Synthesis, characterisation and performance of (TiO2)(0.18)(SiO2)(0.82) xerogel catalysts

The synthesis of high surface area xerogels has been achieved using the sol-gel route. Heptane washing was used during the stages of drying to minimise capillary pressures and hence preserve pore structure and maximise the surface area. SAXS data have identified that heptane washing during drying, in general, results in a preservation of the pore structure and surface areas of up to 450 m(2) g(-1). O-17 NMR showed that Ti is fully mixed into the silica network in all of the samples. XANES data confirm that reversible 4-fold Ti sites are more prevalent in samples with high surface areas, as expected. The calcined xerogels were tested for their catalytic activity using the epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) as a test reaction, with excellent selectivities and reasonable percentage conversions. FT-IR spectroscopy has revealed that the catalytic activity is correlated with the intensity of the Si-O-Ti signal, after accounting for variations in Si-OH and Si-O-Si. The most effective catalyst was produced with heptane washing, a calcination temperature of 500 degreesC, and a heating rate of 5 degreesC min(-1)

Molecular understanding of the catalytic consequence of ketene intermediates under confinement

[Image: see text] Neutral ketene is a crucial intermediate during zeolite carbonylation reactions. In this work, the roles of ketene and its derivates (viz., acylium ion and surface acetyl) associated with direct C–C bond coupling during the carbonylation reaction have been theoretically investigated under realistic reaction conditions and further validated by synchrotron radiation X-ray diffraction (SR-XRD) and Fourier transformed infrared (FT-IR) studies. It has been demonstrated that the zeolite confinement effect has significant influence on the formation, stability, and further transformation of ketene. Thus, the evolution and the role of reactive and inhibitive intermediates depend strongly on the framework structure and pore architecture of the zeolite catalysts. Inside side pockets of mordenite (MOR), rapid protonation of ketene occurs to form a metastable acylium ion exclusively, which is favorable toward methyl acetate (MA) and acetic acid (AcOH) formation. By contrast, in 12MR channels of MOR, a relatively longer lifetime was observed for ketene, which tends to accelerate deactivation of zeolite due to coke formation by the dimerization of ketene and further dissociation to diene and alkyne. Thus, we resolve, for the first time, a long-standing debate regarding the genuine role of ketene in zeolite catalysis. It is a paradigm to demonstrate the confinement effect on the formation, fate, and catalytic consequence of the active intermediates in zeolite catalysis

Structure-activity correlations for Brønsted acid, Lewis Acid, and photocatalyzed reactions of exfoliated crystalline niobium oxides

Exfoliated crystalline niobium oxides that contain exposed but interconnected NbO6 octahedra with different degrees of structural distortion and defects are known to catalyze Brønsted acid (BA), Lewis acid (LA), and photocatalytic (PC) reactions efficiently but their structure–activity relationships are far from clear. Here, three exfoliated niobium oxides, namely, HSr2Nb3O10, HCa2Nb3O10, and HNb3O8, are synthesized, characterized extensively, and tested for selected BA, LA, and PC reactions. The structural origin for BA is associated mainly with acidic hydroxyl groups of edge-shared NbO6 octahedra as proton donors; that of LA is associated with the vacant band position of Nb5+ to receive electron pairs from substrate; and that of PC is associated with the terminal Nb=O of NbO6 octahedra for photon capture and charge transfer to long-lived surface adsorbed substrate complex through associated oxygen vacancies in close proximity. It is believed that an understanding of the structure–activity relationships could lead to the tailored design of NbOx catalysts for industrially important reactions

Achieving ultra‐high rate planar and dendrite‐free zinc electroplating for aqueous zinc battery anodes

Despite being one of the most promising candidates for grid-level energy storage, practical aqueous zinc batteries are limited by dendrite formation, which leads to significantly compromised safety and cycling performance. In this study, by using single-crystal Zn-metal anodes, reversible electrodeposition of planar Zn with a high capacity of 8 mAh cm−2 can be achieved at an unprecedentedly high current density of 200 mA cm−2. This dendrite-free electrode is well maintained even after prolonged cycling (>1200 cycles at 50 mA cm−2). Such excellent electrochemical performance is due to single-crystal Zn suppressing the major sources of defect generation during electroplating and heavily favoring planar deposition morphologies. As so few defect sites form, including those that would normally be found along grain boundaries or to accommodate lattice mismatch, there is little opportunity for dendritic structures to nucleate, even under extreme plating rates. This scarcity of defects is in part due to perfect atomic-stitching between merging Zn islands, ensuring no defective shallow-angle grain boundaries are formed and thus removing a significant source of non-planar Zn nucleation. It is demonstrated that an ideal high-rate Zn anode should offer perfect lattice matching as this facilitates planar epitaxial Zn growth and minimizes the formation of any defective regions

