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1 Introduction
Catalysis lies at the heart of many everyday life processes,
from the academic research lab through living systems to the
large-scale industrial applications. It is also widely recognized
that catalysis plays a relevant role in the solution of one of
the major problems affecting our technological society,
namely, the production and consumption of vast amounts of
energy causing excessive pollution of our environment. By
understanding and carefully using catalysis, many processes
can be made faster, cleaner, and more sustainable.
Catalysis is clearly an interdisciplinary field, both industrial
and academic, across chemistry, chemical engineering, physics,
and materials science. Surface physics, through experiments
and numerical simulations, plays a specific role, yielding
thorough understanding down to the atomic level detail of
many processes, which are fundamental to heterogeneous
catalysis [1-3]. The 2007 Nobel Prize awarded to Gerhard Ertl
emphasizes the importance of the studies of reactions on solid
surfaces [4].
In this work we revise a series of representative studies
starting from the simplest model case of single-crystal
metallic surfaces – namely, Cu(110) and Ni(110) – interacting
with carbon oxides under ultra-high–vacuum conditions.
We show how the interplay between theory and experiment
allows an unambiguous interpretation of the data and a deep
The combination of surface science techniques with
accuratequantum-mechanicalnumericalsimulations
applied to model catalytic systems like single-crystal
samples under ultra-high–vacuum conditions
has allowed understanding and in selected cases
predicting the catalytic behavior of materials,
unveiling the contribution of surface structure,
alloying, and coverage effects. Work is in progress
nowadays to bridge to a certain extent the material
and pressure gaps, dividing the single-crystal studies
in vacuum typical of fundamental physics research
and the investigation of catalysts under working (in
situ and in operando) conditions. In this context
and for the practical purpose of developing efficient
catalysts, nowadays nanoparticles deserve special
attention, due to their reactivity, surface-to-volume
ratio, and peculiar behavior related to finite-size
effects. Furthermore, novel spectroscopic techniques
thatcanbeapplieduptoambientpressureconditions
and even in liquid and, in parallel, new algorithms for
numerical simulations in realistic environments such
as electrochemical cells are under development.
HETEROGENEOUS CATALYSIS
TOWARDS THE NANOSCALE
Combining surface science experiments and
numerical simulations
Maria Peressi1,2
, Erik Vesselli1,2
1
Dipartimento di Fisica, Università di Trieste, Italy
2
CNR-IOM, Istituto Officina dei Materiali, Trieste, Italy

2. 5vol30 / no3-4 / anno2014 >
understanding of the chemical/physical processes. The
extension of this investigation to bimetallic surfaces, and in
particular to the Ni/Cu(110) surface, is discussed with the
aim of understanding the role of alloying, evaluating which
composition is more efficient for specific reactions, to which
extent the catalyst’s structure can be controlled locally,
and to which extent the surface properties can be tailored
and engineered. Experiments and simulations allow also
understanding the effect of the initial state of the reactants:
for instance, using molecular or atomic hydrogen in the
hydrogenation process of carbon dioxide makes a difference,
thus providing useful insight into the role of competitive
reaction mechanisms, namely Langmuir-Hinshelwood and
Eley-Rideal processes.
Further, we will mention nanostructured systems, and in
particular nanoparticles or clusters, which have a very high
active-surface–to–volume ratio and, thanks to the presence
of under-coordinated sites, edges, kinks, may efficiently
activate the adsorbed molecules for further reactions.
Finite-size effects, charge spill-over, and interaction with
the support can also be determinant for their effective
catalytic activity. In particular, as a case study, we will discuss
the possibility of creating regular arrays of nanoparticles
anchored to an ordered alumina ultrathin templating film
grown onto a NiAl alloy termination. The use of nanoparticles
goes in the direction of bridging the material gap between
surface science and real catalysts.
At the end, some directions of ongoing progress in the
experimental and computational tools of the surface science
atomistic approach to catalysis will be sketched. On the one
hand, recently, classical surface science techniques applied
under ultra-high–vacuum conditions have been augmented
by novel techniques, which can be applied also under ambient
gas pressures and liquid reaction conditions, including
electrochemical cells, thus offering in situ and in operando
analytical tools. On the other hand, concerning simulations,
new algorithms are under development for modeling realistic
environments. These are promising steps towards bridging the
pressure and, say, environment gap between studies in surface
science and under realistic conditions.
2 Catalyst help
A catalyst is a substance which alters (typically, speeds up)
the rate of a chemical reaction, and remains unchanged at the
end of the reaction. A catalyst is selective in its action, i.e., it
steers the reaction towards a selected channel or pathway. A
certain energy barrier (activation energy) has to be overcome
for a reaction to occur. A catalyst normally favors the reaction
lowering the activation energy (fig. 1).
Fig. 1 Energy scheme of a
catalysed/uncatalysed reaction:
the presence of a catalyst reduces
the activation energy.

