Abigail Rose

Effect of Ru surface composition on the CO tolerance of Ru modified carbon supported Pt catalysts

A series of ruthenium modified carbon supported catalysts have been prepared by surface organometallic chemistry (SOMC) with the following nominal Ru∶Pt surface ratios, (1∶4)RuPt/C, (1∶2)RuPt/C, (3∶4)RuPt/C and (1∶1)RuPt/C. The catalysts were characterised using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), cyclic voltammetry (CV), and half-cell polarisation studies. The XRD measurements showed that a bulk PtRu alloy was not formed following SOMC modification. However, the EXAFS measurements indicated that a surface alloy is formed upon electrochemical reduction of the Ru modified catalysts. The CV studies show that the electrooxidation of CO on the Ru modified Pt/C catalysts occurs at lower potentials than on the unmodified Pt/C catalysts, but at higher potentials than on an alloyed PtRu/C with a bulk composition of 1∶1 Pt∶Ru. Half cell polarisation measurements in 100 ppm CO in H2 show that the CO tolerance of the SOMC RuPt/C catalysts approached that of the conventional PtRu/C alloy catalyst. The results therefore indicate that a bulk alloy phase is not an essential factor in the improvement in CO tolerance of PtRu/C catalysts over that of Pt/C.

Effects of composition on structure and activity of PtRu/C catalysts

A series of carbon supported PtRu bimetallic catalysts with varying Pt:Ru ratio were prepared and characterised using ex situ and in situ XRD, in situ EXAFS at 0 V vs. RHE, ex situ XPS and monolayer CO stripping voltammetry. Although the catalysts were found to be well mixed/alloyed, with no evidence of unalloyed Ru (oxides) present, the surfaces of the electrocatalyst nanoparticles were found to be enriched with Pt compared to the nominal bulk composition. The methanol oxidation activities of the catalysts were determined in 1.0 mol dm(-3) H2SO4. In agreement with published studies of polycrystalline bulk PtRu alloys the catalyst with a 0.6 surface fraction of Pt was found to give the best methanol oxidation activity at 30 degrees C. However, at 80 degrees C a greater surface fraction of Ru could be tolerated, with some activity at low current densities found for a Pt surface fraction as low as 0.2. The results support the conclusion that a limited amount of methanol dehydrogenation occurs at Ru sites or Ru dominated surface ensembles at 80 degrees C.

In situ time resolved studies of hydride and deuteride formation in Pd/C electrodes via energy dispersive X-ray absorption spectroscopy

The electrochemical formation of β-hydride and β-deuteride phases of a carbon supported palladium nanoparticle catalyst has been investigated using time resolved in situ energy dispersive X-ray absorption spectroscopy (EDE) and the simultaneously collected chronoamperometric data. For the β-hydride formation the rate of the phase transition was found to be limited by the rate of diffusion of H through the bulk of the Pd particles, while for β-deuteride formation the interfacial reduction of D+ was found to contribute more significantly to the kinetics. The composition of the hydride and deuteride phases formed was determined using both the lattice parameters obtained by fitting the EDE data and from linear sweep voltammograms, the former giving compositions of PdH0.83 and PdD0.63, which are consistent with the β-hydride and deuteride phases. Hyperstoichiometric compositions were obtained from the linear sweep voltammograms and the excess attributed to H or D spill-over on to the carbon support or re-oxidation of H2 or D2 trapped in the porous structure of the catalyst electrode.

In Situ XAS Studies of Core-Shell PEM Fuel Cell Catalysts: The Opportunities and Challenges

Core-shell electrocatalysts are of increased interest in PEM fuel cells as a means of improving activity and reducing costs. In situ x-ray absorption spectroscopy (XAS) provides a unique opportunity to provide characterization of the structure of such materials under operating conditions, enabling the effects of the electrochemical environment on the structures to be explored. However, such measurements present numerous challenges in terms of both data collection and analysis. Here we present two case studies that illustrate both the opportunities and challenges; (i) the modification of a Pt catalyst by Ru and the potential dependent formation of a surface alloy and (ii) a Pt shell - Pd core catalyst with varying shell thickness.