Stduy On The High Stability Catalyst For CO₂ Reforming Of CH₄

To deal with the escalating global warming problem,caused by the rising concentration of greenhouse gases?CH₄,CO₂?,the removal,disposal and chemical-utilization of CH₄ and CO₂ have become the focus of research.Carbon dioxide reforming of methane?CDRM?can not only convert these two main greenhouse gases into syngas?H₂/CO?1?,which is suitable for the synthesis of long-chain hydrocarbons or oxygenated compounds.Ni-based catalysts are considered to be the most promising catalysts for CDRM due to their low cost and high activity.However,the catalysts are usually deactivated by sintering and carbon deposition.Therefore,the development of highly stable and carbon-resistant catalysts is the main focus of this work.In the process of CDRM,CH₄ molecule decomposed into CHx species after adsorption and activation on the active sites.This mechanism determines that carbon deposition is inevitable,so it is impossible to prevent carbon deposition completely.Therefore,the key to reducing carbon deposits is to control the rate of carbon removal to be greater than the rate of carbon deposition.However,a large number of researches have focused on how to prevent the formation of surface carbon from the catalyst,ignoring the inevitability of carbon deposition.Based on the results of previous studies,the concept of"preventing-conversion combination"catalyst design was proposed in this paper.The"preventing"is to minimize carbon deposition by various methods and means,such as employing an appropriate method to control Ni crystal size,using metal oxides with strong Lewis basicity as supports or promoters.The so-called"conversion"refers to the introduction of some reactions in the catalyst to convert the carbon deposited on the surface of the catalyst.From the“preventing”perspective,a structurally anti-coking Ni/MgAl₂O₄-MgO complex-support catalysts were designed by combining the merits of MgAl₂O₄ spinel?high thermal stability,good chemical inertness,low acidity and suitable interaction with the active species?and NiO-MgO solid solution?strong adsorption ability for CO₂ and the formation of NiO-MgO solid solution?.From the“conversion”perspective,our previous work reported that the Mo-containing catalysts could promote the carbonation and oxidation balance between Mo₂C and MoOx,which was favorable for coke elimination and improvement of catalytic activity and stability.In our latest study,we already preliminarily verified the feasibility of the coke elimination by the circulation of Ce⁴⁺/Ce³⁺when the reaction was carried out at above 900°C.Also,the incorporation of Zr⁴⁺into the lattice of CeO₂ resulted in the formation of solid solution,which could improve the oxygen storage capacity,redox property and thermal stability of CeO2.Therefore,Ce,Zr and Mo as promoters are added into the above mentioned complex support catalysts to convert the carbon deposits by the reaction of Mo,Ce with carbon deposits and CO₂ in the feed gas to achieve the purpose of"conversion".Based on the above concept of catalyst design mentioned above,here,Ni/MoCeZr/MgAl₂O₄-MgO complex support catalysts were successfully synthesized by tuning the calcination temperatures of the Ce,Zr and Mo promoted NiMgAl-HTLcs precursors.XRD,H₂-TPR,N₂ physical adsorption,CO₂-TPD-MS,CO chemical adsorption,O₂-TPO-MS and TEM were used to analyze the relationship between the catalyst structure and catalytic performance.The effects of calcination temperature,the content of Ce and Zr,the precipitation pH and the effect of Ni loading on the catalytic performance were investigated.And the cylindrical Ni/MoCer/MgAl₂O₄-MgO complex support catalyst was tested in the pilot-scale experiment of coke oven gas and gasification gas reforming to syngas.The detailed research work and results are as follows:?1?Ni/MoCeZr/MgAl₂O₄-MgO complex support catalysts were synthesized by calcination of the Ce,Zr and Mo promoted NiMgAl-hydrotalcite-like precursors at different temperatures?700,800 and 900°C?.The catalytic performance of these catalysts in the carbon dioxide reforming of methane strongly relied on the calcination temperature.The catalyst obtained at 800°C maintained a steady conversion of CH₄ above 95%for 658 h without any remarkable coke deposition(only 0.015 0.015 mg_C·g^-1_cat·h^-1),exhibiting the best activity and stability compared with the other two catalysts calcined at 700 and900°C.Carbon deposition analysis showed that all the catalysts generated negligible coke deposition,indicating that solving the coke deposition problem puzzled researchers for many years is possible.Further analysis revealed that the generation of small Ni nanoparticles?