model catalysts: simulating the complexities of ... files/543_arpc-12_63_265-286.pdfmodel catalysts:...

PC63CH13-Goodman ARI 6 March 2012 8:45

Model Catalysts: Simulating theComplexities of HeterogeneousCatalystsFeng Gao1 and D. Wayne Goodman2

1Chemical and Materials Science Division, Pacific Northwest National Laboratory, Richland,Washington 99352; email: [email protected] of Chemistry, Texas A&M University, College Station, Texas 77842-3012

Annu. Rev. Phys. Chem. 2012. 63:265–86

First published online as a Review in Advance onJanuary 13, 2012

The Annual Review of Physical Chemistry is online atphyschem.annualreviews.org

This article’s doi:10.1146/annurev-physchem-032511-143722

Copyright c© 2012 by Annual Reviews.All rights reserved

0066-426X/12/0505-0265$20.00

Keywords

surface science, heterogeneous catalysis, model catalyst, single crystal,oxide-supported nanoparticle

Abstract

Surface-science investigations have contributed significantly to heteroge-neous catalysis in the past several decades. Fundamental studies of reactivesystems on metal single crystals have aided researchers in understanding theeffect of surface structure on catalyst reactivity and selectivity for a numberof important reactions. Recently, model systems, consisting of metal clus-ters deposited on planar oxide surfaces, have facilitated the study of metalparticle-size and support effects. These model systems not only are usefulfor carrying out kinetic investigations, but are also amenable to surface spec-troscopic techniques, thus enabling investigations under realistic pressuresand at working temperatures. By combining surface-science characteriza-tion methods with kinetic measurements under realistic working conditions,researchers are continuing to advance the molecular-level understanding ofheterogeneous catalysis and are narrowing the pressure and material gapbetween model and real-world catalysts.

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Ultrahigh vacuum(UHV): the vacuumregime characterizedby pressures lowerthan ∼10−9 mbar orTorr

Pressure gap: theorders of magnitude ofpressure differencebetween surface-science studies invacuum and technicalcatalysis at elevatedpressures

1. INTRODUCTION

Heterogeneous catalyst–based processes are wide and varied, including hydrocarbon refining, theproduction of bulk chemicals, automobile pollution control, and fuel-cell technologies (1, pp. 1–57). Typical heterogeneous metal catalysts are constructed of a highly porous, high-surface-areaoxide (e.g., Al2O3, TiO2, SiO2) containing a dispersed metal phase active for catalytic reaction.The performance of heterogeneous catalysts is often defined in terms of their activity, selectivity,and stability, indicators that determine the practical and economic feasibility of a catalyst for use ina particular application (1; 2, pp. 59–80). These key properties can be governed by a multitude offactors, including the type and nature of the active metal, support effects and interactions, particlesize, particle atomic and electronic structure, catalyst promoters, catalyst treatments, and reactionconditions often at elevated pressures (greater than 1 atm) and temperatures. Although a numberof analytical tools (e.g., microscopy and X-ray diffraction) have provided researchers with insightsinto relations between the overall average particle structure and the collective catalytic behavior ofparticles in a reaction system, assessing the effect of the atomic and electronic structure on industrialcatalysts at the molecular level has proven challenging (3, pp. 1–9). As a result, researchers havestrived to develop experimental systems and techniques to understand catalytic behavior at themolecular level and the numerous factors that work together to define catalyst activity, selectivity,and stability.

Over the past several decades, a molecular-level understanding of surfaces and adsorbates hasbeen greatly advanced through the development of an array of surface-sensitive spectroscopic andultrahigh-vacuum (UHV) techniques (4–8). Combined with the ultraclean UHV environment(pressures less than 10−9 Torr), these spectroscopic techniques have facilitated the study of gassurface adsorption, desorption, chemical reactions, surface-adsorbate and adsorbate-adsorbate in-teractions, and other processes in great detail on well-characterized, model catalyst samples. Themodel catalysts typically employed by the surface scientists are metal single crystals, multimetallicthin films, and thin oxide-supported metal catalysts, samples that aim to mimic the active sur-face(s) on industrial catalyst samples. For a number of reaction systems, surface scientists haveshown good correlation between the catalytic behavior of model and industrial catalyst samples(4, 7).

A UHV environment is required to utilize many surface-science spectroscopies and guarantee aclean surface. Industrial catalysts, however, are typically operated at elevated pressures greater than1 atm. The large difference in operating pressures of the two applications, the so-called pressuregap, is a barrier that surface scientists have faced in relating investigations on model catalysts toindustrial catalysts (4). To overcome this barrier without sacrificing UHV feasibilities, they havedesigned systems capable of performing reactions at elevated pressures in a reactor contiguous toa UHV system, allowing sample transfer between the two regimes without exposure to the openair (9–11). These systems have enabled kinetic reaction measurements to be performed at elevatedpressures on clean surfaces prepared and analyzed in UHV, with the surface characterized beforeand after reactions with various surface-science techniques.

Similarly, another barrier encountered with the use of model catalyst samples is the so-calledmaterial gap (4, 7). Industrial catalysts typically are highly porous and have large surface ar-eas, whereas metal single crystals do not provide the opportunity to investigate support effectsand particle-size and structure effects. To this end, over the past two decades, surface scien-tists have developed techniques to create well-characterized model catalyst samples (under UHVconditions) comprising metal clusters supported on ultrathin metal-oxide surfaces. These modelsystems provide a suitable replicate for complex industrial catalysts, enabling investigations ofsupport interactions and particle-size effects (6–8, 12, 13). These ultrathin metal-oxide films are

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Material gap:complexity differencebetween planar modelcatalysts andhigh-surface-areasupported catalysts

STM: scanningtunneling microscopy

thin enough to be conductive, thus enabling the use of surface-science tools, such as scanningtunneling microscopy (STM), without the commonly encountered problems with sample charg-ing for industrial catalysts.

The effort to bridge the gaps between the model and heterogeneous catalyst systems with re-spect to the operating pressures and relative surface characteristics started approximately 30 yearsago. The goal of this article is to discuss the advances made in catalysis research using modelsystems that enable the more complex realistic catalysts to be simplified and the individual fea-tures to be investigated. Select examples from reaction studies on single-crystal model catalystsamples are discussed first, emphasizing the correlation of these results to industrial catalyst dataand the elucidation of the structure sensitivity and insensitivity of these reaction systems. Wethen explore recent studies of model catalyst systems consisting of metal clusters supported on ul-trathin metal-oxide films. These model systems, which are amenable to surface-science analyticaltools, are providing researchers the opportunity to further develop a fundamental, molecular-levelunderstanding of heterogeneous catalysis.

2. METAL SINGLE CRYSTALS AS MODEL CATALYSTS

Early work by several groups addressed the significance of metal single crystals as model catalysts.The methodology relies largely on carrying out catalytic reactions on metal single-crystal surfacesat realistic conditions approaching those typically found in technical applications. The kineticsand surface chemistry of several catalytic reactions have demonstrated the direct relevance ofsingle-crystal studies for modeling the behavior of high-surface-area supported catalysts.

2.1. Structure-Insensitive Reactions

In the following, representative examples are given to structure-insensitive reactions, i.e., reactionswith kinetics independent of active metal particle size and surface orientations.

2.1.1. CO methanation (CO + H2 → CH4). CO methanation has some fundamental signifi-cance as it is an essential component of Fischer-Tropsch synthesis. It is also an essential reactionin the production of synthetic natural gas from hydrogen-deficient carbonaceous materials andin the removal of CO/CO2 from the H2 feed in ammonia synthesis (14). As shown in Figure 1a,the kinetic data for the close-packed Ni(111) (Figure 1b) and more open Ni(100) (Figure 1c) arestrikingly similar with respect to the specific rates and activation energies (15). Furthermore, thesingle-crystal kinetics is virtually identical to that acquired from alumina-supported nickel cata-lysts (16). These extraordinary similarities in kinetics taken under identical conditions demonstrateunequivocally that CO methanation is a structure-insensitive reaction. Further investigations ex-tended the structure insensitivity of this reaction to Ru, W, Rh, Fe, Mo, and Co single crystals(14, and references therein). For structure-insensitive reactions in general, these studies collec-tively provide a good example of the appropriateness of using single crystals as models.

Metal catalysts are often modified by impurity species. Their roles (i.e., whether they functionas poisons or promoters) are of virtual importance in catalysis. For example, the activity of transi-tion metals can be reduced substantially by electronegative species (14). However, the underlyingrelative importance of ensemble (local, site-blocking) versus electronic (long-range) effects is dif-ficult to assess on complex technical catalysts. Such difficulties, however, are easily circumventedon single-crystal model catalysts. For example, kinetics studies carried out on sulfur (more elec-tronegative) and phosphorus (less electronegative) poisoned Ni(100), using CO methanation as amodel reaction, have convincingly proven the dominance of extended electronic effects (17).

www.annualreviews.org • Model Catalysts 267

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a b Ni(111)

c Ni(100)

10

1

10–1

10–2

10–3

10–4

800 K 700 K 600 K 500 K 450 K

1.2 1.4 1.6 1.8 2.0 2.2

25–50% Ni/Al2O3

8.0% Ni/Al2O3

5% Ni/Al2O3

CH

4 f

orm

ati

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TO

F)

1/T × 103 (K–1)

Ni (100)

Ni (111)

H2/CO = 4Ptotal = 120 Torr

Figure 1(a) A comparison of the rate of methane formation [in turnover frequency (TOF), CH4 molecule site−1 s−1]over single-crystal nickel catalysts and supported Ni/Al2O3. The reaction conditions are 120 Torr and aH2/CO ratio of 4. (b) Atomic configuration of an Ni(111) surface. (c) Atomic configuration of an Ni(100)surface. Figure adapted with permission from Reference 15. Copyright (1982) by Elsevier.

