Unveiling the Dual Inhibition Mechanism of Ammonia Slip on VOC Oxidation over CeO₂: From Electronic Perturbation to...

Ammonia (NH₃) slip from upstream SCR units threatens coupled catalytic oxidation systems designed for synergistic control of NO_x and volatile organic compounds (VOCs). While competitive adsorption is often cited, the dynamic electronic impacts of NH₃ on oxidation catalysts remain unclear. Here, we elucidate the molecular mechanism by which slip NH₃ poisons toluene oxidation over a model CeO₂ catalyst. Through in situ spectroscopies and DFT calculations, we reveal a dual-pathway deactivation mechanism. First, NH₃ strongly chemisorbs at Lewis acid sites, fundamentally disrupting the redox cycle by increasing the oxygen vacancy formation energy and suppressing O₂ activation, thereby accelerating the irreversible consumption of reactive oxygen species. Second, NH₃ chemically incorporates into the reaction network, reacting with partial oxidation products to form persistent nitrogenous byproducts (e.g., benzonitrile). These refractory species possess a stronger surface adsorption energy and a higher ring-opening barrier than toluene itself, redirecting the reaction from complete oxidation. Our findings demonstrate that NH₃ poisoning extends beyond site blocking to include persistent electronic modification and recalcitrant intermediate generation. This work underscores that mitigating ammonia slip is a system-level engineering challenge, calling for integrated strategies like smarter NH₃ dosing and zoned catalyst design to ensure the long-term efficacy of multipollutant abatement technologies.