The adsorption, desorption, and reactions of CO and O 2 on Rh

Abstract

The adsorption and desorption of CO, O 2, and CO 2 on polycrystalline Rh have been investigated using flash desorption spectroscopy and adsorption transients. Carbon monoxide adsorbs nondissociatively on clean Rh and obeys first-order Langmuir-type adsorption kinetics with an initial sticking probability of 0.5 at 330 K. It desorbs following first-order kinetics ( E des = 134 kJ mole −1 if v = 10 13sec −1) at about 530 K. Oxygen adsorbs with an initial sticking probability of about 0.1. Two forms of surface oxygen are produced: one is atomically chemisorbed and quite reactive with CO; the other is perhaps a subsurface, interstitial species which requires an activation energy for reaction. These desorb as O 2 at around 1000 K. Carbon dioxide will not adsorb on Rh at 330 K. The carbon monoxide oxidation reaction on Rh has been studied using the transient kinetics of the reaction of CO gas with adsorbed oxygen and the steady-state kinetics of CO 2 production from gas phase CO and O 2. The reaction proceeds via a modified Eley-Rideal path at high temperatures, where chemisorbed CO is negligible. By this path, an impinging CO enters a mobile, transiently adsorbed precursor state with a certain probability which is a function of chemisorbed CO coverage and temperature. Once in this state the CO can visit about 15 sites before reentering the gas phase. If the oxygen coverage exceeds 0.18, CO then has almost unit probability of finding, and reacting with, an oxygen adatom at one of these sites. At lower temperature, the Langmuir-Hinshelwood path becomes important and adsorbed CO strongly inhibits oxygen adsorption.