# Copper-Cobalt Bimetallic Nanoparticles for the Acceptorless Dehydrogenation of Alcohols: A Combined Experimental and Theoretical Study

Ning Huang, Chaima Hammed, Guillaume Wang, Sophie Nowak, Philippe Decorse, Delphine Schaming, Anis Gannouni, Wahiba Najjar, Carine Michel, Lorette Sicard, et al.

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Ning Huang, Chaima Hammed, Guillaume Wang, Sophie Nowak, Philippe Decorse, et al. Copper-Cobalt Bimetallic Nanoparticles for the Acceptorless Dehydrogenation of Alcohols: A Combined Experimental and Theoretical Study. ChemNanoMat, 2024, 10 (11), 10.1002/cnma.202400259. hal-05351671

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Distributed under a Creative Commons Attribution 4.0 International License
Copper-Cobalt Bimetallic Nanoparticles for the Acceptorless
Dehydrogenation of Alcohols: a Combined Experimental and
Theoretical Study.

Ning Huang,¹ Chaima Hammed,² Guillaume Wang,³ Sophie Nowak,¹ Philippe Decorse,¹
Delphine Schaming,¹ Anis Gannouni,² Wahiba Najjar,² Carine Michel,⁴ Lorette Sicard,¹,*
Jean-Yves Piquemal.¹,*

¹Université Paris Cité, CNRS, ITODYS, F-75013 Paris, France

²Laboratory of Materials Chemistry and Catalysis, Faculty of Sciences of Tunis,
University of Tunis El Manar, Tunis, Tunisia

³Université Paris Cité, CNRS, Matériaux et Phénomènes Quantiques, F-75013 Paris,
France

⁴ CNRS, ENS de Lyon, LCH, UMR 5182, 69342, Lyon cedex 07, France

Corresponding authors: lorette.sicard@u-paris.fr, jean-yves.piquemal@u-paris.fr
Abstract

Bimetallic Cu and Co nanoparticles were prepared using the polyol process. The nature of the polyol was found to be decisive to control the distribution of the two metals within the particles, resulting in either core-shell or quasi-alloyed structures, as shown by chemical analysis at the single particle level. The particles can be prepared over the entire composition range with a very good control of the Cu/Co molar ratio. The introduction of Cu in the synthetic system allows decreasing significantly the mean size of the resulting particles. The two sets of particles were used as unsupported catalysts for the solvent-free acceptorless dehydrogenation of octan-2-ol. Very good ketone yields were measured when using the quasi-alloyed particles while lower activities were obtained with core-shell structures. The Cu<sub>25</sub>Co<sub>75</sub> quasi-alloyed catalyst could be recycled at least ten times without a significant loss in catalytic activity and particle-scale chemical analyses confirmed that the recovered particles are still alloyed with no observed demixing. The catalytic results are discussed in the light of different exposed metal surface areas and possible synergetic effects using theoretical predictions.

Keywords: bimetallic, soft chemistry synthesis, catalysis, alcohol dehydrogenation, DFT calculations.
# 1. Introduction

The dependence of human society on energy needs and chemicals has reached an unprecedented level due to the rapid growth of the world's population. To face our current societal and environmental issues, more sustainable and efficient chemical processes have to be developed. Along these lines, catalysis has a prominent role to play. Very often, noble metals are involved in catalytic processes because of their excellent activities. Nevertheless, due to their scarcity and high cost, tremendous efforts have been directed towards the use of base transition metals, albeit with mitigated success. Because base transition metals generally display higher selectivities but lower activities than noble metals,[1] a strategy consists in developing bimetallic catalysts associating noble metals and non-noble or base metals. The combination of two base transition metals is even more appealing provided that synergetic effects can be obtained compared to their monometallic counterparts. The two metals to be combined should ideally possess similar physico-chemical properties such as crystal structure, redox potentials, cohesive energies and surface energies, limiting the combining possibilities.[2] Nevertheless, an increasing number of studies show that the formation of alloys made of reputed immiscible metals at the bulk state can be obtained in nanoparticles synthesized by solution-based methods as was shown recently for e.g. Cu and Ru[3] or Pd and Ru.[4]

Similarly, although Cu and Co are not miscible in the bulk state,[5,6] several studies have claimed the formation of Cu-Co nano-alloys, mainly for catalytic applications,[7–11] while interesting magnetic properties can also be expected.[12,13] Since the first reports by the *Institut Français du Pétrole* (IFP, now IFPEN) in the 1980's,[14] Cu-Co catalysts have indeed been extensively studied because of their high catalytic potential for higher alcohols synthesis (HAS) from syngas (a mixture of CO and H₂).[8,10,15–22] Intimate mixing of the two metals, i.e. the formation of a nano-alloy, is the key to obtain active catalysts. DFT calculations have shown that the addition of Cu (i) controls the Co ensemble size and block the active sites for C-O bond cleavage and (ii) provides undissociated CO/HCO and reduces the barriers for HCO insertion to form oxygenates precursors.[21]

Besides HAS, Cu-Co based metal catalysts are also active for a number of other reactions such as the degradation of pollutants in water using NaBH₄,[23] the hydrolytic dehydrogenation
of amino-borane,[7] the photocatalytic reduction of CO₂ into CO[24] or the electrocatalytic reduction of CO₂ to ethylene.[11] Although some successful achievements have been published these last years, the formation of a Cu-Co alloy still corresponds to an important challenge and several strategies have been implemented for this purpose. A first possibility consists in depositing Cu and Co species onto a support that helps to stabilize the bimetallic phase during subsequent thermal treatments leading to the final bimetallic phase.[7,10,17,19,22,25,26] Another option is to use a solid phase where the two metals are already in intimate contact, expecting that the mixing of the two metals will be preserved during subsequent thermal treatments.[8,20] The formation of the bimetallic phase from solution is more challenging but permits generally a control over sizes and shapes. For soft-chemistry routes, several reaction parameters such as the nature of the metal precursors and of the reducing agent and the presence of capping agents can be judiciously selected to allow the concomitant reduction of the two metals, avoiding the formation of other architectures such as core-shell NPs that are prone to precipitate because of the known immiscibility of the two metals in the bulk.[23]

