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. MOFs can achieve atom-precision in the design of single-atom catalysts (SACs). We elucidated the PROX-reaction mechanism on a well-defined Cu-SAC supported by UiO-66 framework using a multitude of experimental methods and detailed DFT computations.