In Situ Photo-Fenton-Like Tandem Reaction for Selective Gluconic Acid Production from Glucose Photo-Oxidation

In Situ Photo-Fenton-Like Tandem Reaction for Selective Gluconic
Acid Production from Glucose Photo-Oxidation
Jiu Wang,∥
Lin Chen,∥
Heng Zhao,* Pawan Kumar, Stephen R. Larter, Md Golam Kibria,*
and Jinguang Hu*
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ABSTRACT: Biomass photorefining to selectively produce value-
added bioproducts is an emerging alternative biomass valorization
approach to alleviate energy crisis and achieve carbon neutrality.
Here, we demonstrate an efficient and selective glucose photo-
oxidation to gluconic acid via a rationally designed dual-functional
carbon nitride photocatalyst that not only allows H2O2 production
via 2e−
oxygen reduction reaction (2e-ORR) but also realizes in
situ photo-Fenton-like reaction. As a result, the essential oxidative
species (•
O2
−
and •
OH) for glucose oxidation into gluconic acid
are generated that achieves >60% glucose conversion and >60% of
gluconic acid selectivity within 4 h. Density functional theory
calculations demonstrate the superior performance of the photo-
catalyst for •
O2
−
and H2O2 generation. Further experimental
results reveal that the moderate concentration of H2O2 produced by 2e-ORR reaction plays a vital role in regulatinge gluconic acid
selectivity. This work demonstrates a good example to realize selective biomass photorefining through tandem reaction of ORR and
in situ photo-Fenton-like process, which could have profound impact on artificial photoenzyme systems involving moderate H2O2
modulation.
KEYWORDS: glucose photo-oxidation, gluconic acid, tandem reaction, 2e-ORR, in situ photo-Fenton-like reaction, photo-enzyme
Biomass valorization into fuels and chemicals provides an
effective approach to resolve energy shortage and
environmental issues.1,2
The abundant monosaccharide
glucose has already been converted into a diverse range of
chemicals with added value including arabinose, gluconic acid,
glucaric acid, fructose, lactic acid, and 5-hydroxymethylfurfu-
ral.3−7
Among these, gluconic acid together with its salts have
already been widely used in the food and pharmaceutical
industry and will reach a remarkable market value of US$ 1.9
billion by 2028.8−10
The current gluconic acid production
mainly rely on enzyme-catalyzed and chemo-catalyzed
processes, in which the first method involves expensive enzyme
and the second method gives rise to environmental
problems.11
Photocatalysis is an emerging green and
sustainable alternative technology platform for chemical
production,12−14
but simultaneously achieving good substrate
conversion and product selectivity over photocatalysis is still
challenging, especially using water as the solvent.15,16
Previous
studies have reported selective gluconic acid production from
glucose photo-oxidation, however, either on alkaline solution
or with the aid of a noble metal or an oxidant.17,18
Therefore,
rational design of the photocatalyst and the reaction pathway is
highly desired for selective biomass photorefining under mild
reaction conditions.
Fenton reaction has been demonstrated as one of the most
efficient methods for water purification in recent years.19,20
Typically, free hydroxyl radicals (•
OH) are generated during
the Fenton oxidation process as the active species to mineralize
the organic pollutants, while the reaction process and liquid
product selectivity are difficult to be controlled.21,22
Besides,
the addition of excessive H2O2, the Fe2+
ions in water, and the
limited pH range restrained the application of Fenton
process.23
It has been reported that high-value gluconic acid
can be obtained through selective glucose oxidation via
traditional Fenton process assisted by ultrasonication method,
during which •
OH plays an important role in the oxidation of
glucose.24
However, excessive •
OH might lead to over-
oxidation process due to the highly oxidative nature.25,26
Alternatively, in situ photo-induced Fenton-like process with
rational control of active species concentration is foreseen to
light up the selective conversion of glucose into gluconic acid.
Received: December 1, 2022
Revised: January 25, 2023
Research Article
pubs.acs.org/acscatalysis
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2637
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Thus, the key issue is how to design a dual functional
photocatalyst that can both generate H2O2 and decompose it
via in situ photo-Fenton process.
