100
Cu@m-SiO2 1:3 − 3.73 5.0 523 10.2 1.77 26.5 101
Cu/ZnO@m-SiO2 1:3 − 3.73 5.0 523 9.8 4.27 66.6 101
1:3 − 3.73 5.0 543 11.9 4.81 61.8 101
ACE-calcined at 623 K 1:3 − 2.68 1.0 543 ca. 4.0 1.8 59 102
NIT-350 1:3 − 2.68 1.0 543 ca. 2.4 ca. 1.1 63 102
AMM-350 1:3 − 2.68 1.0 543 ca. 2.1 ca. 0.9 60 102
RHT-9 1:3 4000 − 5.0 463 10.7 2.72 81.8 103
CuZnAl@HT(40%) 1:3 − 8.62 3.0 523 6.16 ca. 1.34 74.7 104
CuZnZr@HT 1:3 − 5.60 ca. 3.0 523 ca. 5.3 ca. 2.02 ca. 83 105
1:3 − 5.60 ca. 2.5 523 ca. 4.6 ca. 1.63 ca. 78 105
1:3 − 5.60 ca. 0.5 523 ca. 4.1 ca. 1.31 ca. 72 105
CHT-A 1:3 − 1.87 5.0 523 19.7 10.63 39.7 106
CHT-AMn 1:3 − 1.87 5.0 523 22.3 13.13 43.0 106
CHT-ALa 1:3 − 1.87 5.0 523 23.3 13.75 43.8 106
CHT-ACe 1:3 − 1.87 5.0 523 23.6 14.06 45.9 106
CHT-AZr 1:3 − 1.87 5.0 523 24.7 15.31 48.0 106
CHT-AY 1:3 − 1.87 5.0 523 26.9 16.25 47.1 106
CHT-Y0.05 1:3 − 2.24 5.0 503 17.4 10.31 63.2 107
G

Table 2. continued
W/F/g-cat h mol−1
press/
MPa temp/K
CO2
conv/%
STY/
mol kg-cat−1
h−1
selec/
C-mol % ref
CHT-Y0.1 1:3 − 2.24 5.0 503 20.2 12.19 69.3 107
CHT-Y0.2 1:3 − 2.24 5.0 503 17.8 10.94 70.5 107
CHT-Y0.5 1:3 − 2.24 5.0 503 15.1 9.06 66.6 107
CHT0.24-F 1:3 4000 − 5.0 523 21.1 13.75 53.5 108
CHT-F0.83 1:3 − 2.64 5.0 503 18.0 11.88 68.4 109
CuZnAl-4 1:3 − 14.93 4.0 493 15.0 1.46 58.9 110
1:3 − 14.93 4.0 513 18.3 1.71 56.5 110
LDH30Ga(Cu, 33.5 wt %) 1:3 − 1.24 4.5 543 ca.19 18.44 ca. 48 111
aCuZnZr-LDH 1:3 2000 − 3.0 523 4.9 1.14 78.3 112
Ni(OH)2 1:3 − − 3.2 423 − 141.4 − 113
CoMn LDHs 1:3 − − 3.2 423 − 176.1 − 113
NiTi LDHs 1:3 − − 3.2 423 − 282.6 − 113
NiCo LDHs 1:3 − − 3.2 423 − 335.7 − 113
Cu/cylindrical ZrO2 1:3 − 1.85 3.0 513 17.8 15.63 64.7 114
CuZnAlZr-FA-650 1:3 4000 − 3.0 523 25.88 7.24 49.17 115
CuZnAlZr-USP 1:3 10000 − 3.0 503 22.5 − 22.6 116
RE-CuZnO/SiO2 1:3 — − 3.0 523 11.4 − 35.5 117
30CuZn-ZpH 1:3.89 10000 − 5.0 553 22.2 10.81 34 118
30CuZn-ZM 1:3.89 10000 − 5.0 553 21.0 15.19 34 118
CuZnZr-TPABr 1:3 − 7.47 5.0 523 11.4 ca. 3.26 92.7 119
CuZnZr 1:3 − 7.47 5.0 523 26.7 ca. 4.54 55.2 119
S-Cu-Zn-Zr-600 1:3 3600 − 3.0 513 4.8 − 73.4 120
Cu-Zn-Zr-600 1:3 3600 − 3.0 513 8.1 − 38.6 120
CuZnZr-400 1:3 − 9.33 3.0 513 16.8 ca. 1.64 41.4 121
CuZnTiZr-SG 1:3 − 9.33 3.0 513 17.0 ca. 1.76 44.0 122
CuZnTiZr-SR 1:3 − 9.33 3.0 513 16.2 ca. 1.67 43.7 122
CuZnTiZr-SC 1:3 − 9.33 3.0 513 15.6 ca. 1.54 41.9 122
3DOM-CuZnZr(16) 1:3 3 − 3.0 493 18.9 9.29 80.2 123
CuZnAlZr-573 1:3 4600 − 5.0 503 17.3 − 62.1 124
1:3 4600 − 5.0 543 24.5 − 57.6 124
Cu(ZnGa) 1:3 3000 − 3.0 523 2.61 0.83 37.8 125
1:3 3000 − 3.0 533 3.75 1.21 38.1 125
(CuZnGa)microwave 1:3 3000 − 3.0 523 9.77 3.71 42.8 125
1:3 3000 − 3.0 533 12.7 4.20 36.5 125
CuZnZr-120 1:3 − 9.33 3.0 513 17.4 1.54 37.5 126
Cu/SiO2-AE 1:4 − 1.40 3.0 593 ca. 28 ca. 8.13 21.3 127
CuZnAl-C-1.00 1:3 3600 − 3.0 513 14.6 3.75 63.6 128
CuZnAl-O-1.00 1:3 3600 − 3.0 513 12.1 3.13 62.6 128
CuZnAl-U-1.00 1:3 3600 − 3.0 513 3.2 0.69 53.9 128
CuZnAl-400 1:3.03 − 25.6 4.0 513 59.5 ca. 4.09 73.4 129
CuZnAlZr-5Al-fixed-bed 1:3 − 5.6 5.0 523 25.2 6.55 60.6 130
CuZnAlZr-5Al-slurry bed 1:3 − 5.6 5.0 523 11.8 1.61 63.5 130
CuZnAlZr-fixed-bed 1:3 − 5.6 5.0 523 25.9 6.83 61.5 130
CuZnAlZr-slurry bed 1:3 − 5.6 5.0 523 8.5 1.11 61.1 130
30Cu/Zn/ms-SiO2 1:3 − 11.2 3.0 493 14.1 1.73 57.2 131
M-CZZ ca. 1:3.1
(H2O = 10%)
− 1.24 3.0 493 12.3 18.22 74.8 132
C-CZZ ca. 1:3.1
(H2O = 10%)
− 1.24 3.0 493 11.2 12.48 56.2 132
Pd/Ga2O3 1:3 − 1.24 5.0 523 19.6 ca. 20.28 51.5 133
Pd/Al2O3 1:3 − 1.24 5.0 523 3.4 ca. 1.99 29.9 133
Pd/Cr2O3 1:3 − 1.24 5.0 523 2.1 ca. 0.95 22.4 133
Pd/SiO2 1:3 − 1.24 5.0 523 0.05 ca. 0.10 100 133
Pd/TiO2 1:3 − 1.24 5.0 523 15.5 ca. 1.21 3.9 133
Pd/ZnO 1:3 − 1.24 5.0 523 13.8 ca. 10.40 37.5 133
Pd/ZrO2 1:3 − 1.24 5.0 523 0.4 ca. 0.03 4.3 133
Pd/CNTs-in 1:3 − − 2.0 523 0.77 0.048 48.8 134
Pd/CNTs-out 1:3 − − 2.0 523 0.61 0.011 13.4 134
Pd/SiO2 1:3 − − 2.0 523 0.33 0.013 31.6 134
H

Table 2. continued
W/F/g-cat h mol−1
press/
MPa temp/K
CO2
conv/%
STY/
mol kg-cat−1
h−1
selec/
C-mol % ref
Pd/AC 1:3 − − 2.0 523 0.60 0.027 34.6 134
Pd-Cu/SiO2 1:3 − 6.2 4.1 523 6.6 1.12 34.0 135
Pd-Cu/P25 1:3 − 6.2 4.1 523 16.4 1.80 25.7 136
Pd-Cu/CeO2 1:3 − 6.2 4.1 523 9.9 1.37 28.4 136
Pd-Cu/ZrO2 1:3 − 6.2 4.1 523 15.8 1.87 26.8 136
Pd-Cu/Al2O3 1:3 − 6.2 4.1 523 12.4 1.69 31.4 136
1%Pd/ZnO, SI 1:3 − 6.22 2.0 523 1.7 0.41 76 137
5%Pd/ZnO, SI 1:3 − 6.22 2.0 523 10.7 2.42 60 137
1%Pd/ZnO, IM 1:3 − 6.22 2.0 523 3.2 0.27 22 137
5%Pd/ZnO, IM 1:3 − 6.22 2.0 523 8.7 0.055 1 137
1.0PdZn 1:3 2400 9.3 2.0 493 14.07 5.18 97.2 138
5Pd5ZnZr 1:3 2400 9.3 3.0 503 5.7 ca. 1.41 100 139
0.5Ca5Pd5ZnZr 1:3 2400 9.3 3.0 503 7.2 ca. 1.66 100 139
Ag@Pd-ZnO 1:3 − 2.33 4.5 543 ca. 18 ca. 8.75 ca. 46 140
1:3 − 2.33 4.5 503 ca. 16 ca. 7.50 ca. 62 140
Pd-ZnO 1:3 − 2.33 4.5 543 ca. 12 ca. 6.88 ca. 40 140
1:3 − 2.33 4.5 503 ca. 9 ca. 4.69 ca. 52 140
PdZn/ZnO-3.93Al 1:3 − 3.73 3.0 523 14.2 ca. 4.51 51.6 141
PdZn/ZnO 1:3 − 3.73 3.0 523 5.8 ca. 2.49 69.7 141
0.5Ca5Pd5ZnCeO2 1:3 − 9.33 3.0 493 7.7 ca. 2.06 100 142
5Pd5ZnCeO2 1:3 − 9.33 3.0 493 6.3 ca. 1.69 100 142
37.5PdCuZn/SiC 1:9 − 2.99 0.1 473 − 0.11 80.9 143
