Size effect of gold nanoparticles supported on carbon nanotube as catalysts in selected organic reactions (original) (raw)
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Supported gold nanoparticles catalysts for organic transformations
2019
The research work described in this thesis concerns the synthesis, characterisation and study of the catalytic activity of supported gold nanoparticles (AuNPs) immobilised on various oxide supports, i.e. silica (SiO2), alumina (Al2O3), titania (TiO2) and magnetite (Fe3O4), previously functionalised with [3-(2-propynylcarbamate)propyl]triethoxysilane (PPTEOS). The alkynyl-carbamate moieties anchored on the support were capable of straightforwardly reducing the gold precursor chloroauric acid (HAuCl4) to afford Au/OS@Yne (OS = Oxide Support, Yne = organic functionalisation), without the need of additional reducing or stabilising agents. The resulting materials were characterised by means of several complementary techniques, such as thermogravimetric analysis (TGA), atomic absorption spectroscopy (AAS), transmission electron microscopy (TEM), solid state NMR spectroscopy (SS NMR) and x-rays photoelectron spectroscopy (XPS), in order to investigate their structural and chemical properties. Furthermore, the catalytic activity of the obtained Au/OS@Yne was evaluated first in the oxidation of alcohols and then in the hydroamination of alkynes. Finally, during a six months stay at the Karl-Franzens University of Graz, a second research work was carried out, concerning the study of metal organic frameworks biocomposites. The thesis organisation and the content of each chapter can be summarised as follows: Chapter 1. General introduction on the topic of nanocatalysis, followed by an overview on the synthesis and properties of gold nanoparticles, with a special focus on their application as catalysts. Moreover, a brief discussion on the main advantages of flow chemistry and the exploitation of this technology in heterogeneous catalysis is reported. Chapter 2. The Au/SiO2@Yne, Au/Al2O3@Yne and Au/TiO2@Yne systems were prepared by in situ reduction of a gold precursor in the presence of the alkynyl carbamate moieties grafted on the support surface. The physical and chemical characterisation of the obtained materials was then carried out, alongside an investigation on the mechanism of formation/stabilisation of AuNPs on the functionalised supports. Chapter 3. The catalytic activity of Au/SiO2@Yne, Au/Al2O3@Yne and Au/TiO2@Yne in the oxidation of benzyl alcohol was first evaluated in batch conditions, with the aim of tuning the reaction selectivity by employing different oxidising agents and solvent media. Furthermore, the oxidation of a variety of primary and secondary alcohols catalysed by Au/SiO2@Yne was performed, both in batch and in continuous-flow conditions. Finally, the effect of the oxide support on the catalytic properties of the system was studied in the oxidation of 1-phenylethanol. Chapter 4. A further functionalisation of the silica supported catalyst was carried out following two approaches: grafting with ethoxytrimethylsilane (Au/SiO2@Yne-TMS) and impregnation with thriethylamine (Au/SiO2@Yne-NEt3). The new synthesized materials were thoroughly characterised and employed as catalysts in the hydroamination of phenylacetylene with aniline. Chapter 5. The synthesis of nanomagnetite supports was performed by co-precipitation of iron salts in alkaline media (NH3 and NaOH), then the obtained magnetic nanoparticles were functionalised with PPTEOS and decorated with AuNPs. Furthermore the characterisation of these magnetic systems was carried out and compared with the analogues materials formed by immobilising AuNPs on bare magnetite. Chapter 6. Synthesis and characterisation of zeolitic imidazolate framework-8 biocomposites containing bovine serum albumin (BSA@ZIF-8). The possibility of tuning the ZIF-8 topology by varying the reaction conditions was first investigated in batch conditions, then the synthesis was carried out in a continuous-flow system with a special focus on the control of particle size.
Synthesis of carbon nanotubes over gold nanoparticle supported catalysts
The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO 2 -Al 2 O 3 . Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850°C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550°C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.
Decoration of Carbon Nanotubes with Gold Nanoparticles for Catalytic Applications
MRS Proceedings, 2004
Gold nanoparticles supported on carbon nanotubes (CNTs) were prepared by using electroless plating technique. High-resolution transmission electron microscopy (HRTEM) has shown that spherical gold nanoparticles were homogeneously dispersed on the surfaces of the carbon nanotubes with a distribution of particle sizes sharply at around 3-4 nm in diameter. The results presented in this work will probably provide new catalysts with better performances.
Gold nanoparticle (AuNPs) and gold nanopore (AuNPore) catalysts in organic synthesis
Organic & Biomolecular Chemistry, 2014
Organic synthesis using gold has gained tremendous attention in last few years, especially heterogeneous gold catalysis based on gold nanoparticles has made its place in almost all organic reactions, because of the robust and green nature of gold catalysts. In this context, gold nanopore (AuNPore) with a 3D metal framework is giving a new dimension to heterogeneous gold catalysts. Interestingly, AuNPore chemistry is proving better than gold nanoparticles based chemistry. In this review, along with recent advances, major discoveries in heterogeneous gold catalysis are discussed.
