Alexandra Bouriakova

Pedro S F Mendes

Benjamin Katryniok

Jeriffa De Clercq

Joris W Thybaut
email: joris.thybaut@ugent.be

Robert Wojcieszak

Christophe Detavernier

Insight into the mechanism of glycerol hydrogenolysis to 1,2-propanediol: benefits of Cu promotion by La

published or not. The documents may come

Co-metal induced stabilization of alumina-supported copper: impact on the hydrogenolysis of glycerol to 1,2-propanediol

Introduction

Glycerol has emerged as a biomass-based feedstock in the chemical, solvent and fuel industry. [START_REF] Behr | The Future of Glycerol. New Usages for a Versatile Raw Material[END_REF][START_REF] Bauer | Is there a future in glycerol as a feedstock in the production of biofuels and biochemicals?[END_REF][START_REF] Pagliaro | Glycerol: properties and production, The future of glycerol[END_REF] The majority (60-70 %) of the natural glycerol is co-produced with biodiesel by transesterification, the resulting mixture containing approximately 10 vol.% of glycerol. [START_REF] Pagliaro | Glycerol: properties and production, The future of glycerol[END_REF][START_REF] Ciriminna | Understanding the glycerol market[END_REF] It is one of the key platform chemicals in these industries thanks to its unique chemical versatility originating from the three hydroxyl groups on its C3-backbone. In a hydrogenolysis reaction, the cleavage of a C-O or C-C bond occurs simultaneously with hydrogen addition. [START_REF] Zhou | Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals[END_REF] The cleavage of a primary C-O bond in glycerol results in 1,2-propanediol (i.e. propylene glycol), while that of the secondary C-O yields 1,3propanediol. 1,2-propanediol has a wide range of applications depending on its purity, including food, pharmaceuticals, cosmetics, as a monomer for resins, aircraft deicing fluids, automotive coolants, paints and coatings, liquid detergents, inks, varnishes, tobacco humectants, etc. [START_REF] Sara | Chapter 5 -Propylene Glycol: An Industrially Important C3 Platform Chemical[END_REF] Currently, it is produced from the petroleum derived and explosive propylene oxide, in the presence of an acid catalyst. [START_REF] Sara | Chapter 5 -Propylene Glycol: An Industrially Important C3 Platform Chemical[END_REF] Thus, the valorization of glycerol contributes to the development of safer and sustainable production route for currently petroleum-based 1,2-propanediol.

Cu-based catalysts are often applied in industry due to their lower price compared to noble metals and their enhanced selectivity to C-O bond cleavage, which is higher compared to Co-or Nicatalysts. [START_REF] Nanda | Recent advancements in catalytic conversion of glycerol into propylene glycol: A review[END_REF][START_REF] Wang | Catalytic hydrogenolysis of glycerol to propanediols: a review[END_REF] Initially, many industrially used commercial Cu catalysts have been investigated for glycerol hydrogenolysis. [START_REF] Dasari | Low-pressure hydrogenolysis of glycerol to propylene glycol[END_REF][START_REF] Ma | A non-alkoxide sol-gel route to highly active and selective Cu-Cr catalysts for glycerol conversion[END_REF][START_REF] Xiao | Synergetic effect between Cu0 and Cu+ in the Cu-Cr catalysts for hydrogenolysis of glycerol[END_REF][START_REF] Bienholz | Hydrogenolysis of glycerol over a highly active CuO/ZnO catalyst prepared by an oxalate gel method: influence of solvent and reaction temperature on catalyst deactivation[END_REF][START_REF] Balaraju | Selective Hydrogenolysis of Glycerol to 1, 2 Propanediol Over Cu-ZnO Catalysts[END_REF][START_REF] Wang | Selective hydrogenolysis of glycerol to propylene glycol on Cu-ZnO catalysts[END_REF] Later, other Cu-based supported over MgO [START_REF] Balaraju | Catalytic hydrogenolysis of biodiesel derived glycerol to 1,2-propanediol over Cu-MgO catalysts[END_REF][START_REF] Yuan | Biodiesel derived glycerol hydrogenolysis to 1, 2-propanediol on Cu/MgO catalysts[END_REF][START_REF] Yuan | Hydrogenolysis of glycerol over homogenously dispersed copper on solid base catalysts[END_REF], SiO2 [START_REF] Bienholz | Selective hydrogenolysis of glycerol over copper catalysts both in liquid and vapour phase: Correlation between the copper surface area and the catalyst's activity[END_REF][START_REF] Shan | Identification of the dehydration active sites in glycerol hydrogenolysis to 1,2-propanediol over Cu/SiO2 catalysts[END_REF][START_REF] Vasiliadou | Synthesis and performance of highly dispersed Cu/SiO2 catalysts for the hydrogenolysis of glycerol[END_REF][START_REF] Zhu | A highly efficient and robust Cu/SiO2 catalyst prepared by the ammonia evaporation hydrothermal method for glycerol hydrogenolysis to 1,2propanediol[END_REF][START_REF] Wu | Cu/boehmite: A highly active catalyst for hydrogenolysis of glycerol to 1,2-propanediol[END_REF] or Al2O3 [START_REF] Dieuzeide | Vapor-Phase Hydrogenolysis of Glycerol to 1,2-Propanediol over Cu/Al2O3 Catalyst at Ambient Hydrogen Pressure[END_REF][START_REF] Gandarias | Liquid-phase glycerol hydrogenolysis by formic acid over Ni-Cu/Al2O3 catalysts[END_REF][START_REF] Liu | Glycerol Hydrogenolysis to 1, 2-Propanediol by Cu/ZnO/Al2O3 Catalysts[END_REF][START_REF] Sun | Effect of Ag loading on Cu/Al2O3 catalyst in the production of 1,2propanediol from glycerol[END_REF][START_REF] Vila | Glycerol hydrogenolysis to 1,2propanediol with Cu/γ-Al2O3: Effect of the activation process[END_REF][START_REF] Wang | Insight into the Effect of Dual Active Cu0/Cu+ Sites in a Cu/ZnO-Al2O3 Catalyst on 5-Hydroxylmethylfurfural Hydrodeoxygenation[END_REF][START_REF] Wolosiak-Hnat | Influence of Parameters on Glycerol Hydrogenolysis over a Cu/Al2O3 Catalyst[END_REF][START_REF] Wołosiak-Hnat | Influence of reduction time of copper based catalysts: Cu/Al2O3 and CuCr2O4 on hydrogenolysis of glycerol[END_REF][START_REF] Meher | Catalytic Hydrogenolysis of Glycerol to Propylene Glycol over Mixed Oxides Derived from a Hydrotalcite-Type Precursor[END_REF][START_REF] Deng | Higher alcohols synthesis from syngas over lanthanum-promoted CuZnAl catalyst[END_REF][START_REF] Mane | Cu: Al nano catalyst for selective hydrogenolysis of glycerol to 1, 2-propanediol[END_REF] were also reported by various researchers.

