Dry Reforming CO₂-CH₄ Over Activated Carbon Supported Nickel Catalysts Combined With Non-thermal Plasma

With the acceleration of industrialization,the concentration of CO₂ in the atmosphere has risen sharply due to human activities and industrial production.As a result,the global warming becomes more serious,resulting in serious ecological safety issues such as sea-level rise,more extreme weather and sharp reduction of biodiversity.Non-thermal plasma(NTP)coupled with catalyst reforming CO₂-CH₄ technology can not only consume CO₂ and CH₄ simultaneously at room temperature and pressure,but also produce syngas(H₂and CO mixtures)which can be an important feedstock for the synthesis of high-value chemicals.In view of the current low conversion efficiency of reforming catalyst,low energy efficiency and poor selectivity of target product,a low cost and high activity activated carbon supported nickel(Ni/AC)catalyst was prepared.The properties of Ni/AC prepared at different reduction temperatures were studied,and the effect of reduction temperature on the catalytic performance of Ni/AC coupled NTP was revealed.Under the optimal reduction temperature,promoter and active metal were introduced into Ni/AC,and the catalytic and carbon deposition mechanism of the plasma-catalytic reaction were elucidated.Based on the semi-empirical power law model,the reaction rate expression was established.A series of Ni/AC catalysts were prepared through wet impregnation,and the effects of reduction temperature on the properties and catalytic performance of Ni/AC were revealed.Based on the results of BET,XRD,SEM,FT-IR and TG-DTG,it was found that the high temperature was conducive to the formation of pure nickel element.The optimal specific surface area(502.4 m² g^-1),pore volume(0.35 cm³ g^-1)and mesoporous amount were obtained at 700?.The catalysts prepared under this condition coupled with NTP(45W of discharge power)also showed strong synergistic effects with 58.3%-64.6%CO₂and 62.7%-65.7%CH₄ conversion,which were significantly higher than the sum efficiency of the individual ones.Moreover,the selectivity of H₂ and CO was up to 42.4%and 49.9%,respectively.The Ni/AC catalyst was modified by doping Ce promoter,and the catalysis and carbon deposition mechanism of Ni-Ce/AC coupled NTP were elucidated.The introduction of Ce could not only reduce the Ni particle size and make it better dispersed,but also increase the basic sites and basicity of the catalyst,which was conducive to the CH₄ decomposition and the CO₂ activation.The experimental results indicated that the activity of the catalyst increased first and then decreased with the increasing of Ce amount,reaching the highest value when the Ce mole addition was 1%.Compared with Ni/AC catalyst,the CO₂ and CH₄ conversion increased respectively by 11.5%and 10.2%of Ni-Ce/AC.However,the increase of Ce addition might lead to more carbon deposition,the effect could be ignored when it was less than 1%.In addition,not only the coke deposition could be reduced,but also the carbon support could be avoided to participate in the reaction when the initial molar ratio of CO₂/CH₄ was 1:1.The Ni-M-Ce/AC bimetallic catalysts were prepared through introducing active metals(Co,Mn,Fe)into the Ni-Ce/AC catalysts,the catalysis and coke-resistant mechanism of Ni-M-Ce/AC coupled NTP were elucidated.With the addition of another active metal,although the specific surface area and total pore volume of the catalyst were reduced by 26.0%?27.6%and 7.1%?14.3%,the Ni particles size was also decreased by4.0%?9.7%,and the Nickel particles obtained better dispersion.Compared with Ni-Ce/AC catalyst,the CO₂ and CH₄ conversion rate and energy efficiency were improved respectively by 2.0%?8.2%,5.6%?15.5%and 10.2%?20.3%of the Ni-M-Ce/AC.Among them,the addition of Fe had the best performance,which could be attributed to the reason that metal Fe increased the Ce³⁺content in the catalyst,forming more oxygen vacancy and promoting the activation of CO₂.Furthermore,more O*were adsorbed due to the synergistic effect of the bimetallic catalysts,which could enhance C*gasification and slow down the coke deposition rate.Thus,the coke deposition could be effectively inhibited during the reaction process.The semi-empirical power law model was applied to calculate the kinetic parameters of catalytic-plasma DRM reaction,and the kinetic rate equation and apparent activation energy were obtained.It was found that the introducing of promoter and active metal could improve the reactant conversion.At the same time,the apparent activation energy of CO₂ and CH₄ reaction were deceased.In particular,the Ni-Fe-Ce/AC catalyst showed the lowest CO₂ and CH₄ reaction apparent activation energy,which were only 23.5 k J mol^-1 and 21.6 k J mol^-1.In addition,it was found that increasing the specific energy density and setting an appropriate discharge volume could increase the conversion rate of CO₂ and CH₄,but the effect of discharge frequency was not evident.