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Dive into the research topics where David W. Knight is active.

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Featured researches published by David W. Knight.


Science | 2011

Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles

Lokesh Kesavan; Ramchandra Tiruvalam; Mohd Hasbi Ab Rahim; Mohd Izham Saiman; Dan I. Enache; Robert Leyshon Jenkins; Nikolaos Dimitratos; Jose Antonio Lopez-Sanchez; Stuart Hamilton Taylor; David W. Knight; Christopher J. Kiely; Graham J. Hutchings

A gold- and palladium-based catalyst can be used to oxidize toluene and form a commercially useful ester. Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2 are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2 supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.


Nature Chemistry | 2011

Facile removal of stabilizer-ligands from supported gold nanoparticles

Jose Antonio Lopez-Sanchez; Nikolaos Dimitratos; Ceri Hammond; Lokesh Kesavan; Saul White; Peter John Miedziak; Ramchandra Tiruvalam; Robert Leyshon Jenkins; Albert Frederick Carley; David W. Knight; Christopher J. Kiely; Graham J. Hutchings

Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, we report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.


Angewandte Chemie | 2011

Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions†

Qian He; Ceri Hammond; Peter John Miedziak; Nikolaos Dimitratos; Meenakshisundaram Sankar; Andrew A. Herzing; Marco Conte; Jose Antonio Lopez-Sanchez; Christopher J. Kiely; David W. Knight; Stuart Hamilton Taylor; Graham J. Hutchings

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.


Physical Chemistry Chemical Physics | 2009

Oxidation of glycerol using gold-palladium alloy-supported nanocrystals

Nikolaos Dimitratos; Jose Antonio Lopez-Sanchez; Jinto Manjaly Anthonykutty; Albert Frederick Carley; Rc Tiruvalam; Andrew A. Herzing; Christopher J. Kiely; David W. Knight; Graham J. Hutchings

The use of bio-renewable resources for the generation of materials and chemicals continues to attract significant research attention. Glycerol, a by-product from biodiesel manufacture, is a highly functionalised renewable raw material, and in this paper the oxidation of glycerol in the presence of base using supported gold, palladium and gold-palladium alloys is described and discussed. Two supports, TiO(2) and carbon, and two preparation methods, wet impregnation and sol-immobilisation, are compared and contrasted. For the monometallic catalysts prepared by impregnation similar activities are observed for Au and Pd, but the carbon-supported monometallic catalysts are more active than those on TiO(2). Glycerate is the major product and lesser amounts of tartronate, glycolate, oxalate and formate are observed, suggesting a sequential oxidation pathway. Combining the gold and palladium as supported alloy nanocrystals leads to a significant enhancement in catalyst activity and the TiO(2)-supported catalysts are significantly more active for the impregnated catalysts. The use of a sol-immobilisation preparation method as compared to impregnation leads to the highest activity alloy catalysts and the origins of these activity trends are discussed.


Faraday Discussions | 2010

Oxidation of alcohols using supported gold and gold–palladium nanoparticles

Sankar Meenakshisundaram; Ewa Nowicka; Peter John Miedziak; Robert Leyshon Jenkins; Nikolaos Dimitratos; Stuart Hamilton Taylor; David W. Knight; Donald Bethell; Graham J. Hutchings

A key discovery in the last two decades has been the realisation that gold, when prepared as supported nanoparticles, is exceptionally effective as an oxidation catalyst, particularly for the oxidation of carbon monoxide at sub-ambient temperature but also for a number of organic reactions of synthetic significance. To some extent this observation is counterintuitive since extended gold surfaces do not chemisorb oxygen, nor do they corrode. For some oxidation reactions, the catalytic activity is markedly enhanced by the addition of palladium. This paper is concerned with recent advances in understanding the mechanism of catalysis by gold–palladium alloy nanoparticles of one such organic reaction, the oxidation of alcohols to the corresponding carbonyl compounds by molecular oxygen. We report detailed reaction studies using a high activity catalyst prepared by sol-immobilisation on a titania support. Using solvent-free conditions, benzyl alcohol is oxidised primarily to benzaldehyde but small amounts of toluene are also formed. The origin of these products is explored using initial rate measurements, deuterium labelling and kinetic isotope effects, and by the study of substituent effects. The effect of changing the nature of the catalyst support is also briefly examined. On the basis of all the results, we consider that we have evidence for multiple reaction pathways in this heterogeneous system. We put forward general mechanisms for the overall processes and describe confirmatory experiments in support of these, and we suggest possible reaction intermediates involved in the heterogeneously catalysed reaction.


Dalton Transactions | 2011

Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts

Ceri Hammond; Jose Antonio Lopez-Sanchez; Mohd Hasbi Ab Rahim; Nikolaos Dimitratos; Robert Leyshon Jenkins; Albert Frederick Carley; Qian He; Christopher J. Kiely; David W. Knight; Graham J. Hutchings

The reaction of glycerol with urea to form glycerol carbonate is mostly reported in the patent literature and to date there have been very few fundamental studies of the reaction mechanism. Furthermore, most previous studies have involved homogeneous catalysts whereas the identification of heterogeneous catalysts for this reaction would be highly beneficial. This is a very attractive reaction that utilises two inexpensive and readily available raw materials in a chemical cycle that overall, results in the chemical fixation of CO(2). This reaction also provides a route to up-grade waste glycerol produced in large quantities during the production of biodiesel. Previous reports are largely based on the utilisation of high concentrations of metal sulfates or oxides, which suffer from low intrinsic activity and selectivity. We have identified heterogeneous catalysts based on gallium, zinc, and gold supported on a range of oxides and the zeolite ZSM-5, which facilitate this reaction. The addition of each component to ZSM-5 leads to an increase in the reaction yield towards glycerol carbonate, but supported gold catalysts display the highest activity. For gold-based catalysts, MgO is the support of choice. Catalysts have been characterised by XRD, TEM, STEM and XPS, and the reaction has been studied with time-on-line analysis of products via a combination of FT-IR spectroscopy, HPLC, (13)C NMR and GC-MS analysis to evaluate the reaction pathway. Our proposed mechanism suggests that glycerol carbonate forms via the cyclization of a 2,3-dihydroxypropyl carbamate and that a subsequent reaction of glycerol carbonate with urea yields the carbamate of glycerol carbonate. Stability and reactivity studies indicate that consecutive reactions of glycerol carbonate can limit the selectivity achieved and reaction conditions can be selected to avoid this. The effect of the catalyst in the proposed mechanism is discussed.


