Au-based bimetallic catalysts: How the synergy between two metals affects their catalytic activity

Article abstract of DOI:10.1039/c9ra06001d

Supported bimetallic nanoparticles are particularly attractive catalysts due to increased activity and stability compared to their monometallic counterparts. In this work, gold-based catalysts have been studied as catalysts for the selective base-free oxidation of glucose. TiO₂-supported Au-Pd and Au-Cu series prepared by the sol-immobilization and precipitation-reduction methods, respectively, showed a significant synergistic effect, particularly when the theoretical weight ratio of the two metals was close to 11 (with an actual experimental bulk Au/Pd molar ratio of ca. 0.8 and ca. 0.4 for Au/Cu) in both cases. XPS analysis showed that the presence of Au^δ+, Pd²⁺ and CuOH species played an important role in the base-free glucose oxidation.

Full text of DOI:10.1039/c9ra06001d

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Au-based bimetallic catalysts: how the synergy
between two metals affects their catalytic activity†
Cite this: RSC Adv., 2019, 9, 29888
*
Jin Sha, Sebastien Paul, Franck Dumeignil
and Robert Wojcieszak
´
Supported bimetallic nanoparticles are particularly attractive catalysts due to increased activity and stability
compared to their monometallic counterparts. In this work, gold-based catalysts have been studied as
catalysts for the selective base-free oxidation of glucose. TiO2-supported Au–Pd and Au–Cu series
prepared by the sol-immobilization and precipitation-reduction methods, respectively, showed
a significant synergistic effect, particularly when the theoretical weight ratio of the two metals was close
to 1 : 1 (with an actual experimental bulk Au/Pd molar ratio of ca. 0.8 and ca. 0.4 for Au/Cu) in both
cases. XPS analysis showed that the presence of Aud+, Pd2+ and CuOH species played an important role
in the base-free glucose oxidation.
Received 2nd August 2019
Accepted 16th September 2019
DOI: 10.1039/c9ra06001d
rsc.li/rsc-advances
properties in many reactions when combining gold with other
metals to form new active sites induced by synergistic effects.1,2,6–9
Introduction
While in the case of monometallic gold catalysts, the activity is
mainly inuenced by the particle size and the nature of the support,
in the case of bimetallic catalysts they are secondary factors of
importance.10,11
Since Haruta and Hutchings discovered the peculiarity of gold-
based catalysts in CO oxidation and ethylene hydrochlorination
in the 80's, this metal has reached a very special status in the
eld of catalysis.1,2 Actually, the use of gold-based catalysts has
tremendously expanded. It is now, e.g., widely used in vinyl
acetate monomer synthesis, in the direct synthesis of hydrogen
peroxide from H2 and O2, and in the oxidation of a variety of
hydrocarbons and alcohols.3 However, gold-based catalytic
systems were always affected by a high variation of their cata-
lytic properties depending on the preparation method
employed, which is also related to the kind of support used. In
fact, many studies stated the importance of the preparation
conditions in determining the morphology of the gold particles
and the type of metal-support interactions, both being able to
profoundly modify the activity and/or the selectivity of the
catalyst. For this reason, a lot of attention has been paid to the
preparation of gold catalysts in order to assess as much as
possible the relation between support/preparation method and
the characteristics of the produced catalytic materials.4,5
Unlike Pt or Pd, Au-based catalysts showed very good selectivity
to desired products and high resistance to O2-poisoning when
molecular oxygen is used as the oxidant. However, gold-based
catalysts oen showed low activity compared to Pt or Pd-based
catalysts, and are oen suitable for alcohol oxidation only in alka-
line conditions. However, bimetallic systems could overpass such
limitations, combining the properties associated with the two
constituting metals. There was a great enhancement of catalytic
To enhance catalytic activity and selectivity in carbohydrates
oxidation under non-controlled pH conditions, several combi-
nations of gold with one or more other metals have been re-
ported.12,13 Among various Au–Me combinations, oxidation of
glucose using Au–Pd catalyst gave better conversions than
ꢀ
monometallic gold at 60 C. The advantage of the addition of
a second metal was clearly shown when working in an acidic
medium. However, Comotti et al.14 showed that Pt exhibit
a better selectivity to gluconic acid compared to Pd, when only
this acid is produced. Moreover, some works showed that
higher catalytic activity could be obtained in the presence of an
Ag–Au core–shell structure. It is due to the specic structure of
such bimetallic catalysts, which permits an easy transfer of
charges between the core and the shell leading to oxygen acti-
vation. In this case, the best catalytic activity was obtained for
a 20% Ag and 80% Au mixture. In addition to bimetallic
nanoparticles, the use of trimetallic systems was also reported.
One of the examples is the Au–Pt–Ag catalyst. The highest TOFs
were observed with a mass ratio of 70/20/10.15 Recently, the
Hutchings's group16 studied different gold-based bimetallic
catalysts for the oxidation of carbohydrates (mono and dicar-
bohydrates). As a general conclusion, they found that the
preparation method is a crucial parameter that governs the
catalytic activity of supported Au catalysts in acidic medium.
The catalytic results obtained for gold-based systems conrmed
that the presence of multiple metals has a positive effect on
activity and selectivity. A simple change in the metal-to-metal
weight ratio enables ne-tuning the catalytic activity.17
´
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 – UCCS – Unite de
Catalyse et Chimie Du Solide, F-59000 Lille, France. E-mail: robert.wojcieszak@
univ-lille.fr
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06001d
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Actually, the arrangement between gold and the other metal the physicochemical and catalytic properties of bimetallic
atoms plays the most important role in determining the cata- heterogeneous gold-based catalysts for the base-free oxidation
lytic performance of the as-obtained catalysts.18,19 Therefore, of glucose. Especially, the occurrence of any synergistic effect in
controlling and optimizing the architecture of bimetallic these catalytic systems was studied. In this context, a set of
nanoparticles (noted NPs in the following) constitutes the major different mono- and bimetallic supported gold-based catalysts
goal to get high performance and to merge synthesis with were synthesized, characterized and tested in the base-free
structure–activity relationships (the so-called “catalysis by oxidation of glucose.
design”) that ultimately allows the rational design of efficient
catalysts. The type of morphology obtained strongly depends on
the synthesis protocol, the miscibility of both metals, but also
on post-synthesis treatment.3,13 The bimetallic composition
Experimental
Catalysts' preparation
obviously strongly inuences the catalytic performance as On the basis of literature, sol-immobilization method and
shown in many previous studies.3,10,14,20 The question of which is precipitation-reduction methods were chosen for the synthesis
the best metal ratio for an optimal catalyst for a given reaction of the series of catalysts used in this work.23 By these two
and why such a ratio is actually the optimal one still remain methods, the particle size of the metal nanoparticles could be
unanswered.
kept below 10 nm,16 which is highly important to get a good
In this work, we have chosen the oxidation of glucose as activity of the gold-based catalyst. Especially, in the sol-
a representative reaction to study these factors. Comotti et al.14 immobilization method, by adjusting reaction time, reducing
have studied the activity of Au–Pt and Au–Pd NPs supported on agent concentration, and precursors' concentration, the particle
activated carbon compared to monometallic (Au, Pt, Pd, Rh) size could be very well controlled.23 The support provides the
catalysts in glucose oxidation under controlled and uncon- surface of the catalysts, where the metals are loaded and
trolled pH. Hermans et al.21 have also reported synergistic dispersed. The stability of the support is obviously also very
activity in glucose oxidation at basic pH over Au–Pd/C catalysts important for the nal catalyst. TiO2 was chosen to play this
prepared by impregnation in aqueous solution. The synergistic role, as it is very well known to be stable in base-free conditions
effect was related to high Pd surface content. Benko et al.10 have while providing reasonable specic surface areas.24 The cata-
´
reported SiO2-supported Au–Ag catalysts prepared by sol lysts preparation methods are described in details in the
adsorption method with different metal ratios at pH 9.5. The following sections.
