Hydrogen Production By The Decomposition Of CH₄/C₂H₅OH Over Carbon Materials Under Microwave-heating

As the growing of the energy crisis and the environmental pollution in the word,seeking a clean and non-polluting energy sources has become a new topic. Hydrogenis regarded as the sustainable and clean energy of the future.Recently,thethermo-catalytic decomposition of methane or ethanol has become an effective way toproduce hydrogen-rich gas,attracting more and more attention of the worldwidescientist.Furthermore,in the process of the decomposition, the carbon nanotubes couldbe generated on the surface of catalyst.Therefor, the economic value of the cleavageproduct will be greatly improved.At first,in this paper the microwave-assisted pyrolysis of methane over an activatedcarbon(AC) or pyrolysis residue of sewage sludge(PRSS) under microwave heating ina fixed-bed quartz-tube reactor was studied. the influence of different reactionconditions on the conversion of CH4to H2were investigated.Furthermore, the impactof gas discharge in the process of methane decomposition was also investigated. Themechanisms of the deactivation of catalysts were studied by FTIR, XRD, SEM, EDSand BET analyzer.Then,the decomposition of ethanol over the two catalysts(the AC and PRSS) undermicrowave heating were studied,respectly. The influencing factors, includingmicrowave power and reaction velocity, on the conversion of ethanol to H2wereinvestigated. The influence of different reaction conditions on the generation ofcarbon nanotubes was also investigated by SEM.Finally, the influence of ethanol on the activity of the two catalysts for themethane decomposition was studied.The mechanisms of the impact of ethanol wassimply analyzed by comparative experiments.the following main conclusions and results were obtained:(1) The results of the decomposition of CH4over the AC indicated that the CH4 conversion increases with the increase of microwave power. Due to the impactof gas discharge, the AC under different atmospheric conditions exhibitdifferent temperature characteristics. Under the microwave power of560W,the maximum average temperature of AC was735℃in N2.The averagetemperature in H2is slightly lower than thant in N2and.The lowest was foundin CH4of625℃.The reason is mainly that the number of plasma in N2is morethan H2or CH4. Reunder microwave heating conditions, methane conversiondecreased with the increase of methane partial pressure from0to60%, andincreased with the increase of methane partial pressure from60%to100%.However, under electric heating conditions, methane conversion only increasedwith the increase of methane partial pressure.The reason is probably related tothe “Penning effect”. It was found that iron powder can promote the formationof micro-plasma under microwave heating of AC, leading to the highertemperature and methane conversion. The results of FTIR indicated that theoxygen-containing functional groups may be the activity site of the AC. TheSEM results show that, during CH4decomposition reaction, carbon producedfrom CH4decomposition will deposit on the surface of the catalyst. The spentchar have lower surface area and larger pore diameter than those of fresh ones.This observation suggests that the carbon deposition will prohibit the contactbetween the active center of the char and CH4, leading to the deactivation ofthe AC.(2) The PRSS has a significant activity for the decomposition of CH4. The resultsindicated that the CH4conversion increases with the increase of microwavepower. Due to the impact of gas discharge, the PRSS under differentatmospheric conditions exhibit different temperature characteristics.The orderof the temperature is:TN2＞TH2＞TCH4.It suggested that the atmospher has asignificantlye large influence in the process of the decomposition. the CH4conversion increases with the decreas of particle sizes.There are tworeasons:one is the decrease of particle sizes can reduce the diffusion resistanceof methane;the other one is it can promote the formation of micro-plasma,leading to the higher temperature. The results of FTIR indicated that theoxygen-containing functional groups are probably not the activity site of thePRSS. The SEM results indicated that, during CH4decomposition reaction,carbon produced from CH4decomposition will deposit on the surface of thecatalyst. The spent char have lower surface area and larger pore diameter than those of fresh ones. This observation suggests that the carbon deposition willprohibit the contact between the active center of the char and CH4, leading tothe deactivation of the char.(3) The results of the decomposition of ethane over the PRSS indicated that theselectivity of hydrogen increases with the increase of microwave power.Thereaction velocity has no obvious effect.For the AC, the selectivity of hydrogenincreases with the increase of microwave power and reaction velocity.Only onthe reaction conditon of the microwave power of320W and the reactionvelocity of8.5, many carbon nanotubes was found on the surface of thePRSS.There are probably three reasons:(1)in the lower microwave power,thereaction temperature is not higher,leading to the lower rate of thedecomposition of ethanol and the lack of carbon;(2)a higher power will leadthe rapid decomposition of ethnol,resulting in the few diffusion of carbonatoms and generating carbon deposits;(3)a higher or lower reaction velocitymay has an effect on the decomposition of ethnol and the balance between thedissolution and diffusion of carbon atoms.No carbon nanotubes was found onthe surface of the AC.(4) The introduction of ethanol coulde inhibit the deactivation of the two catalystfor the decomposition of methane.Furethermore,the ethanol can regeneratedthe activity for the methane decompisotion of the two catalysts.The reason isprobably that, the carbon deposits generated from the decomposition of ehanolhave a good activity for the decompisotion of methane.