Clusters, surfaces, and catalysis

Abstract

The surface science of heterogeneous metal catalysis uses model systems ranging from single crystals to monodispersed nanoparticles in the 1–10 nm range. Molecular studies reveal that bond activation (C–H, H–H, C–C, CO) occurs at 300 K or below as the active metal sites simultaneously restructure. The strongly adsorbed molecules must be mobile to free up these sites for continued turnover of reaction. Oxide–metal interfaces are also active for catalytic turnover. Examples using C–H and CO activation are described to demonstrate these properties. Future directions include synthesis, characterization, and reaction studies with 2D and 3D monodispersed metal nanoclusters to obtain 100% selectivity in multipath reactions. Investigations of the unique structural, dynamic, and electronic properties of nanoparticles are likely to have major impact in surface technologies. The fields of heterogeneous, enzyme, and homogeneous catalysis are likely to merge for the benefit of all three.

Metal containing catalysts are clusters of 1–10 nm in size. These are grouped into three types: heterogeneous catalysts that are embedded in high surface area supports, usually oxides, to optimize their surface area and thus the number of molecules they produce per second and also to optimize their thermal and chemical stability. They are used mostly at high temperatures (400–800 K) and in the presence of vapor phase reactants and product molecules. Enzyme catalysts operate in solution, mostly in water near 300 K, and the metal-containing active sites are surrounded by proteins that maintain structural mobility (1). Homogeneous catalysts function in the same solution in which the reactant and product molecules are dissolved, mostly in organic solvents, and used in the 300–500 K temperature range. Because of the different operating conditions and for reasons of history, the three fields of catalysis developed independently and became separate fields of science within different subdisciplines of chemistry. Heterogeneous catalysis was practiced mostly by physical chemists and chemical engineers, enzyme catalysis by biochemists, and homogeneous catalysis by inorganic and organometallic chemists.

Catalytic reactions are distinguished from stoichiometric reactions by having many turnovers per reacting active site producing 10² to 10⁶ molecules per site. Some of these catalytic reactions are very fast. For instance, the catalytic oxidation of carbon monoxide (CO) produces thousands of CO₂ molecules per metal surface site per second above ignition where the exothermicity of the process makes it self-sustaining in temperature and the reaction rate is only limited by the speed of transport of molecules to and from the catalyst surface (2–4). Ethylene hydrogenation, another exothermic reaction, turns over to produce ethane ≈10 times per metal surface site per second at 300 K on Pt(111) (5). The catalytic conversion of n-hexane to benzene or to branched isomers, a complex but important reaction that produces high-octane gasoline, has a turnover rate of 10⁻² product molecules per platinum surface site per second at ≈600 K (6). In general, the more complex the chemical reaction, the slower the turnover as the elementary reaction steps involve more complicated molecular rearrangements.

Over the past 2 decades, molecular studies of the structure and dynamics, during catalytic turnover, of these three types of catalysts, heterogeneous, homogeneous, and enzyme, have revealed features that are similar to all. The catalyst structures change under working conditions, and thus in situ studies are needed to verify the structures of the active, working catalyst. The active metal sites restructure during turnover as they bind, react, and release the adsorbed molecules. The reactive intermediates also must be mobile to free up the active site to be able to carry out the next turnover.

Thus, dynamics of the catalyst structure and that of the reactant molecules and reaction intermediates control both the activity (rates) and the selectivity (product distribution) of the catalysts.

In this overview, we focus on metal heterogeneous catalysts from the physical chemistry perspective. This field developed rapidly by use of model catalysts, first single-crystal surfaces (7–12), then monodispersed nanoclusters deposited on oxide surfaces by lithography techniques (13–17) (Fig. 1). These two-dimensional (2D) catalyst systems could be characterized by a large number of techniques on the molecular level and under reaction conditions. In our laboratory, sum frequency generation (SFG) vibrational spectroscopy was used to monitor the structure and bonding of the adsorbed monolayer under reaction conditions (18–20). Scanning tunneling microscopy (STM) applied at high reactant pressures under reaction conditions was used to monitor adsorbate mobility (21). To observe changes in surface structure of the metal catalysts low-energy electron diffraction (LEED) surface crystallography was used with single crystal metal surfaces.

Fig. 1.

Model surfaces for surface science and catalysis studies. (Upper) Three single crystal surface diagrams representing the (111), (100), and stepped (557) surfaces of a face-centered cubic crystal lattice. (Lower) A 2 × 2-μm atomic force microscopy image of a platinum nanoparticle array supported on a thin film of alumina. This array was fabricated with electron beam lithography.