1%Pt4Co NWs/C 1:3 − − 3.2 423 − 81.4 − 144
1:3 − − 3.2 483 − 147.0 − 144
3%Pt4Co NWs/C 1:3 − − 3.2 423 − 239.5 − 144
Pt4Co NWs/Al2O3 1:3 − − 3.2 425 − 74.9 − 144
Pt4Co NWs/P25 1:3 − − 3.2 425 − 35.0 − 144
Pt4Co NWs/SiO2 1:3 − − 3.2 425 − 30.2 − 144
ZnO-ZrO2 1:3 − 0.93 2.0 573 3.4 7.75 87.0 33
1:3 − 0.93 5.0 593 10 ca. 23.04 ca. 86 33
CdZrOx 1:3 24000 − 2.0 573 5.4 − 80 145
GaZrOx 1:3 24000 − 2.0 573 2.4 − 75 145
In2O3/ZrO2 1:4 16000 1.1 5.0 573 5.2 9.22 99.8 34
1:4 16000 1.1 5.0 503 − ca. 1.30 100 34
In2O3 1:4 16000 1.1 5.0 573 − ca. 6.25 100 34
1:4 16000 1.1 5.0 503 − ca. 0.78 100 34
In0.25/ZrO2 1:4 24000 − 5.0 523 0.3 0.22 46.8 146
In0.5/ZrO2 1:4 24000 − 5.0 523 0.5 0.44 50.3 146
In2.5/ZrO2 1:4 24000 − 5.0 523 0.9 1.09 73.8 146
In5/ZrO2 1:4 24000 − 5.0 523 0.6 0.75 77.9 146
hexagonal-In2O3 1:3 − 1.04 4.0 598 4.4 6.25 67.6 147
Pd-P/In2O3 1:4 − 1.1 5.0 573 20 27.81 70 148
Pd-P/In2O3 1:4 − 1.1 5.0 498 ca. 3 6.01 ca. 95 148
Pd-I/In2O3 1:4 − 1.1 5.0 573 ca. 18 ca. 25.00 ca. 70 148
Pd-I/In2O3 1:4 − 1.1 5.0 498 ca. 2 2.66 ca. 92 148
In:Pd(2:1)/SiO2 1:4 − 0.36 5.0 573 − ca. 2.42 61 149
Pd-In2O3 CP 1:4 − 0.47 5.0 553 − 31.56 78 150
1:4 − 0.93 5.0 553 − 19.06 75 150
Pt/film/In2O3 1:3 − 4.67 0.1 303 37 11.09 62.6 151
Pd/In2O3/SBA-15 1:4 − 1.49 5.0 533 12.6 11 83.9 152
Cu0.25-In0.75-Zr0.5-O 1:3 − 1.24 2.5 523 ca. 1.5 ca. 2.38 ca. 80 153
CuIn-350 1:3 − 2.99 3.0 553 11.4 6.14 80.5 154
In@SiO2 1:3 − 2.99 3.0 553 4.3 2.56 89.0 155
CuIn@SiO2 1:3 − 2.99 3.0 553 12.5 6.55 78.2 155
1:3 − 1.12 3.0 553 9.8 13.7 78.1 155
CuIn/SiO2 1:3 − 2.99 3.0 553 7.7 4.22 81.8 155
1.5YIn2O3/ZrO2 1:4 − 0.43 4.0 573 7.6 13.13 69.0 156
3La10In/ZrO2 1:4 − 0.43 4.0 573 7.7 13.13 66.0 156
Ni5Ga3/SiO2 1:3 5310 − 0.1 483 − ca. 7.50 − 157
I

Bansode et al. have systematically investigated the promoter
effect on CO2 hydrogenation over K and Ba-promoted Cu/
Al2O3 catalysts.81
Both Ba and K significantly improve the
capacity of CO2 adsorption in comparison to the unpromoted
catalysts. However, Ba- and K-promoted catalysts exhibit a
distinct difference in product distribution (Table 2), wherein K
favors CH3OH synthesis (selectivity, 62.2 C-mol %), while Ba is
selective to CO via RWGS (selectivity, 95.8 C-mol %).
Characterization results reveal that K selectively covers the
Al2O3 surface, facilitating the reducibility of Cu oxide to benefit
CH3OH synthesis. Differently, Ba covers both Cu and Al2O3
surface sites, leading to the creation of active sites to stabilize
intermediate species for the RWGS pathway.
Ban et al. have examined the influence of various rare-earth
elements on the performance of the Cu/Zn/Zr catalyst for
CH3OH synthesis.82
La and Ce favor the production of
CH3OH; however, Nd and Pr-modified catalysts exhibit
relatively low activity, even lower than the unmodified Cu/
Zn/Zr catalyst (Table 2). The better catalytic performance of
La- and Ce-promoted catalysts is attributed to their stronger
interaction with the catalyst components, benefiting CH3OH
synthesis via H2 spillover. Monometallic Cu catalysts supported
on CeO2-doped Al2O3 are also prepared, namely Cu/AlCeO,
which exhibit enhanced CH3OH formation rate in comparison
to Cu/Al2O3 (Table 2).86,162
The incorporated CeO2 is
advantageous in controlling the growth of Cu crystallite size,
improving surface Cu+
proportion and surface basicity for CO2
adsorption, as well as lowering the apparent activation barriers
for CO2 activation and subsequent hydrogenation. Furthermore,
CeO2 can strengthen the binding between Cu and CO such that
CO is difficult to desorb from the surface, therefore inhibiting
both RWGS and methanol decomposition (CH3OH ↔ CO +
2H2) while promoting CH3OH synthesis selectively. Cu-Ni
alloys are known to be active for CH3OH synthesis from CO
hydrogenation, including SiO2-163
and Al2O3-supported Cu−Ni
catalysts.164
Cu−Ni/SiO2, prepared by the deposition-copreci-
pitation method, displays a promising CH3OH selectivity as
high as 99.2 C-mol % (CH3OH STY = 20.6 mol kg−1
h−1
, 548 K,
10 MPa) for CO hydrogenation.163
To adapt this bimetallic
catalyst into CO2 hydrogenation to CH3OH, modification is
needed to improve CO2 adsorption. Tan et al. have introduced
CeO2 nanotubes (NTs) into the preparation of supported Cu-
Ni catalysts.87
The regular and polycrystalline CeO2 NTs show a
strong interaction with the alloy through SMSI, resulting in
partial reduction of surface CeO2, Ce4+
→ Ce3+
and
consequently the generation of oxygen vacancies to facilitate
CO2 adsorption and activation. The optimal methanol
formation rate is obtained on the Cu-Ni/CeO2-NT catalyst
with the Ni/(Cu + Ni) atomic ratio = 2/3 (Table 2). CeO2 can
undergo both partial reduction in the presence of H2 and
reoxidation by oxygen from CO2 dissociation, and the effects of
these opposing steps are studied recently by Winter et al. on
CeO2-supported Ni catalysts.165
An oxygen exchange occurs
between gas-phase species and the ceria support beyond the
surface layer during CO2 hydrogenation, and its rate is faster
than CO2 hydrogenation rate.
