Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

Silver is well known to show peculiar catalytic activities in several oxidation reactions. In the present paper, we investigate the catalytic activity of silver catalysts toward CO-gelective oxidation in H-2. XRD, TEM, TPD, and in situ FTIR techniques were used to characterize the catalysts. The pretreatment of the catalysts was found to have great influence on their performance. The pretreatment in 02 improves the activity of the silver catalyst, whereas He pretreatment at 700 degreesC or direct hydrogen pretreatment shows an inverse effect. Silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures (> 500 degreesC), and there is solid evidence for the formation of subsurface oxygen species. The existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for CO oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. Its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. It is observed, for the first time, that adsorbed CO on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., RT), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with CO. This finding provides a new reaction pathway for CO oxidation on silver catalyst. (C) 2004 Published by Elsevier Inc.

Liquid phase oxidation of toluene to benzaldehyde with molecular oxygen over copper-based heterogeneous catalysts

We conducted the liquid phase oxidation of toluene with molecular oxygen over heterogeneous catalysts of copper-based binary metal oxides. Among the copper-based binary metal oxides, iron-copper binary oxide (Fe/Cu = 0.3 atomic ratio) was found to be the best catalyst. In the presence of pyridine, overoxidation of benzaldehyde to benzoic acid was partially prevented. As a result, highly selective formation of benzaldehyde (86% selectivity) was observed after 2 h of reaction (7% conversion of toluene) at 463 K and 1.0 MPa of oxygen atmosphere in the presence of pyridine. These catalytic performances were similar or better than those in the gas phase oxidation of toluene at reaction temperatures higher than 473 K and under 0.5-2.5 MPa. It was suggested from competitive adsorption measurements that pyridine could reduce the adsorption of benzaldehyde. At a long reaction time of 4 It, the conversion increased to 25% and benzoic acid became the predominant reaction product (72% selectivity) in the absence of pyridine. The yield of benzoic acid was higher than that in the Snia-Viscosa process, which requires corrosive halogen ions and acidic solvents in the homogeneous reaction media. The catalyst was easily recycled by simple filtration and reusable after washing and drying.

Factors influencing the catalytic activity of SBA-15-supported copper nanoparticles in CO oxidation

Copper nanoparticles were deposited onto mesoporous SBA-15 support via two different routes: post-grafting method and incipient wet impregnation method. Both XRD and TEM reveal that the post-grafting can make Cu particles very small in size and highly dispersed into channels of SBA-15, while the impregnation method mainly forms large Cu particles on the external surface of SBA-15. TPR experiments show that CuO species formed by the post-grafting method is more reducible than that prepared by the impregnation method. The catalytic activity tests for CO oxidation manifests that the sample prepared by the post-grafting method has a much higher activity than that prepared by the impregnation method, with a lowering of 50 degrees C for T-50, showing a strong dependence of catalytic activity on the size and dispersion of Cu particles. Besides the preparation procedure, other factors including calcination temperature, reduction treatment, copper loading as well as the feed composition, have an important effect on the catalytic activity. The best performance was obtained when the catalyst was calcined at 500 degrees C and reduced at 550 degrees C. The calcination and reduction treatment at high temperature have been found to be necessary to completely remove the organic residue and to generate active metallic copper particles. (c) 2005 Elsevier B.V. All rights reserved.

Reaction path for oxidation of ethylene to acetic acid over Pd/WO3-ZrO2 in the presence of water

Pd-supported on WO3-ZrO2 (W/Zr atomic ratio=0.2) calcined at 1073 K was found to be highly active and selective for gas-phase oxidation of ethylene to acetic acid in the presence of water at 423 K and 0.6 MPa. Contact time dependence demonstrated that acetic acid is formed via acetaldehyde formed by a Wacker-type reaction, not through ethanol by hydration of ethylene.

Benzene electro-oxidation in a PEMFC for phenol and electricity cogeneration

In the present investigation, the electrochemically-assisted oxidation of benzene in a H-2-O-2 proton exchange membrane fuel cell (PEMFC) for electricity and phenol cogeneration is studied. Experiments were carried out in a PEMFC electrochemical reactor using Pd black as cathode electrocatalyst at 60 and 80 degrees C, respectively and 1 atm back pressure. Indeed, it was found that the only product detected under the examined experimental conditions was phenol. The online GC product analysis revealed that it is impossible to produce phenol when the fuel cell circuit is open (I = 0) under all the examined experimental conditions. When the fuel cell circuit was closed, however, the phenol yield was found to follow a volcano-type dependence on the cur-rent of the external circuit. It was found that the maximum phenol yield was 0.35% at 100 mA/cm(2) at 80 degrees C. At the same time, the PEMFC performance was also investigated during the phenol generation process. Furthermore, experiments with the rotating ring disc electrode (RRDE) technique showed that the intermediate oxidation product, i.e. H2O2 existed during the oxygen electro-reduction process. The cyclic voltammograms showed that benzene was strongly adsorbed on the Pd surface, leading to a degradation of the PEMFC performance. (c) 2005 Elsevier B.V. All rights reserved.