Significant efforts have been made in preparing effective and stable Cu-based catalysts in the last two decades. Typically, promoters are required to limit the tendency of Cu to sinter. [START_REF] Twigg | Deactivation of supported copper metal catalysts for hydrogenation reactions[END_REF][START_REF] Miller | Catalyst for polyol hydrogenolysis[END_REF][START_REF] Zhiguo | Method for preparing propylene glycol by glycerin hydrogenolysis[END_REF][START_REF] Mane | Effect of Cometals in Copper Catalysts for hydrogenolysis of Glycerol to 1, 2-Propanediol[END_REF] Undoubtedly, the Cu dispersion and/or interaction between Cu species, the support, and a promotor are the main factors in determining the catalytic activity, selectivity and stability. In our previous work [START_REF] Bouriakova | Co-metal induced stabilization of alumina-supported copper: impact on the hydrogenolysis of glycerol to 1,2propanediol[END_REF], Ba, Ce, Cs and La were investigated as dopants, c.q. promoters, for the hydrogenolysis of glycerol into 1,2-propanediol over a Cu/γ-Al2O3 catalyst. The addition of Ba or Ce increased the initial activity, while La allowed to establish the most stable activity in terms of glycerol conversion over time (relative decrease of ca. 25 % over 68 h time on stream).

Two alternative mechanisms are mainly reported in the literature for the production of 1,2-propanediol from glycerol: the dehydration-hydrogenation mechanism (involving acidic sites) [START_REF] Alhanash | Gas-phase dehydration of glycerol to acrolein catalysed by caesium heteropoly salt[END_REF][START_REF] Vila | Significance of isomeric reaction intermediates in the hydrogenolysis of glycerol to 1,2-propanediol with Cu-based catalysts[END_REF] and the dehydrogenation-dehydration-hydrogenation mechanism (involving basic sites) [START_REF] Montassier | Aqueous polyol conversions on ruthenium and on sulfur-modified ruthenium[END_REF]. It is commonly accepted that γ-Al2O3 has intrinsic acidity [START_REF] Pines | Alumina: Catalyst and Support. I. Alumina, its Intrinsic Acidity and Catalytic Activity1[END_REF], automatically leading to the assumption that the hydrogenolysis of glycerol proceeds according to the dehydration-hydrogenation mechanism on Cu based catalysts. In this case, the dehydration step would occur on an acid site, available on the support, while the hydrogenation step is then catalyzed by a Cu 0 particle. However, various researchers are convinced that Cu 0 is the active site for both steps in the dehydration-hydrogenation mechanism. [START_REF] Vasiliadou | Kinetic study of liquid-phase glycerol hydrogenolysis over Cu/SiO 2 catalyst[END_REF][START_REF] Vasiliadou | Investigating the performance and deactivation behaviour of silicasupported copper catalysts in glycerol hydrogenolysis[END_REF][START_REF] Sato | Vapor-phase reaction of polyols over copper catalysts[END_REF][START_REF] Rajkhowa | A comprehensive kinetic model for Cu catalyzed liquid phase glycerol hydrogenolysis[END_REF] Other researchers believe in the synergetic effect between Cu + and Cu 0 , or in the role of Cu + as active site, i.e., assuming that Cu is not entirely present in the pure metallic state. [START_REF] Xiao | Synergetic effect between Cu0 and Cu+ in the Cu-Cr catalysts for hydrogenolysis of glycerol[END_REF][START_REF] Shan | Identification of the dehydration active sites in glycerol hydrogenolysis to 1,2-propanediol over Cu/SiO2 catalysts[END_REF][START_REF] Zhu | A highly efficient and robust Cu/SiO2 catalyst prepared by the ammonia evaporation hydrothermal method for glycerol hydrogenolysis to 1,2propanediol[END_REF][START_REF] Mane | Cu: Al nano catalyst for selective hydrogenolysis of glycerol to 1, 2-propanediol[END_REF][START_REF] Yin | The Nature of Active Copper Species in Cu-HMS Catalyst for Hydrogenation of Dimethyl Oxalate to Ethylene Glycol: New Insights on the Synergetic Effect between Cu0 and Cu+[END_REF][START_REF] Li | Hydrogenolysis of glycerol over bimetallic CuNi catalysts supported on hierarchically porous SAPO-11 zeolite[END_REF][START_REF] Samson | Influence of ZrO2 Structure and Copper Electronic State on Activity of Cu/ZrO2 Catalysts in Methanol Synthesis from CO2[END_REF] Vila et al. [START_REF] Vila | Glycerol hydrogenolysis to 1,2propanediol with Cu/γ-Al2O3: Effect of the activation process[END_REF] pointed out that Cu + species in a Cu/γ-Al2O3 catalyst improved the glycerol conversion, while the differences in 1,2-propanediol selectivity could be related to the different Cu 0 /Cu + in the catalysts. Generally, Cu catalysts show a high selectivity towards 1,2-propanediol, however, some C-C cleavage to 1,2-ethanediol can be observed. The retro-aldol condensation reaction that leads to the C-C cleavage, requires hydrogen.

Hence, it is commonly accepted that it is catalyzed by a metallic site, in this case Cu 0 . [START_REF] Nakagawa | Perspective on catalyst development for glycerol reduction to C3 chemicals with molecular hydrogen[END_REF][START_REF] Liu | Synergistic effect between copper and different metal oxides in the selective hydrogenolysis of glucose[END_REF][START_REF] Kandasamy | The route towards sustainable production of ethylene glycol from a renewable resource, biodiesel waste: a review[END_REF] However, the effect of strong acidic sites cannot be ruled out in a C-C cleavage. [START_REF] Liu | Synergistic effect between copper and different metal oxides in the selective hydrogenolysis of glucose[END_REF][START_REF] Liu | The Promoting Effect of Ni on Glycerol Hydrogenolysis to 1,2-Propanediol with In Situ Hydrogen from Methanol Steam Reforming Using a Cu/ZnO/Al2O3 Catalyst[END_REF] To elucidate the role of the different Cu species, a well-controlled preparation of catalysts with various Cu + or Cu 0 amounts is necessary. La was found to impact on the presence of Cu + and Cu 0 sites in Cu/SiO2 catalysts. [START_REF] Huang | Modification of Cu/SiO2 Catalysts by La2O3 to Quantitatively Tune Cu+-Cu0 Dual Sites with Improved Catalytic Activities and Stabilities for Dimethyl Ether Steam Reforming[END_REF][START_REF] Zheng | Lanthanum oxide-modified Cu/SiO2 as a highperformance catalyst for chemoselective hydrogenation of dimethyl oxalate to ethylene glycol[END_REF] Insights in this respect on γ-Al2O3 supported Cu catalysts for glycerol hydrogenolysis are, however, still lacking.

In this work, we aim to investigate the effect of La promotion on γ-Al2O3 supported Cu species to gain insight into their catalytic mechanism for glycerol hydrogenolysis. To this end, a series of La-Cu catalysts was prepared by sequential impregnation. By varying the La and Cu loading, twelve different promotion ratios were obtained with various distribution of the Cu + and Cu 0 species. The catalysts were comprehensively characterized in terms of textural and physicochemical properties, crystallographic and morphological structure, reduction behavior and surface species to better understand the effect of the promotion ratio. We selected five La-Cu catalysts, with specific characteristics, to perform liquid phase glycerol hydrogenolysis in a fixed bed reactor operated in trickle flow regime and compared them to a bare Cu catalyst. The effect of the La/Cu mass ratio was investigated by comparing the surface species, catalysts activity and 1,2-propanediol selectivity for these six catalysts. The catalysts stability was investigated by comparing the Cu 0 particle size prior and post reaction.

