Ambient base-free glycerol oxidation over bimetallic PdFe/SiO2 by in situ generated active oxygen species

Article abstract of DOI:10.1007/s11164-020-04333-2

Low temperature oxidation of alcohols over heterogeneous catalysts is exceptionally challenging, particularly under neutral conditions. Herein, we report on an efficient, base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO₂ catalyst at ambient temperature in the presence of gaseous H₂ and O₂. The exceptional catalytic performance was attributed to the in situ formation of highly reactive surface-bound oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe bimetallic catalyst was determined to be significantly more active than corresponding monometallic analogues, highlighting the important role both metals have in this oxidative transformation. Fe leaching was confirmed to occur over the course of the reaction but sequestering experiments, involving the addition of bare carbon to the reactions, confirmed that the reaction was predominantly heterogeneous in nature. Investigations with electron paramagnetic resonance spectroscopy suggested that the reactivity in the early stages was mediated by surface-bound reactive oxygen species; no homogeneous radical species were observed in solution. This theory was further evidenced by a direct H₂O₂ synthesis study, which confirmed that the presence of Fe in the bimetallic catalyst neither improved the synthesis of H₂O₂ nor promoted its decomposition over the PdFe/SiO₂ catalyst.

Full text of DOI:10.1007/s11164-020-04333-2

Research on Chemical Intermediates (2021) 47:303–324
https://doi.org/10.1007/s11164-020-04333-2
Ambient base‑free glycerol oxidation over bimetallic PdFe/
SiO2 by in situ generated active oxygen species
Ricci Underhill1 · Mark Douthwaite1 · Richard J. Lewis1 · Peter J. Miedziak1,2
·
Robert D. Armstrong1 · David J. Morgan1 · Simon J. Freakley1,3
·
Thomas Davies1 · Andrea Folli1 · Damien M. Murphy1 · Qian He1,4
Ouardia Akdim1 · Jennifer K. Edwards1 · Graham J. Hutchings1
·
Received: 17 October 2020 / Accepted: 26 October 2020 / Published online: 5 January 2021
© The Author(s) 2020
Abstract
Low temperature oxidation of alcohols over heterogeneous catalysts is exceptionally
challenging, particularly under neutral conditions. Herein, we report on an efcient,
base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO2 catalyst at ambient
temperature in the presence of gaseous H2 and O2. The exceptional catalytic per-
formance was attributed to the in situ formation of highly reactive surface-bound
oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe
bimetallic catalyst was determined to be signifcantly more active than correspond-
ing monometallic analogues, highlighting the important role both metals have in this
oxidative transformation. Fe leaching was confrmed to occur over the course of the
reaction but sequestering experiments, involving the addition of bare carbon to the
reactions, confrmed that the reaction was predominantly heterogeneous in nature.
Investigations with electron paramagnetic resonance spectroscopy suggested that the
reactivity in the early stages was mediated by surface-bound reactive oxygen spe-
cies; no homogeneous radical species were observed in solution. This theory was
further evidenced by a direct H2O2 synthesis study, which confrmed that the pres-
ence of Fe in the bimetallic catalyst neither improved the synthesis of H2O2 nor pro-
moted its decomposition over the PdFe/SiO2 catalyst.
*
*
Mark Douthwaite
douthwaitejm@cardif.ac.uk
Graham J. Hutchings
hutch@cardif.ac.uk
1
Cardif Catalysis Institute, School of Chemistry, Cardif University, Main Building, Park Place,
Cardif CF10 3AT, UK
2
3
4
School of Applied Sciences, University of South Wales, Pontypridd CF37 4AT, UK
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 2AY, UK
Department of Materials Science and Engineering, Faculty of Engineering, National University
of Singapore, Singapore, Singapore
Vol.:(0123456789)
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Keywords Glycerol · Oxidation · In situ · Ambient · PdFe
Introduction
Demand for bio-derived liquid fuel production is projected to increase steadily over
the coming years. This is due to a global drive towards decoupling economic growth
from fossil fuels and a shift towards renewable, sustainable fuels [1]. Current tech-
nologies for the extraction and refning of fossil fuels are comparably efcient and
economical. As such, further process development is required in order to reduce the
economic defcit separating the fossil fuel and bio-derived fuel industries.
A number of processes for producing bio-derived liquid fuels are currently oper-
ated commercially.1 Of these, the production of biodiesel through transesterifcation
of vegetable fats is perhaps the most promising [2, 3]. However, the product stream
for this process contains approximately 10 wt% yield of glycerol, which is consid-
ered a low-value waste by-product. One possible way to ofset the economic defcit
between bio- and fossil fuels would be the development of efcient technologies for
valorising this waste glycerol.
Catalytic upgrading of glycerol is not a novel concept [4, 5]. Indeed, aerobic oxi-
dation of glycerol over noble metal catalysts is well documented [6, 7]. The highly
functionalised nature of glycerol means that it lends itself well as a model substrate
for studying how physicochemical properties of catalysts drive selectivity in the
oxidation of alcohols and carbonyl species. This is compounded with the numer-
ous products that form during glycerol oxidation, many of which are of commercial
interest. The reaction conditions and catalyst used can dramatically infuence the
product selectivity, lactic acid [8, 9], dihydroxyacetone [10] and glyceric acid [11]
have all been targeted as products for this reaction.
For liquid phase alcohol oxidation, it is widely accepted that the rate determining
step is the C-H bond activation in the alcohol dehydrogenation [9], although to the
best of our knowledge, this has yet to be confrmed experimentally using glycerol as
a substrate. It is, however, evidenced indirectly as the application of stoichiometric
base, such as NaOH, dramatically enhances the rate of reaction [7, 12]. DFT calcu-
lations for ethanol oxidation over Au nanoparticles reveal that Au-hydroxide species
assist with alcohol deprotonation and dramatically reduced the activation barrier for
the subsequent C-H bond activation [13, 14]. Various studies have postulated that
the presence of hydroxide allows for an additional C-H bond activation pathway, in
addition to beta-hydride elimination, through H-abstraction [15, 16]. This is, there-
fore, likely to be why such signifcant rate enhancements are observed for alcohol
oxidation in the presence of base.
Despite progress in this area, reaction rates remain substantially lower under
base-free conditions. For this reason, alternative strategies must be considered, if the
oxidation of glycerol under ambient temperature and base-free conditions is to be
realised on a commercial scale. While O2 utilisation is desirable, use of a more reac-
tive oxidising agent might prove to be an efective way to promote alcohol oxidation
reactions under base-free conditions. An example of such an oxidant being hydrogen
peroxide (H2O2).
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Ambient base-free glycerol oxidation over bimetallic…
305
H2O2 is an environmentally benign oxidant with the sole by-product of its decom-
position being water. This contrasts with common oxidants such as perchlorates,
permanganates and chromate-based reagents, which often produce unwanted, haz-
ardous by-products. These incur a signifcant economic cost, due to downstream
separation of waste streams. Commercial H2O2 synthesis via the anthraquinone oxi-
dation process requires constant replacement of the anthraquinone carrier, due to
unselective and irreversible hydrogenation of the carrier molecule. Therefore, H2O2
production is only economical on a relatively large scale, requiring that H2O2 be
stored or transported at high concentrations, typically in excess of 70 wt%, in the
presence of acid stabilising agents, prior to dilution at point of use. Due to hazard-
ous nature of highly concentrated H2O2 solutions, this has signifcant technologi-
cal and economic impacts [17]. A preferable alternative towards on-site, small scale
H2O2 production would be the direct synthesis from molecular H2 and O2 [18].
Many studies have reported utilisation of preformed H2O2 as oxidant for glyc-
erol oxidation. These studies have focussed on a range of diferent catalysts, includ-
ing various supported metal catalysts [19–25] and metal oxides [26–34]. McMorn
et al. [35] assessed metal-doped silicate (containing Ti, V or Fe) and aluminophos-
phate catalysts (containing Cr, V, Mn or Co) for oxidation of glycerol with H2O2,
reporting low yields of the desired partial oxidation products. Subsequently, double-
layered hydroxide (DLH)-supported transition metal complexes were reported as
active for glycerol oxidation under comparable reaction conditions [10]. Indeed, a
DLH-upported Cr(III) complex was reported to aford reasonable selectivity towards
dihydroxyacetone (DHA) at high glycerol conversions (59.3% selectivity at 85.5%
conversion). Additional investigations by Laurie et al. [36] and Farnetti et al. [37]
attempted to propagate Fenton-type chemistry through combining Fe salts with
H2O2. In both cases, relatively high selectivity towards C3 products was observed,
underlining the potential of using Fe-based species in this reaction.
In this study, a highly active heterogeneous catalytic system is reported for the
oxidation of glycerol under base-free conditions. A 1 wt% PdFe/SiO2 catalyst is
used to synthesise surface-bound active oxygen species directly from H2 and O2
in situ which subsequently assists with the catalytic oxidation of glycerol. To the
best of our knowledge, despite the abundance of literature in this area, this is the frst
example of invoking this strategy for this popular catalytic transformation.
Experimental
Catalyst preparation
Catalysts were prepared using a modifed impregnation procedure based on
that previously reported in the literature [38]. For the typical preparation of a 1
wt%PdFe/SiO2 catalyst (Fe:Pd–0.5:0.5 wt%), the requisite amount of PdCl2 (Sigma-
Aldrich,≥99.999%) solution (6 mg/mL Pd, 0.58 M HCl) and FeCl3.6 H2O (Sigma-
Aldrich,≥99%) solution (6 mg/mL Fe) were added to a 50 mL round bottom fask.
Deionised water was then added to achieve a total solution volume of 16 mL. The
solution was then heated to 60 °C with 1000 rpm stirring, and the required amount
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R. Underhill et al.
of SiO2 (Fisher Scientifc, 60 Å particle size) support was added to produce 2 g of
supported metal catalyst over 8–10 min. After complete addition of the support, the
mixture was heated to 95 °C with stirring (1000 rpm) for 16 h to allow complete
evaporation of the water. The dried catalyst was then recovered and ground using
a pestle and mortar. The ground catalyst was then treated under a fow of 5% H2/
Ar (BOC) at 500 °C for 4 h (10 °C min−1 ramp rate). The treated catalyst was then
allowed to cool to room temperature under a fow of 5% H2/Ar (BOC).
Two corresponding monometallic catalysts were also prepared using the proce-
dure described above. For this, requisite quantities of FeCl3·6H2O and PdCl2 were
used for the synthesis of 0.5wt%Fe/SiO2 and 0.5 wt%Pd/SiO2 catalysts, respectively.
Catalyst characterisation
HR-TEM (high-resolution transmission electron microscopy) and HAADF-STEM
(high-angle annular dark feld scanning transmission electron microscopy) analy-
ses were performed in a JEOL JEM-2100 microscope at 200 kV. Energy dispersive
X-ray spectroscopy (EDS) analysis was carried out using an Oxford Instruments
X-MaxN analyser and Aztec software. Samples were supported onto holey carbon
flm copper grids.
Catalyst testing
Reaction procedure—in situ generated H2O2
Glycerol oxidation reactions were performed in a Parr stainless steel autoclave ftted
with a PTFE liner and a nominal reactor volume of 50 mL. In a typical reaction, the
PTFE liner was charged with an aqueous glycerol solution (10 mL, 0.3 M, Sigma-
Aldrich,≥99.5%) and catalyst (50 mg). The PTFE liner was then inserted into the
reactor and the reactor sealed. The reactor was then purged three times with a dilute
hydrogen mixture (5%H2/CO2, 100 psi). The reactor was then charged with 420 psi
of 5% H2/CO2 and 160 psi of 25% O2/CO2, 160 psi to give a H2: O2 ratio of 1:2. The
reactor was then heated to 30 °C and stirring commenced (1200 rpm) for 6 h unless
otherwise stated. For recharge experiments, gases were vented and recharged at 1 h
intervals. After the desired reaction time, the reactor was vented and solution col-
lected. Following fltration, liquid products were quantifed using high performance
liquid chromatography (HPLC).
Reaction procedure—external addition of H2O2
Glycerol oxidation reactions were performed in a Parr stainless steel autoclave ftted
with a PTFE liner and a nominal reactor volume of 50 mL. In a typical reaction, the
PTFE liner was charged with an aqueous glycerol (Sigma-Aldrich,≥99.5%) solution
(10 mL, 0.3 M), catalyst (0.05 g) and hydrogen peroxide (50 wt% in H2O stabilised,
1 3

