Interactions of hydrogen with alkali promoted Ru/SiO2 catalysts: a proton NMR study

The work completed for this dissertation has endeavored to characterize alkali promoted Ru/SiO[subscript]2 catalysts. Initial work investigated the role of hydrogen spillover to the silica support on the characterization of the active metal sites via a hydrogen chemisorption technique. A strongly bound component of spilled over hydrogen was found in the silica support which interfered with accurate measurements of active metal sites via volumetric strong hydrogen chemisorption techniques. Based on the results of this work, the volumetric chemisorption technique was modified such that typical measurement times were reduced from 12-36 hours to 1 hour for silica supported ruthenium catalysts;The active Ru surface was characterized by observing the changes in the proton spin counts and NMR Knight shifts as a function of alkali loading. Na and K promoters blocked the active surface of the Ru metal. However, Cs was pushed off the metal surface upon hydrogen chemisorption. Invariance of proton Knight shifts as a function of alkali coverage suggested the absence of electron transfer from the alkali promoter to ruthenium metal. Dynamic proton NMR studies indicated that the presence of the alkali promoters restricted hydrogen mobility on both the metal surface and at the metal support interfaces (spillover). This finding was consistent with the previously reported effects of the alkali promoters on the Fischer Tropsch synthesis;The effect of the active metal and the promoter on the support hydroxyl groups were characterized via proton nuclear magnetic resonance ([superscript]1H NMR) spectroscopy. The characterization of the support at different Ru loadings indicated that the OH group density in the silica support decreased with increasing metal and or promoter loading but not on a one-to-one basis. The exchange efficiency of the hydroxyls decreased with increasing atomic size of the alkali metal (Na \u3e K \u3e Cs). An additional downfield proton resonance was detected for all the alkali promoted catalysts which was assigned to the alkali hydroxide species present in the support

On the Limits of Photocatalytic Water Splitting

The major drawbacks on the limited H2 and O2 evolution activities of one-step photocatalytic water splitting systems are given here with the emphasis on charge recombination, back-oxidation reactions, and mass transfer limitations. Suppression of these unwanted phenomena is shown to be possible with the usage of small crystal-sized photocatalysts with low defect concentrations, presence of phase junctions, selection of co-catalyst that would be active for H2 evolution but inactive for O2 reduction, coating of the co-catalyst or the whole photocatalyst with selectively permeable nanolayers, and usage of photocatalytic systems with high solid–liquid and liquid–gas surface areas. The mass transfer limitations are shown to be important especially in the liquid–gas interfaces for agitated and suspended systems with estimated H2 transfer rates in the range of ∼200–8000 μmol/h

CHANGES IN THE CHEMICAL STRUCTURE OF THERMALLY TREATED WOOD

Changes in the chemical structure of hornbeam and uludag fir woods during thermal treatment were investigated at three temperatures (170, 190, and 210 oC) and three durations (4, 8, and 12 hours). After thermal treatment, the extents of degradation in the chemical structure of the samples were determined, and the effects on the chemical composition of hornbeam wood and uludag fir wood were investigated. The data obtained were analyzed using variance analysis, and Tukey’s test was used to determine the changes in the chemical structure of uludag fir and hornbeam woods. The results showed that heating wood permanently changes several of its chemical structures and that the changes are mainly caused by thermal degradation of wood polymers. It was found that decreasing of the cellulose and holocelluloses ratio had a favorable effect on the interaction of the wood with moisture. According to the obtained results, hornbeam wood is affected more than uludag fir wood. For each wood, the maximum decreases of holocellulose and α-cellulose were found at 210oC for 12 hours, and the maximum increase of lignin occurred at the same treatment combination

The effect of H2:N2 ratio on the NH3 synthesis rate and on process economics over the Co3Mo3N catalyst

In this study, the process economics of ammonia synthesis over Co3Mo3N was investigated by searching an optimum feed stoichiometry. By ammonia synthesis rate measurements at atmospheric pressure and 400 oC over Co3Mo3N, it was found that, the rate was independent of H2:N2 stoichiometries above 0.5:1. For H2:N2 stoichiometries below 0.5:1, there was a linear dependency of ammonia synthesis rate on the H2:N2 stoichiometry. Static measurements of hydrogen adsorption isotherms measured at 25, 50, and 100 oC revealed that the adsorbed amounts of the strongly bound hydrogen over Co3Mo3N surface were saturated at around 100 Torr hydrogen pressure. This pressure corresponds to the partial pressure of hydrogen when H2:N2 stoichiometries are around 0.5:1, correlating the role of strong hydrogen in ammonia synthesis. These results were used to modify an existing kinetic expression to be used in a conceptual design, based on a lateness of mixing strategy for the hydrogen stream. This conceptual design and its economical analysis revealed that keeping low hydrogen stoichiometries can cut the investment and operating costs by a factor of 2

Double Perovskite Structure Induced by Co Addition to PbTiO3_3 : Insights from DFT and Experimental Solid State NMR Spectroscopy

