Infrared reflection absorption spectroscopy of carbon monoxide adsorbed on Pd(S)-[n(1 1 1) × (1 1 1)] and Pd(S)-[n(1 0 0) × (1 1 1)] electrodes

The binding positions (terraces, steps) and sites (on-top, bridge-bonded, threefold) of adsorbed CO have been studied on Pd(S)-[n(1 1 1) × (1 1 1)] (n = 2, 3, 5, 9, 20, ∞) and Pd(S)-[n(1 0 0) × (1 1 1)] (n = 2, 5, 9, ∞) electrodes in 0.1 M HClO4 saturated with CO using infrared reflection absorption spectroscopy (IRAS). The band of threefold CO is observed around 1950 cm−1 on Pd(S)-[n(1 1 1) × (1 1 1)] electrodes with terrace atomic rows n ⩾ 5. The band intensity is enhanced with the increase of the terrace width; threefold CO is adsorbed on the terrace of Pd(S)-[n(1 1 1) × (1 1 1)] series. Threefold CO is not found on Pd(S)-[n(1 0 0) × (1 1 1)] series. On-top CO is observed around 2070 cm−1 on both Pd(S)-[n(1 1 1) × (1 1 1)] and Pd(S)-[n(1 0 0) × (1 1 1)] series. The band intensity is enlarged with the increase of the step atom density, supporting that on-top CO around 2070 cm−1 is adsorbed at steps. A high frequency band of on-top CO adsorbed on the terrace is found around 2100 cm−1 on Pd(S)-[n(1 1 1) × (1 1 1)] surfaces with n ⩾ 20. Bridge-bonded CO gives a predominant IRAS band around 1980 cm−1 on all the surfaces. The band intensity of bridge-bonded CO does not correlate with the step atom density; bridge-bonded CO is adsorbed on both terrace and step sites. The on-top CO at steps has higher Stark tuning rate (dν/dE) than the one at terraces on Pd(S)-[n(1 1 1) × (1 1 1)] electrodes.