Kinetic Analysis of the Inhibition of CH4 Oxidation by H2O on PdO/Al2O3 and CeO2/PdO/Al2O3 Catalysts

Addition of CeO 2 to a PdO/Al 2 O 3 catalyst, assessed for the oxidation of CH 4 in the presence of 5% H 2 O at various temperatures, reduces the inhibition effects of H 2 O. A simplified kinetic analysis that assumes Langmuir adsorption of H 2 O on the catalyst active sites is used to model both the dynamic response and the steady-state CH 4 conversion following H 2 O addition to the feed gas and to quantify the effect of CeO 2 addition. The analysis shows that less H 2 O is adsorbed on the 2.9Ce/6.5Pd/Al 2 O 3 than the 6.5Pd/Al 2 O 3 catalyst and that the rate of H 2 O desorption is higher on the Ce-promoted catalyst. Both factors result in reduced inhibition of CH 4 conversion by H 2 O on the 2.9Ce/6.5Pd/Al 2 O 3 catalyst compared to the 6.5Pd/Al 2 O 3 catalyst. The apparent activation energy for CH 4 oxidation over the 2.9Ce/6.5Pd/Al 2 O 3 catalyst (61 ± 11 kJ mol –1 ) is shown to be statistically the same over the 6.5Pd/Al 2 O 3 (56 ± 9 kJ mol –1 ). Catalyst characterization data show minimal changes in catalyst properties after reaction, and removal of H 2 O from the reactant feed gas results in partial recovery of the catalyst activity. The data are consistent with H 2 O adsorption on the catalyst/support that may also inhibit O exchange with Pd-*/PdO site pair (Pd-* is an O-vacancy), the effect of which is reversible.