Walter Sarholz

Influence of catalytic activity on semiconducting metal oxide sensors I. experimental sensor characteristics and their qualitative interpretation

Abstract The influence of the catalytic activity of semiconductive metal oxides, working as chemical gas sensors, on their conductance G is studied in synthetic N2O2CO mixtures and in exhaust gases of engines. Doped and undoped Cr2O3, ZnO, Mn2O3, SnO2 and CeO2 specimens show a correlation between the sensor characteristics (G versus the air fuel ratio λ) and catalytic activity for the oxidation of CO by O2. The results can be interpreted qualitatively by an adsorption-reaction model, which is based on the ratio of the rate of reaction to the rate of adsorption. Sensors with high catalytic activity, i.e., where the reaction rate of the oxidation of CO is larger than the adsorption rates, show an S-shaped G—λ curve with a large step at the stoichiometric point (λ = 1) and with small slopes outside λ = 1. Sensors with low or moderate catalytic activity, i.e., where the reaction rate is smaller than the adsorption rate, show a more or less linearized curve with a slope larger than 0.5 over a large range of λ and do not show a step at λ = 1. Sensors with no or negligible catalytic activity obey a power law relationship G ∼ pi±m with m

Crystal face specificity in ammonia synthesis on tungsten carbide

The synthesis of ammonia from N/sub 2/ and H/sub 2/ using WC as a catalyst was investigated in a flow reactor at normal pressure and 400 to 500/sup 0/C to elucidate the mechanisms of surface reactions. The catalytic behavior of WC seemed to be similar to that of W metal. However, studies of the adsorption of H/sub 2/S on WC indicated that only a fraction of the surface area was reactive. This limited adsorption capacity of WC is discussed in relation to the nature of the exposed crystal faces. A hexagonal model for the WC crystal is presented, and a discussion of possible bonding mechanisms that would explain the metallic behavior of WC is included. (BLM)

Quantenchemische abschätzung von sauerstoffadsorptionsenergien an einigen metallen

Abstract For a better understanding of adsorption processes at fuel cell electrodes we want to apply quantum-chemical methods. Simple semiempirical models, the Huckel method and the extended Huckel method, were used to calculate MeO bond energies for systems consisting of a few metal atoms and an oxygen atom. The results were compared with adsorption energies of atomic oxygen (from experimental oxygen adsorption energies and oxygen dissociation energy). The best values could be obtained for an extended Huckel calculation on metal clusters with one oxygen atom, using a special modification of Coulomb-integral values from HF-SCF tables.