D. W. Goodman

The Nature of the Metal-Metal Bond in Bimetallic Surfaces

The formation of a surface metal-metal bond can produce large perturbations in the electronic, chemical, and catalytic properties of a metal. Recent studies indicate that charge transfer is an important component in surface metal-metal bonds that involve dissimilar elements. The larger the charge transfer, the stronger the cohesive energy of the bimetallic bond. On a surface, the formation of a heteronuclear metal-metal bond induces a flow of electron density toward the element with the larger fraction of empty states in its valence band. This behavior is completely contrary to that observed in bulk alloys, indicating that the nature of a heteronuclear metal-metal bond depends strongly on the structural geometry of the bimetallic system.

Kinetics of the hydrogenation of CO over a single crystal nickel catalyst

A specially designed ultrahigh vacuum system has been used to examine the effect of surface chemical composition on the kinetics of the catalytic methanation reaction over a single crystal Ni(100) catalyst. The surface is characterized using Auger Electron Spectroscopy (AES) in an ultrahigh vacuum chamber, and reaction kinetics are determined following an in vacuo transfer of the sample to a catalytic reactor contiguous to the AES chamber. The kinetics of CO hydrogenation on a clean Ni(100) surface at 450–800 K are compared with kinetic data reported for high-area supported nickel catalysts. Excellent agreement is observed between specific rates, activation energy, and product distribution measured for the supported catalysts and the single crystal Ni(100). The dependence of the specific rate on total pressure (1–1500 Torr) and on H2 and CO partial pressures as well as the product distribution are also reported. These data are consistent with a mechanism in which an active surface carbon species is the dominant route to product.

Comparative kinetic studies of COO2 and CONO reactions over single crystal and supported rhodium catalysts

The kinetics of the COO2 and CONO reactions over single crystal Rh(111) and over alumina-supported Rh catalysts have been compared at realistic reactant pressures. For the COO2 reaction, there is excellent agreement between both the specific rates and activation energies measured for the two types of Rh catalysts. The CONO reaction, on the other hand, exhibits substantially different activation energies and specific reaction rates between the single crystal and supported catalysts. This indicates that the kinetics of the CONO reaction, unlike the COO2 reaction kinetics, are sensitive to changes in catalyst surface characteristics. The kinetic data for the COO2 and CONO reactions over Rh(111) and RhAl2O3 were analyzed using mathematical models which account for the individual elementary reaction steps established from surface chemistry studies of the interactions of CO, NO, and O2 with Rh surfaces. The model, when used with the parameter values similar to those reported in the surface chemistry literature, can quantitatively fit the CO oxidation rate data over both the single crystal and supported Rh catalysts. The kinetics of the CO NO reaction over Rh(111) can also be well described by a reaction model using parameter values taken from surface chemistry studies. However, the rate data for the CONO reaction over supported Rh can be rationalized by assuming that the dissociation of molecularly adsorbed NO occurs much more slowly on supported Rh than on Rh(111).

Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications

Catalytic decomposition of ammonia has been investigated as a method to produce hydrogen for fuel cell applications. The absence of any undesirable by-products (unlike, e.g., COx, formed during reforming of hydrocarbons and alcohols) makes this process an ideal source of hydrogen for fuel cells. In this study a variety of supported metal catalysts have been studied. Supported Ru catalysts were found to be the most active, whereas supported Ni catalysts were the least active. The supports were found to play a profound role in the ammonia decomposition process. The activation energies for the ammonia decomposition process varied from 17 to 22 kcal/mol depending upon the catalyst employed. The activation energies of the supported Ir catalysts were found to be in excellent agreement with our recent studies addressing ammonia decomposition on single crystal Ir.

CO-free fuel processing for fuel cell applications

In view of the stringent CO intolerance of the state-of-the-art proton exchange membrane (PEM) fuel cells, it is desirable to explore CO-free fuel processing alternatives. In recent years, step-wise reforming of hydrocarbons has been proposed for production of CO-free hydrogen for fuel cell applications. The decomposition of hydrocarbons (first step of the step-wise reforming process) has been extensively investigated. Both steam and air have been employed for catalyst regeneration in the second step of the process. Since, PEM is poisoned by very low (ppm) levels of CO, it is essential to eliminate even trace amounts of CO from the reformate stream. Preferential oxidation of CO (PROX) is considered to be a promising method for trace CO clean up. Related studies along with a discussion of catalytic ammonia decomposition (for applications in alkaline fuel cells) will be included in this review.