Three phenomena were discovered that appear to be essential features of active heterogeneous catalysts.

1. The adsorption and bonding of incoming reacting molecules restructures the metal surface around the adsorption site. This process occurs to optimize the adsorbate-metal bonding. Bond activation (C–H, H–H, C–C, CO) leading to bond-breaking occurs as the metal active sites restructure.

2. The strongly adsorbed molecules at the active sites must be mobile along the surface to free up the active sites for continued turnover. This mobility is detectable on metal single crystals by STM. When the catalyst is poisoned by another adsorbate, mobility stops, and the turnover is inhibited.

3. Oxide–metal interfaces are active sites for catalytic turnover. There is evidence from diverse experiments for their unique activity even after the metal sites are deactivated.

Below we give examples that describe these phenomena followed by suggestions of some of the directions of catalysis science for the future.

Model catalysts are used less frequently in the fields of enzyme and homogeneous catalysis. Nevertheless, as the experimental conditions used for the three types of catalysts become similar by creative design, it is likely that the molecular characteristics, which control the activity and selectivity of the different types of catalysts (surface, enzyme, and homogeneous), also become similar. It is our hope that these three fields of catalysis will merge to become one field in the foreseeable future.

C–H Bond Activation of Ethylene over the Platinum (111) Crystal Surface

The activation of C–H bonds is at the heart of the catalytic transformation of organic molecules and the production of hydrogen from light alkanes.

At 52 K, ethylene adsorbs molecularly on the Pt(111) surface, forming a π-bonded ethylene surface species (22). At 200 K on the Pt(111) crystal face and at low pressures of ethylene (<10⁻⁶ torr; 1 torr = 133 Pa), the SFG vibrational spectrum is interpreted as owing to an ethylene molecule that forms di-σ bonds with the platinum surface (23). LEED surface crystallography studies indicate that ethylene occupies a threefold surface site, and the C–C bond is at a 23° angle with respect to the metal surface plane (Fig. 2a) (24). Upon heating, the SFG spectrum changes, indicating the formation of ethylidene (HC–CH₃) at 250 K by intramolecular hydrogen transfer (25). Slightly above this temperature, ethylidyne, C–CH₃, forms as another C–H bond is activated and dissociates, and there is a transfer of a hydrogen atom to the metal surface. SFG spectra show this transformation, and a mechanism is presented in Fig. 3. Ethylidyne remains stable up to 430 K when further loss of hydrogen, as well as C–C bond breaking, occurs with the formation of C₂H (acetylide) and CH (methylidyne) species that turn into graphite at >800 K (26). Ethylidyne on Pt(111) occupies the same threefold fcc metal site as di-σ-ethylene, as shown by LEED crystallography (Fig. 2b) (27). Its C–C bond is normal to the metal surface and the nearest- and next-nearest-neighbor metal atoms change their locations as compared with their positions on the metal surface before C–H bond dissociation occurs. Adsorbate-induced restructuring of the metal surface is, therefore, an important part of C–H activation, even on the (111) crystal face of platinum, which is the closest-packed and lowest surface free-energy plane of this face-centered cubic metal. The thermal activation of the C–H bond in ethylene on the (111) crystal faces of rhodium and palladium is similar to that found for the (111) crystal face of platinum (28, 29). On Rh(111), ethylidyne occupies a hexagonal close-packed hollow site; that is, there is a rhodium atom from the second metal layer directly under the carbon that is bound to the metal surface (30). This different site occupancy of ethylidyne on the Rh(111) surface changes the nature of adsorbate-induced restructuring of the metal surface around the chemisorption bond, as shown by LEED crystallography. On the Pd(111) surface, ethylene dehydrogenates to ethylidyne at 300 K, which reacts to form methylidyne >400 K (29).

Fig. 2.

The structure of adsorbed ethylene and the C–H activation induced restructuring of the platinum surface. (a) The best fit structure of di-σ-bonded ethylene on Pt(111) is a mixture of 60% at fcc sites and 40% at hcp sites. The symbols b₁ and b₂ represent Pt–C bond lengths, and bu represents buckling in the top layer. These data were obtained by LEED surface structure analysis. (b) The restructuring of the Pt(111) crystal face, when ethylidyne forms from ethylene by C–H bond breaking, is obtained by LEED surface structure analysis. Shading distinguishes buckled metal atoms in each layer.

