Development of photochemical and electrochemical cells for operando X-ray absorption spectroscopy during photocatalytic and electrocatalytic reactions

Abstract

Photochemical and electrochemical reactions are highly relevant processes for (i) transforming chemicals (e.g. photoreduction of isopropanol to acetone, electrochemical hydrogenation of benzaldehyde to benzyl alcohol, etc.), and (ii) sustainable energy production (e.g. photoreduction of CO₂ to methanol, electrocatalytic H₂ evolution reaction). It is therefore of importance to monitor the structural changes and to understand the properties of active sites under photocatalytic and electrocatalytic reaction conditions. Operando X-ray absorption spectroscopy (XAS) provides the means to investigate the nature of active sites under realistic reaction conditions. In this contribution, we describe the successful development of photochemical and electrochemical cells for operando XAS measurements during photocatalytic and electrocatalytic reactions. We have used the operando photochemical cell to monitor the formation of Pt nanoparticles on graphitic carbon nitride nanosheets (g-C₃N₄-ns) via photodeposition under visible light illumination and observed the formation of highly dispersed Pt nanoparticles with an estimated size of ∼2.5 nm and >60% dispersion. We have also tested this cell to follow the oxidation state of Pt in Pt/TiO₂ and Pt/g-C₃N₄-ns during H₂ evolution reaction (HER). We observed that Pt predominantly existed as metallic (reduced) Pt⁰ species under HER conditions, and that PtO_x species were partially reduced from Pt^IV to Pt⁰ upon illumination with UV or visible light. The rates of H₂ evolution obtained in the photochemical cell (12.1 mmol g⁻¹ h⁻¹ on Pt/TiO₂ and 1.01 mmol g⁻¹ h⁻¹ on Pt/g-C₃N₄-ns) were comparable to that obtained in a standard top-irradiated photoreactor (16.6 mmol g⁻¹ h⁻¹ on Pt/TiO₂ and 1.76 mmol g⁻¹ h⁻¹ on Pt/g-C₃N₄-ns). The operando electrochemical cell was successfully tested to monitor the changes in the structure and oxidation state of Pd in Pd/C electrocatalyst during electrocatalytic hydrogenation (ECH) of benzaldehyde. It was demonstrated that Pd in Pd/C was present in a partially reduced state (∼80% Pd⁰ and ∼20% Pd^II) and Pd nanoparticles did not degrade upon the application of an external potential under ECH reaction conditions.