Novel Catalyst Composites of Ni- and Co-Based Nanoparticles Supported on Inorganic Oxides for Fatty Acid Hydrogenations

In the quest to develop nanometrically defined catalytic systems for applications in the catalytic valorization of agri-food wastes, small Ni-based nanoparticles supported on inorganic solid supports have been prepared by decomposition of organometallic precursors in refluxing ethanol under H2 atmosphere, in the presence of supports exhibiting insulating or semi-conductor properties, such as MgAl2O4 and TiO2, respectively. The efficiency of the as-prepared Ni-based nanocomposites has been evaluated towards the hydrogenation of unsaturated fatty acids under solvent-free conditions, with high selectivity regarding the hydrogenation of C=C bonds. The influence of the support on the catalytic performance of the prepared Ni-based nanocomposites is particularly highlighted.


Introduction
Despite the fact that the valorization of fatty wastes in the form of fatty esters has long been studied, the corrosive nature of fatty acids makes their transformation into free acid form a more challenging quest in terms of scaling up or intensification for industrial applications provided that robust catalyst solutions withstanding the reaction conditions are found. Since the seminal works of Paul Sabatier regarding nickel reactivity [1,2] and its further applications in catalytic hydrogenations [3,4], the quest for attaining high activity and selectivity while overriding deleterious catalyst passivation pathways has resulted in a number of preparations, with the aim of preserving the catalyst dispersion due to its key implications towards reactivity [5]. Raney nickel was described for the first time in 1925 [6] and Raney cobalt in 1933 [7]; intermetallic nickel-aluminum and cobalt-aluminum alloys, respectively, featuring mesoporous structures upon activation via alkaline treatment, have been widely used as hydrogenation catalysts [5,8], such as for the industrial production of hexamethylenediamine from adiponitrile [9] and margarine both from vegetable oils and animal fats [10]. In particular, the correlation between Raney nickel catalyst microstructure (NiAl 3 vs. Ni 2 Al 3 ) and catalytic performance has been recently established [11], providing new means towards rational catalyst design involving inorganic supports. Later on, Ni-based Urushibara [12] and Kieselguhr hydrogenation catalysts [13,14], as well as cobalt-based ones [15][16][17], have also been reported in the literature, affording new means to attain zero-valent nickel and cobalt phases that play essential roles in catalytic C=C bond hydrogenation reactions. To reduce Ni soap formation, Nanomaterials 2023, 13, 1435 2 of 15 fatty acid hydrogenations are generally carried out at high pressures (20-30 bar) and temperatures in the range of 180-210 • C with pre-dried samples [18].
Nanocatalysis provides innovative solutions towards catalyst enhancements via kinetic stabilization at the nanoscale of small particles, thus providing high surface-metal ratios together with a number of highly reactive metal sites arising from low-coordination defects, as well synergistic effects at the metal-support interfaces for heterogeneized systems [29]. Based on our previous experience [30,31], herein we describe the synthesis of original mono-metallic and bi-metallic Ni and/or Co-based nanocomposites by a onestep methodology, resulting in well-defined metal nanoparticles (MNPs) supported on TiO 2 or MgAl 2 O 4 . Working under smooth conditions, this approach permits a straightforward MNPs incorporation on the support surface, as well as overriding potential structural changes of the supports. These as-prepared catalytic materials exhibited remarkable activity in the hydrogenation processes of fatty acids, including their application in waste valorization.

Materials
All reagents were purchased at the highest commercial quality and used without further purification unless stated otherwise. Halloysite and quinidine were dried in a Schlenk flask under vacuum at 100 • C overnight prior to use. The synthesis of metal nanoparticles was performed in Fisher-Porter bottles. The synthesized catalytic materials were isolated by centrifugation and dried under vacuum at 100 • C overnight; the dried materials were stored in a glove box. Solvents were previously dried on a solvent purifier system and degassed via three freeze-pump-thaw cycles. High-pressure hydrogenation reactions were carried out in stainless steel autoclaves acquired from Parr and Top Industries.
