The Nature of the Active Metal Surface of Catalysts for the Clean Combustion of Biogas Containing Ammonia

Abstract

The nature of the active surface of supported metal catalysts for the clean combustion of ammonia-containing simulated biogas has been investigated to understand the very high N2 selectivity of some catalysts. A comparison of the activities of Al2O3-supported precious group metals has shown that under conventional operating conditions the fuel is fully combusted but NH3 is converted predominantly into NOx. In contrast, it is shown that it is possible by operating under O2-deficient conditions to attain essentially zero emissions of NOx with Rh- or Ir-based catalysts, whereas Pt or Pd gave markedly lower selectivities to N2. These variations in selectivity are attributed to the differences in the state of the metal surface during reaction. For Rh- and Ir-based catalysts it has been shown that under fuel-rich conditions, surface carbon, derived from the dissociative adsorption of CO, results in self-poisoning toward CO and H2 oxidation at low temperatures. However, at higher temperatures a preference toward methanation of the Cads is observed. It is thought that this surface reaction between the Cads and Hads scavenges both reductants so that Oads can react with NH3 in a highly specific manner, to give NOx. The NOx thus formed is subsequently reduced by reaction with excess CO to give N2. A model is presented that shows the way in which the chemistry of the surface of the active metal changes as the reaction parameters are varied.