A comparative study of the behaviour of single-crystal surfaces and supported catalysts in NO reduction and CO oxidation over Pt–Rh alloys

Abstract

The use of Pt–Rh-based three-way catalysts for automotive exhaust-gas control stimulated us to study various Pt–Rh alloy surfaces: well defined single-crystal surfaces and SiO₂-supported catalysts. For the sake of comparison data were also obtained for pure Pt and Rh single-crystal surfaces and catalysts. The surface composition and chemical properties have been studied using AES, FEM, TDS and XPS. The surface composition was studied under various experimental conditions, both in vacuum and in the presence of the relevant gases, NO, O₂, CO and H₂. We discuss the effects of the surface structure and the surface composition on the chemical properties of the Pt–Rh alloys. The surface composition varies easily with changing experimental conditions (temperature and gas-phase composition). Clean Pt–Rh alloy surfaces are enriched with Pt after high-temperature annealing (T 1000 K). However, the surface composition of Pt-rich Pt–Rh alloys is almost bulk-like following low-temperature equilibration (T≈ 800 K). Adsorbates can easily induce segregation of Rh or Pt to the surface, and the chemical properties of the surfaces are changed accordingly. In particular, the NO dissociation, which is often mentioned as the first step in the NO reduction with CO or hydrogen, is extremely sensitive to both the structure and the composition of the surface. For the pure metals the influence of the surface structure is different for Pt and Rh. The NO reduction and CO oxidation reactions were studied at low pressures on the single-crystal surfaces and at atmospheric pressure on the supported catalysts. We show that the chemical properties of the supported catalysts can be understood on the basis of the single-crystal results.