For general experimentation details and characterization techniques, see Supplementary Materials.  O 4 or TiO 2 -P90) and then sealed with a septum inside the glove box. The Fisher-Porter bottle was then removed from the glove box, and the solid mixture was suspended in degassed EtOH (16 mL) under Ar prior to sealing the Fisher-Porter with its head. The system was pressurized with H 2 (3 bar) at room temperature and then heated to 100 • C and stirred for 18 h. A black dispersion was obtained and transferred to a centrifuge tube via cannulation under Ar. Centrifugation was carried out at 3500 rpm for 10 min. After the removal of the supernatant, the solid was dispersed in degassed EtOH (rinsing repeated three times). The obtained black solid was dried under a vacuum overnight and stored in the glove box prior to use. NiCoNP@MgAl 2 O 4 : 450 mg (84% yield); NiCoNP@TiO 2 : 520 mg (97% yield).

Extraction of Metal Nanoparticles from the Inorganic Support with Glycerol
For characterization purposes, 70 mg of catalytic material was dispersed in 5 mL of glycerol. The mixture was stirred overnight at room temperature. A black dispersion was obtained and transferred to a centrifuge tube via cannulation under Ar. Centrifugation was carried out at 3500 rpm for 15 min. The obtained glycerol phase was analyzed by TEM to determine average nanoparticle diameters with size distributions.

Catalytic Hydrogenation Reactions Using NiNP@MgAl 2 O 4
A small glass flask containing 12 mg (0.01 mmol Ni) of nickel nanoparticles NiNP@MgAl 2 O 4 with 285 mg of oleic acid (1 mmol), weighted in a glove box, was introduced in an autoclave under an argon atmosphere. The mixture was then pressurized under 5 bar H 2 and heated at 250 • C with an aluminum heating block for 30 min. At the end of the reaction, the organic products were extracted with dichloromethane (5 × 3 mL), the solution was filtered through a 0.6-µm PTFE syringe filter, and the solvent was evaporated under a vacuum. Conversion and selectivity were determined by 1 H NMR using 4-methylanisole as the internal standard. Reported catalytic results correspond to the mean values obtained for three replicates.

Design and Characterization of Nanocomposite Materials
With the aim of efficiently immobilizing mono-metallic Ni-(NiNP), Co-(CoNP), and bi-metallic NiCo nanoparticles (NiCoNP) on inorganic oxides (MgAl 2 O 4 and TiO 2 ), we used herein a methodology based on the decomposition of organometallic precursors, either bis(1,5-cyclooctadiene)nickel(0), octacarbonyldicobalt(0), or a combination of both under H 2 atmosphere (3 bar) in the presence of quinidine as a stabilizer and a suspension of the solid support in ethanol under stirring at 100 • C overnight, adapting a methodology that has been recently described by our group for the preparation of Ni-based halloysite nanocomposites [31]. Thus, the synthesis of six nanocomposite materials, namely NiNP@MgAl 2 O 4 , CoNP@MgAl 2 O 4 , NiCoNP@MgAl 2 O 4 , NiNP@TiO 2 , CoNP@TiO 2 , and NiCoNP@TiO 2 was achieved ( Figure 1). For the bi-metallic systems, a nickel:cobalt ratio of 1:1 was used. under H2 atmosphere (3 bar) in the presence of quinidine as a stabilizer and a suspension of the solid support in ethanol under stirring at 100 °C overnight, adapting a methodology that has been recently described by our group for the preparation of Ni-based halloysite nanocomposites [31]. Thus, the synthesis of six nanocomposite materials, namely NiNP@MgAl2O4, CoNP@MgAl2O4, NiCoNP@MgAl2O4, NiNP@TiO2, CoNP@TiO2, and NiCoNP@TiO2 was achieved ( Figure 1). For the bi-metallic systems, a nickel:cobalt ratio of 1:1 was used. The sizes of metal NPs could not be directly estimated by TEM from the as-prepared composites due to the difficulty of distinguishing the metal NPs from the support; therefore, metal NPs were extracted from the corresponding support with glycerol for characterization purposes taking advantage of the high