Effects of Phosphorus Addition on the Hydrophobicity and Catalytic Performance in Methane Combustion of θ-Al2O3 Supported Pd Catalysts

A series of xPθ-Al2O3 supports modified with different amounts of phosphorus element were prepared and taken as supports of palladium catalysts for methane catalytic combustion. The impacts of phosphorus additives on the hydrophobicity of Pd/xPθ-Al2O3 and its performance of methane catalytic combustion in the absence of and presence of 8% water were systematically studied. It was found that the hydrophobicity of xPθ-Al2O3 changed with the increase of phosphorus content, which had a significant effect on the activity of methane catalytic combustion. The incorporation of phosphorus replaced the hydroxyl groups on the surface of Al2O3 in the form of phosphates, thus changing the density of hydroxyl groups of Al2O3 support. TGA, NH3-TPD, IR, and XPS were employed to illustrate the process of phosphate replacement. xPθ-Al2O3 with less than 1 wt.% phosphorus content had better hydrophobicity than the unmodified θ-Al2O3 and Pd/xPθ-Al2O3, therefore had better performance for methane catalytic combustion, which was attributed to the substitution of hydroxyl groups on the surface of θ-Al2O3 by PO43− and HPO42−. However, when the phosphorus content of Al2O3 was higher than 1 wt.%, the substitution of H2PO4− began to dominate, which would lead to poorer hydrophobicity and catalytic performance. This work will guide the design of methane catalytic combustion catalysts resistant to water inhibition problem.