ORIGINAL PAPER
Monodisperse Metal Nanoparticle Catalysts: Synthesis,Characterizations, and Molecular Studies Under ReactionConditions
Vladimir V. Pushkarev • Zhongwei Zhu •
Kwangjin An • Antoine Hervier • Gabor A. Somorjai
Published online: 10 October 2012
� Springer Science+Business Media New York 2012
Abstract We aim to develop novel catalysts that exhibit
high activity, selectivity and stability under real catalytic
conditions. In the recent decades, the fast development of
nanoscience and nanotechnology has allowed synthesis of
nanoparticles with well-defined size, shape and composi-
tion using colloidal methods. Utilization of mesoporous
oxide supports effectively prevents the nanoparticles from
aggregating at high temperatures and high pressures.
Nanoparticles of less than 2 nm sizes were found to show
unique activity and selectivity during reactions, which was
due to the special surface electronic structure and atomic
arrangements that are present at small particle surfaces.
While oxide support materials are employed to stabilize
metal nanoparticles under working conditions, the supports
are also known to strongly interact with the metals through
encapsulation, adsorbate spillover, and charge transfer.
These factors change the catalytic performance of the metal
catalysts as well as the conductivity of oxides. The
employment of new in situ techniques, mainly high-pres-
sure scanning tunneling microscopy (HPSTM) and ambi-
ent-pressure X-ray photoelectron spectroscopy (APXPS)
allows the determination of the surface structure and
chemical states under reaction conditions. HPSTM has
identified the importance of both adsorbate mobility to
catalytic turnovers and the metal substrate reconstruction
driven by gaseous reactants such as CO and O2. APXPS is
able to monitor both reacting species at catalyst surfaces
and the oxidation state of the catalyst while it is being
exposed to gases. The surface composition of bimetallic
nanoparticles depends on whether the catalysts are under
oxidizing or reducing conditions, which is further corre-
lated with the catalysis by the bimetallic catalytic systems.
The product selectivity in multipath reactions correlates
with the size and shape of monodisperse metal nanoparticle
catalysts in structure sensitive reactions.
Keywords Nanoparticle catalysts � Molecular studies
under reaction conditions � Monodispersed metal catalysts
1 Catalysts are Nanoparticles
Catalysis is a multidisciplinary field that includes aspects
of chemistry, physics, material science, biology, and
engineering all contributing to its essence of controlling the
rates of chemical reactions via use of substances called
catalysts [1]. Catalysis is traditionally divided onto three
subfields: heterogeneous, homogeneous, and biocatalysis.
In recent decades, the significant progress in the molecular
level understanding of the catalytic phenomena has
prompted the classical distinctions between these subfields
to diminish. The emerging field of nanoscience allows us to
develop novel interdisciplinary approaches for designing
more efficient catalysts and to merge the three subfields
[2, 3]. Recent breakthroughs in colloidal synthesis have
permitted an unprecedented control of composition, struc-
ture, geometric dimensions, shape, and ligand environment
of the transition metal nanoparticles. Not only do the
nanoparticles, which consist of only tens or hundreds of
atoms, present novel reactivity, their heterogeneous nature
also enables them to be effectively recycled from the
V. V. Pushkarev � Z. Zhu � K. An � A. Hervier �G. A. Somorjai (&)
Department of Chemistry, University of California, Berkeley,
CA 94720, USA
e-mail: [email protected]
Z. Zhu � A. Hervier � G. A. Somorjai
Lawrence Berkeley National Laboratory, 1 Cyclotron Rd,
Berkeley, CA 94720, USA
123
Top Catal (2012) 55:1257–1275
DOI 10.1007/s11244-012-9915-y
reaction media. When metal nanoparticles are supported by
oxides, the support materials may also provide an addi-
tional catalytic functionality that further improves their
overall catalytic performance. In situ characterization has
also revealed that the surface structure might undergo
dramatic change under realistic reactions, which will
potentially alter their catalytic behavior. Accordingly, it
has become apparent that these novel nano-engineered
materials are the key to achieving the optimal levels of
activity, selectivity, and stability in catalytic processes in
the 21st century. In this review, we summarize the recent
developments in synthesis, materials characterization and
catalytic performance in industrially significant chemical
reactions of transition metal nanoparticles.
2 Colloidal Synthesis of Transition Metal
Nanoparticles: Effective Control of Particle Size,
Shape and Composition
Transition metal nanoparticles with high uniformity of
their essential properties such as size, morphology (shape),
and composition can be synthesized via colloidal methods.
In this section, we focus on recent progress in colloidal
synthesis of nanoparticles with well-defined size, shape,
and composition for three transition metals: platinum (Pt),
rhodium (Rh), and palladium (Pd). These are our chosen
focus because of their exceptional catalytic properties.
2.1 Size Controlled Metal Nanoparticles
A conventional strategy for colloidal synthesis is the
reduction of metal precursors in polar solvents in the
presence of surfactants that prevent nanoparticles from
aggregating in solution. Alcohols usually play the roles of
both solvents to dissolve metal precursors and reducing
agents to generate metal nanoparticles, while polymers
such as poly(vinylpyrrolidone) (PVP) or hyperbranched
dendrimers can serve as stabilizing agents. In order to
narrow the particle size distribution, nucleation and growth
kinetics should be regulated as well as steric control of
surfactants during synthesis. Rioux et al. synthesized Pt
nanoparticles with a particle size range of 1.7–7.1 nm
using dihydrogen hexachloroplatinate (H2PtCl6) as the Pt
precursor in the presence of PVP in different solvents such
as methanol, ethanol, and ethylene glycol to control the
reduction rate [4]. The seed mediated growth strategy, in
which metal shells deposited on the surface of the exter-
nally added nanoparticle seeds, is also employed to govern
particle sizes. Recently, extremely small metal nanoparti-
cles or clusters less than 3 nm have been synthesized by
using polyaminoamide dendrimer. Huang et al. synthesized
Pt and Rh nanoparticles with in size regimes 0.8–1.6 nm by
using 4th-generation dendrimers [5]. Kuhn et al. controlled
the Pt nanoparticle sizes from 0.8 to 5 nm by using either
dendrimer or PVP as the capping agents and demonstrated
size-dependent selectivity of Pt catalysts for pyrrole
hydrogenation reactions (Fig. 1) [6].
2.2 Shape Controlled Metal Nanoparticles
As the particle shapes also participate in determination of
surface active sites, a number of synthetic strategies have
been developed to simultaneously control the size and
shape of Pt-group nanoparticles. For instance, the nano-
particle shape can be controlled by addition of trace
amounts of secondary transition metal ions into the reac-
tion mixture during the crystal growth stage. Song et al.
used small concentrations of Ag? ions to prepare Pt
nanocrystals with cubic, tetrahedral, and cuboctahedral
shapes in the 10 nm size range [7]. Wang et al. reported the
synthesis of 3.5 nm polyhedral, 7–8 nm cubic and 5 nm
truncated cubic Pt nanocrystals by adding traces of
Fe(CO)5 into the reaction solution as the shape control
agent [8, 9]. The secondary metal species are suggested to
adsorb on certain crystal facets, which directs the further
crystal growth. Nevertheless, the mechanism of how the
addition of secondary metal controls the shape of nano-
particles is not yet fully understood.
