Reverse water–gas shift reaction catalyzed by diatomic rhodium anions

Abstract

The reverse water–gas shift (RWGS, CO₂ + H₂ → CO + H₂O, ΔH₂₉₈ = +0.44 eV) reaction mediated by the diatomic anion Rh₂⁻ was successfully constructed. The generation of a gas-phase H₂O molecule and ion product [Rh₂(CO)_ads]⁻ was identified unambiguously at room temperature and the only elementary step that requires extra energy to complete the catalysis is the desorption of CO from [Rh₂(CO)_ads]⁻. This experimentally identified Rh₂⁻ anion represents the first gas-phase species that can drive the RWGS reaction because it is challenging to design effective routes to yield H₂O from CO₂ and H₂. The reactions were performed by using our newly developed double ion trap reactors and characterized by mass spectrometry, photoelectron spectroscopy, and high-level quantum-chemical calculations. We found that the order that the reactants (CO₂ or D₂) were fed into the reactor did not have a pronounced impact on the reactivity and the final product distribution (D₂O and Rh₂CO⁻). The atomically precise insights into the key steps to guide the reaction toward the RWGS direction were provided.