Redox reactivity of transition metal dioxide anions towards sulfur dioxide in the gas phase

Transition metal oxides (TMO) have long been known as effective catalysts in a variety of chemical transformations [1]. As an example, mineral oxides constituting miniaturized atmospheric aggregates provide reactive surfaces for the oxidation of tropospheric-sourced SO2 [2-4]. This air pollutant represents the key precursor of the sulfate aerosols that are responsible for climate changes, acid rainfalls, and severe haze events, such as the “London fog” episode occurred in 1952 [5]. Interestingly, the oxidative properties of TMOs are predominantly due to the O·- radical on the metal surface, as demonstrated by several studies carried out in the condensed phase [6-8]. Nonetheless, the catalytic features of the active sites can be masked by interfering factors observed in bulk, such as solvent and counter-ion effects, that prevent the elucidation of the reaction mechanism. A successful approach to circumvent this problem is to perform gas-phase studies of mass-selected cluster ions generated at their electronic ground states. The reactivity of these species can be investigated by mass spectrometric techniques under single-collision conditions and the results, thus obtained, complemented by comprehensive computational studies. This strategy allows one to assess the elementary steps of a chemical reaction and investigate the effects of stoichiometry, spin distribution, and charge state on cluster reactivity at a strictly molecular level [9,10]. As a result, we report on the reactivity of the first-row transition metal dioxide anions (CrO2·-, CoO2-, NiO2·-, CuO2- and ZnO2·-) towards SO2 by combining ion-molecule reaction experiments and theoretical calculations. An unprecedented fast and efficient oxidation of SO2 to sulfate radical anion is promoted by dioxide anions of the late transition metals, Cu and Zn. A double oxygen transfer is indicated as the energetically-favoured reaction mechanism that switches to a single oxygen anion transfer, leading to SO3·-, when a water molecule is ligated to ZnO2·-. Interestingly, when the spin density is highly localized on the metal centre, as in the case of CrO2·-, the anion acts as a reductant towards SO2, whereas only the consecutive addition of two SO2 molecules is observed with the earlier transition metals, Ni and Co. In conclusion, owing to the borderline-acid nature of SO2, the spin distribution alternatively located on the metal centre or on the terminal oxygen atoms of MO2-/·- anions is crucial in affecting the redox properties and the gas-phase reactivity of these species. [1] G. J. L. Fierro, Metal oxide: chemistry and applications, Taylor and Francis, 2006, London. [2] X. Zhang, et al., J. Phys. Chem. B 2006, 110, 12588-12596. [3] C. Liu, Q. Ma, Y. Liu, J. Ma, H. He, Phys. Chem. Chem. Phys. 2012, 14, 6957-6966. [4] W. Yang, J. Zhang, Q. Ma, Y. Zhao, Y. Liu, H. He, Sci. Rep 2017, 7, 4550. [5] G. Wang, et al., Proc. Natl. Acad. Sci. 2016, 113, 13630-13635. [6] V. A. Shvets, V. B. Kazansky, J. Catal. 1972, 25, 123-130. [7] G. I. Panov, K. A. Dubkov, E. V. Starokon, Catal. Today 2006, 117, 148-155. [8] H. F. Liu, et al., J. Am. Chem. Soc. 1984, 106, 4117-4121. [9] G. E. Johnson, E. C. Tyo, A. W. Castleman Jr., Proc. Natl. Acad. Sci. 2008, 106, 6136-6148. [10] D. K. Bohme, H. Schwarz, Angew. Chem. Int. Ed. 2005, 44, 2336-2354.