affinity of MNPs for the glycerol phase [32][33][34][35]. As observed by TEM analyses of the glycerol dispersions (direct analyses thanks to the negligible vapor pressure of glycerol), well-dispersed small Ni nanoparticles were obtained from both MgAl2O4 and TiO2 supports (mean diameter: 1.4 ± 0.4 nm and 1.6 ± 0.5 nm for NiNP@MgAl2O4 and NiNP@TiO2, respectively, for 1466 and 2064 particles, respectively; Figure 2), in agreement with our previous contributions involving nickel nanoparticles [30,31]. For the analogous mono-metallic Co systems, well-dispersed small Co nanoparticles were also obtained independently from the support used (mean diameter: 1.2 ± 0.3 nm and 1.3 ± 0.3 nm for CoNP@MgAl2O4 and CoNP@TiO2, respectively, for 1103 and 1493 particles, respectively. Moreover, for the NiCo bi-metallic systems, the glycerol extraction was only efficient for NiCoNP@TiO2 (mean diameter: 1.2 ± 0.4 nm for 3868 particles). The sizes of metal NPs could not be directly estimated by TEM from the as-prepared composites due to the difficulty of distinguishing the metal NPs from the support; therefore, metal NPs were extracted from the corresponding support with glycerol for characterization purposes taking advantage of the high affinity of MNPs for the glycerol phase [32][33][34][35]. As observed by TEM analyses of the glycerol dispersions (direct analyses thanks to the negligible vapor pressure of glycerol), well-dispersed small Ni nanoparticles were obtained from both MgAl 2 O 4 and TiO 2 supports (mean diameter: 1.4 ± 0.4 nm and 1.6 ± 0.5 nm for NiNP@MgAl 2 O 4 and NiNP@TiO 2 , respectively, for 1466 and 2064 particles, respectively; Figure 2), in agreement with our previous contributions involving nickel nanoparticles [30,31]. For the analogous mono-metallic Co systems, well-dispersed small Co nanoparticles were also obtained independently from the support used (mean diameter: 1.2 ± 0.3 nm and 1.3 ± 0.3 nm for CoNP@MgAl 2 O 4 and CoNP@TiO 2 , respectively, for 1103 and 1493 particles, respectively). Moreover, for the NiCo bi-metallic systems, the glycerol extraction was only efficient for NiCoNP@TiO 2 (mean diameter: 1.2 ± 0.4 nm for 3868 particles). Nanomaterials 2023, 13, x 5 of 15 Scanning Transmission Electron Microscopy Bright Field (STEM-BF), mapping on an HRTEM image corresponding to NiCoNP@TiO2 evidenced a homogeneous dispersion of nickel and cobalt over the support surface, suggesting an alloy structure ( Figure 3 and S7 in the Supplementary Materials for another HRTEM image). However, the small size of the as-prepared nanoparticles lies at the limit of current characterization techniques. It is worth mentioning that the insulating nature of MgAl2O4 as support hampered HRTEM analysis due to charging effects observed during acquisition at 200 KV.
ICP-AES analyses of the as-prepared materials were consistent with an efficient metal deposition over both supports: 4.6 wt% Ni for both NiNP@MgAl2O4 and NiNP@TiO2; 4.5 wt% Co for both CoNP@MgAl2O4 and CoNP@TiO2; 3.0 wt% Ni and 1.8 wt% Co for Ni-CoNP@MgAl2O4, as well as 2.8 wt% Ni and 2.6 wt% Co for NiCoNP@TiO2 (expected data: 5 wt% Ni, 5 wt% Co or overall Ni-Co metal content). In addition, the presence of C and N in the as-prepared materials could be determined by elemental analyses, evidencing the presence of quinidine acting as a stabilizer in the final materials (N content: lower than 0.3 wt% for all composite materials, see Table S1 in the Supplementary Materials for further details). Powder X-ray diffraction analyses of the four nanocomposites only exhibited the corresponding peaks of MgAl2O4 and TiO2 supports; probably, the 5 wt% metal loading (of Ni and/or Co) content falling below the limits of detection for this technique, together with potential peak broadening effects arising from the small size of NiNP and NiCoNP, in particular for NiNP@MgAl2O4, NiNP@TiO2, and NiCoNP@TiO2 (see Figures S8-S13 in the Supplementary Materials).