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We focus here on heterogeneous
catalysis, where the catalyst is in a
different physical phase from the
reactants: in particular, the catalyst is
a solid surface and the reactants are
molecules interacting with the surface
from the gas phase (or from a liquid
phase).
The overall reaction occurring on a
catalyst usually consists of a series of
elementary steps (fig. 2). These include
adsorption of the reactants on the
surface of the solid or direct interaction
with pre-adsorbed species, diffusion on
the surface, breaking of some reactant
bonds, and the creation of new ones
to form the product molecules that
eventually desorb from the surface.
Adsorption is therefore the first step of
heterogeneous catalysis, and according
whether it occurs for all the reactants
or not, the reaction proceeds following
the so-called Langmuir-Hinshelwood
(LH) or Eley-Rideal (ER) mechanisms,
respectively (fig. 3). The rate (or velocity)
of a reaction depends on intrinsic
parameters such as energy barrier
and temperature, and is typically well
described by a simple but remarkably
accurate formula known as Arrhenius’
equation: k = Ae–Ea/(RT)
, where T is
3 Modelling to understand
In order to unravel the detailed
mechanisms of catalytic reactions, it is
useful to reduce the complexity with
respect to the real catalytic systems
and to investigate the reaction on
model systems working under simpler
conditions. The first approach taken in
surface science is to study elementary
reactions steps on well-defined
single-crystal surfaces, although they
are only crude models of industrially
used catalysts, and first of all under
ultra-high–vacuum conditions (UHV).
Indeed, this issue generally questions
the transferability of surface science
results to applied catalysis. On the
contrary, modern experimental
surface science approaches make
it possible to characterize the
structure and other properties of
the working catalysts in situ, yielding
the possibility of identifying stable
reaction intermediates, paths, and
routes in operando. In particular,
the main goal is the experimental
identification of reaction intermediates
and surface species, together with a
deep description of the morphology
and chemical state of the catalyst’s
surface. Several techniques have been
Fig. 2 Some typical elementary
steps of a reaction: (a) adsorption
of the reactants on the surface
of the solid, (b) diffusion on the
surface and possible adsorption
the absolute temperature in kelvin,
Ea is the activation energy, R is the
universal gas constant, and A is a pre-
factor depending on the reactants
and the environment. The overall
rate of a reaction is often determined
by the slowest step, known as the
rate-determining step that in case of
heterogeneous catalysis can be the
diffusion of reactants on the surface
and desorption of products from the
surface, or some intermediate step.
Industrial catalysts need to have a
high surface area to be efficient, and
they often consist of mixtures of phases,
some of which have the catalytically
active surface, whereas others support
small particles of active phases or
prevent them from sintering. Typical
surface areas are of the order of several
tens of square meters per gram weight
of catalyst. The complexity of the
overall reaction process and of the real
catalysts makes it a demanding task to
establish an atomic-level understanding
of heterogeneous catalysis, which is the
first step towards improving catalytic
devices by means of a bottom-up
design, engineering, and tailoring
approach.
on active sites, (c) breaking/
forming of some bonds, and
(d) formation of the products that
can eventually diffuse or desorb
from the surface.
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4. 7vol30 / no3-4 / anno2014 >
recently developed in order to bridge
the pressure gap, mainly upgrading
previous surface science spectroscopy
and microscopy approaches. Among
them, we mention X-ray Photoelectron
Spectroscopy (XPS) that nowadays
can be performed up to near ambient
pressure conditions (mbar) thanks to
the development of novel electron
energy analyzers with differential
pumping systems. Moreover, Scanning
Tunneling Microscopy has also been
improved and some microscopes
working under ambient pressure or
even liquid conditions were developed.
On the other hand, a fundamental
description of simple processes
occurring at metal surfaces is possible
with the development of quantum
theoretical methods, which allow
calculating equilibrium geometries,
structural and vibrational parameters,
charge distribution, electronic
structure, adsorption energies, and
energy barriers. Among different
computational approaches, quantum-
mechanical methods are preferred for
modeling catalytic processes since
they adequately describe the chemical
bonding. They are also referred to as
ab initio or first-principles methods
since they are totally based on the
interactions among the elementary
constituents of matter at the atomic
level (nuclei and electrons) treated at
the level of quantum mechanics and
not making use of empirical parameters.
Ab initio methods, however, are not
totally free from approximations. The
complicate many-body Schrödinger
equations involving all the degrees
of freedom of nuclei and electrons
cannot be solved directly but needs
to be simplified. A first simplification
is given by the Born-Oppenheimer
approximation that, assuming that the
nuclear dynamics is much slower than
the electronic one, allows separating
nuclear and electronic degrees of
freedom, and factorizes the solution.
The equation describing the interacting
electrons is still very complicated
and further simplification is required
to achieve any solution. Density
functional theory (DFT), formulated
by W. Kohn who was awarded the
Nobel Prize for that [5], has been a
great step forward, proposing how
the complicate electron many-body
Schrödinger equation can be reduced
to a set of one-electron equations still
reliably describing the ground-state
Fig. 3 Schematic representation
of the Langmuir-Hinshelwood
and Eley-Rideal mechanisms of
chemical reactions on a solid
electron density and total energy.
