## 3 Results and Discussion
Figure 2: XRD patterns from the solution combustion synthesis (SCS) derived materials on the basis of the initial mixture of 66.7% Ni(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O and 33.3% glycine with different amounts of water added in the initial solution (**a**) with and (**b**) without heated stirring. Based on the data obtained by the XRD patterns, the heated stirring appeared to have a major influence on the final products' composition. When heated stirring was applied in the precursor SCS solution up to 70 \({}^{\circ}\)C, the concentration of nickel in the final product was strongly enhanced, thus improving their catalytic activity in the liquid-phase hydrogenation of maleic acid. The pretreatment influenced the nickel nitrate-glycine complexes that were formed in the precursor solution in the form of dendrites, which play a key role in the SCS reaction mechanism and the formation of nickel. When these dendrites existed (with pretreatment) the reaction took place at lower velocity, as it was yielded through their complicated structure, and thus there was less time available for the oxidation of nickel. Furthermore, the heated stirring before SCS had a considerable effect on the nanopowders' microstructure as shown in Figures 4 and 5 that exhibit the calculated crystal lattice spacing and crystallite size for nickel and nickel oxide. The standard deviation for the crystal lattice spacing values was estimated to be \(\pm\)0.0005 A with a crystallite size of \(\pm\)3 nm. Figure 3: Semi-quantitative analysis of the (**a**) Ni and (**b**) NiO phases in the resulting materials. Figure 4: Influence of heated stirring on the crystal lattice spacing of (**a**) Ni and (**b**) NiO. Figure 5: \(Cont.\)  
The pretreatment on the precursor solution appeared to have a more complicated influence on the crystal lattice spacing of nickel and nickel oxide. When 50 and 100 mL of water were added, pretreatment resulted in a reduction of crystal lattice spacing for both phases. On the other hand, when 75 mL of water was added in the initial mixture, the applied heated stirring did not significantly influence the crystal lattice spacing. The observed changes in the lattice spacing suggested alteration of the materials' defect structure, which is a major characteristic concerning their catalytic performance. The influence of pretreatment on the crystallite size of nickel and nickel oxide was also clearly observed. The calculated (using Scherrer's formula) crystallite size of both phases grew when the pretreatment was applied on the initial SCS solution. The existence of dendrites led to the inhibition of oxygen diffusion on the foam-like structure due to the denser structure (i.e., lower height of dendrites in the final product). Accordingly, the oxidation of glycine was not completed, yielding carbon and carbon monoxide, which reduced nickel oxide to nickel. This double thermal treatment led to the occurrence of a sintering process and an increase of crystallite size. Larger crystallites have less surface energy and therefore tend to agglomerate less. The extent of crystallite agglomeration severely influenced the material's specific surface area, as shown in Figure 6. The standard deviation of the surface analyzer's values was estimated to be \(\pm\)0.1 m\({}^{2}\)/g. Figure 5: Influence of water quantity and heated stirring on the (**a**) Ni and (**b**) NiO crystallite size. Figure 6: Influence of water quantity and heated stirring on the specific surface area of the produced materials. The specific surface area (SSA) of the as-burnt materials was also altered by the pretreatment of the precursor solution (refer to Figure 6). The effect of the pretreatment on the material's specific surface area was beneficial, as the SSA increased for both water quantities examined, suggesting its advantageous nature. These results are in accordance with the previous calculated crystallite size for pretreated and not-pretreated materials.