Au–Ag bimetallic effect and its dependence on the Ag/Au molar
Sol-immobilization method (SIM). A series of Au–Pd/TiO2
ratio was studied in glucose oxidation, where a synergistic catalysts was prepared by sol-immobilization method. This
activity increase was observed in the case of the bimetallic method was adapted from the literature with a view to get small
samples with Ag/Au ¼ 50/50 molar ratio when compared to the and narrow dispersed nanoparticles on the support.25 It con-
Au/SiO2 monometallic reference sample. Synergistic effect has sisted in the preparation of Au and Pd nanoparticles stabilized
been reported for the three bimetallic systems at high pH. with polyvinyl alcohol (PVA) and their subsequent immobiliza-
However, this synergistic effect in base-free oxidation of glucose tion on an appropriate amount of support. Titanium dioxide
was seldom studied. Realizing base-free reaction would be of (TiO2, P25 from Degussa, surface area 53 m2 gꢁ1) was chosen as
high interest because the absence of base in the media would a support of the bimetallic catalysts with also a view to reduce
avoid the formation of salts, which must be disposed of at the the agglomeration and to help the dispersion of the nano-
end of the reaction, hence generating wastes. Glucose oxidation particles. For instance, in a typical procedure for preparing Au–
can give various products, but gluconic acid was chosen as the Pd/TiO2 catalyst, 0.1 g of a 1 wt% aqueous solution of PVA was
desired product in this work because the selectivity to this acid rst added to an aqueous solution of tetrachloroauric acid and
strongly depends on the catalyst properties.22 It can therefore to Pd(NO3)2 diluted in distilled water under vigorous stirring to
play the role of a marker, as a discriminating criterion. In the form a 60 mL solution. A freshly prepared solution of NaBH4
absence of a base, isomerization of glucose to fructose is slowed (0.021 g was dissolved in 5 mL H2O) was then added to form
down. Thus, the selectivity to gluconic acid should be a light-red sol. Within 30 min of sol generation, the colloid
enhanced. In addition, this route would be a one-step synthesis (acidied at pH 1 with a 1 mol Lꢁ1 solution of sulfuric acid) was
of gluconic acid from glucose instead of the acidication of immobilized on 0.5 g of TiO2 under vigorous stirring condi-
a salt obtained at high pH. The use of unwanted and environ- tions. The amount of support was calculated to give a total nal
mentally dangerous chemicals such as NaOH could be elimi- metal loading of 1 wt%. Aer 2 h, the slurry was separated by
nated, which is positive from an economical point of view centrifugation (30 min, 4700 rpm), then the solid was washed
(increase of the equipment lifetime due to the elimination of with hot water (2 ꢂ 70 mL) and ethanol (1 ꢂ 70 mL) before
strong corrosive media, elimination of the nal gluconate-acid being dried at 80 C overnight.25
ꢀ
transformation). Moreover, it will be easier to study the occur-
Precipitation-reduction method. An Au–Cu/TiO2 series of
rence of any synergistic effect with a base-free system. Thus, it is catalysts was prepared by the precipitation-reduction method.
important to focus on the development of catalysts that will For example, briey, aqueous solutions of tetrachloroauric acid
have better activity and stability in base-free conditions.
and copper nitrate were added into 10 mL of an aqueous solu-
As a consequence, the main objective of this work was to tion containing 0.5 g of TiO2 and then distilled water was added
achieve a thorough systematic study of the correlation between until a total volume of 60 mL was reached.
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The resulting slurry was stirred vigorously at room temperature at room temperature 3 times before setting the air pressure at 5
for 1 h. Then, 5 mL of an aqueous solution of NaBH4 (0.1 M) were bars. Then, the reactors were sealed and the heating was started
added dropwise to reduce Au and Cu. The as-obtained Au–Cu/TiO2 (the temperature ramp rate was 5 ꢀC minꢁ1). When the desired
catalysts were centrifuged, washed with deionized water until no temperature was reached, stirring was set at 700 rpm and
chloride traces were detected using a 1 wt% AgNO3 solution test, maintained for 5 hours. At 60 ꢀC the nal pressure was around 6
before being nally dried at 80 ꢀC overnight for 24 h.
bars. At the end of the reaction, theꢀreactors were cooled down.
The catalysts samples were denoted as x%Auy%Pd (where When the temperature reached 40 C, the remaining pressure
x% corresponds to the gold wt% and y to the palladium wt%) or was released. Finally, the reaction solution was ltered to
x%Auz%Cu (where x% corresponds to the gold wt% and z to the separate the catalysts from the liquid. The as-obtained solutions
copper wt%).
were used directly for analysis, which was carried out on a Dio-
nex ultimate 3000 HPLC equipped with a xed wavelength UV
detector and a refractive index (RI) detector. As far as the
stationary phase is concerned, a Rezex organic acid H+ 300 ꢂ
7.80 mm column was used. The mobile phase was
a 0.0025 mol Lꢁ1 sulfuric acid solution used at room tempera-
ture with a 0.45 mL minꢁ1 ow rate.
Characterization
The prepared series of catalysts were characterized in order to
determine their physical properties, such as the specic surface
area, the particles size, their composition and structure, etc. The
techniques used in the present work to characterize the cata-
lysts are described in the followings.
Results
Au–Pd/TiO2 catalysts
An Agilent 720-ES ICP-OES was used for determining the
metal loading combined with a Vulcan 42S robot, which is an
automated digestion system combining the two essential steps
in sample preparation prior to analysis by ICP-OES: sample
digestion and sample preparation.
N2 physisorption measurements were carried out on a TriS-
tar II Plus apparatus from Micromeritics. Before measurements,
the samples were degassed on a VacPrep061 equipment
(Micromeritics) at 75 ꢀC under primary vacuum for 80 min and
at 250 ꢀC for 3 h. The specic surface area of TiO2 support was
56 m2 gꢁ1. The specic surface of the Au–Pd and Au–Cu
modied catalysts did not change aer the preparation due to
the very low metal content.
The actual amount of metal loading measured by ICP-OES and
the mean particle size obtained from TEM images (Fig. 1)
analysis are presented in Table 1.
The metals loadings determined from ICP-OES are always
lower than expected (Table 1). This is due to the preparation
method in solution, which involves the deposition of the metals
and several post-preparation steps (ltration and washing).
However, the observed differences are quite reasonable and
allow comparison of the catalysts' series with various metal
compositions.
¨
XRD measurements were carried out on Bruker X-ray
Diffractometer D8 Discover. The samples were scanned in the
2q range of 20ꢀ to 70ꢀ with a step of 0.014 and using CuKa1
radiation source (l ¼ 0.1538 nm).