The acceptorless alcohol dehydrogenation (AAD), allowing to access carbonyl compounds (aldehydes and ketones) from alcohols, is an interesting reaction from an atom economy point of view, since no hydrogen acceptor is needed and only valuable molecular hydrogen is generated as a by-product. However, this reaction suffers from thermodynamic limitations and high temperatures are generally necessary to achieve good activities.[27] Catalysts have a prominent role to play with this demanding reaction. Apart from noble metals (Pd, Pt, Ru, Au, Ag), non-noble transition metal based catalysts, and particularly Co and Cu, have also shown their interest.[28-30] Hence, Co supported on TiO₂ has demonstrated better activity than noble metal based catalysts for the dehydrogenation of cyclodecanol and excellent activity for the secondary alcohols tested.[31] As regards supported catalysts, the choice of the support and of the solvent plays an important role during the catalytic process.[32] Cu supported on oxides[33-36] or carbon[37,38] has also been reported for the dehydrogenation of alcohols and the influence of the oxide support demonstrated.[39] Finally, alloying has also proven to be a good strategy to enhance the activity and stability.[28,29,40] For example, the addition of a very small amount of Ni to Cu (1 ‰ to 1%) resulted in a decrease of the apparent activation energy and a
stabilization of Cu against sintering.[41] While Ni atoms are the active sites for C-H bond cleavage, they are also prone to deactivation by coking. The dispersion of Ni in the Cu matrix inhibited this deactivation.[42]

In our search for active catalysts for AAD, we recently reported the high activity and selectivity of unsupported Co NPs for the qualitative conversion of secondary alcohols while being inactive for primary ones.[43,44] The choice of using non-supported nanoparticles was driven by the aim of understanding the reactivity of the crystallographic facets of Co without interaction with the support which has a considerable influence.[30-32] A strong influence of the crystalline phase and of the facets exposed have been demonstrated on the stability of the catalyst. hcp Co nanorods were found more active than Co cubes constituted of a mixture of hcp, fcc and ε-Co phases and than mixed hcp and fcc-phases Co diabolos. A correlation between the strong binding of carboxylate ligands on the major exposed {1120} facets which prevents from a modification of the crystalline structure and the stability of the Co nanorods was evidenced.[45] Moreover, these NPs were obtained through a seed-mediated route using tiny Ru NPs to trigger Co nucleation and growth.[46] Depending on the reaction conditions, Ru NPs were also found at the surface of Co NPs and conferred superior catalytic stability compared to bare NPs.[47] Along these lines, CoₓRu₁₀₀⁻ₓ alloyed particles were prepared and revealed to be active catalysts with a clear synergetic effect between the two metals.[48]

To the best of our knowledge, the activity of Cu-Co bimetallic NPs has never been evaluated for the AAD. The known activity of both metals for this reaction, the effect of alloying as well as the magnetic properties of cobalt which facilitates the recovery of the catalyst and thus its recycling are yet undeniable assets. In this study, we report the soft-chemical synthesis of Cu-Co bimetallic nanoparticles with two different architectures, i.e. quasi-alloys and core-shell nanoparticles. It is shown that the main parameter driving the formation of the alloy is the nature of the polyol that is used as the solvent and the reducing agent. The two sets of nano-catalysts are tested for the solvent-free acceptorless dehydrogenation of octan-2-ol, a model alcohol substrate. Superior activities are measured for the particles characterized by intimately mixed Cu and Co atoms compared to core-shell objects. A rational is provided by *ab initio* simulations of the AAD of a model alcohol (isopropanol) on model Co-Cu actives sites.
## 2. Experimental

### 2.1. Materials

Copper(II) acetate ($Cu(CH_3COO)_2$, Alfa Aesar, > 98 %), cobalt(II) acetate ($Co(CH_3COO)_2$, Alfa Aesar, > 98 %), polyvinylpyrrolidone (PVP, MW 58000, Aldrich), sodium hydroxide (Alfa Aesar, 99.9 %), 1,2-butanediol (Aldrich, 99 %), diethylene glycol (Aldrich, 99 %) and (±)-octan-2-ol (Alfa Aesar, > 98 %) were used without further purification.

### 2.2. Syntheses

#### 2.2.1. Preparation of $Cu_xCo_{100-x}$ core-shell catalysts

The synthesis procedure was adapted from a previously reported one.[15] Briefly, 0.75 g of PVP and 0.60 g (150 mM) of NaOH were added in a three-neck round-bottomed flask containing 50 mL of diethylene glycol. The suspension was then heated under stirring (200 rpm) to 100 °C to promote the dissolution of NaOH and PVP. In a second step, the Cu and Co acetate precursors in the required molar ratio were quickly added to the orange-yellow solution. Mechanical stirring and ultrasonic dispersion were maintained for 60 min to ensure the complete dissolution of the metal precursors. A dark-green solution with a total metal concentration of 0.10 M was obtained. In order to prevent oxidation, the flask was purged with Ar for more than 30 minutes before starting the heating step and the synthesis was carried out under argon flushing. Finally, the solution was heated to the boiling point at a rate of 10°C/min and after refluxing for 90 min, the solution was cooled down to room temperature. The nanoparticles were recovered by centrifugation at 20,000 rpm and washed twice with absolute ethanol and twice with acetone, then dried in an oven at 50 °C overnight. The samples were labelled $Cu_xCo_{100-x}$-CS, where x is in the range 0-100, indicating the mol % of Cu in the bimetallic sample.

#### 2.2.2. Preparation of $Cu_xCo_{100-x}$ quasi-alloyed catalysts

$Cu_xCo_{100-x}$ quasi-alloyed catalysts were prepared using the same procedure as described above, except that the solvent was changed from diethylene glycol to 1,2-butanediol. The samples were denoted $Cu_xCo_{100-x}$-A, where x is in the range 0-100, indicating the mol % of Cu in the bimetallic sample.

## 2.3 Characterization techniques
Powder X-ray diffraction analyses (XRD) were performed using a PANalytical X'pert Pro diffractometer equipped with a Co anode (λKα = 1.7889 Å) and X'celerator detector. The size of the coherent diffraction domain of the CuₓCo₁₀₀₋ₓ particles was determined using MAUD software that is based on the Rietveld method combined with Fourier analysis and is well suited for broadened diffraction peaks. Instrument broadening was determined using PANalytical standards.

Elemental analyses of the CuₓCo₁₀₀₋ₓ powders were performed by energy dispersive X-ray fluorescence (XRF) using an epsilon 3XL spectrometer equipped with a silver X-ray tube from Panalytical and by ICP-OES using an ICAP 6300 apparatus from ThermoElectron.