With the aim of assessing the presence of quinidine or potential metal oxides on the as-prepared Ni-based materials, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses were carried out (Figure S14-S15 in the Supplementary Materials). For NiNP@TiO2, CoNP@TiO2 NiCoNP@TiO2 absorption bands corresponding to C- Scanning Transmission Electron Microscopy Bright Field (STEM-BF), mapping on an HRTEM image corresponding to NiCoNP@TiO 2 evidenced a homogeneous dispersion of nickel and cobalt over the support surface, suggesting an alloy structure ( Figure 3 and Figure S7 in the Supplementary Materials for another HRTEM image). However, the small size of the as-prepared nanoparticles lies at the limit of current characterization techniques. It is worth mentioning that the insulating nature of MgAl 2 O 4 as support hampered HRTEM analysis due to charging effects observed during acquisition at 200 KV.
ICP-AES analyses of the as-prepared materials were consistent with an efficient metal deposition over both supports: 4.6 wt% Ni for both NiNP@MgAl 2 O 4 and NiNP@TiO 2 ; 4.5 wt% Co for both CoNP@MgAl 2 O 4 and CoNP@TiO 2 ; 3.0 wt% Ni and 1.8 wt% Co for NiCoNP@MgAl 2 O 4 , as well as 2.8 wt% Ni and 2.6 wt% Co for NiCoNP@TiO 2 (expected data: 5 wt% Ni, 5 wt% Co or overall Ni-Co metal content). In addition, the presence of C and N in the as-prepared materials could be determined by elemental analyses, evidencing the presence of quinidine acting as a stabilizer in the final materials (N content: lower than 0.3 wt% for all composite materials, see Table S1 in the Supplementary Materials for further details). Powder X-ray diffraction analyses of the four nanocomposites only exhibited the corresponding peaks of MgAl 2 O 4 and TiO 2 supports; probably, the 5 wt% metal loading (of Ni and/or Co) content falling below the limits of detection for this technique, together with potential peak broadening effects arising from the small size of NiNP and NiCoNP, in particular for NiNP@MgAl 2 O 4 , NiNP@TiO 2 , and NiCoNP@TiO 2 (see Figures S8-S13 in the Supplementary Materials).
termined by elemental analysis for MgAl2O4 spinel composites, the qualitative presence of quinidine in NiNP@MgAl2O4, CoNP@MgAl2O4, and NiCoNP@MgAl2O4 could not be assessed by DRIFTS (see Figure S15 in the Supplementary Materials). In addition, the intense absorption bands obtained for these composites in the region of 900-450 cm −1 corresponding to Al-O and Mg-O bond stretching vibrations of the support, do not permit to exclude the presence of metal oxides (stretching vibrations: 700-600 cm −1 for Ni-O and 680-558 cm −1 for Co-O)see Figure S15 in the Supplementary Materials). Moreover, magnetic measurements were carried out for the Ni-based nanocomposites at 2 K ( Figure 4, Table 1). Magnetic measurements for the mono-metallic NiNP@MgAl2O4 and NiNP@TiO2 systems were carried out, showing a weak ferromagnetic behavior for both materials, with a narrow hysteresis at 2 K with coercive fields of 775 and 947 Oe, respectively, together with moderate remanence magnetization (25.2 and 8.3 emu/gNi, respectively). The saturation magnetization values (Ms) for NiNP@MgAl2O4 (63.7 emu/g) were similar to those reported for bulk nickel (54 emu/g) [36,37]; however, for NiNP@TiO2 the Ms (29 emu/g) was lower probably due to either spin capping effects from stabilizers over the surface of nickel nanoparticles or the presence of nickel oxides. Moreover, magnetic measurements were carried out for the Ni-based nanocomposites at 2 K ( Figure 4, Table 1). Magnetic measurements for the mono-metallic NiNP@MgAl 2 O 4 and NiNP@TiO 2 systems were carried out, showing a weak ferromagnetic behavior for both materials, with a narrow hysteresis at 2 K with coercive fields of 775 and 947 Oe, respectively, together with moderate remanence magnetization (25.2 and 8.3 emu/gNi, respectively). The saturation magnetization values (M s ) for NiNP@MgAl 2 O 4 (63.7 emu/g) were similar to those reported for bulk nickel (54 emu/g) [36,37]; however, for NiNP@TiO 2 the M s (29 emu/g) was lower probably due to either spin capping effects from stabilizers over the surface of nickel nanoparticles or the presence of nickel oxides.  In comparison to the mono-metallic composites, magnetic measurements of the bimetallic systems NiCoNP@MgAl2O4 and NiCoNP@TiO2 showed a superparamagnetic   In comparison to the mono-metallic composites, magnetic measurements of the bimetallic systems NiCoNP@MgAl 2 O 4 and NiCoNP@TiO 2 showed a superparamagnetic behavior, with narrow hystereses at 2 K and coercive field values of 511 and 181 Oe, respectively, together with the lowest remanence magnetizations (6.7 and 2.7 emu/gNiCo, respectively). Moreover, NiCoNP@MgAl 2 O 4 and NiCoNP@TiO 2 presented lower M s values (156.2 and 52.0 emu/gNiCo) than the value for Co-hcp (162 emu/g) [38]. The lower value obtained for NiCoNP@TiO 2 in comparison to the Ni reference (54 emu/g for bulk Ni) could be due to the presence of nickel and cobalt oxides or the presence of capping agents, such as quinidine [39][40][41][42].
