Modulation Of The V⁴⁺/V⁵⁺ Ratio With SCR DeNO_x Activity And Properties Of Catalyst

NO_xis the unignorable air pollution sopurse. Selective catalytic reduction technology isthe most promising method for removing the NO_xemission. The vanadium-based catalysts arecommonly used in DeNO_xprocess at present. Research discoveried that V⁴⁺/V⁵⁺ratio andcatalytic activity maybe have a certain correlation, this thesis put emphasis on adjusting theloading of the metal additives to optimize the V⁴⁺/V⁵⁺ratio of DeNO_xcatalyst and strengthendistribution of atoms’ valence and surface acidity-basicity in catalysts, thereby improvingDeNO_x performance at low temperature and widening the window of temperature.Using the TiO₂as the support, V-W/TiO₂, a single transition oxide and the compositeoxide catalysts were prepared by impregnation. The study explored regularity of V⁴⁺/V⁵⁺ratioof the transition metals Mn, Cu, Ni, Zn and rare-earth element Ce modulation catalyst by XPSmethod, and combined the DeNO_xactivity of Urea-SCR to discuss the relationship betweenV⁴⁺/V⁵⁺ratio and SCR DeNO_xactivity. Catalysts were characterized by using severaltechniques, incluing NH3-TPD, BET-surface area, XRD, H2-TPR, SEM-EDS, and then theresults showed the optimal load: Mn/Ti=0.4, Cu/Ti=0.3, Ni/Ti=0.4，Zn/Ti=0.3.For V-W/TiO₂catalyst, V⁴⁺/V⁵⁺radio was small. After loaded transition metal, theV⁴⁺/V⁵⁺ratio had increased to some extent, and it had shown the trend of first increased andthen decreased. From the ratio of the size and magnitude of change, Mn had been notconductive to modulation of V⁴⁺/V⁵⁺ratio than Cu, Ni, Zn. The V⁴⁺/V⁵⁺radio of each seriescatalysts was the maximum when the corresponding optimal activity of this series was best.However, it was clear that the size of V⁴⁺/V⁵⁺ratio was an important factor that influencingon DeNO_xactivity level on a single transition metal series supported. For V-W-Ti supportedcatalysts, the electrically negative was larger than V, which contained Cu，Ni and Zn hadlower low-temperature activity，while Mn with smaller electrically negative had high activity.It is evolved that the different electron attractive ability of V⁵⁺/V⁴⁺and Mn+plays animportant role in the formation of V⁴⁺. The more Mn+-O-V⁴⁺specie exists，the better theactivity be. For the composite element catalyst, V2p binding energy was observed to move toa lower energy direction, which was due to the larger electronegative of V⁵⁺, it madeelectronic enrichment of V atoms around stronger, and leaded to an increase of electron clouddensity, reducing the sake of binding energy of different valence states of the V. Cu was more conductive to the modulation of V different valence than Mn and Ce elements for thecomposite element catalyst.Three acid sites existed on the surface of Mn serie catalysts, and they had betterlow-temperature activity. In particular, the V⁴⁺/V⁵⁺radio had amplified by loaded Mn and Cusimultaneously, and broadened the temperature scope of DeNO_xactivity.They exhibited theoptimal NO removal efficiency over80%at230-430°C. But Cu, Ni series, Zn-based catalystsurface only existed weak and strong acid sites. V⁴⁺/V⁵⁺ratio were larger and had much largermagnitude of the changes. However, the activity was lower from150to400°C. Consequently,the experiment confirmed that L acid site had a greater contribution to the low-temperatureactivity. The order of factors affecting SCR activity: element catalytic performance＞surfaceacidity and acid category＞V⁴⁺/V⁵⁺ratio＞redox property of metal oxide＞＞BET surfacearea.Ce significantly changed the V⁴⁺/V⁵⁺ratio, and made it show an increasing trend.However, the activity level and the V⁴⁺/V⁵⁺ratio wasn’t completely consistent. The more theloading of Ce was, the more non-stoichmetry vanadium of the catalyst produced, but theamount of active center V⁵⁺=O was smaller, thereby decreasing the catalytic activity.Compared with the Mn-Cu catalyst series, Mn mainly existed to the high valence Mn4+andMn3+, thus Ce promoted surface capability of the active components supported TiO₂. So thatthe activity which Ce/Ti was0.07had increased compared to Mn-Cu series catalysts.