Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 107

FIGURE 6.11 The spillover mechanism: the hydrogen molecules dissociate on the catalyst. Some
hydrogen atoms remain attached to the catalyst, while others diffuse to the catalyst support and subse-
quently penetrate into the metal, where the hydrogen is said to spill over and interact directly with the
metal. Source : Reproduced with permission from Berube et al. [22].
process and increases the resistance to contaminants and air exposure.
In addition, as shown in Figure 6.5, the later kinetics of the hydrogena-
tion process is governed by the diffusion of hydrogen atom through the
β -phase layer formed around the particle. Since the hydrogen diffusion
rate in the β -phase can be significantly smaller than that in the α -phase,
minimizing this effect could promote faster kinetics. Figure 6.12 shows
that for a sufficiently small particle, a closed β -phase layer may not be
formed, so that the hydrogen atoms could have fast pathways to access
the particle core during the hydrogenation reaction or leave the core
upon the dehydrogenation reaction [22]
6.4 HYDROCARBONS FOR HYDROGEN STORAGE
As discussed in Chapter 2, hydrocarbons are organic compounds that contain
hydrogen and carbon atoms. In principle, hydrocarbons could be used for
hydrogen storage. In practice, this is quite challenging, since hydrocarbons
are generally very stable under ambient conditions, and the release of hydro-
gen from hydrocarbons is a highly endothermic process. High temperature
steam reforming is a common process for hydrogen generation from hydro-
carbons, as discussed in detail in Chapter 2.
Upon hydrogen release, either carbon (C) or carbon dioxide (CO 2 ) is
produced as a by-product. For the purpose of hydrogen storage, it would be