Structure sensitivity:dependence ofcatalytic activity on thecatalyst structure(surface orientationand particle size)

2.1.2. CO oxidation (CO + O2 → CO2) on Rh, Pd, and Pt. The catalyzed oxidation of CO byO2 is an important process in pollution control. Since the 1970s, the addition of three-way catalyticconvertors in automobiles, in which Rh, Pd, and Pt are the main active components, has greatlyenhanced air quality and environment protection (4, 5, 18). More recently, the fast developmentof proton-exchange-membrane fuel cells for efficient energy conversion has renewed researchinterest in the preferential oxidation of CO in excess of H2 (19, 20). At near-atmospheric pressures,this reaction has proven to be structure insensitive on Rh, Pd, and Pt catalysts under reducing, evento slightly oxidizing, conditions. As shown in Figure 2, excellent agreement between two modelcatalysts, Rh(111) and Rh(100), and two supported catalysts (21, 22) was found in terms of specificreaction rates and apparent activation energies, factors that are generally applied to determinestructure insensitivity (4). The structure insensitivity of CO oxidation is safely extended to Pd andPt catalysts (21–24).

Reaction kinetics and infrared spectroscopic studies reveal that under the conditions in whichCO oxidation is structure insensitive, the surface of the catalyst is almost entirely covered withCO, and the reaction rate is determined by the rate of CO desorption (21–24). According tothe prevailing Langmuir-Hinshelwood mechanism that describes this reaction (5), continuousturnover relies on CO desorption to create surface open sites for O2 adsorption and dissociation.The behavior of this reaction has been analyzed using a kinetic model established from UHVsurface-science studies of the interactions of CO and O2 with Rh. The results of this model arevirtually indistinguishable from the measured data and thus represent an exceptional example ofthe continuity between UHV surface-science and real-world catalysis (21). More recently Gaoand colleagues (25, 26) conducted research to further prove the continuity of the reaction kineticsfrom UHV to near-atmospheric conditions on Rh, Pd, and Pt single crystals.

2.1.3. Ethylene hydrogenation. Hydrogenation and dehydrogenation of olefins are good ex-amples of reactions that do not show structure sensitivity at elevated pressures (above 1 atm) onboth single-crystal and supported noble metals (e.g., Pt) (4). For ethylene hydrogenation, early ex

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103

102

10

1

10–1

1.2 1.6 2.0 2.4 2.8

10–2

10–3

10–4

Rh/Al2O3

Rh/Al2O3

CO

2 f

orm

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ra

te (

TO

F)

1/T × 103 (K–1)

Rh (100)

Rh (111)

PCO = 8 TorrPO2

= 8 Torr

Figure 2A comparison of the rate of CO2 formation [in turnover frequency (TOF), CO2 molecule site−1 s−1] overRh(111), Rh(100), and supported Rh/Al2O3. Reaction conditions are PCO = 8 Torr and an O2/CO ratio of1. Figure adapted with permission from Reference 22. Copyright (1986) by the American Chemical Society.

SFG: sum frequencygeneration

situ electron spectroscopic measurements after evacuation of the reactants (27) and more recentin situ sum frequency generation (SFG) vibrational spectroscopic studies (4, 28, 29) revealed thegeneration of long-lived carbonaceous deposits (especially ethylidyne) during reaction. It has beenconcluded that ethylene hydrogenation must occur on top of this carbonaceous layer.

2.1.4. Nature of structure insensitivity. For metal catalysts, the relative concentrations of ter-races, steps, kinks, and point defects change with formation methods and particle sizes (4, 5).These various factors modify the coordination of surface atoms and therefore adsorbate bondstrengths, and eventually catalytic activities. In this sense, a structure-sensitive situation is ex-pected to prevail, yet the examples above clearly demonstrate that this is not the case. A commonexplanation of structure insensitivity appears to be a poisoning effect in which surface sites thatmight be responsible for structure sensitivity are poisoned (or screened) during reaction by re-actants/intermediates. These species have been identified as carbidic carbon in CO methanation(15, 17), near-saturation coverage of CO in CO oxidation (24–26), and a full layer of carbonaceousspecies in ethylene hydrogenation (28, 29). One has to keep in mind that the significance of thestudies above is to demonstrate the relevance of kinetics measured on single-crystal surfaces formodeling the behavior of technical catalysts. It is entirely conceivable that some non-rate-limitingsteps in a global structure-insensitive reaction are still structure sensitive.

2.2. Structure-Sensitive Reactions

Next, examples are given to structure-sensitive reactions, i.e., reactions with kinetics dependingheavily on active metal particle size and surface orientations.


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Ni (100)

Ni (111)

1.6 2.01.8 2.2

1

10–1

10–2

10–3

10– 4

CH

4 f

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H2/C2H6 = 100Ptotal = 100 Torr

H2C2H6 CH4

Figure 3A comparison of the rate of CH4 formation [in turnover frequency (TOF), CH4 molecule site−1 s−1] fromethane hydrogenolysis over Ni(111) and Ni(100). Reaction conditions are Ptotal = 100 Torr and anH2/C2H6 ratio of 100. Figure adapted with permission from Reference 30. Copyright (1982) by Elsevier.

2.2.1. Alkane hydrogenolysis. One important example of a structure-sensitive reaction is thehydrogenolysis or cracking of alkanes on metal surfaces. This type of reaction is initiated by thedissociative adsorption of alkane molecules on a metal surface, a step shown to depend stronglyon the surface morphology. Figure 3 displays an example of ethane hydrogenolysis over Ni(100)and Ni(111) surfaces. The more open (100) surface is far more active than the close-packed (111)surface (30). Moreover, at a given temperature, the rate of methane production over an initiallyclean crystal was extremely constant with no apparent induction period; the carbon level duringreaction remained constant at submonolayer coverage (14, 30). The activity difference could beexplained by considering the atomic spacing between high-coordination bonding sites on thesurfaces for which the high-coordination threefold hollow sites are ideally suited for maintainingthe C-C bond intact on Ni(111); however, on Ni(100), the high-coordination sites are furtherapart, facilitating C-C bond cleavage.

A more detailed mechanistic investigation of alkane hydrogenolysis was conducted on Ir singlecrystals using ethane, propane, n-butane, and neopentane as reactants (31). Numerous studies havealso addressed hydrogenolysis over Rh, Pt, Mo, and Ru model catalysts (14). Altogether this largeinventory of single-crystal reaction data shows convincingly that alkane hydrogenolysis in generalis structure sensitive, with the more open surfaces exhibiting considerably greater propensity forbreaking C-C bonds.

2.2.2. CO oxidation by NO on Pd surfaces. The oxidation of CO by NO (CO + NO →CO2 + N2) is important in three-way catalytic conversion. Mechanistically, NO dissociates toN(a) and O(a) on the catalyst surface, whereas CO(a) maintains molecularly. The combination oftwo N(a) gives rise to N2, and the combination of CO(a) and O(a) gives rise to CO2. This reaction

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Planar modelcatalyst: 2D catalysts,including metal singlecrystals, metallic thinfilms, and oxidethin-film supported(metal) particles

10

1

0.1

0.01

Pd (111)

Pd (100)

Single crystal

12.5 nm

6.0 nm

1.5 1.6 1.7 1.8 1.9

Pd (110)

CO

2 f

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1/T × 103 (K–1)

Pd/Al2O3

powder

120 nm(averageparticlediameter)

Figure 4A comparison of the rate of CO2 formation [in turnover frequency (TOF), CO2 molecule site−1 s−1] fromthe CO+NO reaction over Pd single crystals and Pd/Al2O3 powder catalysts. The powder-catalyst datawere taken in the flow reaction mode (4.4/5.2 Torr CO/NO ratio, steady state), and the single-crystal datawere acquired for a batch reaction mode in 1 Torr of each reactant. Figure adapted with permission fromReferences 32–34. Copyright (1995, 1996) American Vacuum Society; copyright (1997) Elsevier.

was studied extensively over Pd single crystals, planar model Pd/Al2O3/Ta(110) catalysts, andconventional high-surface-area Pd/Al2O3 catalysts (32). Figure 4 displays kinetic data acquiredusing single crystals in a batch reactor and data for the conventional supported catalysts takenwith a flow reactor. It is apparent from the single-crystal data that the CO+NO reaction overPd is structure sensitive, with Pd(111) five times more active than the more open (100) and (110)surfaces (32–34). When the activity of the Pd/Al2O3 powder catalysts is compared, it is even moreconclusive: The most active catalyst has an average cluster size of 120 nm and an activity thatis 30-fold higher than that catalyst with an average cluster size of 6 nm. It is also evident thatthe single crystals have higher activities and lower activation energies than the powder-supportedcatalysts, behavior similar to that reported for Rh catalysts (35, 36). The rate-limiting step for thisreaction has been identified as the N(a) combination step to form N2. The structure-sensitivitybehavior is understood by the binding energy difference of N(a) with different surface sites, withlower-coordination Pd sites (open surfaces, smaller particles) binding N(a) stronger, leading tolower activity.

