Mars–van Krevelen mechanism for CO oxidation on the polyoxometalates-supported Rh single-atom catalysts: An insight from density functional theory calculations

Abstract

• The mechanism of CO oxidation by O 2 was studied over two POM-supported SACs (Rh 1 /PTA and Rh 1 /PMA). • The oxidation process involves consecutive oxidation of three CO molecules via a Rh-assisted MvK mechanism. • The second CO molecule was oxidized by the corner O atom of the POM support rather than adsorbed O 2 molecule. • Oxidation state of the Rh center changes among III, II, and I, which promotes electron transfer and the whole reaction. The low and insufficient anchored site on support structure is the bottleneck problem for development of single-atom catalysts (SACs). Owing to the discrete anionic structure, the surface structure of unimolecular polyoxometalates (POMs) in solid is more like the fragments of metal oxide and possesses the unique separate structure, which can be viewed as the separate small “island” and effectively prevent agglomeration of single metal atoms. In the present paper, the mechanism of CO oxidation by O 2 was systematically studied over two POM-supported SACs, Rh 1 /PTA and Rh 1 /PMA (PTA = [PW 12 O 40 ] 3− and PMA = [PMo 12 O 40 ] 3− ) by means of density functional theory (DFT) calculations. The results indicated that this process involves consecutive oxidation of three CO molecules over Rh 1 /PTA and Rh 1 /PMA SACs via a Rh-assisted Mar-van Krevelen (MvK) mechanism. The first CO molecule was oxidized by a bridged O atom of POM support to form CO 2 molecule and an oxygen vacancy. The surface oxygen vacancy was refilled by O 2 molecule, and the second CO molecule was oxidized by a corner O atom of the POM support to form CO 2 molecule and a new oxygen vacancy. Finally, the forming surface oxygen vacancy was replenished by the O 2 molecule to form the key Mo-O-O-Mo unit, which can easily oxidize the third CO to form the CO 2 molecule. Compared with the reported MvK mechanism, the significant difference arises from oxidation of the second CO molecule, in which the second CO molecule was oxidized by the corner O atom of the POM support rather than adsorbed O 2 molecule. This is mainly due to a low reactivity for the adsorbed O 2 molecule, which comes from a bonding interaction between the adsorbed O 2 molecule and the single Rh atom. The atomic charge analysis show that the single Rh atom stabilized on the POM supports promotes the whole catalytic cycle via the change of oxidation state among III, II, and I. All these results show that the presence of single Rh atom on the unimolecular PTA and PMA supports is a key factor for determination of new reaction pathway and high reactivity. We hope our computation results can provide new physical insights to design SACs for CO oxidation by using unimolecular POM cluster. The reaction mechanism of CO oxidation by O 2 was systematically studied over two polyoxometalate-supported single-atom catalysts by means of density functional theory calculations.