Transition metal oxides with amphoteric properties, such as
TiO2 and ZrO2, are popular candidates as both support and
additive. The acid sites can foster CO2 adsorption, while the
basic sites facilitate the hydrogenation of intermediates.83
TiO2
is one of the most studied modifier to decorate the catalyst
surface and tune metal-support interaction. This is because (i)
TiIV
is a Lewis acid, (ii) the reducibility of TiO2 can result in the
formation of oxygen vacancies accompanied with partial
reduction of TiIV
to TiIII
, and (iii) partial coverage of Cu surface
atoms at the periphery of TiOx can lead to SMSI. Noh et al. have
prepared small Cu NP with narrow size distributions supported
on SiO2, which is decorated with isolated TiIV
sites through a
surface organometallic chemistry (SOMC) approach.88
The
resultant Cu/Ti@SiO2 outperforms the benchmark Cu/TiO2
catalysts (Table 2). The in situ 1
H−13
C HETCOR (hetero-
nuclear correlation) spectrum identifies the coexistence of
formate and methoxy during the reaction, and they appear to
correlate to the presence of both Lewis-acid isolated TiIV
sites
and Cu NPs on the catalysts. The same group continues to tailor
the silica support with isolated Zr4+
surface sites, on which Cu
NPs (ca. 3 nm) form.166
The as-prepared Cu/ZrO2/SiO2
catalyst shows comparable activity as Cu/ZrO2. La oxide is
also a candidate, and the effect entails assistance in forming basic
active sites or improving metal surface area.89
Chen et al. have
found that the interfacial area of Cu-LaOx of Cu catalysts
supported on rodlike La2O2CO3 is active for methanol synthesis
(110.2 mol molCu
−1
h−1
, selectivity, 92.5 C-mol %).167
However,
the hydrothermal reaction conditions are detrimental to the
catalyst structure, leading to decreased activity. A confined
growth strategy is proposed to address this issue, in which a
small amount of La oxides is dispersed onto the porous materials
surface, such as SBA-15.89
The Cu−LaOx interface is generated
through the interaction of highly dispersed Cu NPs with LaOx in
the SBA-15 wall. Combined with the improved CO2 adsorption
capacity and Cu dispersion, the stabilized Cu−LaOx interface
accounts for the enhanced methanol synthesis activity (Table
2).
Ce- and Ti-containing bimetallic oxides are promising
promoter candidates, as they modify Cu-Zn catalysts through
tuning the metal-support interaction and surface basicity.
Commonly used oxide combinations include Ti-Zr, Ce-Zr,
and Ce-Ti. Xiao et al. have investigated the effect of bimetallic
Table 2. continued
W/F/g-cat h mol−1
press/
MPa temp/K
CO2
conv/%
STY/
mol kg-cat−1
h−1
selec/
C-mol % ref
Ni5Ga3/SiO2−CP 1:9 3600 − 0.1 473 ca. 1.8 2.53 96.1 158
Ni5Ga3/SiO2/Al2O3/Al-
fiber
1:3 − 7.47 0.1 483 ca. 2.3 0.62 86.7 159
Pd1Ga10-(Pd/CNT-h) 1:3 − 1.24 5.0 523 16.3 17.34 95.7 160
Pd1Ga10-CNT-h 1:3 − 1.24 5.0 523 16.5 16.00 96.2 160
Pd1Ga10-CNT-p 1:3 − 1.24 5.0 523 13.7 13.38 96.6 160
PdZnAl 1:3 − ca. 1.49 3.0 523 0.6 0.55 60.0 161
PdMgGa 1:3 − ca. 1.49 3.0 523 1.0 0.63 47.0 161
a
The unit of the CH3OH formation rate was mol kg Cu−1
h−1
.
J

oxides TiO2-ZrO2 on the catalytic performance of Cu-ZnO
catalyst for CO2 hydrogenation to CH3OH.90
The incorpo-
ration of bimetallic oxides not only reduces the particle sizes of
both CuO and ZnO and enlarges the Cu surface area but also
improves the adsorption capacity toward CO2 and H2, leading to
enhanced CO2 conversion and CH3OH selectivity. Guo et al.
have observed enhanced methanol synthesis activity on
Attapulgite/Ce0.75Zr0.25O2 (ATP-CZO), which originates from
the synergy between well-dispersed Cu sites and strong basic
ZnO sites, as well as that between ATP-CZO composites and
ZnO-CZO surfaces on the catalyst surface.168
Similar promoting
effect is obtained on the CuZnCeTi mixed oxides catalysts,
which can be attributed to the improved CO2 uptake due to the
TiCe-induced strong basic sites.169
Saito et al. have developed metal oxides-mixed Cu/ZnO
catalysts, and the most active catalyst system is the multi-
component Cu/ZnO catalyst, consisting of Al2O3, ZrO2, Ga2O3,
and Cr2O3 as modifiers.133,170,171
The daily capacity in a bench
plant is 50 kg CH3OH for gas-phase methanol synthesis from
CO2/H2.172
Among the metal oxides, Al2O3 and ZrO2 are
suggested to improve the Cu surface area, while the presence of
Ga2O3 and Cr2O3 enables an increase in specific activity by
optimizing the surface Cu+
/Cu0
ratio. Recently, Guo and Mao
have examined the effect of metal oxides from VIB group such as
Cr2O3, MoO3, and WO3 on the activity performance over Cu-
Zn-Zr catalysts.91,173
As listed in Table 2, both MoO3- and WO3-
ptomoted catalysts exhibit better methanol formation rate and
selectivity than the benchmark Cu-Zn-Zr catalyst, while the
Cr2O3-mixed counterpart displays a decrease in these values.
MoO3 and WO3 have positive impacts on the physicochemical
properties including BET surface area, reducibility, surface Cu
area, and CO2 adsorption capacity (especially the adsorbed
species on strong basic sites). Furthermore, the presence of
MoO3 and WO3 can tune the surface ratio of Zn/Cu and fraction
of strong basic sites, enabling the manipulation of methanol
selectivity.
Noble metals Au and Ag are also beneficial for CH3OH
synthesis. Li et al. have observed the dependence of CH3OH
selectivity on Au loading level of Au-CuO/SBA-15 catalysts, and
the maximum CH3OH selectivity is obtained at Au = 2 wt %
(Table 2).174
The interaction between Au and CuO plays a
pivotal role in the promoting effect, as (i) an appropriate Au
loading amount can improve the thermal stability against
sintering and aggregation during the heat treatment, and (ii) the
hydrogen spillover at the Au/CuO interface can improve the
reducibility of CuO. Similarly, Tada et al. have employed Ag as a
promoter for Cu/ZnO2 catalysts.92
The increase of Ag loading
results in a monotonical increase of CH3OH selectivity from 39
(i.e., Cu/ZrO2) to 65 C-mol % (i.e., 5 wt % Ag/Cu/ZrO2)
(Table 2). In this case, the interaction between Cu and Ag
becomes stronger in the form of Ag−Cu alloy, responsible for
the intrinsic activity in comparison to unpromoted Cu/ZrO2.175
Moreover, Ag can reduce the activation energy for CO2
hydrogenation and retain the size of Cu particles.
Main group metal oxides have been explored as modifiers to
tailor the metal−metal oxide interface, improve the reducibility
of catalysts, and tune the adsorption capacity. Phongamwong et
al. have prepared a series of Cu-ZnO-ZrO2-SiO2 catalysts by
reverse coprecipitation of Cu, Zn, and Zr precursors with
dispersed colloidal silica NPs.84
SiO2 (1 wt %)-promoted
CuZnZr catalyst exhibits a significant enhancement in CH3OH
synthesis activity (26%) at 513 K and 2.0 MPa in comparison to
the SiO2-free catalyst. Such promotion originates from the SiO2-
induced dispersion of metal oxides components, resulting in an
increase of metallic Cu surface area. Moreover, the addition of
an appropriate amount of SiO2 adjusts the surface basicity and
microstructure of the resultant catalysts. Combined with the
improved interdispersion of mixed metal oxides, the CO2
adsorption capacity is significantly enhanced. Li et al. have
reported that the addition of a small amount of Ga3+
into the
Cu/ZnO catalyst via a pH-controlled coprecipitation method
enables the thermal reduction of ZnO support to Zn atoms
under hydrogen, thereby forming a Ga-containing spinel
structure, namely ZnGa2O4.176
In conjunction with excess
ZnO phase, this structure features electronic heterojunction,
benefiting the reduction of Zn2+
to Zn0
to form Cu-Zn alloy
particles. The catalytic performance is optimized at 5 mol %
loading of Ga at 603 K and 4.5 MPa. Both activity and selectivity
increase monotonically with the surface Zn0
/Cu ratio,
corroborating the key role of the Cu-Zn alloy in promoting
the CH3OH synthesis activity. DFT calculations indicate that
the alloying of Zn with Cu enables an increase in adsorption
strength toward intermediates such as HCO, H2CO, and H3CO,
which are proposed as key intermediates in CO2-to-CH3OH
conversion on Cu-based catalysts.176
Nonmetal-Modified Cu Catalysts. Witoon et al. have
introduced graphene oxide (GO) into the preparation of
CuO-ZnO-ZrO2 catalysts via a reverse coprecipitation meth-
od.177
An appropriate amount of GO addition enables a higher
STY of CH3OH in comparison to the GO-free counterpart,
which can be associated with the GO-induced enhancement in
CO2 and H2 adsorption capacities. However, an excessive
amount of GO results in a decrease in activity because of the
GO-induced isolation of metal oxide particles and the significant
increase of CuO crystallite size. The addition of GO can also
serve as a bridge between mixed metal oxides, through which the
H2 spillover is facilitated from the Cu surface to the carbon
species adsorbed on the isolated metal oxide particles.
