Low-temperature methane oxidation over oxide-supported Pd catalysts:: inhibitory effect of water vapor

The influence of water vapor on the activity for low-temperature methane oxidation over oxide-supported catalysts such as Pd/Al2O3, Pd/SnO2, and Pd/Al2O3-36NiO was studied. It was found that Pd/Al2O3 was deactivated most significantly due to water vapor, and that Pd/Al2O3-36NiO was most insensitive to water vapor. The catalytic activity of Pd/Al2O3 decreased monotonically as water vapor concentration increased, whereas Pd/SnO2 and Pd/Al2O3-36NiO showed almost constant activity under higher water vapor concentrations. The catalytic activity at high steam concentration was in the following order: Pd/SnO2 > Pd/Al2O3-36NiO > Pd/Al2O3. Kinetic analysis with methane adsorption as the rate-limiting step was applied to evaluate the water inhibiting effect. Pd/Al2O3 displayed the most negative value of the enthalpy of water adsorption, while Pd/SnO2 and Pd/Al2O3-36NiO exhibited similar water adsorption enthalpy. Deactivation and regeneration of Pd/SnO2 and Pd/Al2O3 catalysts were investigated by cyclic feed of water vapor. Both the catalysts were deactivated rapidly upon switching on water feed, and then they regenerated gradually to the initial activity after the water feed was switched off. (C) 2002 Elsevier Science B.V. All rights reserved.