Relationship Between The Denitration Activity And The V^4+?3+?/V⁵⁺ Ratio Of V₂O₅?MO_x?/TiO₂ Catalyst

Nitrogen oxide is greatly harmful to environment. It causes acid rain, photochemical smog and ozone hole etc. environmental issues. Selective Catalytic Reduction ?SCR?technology is the primary way to control nitrogen oxides emission from stationary sources.The key of selective catalytic reduction technology is a high performance catalyst,the commercial application catalyst has been suitable for stationary sources. But for mobile sources,the low temperature and high efficiency catalyst doesn't have a mature product, so this kind of catalyst required to further develop. Studying about the low temperature SCR catalyst, it has been found that the aggregation state of the catalytic active component has an effect on the V⁴⁺/V⁵⁺ ratio, which can result in the variation of catalyst surface acid sites types and content leading to NH₃-SCR denitration performance changes.By changing the oxygen content in the calcining process, the V₂O₅/TiO₂ catalyst is prepared by the impregnation method, the experimental results demonstrate that when the oxygen content reached 15%, the catalyst has a good denitration performance in 300?450?temperature range. Studying on the influence of oxygen content in the calcining process on additives WO₃ and CeO₂, the experimental results show that V₂O₅-WO₃/TiO₂ catalyst and V₂O₅-CeO₂/TiO₂ catalyst have a best denitration efficiency when the oxygen content is 15%.Meanwhile, WO₃ broadens the denitration reaction temperature window ?250?450??, but it has been little affected by the oxygen content in the calcining process. The additives CeO₂ is good for increasing the catalyst denitration efficiency in a low temperature range, and V₂O₅-CeO₂/TiO₂ catalyst has a stable denitration efficiency at high temperature ?500??.It is strongly influenced by the oxygen content in the calcining process. Additionally, XPS results show that the V⁴⁺/V⁵⁺ ratio of V₂O₅/TiO₂ catalysts,V₂O₅-WO₃/TiO₂ catalysts and V₂O₅-CeO₂/TiO₂ catalysts close to 1 when the catalyst has a good denitration performance,and the ratio of Br???onsted acid content and Lewis acid content and the hydrogen consumption are bigger than other catalysts.Active component V of catalyst calcined without oxygen has a strong interaction with the carrier TiO₂ vanadium and producing the VOx, so that the vanadium species on catalyst surface is mainly in the form of isolated state. Because of no oxygen in the calcining process,vanadium species are mainly existed in the form of low-state, the low ratio of Br???nsted acid content and Lewis acid content is not conducive to the denitration reaction. In contrast, the V₂O₅ /TiO₂ catalyst calcined with oxygen has an interaction with adsorbed oxygen to form the aggregate state of vanadium species. With the increases amount of oxygen content, the high vanadium content is increased so that the catalyst surface V⁴⁺/V⁵⁺ ratio decreased, the ratio of Br???nsted acid and Lewis acid and the denitration activity increased. When the oxygen content in the calcining process is up to 20%, more isolated vanadium species are generated causing the V⁴⁺/V⁵⁺ ratio decreases, reducing the catalyst surface acid content and the ratio of Br???nsted acid content and Lewis acid contents, resulting in a decreasing denitration activity.After three hours of denitration, V₂O₅ /TiO₂ catalysts' denitration efficiency stabilized.Compared to the changes in physical and chemical properties of V₂O₅ /TiO₂ catalyst before and after the denitration, XRD and BET test results demonstrated that the specific surface area of V₂O₅/TiO₂ catalysts decreases after denitration three hours, but no crystal phase formation.XPS results show catalyst denitrated at 400? in 3 hours makes the active component V volatilize, and reducing the active site of the catalyst. In addition, the V⁴⁺/V⁵⁺ ratio unchanged before and after the denitration reaction occurring, which means that it is an effective way to control the V⁴⁺/V⁵⁺ ratio show through changing the oxygen content in the calcining process in order to improve the SCR denitration reaction efficiency. NH₃ temperature-programmed desorption test and pyridine adsorption infrared test results also show that, due to the reduced surface active component of the catalyst leads to reduced catalyst surface Br???nsted acid sites and Lewis acid sites content, which in turn lead to the denitration efficiency decrease.