Preparation,Performance And Water Vapor Tolerance Of Metal-Fiber-Structured Pd-based Catalysts For Catalytic Combustion Of Lean Methane

Catalytic combustion of lean methane,a process which can not only reduce air pollution but also improve the energy utilization,has been receiving much attention.However,the application of the widely-studied Pd-based catalysts are greatly restricted to the low thermal stability and poor water vapor resistance as well as the intense heat dissipation and mass transport limitation in practical applications.Therefore,we adopted two technical routes for the fabrication of novel catalysts based on the structured metal micro-fiber utilization technology in our laboratory:1)Microfibrous-structured analogous core-shell catalysts embedded onto Al₂O₃/Al-fiber by one-step coupling-agent-assisted organization;2)In-situ hydrothermal endogenous growth of LDHs on Al-fiber felt and subsequent solvent-assisted incipient wetness impregnation of Pd?OAc?₂.As-prepared metal-fiber-structured Pd-based catalysts were examined in the catalytic combustion of lean CH₄,the major results are as follows:?1?Microfibrous-structured analogous core-shell Pd@SiO₂-ZrO₂?N97?/Al-fiber catalysts for low concentration CH₄ catalytic combustion and their water vapor resistance.The zirconium coupling agent?N97 or N44?was used as a complete or partial alternative of APTES,which acted as the bidirectional bridging to realize one-step self-assembly process of the analogous core-shell structure and AlOOH/Al-fiber support.0.5 wt%Pd@Si O₂-ZrO₂?N97?/Al-fiber catalyst?Pd/N97/APTES=1/0.5/3?was prepared successfully,which has the best catalytic performance among all the catalysts with different Pd/Zr,Pd/Zr/Si molar ratio and zirconium coupling agent.At GHSV of 72,000 m L^-1?g^-1?h^-1,complete CH₄ conversion could be obtained at nearly 400 ^oC.Then,when 3 vol%water vapor was added into the reaction,it was found that although the activity was inhibited and deactivated slowly,it can almost restore to its initial value in the absence of water when water vapor was cut off,and the activity could be stabilized at about 84%finally after the cycle test of 100 h.XPS analysis showed that the introduction of ZrO₂ indeed suppressed the reaction of PdO and water vapor,thus inhibited the formation of Pd?OH?_x,which was usually considered a main cause of deactivation for Pd-based catalyst in the presence of water vapor.Finally,from the phenomenon of increased PdO particles and reduced specific surface area after reaction,we attributed the deactivation of the catalyst to the sintering of Pd nanoparticles.?2?Preparation of microfibrous-structured Pd/MgAl-LDH?T180?/Al-fiber catalysts and its low concentration CH₄ catalytic combustion reaction performance and water poisoning resistance.In order to improve the water poisoning resistance of the catalyst,the Mg promoter was introduced via in-situ growing Mg-Al hydrotalcites onto the Al-fiber surface,after the modification of the MgAl-LDH/Al-fiber carrier,Pd?OAc?₂ was loaded by solvent-assisted incipient wetness impregnation.The catalyst Pd/MgAl-LDH?T180?/Al-fiber with excellent low temperature activity,high stability and strong resistance to water poisoning was successfully prepared by adjusting the catalyst preparation conditions.The optimized catalyst had an ignition temperature of only230 ^oC and could achieve complete oxidation of CH₄ at 350 ^oC for a feed gas of 1 vol%CH₄,in air at a high GHSV of 72,000 mL?g^-1?h^-1.Besides,the catalyst had good stability and resistance to water vapor.There was no sign of deactivation for catalyst reacting under dry condition at 350 ^oC for 120 h.After continuous 500 h stability test with varied water vapor contents?3 vol%¹5 vol%?and temperature,the catalyst still showed good activity maintenance.?3?The origination of high activity and high resistance to water poisoning of the catalyst Pd/MgAl-LDH?T180?/Al-fiber.On the basis of a series of characterizations of the Pd/MgAl-LDH?T180?/Al-fiber catalyst,we attributed its high activity to the following four points:First,the large specific surface area facilitated the dispersion of Pd nanoparticle on the surface of support.Second,the introduction of Mg enhanced the alkalinity of the support,thus changed the electronic state of Pd and promoted the formation of more active species of PdO,and Mg,as an electron donor,facilitated the chemical adsorption of oxygen and promoted the formation of more surface active oxygen species(O_ads).Third,the reduction-reoxidation properties of the active PdO species were improved,in our experiment,to a certain extent.Eventually,the apparent activation energy of the catalyst was decreased and the activity of the catalyst was improved.The improved water vapor resistance was accounted for:First,thanks to the good stability of the catalyst structure under hydrothermal conditions,PdO can be well stabilized and dispersed.Second,effectively stabilized the PdO active species and inhibited the formation of inactive Pd?OH?_x.Third,the enriched reactive oxygen species(O_ads)on the surface promoted the oxidation of Pd and desorption of-OH thereby leading to the improved water vapor resistance of the catalyst.