(509f) Colloidal Synthesis and Characterization of Alumina-Supported Cu/Ni Core-Shell Nanoparticles In Water Gas-Shift Reaction

Recently, the water-gas shift (WGS) reaction has attracted renewed attention because high purity H₂ is needed for the operation of fuel cells.Metallic Cu and Ni have been predicted as highly promising catalysts for the water-gas-shift reaction based on theoretical (DFT) calculations. Supported bimetallic Cu/Ni nanoparticles with a core-shell structure have not been considered to date as WGS catalysts with enhanced catalytic activity and improved selectivity. However, conventional (support impegnation) approaches show poor control over bimetallic structures. Controlled tailoring of various nanostructures using colloidal methods may lead to new electronic and catalytic properties of nanosized metals. Accordingly, the main goal of the present study is to synthesize a series of stable and well-defined bimetallic Cu/Ni nanoparticle catalysts, namely, (a) Cu core and Ni shell (Cu@Ni), (b) Ni core and Cu shell (Ni@Cu), and (c) Cu-Ni mixed alloy nanoparticles supported on alumina, and to investigate their behavior in the WGS reaction. Another important practical aspect of WGS catalysis is the sensitivity of pure Cu and Ni catalysts to the presence of sulfur-containing impurities in the process streams. These catalytic metals readily chemisorb H₂S and form stable surface metal sulfides that poison WGS activity. Thus, bimetallic catalysts with core shell and alloy structure have been also explored with respect to their sulfur tolerance.

The supported bimetallic Cu@Ni, Ni@Cu, and CuNi alloy nanoparticles were synthesized by a successive chemical reduction, a redox-transmetallation method, and simultaneous reduction, respectively. These catalysts were characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) equipped with an HAADF detector, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), CO chemisorption, and UV-vis spectroscopy. The size and shape of supported Cu@Ni and Ni@Cu nanoparticles were determined by HRTEM and STEM. The metal composition in the core and shell regions was characterized by HAADF-STEM. The combination of X-ray diffraction, TGA and UV-vis spectroscopy clearly showed the formation of Cu or Ni core structures distinctly different from CuNi alloy nanoparticles. These catalysts were evaluated for the WGS reaction at 423-673 K under normal atmospheric pressure in a fixed-bed glass reactor connected to a gas chromatograph. Supported Cu@Ni and Ni@Cu nanoparticles showed enhanced WGS activities, as compared to traditional CuNi nano-alloys. Specifically, the Cu core with an approximately 1-2 monolayer thick Ni shell exhibited a greater WGS activity as compared to traditional CuNi nano-alloys. However, bimetallic CuNi catalysts supported on alumina with core shell or alloy structure showed similar sulfur resistance to that of pure Cu and Ni catalysts. On the other hand, the sulfur resistance of the catalytic system was improved when ceria is employed as a support.