Given the fast oxidation of the samples during the introduction to the experimental station, X-ray absorption spectroscopy (XAS) analyses of NiNP@MgAl 2 O 4 , NiNP@TiO 2 , NiCoNP@MgAl 2 O 4 , and NiCoNP@TiO 2 could only confirm the presence of nickel and/or cobalt oxides, mainly attributed to NiO or Ni(OH) 2 ( Figure S16 in the Supplementary Materials) and CoO ( Figure S17 in the Supplementary Materials) [43].
Considering the high catalytic performance of both NiNP@MgAl 2 O 4 and NiCoNP@MgAl 2 O 4 (see below, Section 3.2), further characterization of these composite materials was performed. X-Ray photoelectron spectroscopy (XPS) analyses were carried out to determine the oxidation state of nickel and cobalt species immobilized on MgAl 2 O 4 , as well as the elements present at the surface of the catalytic materials ( Figure 5). The XPS survey spectrum of the MgAl 2 O 4 support showed the expected peaks for Mg, Al, and O. The XPS quantification showed that MgAl 2 O 4 accounts for about 24 at.% while Al 2 O 3 is also present with about 60 at.% in accordance to previous reports concerning solid solutions of Al 2 O 3 phases in MgAl 2 O 4 closer to the composition of stoichiometric spinel [44]. For NiNP@MgAl 2 O 4 , peaks for Ni were also observed, with a Ni content of 13 at.%. Notably, the Ni 2p XPS core level spectra showed three contributions from Ni metal (18%), NiO (26%), and Ni(OH) 2 (56%). For NiCo@MgAl 2 O 4 , peaks for both Ni and Co were also observed, which account for a low content of about 1 at.% each. Significantly, the Ni 2p XPS core level spectra showed three contributions from Ni metal (17%), NiO (53%), and Ni(OH) 2 (50%), and the Co 2p XPS core level spectra showed three contributions from Co metal (16%), CoO (40%), and Co(OH) 2 (44%) (see Figure S18 and Table S2 in the Supplementary Materials). Despite the fact that Co 3 O 4 is thermodynamically more stable than CoO, the coexistence of Co(II) and Co(III) species is difficult to assess given their close binding energies (779.5 and 781.3 eV, respectively) [39][40][41][42]45]. However, a better fitting of the Co 2p 3/2 XPS core level spectra with the envelopes corresponding to Co 2p 3/2 peaks of the following references Co(0), Co 3 O 4 , CoO, Co(OH) 2 was obtained with a slight constraint on the position of each, where the presence of Co 3 O 4 fell to zero. It is noteworthy to highlight that although Co 3 O 4 could only be estimated if CoO was imposed to be zero, Co(0) content was similar for both fittings. Overall, these results highlight that the preparation method for Ni@MgAl 2 O 4 and NiCo@MgAl 2 O 4 is effective in obtaining mono and bi-metallic NPs, albeit with partial oxidation. It is important to mention that due to the rapid oxidation of Ni(0) and Co(0), even under an Ar glove box atmosphere or ultra-high vacuum, XPS analyses of such powder samples were a challenge. Consequently, the real at.% of Ni and Co metal in the samples is probably higher than the values reported herein. ; high-resolution spectrum at the binding energy region of Ni 2p; black, red, and blue continuous traces correspond to Ni(0), NiO, and Co(OH)2 envelopes used to fit the experimental data (dotted line); the fit was carried out on the Ni 2p3/2 binding energy (middle column); high-resolution spectrum at the binding region of Co 2p; black, orange, and blue continuous traces correspond to Co(0), CoO, and Co(OH)2 envelopes used to fit the experimental data (dotted line); the fit was carried out on the Co 2p3/2 binding energy (right column). For the peak fitting procedures, see the experimental section.
