Study On Bifunctional Catalysts For Production Of Hydrogen Via Methane Decomposition

Nowadays, the world is facing two significantly related challenges: an increasing demand for new energy and a stringently environmental concern. The increasingly environmental regulations require the utilization of clean and high quality gasoline and diesel which has a strong dependency upon the hydrotreating process. Thus production of hydrogen at low cost plays a significant role on the development of hydrotreating process. Meanwhile, with the development of Shale Gas, the efficient utilization of the flourishing natural gas has been an intensive research issue. In this study, based on the catalytic decomposition of methane, the cost-effective and energy-efficient catalysts for the low-cost hydrogen production was studied. These findings make practical guidance in the development of industrial catalysts.First, the nickel based catalysts over the conventional supports had been systematically studied. The effect of preparation method, the content of nickel loading, the support type and the textural property on the performance of catalysts in decomposition of methane reaction was investigated. The results indicated that compared with the NiO/Al₂O₃ catalysts prepared by precipitation method, the catalysts prepared by sol-gel method could maintain more Ni species in the form of NiO and suppress the formation of NiAl2O4. However, the catalysts prepared by sol-gel method should be modified to improve the initial conversion of methane. The results also showed that the proper Ni loading was between 20- 30 %, which could provide abundant active center and suppress the formation of graphite with high degree of crystallinity. In the aspect of appropriate support property, the regular and well-defined channel systems, larger pore volume and broader pore diameter was beneficial to the decomposition of methane.The further research were carried out to study the nickel based catalysts modified by second metal in order to enhance the initial conversion of methane. The promoter Ce, Cu showed different effect on the performance of nickel based catalysts. The results indicated that Cu promoter would significantly deteriorate the dispersity of nickel species and reduce the initial conversion of methane, thus Cu species is not an appropriate promoter in this study system. However, Ce species were confirmed to inhibit the sintering of nickel particles and benefit the dispersity of nickel species. In the methane decomposition reaction, Ni-Ce catalysts showed excellent methane conversion. Moreover, during the repeated reaction-regeneration cycles, the existing form of Ce species would influence the hydrogen selectivity of nickel based catalysts. The strong metal-support interaction（SMSI） between Ni and Ce species would provide high hydrogen selectivity for Ni-Ce catalysts.In this study, the catalysts are bifunctional catalysts, the catalysts should not only provide high methane conversion, but produce ungraphitised carbon which is beneficial to the regeneration process. Thus the effect of and regeneration conditions on the performance of nickel based catalysts was also studied. The results showed that the NiO loading under 30 % would inhibit the formation of graphite phases. The degree of graphitization of carbon deposited at catalysts with commercial 1 # precursor of Al₂O₃ was lower than that at catalysts with commercial 5 # precursor. So 1 # precursor of Al₂O₃ is excellent for the Ni based catalysts system. On another aspect, regeneration conditions should avoid excessive regeneration. Alternatively, partial regeneration would provide benefits to hydrogen selectivity.The above research provided practical experience for the development of AMDH catalysts and the design of reaction-regeneration process. The low Ni loading AMDH-10 and high Ni loading AMDH-15 catalysts were developed. The catalysts AMDH-10 and AMDH-15 both could achieve 95 % initial conversion of methane under the reaction condition of 700 oC,1 800 ml/（g-cat h）, simultaneously 90 % hydrogen selectivity could be achieved under partial regeneration condition. The contact time between catalysts and the feed stock had no significant influence on the reaction results of AMDH catalysts, thus there was excellent flexibility for the production process.