TEM electron microscopy images were recorded placing a drop
of the particles in ethanol onto a carbon lm supported on a cooper
grid. The samples were studied using a FEI Tecnai G2 20 micro-
scope with a LaB6 cathode operating at 20 kV and a FEI Titan
Themis 300 with X-FEG emitter operating from 60 to 300 kV.
The XPS spectra were recorded at a residual pressure of 10ꢁ9
mbar on a Kratos Axis UltraDLD electron energy spectrometer
operating with a monochromatic Al-Ka (1486.6 eV) or Mg-Ka
(1253.6 eV) irradiation. The binding energy (BE) scale was cali-
brated on the C 1s peak (BE ¼ 284.8 eV) resulting from surface
contamination. The accuracy of the measure is estimated at
ꢃ0.1 eV. Differential surface charging of the samples was ruled
out by checking the reproducibility of XPS measurement in
repeated scans under different X-ray exposures. Soware
CasaXPS was used for the analysis of XPS spectra, separating
elemental species in different oxidation states, and calculating
relative concentrations of chemical elements.
Base-free aerobic oxidation of glucose
Fig. 1 TEM micrographs of Au–Pd nanoparticles loaded on TiO2
prepared by sol-immobilization and corresponding Au–Pd particle
size distributions. (a) 1%Au/TiO2, (b) 0.9%Au0.1%Pd/TiO2, (c) 0.7%
Au0.3%Pd/TiO2, (d) 0.5%Au0.5%Pd/TiO2, (e) 0.3%Au0.7%Pd/TiO2, (f) 1%
In a standard reaction test, the required amounts of catalyst
were placed in the reactor and then 2 mL of an aqueous glucose
solution (1 wt%) was added. The reactors were ushed with air Pd/TiO2.
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Fig. 1 presents TEM images of the Au–Pd catalysts supported
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on TiO2. At least 300 particles were considered, except for 0.3%
Au0.7%Pd/TiO2, where 109 particles were counted, and the
average particle size and distributions were calculated. A narrow
distribution in particles sizes was obtained in any case with the
majority of the particles being in the 2–3 nm range irrespective
of the catalyst. Unfortunately, it was difficult to observe the
metal particles in the 1%Pd/TiO2 catalyst prepared by sol-
immobilization method. However, in this sample palladium
was actually deposited on the support, as conrmed by ICP-
OES, but the Pd particles were probably too small to be distin-
guished in the TEM images.
It can be concluded from TEM images that Au–Pd nano-
particles prepared by SIM were dispersed homogeneously on
the TiO2 support. It is of importance that the mean sizes of Au– Fig. 2 HAADF image and the corresponding EDX spectrum from
a single Au–Pd nanoparticle supported on TiO2 prepared by the sol
immobilization method (0.5%Au0.5Pd%/TiO2). The HAADF image and
montage of Au, Pd EDX elemental maps shows the co-existence of
both elements within the nanoparticle.
Pd particles in the bimetallic Au–Pd/TiO2 samples with different
metal ratios were quite similar in the 2–3 nm range (Table 1).
Such small Au–Pd particles are believed to be efficient in
glucose oxidation.16,26 This also is interesting for the study of the
effect of gold and palladium structure and ratio on catalytic
performance since the activity will not be governed by the
particle size, but only by the composition of the bimetallic
particles.
Fig. 4 presents the XPS spectra of the Au 4f and Pd 3d XPS
regions in the catalysts. Different Au 4f and Pd 3d photoelectron
peaks corresponding to surface gold and palladium species
Fig. 2 shows the EDX spectrum and mapping of elements of
were found depending on the catalysts' formulation. Because
one particle taken as a signicant example on the 0.5%Au0.5%
Au and Pd can easily form alloys, they can affect the binding
Pd/TiO2 catalyst. It illustrates the alloy-type of the synthesized
energy of each other.28
Au–Pd particles. Note that no monometallic particles were
In all the AuPd catalysts, the Au 4f7/2 photoelectron peak was
detected by EDX conrming a good distribution of the metals in
bimetallic nanoparticles.
located at a BE value between 83.22 and 85.99 eV, as reported in
Table 2. These values are typical of metallic Au0 and oxidized
The X-ray diffraction patterns of catalysts of the Au–Pd/TiO2
Aud+ species, respectively.29,30 The curve tting of the Au 4f core-
were identical. We present here the XRD patterns of the 0.5%
level spectrum was performed by using two spin–orbit split Au
4f7/2 and Au 4f5/2 components, separated by 3.7 eV, in a xed
Au0.5Pd%/TiO2 and 1%Au/TiO2 samples to represent the cata-
lysts series, together with that of the fresh TiO2 support for
intensity ratio (1.33), keeping the same full width at half-
comparison. Due to the limitations of the technique which is
maximum (FWHM) values. The Au 4f curve tting of the 0.7%
not suitable to detect low metal contents (<1 wt%) or small
Au0.3%Pd/TiO2 sample showed Au 4f7/2 components at BE ¼
83.31 and 85.76 eV, which can be assigned to Au0 and Aud+,
respectively.29–31 A small fraction of oxidized Aud+ species were
found among metallic gold and gold-palladium particles as
indicated by the relative distribution of Au species in Table 2.
These metallic gold species were present in the sample as very
small nanoparticles undetectable by XRD, since the diffraction
patterns showed only the reections of the TiO2 support. The
same oxidation states of gold were detected on the surface of the
metal particles (<4 nm)27 and thus due to the probable good and
homogeneous dispersion of the metals on the titania surface,26
X-ray diffraction patterns of the catalysts did not show peaks
originating from the introduced metals (Fig. 3). The patterns
only show the presence of pure TiO2 with anatase and rutile-
crystal structures, which is in good agreement with the stan-
dard data from the JCPDS PDF card 00-021-1272 and 04-003-
0648, respectively.
Table 1 ICP-OES analysis and particles size by TEM analysis of the Au–Pd/TiO2 series of catalysts prepared by sol-immobilization method
Theoretical metal loading (wt%) Measured metal loading (wt%) – ICP-OES
Catalysts
Au
Pd
Au/Pd
Au/Pdb
Au
Pd
Au/Pd
Au/Pdb
Particles size (nm)
1%Au
1
0
—
9
2.33
1
0.43
—
—
0.97
0.83
0.68
0.45
0.27
ND
NDa
0.05
0.19
0.31
0.24
0.67
—
—
2.5 ꢃ 0.6
2.5 ꢃ 0.7
2.7 ꢃ 0.7
2.6 ꢃ 0.7
2.4 ꢃ 0.8
ND
0.9%Au0.1%Pd
0.7%Au0.3%Pd
0.5%Au0.5%Pd
0.3%Au0.7%Pd
1%Pd
0.9
0.7
0.5
0.3
0
0.1
0.3
0.5
0.7
1
4.86
1.26
0.54
0.23
—
16.60
3.58
1.45
1.13
—
8.97
1.93
0.78
0.61
—
a
Not detected. b Molar ratio.
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represented only a small fraction (9.4%) of the total gold in
presence. The other samples were quite similar with the Au 4f7/2
photoelectron peak located at a BE value between 83.22 and
85.99 eV.
