- Title
- Catalytic low-temperature combustion of hydrogen for zero-emission
- Creator
- Kim, Jongho
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2024
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Catalytic hydrogen combustion (CHC) is a promising technology for clean, efficient and safe energy generation with ultra-low emissions in hydrogen-fuelled systems such as fuel cells and passive autocatalytic recombination. Many advances in CHC have been achieved in recent years through fundamental research. The research objectives of this thesis were as follows: a) to prepare bimetallic catalysts and optimise them with parameters (e.g. metal composition; pretreatment temperature) by the impregnation method to achieve well-dispersed active materials on the catalysts for catalytic low-temperature hydrogen combustion; b) to elucidate the kinetics and reaction mechanisms regarding catalytic low-temperature combustion of hydrogen over the bimetallic catalysts in packed-bed experiments and characterisations using various analytical techniques. To obtain research insights, this study investigated CHC over the Pd-Cu/Al2O3 catalysts at a temperature range of 20-600 ℃ to determine the reaction rate law using a differential fixed-bed reactor. The kinetic study and catalyst characterisations were performed to evaluate the reactivity and to understand reaction mechanisms by using various analytical techniques such as transmission electron microscopy energy-disperse X-ray spectroscopy (TEM-EDS), temperature-programmed reduction (TPR) and in situ Fourier transform infrared spectroscopy (FTIR). Metal composition, average particle size and reducibility were found to affect the reactivity of catalysts. The rate law of CHC over the optimised catalyst was determined using non-linear regression based on the experimental reaction rates obtained under different partial pressures of hydrogen and oxygen to provide design parameters for the reactor design of a catalytic hydrogen combustor. The main research findings are mentioned briefly below. The sizes of active metal particles were smaller for the reduced Pd–Cu catalysts than for the calcined catalysts, which resulted in improved catalytic activity. The Pd0.75Cu0.25/Al2O3 catalyst exhibited the smallest active metal size (4.09 nm) and the highest reducibility among all catalysts tested. At elevated reduction temperatures, the Pd0/Pdtotal ratio increased, and the active metal particle size decreased, and reached 43.25% and 4.09 nm, respectively. One contributing factor to the CHC rate was the support–metal interactions, which resulted in the transfer of electrons from the reduced cations of the support to the dispersed metal ions. These changes to the catalyst structure promoted catalytic reactivity during the CHC. The optimised catalyst, Pd0.75Cu0.25/Al2O3 (R600) achieved complete hydrogen conversion at temperatures as low as 60 ℃ and was retained for 40 h at the reaction temperature; this showed superior catalyst stability. Based on the reaction rates measured at 60 ℃, various kinetic models were compared to determine the most suitable model. The Langmuir-Hinshelwood single-site mechanism, in which both Pd and Cu participate in the adsorption of hydrogen and oxygen, was found to be the most accurate rate law (R2=0.967) with the highest kinetic constant. The reaction mechanism included the dissociative adsorption of hydrogen and oxygen, followed by the surface reaction of the adsorbed reactants forming adsorbed OH and then water, and finally the desorption of water as the product of the reaction. The activation energy of the optimised catalyst during the CHC reaction was calculated as 22.02 kJ/mol, which was lower than other alumina-supported Pd monometallic and Pd-Ni bimetallic catalysts. The catalytic reactivities of Pd–Cu/Al2O3 and Cu/Al2O3 under a feed condition (3 vol% H2 in air) were tested to evaluate their feasibility for the hydrogen combustion system at temperatures ranging from 20 to 600 ℃. Cu/Al2O3 achieved 96.5% and 98% hydrogen conversion at 500 and 600 ℃, respectively, which indicated that it can be used as a catalyst in a combustion system in that temperature range. However, Pd–Cu/Al2O3 was more suitable at lower temperatures (e.g., 200 ℃) as it has a higher reactivity than Cu/Al2O3. Both catalysts were tested for 40 h and achieved complete conversion of hydrogen. The effects of hydrogen, oxygen and steam on the reaction rate of CHC were investigated for two catalysts. The changes in the reaction rates for the two catalysts showed similar trends. The partial pressure of steam as the product of the CHC reaction had a significant influence on the decreasing reaction rate, in general. The reaction rates over the catalyst containing Pd under the dry condition were higher than those of the catalyst containing only Cu because of the higher solubility of Pd for hydrogen. However, with an increase in partial pressure of steam at higher temperatures (i.e., 600 ℃), the difference in reactivity between the two catalysts was not significant. Rate equations for both catalysts at various temperatures, including a term for the partial pressure of steam, were developed and these could be used to predict the reaction rates under various conditions with a high accuracy. These equations can be used to design CHC systems and determine the type of catalyst, amount of catalyst and volume of reactor required for practical applications. The OH groups (stretching and bending modes) adsorbed by the catalysts were measured, and their intensity provided mechanistic insights into the qualitatively determined water desorption rate of the catalysts. This analysis showed the greater CHC reactivity of Pd–Cu/Al2O3 than Cu/Al2O3 at lower temperatures. However, their reactivity became similar as the reaction temperature increased, thereby decreasing OH intensity measured using in situ FTIR analysis. The rate-determining steps of Pd–Cu/Al2O3 and Cu/Al2O3 are suggested to be the formation and breaking of metal–oxygen bonding explained by measured OH intensity over Cu/Al2O3, even at the highest temperature. This finding suggests that the bimetallic catalyst, Pd-Cu/Al2O3, which contains a 3:1 Pd:Cu weight ratio, plays a similar role to a noble metal rather than a non-noble metal in terms of the rate limiting mechanism. The main reaction intermediate, the hydroxyl group, measured over Pd–Cu/Al2O3 and Cu/Al2O3 in reducing and oxidising conditions was also compared in this study. The OH intensity was lower under the reducing condition than under the oxidising condition because only the surface oxygen species affected the OH formation. The OH intensity over Cu/Al2O3 remained even at the highest temperature because of the lower reducibility of Cu.
- Subject
- catalytic hydrogen combustion; reaction kinetics; bimetallic catalyst; palladium; copper; FTIR analysis
- Identifier
- http://hdl.handle.net/1959.13/1497745
- Identifier
- uon:54456
- Rights
- Copyright 2024 Jongho Kim
- Language
- eng
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