| Summary: | Heteropoly acids (HPAs) with the Keggin structure have been widely used in NOx removal. Two kinds of catalysts (those with and without the Keggin structure) are prepared for studying the effect of the Keggin structure on the NH3-SCR reaction. A series of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analyses are conducted to investigate the surface-adsorbed species on the catalysts during the SCR reaction. The mechanism for enhancing low-temperature activity of the catalysts is proposed. Furthermore, the effect of NH4+ in the Keggin structure is also investigated. Results indicate that both the Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) mechanisms occurred in the NH3-SCR reaction over the catalyst with the Keggin structure (Cat-A); in addition, when more acid sites are provided, NOx species activity is improved and more NH4+ ions participate in reaction over Cat-A, thus promoting SCR activity.
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