Synthesis And Characterization Of Rare Earth Composite Oxides And Their Catalytic Decomposition Properties For Methyl Mercaptan

Methyl mercaptan(CH3SH)is a highly reactive volatile sulfur compound,which exhibits high levels of toxicity to human health and environnmental safety in view of its low odor threshold.Therefore,the raised environmental awareness has urgently stimulated the importance to find a suitable method to control the emission of CH3SH.A seires of ceria based catalysts and rare earth doped HZSM-5 zeolite catalysts were prepared and investigated for CH3SH catalytic decomposition.Several characterization studies were carried out to provide valuable information to understand the reason for the improved catalytic activity and stability of the modified catalysts.The main results are summarized as follows:(1)A simple and convenient route,which is microwave-assisted sol-gel method,has been employed to synthesize CeO2 nanoparticles as a high-performance catalyst for CH3SH catalytic decomposition.The influence of synthetic conditions on the catalytic performance of the synthesized catalysts has also been comprehensively investigated.Optimum preparation conditions including calcination temperature,calcination time,the molar ratio of cerium salt/citrate and microwave time were optimized in this study.Catalysts with large surface area and small crystallite size can be obtained under the optimum synthetic conditions,and they can provide more structural defects/oxygen vacancies as well as active sites,which are in favor of CH3SH catalytic decomposition.In addition,it has been shown that CeO2 catalyst can easily react with gaseous sulfur species to form cerium sulfide(Ce2S3)and various Ce-O-S phases during the desulfurization processes.The formation of CC2S3 on the spent CeO2 samples mainly accounts for the deactivation.The spent catalysts can be successfully regenerated with air for many times at a short regeneration time of 10 min.(2)A series of Ce0.75RE0.25O2-?(RE= Y,Gd,Sm,Nd,La)solid solutions and pure CeO2 sample were prepared rapidly by citrate-complexation method.These samples were evaluated in terms of catalytic total decomposition of CH3SH.An extensive characterization study was undertaken in order to correlate the structural and surface properties of obtained ceria-based catalysts with their catalytic performance for CH3SH decomposition.More oxygen vacancies associated with improved alkaline properties were exhibited in doped CeO2 samples.Ce0.75Y0.25O2-? and Ce0.75Gd0.25O2-?samples,with appropriate increase of alkaline,possessed more oxygen vacancies,and more structural defects and active sites should be in favor of better catalytic performance.Ce0.75Nd0.25O2-? and Ce0.75La0.25O2-? samples,with stronger basicity,more active sites would be occupied during the reaction processes and more cerium sulfide species(Ce2S3)would accumulate on the catalysts.The accumulation of Ce2S3 mainly caused the deactivation of catalysts.(3)The interaction between oxygen species of cerium-based oxygen carriers and catalytic behaviors was investigated by using CH3SH and Ce1-xYxO2-?(x= 0,0.25,0.50,0.75 and 1.0)solid solutions as examples.A combination of activity and stability experimental studies was carried out to gain insight into the roles for definite surface oxygen in cerium-based oxygen carriers.Two conclusions can be conceivably deduced as follows:surface lattice oxygen should be responsible for the catalytic decomposition of CH3SH;the difference in catalytic activity between CeO2 and Y doped samples would be closely associated with the fact the insertion of Y3+ ion into the lattice of CeO2 caused the losing surface lattice oxygen.Characteration results were employed to demonstrate the migration of bulk lattice oxygen,which was relevant to catalytic stability between CeO2 and the Y doped catalysts.Results were obtained,including:i)direct evidences of the migration of bulk lattice oxygen over cerium-based oxygen carriers;ii)the migration rate of bulk lattice oxygen within Ce0.75Y0.25O2-? is faster compared to the migration rate of bulk lattice oxygen of Ce02;iii)the improvement of catalytic stability should be closely associated with the fact that bulk lattice oxygen should participate in the decomposition of CH3SH via its faster migration to replenish surface lattice oxygen.(4)HZSM-5 zeolite catalysts modified with various amounts of Sm were synthesized and evaluated for CH3SH catalytic decomposition tests.The results illustrated that the addition of Sm improved both catalytic activity and stability of the HZSM-5 catalysts.The optimal content of Sm addition was investigated and Sm/HZSM-5 catalyst with 13 wt%-loading demonstrated high stability with no obvious deactivation during the 80 h test,while the pure HZSM-5 catalyst rapidly deactivated after a relatively short time about 15 h on-stream reaction.Based on the characterization results,increased concentration of basic sites were presented on the modified HZSM-5 catalysts.Moreover,the decrease in the amount of strong acid sites over the Sm doped HZSM-5 catalysts significantly suppressed the formation of coke deposit on the catalysts.Acid-base properties of the catalysts were proved to be closely related to the improved catalytic activity and stability.The spent catalyst can be easily regenerated(at 550 ? and under air condition)for many times.