CO oxidation on the phosphotungstic acid supported Rh single-atom catalysts via Rh-assisted Mans-van Krevelen mechanism

Recently, there are contradictory picture of the surface processes for polyoxometalates (POMs) supported single-atom catalysts (SACs) in CO oxidation, both Langmuir-Hinshelwood (LH) and Mars-van Krevelen (MvK) mechanisms have been proposed based on experimental and theoretical studies. We here present results of density functional theoretical (DFT) calculations on the neutral phosphotungstic acid (PTA) supported Rh SAC (Rh-1/PTA), aiming at the identification of an Rh-assisted MvK mechanism. The main finding of our DFT calculations are: (1) two CO molecules adsorb simultaneously at the Rh site with considerable adsorption energy, which inhibits the adsorptions of O-2; (2) anchoring of single Rh atom on the PTA surface weakens W-O-b/O-c bonds and leads to the charge rearrangement of the surface oxygen species; (3) the anchored Rh atom serves as an electron reservoir along the favorable pathway, as supported by the Rh atom acts as electron acceptor in formation of oxygen vacancy and electron donor in the replenishing step; (4) the difference between O-b and O-c atoms of PTA surface is remarkable, the calculated barrier for vacancy formation by reaction with adsorbed CO and O-b atom is lower than that of O-c atom (25 kcal mol(-1) vs 40 kcal mol(-1), however, previous experimental and theoretical studies neglect this remarkable difference of the O-b and O-c atoms in CO oxidation, which were treated indiscriminately.); (5) although the O-2 molecule was activated by the charge transfer from the anchoring Rh atom to pi* antibonding orbital of O-2, a strong bonding interaction of Rh-O-2 leads to a high barrier of 41 kcal mol(-1). Therefore, the dissociation of the adsorbed O-2 molecule is the rate-determining step for the whole catalytic cycle. Consequences of these findings for the Rh-assisted MvK mechanism on the Rh-1/PTA SAC, can be viewed as a model system for POM-supported SACs.