Core-shell Structured Nanocatalysts:Design, Synthesize And Their Application In Partial Oxidation Of Methane To Syngas

Natural gas conversion and utilization is very important in the fields of petrochemical and alternative energy resources. The conversion of methane to synthesis gas (syngas) will be a key step for value-added products, fuel cell, and power plant applications. Compared with the conventional steam reforming of methane, the partial oxidation of methane (POM) to syngas attracted extensive attentions due to its efficient energy-saving advantage.The application of nanoparticles (NPs) in heterogeneous catalysis is highly desirable due to the intrinsic "surface effects". Unfortunately, NPs are unstable and aggregate easily especially at elevated temperatures. Enwrapping a nano-material in a stable and porous shell can enhance the thermal stability of the core material and in addition, may cause a change in electron charge, reactivity and functionality of enwrapped material. Core-shell structured materials have attracted great attentions because of the unique structural feature and physicochemical properties.In the present thesis, we focused on the design and development of novel core-shell structured nanocatalysts and its application in POM. It was found that the catalytic performance can be modified by tuning the morphology, structure, size, and element composition of catalyst. Together with the characterization of catalysts and evaluation of catalytic performance, the major conclusions can be derived as follows:1. NPs with different size and morphology can be encapsulated with a layer of microporous/mesoporous SiO2by a modified sol-gel method. The metal oxide cores can be in-situ reduced in hydrogen to generate metal cores for catalysis.2. The application of certain surfactants (PVP and CTAB) in encapsulation has a significant impact on the property of the shell material, especially for the porosity of SiO2shell. The relatively homogeneous core-shell morphology with a higher fraction of mesopores can be obtained by adoption of PVP. In the case of CTAB, a higher porosity of SiO2shell can be achieved, with a larger specific surface area of catalyst.3. The catalytic performance of a core-shell catalyst was mainly determined by the size of Ni cores in the POM reaction. And the smaller the size is; the higher activity will be. The structure-performance relationship has been established in the present study. An effective correlation between the core particle size and catalytic activity can also be obtained due to the high stability of core-shell structured catalysts at elevated temperatures. In addition, it was found that less carbon deposition occurred on the smaller sized Ni cores.4. The catalytic activity of POM is strongly dependent on the proportion of Co in the binary Co-Ni cores. The best conversion and selectivity can be obtained over the core-shell catalyst with a molar ratio of Co/Ni=1:2. Co addition to Ni can also greatly suppress carbon deposition, and thus enhance catalyst stability.5. The porosity of SiO2shell showed a certain impact on the catalytic activity. The shell structure of different porosity can play a role in molecule diffusion. When the PVP was adopted in catalyst preparation, the methane conversion changed slightly, while the H2selectivity was increased obviously. When the CTAB was applied in catalyst preparation, a much looser shell structure was generated, which weakens the core-shell interaction and further affects the catalytic activity.6. The Ni-based core-shell catalysts doped with the elements such as La, Ce, Ba, and Cu, etc showed different catalytic behaviors. It was found that the La-doped catalyst outperformed the counterparts. The core-shell catalyst doped with Ce and Cu did not notably enhance the activity. While dopping of Ba and Fe to the Ni-based catalyst showed unfavorable effect, especially for Fe doping. The results of characterization and evaluation suggested that La doping caused less coking as well as anti-sintering of Ni NPs in the POM reaction.7. The superior catalytic activities of the core-shell structured catalysts for POM are thought to be the consequence of the following factors:on one hand, the nano-sized core particles are effectively isolated by the silica shells which prevent aggregation or sintering of core NPs at high reaction temperatures of POM. On the other hand, the core-shell structured catalysts involve a strong core-shell interaction, and also provide the confined enviroment of a microcapsular-like reactor in which the enrichment of reactants would occur.