Doping materials with semiconductor properties can improve
the catalytic performance by tuning the electronic property on
the catalyst surface.176,178,179
Among those reported, g-C3N4 is
advantageous because it is nontoxic, cheaper, and easier to
produce hybrid with ZnO.85
Deng et al. have replaced ZnO in
Cu/Zn/Al with a g-C3N4-ZnO hybrid and prepared a series of
catalysts with various g-C3N4 loading amounts.85
The catalyst
with 5 wt % of g-C3N4-ZnO loading exhibits the highest CH3OH
formation rate and selectivity (5.73 mmol g Cu
−1
h−1
and 38.8 C-
mol %, respectively), which represents an improvement by 5.1%
and 10.5%, respectively, in comparison to those of the
commercial Cu/ZnO/Al2O3 catalyst under the same conditions.
ZnO becomes more electron-rich because of the formation of
type-II staggered gap heterojunction between g-C3N4 and ZnO,
resulting in a stronger interaction between ZnO and Cu and
improved catalytic activity.
2.1.3.2. Support Effect. Among support materials, metal
oxides are widely used, such as Al2O3, ZrO2, CeO2, and SiO2.
The selection of suitable support materials is typically based on
the following aspects including (i) configuration, (ii) capability
of modulating the electronic and structural interaction, (iii)
tunable surface basicity and acidity to affect adsorption and
activation of CO2 and H2, and (iv) tailorable textural property to
facilitate mass transfer. Explorations are also extended to other
support materials such as layered double hydroxides (LDHs),
carbon nanotubes (CNTs), and single-walled tubular structured
silicate.
K

Metal Oxides. Arena et al. have systemically studied the
correlation between the oxide supports with catalytic properties
over Cu-ZnO catalysts.93,180
Al2O3, ZrO2, and CeO2, three
commonly used support materials, are chosen for the
comparative assessment. As listed in Table 2, the activity of
these catalysts decreases in the following order: CuZnZr >
CuZnAl > CuZnCe. Considering the largest surface area, the
biggest pore volume, and the smallest decay upon reduction,
ZrO2 is identified as the most effective textural promoter for Cu-
ZnO catalysts. The quantified metal surface area (MSA) displays
a close relationship with the catalyst surface area (SA), as
illustrated in Figure 6(a). As a key factor on the surface, the oxide
surface area (OSA) correlates to the SA as well, and both MSA
and OSA increase with the increase of SA (Figure 6(a)).
However, the relative proportion of OSA decreases with the
increase of SA, while that of MSA shows an inverted trend and
becomes dominant at higher SA. The MSA, OSA, and SA-
normalized CO2 uptakes all reach the maximum values on the
CeO2-supported Cu-ZnO catalyst. This high CO2 adsorption
capacity originates from the peculiar ability to form surface and
bulk oxygen vacancies at the metal−oxide interface.181
As
depicted in Figure 6(b), the MSA-normalized rate decreases
with the increase of dispersion, while the OSA-normalized rate
barely changes and is independent of dispersion. This is
indicative of the important role of surface OSA in improving
CO2 adsorption capacity.
The structures of metal oxides may alter the metal−support
interaction and tune the Cu electronic state. In addition, tuning
surface oxygen percentage or oxygen vacancies is promising in
altering CO2 adsorption capacity and activation. The activity of
Cu/CeO2 catalysts strongly depends on the nanostructures of
CeO2, in which nanorod CeO2-supported catalyst exhibits a
better CH3OH selectivity than the nanocube- and nanoparticle
CeO2-supported counterparts (Table 2).182
The nanorods
CeO2 with the dominant exposure of (100) and (110) faces
show the strongest interaction with CuO and the highest CuO
dispersion, leading to the highest intrinsic activity. Similarly,
TiO2 is a suitable support material for methanol synthesis
because it has multiple adsorption sites including oxygen
vacancies and different types of undercoordinated Ti and O
atoms.183
To tune these aspects and control the Cu particle size,
Ferrah et al. have prepared Cu/TiO2 catalysts by two chemical
deposition steps.184
The first step involves the deposition of
nanosized (ca. 5 nm) spherical Cu(OH)2 NPs onto highly
oriented pyrolytic graphite (HOPG) by adding HOPG
substrates into a Cu-containing colloidal solution. Physical
vapor deposition is the following step, in which photocatalytic
reduction of [Cu(H2O)6]2+
takes place on a high density of TiO2
NPs grown on HOPG. The as-prepared TiO2 NPs enables
tuning of surface oxygen percentage for CO2 adsorption and
activation, which leads to a high selectivity toward methanol
synthesis. Similar promoting effect can be achieved by
incorporating TiO2 nanotubes (TNTs) as support for CuZnCe
catalysts (Table 2).94
Samson et al. have prepared Cu/ZrO2 catalysts by
impregnation of ZrO2 and complexation with citric acid.95
The polymorphic phases of ZrO2, including tetragonal and
monoclinic phases (t-ZrO2 and m-ZrO2, respectively), are
altered by changing the calcination temperatures, adding the
citric acid, and tuning the acidity and basicity of solution and
precursors. As listed in Table 2, the t-ZrO2-rich (i.e., 71%, Cu/
ZrO2 (III)) and phase-pure t-ZrO2-supported catalysts (i.e.,
100%, Cu/ZrO2(IV)) exhibit a high CH3OH selectivity at 8
MPa and 533 K. The oxygen vacancies on ZrO2 play a crucial
role in the observed activity performance by facilitating the
stabilization of thermodynamically unstable t-ZrO2 with Cu+
cations in the vicinity. In another work, amorphous ZrO2 (a-
ZrO2) is employed to compare with m-ZrO2 and t-ZrO2.185
A
unique inward diffusion of Cu into ZrO2 occurs only in the case
of a-ZrO2, resulting in the formation of Cu−Zr mixed oxides,
CuaZr1−aOb, after calcination. These mixed oxides are conducive
to the formation of Cu NPs on a-ZrO2, resulting in promising
CH3OH synthesis activity.
Metal oxides with desired pore structures may improve
catalytic performance by confining the growth of the active
metals and improving the mass transfer of reagent molecules in
the pores. Witoon et al. have synthesized the hierarchical meso-
macroporous alumina (HAl) as support material. Comparative
studies show that the HAl-supported Cu catalysts outperforms
the benchmark, unimodal mesoporous alumina (UAl)-
supported counterpart (Table 2).96,186
Mass transfer studies
indicate that the Cu/UAl catalyst possesses a longer residence
time for methanol molecules inside the catalyst pellets than that
inside the pores of Cu/HAl, thus increasing the probability of
methanol decomposition. The meso-macropores can also
promote H2O diffusion out of the pore upon desorption,
alleviating the deactivation resulting from the oxidation of Cu0
particles in the presence of H2O. Koh et al. have investigated the
Figure 6. (a) Influence of surface area (SA) on the extent of metal surface area (MSA) and oxide surface area (OSA) and MSA/SA and OSA/SA ratios.
(b) Influence of metal dispersion on the specific rate of CO2 conversion referred to SA, MSA, and OSA at 473 K and 3.0 MPa. Reproduced with
permission from ref 93. Copyright 2013 Elsevier.
L

morphological effect of porous silica on catalytic performance
over Cu−Zn−Mn catalysts, including SBA-15, MCF, and KIT-
6.97
Among them, SBA-15 is a 2D mesoporous silica with long
parallel pore channels in hexagonal arrangement and micropores
in the wall; MCF is a 3D mesoporous silica consisting of large
uniform spherical cells; and KIT-6 features 3D mesopores with a
gyroid cubic structure. Mesopore plugging is evident for both
SBA-15- and MCF-supported catalysts (mesoporous plugging,
15.0 and 33.5%, respectively), especially for the latter, which can
be ascribed to the loss of mesopore volume of long channels and
the small windows that connected the spherical cells,
respectively. However, the KIT-6-supported catalyst appears
to have less pore slugging because of its short channels.
Therefore, KIT-6 functions better in confining the growth of Cu
crystallites and retaining Cu surface area during reaction, leading
to better catalytic performance on CuZnMn/KIT-6 (Table 2).
Diffusion studies (based on Knudsen and bulk diffusion)
demonstrate that CnZnMn/KIT-6 features an efficient transfer
of CO2 molecules (effective diffusivity of CO2, 5.94 × 10−4
cm/
s) within the pore structure in comparison to MCF- (4.91 ×
10−4
cm/s) and SBA-15-supported counterpart (2.33 × 10−4
cm/s). The confinement of the active phase for SBA-15-
supported CuZnZr catalysts is also reported by Mureddu et al.98
Not only can the well-ordered mesoporous channels control the
particle size, dispersion, and morphology, but also they can
facilitate the interaction between active sites and H2 and CO2
inside the channels.
Nanotubes and Single-Walled Tubular Structured Materi-
als. Carbon nanotubes (CNTs) are potential candidates as
support materials because (i) carbon surface chemistry enables
the confinement of particle size and adjustment of metal-support
interactions; (ii) the hydrophobicity of carbon surface can
facilitate H2O desorption, preventing sintering and shifting the
equilibrium to favor CH3OH synthesis; (iii) carbon surface with
functionalized basic groups are beneficial to CO2 adsorption;
and (iv) highly dispersed catalyst can facilitate H2 adsorption.
