NO−C2H4 Reactions on the Surface of Stepped Pt(332)

Abstract

NO−C2H4 interactions on the surface of stepped Pt(332) have been studied using Fourier transform infrared reflection−absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). IR data show that pre-dosed C2H4 molecules suppress the adsorption of NO on the surface of Pt(332) to an extent depending on both C2H4 coverage and the temperatures to which C2H4 pre-adlayers are annealed. At 90 K, the adsorption of NO on step sites is significantly suppressed by C2H4 following exposures greater than 0.32 L. This site-blocking effect persists and is even enhanced when annealing C2H4 pre-adlayers to 200 K, a temperature at which the adsorbed C2H4 molecules are not dissociated. As annealing temperatures are increased beyond 260 K, an ethylidyne species forms and is located on terraces. Consequently, the adsorption of NO on step sites is restored but to an extent smaller than that on a clean Pt(332) surface. The IR spectra also indicate that there are no detectable intermediates resulting from direct chemical reactions between NO and C2H4/C2H4-derived hydrocarbons, which can promote N2 production. The co-adsorption of C2H4- and C2H4-derived hydrocarbons does significantly promote N2 desorption, being dependent on the temperatures to which pre-dosed C2H4 adlayers are annealed. Annealing C2H4 adlayers to temperatures ≤300 K significantly enhances N2 desorption at temperatures below 400 K, giving rise to a peak at about 340−380 K. This low-temperature N2 desorption disappears completely after annealing the C2H4 adlayers to >350 K. N2 desorption at ∼460 K appears to be slightly enhanced. NO dissociation is the rate-limiting step in the reduction of NO by C2H4- and C2H4-derived hydrocarbons. The contribution of C2H4- and C2H4-derived hydrocarbons to N2 desorption is mainly attributed to 1) weakening of N−O bonds through an electron-donation effect; and 2) providing a source of reductants, i.e., H, CHx, C2Hx, and even C, which react with the atomic O from NO dissociation, leaving the surface with more vacant sites for further NO dissociation. The generation of CHx and C2Hx therefore plays a central role in the NO reduction mechanism.