Ambient base-free glycerol oxidation over bimetallic…
307
Fluka, volume stated). The PTFE liner was then inserted into the reactor and reactor
sealed. The reactor was then purged thrice with 25%O2/CO2 at 100 psi. The reactor
was then charged with 580 psi of 25% O2/CO2. The reactor was then heated to 30 °C
and stirring commenced (1200 rpm). After the desired reaction time, the reactor was
vented and solution collected. Following fltration, liquid products were quantifed
using HPLC.
Reaction procedure—direct synthesis of H2O2 from molecular H2
and O2
Hydrogen peroxide synthesis was evaluated using a Parr Instruments stainless steel
autoclave with a nominal volume of 100 mL and a maximum working pressure of
14 MPa. To test each catalyst for H2O2 synthesis, the autoclave was charged with
catalyst (0.01 g), solvent (5.6 g MeOH and 2.9 g H2O). The charged autoclave was
then purged three times with 5% H2/CO2 (100 psi) before flling with 5% H2/CO2 to
a pressure of 420 psi, followed by the addition of 25% O2/CO2 (160 psi). The tem-
perature was then decreased to 2 °C followed by stirring (1200 rpm) of the reaction
mixture for 0.5 h. The above reaction parameters represent the optimum conditions
we have previously used for the synthesis of H2O2 [39]. H2O2 productivity was deter-
mined by titrating aliquots of the fnal solution after reaction with acidifed Ce(SO4)2
(0.01 M) in the presence of ferroin indicator. Catalyst productivities are reported as mol
H2O2kgcat−1 h−1.
Reaction procedure—degradation of H2O2
Catalytic activity towards H2O2 was determined in a manner similar to the direct syn-
thesis activity of a catalyst. The autoclave was charged with MeOH (5.6 g), H2O2 (50
wt% 0.69 g) HPLC standard H2O (2.21 g) with the solvent composition equivalent to a
4 wt% H2O2 solution. Prior to the addition of catalyst (0.01 g) from the solution two ali-
quots of 0.05 g was removed and titrated with acidifed Ce(SO4)2 solution using ferroin
as an indicator to determine an accurate concentration of H2O2 at the start of the reac-
tion. The autoclave was pressurised with 420 psi 5% H2/CO2 and cooled to 2 °C, and
the reaction mixture was stirred at 1200 rpm for 0.5 h. After the reaction was complete,
the catalyst was removed from the reaction solvents and as previously two aliquots of
0.05 g were titrated against the acidifed Ce(SO4)2 solution using ferroin as an indica-
−1
tor. The degradation activity is reported as mol H2O2kgcat h−1. Note: The reaction
conditions used within this study operate below the fammability limits of gaseous mix-
tures of H2 and O2.
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R. Underhill et al.
Product analysis and catalyst evaluation
Both glycerol conversion and reaction products were quantifed by HPLC using an
Agilent 1260 Infnity series HPLC ftted with a MetaCarb 67 h column (column
temp = 50 °C) and both RID and DAD detectors. 0.1% H3PO4 in deionised H2O
(Fisher Scientifc, HPLC grade) was used as the mobile phase, and the fow rate
was fxed at 0.8 mL min−1. Calibrations were performed using known standards.
Nuclear magnetic resonance (NMR) was used for qualitative analysis of the
1
radical trapping compounds. For this, H NMR was performed at ambient tem-
peratures on a 500 MHz Bruker Avance III HD spectrometer ftted with a Prodigy
cryoprobe. Due to the strong solvent peak arising from H2O, H2O suppression
was utilised. To the NMR tube was added fltered reaction solution (0.7 mL) and
deuterium oxide (D2O, 0.1 mL, Sigma-Aldrich). Additionally, a glass ampule
containing 1 wt% TMS/CDCl3 (Sigma-Aldrich) was added as internal standard.
The X-band (ca. 9.5 GHz) continuous wave (CW) EPR spectra were recorded
on a Bruker EMX Micro spectrometer equipped with a Bruker ER4123-D dielec-
tric resonator, operating at room temperature. Before each measurement, sam-
ples coming from the fow reactor where deoxygenated for 20 min under N2 fow.
Spectra were recorded at 298 K using the following instrumental conditions:
3.17·103 receiver gain; 100 kHz modulation frequency; 1.0 Gauss modulation
amplitude and 3.282 mW microwave power. Experimental spectra were simu-
lated using the EasySpin package [40] operating within the Mathworks MATLAB
environment.
The concentration of leached metals in product streams was quantifed through
microwave plasma atomic emission spectroscopy (MP-AES) using an Agilent
MP-AES 4100. Post-reaction solutions were fltered using PTFE syringe flters
(0.45 µm) prior to analysis for Fe and Pd. Emission lines were calibrated using
commercial standards (Sigma-Aldrich).
Results and discussion
We recently demonstrated the potential of using a 1 wt% Pd-Fe/SiO2 catalyst
towards phenol oxidation in wastewater streams [41]. The improved performance of
this catalyst, compared to the well-studied AuPd analogue, was attributed to the for-
mation and utilization of in situ formed H2O2 from H2 and O2. It was proposed that
the material served as a multifunctional catalyst; the Pd component was proposed to
be responsible for the generation of H2O2, while the Fe was responsible for the gen-
eration of the active oxygen species involved in the oxidation of phenol and its inter-
mediates. Given the highly active nature of this catalyst, it was of profound interest
to us to establish whether the catalyst, under similar base-free conditions, could be
used for selective oxidation. The abundance of literature on glycerol oxidation, and
the notion that it is exceptionally difcult to activate at low temperatures and neutral
conditions, it represented an ideal substrate for these investigations.
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Ambient base-free glycerol oxidation over bimetallic…
309
The 1% PdFe/SiO2 catalyst was synthesised via a modifed impregnation meth-
odology, described previously [38]. TEM was used to assess the size of the particles
on the SiO2 support (Fig. 1a and b). These experiments confrmed that the catalyst
comprised a distribution of small bimetallic nanoparticles with mean particle size
of approximately 3.2 nm (SD=0.8). Further investigation by HAADF-STEM iden-
tifed that some larger particles were also present in the catalyst. A representative
40 nm PdFe particle and corresponding EDXS is presented in Fig. 1c and d, respec-
tively. In the larger particles, it is evident that Pd and Fe components are alloyed.
Given the conclusions from our previous work [41], it was important to estab-
lish how efcient this catalyst, and its monometallic counterparts were at catalys-
ing the direct synthesis and subsequent degradation of H2O2. A series of experi-
ments were therefore conducted to investigate this, the results of which can be found
in Table 1. Interestingly, the presence of Fe in the catalyst had very little infuence
on the rate of H2O2 synthesis; the productivity of H2O2 was determined to be 69
−1
and 66 mol H2O2kgcat h−1 for the 0.5% Pd/SiO2 and 0.5%Pd-0.5%Fe/SiO2 cata-
lysts, respectively. The monometallic Fe catalyst unsurprisingly, exhibited no activ-