The computational simplification is
enormous, and, together with the
increased computational power,
allows nowadays the description of
systems with thousands of electrons.
To solve in practice the one-electron
equations, some approximations are
needed, and concern the model used
to describe the system (necessarily
limited in size), the technical details
such as the basis set used to expand
the wave functions, and the treatment
of the interactions. Corrections to
standard DFT implementations are
necessary to describe long-range
dispersion (van der Waals) interactions
that can be important in catalytic
reactions including physisorbed or
non-covalently adsorbed molecules.
Advanced techniques are needed for
the treatment of excited states and
the calculation of optical properties,
requiring many-body corrections.
Concerning the size limitation, it is
worth specifying that it is impossible
to describe ab initio all the atoms in a
catalyst. Two basic approaches exist to
reduce this number: a cluster approach,
describing only a limited cluster of the
surface atoms in the region involved
surface, depending on whether
the adsorption on the surface
occurs for all the reactants or not,
respectively.
M. Peressi, E. Vesselli: HETEROGENEOUS CATALYSIS TOWARDS THE NANOSCALE

5. 8 < il nuovo saggiatore
in the catalytic process, and a periodic
slab approach, where the surface is
described as a slab with a periodic
structure in the surface plane. The size
of the surface unit cell determines the
computational effort, and care has to
be taken for the unit cell to be large
enough to avoid fictitious interactions
between adsorbates (in catalysis:
reactants, intermediates, and products)
in the repeated images.
Several computer codes designed
to perform electronic structure
calculations are available (see for
instance [6] for a representative list).
Among those based on slab models
for periodic DFT calculations, which
are widely used by the computational
physics community, we mention the
Quantum-ESPRESSO package [7].
Cluster models are used in state-of-
the-art quantum chemical codes,
released and updated mainly by the
computational chemistry community.
Finally, it is important to point out
that large-scale calculations can be
viewed as computer experiments.
They complement real experiments in
several ways, sometimes performing
situations that are not fully controllable
or even realizable experimentally,
such as changing the lattice constant
of a substrate or locally modifying its
composition or morphology.
4 Starting from the simplest
cases: single-crystal surfaces and
UHV
As a case study, we focus here on
the catalytic reduction of carbon
mono- and di-oxide. Carbon dioxide,
in particular, is of great technological
interest. Its recovery for subsequent
transformation, besides limiting
greenhouse effects, is a key process
for the organic synthesis of chemicals
like methanol, a promising candidate
as an energy carrier to feed internal
combustion engines or fuel cells. As
the conversion of CO2 into synthetic
fuels is energetically uphill, this process
involves a transfer of energy which can
be provided for instance by hydrogen
in standard catalytic reduction or by
electricity in electrochemical cells. Many
other technologically relevant synthesis
processes involve carbon monoxide.
An important example is the water-gas
shift reaction (WGSR), where CO and
water react to produce hydrogen. The
WGSR process is active, together with its
reverse, in several catalytic technologies
like, again, methanol synthesis, and
methanol steam reforming, ammonia
synthesis, coal gasification. Due to the
great chemical stability of CO2, metal
catalysts able to activate the closed-
shell molecule are required. Methanol
synthesis is generally performed on Cu-
based catalysts (for instance, Cu/ZnO/
Al2O3) in a CO2+H2+CO stream at high
temperature and pressure (about 500 K
and 50–100 bar). Although empirically
proven to be essential, the role of
CO in this process could not be fully
understood.
UHV studies of CO2 hydrogenation
on Cu(100) and Ni/Cu(100) model
catalyst interestingly show that: i) on
pure Cu(100) there is no difference
in the reaction rate with/without CO;
ii) on Ni/Cu(100) the reaction rate for
CO2 hydrogenation is much higher at
Fig. 4 Scanning tunneling
microscopy images of CO2
adsorbed on Ni(110) at 110 K.
(a) three-molecule,
(b) two-molecule, and
(c) single-molecule configurations.
calculated with DFT;
lower row: STM images taken from
experiments (parameters: sample
bias V=−10 mV , It =1 nA). Adapted
from [11].
The region shown in each panel
has dimensions of 1.4×1.0 nm2
.
Upper row: stick-and-ball models
of equilibrium configurations
calculated using DFT;
middle row: STM images

6. 9vol30 / no3-4 / anno2014 >
Fig. 5 Schematic summary of
XPS and HREELS experiments
detecting the important steps of
CO2 hydrogenation on Ni(110). In
the middle: cartoon showing the
possible first intermediates of CO2
hydrogenation (HCOO, CO and
OH, CO and H2O). Left side: XPS
signal of the O 1s core level for the
Ni(110) surface covered by CO2 at
low temperature without (left) and
with (right) coadsorbed hydrogen;
the two peaks correspond to the
physisorbed and chemisorbed
configuration, described by the
ball models; the orange part is a
differential plot of the two cases.