Wang et al. have reported using nitrogen-functionalized
multiwalled carbon nanotubes (N-CNTs-3) as support for
Cu/ZrO2 catalyst.99
The resultant catalyst exhibits better
CH3OH synthesis activity in comparison to the benchmark
Cu/ZrO2 (Table 2). This improvement is attributed to the N-
containing functional groups on the CNTs surface, which can
improve the Cu oxide dispersion and reducibility, as well as H2
and CO2 adsorption capacity. XPS results reveal the presence of
three types of N-containing species on the carbon surface,
including pyrrolic, pyridinic, and graphic nitrogen.187
Among
them, pyridinic nitrogen-rich CNT (CNT-N) contributes to the
strong CO2 adsorption and creation of more active sites, which
appear to be associated with the maximum CH3OH yield.
Differently, pyrrolic nitrogen-rich CNT (CNT-NH2) presents
adsorption capacity toward moderate-bonded CO2 species,
which render the catalyst higher intrinsic activity (TOF) than
others. Similar roles of pyridinic-N on graphene aerogel has been
reported by Deerattrakul et al. (Table 2).100
In addition to CO2
adsorption and activation, the pyridinic-N species can improve
metal dispersion and associated H2 dissociation.
The single-walled tubular structure is characteristic of
maximizing the exposed surface and high metal content, making
it a desirable support for heterogeneous catalysts.188,189
Sheng et
al. have reported general methods for preparing transition metal-
doped Cu catalyst supported on tubular silicate, including
laboratory-scale ion exchange, scale-up, and one-pot synthesis of
a series of single-walled silicate nanotubes consisting of 3d
transition metals such as Mn, Fe, Co, Ni, and Zn.190
Among
them, the Zn-doped CuSiNT (i.e., Cu silicate nanotubes) is
tested for CO2 hydrogenation to CH3OH. The Zn34-CuSiNT
catalyst exhibits a higher CO2 conversion rate than the undoped
CuSiNT by 33% with a higher CH3OH selectivity of ca. 27 C-
mol % in comparison to ca. 22 C-mol % of undoped CuSiNT.
This catalyst even outperforms the commercial CuZnAl catalyst
in terms of CO2 conversion rate by 15%. This is attributed to the
reduced Cu particle size as a result of SMSI with the tubular
silicate support materials. Further doping Ni into Zn30-
CuSiNT, namely Ni4-Zr30-CuSiNT, improves the CH3OH
selectivity to ca. 32.5 C-mol %. It is speculated that the Ni
dopant is concentrated on the surface of Cu forming a dense
oxide layer, preserving the surface Cu species from oxidation
during the reaction.
2.1.3.3. Core-Shell Structure. The core-shell structure
catalysts have been explored for CO2 hydrogenation to
CH3OH, as a major advantage is to provide more Cu-ZnO
interfacial sites which are proposed as active sites responsible for
CH3OH synthesis. The synthetic strategy is to take advantage of
the memory effect (chemical memory) of the carbonate salt after
the catalyst reduction and the selection of precursor.191
Tisseraud et al. have studied the layered hydroxide salts
(LHS) hydroxynitrates as precursor to prepare Cu-ZnO
catalysts, as the layered packing of 2D assemblies is conducive
to achieving large specific surface areas.191
As the Cu content
increases in the precursor, more Zn content migrates through
lixiviation. The Cu surface area follows the same trend. When Zn
is fully migrated, a core-shell structure is formed consisting of Cu
core and CuxZn1−xOy mixed oxide shell. In this structure, the Cu
surface area is optimized, so are the interfacial areas between Cu
and ZnO, leading to enhanced CH3OH synthesis activity. The
core-shell structure is also capable of preventing the
encapsulated core metal NPs from sintering. Yang et al. have
prepared core-shell structured Cu@m-SiO2 and Cu/ZnO@m-
SiO2 catalysts by coating Cu and Cu/ZnO NPs with
mesoporous silica shells.101
The trapped Cu (56.6 g kg cat−1
h−1
) and Cu/ZnO (136.6 g kg cat−1
h−1
) NPs exhibit higher
CH3OH formation rates than the one without (9.8 g kg cat−1
h−1
) at 523 K and 5 MPa (Table 2). Furthermore, the core-shell
structure endows the catalysts with enhanced stability because
the confinement effect of the outer shell prevents the catalyst
from agglomeration.
2.1.3.4. Effect of Hydrotalcite-Like Compounds (HTIcs) as
Catalyst Precursor. The common Cu precursor used to prepare
catalysts via the impregnation method include Cu nitrate (NIT),
Cu acetate (ACE), and Cu ammine complex (AMM).102
Among them, Cu-acetate-based catalyst is reported to be more
active and selective toward methanol synthesis (Table 2) due to
the presence of the surface-dispersed Cu2+
specie after
calcination. The subsequent H2 reduction results in the
formation of well-dispersed Cu0
NPs. Of note, this preparation
method, though widely used, limits the metal loading level.
The hydrotalcite-like compounds (HTIcs) ,
[M2+
1−xM3+x
(OH)2]x+
(An−
)x/n·mH2O (M2+
and M3+
are
divalent and trivalent metal cations, respectively), are one of
the promising candidates as a catalyst precursor because the
HTIcs-based process leads to uniform dispersion of metal
cations. The resultant catalysts feature high stability against
sintering, high specific surface area, and stronger basic
properties.103,192,193
Xiao et al. have studied the effect of
precursor phases during coprecipitation on the methanol
synthesis activity on Cu-Zn-Al-Zr catalysts.103
The evolution
M

of hydrotalcite-like phase and zinc malachite (zM) phase is
identified and found to be dependent on pH values during
coprecipitation. The catalyst, prepared at pH = 9.0 (RHT-9),
possesses smaller Cu particle size and stronger Cu-ZnO
interaction, leading to a high methanol selectivity (81.8 C-mol
%) at low temperature 463 K (Table 2). A high methanol
selectivity is also obtained by Fang et al. on Cu-Zn-Al catalysts
physically mixed with hydrotalcite (Table 2).104
Kim et al. have
investigated the effect of precursors on CH3OH synthesis
activity over Cu/Zn/Al/Zr quaternary catalysts, particularly
focusing on various Al/Zr ratios.194
A coprecipitation-based
synthetic strategy is proposed to adjust metal compositions to
optimize activity for Al-rich and Zr-rich catalysts, as illustrated in
Figure 7. The formation of hydrotalcite phase (HTI,
Cu3Zn3Al2(OH)16CO3) originates from well-mixed Cu2+
and
Zn2+
ions with Al3+
ions at the coprecipitation and aging stages.
The addition of Zr4+
ions can interfere with HTI formation, even
under Al-rich conditions. However, the presence of an excessive
amount of the HTI phase should be avoided due to its negative
impact on CH3OH formation.194
The optimal scenario would
be adjusting the relative concentration of Zn or Al in preparing
the precursor to obtain the desired zM with smaller Cu/Zn
particles, accompanied with the joint presence of an appropriate
amount of HT phase. In addition to the assistance in catalyst
dispersion, HTIcs-based materials also enable enhanced local
concentration of CO2.105
This endows HTI-supported Cu-Zn-
Zr catalysts with comparable CH3OH synthesis activity at
moderate pressures (0.55−2.45 MPa), saving energy con-
sumption for compression (Table 2).105
Modifiers have been employed into the preparation of HTIcs-
derived Cu/Zn/Al catalysts for comparative studies (CHT-
Metal), including Mn, Zr, and rare-earth metals La, Ce, and Y.106
As listed in Table 2, the CH3OH selectivity increases in the
following order: Cu/Zn/Al < Cu/Zn/Al/Mn < Cu/Zn/Al/La <
Cu/Zn/Al/Ce < Cu/Zn/Al/Zr < Cu/Zn/Al/Y, which exhibits
a linear relationship with the proportion of the surface basic
sites. A linear relationship is also observed between CO2
conversion and Cu surface area. These observations demon-
strate that the modifier can tune Cu surface area and surface
basicity of the catalyst. For the best-performance catalyst Cu/
Zn/Al/Y, the activity appears to correlate with the loading
amount of Y and maximizes at Y3+
/(Y3+
+ Al3+
) = 0.1 (Table
2).107
This is because an appropriate amount of Y can prevent
the aggregation of Cu NPs. However, an excess loading
decreases Cu surface area and weakens the interaction between
Cu and ZnO, detrimental to CH3OH synthesis activity.
The acid−base property of HTIcs is tunable by the
incorporation of various anions in the interlayer.192
Gao et al.
have introduced fluorine into the Cu/Zn/Al/Zr catalysts by
using HTIcs as the precursor.108
The HTIcs-derived catalyst
with optimal F content (CHT-0.24F) shows a significant
enhancement in CH3OH synthesis activity (Table 2), which is
associated with the strengthened surface basicity by incorporat-
ing fluorine anions. The same group continues to innovate the
synthesis strategy by substituting (Al(OH)6)3−
octahedra with
(AlF6)3−
, which improves the synthesis efficiency.109
Through
this novel method, fluorine can be easily incorporated into the
interlayer of HTI structure with a wide range of content. The
optimal F content is obtained at F/Al = 0.83 with the maximum
methanol yield (Table 2).
