Design and development of nickel based catalysts for COx free H2 production by catalytic decomposition of methane

摘要

Catalytic decomposition of methane (CDM) is considered as a promising environmental-friendly process for the production of hydrogen. The major promise of this process lies in the potential for simple process design and production of highly desirable fuel cell grade hydrogen (&10 ppm CO) (without the need for complex hydrogen separation processes) and valuable carbon nano materials (CNF/ CNT) as by-products. One of the main challenges in realising the potential of CDM is the development of highly efficient (highly active and stable / easily regenerated) catalysts. There has been a significant amount of research conducted on the development of active and stable catalysts for CDM over the last two decades. Of the different catalysts that have been studied to date it is widely accepted that Ni based SiO2 and Al2O3 catalysts are the most promising for lower temperature (&600 °C) CDM. However Ni based catalysts that have been developed to date suffer from rapid catalyst deactivation due to surface coking and also sintering of the Ni particles. Hence there is a need to develop improved catalysts to enable economic viability of the process. The main aim of this study was to investigate factors that influence the activity and stability of Ni based catalysts in CDM. The first approach involved investigating the influence of the number of Ni active sites present (through the use of promoters and different supports) whilst the second involved investigating factors that influence carbon deposition in CDM (with carbon deposition being known to have a significant influence on activity and stability). In chapter III, initial studies were conducted to study the influence of Fe, Co, Cu, Zn as promoters for Ni supported on Hβ zeolite supports. The influence of support particle size was also investigated for Ni and Ni-Cu supported H-β zeolite catalysts. Characteristics of the catalysts studied were determined using various techniques such as powder XRD, H2-TPR, BET-SA, XPS, SEM/TEM and pulse chemisorption measurements. The results obtained using Ni supported on Hβ catalysts showed that copper was the most effective promoter of the metals studied, whilst the results obtained on the influence of support particle size showed that Ni and Ni-Cu dispersion were significantly higher on the nano H-β zeolite support compared to that observed using a commercial H-β zeolite support. The influence of reaction temperature and the influence of catalyst calcination temperature were also studied over the most active promoted Ni catalyst (Ni-Cu/nanoH-β). The activity results obtained showed that the optimised reaction temperature and also optimised calcination temperature for the Ni-Cu/nanoHβ was 550 °C. In Chapter IV; detailed studies were conducted on the influence of the number of Ni active sites as well as the role of the most active promoter identified. The results obtained showed that the presence of copper minimised the sintering of the Ni particles during the reaction (and hence most likely lead to a slower loss of Ni surface area / Ni active sites due to carbon deposition). Moreover, Raman spectroscopic studies of the deactivated catalysts showed the presence of more graphitic carbon formation with increasing copper loading. A mechanism for carbon deposition was proposed based on the results obtained (activity results and characterisation results from spent catalysts) . In Chapter V, the influence of support type was further investigated by studying Ni-Cu supported on different silicious materials such as non-porous fumed SiO2, microporous (silicalite-1), mesoporous (MCM-41), Al-MCM-41 (Si/Al ratio = 150) and Al-MCM-41 (Si/Al ratio = 75). This represents detailed studies on the influence of pore characteristics (pore volume, pore size) and also on the introduction of Al- into the frame work of one of the supports studied i.e. MCM-41. The results obtained using the different Ni-Cu supoorted on silica based catalysts showed that the Ni-Cu/Al-MCM-41 (Si/Al = 150) was clearly the best catalyst. The high activity / stability of the Ni-Cu/Al-MCM-41 (Si/Al = 150) was most likely due to the presence of framework Al in this catalyst which led to increased Ni and Cu dispersion and Ni-Cu surface enrichment that was explained based on H2 and/or N2O pulse chemisorption and XPS measurments. Raman spectroscopic analysis of the deactivated Ni-Cu/Al-MCM-41 (Si/Al = 150) catalyst also showed a highly ordered carbon with an unprecedented quality of CNFs.