IntroductionSelective catalytic reduction (SCR) technology is widely used due to the advantages of high denitrification efficiency and low operating cost. The existing commercial SCR catalysts are mainly vanadium-based catalysts with V₂O₅ as an active component, metal oxides WO₃ and MoO₃ as additives, and anatase TiO₂ as a carrier. They have a good denitrification activity at 320–420 ℃, but have a poor activity at a low temperature. The denitrification activity of vanadium-based catalysts is mainly related to their redox capacity, acidic sites, and chemisorbed oxygen. In order to improve their denitrification activity, in this work, V-Mo-P/TiO₂ catalysts were prepared by an impregnation-roasting method. The effect of roasting atmospheres (i.e., air, hydrogen, and nitrogen) on the active components and denitrification performance of vanadium-molybdenum-titanium catalysts (V–Mo–P/TiO₂) was investigated.MethodsA certain amount of oxalic acid was added into deionized water and dissolved under stirring., Ammonium metavanadate was then added into the solution, heated at 100 ℃ and stirred until the solution became dark blue. Afterwards, ammonium molybdate and diammonium hydrogen phosphate were added into the solution. TiO₂ was added and impregnated into the solution under stirring for 2 h, and then dried in an oven at 108 ℃ for 12 h to obtain a precursor of V–Mo–P/TiO₂ catalysts. The precursor was ground, pressed, and sieved to 40–60 mesh and placed in a fixed-bed reactor, and then roasted in different gases (i.e., air, nitrogen, and hydrogen) at 450 ℃ for 4 h to obtain different catalyst samples, which were named Air-450 ℃, N₂-450 ℃, and H₂-450 ℃, respectively.The structure of the catalysts was analyzed by a model PANalytical X-Pert PRO MPD X-ray diffractometer (XRD, PANALYTEC Co., the Netherlands). The pore structure of the catalysts was determined by a model NOVA2200e specific surface area analyzer (Kantar Instruments Co., USA). The surface elemental valence of the catalysts was characterized by a model ESCALAB250Xi Instrument (Thermo Co., USA). The elemental valence states on the catalyst surface were determined by an X-ray photoelectron spectroscope (XPS). The redox capacity and surface acidity of the catalysts were characterized with H₂-TPR and NH₃-TPD, respectively, and the NO adsorption on the catalyst surface was determined with NO-TPDe. Pyridine adsorption test (Py-FT IR) was performed using a model PE Spectrum II infrared spectrometer (PerkinElmer Co., USA) to characterize the types and strengths of the acidic sites on the catalyst surface.The catalyst activity was tested in a fixed-bed reactor at a gas flow rate of 1400 mL/min, a NO concentration of 500 mg-Nm³, an O₂ concentration of 8%, and an NH₃:NO volume ratio of 1:1 at 80–300 ℃. The catalysts were used in a fixed-bed reactor with a gas flow rate of 1400 mL/min, a NO concentration of 500 mg-Nm³, an O₂ concentration of 8%, and an NH₃:NO volume ratio of 1:1. The denitrification tail gas was examined by a model OPTIMA7 flue gas analyzer (MRU Co., Germany) to calculate the denitrification efficiency of the catalyst.Results and discussionAt 100–250 ℃, the NO conversion of Air, N₂, and H₂ catalysts increases with the increase of temperature. Clearly, the denitrification performance of the catalysts obtained by roasting in three atmospheres is different, and the denitrification activity in different gases follows an order of Air > N₂ > H₂, in which the catalysts roasted in air have the optimum low-temperature catalytic activity.The ratios of V⁴⁺/(V⁴⁺+V⁵⁺) in the catalysts roasted in air, nitrogen, and hydrogen are 0.440, 0.482, and 0.538, respectively, indicating that more V⁴⁺ is produced in a reducing atmosphere. Roasting in air can oxidize V⁴⁺ to V⁵⁺ to a certain extent, thus increasing the percentage of V⁵⁺ on the catalyst surface and the amount of chemisorbed oxygen O_α. From the results of H₂-TPR reduction, the corresponding integral area sizes are Air-450 ℃> N₂-450 ℃> H₂-450 ℃, indicating that polymerized vanadium is more easily formed in air. From the results of the NH₃-TPD desorption, the NH₃ desorption of the Air-450 ℃ catalyst is more than that of other two catalysts, and the number of acidic sites is in the following order of Air-450 ℃> N₂-450 ℃> H₂-450 ℃. The results of Py-FT IR further show that the acidic sites on the surfaces of the three catalysts are mainly of B-acid and L-acid. Roasting in air can increase the number of B- and L-acidic sites of the catalysts simultaneously, in which the B-acidic sites increased integrated areas of the catalysts roasted in air, nitrogen, and hydrogen are 1695, 1254 and 481, respectively, which are also basically consistent with the results of the NH₃-TPD characterization. The results of NO-TPD desorption indicate that the catalyst surface at Air-450 ℃ is more prone to forming highly active bridged or double-toothed nitrates. The V—O bond and B acidic sites in polymerized vanadium make it easier for the catalyst to adsorb and activate NH₃ species, which then reacts rapidly with the bridged or double-dentate nitrates to generate N₂ and H₂O. The reaction process mainly follows the L–H mechanism. The abundant O_α on its surface accelerates the formation of NO oxidized to NO₂, promoting a rapid SCR reaction. The V–Mo–P/TiO₂ catalysts roasted in air are superior to those roasted in nitrogen and hydrogen in terms of NO removal in the low-temperature section.ConclusionsV–Mo–P/TiO₂ catalysts for low-temperature denitrification were prepared by an impregnation method, and the denitrification activities of V–Mo–P/TiO₂ catalysts roasted in air, nitrogen, and hydrogen atmospheres were investigated. The order of their catalytic activities was as follows: air > nitrogen > hydrogen. The results of H₂-TPR and XPS revealed that the catalysts roasted in air had a larger proportion of exposed V⁵⁺ on their surfaces, and contained abundant chemisorbed oxygen O_α, which further accelerated the oxidation of NO to NO₂, and participated in the “fast SCR” reaction of denitrification. Moreover, the catalysts roasted in air at 450 ℃ were characterized by NH₃-TPD, Py-FT IR and NO-TPD, and the results indicated that the catalysts possessed more abundant B acid sites, and the B acid and polymerized vanadium were more prone to adsorption of activated NH₃ species, which further reacted with the bridged and double-toothed nitrate formed on the surface of the catalysts to generate N₂ and H₂O, thus promoting the SCR denitrification reaction by L-H mechanism.