Effect of Gold Particles Size over Au/C Catalyst Selectivity in HMF Oxidation Reaction

Article abstract of DOI:10.1002/cctc.201901742

A series of gold nanoparticles in the 4–40 nm range were prepared, immobilized on activated carbon and further tested, at low base concentration, in the catalytic oxidation of 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA). Gold particles size variation has no influence on HMF conversion but significantly affects product selectivity and carbon balance. This behavior is ascribed to the thermodynamically favorable oxygen reduction reaction on Au(100) faces. As the gold particle size decreases the Au(100)/Au(111) exposure ratio, estimated by using the van Hardeveld-Hartog model, increases as well as the FDCA selectivity. The smaller the gold particle size the smaller the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to FDCA ratio pointing to the gold size dependent behavior of the oxidation of the alcohol function of the HMF molecule.

Full text of DOI:10.1002/cctc.201901742

Accepted Article
Title: Effect of gold particles size onAu/C catalyst selectivity in HMF
oxidation reaction
Authors: Cristina Megías-Sayago, Alice Lolli, Danilo Bonincontro, Anna
Penkova, Stefania Albonetti, Fabrizio Cavani, Jose Antonio
Odriozola, and Svetlana Ivanova
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ChemCatChem
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Effect of gold particles size over Au/C catalyst selectivity in HMF
oxidation reaction
[
a]
[b]
[b]
[a]
[b]
[b]
C. Megías-Sayago , A. Lolli , D. Bonincontro , A. Penkova , S. Albonetti , F. Cavani , J. A.
[
a]
[a]
Odriozola and S. Ivanova*
Abstract: A series of gold nanoparticles in the 4-40 nm range were
prepared, immobilized on activated carbon and further tested, at low
base concentration, in the catalytic oxidation of 5-hydroxymethyl
furfural (HMF) to 2,5- furandicarboxylic acid (FDCA). Gold particles
size variation has no influence on HMF conversion but significantly
affects product selectivity and carbon balance. This behavior is
ascribed to the thermodynamically favorable oxygen reduction
reaction on Au(100) faces. As the gold particle size decreases the
Au(100)/Au(111) exposure ratio, estimated by using the van
Hardeveld - Hartog model, increases as well as the FDCA selectivity.
The smaller the gold particle size the smaller the 5-hydroxymethyl-2-
furancarboxylic acid (HMFCA) to FDCA ratio pointing to the gold size
dependent behaviour of the oxidation of the alcohol function of the
HMF molecule.
particular interest awakes gold, longtime considered as a metal
with poor ability to activate oxygen. However, more and more
studies demonstrates that gold – oxygen interactions are
controlled by the support nature or by the size of the metal
clusters [9–11], in such a way that the higher the gold dispersion
the higher the overall oxidation reaction rate. For gas phase
oxidation reactions dioxygen activation by the catalyst, either bare
gold or supported on non-reducible oxides or carbon, involves
charge transfer from Au to O
2
. This results in an increase of the
O-O bond distance appearing superoxo-like species (O -
)
2
probably accompanied by partially reduced gold that is very
difficult to detect as purely cationic states since important
electronic structure rearrangements [12,13].
Liquid phase oxidation reactions on gold behave quite differently.
In liquid media, dioxygen-solvent interactions must be considered.
Fristrup et al. [14] studied the gold-catalyzed aerobic oxidation of
aldehydes in methanol. These authors reported that dioxygen
was not directly involved in the oxidation process but indirectly
after reacting with the solvent. Similar results were found in an
aqueous solution, dioxygen reacts indirectly as hydroxide ions
Introduction
Progressive substitution of petroleum-based chemicals by
sustainable bio-based products should be the result of renewable
lignocellulosic biomass valorization through molecules able to
replace those employed nowadays. Lignocellulosic components
are rich sources of platform chemicals available for dehydration,
hydrogenation and/or oxidation reactions in separate or in
consecutive manner. 2,5- furandicarboxylic acid (FDCA) is a very
good example of such chemical. FDCA is a potential substitute of
terephtalic acid, used for polyethylene terephthalate plastics
production, because of their structural similarity and final plastics
properties alikeness [1,2]. Its synthesis route follows a set of
consecutive ractions including biomass depolimerization,
hexoses isomerization and dehydration to 5-hydroxymethyl
furfural (5-HMF) and oxidation to the final FDCA product.
generated upon O
function oxidations occur by H-abstraction in the functional group
to be oxidized mediated by the O -generated hydroxide species.
