Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions

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1 Top Catal (2012) 55: DOI /s y ORIGINAL PAPER Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions Vladimir V. Pushkarev Zhongwei Zhu Kwangjin An Antoine Hervier Gabor A. Somorjai Published online: 10 October 2012 Ó 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 composition 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-pressure scanning tunneling microscopy (HPSTM) and ambient-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 O 2. APXPS is V. V. Pushkarev Z. Zhu K. An A. Hervier G. A. Somorjai (&) Department of Chemistry, University of California, Berkeley, CA 94720, USA somorjai@berkeley.edu Z. Zhu A. Hervier G. A. Somorjai Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA 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 correlated 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, structure, 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

2 1258 Top Catal (2012) 55: reaction media. When metal nanoparticles are supported by oxides, the support materials may also provide an additional 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 nm using dihydrogen hexachloroplatinate (H 2 PtCl 6 ) 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 externally added nanoparticle seeds, is also employed to govern particle sizes. Recently, extremely small metal nanoparticles 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 nm by 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 H 2, 413 K) [6] 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 nanoparticle shape can be controlled by addition of trace amounts of secondary transition metal ions into the reaction 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 nanoparticles 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, catalytically active Rh and Pt nanocubes were synthesized in the presence of Br -, their shape being attributed to the preferential stabilization effect on {100} faces by Br - 11, 12].

3 Top Catal (2012) 55: 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] 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 compounds [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 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

4 1260 Top Catal (2012) 55: Fig. 3 a X-ray photoelectron spectra measured on Rh x Pt 1-x (x = 0-1) nanoparticles on silicon surface. b Plot of Rh composition determined by XPS measurement. c TEM images of the Rh x Pt 1-x nanoparticles [16] 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 Rh x Pd 1-x (*15 nm), Rh x Pt 1-x (8 11 nm), and Pd x Pt 1-x (*16 nm) nanoparticles with various atomic fractions (x = 0.2, 0.5 and 0.8) [15]. Despite the fact that the bimetallic 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 Rh x Pd 1-x and Pd x Pt 1-x nanoparticles. Park et al. also synthesized Rh x Pt 1-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 Rh x Pt 1-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 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 particles 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

5 Top Catal (2012) 55: Fig. 4 Schematic illustrations for preparation of nanoparticle-based heterogeneous catalysts mesoporous supports by sonicating together a colloidal solution of metal nanoparticles and oxide supports. Although the nanoparticles can be dispersed this way within the mesoporous framework [17], a certain fraction of the them is potentially 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 channels. Nanoparticle encapsulation, in which the silica grows around the metal particles, has hence been developed as an alternative approach. Song et al. reported that monodispersed Pt nanoparticles of nm were incorporated into SBA-15 via hydrothermal synthesis, in which Pt particles 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 susceptible 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 subsequently converted to Pt/m-SiO 2 by calcination to generate the metal-core/silica-shell structure, as shown in Fig. 5 [19]. The Pt/m-SiO 2 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-SiO 2 core/shell structure 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 reactions 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 structuresensitive reactions, since the experimental measurements

6 1262 Top Catal (2012) 55: Fig. 5 TEM images of a Pt nanoparticles, b Pt/SiO 2, and c Pt/mesoporous-SiO 2 core/shell structures 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 structureinsensitive. Boudart s explanation of structural sensitivity was based on the active ensemble (geometric) theory of catalysis originally 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 distinguished 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 contrast, 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 monodispersed 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 surfaces 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 structure-sensitive reactions. Kliewer et al. studied adsorption and catalytic hydrogenation 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 hydrogenation 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 hydrogenation 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-butanol, 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

7 Top Catal (2012) 55: 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 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 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]. Reactions 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 spectroscopy 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 cyclohexane is observed [28]. The results on single crystals are in agreement with the kinetics measured over Pt nanoparticles with finely controlled shape. Both cyclohexene and cyclohexane reaction products are formed on Pt nanocrystals 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 CO 2 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 electronic 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 Rh x Pt 1-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 nanoparticle surface under reaction conditions.

