In situ FT-IR investigation on the selective catalytic reduction of NO with CH4 over Pd/sulfated alumina catalyst

In situ Fourier Transform Infrared (FT-IR) spectroscopy was applied to study the mechanism of the selective catalytic reduction (SCR) of NO with methane over 0.1 wt% Pd/sulfated alumina (SA) catalyst. Here, CO adsorption, NO adsorption, NO–O2 co-adsorption and interaction of the intermediates after NO–O2 adsorption with methane or methane/oxygen were investigated. The FT-IR results show that the Pd supported on Al2O3 mainly exists as oxide, such as PdO cluster. While, the Pd supported on SA mainly exists as isolated Pd2+ ions. The addition of Pd can promote the formation of nitro/nitrito and nitrate species on Al2O3 or SA at room temperature. At the same time, Pd also promotes the nitrates decomposition at high temperatures. Support sulfation further decreases the total concentration of the nitrates adsorbed on the surface of catalysts and lowers their thermal stability. Moreover, support sulfation can also stabilize isolated Pd2+ ions, which could catalyze the reaction between nitrate species and methane. Upon co-adsorption of NO and O2 on 0.1 wt% Pd/SA catalyst, the intermediate Pd2+–NO was produced. This intermediate could readily react with CH4 and O2, but almost not react with CH4 over 350 °C.

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► Sulfation of alumina generates strong BrØnsted acid sites which can stabilize Pd as isolated Pd2+ ions.
► The nitrate species formed over sulfated samples can react with methane under the catalysis of Pd2+ ions.
► Co-adsorption of NO and O2 led to the generation of the intermediate Pd2+–NO. This intermediate could readily react with CH4 and O2, but almost not react with CH4 over 350 °C.