Halogen ions, among which the most frequently used is
bromide ion, are the alternative to secondary metal species
for the shape control of Pt-group metal nanoparticles. The
bromide species can be readily washed away after synthesis,
owing to the weak interaction with metal surfaces rather than
strong incorporation into the particles [10]. As a result, cat-
alytically active Rh and Pt nanocubes were synthesized in the
presence of Br-, their shape being attributed to the prefer-
ential stabilization effect on {100} faces by Br- 11, 12].
Fig. 1 Dendrimer- or PVP-capped Pt nanoparticles with controlled
sizes and the size-dependence of their selectivity for pyrrole
hydrogenation (4 torr pyrrole, 400 torr H2, 413 K) [6]
1258 Top Catal (2012) 55:1257–1275
123
Lately, Tsung et al. reported synthesis of Pt nanoparticles
in the shapes of cubes and polyhedra from 5 to 9 nm by
controlling the reduction rate of metal precursors in a one-pot
polyol synthesis, without any use of secondary metal com-
pounds [10]. The oxidation state of Pt precursors determined
the number of nuclei in the nucleation step which in turn
regulated the size of Pt particles. The shapes of Pt were
governed by the reduction rate, in turn controlled by reaction
temperature, as indicated by the scheme and transmission
electron microscopy (TEM) images shown in Fig. 2.
2.3 Composition Controlled Bimetallic Nanoparticles
In many catalytic reactions, bimetallic nanoparticles allow
superior activity and selectivity to be achieved as compared
to their monometallic counterparts. The addition of a
Fig. 2 (top) Schematic illustration of the size and shape control of sub -10 nm Pt nanoparticles. (bottom) TEM and HRTEM images of Pt
nanocubes with sizes of a–b 9 nm, c–d 7 nm, and e–f 6 nm, respectively [10]
Top Catal (2012) 55:1257–1275 1259
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second metal is not only able to alter the surface electronic
or geometric structure of the primary metal, but also add
additional active sites such that the material may perform as
a bifunctional catalyst [13, 14]. Tao et al. synthesized
RhxPd1-x (*15 nm), RhxPt1-x (8–11 nm), and PdxPt1-x
(*16 nm) nanoparticles with various atomic fractions
(x = 0.2, 0.5 and 0.8) [15]. Despite the fact that the bime-
tallic nanoparticles were prepared using a one-step colloidal
method, the X-ray photoelectron spectroscopy (XPS)
results showed a core/shell structure for the as-synthesized
RhxPd1-x and PdxPt1-x nanoparticles. Park et al. also syn-
thesized RhxPt1-x bimetallic nanoparticles with varying
composition in a constant size (9 ± 1 nm) [16]. Figure 3
shows the Rh 3d, Pt 4d and Pt 4f XPS spectra recorded from
Langmuir–Blodgett (LB) films of RhxPt1-x (x = 0–1)
nanoparticles and the corresponding TEM images. The
alloy compositions estimated by integrated peak areas and
sensitivity factors in XPS spectra agreed with the initial
fraction of Rh precursor, x, as demonstrated in Fig. 3b.
3 Preparation of Heterogeneous Catalysts Based
on Colloidal Nanoparticles
Prior to being exposed to catalytic conditions, typically
high temperatures and high pressures, metal nanoparticles
should be supported either on flat substrates or in porous
materials in order to prevent their severe aggregation at
these harsh conditions. A few methods have therefore been
developed to enable the metal nanoparticles to survive and
serve as model catalysts under realistic conditions.
3.1 2-D and 3-D Catalysts Based on Colloidal
Nanoparticles
Applied colloidal metal nanoparticles are mainly classified
into two main types according to the dimension of their
supports: two-dimensional (2-D) and three-dimensional
(3-D) catalysts (Fig. 4). 2-D catalysts are prepared by
nanoparticle assembly on a planar substrate with a LB
trough. Nanoparticles capped with surfactants, floating on a
poor solvent, form a close-packed array on a substrate,
typically a Si wafer, while immersing the substrate into the
solvent. The inter-particle spacing can be tuned by varying
the surface pressure. Thus, the catalytic performance as a
function of size and shape of the nanoparticles can be
studied with such monolayer assemblies. However, many
industrial catalysts require a larger amount of metal parti-
cles than an LB film can provide. The metal nanoparticles
are therefore dispersed into oxide materials or activated
carbons with high surface area, ordered pore structure, and
large pore volume as 3-D catalysts. SBA-15 and MCF-17
mesoporous silicas have been utilized for the preparation of
3-D catalysts by incorporating metal nanoparticles into
their pores [6, 10, 17].
Conventional industrial catalysts with high surface area
are prepared by either ion-exchange or incipient wetness
impregnation, where the difficulties in thermal activation
usually cause a broad size distribution of nanoparticles.
Nowadays, the size and shape controlled metal nanoparticles
are incorporated into mesoporous supports by two methods:
capillary inclusion and nanoparticle encapsulation. In the
capillary inclusion method, nanoparticles are dispersed into
Fig. 3 a X-ray photoelectron spectra measured on RhxPt1-x (x = 0 - 1) nanoparticles on silicon surface. b Plot of Rh composition determined
by XPS measurement. c TEM images of the RhxPt1-x nanoparticles [16]
1260 Top Catal (2012) 55:1257–1275
123
mesoporous supports by sonicating together a colloidal solu-
tion of metal nanoparticles and oxide supports. Although the
nanoparticles can be dispersed this way within the mesopor-
ous framework [17], a certain fraction of the them is poten-
tially located on the outer surface of support granules, but not
inside the pore channels. Moreover, the maximum particle
size incorporated is restricted by the diameter of the chan-
nels. Nanoparticle encapsulation, in which the silica grows
around the metal particles, has hence been developed as an
alternative approach. Song et al. reported that monodi-
spersed Pt nanoparticles of 1.7–7.1 nm were incorporated
into SBA-15 via hydrothermal synthesis, in which Pt parti-
cles are located within the surfactant micelles during silica
formation [18].
3.2 Metal Core/Silica Shell Typed Catalysts
As a number of industrial catalytic reactions are performed
at temperatures above 573 K, the decomposition of organic
capping agents leads the nanoparticles being more sus-
ceptible to aggregation and thereby loss of activity. Thus,
thermal stability of nanoparticle model catalytic systems
should be considered to be as important as catalytic activity
and selectivity. Since the model catalysts with high thermal
stability at elevated reaction temperatures are demanded,
metal-core/inorganic-shell typed structures are designed
with intention of keeping the metal nanoparticles from
aggregating. Joo et al. reported the synthesis of silica shells
over TTAB capped Pt nanoparticles, which were subse-
quently converted to Pt/m-SiO2 by calcination to generate
the metal-core/silica-shell structure, as shown in Fig. 5
[19]. The Pt/m-SiO2 structure showed as high an activity in
ethylene hydrogenation and CO oxidation as the bare Pt
nanoparticles, indicating that the mesoporous shell did not
prevent the reactant molecules accessing the Pt surfaces
during catalytic reactions. The Pt/m-SiO2 core/shell struc-
ture was maintained up to 1023 K with Pt nanoparticles
still being encaged in the silica shell.