8 1264 Top Catal (2012) 55: have been reduced in H 2 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 Fig. 7 Plot of turnover rates and E a in CO oxidation over Rh x Pt 1-x (x = 0 1) nanoparticles as a function of Rh content. The experiments were performed in the K temperature range in a batch reactor using 100 torr O 2 and 40 torr CO initial reactant pressures [16] 5 Effects of the Catalyst Support on Nanoparticle Catalysis As mentioned in the previous sections, once the nanoparticles 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 nanoparticles are ordinarily dispersed on a porous oxide or carbonaceous support in industrial applications [32]. Usually not active on its own, the support materials tend to maintain 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 H 2 at high temperatures, noble metal catalysts supported on TiO 2 almost completely lost their ability to adsorb CO and H 2 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 platinum particles. Hence, the intriguing factors that contributed 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 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 H 2. The authors ruled out a possible explanation of metal encapsulation by the oxide because the effect was reversible 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/TiO 2 catalysts at 973 K and suggested that TiO 2 was reduced to Ti 4 O 7 which subsequently migrated over the Pd surface, based on TEM images and H 2 adsorption results [36]. In a similar experiment, Komaya et al. provided high resolution TEM images of reduced Rh/TiO 2 catalysts, demonstrating that Rh particles 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 TiO 2 agreed with an uptake drop in H 2 adsorption experiments. As TEM resolution has continuously improved, evidence for encapsulation became unquestionable: Fig. 8 illustrates a Rh particle encapsulated with CeTbO x 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. 8 HREM images of a 0.5 % Rh/Ce 0.8 Tb 0.2 O 2-x catalyst reduced at 1173 K [38]

9 Top Catal (2012) 55: that support effects could be observed at reduction temperatures lower than those required to cause encapsulation greatly challenged the explanation [40]. As a consequence, encapsulation is considered not to be the only factor contributing 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 hypothesis 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 GeH 4 to Ge [42]. Taylor later suggested that the Pt surface provided recombination sites for atomic H to form H 2 that readily desorbs [43]. Lately, spillover was directly observed for the first time using scanning tunneling microscopy (STM) on methanol adsorption onto the Pt/TiO 2 (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 TiO 2 surface and the migration along the five fold-coordinated Ti rows away from Pt. TPD measurements indicated that these spots corresponded to CH 3 O(a), even though TiO 2 (110) alone cannot dissociatively adsorb CH 3 OH 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/ TiO 2, for example, dissociatively adsorbed H atoms on Rh can spill over onto the TiO 2 surface, resulting in an overestimate of the number of active Rh surface sites [37] Fig 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 Charge Transfer at the Metal/Oxide Heterojunction 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/TiO 2 (110) surface nm 2 ; V s,?1.0 V; I t, 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 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]

10 1266 Top Catal (2012) 55: support, which was more noticeable for smaller particles. After a high temperature reduction in H 2 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. 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] Fung characterized thin films of Pt on SiO 2 and TiO 2 by XPS before and after reduction in H 2 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 SiO 2.Insteadof the direct reduction of the metal by hydrogen, they ascribed the shift to electron transfer from the TiO 2 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/TiO 2 could be explained by a model that involved two kinds of charge transfer [49]. After a low temperature reduction in H 2 at 473 or 523 K, the metal particles donated electrons to the 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 semiconductor, 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/TiO 2 and Pt/GaN diodes, and H 2 oxidation over Pt/TiO 2 diodes [51, 52]. In both cases, the activation energies measured for the currents were in agreement with the activation energies of the reaction, indicating that the reaction on the surface dissipates 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 prevent 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 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].

11 Top Catal (2012) 55: In the experiments, TiO x films were annealed under different conditions to obtain various stoichiometries, such as TiO 1.7, TiO 1.9, and TiO 2 determined by XPS. Oxygen vacancies in TiO 2 create an electronic state about 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 SF 6 plasma, yielding six types of oxide support: fluorine doped and undoped TiO 1.7, TiO 1.9, and TiO 2. F binds with Ti by filling oxygen vacancies and consequently the electrons filled the vacant midgap states, slightly offsetting the conductivity of the TiO 1.7 and TiO 1.9 films [53]. However, F also acts as an n-type donor, forming donor levels just beneath the conduction band, which increases conductivity of TiO 2 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/ TiO x catalysts. Although turnover increases nearly twofold when stoichiometric TiO 2 is F-doped, no increase is observed with the non-stoichiometric TiO 1.7 and TiO 1.9 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. Nonstoichiometric TiO x 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 conditions used, the three products of the reaction are the total oxidation product CO 2, and partial oxidation products methyl formate and formaldehyde. After fluorine doping in stoichiometric TiO 2, the methanol oxidation occurs significantly faster with the partial oxidation product fraction enhanced from 17 to 35 %. When non-stoichiometric TiO 2 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 Fig. 11 a Turnover frequencies (TOF) for CO oxidation on Pt nanoparticles supported on the six titanium oxide supports: TiO 2, TiO 1.9, and TiO 1.7, each with and without F insertion. Reaction occurred in 40 Torr CO, 100 TorrO 2, 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 measurements for all six titanium oxide supports before Pt nanoparticle deposition. In the case of TiO 2, F insertion increased surface conductivity by a factor of 40 by acting as an extrinsic n-type donor. However, in the case of TiO 1.7 and TiO 1.9, 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 TiO 2 with and without F is magnified by This reflects the insulating nature of TiO 2 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] 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 intermediates, the results obtained with traditional surface science approaches under ultrahigh vacuum (UHV) might not be in general applied to catalyst structure during catalytic reactions. 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, including 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

