CO Adsorption and Oxidation on Pd(110)-c(2 × 4)−O by Reflection−Absorption Infrared Spectroscopy

Abstract

Adsorption and oxidation of CO on Pd(110)-c(2 × 4)−O were investigated by reflection−absorption infrared spectroscopy (RAIRS), low-energy electron diffraction (LEED), and temperature-programmed reaction (TPR). High-frequency CO at 2140 cm-1, which was greatly influenced by adsorbed oxygen atoms, was observed by RAIRS even at low CO coverage when the Pd(110)-c(2 × 4)−O surface was exposed to CO at 100 K. This peak developed along with a broad band around 2000 cm-1 from low CO coverage to the saturation. The reaction of CO and an oxygen atom to form CO2 on the c(2 × 4)−O surface was observed in the range 100−450 K. Below 250 K, two desorption peaks of CO2 were observed in TPR spectra and the corresponding RAIR spectra showed that linear CO species (2140−2070 cm-1) disappeared by desorption as CO or reaction with an oxygen atom to form CO2, while there was little decrease of bridge CO species (2020−1880 cm-1). It is likely that linear CO species mainly reacted to form CO2 on Pd(110)-c(2 × 4)−O below 250 K. We successfully followed the constant-flow reaction of CO and O2 on the Pd(110)-c(2 × 4)−O at 220 K; however, the reaction ceased with an increase of bridge CO at 1992 cm-1. RAIR spectra during the reaction of CO and O above 250 K suggest that CO began to form domains separated from oxygen atoms and the substrate structure of (1 × 2) was gradually converted into (1 × 1) according to the local coverage of CO.