Abstract
Single-cluster catalysts (SCCs) representing structurally well-defined metal clusters anchored on support tend to exhibit tunable catalytic performance for complex redox reactions in heterogeneous catalysis. Here we report a theoretical study on an SCC of Ru(3)@Mo(2)CO(2) MXene for N(2)-to-NH(3) thermal conversion. Our results show that Ru(3)@Mo(2)CO(2) can effectively activate N(2) and promotes its conversion to NH(3) through an association mechanism, in which the rate-determining step of NH(2)* + H* → NH(3)* has a low energy barrier of 1.29 eV. Notably, with the assistance of Mo(2)CO(2) support, the positively charged Ru(3) cluster active site can effectively adsorb and activate N(2), leading to 0.74 |e| charge transfer from Ru(3)@Mo(2)CO(2) to the adsorbed N(2). The supported Ru(3) also acts as an electron reservoir to regulate the charge transfer for various intermediate steps of ammonia synthesis. Microkinetic analysis shows that the turnover frequency of the N(2)-to-NH(3) conversion on Ru(3)@Mo(2)CO(2) is as high as 1.45 × 10(-2) s(-1) site(-1) at a selected thermodynamic condition of 48 bar and 700 K, the performance of which even surpasses that of the Ru B5 site and Fe(3)/θ-Al(2)O(3)(010) reported before. Our work provides a theoretical understanding of the high stability and catalytic mechanism of Ru(3)@Mo(2)CO(2) and guidance for further designing and fabricating MXene-based metal SCCs for ammonia synthesis under mild conditions.