Study of shape effect of Pd promoted Ga2O3 nanocatalysts for methanol synthesis and utilization

The area of methanol synthesis and utilization has been attracting research interests due to its positive impact on the environment and also from energy perspectives. Methanol synthesis from CO2 hydrogenation not only produces methanol which is a key platform chemical and a clean fuel, but can also recycle CO2 which is one of the major greenhouse gases causing global warming. As a mobile energy carrier (particularly as a hydrogen carrier), methanol is a versatile molecule which is able to generate H2 via its decomposition. Catalysis plays a decisive role in the success of both methanol synthesis from CO2 hydrogenation and its reverse decomposition reaction. Pd/Ga2O3 binary catalyst has recently been identified as an active catalyst for the methanol synthesis reaction. In this thesis, it is reported the shape effect of Pd promoted Ga2O3 for this reaction. The catalytic H2 evolution from methanol photodecomposition has also been studied over these catalysts. Three shapes of Ga2O3 nanomaterials (i.e. rod and plate β-Ga2O3, and particle γ-Ga2O3) have been synthesized, followed by doping with Pd metal to form corresponding Pd/Ga2O3 nanocatalysts. It was found that a (002) polar Ga2O3 surface which was dominantly presented on the plate form was unstable, giving a higher degree of oxygen defects and mobile electrons in the conduction band than the other non-polar (111) and (110) surfaces of the rod form. It was shown that a significantly stronger metal support interaction was found between the (002) polar Ga2O3 on the plate form and Pd, which gave higher methanol yield and selectivity. For methanol photodecomposition, it was found that, for pure Ga2O3 catalysts of different shapes, the plate form with a highest degree of defects (unstable polar surface) could encourage a non-radiative catalytic recombination of electron and hole pairs upon irradiation, hence giving a highest photocatalytic activity for H2 production. Once Pd was introduced onto these oxide surfaces, it was noted that there was a fast and readily electron transfer from the conduction band of Ga2O3 to Pd due to the formation of a Schottky junction between the two materials. This produces metal sites for hydrogen production and further enhances the rate of the photocatalytic reaction over the radiative recombination of excitons. However, it was also found that at higher Pd content (>1%), the significantly shortened exciton lifetimes reduce the catalytic rate hence giving an overall volcanic response of activity to increasing Pd content for each shape of Ga2O3. At the higher Pd content, the plate form appeared to sustain a longer lifetime for photocatalysis compared to the other forms at the equivalent Pd loading.This thesis is not currently available in ORA

Structure-reactivity relationship in catalytic hydrogenation of heterocyclic compounds over ruthenium black; Part B: Effect of carbon substitution by heteroatom

The effect of the type of heteroatom in the structure on the recyclability of possible candidate compounds for application as LOC (liquid organic carriers) was studied by comparing the rate and selectivity obtained in hydrogenation of carbazole, dibenzothiophene, dibenzofuran and fluorene. The effect of a partial saturation of the compound on its hydrogenation yield and reaction pathway was also considered by studying hydrogenation of 1,2,3,4-tetrahydrocarbazole. Using Ru black catalyst, the rate of hydrogenation was found to decrease in order; dibenzofuran >1,2,3,4-tetrahydrocarbazole > carbazole >> fluorene. No reaction was observed in the hydrogenation of dibenzothiophene under the conditions studied which was attributed to the immediate poisoning of ruthenium metal by sulphur compounds. The rate of hydrogenation of fluorene was around 3 times lower as compared to carbazole and over 8 times lower as compared to that of dibenzofuran under the same reaction conditions. With the exeption of S containing dibenzothiophene, the presence of the heteroatom in the structure was found to be beneficial in terms of increasing the rate of hydrogen loading step. Additionally, a higher reaction rate was obtained in the hydrogenation of the partially saturated 1,2,3,4-tetrahydrocarbazole as compared to the substrate carbazole. The structure and stability of intermediates was found to be significantly influenced by the type and presence of a heteroatom in the structure. A stable octahydro-intermediate was observed only with N-heterocycles, whereas a stable hexahydro-intermediate was produced in the polyaromatic hydrocarbon-fluorene. Additionally, the theoretically obtained lowest total enthalpies using DFT calculations agreed well with the stable intermediates observed experimentally in the hydrogenation of fluorene. Theoretical DFT differences in enthalpies also indicated the products of hydrogenolysis of perhydro-dibenzofuran to be the most favourable products of its hydrogenation, which agreed well with the experimental observations. Overall, taking into account the recyclability of LOC, substitution of carbon with a N heteroatom was demonstrated to be one of the promising approaches to improve the kinetics of the hydrogen loading step

The rational design of photocatalytic semiconductor nanoparticles

This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb2O5 nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5+ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.This thesis is not currently available on ORA