Experimental

Catalyst synthesis

All the catalysts in this work, indicated as xLa-yCu/γ-Al2O3, in which x and y are the corresponding weight percentage (wt.%), were synthesized using a sequential wet impregnation method. The La and Cu loading varied between 0 and 2.5 wt.%, and 10 and 25 wt.%, respectively. In this method, the support, γ-Al2O3 (Sasol, Puralox SCCa-150/200), was suspended in 10 ml demineralized water per gram of support. At the same time, the promoter precursor, La(NO3)3.6H2O (Sigma-Aldrich, ≥ 99 %), was dissolved in 6.6 ml distilled water per wt.% of La. The suspensions were mixed and stirred at 500 rpm at room temperature for 2 h. Subsequently, the water was evaporated at 363 K, after which the sample was further dried in air at 393 K for 1.5 h and calcined at 673 K, using a ramp of 2 K min -1 in static air for 2 h. In the second step, the x wt.% La/γ-Al2O3 powder was suspended in 150 mL demineralized water, mixed and stirred at 500 rpm for 2 h with the desired amount of metal precursor, Cu(NO3)2.5/2H2O (Sigma-Aldrich, ≥ 99 %), which was dissolved in 75 mL demineralized water. The catalyst was obtained after the same thermal treatment as in the La-loading step. For comparison, a bare Cu/γ-Al2O3 catalyst was also prepared.

Catalyst characterization

The elemental analysis was performed by inductively coupled plasma-optic emission spectroscopy over a 720-ES ICP-OES equipment (Agilent) with axial viewing and simultaneous Charge Coupled Device detection. Before the elemental analysis, the sample was prepared by dissolving 10 mg of dried and grounded sample in concentrated aqua regia solution (HNO3:HCl, 1:3, v:v). The solution was heated to 383 K during 2 h in the autodigestor Vulcan 42 (Questron) and diluted up to 20 ml with ultrapure water before analysis.

The specific surface area and pore volume of the catalyst were measured through N2-sorption experiments at 77 K using a Tristar II 3020 Micrometrics apparatus. The specific surface area, , was determined using the five-point Brunauer-Emmett-Teller method. The total pore volume, , was estimated from the adsorbed nitrogen amount at relative pressure of 0.98. The pore size distribution and average pore size, , were calculated according the Barrett-Joyner-Halenda formula applied to the desorption isotherm.

Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8-AXS Discover device and a linear Vantec detector, using with a CuKα X-ray source (λ = 1.54 Å) under continuous scan mode over a 2θ range of 15° to 75°, with a step size of 0.02°. [START_REF] Rampelberg | In situ X-ray diffraction study of the controlled oxidation and reduction in the V-O system for the synthesis of VO2 and V2O3 thin films[END_REF] The reduction was performed insitu with 5 vol.% H2-He at 1.1 NmL s -1 with a temperature increase up to 723 K (ramp: 8 K min -1 ). For the presentation of the results, the observed intensity was normalized based on the intensity of the highest peak. The Scherrer equation, was used to determine the size of the crystals in the calcined, reduced or spent catalysts.

The dispersion, , the mean Cu surface area, , and the average diameter of the Cu aggregates, ∅ , were determined by dissociative N2O adsorption according to the method of Gervasini et al. [START_REF] Gervasini | Dispersion and surface states of copper catalysts by temperatureprogrammed-reduction of oxidized surfaces (s-TPR)[END_REF] on the Micrometrics Autochem II 2920 equipped with a TCD-detector. This method determines the , and with qualitative and quantitative information by a successive analysis of H2-TPR, a surface oxidation by N2O and the H2-TPR of the freshly oxidized Cu surface (defined as s-TPR). s-TPR allows to distinguish between surface Cu species and bulk Cu. [START_REF] Gervasini | Dispersion and surface states of copper catalysts by temperatureprogrammed-reduction of oxidized surfaces (s-TPR)[END_REF] To desorb any potentially adsorbed species, the temperature was first increased to 473 K (ramp: 90 K min -1 ), held constant for 5 min, and cooled to 323 K. In the next step, the reducibility of the catalysts was evaluated by H2-TPR, pursuing complete reduction of CuO to Cu 0 . A gaseous 10%H2-Ar mixture with a flow rate of 1 NmL s -1 was sent over the catalyst while heating to 723 K at a ramp of 8 K min -1 . Once at 723 K, the temperature was held constant for 30 min during which the Cu x species were reduced to Cu 0 . Next, the temperature was reduced to 323 K. In the second step, a constant flow of 1 vol.% N2O-Ar gas with a flow rate of 0.25 NmL s -1 (Air Liquide) was sent over the catalyst at 323 K for 1 h, during which surface Cu 0 species reacted with N2O and formed Cu + . In the third step again a 10 vol.% H2-Ar gas at a flow rate of 1 NmL s -1 was sent over the catalyst, while heating this time to 1073 K (ramp: 20 K min -1 ) thereby reducing the surface Cu + species to Cu 0 . The amount of consumed H2 was measured by a thermal conductivity detector (TCD). The Cu dispersion, , defined as the ratio of the Cu exposed at the surface to the total amount Cu, was calculated from the amount of H2 consumed in the s-TPR analysis. The mean Cu surface area, [m² g -

= • • 10 • (1)
The average diameter of the Cu aggregates, ∅

[nm], is determined from via Eq. ( 2), where is a constant representing the available surface area of a particle ( = 5) and the density of Cu (8.92 g cm -³).

∅ = 10 • • . % • (2)
For the determination of acid sites, ammonia (NH3) was used as a probe molecule during TPD. For the measurement of the total number of acid sites, ca. 0.2 g of catalyst (or pure support) was diluted with 0.8 g of inert material. First, this diluted sample was heated in an inert atmosphere (Ar, 0.42 NmL s -1 ) up to 473 K (ramp: 90 K min -1 ). After 5 min, the temperature was decreased to 383 K and kept at that temperature for 20 min. Next, the catalyst was reduced using 10 vol.%H2-Ar (1 NmL s -1 ) while heating to 473 K (ramp: 8 K min -1 ) after which the temperature was kept for 15 min. After the reduction, the temperature was decreased to 423 K and the gas flow was switched to 4 vol.% NH3-He with a flow rate of 0.25 NmL s -1 . After 30 min, the gas was switched to Ar (0.42 NmL s -1 ) to remove the physisorbed NH3. Finally, the temperature was increased to 873 K at a heating rate of 10 K min -1 . The amount of desorbed NH3 was detected by a TCD.

The composition and oxidation state of the elements present on the catalysts surface were determined using X-ray photoelectron spectroscopy (XPS). The XPS spectra were recorded at a residual pressure of 10 -9 mbar on a Kratos Axis UltraDLD electron energy spectrometer operating with a monochromatic Al-Kα (1486.6 eV). The pass energy of the hemispherical analyzer was set at 160 eV for the wide scan and 40 eV for narrow scans. In the latter conditions, the full width at half maximum (FWHW) of the Ag 3d5/2 peak of a standard silver sample was about 0.9 eV. Charge stabilization was achieved by using the Kratos Axis device. Peak decomposition was performed using curves with a 70% Gaussian type and a 30% Lorentzian type and a Shirley nonlinear sigmoid-type baseline. The following peaks were used for the quantitative analysis: O 1s, C 1s, La 3d, Cu 2p, and

Cu LMM Auger peak. Molar fractions were calculated using peak areas normalized on the basis of acquisition parameters after a Shirley background subtraction and corrected with experimental sensitivity factors and transmission factors provided by the manufacturer. The binding energy (BE)

scale was calibrated by measuring C 1s peak (BE = 284.8 eV) from the surface contamination and the accuracy of the measurement was ± 0.1 eV. Differential surface charging of the samples was ruled out by checking the reproducibility of XPS measurements in repeated scans under different Xray exposures. Software CasaXPS was used for the analysis of XPS spectra, separating elemental species in different oxidation states, and calculating relative concentrations of chemical elements.