Fig. 3.

Temperature-dependent rearrangement of adsorbed ethylene as monitored by SFG on Pt(111) surfaces. (Left) SFG spectra showing conversion from di-σ-bonded ethylene to ethylidyne with increasing temperature stepwise from 243 to 352 K. These spectra are taken after a 4-langmuir dose of ethylene onto Pt(111) single crystal. (Right) Proposed mechanism for this surface transformation.

When hydrogen is introduced in excess, in the 1–100 torr range, along with ethylene at 295 K, the dehydrogenation of the molecule slows down such that di-σ-ethylene and ethylidyne coexist on the Pt(111) surface (18, 25, 31, 32). The SFG spectrum demonstrating this point is shown in Fig. 4a. Under these reaction conditions, ethylene hydrogenation occurs with a turnover rate of ≈10 ethane molecules being produced per platinum surface site per second. The same rate of catalytic hydrogenation is found for the Pt(100) crystal face because this reaction is a structure-insensitive one (32). Only the di-σ-ethylene to ethylidyne ratio is somewhat different on the two crystal faces. Hence, although C–H dissociation of ethylene is surface-structure sensitive, its catalytic hydrogenation to ethylene is structure-insensitive. Detailed reaction studies of high pressures of ethylene in the presence of excess hydrogen have revealed that ethylidyne remains strongly adsorbed on the platinum surfaces for ≈1 million turnovers (molecules of ethane formed per platinum site per second) (33). Hydrogenation occurs through weakly adsorbed π-bonded species that occupy atop sites (18). This bonding site is available on all crystal planes, which explains the lack of structure sensitivity of the catalytic hydrogenation of ethylene. π-bonded ethylene hydrogenates sequentially to an ethyl intermediate and then to ethane, as both of these weakly adsorbed species have been detected on the Pt surface under reaction conditions by SFG in surface concentrations of 4% of a monolayer (Fig. 4b) (31).

Fig. 4.

The H₂ pressure dependence of ethylene surface species as monitored by SFG on Pt(111) surfaces. (a) SFG spectrum of the Pt(111) surface during ethylene hydrogenation with 100 torr of H₂, 35 torr of ethylene, and 615 torr of He at 295 K. (b) SFG spectrum of the Pt(111) surface during ethylene hydrogenation with 727 torr of H₂ and 60 torr of ethylene.

In Tables 1 and 2, we list the temperatures and pressures at which C–H bond dissociation was first observed in our studies and in those of others. It should be noted that bond dissociation occurs at a certain rate under the experimental conditions used. Thus, its detection by SFG spectroscopy or by the formation of surface carbon is somewhat uncertain and also subjected to detection limits. Nevertheless, it is clear from the inspection of the tables that C–H activation is facile at <300 K for all of the molecules listed.

Table 1.

Surface structure sensitivity of C-H activation

Crystal face	Ethylene		Cyclohexene
	Temp., K	Ref.	Temp., K	Ref.
Pt(111)	250	25	200	34
Pt(100)	275 (1 × 1)	35	220 (1 × 1)	36
	255 (5 × 20)	35	200 (5 × 20)	36
Pt(110)	280	37	–	–

Table 2.

Temperatures of observed C–H activation

Reactant species	Temp., K	Ref.
Alkenes*
Ethylene	250	25
Propylene	230	38
Isobutene	270	39
1-Hexene	250	40
Cyclohexene	200	34
Alkanes†
Methane	250	41
Ethane	275	41
n-hexane	296	40
2-methylpentane	296	40
3-methylpentane	296	40

*Pressure <10⁻⁶ torr.

^†Pressure 1–1.5 torr.

CO Bond Activation over Platinum Single Crystal Surfaces

By using SFG surface vibrational spectroscopy, the interaction of CO with platinum single crystals was investigated at high pressure and high temperatures (42). Under 40 torr of CO, the molecule was found to dissociate on Pt(111), Pt(557), and Pt(100) at 673, 548, and 500 K, respectively. The CO top site frequency was observed to shift to lower frequencies as a function of temperature (Fig. 5). At a particular temperature, dependent on the surface structure, the SFG spectra evolved with time, indicating the surface was being modified. The observed frequency shift before CO dissociation is attributed to a harmonic coupling to the frustrated translational mode. At the dissociation temperature, the frequency shift is attributed to surface roughening. The surface roughening is believed to result from the formation of platinum carbonyl species, which would be a driving force to extract platinum atoms from the surface lattice. For both the (111) and (100) surfaces of platinum, the crystal must be heated to a temperature at which platinum carbonyls are formed to produce step and kink sites, which are needed for dissociation, which deposits carbon on the metal surfaces. The Pt(111) surface exhibits a much higher CO dissociation temperature as compared with Pt(100) because it is the most stable surface for platinum. Pt(557) is essentially a (111) surface with steps already present in the structure (Fig. 1), so the crystal does not need to be heated to a high temperature to produce step sites necessary for CO dissociation and carbon deposition through the production of platinum carbonyls.