The Pd 3d5/2 photoelectron peak is located at a BE value
between 334.55 and 336.93 eV. These values are typical of
metallic Pd0 and Pd2+ species, respectively.32 Pd 3d was well
separated by 5.26 eV by using spin–orbit Pd 3d5/2 and Pd 3d3/2
components, in a xed intensity ratio (1.5), also keeping the
same full width at FWHM values. Pd 3d peaks have asymmetric
peak shape for metal while Pd oxides have symmetric peak
shapes. The Pd 3d curve tting of the 0.5%Au0.5%Pd/TiO2
sample shows Pd 3d5/2 components at BE ¼ 334.80 and
336.16 eV, which can be assigned to Pd0 and Pd2+, respectively.
Their relative distribution reveals a signicant fraction of
oxidized Pd2+ on the sample surface, as reported in Table 2. A
little shi of BE value of Pd 3d5/2 components is observed in the
Pd 3d curve tting of 0.3%Au0.7%Pd/TiO2 sample, with peaks at
335.05 and 337.00 eV that can also be assigned to Pd0 and Pd2+,
respectively.30 The Pd 3d spectrum of 0.9%Au0.1%Pd/TiO2 and
0.7%Au0.3%Pd/TiO2 catalysts consisted of two components
located at BE ¼ 334.69 and 334.68 eV, respectively, which can be
assigned to metallic palladium species, namely Pd0.30 Surpris-
ingly, in 1%Pd/TiO2, no Pd 3d photoelectron peaks were
observed as shown in Fig. 4. This suggests that there is no
palladium on the surface, or that its concentration is very low
(less than 0.01 wt%).
Fig. 3 Diffractograms of selected samples.
0.5%Au0.5%Pd/TiO2 catalyst. The curve tting of Au 4f7/2
spectrum indicates two Au 4f7/2 components at BE ¼ 83.38 eV
and 85.92 eV that also can be attributed to Au0 and Aud+ species,
respectively, with thus a little shi in BE value in comparison
with 0.7%Au0.3%Pd/TiO2. The oxidized Aud+ species
The surface chemical compositions of the x%Auy%Pd/TiO2
catalysts, as determined by XPS, are reported in Table 3.
Prior to the evaluation of the catalytic performance of the
series of catalysts, blank tests were performed under standard
reaction conditions. As shown in Table 4, 6% of glucose
conversion was observed, while the selectivity to gluconic acid
was only 9.0%. In contrast, over TiO2 the conversion of glucose
was only 3.3% and selectivity to gluconic acid raised to 39.6%.
Blank test was also done at 80 ꢀC by adding only the TiO2
support. 3.6% of glucose conversion was observed, and no
gluconic acid was detected. The carbon balance of all the blank
tests was in the 94–98% range.
Glucose oxidation was carried out at 60 ꢀC, 80 ꢀC, 100 ꢀC and
ꢀ
150 C, respectively. Because degradation of glucose occurs at
relatively high temperatures, a lot of by-products were gener-
ated during reaction at 150 ꢀC. In that case, the solutions
became brown aer 2 h of reaction with a very low carbon
balance. Thus, the results of the tests at 150 ꢀC were not
considered in the following discussion. The main product of the
oxidation reaction was gluconic acid. However, in some cases
glucuronic acid or fructose could also be observed. The carbon
balance of the reactions at 60 ꢀC was nearly 100%, while at 80 ꢀC
ꢀ
it was around 90% and at 100 C it was around 75%.
Fig. 5 presents the catalytic performances vs. gold content
(from ICP-OES) at 60 ꢀC, 80 ꢀC and 100 ꢀC, respectively. To
enable comparison of the reactivity results, the catalytic activity
was normalized by expressing it as moles of glucose converted
per mole of golꢀd in the catalyst. Two different trends were
observed. At 60 C, as the introduced quantities of palladium
increased, the catalytic activity increased to a maximum at
Fig. 4 XPS curve of the Au 4f and Pd 3d photoelectron peaks in the
series of catalysts prepared by the SIM method.
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Table 2 Relative surface distribution of gold and palladium species
Catalysts
Au 4f7/2 BE (eV)
Surface Au species
Pd 3d5/2 BE (eV)
NDa
Surface Pd species
ND
1%Au
83.51
85.99
83.49
85.99
83.31
85.76
83.38
85.92
83.22
85.65
ND
Au0 (95.0%)
Aud+ (5.0%)
Au0 (95.9%)
Aud+ (4.1%)
Au0 (94.0%)
Aud+ (6.0%)
Au0 (90.6%)
Aud+ (9.4%)
Au0 (87.9%)
Aud+ (12.1%)
ND
0.9%Au0.1%Pd
0.7%Au0.3%Pd
0.5%Au0.5%Pd
0.3%Au0.7%Pd
1%Pd
334.69
Pd0 (100%)
Pd0 (100%)
334.68
334.80
336.16
335.05
337.00
ND
Pd0 (71.8%)
Pd2+ (28.2%)
Pd0 (87.8%)
Pd2+ (12.2%)
ND
a
Not detected.
0.45 wt% Au, and then decreased. The less active catalyst tested
was the monometallic Pd catalyst, for which only 1.5% conver-
sion of glucose was observed (Table 5). Therefore, a volcano-type
plot was observed across the range of bimetallic catalyst
compositions tested. This conrmed the presence of a syner-
gistic effect between Pd and Au. The selectivity to gluconic acid
was always around 75% except for the 1%Au/TiO2 catalyst,
where the selectivity to gluconic acid was only 53%. The
oxidation reaction at 80 ꢀC exhibited the same activity trend,
across the range of Au–Pd catalyst composition tested. The
selectivity to gluconic acid was maintained at ca. 75% without
signicant inuence of the Pd content in the catalyst. In
contrast, at 100 ꢀC, a linear relation between catalyst activity
and gold content was observed. This could be partly explained
by the fact that, at this temperature, glucose conversion reached
100% over all catalysts. The selectivity to gluconic acid was kept
around 65%, which is less than that observed at lower
temperature, while around 10% selectivity to fructose was
observed. This suggests that increasing the temperature will
increase the conversion of glucose but the selectivity to desired
gluconic acid will be decreased because the isomerization of
glucose to fructose occurred. The activity results for 0.1%
Au0.9%Pd/TiO2 catalyst were not shown in Fig. 5 because the
conversion wasꢀnearly null and very close to the blank test. For
example, at 60 C the conversion of glucose was 4.3% and the
selectivity to gluconic acid was 38.8%.
Au–Cu/TiO2 catalysts
Au–Cu/TiO2 catalysts were prepared by precipitation-reduction
(PR) method. ICP-OES technique was used to determine the
amounts of metal actually present in the synthesized catalysts
(Table 6). The actual loadings were generally close but a little bit
less than the theoretical ones. Comparing to what was obtained
for Pd, Cu seems to be more easily reduced and hence the actual
content is close to the theoretical values. However, as the metal
loading increased, copper was not loaded as well as gold. For
example, for 1 wt% Au in theory, the actual ICP-OES loading was
0.92 wt%, while for the copper loading of 1 wt% in theory, the
actual ICP-OES loading was 0.81 wt%.
