SO₂ Poisoning Mechanism of the Multi-active Center Catalyst for Chlorobenzene and NO Synergistic Degradation at Dry...

The performance of fresh (PdV/TiO₂), sulfur poisoned (Used-S and Used-H), and regenerated (Used-R_S and Used-R_H) 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 SO₃^2- to SO₄^2- is just 0.041 eV. The two oxygen vacancies (VO or TiO) 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/NO by the catalyst for anti-SO₂ poisoning modification and application in the manufacture industry.