Polymeric carbon nitride (CN) has been considered to be an
ideal and eco-friendly photocatalyst along with surpassing
visible light response and promising band positions.27,28
More
importantly, CN has been demonstrated to have highly
selective H2O2 production via the 2e−
oxygen reduction
reaction (2e-ORR).29,30
However, the bulk CN derived from
ordinary thermal polymerization usually suffers from low
efficiency owing to severe recombination of electron−hole
pairs.31
Band gap engineering is a capable approach to improve
the 2e-ORR process of CN-based photocatalysts to form
H2O2.32−34
Additionally, alkalinization with cyano groups has
been proven to effectively decompose H2O2 into •
OH via
photo-Fenton process.35
Inspired by the aforementioned
literature, designing a dual functional CN photocatalyst that
can realize tandem reaction of ORR and in situ photo-Fenton-
like process would be a feasible strategy for selective glucose
photo-oxidation into gluconic acid. To the best of our
knowledge, such a conceptual design applied for selective
biomass valorization into value-added chemicals is yet to be
investigated.
Herein, for the first time, we demonstrate the feasibility of
selective glucose photo-oxidation into gluconic acid by a dual
functional CN photocatalyst. The procedure is achieved via the
fine construction of red CN photocatalyst with codoping of
potassium/oxygen, which can generate H2O2 by 2e-ORR and
in situ decompose into •
OH via photo-Fenton-like process.
Density functional theory (DFT) calculations demonstrate the
superior performance of the modified CN for the generation of
•
O2
−
and H2O2. Accordingly, the well-designed potassium/
oxygen codoped red CN exhibited >60% glucose conversion
and >60% gluconic acid selectivity after 4 h upon light
irradiation. The current study offers an alternative approach for
dual functional photocatalyst design to selectively produce
gluconic acid via tandem reaction of photocatalytic ORR and
in situ photo-Fenton-like process, which could also inspire
artificial photoenzyme systems involving moderate H2O2
modulation.
Figure 1. (a) Synthetic process for K, O codoped red CN through one-step thermal copolymerization. (b) FESEM image and (c) HAADF-STEM
image of RCN and relevant elemental mappings. The colors of red, green, yellow, and blue represent the elemental components of C, N, K, and O,
respectively.
ACS Catalysis pubs.acs.org/acscatalysis Research Article
ACS Catal. 2023, 13, 2637−2646
2638

■ RESULTS AND DISCUSSION
Modified red CN by the codoping of K and O is prepared from
urea and potassium persulfate as precursors through one-step
thermal polymerization approach, which is marked as RCN
(Figure 1a). For comparison, pure CN is synthesized by the
same method without the addition of potassium persulfate
(UCN). Field emission scanning electron microscopy
(FESEM) and transmission electron microscopy (TEM)
images of UCN exhibit a nanosheet-like structure (Figure
S1), while the RCN shows a honeycomb-like morphology
(Figure 1b), which is further demonstrated by the HAADF−
STEM image, and relevant elemental mapping indicates that C,
N, K, and O elements are homogeneously distributed within
the structure of RCN (Figure 1c).
The formation of CN is proved by XRD and FTIR
characterizations. The XRD spectra of UCN and RCN are
depicted in Figure 2a. The XRD signals of UCN at 13.0 and
27.5° are associated with the typical (100) plane and (002)
plane of CN.36
Notably, the (100) peak of RCN nearly
disappears, most likely due to the alteration of the in-plane
graphitic structure by K interaction within the CN matrix.37,38
Additionally, the FTIR spectra of UCN and RCN are shown in
Figure S2. A distinguishable signal at 810 cm−1
is owing to s-
triazine rings. The representative features of C−N in the
heterorings are ascribed to the signals from 1150 to 1750 cm−1
,
revealing the obtainment of CN.39,40
The peak of RCN at
around 2150 cm−1
could be ascribed to the formation of
terminal cyano groups.41
The signals from 2900 to 3500 cm−1
are indicative of O−H or N−H.42
RCN shows an obvious
redshift in the light absorption edge and enhanced visible-light
absorption compared to UCN (Figure 2b), which is a hint of
the alteration of electronic band structure by elemental doping,
thus leading to much stronger light-harvesting ability.43
As
shown in the inset of Figure 2b, the band gap of UCN and
RCN is obtained as 2.75 and 2.58 eV according to Kubelka−
Munk function. Steady-state photoluminescence (PL) spectra
qualitatively reveal the enhanced charge separation of RCN
compared to UCN (Figure S3). To further demonstrate the
better charge separation efficiency of RCN, time-resolved
photoluminescence (TRPL) spectra, photocurrent measure-
ments, and electrochemical impedance spectroscopy (EIS) are
carried out. Regarding UCN, the average lifetime of RCN
Figure 2. (a) XRD spectra, (b) UV−vis DRS and high-resolution XPS spectra of (c) C 1s, (d) N 1s, and (e) O 1s of UCN and RCN. (f) Solid-state
13
C NMR spectra of UCN and RCN.
