1. The success of the CO 2 conversion depends on three factors: catalyst activity, catalyst stability (which depends significantly on the coke formation and
the nature of the coke), and efficient heat transfer operations. While there
are numerous catalysts examined in the literature, it is clear that nickel
catalyst is still the most practical from an economic point of view at the
commercial scale. Noble metal catalysts such as Rh, Ru, and Pt are more
active and perhaps more stable, but they are too expensive to be of commercial value. Future research should be focus on bimetallic catalysts such
as Ni—Ru. Ru is about 40–50 times less expensive than Rh, and therefore,
it will carry more practical viability for the commercial process [57–62].
2. The nature of the catalyst support is also very important [57]. The support
often interacts with metals, and because of that, it is often considered as
part of the catalyst. The best situation is the uniform distribution of very
active metals in small sizes distributed along the support and they do not
migrate or sinter during high-temperature reforming process. Perovskite
support offers special attraction because in this case metals are uniformly
and tightly distributed in the support lattice. The catalyst must be a basic
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Water for Energy and Fuel Production
fragments; oxidative dehydrogenations of ethane and propane also proceed partially. The catalysts are in more reduced state and the activation of the hydrocarbon
is the rate-controlling step. In the case of C 4 and higher hydrocarbons, the first step
of the process is direct hydrogenation of alkanes. Activation of carbon dioxide, but
not the activation of hydrocarbon, is the rate-controlling step. Hydrogen formed
interacts with carbon dioxide and shifts the equilibrium of the dehydrogenation
reaction.
Just as for methane, a required condition for dry reforming of higher hydrocarbon is that the catalyst system adsorbs and activates carbon dioxide. The
acidic property of CO 2 necessitates the choice of a catalyst with basic properties. However, alkali metal and alkaline earth oxides are ineffective because of
strong carbonate formation. Oxides of a moderate basicity are necessary, and
moreover, they must participate in the redox process with CO 2 reduction. While
MnO was used in the earlier studies, its modification by oxides of K, Na, Cr,
and La influences both its acceptor function and the degree of surface oxidation. It controls the mechanism of hydrocarbons and alcohol transformations.
Possible other good candidates are La 2 O 3 , cesium oxides, and praseodymium
oxides. La 2 O 3 showed the greatest interactions among CO 2 , hydrocarbons, and
alcohols. Binary oxide-based support system and dual metals can improve the
performance. Promoters and the method of catalyst preparation also have an
effect on the catalyst performance [57].
As mentioned earlier, the dry reforming of hydrocarbons leads to a variety of
products and the transformation to syngas with different degrees of success depending on the operating conditions and the nature of the catalyst. The major issues with
dry reforming are (1) endothermic nature of reaction requiring high-energy input for
the reaction process, (2) difficulty in igniting the reaction at low temperature (lower
than about 500°C), and (3) requiring very high temperature (>650°C) to reduce coke
deposition on the catalyst. In sum,