Direct Observation of Electron Donation onto the Reactants and a Transient Poisoning Mechanism During CO Electroreduction on Ni Single Atom Catalysts.

Single atom catalysts (SACs) are an important class of materials that mediate chemical reduction reactions, a key subset of which is Ni within a carbon support for the electrochemical CO reduction reaction (CORR). However, how the metal atom/clusters and the carbon-based support act in concert to catalyze CORR is not well understood, with most reports attributing activity solely to the Ni-N/C moieties. To address this gap, we have undertaken a mechanistic investigation, employing in situ X-ray absorption spectroscopy (XAS) coupled with electrochemical studies and density functional theory (DFT) calculations to further understand how Ni single atoms work in conjunction with the nitrogen-doped carbon matrix to promote CORR to CO, and how the presence of impurities such as those present in CO-containing waste flue gases (including NO, and CN) changes the catalyst upon reduction. In contrast to previous works, we do not find strong evidence for a purely metal-based reduction upon application of negative reductive potentials. Instead, we present evidence for an increase in the equatorial vs. axial splitting of Ni, consistent with electrons moving onto the reactants via the Ni single atom 3d orbital. In addition, we demonstrate a transient poisoning mechanism of the Ni SAC by nitrite and thiocyanate, explaining the recovery of activity during CORR. These insights can aid the design of practical CO valorization technologies.