Right side: selected HREEL spectra
of the same Ni(110) surface after
hydrogen coadsorption and
annealing at 230 K; orange regions
indicate the relevant peaks due
to HCOO intermediate formation.
Adapted from [12].
Ni sites than at Cu sites; iii) the source
of carbon and oxygen of methanol
is CO2 and not CO; iv) CO is essential
for promoting Ni segregation to the
surface; v) formate (HCOO) is identified
as the main stable intermediate of the
methanol synthesis reaction but it
was not known whether it is actually
involved in the full CO2 hydrogenation
process or it is just a spectator [8].
These facts point to the necessity of
unraveling the basic adsorption and
activation mechanisms of CO2 on pure
Cu and pure Ni, first of all.
CO2 is only weakly bound to the
copper surface remaining linear
and neutral (physisorption), and its
hydrogenation leads to bidentate
formate that binds to the surface
through the O atoms. This step is
endothermic and the estimated barrier
is as high as 1 eV [9].
At variance with Cu and other metal
surfaces, CO2 is also chemisorbed under
UHV conditions on the single-crystal
Ni(110) surface, in addition to forming
a low-temperature physisorbed state.
Combined experimental and theoretical
investigations have confirmed the
existence of chemisorption states:
the molecule gets a non-negligible
electronic charge from the metal,
bends, and binds to the surface mainly
via the carbon atom, and, being no
longer neutral and linear as in the
physisorbed state, is activated for
further reactions. These findings come
from the calculations of adsorption
energies and barriers compatible with
the measured desorption energy, and
from a clear correspondence that can
be established between the calculated
vibrational frequencies of chemisorbed
configurations and the main peaks
detected by High-Resolution Electron
Energy Loss Spectroscopy (HREELS),
and, on the other hand, between
the calculated STM images for the
most favored configurations and the
images experimentally detected at low
temperature (90 K) [10,11] (fig. 4).
In the activated chemisorbed
state on Ni(110), CO2 is ready for the
reaction with hydrogen. The key step
is a change of the activated molecule
coordination to the metal surface.
When the temperature is increased
and H approaches from the surface
where it is coadsorbed following the
LH mechanism, the H-CO2 complex
is formed; the otherwise saturated
oxygen atoms of the adsorbed CO2
molecule become available for surface
coordination, the complex flips, and
binds to the surface through the two
oxygen atoms, while H binds to the
carbon atom, thus yielding the formate
intermediate which is known to exist
in such a configuration and to be
present also under real catalyst working
conditions [12,13]. The fingerprint of the
HCOO formation is clearly detected by
XPS and HREELS experiments (fig. 5).
As shown in fig. 6 (blue path), the
calculated lowest-energy barrier for
this hydrogenation reaction is relatively
small, 0.43 eV and, most importantly,
smaller than that for CO2 desorption
and that for dissociation into CO + O.
The barrier for CO2 hydrogenation on
Ni(110) is also remarkably smaller than
that on the common Cu metal-based
catalyst, which is, as mentioned above,
of the order of 1 eV. This provides
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7. < il nuovo saggiatore10
Fig. 6 DFT energy diagram
for different possible CO2
hydrogenation pathways on
Ni(110): solid/dashed lines are for
LH/ER mechanisms, respectively.
possible explanations for the peculiar
behavior of the CO2 + CO + H2 reaction
on the NiCu-alloy–based catalysts.
However, HCOO formation is not the
rate-limiting step for CO2 hydrogenation
on Ni(110): the subsequent step cannot
be overcome in UHV coadsorption
experiments. Indeed, DFT calculations
predict very high-energy barriers for
further reactions from formate, as
shown in fig. 6. An alternative CO2
hydrogenation path is followed if the
coadsorbed H reacts with an oxygen
atom of the CO2 molecule instead of the
carbon atom, overcoming a barrier of
about 0.5 eV, in close competition with
CO2 dissociation and desorption.
The reaction mechanism changes
completely when CO2 reacts with
atomic hydrogen impinging from
the gas phase rather than from a
coadsorbed one, that is, following an
ER mechanism. The use of an atomic
hydrogen flux when working in UHV
allows mimicking somehow standard
pressure conditions: the impinging
atoms carry a high kinetic energy
(“hot”atoms) when adsorbing on the
surface. Thus, they mimic the behavior
of the atoms that would populate
the surface emerging from the metal
bulk interstitial absorption sites under
standard pressure conditions, at
variance with the UHV environment.
The availability of“hot”adsorbates
allows accessing other, higher-energy,
reaction pathways.