4 Chemistry of Structural and Compositional
Sensitivity
4.1 Structure Sensitive and Insensitive Reactions
in Catalysis
In his original work ‘‘Catalysis by Supported Metals.’’
M. Boudart introduced a classification of heterogeneous
catalytic reactions based on structure-sensitivity [20]. At the
root of this distinction was the experimental evidence that
specific (turnover) reaction rates of some catalytic reac-
tions depended on the surface structure of active transition
metal catalysts, while such dependence was not observed
for some other reactions. For example, isomerization and
cracking of neopentane on Pt was assigned to structure-
sensitive reactions, since the experimental measurements
Fig. 4 Schematic illustrations for preparation of nanoparticle-based heterogeneous catalysts
Top Catal (2012) 55:1257–1275 1261
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of turnover rates over supported Pt catalysts displayed
strong dependence on Pt particle size. To the contrary, the
reaction rates of catalytic reactions such as hydrogen–
deuterium exchange, hydrogenolysis of cyclopentane, and
hydrogenation of 1-hexene stayed constant with the Pt
particle size and were thus classified as structure-
insensitive.
Boudart’s explanation of structural sensitivity was based
on the active ensemble (geometric) theory of catalysis orig-
inally developed by Kobosev and further by Poltorak [20].
The structure sensitivity of chemical reactions originates
from the availability of surface active sites with particular
geometry and is determined by the material crystalline
structure and particle size. The validity of the theory was
confirmed in a series of surface science studies of catalytic
reactions using several single crystals on which distin-
guished surface sites with a specific geometry are present.
For instance, ammonia synthesis over (111), (100) and (110)
crystal faces of Fe, cyclohexane dehydrogenation and
hydrogenolysis of cyclohexane and cyclohexene over (111),
(100), (557), (25, 10, 7) and (10, 8, 7) crystal faces of Pt, and
thiophene hydrodesulphurization over Re (0001) single
crystal surfaces are all structure sensitive [21–24]. In con-
trast, ethylene hydrogenation over Pt(111) and (100) and
thiophene hydrodesulphurization over Mo(100) surfaces are
structure-insensitive [24, 25]. Studies of catalysis on single
crystals are essential for fundamental understanding of the
chemical mechanisms that affect structural sensitivity in
heterogeneous catalysis. Nevertheless, though being well
described, single crystal surfaces lack the complexity of real
catalysts in that they cannot mimic the effects of metal
cluster size and metal-support interactions on the reaction
rate and selectivity. Thus, the next advancement in basic
research of catalysis phenomena should be carried out using
well characterized model systems constructed of monodi-
spersed nanoparticles supported on two or three dimensional
supports. When nanoparticle size decreases to a certain range
(1–5 nm), the surface structure of metal crystals is expected
to change, because certain configurations of atoms at sur-
faces may no longer be available upon decreasing of the
crystal dimension below specific threshold limits.
4.2 An Overview of Structure-Sensitive Reactions
The majority of structure-sensitive reactions belong to one
of the three following reaction classes: hydrogenation/
dehydrogenation, C–C cleavage/coupling, and oxidation.
Scheme 1 shows examples of recently investigated struc-
ture-sensitive reactions.
Kliewer et al. studied adsorption and catalytic hydro-
genation of furan on the Pt(100) and Pt(111) single crystal
surfaces and on monodispersed Pt nanoparticles with 1,
3.5, and 7 nm particle sizes (Fig. 6) [26]. Furan hydroge-
nation on Pt produces two ring hydrogenation products—
2,3-dihydrofuran (DHF) and tetrahydrofuran (THF), one
ring opening product—n-butanol, and one ring cracking
product–propylene (Scheme 1a). Figure 6 displays the
dependence of the initial reaction product selectivities at
393 K on the Pt particle size and the Pt single crystal
surface orientation, illustrating strong structure-sensitivity.
The selectivity towards propylene, the dominant product
using nanoparticle catalysts, is enhanced from 70 to 83 %
upon increasing the particle size from 1 to 7 nm, while
selectivity towards n-butanol simultaneously decreases
from 22 to 8 %. The selectivities to the two ring hydro-
genation products, DHF and THF, are less dependent on
the Pt particle size than those of the ring opening and
cracking products. Two major products observed during
furan hydrogenation over Pt crystals are THF and n-buta-
nol, while propylene is not detected at all. The absence of
any propylene on Pt single crystal surfaces is possibly due
to the lack of coordinatively unsaturated active sites that
are required for the deep ring cracking. The differences in
the selectivities toward THF and n-butanol between
Fig. 5 TEM images of a Pt nanoparticles, b Pt/SiO2, and c Pt/mesoporous-SiO2 core/shell structures
1262 Top Catal (2012) 55:1257–1275
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Pt(100) and Pt(111) are also well pronounced. Pt(100) is
more active in hydrogenative ring opening that results in
the formation of n-butanol, as compared to Pt(111), which
favors formation of THF under these experimental
conditions.
Reactions that involve a carbon–nitrogen ring opening
step over Pt are also structure-sensitive. For instance, Kuhn
et al. studied hydrogenation of pyrrole over a particle size
dependent series of Pt catalysts, demonstrating that Pt
nanoparticles below 2 nm favored pyrrolidine [6]. Reac-
tions of alkane hydrogenolysis over supported Pt catalysts
are also structure sensitive [4, 18].
Benzene hydrogenation is particularly relevant to the
petrochemical and fine chemical industries. Studies dating
back four decades indicate that benzene hydrogenation
over supported Pt catalysts is structure insensitive [20].
Recent studies of this reaction by a combination of reaction
kinetics and in situ surface sensitive vibrational spectros-
copy on Pt single crystal surfaces and monodispersed shape
controlled Pt nanoparticles have revealed that benzene
hydrogenation can be structure sensitive [27–29]. Indeed,
benzene hydrogenation over Pt(111) single crystal surface
results in formation of two reaction products: cyclohexane
and cyclohexene [27]. On Pt(100), however, only cyclo-
hexane is observed [28]. The results on single crystals are
in agreement with the kinetics measured over Pt nanopar-
ticles with finely controlled shape. Both cyclohexene and
cyclohexane reaction products are formed on Pt nano-
crystals with cuboctahedral shape which exhibited both
(111) and (100) faces [29]. On the other hand, under
similar reaction conditions using Pt nanocubes exposing
solely (100) face, only cyclohexane could be detected.
