Infrared spectroscopy of carbon monoxide at the ordered palladium (110)-aqueous interface: evidence for adsorbate-induced surface reconstruction

Abstract

In situ infrared reflection-absorption spectra in the CO stretching ( ν CO) region for carbon monoxide adsorbed on an ordered Pd(110) electrode in 0.1 M HClO 4 as a function of coverage ( θ CO) and electrode potential ( E) are compared with corresponding θ CO-dependent spectra reported in previous studies for the same surface in ultrahigh vacuum (UHV). The latter system has been shown from electron diffraction to involve an unusual case of CO-induced surface reconstruction which is reflected also in marked changes in the θ CO-dependent ν CO spectra. Detailed comparison of the infrared data obtained in the electrochemical and UHV environments therefore enables the possible occurrence of Pd(110) reconstruction in the former system to be assessed. Indeed, rich θ CO-dependent ν CO spectra were obtained which provide strong evidence that similar surface structural changes also occur at the Pd(110)-aqueous interface. Specifically, dosing CO from dilute (ca. 0.01 mM) solution yields a single ν CO band (at 1810–1840 cm −1) at low coverages. These spectra transform for θ CO values above ca. 0.4 into a complex multiple-band pattern which contains an additional band near 1930–1940 cm −1 and weaker higher-frequency ν CO features along with a peak at ca. 1850–1880 cm −1. Significantly, this spectral pattern, obtained for coverages between ca. 0.4 and 0.75, is similar to that observed in the UHV system over a comparable θ CO range, the latter being diagnostic of the CO-induced formation of a (1 × 2) “missing-row” reconstruction. Increasing the coverage further, from 0.75 up to saturation, θ CO = 1.0, triggers another spectral transition to a single dominant ν CO band at 1960–1965 cm −1, consistent with the formation of a (2 × 1) atop CO adlayer on a (1 × 1) (i.e. unreconstructed) substrate. Further evidence that the freshly prepared surface is unreconstructed prior to CO dosing, as well as in the presence of a saturated adlayer, was obtained from ν CO spectra following CO removal and redosing, and also from energetic considerations along with voltammetric data. Intermediate-coverage CO adlayers formed by solution dosing followed by partial electrooxidative removal exhibit much smaller θ CO-dependent spectral changes, indicative of adsorbate island formation. However, the ν CO spectra for such adlayers transformed within 10–20 min into θ CO-dependent patterns comparable to those obtained upon initial solution CO dosing. This “island dissipation” may be driven by the resultant formation of reconstructed nanoscale domains.