The addition of alkali and alkaline earth metals to Pd/Al2O3 to promote methane combustion. Effect of Pd and Ca loading
Journal article, 2018
Methane catalytic combustion was studied on Pd/Al2O3 catalysts modified with alkali and alkaline earth metals (K, Na, Ca and Ba). Temperature-programmed reactions were performed in flow reactor on washcoated monolithic samples. CO chemisorption was carried out and BET surface area was measured in order to characterize the catalysts. Addition of alkaline metals was either beneficial or detrimental for methane oxidation, depending on the catalyst composition and pretreatment. High loading of alkaline modifiers and cooling in reaction mixture increased the light-off temperature of methane oxidation. However, promotion was achieved by doping with calcium after cooling in inert atmosphere. Furthermore, calcium doping was more beneficial in presence of NO, which inhibits the methane oxidation. Higher Pd loading enhanced CH4 oxidation, whereas the effect of calcium depends on Ca and Pd loading. Indeed, the promotion by calcium was more pronounced on catalysts with 5% Pd than with 2.5% Pd. On 2.5% Pd catalyst, high Ca loading was detrimental for CH4 oxidation. In contrary, on 5% Pd catalyst, even 1.51 wt% calcium (the highest loading used) induced promotion, which demonstrates a correlation between Ca: Pd ratio and promotion/inhibition effect. Samples with a mass ratio of 0.15 showed the best activity.