Ni-based catalysts for the dry reforming of methane
- Ni-basierte Katalysatoren für die trockene Reformierung von Methan
Pegios, Nikolaos; Palkovits, Regina (Thesis advisor); Weßling, Matthias (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2018
Carbon capture and utilization (CCU) can be more advantageous than carbon capture and storage (CCS). Dry reforming of methane (DRM) enables an efficient utilization of two abundant greenhouse gases, CO2 and CH4, by converting them into syngas, a versatile feedstock for chemical synthesis. Depending on its H2:CO ratio, syngas can be used for methanol production or synthetic fuels via the Fischer-Tropsch process. Different catalysts have been examined so far, with noble metals exhibiting the best catalytic results but their expensive price and scarcity make them an unattractive choice as opposed to Ni-based catalysts supported on Al2O3. The challenge they face is deactivation due to sintering and carbon formation. In this study Ni-based catalysts were prepared, and the deactivation mechanisms were investigated. Firstly, novel modular catalysts for DRM were prepared based on chemically modified Ni-foams by a stepwise dip coating deposition using different Al2O3 precursors and MgO and SiO2 as promoters. The coated Ni-foams were thoroughly characterized before and after reaction by means of Kr-physisorption, X-ray fluorescence and X-ray diffraction. Comparing the catalytic performances of the different catalysts in DRM emphasized the major importance of the precursor for the nature of aluminum oxide deposition, catalyst activity, and deactivation degree. The mechanism of catalyst deactivation was thoroughly studied by high-resolution scanning electron microscopy and energy dispersive spectroscopy. Additionally, regeneration profiles were investigated. Overall, the presence of aluminum oxide and its intimate contact with Ni-foam appears to be essential for catalyst activity and the active sites are likely to be at the Ni-Al2O3 interface. Aiming for high catalyst performance and enhanced coke resistance, different preparation techniques of La-promoted Ni/γ-Al2O3 catalysts for DRM were compared, facilitating structure-performance correlations. The studied synthesis techniques comprise incipient wetness impregnation, co-precipitation and spray drying as well as modifications of them. Thorough characterization of the catalysts and the carbon deposits has been carried out. This exhibited clearly that different preparation techniques led generally to very different physical properties, structure, chemical species and anti-coking properties of the catalyst. With a higher amount of species with strong metal-support interactions, higher activity and coking resistivity was observed as well as less graphitization of amorphous carbon. Superior catalytic performance can be reached by catalysts prepared by spray drying and is related to excellent Ni dispersion and strong metal-support interaction. Only 2.7 % of the catalyst weight was found to be carbon deposits after 6 h time on stream with minor sintering, and a Ni-nanoparticle size < 10 nm. Further investigating spray drying, synergistic effects of different promoters were evaluated for seven Al2O3-supported Ni-catalysts tested at 800 °C. MgO, CoOx, CeO2 and La2O3 were utilized as promoters. Rigorous characterization of the catalysts was performed before and after reaction. Generally, high metal dispersion, strong metal-support interactions and high crystallinity were achieved. The co-promoted catalyst, NiCoCe/Al2O3, exhibited the best catalytic results even after six cycles of reaction-regeneration-reduction experiments. This was attributed to its excellent NiO dispersion and strong metal-support interactions. The coking resistivity was improved due to the high amount of basic sites and high mobility of lattice O2, provided by CeO2. NiCoCe/Al2O3 deactivated mainly due to sintering, with Ni-nanoparticle average size of 19.4 ± 3.1 nm after 6 cycles.Catalysts prepared by hard-templating with low and high surface area activated carbon, were tested for DRM at 800 °C. The yielded syngas exhibited a low product ratio of H2:CO [0.04-0.12], due to the reverse water-gas shift reaction, and even after 75 h time on stream (TOS) minimal deactivation of the catalyst can be observed. A rather unusual activity evolution was found involving a sequence of minimum-maximum-plateau. The suggested scheme explains the activity evolution based on the Ni-nanoparticle positioning from being bare or encapsulated by Al2O3. The Al2O3 shell can then crack and undergo restructuring during reaction due to the high temperature providing more active sites for the reaction. Superior metal dispersion was achieved with an average Ni-nanoparticle size at 4.9 ± 1.3 nm. The sintering mechanism was also investigated. Surprisingly, hollow nickel nanoparticles were observed at 25 h TOS due to the nanoscale Kirkendall effect that in turn caused fragmentation of nickel particles to smaller but solid nanoparticles. This diffusion phenomenon between the core, Ni0, and the outer shell, NiO, (Ni2+) leads to severe structural and morphological changes of the catalyst.