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44545-G5
New Approaches to in Situ X-ray Adsorption Spectroscopy of Pt-Ru Eletrocatalysts
The addition of certain metals, particularly Ru, to Pt electrocatalysts is a common approach to improve the performance of fuel cell anodes for oxidation of methanol and CO-contaminated hydrogen. Despite numerous studies in recent years, the mechanism of the promoting activity is little understood. X-ray absorption spectroscopy (XAS) is ideally suited to study such systems, since it can yield information on both electronic and geometric structure of the catalysts in situ, i.e. under the operating conditions of fuel cells. In the past such experiments were hampered by the lack of surface sensitivity in XAS, which did not allow for identification of potential-dependent behavior of Ru surface atoms. We prepared and characterized electrocatalysts consisting of core Pt nanoparticles (10 nm) coated with Ru shells of submonolayer coverage. Ru K-edge X-ray spectra of such materials contains contribution from surface Ru atoms only and show a pronounced potential dependence when acquired in situ.
Ru shell on Pt nanoparticles were prepared by electroreduction of Ru(NO)(NO3)3. in 0.1M HClO4. Depending of the deposition potential either Ru metal or RuO2 can be (co)deposited in submonolayer quantities, as judged by Cu UPD stripping analysis. The RuO2 formation during the electrodeposition starts at 0.50V and at 0.80V it becomes the only detectable Ru species. Metallic Ru was found to provide higher rates of CO/MeOH oxidation compared to RuO2, yet both forms show the promoting effect. The best performing catalyst was produced by electrodeposition at 0.425V vs RHE. It was selected for further X-ray studies, and larger quantities of this catalyst were prepared using suspension electrode. Detailed characterization of this material revealed that ruthenium is present on Pt surface in metal form with the surface coverage of about 30%, and no traces of RuO2 were found.
We studied the speciation of surface Ru adatoms in working fuel cell conditions using in situ Ru-K edge XAS in fluorescence mode at various applied electrode potentials without and with addition of CO and methanol. The measurements were performed in a custom-made liquid electrolyte fuel cell designed to eliminate elastic scattering from carbon support and to maximize the fluorescence signal from the surface Ru atoms.
The acquisition of in-situ XA spectra was started at open circuit potential 0.75 V RHE and the potential was changed stepwise several times to 0.05 V RHE and back to 0.95V with 0.15V steps. The in situ XAS spectra show two isosbestic points between 0.05 and 0.95V, suggesting the presence of two surface Ru compounds interconverting with 1:1 stoichiometry in this potential region. Comparison with the XANES spectra of standard Ru compounds shows that at 0.05 V RHE all Ru adatoms are in reduced form Ru(0) and at 0.95 V RHE the adatoms are reversibly oxidized to Ru(III) form.
Assuming the presence of pure Ru(0) and Ru(III) states at 0.05 and 0.95V RHE, we conducted Principal Component Analysis of several consequent anodic and cathodic potential scans. For each cycle we used two end compounds data as standards for linear combination fits. One can see that reduction of oxidized form and oxidation of metallic ruthenium follows a hysteresis pattern, i.e. Ru(0) ↔ Ru(III) reaction is rather slow yet reversible. Based on this analysis we can state that the Ru(III) appears at 0.30V vs RHE.
Analysis of the EXAFS data at electrode potentials of 0.05, 0.90 and 1.35V yielded the coordination numbers and interatomic distances in first coordination shells of Ru. Low coordination numbers (3) for Ru-Ru neighbors and higher coordination number for Ru-Pt (5) evidence the presence of small two-dimensional ruthenium islands in agreement with the preferred location of Ru adatoms at the Pt nanoparticle step edges and ~30% surface coverage. At 0.95V Ru-Ru neighbors disappear, Ru-Pt coordination number drops to 3 and a new Ru-O shell with coordination number 3 and Ru-O distance of 2.02 ±0.04 Å shows up. This bond length agrees with the assignment of 0.95V species as Ru(OH)3, Extension of the potential to values higher than 1.00V causes irreversible changes in the XAS spectra. A detailed analysis of the new species stable in this potential region identifies them as RuO2
The addition of CO to the electrolyte allowed us to get an insight into the mechanism of its oxidation. XAS shows that CO on Ru is completely oxidized potentiostatically at 0.30V, whereas electrochemical data show the presence of CO on the catalyst at this potential. Our interpretation is that at 0.30V, i.e. the onset of methanol oxidation, Ru sites are free of CO but Pt sites are still poisoned to a large extent.
In the presence of 1M MeOH the onset of Ru(OH)3 formation observed in XAS spectra at higher potentials (> 0.50V) compared to the blank electrolyte (0.30V). The latter suggest that between 0.30 and 0.50V Ru(OH)3 provides OH groups for CO removal in a fast step, thus rendering Ru(0) as the predominant species.
