Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation

Article abstract of DOI:10.1007/s10562-019-02952-y

Abstract: 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. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02952-y

Catalysis Letters
https://doi.org/10.1007/s10562-019-02952-y
Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared
by Sol‑Immobilisation
Laura Abis1 · Nikolaos Dimitratos1 · Meenakshisundaram Sankar1 · Simon J. Freakley1,2 · Graham J. Hutchings1
Received: 16 July 2019 / Accepted: 9 August 2019
© The Author(s) 2019
Abstract
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 afected by the plasmon-
assisted oxidation and there is an interplay between the presence of the stabiliser and the Au nanoparticle size.
Graphic Abstract
Dark
Light
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10
8
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1
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5
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Actual Au loading / wt %
Keywords Gold · Plasmonic · Glycerol oxidation
1 Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02952-y) contains
supplementary material, which is available to authorized users.
Glycerol is a potentially valuable by-product from the manu-
facture of biodiesel. Due to its high functionalisation, can
be used as an important precursor for the fne chemicals
industry [1–3]. 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
* Graham J. Hutchings
hutch@cf.ac.uk
1
School of Chemistry, Cardif Catalysis Institute, Cardif
University, Main Building, Park Place, Cardif CF10 3AT,
UK
2
Department of Chemistry, University of Bath, Claverton
Down, Bath BA2 7AY, UK
Vol.:(0123456789)
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L. Abis et al.
glyceric acid under basic reaction conditions [6], and the
high pH is required to activate the frst 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
fne tune selectivity [7]. 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 sufcient amounts
to provide the basic environment which induces the selective
oxidation [8]. Hence new approaches are required to observe
catalysed base-free oxidation of glycerol.
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% hydro-
lysed), 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 con-
centrated H2SO4 (1 mL). After 1 h of continuous stirring,
the slurry was fltered, and the catalyst was recovered by
fltration and washed thoroughly with deionised water and
dried (110 °C, 16 h). Catalysts were also prepared using the
same procedure with no stabiliser present.
Plasmonic photocatalysis is an emerging feld that can
ofer a new strategy for the oxidation of glycerol. This relies
on the plasmonic properties of noble metal nanoparticles,
used to induce reactivity [9–17]. 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 possi-
bility to harvest visible light to promote chemical reactions.
Several works have been conducted for selective oxidation
reactions of alcohols driven by plasmonic photocatalysis
[18–26]. To date glycerol oxidation has been studied using
traditional photocatalysts [27–31] 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 [33]. 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
fexibility than typical deposition methods. We wanted to
explore the use of Au catalysts prepared using sol-immobi-
lisation, as this method allows to obtain nanoparticles which
can be <5 nm in diameter with a reasonably narrow particle
size distribution.
3 Glycerol Oxidation
Plasmonic oxidation of glycerol was carried out using a reac-
tor design based on that used previously [33], 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, ther-
mocouple 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 Tefon jacket (2.5 cm diameter) and
wrapped in aluminium foil to ensure both thermal contact
and light refection 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, assem-
bled by Elmatic, Cardif). 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 radia-
tion was removed from the output light by a water flter, and
wavelengths below 420 nm were eliminated through a cut-
of flter (Newport Stablife® Technology). The fltered light
was focused on the top of the reactor through a 90° mirror
placed at the end of the flters.
We aimed to examine catalysts containing lower Au con-
centrations, and in this paper we report the results for the
plasmonic photocatalysis of glycerol using sol-immobilised
Au/TiO2 catalysts.
Plasmonic photocatalytic glycerol oxidation at pH 7 was
carried out under dark and illuminated conditions with aque-
ous 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, fushed with 5 bar O2 fve times and fnally pressur-
ised 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 reac-
tion time was 3 h. After reaction, the reactor was rapidly
quenched in an ice bath, the liner weighed and the slurry
2 Experimental
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 solu-
tion of HAuCl4·3H2O (0.8 mL, 12.5 mg Au/mL Sigma
Aldrich, metal content ≥ 49.0%) was added to 400 mL of
1 3

Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation
collected, fltered through a micropore PTFE flter (0.25 µm)
Dark
Light
12
and directly injected to HPLC for analysis. Product analysis
was carried out using an Agilent 1260 Infnity HPLC with
10
a Metacarb 67H column with a 0.1 wt% solution of phos-
phoric acid as mobile phase.
8
3.1 Catalyst Characterisation
6
The plasmon resonance for the Au catalysts was determined
4
using UV–visible spectroscopy. The metal content of the
catalysts was determined inductively coupled plasma–mass
2
spectrometry (Agilent 7900 ICP-MS) following digestion of
the catalyst in aqua regia. Transmission electron microscopy
0
analysis was performed using a JEOL 2100 microscope with
1
2
3
4
5
6
a LaB6 flament 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 flm supported
Actual Au loading / wt %
Fig. 1 Conversion under illumination and dark conditions for the Au/
TiO2-PVA catalysts plotted versus the experimentally determined Au
content of the catalyst
over a 300 mesh copper TEM grid.
4 Results and Discussion
samples, independent of the gold loading, an enhancement in
the conversion under illumination was observed and a clear
maximum is observed for the conversion under both dark
and illuminated reaction conditions.
4.1 Au/TiO2 Catalysts Prepared Using PVA
as Stabiliser
Au/TiO2 catalysts (with nominal compositions 1, 3, 5,
7 wt%) were prepared by a sol–immobilisation method with
and without PVA as the stabilising agent. The catalysts
were analysed by ICP-MS and the measured Au contents
are shown in Table 1 where it was observed that as the tar-
get metal loading was increased the actual metal loading
deviated more from the target composition likely due to
inefcient immobilisation of the Au sol with higher metal
concentrations in the preparation.
All of the samples showed a similar product distribu-
tion, with the selectivity under dark and illumination being
predominantly to GA and DHA. Under illuminated con-
ditions, the nominal 3, 5 and 7 wt% catalysts showed the
formation of glycolic acid (Fig. 2), which was not detected
7% wt - OFF
7% wt - ON
5% wt - OFF
5% wt - ON
3% wt - OFF
3% wt - ON
1% wt - OFF
1% wt - ON
The catalysts were tested for base-free glycerol oxidation
under dark and illumination conditions (wavelengths above
420 nm with IR water flter in place) using constant catalyst
(5 mg) and glycerol concentration (5 mL 0.05 M) and as a
consequence the substrate/catalyst ratio changed for each Au
concentration (Fig. 1) (Gly/Au molar ratio of 984, 357, 234,
186 with increasing metal content respectively). For all the
Table 1 Au content of Au/TiO2 catalysts prepared with and without
0
10
20
30
40
50
60
70
80
90
100
PVA as stabilising agent
Selectivity, Conversion, CMB / %
Nominal Au
wt%
Prepared with PVA
Actual Au wt%
Prepared without PVA
Actual Au wt%
Fig. 2 Catalytic activity of 1, 3, 5, 7 wt% Au/TiO2–PVA. Reaction
conditions: glycerol (5 mL, 0.05 M), catalyst (5 mg), 90 °C, 3 h, (1%)
Gly/Au molar ratio=984 (3%) Gly/Au molar ratio=352 (5%) Gly/
Au molar ratio=234 (7%) Gly/Au molar ratio=186. Key: flled circle
glycerol conversion flled square pyruvic acid (PA) open square glyc-
eric acid (GA) light grey coloured square glycolic acid (GLYA) dark
grey coloured square dihydroxyacetone (DHA)×carbon mass balance
(CMB); remaining product not shown is CO2
1
3
5
7
0.99
2.77
4.26
5.26
0.99
2.81
4.44
5.83
aMetal amount (wt%) determined by ICP
1 3

