## 1 Introduction
Many scientists tried to reproduce these claims, but unfortunately, they were not reproducible [36; 37; 38; 39; 40]. A pessimistic conclusion about the storage limit in CNTs was reported to be 1.0 wt.% or even lower, while any claims above this limit were a form of experimental error [41; 42; 43; 44]. Although the activated microporous carbon was reported to meet the DOE's target [45; 46; 47; 48], and the hydrogen storage claim of 23.76 wt.% was reported by [PERSON] et al. [49], others disputed these claims [34; 50; 51]. The biggest excitement was caused by [PERSON] et al. [52], when he claimed a room-temperature hydrogen storage capacity of 67 wt.% on graphite nanofibers (GNF). [PERSON]'s results were not reproducible, but some others claimed some values up to 10 wt.% [53] and 6.5 wt.% [54]. Such discrepancies, which were found in these reports, could be attributed to the problems of assessing hydrogen uptake and non-standardization of the techniques used for the evaluation, besides the interpretation of the experimental data. The main factors limiting the storage of hydrogen on the surface of carbonaceous materials are the low adsorption energy (~4-8 kJ mol\({}^{-1}\)) for the majority of the porous carbons and the weak interactions among hydrogen molecules, which decrease the possibility formation of more than one layer of hydrogen [55; 56; 57; 58; 59]. Decoration of the graphitic surface with metals may improve the hydrogen adsorption by creating strong interaction sites. Furthermore, these metal centers can facilitate the dissociation of the hydrogen molecules to atomic hydrogen, and, subsequently, making them diffuse onto the graphitic skeleton through the spillover mechanism [28; 48; 60; 61; 62; 63; 64; 65; 66]. However, the spillover effect was a dilemma for long time between theoretical and experimental studies due to hydrides formation [67; 68; 69; 70], in addition to the inactivity of the metal surface due to clustering [71], hampering the spillover effect. Many carbonaceous systems were studied to explore the spillover effect in hydrogen storage and hydrogenation [72; 73; 74; 75; 76; 77; 78]. Modifying carbonaceous structures by hetero-atoms, such as boron or nitrogen, before metal decoration, was suggested to enhance their catalytic effect in comparison to the unmodified ones [79; 71; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89]. Previously, we tested platinum-decorated, ammonia-treated activated carbons for carbon dioxide and methane gas storage [90]. They showed high storage capacities for both of these, with slight improvement upon ammonia treatment and more preference towards carbon dioxide gas storage. The ammonia treatment was used as a doping source for nitrogen, and acted as an erosion agent for enhancing the surface area of the activated carbons in addition to its role in removing impurities and amorphous carbon before the platinum metal decoration step [88; 90; 91]. In this work, we further tested other transition metal-decorated (Pd, Ni, Rh, Ir and Ru) AC samples for hydrogen storage, at cryogenic and ambient temperatures, before and after modification. Furthermore, we aimed to investigate the hydrogen spillover phenomenon and its dependence on the identity of the metal at the studied two temperatures.