From supersonic beams and single crystal microcalorimetry to the control of catalytic reactions

Abstract

Abstract Modern surface science techniques provide a means for a detailed unravelling of the mechanisms of catalytic reactions and the surface processes which accompany them. The focus of the present review is on one surface, Pt{100}, and several important catalytic reactions: CO oxidation, a critical reaction in car exhaust catalysis, NO reduction by H 2 , and NH 3 oxidation, the industrial process used to manufacture NO and hence nitric acid. The {100} surface of Pt is of particular interest, because it can be prepared in two forms, a metastable (1×1) bulk termination structure and the stable hexagonal top layer structure, denoted hex. Using a novel single crystal adsorption calorimeter, we have determined the energy differences between the clean (1×1) and hex surfaces, and also between the two surfaces with adsorbates which lift the hex reconstruction. Molecular beam studies with CO and D 2 revealed the mechanism for this adsorbate restructuring process: it proved to be strongly non-linear. This non-linearity was subsequently shown to be critical in the widespread observation of regimes of sustained oscillations in many catalytic processes, such as CO + O 2 , CO + NO, and NO + H 2 , observed on Pt{100}. Detailed modelling using only experimentally determined kinetic parameters gives remarkably good agreement with experimental measurements of oscillatory existence regimes and periods. Finally, a combination of calorimetric and molecular beam techniques have produced a new mechanism for the industrially important ammonia oxidation reactions (the Ostwald process) over Pt catalysts. This includes a tested recipe for a very substantial improvement in the operating conditions, with high selectivity to the desired product (NO) at low temperatures, and at high rates.