L. Abis et al.
in signifcant amounts under dark conditions. This might
indicate a diferent or accelerated oxidation pathway occur-
ring under illumination and this is considered to be due to
the formation of H2O2 under these conditions as this is a
well-known intermediate in photocatalysed reactions with
Au resulting in C–C scission. Another feature common to
all the samples is the production of CO2, except for the 1%
wt sample under dark reaction conditions, due to the low
conversion observed. It is clear that the formation of CO2 is
a result of the sol-immobilised catalysts being more active
than equivalent catalysts made by impregnation or precipita-
tion which typically contain larger particles [33].
balance decreased slightly possibly due to polymerisation
of reaction products. Initially no glycolic acid was observed
but this was observed at reaction longer than 45 min as a
sequential oxidation product although DHA was observed
in similar selectivity at all reaction times.
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 [34]. 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 sta-
biliser 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.
The catalysts were characterised by TEM (Fig. S1) and
showed that the gold nanoparticles were well-dispersed on
the support and displayed a very similar size distribution,
averaging 2.8 0.7 nm for all the samples. The catalysts
were further characterised using UV–visible spectroscopy
(Fig. S2). All the catalysts showed a plasmonic band at
similar wavelength (around 540 nm), which is expected for
the nanoparticle size observed by TEM. Also as expected,
higher metal loading catalysts demonstrated a more intense
plasmon resonance band. The similarity observed in the
catalyst selectivity can therefore be explained by the similar
morphologies of the catalysts with conversion determined
by the metal loadings (Fig. 3).
The catalysts were characterised by difuse refectance
UV–Vis spectroscopic showed that all the samples present
an absorption band corresponding to the plasmonic reso-
nance, with values increasing with metal loading (Fig. S3).
The shape and position of the plasmonic resonances change
with increasing metal loading, with broader absorptions for
the 3% and 5% samples and a red shift of the associated
wavelengths. Also, both the signals of the 3% and 5% wt
catalysts show a shoulder at higher wavelengths (more evi-
dent for the 5% wt sample) which might indicate a non-uni-
form resonance frequency, correspondent to a non-uniform
particle size distribution [22]. TEM analysis carried out on
the 1–7% wt Au/TiO2-SF catalysts confrmed a broader size
The 3% wt Au/TiO2 catalyst was investigated to deter-
mine the efect of the reaction time on conversion and selec-
tivity under illumination (Fig. 4). Multiple experiments
were carried out for selected times (45, 90, 120, 180 and
300 min). As expected, the conversion increases linearly
with time of illumination showing that illumination does
not induce signifcant deactivation, reaching 20% after 5 h.
As the conversion increased with reaction the carbon mass
40
a
b
c
d
35
30
25
20
15
10
5
2.9 0.7 nm
2.8 0.8 nm
2.7 0.6 nm
2.7 0.7 nm
0
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
6
Particle size / nm
Fig. 3 Nanoparticle size distribution for the 1, 3, 5, 7% (a, b, c, d respectively) wt% Au/TiO2 catalysts derived by counting 100 nanoparticles
from TEM images
1 3

Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation
22
a
b
100
20
90
80
70
60
50
40
30
18
16
14
12
10
8
6
20
10
0
4
2
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
Time / min
Time / min
Fig. 4 Efect on reaction time on a product selectivity and b glycerol
conversion for 3% wt Au/TiO2 under illumination. Reaction condi-
tions: glycerol (5 mL 0.05 M), catalyst (5 mg), 90 °C, Gly/Au molar
ratio=357. Key: flled circle glycerol conversion flled square pyruvic
acid (PA) open square glyceric acid (GA) light grey coloured square
glycolic acid (GLYA) dark grey coloured square dihydroxyacetone
(DHA)×carbon mass balance (CMB); remaining product not shown
is CO2
distribution and larger particle size at higher loadings (Fig-
ure S4 and Fig. 5).
smaller average particle size (7.3 nm vs 2.8 nm). This seems
to suggest a pure dependence of the catalytic activity on the
plasmonic resonance, with the catalytic system switched on
or of under illumination for larger Au structures, which have
a more intense plasmon resonance than sub 3 nm particles.
As the stabiliser may be infuencing the catalyst activity,
we investigated the efect of calcination on catalysts pre-
pared with stabiliser as a method to burn away the residual
organic species which are present on the surface. These can
be detrimental for catalytic activity [35], but it is known
Surprisingly, all the dried catalysts prepared by the stabi-
liser-free method were inactive, due in part to the larger par-
ticles sizes reducing the number of surface metal atoms sig-
nifcantly, suggesting that smaller nanoparticles are required
for this reaction. The 7 wt% Au/TiO2 catalyst showed a little
activity under dark conditions, as shown in Fig. 6. When
illuminated, its activity is switched on and it is as active as
the equivalent catalyst prepared with stabiliser, having much
20
a
b
c
d
5.4 1.6 nm
11.1 3.8 nm
10.6 3.4 nm
7.3 2.6 nm
10
0
2
4
6
8
10
0
2
4
6
8
10 12 14 16 18
0
2
4
6
8
10 12 14 16
0
2
4
6
8
10 12 14 16
Particle size / nm
Fig. 5 Nanoparticle size distribution for the 1, 3, 5, 7% wt (a, b, c, d respectively) Au/TiO2–SF catalysts derived from TEM images
1 3

