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

Abstract

Recent progress in colloidal synthesis of nanoparticles with well-Controlled size, shape, and composition, together with development of in situ surface science characterization tool's, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle Model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir-Blodgett deposition technique. Mild oxidative and reductive characteristics, indicate the formation of surface oxide on the Ru nanoparticles, the thickness of Which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50-200 degrees C under reaction conditions, which appears to he correlated with the higher catalytic, activity of the bigger nanoparticles. We found that the smaller. Ru nanoparticle form bulk RuO2 on their. surfaces, causing the lower catalytic activity As the size of the nanoparticle. increases, the core-shell type RuO2 becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and hence, may allow for rational catalyst designs for practical applications.