Methane Dry Reforming: Designing Potent Coke-resistant Ni-based Catalysts

Nowadays, energy and environment protection become more and moreimportant topics for all of the world. Increasing attention has been paid to thecatalytic methane dry reforming with carbon dioxide (CRM). Dry reforming ofmethane has been considered as a good options, which makes a full use of methaneand at the same time consumes a significant amount of greenhouse gas, carbondioxide. Ni-based catalysts have been widely investigated due to their high activity,abundant source and low price, but there are also still some remained problems. Forinstance, severe coke deposition, which deactivates the Ni active sites and evenblocks the reactor, is a major problem to be solved. It’s very necessary to design acoke-resistant catalyst to industrialize dry reforming process. In this thesis Sn, Ce andNd have been used as promoters to modifiy Ni-based catalysts. With intentionaldesign, a series of coke-resistant catalysts have been obtained and the reasons leadingto the prohibiting of coke formation have been rationalized. These information mayhelp people to design and develop stable dry reforming catalysts for industrialapplication.First，the improvement of coke resistance for Ni/Al2O3by Sn addition wasstudied. Ni-based catalysts doped by a small amount of Sn were prepared usingco-impregnation method. It’s interesting to find that a small amount of Sn addition onNi could effectively decrease the carbon deposit formation with a minor loss ofreforming activity. With the increasing of the Sn/Ni molar ratio up to0.02, the cokingof the catalysts can be completely suppressed but at a reasonable loss of the reformingactivity. No any coke can be detected on the catalyst even after reaction with anextended period of150h.Catalysts with a Sn/Ni molar ratio of0.02were thus intentionally prepared byco-impregnation or two step impregnation, with the expectation to find the role of Snin the enhancement of coke resistant of the catalysts. It was found that the step ofimpregnation had a great influence on the catalytic performance. The catalystsprepared with co-impregnation had the best coke resistant but with the lowestreforming activity. The H2-TPR results show that Sn addition increased the reducibility of the NiAl2O4species formed on the fresh samples. Sn made thedesorption of CO2easier, as confirmed by CO2-TPD. XRD and TEM results showedthat the presence of Sn improved the crystallite and particle sizes of Ni active sites.The formation of Sn-Ni alloy increases the size of metallic Ni, and the results ofSTEM-EDX mapping testified this further. It was concluded that the formation ofsurface Ni-Sn alloys was the main reason for the improved coke resistance of thecatalysts. Based on the information here, reforming catalysts with superior cokeresistance and applicable activity could be designed and developed by adjusting theamount of surface Ni-Sn alloys.Second，Ce and Nd modified Ni/Mg/Al catalysts have been studied. Thehydrotalcite-type precursors with a Mg/Al molar ratio of2.5were prepared usingcoprecipitation method. Then the Ni catalysts were prepared by impregnation methodwith hydrotalcite-type precursors or γ-Al2O3as support. It was found that thecatalysts, which using hydrotalcite-type precursors as support, had a better reformingactivity and coke resistance than the catalysts with γ-Al2O3as support. The XRDresults showed that in Ni/Mg/Al catalysts, Ni was doped into MgO lattices and toform solid solution. The formation of solid solution indeed made Ni more difficult tobe reduced, which was testified by H2-TPR results. The formation of Ni-Mg-O solidsolution also made the catalysts have smaller Ni crystallite sizes than the Ni/Al2O3these may be the major reasons leading to the high coke resistance of the catalysts.The Ni/Mg/Al catalysts, which were modified by Ce and Nd, were prepared byco-impregnation method. It was found that using Ce and Nd as promoters couldfurther enhance the catalytic reforming activity and coke resistance of the catalysts.The addition of Ce was even more effecteve compared with the un-modified NiMgAl.Its coke formation rate drops from30to7mg·gcat.-1.h-1. It’s found that the addition ofCe and Nd enhances the strong interaction between Ni and the support, therefore,smaller Ni crystallite sizes were formed, which is believed to be the major reasonaccounting for the coke resistance improvement.