2.2.3. Ammonia synthesis on Fe (N2 + 3H2 → 2NH3). The adsorption and dissociation ofN2, H2, and NH3 on Fe single crystals have been investigated in detail under UHV conditions toreveal the reaction mechanism of ammonia synthesis. The rate-limiting step has been identifiedto be the dissociation of adsorbed N2 (4, 5). The high-pressure kinetic data presented in Figure 5show that the presence of Fe surface atoms with a coordination number of 7 (C7 coordinationsites, the most highly coordinated surface sites Fe can expose) is far more important than surfaceroughness for catalytic activity (37). The atomically rougher Fe(210) surface has much less activitythan the less open Fe(211) surface; the only possible explanation for this is that the latter surfaceexposes C7 sites. Remarkably, the N2 sticking coefficients derived at vacuum are quite similar


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C4C7 C7

Fe(111) Fe(210) Fe(100) Fe(211) Fe(110)

First layer Second layer Third layer

a

b 14

12

10

8

6

4

2

0(111) (211) (100) (210) (110)

Mo

l N

H3

/cm

2 s

× 1

0–

9

Surface orientation

T = 673 K20 atm 3:1 H2:N2

C5

C7

C4C6

C6 C4C8

C6

Figure 5(a) Schematic representation of five Fe single-crystal surfaces: Fe(111), Fe(210), Fe(100), Fe(211), and Fe(110). (b) Rate of NH3synthesis over the five different faces of Fe. Reaction conditions are Ptotal = 20 atm, an H2/N2 ratio of 3, and T = 673 K. Figureadapted with permission from Reference 37. Copyright (1982) by Elsevier.

to the reaction probabilities for the Fe single crystals determined at 20 bar shown in Figure 5.For structure-sensitive reactions as well, this demonstrates that kinetic data obtained with single-crystal surfaces at vacuum can be transferred across the pressure gap to high pressures and alsoreflect the behavior of real catalysts (5). Studies with Fe single crystals have also shown that therole of aluminum oxide (an additive of technical ammonia-synthesis catalysts) is to restructure andstabilize the most active surfaces (38).

Overall, the above examples clearly show that for structure-sensitive reactions, atomic spacing,coordination numbers, and electronic properties of catalyst surfaces play important roles in therate-limiting kinetics. Specifically, the single-crystal studies demonstrate that the activity of aparticular site or set of sites can be examined and the effects of surface structure explored inatomic detail. In certain cases, the kinetic parameters derived at vacuum are transferable to elevatedpressures as structure-insensitive reactions.

3. OXIDE-SUPPORTED METAL CLUSTERS

Single-crystal surfaces do not accurately mimic the complexity of real-world catalysts for whichissues related to particle size and the support-particle interaction cannot be addressed. This

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XPS: X-rayphotoelectronspectroscopy

LEED: low-energyelectron diffraction

LEIS: low-energy ionscattering

material gap can be bridged via the utilization of planar oxide-supported metal-cluster modelcatalysts. Planar oxide supports can be prepared by two methods: (a) cleaving the bulk oxide singlecrystals, especially ones that can be made sufficiently conductive after vacuum treatment (e.g.,TiO2) (39), and (b) growing thin films of metal oxides on refractive metal substrates, which ismore common. A number of thin (metal) oxide films have been prepared in the past decades,including Al2O3 (40–42), SiO2 (43–48), MgO (49–53), NiO (54–57), Cr2O3 (58, 59), TiOx (60,61), FeOx (62–64), and many others (65–67). These films have been extensively used to study themetal-cluster-support interactions.

3.1. Oxide Thin-Film Supports: SiO2, TiO2, MgO, Al2O3, and TiOx

For supported metal-cluster catalysts, the electronic structure, chemical (catalytic) and thermalbehavior, and the interaction with the support are dependent on the particle size of the clus-ters. The synthesis of oxide supports is the first step toward studying these clusters. Below wepresent examples on the synthesis, characterization, and testing of (a) oxide films and (b) metalclusters deposited on top. We focus on oxide thin films that are thin enough to be conductive andhomogenous for the applications of the spectroscopic methods (44, 68).

Silica (SiO2) is a commonly used inert support for technical heterogeneous catalysts. High-quality, ultrathin, ordered, and flat thin films are achieved by the sequential deposition of 0.5 MLof Si at room temperature on Mo(112) and subsequently oxidizing in 1 × 10−7 mbar O2 at 800 Kfor 5 min (69). The film thickness can be gradually increased using this recipe to a desired level.To achieve extremely flat films, one requires high-temperature annealing up to 1,200 K. X-rayphotoelectron spectroscopy (XPS) and Auger electron spectroscopy have been used to ascertainthe stoichiometry of these films. On the same substrate, a different film-formation recipe involvesoxidizing a half-monolayer of Si at 800 K for 6 min in 5 × 10−6 mbar O2 followed by repeatedgrowth cycles and high-temperature annealing (45, 46). Both ways of synthesizing the films havebeen established to form flat and epitaxial structures. However, there are still debates regardingwhether the film structure is an isolated [SiO4] tetrahedron or a two-dimensional (2D) network(47, 70).

Alumina thin films have been prepared on Mo(110) substrate by depositing aluminum in abackground O2 pressure of 7 × 10−7 Torr at room temperature and were subsequently annealed to1,200 K (42). Similar films have been synthesized on Re(0001) and Ta(110) (40, 41, 71). These filmsgrow two dimensionally before evolving into 3D structures at higher coverages. The long-rangeorder, growth mode, and oxidation state of Al and the vibrational modes of these films were studiedusing standard surface-science techniques, including low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), Auger electron spectroscopy, and high-resolution electron energy-loss spectroscopy. Ultrathin, well-ordered, and flat Al2O3 films can also be developed by oxidizingAl single crystals, AlNi, and Ni3Al (44, 72–74).

Well-ordered MgO thin films (49, 52, 53) were prepared on Mo(100) by the deposition ofMg in an O2 background at 600 K. Subsequently, these films were annealed to 1,100 K to obtainordered structures. These films have been studied using a variety of surface-science techniques,such as temperature-programmed desorption, infrared reflection absorption spectroscopy (IRAS),XPS, and LEED, and with CO and water as probe molecules to investigate their stoichiometry,long-range order, and chemical properties. The synthesis of well-ordered MgO films has alsobeen reported on Fe (75) and Ag (76, 77) single crystals.

Among the many model oxide films investigated, titania thin films have been prominent aca-demically because of their properties, easy preparation methods, and their availability as a con-ducting single crystal (39, 78–83) after sufficient sputtering and annealing in vacuum. Another


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way to use TiO2 is to grow thin films on refractive metals in vacuum. TiO2 films were grown onMo(110) by vapor deposition of Ti at 600–700 K in an O2 background of 2 × 10−7 Torr pressurewith subsequent annealing to 800 K in O2 (78). The thermal stability of these films after annealingto high temperatures was investigated by using LEIS and STM. Surface defects can be introducedinto the films by further annealing them to 1,200 K, as confirmed by XPS.

Well-ordered (8 × 2)-TiOx films were grown on an Mo(112) surface (84, 85). High-qualityfilms were synthesized using an indirect method involving the growth of a well-ordered SiO2 filmfollowed by a TiOx layer. As detailed below, this TiOx film is noteworthy in that Au completelywets the surface of the film, forming mono- and bilayer well-ordered uniform films that displayspectacular catalytic activity for CO oxidation (86).

3.2. Cluster-Deposition Methods

One of the easiest ways to synthesize clusters supported on metal-oxide model catalysts is by vapordepositing the metal. The operating conditions have a pronounced effect on the morphology of theclusters. This effect also varies with the metal, support oxide, and their interactions. But by varyingthe deposition rates and controlling the experimental conditions, we can accurately control thesize, density, and morphology of the clusters. The cluster size can be decreased by the formation ofartificial nucleation centers on the substrate by controlling the defect density or by preadsorbinga reactive adsorbate on the surface before cluster deposition (80). The cluster size distribution inthe vapor-deposition method is usually quite broad.

Ligand-stabilized clusters can be grown in UHV conditions by the use of surfactants (87, 88).The surfactants prevent further nucleation of the clusters after deposition and can be removed byannealing or other methods. Molecular beams have been used along with mass-selective cluster-deposition methods to grow nanoclusters with rigid control over the size and density (89–91).This cluster-synthesis procedure has evolved rapidly after circumventing some challenges due tomass selection, cluster stabilization, and cluster soft landing. The movement of the clusters on thesurface is dependent on the temperature and the number of point defects. This method providesinsights into the metal-support interaction by comparing the chemical reactivity of the supportedclusters with that of the gas-phase clusters. However, the morphology of the cluster deposited ona surface also depends on the approach used to grow it.

Nanometer-range clusters can be synthesized using nanolithography (92–94). This techniqueallows the surface to be patterned into well-defined arrays before cluster deposition. The chemicaland physical properties of clusters, such as sintering and lateral mass transport, can be studied usingthis approach. This method is relatively time-consuming, but well-defined uniform clusters in thesize range of 10–100 nm can be synthesized.

3.3. Cluster Size and Reactivity: Model Catalyst Samples

In heterogeneous catalysis, it is of virtual importance to build structure-reactivity relationships atan atomic level. Whereas this is very difficult to nearly impossible for high-surface-area technicalcatalysts, it is rather achievable for planar model catalysts. In the following, examples are givenon titania-supported Au nanoparticles and thin films in CO oxidation, and silica-supported Rhnanoparticles in ethylene hydroformylation.