The catalytic oxidation of CO to CO2 over platinum
group metals is a structure sensitive reaction that carries
significant industrial and environmental importance
[30, 31]. In additoin to particle size and shape, another
important factor that can tailor the catalyst’s surface elec-
tronic structure involves the surface composition when
multi-component catalysts are utilized. Park et al. studied
the Rh composition dependence of catalytic activity in CO
oxidation on a series of RhxPt1-x (x = 0–1) nanoparticles
of a constant (9 ± 1 nm) size [16]. The reaction kinetics
were studied using two-dimensional nanoparticle LB film
catalysts on Si substrates. The turnover rate measurements
at 453 and 473 K revealed that CO oxidation rates exhibit a
20 ± 4 times increase upon transition from pure Pt to pure
Rh (Fig. 7). A similar trend was also observed for the
apparent activation measured in this temperature range; its
value increased from 25.4 ± 1.2 to 27.1 ± 1.4 kcal/mol
upon increasing Rh content. It is worth mentioning that the
increase of turnover rates of the bimetallic nanoparticles
increased nonlinearly as a function of total Rh content,
coincident with the partial segregation of Pt to the nano-
particle surface under reaction conditions.
Scheme 1 Examples of structure-sensitive reactions: hydrogenation
of a furan and b pyrrole, c ethylene hydroformylation, d CO
hydrogenation (Fischer-Trosch synthesis), e methylcyclopentane ring
rearrangement, dehydrogenation, hydrogenative ring opening and
isomerization, f ethane hydrogenolysis, and g CO oxidation
Fig. 6 Dependence of product selectivities in furan hydrogenation on
the size of Pt nanoparticles encapsulated in a dendrimer (1 nm) and
PVP (3.5 and 7 nm) and on the crystallographic orientation of Pt(100)
and Pt(111) single crystal surfaces. The product selectivity values
were determined at 393 K using 10 torr of furan and 100 torr of
hydrogen in a batch reactor with forced recirculation
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5 Effects of the Catalyst Support on Nanoparticle
Catalysis
As mentioned in the previous sections, once the nanopar-
ticles are solely exposed to the temperatures required for
catalytic reactions, the quick aggregation would lead to a
substantial drop in the turnover frequency owing to the
catalyst surface area vanishing. Therefore, metal nanopar-
ticles are ordinarily dispersed on a porous oxide or car-
bonaceous support in industrial applications [32]. Usually
not active on its own, the support materials tend to main-
tain the metal catalysts in a highly dispersed state during
catalytic applications [33]. However, it has been known for
decades that the choice of support also has dramatic effects
on metal surface chemistry, which is largely referred to as
the ‘‘strong metal support interaction’’ (SMSI).
In the original sense, the term SMSI described a specific
phenomenon observed in catalysts synthesized by the
incipient wetness impregnation method. Tauster and Fung
first observed that upon reduction in H2 at high tempera-
tures, noble metal catalysts supported on TiO2 almost
completely lost their ability to adsorb CO and H2 without
significant change in catalyst surface area [34]. Electron
microscopy and X-Ray diffraction showed that the loss of
adsorption ability was not due to aggregation of the plati-
num particles. Hence, the intriguing factors that contrib-
uted to the unexpected activity loss became an attractive
topic of study.
Despite having been studied for decades with hundreds
of relevant publications, the SMSI phenomenon is still in
need of experimental investigation to fully understand the
effect. Most studies deal with the more general question of
understanding how oxide supports interact with the metal
catalysts, regardless of whether or not the support materials
have been reduced in H2 at high temperatures. The distinct
chemical nature of various supports and the diversity of
metal/oxide interactions further complicate the picture. A
recent example has revealed that gold nanoparticles with
identical sizes exhibited dramatically different behaviors
for CO oxidation reaction depending on the type of oxide
support used [35]. So far, several valid models that are not
mutually exclusive have been proposed in regards to the
effects at metal/oxide interface.
5.1 Decoration/Encapsulation
Tauster and Fung described their catalysts as being in an
‘‘SMSI state’’ after reduction in hydrogen at 773 K, a state
in which there was virtually no adsorption of CO and H2.
The authors ruled out a possible explanation of metal
encapsulation by the oxide because the effect was revers-
ible while the total surface area of the catalyst was
unchanged [34]. However, evidence to the contrary has
been become available since then.
Baker et al. reduced Pd/TiO2 catalysts at 973 K and
suggested that TiO2 was reduced to Ti4O7 which subse-
quently migrated over the Pd surface, based on TEM
images and H2 adsorption results [36]. In a similar exper-
iment, Komaya et al. provided high resolution TEM images
of reduced Rh/TiO2 catalysts, demonstrating that Rh par-
ticles were partially covered by an amorphous titania
overlayer after reduction at 573 K. The titania completely
covered Rh particles upon reduction at 773 K [37]. The
decoration of Rh by TiO2 agreed with an uptake drop in H2
adsorption experiments. As TEM resolution has continu-
ously improved, evidence for encapsulation became
unquestionable: Fig. 8 illustrates a Rh particle encapsu-
lated with CeTbOx after reduction at 1173 K [38].
It is generally agreed that surface tension is the driving
force behind encapsulation. Leyrer et al. showed that the
ability of an oxide to wet the surface of the metal catalyst
correlated with its surface energy [39]. However, the fact
Fig. 7 Plot of turnover rates and Ea in CO oxidation over RhxPt1-x
(x = 0–1) nanoparticles as a function of Rh content. The experiments
were performed in the 453–473 K temperature range in a batch
reactor using 100 torr O2 and 40 torr CO initial reactant pressures
[16]
Fig. 8 HREM images of a 0.5 % Rh/Ce0.8Tb0.2O2-x catalyst reduced
at 1173 K [38]
1264 Top Catal (2012) 55:1257–1275
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that support effects could be observed at reduction tem-
peratures lower than those required to cause encapsulation
greatly challenged the explanation [40]. As a consequence,
encapsulation is considered not to be the only factor con-
tributing factor to these SMSI effects.
5.2 Spillover
Various catalysts differ in their ability to physisorb and
chemisorb gaseous species. By combining two different
surfaces, species adsorbed on one surface are capable of
migrating onto the other, provided that a large enough
interface area is available by a high dispersion of metal
nanoparticles on the oxide supports. Since the first hypoth-
esis of spillover was introduced as early as 1940 by Emmett
[41], many supportive phenomena have been reported.
Kuriacose et al. first reported that the presence of Pt
accelerated the decomposition of GeH4 to Ge [42]. Taylor
later suggested that the Pt surface provided recombination
sites for atomic H to form H2 that readily desorbs [43].
Lately, spillover was directly observed for the first time
using scanning tunneling microscopy (STM) on methanol
adsorption onto the Pt/TiO2(110) catalyst [44]. The
sequential STM images in Fig. 9 showed the formation of
bright spots at the interface between the Pt particles and the
TiO2 surface and the migration along the five fold-coor-
dinated Ti rows away from Pt. TPD measurements indi-
cated that these spots corresponded to CH3O(a), even
though TiO2(110) alone cannot dissociatively adsorb
CH3OH at room temperature. As a result, spillover might
lead reactions to occur through pathways whose activation
barriers are too high without the existence of interfaces.
