## 4 Results and Discussion
### Reversed Flow - Inverse Gas Chromatography (RF-IGC)  
The hydrogen adsorption phenomena can reveal how adsorption properties and surface properties are changed under aging conditions. There can be variations concerning the nature and the different groups of the active sites for hydrogen adsorption over Ni catalysts. Thus, the various adsorption parameters are here presented which are related to hydrogen adsorption in the context of maleic acid hydrogenation in a temperature that allows for the maximum catalytic activity (80 \({}^{\circ}\)C). The local energy of adsorption (\(\varepsilon\)), the local isotherm (\(\theta\)), the local maximum monolayer capacity (c*max), the local adsorbed equilibrium concentration of CO (c*s) as well as the energy distribution function \(\phi\) (\(\varepsilon;\mathfrak{t}\)) are the physicochemical parameters that were determined by RF-IGC, and they are related to the examination of the surface heterogeneity. The time distribution of the lateral local adsorption energies, among the adsorbed species for the different powdered catalyst samples, illustrates the different characteristics of the surfaces of catalysts prepared at different quantities of water (Figure 8a) and aging durations (Figure 8b). Figure 8a depicts the time variation of the lateral local adsorption energies which are the points distinguishing the various types of active sites in each catalyst. The change of activity during aging can be explained by the presence of at least two types of active centers on the catalyst surface (Figure 8b). Some of them are responsible for adsorption reactions, and their number per unit surface of the catalyst does not change during aging. Others catalyze the reaction of hydrogen redistribution, and when the catalyst is aging, their properties and the number of active centers per unit surface significantly change. Figure 8: Time-distribution of the lateral local adsorption energies among adsorbed species for the powdered catalyst samples: **(a)** S-27 (prepared at different quantities of water) and **(b)** S-27 (75 mL H\({}_{2}\)O, 1 day aged). Figure 9a-c depicts the rough aspect of the surface heterogeneity of the powdered catalyst S-27 (75 mL of water in the initial mixture): fresh catalyst and 1 day aged catalyst (first and second experiment) through the distribution density function of the local adsorption energies over energy. The various curves shown in Figure 9, obtained for the hydrogen adsorption over the produced catalysts, illustrate the existence of three types of active sites with respect to adsorption energy \(\varepsilon\). Groups A, B, and C (Figure 9a) indicate the three groups of active sites corresponding to higher, intermediate, and lower energies of adsorption, respectively. In general, Type A active sites, corresponding to high values of \(\varepsilon\), demonstrate a narrower distribution compared to the main curve B, while the type C active sites, which correspond to lower energies of adsorption, form a narrow distribution. After 1 day of aging of the SCS nanocatalyst, the adsorption of hydrogen changed and there was only one group of active centers with respect to the adsorption energy (Figure 9b). This justifies the change in the activity of the Ni nanocatalysts after 1 day of aging. The aged catalyst (first experiment) (Figure 6b) had only one type of active site because of the changing composition and structure as shown in the XPS and TEM results. The aged catalyst (second experiment) (Figure 9c) had only two types of active sites due to the reduction of NiO with hydrogen after the first experiment and which resulted in the nickel providing the second type of active site. A possible explanation for the change in the number of active centers with aging is the structural and composition alterations. The difference between the fresh catalyst and 1 day aged catalyst was that, in the 1 day aged catalyst, the concentrations of NiO and Ni\({}_{2}\)O\({}_{3}\) increased on the surface because of the oxidation process as shown in the XPS analysis.