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

Abstract

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 ZrO_x nodes with the increase in reaction temperature, which correlated with a decrease in PROX selectivity. Under similar conditions, DRIFTS measurements revealed a decay of CO_ad adsorption on Cu (i.e., CO_ad@Cu¹⁺ species) and a corresponding red-shift under PROX conditions compared to CO oxidation, suggesting reduction-mediated charge transfer at the Cu–ZrO_x 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.