On the other hand, spillover can complicate the task of
measuring surface area for calculating turnovers. On Rh/
TiO2, for example, dissociatively adsorbed H atoms on Rh
can spill over onto the TiO2 surface, resulting in an over-
estimate of the number of active Rh surface sites [37]
Fig. 10.
5.3 Charge Transfer
It was proposed early on that certain forms of charge
transfer, which occurred on or within the catalyst, played a
significant role in oxide support effects. However, the
various possible forms of charge transfer lead to a poor
current understanding of such effects.
5.3.1 Charge Transfer at the Metal/Oxide Heterojunction
5.3.1.1 Steady State Charge Transfer It is well known
that when the surfaces of two materials are brought into
contact, the difference in Fermi levels drives electrons to
flow from the one with a high Fermi level to the other until
reaching equilibrium. The phenomenon is the basis not
only for the electronics industry but also for the catalysis at
metal/oxide interfaces.
Fig. 9 Snapshots of sequential STM measurements of the methanol
adsorption process on a Pt/TiO2(110) surface. 6.4 9 6.4 nm2; Vs,
?1.0 V; It, 0.30 nA. Image (a) was captured just after introduction of
methanol vapor into the STM chamber by backfilling. The vapor
pressure was kept at 1.2 9 10-7 Pa during the measurement. Images
(b-h) were acquired 170, 225, 280, 335, 775, 830, and 885 s,
respectively, after image (a) was acquired [44]
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Fung characterized thin films of Pt on SiO2 and TiO2 by
XPS before and after reduction in H2 at 623 and 873 K [45].
The Pt 4f peak was shifted down in binding energy by 1.6 eV
following treatment at both temperatures, whereas no such
shift occurred for the Pt thin film supported on SiO2. Instead of
the direct reduction of the metal by hydrogen, they ascribed
the shift to electron transfer from the TiO2 support to metal.
Nevertheless, contradictory proofs were later reported by
Sexton et al., who pointed out that the surprisingly high
downshift would actually corresponded to the transfer of 1.5
electrons per platinum atom [46]. They found that reduction in
hydrogen led to a small shift in the order of 0.04 eV. The
energy shift was only partially reversible upon re-oxidation,
perhaps on account of sintering of the metal particles. The
reversible contribution to the binding energy downshift was
hence only 0.02 eV, two orders of magnitude smaller than
Fung’s reports.
The suggestion of such a small transfer of electrons to
the metal was met with skepticism by Ponec [47], since the
charge screening length in a metal is approximately the
single bond length [48]. Nonetheless, Resasco and Haller
found that the kinetics of ethane hydrogenolysis and
cyclohexane dehydrogenation on Rh/TiO2 could be
explained by a model that involved two kinds of charge
transfer [49]. After a low temperature reduction in H2 at
473 or 523 K, the metal particles donated electrons to the
support, which was more noticeable for smaller particles.
After a high temperature reduction in H2 at 773 K, the
electron transfer occurred from the oxide to the metal, and
became localized. This amounted to stating that a chemical
bond formed between the oxide and the metal, suggesting
that the oxide covered up the active metal sites, combining
the charge transfer model and the encapsulation model
together.
5.3.1.2 Charge Transfer During a Reaction Work in our
laboratory showed that charge transfer from the metal to
the oxide could also occur as a direct result of the reaction
occurring on the surface. The nanodiode, consisting of a
metal film deposited onto a semiconductor, was employed
as the solid state model catalyst to observe the charge
transfer occurring as a dynamic event.
As explained previously, the mismatch in Fermi levels
leads to charge transfer between the metal and the semi-
conductor, which in turn causes the energy bands to bend at
the interface (Fig. 10). Contacts can either be ohmic, or
Schottky-type with a transport barrier. The barrier serves as
a high energy filter, letting through only hot electrons, i.e.,
electrons with energy significantly higher than the Fermi
level in the case of an n-type semiconductor. The same
concept can also be applied as hot holes when using p-type
semiconductors. By connecting the diode to a circuit, it
becomes possible to measure the flow of hot electrons or
holes under reaction conditions. This chemicurrent is
measurable if the metal film is thin enough for electrons to
reach the Schottky barrier without dissipating their excess
energy, since typical mean free paths for electrons with
excess energies of 1 eV in metals are in the order of a few
nanometers [50].
The mechanism was recognized for two exothermic
reactions, CO oxidation over both Pt/TiO2 and Pt/GaN
diodes, and H2 oxidation over Pt/TiO2 diodes [51, 52]. In
both cases, the activation energies measured for the cur-
rents were in agreement with the activation energies of the
reaction, indicating that the reaction on the surface dissi-
pates energy into the metal by exciting electrons. In a true
catalyst where no circuit is present to shuttle charges back
to the metal, eventually an electrical field appears to pre-
vent any further charge flow. These experiments lead to an
important conclusion that if a reaction leads to a current
between the metal and the oxide, it raises the possibility
that applying certain currents to the catalytic nanodiode
will affect the surface chemistry by the reverse mechanism.
5.3.2 Charge Transfer From the Oxide to the Adsorbate
More recently, we have found evidence of charge transfer
occurring from titanium oxide to surface oxygen during
CO oxidation (Fig. 11) [53] and methanol oxidation [54].
Fig. 10 Scheme for the detection of ballistic hot charge carriers in a
reaction using a catalytic metal semi-conductor Schottky diode.
a Band bending at the interface leads to hot electron collection when
the semi-conductor has a higher Fermi level than the metal. b Hot
holes are collected when the semi-conductor instead has a lower
Fermi level than the metal [51]
1266 Top Catal (2012) 55:1257–1275
123
In the experiments, TiOx films were annealed under dif-
ferent conditions to obtain various stoichiometries, such as
TiO1.7, TiO1.9, and TiO2 determined by XPS. Oxygen
vacancies in TiO2 create an electronic state about
0.5–1.0 eV below the bottom of the conduction band [55].
The midgap state acts as a conduction channel to amplify
the conductivity of the film by orders of magnitude
[56, 57]. Each type of titanium oxide film was also doped
in SF6 plasma, yielding six types of oxide support: fluorine
doped and undoped TiO1.7, TiO1.9, and TiO2. F binds with
Ti by filling oxygen vacancies and consequently the elec-
trons filled the vacant midgap states, slightly offsetting the
conductivity of the TiO1.7 and TiO1.9 films [53]. However,
F also acts as an n-type donor, forming donor levels just
beneath the conduction band, which increases conductivity
of TiO2 by 40-fold, as shown in Fig. 11b.
While both types of electronic structure modification
can increase the film conductivity, the resulting conduction
channels are about 1.0 eV apart in energy. This energy
difference correlates with the surface chemistry of the Pt/
TiOx catalysts. Although turnover increases nearly two-
fold when stoichiometric TiO2 is F-doped, no increase is
observed with the non-stoichiometric TiO1.7 and TiO1.9
films, as demonstrated in Fig. 11a.
