Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation
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Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile sol-immobilisation method and investigated for the surface plasmon-assisted glycerol oxidation under base-free conditions. The Au/TiO2 samples were characterized by UV–vis spectroscopy and transmission electron microscopy. Catalysts were prepared using polyvinyl alcohol as stabiliser as well as in the absence of polymer stabiliser. Both the conversion and the reaction selectivity are affected by the plasmon-assisted oxidation and there is an interplay between the presence of the stabiliser and the Au nanoparticle size.
KeywordsGold Plasmonic Glycerol oxidation
Glycerol is a potentially valuable by-product from the manufacture of biodiesel. Due to its high functionalisation, can be used as an important precursor for the fine chemicals industry [1, 2, 3]. The selective oxidation of glycerol has been extensively studied as a means of converting glycerol to potentially valuable products such as glyceric acid, tartronic acid and dihydroxyacetone [4, 5]. Supported Au catalysts have been shown to be very active for glycerol oxidation to glyceric acid under basic reaction conditions , and the high pH is required to activate the first hydroxyl group of glycerol. It has been found that catalysts that are active at high pH are typically inactive at neutral pH. In addition to the metal, the support can play a role and can be used to fine tune selectivity . Indeed, although there are reports of base-free oxidation of glycerol with Au catalysts, these tended to utilise basic supports such as MgO but these are found to dissolve during the reaction in sufficient amounts to provide the basic environment which induces the selective oxidation . Hence new approaches are required to observe catalysed base-free oxidation of glycerol.
Plasmonic photocatalysis is an emerging field that can offer a new strategy for the oxidation of glycerol. This relies on the plasmonic properties of noble metal nanoparticles, used to induce reactivity [9, 10, 11, 12, 13, 14, 15, 16, 17]. Among them, the gold-containing ones are of particular interest since they show a surface plasmon resonance (LSPR) falling in the visible range of the electromagnetic radiation, resulting in the possibility to harvest visible light to promote chemical reactions. Several works have been conducted for selective oxidation reactions of alcohols driven by plasmonic photocatalysis [18, 19, 20, 21, 22, 23, 24, 25, 26]. To date glycerol oxidation has been studied using traditional photocatalysts [27, 28, 29, 30, 31] and more recently Au catalysts [32, 33]. These earlier studies used monometallic Au catalysts prepared by deposition–precipitation, with a content of gold of typically > 5 wt%, and showed that base-free glycerol oxidation is indeed accelerated under visible light illumination . Colloidal catalyst synthesis methods allow the control nanoparticle size, bimetallic nanoparticle composition and morphology (such as core shell particles) and control of nanoparticle shape in solution with greater flexibility than typical deposition methods. We wanted to explore the use of Au catalysts prepared using sol-immobilisation, as this method allows to obtain nanoparticles which can be < 5 nm in diameter with a reasonably narrow particle size distribution.
We aimed to examine catalysts containing lower Au concentrations, and in this paper we report the results for the plasmonic photocatalysis of glycerol using sol-immobilised Au/TiO2 catalysts.
2.1 Preparation of Au/TiO2 Catalysts by Sol-Immobilisation
Au/TiO2 catalysts were prepared by sol-immobilisation using polyvinyl alcohol (PVA) as a stabilising polymer. In a typical catalyst synthesis (1 g), an aqueous solution of HAuCl4·3H2O (0.8 mL, 12.5 mg Au/mL Sigma Aldrich, metal content ≥ 49.0%) was added to 400 mL of deionised water with stirring, followed by the addition of a polymer stabilizer (PVA 1 wt% aqueous solution (average molecular weight MW = 9000–10,000 g/mol, 80% hydrolysed), polymer/metal = 0.65 by weight). Subsequently, a freshly prepared solution of NaBH4 (≥ 99.99%, Aldrich, 0.1 M) was added (mol NaBH4/mol Au = 5) to form a red sol. After 30 min, the colloid was immobilised by adding 0.99 g of TiO2 (P25 Aeroxide®, Evonik) together with concentrated H2SO4 (1 mL). After 1 h of continuous stirring, the slurry was filtered, and the catalyst was recovered by filtration and washed thoroughly with deionised water and dried (110 °C, 16 h). Catalysts were also prepared using the same procedure with no stabiliser present.