Linear sweep voltammetry measurements were carried out at 25 °C and at 60 °C in the presence of [N(C₄H₉)₄]PF₆ as the supporting electrolyte with a sweep rate of 5 mV·s⁻¹. The working, reference and auxiliary electrodes were respectively a Pt microelectrode of 25 µm internal diameter, a saturated calomel electrode and a Pt wire.

X-ray photoelectron spectroscopy (XPS) measurements were performed using a K-Alpha⁺ spectrometer (Thermo Fisher Scientific, East Grinstead, UK) operated in the constant analyzer energy mode and equipped with a micro-focused and monochromatic Al Kα X-ray source (1486.6 eV, 400 µm, 12 kV, 6 mA) and a flood gun (automatic charge compensation system for insulating samples). The samples were clipped onto the sample holder and outgassed overnight in the fast-entry lock. The step size and pass energy were set to 1 eV/200 eV and 0.1 eV/40 eV for the survey and the high resolution regions, respectively. Data acquisition and processing were done using “Avantage” software, version 5.9902. The surface chemical composition was determined by using the manufacturer sensitivity factors. The peak binding energy positions were calibrated by setting the C 1s component due to hydrocarbon contamination to 285 eV.

The specific surface area of the samples was measured by N₂ physisorption at 77 K using a Belsorp-max apparatus from MicrotracBel. Specific surface area (SSA<sub>BET</sub>) was obtained over the relative pressure range (0.05–0.25) using the Brunauer, Emmet, and Teller (BET) equation. Before analysis, samples were degassed under primary vacuum (<150 mPa) at 393 K for 12 h using a belprep vac module from Microtrac Bel. Assuming a spherical shape, specific surface
areas (SSA<sub>TEM</sub>) can also be calculated using the mean diameter determined by TEM and the following equation : $SSA_{TEM} = \frac{6}{\rho_{Cu,Co_{100-x} \times d_{TEM}}} \rho_{Cu,Co_{100-x}}$ and $\overline{d_{TEM}}$ respectively the density of the alloy and the mean diameter measured by TEM.

Routine transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) characterizations were performed using JEOL ARM 200F (equipped with a cold-field emission gun operated at 200 kV and a CEOS aberration corrector of the objective lens) and JEOL JEM 2010 (LaB<sub>6</sub> filament and operating at 200 kV) transmission electron microscopes. Image analysis was carried out using ImageJ and Gatan Digital Micrograph softwares and allowed to extract the mean particle sizes (d̅<sub>TEM</sub>) and the standard deviations (σ). The samples were prepared by evaporating a drop of a diluted suspension in ethanol on a carbon coated copper grid.

High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) coupled to EDX spectroscopy experiments were performed using a JEOL JEM 2100 Plus microscope operating at an electron beam voltage of 200 keV (for linescan analyses) and a JEOL ARM 300F (for elemental mapping) microscope with a resolution of 0.063 nm. For EDX analyses, the probe current was 23 pA. Image analysis was carried out using gatan Digital Micrograph. The samples were prepared by evaporating a drop of a diluted suspension in ethanol on a carbon coated nickel grid.

## 2.4 Catalytic Testing

The performances of the Cu<sub>x</sub>Co<sub>100-x</sub> nanocatalysts were evaluated for the acceptorless dehydrogenation of octan-2-ol, a model alcohol. Cu<sub>x</sub>Co<sub>100-x</sub> NPs were dispersed directly (i.e. without a solvent) in octan-2-ol in a flask equipped with a condenser and the suspension was heated to 458 K with a heating rate of 10 K/min. The alcohol/(Cu+Co) molar ratio was fixed to 100/1. For the recycling tests, the particles were recovered after 24 h of reaction using a permanent magnet, washed with the substrate and reused under the same operating conditions. Conversion and selectivity were determined at different reaction times by extracting liquid aliquots and analyzing them by gas chromatography coupled to mass spectrometry using a Shimadzu GCMS-QP2010 SE equipped with a SH-Rxi-5MS column (0.25 mm internal diameter and 30 m length) and an automatic AOC-20i sampler. The temperature
program was the following: after 5 min at 313 K, heating to 443 K at 5 K/min ramping rate, then
5 min at 443 K. Given the overall oxidation reaction of the alcohol: CH<sub>3</sub>-CH(OH)-R' → CH<sub>3</sub>-
C(=O)-R' + H<sub>2</sub>, conversion could also be calculated by measuring the volume of generated H<sub>2</sub>
using a Bronkhorst flow meter linked to a micro-computer. The nature of the gas was checked
by analyzing gas aliquots with an Agilent 7820 Gas Chromatograph equipped with a TCD
detector and an Agilent CarboPLOT P7 column (detector temperature: 90 °C, oven
temperature: 35 °C).

**2.5 DFT Computational Details.**

Periodic DFT computations were performed with the Vienna Ab initio Simulation Package (VASP).[49,50] The exchange-correlation energy and potential were calculated with the generalized gradient approximation (GGA) using the PBE functional[51] with the dDsC dispersion correction[52,53] a combination that was proved to be the most accurate to describe the adsorption of molecules on metal surfaces.[54] The projector augmented wave method (PAW)[55,56] as used to describe the electron-ion interactions. A cut-off energy of 400 eV was applied to obtain a tight convergence of the plane-wave expansion. Electronic energies were obtained with a convergence criterion of 10<sup>-6</sup> eV. Spin-polarized calculations were performed using an initial magnetic moment of 1.63 µB per Co atom.[44,57] Close packed monometallic (Cu and Co) have been previously studied by Kazmierczak et al.[57] In this study, Core@Shell (Cu@Co and Co@Cu) and bimetallic (Cu<sub>8</sub>Co<sub>1</sub> and Co<sub>8</sub>Cu<sub>1</sub>) surfaces were modelled by a four-layers slab, separated by at least 10 Å of vacuum and using p (3 x 3) supercell. A dipole correction was included in the z direction perpendicular to the surface. Monkhorst-Pack meshes of 5 × 5 × 1 K-points were used for the Brillouin zone integration.[58] Half of the layers (bottom layers) were fixed in bulk truncated positions (with an optimized metal-metal distance of 2.56 Å and 2.47 Å for Cu and Co respectively, which corresponds to an overestimation of 1% versus experimental values), whereas the remaining half (upper layers) were allowed to relax together with the organic part until the forces were lower than 0.015 eV Å<sup>-1</sup>. Adsorption and reaction processes were realized on the upper surface of the slab. Frequencies were computed numerically within the harmonic approximation. The dimer method allowed us to determine the transition-state (TS) structures.[59,60] All TSs were verified
to present a single imaginary frequency whose normal mode corresponds to the reaction
coordinate. For the species adsorbed on the surfaces, Gibbs free energies were approximated
by the electronic energies. For gas-phase species, translational and rotational entropy
contributions were added at 458 K (the experimental temperature of the catalytic test) within
the perfect gas and the rigid rotator approximations.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** General synthetic scheme for the preparation of CuₓCo₁₀₀₋ₓ-CS and CuₓCo₁₀₀₋ₓ-A samples, respectively in diethyleneglycol and 1,2-butanediol.</figcaption></figure>