ity towards H2O2 synthesis. Similar fndings were observed when H2O2 degrada-
tion was investigated over the three catalysts. The presence of Fe in the bimetallic
catalyst did not meaningfully increase the rate of H2O2 degradation compared to
the monometallic 0.5%Pd/SiO2 analogue with degradation activity of 255 and 276
Fig. 1 Representative bright feld transmission electron micrograph (a) and corresponding particle size
distributions (b) of as prepared 0.5%Pd-0.5%Fe/SiO2 catalyst prepared by a modifed impregnation
methodology (5%H2/Ar, 500 °C, 4 h, ramp rate=10 °C min−1). HAADF-STEM image (c) and the cor-
responding X-ray energy dispersive spectrum (d) of a large 30–40 nm particle evidencing alloying of Pd
and Fe
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R. Underhill et al.
Table 1 Catalytic activity of SiO2 supported monometallic and bimetallic catalysts towards H2O2 synthe-
sis and its subsequent degradation
Catalyst
Productivity/mol H2O2kgcat−1 h−1 H2O2 Degradation/
a
mol H2O2kgcat−1 h−1
b (%)
0.5 wt%Pd/SiO2
0.5 wt%Pd–0.5wt% Fe/SiO2
0.5 wt% Fe/SiO2
69
66
0
255 (6)
276 (6)
177 (4)
aH2O2 direct synthesis reaction conditions: Catalyst (0.01 g), H2O (2.9 g), MeOH (5.6 g), 5% H2/CO2
(420 psi), 25% O2/CO2 (160 psi), 0.5 h, 2 °C 1200 rpm. bH2O2 degradation reaction conditions: Catalyst
(0.01 g), H2O2 (50 wt% 0.68 g) H2O (2.22 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 0.5 h, 2 °C 1200 rpm
−1
mol H2O2kgcat h−1 observed for these two catalysts, respectively. The monome-
tallic 0.5%Fe/SiO2 catalyst was however also found to catalyse H2O2 degradation,
−1
exhibiting an activity of 177 mol H2O2kgcat h−1 for this competitive reaction.
Given that the sum of H2O2 degradation activity over the two monometallic catalysts
is greater than that observed over the corresponding bimetallic catalyst, it appears
as though the presence of Fe is able to inhibit the formation of water, with recent
studies reporting the ability of a range of secondary transition metals to similarly
improve catalytic performance towards H2O, with the modulation of Pd oxidation
state and inhibition of contiguous Pd ensembles typically attributed as the cause for
enhanced catalytic selectivity [42–44].
The performance of the bimetallic 0.5%Pd-0.5%Fe/SiO2 catalyst was subse-
quently assessed for base-free glycerol oxidation, in the presence of H2 and O2.
The results from this experiment are displayed in Fig. 2a. Glycerol conversion was
Fig. 2 Glycerol oxidation over a 0.5wt.%Pd-0.5wt.%Fe/SiO2 with H2O2 generated in situ. a The efect of
introducing hourly of H2 / O2 recharging cycles. b Glycerol conversion, product yields and carbon mass
balance over a 6 h reaction with hourly recharging of H2 and O2. Reaction Conditions: 10 g 0.3 M glyc-
erol solution, 420 psi 5%H2 / CO2, 160 psi 25%O2 / CO2, 50 mg 0.5wt.%Pd-0.5wt.%Fe / SiO2, 1200 rpm
stirring, 30 °C . Black diamond glycerol conversion with no gas recharges Blue diamond glycerol conver-
sion with hourly gas replacement. Green circle glyceraldehyde, Violet circle dihydroxyacetone, Red tri-
angle formic acid, Black square oxalic acid, Blue diamond glycolic acid. Asterisk carbon mass balance.
(Color fgure online)
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Ambient base-free glycerol oxidation over bimetallic…
311
determined to be linear with time for the initial 1 h of reaction, after which no fur-
ther glycerol conversion was observed (up to 6 h on-line). Given that active oxygen
species are considered to be the oxidant in this reaction, the rate at which both glyc-
erol and partial oxidation products (GLD, DHA etc.) are converted, is limited by
the rate in which these species are formed. Furthermore, the stoichiometric excess
of O2 to H2 in the reactant gas could indicate that the lack of catalytic turnover at
times greater than 1 h, is evidence of a H2 limitation. Indeed, this proposal is fur-
ther strengthened by the fact that O2 is more soluble in the reaction solvent (H2O)
than H2 [45, 46]. To determine whether this was the case, an additional experiment
was conducted, whereby the reactor was de-pressurised and replenished with reac-
tant gases every hour for 6 h. The rate of glycerol conversion under these conditions
is also shown in Fig. 2a. When H2 and O2 were regularly replenished, the glycerol
conversion continued to increase linearly after the frst hour of reaction, confrming
that in the absence of gas replenishment, the reaction is indeed likely to be limited
by the availability of H2.
The product distribution when replenishment cycles were applied is displayed in
Fig. 2b. The data indicate that both the primary and secondary alcohols can undergo
oxidative dehydrogenation to produce either glyceraldehyde (GLD) or DHA, respec-
tively. Increased yields of GLD are observed, suggesting that the activation of the
primary alcohol is more favourable than that of the secondary alcohol. It should be
noted, that increased quantities of formic acid (FA) are also observed with time.
This is unsurprising, given that FA is considered to be one of the terminal oxidation
products in this reaction.6,30 It is, however, unusual that comparable quantities of
C2 products are not also observed, suggesting that a direct route for the conversion
of GLD or DHA to FA may exist. Alternately, it is possible that additional uniden-
tifed compounds are formed under reaction conditions. The observed decrease in
carbon mass balance (CMB) with time is consistent with formation of other prod-
ucts, which might include the total oxidation product CO2. Indeed, CO2 has previ-
ously been reported as a product of aerobic glycerol oxidation [47]. Given that FA
is known to readily decompose to CO, CO2 and H2 over supported Pd nanoparticles
[48], it is likely some of the missing carbon represented in observed is present as
either gaseous of CO or CO2. However, the use of CO2 as a diluent gas for both H2
and O2 reactant gases precludes us from determining if this is the case.
To gain a greater understanding of the mechanistic routes which lead to low
CMBs, additional 6 h reactions (with gas replenishment) were conducted using
DHA and GLD as substrates. Results of these experiments are displayed in Table 2
and highlight that there are clear diferences in rates of carbon loss when diferent
starting substrate is utilized. Over a 6 h period, 9.8% and 23.2% of the total car-
bon were unaccounted for with GLD and DHA oxidation, respectively. Interestingly,
no apparent isomerisation between GLD and DHA was observed in either reaction.
Given that this isomerisation is not observed, it suggests that both dehydrogenation
of the primary and secondary alcohol in glycerol can occur. It also suggests that the
unidentifed reaction products can arise from both intermediates, but that the rate of
the unknown pathway arising from DHA is faster. A study by McMorn et al. [35]
illustrated that acetyls and esters can be formed when glycerol is oxidised with H2O2
1 3

