The performance of fresh (PdV/TiO2), sulfur poisoned (Used-S and Used-H), and regenerated (Used-RS and Used-RH) multi-active center catalysts for chlorobenzene catalytic oxidation and selective catalytic reduction (CBCO + SCR) reaction is investigated. The reaction on the catalyst surface is blocked after sulfur poisoning owing to the occupation and deposition of catalyst active centers (mainly Pd centers) by PdSO4 (and/or PdS in a dry environment) and NH4HSO4 species, especially the CBCO process. Sulfates (mainly NH4HSO4) on the sulfur poisoned catalyst surface are partially decomposed after 400 °C thermal regeneration, while the deactivation caused by the formation of PdSO4 species is irreversible. Density functional theory calculation results show that in the PdSO4 and NH4HSO4 generation paths, each step of the elementary reaction has just a small energy barrier to overcome, and the stability of the product for each elementary reaction increases gradually. Even worse, SO2 is easily combined with H2O gas molecules to form H2SO3 in a humid environment, and the energy barrier for conversion of SO32- to SO42- is just 0.041 eV. The two oxygen vacancies (VOx-1 or TiOx-1) provide adsorption sites for CBCO + SCR reaction gas molecules but do not exhibit adsorption properties for SO2, which gives a possible idea for optimization of sulfur resistance. The present work is favorable for further synergistic removal of CB/NOx by the catalyst for anti-SO2 poisoning modification and application in the manufacture industry.
Keywords: CB/NOx synergistic degradation; SO2 poisoning mechanism; competitive adsorption; multi-active center catalyst; oxygen vacancies.