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.

 2 wt% V2O5/WO3/TiO2 calcined at 450°C activated after aging under various conditions  Low temperature SCR activity was maximum after hydrothermal aging at 550°C  Low temperature SCR activity was maximum after thermal aging at 600°C  Activation was attributed to change of V coverage and surface acidity

We report the increased level of mechanistic information that Raman spectroscopy can provide 3 when combined with modulated excitation experimentation. A V 2 O 5 /TiO 2 catalyst for the selective catalytic reduction (SCR) of NO x was subjected to periodic NH 3 pulses while 5 recording time-resolved Raman spectra simultaneously. Through phase sensitive detection, we 6 confirmed unambiguously that both VO x and TiO 2 play a dynamic role in the SCR process. 7 Only a well-defined portion of VO x species (i.e., those that are coordinatively unsaturated) act 8 as catalytic active centers while the majority remain largely unresponsive. Although TiO 2 is 9 not directly involved in the SCR reaction, the phase-resolved Raman spectra demonstrate that 10 it acts as a reservoir of NH 3 molecules that can be mobilized to the active sites. The inherent 11 depth of these molecular insights goes beyond what is currently achievable with Raman 12 experiments under steady-state conditions. Modulated-excitation Raman spectroscopy can 13 therefore advance the fundamental understanding of SCR and other metal oxide-catalyzed 14 reactions.