Filtration of the preferred catalyst for reverse water-gas shift among Rh_n⁻ (n = 3-11) clusters by mass spectrometry under variable temperatures

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 (CO₂ + H₂ → CO + H₂O) 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⁻ + CO₂ and Rh_nO⁻ + H₂) requires to take place was identified. The Rh₄⁻ 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.