## 3 Results and Discussion
### Hydrogen Storage Measurements  
Figures 10-15 show the hydrogen adsorption and desorption curves at both 77 K and 298 K for pristine and ammonia-treated activated carbon and their decoration with Pt, Pd, Ni, Rh, Ir and Ru, respectively. Their excess hydrogen storage capacities are summarized in Table 2.  
Figure 10: Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Pt at (**a**) 77 K and (**b**) 298 K.  
Figure 9: Adsorption average pore widths using the BET for pristine activated carbon (**a**) and ammonia-treated activated carbon (**b**), and both were decorated with Pt (**c**,**d**). Figure 11: Pressure-composition isotherms for AC samples before and after ammonia treatment and decorated with Pd at (**a**) 77 K and (**b**) 298 K.  
Figure 12: Pressure-composition isotherms for AC samples before and after ammonia treatment and decorated with Ni at (**a**) 77 K and (**b**) 298 K.  
Figure 13: Pressure-composition isotherms for AC samples before and after ammonia treatment and decorated with Rh at (**a**) 77 K and (**b**) 298 K.  
At cryogenic temperatures, the isotherm shape may be a hybrid of a Type I isotherm, indicated by the steep uptake at low pressure, and a Type IV isotherm, indicated by gradual increase, beyond the steep region, to the saturation point [102]. At the low-pressure region around the ambient pressure, an initial sharp gas uptake occurred, owing to the strong interaction between hydrogen gas molecules and the narrow micropores, and this indicated their complete filling. The adsorption curves were overlapped for all of the samples at low pressure, indicating similar micropore fillings for all of the samples, which is consistent with the stability of the micropore volume with ammonia treatment and metal decoration. As the micropores are just a portion of the total porosity, the filling of mesopores and macropores will begin after filling of the micropores and is indicated by the gradual increase in adsorption beyond the steep uptake. The fillings of the mesopores and macropores take place through overlapping between the monolayer/multilayer adsorption [102]. The hydrogen storage uptake at 77 K maximized around 7.0 bar, reached saturation, and then decreased with increasing hydrogen gas pressure. The descending behavior after reaching the maximum was experimentally reported [34] as a supercritical adsorption and a typical behavior for the excess isotherm model reported by [PERSON] et al. [103; 104; 105]. It also follows the same approach proposed by [PERSON] et al. [106], who described the decrease in the excess adsorbed gas, defined by Gibbs excess. At the maximum, a complete filling of the adsorbed  
Figure 14: Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Ir at (**a**) 77 K and (**b**) 298 K.  
Figure 15: Pressure-composition isotherms for AC samples before and after ammonia treatment and decorated with Ru at (**a**) 77 K and (**b**) 298 K.  
gas was achieved, and then the excess adsorbed gas would decrease to zero, and a further increase in pressure would result in negative values [97; 107]. At ambient temperature, the adsorption behavior was different in comparison to cryogenic temperature, as the porosity filling was increasing gradually for all samples over the studied pressure range, and the excess amount did not reach the maximum over the applied pressure range due to the very weak adsorption forces at this temperature [59]. These excess hydrogen storage capacities agree with the data reported for several carbonaceous materials with similar textural properties at these conditions [28; 37; 59; 67; 68; 108; 109], and are also reliable with the theoretical hydrogen storage capacity values, calculated by [PERSON] et al. [110], for carbonaceous materials with similar textural properties.