Study On Cobalt And Cerium Modified Mn/TiO₂ Catalysts For Low Temperature SCR Of Nitrogen Oxides With NH₃

Nitrogen oxides（NOx）in the exhaust are toxic to human’s health and have brought environmental problems.The selective catalytic reduction（SCR）of NO x with ammonia is the dominant technology to abate NO x emitted from stationary sources.Low-temperature SCR catalysts were developed to work at the tail en d of the desulfurization and dust removal process.Supported manganese oxides have been proved to possess excellent activity in low-temperature SCR reaction.However,the drawbacks,such as the narrow reaction temperature window and poor SO₂-poisoning resistance,have limited industrial application for the catalysts.In this paper,to solve the above problems,a supported multi-metallic catalyst was developed by co-impregnation method,with Mn oxides as the main active component,Co and Ce oxides as added components and TiO₂ as support.The researches on the physical and chemical properties,low-temperature SCR activities,reaction mechanism and SO₂ resistance of the catalysts were carried out detailedly.A series of Mn-Co/TiO₂ catalysts were prepared by wet co-impregnation method.The influence of the active metal contents,support materials and calcination temperature on the physical-chemical properties and low-temperature SCR activities of the catalysts were evaluated.The changes of surface area,pore structure,crystalline phase,valence states and oxidizing ability by Co addition were studied.The activity test results showed that the optimized preparation conditions are: the Mn content is 10 wt.%,the Co content is 5 wt.%,calcined at 500 ℃.The characterization results indicated that a new electron transport system was formed between Co Ox and Mn Ox.The formation of Co3O4.Co Mn O3 mixed oxides is responsible for enhanced activities towards NO oxidation to NO 2,which is beneficial to low-temperature SCR reaction.The addition of Co enhanced Mn4+ formation and increased chemical adsorbed oxygen amounts,which also accelerated the NOx reduction.The third active metal component Ce was added into the catalyst to further improve catalytic activity.A series of Mn-Co-Ce/TiO₂ termetallic catalysts were prepared by co-impregnation method.The activity test results indicated that the Ce addition further improved catalytic activity and broadened the reaction temperature window toward high temperatures.The Mn-Co-Ce/TiO₂ catalyst,with 10 wt.% Mn,5 wt.% Co and 5 wt.% Ce and calcined at 500 ℃,exhibited the highest NOx conversion of 99% in the temperature range of 160¹80℃.The characterization results indicated that the metal cations existed mainly in the form of Mn4+,Co2+ and Ce4+,respectively.Addition of Co and Ce led to higher surface Mn4+ and Mn3+ atom ratios and chemisorbed oxygen ratio,which were beneficial to SCR reaction.Both Co and Ce addition increased the acid sites on surface of the catalyst,which enhanced NH3 adsorption.The addition of Ce increased the NO adsorbed amounts.Addition of Ce could enhance dispersion of Mn-Co oxides on surface of the catalysts.These changes were responsible for the improvement of SCR activity of the Mn-Co-Ce/TiO₂ catalysts.The adsorbed species of NH3,NO+O2 and co-adsorption of NH3+NO+O2 over the catalysts at different temperatures were investigated by in-situ FTIR spectroscopy,to study the affecting mechanism of Co and Ce on SCR reaction.The results suggested that addition of cobalt and cerium increased the numbers of acid sites.Especially,the cobalt oxide produced lots of Br?nsted acid sites,which favor to the migration of NH4+ to neighbor Lewis acid sites to form actively coordinated NH3 and then accelerated low-temperature SCR reaction.The active NH2 species were formed by Co and Ce addition.A low-temperature SCR mechanistic pathway over the catalyst Mn-Co-Ce/TiO₂ was proposed.At low-temperature,coordinated NH3 reacted with NO2-,and NH2,NH2-reacted with NO（ad）or NO（g）to form N2.It was found that NO2-species formed on Co sites improved the catalytic activity at low temperatures,and NH2-species were generated on Ce sites,which improved the N2 selectivity.The SO₂-poisoning resistances of the Mn-Co-Ce/TiO₂ catalysts were evaluated.The results demonstrated improvement of SO₂ resistance ability by Co and Ce addition.The low-temperature SO₂ poisoning was mainly due to the deposition of（NH4）2SO4 and NH4HSO4 on surface of the catalyst.The activity could be recovered by thermal treatment.In addition,the Mn-Co-Ce/TiO₂ catalyst was modified with H2SO4 solution.The low-temperature SO₂-poisoning resistance of the catalyst was improved by modification with 0.1 mol/L SO 42-and calcined at 400℃.The characterization results indicated that the Cen+–O–Sn+ and Con+–O–Sn+ sulfates trended to form at low concentration of SO42-and calcined at 400℃,resulting in the increase of Co and Ce atom ratios and improved acidity of the catalysts.The formation of the sulfates improved the mobility of lattice oxygen and restrained the SO₂ oxidation,which caused better SO₂-poisoning resistance.The results indicated that in the multimetallic catalytic system of Mn-Co-Ce/Ti,the better SO₂ resistance could be realized by controlling the concentration of SO42-and applying a suitable method to avoid formation ofMn（SO4）x.However,a high concentration of SO42-deposited on surface of the catalyst would decrease the SCR activity.