2
dissolution [15]. Aldehyde and alcohol
2
The HMF oxidation path to FDCA may be schematized
considering three different steps (scheme 1): i) the fast oxidation
of the aldehyde to carboxylic group resulting in 5-hydroxymethyl-
2-furancarboxylic acid (HMFCA) and ii) a two-step oxidation of the
HMFCA species including the slow oxidation of the alcoholic
function to aldehyde given rise to, 5-formyl-2-furancarboxylic acid
(
FFCA) and, in a further fast reaction step, the oxidation of the
dialdehyde to 2,5-furan dicarboxylic acid (FDCA). As long as the
-
OH are present or generated in the media the HMF oxidation
occurs selectively towards the final product.
5-HMF to FDCA selective oxidation occurs either in
homogeneous conditions, using KMnO or metal halides [3–5] or
4
over noble metals based heterogeneous catalysts [6–8]. Within
the group of catalytically active noble metals in oxidation reactions,
[
[
a]
b]
Dr. C. Megías-Sayago, Dr. A. Penkova, Prof. J. A. Odriozola and Dr.
S. Ivanova
Department: Departamento de Química Inorgánica e Instituto de
Ciencia de Materiales de Sevilla
Institution: Universidad de Sevilla-CSIC
Address: Américo Vespucio 49, 41092, Seville, Spain
E-mail: svetlana@icmse.csic.es
Dr. A. Lolli, D. Bonincontro, Prof. S. Albonetti, Prof. F. Cavani
Department Dip. di Chimica Industriale “Toso Montanari”,
Institution: Università di Bologna
Scheme 1. Reaction path for HMF oxidation to FDCA.
In this study HMF is used as model molecule to study the size
dependent ability of gold catalysts on the oxidation of alcohol
Address: Viale del Risorgimento, 4, 40136 Bologna BO, Italy
(
aldehyde) functions. This will provide clues on the dioxygen
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
activation mechanism on gold and clarify its participation in the
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ChemCatChem
10.1002/cctc.201901742
FULL PAPER
reaction. For ensuring that the results are not altered by support
properties or gold loading the same support and similar Au
loadings are used.
Within the series, no matter the gold particle size, all catalysts
lead to full HMF conversion and only HMFCA and FDCA are
detected as products. On the other hand, the product selectivity
and C balance are significantly affected by the particle size. It is
interesting to underline that in the 4-16 nm particle size only minor
changes in selectivity are observed disfavoring FDCA for HMFCA.
However, for particle sizes higher than 35 nm HMFCA is the
primary product. It seems that aldehyde group oxidation to
HMFCA is less size dependent than alcohol group oxidation. The
oxidation rate of alcohol function slightly decreases with particle
size in the 4-16 nm range and dramatically for the highest particle
size tested. A question raises, what is making the difference? Full
HMF conversion is always attained; therefore, its concentration is
not a factor in these conditions. Let’s consider the presence of
base and oxygen.
Results and Discussion
The gold nanoparticles synthesis parameters are chosen in such
a way that a wide range of particle sizes may be obtained. A
selection of gold early prepared and already reported by us are
used in this study [16]. A typical example of the preparation
method is presented in the experimental section and the resulting
gold colloids preparation conditions, gold loadings and average
particle sizes summarized in table 1.
Table 1. Selected samples, preparation, parameters, corresponding gold
-
It is well known that, in basic conditions (1:2 HMF:OH molar ratio
loadings, and particle sizes.
in our experiments), aldehyde groups transform to alkoxide
intermediates trough nucleophilic addition of hydroxide ion and
subsequent proton transfer from water [17,18]. When catalyzed
this reaction goes selectively to carboxylic acid but in absence of
catalysts Cannizaro reaction occurs and the aldehyde (HMFCA)
transforms in the corresponding alcohol and carboxylic acid.
