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Valuable information on adsorption and reaction of NO and NH3 on the rare earth tailings and rare earth tailings/γ-Al2O3 has been provided on the basis of the diffuse reflectance infrared Fourier transform spectroscopy experiments and characterization. The redox ability and acidity of rare earth tailings are enhanced after adding γ-Al2O3. The highest NO conversion achieved at 200 °C is 65% after adding γ-Al2O3, which is increased by 86%. The surface area of rare earth tailings/γ-Al2O3 is 96.3 m2/g. The improvement of NH3-SCR activity is attributed to the increase of the surface acidity, especially Brønsted acid sites. NH3 can be attributed to NH4 + on Brønsted acid sites and coordinated NH3 on Lewis acid sites on the rare earth tailings/γ-Al2O3. Spillover of ammonia species NH4 + between rare earth tailings and γ-Al2O3 exists. These NH4 + species (spillover) are activated to NH3 + and NH2 + on the Brønsted acid sites of rare earth tailings. Spillover of ammonia species NH4 + and activation are extremely significant for NH3-SCR reaction. The NH3-SCR reaction over rare earth tailings/γ-Al2O3 follows by both Eley–Rideal and Langmuir–Hinshelwood mechanisms especially by the Eley–Rideal mechanism. NH3NO and NH2NO are the vital intermediates of the reaction. The obtained results could provide some guidances in the application of catalyst got with rare earth tailings at low cost and application of rare earth tailings. [ABSTRACT FROM AUTHOR]