Influence of modifiers on Palladium based nanoparticles for room temperature formic acid decomposition

Heterogeneous catalysts form a highly important part of everyday life, ranging from the production of fertiliser enabling the growth of crops that sustain much of the world's population to the production of synthetic fuels. They constitute a key part of the chemical industry and contribute towards substantial economic and environmental benefits. Heterogeneous catalysts are also believed to have an important role to play in a future hydrogen economy, reducing our requirements for fossil fuels. To this end, formic acid has been proposed as a potential hydrogen storage material for small portable devices. Additionally, formic acid has historically been used as a probe molecule to study catalyst materials and recent developments in the knowledge of its decomposition pathways and the preferred sites of these reactions, establish a good foundation for further study. This work explores a range of novel modification techniques that alter the activity of Pd nanoparticles to decompose formic acid to H2 and CO2. The methods used are the addition of polymers, attaching various functional groups to the surface of the catalyst support and decoration of nanoparticles with sub-monolayer coverages of another metal. Using a range of characterisation methods including FTIR of an adsorbed CO probe, XRD and XPS coupled with computational modelling, it is found that these methods result in some significant electronic and/or geometric alterations to the Pd nanoparticles. For polymer modification, the nature of the pendent group is highly important in determining the effects of the polymer on the Pd particles, with all the tested polymers resulting in varying degrees of electronic donation to the Pd surface. The geometric modifications caused by the polymers also varied with pendent groups; with amine containing pendent groups found to selectively block low coordinate sites, preventing the undesired dehydration of formic acid which results in poisoning of the Pd catalyst by the resulting CO. Attachment of amine groups to the surface of metal oxide catalyst supports, is demonstrated to result in dramatic electronic promotional effects to the supported Pd nanoparticles, and when an amine polymer is attached to the support surface the geometric modification is again observed. Finally decoration of Pd nanoparticles with a sub-monolayer coverage of a second metal is examined, resulting in some similar electronic and geometric effects on Pd nanoparticle surfaces to those observed with polymer modification with corresponding changes in formic acid decomposition activity. Overall, a number of methods are displayed to tune the catalytic activity and selectivity of Pd nanoparticles for formic acid decomposition, resulting in catalysts with some of the highest reported TOF's at room temperature. These modification methods are believed to be potentially applicable to a wide range of other catalytic reactions that operate under mild conditions.</p

Influence of modifiers on Palladium based nanoparticles for room temperature formic acid decomposition

Heterogeneous catalysts form a highly important part of everyday life, ranging from the production of fertiliser enabling the growth of crops that sustain much of the world's population to the production of synthetic fuels. They constitute a key part of the chemical industry and contribute towards substantial economic and environmental benefits. Heterogeneous catalysts are also believed to have an important role to play in a future hydrogen economy, reducing our requirements for fossil fuels. To this end, formic acid has been proposed as a potential hydrogen storage material for small portable devices. Additionally, formic acid has historically been used as a probe molecule to study catalyst materials and recent developments in the knowledge of its decomposition pathways and the preferred sites of these reactions, establish a good foundation for further study. This work explores a range of novel modification techniques that alter the activity of Pd nanoparticles to decompose formic acid to H2 and CO2. The methods used are the addition of polymers, attaching various functional groups to the surface of the catalyst support and decoration of nanoparticles with sub-monolayer coverages of another metal. Using a range of characterisation methods including FTIR of an adsorbed CO probe, XRD and XPS coupled with computational modelling, it is found that these methods result in some significant electronic and/or geometric alterations to the Pd nanoparticles. For polymer modification, the nature of the pendent group is highly important in determining the effects of the polymer on the Pd particles, with all the tested polymers resulting in varying degrees of electronic donation to the Pd surface. The geometric modifications caused by the polymers also varied with pendent groups; with amine containing pendent groups found to selectively block low coordinate sites, preventing the undesired dehydration of formic acid which results in poisoning of the Pd catalyst by the resulting CO. Attachment of amine groups to the surface of metal oxide catalyst supports, is demonstrated to result in dramatic electronic promotional effects to the supported Pd nanoparticles, and when an amine polymer is attached to the support surface the geometric modification is again observed. Finally decoration of Pd nanoparticles with a sub-monolayer coverage of a second metal is examined, resulting in some similar electronic and geometric effects on Pd nanoparticle surfaces to those observed with polymer modification with corresponding changes in formic acid decomposition activity. Overall, a number of methods are displayed to tune the catalytic activity and selectivity of Pd nanoparticles for formic acid decomposition, resulting in catalysts with some of the highest reported TOF's at room temperature. These modification methods are believed to be potentially applicable to a wide range of other catalytic reactions that operate under mild conditions.This thesis is not currently available in OR