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R. Underhill et al.
Table 2 The oxidation of glycerol (0.3 M), glyceraldehyde (0.23 M) and dihydroxyacetone (0.13 M) over
a 0.5Pd-0.5%Fe/SiO2 under diferent gas compositions
Substrate Reactant gases χ Glycerol Product yields/%
/%
CMB (%)
GLD DHA GLA GLYOX GLO FA OXA
Glycerol H2 +O2
O2
40.6
0.5
40.8
0
14.6 4.2
0.0 0.1
0.0 0.0
0.0 0.0
0.0 0.0
10.0 2.4
0.7
0.0
4.9
–
3.5 0.1
0.0 0.0
14.0 0.7
82.6
99.6
91.2
–
GLD
H2 +O2
O2
–
–
–
–
–
–
DHA
H2 + O2
O2
31.8
0
0.0 0.0
0.0 0.8
7.3
–
0.0 0.5
76.8
–
–
–
–
–
–
–
Reaction conditions: 10 g 0.3 M glycerol solution, 420 psi 5%H2/CO2, 160 psi 25%O2/CO2, 50 mg 0.5
wt%Pd-0.5 wt%Fe / SiO2, 6 h, 1200 rpm stirring, 30 °C. GLY (glycerol), GLD (glyceraldehyde), DHA
(dihydroxyacetone), GLYOX (glyoxylic acid), GLO (glycolic acid), GLA (glyceric acid), FA (formic
acid), OXA (oxalic acid), CMB (carbon mass balance), χ (conversion)
under base-free conditions. A reaction scheme for glycerol oxidation the products
observed is proposed in Scheme 1.
It is proposed that two GLD molecules can partake in an acid-promoted conden-
sation reaction to form acetyl species. This is likely given the mildly acidic reaction
medium; a consequence of CO2, which serves as a diluent for O2 and H2, dissolution
in the aqueous medium [49]. In competition with this pathway, it is proposed that
GLD can either undergo a sequential oxidation to GLA or undergo a C–C scission
to from GLO and FA. GLO can subsequently undergo subsequent oxidative trans-
formation to glyoxylic acid (GLYOX) and oxalic acid (OXA). These reaction path-
ways are well established in the literature [7]; DHA can also undergo C–C scission
reactions in a similar manor to produce GLO, GLYOX, OXA and FA. Interestingly
however, FA was not observed as a product in DHA conversion. As discussed previ-
ously, FA can decompose to CO2 and H2, however given its complete absence in the
product distribution; it’s more likely that it is consumed in an esterifcation reaction
with DHA; the product of which was observed in the previous study by McMorn
et al. [35]
O
O
O
O
GLYOX
O
HO
OH
HO
O
DHA
HPD
H
OH
OH
O
OH
OH
O
HO
OH
Acetyls/Esters
OH
HO
O
O
O
HO
GLY
HPA
OH
GLO
OH
OH
OH
OXA
O
HO
HO
O
CO2
GLD
GLA
OH
Scheme 1 Proposed schematic diagram for the oxidation of glycerol over the 0.5%Pd-0.5%Fe/SiO2
catalyst. Abbreviations: GLY (glycerol), DHA (dihydroxyacetone), GLD (Glyceraldehyde), HPD
(hydroxypyruvaldehyde), GLA (glyceric acid), HPA (hydroxy pyruvic acid), GLO (glycolic acid),
GLYOX (glyoxylic acid), OXA (oxalic acid)
1 3

