Hydrogen is considered as an alternative clean energy source due to the depletion of fossil fuels and related environmental problems. Steam reforming of bio-ethanol, has excellent potential for hydrogen production, with CO2 neutrality. Ni, Pd and Pt are the most active metals for steam reforming of ethanol. Improving catalytic activity of supported Ni catalyst by incorporating small amount of Pd or Pt is a successful method for increasing activity and stability of the catalyst. Development of active and stable catalysts with low coke formation and high hydrogen yield attracted major attention of researchers in recent decades.
MCM-41 supported bimetallic mesoporous catalytic materials containing well dispersed Ni and Pd nanoballs were synthesized following an impregnation procedure. TEM images and XRD analysis of these materials indicated the formation of nickel and palladium nanoballs of 15-25 nm and 5-8 nm respectively, within the synthesized materials. These materials have quite narrow pore-size distributions in the range of 1-5 nm. In the calcined materials, nickel was in Ni+3 and Ni+2 states, however in the reduced sample most of the nickel was in Nio state, together with some NiO. Formation of NiOx crystals takes place after the calcination step. Impregnation of palladium into Ni/MCM-41 caused a decrease in the reduction temperature of NiO for about 50oC. Activity of the catalytic materials were tested in the reaction of steam reforming of ethanol.all the catalysts showed high conversion and quite high hydrogen yield over 400 oC. PdNi impregnated MCM-41 supported catalysts showed higher conversion of ethanol but lower hydrogen yield than Ni impregnated catalysts due to methane formation in the case of Pd incorporation.
However, MCM-41 does not show enough hydrothermal stability for steam reforming of ethanol reaction. SBA-15 has very similar physical properties of MCM-41 with larger pores and high hydrothermal stability. Ceria also has widespread applications in catalysis area with its excellent oxygen buffering capacity. It can be used as catalyst support and also an improving agent for silica supports.
Mesoporous silica SBA-15 with ordered pore structure was synthesized following a hydrothermal procedure and then bimetallic Ni-Ce and Pd-Ni-Ce incorporated mesoporous silica catalysts were prepared and tested in steam reforming of ethanol. On the other hand, ceria enriched silica structures i.e cerium/silicate composites were also synthesized. However the syntheses were unsuccessful due to the thermal sintering of ceria.
Addition of ceria to the support structure decreased coke formation significantly.. According to the thermal gravimetric analysis studies conducted after ethanol steam reformin reaction at 600 oC, coke formation with Ni-SBA-15 (Ni/Si=0.10) catalyst was 40% and it was 10% with NiCe-SBA-15 (Ni/Si=0.10, Ce/Si=0.50) Activity test results obtained with Ni incorporated silica catalysts in steam reforming of ethanol gave high hydrogen yield over 4 (max. 6) and complete conversion of ethanol at 600 oC. SBA-15 found to be more stable catalyst than MCM-41 in the steam reforming of ethanol reaction.
The highest hydrogen yield values were achieved by AlMCM-41 supported Ni-Ce impregnated catalysts. Aluminum in the slica matrix (3% wt.) increased the catalytic activity significantly, by giving acidic properties to the catalyst. Simultaneous and consecutive Ni and Ce impregnation were also examined. AlMCM-41 supported, consecutively Ce and Ni impregnated catalysts with Ce/Si and Ni.Si molar ratio sof 0.10, showed very high catalytic activity (5.8 at the beginning). AlMCM-41 supported catalyst were less stable because of high activity and consequently coke formation.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/12614996/index.pdf |
| Date | 01 September 2012 |
| Creators | Sener, Canan |
| Contributors | Dogu, Timur |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | Ph.D. Thesis |
| Format | text/pdf |
| Rights | Access forbidden for 1 year |
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