ACS Catal. 2023, 13, 2637−2646
2639

decreases from 14.67 to 3.53 ns (Figure S4a and Table S1),
implying the rapid photogenerated electron transfer through
nonradiative pathways from the bulk to interface instead of
recombination, thereby enhancing the separation efficiency of
charge carriers.44,45
As shown in Figure S4b,c, compared with
UCN, RCN shows greater density of photocurrent and smaller
Nyquist plot radius, suggesting better separation efficiency of
charge and reduced resistance to charge migration. According
to Mott−Schottky plots, both UCN and RCN belong to n-type
semiconductors because of the positive slopes (Figure S5a). It
can be measured that UCN and RCN have a flat band position
at −1.09 and −0.68 eV, respectively. The positions of the
conduction bands were estimated at −1.29 and −0.88 eV.46
The sample band structure is provided in Figure S5b. X-ray
photoelectron spectroscopy (XPS) spectra of C 1s show three
peaks at 284.8 eV, 286.1 (286.4) and 288.2 eV, corresponding
to the surface adventitious carbon, sp2
C bonded with the
−NH2 groups, and sp2
C attached to N within the heteroring,
respectively (Figure 2c).47
Additionally, the N 1s is fitted into
four peaks at 398.5, 399.9 (399.4), 401.1, and 404.3 eV, which
are associated with C−N�C, N−(C)3, nitrogen of −NH2, and
charging effects, respectively (Figure 2d).48
Moreover, as
shown in Figure 2e, the O 1s peaks originated from three peaks
at 531.9, 532.6, and 533.4 eV, corresponding to C−O, O−H,
and adsorbed oxygen, respectively.49,50
The RCN shows a
sharper and stronger peak at 531.9 eV, which could be due to
the strong oxidizing property of potassium persulfate to attack
the bi-coordinated N within the CN heterorings during
thermal polymerization, thus leading to the substitution N
sites by O atoms.51
The O doping is further demonstrated by
elemental analysis (Table S2). The ratio of O increases from
3.91% in UCN to 11.64% in RCN, while no noticeable S
content is detected (0.254%). In addition, the signals at 292.8
and 295.6 eV are assigned with K 2p orbitals (Figure S6), while
the former peak could imply the presence of potassium azide
(KN3) and existence of N−K bonds in RCN.52,53
The content
of K in RCN is further determined to be 15.2% by inductively
coupled plasma (ICP) analysis. We adopt solid-state 13
C NMR
spectroscopy to clarify the intrinsic structure of UCN and
RCN (Figure 2f). The peaks of UCN at 156.6 (C2) and 164.6
ppm (C3) are indicative of the CN3 moieties and CN2-(NHx),
respectively.54,55
As compared with UCN, the relative intensity
of the C2 peak alters, suggesting the decrease of C2 atoms in
heptazine units. Additionally, the position of C3 peak shifts 1.2
ppm for RCN, revealing that the coordination environment
around C3 atoms has altered. Combined with the shift of
tertiary nitrogen in XPS, it is possible that potassium ions exist
in the form of coordination within the CN matrix, while the
two signals at 122.1 (C1) and 171.7 ppm (C4) correspond to
the cyano group C atom and neighbor C bonded with the
cyano groups.37,50
When the well-designed RCN is utilized for glucose photo-
oxidation into gluconic acid, it certifies exceptional perform-
ance for gluconic acid production. The RCN shows >60%
glucose conversion after 6 h, while UCN presents negligible
glucose conversion (Figure 3a). In the meanwhile, the gluconic
acid yield gradually grows with time and arrives at the
maximum at 5 h, which accounts for ∼40% gluconic acid yield
by converted glucose. As the reaction time increases, gluconic
acid selectivity of RCN declines, but >60% gluconic acid
selectivity of RCN remains after 6 h reaction (Figure 3b). It is
