Development of electroless deposition bath for Pt-Ru system and characterization for Pt-Ru bimetallic catalysts
Weijian Diao, John Meynard M. Tengco, John R. Regalbuto and John R. Monnier
Department of Chemical Engineering, University of South Carolina, Columbia SC 29208 USA
Bimetallic catalysts have replaced monometallic catalysts for a wide range of catalytic applications. Bimetallic catalysts often exhibit enhanced selectivity, stability, and/or activity relative to their corresponding monometallic components. Many different methodologies are used for bimetallic catalysts preparation [1]. However, the typical methods of co-impregnation and successive impregnation have poor control of metal-metal interaction and surface composition. Electroless Deposition (ED) has been used for the preparation of true bimetallic catalysts with controlled, bimetallic surface compositions [2, 3]. We have recently studied platinum-ruthenium bimetallic catalysts that are widely used in fuel cells and bio-mass conversion [4]. My research has focused on the preparation and characterization of both platinum deposited on carbon-supported ruthenium catalysts (Ru@Pt/C) as well as Ru deposited on carbon-supported platinum catalysts (Pt@Ru/C).
Eletroless deposition is the catalytic or autocatalytic process for deposition of metals by the pre-existing metal (catalysis) or the metal which is being deposited (auto-catalysis). Figure 1 shows a typical electroless deposition process.
Two series of Pt@Ru/C and Ru@Pt/C bimetallic catalysts have been prepared by electroless deposition (ED) method. For Pt@Ru/C preparation, a new ED bath was developed using Ru(NH3)6Cl3 as Ru precursor and HCOOH as reducing agent. Temperature and pH effects were studied by varying temperature from 70°C to 130°C and pH from 2 to 4. A deposition temperature of 110°C (to minimize effects of CO poisoning on Pt surface during deposition) and pH 3 (to avoid strong electrostatic adsorption) were chosen to synthesize Pt@Ru/C catalysts with variable and controlled Ru weight loadings. For Ru@Pt/C preparation, a standard bath using H2PtCl6 and DMAB as Pt precursor and reducing agent, respectively, was employed. Several Ru@Pt/C catalysts with different Pt weight loadings were synthesized by controlling initial Pt concentrations in the ED bath at the preferred conditions of 70°C and pH 10.
The Pt@Ru/C and Ru@Pt/C bimetallic catalysts have been characterized by temperature programmed reduction (TPR), selective chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM). TPR data showed that for Ru@Pt/C catalysts, where Ru was the major component, the peak for the reduction of oxygen pre-covered Ru shifted from 180°C (for monometallic 20 wt% Ru/C) to temperatures between 60°C and 100°C. However, for Pt@Ru/C catalysts, where Ru was the minor component, TPR spectra resembled that for monometallic 20 wt% Pt/C; both oxygen-covered Pt and Ru surface sites underwent reduction at 40°C. Selective chemisorption (H2 titration of oxygen pre-covered surfaces) experiments also confirmed the existence of strong surface interactions between Pt and Ru, which are explained as hydrogen spillover (Pt-assisted reduction of oxygen pre-covered Ru). XPS analyses in Figure 2 showed that binding energies (BE) shifted to lower values for the Ru 3d5/2 peak, and to higher values for the Pt 4d7/2 peak. The directions of the binding energy shifts indicate e- transfer from Pt to Ru on the bimetallic surface, again indicating strong surface interactions between Pt and Ru. There were no obvious differences between the XRD patterns in Figure 3 for the ED catalysts and their corresponding base catalysts, revealing that deposition of the second metal by ED bath formed only thin overlayers of the secondary metal, and not three-dimensional aggregates. In addition, the peaks observed in the XRD patterns were not shifted relative to the standard positions of the primary metals; the similar lattice parameters remain the same, suggesting no alloy formation. Finally, The STEM and XEDS images provided strong, visual evidence of targeted deposition of the secondary metal on the primary metal. The XEDS images confirmed that individual nanoparticles of the catalysts prepared by ED were bimetallic, with excellent association between the primary and the secondary metals. No monometallic Pt or Ru particles were detected for either of the families of bimetallic particles.
References
[1]. Regalbuto, J.R. in "Catalyst Preparation: Science and Engineering" CRC Press, Boca Raton, 2007.
[2]. Schaal, M.T., Pickerell, A.C., Williams, C.T., and Monnier, J.R. J. Catal. 254, 131 (2008).
[3]. Zhang, Y., Diao, W., Williams, C.T., and Monnier, J.R. Appl. Catal. A 469, 419 (2014).
[4]. Hamnett, A. Catal. Today 38, 445 (1997).
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