Pt and Pt/Sn carbonyl clusters as precursors for the synthesis of supported metal catalysts for the base-free oxidation of HMF

Article abstract of DOI:10.1016/j.apcata.2019.117279

In the present work, Pt and Pt/Sn nanoparticles (NPs), synthesized from carbonyl cluster precursors were deposited on TiO₂ and the resulting materials were tested as catalysts in selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The work was mainly focused on the study of the formation of bimetallic or mixed oxide-metal nanoparticles on TiO₂ starting with Pt/Sn carbonyl clusters and on the structure-activity relationship in the reaction of HMF oxidation in base-free conditions. The developed synthesis procedure allowed to obtain very small mono and bimetallic particles characterized by a narrow particle size distribution. Promising results in base-free FDCA production have been achieved using the prepared samples. In particular, the introduction of Sn in an equimolar amount with Pt improved the catalyst activity as well as its time stability upon operation, demonstrating that the modification of Pt electronic configuration by Sn is a key factor for the mastering of functional performances.

Full text of DOI:10.1016/j.apcata.2019.117279

Journal Pre-proof
Pt and Pt/Sn carbonyl clusters as precursors for the synthesis of
supported metal catalysts for the base-free oxidation of HMF
Danilo Bonincontro, Alice Lolli, Alba Storione, Alberto Gasparotto,
Beatrice Berti, Stefano Zacchini, Nikolaos Dimitratos, Stefania
Albonetti
PII:
S0926-860X(19)30434-X
DOI:
https://doi.org/10.1016/j.apcata.2019.117279
APCATA 117279
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
16 July 2019
27 September 2019
28 September 2019
Please cite this article as: Bonincontro D, Lolli A, Storione A, Gasparotto A, Berti B, Zacchini
S, Dimitratos N, Albonetti S, Pt and Pt/Sn carbonyl clusters as precursors for the synthesis of
supported metal catalysts for the base-free oxidation of HMF, Applied Catalysis A, General
(2019), doi: https://doi.org/10.1016/j.apcata.2019.117279
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Pt and Pt/Sn carbonyl clusters as precursors for the synthesis of
supported metal catalysts for the base-free oxidation of HMF
a
a
a
b
a
Danilo Bonincontro , Alice Lolli , Alba Storione , Alberto Gasparotto , Beatrice Berti , Stefano
a
a
a*
Zacchini , Nikolaos Dimitratos , Stefania Albonetti
aDepartment of Industrial Chemistry “Toso Montanari”, Bologna University, Viale Risorgimento 4,
0136 Bologna, Italy
4
bDepartment of Chemical Sciences, Padova University and INSTM, Via Marzolo 1, 35131 Padova,
Italy
Graphical Abstract
Research highlights



Pt-based carbonyl clusters are suitable precursor for small Pt NPs
Cluster derived materials are effective in base-free HMF oxidation
Sn addition enhances catalyst activity and stability
Abstract
In the present work, Pt and Pt/Sn nanoparticles (NPs), synthesized from carbonyl cluster
precursors were deposited on TiO and the resulting materials were tested as catalysts in selective
2
oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The work was
mainly focused on the study of the formation of bimetallic or mixed oxide-metal nanoparticles on
1

TiO starting with Pt/Sn carbonyl clusters and on the structure-activity relationship in the reaction of
2
HMF oxidation in base-free conditions. The developed synthesis procedure allowed to obtain very
small mono and bimetallic particles characterized by a narrow particle size distribution. Promising
results in base-free FDCA production have been achieved using the prepared samples. In particular,
the introduction of Sn in an equimolar amount with Pt improved the catalyst activity as well as its
time stability upon operation, demonstrating that the modification of Pt electronic configuration by
Sn is a key factor for the mastering of functional performances.
Keywords carbonyl cluster, Pt/Sn, 5-hydroxymethylfurfural oxidation, 2,5-furandicarboxylic acid
1
Introduction
The conversion of biomass derived molecules for the development of new processes with low
environmental impact is an appealing topic both from a fundamental and applied perspective. Within
this field, 5-hydroxymethylfurfural (HMF), obtained by renewable biomasses, is one of the most
studied platform molecules for the sustainable production of various fuels and chemicals. [1-7]. The
presence of different functional groups such as hydroxyl, formyl and furan rings enhance HMF
reactivity, allowing different reactions such as reduction, oxidation and esterification to take place.
Over the past two decades, HMF oxidation has been widely studied using different reaction conditions
and heterogeneous catalysts with specific metal content and morphology.[7-11] Selective oxidation
of this molecule leads to the formation of 2,5-furandicarboxylic acid (FDCA), which is one of the
most promising intermediates for the production of poly(ethylene furanoate) (PEF), the furan-based
analogue to poly(ethylene terephthalate) (PET), which is the dominant polymer in the beverage
packaging industries. Recently, it has been reported that the 100% bio-based polymer made up of
FDCA and bio-sourced ethylene glycol is characterised by reduced oxygen and carbon dioxide
permeability compared to PET.[12, 13] Moreover, many companies are genuinely interested in
developing processes for FDCA production as a possible candidate monomer for the synthesis of
polyesters, polyamides and polyurethanes. Consequently, several patents have been published in this
field.[14-17]
In the last few years, a number of pilot scale plants have also been developed for the current
process, such as Du Pont[18] and Synvina,[19] however, technical problems have been reported for
the final commercialisation of the process. Nevertheless, the optimized route for the synthesis of
FDCA has not yet been identified and the current technology for terephthalic acid production using
metal/bromide catalysts seems to be currently used in pilot reactor, to facilitate is direct integration
2

