Preparation Of Highly Dispersed Ni-based Catalysts And Their Application In Methane Dry Reformin

The widespread use of fossil fuels leads to the emission of large amounts of greenhouse gases into the atmosphere,increasing the average temperature of the planet,with CH₄ and CO₂ being the two main contributors to the greenhouse effect.The production of syngas CO and H₂ by dry reforming of methane（DRM）can reduce greenhouse gas emissions and the product syngas is an important intermediate in the Fischer-Tropsch synthesis of energy chemicals such as alkanes and olefins,which are used as liquid hydrocarbons in a variety of chemical products.Nickel-based catalysts are widely used in DRM reactions due to their high activity and low cost.According to thermodynamic analysis,DRM is a highly heat absorbing and reversible reaction requiring high reaction temperatures.Therefore,nickel-based catalysts used under such conditions may be deactivated due to sintering of the active metal.The low activity and selectivity of large-sized metal nanoparticles（above 10 nm）for both H₂and CO also tend to lead to carbon deposition covering the active site,which may further accelerate the deactivation of the catalyst.In this paper,two silica-based molecular sieve carriers were modified and then the active nickel metal was anchored using a clever synthetic method to obtain efficient and stable catalysts for dry reforming of methane.A series of in situ experiments such as in-situ FTIR,in-situ warming XPS and in-situ XRD were used to investigate the interaction between the nickel species and the carrier during the calcination process.In addition,the effect of Ni particle size and metal-carrier interaction forces on the catalytic performance of the catalysts was investigated by XRD,TEM,H₂-TPR,in-situ XPS,UV-Vis-DRS and hydrogen-chemisorption experiments.The specific tasks are as follows:The Ni/Si Beta catalyst was prepared by solid-grinding.The carrier Si Beta was obtained by hydrothermal dealumination of commercial Beta zeolite molecular sieves,which formed a large number of silanol nesting structures（T-vacancies）in the original aluminium atom positions.The Ni/Si Beta showed excellent catalytic performance during the 100-h high temperature DRM reaction,with initial conversions of methane and carbon dioxide reaching 76.5%and 83.2%,respectively,without significant deactivation.Characterisation revealed that the nickel precursor,nickel acetylacetonate,was thermally decomposed during the catalyst preparation and adsorbed as Ni O in the T vacancies of the Si Beta carrier under the action of flowing air,forming fine and homogeneous nickel particles after reduction.The TG test of the catalysts after the reaction revealed that all samples showed very low carbon accumulation in the DRM reaction at 750°C（less than 10.3%）.Therefore,the most important factor affecting the catalyst performance was the particle size and stability of the active metal.Compared to Ni/Al₂O₃ and Ni/Si O₂ catalysts prepared by conventional impregnation,Ni/Si Beta has a relatively moderate metal carrier interaction,which ensures the stability of nickel at high temperatures while maintaining the availability of nickel as an active site.In the DRM catalytic tests,Ni/Si Beta catalyst combines the long-term stability of Ni/Al₂O₃ with the high activity of Ni/Si O₂.The Ni/SBA15-OH catalyst was prepared by the"vapour phase diffusion"synthesis method.The rehydroxylated SBA15-OH carrier was obtained by conditioning ordinary SBA15 molecular sieves in a solution of H₂O₂ at a concentration of 3 wt%,with a higher concentration of surface hydroxyl groups.Under the combined effect of the pore-limiting domain of the carrier and the adsorption of hydroxyl groups on the surface,the decomposition of nickel acetylacetonate into Ni O was uniformly dispersed on the SBA15-OH carrier,which after reduction formed fine nickel particles with an average size of less than 2 nm.In high temperature DRM tests,the initial conversion of Ni/SBA15 to methane and carbon dioxide reached 87.9%and 95.8%,respectively,with essentially no decrease after 150 h.The excellent catalytic activity and stability of the Ni/SBA15-OH catalyst has great potential for practical DRM processes.