Influence of the Carbon–Carbon Bond Order and Silver Loading on the Formation of Surface Species and Gas Phase Oxidation Products in Absence and Presence of NOx over Silver-Alumina Catalysts

Abstract

The influence of carbon–carbon bond order, here systematically represented by prototypical C2H6, C2H4, and C2H2, on the formation of oxidation products and surface species in the absence and presence of NO has been studied for silver-alumina catalysts with different silver loadings (2 and 6 wt %). The catalysts were prepared with a sol–gel method including freeze-drying, which results in small silver species uniformly distributed throughout the alumina matrix. The performance of the catalysts was investigated by temperature programmed extinction-ignition experiments using a continuous gas flow reactor. The evolution of surface species during reactant step-response experiments was studied in situ by diffuse reflection Fourier transform infrared spectroscopy. The results show that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For example, C2H2 shows the highest conversion at low temperatures. Furthermore, the use of hydrocarbons with high bond order, that is, C2H2, as reductant for lean NOx reduction results not only in the highest peak activity but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperature. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. Several types of adsorbates, for example, carbonate, acetate, formate, enolic and isocyanate/cyanide surface species, are present on the catalyst during reaction. Generally the nature of the surface species and likely also the surface processes are more influenced by the carbon–carbon bond order than the silver loading. This needs to be considered when designing catalysts for emission control systems. Especially for applications using homogeneous fuels with short hydrocarbons, this may provide opportunities to tailor the catalyst functionality for the needs at hand.