Ambient base-free glycerol oxidation over bimetallic…
313
The aerobic oxidation of glycerol over Pd-supported catalysts has been reported
[15]. To establish whether the observed glycerol conversion in Fig. 2a was a result
of aerobic oxidation, an additional experiment was performed in the absence of
hydrogen. The results from this experiment are displayed in Table 2 and Fig. 3a and
confrm that no glycerol is converted in the absence of H2. Additional studies were
also conducted under aerobic conditions utilizing GLD and DHA (also displayed in
Table 2), and similarly low conversion was observed.
To investigate the role of each supported metal present in the 0.5%Pd-0.5%Fe/
SiO2 catalyst, monometallic 0.5%Pd/SiO2 and 0.5%Fe/SiO2 catalysts were also syn-
thesised and tested for the oxidation of glycerol under the same reaction conditions.
The results from these experiments are displayed in Fig. 3b. Both monometallic cat-
alysts exhibit a far lower glycerol conversion; the 0.5%Pd/SiO2 was however notably
more active than the 0.5Fe/SiO2 catalyst. Indeed, the sum of glycerol conversion
over the two catalysts, which is representative of the same quantity of Pd and Fe
present in the reaction of the bimetallic catalyst, is still notably lower, indicative of
synergy between the Pd and Fe in the bimetallic catalyst.
As discussed previously, a number of studies have investigated the potential of
using H2O2 as the oxidant for this reaction [10, 35]. These studies, however, used
pre-synthesised H2O2, likely containing acidic stabilising agents, introduced at the
start of the reaction. In order to compare how the addition of commercial H2O2 to
the reaction in this way afected the glycerol conversion under our reaction condi-
tions, additional experiments were conducted (Table 3). To ensure that the observed
reactivity in the ex situ experiments was not limited by available H2O2, a stoichio-
metric excess of H2O2 was used (1.25: 1—mol H2O2: molglycerol). Over a 6 h reac-
tion, the observed glycerol conversion rates were far lower when H2O2 was added
as a reagent, regardless whether the reaction was run in an oxidative or reductive
environment. The slight increase in performance under a reductive atmosphere may
Fig. 3 a The efect of gas composition on glycerol conversion rates. Blue diamond 5%H2/CO2 with
25%O2/CO2 versus Black square 25%O2/CO2. b Glycerol conversion over 0.5 wt.% Pd/SiO2 Violet
square, 0.5 wt.% Fe/SiO2 Red square and 0.5 wt.%Pd-0.5 wt.%Fe/SiO2 Blue diamond catalysts with
H2O2 generated in situ. Reaction Conditions: 10 g 0.3 M glycerol solution, 420 psi 5%H2 / CO2, 160 psi
25%O2 / CO2, 50 mg of catalyst, 1200 rpm stirring, 30 °C. (Color fgure online)
1 3

