Nanostructured Gold And Palladium Catalysts For Methanol And Formic Acid Catalyzed Oxidation

The development of an appropriate fuel cell system is an important research project from both economic and environmental points of view since fuel cells do not create pollution or toxic byproducts, as do fossil fuels or nuclear energy. At present, platinum is the most common catalyst for fuel cells based on the oxidation of small organic molecules. However, a major problem for Pt-based catalysts is the poisoning of Pt by CO-like intermediate species. How to enhance catalytic activity, CO-tolerance and Pt utilization of the Pt-based catalysts and to develop high-efficiency, low-cost non-platinum catalysts are the technological keys to commercialization of fuel cells. As a result, the main research content of this thesis focus on the fabrication of novel Au- and Pd-based electrocatalysts and the characterization of electrocatalytic oxidation of methanol and formic acid over the Au- or Pd-based anode catalysts, which are summarized as follows:(1) Dealloying single phase alloys is known to generate a type of nanostructured porous metals with intriguing properties. Nanoporous gold (NPG) made by dealloying Au-Ag was used as a novel electrode material for studying the methanol electro-oxidation. Compared to bulk Au electrode, oxidation and subsequent reduction of NPG occur at significantly negative potentials in both acid and alkaline solutions. NPG shows great catalytic activity for methanol electro-oxidation, but the structure quickly coarsens upon long time potential cycling. After surface modification with only a tiny amount of platinum, NPG exhibits greatly enhanced electrocatalytic activity toward methanol oxidation in the alkaline solutions, which is exemplified by a broad and high anodic peak during the positive scan and two secondary oxidation peaks in the subsequent reverse scan. SEM observation and long-time potential cycling both prove that NPG-Pt has much enhanced structure stability as compared with bare NPG At the same time, such bimetallic Pt-Au nanostructures exhibit better catalytic activity and stronger poison resistance than commercial Pt-Ru catalysts in acidic solutions. The experimental results and theoretical calculation exhibit that the synergistic effect of the bimetallic compositions may be mainly responsible for the better catalytic activity of the NPG-Pt.(2) Cobalt thin films composed of a large amount of nanopetals were fabricated on the glassy carbon (GC) substrate by electrochemical deposition with cyclic voltrmmetry of Co(II) ions. The use of the hierarchical Co nanostructures as the sacrificial template is the technological key to acquiring the Pd (or Pt) thin film electrocatalysts with hierarchical architecture through galvanic replacement reaction between Co nanopetals and chloropalladite (or tetrachloroplatinate). The as-prepared Pd (or Pt) thin films contain quantities of nanoparticles and many hollow Pd aggregates in the range of submicrometer to micrometer scale. Due to novel structural features, the Pd films display unusual hydrogen absorption properties and unexpected electrocatalytic activity. The hollow Pd aggregates were found to burst in acidic solutions at the potentials more negative than the hydrogen evolution potential since Pd absorbed too much hydrogen. When used as an electrocatalyst for the formic acid oxidation, the Pd thin films presented the much higher catalytic activity than the Pt films with the similar structure. An important reason is that the formic acid oxidation at the Pd nanostructures proceeds via a non-CO reaction pathway, while the reaction at the Pt nanostructures involves formation of CO adsorbed species, which was confirmed by the CO stripping from the Pt architectures. The as-prepared hierarchical Pd architectures are expected to be a promising electrocatalyst in the direct formic acid fuel cells (DFAFCs).