Understanding the Poisoning Mechanisms of Na2O and NaCl on CuO/Al2O3 Catalysts: Toluene Oxidation Performance and Reaction Pathways

Cu-based catalysts have been extensively adopted for catalytic oxidation of volatile organic compounds (VOCs). Nevertheless, the poisoning effect of alkali metals over Cu-based catalysts has received insufficient consideration despite that alkali metals are common components in the coal-fired fuel gas. In this study, the poisoning effect of Na₂O and NaCl on CuO/Al₂O₃ catalysts during toluene oxidation is studied. Experimental results show that Na₂O and NaCl cause an increase in the temperature corresponding to 90% of toluene conversion by 38°C and 87°C, respectively. After being poisoned by Na₂O and NaCl, the CO₂ selectivity decreases by 0.67%–8.15% and 12.76%–42.99%, respectively. The significant inhibition effect arises from the formation of Cu-O-Na and O-Cu-Cl structures. Cu-O-Na structure reduces toluene adsorption capacity, surface acidity, the ratio of surface adsorbed oxygen to total oxygen (marked as O_A ratio), and the quantity of active oxygen species. O-Cu-Cl structure reduces O_A ratio, the quantity of active oxygen species, and low-temperature reducibility. Besides, the following changes in toluene oxidation pathway are identified. Cu-O-Na structure promotes the generation of benzene. O-Cu-Cl structure inhibits the benzaldehyde oxidation and causes the generation of chlorinated aromatics. These results well explain the decrease of CO₂ selectivity after the catalyst is poisoned by Na₂O and NaCl.