X-ray absorption spectroscopy and electrochemical studies of Pt-Sn electrocatalysts

Abstract

The addition of a second metal to form bimetallic electrocatalysts offers a means of enhancing electrocatalytic activity through promotional effects. To establish the structure-activity relationship and ascertain details of the promotional mechanism, it is key to characterise the complex structure of bimetallic nanoparticles and acquire molecular-level information regarding the bonding between the adsorbate and the surface.

This thesis focuses on characterising Pt-Sn bimetallic electrocatalysts using X-ray absorption spectroscopy (XAS) and electrochemical measurements. The work starts with Pt3Sn samples containing both SnO2 and alloyed Sn, where the extent of alloying was examined by a variety of techniques. The presence of Sn did promote CO oxidation, but the extent of alloying showed little effects on CO oxidation activity. To deepen understanding of the promotional mechanism, Pt-Sn samples in which the Sn was present exclusively as either SnO2 or as the Pt-Sn alloy were prepared, the structures of which were ascertained by both XAS and other methods. Both SnO2 and alloyed Sn were found to promote CO oxidation mainly via the bifunctional mechanism, but with different active species and redox couples. Whilst a surface hydroxide of SnO2 (SnIV -OH) generated from a SnII/SnIV reversible redox couple was proposed for Pt-SnO2 using cyclic voltammograms, Pt-Sn-OH from water dissociation on surface Pt-Sn* sites was determined for the surface alloyed Sn using in situ XAS. Finally, through the collection of a library of Pt-Sn XAS data, a method for determination of the contraction of the surface atoms of Pt-Sn alloys was derived, with the contraction found to be proportional to the Pt-Sn coordination number.

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ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

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