noted that some other byproducts (e.g., glucaric acid, fructose,
arabinose, and formic acid) could be detected during the
photo-oxidation process (Figure S7), which can be regarded as
the inherent reason for the decline in gluconic acid selectivity
by over-oxidation process.56
Extra experiment using increased
glucose concentration (10 g/L) is carried out (Figure S8). The
reaction of glucose photo-oxidation to gluconic acid exhibits
Figure 3. (a) Glucose conversion and gluconic acid yield of UCN and RCN under 6 h reaction. (b) Gluconic acid selectivity of RCN upon 6 h
reaction. (c) Stability experiments of RCN of glucose photo-oxidation into gluconic acid upon 18 h. (d) Experiments of scavenger of RCN after
adding BQ, IPA, EDTA-2Na, and NaN3. (e) DMPO-•
O2
−
and (f) DMPO-•
OH ESR spectra of RCN in methanol dispersion and aqueous solution,
respectively. Reaction conditions: 10 mg photocatalyst, 2 g/L glucose solution with a volume of 10 mL (pH ∼ 7), and 300 W Xenon lamp under
air.
ACS Catal. 2023, 13, 2637−2646
2640

∼10% glucose conversion and ∼55% gluconic acid selectivity
after 6 h, suggesting that the higher glucose concentration has
a moderately negative influence on gluconic acid production.
The effect of different K amounts has been studied (Figure
S9), and it is found that RCN-0.5 and RCN-1.5 can achieve
glucose photo-oxidation into gluconic acid with ∼33 and
∼45% glucose conversions, respectively. The gluconic acid
selectivity shows decreasing trend with reaction time, and there
are ∼58 and ∼40% gluconic acid selectivities of RCN-0.5 and
RCN-1.5 within 6 h of reaction, respectively, suggesting that a
moderate amount of K doping would be beneficial for the
gluconic acid production. Additionally, the photocatalytic
performance for K-doped CN and O-doped CN are carried
out (Figure S10). It is found that K-doped CN can achieve
∼30% glucose conversion and ∼18% gluconic acid yield upon
6 h illumination, while O-doped CN realizes ∼10% glucose
conversion and the main product becomes arabinose with a
yield of ∼8% under the same reaction conditions. The results
reveal that K doping plays an important role in gluconic acid
production from glucose photo-oxidation. As shown in Figure
3c, RCN exhibits excellent recyclability in terms of glucose
conversion and gluconic acid selectivity during the 18 h cycling
test. More importantly, the yield and selectivity of gluconic
acid of this work stands out with those reported in the
literature, since it is under mild neutral conditions instead of
harsh conditions (e.g., alkaline solution) and without the help
of any noble metals or additional oxidants (Table S3). As a
result of the scavenger experiments, primary reactive species of
photo-oxidation reaction have been further demonstrated,
where 1,4-benzoquinone (BQ), isopropanol (IPA), ethyl-
enediaminetetraacetic acid disodium salt (EDTA-2Na), and
sodium azide (NaN3) have been utilized as captures of •
O2
−
,
•
OH, h+
, and 1
O2, respectively (Figure 3d). Glucose
conversion decreases from 62.87 to 12.51, 31.66, 43.84, and
51.61% for 6 h reaction with the addition of BQ, IPA, EDTA-
2Na, and NaN3, respectively. It implies that •
O2
−
, •
OH, h+
,
and 1
O2 are favorable for the glucose photo-oxidation into
gluconic acid, while •
O2
−
and •
OH are the major active
species, which could play important roles for glucose photo-
oxidation into gluconic acid. 5,5-Dimethyl-1-pyrroline N-oxide
(DMPO) is adopted as a spin trap to conduct electron spin
resonance (ESR) spectra to further prove the major reactive
species in RCN involved in photo-oxidation. Increased