**3. Results and Discussion**

**3.1. Alloyed vs. core-shell structure of CuₓCo₁₀₀₋ₓ catalysts**

In order to find the optimal reaction conditions allowing preparing Cu-Co nano-alloys, several
syntheses were devised and performed by modifying the main reaction parameters such as
the nature of the metal precursor (acetylacetonate or acetate precursors) and the nature of
the alcohol or polyol (1-heptanol, ethyleneglycol, diethyleneglycol (DEG), 1,2-propanediol,
1,2-butanediol), which acts both as the solvent and a mild reducing agent. Different stabilizing
agents (oleylamine, PVP with different molecular weights) were also introduced in the reaction
medium in order to control the final size and shape of the particles while NaOH was added to
enhance the reduction ability of the polyol (see below). In our hands, the best results in terms
of size and shape control as well as chemical distribution at the atomic scale were obtained
with a procedure (see scheme 1) adapted from Mendes *et al.*[15]. However, using DEG as in the
original paper, core-shell particles with a Cu core and a Co corona were obtained rather than
alloys, as illustrated in Figure 1 for the Co₅₀Cu₅₀ composition (see Figure S1 in SI for the other
compositions). Syntheses carried out in ethyleneglycol, 1,2-propanediol or 1-heptanol, all
other conditions being equal, did not lead to the formation of the alloy either (see Figure S2).
Conversely, with 1,2-butanediol, a more homogeneous distribution of the two metals was
obtained with the formation of a quasi-alloyed phase, as is clearly evidenced with STEM – HAADF coupled to EDX spectroscopy (Figure 1G). Co and Cu are well alloyed in the center of the particle although one can note a Co enrichment in the periphery. The two sets of particles are well-crystallized but the examination of the HRTEM image suggests that sample Cu<sub>50</sub>Co<sub>50</sub>-A corresponds to a polycrystalline particle (see Figure 1F). For the Cu<sub>50</sub>Co<sub>50</sub>-CS sample, the Cu core can be clearly evidenced on the HRTEM image in Figure 1B and the measured distance, 0.210 nm, is in agreement with Cu(111). Another striking difference between the two sets of materials is that the mean particle size is lower in 1,2-butanediol than in DEG (respectively 10.3 nm, σ = 1.4 nm, and 35.5 nm, σ = 9.9 nm, for the Cu<sub>50</sub>Co<sub>50</sub>-A and Cu<sub>50</sub>Co<sub>50</sub>-CS samples; see Figure 2 and S3 for the other compositions). This can be explained by a higher nucleation rate in the former than in the latter, leading to a highest number of nuclei and, consequently, to a lower mean size. These observations are in line with linear sweep voltammetry experiments (see Figure S4). Even if these electrochemical measurements are performed under conditions that differ significantly from the synthesis, they clearly show that the reducing power of 1,2-butanediol is higher than that of DEG (with an oxidation onset observed for a lower applied voltage). Moreover, the presence of NaOH in the reaction medium is a way to increase the reducing power of the polyol. A pre-peak can be seen for the measurement performed in 1,2-butanediol with 150 or 300 mM NaOH, that has already been reported by our team[48] and others.[61,62] This is probably due to trace impurities that are oxidized prior to the 1,2-butanediol molecule but is beyond the scope of this study.

Generally, the possible formation of the alloy is evidenced using XRD by a peak shift with the molar fraction of the elements over the whole range of composition (Cu<sub>x</sub>Co<sub>100-x</sub>, 0 ≤ x ≤ 100). XRD patterns are given in Figure 2. Here, all the samples crystallize with the fcc structure, except pure Co which is a mixture of fcc and hcp phases, the former being the dominant one. The Co hcp phase can also be detected for Cu<sub>10</sub>Co<sub>90</sub> NPs prepared in the two polyols. As expected for quasi-alloyed samples, peaks were shifted from pure Cu to pure Co when increasing the Co content and a Vegard law was observed (see SI, Figure S5A). A Rietveld analysis carried out on the Cu<sub>50</sub>Co<sub>50</sub>-A sample confirmed the formation of a single phase (see Figure S5B). Surprisingly, diffraction peaks were also shifted for core-shell materials and a
Vegard law was satisfied (Figure S5C). For Cu<sub>25</sub>Co<sub>75</sub>-CS, the observed broad peaks are obviously the sum of two contributions originating from pure Co and Cu phases. For the Cu<sub>50</sub>Co<sub>50</sub>-CS sample, Rietveld analysis revealed the same result (see SI, Figure S5D). Consequently, the use of XRD only can be misleading and other techniques such as EDX mapping have to be implemented to assess the formation of an alloyed phase. Note that the same observation was reported recently for CuIr particles, where a broad peak located between those associated with Cu and Ir was observed and interpreted as a combination of Cu(111) and Ir(111), rather than the formation of an alloy, while EDX coupled to HAADF-STEM showed unambiguously the formation of core-shell nano-structures.[63]

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (A,E) TEM images, (B,F) HRTEM images, (C,D) HAADF-STEM, EDX mapping and (D,H) Co and Cu elemental line profiles of Co<sub>50</sub>Cu<sub>50</sub>-CS (A,B,C,D) and Co<sub>50</sub>Cu<sub>50</sub>-A (E,F,G,H) nanoparticles. EDX analyses are performed along the yellow line in C and G. Inset in (F): FFT image corresponding to the yellow box.</figcaption></figure>

Whatever the polyol used, a very good agreement was found between the nominal Cu amount and the amount determined by XRF or ICP (see Figure 2 C and G). As regards the mean particle size, the incorporation of Cu in the synthetic system led to a dramatic decrease of the mean particle size. Indeed, the mean size decreases from hundreds of nm to tens of nm by adding 10 mol% Cu in the system (Figure 2 D and H). This can be explained by the fact that Cu<sup>2+</sup>, which is more easily reduced than Co<sup>2+</sup>, is first reduced and leads to the formation of primary seeds that trigger particle formation through a heterogeneous nucleation pathway. Another
explanation is that the metal with the lowest surface tension, here Co,[2] generally tends to segregate to the surface[64] and stabilize smaller nanoparticles sizes.