Oxidation of alcohols are reported to occur by H-abstraction from
the OH group by hydroxide ions [15,19]. The base concentration
used in these experiments, two base equivalents, is not enough
to complete the reaction unless its concentration were
Sample
PVA[a]:Au:NaBH
ratio
4
[b]
Au loading,
wt.%
Particle size,
a]
nm[
AuC_I
AuC_II
AuC_III
AuC_IV
AuC_V
0.85:1:5
0.85:1:10
0.5:1:5
2.3
2.3
2.4
2.0
2.1
4.8
6.6
8.7
0:1:5
15.4
37.9
0.85:1:10
-
continuously renewed. On reducing the OH concentration (Figure
[
a]
PVA:Au weight ratio, [b] Au:NaBH molar ratio; [c] measured by TEM
4
2) the HMFCA is the main product although FDCA is also found.
The presence of both suggests that base is helping in all
processes and that the reaction rate of aldehyde oxidation over
gold is higher than that of alcohols, this is also confirmed by the
absence of 2,5-Diformylfuran (DFF). It is worth underlining that
HMF conversion and HMFCA production are almost equimolar
suggesting that the first process is using the starting excess of
base before all the other processes and that a full conversion of
Figure 1 summarizes the HMF conversion and product selectivity
as a function of the gold particle size.
-
HMF will be always observed when an initial OH concentration
equal or superior to that of HMF is employed. It should be noted
that HMF is not converted in absence of base.
100
80
60
40
20
0
Figure 1. Gold catalysts screening: HMF conversion and HMFCA, FDCA yields
vs. mean particle size. Reaction conditions: HMF:Au:NaOH molar ratio 1:0.01:2,
0.5
2
2
10 bar O , 70 ºC, 400 rpm, 4h.
NaOH equivalent
Conversion of HMF to FDCA on gold catalysts proceeds through
preferential aldehyde group oxidation to HMFCA and subsequent
alcohol group oxidation to FFCA finally transformed into FDCA.
Aldehyde group oxidation is a rapid process and normally the
FFCA intermediate is absent at the final reaction time.
Figure 2. Conversion/yield dependence on base equivalent over AuC_II sample.
Reaction conditions: HMF:Au:NaOH molar ratio 1:0.01:0.5 or 1:0.01:2,
respectively, 10 bar O , 70 ºC, 400 rpm, 4h.
2
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ChemCatChem
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-
At this point, a new charge of OH is necessary for all other steps.
Here is where oxygen enters in the equations. Davis et al. [20]
found using labled oxygen that it is involved in the oxidation of
alcohols although not directly incorporated from dioxygen but
The 100/111 gold surface ratio decreases exponentially with
particles size as FDCA selectivity does. A 1st order exponential
decay expression fits both tendencies with the same value for the
exponent suggesting a direct relationship between the processes
involved over Au (100) surface and FDCA production. If we
consider that at high 100/111 ratio the 4-electron oxygen
reduction occurs with hydroxide ions as a product, the oxidation
of HMFCA proceeds and the FDCA yield is higher. All processes
-
from OH groups present in the reaction media. They conclude
that the role of dioxygen is indirect as regenerator of the hydroxide
ions. The only way to generate hydroxides from O in aqueous
2
media is its reduction. Two possible ways of oxygen reduction in
alkaline media are reported, 2-electron reduction with the
formation of peroxide ions
need base, O
2
reduction and alcohol/aldehyde oxidation. The Au
-
(100) surfaces must be covered by OH to produce AuOHads in
order to reduce the potential of O
reduction [26]. So, for hydroxide ion generation (O
-
-
nd
2
to HOO and to OH 2 step
activation)
-
-
-
O
2
+H
and 4-electron reduction
+2H
2
O+2e  HOO +OH
2
over gold we need preferential exposure of the Au (100) face, i.e.
particles as small as possible and presence of base in order to
promote the 4-electrons O reduction and to renew continuously
2
-
O
2
2
O+4e  4OH-
the hydroxide ions concentration required to accomplish the
reaction to FDCA. The latter means that for very high base excess
the oxidation reaction may proceed using the hydroxide ions from
the media and probability should not be size dependent. However
in the absence of base, when the oxygen reduction reaction is not
thermodynamically favorable, the FDCA production proceeds via
direct oxidation of HMFCA. The rate of this reaction increases
linearly with the surface gold concentration needed to
dehydrogenate HMFCA and further convert it in FDCA [27].Both
facts, suggest that FDCA production is size dependent at low to
moderate base concentration and independent in the limiting
cases, absence or great excess of base.
