Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 110

FIGURE 6.14 STM images (4 × 4 nm 2 ) of Ru(000l) acquired at T = 6 K. (a) Surface containing
approximately 0.03 mL of C prepared by segregation from the bulk. The C atoms appear as depressions
(black spots). (b) After introducing H atoms (from water dissociation in this experiment), C is converted
to CH (bright protrusion surrounded by a dark ring). A similar transformation occurs with H obtained
from H 2 dissociation. Individual nonreacted H atoms appear as smaller dark spots. Tunneling condition
in (a) is V sample = 50 mV and I t = 295 pA, and (b) V sample = 9 mV and I t = 495 pA. The total z scale is
adjusted to be 50 pm in both images. Source : Reproduced with permission from Shimizu et al. [41].
materials can vary substantially, the fundamental chemistry for their reaction
with hydrogen is similar. As an example, the reaction of single-walled
carbon nanotubes (SWNTs) with hydrogen gas studied in the temperature
range of 400-550°C and at hydrogen pressure of 50 bar showed that hydro-
genation of nanotubes was observed for samples treated at 400-450°C with
about 1/3 of carbon atoms forming covalent C-H bonds, whereas hydrogen
treatment at higher temperatures (550°C) occurred as an etching process,
which was associated with the formation of light hydrocarbons [42]. The
etching reaction of hydrogen on the edges of nanotubes was preferable com-
pared with direct etching of the nanotube wall and starts at lower tempera-
tures (400-450°C). Small hydrocarbon molecules such as CH 4 could be
formed, especially at higher temperatures like 550°C. The reactions were
likely facilitated by Fe nanoparticles that acted as catalysts for hydrogen
dissociation. While this example demonstrates that it is possible to produce
hydrocarbons from reactions of solid carbon such as CNTs with hydrogen,
it is unclear yet if such reactions can become practically useful in storing
hydrogen. Further studies are needed to understand the yield, mechanism,
kinetics, and energy balance.
6.4.3 Reaction between Carbon Dioxide and Hydrogen
CO 2 is usually the major by-product of hydrogen release or generation from
hydrocarbons, as discussed in Chapter 2. In order to recycle or reuse the
carbon, it would be ideal to hydrogenate the CO 2 back to hydrocarbons. This
is a demanding task, similar to hydrogenation of carbon directly. However,