3.3.1. Reactivity of Au/TiO2 in CO oxidation. Au in bulk is chemically inert and has oftenbeen regarded to be poorly active as a catalyst. However, when Au is finely dispersed (with particlediameters below 10 nm), it exhibits surprisingly high activity for many reactions (e.g., CO oxidation

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0.05

0

0.01

0.02

0.03

0.04

–0.05

–0.04

–0.03

–0.02

–0.01

–2.00 –1.00 0.00 1.00 2.00

Bias voltage (V)

Tun

ne

lin

g c

urr

en

t (n

A)

nm

a b

nm

50

40

30

20

10

00 10 20 30 40 50

Cluster (50 × 25 Å)




TiO2(110)

Figure 6(a) A constant current topographic (CCT) scanning tunneling micrograph of Au/TiO2(110)-(1 × 1) as prepared before a CO:O2reaction. The Au coverage is 0.25 ML, and the sample was annealed at 850 K for 2 min. The size of the images is 50 nm × 50 nm.(b) Scanning-tunneling-spectroscopy data acquired for Au clusters of varying sizes on the TiO2(110)-(1 × 1) surface. For reference,scanning-tunneling-spectroscopy data for the TiO2(110)-(1 × 1) substrate are also shown. Figure adapted with permission fromReference 100. Copyright (1998) by the American Association for the Advancement of Science.

and propylene epoxidation) (95, 96). The correlation between the reaction rate and cluster size isexceptionally observed for Au/TiO2 catalysts (80, 95–100).

The preparation of the planar Au/TiO2 catalyst involves the vapor deposition of Au onto theTiO2 single crystal or the thin film of TiO2 grown on Mo(100) described above. XPS and LEIShave been used (101, 102) to understand the growth of Au on TiO2. STM is particularly usefulto precisely follow the 2D-to-3D transition of the metal clusters. This methodology, coupledwith reaction kinetics measurements, gives rise to outstanding structure-reactivity correlations, asshown below.

Figure 6a shows an STM image of 0.25 ML of Au deposited on a TiO2(110) single crystal atroom temperature and subsequently annealed to 850 K for 2 min (80, 100). The bright spots onthe surface correspond to the 3D Au clusters with an average diameter of ∼2.6 nm and a height of∼0.7 nm. Scanning tunneling spectroscopy was used to probe the correlation between the clustersize and the associated electronic structure. In this method the tunneling current (I) is measuredas function of bias voltage (V) in the region of interest to generate an I(V) curve. Figure 6b showsthe corresponding scanning-tunneling-spectroscopy curves for various Au cluster sizes and theTiO2 substrate. For the TiO2 substrate, the I(V) curve shows a wide tunneling gap, consistentwith the semiconducting properties for bulk TiO2. As for the Au particles, the largest band gap isreflected by those undergoing the transition from 2D to 3D structures. This is reminiscent of thetransition between metallic and nonmetallic clusters, which occurs at the critical cluster diameterof 2.0–4.0 nm and a height of approximately two atomic layers. Hence these data evidently showthat the I(V) curves reflect the changes in the cluster properties with the variation in the clustersize.

In the following, we discuss the activity exhibited by the Au/TiO2 catalyst for CO oxidation.As displayed in Figure 7a, this activity is clearly dependent on the cluster size. The maximumreaction rate is observed for the catalyst with a cluster size of approximately 3 nm. Figure 7b plots


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Au/TiO2

Au/TiO2 (110)

2.20

1.80

1.40

1.00

0.60

Act

ivit

y

a

1.50

1.20

0.90

0.60

0.30

0.00

Ba

nd

ga

p (

V)

b

0

15

30

45

60

Po

pu

lati

on

(%

)

0.0 2.0 4.0 6.0 8.0 10.0

c

Cluster diameter (nm)

Au clusters witha band gap of 0.2–0.6 V

measured by STS

Figure 7(a) The activity for CO oxidation at 350 K as a function of Au cluster size supported on TiO2(110)-(1 × 1),assuming total Au dispersion. The CO:O2 mixture is 1:5 at a total pressure of 40 Torr. Activity is expressedas (product molecules) × (total Au atoms)−1s−1. (b) The cluster band gap measured by scanning tunnelingspectroscopy as a function of the Au cluster size supported on TiO2(110)-(1 × 1). The band gaps wereobtained while the corresponding topographic scan was acquired on various Au coverages, ranging from 0.2to 4.0 ML. Filled circles represent 2D clusters; open diamonds are 3D clusters, two atom layers in height;and filled triangles are 3D clusters, three atom layers or greater in height. (c) The relative population of theAu clusters (two atom layers in height) that exhibited a band gap of 0.2 to 0.6 V as measured by scanningtunneling spectroscopy from Au/TiO2(1 × 1). Figure adapted with permission from Reference 100.Copyright (1998) by the American Association for the Advancement of Science.

the band gap as a function of the cluster size. The 2D small clusters (diameter less than 2.0 nm)have a large band gap, and a transition toward a smaller band gap is observed for larger clusterswith a cluster size of approximately 3.5 nm. As the clusters grow larger (to approximately 4.0 nm orgreater), they exhibit metallic properties, and no corresponding band gap can be seen. ComparingFigure 7a,b, it is obvious that there is maximal catalyst-cluster activity for those transitioning froma metal to a nonmetal. This transition effect with the cluster-size variation has also been reportedfor Fe clusters on GaAs(110) (103), Pd on TiO2(110) (104), Ag particles on Al2O3/NiAl(110)(105), and Ag particles in nanopits of graphite (106).

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(1 0)(0 1)

(1 1)

(1 0)(0 1)

(1 1)

(1 0)

(0 1)

(1 1)

Mo

c

Mo

AuAu

Ti

a

MoMo

Ti

Top view

Side view

b

MoMo

AuTi

(8 × 2)

(1 × 1)

(1 × 3)

[1–

10]

[1–1–

1]

Figure 8Low-energy electron diffraction patterns and structural models, top and side views, for (a) Mo(112)-(8 × 2)-TiOx, (b) Mo(112)-(1 × 1)-(TiOx, Au), and (c) Mo(112)-(1 × 3)-(TiOx, Au1.33). The oxygen atoms areomitted from the models for clarity. Figure adapted with permission from Reference 86. Copyright (2004)by the American Association for the Advancement of Science.

Au was deposited on a Mo(112)-(8 × 2)-TiOx support that was grown on a Mo(112) singlecrystal by depositing 1 ML of Ti on a Mo(112)-c(2 × 2)-[SiO4] surface (84–86). Au completelywets the surface, and two well-ordered structures were formed: a Mo(112)-(1 × 1) structure at anAu coverage of 1.0 ML and a Mo(112)-(1 × 3) bilayer structure at an Au coverage of 1.33 ML.The corresponding LEED patterns and the schematic models are displayed in Figure 8.

These two structures were tested for CO oxidation activity. As seen in Figure 9, the catalyticactivity is maximized for the (1 × 3) bilayer structure, whereas the (1 × 1) structure shows muchless activity. The bilayer structure showed activity that was more than 45 times higher than thatreported for any high-surface-area Au/TiO2 catalysts (86, 107). With the further addition of Au,the activity decreases owing to the formation of 3D particles that block the active structure. The


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0

1

2

3

4

0 1 2 3 4 5 6

Au coverage (ML)

TO

F (

S–1

)

(1 × 3)

(1 × 1)

Mo Ti

Au

Mo

Au

Mo TiMo

Au

Figure 9Activity for CO oxidation at room temperature as a function of Au coverage above the monolayer onMo(112)-(8 × 2)-TiOx. The CO:O2 ratio is 2:1, and the total pressure is 5 Torr. The data represent theinitial rates derived by extrapolating the rate data to zero time. The turnover frequency for the (1 × 1) Austructure was calculated with the total number of Au atoms in the structure; the turnover frequency (TOF)for the (1 × 3) structure was computed by dividing the overall rate minus two-thirds the (1 × 1) rate (thosereactive atom sites blocked by the second-layer Au) by the number of Au atoms in the second layer of thestructure; for Au coverages greater than 2.0 ML, the TOFs are based on total Au owing to the formation of3D clusters. (Insets) Schematic models for the (1 × 1)- and (1 × 3)-Au/TiOx surfaces. Figure adapted withpermission from Reference 86. Copyright (2004) by the American Association for the Advancement ofScience.

activity difference between the (1 × 1) and (1 × 3) structures suggests that the presence of the rightsupport does not ensure the complete activation of the catalyst. The presence of the second layerof Au is clearly crucial. As seen in Figure 7, maximized activity was observed for Au particles withtwo-layer thicknesses as well. These two studies clearly demonstrate the ability of surface-sciencemodels not only to replicate the catalytic properties of real-world heterogeneous catalysts but alsoto set achievable targets toward which the high-surface-area catalysts can be tuned to performbetter.

3.3.2. Ethylene hydroformylation on SiO2-supported Rh particles. CO insertion into ad-sorbed alkyl groups is an important catalytic reaction step to form oxygenates (e.g., aldehydes,alcohols). C2H4 hydroformylation (C2H4 + CO + H2) is a well-known reaction for the synthesisof aldehydes via the CO insertion reaction (108). It has been proposed that during heterogeneousC2H4 hydroformylation on Rh surfaces, C2H4 is first hydrogenated to a -C2H5 surface species,which then undergoes CO insertion to form a surface acyl species, followed by hydrogenation toform propionaldehyde (109, 110). Previous investigations on supported Rh have indicated thatundercoordinated Rh surface sites are favorable for CO insertion, in which linearly bound atop COis more reactive than CO bound in a dispersed or gem-dicarbonyl [Rh(CO)2] fashion (111, 112).Additionally, propionaldehyde formation has been found to increase with increased Rh dispersionfor supported Rh systems (112, 113).