2.1.3.5. Layered Double Hydroxides (LDHs) as Both
Catalyst Precursor and Support. LDHs are composed of
metal hydroxide and interlayers. The positively and negatively
charged components render them great potential as both
catalyst precursors and supports. Furthermore, the use of LDH
in catalysis is potentially advantageous in fabricating well-
dispersed NPs with enhanced synergetic effects between
components and improved stability against sintering, as well as
offering surface basic sites.195
Zhang et al. have explored the deposition-precipitation
method to prepare CuZnAl-LDH catalysts.110
The key issue is
to deposit Cu2+
and Zn2+
ions on microspherical Al2O3, which
can be achieved by decreasing the pH from moderately basic
values to neutral through the evaporation of ammonia from an
ammonia/carbonate buffer solution. The as-synthesized catalyst
CuZnAl-4 exhibits high CH3OH selectivity (i.e., 58.9 C-mol %,
Figure 7. Representation of the changes in the phase of the Cu/Zn/Al precursor with the Al composition increasing (bottom left), in the phase of the
Cu/Zn/Zr precursor with the Zr composition increasing (bottom right), in the phase of the Cu/Zn/Al/Zr precursor from the Cu/Zn/Al precursor by
the addition of Zr (top left), and in the phase of the Cu/Zn/Al/Zr precursor from the Cu/Zn/Zr precursor by the addition of Al (top right).
Reproduced with permission from ref 194. Copyright 2018 Elsevier.
N

Table 2) and good stability after 200 h on stream. Another
method to imbed Cu(Zn) metal atoms in the cationic layer of
LDH is to use ultrathin LDH nanosheets, prepared by aqueous
miscible organic solvent treatment method (AMOST), as a
catalyst precursor.111
Acetone is used as the AMO-solvent for
interlayer disruption, which can override the electrostatic
interaction, resulting in increased surface area. Meanwhile, a
critical Ga3+
composition is imperative to obtain consistently
and significantly higher Cu surface area and dispersion. The as-
prepared catalyst with the optimal composition, LDH30Ga,
shows both doubled Cu dispersion (46.0%) and Cu surface area
(99.2 m2
g cat−1
) in comparison to the Cu-Zn catalyst prepared
by the conventional method (21.8% and 43.0 m2
g cat−1
,
respectively), leading to a higher CH3OH formation rate (Table
2).
Fang et al. have incorporated commercialized Mg-Al LDHs
(Pural MG50, SASOL, Germany GmbH) as supports for
CuZnZr through LDH activation, coprecipitation, calcination,
and reduction.112
The LDH-supported CuZnZr catalysts
demonstrate not only a high CH3OH formation rate but also
a higher CH3OH selectivity (78.3 C-mol %) than the
benchmark LDH-free CuZnZr catalyst (Table 2). The enhanced
catalytic performance stems from the LDH-induced large
surface area and improved metal dispersion, as well as the
enhanced CO2 adsorption capacity on LDH-derived amorphous
oxide adjacent to active metal sites. Peng et al. have employed
layered double hydroxides (LDHs) and tested in CO2
hydrogenation to methanol under 423 K and 3.2 MPa.113
The
highest CH3OH formation rate is obtained on NiCo LDHs at
335.7 mol kg−1
h−1
(Table 2).
2.1.3.6. Development of Catalyst Preparation Method.
Precipitation Method. Impregnation method is an easy
operation to prepare Cu-Zn catalysts. However, it is not suitable
to prepare catalysts with higher metal loadings such as >10−
20%.196
The precipitation method provides solution to this
issue. In comparison to the impregnation method, it can help
form well-defined and crystalline precursor compounds and
ultimately yield supported and uniform metal NPs.197,198
Due to
its simplified procedure and economic viability in industry, there
are continuous interests in innovating this method, as illustrated
in Figure 8.
The precipitation method allows higher metal loadings, but it
comes with the challenge of how to control the growth of Cu
particle size and increase the Cu surface area. Dong et al. have
prepared Cu/ZnO/ZrO2 catalysts by using NaBH4 as a reducing
agent and studied the influence of precipitation-reduction
process on the average Cu particle size.199
The content of
NaBH4 enables the control over the exposed Cu surface area and
the Cu0
/Cu+
ratio. An appropriate content of NaBH4 can
enhance the surface basicity, improving CO2 adsorption
capacity. The best CH3OH selectivity is achieved at 66.8 C-
mol % at 503 K and 5.0 MPa for the catalyst with the atomic ratio
of B/Cu = 5. L-Ascorbic acid is also a candidate of reducing
agent, the utilization of which can increase the exposed Cu
surface area and the number of oxygen vacancies in ZnO (Table
2).114
Most recently, formaldehyde (FA) has been employed
Figure 8. Innovation of preparation methods and corresponding effects on activity performance.
O

into the precipitate slurries of CuZnAlZr catalysts as a weak
reducing agent.115
After calcination, the oxidation state of Cu
species of these FA preactivated catalysts is Cu+
in Cu2O, which
can improve the dispersion of Cu0
particles with a weak metal−
support interaction. In addition to the role as reducing agent, FA
can also promote the formation of more crystallized semi-
conductor ZnO phase after calcination in N2 at higher
temperatures. The synergy between these two factors leads to
enhanced catalytic performance (Table 2).
Ramli et al. have prepared CuZnAlZr catalysts via the
ultrasonic spray precipitation technique (USP) to obtain finer
and uniform Cu crystallites.116
Different components in the as-
prepared catalyst exhibit good interaction with one another. The
CuZnAlZr-USP catalyst (CO2 conversion = 22.5%, CH3OH
selectivity = 22.6 C-mol %, CH3OH yield = 0.26 g mL−1
h−1
)
outperforms the conventional precipitation-prepared bench-
mark catalyst (Table 2). Dasireddy et al. have reported that the
ultrasonic-assisted coprecipitation possesses the potential to
tune the surface basicity and metal dispersion, leading to
variations of methanol synthesis activity.200
Chen et al. have
developed a facile way to prepare well-dispersed CuZnO/SiO2
catalysts via a rotary evaporation-assisted deposition-precip-
itation method (RE-CuZnO/SiO2).117
This dynamic drying
process allows the metal salt solution to mix homogeneously and
to disperse on the surface of silica gel, resulting in a narrow Cu
particle distribution (ca. 4.9−9.1 nm) with the optimal catalytic
stability (Table 2).
Angelo et al. have employed microfluidic continuous
coprecipitation to prepare Cu-ZnO-ZrO2 catalysts.118
The
synthesis is completed in a microfluidic system by a continuous
formation of little droplets (e.g., diameter ∼1 mm), resulting
from the solution of nitrates and the precipitating agent in a
constant flux of silicon oil. This method can tune the Cu surface
area by varying several parameters during preparation, including
pH, temperature, molar ratio of carbonate/metal cations, speed
of flow, and droplet size. The as-prepared 30CuZn-ZM catalyst
yields markedly more CH3OH (15.19 mol kg cat−1
h−1
) than the
catalyst prepared by the conventional precipitation method
(10.81 mol kg-cat−1
h−1
) under similar reaction conditions
(Table 2).
To modify the surface structure, Chen et al. have introduced
vapor-phase-treatment to prepare CuZnZr catalysts by using
tetrapropylammonium bromide (TPABr) and H2O as treatment
reagents.119
As shown in Table 2, the TPABr-treated CuZnZr
catalyst significantly improves CH3OH selectivity, as high as
above 90 C-mol %. The post treatment results in the increase of
particle sizes (CuO, ZnO, and ZrO2), the formation of rodlike
structures, and the modification of the surface properties such as
enriching Zn, Zr, and oxygen vacancies on the surface. These
TPABr-induced variations account for the high CH3OH
selectivity with significant suppression of CO formation via
RWGS.
Efforts are also put forth in exploring efficient methods to tune
the metal/metal oxide interaction at the interface for optimized
synergetic effect. Li et al. have used cetyltrimethylammonium
bromide (CTAB) as the surfactant to prepare CuO-ZnO-ZrO2
catalysts via the coprecipitation method.120
The CTAB can not
only tune the metal/metal oxide interaction by forming more
Cu-ZnOx and/or Cu-ZrOx species but also promote the
aggregation and precipitation of precursor sol particles. The
resultant catalyst (S-Cu-Zn-Zr), calcined at 873 K, exhibits the
optimal CH3OH selectivity of 73.4 C-mol %, which is
considerably higher than the surfactant-free Cu-Zn-Zr-600
catalyst (e.g., 38.6 C-mol %, Table 2). Due to its higher
capability of removing heavy metal ions in wastewater, chitosan,
a linear polysaccharide, is used as a precipitating agent instead of
alkaline carbonate solution to prepare Cu-ZnO catalysts.201
During the preparation, chitosan can act as a coordination
Figure 9. Morphological and compositive characterizations of a 3DOM CZZ catalyst (CZZ(16)). (a) SEM image, indicating an average pore size of
120 nm. (b, c) TEM images, which allows the smallest ZnO particle size (15.8 nm) in the 3DOM catalyst series. (d) EDS of different points in (c). (e−
g) HRTEM images. (h) Structural diagrammatic sketch of the macroporous catalysts. Reproduced with permission from ref 123. Copyright 2019
Springer Nature.