Despite the lower conversions obtained for NiCoNP@TiO2 and NiCoNP@MgAl2O4 in comparison to the mono-metallic nickel counterparts at 1 mol% catalyst loadings (en- ; high-resolution spectrum at the binding energy region of Ni 2p; black, red, and blue continuous traces correspond to Ni(0), NiO, and Co(OH) 2 envelopes used to fit the experimental data (dotted line); the fit was carried out on the Ni 2p 3/2 binding energy (middle column); high-resolution spectrum at the binding region of Co 2p; black, orange, and blue continuous traces correspond to Co(0), CoO, and Co(OH) 2 envelopes used to fit the experimental data (dotted line); the fit was carried out on the Co 2p 3/2 binding energy (right column). For the peak fitting procedures, see the experimental section.

Catalytic Hydrogenation of Fatty Acids
The catalytic behavior of the six as-prepared nanocomposites was assessed in the hydrogenation of oleic acid (1) towards stearic acid (2) as a benchmark reaction ( Table 2). Under neat conditions, working at 180 • C, 5 bar H 2 pressure, and 1 mol% of metal loading (for the bi-metallic catalyst, 0.5 mol% Ni and 0.5 mol% Co), titania nanocomposites CoNP@TiO 2 , NiNP@TiO 2 , and NiCoNP@TiO 2 only gave low to moderate conversions (10%, 65%, and 31% conversions, respectively; entries 1-3, Table 2), highlighting the positive impact of mono-metallic NiNPs in terms of catalyst efficiency (entry 2 vs. 1 and 3, Table 2). In agreement with this trend, the analogous MgAl 2 O 4 spinel-based nanocomposite system with CoNPs, CoNP@MgAl 2 O 4 , also exhibited a poor performance (12% conversion; entry 4, Table 2). Nevertheless, NiNP@MgAl 2 O 4 and NiCoNP@MgAl 2 O 4 were more active than titania-derived catalytic materials (96 and 93% conversions, respectively; entries 5-6 vs. 2-3, Table 2), exhibiting both comparable efficiencies at 1 mol% metal catalyst loading (entries 5-6, Table 2). Thus, both mono-metallic and bi-metallic MgAl 2 O 4 -based composites displayed a better performance than the corresponding TiO 2 counterparts. CoNP@MgAl2O4 than NiNP@MgAl2O4 (entries 7-8, Table 2), we decided to pursue the reaction scope studies with the mono-metallic catalyst due to its better activity at 1 mol% Ni loading and easier characterization. It is worth mentioning that no significant loss of fatty acids by adsorption on any of the six catalytic composites was observed, recovering quantitatively the organic compounds. For further optimization parameters, see Table S3 in the Supplementary Materials. Substrate scope studies using other mono-and poly-unsaturated C18 fatty acids were then performed (Table 3). Elaidic acid, the (E)-isomer of oleic acid, was efficiently hydrogenated albeit in lower conversion (72% conversion; entry 1, Table 3) in comparison to oleic acid (96% conversion; entry 5, Table 3). Linoleic acid (4), showing two C=C bonds at C9 and C12, led to 69% of mono-unsaturated acid and 31% of stearic acid with nearly full conversion (entry 2, Table 3). Moreover, α-linolenic acid (5), featuring three C=C bonds at C9, C12, and C15 positions, only gave 86% conversion to mono-unsaturated fatty acids (entry 3, Table 3); longer reaction time (4 h) and higher catalyst loading (2 mol% Ni) were required to completely transform both linoleic and linolenic acids to the fully saturated stearic acid (entries 4-5, Table 3). The determination of metal traces present in the organic extracts after the 4 h benchmark reaction using NiNP@MgAl2O4 catalyst revealed the presence of trace amounts of Ni (1.1 ppm by ICP-AES close to the detection limit of this technique), revealing negligible leaching of the catalyst and pointing to a surface reactivity of the catalyst.  Despite the lower conversions obtained for NiCoNP@TiO 2 and NiCoNP@MgAl 2 O 4 in comparison to the mono-metallic nickel counterparts at 1 mol% catalyst loadings (entries 3 and 6 versus entries 2 and 5, Table 2), the poorest performance of Co could be concluded at this catalyst loading. However, it is worth mentioning that Co is permitted to operate at sub-mol% catalyst loadings with NiCoNP@MgAl 2 O 4 , probably hampering the deactivation of the Ni phase. Whereas the presence of oxidized nickel species such as NiO can be attributed to lower catalyst efficiency towards C=C hydrogenation of fatty acids as they usually require a preactivation step to, in situ, generate catalytically active Ni(0) species [46], cobalt oxides are well-known catalysts promoting the reduction of carboxylic acid function as well as hydrodeoxygenation of the corresponding fatty alcohols, albeit under harsher conditions (H 2 pressure and temperature) [47].
