Probing the chemical state of tin oxide NP catalysts during CO2 electroreduction: A complementary operando approach.

Abstract In this paper we combine two operando methods, Raman spectroscopy and X-ray absorption spectroscopy (XAS), in order to probe reduced graphene-oxide supported tin^IV oxide nanoparticles ( SnO 2 NPs @ rGO) as they are being used to catalyse CO 2 electroreduction. To achieve high reaction rates it is necessary to apply sufficiently cathodic electrode potentials. Under such conditions, however, not only CO 2 is reduced electrochemically, but also the catalyst particles may be transformed from the initial Sn^IV state to Sn^II or, in an extreme case, to metallic Sn. While Sn^II species still favour CO 2 electroreduction, yielding formate as a primary product, on metallic Sn CO 2 reduction is disfavoured with respect to the competing hydrogen evolution reaction (HER). We show that operando XAS, a robust technique yielding information averaged over a large surface area and a relatively large thickness of the catalyst layer, is a very expedient method able to detect the reduction of SnO 2 NPs @ rGO to metallic Sn. XAS can thus be used to establish an optimum potential for the electroreduction in practical electrolysing cells. It takes, however, a complementary method offered by operando Raman spectroscopy, having greater sensitivity at the catalyst/electrolyte solution interface, to probe reduction intermediates such as the Sn^II state, which remain undetectable for ex situ methods. As it is shown in the paper, Raman spectroscopy may also find further use when investigating the recovery of catalyst particles following exposure to extreme reducing conditions. Graphical abstract fx1 Highlights • SnO 2 NP catalysts of CO 2 electroreduction were probed by two operando spectroscopies. • At cathodic potentials, Sn^IV is transformed to Sn^II or, in an extreme case, to Sn. • On Sn^II, the reduction of CO 2 to formate is preferred; Sn favours hydrogen evolution. • Operando XAS and Raman give complementary information on the degradation of SnO 2 NPs. • Catalyst recovery is studied, and safe operating conditions are established by Raman spectroscopy. [ABSTRACT FROM AUTHOR]