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R. Underhill et al.
Table 3 Comparison of in situ active oxygen species with ex situ addition of H2O2
Oxidant
Time/h χ Glycerol /% Product yields/%
GLD DHA GLA GLYOX GLO FA OXA
CMB /%
H2 +O2
H2 +O2
O2 +4%H2O2
H2 +4%H2O2
1
6
6
6
7.5
40.6
7.8
3.7 2.4
14.6 4.2
2.5 0.5
5.5 1.1
0.0
0.0
0.4
0.3
0.0
0.0
0.0
0.0
0.2
0.7
0.8
0.6
0.6 <0.1 99.3
3.5
4.8
2.6
0.1 82.6
0.0 101.1
0.0 96.8
13.5
Reaction conditions: 10 g 0.3 M glycerol solution, 420 psi 5%H2/CO2, 160 psi 25%O2/CO2 (or 580 psi
25%O2/CO2 for test with addition of H2O2), 50 mg 0.5%Pd-0.5%Fe/SiO2, 1200 rpm stirring, 30 °C. GLD
(glyceraldehyde), DHA (dihydroxyacetone), GLYOX (glyoxylic acid), GLO (glycolic acid), GLA (glyc-
eric acid), FA (formic acid), OXA (oxalic acid), CMB (carbon mass balance), χ (conversion)
be attributed to the H2 promotion of the Fenton reaction, likely to be facilitated by
the presence of leached Fe in solution [50] (Fig. 4). Nevertheless, these experiments
clearly indicate that the oxidation of glycerol is far more efcient, when reactions
are conducted in the presence of H2 and O2. Even if all the added H2O2 is utilized
rapidly, given the stoichiometric excess used, the selectivity for its utilization in this
reaction is likely to be very poor in comparison with the in situ approach. This data
clearly illustrate that the activity of the 0.5%Pd-0.5%Fe/SiO2 catalyst is likely gov-
erned by the formation of active surface species from H2 and O2 and not from the
synthesis and consumption of H2O2, as originally suspected.
Fig. 4 a Glycerol conversion Blue diamond and Fe leaching (ppm) Red circle as a function of reaction
time. b The concentration of leached Fe (M) Red circle and OXA concentration (M) as a function of
time. Reaction Conditions: 10 g 0.3 M glycerol solution, 420 psi 5%H2 / CO2, 160 psi 25%O2 / CO2,
50 mg 0.5 wt.%Pd-0.5 wt.%Fe / SiO2, 1200 rpm stirring, 30 °C. (Color fgure online)
1 3

Ambient base-free glycerol oxidation over bimetallic…
315
Comparing the efect of homogeneous versus heterogeneous Fe
on catalytic performance
When employing heterogeneous catalysts in an aqueous medium, leaching of the active
metals from the support can be problematic, especially when organic acids are present
in the reaction mixture. To investigate whether any leaching of Fe and/or Pd occurred
in these reactions, post reaction solutions were analysed via MP-AES. As illustrated
in Fig. 4, while no Pd leaching was detected, signifcant leaching of Fe was observed.
Interestingly, although the degree of Fe leaching increased with time, the rate of glyc-
erol conversion remained unafected. Homogeneous Fe species have been shown to be
efective catalysts for oxidations with H2O2 [51]. It was therefore necessary to consider
whether the contribution from Fe in this reaction was a result of hetero- or homogene-
ous catalysis.
A lack of Fe leaching during the initial 1 h of reaction indicates that leaching of Fe
from the SiO2 support is not the result of inherent instability of the metal-support inter-
face. Indeed, no Fe leaching was observed when the catalyst was stirred in water for 6 h
under reaction conditions. Fe leaching must therefore be due to supported metals inter-
acting with reaction products. We have previously shown this to be the case for sup-
ported Fe catalysts in the degradation of phenol, with increased Fe leaching observed
with increased concentrations of oxalic, acetic and formic acids [41]. Given that OXA
can act as a powerful chelating agent, its concentration in the reaction solution was
correlated with that of leached Fe. This data are illustrated in Fig. 4b. A strong correla-
tion between these concentrations was observed, suggesting that the leaching of Fe into
solution is likely to be primarily attributed to chelation by OXA. This is unsurprising,
given that OXA is known to be an efective chelating agent; it has the ability to form
both Fe2+ and Fe3+ oxalates in solution.
To determine whether the synergistic efect observed for 0.5%Fe-0.5%Pd/SiO2
resulted from catalytically active Fe species in solution, carbon was added to seques-
ter leached Fe from solution. A similar approach was successfully invoked by Arm-
strong et al. [52] to remove leached Pt from solution in the oxidation of glucose. The
results from these experiments are displayed in Fig. 5. Addition of carbon (50 mg) at
the beginning of the reaction led to a fve-fold decrease in the Fe concentration in solu-
tion after 6 h, from 17.5 to 3.3 ppm. The addition of low quantities of carbon in this
way had no signifcant efect upon glycerol conversion rates, indicating that the activ-
ity observed in this reaction is independent of leached Fe concentration. However, the
addition of>50 mg carbon led to decreased rates of glycerol conversion and efectively
sequestered Fe from solution. In the presence of 200 mg of carbon, the glycerol conver-
sion drops to ca. 25%. Given that only trace quantities of Fe were detected in the post
reaction efuent in this reaction, we can confdently propose that the majority of glyc-
erol is converted on the catalyst surface.
1 3

316
R. Underhill et al.
Fig. 5 The efect of adding an increasing mass of Norit SX1G carbon at t=0 upon glycerol conversion
and leached Fe concentration. Reaction Conditions: 10 g 0.3 M glycerol solution, 420 psi 5%H2/CO2,
160 psi 25%O2/CO2, 50 mg 0.5 wt%Pd-0.5% wt.Fe/SiO2, X mg Norit SX1G, 6 h, 1200 rpm stirring,
30 °C. Gases were vented and recharged hourly
Investigating the nature of the oxidising species
It is clear that 0.5%Fe-0.5%Pd/SiO2 catalyst efectively catalyses the oxidation of
glycerol under mild reaction conditions using oxidising species generated catalyti-
cally in situ from H2 and O2. The nature of the oxidising species, however, remains
elusive. Given the reactivity of the chemical components, it is possible that some
contribution to the observed activity may be attributed to two sequential catalytic
processes; (i) direct, Pd-catalysed H2O2 synthesis from H2 and O2 and (ii) Pd/Fe-
catalysed H2O2 conversion to yield oxidising hydroxyl (HO·) and/or hydroperoxy
(HOO·) radicals in solution. An alternative mechanism for oxidation could involve
interaction between the substrate and surface-bound reactive oxygen species formed
from H2 and O2. A number of studies have proposed mechanisms whereby Pd sites
*
catalyse H2O2 formation via a surface-bound OOH intermediate [53, 54]. While it
is possible that such a mechanism is occurring in the reactions discussed, herein the
relatively low rates of conversion observed over 0.5%Pd/SiO2 when compared with
0.5%Fe-0.5%Pd/SiO2 suggests that free radicals play a role in glycerol oxidation. To
determine whether glycerol conversion is mediated by oxygen-based free radicals in
solution, the radical trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was added at
the start of the reaction, and resulting product streams were analysed by EPR.
Control experiments in the absence of a catalyst but containing DMPO, glyc-
erol and the H2/O2 gas charge, yielded an EPR spectrum which is consistent
with formation of the DMPO-OH adduct (Fig. 6a). Under these conditions, no
apparent glycerol conversion was observed. Addition of 0.5%Fe-0.5%Pd/SiO2 to
this reaction led to total loss of the DMPO-OH feature (Fig. 6b), suggesting that
the catalyst is efectively terminating ·OH species. This was confrmed through
addition of catalyst, but not glycerol to the reaction, with the resulting EPR spec-
trum showing no apparent DMPO-OH adduct formation (Fig. 6c). This appears
1 3