3 Glycerol Oxidation
Plasmonic oxidation of glycerol was carried out using a reactor design based on that used previously , and comprised a stainless steel custom designed autoclave reactor built by DG Innovation. The reactor consisted of two parts: a top, with a borosilicate window, equipped with inlet, outlet, thermocouple inlet, pressure gauge and pressure release valve; and a vessel where the reaction liner is located. A glass vial (15 mL) was placed in a Teflon jacket (2.5 cm diameter) and wrapped in aluminium foil to ensure both thermal contact and light reflection inside the reaction vessel. The maximum operating pressure of the reactor, determined by the presence of the window, is 11 bar. The maximum operating volume is 7.5 mL. Heating was provided by a heating mantle designed, connected to a control box (K39 Ascon Tecnologic, assembled by Elmatic, Cardiff). The light source is a 300 W lamp (USHIO) positioned inside a case and connected to a control box (ORIEL OPL-500). For all the experiments the IR radiation was removed from the output light by a water filter, and wavelengths below 420 nm were eliminated through a cut-off filter (Newport Stablife® Technology). The filtered light was focused on the top of the reactor through a 90° mirror placed at the end of the filters.
Plasmonic photocatalytic glycerol oxidation at pH 7 was carried out under dark and illuminated conditions with aqueous glycerol (5 mL, 0.05 M), catalyst (5 mg), Gly/Au = 130 molar ratio. The neutral pH is necessary as it is well known that the reaction of glycerol with gold catalysts is greatly accelerated by the presence of base and therefore in the present study we wanted to minimise any possible thermal reactions. After addition of the reactants the reactor was sealed, flushed with 5 bar O2 five times and finally pressurised with O2 (continuously supplied). The temperature was set to 90 °C and the experiment started when temperature in the heating mantle reached the selected value. Total reaction time was 3 h. After reaction, the reactor was rapidly quenched in an ice bath, the liner weighed and the slurry collected, filtered through a micropore PTFE filter (0.25 µm) and directly injected to HPLC for analysis. Product analysis was carried out using an Agilent 1260 Infinity HPLC with a Metacarb 67H column with a 0.1 wt% solution of phosphoric acid as mobile phase.
3.1 Catalyst Characterisation
The plasmon resonance for the Au catalysts was determined using UV–visible spectroscopy. The metal content of the catalysts was determined inductively coupled plasma–mass spectrometry (Agilent 7900 ICP-MS) following digestion of the catalyst in aqua regia. Transmission electron microscopy analysis was performed using a JEOL 2100 microscope with a LaB6 filament operating at 200 kV. Samples were prepared by dispersing the catalyst in ethanol and allowing a drop of suspension to evaporate on a lacey carbon film supported over a 300 mesh copper TEM grid.
4 Results and Discussion
4.1 Au/TiO2 Catalysts Prepared Using PVA as Stabiliser
Au content of Au/TiO2 catalysts prepared with and without PVA as stabilising agent
Nominal Au wt%
Prepared with PVA
Prepared without PVA
Actual Au wt%
Actual Au wt%
4.2 Au/TiO2 Catalysts Prepared in the Absence of a Stabiliser
It has previously been shown that sol-immobilised catalysts active for glycerol oxidation under basic conditions can be prepared without the use of a stabilising . Therefore an analogous series of catalysts with nominal Au loadings of 1, 3, 5, 7 wt% Au/TiO2 were prepared without the use of a stabiliser in the sol-immobilisation preparation method. The Au content of these catalysts was determined using ICP analysis and the results shown in Table 1 show the concentrations are very similar to those analogous formulations prepared using PVA as stabiliser.
These initial findings show that glycerol oxidation can be accelerated with visible light illumination under base free conditions using catalysts prepared by colloidal methods. Colloidal catalyst synthesis methods allow the control nanoparticle size, bimetallic nanoparticle composition and morphology (such as core shell particles) and control of nanoparticle shape in solution with greater flexibility than typical deposition methods. This provides new avenues for catalyst design based on optimising both the light absorption and catalyst properties in and new modes of operation in catalysts that can be switched on/off with light illumination.
Moles of carbon are calculated by multiplying the moles of product/reactant by the number of carbons in the product/reactant). Unless otherwise stated, the results showed in the present paper are expressed as average of three experiments, and the associated error bars represent the standard deviation of each value.
This work is supported by Cardiff University and the MAXNET Energy research consortium of the Max Planck Society and we thank Georgios Dodekatos and Harun Tüysüz for their advice and assistance with the reactor design.
Compliance with Ethical Standards
Conflict of interest
The authors declare no financial conflict of interest.
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