RhPt/graphite catalysts for CO electrooxidation: Performance of mixed metal and alloyed surfaces

L Fang,S.E Huxter,G.A Attard,F.J Vidal-Iglesias,P.B Wells

doi:10.1016/j.susc.2014.06.026

Abstract

Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) have been used to characterise novel PtRh mixed metal and surface alloyed active phases supported on 5% Pt/graphite catalysts (5% Pt/G). The active phases could be prepared using forced deposition or a combination of forced deposition and thermal annealing in a flowing 5% hydrogen in argon gas mixture at 700 K. The protocols employed originate directly from previous single crystal studies of Rh on Pt{100} whereby Rh overlayers (either singly adsorbed or as a ‘sandwich structure’ of alternate Pt/Rh/Pt layers) could be transformed into surface PtRh alloys by careful thermal annealing. We demonstrate that this method is also successful for the preparation of active supported catalysts for CO electrooxidation whereby a peak potential as low as 0.60 V (Pd/H) for the CO oxidation is reported (0.12 V lower than that on 5% Pt/G). Moreover, the onset potential for CO stripping is lowered to 0.33 V (Pd/H). The presence of alloyed PtRh phases gives rise to a small but reproducible 0.1–0.2 eV shift to higher binding energy of the Pt 4f 7/2 XPS peak together with strongly modified Pt and Rh electrosorption features in the CV which correlate with changes in surface composition. Unusual kinetic behaviour in the CO stripping peaks from the PtRh catalyst as a function of CO coverage is ascribed to competition between electrochemical oxide on Rh and CO-induced blocking of electrochemical oxide formation sites at the highest CO coverage. For PtRh alloys, the onset of electrochemical oxide formation shifts to more positive potentials relative to Rh overlayers and so a different behaviour towards CO electrooxidation is observed. • Novel synthesis method for Rh adlayers and PtRh alloys on graphite supported Pt. • CV and XPS characterisation of PtRh alloys as a function of composition. • CO electrooxidation kinetics on Rh adlayer and RhPt alloys are different. • Marked CO electrooxidation activity for PtRh alloys compared to pure Pt.

Highlights

Twenty years ago, we submitted two papers to the IUVSTA conference in San Benedetto del Tronto concerning the electrochemical and UHV characterisation of metal deposition on single crystal electrodes [1,2]
Growth of rhodium adlayers on 5% Pt/graphite catalysts (5% Pt/G) substrates and CO oxidation
The potential ranges around the principal Rh hydrogen underpotential deposition (H UPD) (Fig. 1(a)) and Pt H UPD (Fig. 1(b)) peaks are expanded for clarity

Summary

Introduction

We submitted two papers to the IUVSTA conference in San Benedetto del Tronto concerning the electrochemical and UHV characterisation of metal deposition on single crystal electrodes [1,2]. One of these systems was Rh on Pt{111} whereby strong deviations from Frank-van der Merwe growth were observed together with surface alloying at elevated temperatures [2]. More importantly at that time correlations between the potentials at which various electrosorption states occurred and first/second layer/bulk Rh (as measured using Auger electron spectroscopy) were noted This meant that certainly, for Rh in a monolayer directly bonded to the Pt{111} surface, a specific electrochemical response could be measured that was proportional to surface coverage. Many groups have used the variation in single crystal CV peak positions and intensity to characterise

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Experimental

Results and discussion

Conclusion