Study On The Surface Acid-base Property Of Pd/X-OMA?X:La Ce Zr Mg?Catalyst And Its Performance In Methane Catalytic Combustion

Natural gas is an important part of the energy available at present,and the main component of natural gas is methane.However,the emission of unburned methane into the atmosphere will give rise to serious greenhouse effect in the process of natural gas utilization.The catalytic combustion technology is considered to be the most effective method to solve the problems.?-Al₂O₃ supported palladium catalysts have been widely used in methane combustion due to their excellent catalytic performance.However,?-Al₂O₃ materials prepared by general method have the disadvantages of structural properties such as disordered pore channels and wide pore size distribution,resulting in easy migration,aggregating and sintering of the active component over?-Al₂O₃ supported palladium catalysts,and hence the stability of the catalyst is not ideal at high temperature.Therefore,the development of the durable Pd based catalysts with high activity at low temperature is of great significance.Ordered mesoporous alumina?OMA?materials demonstrate many desirable textural properties including large specific surface area,tunable pore dimension and highly uniform channels,which are conducive to the dispersion of active components and mass and heat transfer,hence contributing to the improvement in catalytic activity and stability of the OMA supported catalysts.At present,OMA materials are mainly synthesized through sol-gel method and nanocasting method,of which sol-gel self-assembly process has been more widely used owing to its simple synthesis process.However,the synthesis of sol-gel reported in the literature took a relatively long time,normally exceeding dozens of hours.This paper focuses on the modification of the existing synthesis technology of sol-gel by shortening the synthesis time of OMA.The surface acid-base properties of supported noble metal catalysts have an important effect on methane catalytic combustion.Therefore,the existing sol-gel method has been further modified in order to rapidly synthesize OMA in this work.The surface acid-base properties of OMA were tuned by the doping or loading of metal element X?X=La,Ce,Zr,Ce-Zr,Mg?.Pd-based catalysts were synthesized taking as-obtained OMA materials as carriers and the influence of acid-base properties of catalysts on the catalytic activities toward methane combustion were investigated.The relationship between the structure,surface acid-base properties and the catalytic performance of the catalysts were also studied.The main research work is as follows:1.With P123 as template,ethanol?EtOH?and isopropanol?IPO?as solvent,HCl as acidity regulator,isopropanol aluminum?Al?OiP?₃?as aluminum source,X-doped OMA?X-OMA?materials exhibiting different surface acidity were synthesized through tuning the molar ratio of the solvent to regulate the formation rate of gelatum.2.OMA was also synthesized using the same method mentioned above and X-loaded OMA?X/OMA?materials were prepared by impregnation method.3.The method to rapidly synthesize X-OMA materials was explored in the non-inorganic acid system through regulating the hydrolysis rate of aluminum isopropanol in the sol-gel system.4.The effects of texture properties,redox properties and surface acid-base properties on the performance of the Pd-based catalysts supported on X-OMA and X/OMA materials were studied.Through the above research content,the following results are obtained:?1?In comparison with undoped OMA,the X-doped OMAs synthesized in sol-gel system containing HCl possessed higher specific surface area,more ordered pore structure and higher thermal stability.The specific surface area and pore volume of Ce-Zr-OMA calcined at 1000?were still 159.1 m²g^-11 and 0.59 cm³g^-1,respectively.The Pd/Ce-Zr-OMA catalyst showed the highest catalytic activity in CH₄ combustion,which could be ascribed to its larger specific surface area,better reduction performance and stronger surface basicity among the tested catalysts.?2?Compared with OMA,the specific surface area and pore volume of X/OMA samples were slightly decreased after the loading of La,Ce,Zr,Mg on the synthesized OMA,but X/OMA samples still maintained the ordered pore structure.Pd-based catalysts supported on X/OMA showed slightly lower catalytic activity in CH₄ oxidation reaction than Pd/OMA.This may be due to the lower specific surface area and pore volume of X-OMA samples,which was not conducive to the dispersion of the active components and the mass transfer and heat transfer in the reaction process.Taking X/OMA materials loaded with X after calcined at 1000?as carriers,the catalysts with weaker surface acidity and stronger surface basicity exhibited superior catalytic activity for CH₄combustion.?3?X-doped OMA samples were rapidly synthesized by adjusting the hydrolysis rate of Al source in the sol-gel system in which HCl was replaced by HAc.Compared with undoped OMA,X-OMAs had higher specific surface area,better pore ordering and higher thermal stability.The superior catalytic activity of Pd/La-OMA and Pd/OMA prepared on the supports calcined at 1000?can be demonstrated,owing to their better reducibility,relatively stronger surface basicity and weaker surface acidity.?4?The results proved that both of the weak surface acidity and strong surface basicity of the catalysts were beneficial to the methane catalytic combustion.Although higher calcination temperature of the carriers resulted in the lower specific surface area and pore volume of the corresponding supported palladium catalysts,the catalysts demonstrated significantly enhanced metal-support interaction,more suitable surface acid-base properties,hence higher activity.Among them,for the Pd/La-OMA catalyst prepared in acetic acid system,the total surface acid amount and alkali amount are 95.3umol/g and 136.9 umol/g,respectively,and the catalyst exhibited higher activity,which can completely convert methane at 385?.