Highly Active Gold and Gold–Palladium Catalysts Prepared by Colloidal Methods in the Absence of Polymer Stabilizers

Catalytic properties and stability of Au nanoparticles are sensitive to factors such as the dimensions, shape and composition of the metal nanoparticles. Although colloidal methods provide an easy and quick way to synthesize supported metal catalysts, they typically involve using polymers such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) as steric stabilizers, which can sometimes be detrimental in subsequent catalytic reactions. Here we report the synthesis of supported gold and gold-palladium nanoparticles without the addition of stabilizing polymers. The catalysts prepared with and without the addition of polymers performed very similarly in the selective oxidation of glycerol and benzyl alcohol suggesting that polymers are not essential to make active catalysts for these reactions. Thus, this new stabilizer free method provides a facile and highly effective way of circumventing the inherent problems of polymer stabilizers when preparing gold and gold palladium catalysts

Highly-dispersed nickel nanoparticles decorated titanium dioxide nanotube array for enhanced solar light absorption

Honeycomb titanium dioxide nanotube array (TiO2-NTA) decorated by highly-dispersed nickel nanoparticles (Ni-NPs) has been grown under control on Ti foil by anodization and subsequent electrodeposition. The pore diameter and length of TiO2-NTA, and the size and quantity of Ni-NPs can be controlled via modulating the variables of the electrochemical processes. It has been found that the pretreatment of TiO2-NTA in the Cu(NO3)2 solution and further annealing at 450 °C in air could greatly improve the dispersion of the electrodeposited Ni-NPs. Absorption of the light in the solar spectrum from 300 to 2500 nm by the Ni-NPs@TiO2-NTA is as high as 96.83%, thanks to the co-effect of the light-trapping of TiO2-NTA and the plasmonic resonance of Ni-NPs. In the water heating experiment performed under an illuminating solar power density of ∼1 kW m−2 (AM 1.5), the ultimate temperature over 66 °C and an overall efficiency of 78.9% within 30 min were obtained, promising for applications in photothermal conversion and solar energy harvest

Copper-Cobalt-Based Catalysts and Gold/Titania Nanostructures for Thermocatalytic and Surface Plasmon-Assisted Glycerol Oxidation

In this thesis, the selective oxidation of glycerol in the aqueous phase over heterogeneous catalysts is investigated. The work is divided in to two main parts, which are based on conventional thermocatalysis and surface plasmon-assisted photocatalysis, respectively. In the first part, the study focuse s on the use of non-precious Copper-Cobalt-based catalysts for the conventional thermocatalytic oxidation reaction. The second part is devoted to the exploitation of the plasmonic properties of TiO2 supported gold nanoparticles in order to assist the thermocatalytic reaction by plasmonic photocatalysis, i.e., photon energy is used together with thermal energy to accelerate the reaction. In the first part of the thesis (Chapter 4), the use of copper-cobalt-based materials for the glycerol oxidation in the aqueous phase is described. Two different synthesis procedures were employed for the catalyst preparation. For the first procedure, metallic copper particles supported on ordered mesoporous cobalt monoxide were prepared through a novel approach by using a nanocasting method with subsequent post-treatments. A 3D ordered mesoporous silica, KIT-6, was employed as removable template for the preparation of ordered mesoporous copper cobalt oxide spinels. A mild reduction heat treatment with ethanol vapor was used to reduce the copper-cobalt oxide spinel to form metallic copper nanoparticle supported on ordered mesoporous cobalt monoxide. As a counterpart, non-ordered cobalt monoxide supported copper particles were prepared by the second procedure presented here in, consisting of a facile co-precipitation method with subsequent post-treatments. Both catalyst types were investigated for the glycerol oxidation reaction. Catalysts prepared by the facile co-precipitation method exhibited a superior catalytic performance with decent recyclability compared to the corresponding mesostructured samples. Careful characterization of the materials before and after the reaction provided important insights into the potentially active crystalline phases of the materials and explained the distinguished differences observed in the catalytic performance between the mesostructured and non-structured catalysts. It was shown that the in situ generation of cobalt oxyhydroxide in close contact with the formed copper oxide has a decisive impact on the catalytic activity. The synergy between these two crystal phases seemed to enhance the catalytic activity – whereas each phase alone showed strongly diminished glycerol conversions. Furthermore, different solvents (apart from water) were introduced for the oxidation reaction and it was shown that certain alcohols as co-solvents have a beneficial impact on the catalytic activity of the materials. The second part of this thesis (Chapter 5) is dedicated to the relatively new field of plasmonic photocatalysis and its efficient integration into the well-studied thermocatalytic process of selective glycerol oxidation over supported gold catalysts. The photocatalytic part of this reaction process supports the thermocatalytic oxidation reaction, augmenting the overall catalytic performance. Hence, the term surface plasmon-assisted glycerol oxidation is coined in this work. Titania supported gold catalysts with two different morphologies were investigated for the reaction process. It was shown that a core-shell morphology with a gold core and a titania shell exhibited no photocatalytic enhancement for the glycerol oxidation. On the other hand, small gold nanoparticles deposited on commercial titania showed a several fold increase in conversion for reactions illuminated by visible light compared to the analogous reactions conducted in the dark. Generally, high selectivities toward dihydroxyacetone, a highly desired product used in the cosmetic industry, were obtained. Moreover, hydrogen peroxide was identified as a key intermediate and essential for obtaining improved catalytic activities with visible light irradiation