Fig. 5.

CO top-site frequency as a function of temperature for Pt(111), Pt(557), and Pt(100) under 40 torr of CO. The observed frequency redshift before CO dissociation is attributed to a harmonic coupling to the frustrated translational mode.

Mobility of Adsorbed Molecules During Catalytic Turnover: Poisoning of Reactions and Adsorbate Mobility by CO

STM studies indicate that ethylidyne is mobile on the Pt(111) crystal face at 300 K, both at high and low pressures, and with or without hydrogen, during the catalytic reaction (21, 26). Ethylidyne could only be slowed to the time scale of the STM scanning rate and observed upon cooling to180 K or lower. Extended Hückel calculations indicate that the activation energy barrier to ethylidyne surface diffusion is low (i.e., 0.1 eV; 1 eV = 1.602 × 10⁻¹⁹ J) (43). This mobility is essential for maintaining the catalyst activity. For the reaction to occur under high pressures where the surface is at nearly saturated coverage, statistical fluctuation of adsorbate density must be maintained to open up active sites crucial for the catalytic reaction. Adsorbate mobility is necessary so that favorable surface metal sites can be accessed by reactants for adsorption and diffusion to sustain the hydrogenation reaction. We tested this concept by adding CO to poison the reaction. By using high-pressure STM and mass spectrometry, we find that the catalytic activity of the Pt(111) and Rh(111) catalysts stops suddenly when CO is coadsorbed (21). STM results suggest that in the presence of CO, the adsorbed species become locked into static-ordered structures. Surface mobility is suppressed to inhibit the surface density fluctuation necessary for the availability of reactive sites, and the catalytic reaction is poisoned. Fig. 6 displays STM images showing this behavior on the Rh(111) surface.

Fig. 6.

Shown are 100 × 100-Å STM images of a Pt(111) single crystal after the sequential addition of 20 mtorr of H₂ (Left Upper), 20 mtorr of C₂H₄ (Center Upper), and 5.6 mtorr of CO (Right Upper and Lower). The catalytically active mobile adsorbate layer becomes immobile upon catalyst deactivation caused by coadsorption of CO.

High-pressure STM studies suggest that mobility within the adsorbed layer is also key for hydrogenation and dehydrogenation of cyclohexene to cyclohexane and benzene, respectively, on the Pt(111) surface (M.M., M. Salmeron, and G.A.S., unpublished results). At 300 K and above and with high pressures of cyclohexene (≥20 mtorr) and hydrogen (≥200 mtorr), surface species present on Pt(111) are disordered and cannot be imaged on the STM time scale (Fig. 7a). SFG studies indicate π-allyl (C₆H₉) species are present on the surface (32, 44). The addition of CO causes all catalytic activity to cease and orders the surface (Fig. 7b). These results indicate that the C₆ reaction intermediates that include cyclohexadienes and π-allyl are mobile during the dehydrogenation and hydrogenation reaction. There is evidence from both SFG and STM studies that formation of the π-allyl (C₆H₉) surface intermediate may be critical for the hydrogenation of cyclohexene to cyclohexane to proceed. Just as for ethylene hydrogenation, introduction of CO stops the mobility and results in the formation of ordered surface structures and the total poisoning of catalytic activity.

Fig. 7.

STM images of active and CO-poisoned Pt(111) catalyst surfaces during cyclohexene hydrogenation/dehydrogenation. (a) A 75 × 75-Å image of Pt(111) in the presence of 200 mtorr of H₂ and 20 mtorr of cyclohexene at 300 K. No discernable order is present. (b) A 90 × 90-Å STM image of Pt(111) in the presence of 200 mtorr of H₂, 20 mtorr of cyclohexene, and 5 mtorr of CO at 300 K. The surface forms an ordered CO structure, and the catalyst is deactivated.

