Hydrophobic Modification of Small-Pore Pd-SSZ-13 Zeolites for Catalytic Methane Combustion

Catalytic oxidation is an effective solution for the control of methane (CH4) emission in exhausts from natural gas vehicles. Pd-based small-pore zeolites (such as Pd-SSZ-13) are considered to be the most active catalysts for CH4 oxidation, but H2O in the exhausts tends to induce deactivation of Pd catalysts. In this work, we tuned the hydrophobicity of Pd-SSZ-13 as a representative to improve its H2O resistance in CH4 oxidation. Pd-SSZ-13 catalysts with different Si/Al ratios were obtained by dealuminizing the pristine SSZ-13 zeolite with acid followed by Pd ion exchange, and a reduction of T50 (i.e. the temperature to reach 50% conversion of CH4) by 20 ℃ was achieved in CH4 oxidation in the presence of 10 vol.% H2O. Detailed physicochemical characterizations showed that the fraction of highly dispersed PdO species (highly active in CH4 oxidation) increased, whereas that of less inactive PdOx clusters decreased, in the Pd-SSZ-13 after acid modification. In addition, the increase of zeolite hydrophobicity after acid modification alleviated the H2O inhibition effect on the active PdO phase, leading to a less activity loss of Pd-SSZ-13 in CH4 oxidation. The improved hydrophobicity also favored C3H8 combustion over Pd-SSZ-13. These results suggested that simple acid modification could tune effectively the Si/Al ratio and hydrophobicity of zeolite supports, and eventually the physicochemical properties and oxidation performance of the supported Pd catalysts.