| Carbon dioxide reforming of methane to synthesis gas has received worldwide interesting,because it converts two greenhouse gases into valuable syngas,which is an important feedstock for Fischer-Tropsch synthesis and methanol synthesis.Ru catalyst is a potential choice for the CH4-CO2 reforming considering its high activity and low cost compared with other noble metals.The use of nickel as catalyst is also a preferable choice because of its relatively high activity and low cost.However,the major problem of these catalysts is the deactivation due to coke deposition and/or sintering of metal particles.Therefore,the development of catalyst with high activity and stability is highly desirable.In this thesis,various supported Ru and Ni catalysts have been prepared by using Mg-Al layered double hydroxides(LDHs)as catalyst precursor or LDHs-derived Mg(Al)O mixed metal oxide as support,and their catalytic activity and stability for the CH4-CO2 reforming were investigated.The catalysts before and after reaction were well characterized by various techniques including inductively coupled plasma(ICP),N2 physical adsorption,powder X-ray diffraction(XRD),temperature-programmed techniques(H2-TPR,O2-TPO,and CO2-TPD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),CO chemisorption,Fourier transform infrared spectroscopy of CO adsorption(CO-FTIR),X-ray photoelectron spectroscopy(XPS),thermogravimetric analysis(TG),and Raman spectroscopy.The main results obtained are as follows:(1)Ru catalysts supported on y-Al2O3,MgAl2O4,Mg3(Al)O,and MgO were prepared by incipient impregnation method to investigate the effect of support on the Ru metal dispersion and the catalytic activity and stability for the CH4-CO2 reforming.The characterization results reveal that Ru metal was highly dispersed on the Mg3(Al)O support obtained from Mg-Al LDH,probably existing in very small nanoparticles(<1 nm)and/or clusters.The Ru metal dispersion decreased in the order of Ru/Mg3(Al)O>Ru/MgO>Ru/MgAl2O4>Ru/?-Al2O3.Both Ru/MgO and Ru/Mg3(Al)O showed higher activity than Ru/MgAl2O4 and Ru/?-Al2O3,which might be related to the strong base intensity of support and more accessible surface RuO atoms,respectively.When tested at 1023 K for 30 h,a significant deactivation was observed on Ru/y-Al2O3,Ru/MgAl2O4,and Ru/MgO due to the sintering of Ru metal particles.In contrast,Ru/Mg3(Al)O showed superior stability and no significant sintering of Ru metal was observed,suggesting that the highly dispersed Ru metal was stable.A 300 h long-term test further demonstrates the excellent stability of Ru/Mg3(Al)0.Irn addition,the influence of Ru loading and calcination temperature on the catalytic performance of Ru/Mg3(Al)O was investigated.The optimal Ru loading is 2 wt%and the optimal calcined temperature is 773 K.(2)Supported Ni/Mg/Al catalysts with Ni loadings of 3-18 wt%were prepared from Ni-Mg-Al LDHs to investigate the influence of Ni loading on the catalytic activity and stability.It is found that the catalyst activity increased with increasing the Ni loading.However,the CH4 decomposition and/or CO disproportionation became significant as the Ni loading increased.The long-term stability of the Ni/Mg/AI catalysts was influenced by the Ni loading as well as the reaction temperature.At 1023 K,the catalyst stability increased with increasing the Ni loading,whereas it decreased with increasing the Ni loading at 873 K.The characterization on the spent catalysts showed that the agglomeration of Ni metal particles increased with increasing the Ni loading,and it became more severe at 873 K than at 1023 K.On the other hand,the coke deposition at 1023 K tended to decrease with increasing the Ni loading,and an opposite tendency was observed at 873 K.The coke deposition particularly the encapsulating carbon was considered to be responsible for the catalyst deactivation.A Ni loading of 12 wt%would be suitable for the Ni/Mg/Al catalyst considering its high activity,good stability,and less coke deposition at high temperature. |
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