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.