## 4 Results and Discussion
### Reversed Flow - Inverse Gas Chromatography (RF-IGC)
The difference between the first and second experiment for the 1 day aged catalyst was the nickel concentration on the surface, due to the reduction in the nickel oxide after the first experiment. Figure 9: _Cont._  
Figure 10 shows the aging effect's influence on the non-adsorbed hydrogen concentration on the fresh catalyst as well as on the 1 day aged catalyst experiments. The time-variation curves of the equilibrium adsorbed concentration (\(c_{y}\)) are depicted in Figure 10, where a linear regression fit was attempted in each case. It appears that the highest gradient appeared for the 1 day aged catalyst from the second experiment and the lowest for the 1 day aged catalyst from the first experiment. This fact, as well as the fact that the highest values for the equilibrium adsorbed concentration were measured for the catalysts, allows for determining the apparent activity ranking of these types of catalysts as follows: fresh \(>\) 1 day aged first experiment \(>\) 1 day aged second experiment. Rapid reduction of the non-adsorbed hydrogen concentration in the second experiment in comparison with first experiment can be connected to the different compositions of the catalysts because of the partial reduction NiO to Ni during the first experiment. It is known that the oxide reduction temperature depends on the particle size, and the usual temperatures for reduction are 300-500 \({}^{\circ}\)C. Nano nickel oxide can be reduced at temperatures even less than 100 \({}^{\circ}\)C [41; 42]. Figure 10: The aging effect on the non-adsorbed hydrogen concentration on the catalyst surface of fresh catalyst, 1 day aged catalyst (first experiment), and 1 day aged catalyst (second experiment) concerning the system H\({}_{2}\)/Ni (75 mL of water in the initial mixture). Figure 9: The energy distribution function \(\phi\left(\varepsilon;t\right)\) versus the local adsorption energy, \(\varepsilon\), at the temperature of the maximum catalytic activity (80 \({}^{\circ}\)C) for the adsorption of hydrogen on Ni nanocatalysts (sample 27): (**a**) fresh catalyst, (**b**) 1 day aged catalyst ( first experiment), and (**c**) 1 day aged catalyst (second experiment). Thus, the reduction at 80 \({}^{\circ}\)C could be originated to the size of the nanoparticles and the localized temperature raise; it is possibly connected to local overheating due to the exothermic reaction O\({}_{2}\) + H\({}_{2}\). The O\({}_{2}\) can stay in the pores even after the vacuum treatment in cases where the pores have a complicated shape. If the partial reduction of a catalyst is possible, then after the first experiment with the fresh catalyst, the catalyst would possess a higher concentration of nickel on the surface, and that explains the quicker adsorption of hydrogen than on NiO (first experiment) and the lower concentration of non-adsorbed hydrogen in the second experiment. Figure 11 exhibits the influence of aging on local monolayer capacities, and it describes how Cs\({}_{\text{max}}\) changes with the catalyst aging. Thus, in the case of fresh catalyst and aged catalyst, the local monolayer changed 2-3 times. The t\({}_{\text{max}}\) in the latter case was 50, 40, and 30 min, respectively. Therefore, the monolayer capacity and the time of its maximum was a function of aging time. It is possible that when the catalyst is aged, the nickel crystal lattice alters and it is better suited for hydrogen adsorption, explaining why the local monolayer was finalized more quickly compared to the fresh catalyst. After the first experiment, where NiO was partially reduced to Ni, the hydrogen adsorption was quicker and the monolayer originated more quickly at the second experiment in comparison with the first one. Figure 12 displays the role of catalyst aging in the local adsorption energy.It appears that the energy of adsorption decreased because of the aging effect. This connected with the origination of more suitable crystal lattice parameters for hydrogen adsorption during aging.