12 1268 Top Catal (2012) 55: 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 fundamental 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 elevated 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 Lawrence Berkeley National Laboratory focused the photoelectrons 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 process, 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 performed on Rh nanoparticles with diameters from 2 to 12 nm under CO oxidation conditions, in order to investigate 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 modified 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 differences of chemical potentials at surfaces compared with the bulk. We employed APXPS to investigate such behaviors using Rh x Pd 1-x, Rh x Pt 1-x, and Pd x Pt 1-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 Rh x Pd 1-x and Pt x Pd 1-x nanoparticles were observed to be Rh-rich and Pd-rich at the surface, respectively, while as-synthesized Rh x Pt 1-x nanoparticles possessed a homogeneous alloy phase. The surface composition variation at 0.7 nm sample depth was subsequently studied under oxidizing (O 2 or NO), reducing (CO or H 2 ) and reaction (NO? CO) conditions at 573 K. It was found that Rh segregated to surface in the presence of oxidizing gases for Rh x Pd 1-x and Rh x Pt 1-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 Pd x Pt 1-x nanoparticles no matter how the chemical environment changed. Differences 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

13 Top Catal (2012) 55: 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 O 2 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] the highest stability of rhodium oxide drove Rh to surface under oxidizing conditions. The gas driven migration was not observed in Pd x Pt 1-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 equilibrium 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 Rh 0.5 Pd 0.5 bimetallic nanoparticles in CO oxidation [78]. Preferential adsorption of CO on Pd atoms and spillover of oxygen atoms dissociated on Rh together contributed to the superior activity as compared to the monatomic counterparts (Fig. 15). Later comparisons of surface segregation effects between Rh 0.5 Pd 0.5 bulk crystal and Rh 0.5 Pd 0.5 nanoparticles 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 multicomponent 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

14 1270 Top Catal (2012) 55: Fig. 14 Depth profiles of as-synthesized Rh 0.5 Pd 0.5,Pd 0.5 Pt 0.5 nanoparticles showing a core shell structure and as-synthesized Rh 0.5 Pt 0.5 nanoparticles exhibiting a homogeneous alloy phase investigated by synchrotron based XPS [15, 77] Fig. 15 Changes of surface atomic fractions of Rh 0.5 Pd 0.5,Rh 0.5 Pt 0.5, and Pd 0.5 Pt 0.5 nanoparticles at 573 K under oxidizing (O 2 or NO), reducing (CO or H 2 ) and catalytic reaction (NO? CO) conditions. 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 changes 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 characterization 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 to several The photoelectrons have a kinetic energy of *300 ev, which corresponds to an inelastic mean free path of 0.7 nm [15] 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 influences on catalytic turnovers. During the hydrogen and deuterium exchange reaction on Pt(111) at room temperature, no distinguishable order could be discerned upon dosing 200 mtorr of H 2 and 20 mtorr of D 2 in the STM chamber, which implied that adsorbates diffused much faster than piezotube

15 Top Catal (2012) 55: 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 Å 9 90 Å STM images of Pt(111) in the presence of 200 mtorr H 2 and 20 mtorr D 2 at 298 K. The Pt(111) surface is catalytically active producing HD. b 90 Å 9 90 Å STM images of Pt(111) in the presence of 200 mtorr H 2, 20 mtorr D 2 and 5 mtorr CO at 298 K. The HD production stopped accompanied by the ordered structure under STM [69] 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 diffusion or even adsorption of reactants, which forced the H 2 /D 2 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/dehydrogenation [93], and ethylene hydrogenation [94], delineated 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

16 1272 Top Catal (2012) 55: Fig. 18 The structure changes of Pt(110) surface induced by (top) 1.7 atm of H 2,(middle) 1 atm of O 2, and (bottom) 1 atm of CO at 425 K [84] Fig. 19 STM images of Pt(557) surface in the presence of a torr of CO; b 1 torr of CO; and c evacuating from (b) to10-8 torr. The cluster formation induced by high pressure of CO is reversible [98] 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 H 2 and O 2 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 resolution: the preferential bonding between CO and lowcoordinated 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 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