illumination time enhances the typical signal of •
O2
−
,
suggesting more •
O2
−
are generated (Figure 3e). As such,
dark condition cannot produce a signal of •
OH, but light
illumination produces an increasing signal of DMPO-•
OH
(Figure 3f), revealing more •
OH are produced during the
Figure 4. (a) Potential Gibbs free energy landscape for 2e-ORR over K, O-codoped CN system (black line). For comparison, the reactions are also
calculated over pristine (green), K-doped (red), and O-doped (blue) CN systems. The structures for each step are shown below the potential
Gibbs free energy landscape. (b) Charge density difference Δρ(r) for adsorbed O2 over each system is shown. The upper figure shows the top view,
and the down figure shows the side view. Cyan and yellow contours represent regions of electron depletion and accumulation, respectively
(isovalue of ±0.002 electron Å−3
). (c) Bader charge of the atoms in the surface layer is marked on each atom. All colors of the frames for structures
in (a), Δρ(r) in (b), and structures with Bader charges in (c) are consistent with the color in the reaction landscapes. Atom color codes: C (gray),
N (blue), O (red), and K (purple).
ACS Catal. 2023, 13, 2637−2646
2641

photocatalytic reaction. The results of ESR demonstrated the
presence of •
O2
−
and •
OH within the photo-oxidation process.
The presence of •
O2
−
is ascribed to the ORR property of
RCN, and H2O2 is indeed detected during the photocatalytic
reaction (Figure S11). However, the valence band position of
RCN is insufficient to directly oxidize H2O into •
OH
according to the band structure analysis (Figure S5b).57
Thus, the detected •
OH most possibly comes from the in situ
decomposition of H2O2 via photo-Fenton-like process, which
indicates the dual functionality of RCN: generating and
decomposing H2O2.
Then, DFT calculations are carried out to further under-
stand the enhanced 2e-ORR activity of RCN codoped with K
and O atoms. The heptazine-based CNs are considered to be
the model structure as it is determined experimentally. It is
found by geometry optimization that the potassium ion prefers
to intercalate into the space between the CN interlayer via
bridging the layers, which is similar to other reported
works.38,58
Since the sp2
-hybridized nitrogen atom is replaced
by the oxygen atom for O-doping structure as demonstrated
experimentally, the 2e-ORR activity over K, O-codoped CN as
well as over pristine, K-doped and O-doped CN is investigated.
The potential free energy landscapes are shown in the upper
panel of Figure 4a. Along the reaction coordinate, the reference
state in the first stage refers to each CN with H2 and O2 in the
gas phase with 1 bar pressure. Oxygen adsorption step behaves
differently within the four systems. For pristine (green) and K-
doped (red) CN, the oxygen molecule can only be physically
adsorbed over the surface, and both adsorption steps proceed
endothermically. It can be explained by the charge density
difference from Figure 4b (green frame) that the π orbital
electrons deplete toward the surface of pristine CN, which
implies that it induces repulsion between O2 and CN. The
electron redistribution of O2 adsorption on K-doped CN (red
frame) is slightly larger than on pristine CN, and the π orbital
electrons accumulate toward the surface. However, the
interaction between O2 and the surface of K-doped CN is
still weak. It should be noted that O2 chemically adsorbs at the
C site on O-doped and K, O-codoped CN by forming
superoxide radicals (•
O2
−
), which can act as an important
intermediate for glucose photo-oxidation into gluconic acid.