into existing terephthalic acid production lines.[20] One drawback of these catalytic systems is the
use of corrosive media and dangerous compounds, which make the process polluting. Thus, the
commercially viable production of FDCA has not yet realized.
Currently, the recent focus is on the search for efficient catalysts and process condition to prepare
FDCA in aqueous media and as a consequence, the preparation of active and stable supported metal
catalysts, eventually combining two metals for improving activity, selectivity and long-term stability,
is attracting a great interest.[21, 22] In this regard, Pt[8, 23-27] is one of the most commonly used
metals, together with Au[28-32] and Pd.[33-36]
Moreover, the tailoring of metal nanoparticle morphology for the preparation of mono and
bimetallic systems, in terms of nanoparticle size and shape, and structure (alloy versus core-shell
bimetallic systems) can be a useful tool to improve catalyst activity, minimising leaching, poisoning
and therefore enhancing long term stability. Recent studies have reported that Au/Cu,[37-39] Au/Pd
[40-47] bimetallic nanoparticles either in the form of alloy or as core-shell structure were active in
HMF selective oxidation, showing superior catalytic performance with respect to their monometallic
counterparts. However, the necessity of basic conditions to activate and stabilize these catalysts is
still the main concern from the industrial point of view. Remarkable results in base-free FDCA
production have been achieved by Ebitani and coworkers[48] using Pd/Pt bimetallic nanoparticles
supported on hydrotalcite. Nevertheless, the potential contribution of solubilized inorganic basic
support and concerns on catalyst stability remain hotly debated.
Recently, bimetallic Pt/Sn supported catalysts attracted renewed interest since they exhibit high
activity and stability in liquid phase hydrogenation and oxidation of biomass-derived molecules.[49-
5
2]. The addition of Sn to Pt seems to lead to the formation of unique active sites and the different
formed phases were demonstrated to facilitate different reactions. As an example, Zhao and co-
workers, demonstrated that the synergy between Pt and Sn species was remarkable and resulted in
excellent activity in the acetic acid hydrogenation and ethanol selectivity [50] while Davis and co-
workers [49] studied the aqueous phase oxidation of 1,6-hexanediol using a series of carbon-
supported Pt/Sn materials and proved the importance of the formed species on the oxidation activity.
On the bases of this information the use of bimetallic Pt/Sn catalysts with controlled particle size
and metal composition may be a promising way to improve catalyst activity and stability in the base-
free oxidation of HMF.
Pt/Sn catalysts are generally prepared by impregnation method, leading to a quite limited control
of the structural parameters, while the use of molecular metal clusters has been reported to be a
suitable methodology for the synthesis of well defined nanosystems,[53-55] and thus, it could be
considered as well a promising route for the synthesis of well-defined Pt/Sn systems with controlled
3

metal composition, particle size and structure, as also reported for other supported bimetallic
nanoparticles.[56-60]
In view of the above considerations, the present work has focused on the synthesis and
characterization of TiO
2
-supported Pt and Pt/Sn nanoparticles obtained from carbonyl cluster
decomposition. The described synthetic route has not investigated so far for the designing of Pt-based
2
-
nanoparticles for the oxidation of HMF. In particular, [Pt12(CO)24] was used as precursor for the
4
-
preparation of homometallic particles,[61-63] while the clusters [Pt
6
(CO)
6
(SnCl
2
)
2
(SnCl
3
) ] and
4
2
-
[Pt
6
(CO)
8
(SnCl
2
)(SnCl
)
3 2
(PPh )2] , having a specific Pt/Sn molar ratio of 1 and 2 respectively, were
3
used for the preparation of bimetallic systems. The catalytic performance of the obtained catalysts
was evaluated in the aqueous phase oxidation of HMF to FDCA in base-free conditions and compared
with analogous systems prepared by metal salt impregnation method using the same metal loading
and Pt/Sn molar ratio. All prepared catalysts have been thoroughly characterized by means of High
Resolution Transmission Electron Microscopy, High Angle Annular Dark Field imaging
(
HRTEM/HAADF), Temperature Programmed Reduction (TPR), X-ray Fluorescence (XRF), X-ray
Photoelectron Spectroscopy (XPS), CO chemisorption and Fourier Transform InfraRed Spectroscopy
FTIR) techniques investigating in detail their structure-property relationships.
(
2
Experimental
2
.1 Catalyst preparation
The metal carbonyl clusters employed in this work have been prepared following the
experimental procedures reported in the literature.[64, 65] For sake of completeness, some details are
given below. The as-synthesized mono and bimetallic Pt-based clusters were, then, supported onto
commercial titania (DT-51 Millennium Chemicals, now Tronox, pure anatase phase with surface area
2
of 84 m /g). Different thermal treatments were carried out on the prepared samples in order to
understand the effect produced (i) in terms of structural modification and (ii) in terms of catalytic
activity and stability.
2
.1.1 Synthesis of [PPh
The preparation of Chini clusters is well known[62] therefore, only a brief description of the
preparation method will be presented here. Na PtCl · 6H O (2.00 g, 4.39 mmol) and CH COONa
2.00 g, 24.4 mmol) were dissolved in methanol (30 mL) and stirred under CO atmosphere
atmospheric pressure) overnight, resulting in the formation of a silky brown suspension of
[Pt3n(CO)6n] (n  30). A methanol solution of NaOH was then added dropwise in the sealed glass
4
]
2
[Pt12(CO)24] (named as Pt12)
2
6
2
3
(
(
Na
2
reactor under the presence of CO atmosphere over 1 h until a green solution of Na [Pt12(CO)24] was
2
4

