Cu Loading Dependence of Fast NH3-SCR on Cu/SSZ-13

The fast NH3-SCR kinetics are studied on Cu/SSZ-13 with different Cu loadings, under reaction conditions (temperature and space velocity) where both steady-state and differential NOx conversions are readily achieved. Cu loading is found to greatly influence low-temperature NOx turnover rates, however not to substantially affect apparent activation energies. The high reaction apparent activation energies (~ 160 kJ/mol) suggest that the rate-limiting kinetics for fast SCR involve the participation of NH4NO3, an intermediate that is of considerable stability in small-pore zeolites. The energetically demanding NH4NO3 + NO reactions make low-temperature fast SCR much slower than standard SCR over Cu-exchanged small-pore zeolite catalysts, in drastic contrast to vanadium and medium/large pore zeolite-based SCR catalysts.