Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 112

limited because it requires eight electrons to fully oxide carbon in CO 2 to
methane, which requires catalysts to achieve acceptable rates and selectivi-
ties [48]. Common catalysts for this reaction include Ni, Ru, and Rh sup-
ported on various oxides such as SiO 2 , TiO 2 , Al 2 O 3 , and CeO 2 . Figure 6.15
shows representative TEM and EDS images of Pd-Mg/SiO 2 catalysts [48].
The results show that the as-synthesized Pd-Mg/SiO 2 catalyst after calcina-
tion at 550°C in air for 6 hours contains well-dispersed electron-dense par-
ticles identified as containing Pd by EDS within a matrix of less dense,
noncrystalline silica shells, with a certain degree of aggregation of Pd par-
ticles. After reaction with CO 2 and hydrogen for 10 hours, the particles
remain well dispersed even though more larger Pd particles appeared, indi-
cating sintering of some particles that are probably not fully encapsulated by
the Mg/Si oxide.
The reaction mechanism is still not well established and the proposed
mechanisms fall into two general categories. The first one involves the con-
version of CO 2 to CO before methanation [49-51], while the second one
involves direct hydrogenation of CO 2 to methane without forming CO as an
intermediate [52, 53]. Even for mathanation of CO, there is no consensus on
the kinetics and mechanism. It has been suggested that the rate-limiting step
is either the formation of a CH x O intermediate and its hydrogenation or the
formation of surface carbon via CO dissociation and its interaction with
hydrogen [51, 54].
To gain insight into the reaction mechanism, it is useful to determine
reaction intermediates. For instance, kinetic studies based on steady-state
transient measurements conducted on Ru/TiO 2 catalyst have identified
several reaction intermediates [50]. Hydrogenation of CO as a key intermedi-
ate leads to methane formation. Formates, as intermediates for CO formation,
are bound strongly on the support in equilibrium and become active
species at the metal and support interface. A proposed mechanism for
the formation of the formate thorough a carbonate species is shown in
Figure 6.16.
Similarly, other hydrocarbons can be produced based on reaction between
CO 2 and hydrogen. The reactions are often divided into two categories:
methanol mediated and nonmethanol mediated. In the methanol-mediated
approach, CO 2 and H 2 react over Cu-Zn-based catalysts to produce metha-
nol, which is subsequently converted into other hydrocarbons such as gaso-
line. Light alkanes are usually generated as major products due to further
catalytic hydrogenation of the alkenes. In the nonmethanol-mediated
approach, CO 2 hydrogenation proceeds in two steps: RWGS reaction, which
converts CO 2 into CO, and then the Fisher-Tropsch (FT) reaction, which
converts CO into hydrocarbons via further hydrogenation.