The dramatic electron redistribution between O2 and the
surface of O-doped (blue frame) and K, O-codoped (black
frame) CN in the charge density difference figure denotes the
strong interaction between O2 and the surface. O2 adsorbs
exothermically on K, O-codoped CN but still endothermically
on O-doped CN. By analyzing the Bader charge (Figure 4c), it
is found that the dopant O atom induces the charge increase
for the neighboring C atom from +1.47 e in pristine CN to
+1.66 e in the O-doped system and to +1.64 e in the K, O-
codoped system. The stronger electronegativity for the C atom
explains the lower adsorption energies of O2 on O-doped and
K, O-codoped systems. However, the charge of nitrogen atom
coordinated with the active C atom increases from −1.21 e in
the O-doped CN to −1.06 e in the K, O-codoped CN, which
makes the repulsion of N with the adsorbed O2 weaker in the
codoped system. The following sequential hydrogenation steps
with the formation of OOH and H2O2 can proceed
exothermically compared with the reference state level. The
effect from the coverage of adsorbed O2 is also studied (Figure
S12), and it is found that the adsorption of two oxygen
molecules at the same time is more difficult. To sum up, the K,
Figure 5. (a) Concentration of in situ-generated H2O2 curve of RCN during the 6 h glucose photo-oxidation into gluconic acid process. (b)
Gluconic acid selectivity and glucose conversion of RCN under different H2O2 concentrations in Ar atmosphere for 1 h irradiation. (c) Proposed
mechanism of glucose photo-oxidation into gluconic acid on RCN.
ACS Catal. 2023, 13, 2637−2646
2642

O-codoped CN, namely, RCN, possesses superior performance
for the generation of •
O2
−
and H2O2, which could play
significant roles during glucose photo-oxidation into gluconic
acid.
In order to further explore the effect of in situ generated
H2O2 on the gluconic acid selectivity from glucose photo-
oxidation, the concentrations of H2O2 during the photo-
catalytic reaction were measured (Figure 5a). It can be found
that the H2O2 concentration gradually increases over time and
arrives at a peak of 1535 μM at ∼1 h and gradually decreases
until the end of 6 h photocatalytic reaction, most likely due to
the exhaustion of oxygen in the reactor. Then, we performed
experiments with a series of concentration gradients of H2O2
to figure out the effect of H2O2 concentrations on the
selectivity of gluconic acid from glucose photo-oxidation
process (Figure 5b). The selectivity of gluconic acid first
increases and then decreases with the increasing concentration
of H2O2 while achieving the highest selectivity of ∼80% at a
H2O2 concentration of 1500 μM for 1 h irradiation under Ar
atmosphere. The result indicates that •
OH generated from
H2O2 decomposition via photo-Fenton-like process could
realize selective gluconic acid production, which also reveals
that the moderate concentration of H2O2 produced by 2e-
ORR plays a vital role to regulate gluconic acid selectivity. It is
noted that the control experiment under the same reaction
condition without photocatalyst shows no glucose conversion,
indicting that the photo-Fenton-like process is indeed triggered
by the rationally designed CN rather than the homogeneous
catalysis process. In order to demonstrate the photo-Fenton-
like function of RCN, ESR experiments adding H2O2 under a
N2 atmosphere are performed (Figure S13). It is found that
increased illumination time enhances the signal of the
characteristic peaks of •
OH when adding H2O2, while
negligible •
OH signal is detected without H2O2, indicating
more •
OH could be generated from the H2O2 decomposition
by RCN via photo-Fenton-like process. A detailed mechanism
is then proposed based on experimental and theoretical results
for the glucose photo-oxidation into gluconic acid over RCN
under light irradiation (Figure 5c). On the one hand, in situ-
generated H2O2 and •
OH can be realized via 2e-ORR and
photo-Fenton-like process, respectively. In detail, oxygen can
first combine with the electron to produce •
O2
−
, followed by
the formation of the immediate radical (•
OOH). Then, •
OOH
can react with the proton and the electron to generate H2O2,
followed by the decomposition of H2O2 into •
OH in situ via a
metal-free photo-Fenton-like process. On the other hand,
glucose is first converted into the intermediate state by the
deprotonation process, accompanied by the release of H2O
molecule. After that, the intermediate will combine with the
•
OH derived from H2O2, thus leading to the formation of the
final product, gluconic acid. Herein, this work demonstrates
the great potential of dually functional photocatalyst for
glucose photo-oxidation into gluconic acid over the tandem
reaction of ORR and in situ photo-Fenton-like process.