formed as confirmed by IR analysis. [PPh
by addition of a saturated solution of [PPh
washed with isopropanol and extracted in acetone. A microcrystalline powder of [PPh
Figure S1) was eventually obtained after removal of the solvent under reduced pressure from the
acetone solution.
4
]
2
[Pt12(CO)24] was precipitated from the reaction mixture
4
]Br in isopropanol. The solid was recovered by filtration,
4
2
] [Pt12(CO)24]
(
2
.1.2 Synthesis of [PPh
SnCl ·2H O (0.371 g, 1.64 mmol) and [PPh
to a solution of [PPh [Pt12(CO)24] (1.00 g, 0.138 mmol) in acetone (30 mL). The solution turned
4
]
4
[Pt
6
(CO)
6
(SnCl
2
)
2
(SnCl
3
)
4
] (Pt/Sn molar ratio 1, labeled as Pt
1
Sn
1
)
2
2
4
]Cl (0.413 g, 1.10 mmol) were added as solids
]
4 2
from green to brown and it was stirred at room temperature for 2 h. The solvent was removed in vacuo
and the residue washed with water (40 mL), iso-propanol (40 mL) and acetone (20 mL), and the green
product
PPh [Pt
under reduced pressure from the CH
extracted
in
CH
] (Figure S2) was eventually obtained after removal of the solvent
CN solution.
3
CN
(20
mL).
A
microcrystalline
powder
of
[
4
]
4
6
(CO) (SnCl
6
2
)
2
(SnCl
)
3 4
3
2
.1.3 Synthesis of [PPh
Pt Sn
PPh
[Pt (CO)
4
]
2
[Pt
6
(CO)
8
(SnCl
2
)(SnCl
3
)
2
(PPh
3
) ] (Pt/Sn molar ratio 2, labeled as
2
2
1
)
3
(0.084 g, 0.321 mmol) was added as a solid to
a
solution of
[PPh
4
]
4
6
6
(SnCl (SnCl ] (0.520 g, 0.161 mmol) in CH CN (20 mL) and the mixture was
2
)
2
3
)
4
3
stirred at room temperature under CO atmosphere for 2 h. Then, iso-propanol (80 mL) was added
under CO atmosphere up to complete precipitation of a dark-brown solid, which was recovered by
filtration, washed with iso-propanol (40 mL) and extracted with CH
atmosphere. A microcrystalline powder of [PPh [Pt (CO) (SnCl )(SnCl
3
CN (20 mL) under CO
(PPh ] was eventually
CN solution under CO
4
]
2
6
8
2
3
)
2
)
3 2
obtained (Figure S3) by addition of iso-propanol (80 mL) to the CH
3
atmosphere, recovered by filtration and dried under reduced pressure.
2
.1.4 Preparation of supported catalysts
The prepared carbonyl clusters have been supported on TiO (2 wt. % total metal loading, i.e.
2
Pt and Sn, with respect to titania, as confirmed by XRF analyses) by incipient wetness impregnation.
The clusters were dissolved in CH CN and the solutions were slowly added to the TiO support under
3
2
nitrogen and stirred for 2h. Then, the solvent was removed in vacuo and the resulting powder was
dried at 120°C for 2h using an oven. Some selected samples have been also thermally treated up to
4
50°C for 2 hours (2°C/min) in controlled atmosphere (air or H2, 50 ml/min).
5

The samples prepared via cluster impregnation on TiO are listed in Table 1:
2
The catalysts prepared using metal salt impregnation have been synthesized by incipient wetness
impregnation of TiO using Pt and Sn precursor solutions (H PtCl and SnCl , respectively). The
2
2
6
2
concentration of metal ions has been calculated in order to obtain materials having the same Pt/Sn
molar ratio and metal loading as the supported cluster materials. The metal salt impregnated solids
were dried at 120°C in air and thermally treated at 450°C in flowing H
2
as reported for the supported
cluster materials. The list of metal catalysts prepared by salt impregnation is reported in Table 2.
2
.2 Catalyst characterisation
HRTEM analyses were performed with a TEM/STEM FEI TECNAI F20 instrument, using a
high-angle annular dark field imaging mode at 200 kV and with an EDS PV9761N SUTW energy
dispersive X-ray spectrometer for X-ray microanalysis. The analyses of suspension were performed
by diluting them in water and then the obtained suspensions were dispersed on a holey carbon film.
Calculation of mean particle size was carried out using over 300 nanoparticles.
TPR (Temperature Programmed Reduction) experiments were performed using a
Micromeritics ChemSorb2750 equipped with a TCD to evaluate H
carried out by pretreating the samples for 30 minutes at 450°C under He flow (heating ramp
0°C/min, gas flow rate 30 mL/min). Then the samples were cooled down to 25°C, the gas flow was
switched to H (5% in He v/v), and the temperature was increased to 650°C (heating ramp 10°C/min,
2
consumption. All the tests were
1
2
gas flow rate 30 mL/min) for the analysis.
FTIR spectra were recorded with a Perkin-Elmer Spectrum One interferometer using CaF cells.
2
XPS spectra were recorded on a Perkin Elmer  5600ci spectrometer, using a non-monochromatized
AlK excitation source (1486.6 eV). Charging effects were corrected by assigning to the C1s line of
adventitious carbon a binding energy (BE) value of 284.8 eV. Atomic compositions were evaluated
using sensitivity factors provided by  V5.4A software. Peak fitting was performed by a least-
squares procedure, using Gaussian-Lorentzian peak shapes.
CO chemisorption tests were performed using a Micromeritics AutoChemII instrument. In a typical
experiment, the materials were pretreated under H
2
/Ar flow (50mL/min, H concentration 5%) from
2
room temperature to 200°C (temperature ramp 2°C/min, isotherm 2h). After cooling to room
temperature, gas was switched to He (50mL/min) and the sample was heated up to 120°C
(temperature ramp 2°C/min, isotherm 2h) to remove the hydrogen species eventually adsorbed on the
6