P

compound to facilitate the homogeneous combination of CuO-
ZnO nanocomposite, as well as a soft template responsible for
the formation of hollow nanospheres. These features are tunable
and dependent on the concentration of chitosan during the
preparation. Complexing agents can also be used to improve
CuO dispersion and metallic surface area, and citric acid and
oxalic acid are common options.202
Samson et al. have reported
that the citric-acid-induced complexes, formed on t-ZrO2 and
built from Cu+
and oxygen vacancies, are the acid sites
responsible for CH3OH synthesis on Cu/ZrO2 catalysts.95
Similar results have been reported by Witoon et al.203
Another
citric acid-involved method is the polymeric precursor
method.121
The first step is to drop citric acid into metallic
cations for chelation, followed by using ethylene glycol to
promote the polymerization and consequent polyesterification.
The CuZnZr-400, calcined at 673 K, displays the optimal
CH3OH formation rate and conversion, which can be attributed
to the smaller crystallite size and large Cu surface area (Table 2).
Of note, choosing an appropriate temperature range for
calcination is of significance because the decomposition of citric
acid during calcination may release strong heat, resulting in
sintering of the catalyst components.202
Other Laboratory-Scale Methods. Relying on the well-
established catalyst composition−structure−activity relation-
ship, various catalyst preparation methods have also been
explored, such as the sol−gel method, liquid-reduction method,
microwave method, and ammonia-evaporation method. Sim-
ilarly, these methods target the manipulation of catalytic activity
by turning the Cu surface area, metal dispersion, particle size,
metal/metal oxide interaction at the interface, and metal−
support interaction (Figure 8).
Sol−gel method: Chen et al. have prepared CuO-ZnO-TiO2-
ZrO2 catalysts by sol−gel (SG), solid-state reaction (SR), and
solution-combustion (SC), among which the sol−gel-prepared
catalyst (Cu-Zn-Ti-Zr-SG) exhibits the highest CO2 conversion
(17.0%) and CH3OH selectivity (44.0 C-mol %) at 513 K and
3.0 MPa (Table 2).122
This method enables a larger metallic Cu
surface area and improved H2 adsorption capacity, leading to
better catalytic performance.
Colloidal crystal template method: Wang et al. have prepared
Cu-Zn-Zr (CZZ) catalysts by the colloidal crystal template
method, in which the uniformly monodispersed poly methyl
methacrylate (PMMA) spheres are synthesized by emulsifier-
free emulsion polymerization and introduced as template for
Cu-Zn-Zr catalysts.123
The as-synthesized catalysts feature
three-dimensional ordered macropores (3DOM) (Figure 9)
and tunable ZnO particle size via altering the ramp rate during
calcination. As shown in Table 2, CZZ(16), with the smallest
ZnO particle size, presents the highest CH3OH synthesis
activity (9.29 mol kg−1
h−1
) and selectivity (80.2 C-mol %) at
relatively lower temperature 493 K. Cu−ZnO or Cu−ZrO2
interface is responsible for dissociative H2 adsorption, while
CO2 adsorption, activation, and subsequent hydrogenation take
place at the ZnO−ZrO2 interface.
Liquid-reduction method: an alternative to form smaller Cu
particles with highly reduced states is the liquid reduction
method. Dong et al. have prepared CuZnAlZr catalysts using
NaBH4 as a reducing agent.124
The catalyst calcined at 573 K
Figure 10. Schematics of (a and b) two-nozzle flame spray pyrolysis (2-FSP) and (c) one-nozzle flame spray pyrolysis (1-FSP). The CuO clusters in
(a) are bigger compared to those in (b), since in (b) a higher O2 dispersion flow is used on the CuO side, cooling the CuO-producing-flame which leads
to a shorter residence time of the CuO clusters in the hot zone of the flame. STEM-EDS maps of (d) CuO/ZrO2-A and (e) CuO/ZrO2-B. Highlighted
areas are Cu-RICH ZONES FROM EDS mapping; particles not highlighted have been identified as ZrO2. (f) Methanol production rate for the three
types of catalysts Cu-ZrO2, CuO (FSP), and CuZnAl. Reaction conditions: CO2/H2/N2 = 1:3:1, 543 K, 2 MPa. Before the reaction test, all catalysts are
reduced at 573 K in 17% H2/N2 (60 mL min−1
) for 30 min under 0.1 MPa. Reproduced with permission from ref 205. Copyright 2018 Royal Society of
Chemistry.
Q

(CZAZ-573) displays the highest CO2 conversion and CH3OH
selectivity within the temperature range examined (Table 2), as
well as good stability. The enhanced catalytic performance is
associated with the highly exposed Cu surface area and a high
ratio of surface Cu+
/Cu0
, resulting from the combined effects of
liquid reduction method and the selection of appropriate
calcination temperature.
Microwave method: the microwave method is able to tune the
metal dispersion and the mean basic site strength.125
The
microwave-method-induced SMSI is effective in preventing Cu
from sintering and is advantageous in the intimate contact
between the highly dispersed Cu species and ZnO with strong
basic ZnO species.125
Cai et al. have prepared CuZnGa catalysts
using the microwave-assisted method and obtained a consid-
erably higher CH3OH STY than the same counterparts prepared
by incipient-wetness impregnation and coprecipitation methods
(Table 2)125
Similarly, Huang et al. have prepared CuO-ZnO-
ZrO2 catalyst by a facile microwave-assisted hydrothermal
synthesis method.126
The catalyst treated at 393 K shows CO2
conversion and CH3OH selectivity with 17.4% and 37.5 C-mol
%, respectively (Table 2).
Ammonia-evaporation method: Wang et al. have prepared
Cu/SiO2 catalysts by the ammonia-evaporation method in an
attempt to control the composition evolution during catalyst
preparation.127
With the increase of calcination temperature
from room temperature to 973 K, the composition evolution
undergoes amorphous Cu species → Cu phyllosilicate →
CuO.204
This method allows one to tune the Cu+
/(Cu+
+ Cu0
)
ratios on the surface and the metal−support interaction derived
from Cu phyllosilicate. The CH3OH selectivity (21.3 C-mol %)
of the as-prepared catalyst surpasses the equilibrium value (6.6
C-mol %) at 593 K and 3.0 MPa (Table 2).
Methods for Practical Applications. From a practical point
of view, the procedure for catalyst preparation must be simple,
scalable, repeatable, and economically viable. The physical
strength of catalysts should also be taken into consideration for
the use with different reactors other than the fixed-bed reactor,
such as the slurry-phase reactor. To bridge the lab-scale
achievements with practical viability, combustion and pyrolysis,
ball-milling process, and the spray drying method are examined
to prepare Cu-based catalysts (Figure 8).
Combustion and pyrolysis. Lei et al. have prepared CuZnAl
catalysts by direct combustion of metal precursors using citric
acid, oxalic acid, or urea as a fuel.128
Among all catalysts, the
citric acid-treated one exhibits the highest CuO dispersion and
surface Cu area and consequently better catalytic performance
(Table 2). Tada et al. have explored the flame spray pyrolysis (2-
FSP) approach for preparing Cu/ZrO2 catalysts, as this process
allows the scale-up production of complex nanoparticle
assemblies in industry (e.g., several kg per hour).205
As
illustrated in Figure 10(a,b), Cu/ZrO2-A and Cu/ZrO2-B are
prepared with a two-nozzle flame spray pyrolysis reactor but
with different O2 dispersion gas flow on the CuO side, namely 10
and 5 L min−1
, respectively. For comparison, the CuO cluster is
prepared using a one-nozzle 1-FSP reactor (Figure 10(c)). Cu/
ZrO2-B exhibits a smaller average size of CuO in comparison to
Cu/ZrO2-A (Figure 10(d,e)), while the ZrO2 size barely
changes, nor does the specific surface area. As presented in
Figure 10(f), the 2-FSP-prepared catalysts outperform the
commercial CuZnAl catalysts under the same reaction
conditions. By varying the Cu metal loadings, the amount of
active sites and interaction between Cu-ZrO2 are tunable,
leading to an optimized methanol yield and selectivity at
relatively high Cu loading levels (i.e., 60 wt %).206
In the FSP
method, the precursor feed rate is a determining factor to control
the crystallite size of ZrO2 for Cu/ZrO2 catalysts.207
Within the
optimal range of feed rate, ZrO2 with small crystalline size is
obtained, which is conducive to stabilizing small Cu particles at
the interface between Cu and ZrO2 for enhanced methanol
synthesis.
