The Preparation And NH₃-SCR Performance Study Of NiTi-based Mixed Oxides

Selectively catalytic reduction with NH₃?NH₃-SCR?has been demonstrated to be one of the most widely used flue gas denitration?DeNO_x?technologies due to its high NO_x removal efficiency,large gas handling capacity and easy control of reaction conditions.In this field,the V-W-Ti catalysts has been widely applied in the coal-fired power plants,whereas the active temperature window of such catalysts are too high for coking and cement plants with relatively lower exhaust gas temperature.Besides,the active vanadium specie in the V-W-Ti catalysts is biologically toxic and can cause secondary pollution to environment during post-treatment which has been incorporated into hazardous waste for management.Therefore,it is of great theoretical and practical significance to develop high-efficiency,stable,inexpensive and eco-friendly DeNO_x catalysts that match the actual working conditions.Ni-based oxide catalysts have drawn tremendous attention in the field of NH₃-SCR due to their suitable redox capacity,abundant acidic sites,wide sources and low price.TiO₂ possesses the advantages of low cost,non-toxicity and non-corrosion,which can promote the formation of oxygen vacancies in the crystal lattice if it mixes with other metals.Moreover,TiO₂ has large specific surface area and good SO₂ resistance,which is usually used as a carrier for the preparation of excellent SCR catalysts.Herein,in order to give full play to the both virtues of NiO and TiO₂,the strategy of NiTi hydrotalcite-like?LDH?as precursor template was adopted to assemble Ni and Ti species.Subsequently,the physicochemical properties of NiTi catalysts were further optimized by modulating the element composition,molar ratios and calcination temperature of LDHs to try to prepare mixed oxide catalysts with excellent SCR performance.With the aid of a series of modern instrumental analysis methods?TG-MS,XRD,BET,TEM,SEM,EDX-Maping,XPS,NH₃-TPD,H₂-TPR,etc.?,the structural features of prepared catalysts were obtained.The intrinsic reasons between structure and SCR performance were revealed.Meanwhile,the dominant preparation conditions of the NiTi mixed oxide catalysts were discussed.This paper would provide a theoretical basis for the preparation of high-performance and eco-friendly mixed oxides catalysts with hydrotalcite-like as precursor template.The specific research contents and main conclusions are as follows:?1?The NiTi precursors were prepared via conventional impregnation method,one-pot sol-gel method and urea homogeneous coprecipitation method?using hydrotalcite-like as precursor template?,respectively,which can further transform into NiO&TiO₂ mixed oxides catalysts after calcining at 400??NiTi-IMP,NiTi-SOL,NiTi-LDO?.The results showed that calcinating hydrotalcite-like precursor was the optimal NiO&TiO₂ assembly strategy in comparison with the other two precursors.Under the assistance of hydrotalcite-like precursor possessing regular morphology and lattice limiting effect,the derived NiTi-LDO catalysts exhibited a larger specific surface area(224.6 m² g^-1),much better component dispersion and more abundant acid sites.Thus the NiTi-LDO catalysts showed the best NH₃-SCR performance.?2?A series of NiTi mixed oxides catalysts?Ni₂Ti₁-LDO,Ni₄Ti₁-LDO,Ni₆Ti₁-LDO?were achieved by calcining hydrotalcite-like with different Ni/Ti molar ratios?2:1,4:1,6:1?at 400?,which displayed significantly various SCR performance.The results showed that more Ni species in the catalytic system were beneficial for the enhancement of redox ability and the increase of surface acid amounts,which could lead to a better low-temperature SCR activity.The existence of Ti species in the catalytic system efficiently inhibited the formation of NiSO₄.Besides,more Ti species induced larger the specific surface area and better the dispersibility of the active species,thus better high-temperature SCR activity was achieved.When the Ni/Ti molar ratio was 4:1,the both advantages of Ni and Ti species were gave full play in the catalytic system,where more Ni³⁺species were induced via the excellent interaction between Ni and Ti species.Thus,the Ni₄Ti₁-LDO catalyst showed the ideal low/high-temperature SCR activity?NO_x conversion exceeds 90%in the temperature range of 240-360??.?3?The calcination temperature had a significant effect on the phase composition,structure and catalytic performance of the NiTi catalysts derived hydrotalcite-like.Tracking the thermal decomposition process of NiTi-LDH,it can be found that the catalysts calcined at 400? and 500? were composed of NiO and anatase TiO₂.As for the catalysts calcined at 600? and 700?,the rutile TiO₂ and NiTiO₃ were formed,which were unfavorable for NH₃-SCR reaction.As the calcination temperature increased,the specific surface area became much smallar,the surface acid amount became much less and the redox ability became worse,thus the SCR activity was gradually decreased.?4?NiCuTi-LDO and NiMnTi-LDO catalysts were prepared in situ by using NiCuTi-LDH and NiMnTi-LDH as precursors template,in which Cu or Mn played the role of synergistic functional components.The results showed that the in-situ doping of Cu and Mn effectively increased the reducible species contents,improved the redox capacity and the low-temperature SCR activity?NiCuTi-LDO catalyst displayed over 90% NO_x conversion within the temperature region of 180-330?;NiMnTi-LDO catalyst showed exceeds 90% NO_x conversion in the temperature range of 150-360??,but the N₂ selectivity and SO₂/H₂O resistance of the catalysts were reduced.