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Silica-Encapsulated Catalysts for Lean Methane Combustion
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- Author / Creator
- Habibi, Amir Hossein
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Due to its low cost, abundance and lower emissions compared to gasoline and diesel, there is a resurgence of interest in using natural gas as a transportation fuel. However, although the combustion of natural gas produces comparably less greenhouse gases (GHGs) and pollutants, CH4 itself is a potent GHG and its release from natural gas vehicles’ (NGVs) exhaust into the atmosphere should be avoided. The catalytic abatement of methane from exhaust emissions faces major challenges for several reasons. CH4 has the strongest C-H bond among alkanes and is difficult to oxidize catalytically. Additionally, in the catalytic converter of a lean-burn NGV, small concentrations of CH4 (500-1500 ppm) are to be oxidized in the presence of high concentrations of H2O and at relatively low exhaust temperatures (below 550 C). Pd catalysts (which have the highest CH4 oxidation activity in lean conditions) are known to deactivate under these conditions. In an attempt to develop stable combustion catalysts, a bimetallic Pd-Pt silicon dioxide-encapsulated catalyst was designed, synthesized, and evaluated in this work. The catalyst design was aimed to benefit from the advantages of encapsulation and effects of bimetallicity.
In the first step, the effect of two synthesis procedures on the accessibility of the Pd nanoparticles after encapsulation in silica was studied. Dry catalytic combustion of CH4 and surface area measurements were used to identify an optimal synthesis formulation for a high-loading, high-porosity monometallic Pd@SiO2 catalyst. It was shown that the application of poly(vinylpyrrolidone) (PVP) alone as a Pd stabilizer and a potential porogen was inadvisable as it lead to non-porous catalysts. By using a suitable pore-inducing agent, Pd@mSiO2 catalysts with high metal loading (~ 6 wt.%) and high surface area (~700 m2g−1) were synthesized. This structure was thermally stable at 550 C and exhibited turnover frequencies similar to those of traditional catalysts. Interestingly, 2/3 of the surface of the Pd nanoparticles was estimated to have been blocked by the shell material even in the highly porous catalysts.
To increase the catalyst activity in the presence of H2O, bimetallic PdPt nanoparticles (7 nm in diameter) were synthesized and encapsulated in porous silica shells (60 nm in diameter) similarly to the Pd@mSiO2 catalyst. The developed catalyst (PdPt@SiO2), had a high metal loading (4 wt.% Pd, 7 wt.% Pt) and high surface area (600 m2g−1) and was evaluated in lean methane combustion in the presence of 5 mol% water at up to 550 C. This structure showed a stable methane conversion during the hydrothermal ageing (HTA) test which was two- and ten-fold higher than the conversion for the impregnated Al2O3 and SiO2-supported catalysts of the same metal loading, respectively. After the HTA, while the surface area and pore size distribution of the shell remained unaffected, an increase in the metal dispersion and some changes in the morphology of the PdPt nanoparticles were observed.
As a next step in the catalytic technology development, a kinetic study of methane combustion on the “aged” PdPt@SiO2 catalyst at varying methane concentrations, temperatures and in the absence/presence of 5 mol% water was performed. The kinetic data of the dry reactions were correlated using an existing rate expression that is first order in methane and negative one order in water. Since this model failed to correlate the kinetic data in wet conditions, an alternative mechanism for wet CH4 combustion was suggested. This mechanism was built up on the basis of previous experimental observations of the prevailing chemical state of Pd in wet feed, the ability of Pt to activate methane in oxygen-deficient atmospheres, and the inhibitory effect of water on the support-mediated oxygen exchange. To the best of our knowledge, this is the first time that the effect of support was incorporated in the wet CH4 combustion mechanism. The resulting rate expression successfully predicted the activity of the PdPt@SiO2 catalyst with wet feed (5 vol.% water) in the temperature range of 550 to 750 K. Additionally, the internal mass transfer across the silica shell was studied. It was shown that for the catalysts used here, the diffusion resistance across the shell was negligibly small. -
- Graduation date
- Fall 2018
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.