Study On Stability Of Ni-based Catalyst For Partial Oxidation Of Methane To Syngas

The catalytic partial oxidation of methane (POM) to syngas had been extensively investigated, because of low cost , low consumption of energy and the syngas produced with a proper H2/CO molar ratio of 2/1, which could be directly used as feed for methanol or Fischer-Tropsch synthesis. The main factor holding back this technology from being industrialized is the inability to develop a catalyst of high activity and stability. In the early stage of the research,the POM process was investigated in a fixed bed micro-reactor system. The Ni-CeO₂ and Ni-Ce_0.95Zr_0.05O₂ bulk catalyst were prepared by optimal conditions. In this work the stability of Ni-CeO₂ and Ni-Ce_0.95Zr_0.05O₂ catalyst were studied by means of long period activity evaluation and a series of characterization methods, in order to explorate several factors which affect the catalyst stability and provide useful information for the improvement of the catalyst stability.At first the Ni-CeO₂ catalyst was evaluated for 82h and the results were CH4 conversion 99.6%, CO selectivity 96%, H2 selectivity 93%. The reaction performance of the Ni-CeO₂ catalyst was stable. But at the eighty-second hour the sintering of Ni-CeO₂ catalyst was found. Under the same reaction condition the Ni-Ce_0.95Zr_0.05O₂ catalyst had been used for 87 hours and the results were CH4 conversion 99.4%, CO selectivity 97%, H2 selectivity 93%. Experimental results clearly proved that Ni-Ce_0.95Zr_0.05O₂ catalyst provided greater stability as compared to the Ni-CeO₂ catalyst , due to the addition of ZrO₂ promoter.The Ni-CeO₂ and Ni-Ce_0.95Zr_0.05O₂ catalyst were characterized by means of XRD, XPS, SEM, TGA-MS. The result showed that the accumulation of the lattice oxygen in the Ni-CeO₂ catalyst aggravated the rising of the temperature till the hot spot appeared. It caused the catalyst sintering. The ZrO₂ promoter decreased the accumulation rate of lattice oxygen. The stability of the Ni-Ce_0.95Zr_0.05O₂ catalyst was better than that of the Ni-CeO₂ catalyst. Otherwise some metallic carbide was found on the Ni-CeO₂ catalyst but no carbide specy was found on the Ni-Ce_0.95Zr_0.05O₂ catalyst. The metallic carbide generated at the high temperature and was stable in the reaction. So the capability of the carbon resistant of the Ni-Ce_0.95Zr_0.05O₂ catalyst was even better.The carbon deposition in the partial oxidation of methane was reseached by means of the SEM, TGA-MS. The results proved that in the process of POM , some whisker tubular carbon was found on the Ni-CeO₂ catalyst. Though it did not affect the activity of the catalyst but a large amount of it might make the catalyst pulverize, which would block the reactor. Therefore, the formation of the whisker tubular carbon should be inhibited. The whisker tubular carbon was not found on the Ni-Ce_0.95Zr_0.05O₂ catalyst after 87h reaction. When the Ni-CeO₂ catalyst was used in the POM, the initial amount of carbon was only about 4%. As the experiment continued for a period of 4h, the carbon content increased to 10%.