-6
sample. Then, after cooling the sample to room temperature, 20 pulses of CO/He (3.61*10 moles
each) were send to the sample.
2
.3 Catalytic test
The oxidation of HMF was carried out using an autoclave reactor of 100 mL capacity, equipped
with a mechanical stirrer and measurement tools for temperature and pressure. The reactor was
charged with an aqueous solution (25 mL distilled water) containing the appropriate amount of 5-
(
hydroxymethyl)furfural (99.999%, AvaBiochem) and catalyst (HMF/total metal molar ratio = 50).
The autoclave was purged 3 times with O (5 bar) and then pressurized at the desired pressure (10
2
bar). If not indicated otherwise, the temperature was increased to 110ºC and the reaction mixture was
stirred at 400 rpm for the indicated reaction time (0.5-3 h). Initial time (time zero) for the reaction
was taken when the set point temperature was reached. At the end of the reaction, the reactor was
cooled down to room temperature and the aqueous solution was centrifuged and then filtered. Then,
the reaction mixture was diluted 5 or 10 times before analysis with an Agilent Infinity 1260 liquid
chromatograph (HPLC-DAD) equipped with an Aminex HPX 87-H 300 mm 7.8 mm column using
a 0.005 M H
2
SO solution as the mobile phase. An external calibration method was used for the
4
identification, quantification of reactants and products using reference commercial samples.
Particular care was devoted to the determination of the C balance, which was found to always fall
between 95 and 105% (calculated as the comparison between converted HMF and the sum of the
product yields).
3
Results and discussion
3
.1 Preparation and characterization of the supported catalysts
Study of the cluster impregnation by means of FTIR technique
Three different cluster precursors have been employed for the preparation of the catalysts, that is
[
PPh
4
]
2
[Pt12(CO)24] (labeled as Pt12), [PPh
4
]
4
[Pt
6
(CO)
6
(SnCl
2
)
2
(SnCl
3
)
4
] (Pt/Sn molar ratio 1, labeled
Sn ). The
as Pt
1
Sn ) and [PPh [Pt (CO) (SnCl )(SnCl
1
4
]
2
6
8
2
3
)
2
(PPh
3
)
2
] (Pt/Sn molar ratio 2, labeled as Pt
2
1
impregnation procedure of the synthesized clusters has been studied by means of FTIR technique
using the CO bands of the different clusters as probes in order to detect the occurrence of any reactions
during catalyst preparation. Particular focus has been given to the three crucial phases of such
procedure, namely, in solution before impregnation, during impregnation (i.e. when a suspension of
TiO and cluster solution is present) and after solvent removal. Indeed, the interest of using molecular
2
cluster precursors for catalyst preparation relies on the possibility to carefully control and tune the
nanoparticle size, structure and (in the case of bimetallic systems) the composition of the final
7

nanostructured active materials. In order to achieve these goals, the intimate molecular nature of the
precursor should be, in theory, retained up to thermal treatment step. Thus, the occurrence of any
(uncontrolled) reaction prior to thermal decomposition of the precursor might be detrimental and
therefore should be investigated.
The spectrum recorded for the Pt12 cluster solution (black line, Figure 1a) shows the carbonyl signals
-1
of the [PPh
4
]
2
[Pt12(CO)24] cluster (2047cm for the terminal carbonyls, 1897, 1881, 1865 and
833cm for the bridging ones). After the addition of TiO , the spectrum of the resulting suspension
red line in Figure 1a) remained almost unchanged, suggesting that no reaction occurred at this stage.
-1
1
2
(
Conversely, after removal of the solvent under reduced pressure, the spectrum of the cluster
impregnated powder (green line in Figure 1a) showed CO band shifted to higher wavenumbers in
2
-
agreement with the formation of [Pt18(CO)36] . This might be due to cluster oxidation as a
consequence of the intimate contact between the cluster and TiO2. It must be remarked that the
reversible) chemical oxidation of [Pt12(CO)24] to afford [Pt15(CO)30]2- and [Pt18(CO)36]2- is a well-
known process.[61, 63, 66]
As far as the Pt Sn cluster impregnation procedure is concerned, this cluster displays in solution a
unique CO band at 2037 cm due to its terminal CO ligands (black line, Figure 1b). After the addition
2-
(
1
1
-1
-1
of TiO2, this CO band remains the major one, even if a shoulder at 2010 cm appears (red line, Figure
b). This suggests that Pt Sn is still the main species present in the suspension, accompanied by a
1
1
1
minor amount of an unknown side product. This situation remains identical after removal of the
solvent in vacuo, as indicated by the fact that the IR spectrum of the cluster impregnated powder
(
green line, Figure 1b) shows almost the same IR pattern, apart from some broadening due to the solid
state nature of the latter sample.
The carbonyl cluster Pt Sn dissolved in acetonitrile (black line in Figure 1c) showed the expected
CO bands due to terminal (2043 and 2028 cm ) and edge bridging CO ligands (1882 and 1831 cm
2
1
-1
-

1). The presence of a shoulder at 2012 cm-1 could indicate the formation of traces of
2
-
[
Pt
atmosphere. Indeed, Pt
slowly loses CO resulting in the formation of [Pt
6
(CO)
6
(SnCl
2
)
2
(SnCl
3
)
2
(PPh
3
)
2
] , due to partial decarbonylation of Pt
2
Sn under nitrogen
1
2
Sn is stable under CO atmosphere, whereas under nitrogen atmosphere
1
2
-
6
(CO)
6
(SnCl
2
)
2
(SnCl
3
)
2
(PPh
3
) ] . This is not a
2
problem, since the two clusters have the same Pt/Sn composition. Taking into account the
aforementioned results, the described impregnation steps did not lead to significant changes in the

CO bands, indicating that Pt
2
Sn did not undergo any structural modification during the impregnation
1
procedure. Moreover, XRF analyses on all supported catalysts (Table 1) indicate that the real metal
content is equal to the nominal one based on the amount of cluster deposited on support.
8

