The hydrogenation of carbon dioxide into value-added chemicals is of great importance for CO 2 recycling. However, the underlying mechanism of CO 2 hydrogenation remains elusive owing to the lack of experimental evidence for the formation of the C−H bond. Herein, the gas-phase reaction of copper hydride anion Cu 2 H 2 − with CO 2 at variable temperatures (∼300−560 K) was investigated. Metal hydrides are the ideal models to study the nature of C−H bond formation in CO 2 hydrogenation, while the related studies are scarcely reported, particularly for the hydrogenation reactions at temperatures above 300 K. The generation of formate (HCO 2 − ) attached on product CuH 2 CO 2 − was identified by temperature-dependent mass spectrometric experiments and density functional theory calculations. Temperature played crucial roles to fine-tune the product selectivity, from Cu 2 H 2 CO 2 − that dominates the room-temperature reaction into CuH 2 CO 2 − at elevated temperatures. The nature behind the temperature-dependent product selectivity and the mechanism of CO 2 hydrogenation has been interpreted by using theoretical calculations. The combined experimental and computational studies have provided solid evidence for the formation of formate attached in CuH 2 CO 2 − .

Titanium dioxide (TiO2) is an important oxide material owing to its extraordinary catalytic reactivity in a wide range of applications. Different species on the surface of TiO2 have been proposed to contribute to its reactivity, while the intriguing catalytic role of a Ti3+ ion has not been substantiated to date. Herein, benefiting from state-of-the-art mass spectrometry and quantum chemical calculations, we demonstrated that an exposed single Ti3+ ion on stoichiometric titanium oxide clusters (TiO2) n – (n = 1–11) works independently to catalyze NO reduction by CO. The single-electron mechanism to reduce NO into N2O was discovered, and an atomic oxygen radical (O•–) on products (TiO2) n O– that is highly reactive (e.g., in CO oxidation) was created. This finding is pivotal for providing a fundamental strategy to utilize an isolated Ti3+ ion on the surface of TiO2 and points out that this catalytic behavior can be a potential pathway in the atmosphere for pollutant removal.