Performance Of Ordered Mesoporous P-doped Alumina Supported Palladium Catalysts In Methane Oxidation Reaction

Natural gas as a clean fuel is becoming one of the main energy sources for China.However,the emission of low concentration of CH₄during the gas exploitation and utilization has exacerbated the greenhouse effect.Methane catalytic combustion is considered as an effective path for abating the release of methane.The key point is the design and development of highly efficient catalysts.In general,noble metal catalysts,especially alumina supported palladium catalysts（Pd/Al₂O₃）demonstrate excellent low-temperature activity towards methane combustion.However,thanks to their low Tammann temperature and high surface energies,palladium nanoparticles are prone to migrate,sinter or agglomerate during reaction,especially under elevated temperatures,eventually resulting in unsatisfactory stability.In addition,palladium-based catalysts are prone to interact with water,leading to the poisoning and deactivation of catalysts.It is necessary to develop highly efficient and stable catalysts,as our country pay more attention to environmental protection,and energy conservation and pollution reduction.In this paper,aiming at the existing problems of Pd/Al₂O₃,ordered mesoporous phosphorus-doped alumina（P-OMA）with excellent structure and performance was synthesized to construct highly efficient and stable catalysts for CH₄oxidation,which was based on the feature of rich electrons for phosphorus and advantages of high specific surface area,ordered pore structure as well as adjustable pore size for ordered mesoporous alumina（OMA）.The major research works manifested in the following several aspects:（1）P-OMA was synthesized via a sol-gel method,taking aluminum isopropoxide as aluminum source,P123 as template agent,H₃PO₄as phosphorus source,HCl as acidity regulator and HAc as interfacial protective agent.The thermal stability of P-OMA,the dispersion of palladium species,redox properties,as well as adsorption properties of CO and CO₂over Pd/P-OMA catalysts prepared by P-OMA were investigated.And the key factors affecting the performance of catalysts were analyzed.（2）Based on the above,the facile synthesis route of P-OMA was explored,taking（NH₄）H₂PO₄as phosphorus source.The influence of heat treatment and P-doping on textural properties and surface acidity of P-OMA were investigated.The effect of surface acidity of carriers,metal-support interaction,Pd²⁺/Pd⁰ratio and redox properties of catalysts on catalytic activity was discussed.The water-resistance and anti-sintering ability of catalysts was also studied.（3）Ultrasound was introduced into sol-gel system to obtain P-OMA,using different phosphorus precursors.The commonality and differences in structural performance of the as-synthesized catalysts were explored.The structure-activity relationship between catalytic performance,and microstructure and performance of catalysts was elaborated.（4）Based on the above,OMA co-doped with phosphorus and metal element（La,Ba or Zr）from different groups was synthesized.The catalytic performance of the as-synthesized catalysts towards CH₄oxidation was investigated.Moreover,the essential reason for the synergistic effect of phosphorus and metal elements influencing the structure and performance of catalysts was revealed.The main findings were listed as follows:（1）It revealed that P-OMA presented higher formation temperature ofγ-crystal phase of Al₂O₃andγ-to-αphase transformation temperature compared with the undoped OMA.The P-OMA exhibited more ordered mesoporous structure and more excellent sintering-resistance.Palladium particles were more highly dispersed over Pd/P-OMA catalysts taking P-OMA as supports.Pd/P-OMA showed better adsorption performance to CO.Compared with Pd/OMA,the surface basicity of Pd/P-OMA was weakened,which was beneficial to the desorption of CO₂.All these factors made Pd/P-OMA with higher catalytic activity for CH₄oxidation than Pd/OMA.（2）Nano-sized clubbed P-OMA was facilely prepared via a modified sol-gel approach,which showed stronger surface acidity than un-doped OMA.The as-synthesized catalysts taking P-OMA as carriers exhibited higher ratio of Pd²⁺/Pd⁰in comparison with Pd/OMA.Among them,Pd/ADP-OMA-6-1000 which was prepared by P-doped OMA（The content of P is 6wt.%）calained at 1000oC,exhibited relatively E_avalue(84 k J mol^-1),in line with its superior activity.This could be attributed to the higher reducibility of Pd O,adequate proportion of Pd²⁺/Pd⁰over catalyst and suitable surface acidity of support,which benefited in the adsorption and activation of CH₄.Encouragingly,the complete conversion of CH₄could be achieved at 321oC over the catalyst when pre-activated with H₂atmosphere.Moreover,it demonstrated excellent thermal stability in the isothermal reaction for 50 h.（3）P-OH groups formed over P-doped OMA supported palladium catalyst could be transformed into P=O groups under high temperature,which would rehydrate into P-OH groups in the presence of water.This could mitigate the adsorption of water over the adsorption sites of catalysts,leading to the enhanced water-resistance for Pd/ADP-OMA as compared with Pd/OMA.（4）Cluster-like P-OMA was obtained through an efficient ultrasound-assisted sol-gel process.Results demonstrated that the intervention of ultrasonic cavitation effect in the sol-gel system promoted the hydrolysis of aluminum salt,which further shortened the synthesis time of OMA materials.It was demonstrated that catalysts obtained from diverse phosphorus precursors performed significantly different activity.Pd/ADP-OMA and Pd/OP-OMA catalysts,which was prepared by OMA doped with phosphorus from（NH₄）H₂PO₄and H₃PO₃,respectively,behaved with higher TOF values and catalytic activity for CH₄oxidation.They exhibited significantly lower full-conversion temperature(T₉₉)compared with Pd/OMA.This may be due to the stronger metal-support interaction of catalysts.In addition,surface oxygen/hydroxyl species over the catalysts presented higher reducibility,in coincide with higher oxygen mobility,which could accelerate the oxygen exchange rate between support and Pd O phase,and increase the capacity of catalysts to convert intermediate CO into final products.（5）In comparison with Pd/ADP-OMA,catalysts prepared by OMA co-doped with phosphorus and metal element from different groups（Ba,La or Zr）demonstrated superior catalytic performance and water-resistance.This could be assigned to the synergistic effect of phosphorus and metal element（Ba,La or Zr）,which reduced the distortion of Pd O crystal lattice,weakened the bond strength of Pd-O and made Pd O more easily reduced.In addition,the synergy increased the number of exposed step sites of palladium nanoparticles,made the activation barrier of the reaction reduced and accelerated the oxygen mobility.Among them,Pd/ADP-La-OMA catalyst performed the best performance with the nearly complete CH₄conversion temperature as low as 305℃after the isothermal activation of 60 h and subsequent activation via methane reduction.The methane oxidation process over the catalyst followed the M-v K mechanism,and the dissociation of CH₄is the rate-determining step.