<figure><img src="image_4.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>E</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>F</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>G</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>H</figcaption></figure>

<figure><img src="image_11.png" /></figure>

**Figure 2.** (A,B,E,F) X-ray diffraction patterns, (C,G) Cu amounts measured by XRF and ICP and (D,H) mean diameter determined by TEM of Co<sub>50</sub>Cu<sub>50</sub>–CS (A,B,C,D) and Co<sub>50</sub>Cu<sub>50</sub>–A (E,F,G,H) samples. B and F correspond to magnification of the zones indicated by dashed red boxes in A and E. Error bars in D and H correspond to standard deviation values.
XPS is a powerful characterization technique to analyze the chemical state of transition metals. However, it is reputed challenging for Cu because of the overlapping of shake-up components originating from Cu(II) species as well as of the overlapping of Cu(0) and Cu(I) peaks (that display very close binding energies).[65] As regards bimetallic particles, variations in binding energies relative to the pure elements are very often interpreted as the existence of electronic interactions between the metals but discordant results have been published these last years concerning nanostructures where Cu and Co are intimately mixed. For instance, with Pd<sub>59</sub>Cu<sub>30</sub>Co<sub>11</sub> alloys, the binding energy of the Cu 2p<sub>3/2</sub> peak was found to decrease from 932.6 eV for pure Cu to 931.9 eV for the alloy[66] while this peak shifted in the opposite direction, from 931.9 eV for pure Cu deposited onto Al<sub>2</sub>O<sub>3</sub> to 932.4 eV upon alloying with Co.[67] XPS survey spectra and high resolution Co and Cu 2p peaks are given in Figures S6 and 3 for the Cu<sub>50</sub>Co<sub>50</sub> composition. As regards Co, the spectra of the Cu<sub>50</sub>Co<sub>50</sub>-CS and Cu<sub>50</sub>Co<sub>50</sub>-A samples are dominated by signals corresponding to 2p<sub>3/2</sub> and 2p<sub>1/2</sub> components and satellites that can be assigned to Co(II) species, similarly to what was reported for pure Co particles prepared in polyols.[45,47] For the Cu<sub>50</sub>Co<sub>50</sub>-CS sample, a very weak signal at 778.1 eV is hardly detected and is attributed to Co(0). The presence of a small amount of Co metal for Cu<sub>50</sub>Co<sub>50</sub>-CS can be explained by the higher mean size for these particles compared to Cu<sub>50</sub>Co<sub>50</sub>-A ones; the Co corona is large enough to prevent total Co(0) oxidation when extracting the particles from the reaction mixture. Note that Co(II) species can be easily reduced back to Co(0) when placing them in a reducing environment such as octanol at high temperature, as was shown previously.[47]
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (A,B,E,F) High-resolution Cu and Co XPS spectra of samples Cu₅₀Co₅₀-CS (A,B) and Cu₅₀Co₅₀-A (E,F). (C,D,G,H) magnifications corresponding respectively to the dashed boxes in A, B, E and F. Co(DEG) and Cu(DEG) stand for Co and Cu particles prepared in diethyleneglycol while Co(BEG) and Cu(BEG) for particles prepared in 1,2-butanediol.</figcaption></figure>

The high resolution Cu 2p spectra exhibit different peaks that are attributed to Cu(0) and/or Cu(I) as well as Cu(II) species (see Figure 3). The shake-up satellite centered at ca. 943 eV and seen for the two types of samples is from Cu(II) only.[65] For the Cu₅₀Co₅₀-A sample, there is a significant increase, about 0.4 eV, of the Cu 2p<sub>3/2</sub> binding energy compared to pure Cu, suggesting an electron density migration from Cu to Co, in agreement with the formation of
an alloyed phase.[67,68] Such an increase was not observed for the Cu₅₀Co₅₀-CS sample (compare Figure 3 D and H). The evolution of the Cu 2p₃/₂ binding energy value as a function of the mol. % Cu amount is given in Figure S7. One can see a clear continuous decrease in binding energy when increasing the amount of Cu in CuₓCo₁₀₀-x-A materials.

Surprisingly, the expected upshift of the Co 2p₃/₂ binding energy that is normally associated with the downshift of the Cu2p₃/₂ peak is not observed for Cu₅₀Co₅₀-A when alloying with Cu (see Figure 3G and Figure S7).[67] This is tentatively explained by considering that for this non-perfect alloyed phase, there is a Co concentration gradient (the periphery is richer in Co) while the analysis depth of XPS is considered to be 5-10 nm.

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Effect of (A) the presence of different additives and (B) the NaOH concentration on the resulting XRD patterns, mean size and particle shape for the system Co(CH₃CO₂)₂/Cu(CH₃CO₂)₂/1,2-butanediol with the Cu₅₀Co₅₀ composition. The error bars given for the mean sizes correspond to standard deviation values.</figcaption></figure>

