Selective Catalytic Reduction of NO with NH3 on Cu-SSZ-13: Deciphering the Low and High-temperature Rate-limiting Steps by Transient XAS Experiments

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 degrees C, ammonia hinders reoxidation of solvated Cu-I species thereby inhibiting reduction of NO. Whilst at temperatures of 283 degrees C and above, the reduction of zeolite bound Cu-II(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 Cu-I at low temperatures and the oxidation of ammonia over Cu at temperatures in excess of 283 degrees C. Between 250 degrees C and 350 degrees C, both side reactions may be present and could explain the dip in the SCR activity typically denoted by the seagull shape.