Experiments indicate that the
exposure of a preadsorbed CO2/
Ni(110) layer to an atomic H flux gives
conversion of chemisorbed CO2 into
intermediate species that at the end
decompose into CO2, H2, and CO and
water production. DFT results yield
a straightforward interpretation of
the experimental data: under these
conditions, there is quite a large amount
of energy available (the adsorption
The energies are referred to
the gas-phase (g) atomic H and
chemisorbed CO2. In the models
of reactants and intermediate
products, only Ni atoms of the
energy of atomic H on Ni(110) is 2.6 eV)
and the hydrogenation of CO2 occurs
through the bonding of the impinging
H with one O atom, an exothermic
process with a very low barrier, of
the order of 0.1 eV. The product is a
hydrocarboxyl intermediate (HOCO)
that in turn yields CO, and OH or water.
Then the reaction proceeds from these
intermediates, and this explains why the
carbon detected in the final product,
methanol, comes from CO2 and not
from the original CO of the CO2+H2+CO
stream [13]. In fig. 6 also these paths are
sketched.
5 Mixing is not just averaging
We have reviewed the interaction
of CO2 with Ni with the aim of
understanding the higher CO2
conversion efficiency of NiCu alloys with
respect to pure Cu for the methanol
synthesis reaction. The results indicate
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surface layers are shown. HOCO*
indicates a“hot”hydrocarboxyl
intermediate. Adapted from [13].

8. 11vol30 / no3-4 / anno2014 >
Fig. 7 Temperature programmed
photoelectron spectroscopy
measurement of the Ni 3p
core level. (a) The temperature
derivative of the XPS signal
intensity providing evidence for
the presence of the steps involved
in the process, separated by the
vertical color shaded areas.
(b) experimental signal intensities
(dots) and results from a simple
diffusion/segregation model with
average energy barriers inferred
from the experiments (lines).
(c) DFT energy diagram of selected
diffusion/segregation paths for a
single Ni atom initially adsorbed
on the Cu(110) surface in case of
low local Ni concentration: the
barriers for Ni ad-dimer formation,
Ni adatom diffusion/segregation
and Ni exchange with a surface
Cu atom have the same value,
whereas higher barriers govern the
Ni diffusion towards deeper layers.
Black arrows indicate the position of
the segregating/diffusing Ni atom.
Adapted from [15].
that Ni makes CO2 more active for the
direct reduction, but this is not enough
to understand the peculiar behavior of
the alloy.
Bimetallic surfaces can have different
local composition depending on how
they are obtained (e.g., by metal-
by-metal deposition or from bulk
alloys) and on surface segregation
thermodynamics and kinetics. They
exhibit peculiar electronic and reactivity
properties, dependent on their
particular local composition, different
from the surfaces of the constituent
metals and even different from a linear
interpolation of those of the pure
constituents. This has been found, for
instance, for adsorption energies [14].
Understanding the mechanisms that
govern the arrangement of the surface
atoms at bimetallic surfaces and the
composition profile of the first layers
opens the way to the possibility of
controlling these systems, and hence of
tuning their reactivity. For this reason,
bimetallic surfaces play a decisive role
in heterogeneous catalysis.
The Ni/Cu system is a very interesting
case. In case of Ni deposited on Cu(110),
at low temperature Ni stays at the
topmost layer; by annealing, a surface
alloy is formed; further, a Cu layer pops
on-top of the Ni film; finally, Ni diffuses
into the bulk of the Cu crystal. Using
in situ time-resolved photoelectron
spectroscopy and DFT calculations it
is possible to investigate the different
steps of the segregation mechanism
in Ni-on-Cu(110) at the atomic scale,
identifying the rate-determining steps
and the associated barriers, explaining
the mechanisms of Ni island formation,
faceting, surface-to-bulk dissolution,
and bulk-to-surface segregation as a
function of temperature.
A first set of experiments consists in
starting from a fixed sub-monolayer
Ni coverage of Cu(110), then suddenly
raising the sample temperature to a
selected target value, and analyzing the
dependence of the Ni and Cu core level
signal intensities in X-ray photoelectron
spectroscopy as a function of time.
Upon diffusion of Ni atoms into the Cu
bulk, the Ni signal decreases, screened
by the atomic layers above, while the
Cu signal intensity raises, as the Cu
concentration at the surface increases.
Analyzing the two signal intensities,
the composition profile of the sample
in the direction perpendicular to the
surface can be recovered and, using a
parametric model for the segregation
mechanism, the most relevant average
energy barriers can be inferred from
the experimental data. Barriers of
increasing height are obtained when
going deeper into the Cu(110) sample.
In parallel, DFT modeling provides
energy barriers of the most relevant
steps of diffusion and segregation
processes. The combined findings
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9. 12 < il nuovo saggiatore
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indicate that Ni-Ni interaction plays an
important role in the Ni-Cu exchange
process, which is at the basis of Ni
migration into the bulk and segregation
to the surface, and that energy barriers,
beyond energy differences (i.e., kinetics
and not just energetics), determine the
segregation process. Furthermore, the
Ni segregation barriers are strongly
influenced by coverage and local
concentration. The DFT energy diagram
of these processes and the XPS data are
shown in fig. 7 [15].
