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The key to optimizing energy-consuming catalytic conversions lies in acquiring a fundamental understanding of the nature of the active sites and the mechanisms of elementary steps at an atomically precise level, while it is challenging to capture the crucial step that determines the overall temperature of a real-life catalytic reaction. Herein, benefiting from a newly-developed high-temperature ion trap reactor, the reverse water-gas shift (CO 2 + H 2 → CO + H 2 O) reaction catalyzed by the Rh n - ( n = 3-11) clusters was investigated under variable temperatures (298-783 K) and the critical temperature that each elementary step (Rh n - + CO 2 and Rh n O - + H 2 ) requires to take place was identified. The Rh 4 - cluster strikingly surpasses other Rh n - clusters to drive the catalysis at a mild starting temperature (∼440 K). This finding represents the first example that a specifically sized cluster catalyst that works under an optimum condition can be accurately filtered by using state-of-the-art mass spectrometric experiments and rationalized by quantum-chemical calculations.

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO 2 catalytic conversion and bimetallic carbides generated at the interface were proposed to contribute to the observed activity. Heteronuclear metal carbide clusters (HMCCs) that compositionally resemble the bimetallic carbides are suitable models to get a fundamental understanding of the reactivity of the related condensed-phase catalysts, while the reaction of HMCCs with CO 2 has not been touched in the gas phase. Herein, benefiting from the newly designed double ion trap reactors, the reaction of laser-ablation generated and mass-selected RhTaC 2 - clusters with CO 2 was studied. The experimental results identified that RhTaC 2 - can reduce four CO 2 molecules consecutively and generate the product RhTaC 2 O 4 - . The pivotal roles of Rh-Ta synergy and the C 2 ligand in driving CO 2 reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC 2 O 4 - was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO 2 atmosphere at elevated temperatures.