Ball-milling process: inspired by the mutual replacement of
cations between solid-phase materials during the mechanical-
force-driven ball-milling process, Wu et al. have prepared Cu/
Zn/Al catalysts via the same synthetic strategy.129
The as-
prepared catalyst (CuZnAl-400) exhibits comparable CO2
conversion and CH3OH selectivity as the commercial CuZnAl
catalyst under the same reaction conditions (Table 2). Such Cu/
Zn hydroxy carbonate precursors simplify the preparation
procedure and have potential to replace the traditional
coprecipitation method.
Spray drying method: different from a fixed-bed reactor, the
attrition resistance of catalysts is a determinant in affecting
performance in the case of a slurry-phase reactor. To improve
the physical strength of the catalysts, a spray drying method has
been used in the preparation of microspherical CuZnAlZr
catalysts.130
The Cu surface area and interaction between Cu
and ZnO can be tuned by altering the loading amount of alumina
sol. The as-prepared catalyst with 10 wt % alumina sol presents
prominent catalytic performance in both fixed-bed and slurry-
phase reactors, as well as good stability with ca. 180 h on stream
(Table 2). To take full advantage of the pore structure and the
availability of functional components, Jiang et al. have prepared
microspherical SiO2 support (ms-SiO2) with good attrition
resistance by the spray drying method.131
The as-prepared
support not only allows a high loading level of Cu and Zn but
also increases the metal surface area. The optimal CH3OH
formation rate is obtained at metal loading = 28.2 wt % (Table
2).
2.1.4. Reactor Design and Optimization. Membrane
Reactor. The roles of H2O in CO2 hydrogenation are in debate,
and both positive and negative impacts have been reported in
the literature, such as acting as catalyst poison, inhibitor,
promoter, and intermediate.208−210
For Cu-based catalysts, H2O
can cause catalyst deactivation via hydrothermal sintering, which
necessitates its removal from the reaction mixture, especially at
high CO2 conversion.208
Wu et al. have reported that an internal
cooling system helps condense the produced H2O and CH3OH
from the product gas mixture, leading to an improved catalytic
performance.129
Alternatively, a membrane reactor could satisfy
this requirement by selectively removing condensable products
and is also promising for shifting equilibrium limited reaction
toward desirable products.
The kinetic diameters of H2O, CH3OH, CO2, and H2
molecules are 0.30, 0.38, 0.33, and 0.29 nm, respectively,211
making it challenging to achieve the selective H2O permeation.
Other challenges include the thermal, chemical, and structural
stability of the membrane at relatively harsh reaction conditions
for CO2 hydrogenation to CH3OH (450−573 K and 3−5 MPa).
Polymeric materials are common materials for the membrane;
however, they cannot withstand harsh reaction condi-
tions.212,213
Zeolite-based membrane is a potential candidate
in this regard due to its thermal and chemical stability.214
Moreover, zeolites feature uniform, well-defined pores in
molecular size, high porosity, unique shape selectivity, and
adsorption property.
R

A Nafion membrane has been integrated into the membrane
reactor for CO2 hydrogenation to methanol by Struis et al.212,213
The reactor consists of a tubular membrane fitted into a
concentric tube shell, and Cu/Zn is used as the catalyst. The feed
gas is introduced into the inner shell of the membrane tube, and
the outer shell volume is swept by an inert gas stream to remove
permeating species.215
A counter flow is performed in the outer
shell volume, as it can maintain an optimal difference in partial
pressure of permeating species. The CH3OH yield in the
presence of the membrane is higher than that in the absence of
the membrane in the whole range of GHSV at milder reaction
conditions, 473 K and 0.43 MPa. However, the yield is not
comparable to the state-of-the-art catalyst. The poor stability of
polymer-based membrane restrains its implementation for
further improvement, and how to preserve the structure and
retain the functionality at relatively higher pressures is
challenging. Tran et al. have found that in the mixture of
reagents and products including CH3OH, H2O, N2, CO2, and
H2, the NaA membrane presents lower permeability but higher
selectivity toward condensable CH3OH and H2O than the MFI
membrane.216
The incorporation of the NaA membrane shows
higher methanol yield and selectivity at all reaction conditions
than the traditional reactor, even at pressurized conditions
(Table 3). Gallucci et al. have successfully demonstrated that the
integration of a zeolite membrane reactor can help achieve
higher conversion than the traditional reactor,217
and the
improved performance can be retained at 2 MPa (Table 3).218
A
similar enhancement of CO2 conversion has been reported by
Tavolaro et al., in which a LTA zeolite composite membrane
with hydrophilicity is incorporated into the membrane
reactor.219
At 483 K, CO2 conversion from the membrane-
assisted reactor reaches up to 17%, which surpasses the
equilibrium value of 6%.
The membrane reactor has been applied in an indirect route
to produce methanol, namely RWGS + CO hydrogenation.215
The membrane-assisted CAMERE (carbon dioxide hydro-
genation to methanol via RWGS) process exhibits a comparable
CH3OH formation rate as the CAMERE process, with an
increase of ca. 20% in comparison to the conventional reactor.
More importantly, the H2O production rate is decreased by 38%
compared to the CAMERE process. This drop in H2O
production is particularly promising in prolonging the catalyst
lifetime.
Ongoing studies are focusing primarily on the separation of
water-permanent gas mixtures under high-temperature and
pressurized conditions.211,220−222
The research also extends to
the separation of higher alcohol mixtures using zeolite MFI
membranes.223
The advancement of this field is more promising
with economic benefits, as the synthesis of higher alcohols are
thermodynamically more favorable than CH3OH synthesis
under the same conditions.224,225
2.1.5. Magnetic Field-Assisted Reactor. Due to the poor
kinetic rates at lower temperature, the thermocatalytic process of
CO2 conversion to CH3OH requires a great amount of energy
input to maintain higher activity, which offsets the environ-
mental benefits. Reducing the energy consumption of the
thermocatalytic process is highly desired and is a promising
research topic that attracts great attention. The integration of
external magnetic field is one of the potential approaches. The
external magnetic field is advantageous in preventing agglom-
eration of magnetic particles, eliminating slugging and
channeling, and reducing the apparent activation energy.226
Donphai et al. have integrated the external magnetic field into
the thermocatalytic CO2 conversion to CH3OH over CuZnZr
catalysts.227
The external magnetic field varies with different
magnetic field intensities (i.e., 0−27.7 mT) and orientations
(north-to-south, N−S, and the inverted orientation, S−N). The
assistance of magnetic field enhances the reaction rate compared
with the magnetic field-free condition, especially at temperatures
higher than 473 K. The optimal activity is obtained when the
magnetic intensity and orientation are 20.8 mT and S−N,
respectively. The comparable activity with the assistance of
magnetic field can also reduce the energy input, which should
alleviate CO2 emissions (Table 4). The enhancement of activity
performance originates from a magnetic field-induced improve-
ment in CO2 adsorption and a reduction in activation barrier.
The same group continues to explore the magnetic field-assisted
CO2 conversion over the Cu-Fe/ZSM-5 catalyst and observes a
similar enhancement.228
These results substantiate the viability
of the incorporation of magnetic field into thermocatalytic CO2
conversion. Future studies should primarily focus on advancing
the knowledge of the correlation of magnetic field and surface
chemistry, which offers a guideline to engineer catalytic
materials and the optimization of reaction system.
2.2. Precious Metal-Based Catalysts (Pd and Pt)
Supported catalysts of precious metals such as Pd and Pt have
been reported to be active for CH3OH formation from CO
hydrogenation at low temperatures.229
Pd catalysts supported
on La2O3,230
Nd2O5,230
and CeO2
231
can selectively convert CO
to CH3OH. Matsumura et al. have demonstrated that Pd/CeO2
yields 300 g kg-cat−1
h−1
of CH3OH from syngas at 443 K and
3.0 MPa, whereas the Cu-ZnO catalyst requires higher
temperature (e.g., 503 K) to yield comparable amounts of
CH3OH.231
On the basis of these results, the precious metal
catalysts should be potential candidates for low-temperature
CO2 hydrogenation to methanol, where the methanol
production is more favorable thermodynamically. However,
early computational and experimental results indicate that Pt NP
Table 3. Comparison of Activity Performance between Membrane Reactor (MR) and Traditional Reactor (TR) in CO2
Hydrogenation (CO2/H2 = 1:3) to CH3OH
CH3OH syn. activity
catalyst reactor
zeolite
layer temp/K press/MPa
GHSV/mL
g−1
h−1
H2/
CO2
CO2 conv/
%
STY/mol
kg-cat−1
h−1
selec/C-mol % ref
CuO-ZnO-CeO2-
Al2O3
MR NaA 513 0.7 18000 3 − 10.72 84 216
MR NaA 513 0.5 18000 3 − 9.25 73 216
TR − 513 0.7 18000 3 − 7.72 77 216
TR − 513 0.5 18000 3 − 6.16 67 216
Cu/ZnO/Al2O3 MR A-type 479 2 6000 3 11.6 ca. 5.83 75 218
TR − 483 2 6000 3 5.0 ca. 1.61 48 218
S

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