Figure 10a shows a plot of propionaldehyde turnover frequency versus average Rh particle size〈dp〉, with propionaldehyde formation clearly dependent on 〈dp〉 and maximum activity occurring at〈dp〉 ∼ 2.5 nm (114, 115). Reactivity measurements obtained on an Rh(111) single crystal revealed

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a

b

0.00 2 4 6 8

0.1

0.2

0.3

0.4

Rh(111)

Pro

pio

na

lde

hy

de

TO

F(C

3H

6O

mo

lecu

les

site

–1 s

–1)

Average Rh particle diameter <dp> (nm)

Rh/SiO2

T = 500 K

t = 1 h

PCO = PC2H4 = 50 Torr

PH2 = 400 Torr

O

H

2,200 2,100 2,000 1,900 1,800 1,700

t = 60 min

t = 50 min

t = 40 min

t = 30 min

t = 20 min

T = 500 K, t = 0

0.01

t = 10 min

T = 300 K

Diff

ere

nce

sig

na

l

Wave number (cm–1)

0.5-ML Rh/SiO2

<dp> = 1.6 nm

Trxn = 500 K2,0342,106

2,0352,066

2,200 2,100 2,000 1,900 1,800 1,700


Diff

ere

nce

sig

na

l

2,034

2,073

0.025

2,101

2,048

t = 60 min

t = 50 min

t = 40 min

t = 30 min

t = 20 min

T = 500 K, t = 0

t = 10 min

T = 300 K

1.0-ML Rh/SiO2

<dp> = 2.9 nm

Trxn = 500 K

2,200 2,100 2,000 1,900 1,800 1,700


Diff

ere

nce

sig

na

l

2,036

2,075

0.032

2,105

2,048

t = 60 min

t = 50 min

t = 40 min

t = 30 min

t = 20 min

T = 500 K, t = 0

t = 10 min

T = 300 K

4.0-ML Rh/SiO2

<dp> = 3.7 nm

Trxn = 500 K

Figure 10(a) Propionaldehyde turnover frequency (TOF) versus average Rh particle size (nm). Reaction conditions are 50 Torr CO, 50 TorrC2H4, and 400 Torr H2 at T = 500 K for 1 h. ± σ y error bars represent the error of repeated reactivity measurements, and ± σ x errorbars represent the sigma of the Rh particle-size distribution as determined from scanning tunneling micrography measurements. Thedotted-dashed line represents reactivity measurements obtained on an Rh(111) single crystal. The smooth line is present to guide theeye. (b) Polarization-modulation infrared reflection absorption spectroscopy on Rh/SiO2 model catalyst surfaces under reactionconditions 50 Torr CO, 50 Torr C2H4, and 400 Torr H2. Reactant mixtures are introduced at T = 300 K, and a rapid infrared scan(20 scans) is obtained. The sample is then heated to T = 500 K, and another rapid infrared spectrum is obtained (20 scans). The sampleis then held at reaction temperature, and spectra (692 scans) are obtained every 10 min. Figure adapted from Reference 114.


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PM-IRAS:polarization-modulation infraredreflection absorptionspectroscopy

much less activity. As the average Rh particle size 〈dp〉 is decreased from the planar Rh(111)surface to the largest-particle-size Rh/SiO2 surface (〈dp〉 = 7.1 nm), a nearly order-of-magnitudeincrease in propionaldehyde formation is observed. Considering the similarity in the total Rh sitedensity of Rh(111) and the 7.1-nm Rh/SiO2 surface, and the relatively small expected increase inundercoordinated (step-like) site density between these surfaces, the SiO2 support evidently playsan important role in the CO insertion reaction. This likely indicates that the hydrogen present onthe SiO2 support under reaction conditions plays a role in hydrogenating the acyl surface groupspresent on the Rh particle (110). When the Rh particle size decreases from 〈dp〉 = 7.1 nm to2.5 nm, a roughly fivefold increase in turnover frequency is observed. This increase in rate,conversely, results mainly from an increase in the number of undercoordinated step-like surfacesites on the Rh surface (114, 115).

However, as Rh particles continue to decrease below 2.5 nm, the number of undercoordinatedRh sites must continue to increase; the decrease in propionaldehyde turnover frequency indicatesthat another factor must be at play for smaller Rh particles. Polarization-modulation (PM-)IRASmeasurements were obtained under reaction conditions as a function of average Rh particle size(〈dp〉 = 1.6, 2.9, and 3.7 nm) to reveal the underlying origin, as shown in Figure 10b. As theinfrared spectra indicate, a single spectral feature associated with linearly bound CO is observedfor surfaces with average Rh particle sizes above 〈dp〉 = 2.5 nm. In contrast, PM-IRAS spec-tra obtained for the Rh/SiO2 surface with 〈dp〉 = 1.6 nm exhibit two prominent vibrationalfeatures: one associated with linearly bound CO (2,066 cm−1) and one consistent with an Rhcarbonyl hydride species [Rh(CO)H] (2,035 cm−1) (116–121). Taken together, the kinetic andPM-IRAS data suggest that the observed particle-size effect of CO insertion for ethylene hy-droformylation on Rh/SiO2 surfaces is driven by two factors: (a) the promotion of CO inser-tion on undercoordinated Rh surface sites as the initial average Rh particle size is decreased to∼2.5 nm and (b) a decrease in propionaldehyde formation on small Rh particles, which correlateswith the presence and formation of Rh carbonyl hydride species under reaction conditions asobserved via PM-IRAS. Comparison with Rh(111) reactivity data suggests that the SiO2 supportplays a role in the propionaldehyde formation on Rh/SiO2.

4. CONCLUSIONS AND OUTLOOK

One of the primary goals of surface scientists a few decades ago was to explain the basic concepts ofthe structure sensitivity and insensitivity of reactions on heterogeneous catalysts. This was suitablyaddressed by employing single crystals as model catalysts and working with unique analyticalmethods that combine UHV equipment with a high pressure reactor. Single crystals as modelcatalysts also aided our understanding of the effects of additives (poisons and promoters) oncatalyst activity. These studies have shown that the mechanistic information of the reactions onthe heterogeneous catalysts can be obtained and that the relative importance of electronic andgeometric effects in the overall behavior of the catalyst can be discerned.

Single crystals as model catalysts do not accurately mimic the subtleties of the heterogeneouscatalysts, and as established through examples above, there could be a significant disparity inthe chemical property of a catalyst, depending on the operating conditions (in terms of pressureand temperature) and the presence of the support, for example. Hence these investigations wereextended to engage more complex systems such as multimetallic systems and metal clusters onthin oxide films. Further advancement in the field of surface science has been possible owing to thepreparation of thin model oxide films. A wide array of thin films were produced that encompassall the oxide supports used in the synthesis of heterogeneous catalysts and accurately depict thestructural and electronic properties of the bulk oxide surfaces. Metal clusters deposited on these

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planar oxide surfaces permit remarkable control over the growth properties and the physicalstructure and offer the prospect of investigations in molecular detail. These model systems notonly are applicable to kinetic reactions, but they are also amenable to many surface spectroscopictechniques, particularly the scanning probe techniques under realistic pressures and at workingtemperature conditions. In the meantime, many novel surface spectroscopic techniques, such asPM-IRAS, SFG, and elevated-pressure XPS, were developed to decipher the nuances of the simpleyet complex metal-cluster catalysts on metal-oxide surfaces.

In the past few decades, there has been great advancement in the development of novel modelcatalysts, and consequently a seamless transition has been established in many instances from thesurface-science models to the real-world catalysts. Cooperation between the two fields is criticalfor their development. Equipped with novel spectroscopic techniques and an extensive range ofmethods to produce catalysts, we can look forward to solving numerous long-standing questionsabout catalytic processes.

SUMMARY POINTS

1. Planar model catalysts (including metal single crystals) are useful models to mimic high-surface-area supported catalysts.

2. The pressure and material gaps can be successfully bridged by using planar model catalystsand UHV-high-pressure reactor systems.

3. Metal clusters deposited on planar oxide surfaces permit remarkable control over growthproperties and physical structure and offer the prospect of molecular-detail investigations.

4. The development of novel spectroscopic techniques (e.g., PM-IRAS, SFG, elevated-pressure XPS) allows surface scientists to continue to solve long-standing questions aboutcatalytic processes.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

We gratefully acknowledge the support of this work by the Department of Energy, Office of BasicEnergy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences (DE-FG02-95ER-14511), and the Robert A. Welch Foundation (A-0030).

LITERATURE CITED

1. Bartholomew CH, Farrauto RJ. 2006. Fundamentals of Industrial Catalytic Processes. New York: Wiley &Sons. 966 pp. 2nd ed.

2. Hagen J. 2006. Industrial Catalysis: A Practical Approach. Weinheim: Wiley-VCH. 503 pp. 2nd ed.3. Niemantsverdriet JW. 2007. Spectroscopy in Catalysis. Weinheim: Wiley-VCH. 346 pp.4. Somorjai GA, Li YM. 2010. Introduction to Surface Chemistry and Catalysis. New York: Wiley & Sons.

771 pp.5. Ertl G. 2009. Reactions at Solid Surfaces. New York: Wiley & Sons. 207 pp.6. Henry CR. 1998. Surface studies of supported model catalysts. Surf. Sci. Rep. 31:235–325


Ann

u. R

ev. P

hys.

Che

m. 2

012.

63:2

65-2

86. D

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d fr

om w

ww

.ann

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ws.

org

by T

exas

A&

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- C

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2. F

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nly.