Ambient base-free glycerol oxidation over bimetallic…
317
Fig. 6 X-band CW-EPR spectra (black traces) recorded at 298 K for a 5% H2/CO2 and 25%O2/CO2 in
water in the presence of DMPO and absence of the catalyst; b 5% H2/CO2 and 25%O2/CO2 in water
in the presence of DMPO and 50 mg of 0.5 wt%Pd-0.5 wt%Fe/SiO2; c 5% H2/CO2 and 25%O2/CO2 in
a 0.3 M aqueous solution of glycerol in the presence of DMPO and 50 mg of 0.5 wt%Pd-0.5 wt%Fe/
SiO2. EPR simulations are shown in red. The 1:2:2:1 signals marked with an asterisk, *, are indica-
14
tive of the presence of DMPO-OH adduct: giso =2.0057 0.001; aiso
(
N)=41.62 0.95 MHz;
a
iso(1H)=42.57 0.95 MHz. The 1:1:1:1:1:1 signals marked with an x are indicative of the pres-
ence of a DMPO adduct generated by trapping a C-based radicals (most likely present as an impurity):
14
g
iso =2.0057 0.001; aiso
(
N)=44.34 0.95 MHz; aiso(1H)=64.01 0.95 MHz. Finally, the 1:1:1
signals marked with an o are due to DMPO decomposition products, often present when DMPO spin
14
trapping experiments are performed: giso =2.0057 0.001; aiso
(
N)=41.76 0.95 MHz *Prior to EPR
measurements, the reagents and catalyst were stirred together in an autoclave under reaction conditions
for 5 min
counterintuitive, given that signifcant glycerol conversion is observed under
these conditions. To rule out potential oxidative decomposition of the DMPO
1
spin trap, quantitative H NMR spectra were collected for the same solutions,
1
with resonances normalised to an internal TMS standard at δ H = 0.0 ppm. It is
clear from spectra in Fig. 7 and integrated peak areas in Table 4 that no signif-
cant DMPO conversion occurs during the 5 min reaction period. Formation of a
DMPO-OH adduct would be expected to result in a downfeld shift of resonance
(d) and would lead to a change in the ratio of b, c, d: a. This was not observed,
1
though appearance of an unknown species (δ H = 2.43 ppm) correlates with
addition of 0.5%Fe-0.5%Pd/SiO2 to the reaction (Figs. 6iii and 6iv). Given that
the EPR/NMR experiments suggest a lack of DMPO-OH adduct formation in
the catalysed reaction (Figs. 5b and 6iv), it is unlikely that glycerol oxidation is
mediated by oxygen- based free radicals in solution. Rather, these data suggest
formation of reactive surface-bound oxygen species and that it is through inter-
action with these that glycerol undergoes partial oxidation.
1 3

318
R. Underhill et al.
c
a
d
b
(iv)
(iii)
(ii)
(i)
7.5
3.0
2.5
2.0
1.5
1.0
δ Chemical Shift / ppm
Fig. 7 1H NMR spectra of spin trap product solutions, inc. nmr assignment for DMPO. i 10 μL DMPO in
10 ml H2O, ii 10 μL DMPO in 10 ml H2O after 5 min reaction with H2 and O2, iii 10 μL DMPO in 10 ml
H2O after 5 min reaction with H2, O2 and 0.5 wt%Pd-0.5 wt%Fe/SiO2, iv 10 μL DMPO in 10 ml 0.3 M
glycerol solution after 5 min reaction with 5% H2/CO2, 25%O2/CO2 and 0.5 wt%Pd-0.5 wt%Fe/SiO2
Discussion
The experimental results presented herein, particularly from the EPR experiments,
are somewhat surprising and indicate that the observed reactivity does not stem
from the formation of ·OH in situ. The data presented in Fig. 3 confrm that H2 is
required in the medium to convert glycerol under these conditions, thus, any con-
tribution from an aerobic oxidation pathway, at least in a traditional sense, can be
dismissed. As discussed previously, the RDS in this reaction is the alcohol dehydro-
genation, deprotonation of the alcohol and subsequent activation of an adjacent C–H
bond. The presence of an adjacent surface-bound hydroxy species on a model Au
catalyst was proposed to dramatically reduce the activation barriers, for both these
process [14]. The authors speculate, with evidence from DFT calculations, that this
reduction in activation energy is attributable to the formation of superoxide (*OOH)
and hydroxide (*OH) surface intermediates. It is therefore plausible to suggest that
these intermediates are also responsible for the activity we observe over the 0.5%Pd-
0.5%Fe/SiO2 catalyst in the presence of H2 and O2.
The reduction of O2 by H2O, which leads to the formation of these intermedi-
ates, is likely to be rather energy intensive, which could explain why in the absence
of base such poor activity is observed over heterogeneous catalysts. One can how-
ever envisage that O2 reduction by dissociatively adsorbed hydrogen would be far
more favourable; this is considered to be a key step in the direct synthesis of H2O2
which is commonly practiced at temperatures as low as 2 °C [55]. Based on this
fact and through reference to previous work [14, 54], we can propose a mechanism
for these transformations (Scheme 2) and highlight the importance of simultaneous
oxygen reduction reactions. First, dissociative adsorption of H2 occurs on a Pd site
(Scheme 2, step 1), which is followed by heterolytic H* oxidation (Scheme 2, step 2)
1 3

