Model catalysts for the metal nanoclusters supported on metal oxide have been studied from the view point of the unique catalytic reactivity of metal nanoclusters different from bulk materials. One of the most popular methods to prepare metal nanoclusters on oxide surfaces has been evaporation of metal. Techniques to control nano structures such as nanolithography and supporting nanoclusters synthesized in the gas or liquid phase have been also applied to surface chemistry especially during the past two decades. Several strategies to prepare model catalysts of metal nanoclusters supported on oxide, and two studies with Pt/TiO2(110) model catalysts provoking discussion about the mechanisms of strong metal-support interaction (SMSI) and photocatalytic reactions are outlined.
Gold is a chemically stable metal, however the gold nano-clusters (2−3 nm) supported on metal oxides, such as titanium dioxie, show high catalytic activity for CO oxidation at low temperatures. The origin of the catalytic activity is still in debate. So far, there are several hypotheses about the reactions on Au catalysts to explain the generation of the catalytic activity: 1. reaction at Au-TiO2 perimeter, 2. reaction on Au atoms of Au nano particles (low-coordination Au atoms, negative charging Au atoms, Au atoms having specific electronic structure). In this review, we summarize recent investigations relating the hypotheses on Au/TiO2 model catalysts. Also, we state several problems and issues for the future investigations.
Recent progress in angle-resolved measurements of desorbing surface reaction products is reviewed. The angular and velocity distributions of desorbing products deliver the most direct structural information of the reaction site when the products are repulsively desorbed. These distributions yield the symmetry and slope of the product formation site as well as the orientation of the intermediate species emitting the product. This method works well even when the overall reaction rate is controlled by the reactant adsorption and the interaction between adsorbed species is obscured in kinetic studies at the steady-state conditions. However, information about the reaction mechanism is requisite for its application because the method is linked directly to the reaction itself. Analysis of the product formation site and its switchover is exemplified in a steady-state CO + O2 reaction on platinum, and the product emission in the NO reduction on palladium is described.
“Adsorption precursor” has been regarded as an important concept to understand gas-surface interactions since 1930s. In recent years, the adsorption precursors have been directly observed using scanning tunneling microscopies and surface spectroscopies. Some of the adsorption precursors have been found to contribute to chemical reactions with surface adsorbates. In this review article, recent studies on adsorption precursors and their contribution to surface reactions are introduced.
Palladium absorbs hydrogen exothermically. Under the industrial conditions for hydrogenation reactions over palladium catalysts, palladium particles should be working as palladium hydride α'-PdHx (x > 0.6), which suggests the enrollment of absorbed hydrogen in the hydrogenation reaction. In this article, after the energetics, kinetics, and microscopic mechanism of the hydrogen absorption and release processes on palladium surface is reviewed, the recent research on the role of absorbed hydrogen on the alkene hydrogenation reaction on palladium is briefed.
A review will be given of noncontact atomic force microscopes (NC-AFM) used in the studies related to surface catalysis, demonstrating that the NC-AFM is a promising probe microscope for catalyst research. In the NC-AFM measurement, weak attractive forces between the tip end and the sample are detected and used to regulate the tip-surface distance. In principle, atomically resolved images can be obtained for every material irrespective of conductivity. This is important because many industrial catalysts employ insulating materials such as alumina and silica as supports. Thus the NC-AFM can bridge the “material gap” between industrial catalysts and model catalysts by using realistic model surfaces. Furthermore, atom-scale NC-AFM imaging has been successfully demonstrated in air and water as well as in vacuum, indicating the potential of NC-AFM also to conquer the “pressure gap”.
We review recent trends in the first-principles theoretical studies on catalysis. We first see the accuracy of first-principles calculations, especially, the accuracy of generalized gradient approximation (GGA). Then we see recent developments of ab initio thermodynamics and statistical mechanics to predict surface phase diagrams as functions of chemical potentials or pressures of gas phase molecules. We also review recent development of micro-kinetic modeling for complex catalytic reactions. Finally, we describe our recent studies on HCOOH decomposition on the TiO2(110) surface and simulation of electrode surfaces.