Scanning (Transmission) Electron Microscopy was performed with a Cs corrected JEOL JEM-2200FS

instrument [START_REF] Jeol | FS Field Emission Electron Microscope[END_REF] equipped with a Schottky-type field-emission gun and JEOL JED-2300D energydispersive X-ray (EDX) detector, at 200 kV. Specimen preparation consisted of immersing a lacey carbon film supported on a Ni grid into the sample powder. Dark-field STEM images give the best contrast to the identification of particles. For the spent 1.5La-10Cu, 1.5La-20Cu, 2.0La-10Cu and 2.0La-20Cu catalysts, the particle size distribution of the large particle population (> 10 nm) was determined based on almost 500 particles. For the 1.0La-10Cu catalyst, the particle size distribution was established on 117 particles, due to a limited amount of STEM images. For the 10Cu catalyst, only 30 particles were measured as sintering was clear on every image.

Catalyst performance testing

The performance of the catalysts was assessed for liquid phase glycerol (99.5%, Sigma-Aldrich) hydrogenolysis in a high-throughput kinetics mechanistic investigation set-up. [START_REF] Van Der Borght | Information-Driven Catalyst Design Based on High-Throughput Intrinsic Kinetics[END_REF] The reaction was performed within the intrinsic kinetic regime, which was assessed by typical correlations [START_REF] Carberry | CHEMICAL REACTION ENGINEERING -Inter-lntraphase Diffusion and Chemical Reaction[END_REF][START_REF] Weisz | Interpretation of measurements in experimental catalysis[END_REF] and experimentally proven in previous work [START_REF] Rajkhowa | A comprehensive kinetic model for Cu catalyzed liquid phase glycerol hydrogenolysis[END_REF], under steady state conditions in a fixed bed reactor operating in a trickle-bed regime.

The reactor effluent was analyzed using a gas chromatograph equipped with a flame ionization detector and an Agilent CP-Was 57 CB glycols column using 1,4-butanediol as an external standard.

The calibration factors are listed in Table SI.1.

Conversion was defined according to equation ( 3), where " # and " represent respectively the molar in-and outlet flow rates of glycerol (mol s -1 ).

$ " = " # -" " # (3)
The selectivity towards a component & was calculated on a molar carbon basis according to equation ( 4), with ' (," the number of carbon atoms in glycerol, * # and * the molar in-and outlet flow rates of a component & (mol s -1 ) respectively.

*," = ' (,* • + * -* # , ' (," • -" -" # . ( 4
)
The activity is defined as the ratio of the mass flow rate in which glycerol is converted, /0 ",(12345647 (kgG s -1 ), to the total amount of catalyst used, 8 (96 (kgcat).

': &;& < = /0 ",(12345647 8 (96 (5)

Results and discussion

Industrially, a high density of active sites on the catalysts, which is mostly achieved at high metal loadings, is desirable to maximize the volumetric activity. However, to prove relationships between catalyst structure and catalytic performance, more moderate metal loadings (< 20 wt.%) are advisable [START_REF] Beerthuis | Particle size effects in copper-catalyzed hydrogenation of ethyl acetate[END_REF][START_REF] Krijn | Synthesis of Solid Catalysts[END_REF]. In our previous work [START_REF] Bouriakova | Co-metal induced stabilization of alumina-supported copper: impact on the hydrogenolysis of glycerol to 1,2propanediol[END_REF], it was clear that a small amount of 1.0 wt.% of La sufficed to increase the stability of a 10 wt.% of Cu/Al2O3 catalyst. In this work, the La/Cu mass ratio was varied around that central point, aiming at changing the distribution of the Cu + and Cu 0 species.

Therefore, a series of La-Cu/γ-Al2O3 catalysts with nominal La and Cu values: x wt.% La -10 wt.% Cu (x = 0, 1.0, 1.5, 2.0), x wt.% Lay wt.% Cu (x = 1.5, 2.0, 2.5 and y = 15, 20, 25) were prepared.

No lanthanum was leached during the synthesis as the La loading in the intermediate La/γ-Al2O3 materials was similar as for the La-Cu/γ-Al2O3 catalyst (Table SI.2). The actual La and Cu loadings in the calcined catalysts were systematically lower than the nominal ones, Table SI.3. This was attributed to the hygroscopic nature of the precursors, leading to an overestimation of the actually introduced amounts during the synthesis.

To understand the nature of the Cu species present on the catalyst's surface and their evolution, the catalysts were characterized in three states: after calcination, after reduction and post-reaction (i.e. spent catalyst). On that ground, the role of Cu species in the catalytic performance for glycerol hydrogenolysis was investigated.

Catalyst characterization

Textural properties

The support and the catalysts were characterized by N2 sorption. A type-IV N2-adsorption isotherm is observed for all catalysts, see Figure 1 and Figure SI.1, which is expected since the support (PURALOX ® γ-Al2O3) is mesoporous. [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF] The presence of mesopores (2 -50 nm) is confirmed by the pore size distribution (Table SI.3). The hysteresis loops are classified as type H1, which is typical for materials with cylindrical pore channels. [START_REF] Sing | Physisorption Hysteresis Loops and the Characterization of Nanoporous Materials[END_REF] The BET surface area exhibits a small decrease upon the La and Cu impregnation without a clear trend as a function of metal loading or promotor ratio.

A similar observation was made for the mesoporous volume and the mean pore volume or size. The BET surface area ranged between 164 and 192 m 2 g -1 support (Table SI.3) while the mesoporous volume and mean pore size varied between 0.38 -0.47 cm 3 g -1 support and 6.4 -8.6 nm, respectively (Table SI.3). Therefore, it is concluded that the promotion has no significant impact on the textural properties of the support.

Crystallographic and morphological structure

The crystalline phases of the support and the calcined and reduced catalysts were determined by XRD, as shown in Figure 2 while for the 1.0La-10Cu even the presence of cylindrical channels, as evidenced by N2-sorption, was observed. For both catalysts, it is clear that La and Cu remain highly dispersed over the γ-Al2O3 support. (Figure 4E and F) after reduction. For the 2.0La-10Cu catalyst (Figure SI.4), a catalyst grain with larger Cu particles was found, however, the number was too small to obtain a particle size distribution. In most images, no well-defined Cu-particles were identified, leading to the hypothesis that most Cu particles are highly dispersed nanoparticles. The presence of a few larger Cu particles is in agreement with the Cu 0 crystallites detected by XRD (Table SI.4).

Active sites related properties

As Cu 0 is needed for the hydrogenation reaction to achieve 1,2-propanediol, the catalysts have to be reduced prior to reaction. Hence, a more elaborate characterization of the reduced catalysts is needed to gain insights into the catalyst's surface and the role of Cu 0 .

The mean surface area of the Cu aggregates, MSA @A , and the average diameter of the aggregates, ∅ @A in the reduced catalysts were determined via N2O-adsorption. The MSA @A was found to be between 17 and 34 m² g -1 cat, which is on the higher side of the range reported for typical supported Cu-based catalysts in literature with similar Cu loading (10-30 m² g -1 cat). [START_REF] Menon | Exposure of metallic copper surface on Cu Al2O3-carbon catalysts[END_REF][START_REF] Budiman | Design and preparation of high-surface-area Cu/ZnO/Al2O3 catalysts using a modified co-precipitation method for the water-gas shift reaction[END_REF][START_REF] Zhao | Efficient tuning of surface copper species of Cu/SiO2 catalyst for hydrogenation of dimethyl oxalate to ethylene glycol[END_REF] The average diameter, ∅ @A , increases with the Cu loading (Figures SI.5A). This is expected, since a higher Cu loading enhances the probability of agglomeration and/or growth of the Cu particles during synthesis. [START_REF] Ayastuy | Effect of copper loading on copper-ceria catalysts performance in CO selective oxidation for fuel cell applications[END_REF] The catalysts exhibited a dispersion, D @A , between 4 % and 20 %. The dispersion decreases with increasing Cu loading (Figure SI.6B), as it is inversely related to ∅ @A , see Table SI.4.

