Modification And Catalytic Performance Study Based On Improving Reactivity Of Nickel Based Catalysts For Methane Reforming

Dry reforming of methane(DRM)was a chemical process for converting methane and carbon dioxide to syngas(mixture of H₂ and CO),which provided a promising way to utilize surplus natural gas and reduce the concentration of carbon dioxide in the atmosphere.Among the catalysts available currently,Ni-based catalysts has attached great attention owing to the relatively high activity,wide-accessibility and low-cost.However,the inferior anti-coke and anti-sintering performance was the main obstacle for its industrial and commercial application.Therefore,extensive researches have taken every effort to develop highly active and stable Ni-based catalysts with excellent anti-coke and anti-sintering performance.In this context,the main objective of this study was to prepare highly active and stable Ni-based catalysts.Considering the acceleration effect of hierarchical porous structure on mass transfer,the promotion effect of lanthanide oxide on carbon tolerance and synergism between metallic Ni and Co,lanthanide oxide modified Ni-based catalysts and bimodal porous alumina supported Ni-Co alloy catalysts were synthesized and employed to promote DRM reaction.The effect of La,Ce and Co on the physiochemical properties,carbon tolerance and catalytic performance was investigated in detail.Moreover,on the basis of verifying the promotion effect of large-size pore structure on improving anti-coke performance,the catalytic performance of bimodal porous hydroxyapatite supported Ni catalysts was investigated.The detailed works were described as following:(1)In this work,a series of silica supported nickel catalysts containing several lanthanide oxides(Ni La/Si O₂,Ni Ce/Si O₂,Ni Sm/Si O₂,Ni Gd/Si O₂)were prepared via“one-pot”method and employed to promote dry reforming of methane.The effect of the lanthanide oxide on carbon tolerance and catalytic performance was investigated in detail.The presence of La,Ce on bulk Si O₂ could effectively prevent the agglomeration of Ni nanoparticles,thus,Ni La/Si O₂ and Ni Ce/Si O₂ exhibited great anti-sintering ability.Moreover,Ni La/Si O₂ and Ni Ce/Si O₂ showed better anti-coke capacity with lower carbon amount(0.7%,0.2%)and graphitization degree.Kinetic study confirmed that adding lanthanide metal oxide significantly affected the activation energy of CH₄ dissociation(E_{a CH4})and CO₂ dissociation(E_{a CO2})compared to monometallic Ni/Si O₂(E_{a CH4}=95.6 k J/mol,E_{a CO2}=88.6 k J/mol).The presence of Sm promoted CH₄decomposition(82.8 k J/mol)and inhibited CO₂ dissociation(133 k J/mol).With almost same E_{a CH4} as Ni/Si O₂,Ni Gd/Si O₂ showed higher E_{a CO2}(132.7 k J/mol),suppressing CO₂dissociation.Ni La/Si O₂ exhibited opposite trend toward Ni Gd/Si O₂ that adding La accelerated CO₂ dissociation(70.9 k J/mol).Introduction of Ce into Ni-based silica catalyst effectively promoted CH₄ decomposition(79.8 k J/mol)and CO₂ dissociation(67.2 k J/mol).Therefore,the addition of lanthanide oxide altered the CH₄ decomposition and CO₂ dissociation,indirectly affecting the carbon generation and oxidation process,which was mainly responsible for the discrepancy of anti-coke performance between the four lanthanide-promoted silica supported nickel catalysts.(2)In this work,ceria-modified hierarchical porous silica supported nickel catalysts(Ni Ce X-Y/HS)were prepared by“one-pot”method and employed to promote DRM reaction.The physiochemical properties,anti-coke ability and catalytic activity of the resulting catalysts was investigated.Ce O₂ surrounded on Ni nanoparticle could efficiently prevent the sintering of Ni nanoparticle.The number of active oxygen species gradually increased with increasing Ce O₂loading,which was beneficial to improve the anti-coke ability by means of participating in the oxidization of carbon.Therefore,Ni Ce X-Y/HS catalysts exhibited lower carbon deposition amount and graphitization degree.Ni Ce5-2/HS and Ni Ce5-3/HS possessed strong anti-sintering ability and carbon resistance,thereby showing excellent stability that CH₄ conversion decreased from 81.3 to 75.6%and 82.2 to 78.2%over 24 h time on stream.