Ni-based Core-shell Catalysts For Dry Reforming Of Methane: Preparation And Catalytic Evaluation

Dry reforming of methane（DRM）can convert two greenhouse gases（CO₂ and CH₄）into syngas（H₂ and CO）,which are the primary chemical feedstock for the processes of useful chemicals synthesis.Ni-based catalysts have been widely investigated and considered to be promising for the industrial utilization in DRM reactions due to the high activity and low cost compared to the noble metal-based catalysts.However,suffers severe catalytic deactivation caused by the sintering of active Ni particles and carbon deposition.Therefore,the key to industrialization is to improve the stability and carbon-resistance of Ni-based catalyst in DRM reaction.The catalysts with the core-shell structure have been intensively studied during the past several years for their remarkable advantages against agglomeration and sintering of active metals in high temperature reaction.In this paper,the core-shell structured Ni-based catalysts were prepared by use the shell of SiO₂,CeO₂ and MgAl₂O₄,respectively,and its catalytic performance in the DRM reaction was evaluated.1.The Ni NPs have been synthesized via the solvent-thermal polyol process,which can automatically align into chains and have a good crystal structure with an average diameter about 30 nm.Moreover,they can be finely dispersed in ethanol and water.PVP plays multiple roles in the preparation process,which could as a stabilizer better dispersing the Ni nanoparticle and protect Ni NPs from oxidizing to NiO.2.The Ni@SiO₂ core-shell catalysts have been synthesized via St?ber’s method using TEOS.Compared to the traditional supported Ni/SiO₂ catalysts,the core-shell Ni@SiO₂catalysts display much better stability at high temperature reaction.The protective effects of SiO₂ shell can inhibit metallic nickel sintering and carbon deposition,hence,maintaining more active centers on the catalyst.The TGA analysis showed that the accumulated carbon deposition on the on the Ni@SiO₂ catalyst was significantly lower than that on the traditional Ni/SiO₂ catalyst after the DRM reaction of 24h,demonstrating that the core-shell Ni@SiO₂ catalyst have a better carbon-resistance ability and a better catalytic activity.3.The Ni@CeO₂ core-shell nanocatalysts were synthesized via the hydrothermal synthesis method using Ni NPs and Ce Cl₃ as the feedstock at 90°C in 50%C₂H₅OH-50%H₂O system.The results of DRM reaction showed that the catalytic activity and high temperature stability of Ni@CeO₂ were significantly higher than that of Ni@SiO₂.Combining the O₂-TPO and TG results,we found that less carbon was deposited on the Ni@CeO₂catalyst during the DRM reaction,and more importantly,the carbon deposition on the Ni@CeO₂ catalyst is more reactive and easily to be removed by oxidation.Ni@Ce O₂catalyst have a higher temperature stability and a better carbon-resistance ability during the reaction of DRM due to more active oxygen could react with the carbon species on the surface of catalysts and the CO₂ could be easier to activate as the more lattice oxygens and vacancies afforded by the CeO₂.4.The nickel-embedded carbon sphere was synthesized firstly via the adsorption mechanism of carbon adsorption,and using glucose and NiCl₂ as the raw material.What’s more,the Ni@Mg Al₂O₄ catalysts was synthesized by hydrothermal synthesis.The results showed that the existence of NiCl₂ have a high effect on the size of nickel-embedded carbon sphere;although the catalytic activity of Ni@Mg Al₂O₄decreased along with the decreasing of Ni,the thermal stability and the selectivity of H₂and CO were both improved.5.The Ni@CeO₂ catalyst and DBD plasma hybrid reaction catalysis for the dry reforming of methane have been studied in this part.The results showed that the conversion rates of CH₄ and CO₂ and the selectivity of H₂ and CO were both increased along with the increased input energy of plasma.Although the CH₄ and CO₂ conversion rate of the synergetic catalytic reaction was between in the Ni@CeO₂ alone under 973K and the of Ni@CeO₂ alone under 773K,the reaction temperature of the synergetic catalytic reaction was far lower than 773K,demonstrated that the plasma synergetic catalytic reaction have overcome the problem of high energy consumption by DRM reaction.In addition,the results also showed that the conversion rates of CH₄ was higher than that of CO₂ in the plasma synergetic catalytic reaction,which was contrary to the Ni@CeO₂ alone under high temperature,and the carbon deposition was smaller than that of Ni@CeO₂ alone under 24-hour reaction.These studies indicate that,compared to the supported Ni-based catalyst,core-shell Ni Base catalyzed by Nano-catalyst in the reaction of DRM have a better stability and a better carbon deposition resistance,these are due to the core-shell structure of"encapsulation effect"inhibits migration of Ni nanoparticles in the reaction of high-temperature,thus preventing active metals Ni inactivation as the high temperature sintering.In addition,using CeO₂ as the lattice shell structure can provide a lot of oxygen,makes DRM have more activity in the reaction of oxygen reacts for the carbon species on the surface of catalysts to be oxidized to CO,and at the same time,the oxygen holes make it easier for the activation of CO₂ to be CO and active oxygen,leading to improving anti-coking performance of Ni@CeO₂ catalyst significantly.