FTIR analyses have been conducted also during the thermal treatment of the supported Pt
1
Sn
1
material. Cluster decomposition begins after drying in oven at 120°C for 2 h, since the CO bands of
the spectra recorded before (green line in Figure 1d) and after such treatment (grey line in Figure 1d)
are completely different. Then, complete cluster decomposition occurs after the thermal treatment at
4
50°C, irrespective of the atmosphere used, as indicated by the disappearance of all the CO bands in
both 2-Pt Sn -H and 2-Pt Sn -O samples (yellow and orange lines, Figure 1d).
1
1
2
1
1
2
Effect of thermal treatment on Pt/Sn supported materials
FTIR analyses demonstrated that complete cluster decomposition occurs after thermal
treatment at 450°C, irrespective of the atmosphere used. Since, the interest of using molecular cluster
precursors for catalyst preparation relies on the possibility to carefully control and tune nanoparticle
size, structure and composition, the effect of thermal treatment of 2-Pt
1
Sn
1
dried sample in different
and the
-TPR
atmosphere was studied. In particular, dried sample was treated at 450°C in flowing air or H
2
obtained materials were characterized thoroughly by means of BET, HRTEM/STEM, EDX, H
2
and XPS (Table 3).
BET surface area results obtained for all catalysts point out that Pt/Sn cluster deposition has a little
2
influence on the textural properties of bare titania (surface area 84 m /g). Nevertheless, the dried
sample show a significant surface area decrease, indicating that cluster residues can partially block
the pores of the support. On the contrary, catalyst treatments with H or air, lead to surface area values
2
similar to the bare support, indicating that the complete cluster decomposition frees the porosity,
being this effect more evident in the presence of air.
HRTEM/STEM analysis was used to obtain information on the morphology in terms of mean particle
size, particle size distribution and composition of the metallic species after the treatment at 450°C
both in air (2-Pt
1
Sn
1
-O2 – Figure 2a) and H
2
atmosphere (2-Pt
1
Sn
1
-H Figure 2b). Representative
2
HRTEM images and particle size distribution histogram for the dried sample are reported in Figure
S4. For this material, the analysis shows a mean particle size of 1.2 nm, with narrow particle size
distribution and with relatively good dispersion of PtSn nanoparticles onto TiO .
2
Figures 2 shows that, the treatment at 450°C, both in air and H2, caused a small increase of
nanoparticles dimension and the average diameter was only slightly influenced by the adopted
thermal treatment atmosphere (2.1 nm in H versus 2.6 nm in Air). However, the skewness of the
2
Gaussian distribution significantly changed depending on the used atmosphere. Indeed, when
hydrogen was used, the distribution shifted towards smaller particle diameter and narrower particle
size distribution; moreover, in air treated sample some large nanoparticles were observed.
9

On the other hand, EDX analyses and mapping evidenced that the distribution of the two metals over
support surface is strongly affected by the used atmosphere. In fact, in the H -treated sample, high
concentrations of Pt and Sn are detected in the same area of the support surface (Figure 3a) while in
the air-treated sample Sn seems to be evenly spread on the TiO surface and Pt is mainly present as
2
2
segregated nanoparticles (Figure 3b). This feature suggests that reducing atmosphere prevents the
well-known out-diffusion of Sn from the cluster structure[67, 68] and bimetallic Pt/Sn nanoparticles
could be obtained in such reducing conditions.
XRD patterns obtained over 2-Pt
1
Sn samples did not allow to identify the presence of Pt/Sn
1
species formed on the different catalysts due to the very low amount of metals and their small
crystallite size, below 5 nm. Nevertheless, the X-ray diffraction pattern of a similar sample prepared
with a total metal loading of 4% w/w, reported in Figure S5, confirmed the presence of the PtSn alloy
(reflexes at 2 = 30, 42, 44, reference code 00-008-0192) in the sample treated in H2 at 450°C. This
specie was not evident in the sample treated in air.
In order to gain further information on the surface features of the thermally treated Pt
1
Sn -
1
based materials, XPS analysis has been performed and compared with the dried Pt
1
Sn material. The
1
most relevant results, summarized in Table 3, (see also Figures S6 and S7 and Table S1), showed that
the surface Pt/Sn molar ratios were appreciably lower than the corresponding bulk values (close to 1
for all the three samples, as revealed by XRF analyses, see Table 1). This difference could be due to
a possible surface segregation of Sn that results in an enrichment of this element with respect to Pt,
as far as the XPS sampling depth is concerned.[69] It could be hypothesized that the driving force for
Sn out-diffusion might be its higher tendency to oxidation compared to Pt, in agreement with previous
works on similar Pt-Sn systems.[49, 70] In fact, analysis of the Sn3d peak revealed two bands arising
from spin-orbit splitting phenomena, located at 486.7 eV (Sn3d5/2) and 495.1 eV (Sn3d3/2) irrespective
of the considered sample (see also Figure S6). Such BE values indicate the absence of Sn(0) and Sn-
Pt metallic alloys in appreciable amounts, and are compatible with Sn(II) and/or Sn(IV) oxidized
species.[71-74] At variance from Sn, the Pt oxidation state was affected by the thermal treatment
atmosphere. A careful analysis of the Pt4f peak revealed indeed the co-presence of Pt(0), Pt(II), and
Pt(IV) species for all specimens[72-74] but a Pt(0)/[Pt(II)+Pt(IV)] content much higher for sample 2-
Pt
1
Sn
1
-H than for the other two systems. Overall, XPS data indicate that during the drying procedure
2
the majority of the Pt is oxidized from its original oxidation state (i.e. metallic Pt), a situation that is
not appreciably changed by the thermal treatment in air. Conversely, the use of a reducing atmosphere
partially restores the oxidation state of Pt, yielding a Pt(0)/[Pt(II)+Pt(IV)] value of 0.95. Interestingly,
the latter effect is also paralleled by the highest Pt/Sn molar ratio (Table 3). In other words, the use
10