Plasmonic Au/TiO₂ nanostructures for glycerol oxidation

Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile deposition-precipitation method with urea and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. Au/TiO2 samples were characterized in detail by X-ray diffraction, UV-vis spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The adopted synthetic methodology permits deposition of Au nanoparticles with similar mean particle sizes up to 12.5 wt% loading that allows for the evaluation of the influence of the Au amount (without changing the particle size) on its photocatalytic performance for glycerol oxidation. The reaction conditions were optimized by carrying out a systematic study with different Au loadings on TiO2, reaction times, temperatures, catalyst amounts, O2 pressures and Au particle sizes for photocatalytic reactions as well as traditional heterogeneous catalysis. It has been shown that visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol where the best catalytic results were observed for 7.5 wt% Au loading with an average particle size of around 3 nm. The main product observed, with selectivities up to 63%, was high-value dihydroxyacetone that has important industrial applications, particularly in the cosmetic industry

Effect of Post‐Treatment on Structure and Catalytic Activity of CuCo‐based Materials for Glycerol Oxidation

A series of CuCo-based materials prepared by co-precipitation with varied Co/Cu ratios and different post-treatments were applied in the selective oxidation of glycerol in the aqueous phase under basic conditions. The influence of the post-treatment on the structure of the materials and the catalytic performance was investigated in detail. As-prepared materials without calcination and materials calcined under air with subsequent reduction under ethanol/N2 gas stream showed higher conversion of glycerol compared to samples solely calcined under air or to samples calcined under air with subsequent reduction under H2/Ar gas stream. Main products identified in the liquid phase were glyceric, glycolic and formic acids. Systematic catalytic studies for differently prepared samples with varied Cu content and characterization of the materials by N2-physisorption, XRD, TEM, and EDX allowed the identification of CoO(OH) in contact with CuO as the potentially active phases

