Understanding the structure–activity relationships of different double atom catalysts from density functional calculations: three general rules for efficient CO oxidation

Abstract

CO oxidation is considered to be a model reaction to explore the relationships between structure and catalytic performance. In this work, several double-atom catalysts (DACs) M/UiO-66 (M = Pd, Pt, Cu, Ag, Au, Zn, or Cd, UiO-66 is a classical metal–organic framework) for CO oxidation have been investigated by density functional theory (DFT) calculations. DACs were obtained by doping metal atoms into the defect sites at the zirconium oxide clusters of UiO-66. All possible mechanisms (Eley–Rideal (ER), Langmuir–Hinshelwood (LH), and termolecular ER (TER) mechanisms) for CO oxidation were considered in detail. Compared with pristine UiO-66, M/UiO-66 remarkably promoted the CO oxidation performance. Among them, the superior catalysts were Zn and Ag-DACs, whose rate-limiting energy barriers (E_bar) are as low as 0.14 and 0.22 eV, respectively. More importantly, three general rules affecting CO oxidation E_bar were proposed: (i) from the aspect of geometric structure, the corresponding E_bar is negatively correlated with the OC–O distance of the transition structure within a certain range; (ii) from the aspect of electronic structure, when the d-band center difference ratio of DACs is in the range of −15% to 15%, the corresponding E_bar is less than 1.08 eV for most cases; (iii) if the E_ads of individual O₂ and CO molecules are similar, or the E_ads of an individual O₂ molecule is extremely large, the catalytic performance is expected to be satisfactory. Therefore, this work provides unique insights into the design of excellent DACs for CO oxidation, and the proposed rules can be extended to other complex reactions.