Au–Cu nanoparticles were dispersed relatively homoge-
neously in all samples. The mean size of the Au–Cu nano-
particles in 0.9%Au0.1%Cu/TiO2, 0.7%Au0.3%Cu/TiO2, 0.5%
Au0.5%Cu/TiO2 and 0.3%Au0.7%Cu/TiO2 samples were found
very close, in the range of 2.7 to 3.4 nm. However, the mean size
of gold nanoparticles in 1%Au/TiO2 catalyst was a bit higher
with a value of 4.3 nm (Table 6).
TEM was used to investigate the metal particle size and their
distributions, as the size of Au–Cu bimetallic particles is a key
parameter determining the catalytic performance. Fig. 6 pres-
ents TEM images for different Au/Cu ratios supported on TiO2.
At least 300 particles were counted. Average size of the
particles and distributions of the size were calculated. As
observed, the metal nanoparticles were well distributed on the
Table 3 XPS analyses of the Au–Pd/TiO2 series of catalysts prepared by the sol-immobilization method. Elemental concentrations are expressed
as atomic percentage (at%)
Catalysts
Au 4f7/2
Pd 3d5/2
Au/Pd
Au/Pda
Au/Pdc
Ti
O
C
1%Au
0.25
0.17
0.13
0.10
0.05
ND
NDb
0.08
0.07
0.04
0.06
ND
—
—
—
15.05
10.80
10.92
11.77
11.93
7.28
42.72
32.70
32.03
33.57
35.34
26.56
41.96
56.23
56.84
54.51
52.61
66.14
0.9%Au0.1%Pd
0.7%Au0.3%Pd
0.5%Au0.5%Pd
0.3%Au0.7%Pd
1%Pd
2.11
1.79
2.30
0.86
—
3.91
3.31
4.23
1.59
—
16.60
3.58
1.45
1.13
—
a
Weight ratio between gold and palladium. b Not detected. c Weight ratio by ICP-OES.
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Table 4 Blank test for the oxidation of glucose
Temperature/ꢀC
Glucose conversion%
Gluconic acid select%
Glucuronic acid select%
Fructose select%
Carbon balance%
60a
60b
80b
6.0
3.3
3.6
9.0
39.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
94.5
97.6
95.9
a
Blank test without adding catalyst. b Blank test adding only support TiO2. Reaction conditions: 2 mL of 1 wt% glucose aqueous solution, catalyst
(21.9 mg), air pressure ¼ 5 bar, 5 h of reaction.
support in most of the cases. However, in case of 0.1%Au0.9% between 83.08 and 85.74 eV. These values can be assigned to
Cu/TiO2 (Fig. 6f) and 1%Cu/TiO2 (Fig. 6g) it was very difficult to metallic Au0 and oxidized Aud+ species, respectively.29,30 The
distinguish the nanoparticles. This is most probably because curve tting of the Au 4f core-level spectrum was performed as
the molar masses of Ti and Cu are very close, so the contrast aforementioned. In Fig. 9, different Au 4f and Cu 2p photo-
between each element is not good. Another possibility is that electron peaks of surface gold and copper species were found
the metal nanoparticles were too small to be observed. ICP-OES depending on the different Au/Cu ratio. Some shis were also
results showed that there was 0.09 wt% of gold and 0.76 wt% of detected. That is because these two metals can easily formed
copper in 0.1%Au0.9%Cu/TiO2 catalysts and 0.81 wt% of Cu in alloys, which affects their respective binding energies.28,33 For
1%Cu/TiO2 catalyst. It suggests that the size of the nano- example, in the 1%Au/TiO2 sample, the Au 4f curve tting
particles had reached the resolution limitation of the TEM shows Au 4f7/2 components at BE ¼ 83.08 and 85.74 eV. These
technique.
two values are in good agreement with those reported in the
XRD analysis of the samples was carried out to determine literature for Au0 and Aud+, respectively.29–31 In the 0.7%Au0.3%
potential crystallinity of the gold and copper species. 0.5% Cu/TiO2 sample, the curve tting of Au 4f7/2 spectrum indicated
Au0.5%Cu/TiO2, prepared by precipitation-reduction method two Au 4f7/2 components at BE ¼ 83.15 and 84.97 eV that could
and fresh support TiO2 as a benchmark were chosen (Fig. 7). also be attributed to Au0 and Aud+ species, respectively. But
However, due to the sensitivity of the technique, only the there was a shi of BE values in comparison with the gold
presence of pure TiO2 could be observed (metal content is a bit monometallic catalyst. As more Cu and less gold were intro-
too low compared to the detection limit).
duced in the catalysts, the shis of BE values became bigger.
The HAADF image and EDX mapping showed the co- The Cu 2p XPS spectrum of 0.1%Au0.9%Cu/TiO2 showed
existence of alloyed Cu and Au within the nanoparticles in a main signal and a satellite peak. The main signal could be
0.5%Au0.5%Cu/TiO2 catalyst (Fig. 8).
deconvoluted into four contributions, indicating different
The bimetallic Au–Cu catalysts with different ratio between oxidation states of copper. The contribution at 932.41 eV could
gold and copper supported on TiO2 were characterized by X-ray be assigned to the presence of CuOH, the peak at 931.54 eV
photoelectron spectroscopy (XPS) to get information about could be assigned to Cu1+ and metallic copper, and the last one
chemical state of the surface species.
at 934.22 eV could be attributed to the CuO species.34,35 The
Fig. 9 shows the XPS spectra of Au 4f and Cu 2p levels of the satellite peak conrmed the presence of CuO species on the
bimetallic Au–Cu/TiO2 series. Tables 7 and 8 show the results of surface of this sample. The satellite peak arises when the
chemical composition of all the investigated catalysts in details. emitted photoelectron lost part of its kinetic energy to excite
As an example, Fig. S1 and S2 (see ESI†) show the XPS peak a valence electron to an unoccupied d orbital of Cu. The pres-
tting of 0.3%Au0.7%Cu/TiO2. In all the Au–Cu/TiO2 catalysts ence of satellite peak was, therefore, an indication of the exis-
prepared by precipitation-reduction method, the Au 4f7/2 tence of d9 partially lled orbitals in Cu fundamental state, and
photoelectron peak was located at a binding energy (BE) value thus it appeared only when Cu2+ species was present.33,35–38
Fig. 5 Oxidation of glucose over a series of mono- and bi-metallic catalysts containing Au and Pd supported on TiO2, reaction conditions: 2 mL
of a 1 wt% glucose aqueous solution, 1 wt% bimetallic catalyst (21.9 mg), air pressure ¼ 5 bar, 5 h of reaction (details in ESI Tables S1–S3†).
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Table 5 Effect of temperature on the catalytic performance of 1 wt% Pd/TiO2 for the oxidation of glucosea
Temperature/ꢀC
Glucose conversion%
Gluconic acid select%
Glucuronic acid select%
Fructose select%
Carbon balance
60
80
100
1.2
3.8
13.5
35.3
53.6
53.6
0.0
0.0
0.0
0.0
0.0
13.2
98.9
98.2
95.5
a
Reaction conditions: 2 mL of a 1 wt% glucose aqueous solution (metal : glucose ratio of 100), catalyst (21.9 mg), air pressure ¼ 5 bar, 5 h of
reaction.
