Intrinsic Relation between Catalytic Activity of CO Oxidation on Ru Nanoparticles and Stability of Ru Oxides Uncovered with Ambient Pressure XPS

Abstract

Recent progress in controlled shape and sizes of nanoparticles using colloidal synthesis technique together with development of in-situ surface science characterization tools such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has resulted in enhanced understanding of surface structure of working catalysts. We report AP-XPS study of Ru nanoparticles to monitor surface oxidation states under oxidizing, reducing and CO oxidation reaction conditions. Ru 2.8 and 6 nm nanoparticles were synthesized in the presence of poly(vinyl pyrrolidone) and deposited onto Si wafers by Langmuir-Blodgett deposition as model catalysts. It was found that Ru nanoparticle exhibited mild oxidative and reductive characters, suggesting presence of an oxide overlayer around metallic core. AP-XPS showed that such oxide formation is dependent on Ru nanoparticle size and we found higher oxide formation for Ru 6 nm nanoparticles at all temperatures. The catalytic activity of Ru nanoparticles increases as increasing nanoparticle size in the range of 2 – 6 nm. We suggest that enhancement in catalytic activity for larger nanoparticles are attributed to a stable surface oxide of core-shell nanoparticles. Such in-situ observations of Ru nanoparticles are useful in identifying active state of the catalysts during use and hence may allow for rational catalysts design for practical application.