Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H2 production from water

Water activation and splitting by single anionic iridium atoms

Mass spectrometric analysis of anionic products that result from interacting Ir^- with H₂O shows the efficient generation of [Ir(H₂O)]^- complexes and IrO^- molecular anions. Anion photoelectron spectra of [Ir(H₂O)]^-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir^-(H₂O), as well as the intermediates, [H-Ir-OH]^- and [H₂-Ir-O]^-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir^- + H₂O → IrO^- + H₂ is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir^- anions.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

CO2 Activation and Hydrogenation by Palladium Hydride Cluster Anions

Mass spectrometric analysis of the anionic products of interaction between palladium hydride anions, PdH^-, and carbon dioxide, CO₂, in a reaction cell shows an efficient generation of the PdHCO₂^- intermediate and isolated formate product. Multiple isomers of the PdHCO₂^- intermediates are identified by a synergy between negative ion photoelectron spectroscopy and quantum-chemical calculations. It is shown that a direct mechanism, in which the H atom in PdH^- directly activates and hydrogenates CO₂, leads to the formation of the formate product. An indirect mechanism, on the other hand, leads to a stable HPdCO₂^- structure, where CO₂ is chemisorbed onto the Pd atom.