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Calculations of oxygen reduction reaction on nanoparticlesTang, Wenjie, 1982- 16 September 2010 (has links)
Proton exchange membrane fuel cells are attractive power sources because they are highly efficient and do not pollute the environment. However, the use of Pt-based catalysts in present fuel cell technologies is not optimal: Pt is rare and expensive, and even the best commercial Pt cathodes have high overpotentials due to slow oxygen reduction kinetics. As a result, much effort has gone toward developing cheaper, more effective catalysts.
Nanoparticles are attractive because they have different catalytic properties than analogous bulk systems, require less material, and have tunable reactivities based on their composition and size. It is important to perform detailed studies of nanoparticle catalysts since composition and size effects are poorly understood. Computational simulations of such materials can provide useful insights and potentially aid in the design of new catalysts.
Here, I examine composition and size effects in nanoparticle catalysts using computational methods. Two bimetallic systems are investigated to explore composition effects: Pd-shell particles with several different core metals, and Pd/Cu random alloy particles. Depending on how the two metals are mixed (core-shell or random alloy), charge transfer and strain due to alloying are found to have different contributions to the catalytic activity. Size effects are studied for pure Pt particles, where corner and edge sites are found to play an important role. The binding geometries of molecular oxygen to corner and edge sites lead to peroxide formation instead of water on small Pt particles. Results form these calculations can provide useful information for designing novel catalysts in the future. By changing the composition and/or size of nanoparticles in the proper way, the interaction between the adsorbate and catalyst can be optimized, and better catalysts can be obtained. / text
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Reflection-absorption infrared spectroscopy of adsorbates on Ni{110} and nickel oxide surfacesSanders, Helen Elizabeth January 1994 (has links)
No description available.
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Catalytic and selective transition metal mediated isomerisations of allylic alkoxides to enolatesRobinson, Simon Jonathan January 1998 (has links)
No description available.
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High temperature COâ†2 permselective planar membranesBjoerkert, U. Stefan January 1999 (has links)
No description available.
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The stereoselective synthesis of alcoholsPalmer, Matthew Jon January 1997 (has links)
No description available.
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Studies towards the asymmetric Baylis-Hillman reactionDozzo, Paola January 1999 (has links)
No description available.
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Studies on the cytochrome c peroxidase of Pseudomonas aeruginosaFoote, N. January 1985 (has links)
No description available.
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The behaviour of potassium and sodium species during the thermal treatment of a demineralized Highveld coal / Lucinda KlopperKlopper, Lucinda January 2011 (has links)
A series of experiments was conducted to investigate the potential influence of pre- and post adding of catalysts to a demineralized coal char. The catalysts were chosen according to yield better catalytic activity and be inexpensive. CO2 gasification was conducted on the samples in a temperature range of 500 °C to 900 °C. The coal chosen was a high-inertinite, high-ash, Highveld bituminous coal. The catalysts chosen were sodium carbonate, potassium carbonate, and a mixture of the two catalysts. Different methods were used to investigate the factors influencing the reactivity of the demineralized coal char, and the extent of the influence from the catalysts. Proximate analysis, ultimate analysis and ash yields were conducted on the starting material to determine the change the demineralization had on the coal. Ash fusion temperatures of the samples were also obtained. The results indicated that demineralization lowered the ash content, as well as the ash fusion temperatures, but the ultimate analysis showed consistency in both sets of samples. Mass losses obtained during the thermal treatment experiments under CO2 atmosphere showed an increase in mass loss in the order of samples without addition of catalysts to the smallest amount of addition. Potassium carbonate showed the largest increase in mass loss during CO2 thermal treatment, together with the mixture of the two catalysts. Samples with pre-added catalysts also had a larger mass loss than samples with post-added catalysts. According to the XRD and QEMSCAN results, some potassium species are retained in the ash, which is confirmed by XRF results. The XRF results also showed that the amount of alkali species retained is quite large. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011
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Design of new asymmetric copper(I)-catalysts for conjugate addition chemistryBennett, Malcolm William January 1999 (has links)
No description available.
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Synthesis and functionalization of gallium nitride nanostructures for gas sensing and catalyst supportKente, Thobeka 10 January 2014 (has links)
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Doctor of
Philosophy. October 2013.
Johannesburg, South Africa. / We report the role of a double step heat treatment process in the synthesis of
novel GaN nanostructures (NSs) using a two stage furnace following a catalyst
free vapour-solid growth mechanism. Morphological analysis revealed that GaN
NSs were composed of rod-like structures with average diameter of 250 nm and
accumulated particulates of GaN with diameter of ~ 12 – 16 nm providing
enhanced surface area. The wurtzite phase of GaN nanorods of agglomerated
nanoclusters was synthesized at temperatures as low as 750 °C. An X-ray
photoelectron spectroscopic study confirmed formation of GaN. The surface areas
of the GaN NSs were high at ~20 m2/g with respect to that expected for solid
nanorod structures. The GaN NSs were of high crystallinity and purity as revealed
by structural studies. Raman spectral analysis showed stronger intensity of the
A1(LO) mode with respect to that for E2(high) mode indicating the high electronic
quality of the sample. A photoluminescence study revealed the dominant presence
of a defect band around 1.7-2.1 eV corresponding to nitrogen di-vacancies.
Subsequent annealing in NH3 has demonstrated a compensation of the defect state
and evolution of a band edge peak with possible hydrogen compensation of
surface states.
We also report the role of activated carbon on Ga2O3 to make GaN/C
nanostructure composites using a single stage furnace. TEM analysis showed that
GaN/C nanostructures gave different morphologies with different ratios of
GaN/C. The surface areas of these materials showed an increase as the ratio of
activated carbon was increased. PXRD showed that a ratio of Ga2O3: C of 1:0.5
(w/w) was sufficient to form GaN. TGA revealed that the ratios of Ga2O3: C of
1:0.5 – 1:2 gave materials that were thermally stable. Raman spectra showed that
the material had excellent electronic properties. The material with a Ga2O3/C 1:2
ratios showed a poor gas response due to the change in reference value of
resistance with the variation of hydrogen concentration.
iv
This study also provides the first investigation of GaN as a catalyst support in
hydrogenation reactions. The GaN NSs were synthesized via chemical vapour
deposition (CVD) in a double stage furnace (750 ºC) while nitrogen doped carbon
spheres (NCSs) were made by CVD in a single stage furnace (950 ºC). TEM
analysis revealed that the GaN NSs were rod-like with average diameters of 200
nm, while the NCSs were solid with smoother surfaces, and with diameters of 450
nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid
functionalized GaN NSs and NCSs. The Pd nanoparticles had average diameters
that were influenced by the type of support material used. The GaN NSs and
NCSs were tested for the selective hydrogenation of cinnamaldehyde in
isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of
the activity of the nanostructured materials revealed that the order of catalyst
activity was 3% Pd/GaN >3% Pd/NCSs > 1% Pd/NCSs > 1% Pd/GaN. However,
100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1%
Pd/GaN at reasonable conversion rates.
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