Study On The Influence Of Ni Element On The Mechanism Of N₂O Formation Over Pt-TiO₂ Catalysts

Nitrous oxide is well known as "laugh gas".It is also a greenhouse gas which can indirectly destroy the ozone layer,and its single molecule warming potential is 298 times higher than that of CO2.Over 80%of N2O in the atmosphere comes from the nitrification and denitrification of microorganisms and the emission of industry exhaust,however such N2O is difficult to be collected for further use due to the low concentration.But if high concentration N2O can be prepared,after purification and collection,it will have wide potential applications in many fields such as military industry,medicine,food,and chemical industry.The traditional methods for N2O industrial production include ammonium nitrate thermal decomposition,ammonia contact oxidation method,nitrate-concentrated hydrochloric acid-urea method,and NO reduction method,but all the above methods have deficiencies.For the ammonium nitrate thermal decomposition method,it is prone to explosion at high temperature;while the conversion of the ammonia oxidation method is low;the nitrate-concentrated sulfuric acid-urea method has great potential to environmental pollution;and the NO reduction method cost highly for production.Since there is no efficient,safe,cheap and environmental friendly N2O preparation method,inspired by the formation of by-product N2O in the SCR denitration reaction,our group found that N2O can be prepared through catalytic reaction of NH3,NO and O2 over Pt-CeO2 and Pt-TiO2 catalysts,here in this thesis,we further explored the optimization of the catalytic performance of N2O yield over Pt-TiO2 catalyst with the addition of Ni element.Moreover,the effects of the morphology of TiO2 support,the loading of Ni element,the atomic ratio of Pt-Ni,the composition and ratio of reaction gases,and the reaction temperatures on the N2O yield were studied.The catalysts were characterized and analyzed by XRD,TEM,XPS,TPR,TPD and other methods,combined with catalytic performance test,the formation mechanism of N2O and the influences of Ni element were discussed.The main contents of this thesis are as follows:1.Three anatase TiO2 samples with different morphologies were prepared to load Pt and Ni elements.All the three samples were able to catalyze NH3 oxidation and generate large amount of N2O.The generation of N2O was attributed to the reaction between NO and�NH or�N species.Such species came from the deep dehydrogenation of NH3,while NO resulted from ammonia deep oxidation.The yield of N2O showed a volcanic curve versus temperature,while increasing the amount of NO in the reaction gas can improve the output of N2O and reduce the optimal reaction temperature at the same time.For O2,there exists an optimal concentration for N2O production,and either the O2 concentration is too high or too low will cause a significant decrease in N2O yield.It is found that with Nz as a carrier gas,the best experimental parameters for N2O preparation are that the total gas flux was 616 sccm(NH3:1791 ppm,NO:446 ppm,O2 concentration:3%)and the nominal loadings of both Pt and Ni were 1%.At about 250?,85%N2O yield can be maintained.2.The effects of the loading ratios of Pt and Ni on TiO2 supports on the N2O yield were investigated.Without deterioration of N2O yield,the addition of Ni element to Pt/TiO2 catalyst can reduce the loading of Pt from 2%to 1%.The addition of Ni element introduced Ni element and Ni oxide.The hydrogen spillover phenomenon between Pt and Ni/Ni oxide enhanced the ability to activate NH3,resulting in more�NH and�N species,which is beneficial for N2O formation at low temperature.When there was excess loading of Ni element,more�N intermediates could be produced,which promoted the selectivity of NO and Nz and prohibited the formation of N2O.With N2 as the carrier gas,when the total gas flux was 616 sccm(NH3:1791 ppm,NO:134 ppm,and O2 concentration:3%),and the nominal loadings of both Pt and Ni were 1%,the yield of N2O at 275? is the highest,which reaches 87%.