of a reducing atmosphere also promotes an enhanced Pt/Sn intermixing, a phenomenon already
evidenced by EDX data.
The above considerations on Pt and Sn oxidation states for 2-Pt
1
Sn
1
-H
2
, 2-Pt
1
Sn
1
-O
2
and 2-Pt
1
Sn
1
samples have been further investigated and confirmed by TPR analyses. In fact, the former two
samples revealed a very similar TPR behavior, characterized by two main peaks at 170 and 360°C
(
Figure 4), indicating once again that the oxidation state of the metals does not substantially change
after an increase of the temperature treatment in the same oxidizing atmosphere. The first of the two
peaks could be attributed to the reduction of Pt-O-Sn species, where Pt(II)/Pt(IV) and Sn(IV) are
reduced to Pt(0) and Sn(II), respectively, while the second could be due to the subsequent Sn(II)
reduction to Sn(0).[75] Such peculiar behavior revealed that the use of bimetallic cluster as precursor
leads to an interaction between the two metals from the initial catalyst synthesis step which is an
essential parameter for achieving the synthesis of bimetallic random alloyed nanoparticles. On the
contrary, the reducing atmosphere leads to an almost complete reduction of the metal species, as could
be seen from the 2-Pt
1
Sn
1
-H profile. The presence of the peak at 175°C underlines that some
2
oxidized species are still present after the reduction process, as already revealed by XPS analysis.
The presence of some Sn(IV) even after the reducing treatment could be caused by the tendency of
this metal to stay in the oxidized form.
Effect of Pt/Sn ratio on supported materials
The effect of metal composition was studied preparing and characterizing a monometallic catalyst (2-
Pt-H
2
) and a bimetallic material with different Pt/Sn ratio (2-Pt
2
Sn
1
-H ). For comparison, catalysts
2
with the same metal molar ratio of the cluster-based materials were prepared by incipient wetness
impregnation of the metal salts (Table 4). Moreover, Pt dispersion is presented in Table 4 for selected
samples.
TEM/STEM and EDX analyses were carried out and compared to the previously reported samples to
verify the effect of Pt/Sn molar ratio on the dispersion of active phase over the support after high
temperature (450°C) treatment under H .
2
A strong effect of Sn content on the final formation of the metal nanoparticle size in the three samples
was found comparing the average nanoparticle size for 2-Pt-H , 2-Pt Sn -H and 2-Pt Sn -H (Table
; Figure 5a and 5b). Sn introduction seems to decrease the mean metal particle size only at low
content, with sample 2-Pt Sn -H . On the other hand, 2-Pt Sn -H showed the same mean particle
size as the monometallic material (2-Pt-H ). It is worth noting that, even if the mean particle size of
those two samples (i.e. 2-Pt Sn -H and 2-Pt-H ) is at similar range, the measured Pt dispersion of
2
2
1
2
1
1
2
4
2
1
2
1
1
2
2
1
1
2
2
11

the bimetallic sample is lower than the monometallic one (12.7 and 20.5%, respectively), probably
due to Sn-enrichment of the Pt nanoparticle surface, as detected by XPS. The aforementioned
observations indicate that the preparation of Pt/Sn bimetallic nanoparticles from carbonyl cluster
precursors is influenced by Sn content, and the right amount of Sn to obtain smaller mean particle
size and controlled Pt surface exposure could be therefore optimized.
In addition to this, EDX mapping for the 2-Pt
support surface, while Pt is mainly present as segregated nanoparticles (Figure 5c). This feature is in
contrast with what previously reported for the 2-Pt Sn -H , since in the latter, Pt/Sn close proximity
2
Sn
1
-H
2
sample showed that Sn is dispersed over the
1
1
2
were observed, (Figure 2). Therefore, even if the lower amount of Sn lead to the formation of smaller
nanoparticles, their composition could not be considered as stable since it segregated during the
reductive thermal treatment.
For comparison, catalysts with the same metal molar ratio of the cluster-based materials were
prepared by incipient wetness impregnation of the metal salts. TEM/STEM analyses have been
performed on samples thermal treated in H
protocols have a significant effect on the structural features of the synthesized materials. TEM
analysis of the 2-Pt-s-H sample showed a poor Pt dispersion over the support surface (Figure S8).
Indeed, Pt is present as 4-5 nm nanoparticles and with some large Pt aggregates. On the contrary,
TEM/STEM analysis of the 2-Pt Sn -s-H sample showed that the co-impregnation of the two metals
2
, with the aim to understand if the different synthetic
2
1
1
2
led to an improved NPs dispersion, confirming that the presence of Sn favors the formation smaller
nanoparticles also in the case of the incipient wetness impregnation (Figure S9). Moreover, EDX
analysis performed on this sample confirmed that, in contrast to 2-Pt
1
Sn
1
-H cluster derived sample,
2
Sn tend to disperse rather than react with Pt nanoparticles (Figure 6b). Taking everything into account,
it is possible to state that metal salt impregnation protocol cannot ensure the same metal particle size
control and surface metal distribution obtained via controlled metal cluster decomposition.
3
.2 Catalytic tests
The catalytic performance of all prepared materials was tested in the aqueous phase oxidation of HMF
to FDCA in base-free conditions. The general reaction pathway for HMF oxidation is presented in
Scheme 1. The first oxidation reaction can either involve the aldehyde group producing 5-
hydroxymethyl-2-furancarboxylic acid (HMFCA) or the alcoholic group. In the latter case, the
formation of 2,5-diformylfuran (DFF) occurs but it is mainly observed in the absence of an added
base and with metals other than Au.[76-78] Then, a second oxidation step takes place and 5-formyl-
2
-furancaboxylic acid (FFCA) is obtained. Consecutive oxidation of this molecule leads to the
12

formation of 2,5-furandicarboxylic acid (FDCA). These molecules were the only detected products
in this study.
3
.2.1 Preliminary screening
At first, reference catalysts prepared using metal salts impregnation, were tested for HMF
oxidation. Figure S10 depicts the catalytic activity and product distribution over the monometallic 2-
Pt-s-H and 2-Sn-s-H . Such preliminary tests revealed that among the two metals, just Pt is active
2
2
toward HMF oxidation in base-free conditions in agreement with previous reports. In detail, Pt is able
to convert HMF into HMFCA and DFF, being the latter more abundant than the former (2 and 18%,
respectively). However, HMF conversion and product yields were very low.
This result is in good agreement with the literature, since, under base-free conditions, DFF formation
is known to be favored over HMFCA, especially using Pt based catalysts. Both products of
consecutive oxidation, i.e. FFCA and FDCA were detected, and carbon balance was always higher
than 95%.
Superior results in terms of HMF conversion and target product (FDCA) yield were obtained by using
the materials prepared via cluster impregnation, indicating that such synthetic strategy has a positive
effect in determining and improving the catalytic properties of the material. This feature could be
ascribed to the higher dispersion and smaller mean metal particle size of the cluster derived material
(
2-Pt-H
2
) with respect to its metal salt impregnated counterpart (2-Pt-s-H ).
2
3
.2.2 Sn content effect on the catalytic activity
Comparison of catalytic performance of monometallic Pt and bimetallic Pt-Sn catalysts via the metal
cluster decomposition route is presented in Figure 7.
The catalytic results show that a Pt/Sn molar ratio equivalent to 2 (as in 2-Pt
detrimental for both HMF conversion and FDCA yield, while the equimolar catalyst provides higher
FDCA yield if compared to the monometallic catalyst (13 and 9% for 2-Pt Sn -H and 2-Pt-H
respectively). This result is of particular interest taking into account that Pt, which has been proven
to be the active metal, is more diluted in 2-Pt Sn -H than in the other two samples, thus the best
result in term of FDCA yield were obtained with the catalyst having the lowest active metal content
Table 5). In addition, by calculating and comparing the TON (turnover number) of the three samples
it is evident that the catalytically active species formed in the 2-Pt Sn -H sample are the most active
2
Sn
1
-H
2
) seems to be
1
1
2
2
,
1
1
2
(
1
1
2
in the studied reaction, since the TON related to this sample is one of order of magnitude higher than
the ones calculated for the other two samples.
13

