Study On The Mechanism Of Acid Gas Enhanced Activated Carbon Catalytic Methane Cracking For Hydrogen Production

With the proposal of the dual carbon goal and the continuous advancement of the CO₂emission reduction strategy,it is urgent to develop clean and renewable energy to realize energy transformation.Hydrogen has attracted extensive attention as a clean new energy with high energy density,and choosing a suitable hydrogen production process is the key to the development of the hydrogen energy industry.Hydrogen production from methane cracking is a clean hydrogen production process that does not generate CO₂and is a transitional process connecting fossil fuels and renewable energy.However,the development of suitable catalysts is one of the main problems faced by this process,compared with metal catalysts,activated carbon has the advantages of low price and renewable,and the acid gas impurities in natural gas may affect the efficiency of methane cracking to hydrogen production.In this thesis,from the perspective that the acid gas impurities CO₂and H₂S contained in natural gas may affect the efficiency of activated carbon（AC）catalytic methane cracking for hydrogen production,the mechanism of acid gas affecting AC catalytic methane cracking is obtained by using reasonable experimental design and simulation research,which provides a theoretical research basis for realizing the industrialization of AC catalytic natural gas cracking for hydrogen production.Firstly,the catalytic properties of the AC catalyst itself were studied.AC was treated with 10%,30%and 50%nitric acid by mass concentration to study the effect of changes in specific surface area,pore structure and surface functional groups on its performance.AC before and after acid treatment was characterized and analyzed,and the key factors affecting its catalytic performance were evaluated.After acid treatment,the specific surface area and pore volume of AC decreased and the number of oxygen-and nitrogen-containing functional groups increased.The increase of oxygen-containing functional groups would increase the number of AC surface active sites,but experiments showed that the specific surface area and pore volume play a greater role in its performance,which was approximately linearly related to methane conversion.More micropores in AC can promote the adsorption of methane molecules on AC,and the reduction of specific surface area and pore volume is mainly related to the elimination of micropores,which reduces the catalytic performance of AC.Secondly,combined with the actual acid gas concentration in natural gas,the effects of 0-7.2%CO₂and 0-300 ppm H₂S on methane decomposition efficiency at850°C,900°C and 950°C were studied.The results showed that CO₂has a promoting effect on the catalytic performance of AC,mainly in the early stage of the reaction.The methane decomposition efficiency could be improved by the concentration of 0-7.2%CO₂,and the concentration of 4.8%CO₂was the most favorable.CO₂was more easily adsorbed in AC than methane,but it didn’t affect the adsorption performance of methane molecules.Addition of 0-300 ppm H₂S could improve the methane conversion rate and delayed the deactivation time of the AC catalyst.The positive effect of H₂S was achieved by changing the carbon deposition morphology,thereby changing the pore width distribution and specific surface area of the AC.Under the action of H₂S,the AC had more micropores during the reaction process,which promoted the adsorption of methane molecules on the AC.Finally,the mechanism of CO₂and H₂S promoting methane cracking was studied in depth.The following conclusions were drawn:after CO₂was introduced into the system,it first reacted with the AC catalyst itself,releasing more oxygen-containing functional groups and increasing active sites,thereby improving the methane conversion rate.After the deposited carbon gradually accumulated until the AC was completely covered,CO₂could not react with the AC itself,and it was difficult to react with the deposited carbon,and the promoting effect disappeared.H₂S alters the CH₄decomposition pathway by promoting the first step in methane decomposition.H₂S first dissociates into HS-,and then unsaturated S atom attract CH₄to make H atom in CH₄and HS-form new H₂S molecule,and then H₂S continues this cycle.H₂S didn’t change the reaction steps in the remaining dehydrogenation of CH₃→C.There are 44 figures,9 tables and 132 references in this thesis.