L. Abis et al.
20
18
16
14
12
10
8
to>6.5 nm for the calcined catalysts. However, in all cases
an enhancement under light illumination as still observed
after calcination.
100
90
80
70
60
50
40
30
20
10
0
5 Conclusions
These initial fndings 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 fexibility 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/of with light illumination.
5.1
6
4
2
0.4
0
ON
OFF
Light
Fig. 6 Glycerol oxidation using 7% wt Au/TiO2-SF. Reaction condi-
tions: glycerol (5 mL 0.05 M), catalyst (5 mg), 90 °C, 3 h. Gly/Au
molar ratio=140. Key: flled circle glycerol conversion flled square
pyruvic acid (PA) open square glyceric acid (GA) light grey coloured
square glycolic acid (GLYA) dark grey coloured square dihydroxyac-
etone (DHA)×carbon mass balance (CMB); remaining product not
shown is CO2
6 Defnitions
moles of reactant converted (carbon)
that the polymers can be removed efectively by calcina-
tion although this invariably increases the mean nanoparti-
cle size. The 3% and the 5% wt Au/TiO2 catalysts prepared
using PVA were calcined at 500 °C in static air for 3 h which
is known to efectively remove the polymer, and tested under
the same conditions as the non-calcined catalysts [35]. The
efect of calcination had a deleterious efect on the conver-
sion (Fig. 7) but selectivity was not markedly afected. The
loss of conversion is possibly due to the increase in particle
size as shown by TEM (Figure S5) as the mean Au parti-
cle size increased from<3 nm for the uncalcined catalysts
Carbon conversion% =
× 100
× 100
moles of fed reactant (carbon)
moles of product (carbon)
Carbon selectivity% =
moles of reactant converted (carbon)
Carbon mass balance %
ꢀmoles of liquid phase product (carbon)
=
× 100
moles of fed reactant (carbon)
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.
5% wt calc - OFF
5% wt calc - ON
5% wt - OFF
5% wt - ON
Acknowledgements This work is supported by Cardif 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.
3% wt calc - OFF
3% wt calc- ON
3% wt - OFF
3% wt - ON
Compliance with Ethical Standards
0
10
20
30
40
50
60
70
80
90 100 110
Conflict of interest The authors declare no fnancial confict of inter-
Selectivity, Conversion, CMB / %
est.
Fig. 7 Calcination efect on the 3% and 5% wt Au/TiO2–PVA cata-
lysts, compared to the analogous dried catalysts. Reaction conditions:
glycerol (5 mL, 0.05 M), catalyst (5 mg), 90 °C, 3 h, (3%) Gly/Au
molar ratio=330 (5%) Gly/Au molar ratio=200. Key: flled circle
glycerol conversion flled square pyruvic acid (PA) open square glyc-
eric acid (GA) light grey coloured square glycolic acid (GLYA) dark
grey coloured square dihydroxyacetone (DHA)×carbon mass balance
(CMB); remaining product not shown is CO2
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
1 3

Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation
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