Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

Abstract

The catalytic oxidations of acetylene, ethylene, and propylene under transient and steady-state conditions were studied using model Pd/Al₂O₃ and Pd/CeO₂–ZrO₂ monolith catalysts. Kinetics and light-off measurements highlight the differences between the oxidation of each hydrocarbon. Acetylene is strongly self-inhibiting and −1 order while the reaction order with respect to ethylene is +1, both at temperatures below light-off. Ethylene oxidation is moderately inhibited by oxygen at low temperatures and ethylene at high temperatures. Mixture light-off experiments reveal acetylene inhibits the oxidations of ethylene, propylene, and CO. Light-off of the other species follow acetylene oxidation, which is nearly unaffected by the other species. The Pd/CeO₂–ZrO₂ catalyst gives consistently higher conversion compared with Pd/Al₂O₃ at the same Pd loading for each of the individual oxidation reactions, although the negative-order kinetics of acetylene is sustained. Oxidation of acetylene and mixtures containing acetylene are enhanced with Pd/CeO₂–ZrO₂ compared to Pd/Al₂O₃. The effects of oxidative or reductive pretreatment of the catalysts on species oxidations are also evaluated. Mechanistic implications for the reactions are described to guide kinetic model development. The findings highlight and clarify inhibition effects during single and co-oxidation and offer insights into optimizing the catalyst composition and its operation.