Practical catalysts often operate under dynamic conditions of temperature variations and sudden changes of feed composition that call for understanding of operation and catalyst structure under analogous experimental conditions. For instance, the copper exchanged small pore SSZ-13 catalyst used currently in the selective catalytic reduction of harmful nitrogen oxides from the exhaust gas of diesel fuelled vehicles operates under recurrent ammonia dosage. Here we report the design of unsteady state experiments that mimic such dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation we were able to capture an ammonia inhibition effect on the rate-limiting Cu re-oxidation at low temperature. The practical relevance of this observation was demonstrated by the optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and the increase of NO conversion at low temperature.

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Cu‐exchanged small‐pore SSZ‐13 catalysts have found wide use for the selective catalytic reduction (SCR) of nitrogen oxides from automotive exhaust gases. The transient working environment of the Cu−SSZ‐13 catalyst during NH3‐SCR requires studying the rate limiting steps under the different operation conditions this catalyst is exposed to. By exploiting time‐resolved operando X‐ray absorption spectroscopy in combination with multivariate analysis we followed the transient speciation of Cu during unsteady state conditions. The results reveal that depending on operating temperature two different rate limiting behaviours inhibit the reduction of NO. At temperatures below 283 °C, ammonia hinders reoxidation of solvated CuI species thereby inhibiting reduction of NO. Whilst at temperatures of 283 °C and above, the reduction of zeolite bound CuII(OH−) is the rate limiting step in the SCR reaction. The results also reveal the presence of two detrimental side reactions occurring, the direct oxidation of zeolite bound CuI at low temperatures and the oxidation of ammonia over Cu at temperatures in excess of 283 °C. Between 250 °C and 350 °C, both side reactions may be present and could explain the dip in the SCR activity typically denoted by the seagull shape.