Hydrogen Production From Catalytic Methane Decomposition Over Coal-Based Activated Carbon

Methane catalytic decomposition is considered as a promising process for hydrogen production becauSe of its simple process,easy separation of products,and no COx production.Transition metals and carbon materials are two common catalysts for methane decomposition,especially activated carbon(AC),which attracts more and more attention owing to some advantages,such as low cost,wide sources of raw material,high temperature resistance,sulphur-tolerance,the resistance of other toxic impurities and no formation of metal carbides.To obtain the carbon catalysts with high activity in hydrogen production from methane decomposition,in this thesis,the effects of different preparation conditions of coal-based AC by KOH activation on the textural properties of AC and its catalytic performance in methane decomposition were investigated.The main works were as follows:(1)The pre-oxidation treatment of Shenmu coal(SM)with air as the oxidant was combined with KOH activation to prepare AC at 900 ℃.The effects of pre-oxidization on the surface chemical properties,structural properties and catalytic performance of AC in methane decomposition were examined.The results show that the pre-oxidization of coal changes the chemical and pyrolysis characteristics of coal,which in turn affect the pore structure and surface chemistry of the resultant AC.The pre-oxidization of coal increases the amount of oxygen-containing functional groups like ketones,quinones and lactones,specific surface area and mesoporosity of the resultant ACs.The prepared AC from pre-oxidized coal at 300℃ for 4 h has the highest amount of oxygen-containing functional groups.When SM is pre-oxidized at 100 ℃ for 12 h,the BET specific surface area and the mesoporous surface area of resultant AC are 2407 m2/g and 1910 m2/g,respectively,which are 1.3 times and 2.0 times as AC prepared from raw coal.When the ACs are used as catalysts for methane decomposition to hydrogen,the initial activity is related to the amount and type of surface oxygen-containing functional groups;the ACs with higher mesoporosity and mesoporous specific surface area have better stability in catalytic methane decomposition to hydrogen.(2)The role of minerals in coal on the preparation of AC was investigated by adjusting the content and composition of minerals with different acid washing.Some minerals in coal,or the produced salts via the reaction with KOH can be used as the template in the preparation of AC.However,the high mineral content in coal is not conducive to the preparation of AC because the minerals can consume part of KOH,leading to the weakeness of the activation of coal by KOH.The remove of HCl-soluble mineral matters,such as Ca and Fe,is harmful to the development of specific surface area and pore in AC.The effect of calcium salts as additives to coal on the structural properties of resultant AC and its catalytic performance in methane decomposition to hydrogen were studied.The results show that the pore structure of AC can be adjusted by the amount of calcium in coal.When the added amount of calcium is less than 10 wt.%of coal mass,the resultant ACs have higher microporisity.However,the mesoporisity of the resultant AC is remarkably increased when the mass ratio of calcium to coal is more than 10 wt.%.The kind of calcium salts will influence the properties of the prepared AC samples.Compared with the AC samples prepared by adding CaC12 or CaO to SM coal,the AC prepared from the coal with Ca(NO3)2 addition has higher BET specific surface area and mesoporous surface area.Higher mesoporous specific surface area and mesoporosity are the main reasons for the higher activity and better stability of ACs in methane decomposition.(3)The effect of hydrogen addition in reactants on methane decomposition over AC catalyst was investigated.The results show that addition of proper amount of hydrogen to methane increases the stability of AC catalyst in methane decomposition,but the excessive hydrogen will suppress the methane decomposition.When H2 flowrate is quarter flowrate of methane,the methane conversion is the highest after the reaction of 370 min,which is four times than that of the case where without the addition of hydrogen.The addition of hydrogen also promotes the formation of filamentous carbon and inhibits the production of aggregated carbon from methane decomposition over AC.The isotopic tracing of CD4 indicates that hydrogen takes part in the free radical reaction of methane decomposition,inducing methane decomposition and the formation of filamentous carbon.(4)Using SM coal as the precursor,Fe(NO3)3 as additive,and KOH as active agent,Fe-doped AC catalyst can be one-step prepared by utilizing the carbon reducibility,which simplifies the preparation process.The activity and stability of the Fe-doped AC in methane decomposition were increased.Especially when the addition amount of Fe in coal is up to 30 wt.%,the activity of the Fe-doped AC continues to increase within 9 h.The prepared Fe-doped ACs at temperatures of 800 and 850 ℃ have higher activity and better stability in methane decomposition to hydrogen.However,higher activation temperature causes the aggregation of Fe particles,resulting in lower activity of Fe-doped AC in methane catalytic decomposition.