## 3.2. Role of the additives

The importance of the reducing ability of the two solvents on the final distribution of Co and Cu inside a NP has already been evidenced above. However, with this multi-parameter system, the question of the role of NaOH and PVP additives is also relevant. To have a better view on the influence of these reaction parameters on the crystal structure, size and shape of the resulting particles, several experiments were devised in 1,2-butanediol, adding 0, 1 (PVP or NaOH) or 2 (PVP and NaOH) additives. The obtained results are gathered in Figure 4. The
syntheses are performed with the Cu₅₀Co₅₀ composition. With only the two metal salts in 1,2-
butanediol, particles with a very broad size distribution are isolated (Figure 4A). HAADF-STEM
analyses reveal that copper particles are generated with Co that precipitates at the periphery,
albeit without generating a continuous shell (see Figure S8). Adding PVP in the system, but
without NaOH, a more homogeneous size distribution was obtained with a higher mean
particle size than in the absence of PVP (d̅<sub>TEM</sub> = 159.3 nm; σ = 36.0 nm). In this case, a clear
core-shell structure is obtained with Cu surrounded by a Co corona. With NaOH (300 mM) but
without PVP, the mean size was significantly decreased to what is obtained with PVP only but
with a broad size distribution (d̅<sub>TEM</sub> = 21.3 nm; σ = 11.9 nm). The elemental mapping
displayed in Figure S8 shows that in this case, the two metals are intimately mixed. Finally with
the reference synthesis, corresponding to the addition of 300 mM NaOH and PVP, small and
very monodisperse nano-crystals were recovered (d̅<sub>TEM</sub> = 10.3 nm; σ = 1.4 nm). As regards
the crystal structure, it is only with the last synthetic system that a single alloyed phase was
obtained. In the other cases, i.e. without NaOH and/or PVP, fcc Cu, fcc Co and hcp Co phases
were detected. One can also note that the XRD peaks are much broader with both PVP and
300 mM NaOH, in agreement with smaller coherent diffraction domains. These results clearly
stress out that both PVP and NaOH are necessary not only to control the size and the shape of
the nanocrystals but also their crystal structure and that NaOH seems to have the most notable
impact on the particle size. In agreement with the data presented in Figure S4, this can be
explained by the improved reducing ability of the solvent in basic media, hence favoring the
formation of more nuclei. Note that PVP is also known to display reducing properties,[⁶⁹] but
this effect can be neglected here, considering the higher reducing ability of the polyol. To go
further, the influence of NaOH concentration was evaluated (Figure 4B). With the
Co''(OAc)₂/Cu''(OAc)₂/1,2-butanediol/PVP system, a minimal NaOH concentration of 300 mM
is necessary to promote the formation of well-defined nano-objects. Below 300 mM, ill-
defined particles with a broad size distribution are recovered while with 300 mM NaOH and
for higher concentrations, quasi-spherical nano-crystals are generated. An increase of the
basicity through NaOH addition could promote the formation of hydroxyacetate or alcoxide
phases during the course of the reaction as was reported for pure Co.[⁷⁰] However, no
intermediate phase could be observed in our syntheses, in contrast with our previous work on
Co NPs.[71]

Some general guidelines have been defined here to promote the formation of CuCo NPs with controlled sizes and shapes as well as a good distribution of the two metals within the nano-crystals.

<figure><img src="image_14.png" /><figcaption>**Figure 5.** Catalytic performances towards the acceptorless dehydrogenation of octan-2-ol with (A,B) CuₓCo₁₀₀-ₓ-CS and (C,D) CuₓCo₁₀₀-ₓ-A NPs. (A,C) octan-2-ol conversion determined by GCMS (circle symbols) and by volumetry (line). (B,D) Ketone yield as a function of the composition. See experimental part for details and reaction conditions.</figcaption></figure>

### 3.3. Catalytic activity

The two sets of CuCo NPs were next tested for an important reaction: the acceptorless dehydrogenation of alcohols, using octan-2-ol as a model substrate. Interestingly, from an atom economy point of view, the catalytic tests can be performed without a solvent, the alcohol substrate being also the liquid reaction medium. However, this is a particularly demanding reaction that necessitates to operate at high temperature in order to enhance the entropic driving force via the -TΔS component,[27] while the equilibrium can be shifted to the formation of the products by continuously removing the molecular hydrogen by-product. The data gathered in Figure 5 show that the two types of nano-structures are active but higher
yields towards the carbonyl compound are obtained with CuₓCo₁₀₀-ₓ-A samples. Irrespective of the sample type, the ketone selectivity is always very close to 100 %. Since the evolved gas during the course of the reaction is found to be H₂ with a very high purity (see SI, Figure S9), conversion, hence the ketone yield, can also be determined either from the volume of gas generated during the course of the reaction, assuming that the perfect gas law is verified. A good correlation is found between the alcohol conversion determined by GCMS and by gas volumetry, showing unambiguously that the reaction proceeds through an acceptor-less dehydrogenation pathway.[⁴³,⁴⁸]

### 3.4. DFT calculations of iPrOH dehydrogenation

To rationalize the catalytic activity/bi-metallic structure relationship, we determined the dehydrogenation pathways of a model alcohol (iso-propanol iPrOH) on model surfaces that represent possible active sites found in alloyed and core-shell Cu-Co systems. The two possible pathways are schematically represented in Figure 6. The alkoxy path starts with the O-H bond breaking (TS<sub>OH</sub>), yielding an alkoxy intermediate (iPrO), from which the C-H bond scission takes place (TS<sub>OH-CH</sub>). The hydroxyalkyl path starts with the C-H bond dissociation (TS<sub>CH</sub>), leading to the formation of a hydroxyalkyl intermediate, and continues with the O-H scission (TS<sub>CH-OH</sub>).

<figure><img src="image_15.png" /><figcaption>**Figure 6.** Possible dehydrogenation pathways for iPrOH.</figcaption></figure>

Regarding the active site models, two extreme bimetallic cases were considered: core@shell (Cu@Co and Co@Cu), as well as single-atoms (Cu₁Co₈ and Co₁Cu₈). In the core@shell slab, the shell is represented by one layer over 3 core layers (e.g. one layer of Cu on three layers of Co for Co@Cu). In the single atom model, one surface atom is replaced, e.g. Co₈Cu₁ is a Co(111) slab with one surface Co atom substituted by a Cu atom, mimicking a possible site of a Co-rich
surface. The alkoxy path is systematically preferred for the alcohol dehydrogenation for all surfaces we considered. In agreement with our characterizations, we will focus mainly on (i) the monometallic surfaces (ii) Cu@Co, taken as a model of Cu<sub>50</sub>Co<sub>50</sub>-CS (iii) Cu<sub>1</sub>Co<sub>8</sub> as a model of Cu<sub>50</sub>Co<sub>50</sub>-A, in agreement with the literature.[57] The corresponding Gibbs free energy profiles are shown in Figure 7. The results obtained for the two remaining bimetallic models, Cu<sub>9</sub>Co<sub>1</sub> and Co@Cu, which significantly differ from the NPs prepared in this study but are nevertheless worthwhile, are presented in SI (Figure S10).