Oxidation Catalysis by Supported Gold Nano-Clusters

The historical notion regarding the inability of gold to catalyze reactions has been discarded in view of recent studies, which have clearly demonstrated the high catalytic efficiency of supported nano-gold catalysts. Although nano-Au catalysts are known to catalyze a variety of reactions, the major focus has been on CO oxidation catalysis. In this work we focus on the important aspects related to the CO oxidation reaction. Special emphasis is placed on the studies undertaken on model nano-Au systems as these studies have considerably enhanced the understanding of the oxidation process. Gold in a highly dispersed state can selectively oxidize CO in the presence of excess hydrogen (of tremendous interest to state-of-the-art low-temperature fuel cells); related studies are addressed in this review. The nano-gold catalysts have also been investigated for the direct vapor-phase oxidation of propylene to propylene oxide in the presence of molecular oxygen; these investigations are highlighted in this work.

Structure sensitivity of CO oxidation over model Au/TiO22 catalysts

Model catalysts of Au clusters supported on TiO2 thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO2 catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)-1 × s-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO2(001)/Mo(100) is structure sensitive.

Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

Abstract A brief review of our laboratorys recent scanning tunneling microscopy (STM) studies on nanoclusters supported on TiO2(110) is presented. Particular emphasis is placed on the system Au TiO 2 (110) . The nucleation and growth of the clusters, which were vapor-deposited on TiO2(110) under ultra high vacuum (UHV) conditions, were investigated using STM. It was found that Au, Pd, and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashion on TiO2(110), but that at low coverages, quasi-two dimensional (quasi-2D) Au and Pd clusters were observed. These quasi-2D clusters are characterized by heights of 1–2 atomic layers. Annealing studies show that Au and Pd clusters form large microcrystals with well-defined hexagonal shapes. Al clusters, which have a strong interaction with the substrate, are oxidized upon deposition, “wetting” the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clusters have been studied using scanning tunneling spectroscopy (STS) to measure the cluster band gaps. The electronic structure was found to be cluster size-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties correlates with the transition from quasi-2D to 3D cluster growth.

Monolayer and multilayer growth of Cu on the Ru(0001) surface

Abstract The adsorption and growth of Cu films on the Ru(0001) surface were studied by work function measurements, low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal programmed desorption (TPD). The results indicate that for submonolayer depositions at 100 K the Cu grows in a dispersed mode forming 2D islands pseudomorphic to the Ru(0001) substrate upon annealing to 300 K. This behavior is seen to continue to the 1 monolayer (ML) level. Additional Cu deposition to 2 ML shows a similar 2D island growth but with an epitaxial Cu(111) structure. Subsequent annealing in both these cases to 900 K enhances the 2D character of the films but does not affect the overall structure. AES and LEED results show that a 900 K anneal of Cu films in excess of 2 ML leads to three-dimensional Cu(111) island formation exposing areas of the surface covered by the original Cu bilayer — one pseudomorphic and one epitaxial. The effects of Cu on the chemisorptive properties of Ru(0001) toward CO were also studied by TPD. It was found that Cu attenuates the CO adsorption relative to the open Ru(0001) sites on approximately a one-to-one basis. In addition, at the 1 ML level the TPD spectrum shows features which are intermediate between those for the tightly bound CO/Ru system and the weakly bound CO/Cu case. A feature in the TPD spectra of CO on submonolayer Cu deposits is identified with mixed Cu/Ru sites, i.e. at the 2D Cu island edges, and allows an estimate of the 2D Cu island sizes to be made. The results and conclusions of this study differ markedly from previous single-crystal studies but are consistent with recent observations of Cu adsorbed onto an epitaxial Ru(0001) film grown on a Mo(110) surface.

A surface science investigation of the role of potassium promoters in nickel catalysts for CO hydrogenation

Abstract The role of potassium promoters in model Ni(100) catalysts for CO hydrogenation has been studied. High-pressure kinetic measurements of the H 2 + CO reaction on Ni(100) containing well-controlled submonolayer quantities of potassium adatoms have been combined with detailed surface analysis performed before and after reaction. Potassium addition decreases the steady-state rate of methane formation and increases that for higher hydrocarbons relative to clean Ni(100). These same results are reported for supported, high-surface-area Ni catalysts, indicating that metal/support interactions are not necessary in achieving the promoter effect. The activation energy for methanation ( ∼ 25 kcal mole −1 ) does not depend upon potassium coverage, suggesting that K changes neither the reaction mechanism nor the rate-limiting step. Surface carbide, a vital reaction intermediate, increases sharply in coverage upon the addition of 0.10 monolayer potassium. This is shown to result from a marked decrease in the activation energy for CO dissociation effected by potassium. The catalyst activity and selectivity are discussed in light of these results.