Kinetics of carbon monoxide oxidation on single-crystal palladium, platinum, and iridium

Abstract

The activity of Pt(100), Pd(110), Ir(111), and Ir(110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O/sub 2/ and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O/sub 2/ partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed bymore » deliberate oxidation and could be detected as CO/sub 2/ desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable at 475 K on Pd(110) in an 11:1 O/sub 2/:CO mixture and to 500 K on Ir(111) in an 80:1 O/sub 2/CO:CO mixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker.« less