The acidity of the reduced catalysts was compared with that of the bare support, see Figure 5. The addition of Cu resulted in a slight increase of the total acidity (Table SI.5). Compared to the calcined support, the addition of Cu resulted to the formation of weak and strong acid sites (Figure 5A).

Moreover, it is clear that the interaction between La and Cu resulted in a different NH3-TPD profile compared to that obtained for the Cu-only catalyst. All La-promoted catalysts exhibited a significant increase in the signal in the weak-moderate acid site region, compared to the support, and a lower signal in the region of the strong acid sites.

The effect of the promotor ratio on the distribution of acid site types in the catalyst is depicted in Figure 5E. Promotion slightly increased the fraction of weak-moderate acid sites and decreased the number of strong acid sites. Lee et al. [START_REF] Lee | Catalytic cracking of C5 raffinate to light olefins over lanthanum-containing phosphorous-modified porous ZSM-5: Effect of lanthanum content[END_REF] reported that a higher La content decreased the strong acidity in zeolites, due to the preferential interaction between La and strong acid hydroxyl groups on the surface. XPS (vide infra) showed an increased amount of surface Cu + , which correspond to Lewis acidity species because of the electron affinity [START_REF] Shan | Identification of the dehydration active sites in glycerol hydrogenolysis to 1,2-propanediol over Cu/SiO2 catalysts[END_REF][START_REF] Zhu | A highly efficient and robust Cu/SiO2 catalyst prepared by the ammonia evaporation hydrothermal method for glycerol hydrogenolysis to 1,2propanediol[END_REF][START_REF] Wang | Insight into the Effect of Dual Active Cu0/Cu+ Sites in a Cu/ZnO-Al2O3 Catalyst on 5-Hydroxylmethylfurfural Hydrodeoxygenation[END_REF][START_REF] Zhu | Efficient Cu catalyst for 5-hydroxymethylfurfural hydrogenolysis by forming Cu-O-Si bonds[END_REF][START_REF] Chimentão | Selective dehydration of glycerol on copper based catalysts[END_REF][START_REF] Ye | Recent progress in improving the stability of copper-based catalysts for hydrogenation of carbonoxygen bonds[END_REF][START_REF] Yue | An Alternative Synthetic Approach for Efficient Catalytic Conversion of Syngas to Ethanol[END_REF], upon La-promotion (Figure SI.10B). It was possible to estimate the distribution of the acid sites induced by the Cu + (Table SI.5).

Overall reduction behavior

The effect of promotion on the Cu species of the calcined catalysts and their reduction behavior was investigated by H2-TPR, of which the reduction profiles are shown in Figure 6. This analysis provides insight into the overall surface species and catalyst reducibility, while an XRD measurement (section 3.1.2) provides only information about the reduction of crystalline phases. It can be clearly seen that the addition of La to the 10Cu catalyst (Figure 6A) decreased the intensity of the α peak, while that of the β peak increased. The first reduction peak, α, might be ascribed to highly dispersed

CuO species [START_REF] Behrens | Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO: Al[END_REF] and/or (Cu-O-Cu) 2+ species [START_REF] Yin | The Nature of Active Copper Species in Cu-HMS Catalyst for Hydrogenation of Dimethyl Oxalate to Ethylene Glycol: New Insights on the Synergetic Effect between Cu0 and Cu+[END_REF], the second reduction peak, β, to the reduction of CuO particles with weak support interactions [START_REF] Huang | Highly dispersed silica-supported copper nanoparticles prepared by precipitation-gel method: a simple but efficient and stable catalyst for glycerol hydrogenolysis[END_REF][START_REF] Dieuzeide | Vapor phase hydrogenolysis of glycerol to 1,2-propanediol at atmospheric pressure over copper catalysts supported on mesoporous alumina[END_REF]. The 1.0La-10Cu catalyst did not show any bulk CuO, corresponding to the γ-peak [START_REF] Behrens | Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO: Al[END_REF][START_REF] Huang | Highly dispersed silica-supported copper nanoparticles prepared by precipitation-gel method: a simple but efficient and stable catalyst for glycerol hydrogenolysis[END_REF][START_REF] Dieuzeide | Vapor phase hydrogenolysis of glycerol to 1,2-propanediol at atmospheric pressure over copper catalysts supported on mesoporous alumina[END_REF], but had a higher number of CuO particles with strong support interactions [START_REF] Huang | Highly dispersed silica-supported copper nanoparticles prepared by precipitation-gel method: a simple but efficient and stable catalyst for glycerol hydrogenolysis[END_REF][START_REF] Dieuzeide | Vapor phase hydrogenolysis of glycerol to 1,2-propanediol at atmospheric pressure over copper catalysts supported on mesoporous alumina[END_REF], corresponding to β'. This species could correspond to the CuAl2O4 crystallites which were not clearly detectable with XRD in section 3.1.2. For the other catalysts (Figure 6B to D), three main reduction peaks were distinguished. At higher Cu-loadings, the reduction peaks shifted to higher temperature. The reduction peak γ increased with higher Culoading, as a result of larger Cu particles, see Table SI.4. For all catalysts, the hydrogen consumption during H2-TPR was somewhat lower than the stoichiometric value for complete reduction from CuO to Cu 0 , resulting in a degree of reduction between 76 % and 98 % (Table SI.4). Most likely, the remaining amount of non-reduced Cu δ+ species have a stronger interaction with the support, such as CuAlO2 or Cu2O. This value related to the amount of Cu 2+ in the calcined sample is in agreement with the determined amount of Cu 2+ on the calcined catalysts by XPS (Table SI.6). In short, it is plausible that in addition to Cu 0 species, a lower amount of Cu δ+ species remains present after reduction, as also indicated by the in situ XRD in Figure 3.

Identification of the surface species

The effect of the promotion on the chemical surface composition and oxidation states in the [84] The latter can lead to the formation of Cu species with a lower oxidation state, such as Cu + , which is in agreement with the observations by in situ XRD (Figure 3). As such, the relative amount of Cu 2+ in the calcined catalysts decreases with the percentage of surface La and varies between 100 % and 80 % (Figure SI.9B, Table SI.6). For the reduced catalysts, the Cu/Al ratio decreased approximately 50 % compared to the calcined samples (Table SI.7, cf. Table SI.6). As the reduction temperature is not sufficiently high to allow the diffusion of Cu into the Al2O3 structure [START_REF] Vila | Glycerol hydrogenolysis to 1,2propanediol with Cu/γ-Al2O3: Effect of the activation process[END_REF][START_REF] Castro | XPS study of copper dispersion in CuO/Al2O3 catalysts[END_REF], this indicates a lower dispersion (and, hence, sintering) of the Cu particles. This is in agreement with the change in Cu crystallite size observed by XRD (Table SI.4, cf. Table SI.3). There was no satellite peak between 942 and 944 eV ascribed to Cu 2+ , implying reduction of Cu 2+ to Cu + and/or Cu 0 (Figure 7). Comparing the calcined to the reduced catalysts (Table SI.6, cf. Table SI.7), a shift to lower BE was observed for all catalysts. The largest shift, from 934.6 eV (Cu 2+ ) to 932.3 eV (Cu 0 ), was observed for the unpromoted Cu catalyst, while the La-Cu catalysts showed a smaller shifts, due to the strong interaction between La and Cu. One can presume that the larger shift for the unpromoted Cu catalysts is either to the larger particle size or less Cu + .

