Zinc And Alkaline Earth Metal Doped Alumina Supported Palladium Catalysts And The Performance In Methane Combustion

Natural gas has advantages of abundance in resource and thermally efficiency.Compared with traditional energies such as gasoline or diesel,natural gas as one of the clean energy sources gives lower release of CO₂,NO_xand other pollutants,so it has been widely applied in power plants and automobiles.However,the direct emission of unburned methane as a greenhouse gas will result in serious greenhouse effect.It is urgent to eliminate the unburned methane from the exhaust gas.Catalytic oxidation of methane has been regarded as an efficient and environmentally friendly technology for automobile emission control,which can realize the conversion of low-concentration methane under lower temperatures.Among of all catalysts,noble metal-based catalysts exhibit higher purification efficiency towards methane oxidation.Especially,supported palladium nanocatalysts have been extensively applied in methane catalytic combustion,due to their excellent ability to reduce the energy barrier of C-H bond cleavage.However,their low-temperature（<400°C）activity is not ideal,and the active Pd O species are prone to decompose or sinter at high temperature,resulting in deactivation of catalysts.Thus,it is urgently needed to improve their low-temperature activity and thermal stability.For palladium-based catalysts,the catalytic performance largely depends on the state of active phase（particle size,morphology,exposed crystal planes,and particle structure）,the nature of support,and the metal-support interaction.Effective strategies must be adopted to obtain methane combustion catalysts with excellent low-temperature activity,water resistance and anti-sintering ability.To solve the above problems for palladium-based catalysts,Zn and alkaline-earth metal were co-introduced into alumina in view of the feature including the ability of amphoteric oxide Zn O to form Zn Al₂O₄phase,and the electron-donating effect from alkaline-earth metal and their capability to react with alumina to create aluminate spinel phase.In this way,Pd/XZn-MA catalysts（X:Mg,Ca,Sr,Ba;MA:mesoporous alumina）with excellent low-temperature activity,water-resistance and anti-sintering ability were constructed based on the electronic effect and interface interaction.（1）Zn and alkaline earth metal co-doped alumina materials with mesoporous structure were synthesized through an sol-gel method,which were taken as carriers to obtain supported palladium catalysts.The differences and similarities in the structural properties of catalysts were explored,including the textural properties,redox performance,the dispersion of palladium particles,the surface-concentration and stability of active Pd O phase,the distribution of surface oxygen species and the surface acid-base properties.The structure-activity relationship of catalysts was clarified.（2）Based on the work above,Zn and alkaline-earth metal co-included alumina supported palladium catalyst with superior catalytic performance was selected as the main research object,the differences and similarities in the structural properties and catalytic behavior from the catalysts that were synthesized through different preparation methods were further investigated,offering new research ideas to fabricate highly efficient alumina supported palladium catalysts towards methane oxidation.The main finding are given as follows:（1）The Pd/XZn-MA（X:Mg,Ca,Sr and Ba）catalysts gave obvious difference in the structure and properties,due to the distinction in the electron-donating effect and the ability to react with alumina to form aluminate spinel from alkaline-earth metal.Amongst,Pd/Mg Zn-MA afforded higher initial activity,showing lower apparent activation energy for methane oxidation.Compared with that in Pd/Zn-MA and Pd/MA,T₉₀on Pd/Mg Zn-MA was decreased by 43 and 68°C,respectively.The result could be attributed to the higher ability of the catalyst to activate methane and the more efficient supplement of oxygen vacancy,which resulted from the higher ratio,stability and reduciblity of Pd O species,and higher surface-concentration of active oxygen species due to the higher ability to adsorb gaseous oxygen,as well as the appropriate density of surface acid-base sites.90%methane conversion could be realized at 340°C on Pd/Mg Zn-MA,when undergoing thermostatic activation in stream and then reduction activation with methane.Furthermore,Pd/Mg Zn-MA and Pd/Ba Zn-MA both demonstrated superior anti-deactivation properties when prolonged exposure to stream not only under dry but also wet conditions.（2）Different methods were applied to synthesize Zn/Mg co-doped alumina materials,and the resultant palladium catalysts offered obviously different structure and performance.Mg could react with alumina synthesized by different ways to form Mg Al₂O₄,but the crystallinity of Mg Al₂O₄in each case was different,giving rise to different interface interaction between support and palladium particles.And thus,the catalysts gave different dispersion,electronic property and particle size of palladium species,and different fraction of palladium nanoparticles with different coordination number.Also,the reducibility and stability of palladium species,surface acid-base properties and the adsorption performance of intermediate products CO among the catalysts were different.As a result,the catalysts afforded different catalytic performance and anti-deactivation property during long-term thermostatic reaction.Among of all cases,when the carrier was obtained with the assistance of ultrasonic cavitation,the corresponding Pd/Zn Mg-MA-Ultrasound catalyst showed higher catalytic activity(T₉₀=405°C)and superior anti-deactivation ability during the 50 h on-stream reaction.