Interactions among 18O2, C2H4, and NO on the surface of stepped Pt(332)

Abstract

The influence of co-adsorbed 18O2 (18O) on NO/C2H4 reactions on the surface of stepped Pt(332) has been investigated using Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of 18O2 (18O) results in changes in C2H4 dissociation behavior, with formation of ethylidyne taking place at surface temperature much higher than that in the absence of 18O2 (18O). Pre-annealing 18O2/C2H4 co-adlayers to 250 and 300 K does not lead to significantly different IR spectra, but a variety of spectra are observed when the 250 K and 300 K 18O/C2H4 co-adlayers are further exposed to 0.8 L NO at 90 K, depending on the 18O2 pre-exposure. NO adsorption in bridge sites, both on steps and on terraces is more significantly suppressed for the co-adlayers in which 18O2/C2H4 is pre-annealed to 250 K. This site-blocking effect is enhanced with increasing 18O2 exposure. However, no new surface species, which are intermediates for N2 production, are detected. Thermal desorption spectra indicate that various species are produced, but only N2 and H2 desorption have intensities that can be reliably analyzed (that is to be able to quantitatively elucidate how the yields of these two species vary with change in the ratios of NO to C2H4 and 18O2). Desorption of both N2 and H2 is more strongly dependent on 18O2 exposure than on the temperature to which 18O2/C2H4 adlayers are pre-annealed. The presence of 18O2, irrespective of the dosing sequence, suppresses N2 desorption, but this effect is much weaker when 18O2 is post-dosed. For the case with 18O2 pre-dosed, irrespective of the annealing temperature (250 K or 300 K), N2 desorption is greatly suppressed at an 18O2 exposure of 0.2 L, but thereafter remains almost unchanged with increasing 18O2 exposure from 0.4 to 1.6 L. This feature of N2 desorption is explained by the restoration of the adsorption of NO onto steps and the subsequent NO dissociation on these sites. In contrast, H2 desorption decreases continuously and disappears at 0.8 L 18O2 and higher. It is concluded that the presence of 18O2 in the reaction of NO with C2H4 on the surface of Pt(332) does not play any role of activating the surface reactants.Key words: NO, platinum, C2H4, deNOx, hydrocarbon, selective catalytic reduction.