Due to the similar BE between Cu + (932.2 ± 0.1 eV) and Cu 0 (932.6 ± 0.3 eV), it is hard to discriminate between those Cu species. [START_REF] Biesinger | Advanced analysis of copper X-ray photoelectron spectra[END_REF] To clearly distinguish between Cu + and Cu 0 in the reduced catalysts, a deep analysis of the Auger Cu LMM region was performed. The Cu LMM spectrum shows larger differences (1.8 eV) between the kinetic energy (KE) of the Cu species compared to the BE. As shown in Figure 8, the Cu LMM spectra of the reduced La-Cu catalysts present an asymmetrical broad peak, suggesting the coexistence of Cu + and Cu 0 on the catalysts surface. [START_REF] Biesinger | Advanced analysis of copper X-ray photoelectron spectra[END_REF] The higher the Cu loading, the more pronounced the Cu 0 and Cu + peaks are. Even if the intensity of the Auger peaks is low, the concentration of the individual species could be determined by the use of Cu standards. Notwithstanding that, caution is needed when interpreting the results as small shifts in KE could result in different fractions of Cu 0 and Cu + . When looking carefully at the individual Cu 0 and Cu + species present on the surface of the reduced catalysts, the strong La-Cu interaction is again noticeable. The higher the La/Cu surface mass ratio, the higher the percentage of Cu + species remained present after the reduction (vide supra, In summary, the La promotion has no pronounced effect on the textural properties of the calcined catalysts. The calcined catalysts exhibit a homogenous distribution of dispersed La and Cu particles.

The La-Cu interaction resulted in Cu species with lower oxidation states, but the major part of the species remained in the Cu 2+ oxidation state on the calcined catalysts. After reduction, the Cu agglomerates remain well dispersed and the dispersion of Cu 0 on the reduced catalysts decreases with increasing Cu loading. Most of the Cu 0 particles sizes are between 1-10 nm while some larger bulk Cu 0 crystallites are also present. Due to the important La-Cu interaction, a significant amount of non-reduced Cu + species remains present after the reduction -most probably at the surface of the Cu 0 particles -whereby, the acidity of the catalysts also slightly increases, as Cu + species are considered as moderate (Lewis) acids.

Glycerol hydrogenolysis

To investigate the effect of La/Cu mass ratio on the catalytic performance, i.e. the catalyst's stability, activity and selectivity, five La-Cu catalysts were selected and compared to the 10Cu catalyst. A La-Cu catalyst with the similar as the 10Cu catalyst but different Cu particle sizes, i.e. 1.5La-20Cu was selected. The 1.0La-10Cu and 2.0La-20Cu catalysts were also selected as those exhibited the same promoter ratio, however, with different Cu particles. Finally, the La-Cu catalysts with respectively the highest and lowest , i.e. 1.5La-10Cu and 2.0La-10Cu, were also selected.

Catalytic performance

Figure 9A shows the effect of the La/Cu mass ratio on the catalyst stability, expressed as the relative decrease in glycerol conversion over 68 h TOS (Table SI.8 till Table SI.13). It is clear that La-Cu catalyst exhibited a better catalyst stability compared to the pure Cu catalyst. The increased stability was attributed the a lower amount of sintering (vide supra, section 3.2.2).

The effect of the promotion on the catalyst activity is shown in Figure 9B. La-promotion did not increase the activity, in agreement with our previous work [START_REF] Bouriakova | Co-metal induced stabilization of alumina-supported copper: impact on the hydrogenolysis of glycerol to 1,2propanediol[END_REF]. For a constant MSA @A , the activity even rather decreases upon La 'promotion'. This indicates that a tradeoff will be needed between stability and activity. The pure Cu catalyst has a higher activity even though no acidic Cu + is present on the surface (Table SI.7). However, a small acid contribution of Cu was estimated (Figure 9D), which could be the result of some Cu-Al2O3 interactions. Therefore, it seems that the active sites on the pure Cu and the La-Cu catalysts are different. [START_REF] Rajkhowa | A comprehensive kinetic model for Cu catalyzed liquid phase glycerol hydrogenolysis[END_REF], and consequently resulted in higher 1,2-propanediol selectivity.

A minor amount of 1,3-propanediol or degradation products, such as 1-propanol and 2-propanol, were also observed over some catalysts (Table SI.8 till Table SI. [START_REF] Balaraju | Selective Hydrogenolysis of Glycerol to 1, 2 Propanediol Over Cu-ZnO Catalysts[END_REF]). For all catalysts, the undesired C-C cleavage, forming 1,2-ethanediol was observed. The selectivity of 1,2-ethanediol was not directly related to the number of strong acid sites (Figure SI.11C), which are generally assumed to promote C-C cleavage. This could be related to the low selectivity towards the product and a higher experimental error. However, the La-Cu catalysts exhibited an increase in 1,2-ethanediol selectivity with increasing total number of acid sites per Cu 0 site (Figure SI.11D). This indicates that for the direct hydrogenolysis of glycerol to 1,2-ethanediol and methanol both an acid and a metal site are required, rather than only a metal site.

Spent catalyst characterization

During reaction, the catalyst can undergo changes in particle size, oxidation state and morphology.

Therefore, the six investigated catalysts were also thoroughly characterized post reaction (68 h TOS).

The La-Cu catalysts are not subject to leaching (Table SI.14 cf. Table SI.2). From the XRD patterns of the spent catalysts, shown in Figure 10, one can see that the CuO signal reappeared, especially in the case of the 10Cu catalyst. This indicates that some amount of the Cu species reoxidized or some CuO sintered during the reaction. Further, it can be observed that the diffraction peaks of Cu 0 are very sharp, indicating larger crystallite sizes. Based on the Scherrer equation, Cu 0 crystallites in the range of 50 to 150 nm were observed on the spent catalyst, see Figure 11, but this size decreased with increasing La/Cu mass ratio. This trend was also confirmed by dissociative N2O adsorption on the spent catalysts. To investigate the morphology of the spent catalysts, STEM images of the spent catalysts, without any pretreatment, were made and a particle size distribution was determined (Figure SI.12). Based on dark-field STEM, the stabilizing effect of La was evident. The effect of the promotor ratio was less straightforward to interpret (Figure 11) as compared to other characterization method. Thus, La-promotion inhibited 29 sintering of not only Cu 0 , but of all Cu species in the catalyst as it is impossible to distinguish different oxidation states in STEM. As discussed in section 3.1.4, the La-Cu interactions also increased the amount of Cu + species, reducing again the extent of Cu species sintering. The surface state of the spent Cu and La-Cu catalysts was probed by XPS. A small satellite peak between 942 and 944 eV was detected and ascribed to Cu 2+ , see Figure SI.13, implying some reoxidation compared to the reduced catalysts, as was observed with XRD (Figure 10). The percentage of (Cu 0 +Cu + ) present in the spent catalysts (Figure SI.14), increased with the La/Cu mass ratio. Due to the strong La-Cu interactions, it was assumed that Cu + was present on the surface of the reduced catalyst, as discussed in section 3.1.5, and remained in the spent catalysts.