Despite the reasonably better performance of the bi-metallic composite NiCoNP@MgAl 2 O 4 than NiNP@MgAl 2 O 4 (entries 7-8, Table 2), we decided to pursue the reaction scope studies with the mono-metallic catalyst due to its better activity at 1 mol% Ni loading and easier characterization. It is worth mentioning that no significant loss of fatty acids by adsorption on any of the six catalytic composites was observed, recovering quantitatively the organic compounds. For further optimization parameters, see Table S3 in the Supplementary Materials.
Substrate scope studies using other mono-and poly-unsaturated C 18 fatty acids were then performed (Table 3). Elaidic acid, the (E)-isomer of oleic acid, was efficiently hydrogenated albeit in lower conversion (72% conversion; entry 1, Table 3) in comparison to oleic acid (96% conversion; entry 5, Table 3). Linoleic acid (4), showing two C=C bonds at C 9 and C 12 , led to 69% of mono-unsaturated acid and 31% of stearic acid with nearly full conversion (entry 2, Table 3). Moreover, α-linolenic acid (5), featuring three C=C bonds at C 9 , C 12 , and C 15 positions, only gave 86% conversion to mono-unsaturated fatty acids (entry 3, Table 3); longer reaction time (4 h) and higher catalyst loading (2 mol% Ni) were required to completely transform both linoleic and linolenic acids to the fully saturated stearic acid (entries 4-5, Table 3). The determination of metal traces present in the organic extracts after the 4 h benchmark reaction using NiNP@MgAl 2 O 4 catalyst revealed the presence of trace amounts of Ni (1.1 ppm by ICP-AES close to the detection limit of this technique), revealing negligible leaching of the catalyst and pointing to a surface reactivity of the catalyst. waste supplied by the enterprise SAPOVAL was used for this study. Thus, NiNP@MgAl2O4 showed high efficiency in the hydrogenation of the agri-food waste, mainly constituted 76% of unsaturated fatty acids and 24% of saturated ones (stearic acid, 20%; palmitic acid, 4%). At 180 °C for 4 h using 2 mol% Ni under 5 bar H2, the hydrogenation of the agri-food waste took place quantitatively, giving a highly enriched stearic acid sample (entry 6, Table 3).

Conclusions
In this contribution, a general strategy encompassing the preparation of first-row nanocomposites based on Co and Ni nanoparticles supported on MgAl2O4 spinel or TiO2 and their use as sustainable catalytic materials towards selective hydrogenation processes of fatty acid substrates is reported. Thus, six original and well-defined mono-metallic (NiNP@MgAl2O4, CoNP@MgAl2O4, NiNP@TiO2, and CoNP@TiO2) and bi-metallic (Ni-CoNP@MgAl2O4 and NiCoNP@TiO2) nanocomposite materials were prepared by onestep procedure following an organometallic bottom-up approach under smooth conditions, and fully characterized by (HR)TEM, ICP, PXRD, FTIR, XPS, XAS, and magnetization. The catalytic performance of MgAl2O4 and TiO2-based composites towards fatty acid hydrogenation was evaluated, revealing the superior role of the former as support for the From an application viewpoint, we assessed the efficiency of NiNP@MgAl 2 O 4 towards the valorization of fatty wastes coming from agri-food industries via selective C=C double-bond hydrogenation; the fully saturated products, namely stearic and palmitic acids and mixtures thereof are found in a large variety of products (e.g., food supplements, emulsifiers, surfactants, cosmetics, and plastics) [48]. In particular, a sample from duck fat waste supplied by the enterprise SAPOVAL was used for this study. Thus, NiNP@MgAl 2 O 4 showed high efficiency in the hydrogenation of the agri-food waste, mainly constituted 76% of unsaturated fatty acids and 24% of saturated ones (stearic acid, 20%; palmitic acid, 4%). At 180 • C for 4 h using 2 mol% Ni under 5 bar H 2 , the hydrogenation of the agri-food waste took place quantitatively, giving a highly enriched stearic acid sample (entry 6, Table 3).

