Preparation Of Nickel-based Catalyst By Solution Combustion And Its Catalytic Methanation Performance

The heavy emission of SO_x,NO_x and VOC,largly resuled from direct burning of coal,has caused severe environmental pollution during the past decades.Most areas of China are suffering haze weather,as reflected by the air quality index,which increased the concern of pollution issue for public.It is a huge and emergent challenge for China to develop sustainable and friendly.Coal to natural gas via gasification,cleaning,conditioning and methanation is an effective way for clean and efficient utilization of coal,among which,CO methanation is a vital process.Methanation catalyst plays a key role in this reaction,which not only affects the design of reactor,the technological processes are also decided by the catalytic activity.Up to now,nickel-based catalysts,normally prepared by impregnation or precipitation methods,were widely adopted in industrial methanation reaction,due to its readily accessibility and benign activity.However,these catalysts were usually suffering from deactivation by sintering and carbon deposition on poorly dispersed and large particles.In addition,its poor low-temperature activity restricted the industrial application.Solution combustion synthesis is a rapid and effective technology for preparation various nanosized materials with small size,regular shape and high dispersion.Thus,this method enables preparation of diverse catalytic materials with high activity.In this research,nickel-based catalysts were successfully synthesized by solution combustion method.The key factors that controlling the solution combustion process,including combustion enthalpy,gases evolved and chemical environment of precursor solution,were thoroughly investigated in terms of fuel types,fuel ratio,fuel additives and soluble salts.These factors corporately influenced on the structure of nickel catalysts and further on the catalytic activities and stabilities during CO methanation.The major conclusions are listed below:（1）Among the four fuels used,i.e.,urea,glycine,ethylene glycol and citric acid,combustion generated the minimum amount of heat and released the largest quantity of gases when urea was used as fuel.Meanwhile,due to a majority of urea,Ni（NO3）2 and Al（NO3）3 decompose simultaneously and occur in extensive overlapped temperature intervals during heating,a moderate and sustained combustion reaction is initiated,which is conducive to the heat dissipation.As a result,the NiAl–U catalyst exhibited the smallest size of Ni particles of 9.2 nm;the BET area and Ni surface area reach up to 256.2 m2/g and 34.6 m2/g,respectively,and more easily reducible and highly dispersive Ni species were formed.In the slurry methanation of CO,the sequence of catalytic activity is in good agreement with the results of physicochemical properties,and the CO conversion and selectivity of CH4 over the Ni Al–U catalyst reach up to 95.7% and 96.2%,respectively,at 300 °C;moreover,it does not exhibit deactivation within 200 h,which is superior to the properties of other combustion–synthesized Ni–Al₂O₃ catalyst and commercial catalyst.（2）The ratio of reducing valences to oxidizing valences（RV/OV）in redox systems of metal nitrates and urea solutions plays a key role in the combustion process to prepare nanosized Ni–Al₂O₃ catalysts.In precursor solution,Ni²⁺ is coordinated with urea to form nickel ammine,which enhances diffusion of Ni²⁺,and the increase of RV/OV ratio results in more urea coordinate with Ni²⁺.During combustion process,the combustion enthalpy and gases act synergistically to achieve a controlled physicochemical properties of yielded catalysts.When RV/OV≤0.75,the effect of gases is critical to form high surface area to disperse Ni nanoparticles.Whereas the combustion enthalpy progressively increases with increasing RV/OV,when RV/OV≥0.75,the combustion enthalpy produce high temperature,facilitating NiO migrate into Al₂O₃ matrix to form low active precursor NiAl2O4 spinel.As a result,the catalyst with RV/OV=0.75 exhibits the maximum nickel surface area and the smallest Ni particles size of 62.6 m2/g and 10.8 nm,respectively.The conversion of CO and selectivity for CH4 over the optimized catalyst reach up to 94.5% and 91.3% at 280 °C,1.0 MPa and 3000 mL/gcat·h.（3）A procedure for fabricating Ni–Al₂O₃ catalysts consisting of small and dispersive nickel nanoparticles was introduced,which was achieved by adding fuel additives in urea–nitrates solution combustion system.In precursor solution,the acetate and amino ligands containing ammonium acetate exhibits a stronger complexation ability than other fuel additives,which effectively facilitates the diffusion of Ni²⁺ cation.During combustion process,the addition of ammonium acetate result in numerous ignition points and uniform propagation of flames in gas–phase.On the contrary to starch and PEG,the decomposition of ammonium acetate during heating process is endothermic and synchronous with urea and metal nitrates,which largely lowers the combustion temperature to produce small and homogenous nickel particles.Moreover,amino group containing ammonium acetate is a more reactive fuel additive than starch and PEG that involve hydroxyl that drives the reaction to be complete instantly.As demonstrated,the optimum Ni–Al₂O₃ catalyst exhibits a promising catalytic activity and stability for methanation of CO to methane in a slurry reactor.（4）A novel solution combustion synthesis of Ni–Al₂O₃ catalyst has been developed by introducing alkali/alkaline earth metal chlorides in the combustion redox under microwave irradiation.Among the investigated metal chlorides,i.e.,LiCl,NaCl,KCl,MgCl2 and CaCl2,the addition of NaCl minimized the combustion temperature to decrease the coarsen and agglomeration of NiO particles.In addition,the melting point of NaCl was close to the adiabatic temperature of NaCl-added system,the molten NaCl exerted a steric hindrance effect by forming a layer of NaCl crust on the surface of the newly formed nanoparticles to restrain the agglomeration of the grains and stabilize the derived nanoparticles.The synthesized Ni–Al₂O₃ catalysts were examined towards slurry phase methanation of carbon monoxide,among which the NaCl modified catalyst showed the best activity.In a 100 h lifetime test under harsh condition,the CO conversion over the microwave treated Ni Al–Na catalyst kept stable at ca.87%.The significant activity and stability mainly because of the small and dispersive Ni particles resulted from the low combustion temperature and confinement effect by the melting NaCl.Also,the homogenous heating environment during microwave radiation was making considerable contribution to enhance the metal dispersion.These studies thus offer a new strategy to combustion synthesis of supported metal catalyst by introducing metal chlorides and utilizing microwave as heating source.