On Cu(111) (black profile, Figure 7), the adsorption of most molecules and intermediates is endergonic, the first O-H scission exhibits a high activation barrier (0.99 eV), resulting in a high energy span (1.67 eV). Those sites are not very active, in agreement with the low activity observed experimentally for the pure Cu catalysts. On Co (111) (grey profile, Figure 7), the iPrO* intermediate is strongly chemisorbed in a hollow site building three Co-O bonds (198 pm). As a result, the activation barrier for the O-H scission (0.74 eV) is lower than on Cu while the activation barrier for the C-H scission is higher (0.93eV). When a shell of Co is covering a Cu slab (Cu@Co model), the alkoxy intermediate and H* are even more strongly bonded to the catalytic surface. The chemisorption of the alkoxy shifts from -0.23 on Co(111) to -0.62 eV on Cu@Co, and a 0.39eV stabilization that is detrimental to the catalytic activity. In addition, the desorption of the produced H<sub>2</sub> is more endergonic (+0.57eV vs. +0.46eV on Co(111)), which may be a source of surface poisoning. As a result, the overall energy span raises to 1.68~eV. In other words, a thin shell of Co strained by an underlying Cu is clearly not a good active site. Conversely, when substituting one Co surface atom by a Cu atom in a Co(111) surface (Cu<sub>1</sub>Co<sub>8</sub>) stabilizes TS<sub>O-H</sub> scission in a greater extent than the iPrO intermediate (0.07 eV vs. 0.04 eV), yielding to a decrease in the energy span to 1.55~eV, the lowest of the four sites under consideration. Thus, a single atom of Cu in Co will tend to favor a greater catalytic activity in comparison with the monometallic and core-shell counterpart. According to our characterizations, those sites are more likely to be found in Co<sub>50</sub>Cu<sub>50</sub>-A than in Co<sub>50</sub>Cu<sub>50</sub>-CS (see the EDX profiles in Figure 1). And indeed, the alloy configuration is clearly more active catalytically than the core-shell one (see the yields in Figure 5). Besides, it is expected that increasing the amount of Cu towards 100% of Cu reduces the number of isolated sites through
aggregation while reducing the amount of Cu towards 0% limits the exposure of Cu. The Cu<sub>1</sub>Co<sub>8</sub>

is similar to a single metal alloy,<sup>[72]</sup> but reverse in its architecture since the group 11 metal
(here Cu) is diluted in the group 8, 9, 10 metal (here Co). The chemisorbed surface species
build bonds with Co atoms only, but this interaction is nevertheless tuned by the presence of
a neighbor Cu atom. For instance, in the TS<sub>OH</sub>, the O-H distance is increased from 1.26 Å on
Co(111) to 1.33 Å on Cu<sub>1</sub>Co<sub>8</sub>, with a Cu-H of 2.03 Å (see Figure 8).

<figure><img src="image_16.png" /></figure>

**Figure 7.** Gibbs free energy profiles of the iPrOH dehydrogenation via the alkoxy pathway on Cu(111) (black), Co(111) (grey) Cu<sub>1</sub>Co<sub>8</sub>(111) (pink), and Cu@Co(111) (orange). Gibbs energies are given in eV at T=458K. The corresponding electronic energies are computed at the PBD-dDsc level on a p(3x3) slab; the * symbol means adsorption on the surface.

<figure><img src="image_17.png" /></figure>

**Figure 8.** Optimized structures of TSOH on Co(111) and Co<sub>8</sub>Cu<sub>1</sub>. Co in pink, Cu in orange, C in grey, O in red, H in white. Distances are indicated in Å.

**3.5. Recycling**

Recycling is a very important point to consider with metal NPs, since with these unsupported
nano-catalysts, morphology evolutions can arise, that in turn can have an impact on the
catalytic performances. Recycling tests were thus performed with CuₓCo₁₀₀-ₓ-A with x = 25, 50 and 75 since they gave the best catalytic results. Recycling was not performed for core-shell catalysts since i) they display lower initial performances and ii) they all present more or less the same exposed Co surface. The possible size, shape and crystal structures evolutions were monitored after catalysis. All the results are presented in Figure 9.

<figure><img src="image_18.png" /><figcaption>**Figure 9.** (A,D,G) Catalytic performances, (B,E,H) X-ray diffraction patterns and (C,F,I) TEM images and size distribution after the 4th catalytic cycle of (A,B,C) Cu₂₅Co₇₅-A, (D,E,F) Cu₅₀Co₅₀-A and (G,H,I) Cu₇₅Co₂₅-A NPs.</figcaption></figure>

For each nano-catalyst, there is a significant drop in ketone yield after the first test, albeit in different proportions, depending on the used catalyst. More importantly, while the octan-2-one yield continuously decreases for Cu₅₀Co₅₀-A and Cu₇₅Co₂₅-A samples through continuous recycling, the activity is found to be more or less constant after the second test for the Cu₂₅Co₇₅-A catalyst. Whatever the composition, the XRD patterns are very similar for fresh and used catalysts, indicating that no phase transition occurred during catalysis. However, one can note that, contrary to what is observed for Cu₂₅Co₇₅-A, there is a clear sharpening of the diffraction peaks after the 4th test for Cu₅₀Co₅₀-A and Cu₇₅Co₂₅-A samples suggesting an increase of the size of the coherently scattering domain (see Figure 9 B, E and H). After the 4th
test, samples were further analyzed by TEM to detect possible size and/or morphological
evolutions. While before test, all the catalysts correspond to quasi-rounded particles with a
homogeneous size distribution (see Figure 1 and Figure S2B, SI), after test, important
morphological evolutions were detected for Cu<sub>50</sub>Co<sub>50</sub>-A and Cu<sub>75</sub>Co<sub>25</sub>-A samples. After the 4<sup>th</sup>
test, a bimodal population is observed for the two sets of particles. This indicates that
aggregation or dissolution-reprecipitation mechanisms occurred during the test. This has
already been observed with pure Co particles.[47] Because of the reducing conditions
corresponding to the catalytic test (the substrate is a reducing agent and H<sub>2</sub> is generated) the
Cu and Co species that are leached from the particles are reduced back to metal and adsorb
on the particles. Aggregation and ripening processes are driven by a minimization of the
surface energy that is higher for the smallest nanoparticles. These phenomena are not
observed for the Cu<sub>25</sub>Co<sub>75</sub>-A sample, probably due to its bigger size (17.3 nm vs 10.3 nm and
12.9 nm for the Cu<sub>50</sub>Co<sub>50</sub>-A and Cu<sub>75</sub>Co<sub>25</sub>-A samples, respectively) and thus lower surface
energy. Also, when increasing the Cu content, a diffusion of these atoms towards the
surface of the nanoparticles during the catalytic tests may occur, leading to ripening.