Laser desorption/ionization mass spectrometry on porous silicon for metabolome analyses: influence of surface oxidation

Jenkins, Tudor; Vaidyanathan, S.; Jones, D.G.; Ellis, J., (2007) 'Laser desorption/ionization mass spectrometry on porous silicon for metabolome analyses: influence of surface oxidation', Rapid Communications in Mass Spectrometry 21(13) pp.2157-2166 RAE2008

Investigating the influence of the sulfur oxidation state on solid state conformation

Design, synthesis and structural characterization of a series of diphenylacetylene derivatives bearing organosulfur, amide and amine moieties has been achieved in which the molecular conformation is controlled through variation of the hydrogen bond properties on alteration of the oxidisation level of sulfur.

Mammalian cell cultures as a model system for the determination of cytotoxicity induced by cholesterol oxidation products

Oxysterols are products of cholesterol oxidation, which may be produced endogenously or may be absorbed from the diet where they are commonly found in foods of animal origin. Oxysterols are known to be cyctotoxic to cells in culture and mode of toxicity has been identified as apoptosis in certain cell lines. The cytotoxicity of the oxysterols 25-hydroxycholesterol (25-OH) and 7β-hydroxycholesterol (7β-OH) was examined in two human cell lines, HepG2, a hepatoma cell line, and U937, a monocytic cell line. Both 25-OH and 7β-OH were cytotoxic to the HepG2 cell line but apoptotic cells were not detected and it was concluded that cells underwent necrosis. 25-OH was not cytotoxic to the U937 cell line but it was found to have a cytostatic effect. 7β-OH was shown to induce apoptosis in the U937 line. The mechanism of oxysterol-induced apoptosis has not yet been fully elucidated, however the generation of an oxidative stress and the depletion of glutathione have been associated with the initial stages of the apoptotic process. The concentration of cellular antioxidant enzyme, superoxide dismutase (SOD) was increased in association with 7β-OH induced apoptosis in the U937 cell line. There was no change in the glutathione concentration or the SOD activity of HepG2 cells, which underwent necrosis in the presence of 7β-OH. Many apoptotic pathways center on the activation of caspase-3, which is the key executioner protease of apoptosis. Caspase-3 activity was also shown to increase in association with 7β-OH-induced apoptosis in U937 cells but there was no significant increase in caspase-3 activity in HepG2 cells. DNA fragmentation is regarded as the biochemical hallmark of apoptosis, therefore the comet assay as a measure of DNA fragmentation was assessed as a measure of apoptosis. The level of DNA fragmentation induced by 7β-OH, as measured using the comet assay, was similar for both cell lines. Therefore, it was concluded that the comet assay could not be used to distinguish between 7β-OH-induced apoptosis in U937 cells and 7β-OH-induced necrosis in HepG2 cells. The cytotoxicity and apoptotic potency of oxysterols 25-OH, 7β-OH, cholesterol- 5a,6a-epoxide (a-epoxide), cholesterol-5β,6β-epoxide (β-epoxide), 19-hydroxy-cholesterol (19-OH), and 7-ketocholesterol (7-keto) was compared in the U937 cell line. 7 β-OH, β-epoxide and 7-keto were found to induce apoptosis in U937 cells. 7β-OH-induced apoptosis was associated with a decrease in the cellular glutathione concentration and an increase in SOD activity, 7-keto and β-epoxide did not affect the glutathione concentration or the SOD activity of the cells.a-Epoxide, 19-OH and 25-OH were not cytotoxic to the U937 cell line.

