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天然气储量巨大,被广泛应用于发电和工业窑炉等.甲烷作为天然气中最主要的成分,是氢碳比最高的碳氢化合物,其温室效应显著.因此,不完全燃烧所引起的CH_4排放,不仅导致能源浪费,同时也可造成环境污染.与传统火焰燃烧相比,CH_4催化燃烧具有更高的燃烧效率,并可显著地减少大气污染物(CO,NOx和未完全燃烧的烃类)的排放.贵金属Pd催化剂对CH_4催化燃烧表现出优异的催化性能,其中Pd颗粒的尺寸、Pd的化学状态、载体性质及其与Pd之间的相互作用等对其活性有显著影响.本文以不同温度(600,800,1000和1200 oC)焙烧所得SnO_2为载体,通过等体积浸渍法制备了Pd/SnO_2催化剂,研究了SnO_2焙烧温度对CH_4催化燃烧性能的影响.结果表明,所制备的SnO_2均为锐钛矿结构,并且随着SnO_2焙烧温度的升高,晶型愈加完美,晶粒尺寸显著增大.催化剂中引入的Pd以高分散形式存在,CH_4催化燃烧反应活性随着载体SnO_2焙烧温度的升高而显著提高,其中Pd/SnO_2(1200)表现出最高的CH_4燃烧活性,起燃温度和最低全转化温度分别为265和390 oC.在反应温度为300 oC时,Pd/SnO_2(1200)上甲烷的反应速率是Pd/SnO_2(600)的36倍.XPS等结果表明,随着SnO_2焙烧温度的升高,Pd的化学状态也有所差异:对于低温焙烧的SnO_2(<800 oC),Pd以Pd4+的形式进入到SnO_2晶格内;随着焙烧温度的升高(>1000 oC),Pd以Pd2+物种的形式存在于载体表面.结合活性评价结果推测,Pd的化学状态可能并非是影响催化剂活性的最关键因素.TEM等结果表明,Pd/SnO_2(1000)上Pd O的(101)晶面与载体SnO_2的(101)晶面相近,分别为0.2641 nm和0.2638 nm.O_2-TPD和CH_4-TPR结果表明,Pd/SnO_2(1200)催化剂上单位Pd原子上O_2的脱附量是Pd/SnO_2(600)的3倍,单位Pd原子上CH_4的消耗量比催化剂Pd/SnO_2(600)高出45%.因此,Pd O和SnO_2在构型上存在的晶面匹配可提高催化剂对O_2的活化能力.综上所述,SnO_2和贵金属之间的晶格匹配有利于氧在Pd-SnO_2界面的活化,同时载体SnO_2中的晶格氧亦可以通过“氧反溢流机理”补充到表面Pd O/Pd上,从而增强催化剂对O_2的吸附和活化能力,并提高CH_4催化燃烧反应性能.升高SnO_2的焙烧温度可强化SnO_2和贵金属之间的晶格匹配,从而使催化剂活性随着SnO_2焙烧温度升高而增大.
Natural gas reserves are huge and are widely used in power generation and industrial kilns, etc. Methane, as the most important component of natural gas, is the hydrocarbon with the highest ratio of hydrogen to carbon and has obvious greenhouse effect, therefore CH4 emission caused by incomplete combustion, Not only leads to energy waste but also causes environmental pollution.Compared with the traditional flame combustion, CH 4 catalytic combustion has higher combustion efficiency and can significantly reduce the atmospheric pollutants (CO, NOx and incomplete combustion of hydrocarbons) Emission.The noble metal Pd catalyst showed excellent catalytic performance for CH 4 catalytic combustion, in which the Pd particle size, the chemical state of Pd, the nature of the support and its interaction with Pd have a significant impact on its activity.In this paper, (600, 800, 1000 and 1200 oC), the Pd / SnO 2 catalyst prepared by the same volume impregnation method was used to study the influence of the calcination temperature of SnO 2 on catalytic combustion of CH 4. The results showed that the prepared SnO 2 were anatase Ore structure and the crystal form is more perfect with the increase of the calcination temperature of SnO 2, the size of the grain is significantly increased.The Pd introduced in the catalyst exists in a highly dispersed form, the catalytic combustion of CH 4 The activity of Pd / SnO 2 (1200) showed the highest CH 4 combustion activity with the increase of the calcination temperature of SnO 2 carrier, the ignition temperature and the lowest total conversion temperature were 265 and 390 ° C. The reaction temperature was 300 oC , The reaction rate of methane on Pd / SnO 2 (1200) is 36 times that of Pd / SnO 2 (600) .XPS results show that the chemical state of Pd varies with the increase of the calcination temperature of SnO 2: SnO_2 (<800 oC), Pd entered the lattice of SnO_2 in the form of Pd4 +, and Pd was present on the surface of the support as Pd2 + species with the increase of calcination temperature (> 1000 oC) .Based on the activity evaluation results, (101) plane of PdO on Pd / SnO 2 (1000) is close to that of (101) plane of SnO 2 support, which are respectively 0.2641 nm and 0.2638 nm. The results of O 2 -TPD and CH 4 -TPR show that the desorption amount of O 2 on the Pd atom of Pd / SnO 2 (1200) catalyst is three times that of Pd / SnO 2 (600) The catalyst Pd / SnO 2 (600) is 45% higher. Therefore, the crystal plane matching of Pd O and SnO 2 in the configuration can increase the activity of the catalyst on O 2 In conclusion, the lattice matching between SnO 2 and noble metal is beneficial to the activation of oxygen at the Pd-SnO 2 interface, and the lattice oxygen in SnO 2 support can also be replenished to the surface through the “Oxygen Anti-Overflow Mechanism” Pd O / Pd to enhance the adsorption and activation ability of O_2 on the catalyst and improve the catalytic activity of CH_4. The increase of the calcination temperature of SnO_2 can enhance the lattice matching between SnO_2 and the noble metal, Calcination temperature increases and increases.