Unveiling the Dual Inhibition Mechanism of Ammonia Slip on VOC Oxidation over CeO 2 : From Electronic Perturbation to Byproduct Trapping

Ammonia (NH 3 ) 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 NH 3 on oxidation catalysts remain unclear. Here, we elucidate the molecular mechanism by which slip NH 3 poisons toluene oxidation over a model CeO 2 catalyst. Through in situ spectroscopies and DFT calculations, we reveal a dual-pathway deactivation mechanism. First, NH 3 strongly chemisorbs at Lewis acid sites, fundamentally disrupting the redox cycle by increasing the oxygen vacancy formation energy and suppressing O 2 activation, thereby accelerating the irreversible consumption of reactive oxygen species. Second, NH 3 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 NH 3 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 NH 3 dosing and zoned catalyst design to ensure the long-term efficacy of multipollutant abatement technologies.