Influences On Modulation Of The V⁴⁺/V⁵⁺ Ratio And Rate Of V⁴⁺ And V⁵⁺ Redox And SCR DeNO_x Activity

NO_x which is released from all kinds of vehicle tail gasis and the fuel combustion in factories is one of the major sources of pollution in the atmosphere. A lot of NOx in the atmosphere has caused a series of environmental problems and have much bad effect on human health. At present, selective catalytic reduction （SCR） is the most promising denitration technology and catalysts are the core of the SCR denitration process. In recent years,many reserchers have come to a discover that a correlation was identified between the ratio of V⁴⁺/V⁵⁺ and the catalyst NOx conversion rate. This paper aims to raserch the correlation between the ratio of V⁴⁺/V⁵⁺ and the catalyst NOx conversion rate through theV/TiO₂ catalysts by the impregnation method which were disposed by adding to Mn、Ce and W ,and aging vanadium precursor solution for V（13wt%）/TiO₂. It comes to conclusion that the best atomic ratios of V and Mn、V and Ce、V and W are 1.48、0.75、0.5 and the optimal aging time is10h.Meanwhile, this paper also researched the ratio of V4/V⁵⁺ played the role in the SCR mechanism and transition metal element W produced the modulation function for vanadium. We can make the catalysts which have higher activity and a wider temperature window by researching those questions.For the prepared catalysts which were disposed by adding to Mn and W ,and aging vanadium precursor solution for V（13wt%）/TiO₂, we discovered that the ratio of V⁴⁺/V⁵⁺showed a trend of decrease after the first increase by XPS.For V-Ce/TiO₂ catalysts, the ratio of V⁴⁺/V⁵⁺ gradually reduce. And from the perspective of numerical t variation, transition metal elements Mn、Ce、W are more advantageous to form the larger ratio of V⁴⁺/V⁵⁺ On the surface of the disposed V/TiO₂ catalyst ,especially Ce is best. Combined with SCR denitration, we can see that the catalyst has the largest ratio of V⁴⁺/V⁵⁺ corresponding to the best active in the same series of catalysts. So we can reach a conclusion that the ratio of V⁴⁺/V⁵⁺ plays an important role in the SCR denitration activity in the case of having almost equal number of V atom and similar acid sites.The V/TiO₂ catalysts and the V（13wt%）/TiO₂ catalysts of different aging time were characterized by XPS with V2p and Ols. It is concluded that V⁴⁺ and V⁵⁺、chemical adsorption oxygen Oα and lattice oxygen Op co-exist on the surface of the catalysts. Chemical adsorption oxygen Oα had higher catalytic active and an important effect on the SCR active.According to the ratio of Oα/ Oβ, we can markedly see that the ratio of Oα/Oβshowed a trend of decrease after the first increase analogous to the ratio of V⁴⁺/V⁵⁺. It is demonstrated that the increased amount of V⁴⁺ could also cause chemical adsorption increased oxygen Oα in the surface of the catalyst.The above prepared catalysts were charactersized by other physio-chemical tests, such as X-ray diffraction （XRD）, BET surface area, NH3 temperature desorption （NH3-TPD）, H2 temperature programmed reduction （H2-TPR）. The results showed that the biggest ratio of V⁴⁺/ V⁵⁺ in the same series had better performance.This paper has also researched why the big ratio of V⁴⁺/V⁵⁺ can cause higher catalytic active for the V-W catalysts through the XPS、H2-TPR and O₂-TPO tests. The results showed that the reduction rate of the process of V⁵⁺=O to V⁴⁺-O-H is quicker than the oxidation rate of V⁴⁺-O-H to V⁵⁺=O in the process of catalytic mechanism which . Topsoe put forward. So oxidation process of V⁴⁺-O-H to V⁵⁺=O became the speed step. When the catalysts had well-balanced V⁴⁺/V⁵⁺,it had higher catalytic active. Because abundant numbers of V⁴⁺facilitate the oxidation rate V⁴⁺-O-H to V⁵⁺=O,the active was greatly improved in the process of catalytic mechanism Topsoe put forward. Meanwhile, the V-W catalysts were characterized by Raman tests. The result showed that more and more V-O-W keys exited on the surface of the catalyst with the increase of the amount of tungsten. The transition elements W played an important role in the process of modulation for V through V-O-W keys.