Reactivity of Oxide–Metal Interfaces

Effect of Oxide Monolayers on Rhodium-Catalyzed CO Hydrogenation Reactions.

CO and CO₂ hydrogenation to methane over a Rh foil decorated with submonolayer quantities of TiO_x, VO_x, ZrO_x, NbO_x, TaO_x, and WO_x were carried out (45). The rate of methane formation was measured at 1 atm (1 atm = 101.3 kPa), and the state of the working catalyst was characterized by x-ray photoelectron spectroscopy immediately after reaction. Each of the oxides was found to enhance the rate of CO methanation relative to that observed over the Rh metal alone in the absence of any of the oxides. The maximum degree of rate enhancement occurs at an oxide coverage of approximately half a monolayer, which corresponds to an optimum oxide–metal interface area. Fig. 8 shows that niobium oxide, titanium oxide, and tantalum oxide show the largest increase in turnover rates, on the order of a 12-fold increase with respect to the clean rhodium foil. Clearly the oxide–metal interface is implicated in increasing the activity of rhodium of these hydrogenation reactions. It also should be noted that the oxides alone are not active for these reactions.

Fig. 8.

Model catalyst system to study the effect of the oxide–metal interface on CO₂ hydrogenation. (a) Diagram of submonolayer metal oxide islands formed on Rh foil. (b) Effect of different metal oxides, as a function of coverage, on the rate of methane formation from CO₂ and H₂ over Rh foil.

Platinum Nanoclusters on Silica and Alumina: CO Does Not Poison Ethylene Hydrogenation Activity at Oxide–Metal Interfaces.

When ethylene hydrogenation is carried out on platinum single crystal surfaces and platinum nanoclusters deposited by electron beam lithography on silica or alumina supports (Fig. 1), the two types of systems yield roughly equal turnover rates at 300 K (17, 46). However, major differences in catalytic behavior emerge when the platinum catalysts are poisoned by the addition of CO. CO poisoning of the platinum single crystal increases the activation energy to 20 kcal/mol from ≈10 kcal/mol and decreases the turnover rate at 300 K by 7 orders of magnitude to ≈10⁻⁶ s⁻¹.

Upon CO adsorption, the platinum nanoparticle arrays show dramatically different behavior than the Pt(111) single crystal. The CO-poisoned activation energy for ethylene hydrogenation on alumina and silica is 11.4 and 15.6 kcal/mol, respectively (17). These values are much lower than for the Pt single crystal. The turnover rates remain in the range of 5 × 10⁻² s⁻¹, which are orders of magnitude greater than for the single crystal surface. It appears that on these platinum nanoparticle arrays deposited on the oxides there are reaction sites that do not deactivate for ethylene hydrogenation in the presence of coadsorbed CO. The unpoisoned turnover frequencies for the Pt nanoparticle samples on alumina and silica are 7.3 and 5.3 s⁻¹, respectively, assuming that all available platinum surface atoms are active for the reactions. By using the same assumption, the CO-poisoned turnover frequencies for the alumina and silica supported samples are 0.071 and 0.041 s⁻¹, respectively. However, if the oxide–platinum interface sites are considered to be the only active sites for reaction during CO poisoning (Fig. 9), and the turnover frequency is calculated from just these sites alone, the alumina- and silica-supported samples have turnover frequencies of 7.1 and 4.2 s⁻¹, respectively. These turnover frequencies are almost identical to those of the unpoisoned samples. Although not conclusive, these results indicate that oxide–metal interface sites remain active under poisoning conditions.

Fig. 9.

The oxide–metal interface (highlighted by the red and black arcs) is catalytically active.

Future Directions of Catalysis Science

We need chemical processes that produce only the desired molecules without any by-products that need disposal as waste. We call this green chemistry, and it requires catalysts that exhibit 100% selectivity toward needed products for multipath reactions where each reaction channel is thermodynamically feasible. We need catalysts that will help us to achieve this goal. However, our understanding of the molecular ingredients of a catalyst system that control selectivity is not as well developed as our knowledge of the molecular properties that control activity (turnover rates). The size, surface structure, and shape of the metal cluster catalyst are known to influence reaction selectivity. For this reason, there are investigations in many laboratories, including our own, to synthesize 2D and 3D catalyst systems with monodispersed metal clusters with controlled size, surface structure, and shape.

