## 3 Results
### Phase Analysis  
The prepared catalysts were analysed using both XRD and Raman to identify the nature of the metal and carbon contents, respectively. Figure 4 displays XRD patterns of B@CuNi and of the reference materials Biochar and B@Ni. Those of B@Ni and B@Cu (not shown) correspond very well to the structures of nickel and copper-face-centered cubic metals (ICDD n\({}^{\circ}\)98-004-1508 and 98-005-2256), respectively. The XRD pattern of B@CuNi exhibits the same type of diffraction lines but at 20 positions located between those of pure Cu and pure Ni phases, suggesting that the biochar serves as a support of a Ni\({}_{1-x}\)Cu\({}_{x}\) metallic solid solution. This is a strong supporting evidence that copper and nickel were alloyed during the one-step pyrolysis process. Moreover, it is noteworthy that all peaks are broadened, which is in line with the production of small crystals that were imaged by SEM (see Figure 2b,c). However, as the technique probes the bulk, no surface oxidation of the nanocatalysts was detected. This would best be accomplished using the more specific technique X-ray photoelectron spectroscopy (XPS). Indeed, elsewhere, surface oxidation of nickel [21], copper [21; 22] and iron [14] loaded on biochar was detected, as well as the partial oxidation of palladium nanoparticles loaded on organo-functionalized mesoporous silica [23]. The formation of metallic Ni and CuNi particles was also confirmed by measuring the variation in the sample magnetization M as a function of the magnetic field H at room temperature. Indeed, by cycling the magnetic field H between \(-\)20.0 and +20.0 kOe, typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity [24] were recorded on both B@CuNi and B@Ni samples (Figure 5). This feature must be underlined since it accounts for the ability of the designed catalysts to be reversibly attracted by an external magnetic field, making their recovery and reuse easy to achieve. Interestingly, as the copper alloyed nickel particles are usually less magnetic than the pure nickel ones, it may be noticed on the recorded M(H) plots that the saturation magnetization of B@Ni is much more quickly reached than that of B@CuNi. To complete these structural characterizations, Raman spectroscopy was carried out on the prepared sample, with a special emphasis on their carbon signature. The spectrum recorded in the 1000-2000 cm\({}^{-1}\) energy region for the metal-free Biochar is shown in Figure 6 to illustrate this purpose. It is fitted with five components assigned to 5 (1262 cm\({}^{-1}\); sp\({}^{2}\)-sp\({}^{3}\)), D (1380 cm\({}^{-1}\); sp\({}^{2}\), C-C between aromatic cycles and highly ordered carbon material), V (1506 cm\({}^{-1}\); sp\({}^{2}\)-sp\({}^{3}\), amorphous carbon structures), G (1596 cm\({}^{-1}\); graphite, sp\({}^{2}\)) and G\({}_{\rm{L}}\) (1696 cm\({}^{-1}\); sp\({}^{2}\), C-O) [25; 26]. Particularly, the D and G bands are assigned to disordered and ideal graphitic lattice, respectively. The D/G peak height and area ratios equal 0.66 and 0.87, respectively. The spectrum indicates, therefore, the presence of C  
Figure 3: EDX spectra of (**a**) olive pit powder particles, OPs; (**b**) OPs impregnated with metal salts (copper and nickel nitrates), and (**c**) B@CuNi. SEM images show spots probed for the acquisition of EDX spectra. EDX quantification: (**d**) elemental analysis of C, O, Cu and Ni detected at 15 spots of B@CuNi surface; and (**e**) (Cu+Ni)/(C+O) and Cu/Ni atomic ratios averaged over 15 spots. sp\({}^{2}\) atoms but no excess of amorphous sp\({}^{3}\) hydrocarbons. It is noteworthy that the D/G ratio, referring to the degree of defects in the carbon material, is much lower than those reported for hydrothermally synthesized graphene quantum dots [27].