Kinetics of CO Poisoning in Simulated Reformate and Effect of Ru Island Morphology on PtRu Fuel Cell Catalysts As Determined by Operando X-ray Absorption Near Edge Spectroscopy

Abstract

In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS), were carried out on commercially produced Pt and PtRu bimetallic electrocatalysts as well as on a mechanically mixed PtRu bimetallic electrocatalyst in an operating fuel cell in H 2 doped with 150 ppm CO. By use of the novel ∆XANES technique, the coverages of CO and ontop and n-fold H (overpotential deposited and underpotential deposited hydrogen) are obtained and compared for the three catalysts, and the results are correlated with PtRu cluster morphology. The mechanical mixing process used to create the bimetallic PtRu catalyst is found to maximize CO tolerance, although the PtRu commercial electrocatalyst exhibits an increased electronic effect, most probably due to the presence of Ru(O) x islands at the catalyst surface. The mobility of the CO on both Ru and Pt is found to be sharply dependent on the CO coverage, decreasing dramatically beyond 0.4 fractional coverage.

Operando XAS Techniques: Past, Present, and Future

“…Assuming that both the Pt and M atom XAS data were measured, one can then obtain four coordination numbers, two at each edge, namely, the Pt-Pt, Pt-M, M-Pt, and M-M coordination numbers. These are the critical parameters that have been used to propose an average nanoparticle morphology [64,68] and given below for one case.…”

“…This change is Figure 17.9, as a function of fuel cell current. The Hopd experimental coverage estimates were obtained from the Dm XANES analysis described previously [68]. Model coverages were obtained from the kinetic model in the text and scaled for optimum agreement with experiment.…”

“…However, the sensitivity of the Dm XANES technique is sufficient to distinguish adsorbed atop OH species from n-fold O, which is critical in fuel cell work. Figure 17.7 also shows how the amplitude of the Dm signature increases with increasing O coverage, and for the most part we assume that the Dm magnitude is representative of relative adsorbate coverage, although we have discussed deviations from this previously [64,68].…”

“…Consequently, the experimental Dm observed in reformate operation needs to be fit to all three species, H n-fold, CO, and H ontop, with linear regression in the range 0-35 eV. Details on this regression technique have been presented elsewhere [68]. Results from this work are shown in Figures 17.9 and 17.10.…”

“…The latter was produced by mechanically mixing carbon-supported Pt and carbon-supported Ru (both from E-TEK) in an agate mortar, resulting in Pt nanoparticles with a small Ru surface component and similarly composed Ru particles with some Pt on the surface. The corresponding estimated morphologies presented in Figure 17.9 were obtained from the calculated Pt-Pt, Pt-Ru, Ru-Pt, and Ru-Ru coordination numbers using the EXAFS cluster modeling technique described elsewhere [68,86].…”