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

ABSTRACT 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 Na 2 O and NaCl on CuO/Al 2 O 3 catalysts during toluene oxidation is studied. Experimental results show that Na 2 O 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 Na 2 O and NaCl, the CO 2 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 CO 2 selectivity after the catalyst is poisoned by Na 2 O and NaCl.