■ CONCLUSIONS
In conclusion, we have successfully synthesized a dually
functional CN photocatalyst with codoping of potassium/
oxygen. The rationally designed red CN not only achieves
much stronger visible light absorption and highly efficient
charge separation but also realizes tandem reaction of ORR
and in situ photo-Fenton-like process. DFT calculations
further demonstrate the superior performance of the modified
CN for the generation of •
O2
−
and H2O2. Accordingly, the
finely designed CN presents favorable glucose conversion
(>60%) and gluconic acid selectivity (>60%) in the presence
of only water as the solvent (without base or any additional
oxidant). This work sheds new light for the photocatalyst
design to selectively produce gluconic acid from glucose
photo-oxidation under mild conditions through in situ photo-
Fenton-like process.
■ METHODS
Synthesis of UCN and RCN. RCN was prepared as
follows: 10 g of urea and 1 g of potassium persulfate were
grinded completely in a mortar to obtain a homogeneous
mixture. Then, the mixture was put in a 100 mL crucible
covered by a lid and calcined at 550 °C for 2 h with a rate of 5
°C/min in air. Afterward, the solid sample was put in 80 °C
hot water with continuous stirring overnight. After that, the
powder was completely washed and centrifuged several times
to remove the soluble substance and dried at 60 °C for 12 h.
Finally, the final red powders were labeled as RCN. Control
samples with different K amounts were synthesized by the
same method with the addition of 0.5 and 1.5 g of potassium
persulfate, which were named as RCN-0.5 and RCN-1.5,
respectively.
For comparison, UCN was synthesized by one-step
calcination process of urea as the precursor. 10 g of urea was
heated at 550 °C for 2 h under air, and the rate of heating was
5 °C/min. Final obtained powders were labelled as UCN.
Detailed process of K-doped CN and O-doped CN
preparation can be found in the Supporting Information.
Characterizations. A Bruker D8 ADVANCE diffractom-
eter was used to characterize the XRD patterns. FTIR spectra
was obtained via a Nicolet iS 50 spectrometer. A PerkinElmer
(Lambda) spectrometer was utilized to obtain the UV−vis
spectra. The PL and TRPL spectra were obtained on F-4700
and FLS920 instruments with excited wavelength at 350 nm.
XPS was performed by an equipment (Escalab, 250Xi), and
284.8 eV was adopted as the calibrated binding energy. NMR
spectra were acquired via a Bruker AVANCE 600 MHz
spectrometer. JSM 7500 and Talos 200 microscopes were
utilized to obtain the FESEM and HRTEM images. The
electrochemical measurements were obtained by a CHI660D
workstation. Additionally, the sample-loaded FTO glass, Ag/
AgCl, and Pt were considered as the working, reference, and
counter electrodes, which were placed in 0.1 M Na2SO4
aqueous solution. The elemental analysis was conducted on a
Vario EL Cube (Germany) analyzer. The ESR spectra were
obtained by the ESR spectrometer (JES-X320, JEOL) adopting
DMPO as the spin trap. The ESR detection experiments were
conducted in methanol solution for superoxide radicals
(DMPO-•
O2
−
) and in aqueous solution for hydroxyl radicals
(DMPO-•
OH).
Photocatalytic Measurement. The photocatalytic tests
were carried out in a 20 mL glass vial. Typically, 10 mg of the
photocatalyst was distributed uniformly in 10 mL of glucose
solution (2 g/L). The tightly sealed reactor was placed in dark
condition with constant stirring for 1 h. Afterward, a 300 W
Xenon lamp was used to initiate the photocatalytic reaction.
High-performance liquid chromatography (1200 Agilent) was
utilized to analyze glucose and other reaction products with an
Aminex HPX-87H column and a refractive index detector. The
flow rate of 0.5 mL/min was used for the mobile phase of 5
mM sulfuric acid. A three-time repeat of each experiment was
ACS Catal. 2023, 13, 2637−2646
2643

also conducted in order to determine the error bar. The
glucose conversion, gluconic acid selectivity, and gluconic acid
yield are obtained as follows
glucose conversion
glucose glucose
glucose
100 %
O T
O
=
[ ] [ ]
[ ]
×
gluconic acid selectivity
gluconic acid
glucose glucose
100 %
T
O T
=
[ ]
[ ] [ ]
×
gluconic acid yield
gluconic acid
glucose
100 %
T
O
=
[ ]
[ ]
×
where [glucose]O
and [glucose]T
correspond to the molar
concentrations of original glucose solution and at time T
during the reaction. [gluconic acid]T
represents the molar
concentration of gluconic acid at time T during the reaction.
