Characterization of active sites on Rh/SiO(2) model catalysts

Rh/SiO(2) model catalyst surfaces are prepared under ultra-high vacuum conditions and examined in situ using scanning tunneling microscope and CO infrared reflection absorption techniques, to quantify the number and kinds of active Rh surface sites available for kinetic reaction (CO oxidation) as a function of Rh particle size. The results are compared against CO desorption measurements and elevated pressure CO oxidation reaction kinetics, to evaluate the extent of the correlation between the low and elevated pressure site characterization techniques. Data demonstrate that estimates of Rh active sites exhibit good agreement between the characterization methods and illustrate the utility of low pressure surface science characterization techniques in understanding elevated pressure reaction kinetics on model catalyst surfaces.

Ultrathin, ordered oxide films on metal surfaces

Metal oxides and oxide thin films are extensively used as active catalysts and catalytic supports, as well as in many other important technical applications. Unlike TiO(2), which is a semiconductor and can be investigated using a variety of surface science techniques, most metal oxides are insulators, which seriously restricts their use as model surfaces with modern surface science techniques. This difficulty can be circumvented by synthesizing ultrathin oxide films a few nanometers in thickness with well-defined structures, that mimic the corresponding bulk oxides yet are thin enough to be sufficiently conducting. In this review, preparations, structures, electronic and chemical properties of four representative oxides, alumina, magnesium oxide, silica, and titania, are addressed. Of these MgO is found to grow in a layer-by-layer fashion, allowing preparation of crystalline thin film structures with varying thicknesses. Crystalline TiO(2) and Ti(2)O(3) can also be synthesized, whereas SiO(2) and Al(2)O(3), although amenable to synthesis as well-defined monolayer structures, have only been grown to date as amorphous multilayers.

NO Adsorption and Dissociation on Rh(111): PM-IRAS Study

We have used in situ polarization-modulation infrared reflection absorption spectroscopy to study the adsorption/dissociation of NO on Rh(111). While these studies have not been conclusive regarding the detailed surface structures formed during adsorption, they have provided important new information on the dissociation of NO on Rh(111). At moderate pressures (< or =10(-6) Torr) and temperatures (<275 K), a transition from 3-fold hollow to atop bonding is apparent. Data indicate that this transition is not due to the migration of the 3-fold hollow NO but rather to the adsorption of gas-phase NO that is directed toward the atop position due to the presence of NO decomposition products, particularly chemisorbed atomic O species at the hollow sites. These results indicate that NO dissociation occurs at temperatures well below the temperature previously reported. Additionally, high pressure (1 Torr) NO exposure at 300 K results in only atop NO, calling into question the surface structures previously proposed at these adsorption conditions consisting of atop and 3-fold hollow sites.

Preparation and Characterization of Silica Supported Au−Pd Model Catalysts

Au-Pd bimetallic model catalysts were synthesized as alloy clusters on SiO2 ultrathin films under ultrahigh vacuum (UHV) conditions. The surface composition and morphology were characterized with low energy ion scattering spectroscopy (LEIS), infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). Relative to the bulk, the surface of the clusters is enriched in Au. With CO as a probe, IRAS and TPD were used to identify isolated Pd sites at the surface of the supported Au-Pd clusters. Ethylene adsorption and dehydrogenation show a clear structure-reactivity correlation with respect to the structure/composition of these Au-Pd model catalysts.

The Composition and Structure of Pd−Au Surfaces

Pd, Au, and Pd-Au mixtures were deposited via physical vapor deposition onto a Mo(110) substrate, and the surface concentration and morphology of the Pd-Au mixtures were determined by low-energy ion scattering spectroscopy (LEISS), infrared absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Pd-Au mixtures form a stable alloy between 700 and 1000 K with substantial enrichment in Au compared to the bulk composition. Annealing a 1:1 Pd-Au mixture at 800 K leads to the formation of a surface alloy with a composition Au(0.8)Pd(0.2) where Pd is predominantly surrounded by Au. The surface concentration of this isolated Pd site can be systematically controlled by altering the bulk Pd-Au alloy concentration.

