Group VIII transition metal alloy clusters are effective catalysts for the dissociation and formation of C-H bonds during methane reforming and methanation reactions, respectively. During these reactions, the chemical compositions, structure, and shape of these clusters vary with the oxygen and carbon chemical potentials at the their surfaces. Here, we probe the structural dynamics of a series of first-row transition metal or alloy clusters, relating them to the instantaneous oxygen and carbon chemical potentials. These chemical potentials and their relative values dictate the surface oxygen and carbon contents and in turn the reactivities during methane reforming and methanation reactions. On clusters with higher carbon affinities (e.g., Ni), the surfaces are partially covered with carbonaceous intermediates and the C-H bond dissociation and formation occur on metal site pairs that leads to H* formation and to H* as the H donor, respectively. In contrast, clusters with higher oxygen affinities retain reactive oxygen species, which assist with the C-H bond dissociation via an oxygen assisted H abstraction route and with the reverse C-H bond formation via a proton donation route, as required by the microscopic reversibility of the catalytic steps. In both of these reactions, the oxophilicity of the clusters dictates the surface densities and also the basicity of the reactive oxygen species and in turn the charge of the H, the identity of the kinetically relevant step, and the kinetic dependencies during the C-H bond activation and formation steps. These consequences of reactive oxygen adatoms resemble those previously found on combustion reactions on 2<sup>nd</sup> and 3<sup>rd</sup> row transition metals.

Dynamics of first row transition metal or alloy clusters and their catalytic consequences for C-H bond dissociation and formation steps

255th National Meeting (2018)

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