The conclusion is that by choosing
proper Ni doping of a Cu model catalyst
surface, the local composition can be
effectively controlled together with
adsorption sites of the adsorbates
and their adsorption and desorption
energies.
Desorption energies can be
Fig. 8 (a) Experimental data of the
adsorbates coverage obtained
from temperature-programmed
desorption (TPD) experiments
on a Ni/Cu(110) surface covered
by CO2. Both desorption and
decomposition take place upon
heating, so that both CO2 and CO
are detected by TPD. Changing the
Ni coverage, the decomposition/
desorption ratio changes: total
carbon oxides coverage (blue
markers), direct desorption
vs. decomposition (black and
orange markers, respectively),
and conversion rate (empty
circles). Lines are drawn to guide
the eye. (b) Energy diagram of
CO2 dissociation into CO+O on
Cu(110) (orange), Ni(110) (blue),
and 1 ML Ni/Cu(110) (dashed).
Ball models of the corresponding
initial, transition, and final states
as obtained from DFT calculations
are also shown. All energies are
referred to the gas phase CO2.
Atom color legend: Cu (orange),
Ni (blue), oxygen (red), carbon
(yellow). The energy diagram
clearly shows that Ni doping
promotes decomposition.
Adapted from [15].
measured experimentally via
temperature programmed desorption
(TPD) experiments. They have been
performed after the adsorption of CO2
at liquid-nitrogen temperature under
UHV on the Ni film grown on a Cu(110)
surface. Increasing progressively the
temperature, CO2 partially desorbs
and partially decomposes giving CO
that desorbs at higher temperature.
The experiments yield straightforward
information about the desorption
energies of CO2 and CO, together with
the CO2 conversion efficiency, as a
function of the Ni coverage. As shown
in fig. 8a, the thickness of the Ni film
affects both the saturation coverage of
chemisorbed CO2 and its dissociation/
desorption ratio. Remarkably, a very
different behavior is observed: for
increasing thickness of the Ni film, CO
is progressively stabilized. Concerning
CO2, it is found that Ni doping stabilizes
a chemisorbed state, whose binding
energy decreases for thicker Ni films, at
variance with what is observed for CO
[16].
The results of the UHV experiments all
together provide a simple explanation
of the behavior observed for the CO2
reduction to methanol on Ni/Cu(100)
under high-pressure conditions, where
Ni doping makes the reaction faster:
it can be concluded that the reaction
occurs at Ni sites and that CO has the
effect of promoting surface segregation
of Ni.
The DFT calculations give a clear
rationale: Ni adatoms (even a single
one) at the Cu(110) surface lead to
a strengthening of the molecule-Ni
bond with respect to pure Ni or Cu

10. 13vol30 / no3-4 / anno2014 >
substrates. An over-binding effect is
found also for CO at low coverage. Ni
doping of Cu not only stabilizes the
reactants and products with respect
to the pure metal surfaces, but also
lowers the CO2 dissociation energy
barrier. The comparison of energies
and barriers shown in fig. 8b yields a
complete picture: in the case of pure
Cu, CO2 physisorption is energetically
even more convenient than adsorption
of the CO + O products; for the doped
surface, dissociation of CO2 is strongly
favored with respect to desorption;
for pure Ni, the two processes
compete. The concentration of Ni at
the surface is therefore an appropriate
and easily accessible parameter
to tune the reactant and product
adsorption energies, thus changing the
dissociation/desorption ratio [16].
6 Towards the nanoscale
Metal nanoparticles (NPs) present
a large surface-to-volume ratio and a
significant number of low-coordination
sites. For this reason they have unique
physical and chemical properties,
often more convenient than bulk
materials and solid surfaces [17]. NPs
are promising for intensive use in solar
cells for improved energy harvesting,
in hydrogen storage for future fuel cell
devices, and in catalysis. It is known
in general that low-coordination sites
are particularly active for catalysis
and might favor reaction paths with
a lower reaction barrier. Surprisingly,
it has been found that even a metal
like Au that is usually inert, becomes
catalytically active for several chemical
reactions when nanostructured into
nanoparticles less than 3–5 nm in
Fig. 9 Side and top view of ball-
and-stick models of a 3-atom
seed and a 15-atom Cu cluster
anchored on a defect (hole) of
diameter [18].
For most applications, NPs need to be
supported on a substrate. Often, oxide
films are used as a support, and now the
use of carbon derivatives (amorphous
carbon, graphite, and graphene) as
novel materials is increasing. Although
several aspects are still under debate,
it is clear that the support plays an
important role. The support can act as
a deposition template for the growth of
regular arrays of NPs and can prevent
sintering at high temperature that
would yield deactivation of the catalytic
devices.
Because of these and other reasons,
the synthesis of ordered arrays of
well-defined equally sized and shaped
nanoparticles is the goal of many
efforts. Oxide films are optimal matrices
for this purpose, making it possible to
M. Peressi, E. Vesselli: HETEROGENEOUS CATALYSIS TOWARDS THE NANOSCALE
alumina supported on Ni3Al alloy.