Since CO oxidation on platinum is limited by activation
of the Pt–O bond, the increase in turnover rate may be
attributed to electron transfer from the oxide to surface O,
which is an activating factor for reaction with CO. Non-
stoichiometric TiOx does not show similar effects because
the conduction channel formed by midgap states is much
lower in energy. Electrons in those states have insufficient
energy to transfer to surface O. Similar work was then
carried out for methanol oxidation [54]. Under the condi-
tions used, the three products of the reaction are the total
oxidation product—CO2, and partial oxidation products—
methyl formate and formaldehyde. After fluorine doping in
stoichiometric TiO2, the methanol oxidation occurs sig-
nificantly faster with the partial oxidation product fraction
enhanced from 17 to 35 %. When non-stoichiometric TiO2
was used, fluorine doping decelerates catalytic turnovers,
while the selectivity toward partial oxidation becomes less
favored or unchanged depending on the oxygen vacancy
concentration. All of the experimental results suggest that
modifying the electronic structure of the support, in this
case by fluorine doping, tunes both the activity and the
selectivity of a catalyst through charge transfer.
6 In Situ Characterization Techniques
Although extensive care has been taken to control the size,
shape, composition of nanoparticles and the interaction
with supports which play an important role in catalytic
reactions, the chances are that the properties change when
varying reaction environments. Owing to the inherent
complexity of real systems stemming from the presence of
various active sites, high adsorbate mobility, diverse
interactions at surfaces, and disparate reaction intermedi-
ates, the results obtained with traditional surface science
approaches under ultrahigh vacuum (UHV) might not be in
general applied to catalyst structure during catalytic reac-
tions. The 13 orders of magnitude pressure difference
between UHV studies and real catalysis at atmospheric
pressure is referred to as the ‘‘pressure-gap’’ [58, 59]. Since
understanding the fundamental reaction processes, includ-
ing the adsorption, dissociation, diffusion and turnover of
reactants as well as desorption of products, has always been
the ultimate objective of surface science, design and
improvement of in situ techniques gives a strong impetus
toward studying the active phases and structural evolution
during reactions. A great deal of effort has been devoted to
bridge the ‘‘pressure-gap’’ with several techniques such as
Fig. 11 a Turnover frequencies (TOF) for CO oxidation on Pt
nanoparticles supported on the six titanium oxide supports: TiO2,
TiO1.9, and TiO1.7, each with and without F insertion. Reaction
occurred in 40 Torr CO, 100 TorrO2, and 620 Torr He at 443 K. TOF
data reflect the stable rate after *30 min of deactivation. Error bars
represent 95 % confidence intervals. b Surface conductivity mea-
surements for all six titanium oxide supports before Pt nanoparticle
deposition. In the case of TiO2, F insertion increased surface
conductivity by a factor of 40 by acting as an extrinsic n-type donor.
However, in the case of TiO1.7 and TiO1.9, F insertion slightly
decreased the conductivity because F binds to Ti at O vacancy sites,
resulting in the removal of subgap states that act as a transport
channel in these samples. Note that TiO2 with and without F is
magnified by 106. This reflects the insulating nature of TiO2 without
the presence of a sub-band conduction channel. Comparison of panels
A and B shows a surprising similarity between the effect of F on the
TOF and on the surface conductivity [53]
Top Catal (2012) 55:1257–1275 1267
123
X-ray adsorption spectroscopy, sum frequency generation,
and infrared spectroscopy to monitor the physical and
chemical behaviors of catalysts under reactions [60–63].
Here we review two important techniques we normally rely
on, ambient-pressure X-ray photoelectron spectroscopy
(APXPS) and high-pressure scanning tunneling microscopy
(HPSTM), which detect the changes in electronic structure
and morphology of catalysts in response to gas condition
changes.
6.1 Ambient-Pressure X-ray Photoelectron
Spectroscopy
Photoelectron spectroscopy techniques have contributed
vastly to the particularly large surface electronic structure
database, which benefits our understanding the fundamen-
tal reaction processes. These techniques were confined to
use in vacuum for decades because of the strong interaction
between the emitted electrons and gas molecules at ele-
vated pressures. However, it is still desired to carry out
photoelectron experiments at high pressures to benefit from
the specific surface sensitivity of the photoelectron based
methods. In order to attenuate the severe electron scattering
by gases, a differential pumping system was first utilized to
perform XPS experiment at pressures up to 1 mbar [64,
65]. An assembly of electron pre-lens with differential
pumping designed at the Advanced Light Source in Law-
rence Berkeley National Laboratory focused the photo-
electrons that passed through a small aperture, which
therefore heavily increased the number of electrons
accepted by the hemispherical analyzer, as shown in the
schematics in Fig. 12 [65, 66]. The photoelectron emission
was further amplified by 10 times via recent modification
of the lens geometry in 2010 [67]. The accordingly shorter
time scale for data acquisition remarkably increased time
resolution. Moreover, as the upgraded system does not
require any nodes while focusing the electrons, spatial and
angular information with spatial resolution of 16 lm and
angle resolution of 0.5� can currently be recorded, which
opens new possibilities of mapping the catalyst electronic
structure during catalysis [67, 68]. It is worth noting that
even if the pressure drops by over six orders of magnitude,
the pressure at the sample is still at least 95 % of the
chamber pressure, thus guaranteeing the validity of APXPS
experimental results.
APXPS has gained considerable attention owning to its
specific ability to detect surface species such as reactants,
products, intermediates, spectators, poisonous species, and
contaminants during the surface’s interaction with the gas
phase [69–72]. APXPS can also probe the oxidation state
changes of catalyst surface involved in the reaction pro-
cess, which is expected to be connected with activity and
selectivity of heterogeneous reactions [73–76]. For
example, a series of APXPS and kinetic studies were per-
formed on Rh nanoparticles with diameters from 2 to
12 nm under CO oxidation conditions, in order to investi-
gate the relation of the superior turnover frequency
exhibited by 2 nm Rh nanoparticles to the distinctly small
size [74]. It was demonstrated by gas chromatography that
2 nm Rh nanoparticles were seven times as active as 12 nm
nanoparticles and 28 times as reactive as Rh foils. APXPS
studies shed light on the activity results by illustrating that
at both 423 K and 473 K, the surface concentration of
oxidized Rh in 2 nm nanoparticles was much higher than
nanoparticles of other sizes (Fig. 13). The special activity
of small particles was therefore attributed to a thicker shell
of rhodium oxide in 2 nm nanoparticles that participated in
the reaction. This was also supported by the appearance of
a unique feature in the 0 1 s spectra not observed under
pure oxygen treatment. For the first time this provided
evidence of rhodium oxide as the active phase of the Rh
catalyst under CO oxidation.