Synthesis and Characterization of 2D and 3D Metal Nanoclusters

We synthesize platinum and rhodium nanoparticles in the 1–10 nm range in colloidal solutions in the presence of polymers (47, 48). As the metal clusters form, they are capped with the polymer that inhibits their aggregation but still permits their growth and to maintain their structure and chemical stability. They are characterized by transmission electron microscopy, small-angle x-ray scattering, and x-ray diffraction (Fig. 10) (47).

Fig. 10.

Transmission electron microscopy of platinum nanoparticles synthesized by two different techniques to obtain size and shape control of the particles. (a) Monodispersed platinum nanoparticles in the 1–8 nm range are synthesized in solution. They are capped with a polymer coating (polyvinylpyrrolidone in this case) that prevents their aggregation. (b) In the presence of silver ion, the shape of the platinum nanoparticles is altered because of preferential adsorption on one of the crystal surfaces.

To deposit 2D arrays of nanoparticles the polymer-coated metal clusters are dispersed on a liquid surface in a Langmuir–Blodgett trough and are captured at a given surface pressure on an oxide surface (49). The surface density can be altered by changing the applied surface pressure as shown in Fig. 11.

Fig. 11.

The polymer-capped monodispersed platinum nanoparticles are compressed using a Langmuir–Blodgett trough and captured on an oxide surface to form 2D arrays of different density. The surface density of nanoparticles is controlled by the surface pressure.

These particles are subjected to low-temperature oxidation and reduction treatment to remove the polymer and covalently bind the nanoparticles on the oxide surfaces. These particles can readily be used for catalysis and monitored by the characterization techniques used in surface science such as Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy (47, 50).

The 3D monodispersed platinum nanoparticles can be produced by encapsulating polymer-coated platinum or rhodium nanoparticles in mesoporous silica. The mesoporous silica is actually synthesized around the nanoparticle. In this circumstance, usually no more than one platinum nanoparticle is embedded in a mesoporous channel (Fig. 12). After oxidation and reduction treatments to remove the polymer that coats the metal nanoparticles, catalytic reactions can be carried out where the variables are the size and shape of the cluster. We have studied ethylene hydrogenation, ethane hydrogenolysis, cyclohexene hydrogenation, and other multipath reactions to monitor changes of reaction selectivity (R.M.R., H. Song, S. Habas, M. Grass, K. Nietz, J. Hoefelmeyer, P. Yang, and G.A.S., unpublished results).

Fig. 12.

Monodispersed platinum nanoparticles are encapsulated in mesoporous silica with a channel structure (SBA-15) to form a 3D model catalyst system. The particle size is varied while keeping the platinum loading at 1%.

Our model studies of heterogeneous catalysis that started with the use of metal single crystal surfaces are being continued using monodispersed metal nanoparticles.

Studies of Structural and Electronic Properties of Metal Nanoclusters

As nanoscience becomes more developed, many interesting physical-chemical size-dependent properties are being uncovered that are important for understanding surfaces and catalysts. The melting temperatures of metal clusters is size dependent (51). Pressure-induced structural transformations are more facile for smaller clusters and have lower activation energy (52). The nanocrystals are more perfect, because they cannot support dislocations because of their small size (51). The 2D phase diagram that is applicable to surface systems permits miscibility of metals to form solid solutions that are immiscible in 3 dimensions (53). There are several structural arrangements for a given size cluster, all of them thermodynamically equally stable, and there is possibility of facile rearrangement among these structures (54).

The electronic properties of nanoclusters are equally interesting. Because the electron mean free path in metals is in the 5–15 nm range, the smaller nanoclusters permit unscattered electron flow (55). The plasma frequency of electrons can shift with size, giving rise to color change in gold, for example (56). The rough edges of clusters exhibit large electric fields leading to resonance enhancement observed in Raman spectroscopy (57). The band gap of insulator and semiconductor nanoparticles increases with decreasing cluster size (58). These and other properties yet to be discovered impart unique opportunities for applications in surface technologies ranging from catalysis and microelectronics to information storage and sensors.

Let us close with the expectation with which we started this work. Heterogeneous, enzyme, and homogeneous catalysis share so many characteristics on the molecular level that we hope they become one field of science that permits deeper understanding of the molecular ingredients of structure and dynamics that makes them function to obtain high reaction selectivity. The learning across these broad fields that would follow would benefit our capability to generate and convert energy, to maintain and enhance environmental quality, and increase the quality and length of human life.

Abbreviations

LEED

low-energy electron diffraction

SFG

sum frequency generation

STM

scanning tunneling microscopy.