Study Of V₂O₅/TiO₂ SCR Catalysts Deactivation By Potassium And Adsorption Of Ammonia On CuO/Al₂O₃ Catalysts

Nitrogen oxide?NO_x?is one of the major pollutants in air.Selective catalytic reduction?SCR?of NO_x with ammonia is an efficient method to reduce NO_x emission from flue gas of stationary sources.V₂O₅/TiO₂ based catalysts have been widely used due to their high activity and selectivity.The catalysts are usually installed at the upstream of dust precipitators.This makes the catalysts prone to deactivation by fly ash,especially the alkali metal-induced deactivation.It has received much attention in recent years.Many researches have been done using different poisoning methods to understand the deactivation behavior of catalyst in industry,but the results were somewhat inconsistent.In addition,although the V₂O₅/TiO₂ catalyst has been widely used,there are also some shortages with it.For instance,its active component V is toxic,so the spent industrial V₂O₅/TiO₂ catalyst might cause a secondary pollution.In the long run,the environmental friendly catalysts are necessary.CuO/Al₂O₃ catalyst is one of the potential SCR catalysts,which is concerned by researchers.However,the SCR mechanism of CuO/Al₂O₃ catalyst needs a deep understanding in molecular level.This work compares the poisoning methods-wet impregnation,solid diffusion and vapor deposition from the viewpoint of deactivation rate and deactivation mechanism using V₂O₅-WO3/TiO₂ as the catalyst and KC1 as the poisoning substance.In addition,the K-poisoning mechanism of V₂O₅/TiO₂ catalyst and the adsorption of NH₃ and SO₂ on CuO surface were studied by density functional theory.The influences of SO₂ in NH₃ adsorption on Cu-based catalyst were also investigated.The main results as follows:1.For V₂O₅ catalyst,results indicate that deactivation extent of the catalyst depends not only on the quantity of KCl but also on how KCl is introduced to the catalyst,following an order of vapor deposition>>solid diffusion>wet impregnation.The presence of V2O3 species due to interaction of K with oxygen in vanadium oxides is mainly responsible for the catalyst deactivation by wet impregnation.The formation of K₂S₂O₇-V₂O₅ eutectic is mainly responsible for the catalyst deactivation by solid diffusion at a higher K/V ratio and vapor deposition due to the significant decrease of NH₃ adsorption and increase in NH₃ oxidation activity.2.The adsorption of KC1 on V₂O₅/Ti02?001?surface is most stable in the form of K+.K+ adsorption will cause reduction of the surface acidity.And K+will be preferentially adsorbed on the surface acid site,which caused NH₃ adsorption decreased and NH₃ over-oxidation.These lead to a low catalytic activity in SCR process.3.For copper-based catalysts,NH₃ are adsorbed on the surface Lewis acid site CuT in molecular form.When the coverage is low,SO₂ promotes the adsorption of NH₃.And the adsorption heat decreases significantly with the increase of the coverage.The reason for the competitive adsorption of NH₃ and SO₂ is the competition for adsorption sites.The formation of CuSO4 can promote the adsorption of NH₃ on catalyst surface.