<figure><img src="image_19.png" /><figcaption>**Figure 10.** (A) Catalytic recycling of the Cu<sub>25</sub>Co<sub>75</sub>-A sample. (B) X-ray diffraction patterns of the native Cu<sub>25</sub>Co<sub>75</sub>-A sample and after ten consecutive tests. (C) HAADF-STEM image, Co and Cu elemental mapping and Co and Cu line profiles for Cu<sub>25</sub>Co<sub>75</sub>-A NPs. EDX</figcaption></figure>
analyses are performed along the yellow line in the HAADF-STEM image.

Since the Cu<sub>25</sub>Co<sub>75</sub>-A catalyst eventually appears to be the most stable one, its stability was tested for a much more prolonged time. Ten recycling cycles were carried out, corresponding to a total reaction time of 240 h. A very good catalytic stability can be noticed (see Figure 10A). Again, the used catalyst was recovered and analyzed by X-ray diffraction. Structural evolutions were evidenced after the 10<sup>th</sup> test (Figure 10B) since a mixture of fcc and hcp phases was obtained. The particles were subsequently analyzed by HAADF-STEM to evidence possible intra-particle composition evolutions (Figure 10C). Depending on where the analysis is located, different Cu and Co line profiles were obtained indicating different Cu/Co molar ratios. These variations in chemical composition from one particle to another are known and have already be described in the literature for different bimetallic systems.[48,73-75] More importantly, no phase segregation was evidenced and the particles still corresponded to well-alloyed nano-structures after catalysis, in line with a very good catalytic stability.

### 3.6. Discussion

Our results show that the quasi-alloyed NPs display better catalytic performances than the Cu@Co core-shell catalysts, suggesting at first sight a synergetic effect between Co and Cu. Co-Cu catalysts have been extensively studied for the synthesis of higher alcohols from syngas and it is admitted that an intimate mixing of the two metals is key to obtain the best catalytic performances.[21] In this study, all the tests have been performed using the same alcohol/[Co+Cu] molar ratio. Consequently, if the catalytic activity is calculated by considering the amount of metal introduced in the system, Cu<sub>x</sub>Co<sub>100-x</sub>-A catalysts appear to be the best ones. On the one side, calculating turnover numbers from the amount of substance of the catalyst is important from an economic point of view and is commonly used in olefin oligomerisation and polymerisation reactions.[76] On the other, it is important to access the activity per active site. However, in this work, the nature of the active site is not known and it is thus not possible to easily calculate the turnover number expressed as mol of substrates converted by mol of active sites.

Nevertheless, the two type of samples have different particle sizes (see Figure 2 and S2), hence
different specific surface areas. This could explain the observed differences in activity. The two
set of samples were thus characterized by N₂ physisorption at 77K. All the materials display a
type II isotherm according to the IUPAC classification (see Figure S11), characteristic of non-
porous samples.[77] The specific surface area values determined using the BET equation, are
reported in Figure S12 together with those calculated using the mean diameter extracted from
TEM images and assuming a spherical shape. There is a good agreement between the two sets
of data confirming that the particles are not aggregated.

<figure><img src="image_20.png" /><figcaption>**Figure 11.** Octan-2-one yield as a function of the specific surface area for CuₓCo₁₀₀-ₓ-A and CuₓCo₁₀₀-ₓ-CS samples. The dotted line is a guide to the eye.</figcaption></figure>

The octan-2-one yield has been plotted as a function of the specific surface area in Figure 11 for the two sets of samples. One can note that a linear correlation is clearly observed: the higher the specific surface area, the higher the catalytic activity. This indicates that the main factor governing the catalytic activity is the number of surface atoms, irrespectively of their nature. Finally, adding Cu in the system is a way to decrease the particle mean size and to optimize metal resources.

**4. Conclusion**

To sum up, CuₓCo₁₀₀-ₓ NPs were prepared by soft chemistry in two polyols with different
reducing abilities. Using the commonly used diethyleneglycol, core-shell particles with a Cu
core and a Co corona were obtained. With 1,2-butanediol, a quasi-alloyed structure was
generated despite Co and Cu are reputed immiscible in the bulk, although with some
concentration gradients. Electrochemical measurements clearly show that the ether glycol has a lower reducing ability than the α diol. This can explain the observed results but other effects cannot be discarded, such as different reactivities of the dissolved metal species due to the possible complexation of the polyol molecule to the metal center. A multi-parameter study aiming at identifying the main parameters governing the formation of the desired phase was performed. It appears that PVP and NaOH are both required to obtain well-defined particles with unimodal size distribution. Moreover, PVP helps to control the shape and size dispersity of the nano-crystals while NaOH allows to increase the reducing power of the polyol, hence to decrease the particle mean size. For the first time, CuₓCo₁₀₀⁻ₓ NPs were used and shown to be active for the catalytic acceptorless dehydrogenation of octan-2-ol. For a same alcohol/[Co+Cu] molar ratio, the results show that core-shell nanocatalysts display lowest yields than those measured with quasi-alloyed nano-structures. Those catalysts may exhibit single metal atom alloys with Cu diluted in a Co surface, which were predicted being more active than pure Cu or Co using periodic DFT computations. The best catalyst, corresponding to the Cu₂₅Co₇₅ composition, could be recycled at least ten times without a significant decrease in activity while keeping its nano-alloyed architecture. However, for a given molar quantity of metals, our results also show that the catalytic activity is intimately related to differences in specific surface areas rather than differences in surface reactivities. Adding Cu in the system helps to decrease the particle size, hence leading to an increase in specific surface area. This leads to an optimization of the use of the two metals added in the system, optimizing the use of metal resources. The catalysts developed in this work will be further tested for the conversion of bio-sourced alcohols and polyols, such as glycerol.

## Acknowledgements

The work was done as a part of the ANR GLYNANO project (ANR-21-CE43-0016). XPS equipment was funded by the Région Ile-de-France, convention SESAME n°16016303 and the Labex SEAM (Science and Engineering for Advanced Materials and devices). Ning Huang thanks the Chinese Scholarship Council for financial support. DIM Respore is gratefully acknowledged for funding the GC-MS equipment. The authors would like to thank Esther
Nublet from the *Laboratoire d'Electrochimie Moléculaire* for GC analysis of the gas phase aliquots.

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