The smallest subnanocluster models of platinum colloid (Ptn) are supposed to diffuse in aqueous media in order to examine their behaviors when they are subjected to electrocatalytic hydrogen evolution reaction under the zero overpotential condition at pH 0. The DFT approach allows us to clarify the natures of individual proton transfer (PT) and electron transfer (ET) processes together with the importance of relying on concerted proton-electron transfer (CPET) pathways to promote the majority of H* adsorption processes by the Ptn subnanoclusters. Although the CPET processes are closely correlated with the Volmer steps (Pt + H+ + e− --> Pt-H*) described so far in electrochemistry, our study for the first time points out the essential capability of the Ptn clusters to promote the multiple PT steps without the need to transfer any electrons, revealing the fundamentally high basicity of the naked Ptn clusters (pKa = 27-28 for Pt4, Pt5, and Pt6). The discrete cluster models adopted herein avoid the structural constraints forced by the standard slab models and enable us to discuss the drastic alterations in the geometric and electronic structures of the intermediates given by the consecutive promotion of multiple CPET steps. The weakening in the Pt-H* bond strength with the increasing number of CPET steps is well rationalized by carefully examining the changes in the ν(Pt-H*) vibrational frequencies, the hydricity, and the H2 desorption energy. The behaviors are also correlated with the underpotential and overpotential deposited hydrogen atoms (HUPD and HOPD) discussed in the electrochemical studies for many years.
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