Prepartion And Experimental Study On The Ce-Doped Ti-Zr Solid Solution Catalyst For Selective Catalytic Reduction Of NO_x

The wide temperature selective catalytic reduction of NO_x with NH₃ has obvious industrial value in the field of thermal power, and rare earth doped catalysts attract much attention because of the better activity in medium and low temperature. In this paper, V2O5/TiO₂ was modified by ceria and zirconium dioxide, the physicochemical properties of the catalysts were characterized by HRTEM, BET, XRD, XRF, NH₃-TPD, H2-TPR, DRIFTS. At the same time, the mechanism of reaction and SO₂ and H₂O poisoning were further researched and discussed. The result suggested that ZrTiO4 composite oxide was amorphous and displayed higher specific surface area. CeO₂ could significantly increase the reduction ability and promote the reduction peaks move to lower temperature for catalysts, NH₃-PTD exhibited TiO₂-ZrO₂ and V2O5/TiO₂ existed medium-to-strong and strong acid desorption, whereas Ce-doped catalysts featured weak acid sites for the desorption of NH₃. A small amount of V2O5 loading made the catalysts keep good activity in the high temperature and V-0.2Ce/Ti-Zr catalyst exhibited the highest activity. The presence of CeO₂ could increase the amount of both the Bronsted and Lewis acid sites, and generate the active substances of-NH2, which was benefit to the NH₃-SCR reaction.The catalytic performance of V-0.2Ce/Ti-Zr found that NO₂ was more easily to be reduced, the presence of O₂ promoted the oxidation of NO, however, the adsorption peaks of NO_x on the catalyst were enhanced, the formation of nitrate and nitro species were difficult to desorption. NH₃ showed better competitive adsorption capacity than NO. If NO+O₂ was pre-adsorbed, the activity of catalyst continued to decline. On the contrary, after pre-adsorption of NH₃, the activity would gradually restore to its original level, presumably nitrate and nitro species occupied the active position, the NH₃ adsorption was weak, which would greatly hinder the SCR reaction. All the conclusions suggested that the catalyst mainly followed the E-R reaction, the adsorption of NH₃ was vital for the whole reaction. Therefore, the pre-adsorption of NH₃ could hinder the NO_x adsorption, prevent the formation of nitrate and nitro species, and avoid the decrease of catalyst activity.The presences of SO₂ and H₂O had great influence on the catalyst activity. When the concentration of SO₂ was low, V-0.2Ce/Ti-Zr catalyst was relatively stable, the NO_x conversion did not change, SO₂ content was higher, the catalyst activity first desreased and then increased. If SO₂ and H₂O both existed, the catalyst activity decreased significantly, and couldn’t return to original level. The NO_x conversion at 200℃-450 ℃ over SO₂-and H₂O-poisoned catalysts was examined for V-0.2Ce/Ti-Zr and V-0.3Ce/Ti-Zr, the activity of catalysts increased within a temperature range of 300℃-450 ℃, but the NO_x conversion was decreased at low temperature. XRD result showed that the formation of Ce（SO4）2 adversely affected reduction capacity of CeO₂, V2O5 acted as the active component after 300 ℃, the impact on the medium and low temperature activity was smaller if CeO₂ content was rich. BET results found that the pore diameter distribution moved to mesoporous and macroporous range when catalysts were poisoned, leading to the decrease of specific surface area of catalyst, reducing the NH₃ adsorption capacity, catalysts showed bad performance. Compared with fresh catalyst, DRIFTS result revealed NH₃ adsorption ability was weak for poisoned catalysts. With increased of SO₂ content, NH₃ adsorption of poisoned catalysts enhanced, the pre-adsorption of SO₂+O₂ on fresh catalyst found NH₃ adsorption ability greatly enhanced, supposing SO₂ promoted the conversion of Ce4+to Ce3+, leading to an increase of oxygen vacancy. NO_x was easier to adsorb on the surface of catalysts when SO₂ existed, however, the DRIFTS spectra of NH₃+NO+O₂+SO₂ showed NH₃ adsorption ability was still stronger than NO_x if the catalyst was pretreated by SO₂+O₂. Macro transient experiment also found that the NO_x conversion had a little difference between the catalysts were pretreated by SO₂+O₂ and NH₃. Steady-state infrared microscopic study found that the SO₂ adsorption ability was weak at the room temperature （25 ℃）, When SO₂ concentration was high, Ce3+appeared, then NH₃ adsorption became strong with the increase of temperature, which could explain the reason that NO_x conversion rate of 600ppm SO₂ poisoning was better than 400ppm SO₂. All the conclusions found the pretreatment by SO₂+O₂ would improve the ability to resist SO₂ poisoning for catalysts. If H₂O existed, a part of CeO₂ would be sulfated and the formation of （NH4）2SO4 would block the catalyst, leading to the decrease of the denitration catalyst efficiency.