Chemistry: A European Journal | 2011

Controlling the Duality of the Mechanism in Liquid‐Phase Oxidation of Benzyl Alcohol Catalysed by Supported Au–Pd Nanoparticles

Meenakshisundaram Sankar; Ewa Nowicka; Ramchandra Tiruvalam; Qian He; Stuart Hamilton Taylor; Christopher J. Kiely; Donald Bethell; David W. Knight; Graham J. Hutchings

In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.


Green Chemistry | 2009

Selective formation of lactate by oxidation of 1,2-propanediol using gold palladium alloy supported nanocrystals

Nikolaos Dimitratos; Jose Antonio Lopez-Sanchez; Sankar Meenakshisundaram; Jinto Manjaly Anthonykutty; Albert Frederick Carley; Stuart Hamilton Taylor; David W. Knight; Graham J. Hutchings

The use of bio-renewable resources, such as glycerol, a by-product from bio-diesel manufacture, can provide a viable way to make valuable products using greener technology. In particular, glycerol can be reduced to give 1,2-propanediol that can then be selectively oxidised to lactate, which has immense potential as a monomer for the synthesis of biodegradable polymers. We show that gold-palladium alloy catalysts can be very effective for the selective oxidation of 1,2-propanediol to lactate. Two supports, TiO2 and carbon, and two preparation methods, wet impregnation and sol-immobilisation, are contrasted. The addition of palladium to gold significantly enhances the activity and retains the high selectivity to lactate using O2 as oxidant (we observe 96% lactate selectivity at 94% conversion). Use of hydrogen peroxide is also possible but lower activities are observed as a result of the reaction conditions, but in this case no marked enhancement is observed on addition of palladium to gold. Comparison of the activity for C3 alcohols shows that the reactivity decreases in the order: glycerol > 1,2-propanediol > 1,3-propanediol ∼ 1-propanol > 2-propanol. The use of a sol-immobilisation preparation method as compared to impregnation leads to alloy catalysts with the highest activity for lactate formation from the oxidation of 1,2-propanediol; the origins of these activity trends are discussed.


ACS Nano | 2012

Synthesis of Stable Ligand-free Gold–Palladium Nanoparticles Using a Simple Excess Anion Method

Meenakshisundaram Sankar; Qian He; Moataz Morad; James Charles Pritchard; Simon J. Freakley; Jennifer K. Edwards; Stuart Hamilton Taylor; David John Morgan; Albert Frederick Carley; David W. Knight; Christopher J. Kiely; Graham J. Hutchings

We report a convenient excess anion modification and post-reduction step to the impregnation method which permits the reproducible preparation of supported bimetallic AuPd nanoparticles having a tight particle size distribution comparable to that found for sol-immobilization materials but without the complication of ligands adsorbed on the particle surface. The advantageous features of the modified impregnation materials compared to those made by conventional impregnation include a smaller average particle size, an optimized random alloy composition, and improved compositional uniformity from particle-to-particle resulting in higher activity and stability compared to the catalysts prepared using both conventional impregnation and sol immobilization methods. Detailed STEM combined with EDX analyses of individual particles have revealed that an increase in anion concentration increases the gold content of individual particles in the resultant catalyst, thus providing a method to control/tune the composition of the nanoalloy particles. The improved activity and stability characteristics of these new catalysts are demonstrated using (i) the direct synthesis of hydrogen peroxide and (ii) the solvent-free aerobic oxidation of benzyl alcohol as case studies.


Topics in Catalysis | 2006

New approaches to designing selective oxidation catalysts: Au/C a versatile catalyst

Graham J. Hutchings; Silvio Carrettin; Philip Landon; Jennifer K. Edwards; Dan I. Enache; David W. Knight; Yi-Jin Xu; Albert Frederick Carley

Selective oxidation is of key importance in the synthesis of chemical intermediates. For many years a number of oxides and supported metal catalysts have been used. The key questions involved in the design of selective oxidation catalysts are discussed in the initial part of this paper. One of the most exciting recent developments in the field of selective oxidation has been the discovery that supported gold catalysts are active. The second part of the paper discusses Au/C catalysts, which are shown to be particularly versatile for oxidation reactions. Four examples of selective oxidation are described using molecular oxygen as oxidant: (a) selective oxidation of glycerol to glycerate in the presence of base; (b) the oxidation of cyclohexane to cyclohexanol and cyclohexanane in the presence of a radical initiator; (c) the oxidation of hydrogen to hydrogen peroxide, and (d) the oxidation of benzyl alcohol to benzaldehyde under solvent free conditions. In contrast, the Au/C catalysts are not active for oxidation of carbon monoxide at ambient temperature. These examples demonstrate that there exists a rich potential for Au/C as a selective oxidation catalyst and that research efforts should now be focussed on selective oxidation using supported gold catalysts.

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Sean P. Bew

University of East Anglia

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