Copper-catalysed asymmetric sulfide oxidation

The focus of this thesis is the preparation of enantiopure sulfoxides by means of copper-catalysed asymmetric sulfoxidation, with particular emphasis on the synthesis of aryl benzyl and aryl alkyl sulfoxides. Chapter 1 contains a review of the methods employed for the asymmetric synthesis of sulfoxides, compounds with many applications in stereoselective synthesis and in some cases with pharmaceutical application. Chapter 1 describes asymmetric oxidation, including metal-catalysed, non metal-catalysed and enzyme-catalysed, in addition to synthetic approaches via nucleophilic substitution of appropriately substituted precursors. Kinetic resolution in oxidation of sulfoxides to the analogous sulfones is also discussed; in certain cases, access to enantioenriched sulfoxides can be achieved via a combination of asymmetric sulfoxidation and complementary kinetic resolution. The design and synthesis of a series of sulfides to enable exploration of the substituent effects of the copper-mediated oxidation was undertaken, and oxidation to the racemic sulfoxides and sulfones to provide reference samples was conducted. Oxidation of the sulfides using copper-Schiff base catalysis was undertaken leading to enantioenriched sulfoxides. The procedure employed is clean, inexpensive, not air-sensitive and utilises aqueous hydrogen peroxide as oxidant. Extensive investigation of the influence of the reaction conditions such as solvent, temperature, copper salt and ligand was undertaken to lead to the optimised conditions. While the direct attachment of one aryl substituent to the sulfide is essential for efficient enantiocontrol, in the case of the second substituent the enantiocontol is dependent on the steric rather than electronic features of the substituent. Significantly, use of naphthyl-substituted sulfides results in excellent enantiocontrol; notably 97% ee, obtained in the oxidation of 2-naphthyl benzyl sulfide, represents the highest enantioselectivity reported to date for a copper-mediated sulfur oxidation. Some insight into the mechanistic features of the copper-mediated sulfur oxidation has been developed based on this work, although further investigation is required to establish the precise nature of the catalytic species responsible for asymmetric sulfur oxidation. Full experimental details, describing the synthesis and structural characterisation, and determination of enantiopurity are included in chapter 3.

Crystal engineering: exploiting variation of sulfur oxidation for the control of the solid state structure of organosulfur compounds

This thesis is focused on the design and synthesis of a diverse range of novel organosulfur compounds (sulfides, sulfoxides and sulfones), with the objective of studying their solid state properties and thereby developing an understanding of how the molecular structure of the compounds impacts upon their solid state crystalline structure. In particular, robust intermolecular interactions which determine the overall structure were investigated. These synthons were then exploited in the development of a molecular switch. Chapter One provides a brief overview of crystal engineering, the key hydrogen bonding interactions utilized in this work and also a general insight into “molecular machines” reported in the literature of relevance to this work. Chapter Two outlines the design and synthetic strategies for the development of two scaffolds suitable for incorporation of terminal alkynes, organosulfur and ether functionalities, in order to investigate the robustness and predictability of the S=O•••H-C≡C- and S=O•••H-C(α) supramolecular synthons. Crystal structures and a detailed analysis of the hydrogen bond interactions observed in these compounds are included in this chapter. Also the biological activities of four novel tertiary amines are discussed. Chapter Three focuses on the design and synthesis of diphenylacetylene compounds bearing amide and sulfur functionalities, and the exploitation of the N-H•••O=S interactions to develop a “molecular switch”. The crystal structures, hydrogen bonding patterns observed, NMR variable temperature studies and computer modelling studies are discussed in detail. Chapter Four provides the overall conclusions from chapter two and chapter three and also gives an indication of how the results of this work may be developed in the future. Chapter Five contains the full experimental details and spectral characterisation of all novel compounds synthesised in this project, while details of the NCI (National Cancer Institute) biological test results are included in the appendix.

Chamber investigations of atmospheric mercury oxidation chemistry

Mercury is a potent neurotoxin even at low concentrations. The unoxidised metal has a high vapour pressure and can circulate through the atmosphere, but when oxidised can deposit and be accumulated through the food chain. This work aims to investigate the oxidation processes of atmospheric Hg0(g). The first part describes efforts to make a portable Hg sensor based on Cavity Enhanced Absorption Spectroscopy (CEAS). The detection limit achieved was 66 ngm−3 for a 10 second averaging time. The second part of this work describes experiments carried out in a temperature controlled atmospheric simulation chamber in the Desert Research Institute, Reno, Nevada, USA. The chamber was built around an existing Hg CRDS system that could measure Hg concentrations in the chamber of<100 ngm−3 at 1 Hz enabling reactions to be followed. The main oxidant studied was bromine, which was quantified with a LED based CEAS system across the chamber. Hg oxidation in the chamber was found to be mostly too slow for current models to explain. A seven reaction model was developed and tested to find which parameters were capable of explaining the deviation. The model was overdetermined and no unique solution could be found. The most likely possibility was that the first oxidation step Hg + Br →HgBr was slower than the preferred literature value by a factor of two. However, if the more uncertain data at low [Br2] was included then the only parameter that could explain the experiments was a fast, temperature independent dissociation of HgBr some hundreds of times faster than predicted thermolysis or photolysis rates. Overall this work concluded that to quantitatively understand the reaction of Hg with Br2, the intermediates HgBr and Br must be measured. This conclusion will help to guide the planning of future studies of atmospheric Hg chemistry.