Recent Advances in Thermo-, Photo-, and Electrocatalytic Glycerol Oxidation

Glycerol is a highly versatile molecule because of its three hydroxyl groups and can be transformed to a plethora of different value-added fine chemicals and products. It is an important byproduct in biodiesel production and, hence, produced in high amounts, which resulted in a high surplus flooding the market over the last decades. Thus, glycerol is regarded as a potential platform chemical, and many research efforts were devoted to find active catalysts to transform glycerol to various products via different catalytic processes. The selective oxidation reaction is one of the most promising reaction pathways to produce valuable fine chemicals used in the chemical and pharmaceutical industry. This Review describes the recent developments in selective glycerol oxidation to value-added products over heterogeneous catalysts. Particular emphasis is placed not only on newly developed catalysts based on supported noble-metal nanoparticles but also on catalysts containing nonprecious metals. The idea of using cost-efficient non-noble metals for glycerol oxidation is appealing from an economic point of view. Numerous parameters can influence the catalytic performance of the materials, which can be tuned by various synthetic approaches. The reasons for enhancements in activity are critically examined and put into perspective among the various studies. Moreover, during the past decade, many research groups also reported photocatalytic and, more scarcely, electrocatalytic pathways for glycerol oxidation, which are also described in detail herein and have otherwise found little attention in other reviews

Copper-cobalt-based catalysts and gold/titania nanostructures for thermocatalytic and surface plasmon-assisted glycerol oxidation

Diese Arbeit handelt über die selektive Oxidation von Glycerin in der Flüssigphase an heterogenen Katalysatoren und ist in zwei Hauptteile unterteilt. Im ersten Teil werden Kupfer-Kobalt-basierte Katalysatoren als eine potentielle Alternative zu den sonst verwendeten Edelmetallkatalysatoren für die Glycerinoxidation untersucht. Diese Materialien wurden über eine harte Templatierungs- oder eine simple Co-Fällungsmethode hergestellt und für die thermische Katalyse erforscht. Die Aktivität und Stabilität konnte auf die $\textit {in situ}$ Bildung von aktiven Phasen zurückgeführt werden. Im zweiten Teil wird die thermische Katalyse in Kombination mit der plasmonischen Photokatalyse als neues Reaktionskonzept für die Glycerinoxidation untersucht. Dabei werden sowohl die plasmonischen als auch die katalytischen Eigenschaften von auf Titandioxid-geträgertem Gold genutzt, um die Photonenenergie zusammen mit der thermischen Energie zu nutzen und somit die katalytische Aktivität zu erhöhen

Surface Plasmon-Assisted Solar Energy Conversion

The utilization of localized surface plasmon resonance (LSPR) from plasmonic noble metals in combination with semiconductors promises great improvements for visible light-driven photocatalysis, in particular for energy conversion. This review summarizes the basic principles of plasmonic photocatalysis, giving a comprehensive overview about the proposed mechanisms for enhancing the performance of photocatalytically active semiconductors with plasmonic devices and their applications for surface plasmon-assisted solar energy conversion. The main focus is on gold and, to a lesser extent, silver nanoparticles in combination with titania as semiconductor and their usage as active plasmonic photocatalysts. Recent advances in water splitting, hydrogen generation with sacrificial organic compounds, and CO2 reduction to hydrocarbons for solar fuel production are highlighted. Finally, further improvements for plasmonic photocatalysts, regarding performance, stability, and economic feasibility, are discussed for surface plasmon-assisted solar energy conversion

Mesoporous Silica Supported Au and AuCu Nanoparticles for Surface Plasmon Driven Glycerol Oxidation

Herein, we report for the first time the visible-light-assisted rate enhancement for glycerol oxidation using direct plasmonic photocatalysis. Au nanoparticles were loaded on various mesoporous SiO₂ supports, and the catalytic performance was investigated with and without visible-light illumination. Monodispersed mesoporous silica spheres loaded with Au nanoparticles demonstrated a superior photoassisted catalytic rate enhancement compared to Au loaded ordered mesoporous silica (SBA-15, KIT-6, and MCM-41). The enhancement is attributed to the particle size of the Au nanoparticles and better light interaction resulting from the small SiO₂ domains. Au loaded monodispersed mesoporous silica spheres exhibit a constant and remarkably small particle diameter of 2 nm at Au loadings of up to 15 wt % as a result of the support’s small domain size and efficient pore confinement. The performance of the Au catalyst could be further improved by preparing bimetallic AuCu nanoparticles. Synergistic effects between Au and Cu improved the glycerol conversion by a factor of 2.5 and the dihydroxyacetone selectivity from 80% to 90% compared to monometallic Au catalysts