It is concluded that the La-Cu catalysts showed a better stability (see Figure 9A) as result of the strong La-Cu interactions, which reduced the extent of Cu sintering.

Proposed reaction mechanism over La-Cu/γ-Al2O3 catalysts

As mentioned in the introduction, the most accepted mechanism for the formation of 1,2propanediol is the dehydration-hydrogenation one. In the first step, glycerol is dehydrated over an acid site, induced by the support, to acetol, while in the second step, these intermediates undergo hydrogenation, which is catalyzed by a metallic site, to 1,2-propanediol. Combining the observed characteristics and catalytic performance, our work allows to propose a more specific mechanism for glycerol hydrogenolysis over La-Cu/γ-Al2O3. This mechanism, hypothesizes that Cu particles, rather than the support, are the active sites for the dehydration to acetol. Figure 13 illustrates the role of the different surface Cu species in this reaction mechanism.

As evidenced by STEM, both the La and Cu agglomerates are well dispersed in the reduced catalysts. The Cu-loading affected mostly the size of the Cu 0 particles (1.3 -5.5 nm). XPS indicated the co-existence of Cu 0 and Cu + , as represented in Figure 14. At higher La/Cu mass ratios, more Cu + is available on the surface. As shown in Figure 13, H2 adsorbs dissociative on any Cu 0 surface with the formation of two adsorbed hydrogen atoms. The adsorption of glycerol then occurs on Cu + , induced by the La-Cu interaction. It is assumed, inspired by the mechanism proposed by Alhanash et al. [START_REF] Alhanash | Gas-phase dehydration of glycerol to acrolein catalysed by caesium heteropoly salt[END_REF], that a primary hydroxyl group of glycerol interacts with the Cu + , which has Lewis acidity, and an O atom of Cu2O abstracts a proton on the adjacent carbon. Cu + weakens the electron density of the C-O bond [START_REF] Wang | Insight into the Effect of Dual Active Cu0/Cu+ Sites in a Cu/ZnO-Al2O3 Catalyst on 5-Hydroxylmethylfurfural Hydrodeoxygenation[END_REF][START_REF] Gong | Synthesis of ethanol via syngas on Cu/SiO2 catalysts with balanced Cu0-Cu+ sites[END_REF], accelerating its cleavage; yielding acetol and water by regeneration of the active site (dehydration). Finally, acetol reacts with two of the hydrogen atoms on the Cu 0 surface, leading to the formation of 1,2-propanediol. An acid site in the proximity of a Cu 0 site contributed to side product formation.

Conclusions

In this work, the effect of La-promotion on γ-Al2O3 supported Cu catalyst for glycerol hydrogenolysis towards 1,2-propanediol was assessed. Different promoter ratios were employed, leading to varying amounts of Cu + and Cu 0 species.

The La/Cu mass ratio had no significant effect on the textural properties (e.g. surface area, average pore size and pore volume) of the calcined catalysts. After the reduction of the catalysts, Cu + and Cu 0 co-existed. It was found that La/Cu mass ratio, affected the amount of Cu + and consequently the acid site distribution.

The activity seemingly decreased with promotion ratio, causing a tradeoff between stability and activity. Sintering of the Cu particles occurred, but its extent was reduced by the presence of La thanks to the La-Cu interaction. It was shown that product selectivity in glycerol hydrogenolysis on La-Cu/Al2O3 catalysts depends on both acid and metal sites. The weak-moderate Cu + acid sites in La-Cu catalysts, rather than the support, enhanced the dehydration of acetol, and consequently to the 1,2-propandiol selectivity. The insights gained in this work provide a new view on the role of Cu + and therefore the complexity of glycerol hydrogenolysis. Additional operando characterization could provide fundamental understanding of the glycerol conversion and facilitating rational catalyst design.

Figure 1 .

 1 Figure 1. N2-sorption isotherms and differential pore size distribution of the support (green), 10Cu catalyst (black) and 1.0La-10Cu (yellow) catalysts, as representative for the La-Cu catalysts.

Figure 2 .

 2 Figure 2. XRD pattern of the support (green) and the calcined (full line) and reduced (dashed lines) 10Cu catalyst (black) and 1.0La-10Cu (yellow) catalysts, as representative for the La-Cu catalysts. Phases patterns from JCPDS: with LaO ( , 00-033-0716), La2O3 ( , 03-065-3185), CuLaO2 ( , 00-35-1403), CuO ( , 00-045-0937), Cu2O ( , 00-005-0667), Cu 0 ( , 00-004-0836), γ-Al2O3 ( , 00-050-0741), CuAl2O4 ( , 00-33-0448) and CuAlO2 ( , 01-075-1792)To study the evolution of the Cu oxidation state during the reduction, the 1.0La-10Cu catalyst was

Figure 3 .

 3 Figure 3. Contour plots of the diffraction peaks as function of temperature between 32° and 52° during reduction with 1.1 NmL s -1 5 vol.% H2-He of Cu oxides present in the 1.0La-10Cu catalyst. The material's morphology was further investigated via STEM. In calcined (Figure SI.3) and reduced

Figure 4 .

 4 Figure 4. Dark-field STEM image of 10Cu (A) and 1.0La-10Cu (B) ex-situ reduced at 723 K using 10 vol.% H2-Ar. The corresponding EDX elemental map with the distribution of Al (blue), Cu (green) and La (red) is shown in C and D. The individual EDX maps of Cu and La are shown in E, corresponding to the dark-field image A, and in F, corresponding to the dark-field image B.

Figure 5 .

 5 Figure 5. NH3-TPD profiles of the calcined support (green) and (A): 10Cu (black), 1.0La-10Cu (yellow); (B): 1.5La-yCu; (C): 2.0La-yCu and (D): 2.5La-yCu catalysts. Intensities were normalized per support mass, such that additional acidity generated by metal deposition can be visually identified. (E) Effect of the bulk La/Cu mass ratio on the fraction of amount of weak-moderate (○) and strong (•) acid sites, as determined by NH3-TPD.

Figure 6 .

 6 Figure 6. TPR profiles of the calcined catalysts with (A): 10Cu (black), 1.0La-10Cu (yellow); (B): 1.5La-yCu; (C): 2.0La-yCu and (D): 2.5La-yCu.

Figure 7 .

 7 Figure 7. Cu 2p3/2 spectra of calcined (full line) and reduced (dashed line) 10Cu catalyst (black) and (A): 1.0La-10Cu (yellow); (B): 1.5La-yCu; (C): 2.0La-yCu and (D): 2.5La-yCu.

  Figure SI.10). The values are summarized in TableSI.7. It was also evidenced by the Wagner plot[START_REF] Biesinger | Advanced analysis of copper X-ray photoelectron spectra[END_REF] that the surface of the reduced La-Cu catalysts is covered with Cu + rather than with Cu 0 (Figure SI.11). Only the bare Cu catalyst showed more Cu 0 than Cu + on the reduced catalyst surface.

Figure 8 .

 8 Figure 8. Cu LMM spectra of the catalysts after reduction at 623 K using 100 %H2 at 0.33 NmL s -1 . (A): 10Cu (black), 1.0La-10Cu (yellow); (B): 1.5La-yCu; (C): 2.0La-1yCu and (D): 2.5La-yCu.

Figure 9 .