Bimetallic systems, whose electronic structure is modi-
fied by addition of a second metal, often show enhanced
reactivity in various catalytic processes, paving another
way for engineering the catalysts. Not only do the surface
oxidation states respond to different gas reactants alteration
as expected, but also the surface composition of bimetallic
systems undergoes dramatic change owing to the differ-
ences of chemical potentials at surfaces compared with the
bulk. We employed APXPS to investigate such behaviors
using RhxPd1-x, RhxPt1-x, and PdxPt1-x nanoparticles
as the model catalysts [15, 77]. Figure 14 illustrates that
with assistance of synchrotron X-ray radiation that can
continuously tune the incident excitation energy to
probe photoelectrons with different escaping depths, the
as-synthesized RhxPd1-x and PtxPd1-x nanoparticles were
observed to be Rh-rich and Pd-rich at the surface, respec-
tively, while as-synthesized RhxPt1-x nanoparticles
possessed a homogeneous alloy phase. The surface com-
position variation at 0.7 nm sample depth was subse-
quently studied under oxidizing (O2 or NO), reducing (CO
or H2) and reaction (NO ? CO) conditions at 573 K. It was
found that Rh segregated to surface in the presence of
oxidizing gases for RhxPd1-x and RhxPt1-x nanoparticles,
whereas under reducing and reacting atmospheres Pd and
Pt tended to occupy the surface region. The change in
surface concentration was illustrated as being reversible by
the phenomenon that switching back the reaction mixture
to NO, Rh was enriched in the shell again. In contrast, Pd
always remained at the surface of PdxPt1-x nanoparticles
no matter how the chemical environment changed. Dif-
ferences in surface energy of metals and oxides can
account for the surface segregation phenomena. Both Pd
and Pt whose surface energies were lower than Rh diffused
to the surface when nanoparticles were reduced, whereas
1268 Top Catal (2012) 55:1257–1275
123
the highest stability of rhodium oxide drove Rh to surface
under oxidizing conditions. The gas driven migration was
not observed in PdxPt1-x nanoparticles because the surface
energy of Pd is lower than Pt and palladium oxide is more
stable than platinum oxide, therefore Pd is always more
stable than Pt at surface. Furthermore, it is noteworthy that
the surface redistribution could only occur at as high a
temperature as 573 K. The inability to reach the equilib-
rium phase at 313 K was due to the insufficient energy to
overcome the migration energy barrier. Therefore the Pd
enrichment at the surface was found to be correlated with
the observed synergetic effect of Rh0.5Pd0.5 bimetallic
nanoparticles in CO oxidation [78]. Preferential adsorption
of CO on Pd atoms and spillover of oxygen atoms disso-
ciated on Rh together contributed to the superior activity as
compared to the monatomic counterparts (Fig. 15).
Later comparisons of surface segregation effects
between Rh0.5Pd0.5 bulk crystal and Rh0.5Pd0.5 nanoparti-
cles were studied [79]. The nanoparticles were more
readily oxidized at the surface, therefore the surface Rh
concentration was higher than the bulk crystals of the same
nominal composition. Additionally, the nanoparticles
underwent more dramatic changes in surface concentration
than the single crystals under identical conditions. The
faster segregation for nanoparticles also suggested the
superiority of catalysts in the nanometer scale. APXPS
therefore provided a way for us to learn how the multi-
component catalysts behave in the nanometer scale and
subsequently control the catalytic behaviors of these
catalysts.
6.2 High-Pressure Scanning Tunneling Microscopy
Ever since the milestone invention in 1981 [80], STM has
become an extremely powerful technique to probe surface
electronic structure at the molecular level especially after
atoms were resolved on both semiconductor [81] and metal
surfaces [82, 83], which allows STM to keep standing at
the frontier of rapid developments in surface science.
Although STM works on the basis of electron moving
between a sharp tip and a conductive sample, the technique
Fig. 12 Schematics of APXPS showing the differential pumping stages and the electromagnetic lensing (left), the conical nozzle (top right), and
the hemispherical analyzer (bottom right) [2]
Fig. 13 Rh 3d XPS spectra of 2 and 7 nm nanoparticles in the
presence of 200 mTorr CO and 500 mTorr O2 at 423 and 473 K
detected by APXPS with photon energy of 510 eV. The spectra of
2 nm Rh nanoparticles showed a much higher concentration of
oxidized rhodium at both temperatures [74]
Top Catal (2012) 55:1257–1275 1269
123
is not limited solely in vacuum use because the electrons
only need to tunnel through an exceedingly narrow region
without being subject to scattering by background gases.
Among all the in situ tools, HPSTM has the greatest
potential to provide information regarding structure chan-
ges in the molecular realm. Since the first demonstration in
our group [84], HPSTM has proved its unique superiority
in that it can investigate surface structural evolution
invoked by high pressures of gases, most of the changes
distinct from those seen in UHV studies, thus bridging the
‘‘pressure-gap’’. A few HPSTM systems were also
designed in several groups to apply surface characteriza-
tion to high pressures [85–88]. The HPSTM in our group
was lately improved in 2007, with a gold-coated high
pressure STM cell which could work from 10-13 to several
bars and at temperature up to 700 K integrated into a UHV
chamber, as the schematics shown in Fig. 16 [89].
Despite the fact that some practical difficulties exist at
high pressures, such as decreased stability of the tip, stronger
tip-induced effects, and more severe thermal drift, HPSTM is
still able to uncover the surface electronic structure and
morphology at the molecular level. In addition to imaging
adsorbate patterns of various systems at high pressures
[90–92], STM revealed the relationship between adsorbate
mobility and catalyst poisoning, and the subsequent influ-
ences on catalytic turnovers. During the hydrogen and deu-
terium exchange reaction on Pt(111) at room temperature, no
distinguishable order could be discerned upon dosing 200
mTorr of H2 and 20 mTorr of D2 in the STM chamber, which
implied that adsorbates diffused much faster than piezotube
Fig. 14 Depth profiles of as-synthesized Rh0.5Pd0.5, Pd0.5Pt0.5 nanoparticles showing a core–shell structure and as-synthesized Rh0.5Pt0.5
nanoparticles exhibiting a homogeneous alloy phase investigated by synchrotron based XPS [15, 77]
Fig. 15 Changes of surface atomic fractions of Rh0.5Pd0.5, Rh0.5Pt0.5,
and Pd0.5Pt0.5 nanoparticles at 573 K under oxidizing (O2 or NO),
reducing (CO or H2) and catalytic reaction (NO ? CO) conditions.
The photoelectrons have a kinetic energy of *300 eV, which
corresponds to an inelastic mean free path of 0.7 nm [15]
1270 Top Catal (2012) 55:1257–1275
123
scanning of the instrument [69]. In contrast, addition of 5
mtorr of CO resulted in an ordered structure, similar to the
structure of pure CO on Pt(111), while production of HD
ceased in the meantime (Fig. 17). The stronger adsorption of
CO on Pt than hydrogen and deuterium impeded the diffu-
sion or even adsorption of reactants, which forced the H2/D2
exchange reaction to stop. Heating the crystal to 345 K
restarted the exchange reaction but at a slow rate, which was
attributed to partial CO desorption that permitted H and D
adatoms to diffuse and collide. Plus, coincident with the
reoccurrence of reaction, the surface structure became
invisible again under STM. The HPSTM results, along with
similar observations in cyclohexene hydrogenation/dehy-
drogenation [93], and ethylene hydrogenation [94], delin-
eated that a highly mobile surface is needed for catalytic
reactions to take place.
