Surface chemistry on transition metal carbides

The carbides of Groups IVB-VIB early transition metals have been shown to exhibit catalytic properties similar to those observed for Pt-Group metals in reactions with hydrocarbon molecules. Various electrochemical studies revealed tungsten carbides (WC and W₂C) to be remarkably resistant to acidic corrosion at anodic potentials, though their effectiveness as electrocatalysts remained inconclusive. The first part of this research therefore focused on the fundamental mechanistic studies of the reactivity of the clean and carbide-modified W(111), W(110), and Mo(110) surfaces toward the direct methanol fuel cell (DMFC) molecules, methanol, water, and carbon monoxide. Temperature-programmed desorption and electron energy loss spectroscopy results showed that the carbide-modified surfaces were highly active toward the decomposition of methanol and water. Furthermore, the desorption of CO from both carbide surfaces occurred at near room temperature.; To better characterize the carbide surfaces, the second part of this research concentrated on identifying the various parameters influencing the surface chemistry of carbides. Studies comparing the reaction of ethylene on carbide-modified W(111) and W(110) showed that, though both surfaces readily decomposed ethylene, only the C/W(110) surface was able to form the ethylidyne intermediate commonly observed on Pt-group metals. Additionally, the Pt-like reactivity of C/Mo(110) was modified after exposure to oxygen. Similar to many carbide-modified early transition metal surfaces, C/Ti(0001) also demonstrated Pt-like reactivity toward cyclohexene and ethylene. Lastly, the effects of carbon-modification was examined on Ni(111), a late transition metal surface. Although the carbide-modified Ni(111) surface was similar to other early transition metal carbide surfaces in the selective dehydrogenation of cyclohexene to benzene, the carbide layer converted to graphitic carbon at higher temperatures.; This dissertation showed that tungsten carbide materials may be considered as a potential electrocatalyst for the direct methanol fuel cell on the basis that, when compared to Pt-group metal surfaces, they exhibited higher oxidation activity toward methanol and water, and that they were able to desorb carbon monoxide at lower temperatures. In addition, this study also demonstrated that substrate structure, surface modifications, and the position of the transition metal in the periodic table could influence the reactivity of the carbide surfaces.