The effects of Co addition on the chemical and electronic structure of PbTiO$_3$ were explored both by theory and through experiment. Cobalt was incorporated to PbTiO$_3$ during sol gel process. The XRD data of the compounds confirmed the perovskite structure for the pure samples. The XRD lines broadened and showed emerging cubic-like features as the Co incorporation increased. The changes in the XRD pattern were interpreted as double perovskite structure formation. $^{207}$Pb NMR measurements revealed a growing isotropic component in the presence of Co. In line with the experiments, DFT calculated chemical-shift values corroborate isotropic coordination of Pb suggesting the formation of cubic Pb$_2$CoTiO$_6$ domains in the prepared samples. The state-of-the-art hybrid functional first-principles calculations indicate formation of Pb$_2$CoTiO$_6$ with cubic structure and confirms that Co addition can decrease oxygen binding energy significantly. Experimental UV-Vis spectroscopy results indicate that upon addition of Co, the band gap is shifted towards visible wavelengths which was confirmed by the energy bands and absorption spectra calculations. The oxygen binding energies were determined by temperature programmed reduction (TPR) measurements. Upon addition of Co, TPR lines shifted to lower temperatures and new features appeared in the TPR patterns. This shift was interpreted as weakening of oxygen cobalt bond strength. The change in the electronic structure by the alterations of oxygen vacancy formation energy and bond lengths upon Co insertion are determined by DFT calculations.Comment: 22 pages, 8 figures, 4 table

Interactions of hydrogen with alkali promoted Ru/SiO2 catalysts: a proton NMR study

The work completed for this dissertation has endeavored to characterize alkali promoted Ru/SiO[subscript]2 catalysts. Initial work investigated the role of hydrogen spillover to the silica support on the characterization of the active metal sites via a hydrogen chemisorption technique. A strongly bound component of spilled over hydrogen was found in the silica support which interfered with accurate measurements of active metal sites via volumetric strong hydrogen chemisorption techniques. Based on the results of this work, the volumetric chemisorption technique was modified such that typical measurement times were reduced from 12-36 hours to 1 hour for silica supported ruthenium catalysts;The active Ru surface was characterized by observing the changes in the proton spin counts and NMR Knight shifts as a function of alkali loading. Na and K promoters blocked the active surface of the Ru metal. However, Cs was pushed off the metal surface upon hydrogen chemisorption. Invariance of proton Knight shifts as a function of alkali coverage suggested the absence of electron transfer from the alkali promoter to ruthenium metal. Dynamic proton NMR studies indicated that the presence of the alkali promoters restricted hydrogen mobility on both the metal surface and at the metal support interfaces (spillover). This finding was consistent with the previously reported effects of the alkali promoters on the Fischer Tropsch synthesis;The effect of the active metal and the promoter on the support hydroxyl groups were characterized via proton nuclear magnetic resonance ([superscript]1H NMR) spectroscopy. The characterization of the support at different Ru loadings indicated that the OH group density in the silica support decreased with increasing metal and or promoter loading but not on a one-to-one basis. The exchange efficiency of the hydroxyls decreased with increasing atomic size of the alkali metal (Na > K > Cs). An additional downfield proton resonance was detected for all the alkali promoted catalysts which was assigned to the alkali hydroxide species present in the support.</p

Adsorption calorimetry in supported catalyst characterization: Adsorption structure sensitivity on Pt/-gamma-Al2O3

In this study, the structure sensitivity of hydrogen, oxygen and carbon monoxide adsorption was investigated by changing the metal particle size of Pt/Al2O3 catalysts. The 2% Pt/Al2O3 catalysts were prepared by incipient wetness method; the particle size of the catalysts was modified by calcining at different temperatures. The differential heats of adsorption of hydrogen, carbon monoxide and oxygen were measured using a SETARAM C80 Tian-Calvet calorimeter. Hydrogen chemisorption sites with low and intermediate heats were lost when the particle size increased consistent with the previous reports in the literature. No structure dependency was observed for hydrogen, carbon monoxide or oxygen initial heats of adsorption. The adsorbate:total metal stoichiometries at saturation systematically decreased with increasing particle size. While the hydrogen site energy distribution changed with increasing particle size, oxygen and carbon monoxide adsorption site energy distributions did not change appreciably with the metal particle size

Selective methane bromination over sulfated zirconia in SBA-15 catalysts

Methane activation via bromination can be a feasible route with selective synthesis of mono-bromomethane. It is known that the condensation of brominated products into higher hydrocarbons can result in coking and deactivation in the presence of di-bromomethane. In this study, selective production of methyl bromide was investigated over sulfated ZrO2 included SBA-15 structures. It was observed that the higher the ZrO2 amounts the higher the conversion, while the catalyst remained >99% selective for the monobrominated methane. Over 25 mol.% ZrO2 included SBA-15 catalyst with a BET surface area of 246 m2/g, methane was brominated with 69% conversion at 340 °C and only CH3Br was selectively produced