A Mechanistic Study On The Rational Design Of SCR Catalyst With Wide Operation Temperature Window

With the rapid development of social economy,the energy consumption increased year by year.In some key areas like Beijing-Tianjin-Hebei,Yangtze delta and Pearl River delta,the energy consumption intensity was much higher than that of the average level in China,which thereby resulted in a serious environmental pollution problems.Nitrogen oxides?NOx?are typical air pollutants,the emission of which could lead to acid rain and photochemical smog and negatively affect the environment and human health.The selective catalytic reduction?SCR?is regarded as the most effective technology for NOx removal,in which the catalyst is the key issue.However the adaptation of load fluctuation was the one of the major challenges during the industrialization of this technology.In China,the load of the boiler always changed.In coal-fired power plant,the traditional V2O5,WO3/TiO2 catalysts exhibit a low activity during the period of low-load operation,resulting in the excess NOx emissions,ammonia slip and even the failure of the environmental protection equipment.Traditional way to solve this problem was to reform the economizer,which cost much and led to the decline of boiler efficiency.We are trying to solve this challenge in this thesis.A strategy to design the SCR catalyst with wide operation window was proposed.We systematically investigated the relationship between the structure,physicochemical properties and the catalytic performances,the reaction mechanism,SO₂-tolerance and the decomposition of ammonia bisulfate.Finally,we obtained the catalyst with a wide operation temperature which was stable during the long run in practical conditions at low temperatures.The main contents are as follows.1.The structure-activity relationship.Based on the SCR reaction mechanism,we design binary oxide with the combination of cerium oxide with good redox properties and niobium oxides with strong acid properties.We synthesized series Ce-Nb mixed oxide catalysts through the co-precipitation method.When the molar ratio of Ce/Nb reaches 1/1,the catalyst exhibited over 80%NO conversion between 200? and 450?.We established the relationship between the structure,physicochemical properties and the catalytic performances.The short-range activation effect of doping niobium species to cerium oxide species increased the amount of surface unsaturated oxygen species,which was found for the first time to be critical for the catalytic activity at low temperatures.2.The reactive species and reaction pathway.We found that the reaction could proceed via both Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism.In L-H mechanism,NO₂ and monodentate nitrate species were found to be reactive while bridging and bidentate nitrate species were inert.In E-R mechanism,Lewis acid sites play an important role.We quantitatively calculated the contribution of L-H mechanism and E-R mechanism to the reaction and found that the L-H mechanism was more important for low-temperature activity while E-R mechanism responsible for high-temperature activity.Based on the results obtained,we proposed a theory for the rational design of SCR catalyst with wide temperature window and designed and tested a series of catalysts.The activities of these catalysts at 250? were obviously higher than that of commercial catalyst.At 400? all the catalysts showed activity higher than 97%.These results confirmed the universality of this theory.3.The SO₂-tolerance.It is found that Ce-Nb catalyst was easily affected by SO₂.The addition of 200 ppm SO₂ led to a serious deactivation of Ce-Nb catalyst;the catalytic activity decreased by 40%after 20 h.The addition of iron oxide could effectively improve Ce-Nb catalyst SO₂-resistance.The catalytic activity remained around 92%after 20 h.Characterization results illustrate that the as-formed sulfate species on the Fe-containing catalyst surfaces showed a lower stability.The contribution of L-H mechanism over sulfated Ce-Nb-Fe catalyst remains as high as 32%,much higher than that over sulfated Ce-Nb catalyst?6%?.These results indicate that the addition of iron oxide could effectively guarantee the L-H mechanism,which is confirmed to be the key factor in the low-temperature activity.4.The decomposition of ammonia bisulfate.Faced with the NH₄HSO₄ deposition problems at low temperatures,we synthesized TiSi mixed support together with Ti support,and studied the NH₄HSO₄ decomposition behavior on the TiSi mixed support surface.N and S species exhibited as NH₄⁺ and bidentate SO₄^2-on the catalyst surface,respectively.During the heating process,the consumption of NH₄⁺ occurred prior to that of SO₄^2-.The onset temperature for the NH₄⁺ consumption behavior on TiSi mixed support was much lower than that on Ti support,while the temperature for the release of SO₂ was higher.Based on these results,we designed a series of mixed support,on which the decomposition behavior of NH₄HSO₄ was obviously promoted.Based on the results in this thesis,we successfully designed the catalyst with wide operation temperature window and utilized it in the 300 MW unit in Wenzhou Power Plant.The NOx emissions at 35%load and 100%load were both less than 50 mg/m³.