(3)In this work,a series of hierarchical porous silica supported nickel catalysts containing different lanthanum loading(Ni La5-0.5/HS,Ni La5-1.5/HS,Ni La5-3/HS,Ni La5-4.5/HS)were prepared via“one-pot”method and used to promote dry reforming of methane(DRM)reaction.The effect of La content on physiochemical properties,carbon tolerance and catalytic performance was investigated in detail.All catalysts possessed hierarchical porous structure,including small-size mesopore at about 3 nm,large-size mesopore at 40 nm and macropore>50 nm.However,the presence of La weakened the metal-support interaction compared to un-modified silica supported nickel catalyst(Ni/HS)and did not effectively prevent the metal sintering because addition of La suppressed the formation of strong-bond 1:1 Ni-phyllosilicate on catalysts.In spite the negative influence on metal-support interaction,the catalysts modified by La exhibited excellent anti-coke performance and the coke tolerance significantly enhanced with increasing La content.The rapid deactivation for Ni/HS was mainly attributed to abundant carbon nanotubes formed on catalyst surface.Similarly,the relatively high carbon deposition was responsible for the decay of activity for Ni La5-0.5/HS.The deactivation for Ni La5-1.5/HS was caused by the serious decrease in surface area and carbon deposition.As for Ni La5-3/HS and Ni La5-4.5/HS,the dramatic decrease in surface area and sintering of Ni were the significant factors for the deactivation.(4)In this work,bimodal alumina supported Ni-Co alloy catalyst(Ni Co/MMAl)was prepared via evaporation-induced self-assembly(EISA)method using P123 as well as monodisperse polystyrene spheres as template and employed to promote DRM reaction.The promotion effect of Ni-Co alloy with bimodal porous structure on the anti-coke performance was investigated.N₂ adsorption-desorption result revealed that the resulting catalyst(Ni Co/MMAl)possessed two different porous structure,including small-size mesopore at 10 nm and large-size mesopore at 40 nm.XRD,TEM and XPS results clearly proved the presence of Ni-Co alloy on bulk alumina.As expected,Ni Co/MMAl exhibited enhanced coke tolerance that only 2.3%carbon formed on catalyst surface,lower than that on Ni Co alumina catalyst without large-size mesoporous structure(Ni Co/MAl,12%)and monometallic Ni alumina catalyst(Ni/MAl,24%).Kinetic study indicated that alloying Ni with Co substantially increased the activation energy of CH₄ dissociation compared to monometallic Ni,suppressing CH₄ dissociation,which was significantly responsible for the promotion effect of Ni-Co alloy on improving coke resistance ability.The presence of large-size mesoporous structure(40 nm)on bulk Ni Co/MMAl,with the properties of enhancing mass transport,which was partly contributed to improve anti-carbon performance.Therefore,the enhanced carbon tolerance for Ni Co/MMAl was not only attributed to the inhibition of CH₄ dissociation by Ni-Co alloy but also to the promotion effect of large-size mesopore structure on enhancing mass transport.(5)In this work,hydroxyapatite(HAP)supported nickel catalysts were prepared and employed to promote the dry reforming of methane(DRM).The proportion of mesopore/macropore in HAP was adjusted by the heating temperature(20,50,80,120°C)during the preparation process.The pore structure played an important role in regulating the physiochemical properties and thus affecting the catalytic performance in DRM.With increasing the proportion of mesopore on HAP,Ni preferentially dispersed on mesopore channel to form small-size particles.High proportion of macropore led to large-size Ni nanoparticle,while high proportion of small-size mesopore at 2-5 nm resulted in the complete blockage of these pore structures.The above mentioned two situations led to relatively weak interaction between nickel particles and the support,low metal dispersion and hence low activity as well as rapid deactivation in the DRM evaluation.Ni nanoparticles on HAP-80 and HAP-120 supports possessed relatively higher dispersion,which imparted these two catalysts with the function of strong anti-sintering ability and carbon resistance,thereby exhibiting better catalytic activity and stability in DRM evaluation.The deactivation of all catalysts during the reforming reaction was mainly ascribed to the sintering as well as encapsulation of Ni nanoparticles by graphitic carbon.