 9 Figure 9. Effect of bulk La/Cu mass ratio on (A) the stability, (B) activity and (C) 1,2-propanediol selectivity. Effect of the weak and moderate Cu acid sites on (D) the activity and (E) 1,2-propanediol selectivity. Activity and selectivity are determined after 12 h time on stream at 125 kgcat s mol -1 glycerol, 473 K, a total H2 pressure of 7.5 MPa and a molar H2 to glycerol ratio of 7 for the 10Cu (black), 1.5La-20Cu (dark grey), 2.0La-20Cu (dark blue), 1.0La-10Cu (yellow), 1.5La-10Cu (light grey) and 2.0La-10Cu (light blue).

Figure 10 .

 10 Figure 10. XRD pattern of the spent 10Cu (black), 1.5La-20Cu (dark grey), 2.0La-20Cu (dark blue), 1.0La-10Cu (yellow), 1.5La-10Cu (light grey) and 2.0La-10Cu (light blue) catalyst with CuO ( ), Cu2O ( ) and Cu 0 ( ).

Figure 11 .

 11 Figure 11. Effect of La/Cu mass ratio on the spent Cu particle size, dCu0, determined by dissociative N2O adsorption (full bars), Scherrer equation (striped bars) and STEM (dotted bars) of 10Cu (black), 1.5La-20Cu (dark grey), 2.0La-20Cu (dark blue), 1.0La-10Cu (yellow), 1.5La-10Cu (light grey) and 2.0La-10Cu (light blue) catalyst.

Figure 12

 12 Figure12shows the dark-field STEM images of the spent 10Cu and spent 1.0La-10Cu catalyst.

Figure 12 .

 12 Figure 12. Dark-field STEM image of spent 10Cu (A) and 1.0La-10Cu (B) catalyst. The corresponding EDX elemental map with the distribution of Al (blue), Cu (green) and La (red) is shown in C and D. The individual EDX maps of Cu and La are shown in E, corresponding to the dark-field image A, and in F, corresponding to the dark-field image B.

Figure 13 .

 13 Figure 13. Schematic representation of proposed role of La-Cu/γ-Al2O3 in glycerol hydrogenolysis. (A) La-Cu/γ-Al2O3 surface and the interaction between Cu + and Cu 0 in 1,2-propanediol formation.

  For none of the catalysts, the most characteristic peak of Cu2O(111) at 36.4° could be detected. Therefore, it can be deduced that, if present, Cu2O was either highly dispersed or amorphous, or in a very little amount. As the La-loading was below 8.5 µmolLa m -2[START_REF] Bettman | Dispersion studies on the system La2O3γ-Al2O3[END_REF], the After reduction at 723 K, the representative Cu 0 XRD peaks appeared at 43.3° for Cu 0 (111) and at 50.5° for Cu 0 (200). No characteristic peaks of CuO remained and, again, no characteristic peak of Cu2O at 36.4° was identified, impeding any conclusion on Cu2O. Based on the Scherrer equation, the Cu 0 crystallite sizes were in the range from 40 nm to 80 nm (TableSI.4), while on the calcined, i.e. prior to the reduction, catalysts the CuO crystallite sizes were calculated in the range of 20 to 40 nm (TableSI.2).e. This indicates that during the reduction, the Cu crystallites doubled in size compared to the initial CuO.

	and Figure SI.2. Broad diffraction peaks for γ-Al2O3 at 31.8°, 37.5°, 39.2°,
	and 45.7°, respectively corresponding to the Al2O3(220), Al2O3(311), Al2O3(222), Al2O3(400) facets are
	detected for all catalysts, see Figure 2. This indicates a limited crystallinity of the support. The
	calcined La-Cu catalysts exhibited main characteristic diffraction peaks of CuO(002) at 35.4° and
	CuO(111) at 38.6°, and minor diffraction peaks at 32.5° (CuO(-110)), 48.7° (CuO(-202)), 51.4° (CuO(112)) and
	53.5° (CuO(020)).

characteristic peaks of lanthanum oxides (La2O3 and LaO) were not observed. The main diffraction peaks of the mixed Cu-Al phase, CuAl2O4(400) at 36.8° and CuAlO2(012) at 37.1°, overlap with the broad diffraction peak for Al2O3(311). Most likely, those phases were not present, as they only form above 773 K, according to the CuO-Al2O3 phase diagram.

[START_REF] Mimani | Instant synthesis of nanoscale spinel aluminates[END_REF][START_REF] Jacob | Thermodynamics of CuAlO2 and CuAl2O4 and Phase Equilibria in the System Cu2O-CuO-Al2O3[END_REF]

  calcined and reduced La-Cu catalysts was analyzed via XPS. Being a surface technique, the representativeness of the results to the whole catalyst should be verified by comparing the La/Al and Cu/Al mass ratios, in the calcined catalysts, determined by XPS (surface composition) to those determined by ICP (bulk composition), see TableSI.3. Note that the La surface composition almost corresponds to that of the bulk, even though Cu is deposited after La (Figure SI.8A). For all calcined catalysts, the low Cu/Al ratios in XPS clearly indicate that quite some Cu is present as bulk Cu, rather than being exposed at the surface (Figure SI.8B). The photoelectron peaks at 932.6 eV were assigned to the binding energy (BE) of Cu 2p3/2, as illustrated in Figure7. The presence of a pronounced satellite peak between 942 eV and 948 eV was ascribed to a contribution by Cu 2+ . Due to the asymmetry of the Cu 2p3/2 peak, the Cu 2+ species in the calcined catalysts are ascribed to CuO (933.5 eV) and CuAl2O4 (935 eV).[START_REF] Li | Facile Preparation of Cu-Al Oxide Catalysts and Their Application in the Direct Synthesis of Ethanol from Syngas[END_REF] The latter is apparently in contrast with the XRD diffraction pattern (section 3.1.2). However, XPS being a surface analysis technique, this can be interpreted as a difference between the surface and bulk composition. When comparing the Cu 2p3/2 BE of the Cu-only calcined catalysts to the La-Cu catalysts, a shift to lower BE was observed (Figure SI.9A, TableSI.6). This may indicate that as La is added first, the Cu particles are preferentially located next to the La particles rather than covering them, as the bulk La/Al corresponded well with the surface

La/Al ratio (Figure SI.8A). The shift to lower BE for Cu 2p3/2 points to the overlap of the d-orbitals of La and Cu, indicating a strong La-Cu interaction.

  The 1,2-propanediol selectivity seemed to increase up to a La/Cu mass ratio of 0.088 (i.e. the 1.5La-10Cu catalyst), see Figure9C. The 2.0La-10Cu catalyst, which has the largest La/Cu mass ratio, showed a lower 1,2-propanediol selectivity. The latter has almost the same total acidity compared to the 1.5La-10Cu catalyst (TableSI.5) but exhibited different acid site distribution (Figure5E). To better understand this effect, Figure9Edepicts selectivity as a function acid sites introduced by the Cu loading, i.e. the concentration of weak and moderate sites that exceed the ones of the bare support (TableSI.5). It is clear that the increasing number of weak-moderate Cu acid sites contributes to the increase in selectivity, where that is not the case for the support acid sites (Figure SI.11A). It seems that the main difference between a pure Cu/Al2O3 and the La-Cu/Al2O3 catalysts is attributed to the Lewis acidity of Cu + , as results of the La-Cu interactions. The weak-moderate Cu + acid sites in La-Cu catalysts, enhanced glycerol conversion to acetol (Figure SI.11B), which is the rate determining step

Acknowledgements

This work was supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) through ERC grant agreement no. 615456 i-CaD. The authors would like to thank K. Vandenbroele for the contribution in the experimental work, dr. L.

Buelens for the S(T)EM measurements and dr. P. Simon for the XPS measurements.