Single-Molecule Nanocatalysis Via the Support Effect of Gold Nanoparticles on Carbon Nanotubes (original) (raw)
Related papers
2017
Gold nanocomposites based on three supports (one novel and two industry-standard) were prepared using an in-situ reduction technique and characterised thoroughly to determine their physical properties. They were then tested for their suitability as heterogeneous catalysts in the green aerobic oxidation of one secondary and two primary aromatic alcohols of industrial importance. For all reactions, the use of single walled carbon nanotubes as supports (SWCNTs) was seen to result in superior reaction efficiency and specificity compared to gold nanoparticles supported on Titanium dioxide and amorphous carbon. In addition, the gold nanocatalysts used in this work can be reused over several reaction cycles with minimal degeneration in catalytic activity There have been several discussions outlining the potential reasons why gold nanoparticle (Au NP) catalysts are so active, including particle shape, size and properties of the supports but there has been very little expansion on the optimi...
Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts
Chemical Reviews, 2020
In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO 2 hydrogenation, C−C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts. CONTENTS 1. Introduction 3890 2. The Role of the Support during Catalyst Synthesis 3891 2.1. Preparing Au Catalysts on Oxides and Other Conventional Supports 3891 2.2. Preparing Au Catalysts on "Engineered"
Reaction Kinetics, Mechanisms and Catalysis, 2019
Monodisperse gold particles (ca. 2 nm) were prepared and deposited on various supports (SiO 2 , Al 2 O 3 , HAP, MgAl 2 O 4 and MgO). The acid/base properties of supports were characterized by NH 3 and CO 2 sorption. The size of the gold nanoparticles spans in the 1.7-6.5 nm mean diameter range after calcination as determined from TEM measurements. The amounts of accessible surface sites were estimated by binary concentration pulse chromatography of CO with Kr adsorption. The data are in agreement with the results of CO adsorption obtained by DRIFT spectroscopy. The activities of the catalysts were compared in the oxidation of benzyl alcohol in stirred batch reactors under two different conditions: in xylene solvent with atmospheric oxygen at 60 °C (in presence and in absence of K 2 CO 3), and in a solvent-free mixture at elevated pressure and temperature (5 bar O 2 , 150 °C, 5 h). The activities of catalysts in benzyl alcohol conversion are described in two variants, namely related to (i) active catalytic sites (ASNA), and (ii) number of Au atoms on the geometric surface of particles (GSNA). The activities of catalysts in xylene solvent at 60 °C were excellent, with 0.28-1.11 s −1 characteristic GSNA ini values (initial reaction rates related to surface Au atoms, Au surf) in presence of K 2 CO 3. The observed order of activities under these conditions is Au
Tetrahedron, 2014
Carbon nanotube-supported gold nanoparticles of different sizes (diameter of 3 or 20 nm) were evaluated as catalysts in four selected organic transformations. The nanohybrids were shown to efficiently catalyze the investigated reactions, regardless of the size of the supported gold nanoparticles. However, some differences were observed as regards turnover frequency values although size effect turned out to be less significant when only gold surface atoms were considered.
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.
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.
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.
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.