Preparation Of NiO-based Catalysts With Different Morphologies: On Their Catalytic Properties For Methane Deep Oxidation

Study on catalytic methane deep oxidation is not only beneficial to solve environmental problems but also promote the utilization of energy. Development of low cost, high performance catalysts for methane deep oxidation is a hot research topic. Therefore, in this work, to find an active and stable catalyst for the reaction, a series of NiO catalysts with varied morphologies were prepared by different methods. With various characterization techniques, the bulk structure and surface properties of the samples were characterized and correlated with their reaction performance. And the important results are as follows:1. A series of polycrystalline NiO catalysts with different morphologies have been prepared in this study with a simple precipitation method by changing the precipitants, and used for CH4 deep oxidation. NiO-NaOH, a sample prepared with NaOH solution, displays the highest CH4 oxidation activity among all the catalysts, which is competitive to that of 0.5 wt.% Pd/Al₂O₃, especially at high WHSV. SEM demonstrates that it consists of uniform nano-sheets with an average thickness around 23 nm. Compared with other two samples composed of irregular nano-particles, this nano-sheet mesoporous sample possesses higher surface area, larger pore volume and pore size. H₂-TPR and XPS demonstrate that it also contains less Ni3+ cations, which are found to be harmful to the activity. Furthermore, it is revealed that more active oxygen species has been formed on the surface of this nano-sheet sample. As a result, NiO-NaOH shows the highest CH4 oxidation activity among all the NiO catalysts. Water vapour has no any negative effect on the activity of NiO-Na OH, as testified by the long-term stability test in the presence of water vapour.2. A series of NiO catalysts with various morphologies have been prepared by different methods, and used for methane deep oxidation. NiO-PEG, a sample prepared by hydrothermal method with PEG as template, consists of nano-flower morphology. It displays the highest catalytic activity, which is competitive to that of 1wt.% Pd/Al₂O₃. The nanoflower-like NiO-PEG with mesoporous structure possesses the highest surface area, largest pore volume. H₂-TPR, O₂-TPD and XPS demonstrate that there is more active oxygen species on the surface of NiO-PEG, which accounts for its superior activity for methane deep oxidation in comparison with other NiO samples. Water vapour has no any negative effect on the activity of NiO-PEG, as testified by the long-term stability test in the presence of water vapour.3. A series of Ni_1-xAl_xO catalysts were prepared by coprecipiation methods, it is found that the addition of Al₂O₃ can decrease the methane oxidation activity. The possible reason is due to the strong interaction between NiO and Al₂O₃. To elucidate this, a series of x wt.% NiO/Al₂O₃ catalysts were synthesized by impregnation methods, it is found that increasing NiO loading can increased its activity, which suggests that NiO is the activity center for the reaction. In addition, a series of Ni: M?Al and Si? = 2:8 catalysts were prepared by different methods. The activity follows the order of NiO-NaOH>Ni0.8Si0.2-PM?Ni0.8Si0.2-IPM>Ni0.8Al0.2-PM>Ni0.8Al0.2-IMP > Ni0.8Al0.2-CP. It is revealed that NiO active sites has a weak interaction with SiO₂, thus its activity is not influenced. In contrast, very strong interaction between NiO active sites and Al₂O₃ support occurred, which reduce the redox properties of NiO, thus degrading the activity of the prepared catalysts.