Reverse water-gas shift catalyzed by RhnVO3,4- (n = 3-7) cluster anions under variable temperatures

A fundamental understanding of the exact structural characteristics and reaction mechanisms of interface active sites is vital to engineering an energetic metal-support boundary in heterogeneous catalysis. Herein, benefiting from a newly developed high-temperature ion trap reactor, the reverse water-gas shift (RWGS) (CO2 + H2 → CO + H2O) catalyzed by a series of compositionally and structurally well-defined RhnVO3,4- (n = 3-7) clusters were identified under variable temperatures (298-773 K). It is discovered that the Rh5-7VO3,4- clusters can function more effectively to drive RWGS at relatively low temperatures. The experimentally observed size-dependent catalytic behavior was rationalized by quantum-chemical calculations; the framework of RhnVO3,4- is constructed by depositing the Rhn clusters on the VO3,4 "support", and a sandwiched base-acid-base [Rhout--Rhin+-VO3,4-; Rhout and Rhin represent the outer and inner Rh atoms, respectively] feature in Rh5-7VO3,4- governs the adsorption and activation of reactants as well as the facile desorption of the products. In contrast, isolated Rh5-7- clusters without the electronic modification of the VO3,4 "support" can only catalyze RWGS under relatively high-temperature conditions.

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

Multiple CO2 reduction mediated by heteronuclear metal carbide cluster anions RhTaC2. Dalton Trans 2022;51:11491-11498. [PMID: 35833563 DOI: 10.1039/d2dt01612e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO₂ 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 CO₂ 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 RhTaC₂^- clusters with CO₂ was studied. The experimental results identified that RhTaC₂^- can reduce four CO₂ molecules consecutively and generate the product RhTaC₂O₄^-. The pivotal roles of Rh-Ta synergy and the C₂ ligand in driving CO₂ reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC₂O₄^- was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO₂ atmosphere at elevated temperatures.

Reverse water-gas shift reaction catalyzed by diatomic rhodium anions

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.