Page 230 - Advances in bioenergy (2016)

3+
3+
activation takes place on a surface Ce site with formation of CO, whereas Ce is oxidized to
4+
Ce . Oxygen vacancies act as ‘additional driving force’ for the reduction of CO to CO. 88
2
Adsorption and dissociation of the CO molecule has been observed to be faster over
2
Ni/La O than over Ni/Al O . Over Ni/La O , the CO molecule interacts with the carrier to
2 3
2
2 3
2 3
form La O CO species, which decompose to produce CO and oxygen species, important for
3
2 2
the DRM reaction mechanism. The much higher affinity of Ni/La O catalyst for CO 2
2 3
chemisorption could be attributed to the higher basicity of lanthana as compared with that of
alumina. 82
Primary elementary steps of the DRM reaction are the adsorption and decomposition of CH 4
on active sites on the metallic surface, forming hydrogen and methyl-like adsorbed species and
the dissociative adsorption of CO on the metal oxide surface, preferably on the metal–support
2
interface, forming CO and oxygen adsorbed species (Figure 9.4a). Once methane and carbon
dioxide are adsorbed, many surface reactions occur, leading to desired or undesired products.
Most of the reaction steps are fast and arrive at equilibrium, e.g. desorption of CO from the
support and of hydrogen from the metallic surface (Figure 9.4b). Kinetic investigations of
DRM have shown that the WGS reaction is near equilibrium over a wide range of
temperatures. The quasi-equilibrium of the WGS reaction implies that the surface reaction
steps related to this reaction are fast. Most kinetic models predict hydrogen spillover from the
metal surface to the support where it reacts with oxygen species forming hydroxyl groups
(Figure 9.4c), while oxygen spillover from the support to the metal also occurs (Figure 9.4d).
However, at temperatures higher than 1073 K, the presence of hydroxyl groups on the support
is not likely. Oxygen migrating on the metal surface reacts with hydrogen depleted S CH
1 x
species (0 ≤ x ≤ 3) forming S CH x O species (Figure 9.4d). It has been suggested that H O,
1
2
produced on the support and migrating to the metal–support interfacial region, participates in
the formation of CH O. The formation and/or the decomposition of S CH x O species to CO
x
1
and H are considered as RDS. 82,84 What seems to be crucial is the relative rate of oxidation of
2
S CH x species as compared to its dissociation: higher rates of oxidation means x > 0 in
1
S CH x O species, whereas higher S CH x decomposition rate leads to the complete
1
1
decomposition of the S CH x species forming surface carbon (S C). If the rate of oxidation
1
1
of S C is not fast enough, carbonaceous species accumulate, leading to catalyst deactivation.
1