Spatial distributions of desorbing products in steady-state NO and N2O reductions on Pd(110)

Abstract

The angular and velocity distributions of desorbing product N 2 were examined over the crystal azimuth in steady-state NO + CO and N 2 O + CO reactions on Pd͑110͒ by cross-correlation time-of-flight techniques. At surface temperatures below 600 K, N 2 desorption in both reactions splits into two directional lobes collimated along 41°-45°from the surface normal toward the ͓001͔ and ͓001͔ directions. Above 600 K, the normally directed N 2 desorption is enhanced in the NO reduction. Each product desorption component, as well as CO 2 , shows a fairly asymmetric distribution about its collimation axis. Two factors, i.e., the anisotropic site structures and the reactant orientation and movements, are operative to induce such asymmetry, depending on the product emission mechanism.

An Experimental Approach to Transition States of Surface Reactions; Energy Partitioning in Repulsive Desorption

“…Spatial distributions very similar to the results in Fig. 1(b) are found for desorbing N 2 in a steady state NO+CO or NO+D 2 reaction on Pd(110) below about 550 K [9]. At higher temperatures, the combinative desorption of N(a) into N 2 (g) becomes predominant, yielding less anisotropy.…”

“…In these processes, the product desorption flux and its translational energy show remarkable anisotropy (Fig. 1) [8,9]. In the CO oxidation step, the product CO 2 is emitted immediately after the strong bond of Pd-O(a) in TS is broken, and the resultant spatial distribution of desorbed CO 2 is affected by the slope of the O(a) adsorption site and the symmetry around it [10].…”

“…1: Three-dimensional spatial distributions on Pd(110) of (a) desorbing product CO2 in CO oxidation under TPD conditions [8] and of (b) N2 in a steady-state N2O+CO reaction at 520 K [9].…”

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

“…The angular distribution of the inclined desorption is broader than that in the N 2 O reduction on Pd͑110͒ showing a cos n ͑ −45͒ form with n = 14-28. 28 The CO 2 desorption follows a simple cosine distribution ͓Fig. 2͑d͔͒, consistent with the results in the CO oxidation under reducing conditions, i.e., in the region inhibited by CO. …”

“…shows a cos 10 ͑ ±24͒ form and a translational temperature of approximately 1500 K, 29 and the product N 2 in the N 2 O reduction on Pd͑110͒ yields a cos [18][19][20][21][22][23][24][25][26][27][28] ͑ ±45͒ form with a translational temperature of approximately 3000 K. 30 Roughly speaking, the sharper the angular distribution is, the higher the translational temperature is expected to be. 9,12 The broad N 2 distribution with a high translational temperature on clean Rh͑110͒ suggests the presence of two or more components with sharper distributions.…”