7. Goodman DW. 1995. Model studies in catalysis using surface science probes. Chem. Rev. 95:523–368. Campbell CT. 1997. Ultrathin metal films and particles on oxide surfaces: structural, electronic and

chemisorptive properties. Surf. Sci. Rep. 27:1–1119. Blakely DW, Kozak EI, Sexton BA, Somorjai GA. 1976. New instrumentation and techniques to monitor

chemical surface reactions on single crystals over a wide pressure range (10−8–105 Torr) in the sameapparatus. J. Vac. Sci. Technol. 13:1091

10. Provides a goodsummary of UHV andhigh-pressure-reactorcoupled systems.

10. Campbell CT. 1989. Studies of model catalysts with well-defined surfaces combining untrahigh-vacuum surface characterization with medium-pressure and high-pressure kinetics. Adv. Catal.36:1–54

11. Szanyi J, Goodman DW. 1993. Combined elevated pressure reactor and ultrahigh vacuum surface anal-ysis system. Rev. Sci. Instrum. 64:2350–52

12. Baumer M, Freund HJ. 1999. Metal deposits on well-ordered oxide films. Prog. Surf. Sci. 61:127–9813. Freund HJ. 2010. Model studies in heterogeneous catalysis. Chem. Eur. J. 16:9384–9714. Goodman DW. 1996. Correlations between surface science models and “real-world” catalysts. J. Phys.

Chem. 100:13090–10215. Kelley RD, Goodman DW. 1982. Catalytic methanation over single crystal nickel and ruthenium: reac-

tion kinetics on different crystal planes and the correlation of surface carbide concentration with reactionrate. Surf. Sci. Lett. 123:L743–49

16. Vannice MA. 1976. The catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen.Catal. Rev. Sci. Eng. 14:153–91

17. Goodman DW. 1984. Chemical modification of chemisorptive and catalytic properties of nickel.Appl. Surf. Sci. 19:1–13

18. Shelef M, McCabe RW. 2000. Twenty-five years after introduction of automotive catalysts: what next?Catal. Today 62:35–50

19. Choudhary TV, Goodman DW. 2002. CO-free fuel processing for fuel cell applications. Catal. Today77:65–78

20. Kahlich MJ, Gasteiger HA, Behm RJ. 1997. Kinetics of the selective CO oxidation in H2-rich gas onPt/Al2O3. J. Catal. 171:93–105

21. Provides a goodexample that kineticmodels established fromUHV surface-sciencestudies can besuccessfullyextrapolated to elevatedpressures.

21. Oh SH, Fischer GB, Carpenter JE, Goodman DW. 1986. Comparative kinetic studies of CO-O2

and CO-NO reactions over single crystal and supported rhodium catalysts. J. Catal. 100:360–7622. Goodman DW, Peden CHF. 1986. CO oxidation over Rh and Ru: a comparative study. J. Phys. Chem.

90:4839–4323. Berlowitz PJ, Peden CHF, Goodman DW. 1988. Kinetics of CO oxidation on single-crystal Pd, Pt, and

Ir. J. Phys. Chem. 92:5213–2124. Xu X, Goodman DW. 1993. An infrared and kinetic study of CO oxidation on model silica-supported

palladium catalysts from 10−9 to 15 Torr. J. Phys. Chem. 97:7711–18

25. Demonstrates thatno pressure gap exists inthe CO oxidationreaction over Pd, Pt,and Rh catalysts.

25. Gao F, McClure SM, Cai Y, Gath KK, Wang YL, et al. 2009. CO oxidation trends on Pt-groupmetals from ultrahigh vacuum to near atmospheric pressures: a combined in situ PM-IRAS andreaction kinetics study. Surf. Sci. 603:65–70

26. Gao F, Goodman DW. 2010. Reaction kinetics and polarization modulation infrared reflection ab-sorption spectroscopy investigations of CO oxidation over planar Pt-group model catalysts. Langmuir26:16540–51

27. Davis SM, Zaera F, Somorjai GA. 1982. The reactivity and composition of strongly adsorbed carbona-ceous deposits on platinum: model of the working hydrocarbon conversion catalyst. J. Catal. 77:439–59

28. Cremer PS, Su XC, Shen YR, Somorjai GA. 1996. Ethylene hydrogenation on Pt(111) monitored insitu at high pressures using sum frequency generation. J. Am. Chem. Soc. 118:2942–49

29. McCrea KR, Somorjai GA. 2000. SFG-surface vibrational spectroscopy studies of structure sensitivityand insensitivity in catalytic reactions: cyclohexene dehydrogenation and ethylene hydrogenation onPt(111) and Pt(100) crystal surfaces. J. Mol. Catal. A 163:43–53

30. Goodman DW. 1982. Ethane hydrogenolysis over single crystals of nickel: direct detection of structuresensitivity. Surf. Sci. Lett. 123:L679–85

31. Engstrom JR, Goodman DW, Weinberg WH. 1988. Hydrogenolysis of ethane, propane, n-butane, andneopentane on the (111) and (110)-(1 × 2) surfaces of iridium. J. Am. Chem. Soc. 110:8305–19

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32. Rainer DR, Vesecky SM, Koranne M, Oh WS, Goodman DW. 1997. The CO+NO reaction over Pd: acombined study using single-crystal, planar-model-supported, and high-surface-area Pd/Al2O3 catalysts.J. Catal. 167:234–41

33. Vesecky SM, Xu X, Chen PJ, Goodman DW. 1995. Evidence for structure sensitivity in the high pressureCO + NO reaction over Pd(111) and Pd(100). J. Vac. Sci. Technol. A 13:1539–42

34. Vesecky SM, Rainer DR, Goodman DW. 1996. Basis for the structure sensitivity of the CO + NOreaction on palladium. J. Vac. Sci. Technol. A 14:1457–63

35. Oh SH, Eickel CC. 1991. Influence of metal-particle size and support on the catalytic properties ofsupported rhodium: CO-O2 and CO-NO reactions. J. Catal. 128:526–36

36. Cho BK. 1991. Chemical modification of catalyst support for enhancement of transient catalytic activity:nitric oxide reduction by carbon monoxide over rhodium. J. Catal. 131:74–87

37. Strongin DR, Carrazza J, Bare SR, Somorjai GA. 1987. The importance of C7 sites and surface roughnessin the ammonia synthesis reaction over iron. J. Catal. 103:213–15

38. Strongin DR, Bare SR, Somorjai GA. 1987. The effects of aluminum oxide in restructing iron single-crystal surfaces for ammonia synthesis. J. Catal. 103:289–301

39. Xu C, Lai X, Zajac GW, Goodman DW. 1997. Scanning tunneling microscopy studies of the TiO2(110)surface: structure and the nucleation growth of Pd. Phys. Rev. B 56:13464–82

40. Chen PJ, Goodman DW. 1994. Epitaxial growth of ultrathin Al2O3 films on Ta(110). Surf. Sci. Lett.312:L767–73

41. Lai X, Chusuei CC, Luo K, Guo Q, Goodman DW. 2000. Imaging ultrathin Al2O3 films with scanningtunneling spectroscopy. Chem. Phys. Lett. 330:226–30

42. Wu MC, Goodman DW. 1994. Particulate Cu on ordered Al2O3: reactions with nitric oxide and carbonmonoxide. J. Phys. Chem. 98:9874–81

43. He JW, Xu X, Corneille JS, Goodman DW. 1992. X-ray photoelectron spectroscopic characterizationof ultra-thin silicon oxide films on a Mo(100) surface. Surf. Sci. 279:119–26

44. Santra AK, Goodman DW. 2002. Oxide-supported metal clusters: models for heterogeneous catalysts.J. Phys. Condens. Matter 14:R31–62

45. Schroeder T, Adelt M, Richter B, Naschitzki M, Baumer M, Freund HJ. 2000. Epitaxial growth of SiO2

on Mo(112). Surf. Rev Lett. 7:7–1446. Schroeder T, Giorgi JB, Baumer M, Freund HJ. 2002. Morphological and electronic properties of

ultrathin crystalline silica epilayers on a Mo(112) substrate. Phys. Rev. B 66:16542247. Weissenrieder J, Kaya S, Lu JL, Gao HJ, Shaikhutdinov S, et al. 2005. Atomic structure of a thin silica

film on a Mo(112) substrate: a two-dimensional network of SiO4 tetrahedra. Phys. Rev. Lett. 95:07610348. Xu X, Goodman DW. 1992. New approach to the preparation of ultrathin silicon dioxide films at low

temperatures. Appl. Phys. Lett. 61:774–7649. Wu MC, Corneille JS, Estrada CA, He JW, Goodman DW. 1991. Synthesis and characterization of

ultra-thin MgO films on Mo(100). Chem. Phys. Lett. 182:472–7850. Wu MC, Goodman DW. 1992. Acid-base properties of MgO studied by high resolution electron energy

loss spectroscopy. Catal. Lett. 15:1–1151. Wu MC, Truong CM, Coulter K, Goodman DW. 1992. Role of F centers in the oxidative coupling of

methane to ethane over Li-promoted MgO catalysts. J. Am. Chem. Soc. 114:7565–6752. Wu MC, Corneille JS, He JW, Estrada CA, Goodman DW. 1992. Preparation, characterization and

chemical properties of ultrathin MgO films on Mo(100). J. Vac. Sci. Technol. A 10:1467–7153. Corneille JS, He JW, Goodman DW. 1994. XPS characterization of ultra-thin MgO films on a Mo(100)

surface. Surf. Sci. 306:269–7854. Wu MC, Truong CM, Goodman DW. 1993. Interactions of ammonia with a NiO(100) surface studied

by high resolution electron energy loss spectroscopy and temperature programmed desorption spec-troscopy. J. Chem. Phys. 97:4182–86

