Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic Acid: Structural Features and Catalytic CO Oxidation Properties

Atomically dispersed Rh catalysts supported on phosphotungstic acid (PTA) with Rh loading up to 0.9 wt % were prepared through a self-assembly method. Rh stays exclusively as single atom species coordinated to six oxygen atoms, plausibly located at the 4-fold hollow site on one phosphotungstic acid (PTA) molecule together with a chemically adsorbed O-2 molecule. The catalyst is active in CO oxidation affording turnover frequencies between 0.2 and 1.7 s(-1) from 165 to 195 degrees C, with an apparent activation energy of 127 kJ/mol. The catalyst is highly stable, well maintaining the Keggin structure of PTA as well as the single-atom identity of Rh after three catalytic cycles (50 to 400 degrees C). CO activation is mainly achieved on Rh via the formation of dicarbonyl species, while oxygen activation and transfer mainly occur through PTA. The proposed catalytic cycle consists of an alternation of Rh(CO)(2)(3+) species and Rh(CO)(2)(1+) species on PTA, during which CO transforms into CO2 with oxygen activation being rate-determining.