The latter just observed on Au (100) surfaces and its vicinals. This
reaction is of particular interest since Au (100) surfaces are the
most active known catalyst for the 4-electron O
2
reduction [21–
2
4] and relative exposure of the (100) face of FCC nanoparticles
is size dependent. The application of electrochemical potential is
generally required for running this reaction, although on small gold
nanoparticles, i.e. with high Au (100) surface exposure and high
OH-
coverage,
the
oxygen
reduction
reaction
is
thermodynamically favorable. In addition, the oxidation of
aldehydes (2-electron oxidation) and that of alcohols (4-electron
oxidation) to carboxylic acid assures the necessary electrons.
Considering that gold is an fcc metal and expose (111) and (100)
surfaces (Figure 3 inset), every change in its size will produce a
change in its (100)/(111) surface ratio. Using the model of van
Hardeveld and Hartog [25] the surface atoms and sites
corresponding to both surfaces and their relation with the
experimental average particle size can be estimated (Figure 3).
Two samples were selected, AuC_II and AuC_III to analyze the
stability of the catalysts. Both samples are submitted to 4 reaction
cycles keeping the HMF:Au:NaOH ratio constant (Figure 4). Our
previous study on the deactivation behavior of these systems [28]
in the glucose to gluconic acid oxidation reveals that a particle
size increase occurs after the reaction 1st cycle remaining then
constant till the 4th cycle. The same behavior is observed in this
study during the cycles of HMF oxidation. The gold particle size
of the AuC_II catalyst increases from 6.6 to 8.8 nm and that of
AuC_III one from 8.7 to 12.2 nm. Figure 5 shows TEM images
and particle size distributions for both catalysts after the 4th cycle.
Figure 3 100/111 surface ratio for the studied samples and FDCA selectivity
dependence on particle size.
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ChemCatChem
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continue losing selectivity to FDCA. This change in the oxidation
selectivity is independent of the gold particle size and could be
ascribed to a continuous change in the symmetry of the Au (100)
surface from four-fold to six-fold layers [28]. This change causes
1
00
AuC_I I
8
0
6
0
-
4
0
a decrease in the OH adsorption on Au (100) inhibiting the 4-
20
electron reduction, i.e. Au (100) surface starts to behave as Au
(
111) surface. Hausen et al. [29] reported a higher reconstruction
0
I use
II use
III use
IV use
rate for Au (100) surfaces than for Au (111) ones. This
reorganization, described as network of dislocations, produces
important changes on surface’ electric potential altering its
electronic properties and negatively affect OH- adsorption
behavior and reactivity. The concentration of AuOHads species
100
80
6
0
decreases as O
does. It appears that subtle change in particle size greatly affects
gold behavior for O activation being much more noticeable for
2
reduction reaction rate and FDCA selectivity
40
20
2
AuC_I II
the smallest gold particle size catalysts. Au 4f level XPS spectra
of AuC_I and AuC_V catalysts, as representatives for the lowest
and highest particle size samples, are plotted in Figure 6. The
binding energy of 4f5/2 and 4f7/2 levels peak at 88.2 and 84.5 eV,
respectively, indicating the presence of metallic gold. Changes
inn the bining energy for the spent catalysts are hardly noticeable,
but the sintering behavior of the gold particles upon reaction is
clearly seen by the intensity decrease observed for the spent
AuC_I catalyst. Figure 7 plots XPS difference spectra. The high-
resolution O 1s XPS spectrum of fresh samples is subtracted from
the one of the spent catalyst. An increase of the number of surface
oxygen species is observed after reaction for both AuC_I and
AuC_V catalysts. The concentration of O-containing species on
the catalyst surface is lower for the catalyst with smaller gold
particle size. The differences result in broad signals suggesting a
wide variety of adosrbed species. The XPS difference spectrum
of gold catalyst with the biggest particle size, AuC_V, presents a
main contribution at ~ 531 eV with a second maxima of lower
intensity at ~534 eV. The difference spectrum of the catalyst with
the smallest particle size, AuC_I, presents just one maximum
peaking at ~532 eV, although its complex shape suggests the
existence of more than one species. All the signals in this energy
range may be ascribed to the presence of C-O, C=O and/or C-OH
bonds in the adsorbed species [31, 32], i.e. sodium carboxylates
or esters present bonds that typically appears in this region
suggesting the presence of intermediates and/or products on the
catalyst surface after reaction [33,34]. The latter should be
expected since for AuC_V, the biggest gold particle size catalyst,
the C-balance is the lowest observed. This suggests that the
adsorption/desorption behavior on the catalyst surface is also
affected by gold particle size. Higher the gold particle size lower
the reaction rate and higher the products/intermediates
adsorption.
