Environmental Engineering Reference
In-Depth Information
α
phase
β
phase
β
phase
α
phase
hydriding reaction
dehydriding reaction
(a)
(b)
FIGURE 6.12
Comparison of the hydrogenation and dehydrogenation of a large and a small particle:
(a) for hydrogenation in a large particle (upper raw), multiple nucleation sites at the surface will merge
and form a closed layer that prevents fast diffusion of hydrogen to the core of the particle, slowing down
the kinetics of the
α
- to
β
-phase transition considerably. If the particle is small (lower raw), fast diffusion
of the hydrogen through the
α
-phase remains possible for a larger fraction of the
α
- to
β
-phase transition
and (b) for desorption in a large particle (upper raw), hydrogen has to diffuse through a thicker layer of
the
β
-phase before being released, while hydrogen rapidly reaches the surface for a smaller particle (lower
raw).
Source
: Reproduced with permission from Berube et al. [22].
necessary to hydrogenate carbon or CO
2
back to hydrocarbons if a sustain-
able and recyclable system is needed. The reaction of hydrogen with either
carbon or CO
2
is complex and requires special conditions, such as high pres-
sure and high temperature or assistance with catalysis. In the next section,
we will discuss the two reactions separately.
6.4.1 Reaction between Carbon Atom and Hydrogen
This seemingly simple reaction is in reality quite complex. Interestingly, the
study of the reaction between C and H or H
2
is very limited to date, partly
due to the need to produce C and H in a well-controlled environment. One
of the earlier studies found that the main products of the reaction between
carbon vaporized in a high intensity arc reactor operating at 2800 K and
hydrogen passing through the arc were acetylene, hydrogen, and condensed
carbon when the hot reaction mixture was sampled under fast-quenching
conditions [39]. Acetylene contents as high as 18.6 V% at 1 atm in the
quenched gas were obtained without diluent, and as high as 23.8 V% with
63.6% helium diluent. It was suggested that the C
2
H radicals existed in the
hot gas mixture in appreciable concentrations, which were generated from
dissociation of C
2
H
2
resulting from the reaction between carbon and hydro-
gen. The reaction products were sensitive to both pressure and temperature,
as shown in Figure 6.13. In another study, a thermally produced beam of
atomic hydrogen was reacted on a carbon target at temperatures between 30
and 950°C [40]. The reaction products isolated on a liquid helium finger and
analyzed by gas chromatography were found to be mostly CH
4
with a small
Search WWH ::
Custom Search