Page 229 - Advances in bioenergy (2016)

Attempts have been made to define the number of hydrogen atoms in CH species adsorbed on
x
transition metal surfaces. Such species have been detected using both steady-state and transient
isotopic tracing experiments, the latter being more representative of phenomena on working

metal surfaces. 81,85 The dissociation of CH is thought to yield a distribution of CH species
4
x
with x depending on the metal and the support. 81
Like methane, the dissociation and reduction of CO are also reported to be structure sensitive,
2
81
promoted at defect sites, such as corner atoms. Ab initio valence-bond calculations and
spectroscopic data indicate that CO can adsorb dissociatively on various metal surfaces (Pt,
2
Pd, Rh, Re, Ni, Fe, Cu, Ag, Al, and Mg), involving electron transfer to the CO moiety, which
2
is accompanied by an elongation of the CO bond with respect to the free molecule. 12

In the presence of methane, the dissociation of CO is promoted, although there are no
2
86
indications for the formation of any surface complexes between the two reactants. The
enhanced dissociation of CO is attributed to the presence of surface hydrogen species and the
2
consecutive formation of carbonyl-hydride species. The effect of the support for the
dissociation of CO to CO is crucial. For example, for Pd, the order of activity for the
2
86
dissociation at 773 K: Pd/TiO > Pd/A1 O > Pd/SiO > Pd/MgO. As the DRM activity of
2 2 3 2
the catalysts, based on turnover frequencies, follows the same order. Enhanced carbon dioxide
decomposition results in higher surface concentration of reactive oxygen species. As the latter
are required for the activation of methane, their increase leads to enhancement of the DRM
rate. As more oxygen vacancies are present on the titania surface, promoting the adsorption and

the dissociation of carbon dioxide. 86

Differences in carbon dioxide adsorption behavior related to the support have been reported
for various catalytic systems. The chemisorption of carbon dioxide on the Ni/TiO catalyst
2
occurred with a heat of adsorption in the order of 1 kcal/mol, indicative of weak adsorption.
On Ru/SiO , both CH and CO are activated on the metallic phase, whereas a bifunctional
4
2
2
87
mechanism is proposed for Ru/Al O . Although Ru is able to activate and dissociate CO , in
2 3
2
the presence of a support such as alumina, a bifunctional mechanism takes place. While
methane adsorbs on ruthenium, an alternative and more effective path for CO activation is
2
−
followed comprising the formation of HCO on the alumina surface and its decomposition to
2
CO and hydroxyl groups on the support. The latter diffuse toward the metal particles where
oxidation of the carbonaceous adspecies, located on the metallic surface, takes place. 87

An important category of oxide materials, which are used as catalyst components for the DRM,
are the oxides of rare earths, particularly ceria and lanthana. There are good reasons for that,
one being the high activity of these materials for the adsorption and activation of CO . 82,85,88 A
2
mechanism of interaction between M/CeO (M = Rh, Ru, Pt, Pd, and Ir) and CO influenced by
2
2
88
reduction temperature. By increasing reduction temperature, a progressive reduction of bulk
CeO takes place, which is not promoted by the presence of the metal. CO adsorption and
2
2