<9 nm?,strong basicity and intimated metal-support interactions coupled with the high reaction temperature might be the reason for the stable coke-free process.?2?The ratio of Ce/Zr in Ni/MoCeZr/MgAl₂O₄-MgO catalyst was studied.The results reveal that the catalytic performance of Ni/Mo/MgAl₂O₄-MgO complex support catalysts are related to the ratio of Ce/Zr,and the catalytic performance is the best when Ce/Zr=4.The coexistence of Ce,Zr and Mo is the necessary condition for the optimal stability of the catalyst.The characterization results show that the addition of CeO₂ is more beneficial to improve the dispersion of active Ni and the elimination of graphite carbon.The addition of Zr species is more beneficial the increase of the specific surface area and the reduction of Ni.The coexistence of Mo,Ce and Zr improve the stability of Ni/MgAl₂O₄-MgO complex support catalysts significantly because of the coexistence of Ce,Zr and Mo strongly favored for the adsorption of CO₂ and the removal of carbon deposition.?3?The influence of mass fractions of Ce_0.8Zr_0.2O₂ on the catalytic performance of Ni/MoCeZr/MgAl₂O₄-MgO catalyst was investigated thoroughly.Experimental results indicated that the catalytic performance was related to the content of Ce0.8Zr0.2O2,and catalysts promoted with 1wt.%Ce0.8Zr0.2O2 showed the highest activity as well as stability.Furthermore,the promoted effect of Ce_0.8Zr_0.2O₂ decrease with increasing of addition content,due to the superfluous content of Ce_0.8Zr_0.2O₂ damage the simultaneity of precipitation of different salts,resulting in the weak metal-support interaction and lower Ni dispersion.?4?The precipitation pH value in the preparation process of the catalyst was investigated.The results indicated that the catalyst showed better catalytic activity and stability when the pH is 9.15 and 9.5.Moreover,less amount of carbon deposition was obsved on these catalysts after subjected to CDRM reaction for658 and 432 h,which was related to the relatively smaller Ni grains and stronger metal-support interaction.?5?HTLcs-derived Ni-MoCeZr-MgAl?O?mixed oxide catalysts were successfully prepared using the simple hexamethylenetetramine-assisted hydrothermal method by tuning the molar ratio of n?HMT?/n?NO₃^-?,and the conversion of CH₄ fluctuated around 95%continuously for 610 h under GHSV of60,000 mL·gcat^-1·h^-1.It was found that the optimal molar ratio of n?HMT?/n?NO₃^-?was equal to 1.Secondly,a series of Ni-MoCeZr-MgAl?O?mixed oxide catalysts with different Ni contents?3,4.8,6,9 and 12%?was prepared under the optimum n?HMT?/n?NO₃^-?=1 conditions.Research on the Ni content indicated that 6%Ni loading was optimal for this series of catalysts.The larger number of the accessible surface Ni sites,the higher dispersion of Ni metal particles?14.9%?and the smaller nickel crystallites?5.4 nm?as well as the relatively less carbon deposition and the lower sintering degree were responsible for its superior activity.Besides,analysis results revealed that sintering and coking were more likely to happen for the catalyst with higher Ni content,and encapsulating graphitic carbon was the main type of carbon deposition when the content of Ni was not more than9%,while CNFs were easily formed on the catalyst with higher Ni content?CAT-12?.?6?The cylindrical Ni/MoCeZr/MgAl₂O₄-MgO complex support catalyst was tested in pilot-scale reforming of coke oven gas and gasification gas,and the conversion of CH₄ can be maintained over 90%for 80 h,which achieved the target required in the project.The regenerated catalyst showed a stable CH₄ conversion?above 94%?for 260 h(900oC,GHSV=30,000 mL.g^-1_cat.h^-1),indicating that this catalyst has a certain ability to regeneration.