Ambient base-free glycerol oxidation over bimetallic…
319
1 3

320
R. Underhill et al.
Scheme 2 Proposed series of
steps for glycerol (GLY) oxida-
tion to glyceraldehyde (GLD) in
the presence of H2 and O2. This
example highlight the role of the
*OOH intermediate in promot-
ing proton abstractions from
the alcohol moiety and adjacent
α-carbon in the primary alcohol.
Note: * corresponds to a surface
active site
H2
H*
**
2H*
(1)
(2)
H+ + e- + *
O2**
O2
+
(3)
(4)
(5)
(6)
(7)
H+ + O2** + e-
H+ + OOH** + e-
GLY* + 2OOH**
H2O2**
OOH**
H2O2**
GLD+2H2O2**
H2O2 + **
and simultaneous adsorption of molecular oxygen (Scheme 2, step 3) sites. The pro-
ton formed from heterolytic oxidation of H* is subsequently transferred to molecular
oxygen (O2**) via proton–electron transfer (Scheme 2, step 4). This peroxide inter-
mediate (HOO**) can then, either abstract a proton from the Pd in a subsequent
proton–electron transfer to produce H2O2** (Scheme 2, step 5) or abstract a proton
from either the alcohol moiety or from the α-carbon in an glycerol alkoxy interme-
diate also producing H2O2** (Scheme 2, step 6). H2O2** subsequently desorbs to
liberate the two active sites (Scheme 2, step 7). We propose that these two processes
are in direct composition, and their relative rates are dependent on surface coverage.
It is also important to consider the synergistic efect observed from introducing
Fe into the Pd particles. McEwen and co-workers demonstrated that the deposition
of Pd onto a Fe2O3 support could signifcantly enhance the reducibility of Fe3+ [56].
We therefore propose that in the 0.5%Pd-0.5%Fe/SiO2 catalyst, the hydrogen formed
from dissociative adsorption on Pd sites can spill over and reduce adjacent Fe2O3 to
form Fe-OH/Fe-OOH species. We propose that this species also assist with deproto-
nation and activation of the C-H bond in the glycerol alkoxy adduct. This theory is
further strengthened by the fact that an electrochemical study investigating alcohol
oxidation over PdNi bimetallic catalysts concluded that the enhanced performance
was attributed to the presence of *OOH species in the base metal component [57].
The preceding discussion provides an explanation into the activity observed from
a heterogeneous perspective, but a small contribution from homogeneous Fenton
chemistry cannot be excluded. This is most pertinent, especially given the ability of
this catalyst to produce H2O2 directly from H2O2. In the reaction where almost all
the Fe were successfully sequestered by carbon (Fig. 5), a notable drop in glycerol
conversion was observed, glycerol conversion drops from 41% without carbon to
26% in the reaction with 200 mg of carbon. While the EPR experiments confrm
that there is no suggestion of.OH species in solution, these measurements were con-
ducted at short reaction times (5 min), where the concentration of leached Fe is min-
imal (Fig. 4). We therefore speculate that the high catalytic performance observed
over the 0.5%Pd-0.5%Fe/SiO2 catalyst is attributable to both heterogeneous and
homogeneous processes.
From the results acquired, it is difcult to assess how selective this catalyst is
for glycerol oxidation. Fairly high concentrations of glyceraldehyde are produced,
but experiments conducted using this as a substrate demonstrate that it is consumed
under reaction conditions. Given the large quantities of formic acid it is plausible to
1 3

Ambient base-free glycerol oxidation over bimetallic…
321
suggest that this catalyst reacts with many of the high value products of this reac-
tion; only trace quantities of glyceric and tartronic acid are observed post reaction.
Further work is required to explore whether there is opportunity to oxidise glycerol
selectively using this methodology.
Conclusion
In conclusion, 0.5%Fe-0.5% Pd/SiO2 prepared by a modifed impregnation method
is highly active for the base-free oxidation of glycerol at 30 °C. The reactivity of this
catalyst is primarily attributed to the formation of active oxygen surface species;
formed from the reduction of O2 by dissociatively adsorbed hydrogen on Pd sites.
The active oxygen species is proposed to be *OOH species, situated on both Fe and
Pd sites. We speculate that some activity arises from homogeneous Fenton chemis-
try, despite the lack of evidence from EPR and NMR. Leaching of Fe species over
the course of the reaction was observed, but sequestering experiments confrmed
that the reaction was predominantly heterogeneous in nature. While this catalyst
does not appear selective towards a given product, to the best of our knowledge,
the observed rates of glycerol conversion are unparalleled, particularly under such a
mild temperature and base-free condition. Future catalyst design studies might target
improved reaction selectivity, while the system may also fnd application in the total
catalytic oxidation of organic contaminants.
Memories of Michel Che
Michel was a great mentor to myself through since 1997 when I became Head of
the School of Chemistry at Cardif University. Michel had a connection with Cardif
through Damien Murphy who then had just been appointed to a lectureship. Previ-
ously Damien had worked with Michel and through this connection I got to know
Michel.
Michel invited me as a visiting Professor to his University in Paris in June 2004.
As I was still Head of School, I could only be in Paris part time and so I visited
Paris early Monday until late Wednesday and was in Cardif for the rest of the week.
Michel was a perfect host. He would let me think about my science in the morning
until about 11.30 and then we would walk together around Paris fnding a wonder-
ful restaurant for lunch and then arriving back at the lab at about 4 pm. This was a
wonderful time as Michel was so knowledgeable and we had such wonderful walks
around Paris. This time was important for me as I decided to concentrate more of
my efort on gold catalysis, and Michel was an enthusiastic mentor in these eforts. I
now look back to that time with Michel as a key point in my research career.
Michel played a leading part in the development of the Cardif Catalysis Institute
when we started in 2008. He served as the Chair of the external advisory board until
January 2019 when I stepped down as the director and he then decided to withdraw
from the board. His advice as chair was so important to us. He gave his time so
freely to ensuring that others could succeed and we all appreciated this quality—he
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R. Underhill et al.
just knew the right way to ensure progress could be made even when it was not
apparent how this could be done at the start.
Michel loved Rugby Union and was a loyal French supporter. I was often able to
get tickets to the Wales-France Six Nations matches when played in Cardif. Michel
would really enjoy these trips. Cardif takes on a magical atmosphere on these match
days and we had a great time. Sadly, in all the matches we enjoyed together we never
once witnessed a French victory!
It goes without saying that Michel was a leading catalysis scientist, and this was
recognised in the UK by the award of the 2009 RSC Centenary Prize. But, the most
signifcant recognition in my opinion was the award of the RSC Faraday Lectureship
and Prize in 2014. This is the oldest award of the RSC and is awarded only every
two years. One of my fondest memories of Michel is having the opportunity to pre-
sent him with the medal for this prize as I was President of the Faraday Division at
the time (see the happy photo).
As I write this, I can hear his voice talking to me, reminding me of the walks we
had, the rugby we watched and the science we discussed. We all miss him greatly.
Graham Hutchings.
Acknowledgements The authors wish to acknowledge the contribution of Selden Research Limited. We
would also like to thank the Cardif University TEM facility for the electron microscopy.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as
you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com-
mons licence, and indicate if changes were made. The images or other third party material in this article
are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons licence and your intended use is
not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission
directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licen
ses/by/4.0/.
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