The lower catalytic activity of 2-Pt
2
Sn
1
-H could be ascribed to the higher tendency of smaller Pt-
2
enriched nanoparticles to undergo overoxidation in the presence of high oxygen pressure and/or
chemical poisoning, leading thus to a less active catalyst.[79-83]
A similar catalytic behavior was displayed by the reference catalysts prepared from metal salts
impregnation being the best catalyst the material obtained with the equimolar loading of the two
metals. On the contrary, the sample having Pt/Sn molar ratio equal to 2 led to poor catalytic
performance (Figure S11). Furthermore, comparing the results of salts derived catalysts with those
obtained via cluster impregnation (Figure 8) demonstrated that better catalytic performance was
always obtained using the catalyst prepared from carbonyl cluster decomposition. This confirmed,
once again, the higher potential of cluster impregnated catalysts than the metal salt impregnated ones.
Finally, the effect of the thermal treatment on the catalytic behavior has been studied. In Figure 9, the
comparison of the catalytic results obtained using the Pt
1
Sn materials thermally treated under
1
different conditions is reported.
The catalytic results showed that thermal treatment plays a key role in terms of catalyst activity and
yield. Indeed, reducing atmosphere leads to higher catalytic performances if compared to those
obtained by using dried and calcined catalysts, indicating that not only the appropriate Pt/Sn ratio is
required to obtain active catalysts, but also control of Pt oxidation state (metallic versus oxidized Pt
species) and/or Pt/Sn aggregation play a key role in determining catalyst activity. Based on the
aforementioned catalytic and characterization data, it is evident that the presence of Sn with a specific
metal molar ratio of 1/1 has a significant influence and improvement in terms of activity. Moreover,
in terms of mean particle size, the presence of Sn in bimetallic Pt/Sn supported nanoparticles, via the
metal impregnation decomposition route and heat treatment under H
mean metal particle size of 2 nm and narrow particle size distribution.
In conclusion, by comparing the catalytic results showed by the most performing catalyst studied in
this work (i.e. 2-Pt Sn -H ) with the ones reported in the literature for similar catalytic systems, it is
2
forms small nanoparticles with
1
1
2
evident that the catalytic performance in terms of activity and yield of the presented bimetallic system
is at similar level with comparable catalytic systems operating at similar operative conditions.[8, 23,
2
4] On the other hand, if the same comparison is made by taking into account as well Pt-based systems
operating under basic conditions, the 2-Pt Sn -H system is outperformed by other catalytic systems,
such as Pt-Bi/TiO , Pt/ZrO [26] or platinum nanoparticles on a cerium coordination polymer.[25]
1
1
2
2
2
14

3
.2.3 Catalyst stability and reproducibility
To determine long-term catalytic performance, the catalyst stability has been studied, by reusing the
spent catalysts in the case of 2-Pt-H and 2-Pt Sn -H under the same operative conditions. Such
tests demonstrated that 2-Pt-H catalyst suffers from significant deactivation, since HMF conversion
dramatically dropped from 63 to 24% after just one cycle, (Figure 10).
On the other hand, the 2-Pt Sn -H showed higher stability, since the catalyst second cycle
2
1
1
2
2
1
1
2
guaranteed the same HMF conversion and products yield respect to first use (Figure 11). The third
cycle of this catalyst showed lower conversion (49%) and lower FDCA yield (11%); nevertheless,
these values still remained higher than the monometallic counterpart.
In order to understand the reasons of the different deactivation trends of the two materials, TEM
analyses have been performed over the two used catalysts and mean metal particle size and metal
particle size distribution have been evaluated. Comparison of the mean metal nanoparticle size before
and after the reaction, shows that Pt nanoparticles in 2-Pt-H catalyst suffered from sintering during
2
the reaction, since mean Pt particle size increased from to 2.0 to 5.5 nm. In addition, some
nanoparticles having diameter higher than 10 nm were detected (Figure S12). On the other hand, 2-
Pt
1
Sn
1
-H did not undergo significant sintering, since mean metal particle size did not vary
2
significantly (Figure S13). Moreover, it seems that the Pt/Sn nanoparticles did not undergo any
change in chemical composition, since the EDX maps on the used catalyst showed that the two metals
are still present and in close proximity even after the reaction, as detected in the fresh sample (Figure
S12). Chemical analysis of the filtrate indicated that the amount of platinum and tin leached out at
the end of the reaction cycles was less than 1% of the original nominal loading confirming the high
relative stability of this catalyst formulation.
To better elucidate deactivation process, XPS analysis has been performed on the used catalyst (Table
S1). The analysis revealed that the Pt(0) species content drops from 49% in the fresh catalyst to 36%
(
i.e. by a factor of 0.73) after recycling tests in agreement with the decrease by a factor of 0.72 of
HMF conversion (from 68 to 49%) after 3 uses. Moreover, comparing these results with the ones
obtained for the series of Pt Sn -based catalysts (dried and heat treatment under air or H ) it is evident
1
1
2
that the concentration of Pt(0) species is strongly correlated to the catalyst ability to convert HMF,
since HMF conversion was found to be linearly correlated with the presence of metallic Pt species
(Fig. 12). In light of all these observations, it has been hypothesized that Sn enhances structural
nanoparticle stability, which is responsible for the higher 2-Pt Sn -H catalytic stability (maintaining
1
1
2
15