0
I use
II use
III use
IV use
Run #
Figure 4. Reuse of AuC_II and AuC_III sample. Reaction conditions:
HMF:Au:NaOH molar ratio 1:0.01:0.5 or 1:0.01:2, respectively, 10 bar O , 70 ºC,
2
4
00 rpm, 4h.
AuC_II spent
1
4
1
2
1
0
8
6
p
d = 8.8 ± 4.0 nm
4
2
0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33
1
0 nm
Particle diameter (nm)
AuC_III spent
1
0
9
8
7
6
5
4
3
2
1
0
dp = 12.2 ± 5.3 nm
1
4
7
10 13 16 19 22 25 28 31 34 37 40 43
1
0 nm
Particle diameter (nm)
Figure 5. Representative TEM images and gold particle size distributions of
AuC_II and AuC_III spent samples after 4 cycles.
If our theory for size dependent O
2
reduction and related FDCA
st
formation is true all reaction cycles after the 1 one; should give
similar FDCA selectivity to those measured for samples with
similar particle sizes whatever the initial size and reaction cycle.
The spent AuC_II catalyst (8.8 nm) and the fresh AuC_III one (8.7
nm) present the same selectivity to FDCA, 90 and 89%,
respectively. For catalysts AuC_III (spent, 12.2 nm) and AuC_IV
(
7
fresh, 15.4 nm) the selectivities to FDCA are also similar, 86 and
5 % respectively. However, in the 3 and 4 cycle the catalysts
rd
th
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ChemCatChem
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8
4,5
Au 4f
7
O 1s
530,8
3
2
1
0
8
8,2
5
33,6
6
5
4
3
2
1
AuC_I
5
32,2
AuC_V
5
28
530
532
534
536
538
9
2
90
88
86
84
82
Binding Energy, eV
Binding Energy, eV
Figure 6. XPS spectra of Au 4f of fresh and spent.
Figure 7. XPS spectra difference of O 1s (spent – fresh sample) for AuC_I and
AuC_V catalysts.
The activity of gold in low base environments is conditioned by
the presence of hydroxide ions, either added as base or produced
via 2e or 4e O reduction. This reaction is thermodynamically
2
Conclusions
-
-
By studying as a function of gold particle size the catalytic
oxidation of HMF to FDCA at low base concentration using O as
2
oxidant, the structure sensitivity of this reaction is analyzed.
Whatever the gold particle size, full conversion of HMF is attained.
However, product selectivity and carbon balance depend on gold
particle size. The catalytic behavior of gold nanoparticles in the 4-
favorable over small size gold nanoparticles having the highest
percentage of Au (100) surface faces exposed. That surface
accelerates the 4-electron oxygen reduction reaction, increases
significantly the hydroxide ion concentration, and influences
positively the alcohol/aldehyde oxidation rate. It is important to
underline that this model applies only for particles within the 4-20
nm range. Bigger particles do not fulfill this relationship and other
mechanisms should be applied. The latter is also confirmed by
the important change of the C balance for the sample AuC_V.
This C-loss is either produced by covering gold surface with
intermediates or by Cannizaro reaction products (although not
detected during the analysis).
40 nm size range may be split in two groups, selectivity to FDCA
and carbon balances above 85% that slightly decrease on
increasing the particle size are observed for the smaller gold
particles (4-16 nm range). Above 16 nm gold particles the carbon
balance and selectivity strongly decrease. The same tendency is
observed for the dependence of the FDCA yield and the
Au(100)/Au(111) exposure ratio on the gold particle size that fit
an exponential decay trend on increasing particle size. The
obtained results evidence that the oxidation of the alcohol function
of the HMFCA intermediate depends on gold particle size
sensitive and, therefore, structure-sensitive. The structure
sensitivity of this reaction is directly related to the oxygen
reduction ability and base concentration. The reaction yield to
FDCA is higher for the smallest gold particle sizes since the 4-
electron oxygen reduction is thermodynamically favorable.