ꢀ
When Cu loading was less than 0.5 wt%, the satellite peak was
The same observation was done at 80 C but as the temper-
almost not observed because the signal intensity was very low, ature increased, the activity and selectivity increased as well.
but CuO existed in most samples. Table 8 also reveals that The trend of activity was very close to that observed at 60 ꢀC
CuOH was present when Cu loading was less than 0.9 wt%. In except for 0.1%Au0.9%Cu/TiO2, for which there was a remark-
the case of 1%Cu/TiO2 catalyst, only Cu1+/Cu0 and CuO could be able increase of activity when the temperature was increased
observed.
from 60 to 80 ꢀC. As reaction temperature was increased to
The Au/Cu atomic ratio on the surface of 0.9%Au0.1%Cu/ 100 ꢀC, an almost linear inverse relation between catalyst
TiO2 was 1.44 (Table 7), which indicated that the bulk of the activity and gold content was observed. This was because at this
particles was rich in gold. On the contrary, other catalysts were temperature, the glucose conversion was close to or reached
rich in copper. This is in good agreement with the results ob- 100% over all the catalysts. The selectivity to gluconic acid was
tained by ICP-OES (Table 6). Comparing the value of Au/Cu bulk maintained at roughly 60%. When the gold content was more
weight ratio, the Au/Cu ratio on the surface was smaller. It than 0.1 wt%, the selectivity to gluconic acid was maintained
reveals that more copper stayed on the surface of the catalysts more or less at the same level irrespective of the Au content in
rather than in the bulk of the catalyst.
the catalyst at a given temperature. This suggests that gold
Similarly to what was done with the Au–Pd series catalysts, played a decisive role in the selectivity to gluconic acid in the
catalytic tests for the Au–Cu/TiO2 series catalysts were carried oxidation of glucose. This effect was not obvious over 0.1%
ꢀ
out at 60, 80 and 100 C, respectively (Fig. 10). The other reac- Au0.9%Cu/TiO2 because the content of gold was too low to be
tion conditions were the same as those used for the Au–Pd the only key factor.
series.
At 60 ꢀC, as the introduced quantity of Au was increased, the
catalytic activity also increased to reach a maximum at 0.3–
0.45 wt% of Au before decreasing with further increase in the Au
proportion. The least active catalyst tested was the mono-
metallic Cu catalyst, which exhibited only 1.5% conversion of
glucose, forming a volcano plot across the range of bimetallic
catalyst compositions tested. This conrmed the presence of
a synergistic effect. The selectivity to gluconic acid was always
around 50% except over the 0.1%Au0.9%Cu/TiO2 catalyst for
which selectivity to gluconic acid was roughly ca. 10 points
lower.
Mechanism
The mechanism of carbohydrates oxidation in basic medium is
already well known. Several roles for O2 during the oxidation
reactions in water at high pH have been already proposed. The
most important of them include (i) the formation and direct
participation of atomic O during reaction, (ii) the reduction of
atomic O to water through H atoms adsorbed on the surface,
(iii) the direct oxidation of the intermediates with atomic O to
form the O-insertion product acids, (iv) and the removal of
strongly bound organic adsorbates from the surface.39
Table 6 ICP-OES analysis and particles size determined by TEM analysis of the Au–Cu/TiO2 series of catalysts prepared by precipitation-
reduction method
Theoretical metal loading (wt%)
Measured metal loading (wt%) – ICP-OES
Catalysts
Au
Cu
Au/Cu
Au/Cud
Au
Cu
Au/Cu
Au/Cud
Particles size (nm)
1%Au
1
0
—
9
—
0.92
0.86
0.72
0.45
0.30
0.09
ND
NDc
0.09
0.29
0.41
0.64
0.76
0.81
—
—
4.3 ꢃ 1.9
3.3 ꢃ 1.0
3.1 ꢃ 1.0
3.4 ꢃ 1.0
2.7 ꢃ 1.0
2.4 ꢃ 1.0
2.2 ꢃ 0.5
0.9%Au0.1%Cu
0.7%Au0.3%Cu
0.5%Au0.5%Cu
0.3%Au0.7%Cu
0.1%Au0.9%Cua
1%Cub
0.9
0.7
0.5
0.3
0.1
0
0.1
0.3
0.5
0.7
0.9
1
2.90
0.75
0.32
0.14
0.04
—
9.56
2.48
1.10
0.47
0.12
—
3.08
0.80
0.35
0.15
0.04
—
2.33
1
0.43
0.11
—
a
59 particles counted. b 18 particles counted. c Not detected. d Molar ratio.
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Fig. 8 EDX spectrum and HAADF images of 0.5%Au0.5%Cu/TiO2
catalyst from a single Au–Cu bimetallic nanoparticle.
a metal hydride (or a water molecule, if metal hydroxide species
are involved). Aer this adsorption, a carbonyl species and
a metal hydride are produced via b-hydride elimination. The
nal step is the oxidation of metal hydride by O2 to regenerate
the metal surface (or metal hydroxide species). As reported, this
last step may lead also to the production of water (on metals
Fig. 6 TEM micrographs of Au–Cu nanoparticles loaded on TiO2
prepared by precipitation-reduction method and the corresponding
Au–Cu particle size distribution. (a) 1%Au/TiO2, (b) 0.9%Au0.1%Cu/
TiO2, (c) 0.7%Au0.3%Cu/TiO2, (d) 0.5%Au0.5%Cu/TiO2, (e) 0.3%Au0.7%
Cu/TiO2, (f) 0.1%Au0.9%Cu/TiO2, (g) 1%Cu/TiO2.
As reported in the study of alcohols oxidation mechanisms
over heterogeneous catalysts reviewed by Davis et al.,40 oxida-
tion of an alcohol to an aldehyde on metal catalyst occurs in
three main steps. The rst step is the alcohol adsorption on the
metal surface, which produces an adsorbed metal alkoxide and
Fig. 9 XPS curves of the Au 4f and Cu 2p photoelectron peak in the
Fig. 7 Diffractograms of TiO2 and 0.5%Au0.5%Cu supported on TiO2. AuCu catalysts series.
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Table 7 XPS analyses of the Au–Cu/TiO2 series of catalysts prepared by the precipitation-reduction method. Elemental concentrations are
expressed as atomic percentage (at%)
Catalysts
Au 4f7/2
Cu 2p3/2
Au/Cu
Au/Cua
Au/Cub
Ti
O
C
1%Au
0.26
0.24
0.18
0.17
0.12
0.05
—
—
—
—
—
24.75
23.32
22.37
22.94
23.59
18.41
17.20
57.34
57.04
56.91
56.91
57.65
51.35
49.96
17.65
19.23
19.96
19.15
17.36
28.97
31.67
0.9%Au0.1%Cu
0.7%Au0.3%Cu
0.5%Au0.5%Cu
0.3%Au0.7%Cu
0.1%Au0.9%Cu
1%Cu
0.17
0.58
0.83
1.27
1.22
1.16
1.44
0.31
0.21
0.09
0.04
—
4.45
0.96
0.64
0.29
0.13
—
9.56
2.48
1.10
0.47
0.12
—
a
Weight ratio between gold and copper. b Weight ratio by ICP-OES.
