Preparation,Characterization,and Catalytic Performance Of Ni-Cu Alloy Catalysts For Hydrocarbon Reforming

Synthesisi gas(CO + H2)is an important feedstock for Fischer-Tropsch synthesis and methanol synthesis to produce clean liquid fuels and chemicals.Catalytic reforming of hydrocarbons such as steam reforming of biomass tar and carbon dioxide reforming of methane is the most effective method to produce synthesis gas.Owing to their low cost and relatively high activity,supported Ni catalysts have been widely investigated for the reaction.However,the main problem of Ni catalyst is the deactivation caused by coke deposition and sintering of Ni metal particles.Therefore,development of Ni catalyst with high activity and stability is highly desirable.In this thesis,various Ni-Cu alloy catalysts with high dispersion and uniform composition have been successfully prepared from Ni-Cu-Mg-Al hydrotalcite(HT)precursors by co-precipitation method,and their catalytic performance for the steam reforming of biomass tar and carbon dioxide reforming of methane are investigated under different reaction conditions.The catalysts before and after reaction were well characterized by various techniques including powder X-ray diffraction(XRD),temperature-programmed techniques(H2-TPR,O2-TPO,and TPSR-MS),H2 chemisorption,Fourier transform infrared spectroscopy of CO adsorption(CO-FTIR),scanning electron microscopy(SEM),scanning transmission electron microscopy-enery dispersive X-ray(STEM-EDX),thermogravimetric analysis(TG),and Raman spectroscopy,in order to understand the effect of Ni-Cu alloying on the catalyst structure and catalytic performance.The main results obtained are as follows:(1)All of the as-synthesized precursors exhibit a pure hydrotalcite phase.Upon calcination at 1073 K,Ni-Cu-Mg-Al HT is decomposted to form Mg(Ni,Cu,Al)O solid solution,and by reduction treatment at 1073 K well-dispersed Ni-Cu alloy nanoparticles are formed.STEM-EDX analysis reveals that the alloy particle size decreases with the increase of Cu/Ni ratio,and the alloy particles have a uniform composition;moreover,H2 chemisorption and CO-FTIR indicate that Cu has a tendency to be enriched on the particle surface.The formation of composition-uniform Ni-Cu alloy nanoparticles is attributed to the homogeneous distribution of Ni2+ and Cu2+ in the HT precursor and the resulted Mg(Ni,Cu,Al)O solid solution.This result highlights that using Ni-Cu-Mg-Al HT as precursor is effective for preparing Ni-Cu alloy catalyst.(2)The result of steam reforming of 1-methylnaphalene(1-MN)as tar model compound reveals that the alloy catalysts show a volcano-type dependence of activity on the Cu/Ni ratio,where the highest activity is obtained at Cu/Ni = 0.25.Moreover,this optimum alloy catalyst shows higher catalytic stability and higher resistance to coke deposition than the Ni catalyst.The kinetic study indicates that 1-MN is strongly adsorbed on both the Ni and Ni-Cu catalysts,whereas the adsorption of steam is promoted by the alloying of Ni with Cu.This result is further confirmed by H2O-TPSR of deposited coke on the spent catalysts.It is suggested that the Ni-Cu alloy nanoparticles may promote the dissociative adsorption of steam to form more adsorbed oxygen species,which enhances the gastification of carbonaceous species,leading to higher activity and lower coke deposition.In addition,the particle size effect and geometry effect induced by Ni-Cu alloying may also contribute to the improved coke-resistance.(3)Carbon dioxide reforming of methane over the catalysts shows that the catalyst with Cu/Ni = 0.25 has higher performance in terms of activity,stability,and coke-resistance.Under the conditions of CH4/CO2/N2 = 1/1/2,WHSV = 60,000 mL g-1 h-1,T= 873 K,and 25 h,the coke deposition on the optimum alloy catalyst is only?1/48 times that on the Ni catalyst,suggesting that the coke deposition was significantly suppressed on the Ni-Cu alloy particles.CH4-TPSR and CO-TPSR indicate that alloying Ni with Cu can inhibit the CH4 decomposition and CO disproportionation,both of which are mainly responsible for the coke deposition;this would be benefitful to reduce the formation rate of carboneous species on the metal surface.Furthermore,CO2-TPSR/H2-TPR and CH4-TPSR/O2-TPO suggest that Ni-Cu alloy may promote the dissociation adsoption of CO2,and the oxygen species adsorbed on Cu metal have higher reactivity,which may enhance the gasification of carboneous species,leading to less coke deposition.