HREELS and TDS Studies of NO+H₂ and NH₃+O₂ Reactions on Pt(111)

Abstract

The reactions of NO+H₂ and NH₃+O₂ on Pt(111) surface were studied at low pressures in the temperature range of 300–350 K. The data obtained indicate that both reactions proceed through the same surface intermediates, namely N _ads and HNO _ads . HREELS spectra give evidence that the HNO intermediates are adsorbed in the side-on state. These species react with hydrogen at 350 К to form N _ads . The latter may be easily removed from the surface by exposure in hydrogen at 400 K. The catalytic reactions of ammonia oxidation and nitric oxide hydrogenation on platinum group metals are of practical importance. Study of their mechanisms is also of interest from the viewpoint of fundamental metal. The reaction pathway and selectivity of these reactions depend on the properties of surface species formed during the catalytic reaction. The known kinetic schemes include such intermediates as NH _ads , HNO _ads etc., but experimental evidence is scarce. This paper reports further on our studies of the adsorption and catalytic properties of Pt(lll) surface by means of High Resolution Electron Energy Loss Spectroscopy (HREELS) and Thermal Desorption Spectroscopy (TDS) [1, 2]. The aim of the present work is to identify and to characterise the intermediates of NO+H₂ and NH₃+O₂ reactions on Pt(lll). The experiments were performed in a VG ADES-400 Spectrometer with a residual pressure of 2·10^-9 Pa. A monochromatic electron beam of 2.3 eV energy, the primary current of 5·10^-11 A and an incidence angle of 45° with respect to the surface normal were used. Energy resolution of the elastically reflected beam was ca. 80 cm^-1 at an intensity of 105 cps. Spectra were recorded in the specular direction. The TD spectra were registered by means of QMS VG QXK-400, a heating rate of lOK/s (DC) was used. Characteristics of the Pt(111) crystal and the procedure* of surface cleaning were described elsewhere [1,2].