In-situ FT-IRAS study of the CO oxidation reaction over Ru(001): I. Evidence for an Eley-Rideal mechanism at high pressures?

Abstract

Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction on a Ru(001) surface in-situ at high pressures. The vibrational spectra allow us to characterize the nature of the surface during reaction and qualitatively determine steady-state coverages of CO and oxygen. Under oxidizing conditions ( CO O 2 ratios < 1) where the reaction rate is highest, the surface is found to be covered with a monolayer ( ML) of oxygen [ O−(1 × 1)− Ru(001)]. Little, if any CO ( θ Co < 0.01) was found to adsorb at these surfaces under high-pressure steady-state conditions at 500 K, or at 85 K. in UHV. From this latter result, we estimate an upper limit of E ads(CO) < 10 kcal mol , and for high-pressure reaction at 500 K, CO coverages of θ Co < 10 −5 and residence times of τ co < 10 −11 s. Under reducing conditions ( CO O 2 ratios > 2) , the steady-state coverage of oxygen decreases with decreasing oxygen partial pressure concurrent with a large reduction in reaction rates. Vibrational data reveal a steady-state coverage of CO (θ Co ≃ 0.11, E ads(CO) ≃ 25 kcal mol ) adsorbed on an O−(2 × 1)− Ru(001) surface ( θ O = 0.5). Severely reducing conditions lead to low steady-state oxygen coverages < 1 2 ML and island formation of oxygen. Th implications of the various CO species and of the oxygen island formation are discussed in relation to the reaction mechanisms suggested by our previous kinetic study. In particular, we propose for reaction under oxidizing conditions an Eley-Rideal mechanism involving reaction between gas-phase or weakly adsorbed CO and the O−(1 × 1)− Ru(001) surface . Under reducing conditions on the O−(2 × 1) surface the reactions proceed via a Langmuir-Hinshelwood mechanism between chemisorbed CO and oxygen.