Mechanism and selectivity of MOF-supported Cu single-atom catalysts for preferential CO oxidation

Zr-based UiO-66 metal–organic frameworks are ideal platforms for the design and development of heterogeneous single-atom catalysts (SACs) because of their thermal and chemical stability with the presence of structural defects, enabling the introduction of isolated metal atoms. Elucidating the structure–reactivity relationships and understanding reaction mechanisms for these catalysts are crucial for their industrial applications. We focus here on these aspects for a technically important reaction, preferential CO oxidation (PROX) on a UiO-66-supported Cu SAC by following temperature perturbations in catalytic performance in correlation with changes in the electronic and adsorption properties, which are validated by comprehensive DFT computations. In situ DR-UV-VIS, XANES and NAP-XPS measurements indicated an increase of Cu1+-like states and partial reduction of ZrOx nodes with the increase in reaction temperature, which correlated with a decrease in PROX selectivity. Under similar conditions, DRIFTS measurements revealed a decay of COad adsorption on Cu (i.e., COad@Cu1+ species) and a corresponding red-shift under PROX conditions compared to CO oxidation, suggesting reduction-mediated charge transfer at the Cu–ZrOx interface. In contrast to the CO oxidation cycle which commences by CO adsorption on Cu1+-like sites, DFT computations revealed that the H2 oxidation cycle starts with the reaction of H2 with a pre-adsorbed O2 molecule on Cu1+-like sites, resulting in the generation of a H2O molecule and Cu2+-like sites, which are subsequently reduced to Cu1+-like sites through a successive reaction with a second H2 (or CO) molecule. Adsorption configurations and energies of CO and H2O co-adsorption indicated a competitive adsorption phenomenon on Cu species, which depends on the oxidation state of the Cu ion with a preference for CO adsorption on Cu1+-like sites, while H2O exhibits a stronger affinity for Cu2+-like sites. These results are discussed in terms of the reaction mechanism and PROX selectivity in Cu SAC catalysts and present a model for understanding the catalytic phenomena on MOF-supported SACs.