## 3 Results
### Thermal Stability  
Thermogravimetric analysis (TGA) was utilized as a substantial technique to monitor the thermal stability of biochar and match it with CuNi-decorated biochar. For comparison, we have also plotted the thermograms of B@Cu and B@Ni prepared with single metal ion nitrate. The sequence of residual weight % at a high temperature is Biochar < B@Ni < B@Cu < B@CuNi. This is a logical sequence as, under air, biochar is burnt out, whereas, with nickel, the final biochar is loaded with metallic nickel, which converts to NiO. Copper converts to CuO, and, as both copper and nickel nitrates were impregnated at an equal number of moles per gram of biomass, it is logical to obtain a slightly higher weight % for B@Cu compared to B@Ni. With an increased final number of moles per gram of biochar (see Table 1), biochar burns out, but not the CuNi alloy, which converts to oxides, hence the higher weight % for B@CuNi (lower weight loss). In the literature, carbon@Cu and carbon@Ni prepared by the annealing of the corresponding metal organic frameworks showed only a slight decrease in weight loss because TGA was conducted under an inert atmosphere [20]. In contrast, biochar loaded with a copper complex catalyst exhibited substantial weight loss under air, with a steady state reached at around 500 \({}^{\circ}\)C and a final 50% weight [31]. As far as the nickel-biochar-based magnetic catalyst is concerned, a weight loss of 49.8% was reported at 800 \({}^{\circ}\)C, presumably under air, but this was not specified [32]. Returning to the main hybrid B@CuNi and reference material Biochar, both samples show a comparable thermal stability behaviour until reaching 240 \({}^{\circ}\)C for steady samples withstanding temperature, as shown in Figure 7. Thermogravimetric analysis was carried out under air. A slight increase in the weight of the pristine biochar sample is observed at 280 \({}^{\circ}\)C; this is correlated with the oxidation of both biochar and the metal alloy. It might have influenced the biochar specimen and led to a slight increase in its weight %. At higher temperatures, biochar started to decompose at 330-340 \({}^{\circ}\)C until reaching 500 \({}^{\circ}\)C.  
For B@CuNi, a plateau region was reached and corresponds to 42% residual weight of the original sample. The sample retained its thermal stability up to 270-280 \({}^{\circ}\)C, then a decomposition step occurred at elevated temperatures to reach a steady state at 455 \({}^{\circ}\)C with a residual weight of 45% of the original weight, higher than that of biochar due to loaded metallic nanoparticles [20]. Figure 6: Peak-fitted Raman spectrum of olive pit biochar. From a quantitative viewpoint, the thermograms were obtained under air, which suggests the formation of CuO and NiO, and the weight % differences between pristine and decorated biochar should account for the oxides. B@Cu has a residual weight of 32.6%, and that of biochar is 12.5%. The difference of 20.1% accounts for CuO, which is 0.201 g CuO for 1 g of hybrid biochar B@C, which corresponds to 2.53 mmol Cu per gram of hybrid biochar. This matches the value of 2.489 mmol/g reported in Table 1, corresponding to an error of 1.6%. With the same reasoning, and by subtracting the residual mass of biochar from that of B@Ni, one obtains 2.45 mmol nickel per gram of hybrid biochar, which is 6% less than the value of 2.608 mmol nickel/g B@Ni. Finally, concerning B@CuNi and assuming the initial exact number of moles of nickel and copper nitrates (they differ by only 1%, see Table 1), the weight % difference with biochar is 44.5\(-\)12.5 = 32%, i.e., 0.32 g of the NiO/CuO mixture (for Ni/Cu equimolar ratio) per 1 g of hybrid biochar. Assuming a hypothetical oxide CuNiO\({}_{2}\) (molar mass = 154.2 g), one would receive 4.15 mmol of copper-nickel mixture, which is 25% less than the total number of moles compared to the expected value reported in Table 1 (5.521 mmol/g).