The Growth of Silver on an Ordered Alumina Surface

The growth of Ag on an ordered Al2O3 surface was studied by low energy ion scattering spectroscopy (LEIS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Three-dimensional (3D) growth of Ag clusters was observed with STM and LEIS, with the cluster size increasing with Ag coverage. The XPS core level binding energies and the Auger parameters indicate a weak interaction between the Ag clusters and the Al2O3 support. Final state effects are determined to be the primary contribution to the Ag core level binding energy shift. Nonzero order kinetics was observed for Ag desorption in TPD with the Ag sublimation energy decreasing with decreasing cluster size.

CO−NO and CO−O2 Interactions on Cu(100) between 25 and 200 K Studied with Infrared Reflection Absorption Spectroscopy

Infrared reflection absorption spectroscopy (IRAS) has been used to study CO-NO and CO-O(2) interactions on Cu(100) between 25 and 200 K. A strong repulsive interaction between CO and NO on Cu(100) at 25 K causes tilting of the CO molecules away from the surface normal and a blue-shift of the CO vibrational frequency. Upon warming and decomposition of the NO, the CO molecules return to a bonding position normal to the surface plane. The vibrational frequency of CO blue-shifts from 2087 to 2136 cm(-1) upon coadsorption with N and O atoms formed from NO decomposition. On the other hand, the interaction of CO with O(2) at 27 K on Cu(100) in the submonolayer regime induces a red-shift of the CO vibrational frequency. Atomic oxygen, formed on Cu(100) by dissociation of O(2) at 95 K, induces a blue-shift of the vibrational frequency of coadsorbed CO to 2116 cm(-1). The CO vibrational frequency shifts to 2091 cm(-1) when the surface is annealed to 140 K, implying a change in the adsorption geometry of the oxygen atoms on Cu(100).

The structure of catalytically active gold on titania

The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We have created well-ordered gold mono-layers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer.

In situ scanning tunneling microscopy of oxide-supported metal clusters: nucleation, growth, and thermal evolution of individual particles

An experimental approach was developed for imaging the nucleation and growth of individual oxide-supported nanoparticles and their subsequent in situ chemical and thermal treatments by scanning tunneling microscopy (STM). The potential of the method is demonstrated for Au nanoparticles supported on a reduced TiO(2) substrate where a cluster-by-cluster comparison is made of the morphological evolution and stability of nanoparticles during their nucleation and thermal annealing. Using this methodology the details of the nucleation and growth kinetics can be directly observed.

Catalysis research of relevance to carbon management: progress, challenges, and opportunities

The goal of the "Opportunities for Catalysis Research in Carbon Management" workshop was to review within the context of greenhouse gas/carbon issues the current state of knowledge, barriers to further scientific and technological progress, and basic scientific research needs in the areas of H2 generation and utilization, light hydrocarbon activation and utilization, carbon dioxide activation, utilization, and sequestration, emerging techniques and research directions in relevant catalysis research, and in catalysis for more efficient transportation engines. Several overarching themes emerge from this review. First and foremost, there is a pressing need to better understand in detail the catalytic mechanisms involved in almost every process area mentioned above. This includes the structures, energetics, lifetimes, and reactivities of the species thought to be important in the key catalytic cycles. As much of this type of information as is possible to acquire would also greatly aid in better understanding perplexing, incomplete/inefficient catalytic cycles and in inventing new, efficient ones. The most productive way to attack such problems must include long-term, in-depth fundamental studies of both commercial and model processes, by conventional research techniques and, importantly, by applying various promising new physicochemical and computational approaches which would allow incisive, in situ elucidation of reaction pathways. There is also a consensus that more exploratory experiments, especially high-risk, unconventional catalytic and model studies, should be undertaken. Such an effort will likely require specialized equipment, instrumentation, and computational facilities. The most expeditious and cost-effective means to carry out this research would be by close coupling of academic, industrial, and national laboratory catalysis efforts worldwide. Completely new research approaches should be vigorously explored, ranging from novel compositions, fabrication techniques, reactors, and reaction conditions for heterogeneous catalysts, to novel ligands and ligation geometries (e.g., biomimetic), reaction media, and activation methods for homogeneous ones. The interplay between these two areas involving various hybrid and single-site supported catalyst systems should also be productive. Finally, new combinatorial and semicombinatorial means to rapidly create and screen catalyst systems are now available. As a complement to the approaches noted above, these techniques promise to greatly accelerate catalyst discovery, evaluation, and understanding. They should be incorporated in the vigorous international research effort needed in this field.