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ChemCatChem
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shows that in the 7-15 bars pressure range no mass transfer limitations
occur being 10 bars sufficient to assure oxygen availability without
influencing the product distribution. After pressurizing, the temperature
was raised to 70ºC and the reaction mixture was stirred at approximately
Experimental Section
Catalyst series
4
00 rpm for 4 hours. Then the reactor was introduced into an ice bath and
The samples used in this study are selected from a series of previously
the reaction mixture was centrifuged and filtered. Afterwards, a sample
was taken and diluted before the test in an Agilent Infinity 1260 liquid
chromatograph equipped with a DAD detector and an Aminex HPX-87H
reported catalysts [16]. In a typical preparation, 5.10-4 M HAuCl
(Johnson
4
Matthey) aqueous solution is stirred together with a polyvinyl alcohol
solution (PVA, 1 wt.% in water), used as stabilizer, for 20 min at 600 rpm
2 4
300 mm × 7.8 mm column using 0.005 M H SO as eluent. The stability
(room temperature). Afterwards, an appropriate amount of 0.1 M freshly
was studied over the spent samples recovered from the post-reaction
mixture and dried at 120ºC overnight. The samples were used directly after
drying without any further treatment in a newly charged reactor with
HMF:Au:NaOH molar ratios kept constant. Conversion, selectivity and
yields are calculated after calibration using as reference commercial
samples, according to the following equations:
prepared NaBH solution was added to reduce the gold precursor. The
4
preformed colloid (2wt% Au nominal value) is immobilized on commercial
activated charcoal Darco© (Sigma Aldrich, 100 mesh). After ageing for 45
min, the resulting solid was recovered by centrifugation, filtered, dried and
finally calcined in static air at 300°C for 2 h. The gold particle size is
4
modified by wisely selecting different Au:PVA:NaBH ratios. This allows
selecting a series of catalysts s with mean particle size in the 4-40 nm
range. The selected catalysts, the Au:PVA:NaBH4 ratios used for their
preparation, their mean particle size and actual gold loadings are
summarized in table 1.
[
HMF]ꢃ−[HMF]I
Coꢂversioꢂ (%) =
x ꢄ00
(2)
(3)
(4)
[HMF]ꢃ
FDꢇꢈ mꢉꢊꢋ
ꢅꢆCA Selectivity (%) =
x ꢄ00
HMF mꢉꢊꢋI−HMF mꢉꢊꢋꢃ
2
High specific surface area (968 m /g) activated charcoal is used in this
ꢇꢉꢌꢍꢎꢏꢋꢐꢉꢌ
ꢅꢆCA Yield (%) =
x Selectivity
study with dominant microporosity. The activated charcoal presents low to
moderate acid character, which decrease with gold metal addition [35]. A
blank test of the reaction over this charcoal does not produce any products
but an adsorption of around 8 % of the initial HMF.
ꢑꢒꢒ
Acknowledgements
Characterization
Financial support has been obtained from the Spanish Ministerio
de Ciencia, Innovación y Universidades (ENE2017-82451-C3-3-
R) co-financed by E.U. FEDER funds.
Gold loadings were measured by using a Horiba Jobin Yvon ICP
spectrometer.
Transmission electron microscopy (TEM/STEM) studies on particle size
and dispersion of the catalysts were performed on a FEI TECNAI F20
microscope operating at 200keV. The average gold particle size was
estimated based on surface distribution calculations as shown in equation
Keywords: HMF oxidation, 2,5-furandicarboxylic acid, gold
catalysts, size/selectivity dependence
1:
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ꢀ
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Gold particle size strongly influences
product selectivity and C balance
during HMF oxidation reaction.
Considering that oxygen reduction is
thermodynamically favourable over Au
C. Megías-Sayago, A. Lolli, D.
Bonincontro, A. Penkova, S. Albonetti,
F. Cavani, J. A. Odriozola and S.
Ivanova*
-
O +2H O+4e- 4OH
2
2
HMFCA
(
100) face, Au 100/111 exposure ratio
Page No. – Page No.
Size
sensitivity
Favoured at maximal
Au 100/111 ratio
is correlated with gold particle size
and related with the observed FDCA
selectivity, showing an indirect and
clear dependence of the alcohol
function oxidation with gold
structure/size.
Effect of gold particles size over Au/C
catalyst selectivity in HMF oxidation
reaction
O2
111
100
FFCA
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