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: somorjai@berkeley.edu

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

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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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