Support and water effects on palladium based methane combustion catalysts

The effect of the support (gamma -alumina and zirconia) and of the presence of water on the catalytic activity of palladium particles was investigated both on oxidized and reduced states using temperature programmed reaction (TPR). The support was found to influence the onset and the rate of the reoxidation process. The particle size of the palladium phase, however, strongly influenced the reduction temperature and the activity decay and recovery during TPR experiments. In the metallic form, the catalyst is not affected by the water presence; in oxide state we found that, independent of the support, the inhibitory effect of the water molecules is stronger at lower temperatures. Consequently, the reaction may take place in two different regimes: one in which the rate limiting step is water desorption from the catalyst surface, which is favored by low temperatures (<500<degrees>C) and high conversions and the reaction order with respect to water is approximately -1, and the second, at higher temperatures, in which the rate limiting step is the methane activation. For the last regime, when working with dry feed, the reaction order with respect to water becomes 0. An important observation is that the water poisoning effect is only slowly reversible, so TPR experiments predict water poisoning to be important to much higher temperature than pulse experiments. (C) 2001 Elsevier Science B.V. All rights reserved.