Ammonia (NH3) slip from upstream SCR units threatens coupled catalytic oxidation systems designed for synergistic control of NOx 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.
Zeng et al. (Thu,) studied this question.