The concentration of H2O2 during the glucose photo-
oxidation into gluconic acid was determined by iodometry.
After a certain time of reaction, the sample solution was
collected, centrifuged, and filtered. After that, 0.1 mol L−1
solution of potassium hydrogen phthalate (C8H5KO4) and 0.4
mol L−1
solution of potassium iodide (KI) were ready for use.
Then, these three solutions were mixed in a ratio of 1:1:1 and
left to stand for 2 h, where H2O2 could react with iodide
anions (I−
) under acidic condition to produce I3
−
(H2O2 + 3I−
+2H+
→ I3
−
+2H2O). The amount of I3
−
was then measured
by a UV−visible spectrometer (Lambda, PerkinElmer) based
on the characteristic absorption at 350 nm.
Computational Details. DFT calculations are carried out
with the Vienna Ab-initio Simulation Package (VASP).59−61
The Kohn−Sham orbitals are expanded with plane waves using
a 450 eV energy cutoff, and the interaction between the
valence electrons and the cores is described with the plane
augmented wave approach.62,63
The number of valence
electrons considered in the calculations are 7 (K), 6 (O), 5
(N), 4 (C), and 1 (H). The exchange−correlation effects are
described within the generalized gradient approximation
according to Perdew, Burke, and Ernzerhof.64
The D3
approach proposed by Grimme and co-workers is added to
describe the vdW interactions.65,66
Structures are optimized
with the conjugate gradient method, and geometries are
considered to be converged when the electronic energy
difference between subsequent steps is lower than 1 × 10−5
eV and the largest force is lower than 0.03 eV/Å. The pressure
for H2 and O2 is set as 1 bar. The charge density difference plot
is performed via VESTA.67
The Bader charge was calculated by
using the algorithm developed by G. Henkelman’s group.68,69
■ ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.2c05931.
Preparation process; FESEM image; TEM image; FTIR
spectra; steady-state PL spectra; TRPL spectra; photo-
current measurements and EIS Nyquist plots; Mott−
Schottky plots; XPS spectra; HPLC measurements;
glucose conversion and gluconic acid selectivity of
higher glucose concentration and different K amounts;
glucose conversion and gluconic acid yield of K-doped
CN and O-doped CN; photograph of the H2O2 test
paper; potential Gibbs free energy landscape; ESR
spectra; fluorescence lifetimes; ICP and elemental
analysis; and comparison of gluconic acid yield with
those in the literature (PDF)
■ AUTHOR INFORMATION
Corresponding Authors
Heng Zhao − Department of Chemical and Petroleum
Engineering, University of Calgary, Calgary, Alberta T2N
1N4, Canada; Email: heng.zhao1@ucalgary.ca
Md Golam Kibria − Department of Chemical and Petroleum
1N4, Canada; orcid.org/0000-0003-3105-5576;
Email: md.kibria@ucalgary.ca
Jinguang Hu − Department of Chemical and Petroleum
1N4, Canada; orcid.org/0000-0001-8033-7102;
Email: jinguang.hu@ucalgary.ca
Authors
Jiu Wang − Department of Chemical and Petroleum
1N4, Canada
Lin Chen − Department of Physics and Competence Centre for
Catalysis, Chalmers University of Technology, SE-412 96
Göteborg, Sweden; orcid.org/0000-0002-7905-9587
Pawan Kumar − Department of Chemical and Petroleum
1N4, Canada; orcid.org/0000-0003-2804-9298
Stephen R. Larter − Department of Geosciences, University of
Calgary, Calgary Alberta T2N 1N4, Canada
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.2c05931
Author Contributions
∥
J.W. and L.C. contributed equally in this work.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Canada First Research
Excellence Fund (CFREF).
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