Supported Gold Nanoparticles as Promising Catalysts
Catalytic Application of Nano-Gold Catalysts, 2016
In recent times, gold nanoparticles (AuNPs) either in the form of colloids or as supported nanoparticles are being extensively used as efficient redox catalyst materials. Catalysis particularly using supported gold nanoparticles (AuNPs) has attracted immense research interest due to their unique properties and greater potentiality that is directly related to their particle size. The primary objective of this chapter is to provide comprehensive overview about gold metal nanoparticles (AuNPs) and their application as promising catalysts. This chapter contains six sections in total. Section 1 starts with a general introduction, recent progress, and brief summary of the application of supported AuNPs as promising catalysts for different applications. Section 2 briefs the properties and stability of gold nanoparticles. Section 3 reviews the preparation methods of supported AuNPs for a wide range of catalytic applications. Section 4 describes briefly some of the most commonly reported supported AuNPs for different applications. Section 5 concentrates on our own results related to the application of supported AuNPs in heterogeneous catalysis. In this section, the oxidation of cyclohexane (CH) and benzyl alcohol (BA) to adipic acid (AA), benzaldehyde (BAl), and ammoxidation of 2-methylpyrazine to 2-cyanopyrazine are discussed. Finally, Section 6 describes, main points and outlook are summarized.
Single-Molecule Nanocatalysis Via the Support Effect of Gold Nanoparticles on Carbon Nanotubes
Bulletin of The Korean Chemical Society, 2018
The catalytic properties of supported catalysts are greatly influenced by the nature of the support. Here, by monitoring the nanocatalysis of gold (Au) nanoparticles deposited on the outer surface (Au/CNTs-out) and inner surface (Au/CNTs-in) of CNTs at the single-molecule level, the support effect of CNTs was observed in both product formation process and product dissociation process. The results indicated that Au/CNTs-out possessed higher intrinsic catalytic activity than Au/CNTs-in in reductive N-deoxygenation reaction of resazurin to resorufin. For the product dissociation process, the product molecules on both Au/CNTs-out and Au/CNTs-in preferred the reactant-assisted dissociation pathway. Furthermore, product molecules dissociated faster on Au/CNTs-out than Au/CNTs-in. These differences were attributed to the support effect of CNTs. This study shows a better understanding of support effect for supported catalyst and gives deeper insight into heterogeneous catalytic process.
Journal of Nanoscience and Nanotechnology, 2019
In this paper, we study the thermal activation of CO 2 on the surface of small Au nanoparticles supported on TiO 2 and titanate nanotube. We characterize the catalysts with high resolution transmission electron microscopy (HR-TEM) and total gold content measurement. We performed catalytic test in flow reactors then we investigate the surface of the catalysts during the adsorption and reaction processes by diffuse reflectance infrared spectroscopy (DRIFTS). The size of gold nanoparticles on the surface has been found to have the most important effect on the final activity of the studied catalysts. Significantly higher TOF values were obtained when the size of Au were smaller on both TiO 2 and titanate nanotube supports. The size of the Au nanoparticles with the method of their preparation was controlled. The gold adatom promotes the adsorption and scission of CO 2 , but the nature of the support has got important effect, too. The explored reaction schemes may pave the way towards novel catalytic materials that can solve challenges associated with the activation of CO 2 and thus contribute to a greener chemistry related to it.
The Journal of Physical Chemistry C, 2007
Core-shell nanocomposites (R-Au) bearing well-defined gold nanoparticles as surface atoms of variable sizes (8-55 nm) have been synthesized exploiting polystyrene-based commercial anion exchangers. Immobilization of gold nanoparticles, prepared by the Frens method, onto the resin beads in the chloride form is possible by the ready exchange of the citrate-capped negatively charged gold particles. The difficulty of nanoparticle loading, avoiding aggregation, has been solved by stepwise operation. Analysis of the gold particles after immobilization and successive elution confirm the unaltered particle morphology while compared to those of the citrate-capped gold particles in colloidal dispersion. It was observed that the rate of the reaction increases with the increase in catalyst loading, which suggests the catalytic behavior of the gold nanoparticles for the reduction of the aromatic nitrocompounds. The rate constant, k, was found to be proportional to the total surface area of the nanoparticles in the system. Kinetic study for the reduction of a series of aromatic nitrocompounds reveals that the aromatic nitrocompound exclusively adsorbs to atop sites of gold particles and that the rate of the reduction reaction increases as the particle size decreases. Similar reaction kinetics was observed involving gold sol of variable size (homogeneous catalysis) as catalyst. The induction time and the activation energy of the reaction decreases with decrease in particle size indicating the decrease in activation energy for the smaller particles, which also speaks for the increase of surface roughness with decrease in particle size. The observed rate dependence, in relation to particle size, is attributed to a higher reactivity of the coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in larger particles.