275792 Tailoring Nanoarchitecture of Bimetallic Cu-Ni Catalysts for Water Gas-Shift Reaction
The application of nanoscience and nanotechnology in the design, preparation, and characterization of bimetallic catalysts is an important part of recent progress in catalysis. Catalytic reforming, CO and partial alkene oxidation, CO hydrogenation, fuel cell electrocatalysis, are a few examples of important technology that rely on bimetallic systems developed over past few decades. The ability to control the composition, shape, and architecture of bi-metallic nanoparticle systems is of increasing importance in tailoring the resulting catalytic properties. The majority of bimetallic nanoparticle catalysts are prepared by impregnation/deposition methods that often give well dispersed systems, but the details of structure and local composition are difficult to ascertain. Bimetallic nanoparticles of the same size with different architectures, such as alloy, core-shell, or monometallic mixtures, show significant differences in activity and selectivity. Rational design and control of the particle activity requires precise synthetic methodology and full knowledge of structure/composition.
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. 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.
The supported bimetallic Cu@Ni, Ni@Cu, and CuNi alloy nanoparticles were synthesized by a successive chemical reduction and simultaneous reduction. 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, CO chemisorption and UV-vis spectroscopy clearly showed the formation of Cu or Ni core structures distinctly different from those in CuNi alloy nanoparticles. These catalysts were evaluated in 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 nanoparticles showed similar WGS activity to supported Ni nanoparticles, but lower methanation activity. Suppressed methanation activity observed for the Cu@Ni nanoparticles may be due to Cu segregation to the surface. Supported Ni@Cu nanoparticles displayed WGS activity comparable to supported Cu nanoparticles. The CuNi alloy prepared by co-reduction showed higher specific reaction rate per active metal surface area as compared to monometallic and core-shell catalysts. Higher irreversible CO uptake was observed for the Cu5Ni5 catalyst, which is explained by the surface alloying responsible for high reaction rate. The WGS activity was not significantly affected by the formation of core-shell structures due to a small mismatch (2.5%) in lattice parameters of Cu and Ni.