mean particle size and to a lesser extent Pt oxidation state), and high Pt(0) concentration is
fundamental to achieve and maintain high catalyst activity.
In addition to the aforementioned points and with the aim to validate the reproducibility of the
synthetic protocol, two additional batches of the 2-Pt
1
Sn
1
-H were prepared and tested. These two
2
additional tests showed that the experimental procedure employed for the synthesis of the catalyst
can afford a good level of catalyst reproducibility in both terms of catalytic performances and carbon
balance, being the latter >99% (see Figure S15).
4
Conclusions
The present work demonstrated the possibility to develop Pt-based active and stable catalysts
able to oxidize HMF in base-free aqueous conditions. The synthesis of TiO -supported Pt and Pt/Sn
2
nanoparticles from carbonyl cluster decomposition with a controlled structure shows the crucial role
played by the synthetic strategy in the preparation of active and stable Pt-based supported catalysts
and this preparation method has not reported previously for the specific reaction. In particular, the
use of carbonyl clusters as precursors for the synthesis led to the formation of metal particles having
well controlled dimension and relatively narrow particle size distribution, whose catalytic
performances were significant higher respect to the ones obtained by their counterparts prepared via
metal salt impregnation. Moreover, the addition of Sn to Pt in an equimolar amount was demonstrated
to lead to the formation of unique active sites with significant improved stability in the base-free
oxidation of HMF. Finally, the influence of heat treatment and usage of H showed to be beneficial
2
for the synthesis of active supported bimetallic Pt/Sn nanoparticles. In the future, we will focus on
the optimization of the preparation method (i.e. by investigating more sustainable solvent for cluster
production and/or choosing less expensive active metals) and experimental conditions to improve the
yield towards FDCA and overall process sustainability.
16

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Figures
Figure 1. FTIR spectra recorded during the Pt12 (a), Pt
1
Sn
1
(b), Pt
2
Sn (c) impregnation protocol and
1
recorded on 2-Pt
1
Sn derived-materials thermally treated under different conditions (d).
1
Figure 2. HR-TEM images and particle size distribution histograms of the 2-Pt
in air (a) and H (b) at 450°C.
1
Sn catalyst treated
1
2
Figure 3. STEM image along with the corresponding Pt-Mα, Sn-Lα and (Pt-M & Sn-L) elemental
maps from the same area regarding 2-Pt Sn catalyst treated in H (a) and air (b). (For interpretation
of the reference to colour in this figure, the reader is referred to the Web version of the article).
1
1
2
Figure 4. H
2
-TPR profiles of 2-Pt
1
Sn sample thermally treated in different conditions. (a) dried
1
sample, (b) treated at 450°C in air, (c) treated at 450°C in H
2
.
Figure 5. TEM images and particle size distribution histograms of the 2-Pt-H
samples; STEM image along with the corresponding Pt-Lα, Sn-Kα and (Pt-L & Sn-K) elemental
maps from the same area regarding 2-Pt Sn catalyst (c).
2
(a) and Pt
2
Sn
1
-H (b)
2
1
1
Figure 6. Comparison of Pt samples prepared by salts or cluster impregnation. Operative conditions:
temperature 110°C, time 4 h, O pressure 10bar, molar ratio HMF/total metal=50.
2
Figure 7. Catalytic performances of cluster impregnated catalysts. Operative conditions: temperature
10°C, O pressure 10bar, molar ratio HMF/total metal=50.
1
2
Figure 8. HMF Conversion and FDCA yield according the catalyst Sn/Pt molar ratio (solid line
cluster impregnated catalysts, dashed line metal salt impregnated catalysts). Operative conditions:
temperature 110°C, time 4 h, O pressure 10bar, molar ratio HMF/total metal=50.
2
Figure 9. Effect of the thermal treatment on the catalysts prepared via Pt
1
Sn impregnation. Operative
1
conditions: temperature 110°C, time 4 h, O pressure 10bar, molar ratio HMF/total metal=50.
2
Figure 10. 2-Pt-H
2
reuse tests. Operative conditions: temperature 110°C, O pressure 10bar, molar
2
ratio HMF/total metal=50.
22

Figure 11. 2-Pt
1
Sn
1
-H
2
reuse tests. Operative conditions: temperature 110°C, time 4 h, O pressure
2
1
0bar, molar ratio HMF/total metal=50.
Figure 12. Correlation between HMF conversion and Pt(0) content revealed by XPS analyses of the
Pt1Sn1-derived samples
23

Figure 1. FTIR spectra recorded during the Pt12 (a), Pt
1
Sn
1
(b), Pt
2
Sn (c) impregnation protocol and
1
recorded on 2-Pt Sn derived-materials thermally treated under different conditions (d).
1
1
24

Figure 2. HR-TEM images and particle size distribution histograms of the 2-Pt
in air (a) and H (b) at 450°C.
1
Sn catalyst treated
1
2
25

Figure 3. STEM image along with the corresponding Pt-Mα, Sn-Lα and (Pt-M & Sn-L) elemental
maps from the same area regarding 2-Pt Sn catalyst treated in H (a) and air (b). (For interpretation
of the reference to color in this figure, the reader is referred to the Web version of the article).
1
1
2
26

Figure 4. H
2
-TPR profiles of 2-Pt
1
Sn sample thermally treated in different conditions. (a) dried
1
sample, (b) treated at 450°C in air, (c) treated at 450°C in H
2
.
27

Figure 5. TEM images and particle size distribution histograms of the 2-Pt-H
samples; STEM image along with the corresponding Pt-Lα, Sn-Kα and (Pt-L & Sn-K) elemental
maps from the same area regarding 2-Pt Sn catalyst (c).
2
(a) and Pt
2
Sn
1
-H (b)
2
1
1
28

Figure 6. Comparison of Pt samples prepared by salts or cluster impregnation. Operative conditions:
temperature 110°C, time 4 h, O pressure 10bar, molar ratio HMF/total metal=50.
2
29