that dissociatively adsorbed oxygen) or hydrogen peroxide (on catalyst surface. It may then undergo a b-hydride elimination to
metals that do not dissociatively adsorb oxygen such as form gluconic acid. It was showed, that aer gluconic acid
gold).36,37 Both molecules would desorb to regenerate the active desorption, both the non-dissociatively adsorb oxygen species
metal surface. In this way, the carboxylic acid is produced by and a metal hydride reacted to form an adsorbed peroxide
subsequent oxidation of the aldehyde through a geminal diol species. This peroxide species desorbs subsequently from the
intermediate. Indeed, as reported in the literature,36 aldehydes catalyst surface.5,41 In the same way the adsorption, of sugar
may undergo reversible hydration to geminal diols. This molecules on a metal surface is facilitated by surface hydroxyl
hydrated diol can then also adsorb to a metal surface and species. It is possible due to the hydrogen bonds with the
undergo a b-hydride elimination to form a carboxylic acid. It aldehyde (or geminal diol intermediate). As could be expected
was observed that the presence of the surface adsorbed various transformations are involved in the process. The
hydroxides, similarly to the use of an alkaline solution, facili- formation of enediol intermediate is with no doubt, crucial for
tates both the formation of an adsorbed alkoxide (or geminal more further oxidations of glucose.42 A lot of attention has been
diol), and the elimination step of a b-hydride towards the also paid to the mechanism of the oxidation of alcohols during
production of aldehyde and carboxylic acid.36,37 A similar homogeneous catalytic reaction using gold catalysts. Rossi
mechanism corresponding to a modied oxidative dehydroge- et al.43 developed one of the most sophisticated molecular
nation mechanism was also suggested as the most probable for models for carbohydrates oxidation in the presence of gold
glucose oxidation to gluconic acid using gold catalysts.5,41 catalysts. Since, as showed above, the oxidation of an aldehyde
Concerning the reaction of glucose oxidation, a deprotonated to a carboxylic acid proceeds via an aldehyde hydrate, it should
geminal diol formed in alkaline solution is adsorbed at the be the same during the oxidation of an alcohol. On the basis of
Table 8 Relative surface distribution of gold and copper speciesa
Catalysts
Au 4f7/2 BE (eV)
Surface Au species
Cu 2p3/2 BE (eV)
Surface Cu species
1%Au
83.08
85.74
83.07
Au0 (93.5%)
Aud+ (6.5%)
Au0 (100%)
—
—
0.9%Au0.1%Cu
0.7%Au0.3%Cu
931.45
932.34
931.44
932.51
934.13
931.46
932.48
934.21
931.42
932.40
934.04
931.54
932.41
934.22
932.05
934.23
Cu0/Cu1+ (56.0%)
CuOH (44.0%)
Cu0/Cu1+ (65.0%)
CuOH (30.7%)
CuO (4.3%)
83.15
84.97
Au0 (92.1%)
Aud+ (7.9%)
0.5%Au0.5%Cu
0.3%Au0.7%Cu
0.1%Au0.9%Cu
1%Cu
83.34
84.91
Au0 (92.0%)
Aud+ (8.0%)
Cu0/Cu1+ (52.4%)
CuOH (34.0%)
CuO (13.6%)
83.45
85.06
Au0 (93.6%)
Aud+ (6.4%)
Cu0/Cu1+ (49.9%)
CuOH (36.0%)
CuO (14.1%)
83.64
85.56
Au0 (87.2%)
Aud+ (12.8%)
Cu0/Cu1+ (36.5%)
CuOH (40.3%)
CuO (23.2%)
—
—
Cu0/Cu1+ (85.0%)
CuO (15.0%)
a
d represents 1 and/or 3.
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Fig. 10 Base-free oxidation of glucose over a series of mono- and bi-metallic catalysts containing Au and Cu supported on TiO2, reaction
conditions: 2 mL of a 1 wt% of glucose aqueous solution, 1 wt% catalyst (21.9 mg), air pressure ¼ 5 bar, 5 h of reaction (details in Tables S4–S6†).
various kinetic results obtained for oxidation of glucose to alcohols to aldehydes or ketones using complexes of gold
gluconic acid, an activation energy of 47 ꢃ 1.7 kJ molꢁ1 was soluble in toluene. This reaction also works under air atmo-
obtained, which is quite similar to the value of 49.6 ꢃ sphere; for example, with benzyl alcohol, a 100% conversion
4.4 kJ molꢁ1 obtained in the case of the enzymatic oxidation. and a 99% selectivity towards benzaldehyde were obtained.44
Moreover taking into account that this reaction is considered as
All of these routes require O2 dissociation on the catalyst
being of the rst order reaction (relative to oxygen), the authors surface. This process is actually not favored on Au catalysts.45,46
suggested a fast adsorption of glucose on gold. In this way, the The production of peroxide during oxidation reactions over
rate-determining step would be glucose oxidation by oxygen Au47,48 indicates that O2 is rst reduced during these reactions
dissolved in the liquid (aqueous) phase. Hydrogen peroxide before dissociation. Then, the activation of O2 should occur
could be detected, but it decomposed quickly under reaction through dissociation of peroxide (OOH*) and hydrogen
conditions. Such glucose oxidations can reach an initial TOF of peroxide (HOOH*) intermediates.39 The steady-state coverage of
50 120 hꢁ1. In a subsequent investigation, Rossi et al.5,43 showed hydroxide on the surface is probably limited by the ability of the
that these oxidations do not proceed via a radical pathway, but metal to accommodate the excess negative charge. As demon-
deliver gluconate and hydrogen peroxide by a two-electron strated by Davis et al.,39 the peroxide that forms can subse-
mechanism. Hydrogen peroxide decomposes because of the quently dissociate on Au to form atomic O and hydroxide, with
basic medium before it reaches a concentration at which it an energetic barrier of 83 kJ molꢁ1. However, as the authors
would efficiently compete with oxygen as an oxidant. Scheme 1 concluded, the more probable step is the further reduction of
shows the mechanism for this oxidation reaction. In aqueous OOH* to H2O2*, with an energetic barrier of only 48 kJ molꢁ1
.
medium the two cyclic diastereomers of glucose and the acyclic The decomposition of hydrogen peroxide over Pd has an ener-
aldehyde form are in equilibrium. Nucleophilic addition of getic barrier of only 5 kJ molꢁ1. Such a low value for peroxide
hydroxide will form (a), which is adsorbed onto the catalyst (b). decomposition on Pd as compared to Au could explain the
Oxygen then reacts with (a) to form the peroxy complex (c), and synergistic effect observed in our work for Au–Pd catalysts
the hydrogen atom of the coordinated aldehyde hydrate is tested under identical reaction conditions.
eliminated as a proton. The produced gluconic acid (e) then
The role of O2 during selective oxidation is an indirect one
leaves the catalyst and hydrogen peroxide is also released. With and does not involve incorporation into the acid product but it
gold colloids with a mean particle diameter of 3.6 nm lie on the participates in the regeneration of the hydroxide ions. Zope
border between heterogeneous and homogeneous catalysis, Shi et al.47,49 proposed that O2 played an indirect role in the mech-
and co-workers recently described the selective oxidation of anism by removing electrons from the metal surface generated
by substrate oxidation.50 Thus, O2 closes the catalytic cycle by
serving as an electron scavenger and regenerating hydroxide
ions that are consumed in the reaction to produce acid prod-
ucts. Obviously, the role of the basic site on the catalyst surface
is important. It activates O–H and C–H bonds. Thus, the activity
of the catalysts will be related to the optimum number of metal-
support interfacial sites, which may contain hydroxyl groups
(surface basic sites). If hydroxyl groups are supplied through the
solution medium (basic solution), the metal-support interface is
much less important so that even bulk gold becomes an active
catalyst. The mechanism of the oxidation in base-free condi-
tions is still unknown and under debate. Adapted advanced
characterization methods and operando studies would be
needed to elucidate it.
