An Experimental And Theoretical Study On The Anti-poisoning Of The Titania-based SCR Catalyst

Nitric oxides are widely thought to be one of the main air pollutants all over the world. Its emission has contributed significantly to the severe air pollutions in China including acid rain, dust-haze and smog. Coal has maintained to be the major primary energy in our country. Combustion of giant amount of coal has lead to the massive emission of NOx. Based on analysis from another institute, coal-fired power plants have contributed35～40%to the overall NOx emission. Selective catalytic reduction of NO using NH3as reductant is the most mature and efficient technologies for NOx abatement. Catalyst is the essential factor of the SCR system. Nowadays, the design of a SCR catalyst is based on the trial-and-error experiments. There is no doubt that to be able to rationally design catalysts is one of the most attractive goals. This study has proposed a method based on theoretical DFT calculations to predict the reactivity of supported metal oxide for SCR of NO with NH3. Further, the commercial V-W-Ti oxide catalyst has accounted several issures during its industrial applications including the low activity at low temperatures and being easily-poisoned by alkali metals. This Ph.D. program has systematically studied the mechanism of alkali-poisoning on the V2O5-based catlayst and proposed an alkali-resistant catalyst. For the ceria oxide based catalyst which has better SCR reactivity at low temperature than vanadia-based catalyst, we have comprehensively studied its performance at the presence of SO2, H2O and alkali metals. The catalyst has been modified and optimized to adjust to the practical condition.The first part is focusing on developing a theoretical method to determine the SCR reactivity of a metal oxide. LUMO energy was used to evaluate the acidity of the oxide and exothermic energy of hydrogenation was used to evaluate the oxidability. For the reduced catalyst, HOMO energy was used to determine the reoxidability. Based on these discussion,22metal oxides were calculated and compared with the experimental results. We have separated these metal oxides into three categories which can possibly act as’active component’,’promoter’ and ’inactive component’ respectively. Experiments were performed to testify our theoretical conclusions. The comparison between experimental and theoretical results has lead to an excellent match, which has proven our method to be an effective way to evaluate the reactivity of supported metal oxide for SCR of NO with NH3.The second part focuses on the discussion of the poisoning effect of V2O5/TiO2by alkali metals and trying to develop a alkali-resistent catalyst. When doping with alkali oxide at the rate of K,Na/V=0.5, the SCR rate constant of the catalyst dropped by more than50%. Theoretcial simulation showed K/Na interaction increased the LUMO energy of the catalyst. Meanwhile, the ESP data around the V=O species turmed from negative to positive after doping with alkali metals, which mean the approaching of H cation to the V=O sites becomes harder and thus formation of the Bronsted acid (V-OH) becomes harder. The NH3-TPD experiment has comfirmed this conclusion. For another key property for SCR reaction, the oxidability has also been influenced by doping of alkali metal. The exothermic energy of hydrogenation of the V=O site decreased due to its interaction with K/Na. H2-TPR has confirmed this conclusion. Based on the discussion above, we have built a method to evaluate the poisoning effect of vanadia catalyst. ESP and hydrogenation energy in calculations and NH3-TPD and H2-TPR in experiments for acidity and oxidability respectively. Based on the poisoning mechanism, metal sulfate was doped into the V2O5/TiO2to resist alkali poisoning. This catalyst was found to better perform than commercial V-W-Ti catalyst at the doping of0.5wt%K.In the third part, coprecipitation method was used to prepare a Ce-Ti mixed oxide. This catalyst can achieve a NO conversion of98%at250℃and a GHSV of96,000ml/(g*h),which is much better than V-W-Ti catalyst. While, the SO2test has shown that the NO conversion dropped by30%after injection of1000ppm SO2for10hours at350℃. XPS patterns showed Ce oxide would turn into Ce2(SO4)3,which has much lower oxidability. Cu doping into the Ce-Ti oxide has greatly enhance its resistence to SO2. After interaction with SO2, there were still a great proportion of tetravalent Ce oxide in the catalyst. This indicated that SO2prefers to react with Cu site and Ce oxide can maintain at its original state. The SCR test showed that after10hours of SO2injection, the NO conversion of Ce-Cu-Ti oxide catalyst still maintained at more than98%.The fourth part mainly focused on the influence of water vapor on the Ce-Cu-Ti catalyst. An interesting phenomenon has been found that influence of H2O showed an temperature-depend profile. At temperature lower than300℃, the catalyst activity dropped because of H2O. While at higher temperature, H2O can promote the catalyst performance. Experiments and theoretical calculations show that H2O would compete with NH3to adsorb on the catalyst. This caused the inhibition of the catalyst performance. On the other hand, the presence of H2O can also surpress the NH3oxidation and thus more NH3can be used for NO reduction.The fifth part has studied the influence of alkali metals on the ceria oxide-based catalyst and tried to find a new alkali-resistent catalyst. K and Na atoms will interact with the oxygens of Ce-O-M (M is Ce or Ti). Thereafter, these oxygens were further ’fixed’and harder to be activated. Ce atoms became harder to be reduced and thus the oxidability of catalyst decreased. On the other hand, alkali metals will also neutralize the acidity of the catalyst, which will cause the decrease of NH3adsorption. To promote the alkali-resistence of the ceria catalyst, CuSO4was doped into the Ce-Ti oxide. The activity test showed the NO conversion of Ce-Ti catalyst dropped to40%after doping with lwt%K2O, while the CuSO4/Ce-Ti catalyst still possessed a NO conversion of89%at350℃. This CuSO4/Ce-Ti catalyst was further test in the presence of SO2and H2O. It is found that the NO conversion maintained at95%after injecting SO2and H2O in the gas for240min.