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

Abstract

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 PdO_x 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.