Conclusions
In this contribution, a general strategy encompassing the preparation of first-row nanocomposites based on Co and Ni nanoparticles supported on MgAl 2 O 4 spinel or TiO 2 and their use as sustainable catalytic materials towards selective hydrogenation processes of fatty acid substrates is reported. Thus, six original and well-defined monometallic (NiNP@MgAl 2 O 4 , CoNP@MgAl 2 O 4 , NiNP@TiO 2 , and CoNP@TiO 2 ) and bimetallic (NiCoNP@MgAl 2 O 4 and NiCoNP@TiO 2 ) nanocomposite materials were prepared by one-step procedure following an organometallic bottom-up approach under smooth conditions, and fully characterized by (HR)TEM, ICP, PXRD, FTIR, XPS, XAS, and magnetization. The catalytic performance of MgAl 2 O 4 and TiO 2 -based composites towards fatty acid hydrogenation was evaluated, revealing the superior role of the former as support for the immobilization of mono-and bi-metallic Ni and Co nanoparticles, where the choice of MgAl 2 O 4 support presenting both acidic and basic sites might play a better role than the amphoteric properties of TiO 2 , which can induce catalyst deactivation such as the formation of Ni soaps known to block the active catalyst surface [18].
Despite their prone oxidation, XPS analysis was instrumental in assessing the content of zero-valent nickel and cobalt in the as-prepared nanocomposites. Their efficacy towards the C=C bond hydrogenation of fatty acids was evaluated. The two nickel-based systems supported on MgAl 2 O 4 , namely the mono-metallic NiNP@MgAl 2 O 4 and the bimetallic NiNP@MgAl 2 O 4 , showed the best catalytic performances operating at 1 mol% catalyst loading and 180 • C for 30 min under 5 bar H 2 pressure. Despite the slightly lower conversion obtained for NiCoNP@MgAl 2 O 4 in comparison to the mono-metallic NiNP@MgAl 2 O 4 at 1 mol% catalyst loadings (93 and 96% conv., respectively), this bimetallic composite permitted to work at sub-mol% catalyst loadings (0.6 mol%) maintaining a good conversion and exclusive selectivity towards stearic acid. NiNP@MgAl 2 O 4 was successfully applied in the selective C=C bond hydrogenation of mono-and polyunsaturated C 18 fatty acids, including waste from agri-food industrial residues. Overall, the NiNP@MgAl 2 O 4 catalyst offers great promise to carry out free fatty acid hydrogenations under milder H 2 pressures (5 bar) and shorter times (30 min to 4 h) than the literature precedents (operating up to 70 bar and 300 • C, see Table S4 in the Supplementary Materials for a selection of recent contributions).  Figure S16: Nickel L 2,3 XAS spectra of NiNP@TiO 2 (orange), NiNP@MgAl 2 O 4 (blue), NiCoNP@TiO 2 (green), and NiCoNP@MgAl 2 O 4 (red) with references of NiO (black), Ni(OH) 2 (black dotted line) and Ni(0) (grey); Figure S17: Cobalt L 2,3 XAS spectra of NiCoNP@MgAl 2 O 4 (red), NiCoNP@TiO 2 (green), Co(0) reference (grey), and an atomic multiplet simulation of Co(II) (black dotted line); Figure S18: XPS analyses: Ni 2p XPS core level spectra of Ni metal, NiO and Ni(OH) 2 and corresponding envelopes from the Ni 2p 3/2 fit (left); Co 2p XPS core level spectra of Co metal, Co 3 O 4 , CoO and Co(OH) 2 and corresponding envelopes from the Co 2p 3/2 fit (right); Table S1: Elemental and ICP-AES analyses;  Disclaimer/Publisher's Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.