Comparative Studies of Phosphate-Modified CeO2 and Al2O3 for Mechanistic Understanding of Dichloromethane Oxidation and Chloromethane Formation

A phosphate-modified CeO2 nanosheet as a promising catalyst presenting high activity, durability, and selectivity for catalytic oxidation of chlorinated volatile organic compounds (CVOCs) was used to investigate mechanisms of dichloromethane (DCM) oxidation and monochloromethane (MCM) formation by comparison with Al2O3-based catalysts, and CeO2-based catalysts showed a higher activity for DCM oxidation and lower selectivity for MCM. A series of well-designed experiments including various isotopic experiments revealed that an acid-base pair catalysis was involved, that is, DCM mainly dissociated on Lewis acid sites and then dehydrochlorinated through hydroxyl groups/Bronsted acid sites, while the basicity was intrinsic to the generation of MCM via a hydride transfer reaction between DCM activated on basic sites and DCM dissociated on Lewis acid sites. Moreover, the superior redox ability could suppress the formation of MCM by a rapid catalytic oxidation but prompt the possible formation of Cl-2 and polychlorinated byproducts.