Co Cation Effects on Activity and Stability of Isolated Pd(II) Cations in Zeolite Matrices for Selective Catalytic Reduction of Nitric Oxide with Methane

Palladium ion-exchanged zeolites (ZSM-5, mordenite, ferrierite, Y, beta) were tested to evaluate catalytic activity for selective reduction of nitric oxide with methane (CH4-SCR) and stability in the presence of 10% water vapor at a relatively high temperature of 500°C. Deactivation of the catalytic activity was inevitable on Pd/HZSM-5 with a Si/Al ratio of 19.8 under the reaction conditions. When the Si/Al ratio decreased, the durability was slightly enhanced. Among the zeolites tested, palladium supported on mordenite showed stable NO conversions. The addition of cobalt effectively stabilized the activity of Pd–H-zeolites, particularly with the addition of 3.3 wt% cobalt onto Pd/HZSM-5, of which no deactivation was noticeable for at least 50 h. Isolated Pd2+ cations on HZSM-5 were the active and selective sites for CH4-SCR. The isolated Pd2+ cations could not be exchanged by Na+ cations in NaCl titration test after the addition of 3.3 wt% cobalt, while NH4NO3 titration revealed that the isolated Pd2+ existed in the sample. This means that Co2+ changes the ion-exchange property of the isolated Pd2+, leading to a much higher stability than Pd/HZSM-5 with regard to durability. The effect of Co2+ on the stability of the isolated Pd2+ cations and their sitings in zeolite cavities is discussed.