Scheme 1 Mechanistic model by Rossi et al. for glucose oxidation.
Adapted from ref. 5.
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Cu, yielding an increase in the Au 4f7/2 BE. Specically, the Aud+
character increased with the increase of the Cu content.
Previous experimental and theoretical investigations on sup-
ported Au catalysts have indicated that the most catalytically
active sites in oxidation reactions correspond to surface Aud+
species,56–58 making this material particularly attractive. It is
important to emphasize that the Au–Cu alloys presented the
highest concentration of this species in comparison to the other
studied supported Au–Me bimetallic catalysts displaying good
performances towards oxidation transformations.
Moreover, as generally accepted, the catalytic rate exponen-
tially depends on the adsorption energy between the active sites
and reactants. Adsorption energy is determined by the compo-
sition, size and facet of the nanocrystals. In addition, ligands
and metal alloy may undergo a charge transfer process, tuning
the surface electronic structure of the nanoparticles, and
therefore affecting their catalytic activity.59 Also, the inuence of
crystal structure and atomic ordering (intermetallic vs. random
alloy) on the catalytic activity is very important. It has been
demonstrated that ordered Au–Cu alloys exhibited enhanced
catalytic properties.60 However, the mechanism of how atomic
ordering affects catalytic performance still needs to be
elucidated.
In the case of the Au–Pd bimetallic catalysts, the remark-
able increase in catalytic oxidation activity is already well
established.3,61–65 For alcohols oxidation, Au–Pd nanoparticles
were found to be twice as active as Pd alone. It was proposed
that Au is an electronic promoter of Pd.61 Composition effect
was also reported, where a progressive increase in catalytic
activity was observed upon increasing the Pd content, with an
optimum Au : Pd ratio of ca. 1 : 1.86, and a progressive
decrease in activity with further increase in the Pd content.66 In
our case, from the XPS data it could be deduced that gold was
mostly reduced and the metallic character decreased with the
increase in Pd content in the samples. In the same time the
oxidation state of Pd progressively moved from Pd0 to Pd2+
with the increase of gold content in the sample. It could be
concluded that there should be an optimum ratio between Pd0
and Pd2+ species that will maintain a high activity as well as
high selectivity.
Discussion
In case of Cu-containing samples the activity was much higher
for Au-rich catalysts. Au–Cu alloy systems supported on
different materials were already explored as catalysts for
oxidation reactions.51,52 Generally, Au0 is believed to be the
active site while, Cu0 plays an important role in activation of
oxygen molecules.53 However, it was observed that Au0–Cu0
nanoparticles alone are not catalytically active in their original
form. The formation of an Au–CuOx structure has been found to
be essential to obtain high activity. As suggested in the litera-
ture54 the Cu0 component enriched the surface of the alloy
nanoparticles while CuOx can provide active oxygen species.
Moreover, Zhan et al.55 have also pointed out that the thickness
of the CuOx layer on the surface is also crucial because a thick
CuOx layer will prevent O2 from approaching Au0 active sites
and slowing down activation. In our case, four different Cu
species were detected (Table 8 and Fig. 9): Cu0, Cu+, CuOH and
CuO. For all catalysts the Cu is mostly in Cu0/Cu+ state. Indeed,
the BE energy corresponds well to the metallic and/or Cu+
states. However, due to the very low Cu loadings, it was
impossible to properly record and discuss the Auger Cu LVV
region. The discussion was made using the CuOH and Cu2+
species that are considered as active sites in the oxidation
reaction (as discussed above). As presented in Fig. 11, glucose
conversion increased when the total% of oxidized Cu increased
(i.e., considering CuOH and CuO).
The presence of CuOH on the catalyst surface was conrmed
by the absence of the satellite peaks and the presence of the
third component in the Cu 2p3/2 region (Fig. 9).
One can expect that the higher content of surface Cu
oxidized species will affect the metallic character of the gold due
to the electron transfer from Au to Cu (Fig. 12).
Indeed, an almost linear correlation was found between
glucose conversion and the chemical state of gold (expressed as
BE of the Au 4f7/2 level). The 4f7/2 BE of gold shied progres-
sively to higher values with the increase of the Cu content in the
sample. This could be explained by the strong interaction
between Au and Cu in the formed alloy. Higher contents of Cu
enable formation of a larger quantity of Cu oxidized species on
the surface. This induces the transfer of electrons from Au to
Fig. 11 Conversion of glucose at 60 ꢀC versus total% of oxidized
copper (CuOH + CuO) as obtained from XPS.
Fig. 12 Glucose conversion at 100 ꢀC versus BE of Au 4f7/2 region.
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´
´
´ ´
10 T. Benko, A. Beck, K. Frey, D. F. Sranko, O. Geszti, G. Safran,
The understanding of the structure and composition of the Au–
Pd bimetallic catalysts is key for designing more active catalysts
and to nd robust structure–activity relationships. However, the
exact characterization of these samples is oen difficult due to
interference of the support (metal support interactions) or low
metal loadings. For glucose oxidation, the activity of the Au–Pd
nanoparticles depends on the composition: gold-rich nano-
particles presented a lower activity when compared with the
palladium-rich nanoparticles, but a maximum of activity (at
60 C and 80 C) was reached with 0.5wt%Au0.5wt%Pd bime-
tallic composition. It could be rstly concluded that the gold-
rich samples continue in their alloyed forms with a small
increase in surface Pd content, as deduced from the XPS data.
Secondly, the Pd-rich samples present metal segregation (Pd
rich surface) with an increasing presence of palladium oxide.
Surface Pd helps in the adsorption of glucose.13
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ꢀ
ꢀ
´
´
18 D. Ferrer, A. Torres-Castro, X. Gao, S. Sepulveda-Guzman,
In case of Au–Cu/TiO2 samples the synergetic effect was also
observed with the highest activity observed for 0.5%Cu0.5%Au/
TiO2 sample at 60 ꢀC. Generally, Au0 is believed to be the active
site while, Cu0 plays an important role in activation of oxygen
molecules. In our case, four different Cu species were detected.
For all Au–Cu bimetallic catalysts the Cu was mostly in Cu0/Cu+
state. A linear correlation between catalytic activity and CuOH
and Cu2+ species content was observed which strongly indicated
that CuOH species are active sites in the glucose oxidation
reaction.
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Conflicts of interest
24 Y. Wang and H. Gao, J. Phys. Chem. B, 2017, 121, 2132–2141.
25 R. Wojcieszak, I. M. Cuccovia, M. A. Silva and L. M. Rossi, J.
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There are no conicts of interest to declare.
Acknowledgements
We gratefully acknowledge Dr Ahmed Addad for providing with
HAADF images and Dr Pardis Simon for the XPS analysis. We
would like to thank the China Scholarship Council (CSC,
201504490079) for nancial support. We also thank the REAL-
CAT platform for providing with reaction and analysis
equipment.
´
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