The Potential of Zero Total Charge As a Descriptor of Catalytic Activity of Ru@Pt Core-Shell Catalysts

Abstract

Ru@Pt core-shell catalytic particles consist of a thin Pt shell covering a Ru core, and may be supported on carbon to provide catalysts with properties intermediate between those of Pt and RuPt alloy particles of similar size [1]. Whereas carbon-supported Pt typically oxidizes adsorbed CO in stripping experiments with a peak potential around 0.8 V vs. a reversible hydrogen reference electrode (RHE) in 0.5 mol dm-3 HClO4(aq) the current for the core-shell catalysts peaks around 0.6 V. The value for the alloy is around 0.5 V. However, although the catalytic properties for for example electrooxidation of CO may not be quite as good as those of the alloy, the core-shell particles display superior stability. These core-shell catalysts should therefore be of interest in applications such as direct methanol fuel cells [2] or as CO-tolerant catalysts in PEM fuel cells. The potential of total zero charge (pztc), the potential corresponding to zero electrode charge (including both free charges and bonded charged species), may be assessed through the CO-displacement technique by Climent et al. [3] and shown by Mayrhofer et al. [4] to correlate with catalytic acticity. The method is based on CO displacing more weakly adsorbed cations and anions at the electrode or catalyst surface. The pztc has been found to correlate with the work function of metal electrodes, as has also the enthalpy of adsorption of CO and O2 [5]. This suggests that it should be possible to infer the enthalpy of adsorption for these species directly from the pztc. Moreover, scaling relations established by density-functional theory (DFT) calculations [6] imply that it is possible to estimate adsorption enthalpies for a number of other adsorbates relevant for for example the methanol oxidation reaction (MOR) or the oxygen reduction reaction (ORR), if the adsorption enthalpies of CO and oxygen are known. With a knowledge of adsorption enthalpies for a given reaction mechanism involving reactions of the type A + 1/2 H2 → AH the free energy of the steps may be inferred as explained by Rossmeisl and co-workers [7,8]. Through this sequence of correlations we have been able to infer the rate-determining step and rank the catalytic activity from the pztc. The pztc measurements thus predict that the onset potentials for the ORR should increase in the order RuPt < Ru@Pt < Pt, whereas that of CO-stripping and the MOR should decrease in the same order, RuPt > Ru@Pt > Pt, in close agreement with experimental results, as shown in the the figure; the figure shows the dependence of the specific activity of the ORR and the apparent rate constant, the exponent of -FEp/RT [9] where Epis of the peak potential and the other symbols take their usual meaning, for CO-stripping to the potential of zero total charge vs. RHE. From left to right are PtRu, Ru@Pt, and Pt, respectively. Moreover, this combination of CO-displacement measurements and DFT results appears to predict the right order of magnitude differences in the onset potential between the catalysts as well. Akcnowledgement This work was funded by NTNU.