Studies On Stability And Coke-Resistant Properties Of Ni-based Surface Alloy Catalysts For Dry Reforming

The greenhouse gas, carbon dioxide, combined with the main ingredient of natural gas, methane, can be utilized by CH4-CO2dry reforming, which is suitable for natural gas reforming reaction as natural gas is rich for CO2. It is an environmental and economical utilization route for CO2to prepare the extensively useful syngas. Compared with the industrialized steam reforming of methane, dry reforming has the following advantages:low energy consumption, high CO selectivity and low H2/CO ratio.The catalysts used in dry reforming can be divided in two kinds:noble metal and Ni-based catalysts. In spite of excellent resistance to coke formation, noble metal catalysts afford too much. The cheap Ni catalysts with high activity are more inclined to be employed in industrial application. However, coke can easily deposit on Ni catalysts, resulting in the decrease of catalytic activity. Aimed at this practical issue, we designed two catalyst systems to develop the coke-resistant properties through doping additives with Ni to prepare surface alloy catalysts in the present study.At the very start, we developed a bottom-up approach called methanol-thermol through a topology chemistry reaction to synthesize Sn, Ce, Mn, or Co promoted Nio.10Mgo.90O catalysts enclosed by Tasker III type polar (111) facets. Sn and Co were evenly distributed on Nio.ioMgo.9oO, whereas Ce and Mn segregated as separate oxides. Mn didn’t show obvious enhancement to catalytic stability towards Cp formation, and a lower activity was observed as it separated from catalyst surface and caused agglomeration of Nio.ioMgo.9oO(111) platelet catalyst. Cp and whisker carbons were found on Ce promoted Nio.ioMg0.90O(111) catalyst, for Ce segregated as CeO2. Both Sn and Co are distinguished to enhance the coke-resistant properties of Nio.10Mgo.90O catalyst in CH4-CO2dry reforming, as they inhibited the formation of Cp. Kinetic studies show that Sn inhibits the nucleation of carbon while Co increases the elimination rate of coke.Thereafter, a series of Ag promoted Ni/CeO2catalysts have been explored in CH4-CO2dry reforming reaction. The temperature-dependent catalytic activities and time-on-stream catalytic performances at760℃under different feeding conditions were evaluated. Pristine Ni/CeO2was not stable for coke deposition, morphology change under H2reduction pretreatment and reaction conditions. Co-feeding of H2can adversely influence the long term stability of Ni/CeO2. Ag severely reduced the catalytic activity in dry reforming of methane, while enhanced the long term stability by diminishing coke deposition. Ag also altered the type of formed coke from recalcitrant whisker carbon and graphitic encapsulated carbon to easily gasifiable amorphous carbon. Ag was applicable as a promoter towards feedstock variations from carrier gas diluted feeding to H2co-feeding and carrier gas free conditions. Kinetic studies show that Ag elevates the activation energy from91to ca.140kJ/mol by a loading of0.3mol.%, and excessive Ag loading does not affect the activation energy. The role of Ag is to block step sites on Ni surface associated with carbon nucleation and growth, and promote gasification of formed coke.