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Strongly bound water and the anomalous pH effect of platinum group metal step-edges



Kathleen A. Schwarz, Bingjun Xu, Yushan Yan, Ravishankar Sundararaman


The unexpected relationship between pH and the electrocatalytic activity of nanoparticles and polycrystalline metals is a longstanding electrochemical puzzle. Here, we provide a solution to the puzzle: strongly adsorbed water molecules on the ridge sites of nanoparticles and step edges induce an electron density reduction that is responsible for the observed anomalous pH effects. We use density functional theory (DFT) coupled with charge and energy-based arguments to describe the unexpected behavior of water on the ridge sites of these catalysts, and to demonstrate that water is the adsorbate across most of the relevant voltage range, rather than other species such as hydroxyl groups. We then show that other adsorbates such as H+ and OH- can displace the strongly bound water at low and high voltages, respectively. We combine theory and experimental evidence to illustrate that the charge caused by this strongly bound water accounts for the anomalous pH effects seen for four supported platinum-group-metal catalysts (Pt/C, Ir/C, Pd/C and Rh/C) over a broad pH range (0-13), for the onset of CO-stripping and underpotential deposition of hydrogen. This strongly bound water significantly impacts the behavior of these catalysts throughout a wide voltage range.
Physical Chemistry Chemical Physics




Schwarz, K. , Xu, B. , Yan, Y. and Sundararaman, R. (2016), Strongly bound water and the anomalous pH effect of platinum group metal step-edges, Physical Chemistry Chemical Physics (Accessed May 29, 2023)
Created May 26, 2016, Updated January 8, 2018