55. Wu MC, Truong CM, Goodman DW. 1993. Interactions of alcohols with a NiO(100) surface studiedby high-resolution electron energy loss spectroscopy and temperature-programmed desorption spec-troscopy. J. Phys. Chem. 97:9425–33

56. Truong CM, Wu MC, Goodman DW. 1992. Adsorption and reaction of formic acid on NiO(100) filmson Mo(100): TPD and HREELS studies. J. Chem. Phys. 97:9447–53


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57. Truong CM, Wu MC, Goodman DW. 1993. Adsorption of formaldehyde on nickel oxide studied bythermal programmed desorption and high-resolution electron energy loss spectroscopy. J. Am. Chem.Soc. 115:3647–53

58. Cho B, Choi E, Chung S, Kim K, Kang T, et al. 1999. A novel Cr2O3 thin film on stainless steel withhigh sorption resistance. Surf. Sci. Lett. 439:L799–802

59. Dong QZ, Hu JD, Guo ZX, Lian JS, Chen JW, Chen B. 2002. Surface morphology study on chromiumoxide growth on Cr films by Nd-YAG laser oxidation process. Appl. Surf. Sci. 202:114–19

60. Oh WS, Xu C, Kim DY, Goodman DW. 1997. Preparation and characterization of epitaxial titaniumoxide films on Mo(100). J. Vac. Sci. Technol. A 15:1710–16

61. Guo Q, Oh WS, Goodman DW. 1999. Titanium oxide films grown on Mo(110). Surf. Sci. 437:49–6062. Corneille JS, He JW, Goodman DW. 1995. Preparation and characterization of ultra-thin iron oxide

films on a Mo(100) surface. Surf. Sci. 338:221–2463. Lemire C, Meyer R, Henrich VE, Shaikhutdinov SK, Freund HJ. 2004. The surface structure of

Fe3O4(111) films as studied by CO adsorption. Surf. Sci. 572:103–1464. Shaikhutdinov SK, Meyer R, Lahav D, Baumer M, Kluner T, Freund HJ. 2003. Determination of atomic

structure of the metal-oxide interface: Pd nanodeposits on an FeO(111) film. Phys. Rev. Lett. 91:07610265. Franchy R. 2000. Growth of thin, crystalline oxide, nitride and oxynitride films on metal and metal alloy

surfaces. Surf. Sci. Rep. 39:199–29466. Provides a detailedsummary of (metal)oxides at an atomic levelas materials, catalysts,and catalyst supports.

66. Freund HJ, Kuhlenbeck H, Staemmler V. 1996. Oxide surfaces. Rep. Prog. Phys. 59:283–34767. Chambers SA. 2000. Epitaxial growth and properties of thin film oxides. Surf. Sci. Rep. 39:105–8068. Street SC, Xu C, Goodman DW. 1997. The physical and chemical properties of ultrathin oxide films.

Annu. Rev. Phys. Chem. 48:43–6869. Chen MS, Santra AK, Goodman DW. 2004. The structure of thin SiO2 films grown on Mo(112). Phys.

Rev. B 69:15540470. Chen MS, Goodman DW. 2006. The structure of monolayer SiO2 on Mo(112): a 2-D [Si-O-Si] network

or isolated [SiO4] units? Surf. Sci. Lett. 600:L255–5971. Wu YT, Garfunkel E, Madey TE. 1996. Growth of ultrathin crystalline Al2O3 films on Ru(0001) and

Re(0001) surfaces. J. Vac. Sci. Technol. A 14:2554–6372. Chen JG, Colaianni ML, Weinberg WH, Yates JT. 1992. The Cu/Al2O3/Al(111) interface: initial film

growth and thermally induced diffusion of copper into the bulk. Surf. Sci. 279:223–3273. Bardi U, Atrei A, Rovida G. 1992. Initial stages of oxidation of the Ni3Al alloy: structure and composition

of the aluminum-oxide overlayer studied by XPS, LEIS and LEED. Surf. Sci. 268:87–9774. Ceballos G, Song Z, Pascual JI, Rust HP, Conrad H, et al. 2002. Structure investigation of the top-

most layer of a thin ordered alumina film grown on NiAl(110) by low temperature scanning tunnelingmicroscopy. Chem. Phys. Lett. 359:41–47

75. Park Y, Fullerton EE, Bader SD. 1995. Epitaxial growth of ultrathin MgO films on Fe(001) seed layers.J. Vac. Sci. Technol. A 13:301–4

76. Schintke S, Messerli S, Pivetta M, Patthey F, Libioulle L, et al. 2001. Insulator at the ultrathin limit:MgO on Ag(001). Phys. Rev. Lett. 87:276801

77. Wollschlager J, Erdos D, Goldbach H, Hopken R, Schroder KM. 2001. Growth of NiO and MgO filmson Ag(100). Thin Solid Films 400:1–8

78. Lai X, Guo Q, Min BK, Goodman DW. 2001. Synthesis and characterization of titania films on Mo(110).Surf. Sci. 487:1–8

79. Kolmakov A, Goodman DW. 2002. In situ scanning tunneling microscopic studies of supported metalclusters: growth and thermal evolution of individual particles. Chem. Rec. 2:446–57

80. Lai X, St. Clair TP, Valden M, Goodman DW. 1998. Scanning tunneling microscopy studies of metalclusters supported on TiO2(110): morphology and electronic structure. Prog. Surf. Sci. 59:25–52

81. Bowker M, Stone P, Morrall P, Smith R, Bennett R, et al. 2005. Model catalyst studies of the strongmetal-support interaction: surface structure identified by STM on Pd nanoparticles on TiO2(110).J. Catal. 234:172–81

82. Bowker M, Stone P, Bennett R, Perkins N. 2002. CO adsorption on a Pd/TiO2(110) model catalyst.Surf. Sci. 497:155–65

284 Gao · Goodman

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83. Diebold U, Pan JM, Madey TE. 1995. Ultrathin metal-film growth on TiO2(110): an overview. Surf.Sci. 331:845–54

84. Chen MS, Goodman DW. 2005. An investigation of the TiOx-SiO2/Mo(112) interface. Surf. Sci.574:259–68

85. Chen MS, Wallace WT, Kumar D, Yan Z, Gath KK, et al. 2005. Synthesis of well-ordered ultra-thintitanium oxide films on Mo(112). Surf. Sci. Lett. 581:L115–21

86. Demonstrates theunique catalyticproperty of an Aubilayer structure.

86. Chen MS, Goodman DW. 2004. The structure of catalytically active Au on titania. Science306:252–55

87. Schon G, Simon U. 1995. A fascinating new field in colloid science: small ligand-stabilized metal clustersand possible application in microelectronics. 1. State-of-the-art. Colloid Polym. Sci. 273:101–17

88. Schon G, Simon U. 1995. A fascinating new field in colloid science: small ligand-stabilized metal clustersand possible application in microelectronics. 2. Future directions. Colloid Polym. Sci. 273:202–18

89. Heiz U, Vanolli F, Trento L, Schneider WD. 1997. Chemical reactivity of size-selected supportedclusters: an experimental setup. Rev. Sci. Instrum. 68:1986–94

90. Bromann K, Felix C, Brune H, Harbich W, Monot R, et al. 1996. Controlled deposition of size-selectedsilver nanoclusters. Science 274:956–58

91. Kleyn AW. 2003. Molecular beams and chemical dynamics at surfaces. Chem. Soc. Rev. 32:87–9592. Avoyan A, Rupprechter G, Eppler AS, Somorjai GA. 2000. Fabrication and characterization of the Ag-

based high-technology model nanocluster catalyst for ethylene epoxidation manufactured by electronbeam lithography. Top. Catal. 10:107–13

93. Jacobs PW, Ribeiro FH, Somorjai GA, Wind SJ. 1996. New model catalysts: uniform platinum clusterarrays produced by electron beam lithography. Catal. Lett. 37:131–36

94. Schildenberger M, Bonetti Y, Aeschlimann M, Scandella L, Gobrecht J, Prins R. 1998. Preparation ofmodel catalysts by laser interference nanolithography followed by metal cluster deposition. Catal. Lett.56:1–6

95. Haruta M. 2001. Catalysis of gold nanoparticles deposited on metal oxides. Cattech 6:102–1596. Summarizes therecent advances in Aucatalysis.

96. Hashmi ASK, Hutchings GJ. 2006. Gold catalysis. Angew. Chem. Int. Ed. Engl. 45:7896–93697. Valden M, Pak S, Lai X, Goodman DW. 1998. Structure sensitivity of CO oxidation over model Au/TiO2

catalysts. Catal. Lett. 56:7–1098. Iizuka Y, Fujiki H, Yamauchi N, Chijiiwa T, Arai S, et al. 1997. Adsorption of CO on gold supported

on TiO2. Catal. Today 36:115–2399. Lai X, Goodman DW. 2000. Structure-reactivity correlations for oxide-supported metal catalysts: new

perspectives from STM. J. Mol. Catal. 162:33–50100. Demonstrates thestrong structure-reactivity relationship ofAu/TiO2 catalysts inCO oxidation.