The model in the left lower corner
is a top view of the empty hole.

11. 14 < il nuovo saggiatore
scienza
in primo
piano
grow in a controlled way and under
UHV conditions model systems of
supported catalysts. Among the oxide
templates, it is worth to mention MgO
and alumina (Al2O3).
Ultrathin ordered films of non-
stoichiometric alumina can be obtained
by means of high-temperature
oxidation of nickel and aluminum
alloys. The complex Al oxide film grown
onto the Ni3Al(111) substrate has been
recently grown and characterized
[19, 20]. The surface structure of the
alumina film shows a periodic structure
characterized by a hexagonal lattice
of 4.15 nm spacing, which is visible
under particular scanning tunneling
microscopy conditions, with atomic
force microscopy, and by means of
electron diffraction techniques. This
periodicity has been related to the
presence of holes in the alumina
film down to the Ni3Al(111) support,
that act as preferential nucleation
and anchoring sites for equidistant,
regularly distributed metallic clusters
[21]. Figure 9 shows a model obtained
from numerical simulations for a small
Cu cluster [22].
The rapid progress in the use of
engineered and tailored nanoparticles
as catalytic systems is a great promise
for industrial applications. It is
impossible to give here even a very
limited review of example of synthesis,
characterization, and first applications.
We address the interested reader to
some recent reviews devoted to this
argument [23,24].
7 Novel experimental
techniques …
With the purpose of reducing the
material and pressure gaps for a
comprehensive understanding and
future large-scale use, the model
systems for catalysis become more
and more refined and complex, going
towards the nanoscale on the one
hand and towards the atomic-level
characterization in an environment
different from the model UHV
conditions. This poses new challenges
to experimentalists and theoreticians.
Novel methods are already under
development and we discuss a few
examples in this section.
Besides the upgrade to previous
surface science spectroscopy and
microscopy approaches previously used
in UHV only, two extremely innovating
techniques allow gathering information
from the catalyst’s surface up to 1 bar or
even in liquid, being almost insensitive
to the homogeneous medium above
the surface: polarization-modulation
infrared adsorption spectroscopy
(PM-IRAS) and visible-infrared sum
frequency generation (vis-IR SFG)
spectroscopy. We concentrate on the
latter, since, at variance with the former,
besides the information about the
vibrational modes of stable adsorbed
species and reaction intermediates, it
also provides indirect details about the
electronic structure of the surface, thus
delivering useful insight into the active
surface properties (metallic, oxidic,
graphitic, carbidic phases).
Vis-IR SFG spectroscopy exploits
the second-order non-linear optical
susceptibility. The tensor describing
this quantity can be non-zero only in
systems without inversion symmetry,
thus making the technique extremely
sensitive to interfaces and seeing
homogenous media like gas phases
and liquids as transparent. Two pulsed-
laser beams (with visible and an
infra-red wavelengths, respectively) are
spatially and temporally overlapped
at the catalyst’s surface. A sequence of
electronic and vibrational transitions is
excited (similarly to a Raman process)
and a photon with an energy equal to
the sum of the two incident photons is
emitted when the IR beam wavelength
matches a vibrational resonance of
a bond in an adsorbed species. The
visible beam essentially samples
surface electronic plasmons giving
origin to a non-resonant background
that interferes with the vibrational
signal, yielding the final SFG spectrum.
We remind that this picture is over-
simplified, since selection rules apply
for the signal generation. In addition,
the cross section is very low and
intense laser beams are needed to
generate a detectable SFG signal.
Many approaches can be adopted. In
particular, scanning and broadband
SFG setups can be developed. In the
former case the IR wavelength is swept
across the resonance as the spectra are
collected, while in the latter case non-
monochromatic IR photons are used
in order to privilege time resolution.
In 2D-SFG spectroscopy also the
wavelength of the visible beam can be
tuned in order to gather information on
the electronic structure of the surface.
Finally, a third laser, a second pulsed IR
beam, can be used to perform pump-
and-probe experiments.
8 … and theoretical approaches
We have shown with a review of
selected examples the power of ab initio
simulations that allow very accurate
calculations of equilibrium structures,
adsorption energies, vibrational
frequencies of reactants, intermediates,

12. 15vol30 / no3-4 / anno2014 >
M. Peressi, E. Vesselli: HETEROGENEOUS CATALYSIS TOWARDS THE NANOSCALE
and products, and relevant barriers
for simple model systems in UHV
conditions. The increasing complexity
and the refinement of model catalysts
require accounting for different
approaches, since a direct quantum-
mechanical approach is not convenient.
For instance, in the case of
nanoparticles, ab initio simulations have
to be combined with complementary
approaches aimed at determining
their lowest-energy structures. Hybrid
approaches using many-body semi-
empirical potential models allow to
generate many structures with modest
computational effort and to determine
the most favorable ones using global
optimization Monte Carlo searches. The
effect of growth temperature and of the
formation kinetics can be tackled by
Molecular Dynamics simulations. These
approaches are up to now limited to a
restricted number of materials and must
be enlarged to deal with the systems
of major interest in applications.
