Time-Resolved Dynamics During the Initial Decline in Catalytic Activity of the CO + NO Reaction over Platinum – Elucidating the Role of NCO

The reduction of NO by CO over a powdered Pt/Al₂O₃ catalyst in flow reactors is investigated as a function of temperature and stoichiometry at low concentrations typical for emission control. The study investigates the initial decline in conversion over pre-reduced catalysts and its correlation with the formation (and decomposition) of isocyanate (–NCO) on the support, tracked by time-resolved operando and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Free sites on noble metal particles are necessary for both isocyanate formation and decomposition. CO adsorption and isocyanate formation occur on different time scales, but the amount of isocyanate equilibrates with CO coverage. In the presence of hydrogen, isocyanate decomposes rapidly, producing NH₃ . The timescales for -NCO formation and activity decline are similar, suggesting a direct link or influence by a common process. Findings align with models that show a) isocyanate formation might aid NO dissociation by countering the inhibiting effect of CO adsorption, and/or b) changes in the oxidation state affect both catalytic activity and isocyanate formation.