Hydrophilic versus hydrophobic coadsorption: Carbon monoxide and water on Rh(111) versus Pt(111)

Abstract

Carbon monoxide is a major poison to anodic reactions in aqueous fuel cells. To ascertain the effects of a controlled aqueous environment on the adsorption of CO on electrocatalytically active metals, the coadsorption of CO and water on Rh(111) and Pt(111) surfaces at 100 K was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). On Rh(111) low coverages of CO shift monolayer water desorption from 182 to 207 K, indicating net attractive COH 2O interactions. Water shifts the CO stretching frequency from 2020 to 1620 cm −1, suggesting a displacement of CO from atop to three-fold hollow sites. A site shift is corroborated by XPS data. Concurrent changes in water vibrational features suggest formation of a mixed phase in which CO and water occupy adjacent sites. It has been hypothesized that such an adsorption geometry would facilitate the normally slow electrooxidation of CO and of fuels such as methanol. On Pt(111), in contrast, all coverages of CO decrease the desorption temperature of water, indicating net repulsive COH 2O interactions. Water splits the single 2110 cm −1 CO stretch of low coverages of CO into two peaks corresponding to the atop and bridging species also seen for higher coverages of CO on the water-free surface. This result and the lack of a change in water vibrational features suggest that on Pt(111) the coadsorbates separate into incompressible islands containing only water and compressible, internally repulsive, patches containing only CO. If the disparate behavior of Rh(111) and Pt(111) also occurs in room temperature aqueous solutions, comparison of their electrooxidation activity in the presence of CO could provide a conclusive test of whether or not rational provision of adjacent CO and water binding sites is likely to be a productive strategy for the development of improved electrooxidation catalysts.