Study On Potassium And Water-sulfur Poisoning Resistance Of Ce-based SCR Catalysts

Selective catalytic reduction（SCR）is an effective and feasible technology for NO_xremoval in the flue gas at present.And the catalysts act as a leading role in the whole SCR system,which are directly related to the NO_x removal rate.Considering the excellent catalytic performance and application prospect of Ce-based SCR catalysts,they are very likely to replace commercial V-based catalysts in the industrial field.However,the toxic inhibition of alkali metals（e.g.K）,H₂O and SO₂ in the flue gas is a big challenge for the research and development of Ce-based catalysts.In this paper,various transition metal elements were used to modify the Ce-based catalysts and the toxic effects of K₂O,H₂O and SO₂ on the catalysts were studied.And the physical and chemical properties of the catalysts as well as the activity of the reaction species were explored using a series of characterizations（BET,XRD,XPS,H₂-TPR,NH₃-TPD and TG）.Furthermore,in situ DRIFT was used to study the reaction mechanism of various catalysts in order to explore the resistance mechanism of the modified catalysts.Firstly,CeO₂/TiO₂ and Ce O₂-Nb₂O₅/Ti O₂ catalysts were synthesized by a sol-gel method.It was found that the catalysts modified by Nb had a broader active temperature window（250-450℃）and more excellent K₂O resistance.The specific reasons were as follows:（1）Nb increased the specific surface area and total pore volume of the catalysts and partly relieved the effect of K₂O on the pore structure.The introduction of Nb also reduced the surface crystallization degree of the catalysts and eliminated the Ti O₂ lattice expansion caused by K₂O.（2）Nb doping inhibited the decrease of Ce³⁺and chemisorbed oxygen content by K₂O to a certain extent,which not only improved the redox performance of fresh CT,but also effectively alleviated the reduction of redox performance by K₂O.（3）Nb increased the number of Br（?）nsted acid sites on the surface of CT,which inhibited the decrease of NH₃ adsorption by K₂O.As for the reaction mechanism,K₂O poisoning did not change the reaction mechanism of CT and CNT catalysts,which was a combination of L-H and E-R mechanism.Based on the high K₂O resistance of Ce O₂-Nb₂O₅/Ti O₂ catalyst,CNT catalyst was modified by different additives（including WO₃,Zr O₂ and Mo O₃）to discover the catalyst with high K₂O as well as H₂O and SO₂ resistance.It was found that Ce O₂-Mo O₃-Nb₂O₅/Ti O₂ catalyst could still maintain high catalytic activity under the condition of K₂O and 10%H₂O+200 ppm SO₂.And the reasons were as follows:（1）Mo doping reduced the deposition of sulfate and ammonium sulfate on the surface of CNT catalyst,thus the catalysts still maintained a large specific surface area.（2）Mo protected the active center of the catalysts and inhibited the over-sulfation of active component Ce,which could reduce the generation of excessive metal sulfate,thus the catalyst still maintained excellent oxidation reduction and surface acidity.In terms of reaction mechanism,the introduction of K₂O,H₂O and SO₂ changed the reaction path on CNT catalyst surface,which was E-R mechanism.However,the SCR reaction of Mo modified catalyst was still carried out by the combination of E-R and L-H mechanisms after compound poisoning of K₂O,H₂O and SO₂.