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 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.

On the Mechanism of Carbon Dioxide Reduction on Sn-Based Electrodes: Insights into the Role of Oxide Surfaces

“…Probably, the maintenance of these metastable oxides under in-situ conditions is given by the OH species that are formed during the CO 2 conversion and by the parallel water reduction. In particular, for the case of Sn, this was nicely evidenced by in-situ Raman experiments [24,25]. Dutta et al [24,25] have shown that the Raman signal ascribed to tin oxides is still present in the potential domain where the CO 2 reduction to formate takes place.…”

“…In particular, for the case of Sn, this was nicely evidenced by in-situ Raman experiments [24,25]. Dutta et al [24,25] have shown that the Raman signal ascribed to tin oxides is still present in the potential domain where the CO 2 reduction to formate takes place. Therefore, even having different initial amounts of surface oxides, Sn-1 and Sn-2 electrodes may have a surface with very similar structure/composition (similar surface metastable oxide amount) after the first scan in Ar-saturated electrolyte.…”

“…Nevertheless, Cui et al [21] have shown that the CO 2 adsorption on the SnO/Sn(112) surface is feasible [21], where a tin oxide film would rapidly grow after the exposition to ambient air [22,23]. In fact, in-situ Raman measurements show that superficial tin oxides are present (metastable) at the CO 2 reduction onset potential, but the reaction still proceeds at lower potentials where metallic tin atoms are formed [24,25]. In another work, Chen and Kanan [16] have employed Sn foil and deposited Sn/SnO x thin-film electrodes as electrocatalysts, reporting that the current densities for the Sn foil with an exposed Sn 0 surface is mainly related to the H 2 evolution, whereas Sn/SnO x thin-film lead to higher faradaic efficiency for HCOO − formation.…”

Leaded Bronze Alloy as a Catalyst for the Electroreduction of CO2

“…The respective efficiencies are FE H2 ∼60 % and FE CO < 5%. Note that the missing contribution to reach 100 % total efficiency might be ascribed to the sluggish reduction of metastable tin and/or lead oxides at such low potentials ,,. Upon increase of the applied cathodic potentials, the overall efficiency of the HER decreases to a quasi‐steady value around 40 %, that one of CO slightly increases and remains relatively constant without exceeding 10 % and the one of formate increases steeply and remains in the 50‐60 % range in the potential window from −1.0 to −1.2 V. A very minor selectivity for methane is found at highest applied potentials (max FE CH4 =2.6 % at −1.17 V).…”

Polyoxometalate-based electron transfer modulation for efficient electrocatalytic carbon dioxide reduction

“…14 As another important factor, the electron transfer process has vital inuences on the thermodynamics, kinetics and various reaction pathways of the electrocatalytic reaction. 15,16 For instance, the number of transfer electrons can greatly determine the products of the electrocatalytic reaction (e.g. CO, 2e; CH 4 , 8e).…”