Moreover, some other methodological
improvements are also necessary, such
as to increase the efficiency of sampling
techniques.
Similarly, standard ab initio methods
are not fully adequate to describe
model systems acting in more realistic
environments, such as electrochemical
cells. A natural,“brute-force”method
is to explicitly include the solvent
molecules in the system. This approach
could be in principle applied for few
selected cases when the molecules
involved are not excessively large and
the simulation cells can be kept within
a reasonable size, but it becomes
prohibitively expensive if we want to
use it systematically for a good statistics.
Alternative approaches are those
describing the solvent as a continuum
dielectric medium surrounding a
quantum-mechanical solute, combined
with a first-principles formulation [25].
A further, additional approach is
offered by hybrid quantum-mechanical/
molecular-mechanical (QM/MM)
techniques. These methods treat
quantum-mechanically just the reacting
part of the system, and use molecular
mechanics for the surroundings.
They use a combined Hamiltonian
for the system. The partitioning of
the system into QM and MM parts
and their interaction, beside the
“standard”problems in the QM and
MM subsystems individually, requires
particular care.
9 Conclusions
In conclusion, we have shown
through some selected examples,
mainly focused on the hydrogenation
reaction of carbon dioxide, that
combining surface science experiments
and numerical simulations gives insight
into the atomic-scale mechanisms
that govern a heterogeneous catalytic
process. Understanding catalysis starts
from the investigation of very simple
model systems, where the catalyst is a
single-crystal surface. Basic questions
arise, such as: What are the parameters
of a surface that determine the
adsorption of a species, diffusion and
activation? Why are some adsorbates
more reactive than others on a given
surface? Which are the elementary steps
in a given reaction and which are the
rate-limiting steps? Which are the best
catalysts for steering a reaction into one
direction rather than another?
The question of the transferability
of these results to realistic conditions
is well known. A step forward is given
by the investigation of model catalytic
systems progressively more complex
and refined, going for instance
towards the nanoscale and towards
conditions of high pressure and
liquid environments. Nanoparticles
grown on thin oxide films on metal
substrates represent an interesting
and promising material combination,
showing peculiar properties and
catalytic behavior different from single
crystals. Novel experimental techniques
and significant improvements in
theoretical approaches allow tackling
the complexity of model systems acting
in conditions other than UHV.
The capability of describing the
mechanisms at the atomic level
gives important insights into the
understanding of catalytic processes
that could be used as a guideline to
predict new features and to design
novel efficient and selective catalysts.
Acknowledgments
We would like to thank all our
co-workers for their excellent work
on this subject. Most of their names
appear in the references cited here. We
acknowledge financial support from:
the Italian Ministry of University and
Research through Futuro in Ricerca, FIRB
2010 project RBFR10J4H7; the Italian
Ministry of Foreign Affairs, Directorate
General for the Country Promotion,
through the Executive Program with
Argentina; the Consortium for Physics of
Trieste, Italy; the Fondazione Kathleen
Foreman Casali; the Beneficentia
Stiftung; the Computational resources
from CINECA, partly within the
agreement between the University of
Trieste and CINECA.

13. Maria Peressi
Maria Peressi is Associate Professor of Condensed Matter Physics at the
Physics Department of the University of Trieste. Her research activity
is mainly devoted to the theoretical/computational investigation
of the electronic structure of materials with first-principles atomic-
scale simulations. The materials studied include semiconductors,
heterostructures, amorphous systems, materials for spintronics, and, in
the last years in particular, metallic surfaces, adsorbates, nanostructures.
She is presently leading a project about Nanoparticles for catalysis within
the Scientific and Technological Collaboration Executive Programme of
the Italian Ministry of Foreign Affairs with Argentina. She is Director of the
Consortium for Physics in Trieste since 2012 and Expert Member of COST
(European Cooperation in Science and Technology) for Materials, Physics
and Nanosciences domain since 2011.
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Erik Vesselli
Erik Vesselli is fixed-term Assistant Professor at the Physics Department
of the University of Trieste. After graduating and successively obtaining
the PhD in Physics in 2005, he has been working on the issue of bridging
the pressure and material gaps in surface science. By using state-of-the-
art experimental investigation methodologies, including synchrotron
radiation techniques, he devoted particular effort to the study of the
mechanisms involved in hydrogenation reactions. For this reason he
was awarded in 2006 the Energy and Environment ENI-Italgas Prize
for debut in research. Presently, he is coordinating several research
projects, including a FIRB grant focused on the carbon dioxide reduction
chemistry and based on a multidisciplinary approach including surface
science, chemistry, and theoretical techniques. At the University ofTrieste
he has just commissioned a laboratory dedicated to the investigation
of (electro)-catalytically active phases by means of vibrational sum-
frequency generation spectroscopy performed in situ and in operando.
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