100. Valden M, Lai X, Goodman DW. 1998. Onset of catalytic activity of gold clusters on titania withthe appearance of nonmetallic properties. Science 281:1647–50

101. Parker SC, Grant AW, Bondzie VA, Campbell CT. 1999. Island growth kinetics during the vapordeposition of gold onto TiO2(110). Surf. Sci. 441:10–20

102. Cosandey F, Madey TE. 2001. Growth, morphology, interfacial effects and catalytic properties of Au onTiO2. Surf. Rev. Lett. 8:73–93

103. First PN, Stroscio JA, Dragoset RA, Pierce DT, Celotta RJ. 1989. Metallicity and gap states in tunnelingto Fe clusters on GaAs(110). Phys. Rev. Lett. 63:1416–19

104. Xu C, Lai X, Zajac GA, Goodman DW. 1997. Scanning tunneling microscopic studies of the TiO2(110)surface: structure and the nucleation/growth of Pd. Phys. Rev. B 56:13464–82

105. Nilius N, Kulawik M, Rust HP, Freund HJ. 2004. Quantization of electronic states in individual oxide-supported silver particles. Surf. Sci. 572:347–54

106. Hovel H, Grimm B, Bodecker M, Fieger K, Reihl B. 2000. Tunneling spectroscopy on silver clusters atT = 5 K: size dependence and spatial energy shifts. Surf. Sci. Lett. 463:L603–8

107. Chen MS, Cai Y, Yan Z, Goodman DW. 2006. On the origin of the unique properties of suppored Aubilayers. J. Am. Chem. Soc. 128:6341–46

108. Kamer PCJ, Reek JNH, Van Leeuwen PWNM. 2002. Rhodium phosphite catalysts. In Rhodium CatalyzedHydroformylation, ed. PWNM Van Leeuwen, C Claver, pp. 35–59. New York: Kluwer Acad.

109. Balakos MW, Chuang SSC. 1995. Transient response of propionaldehyde formation duringCO/H2/C2H4 reaction on Rh/SiO2. J. Catal. 151:253–65


Ann

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110. Chuang SSC, Stevens RW Jr, Khatri R. 2005. Mechanism of C2+ oxygenate synthesis on Rh catalysts.Top. Catal. 32:225–32

111. Chuang SSC, Pien SI. 1992. Infrared study of the CO insertion reaction on reduced, oxidized, andsulfided Rh/SiO2 catalysts. J. Catal. 135:618–34

112. Huang L, Xu Y, Guo W, Liu A, Li D, Guo X. 1995. Study on catalysis by carbonyl cluster-derivedSiO2-supported rhodium for ethylene hydroformylation. Catal. Lett. 32:61–81

113. Hanaoka T, Arakawa H, Matsuzaki T, Sugi Y, Kanno K, Abe Y. 2000. Ethylene hydroformylation andcarbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts. Catal.Today 58:271–80

114. Demonstrates theutilization of PM-IRASas a surface-sensitivespectroscopic tool tomonitor catalyticprocess at elevatedpressure andtemperatures.

114. McClure SM, Lunddwall MJ, Goodman DW. 2011. Supported metal clusters as models forcatalysis: silica supported Rh nanoparticles. Proc. Natl. Acad. Sci. USA 108:931–36

115. McClure SM, Goodman DW. 2011. Simulating the complexities of heterogeneous catalysis with modelsystems: case studies of SiO2 supported Pt-group metals. Top. Catal. 54:351–62

116. Solymosi F, Pasztor M. 1986. Infrared study of the effect of hydrogen on carbon monoxide–inducedstructural changes in supported rhodium. J. Phys. Chem. 90:5312–17

117. Yang AC, Garland CW. 1957. Infrared studies of carbon monoxide chemisorbed on rhodium. J. Phys.Chem. 61:1504–12

118. Henderson MA, Worley SD. 1985. An infrared study of the hydrogenation of carbon dioxide on sup-ported rhodium catalysts. J. Phys. Chem. 89:1417–23

119. Worley SD, Mattson GA, Caudill R. 1983. An infrared study of the hydrogenation of carbon monoxideon supported rhodium catalysts. J. Phys. Chem. 87:1671–73

120. Solymosi F, Erdohelyi A, Bansagi T. 1981. Methanation of CO2 on supported rhodium catalyst. J. Catal.68:371–82

121. Solymosi M, Pasztor M. 1987. Analysis of the IR-spectral behavior of adsorbed CO formed in H2 +CO2 surface interaction over supported rhodium. J. Catal. 104:312–22

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PC63-FrontMatter ARI 27 February 2012 8:1

Annual Review ofPhysical Chemistry

Volume 63, 2012Contents

Membrane Protein Structure and Dynamics from NMR SpectroscopyMei Hong, Yuan Zhang, and Fanghao Hu � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1

The Polymer/Colloid Duality of Microgel SuspensionsL. Andrew Lyon and Alberto Fernandez-Nieves � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �25

Relativistic Effects in Chemistry: More Common Than You ThoughtPekka Pyykko � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �45

Single-Molecule Surface-Enhanced Raman SpectroscopyEric C. Le Ru and Pablo G. Etchegoin � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �65

Singlet Nuclear Magnetic ResonanceMalcolm H. Levitt � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �89

Environmental Chemistry at Vapor/Water Interfaces: Insights fromVibrational Sum Frequency Generation SpectroscopyAaron M. Jubb, Wei Hua, and Heather C. Allen � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 107

Extensivity of Energy and Electronic and Vibrational StructureMethods for CrystalsSo Hirata, Murat Keceli, Yu-ya Ohnishi, Olaseni Sode, and Kiyoshi Yagi � � � � � � � � � � � � � � 131

The Physical Chemistry of Mass-Independent Isotope Effects andTheir Observation in NatureMark H. Thiemens, Subrata Chakraborty, and Gerardo Dominguez � � � � � � � � � � � � � � � � � � 155

Computational Studies of Pressure, Temperature, and Surface Effectson the Structure and Thermodynamics of Confined WaterN. Giovambattista, P.J. Rossky, and P.G. Debenedetti � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 179

Orthogonal Intermolecular Interactions of CO Molecules on aOne-Dimensional SubstrateMin Feng, Chungwei Lin, Jin Zhao, and Hrvoje Petek � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 201

Visualizing Cell Architecture and Molecular Location Using SoftX-Ray Tomography and Correlated Cryo-Light MicroscopyGerry McDermott, Mark A. Le Gros, and Carolyn A. Larabell � � � � � � � � � � � � � � � � � � � � � � � � � 225

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Deterministic Assembly of Functional Nanostructures UsingNonuniform Electric FieldsBenjamin D. Smith, Theresa S. Mayer, and Christine D. Keating � � � � � � � � � � � � � � � � � � � � � 241

Model Catalysts: Simulating the Complexitiesof Heterogeneous CatalystsFeng Gao and D. Wayne Goodman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 265

Progress in Time-Dependent Density-Functional TheoryM.E. Casida and M. Huix-Rotllant � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 287

Role of Conical Intersections in Molecular Spectroscopyand Photoinduced Chemical DynamicsWolfgang Domcke and David R. Yarkony � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 325

Nonlinear Light Scattering and Spectroscopy of Particlesand Droplets in LiquidsSylvie Roke and Grazia Gonella � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 353

Tip-Enhanced Raman Spectroscopy: Near-Fields Actingon a Few MoleculesBruno Pettinger, Philip Schambach, Carlos J. Villagomez, and Nicola Scott � � � � � � � � � � � 379

Progress in Modeling of Ion Effects at the Vapor/Water InterfaceRoland R. Netz and Dominik Horinek � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 401

DEER Distance Measurements on ProteinsGunnar Jeschke � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 419

Attosecond Science: Recent Highlights and Future TrendsLukas Gallmann, Claudio Cirelli, and Ursula Keller � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 447

Chemistry and Composition of Atmospheric Aerosol ParticlesCharles E. Kolb and Douglas R. Worsnop � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 471

Advanced NanoemulsionsMichael M. Fryd and Thomas G. Mason � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 493

Live-Cell Super-Resolution Imaging with Synthetic FluorophoresSebastian van de Linde, Mike Heilemann, and Markus Sauer � � � � � � � � � � � � � � � � � � � � � � � � � � 519

Photochemical and Photoelectrochemical Reduction of CO2

Bhupendra Kumar, Mark Llorente, Jesse Froehlich, Tram Dang,Aaron Sathrum, and Clifford P. Kubiak � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 541

Neurotrophin Signaling via Long-Distance Axonal TransportPraveen D. Chowdary, Dung L. Che, and Bianxiao Cui � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 571

Photophysics of Fluorescent Probes for Single-Molecule Biophysicsand Super-Resolution ImagingTaekjip Ha and Philip Tinnefeld � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 595

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Ultrathin Oxide Films on Metal Supports:Structure-Reactivity RelationsS. Shaikhutdinov and H.-J. Freund � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 619

Free-Electron Lasers: New Avenues in Molecular Physics andPhotochemistryJoachim Ullrich, Artem Rudenko, and Robert Moshammer � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 635

Dipolar Recoupling in Magic Angle Spinning Solid-State NuclearMagnetic ResonanceGael De Paepe � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 661

Indexes

Cumulative Index of Contributing Authors, Volumes 59–63 � � � � � � � � � � � � � � � � � � � � � � � � � � � 685

Cumulative Index of Chapter Titles, Volumes 59–63 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 688

Errata

An online log of corrections to Annual Review of Physical Chemistry chapters (if any,1997 to the present) may be found at http://physchem.AnnualReviews.org/errata.shtml

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model catalysts: simulating the complexities of ... files/543_arpc-12_63_265-286.pdfmodel catalysts:...

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