Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 111

in principle, this can be accomplished, usually assisted with catalysis. Prod-
ucts of reaction between CO 2 and hydrogen include hydrocarbons such as
methane and alcohols, for example methanol. An excellent review on this
topic has appeared recently [43]. The basic steps for catalytic hydrocarbon
generation involve conversion of CO 2 into CO followed by dissociation of
CO into C and O and finally step-wise attachment of H to C. However, the
detailed reaction mechanisms and intermediates are still not well understood,
despite extensive studies.
For example, hydrogenation of CO 2 into CO is an important first step
towards CO 2 reduction and hydrogen storage. This can be accomplished via
the reserve water gas shift (RWGS) reaction:

− 1

CO + = H CO H O H + , ∆ = 41 2 . kJ mol ⋅ .

(6.15)

2 2 2 298 K

This reaction is effective mainly with assistance of catalysts, such as Ni/
CeO 2 , and supported Cu, Fe, Pt, Ru, and Rh. The reaction mechanism has
been examined extensively for Cu-based catalysts, which in itself is still
controversial [43]. Two major mechanisms have been proposed: redox and
formate decomposition. The redox mechanism is modeled by the following
reactions:

CO + 2 Cu

0

→ Cu O

(6.16)

2 2

0

H + Cu O → 2 Cu + H O .

(6.17)

2 2 2

Cu 0 atoms are active in CO 2 dissociation and the reduction of the oxidized
Cu catalyst has to be faster than the oxidation process. Hydrogen is proposed
to be the reducing reagent without direct participation in the formation of
intermediates [44]. The second model based on formate decomposition sug-
gests that CO is formed from decomposition of formate intermediates derived
from the association of hydrogen with CO 2 [45,46]. Different mechanisms
have been proposed for the reaction with Pd- or Pt-based catalysts. For
example, in a study involving Pd/Al 2 O 3 catalysts and supercritical mixture
of CO 2 and H 2 , the formation of surface species such as carbonate, formate,
and CO has been indicated based on infrared spectra [47].
Another important relevant reaction is methanation of CO 2 , which is ther-
modynamically favorable with ΔG 298 K = −130.8 kJ·mol −1 :

− 1

CO + 4 H → CH + 2 H O H , ∆ = 252 9 . kJ mol ⋅ .

(6.18)

2 2 4 2 298 K

This reaction is ideal since it produces CH 4 , which can be easily used with
current infrastructure and the by-product is water. The reaction is kinetically