Effect of pH and Alkali Cations on Hydroxide Adsorption on Ir, Pt, and Au (111)
Authors: McCrum Ian T. , Janik Michael J. 
Published: February 2020

Adsorbed hydroxide is an active intermediate or spectator which can block active sites in many electrocatalytic reactions, including CO oxidation [1, 2], methanol oxidation [3], hydrogen oxidation [4], and oxygen reduction [5, 6]. Therefore, understanding the thermodynamics of hydroxide adsorption are important for designing highly active catalysts for these reactions. Experimental measurements, namely cyclic voltammograms, have shown that pH can alter the onset potential and coverage of hydroxide adsorption on Pt(111) [7]. Recent work further shows that the identity of the alkali metal cation (Li, Na, K, Cs) present in an alkaline electrolyte may also impact the thermodynamics of hydroxide adsorption [8, 9]. We use density functional theory to examine the adsorption of hydroxide onto Ir(111), Pt(111), and Au(111) in the absence (acid) and presence (alkaline) of near-surface or adsorbed alkali metal cations. The predicted hydroxide adsorption potentials compare well with experiment. On Ir(111) and Pt(111), the presence of an alkali metal cation in a high pH electrolyte weakens low coverage hydroxide adsorption and strengthens high coverage adsorption. This acts to shift the onset of hydroxide adsorption to more positive potentials, increase the adsorbed hydroxide coverage, and sharpen the experimentally measured cyclic voltammetric peak in an alkaline electrolyte, relative to an acid electrolyte. We can further capture the major differences between the alkali metal cations, including the trend in hydroxide adsorption potential on Ir(111) (Li<Na<K<Cs) and lower peak potential of hydroxide adsorption in LiOH relative to the other alkali metal cations on Pt(111). On Au(111), the coverage dependence of hydroxide adsorption is significantly different than on Ir(111) and Pt(111), and the presence of a near-surface cation may have a slight promoting effect on low coverage hydroxide adsorption. These results are important for understanding the effects of pH and alkali cation on many electrocatalytic reactions which involve adsorbed hydroxide as either a spectator species which blocks active sites or an active intermediate.
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