Structure alteration at high pressures is not only limited
to adsorbates; under reaction conditions gas molecules can
facilitate substrates in reconstructing [95–100] and new
active phases can form [101, 102]. Such phenomena that
Fig. 16 Schematics of the recently designed HPSTM system: (1) view window, (2) mounting framework, (3) docking scaffold, (4) docking disk,
(5) high pressure reactor (STM body housed within), (6) bayonet seal, (7) guide rod of docking scaffold, (8) sample/tip load-lock system, (9)
transfer rod, (10) gate valve, (11) four-finger sample stage, and (12) sputtering ion gun. Inset: a real picture of the high pressure STM reactor [89]
Fig. 17 High pressure STM
images showing the surface
mobility and the poisoning by
CO. a 90 A 9 90 A STM
images of Pt(111) in the
presence of 200 mTorr H2 and
20 mTorr D2 at 298 K. The
Pt(111) surface is catalytically
active producing HD.
b 90 A 9 90 A STM images of
Pt(111) in the presence of
200 mtorr H2, 20 mTorr D2 and
5 mTorr CO at 298 K. The HD
production stopped
accompanied by the ordered
structure under STM [69]
Top Catal (2012) 55:1257–1275 1271
123
metal atoms could be rearranged at high pressures of gases
were observed as early as the first design of HPSTM, in
which we illustrated that the overall corrugation Pt(110)
surface was largely increased in the presence of H2 and O2
while heating to 425 K, as depicted in Fig. 18 [84]. CO
exposure at the same temperature was able to lift the
(1 9 2) missing-row structure of Pt(110) with formation of
multiple height steps. Later on the lifting mechanism was
proposed by the Besenbacher group with improved reso-
lution: the preferential bonding between CO and low-
coordinated Pt atoms promoted the local displacement of Pt
[103]. CO was also capable of eliminating the hexagonal
overlayer on the Pt(100) surface, 10-5 torr of CO was
found to be sufficient to remove the 20 % excess Pt atoms
on the topmost layer, creating small islands that covered
around 45 % of the Pt(100) surface [99].
We subsequently concentrated on the interaction between
CO and stepped Pt surfaces. Studies on stepped Pt single
crystals are of great interest in the sense that the high density
of low-coordinated step atoms outstandingly mimics the
surface structure of real catalysts, which comprise small
particles within nanometer size. Pt(557) and (332) surfaces,
consisted of six atom wide (111) terraces separated by
monatomic steps in (100) and (111) orientation respectively,
were selected as models for CO adsorption. As shown in
Fig. 19, when introducing 5 9 10-8 torr of CO into STM
reactor, the initial straight step edge of Pt(557) turned
crooked, along with doubling of the step heights and terrace
Fig. 18 The structure changes
of Pt(110) surface induced by
(top) 1.7 atm of H2, (middle)
1 atm of O2, and (bottom) 1 atm
of CO at 425 K [84]
Fig. 19 STM images of Pt(557) surface in the presence of a 5 9 10-8 torr of CO; b 1 torr of CO; and c evacuating from (b) to 10-8 torr. The
cluster formation induced by high pressure of CO is reversible [98]
1272 Top Catal (2012) 55:1257–1275
123
widths, apparently at odds with the fact that the step structure
of Pt(332) remained unchanged at the same pressure [98].
After increasing CO pressure to 1 torr, both surfaces broke
into small clusters around 2 nm in size but in different
shapes. On Pt(557) triangular nanoclusters appeared with
vertices pointing toward the lower terrace on the basis of a
diatomic step height, whereas clusters in roughly parallelo-
gram shape formed on Pt(332). The formation of clusters was
ascribed to the strong repulsion between CO molecules at
coverages close to unity under 1 torr, which was then relaxed
by increasing the spacing between Pt atoms through surfaces
breaking into clusters, as supported by density functional
theory calculations. Moreover, it is noteworthy that pumping
out CO to 10-8 torr accompanied with CO coverage
decreasing allowed the dense Pt packing and thereby the
steps to return on both facets, although the fluctuation at step
edges increased (Fig. 19). The reversible cluster formation at
stepped Pt surfaces highlights the diffusion of substrate
atoms at high pressure of gases and also implied the coverage
dependence of the surface reconstruction.
7 Summary and Outlook
Designing novel catalysts with high activity, selectivity and
stability is the goal of catalysis in the 21st century. We have
demonstrated in this paper that the incorporation of nano-
technology into catalysis enables us to rationally control the
size, shape and composition of nanoparticle catalysts which
ultimately determine the surface electronic structure and
activation sites in catalytic reactions. Oxide support mate-
rials are also employed not only to stabilize metal nanopar-
ticles under working conditions but also to tune the catalytic
behaviors by creating new active sites at metal-oxide inter-
faces. Oxide encapsulation, adsorbate spillover, and charge
transfer are the possible contributing factors that change the
catalytic performances. Electronic structure modification of
semiconductors by creating vacancies or impurity doping
could significantly alter the conducting rate, turnover fre-
quency and selectivity of the reactions. More active sites can
be created owing to substrate surface reconstruction or the
active sites may be blocked by intermediates and poisonous
species that strongly adsorb on catalysts under reaction
conditions. As a result, it is of great importance to develop
in situ techniques that allow molecular level studies of the
catalyst surface structure during reactions, in order to reveal
the crucial factors which dominate the turnover rate and
selectivity towards certain products. APXPS monitors the
surface composition and oxidation state changes in response
to switching chemical environment between oxidizing and
reducing conditions. HPSTM studies the surface mobility
with respect to turnover rate as well as the poison effect, and
the metal substrate restructuring induced by gas adsorption.
Further enhancement of the catalytic properties requires
development of several new strategies in the near future. One
promising method is hybrid catalysts, which originates from
a combination of homogeneous, heterogeneous and enzy-
matic catalysts, since the nanoparticle essence of all catalysts
implies similar determinative molecular factors for reac-
tions. One major challenge in immobilization of homoge-
neous and enzymatic catalysts is the activity degrading
owing to catalyst leaching. Sub-nanometer metal clusters
with narrow size distribution could be converted into
homogeneous catalysts after being dispersed in supports,
showing high activity and stability [104, 105]. Accordingly,
various hybrid catalysts are likely to emerge as a result of
their potentially superior performances in reactions. Another
powerful method involves the development of a nanocrystal
bilayer ‘‘tandem catalyst’’. The concept was tested in het-
erogeneous catalysis with the assembly of CeO2-Pt nano-
cube bilayer on SiO2 in methanol decomposition and the
subsequent ethylene hydroformylation, which selectively
produced propanal [106]. The multiple interfaces could be
correlated with specific activation sites and interaction with
reactants at each interface that finally leads to sequential
reactions with enhanced activity and selectivity to final
products.
Acknowledgments This work was supported by the Director, Office
of Science, Office of Basic Energy Sciences of the U.S. Department
of Energy under Contract No. DE-AC02-05CH11231, and a grant
from Chevron Corp.
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