358
W. Piskorz and F. Zasada
2 CO → C + CO 2 ) and through a COH intermediate. Nucleation and subsequent
oligomerisation of carbon deactivate the catalyst.
In the review by Tang et al. [161], there are discussed three important factors
governing initial steps of the NH 3 -SCR process, namely the interaction of process
reactants—NO, O 2 , and NH 3 —with ceria. The presence of O 2 relevantly increases
the efficiency of NO reduction, although in the absence of O 2 the reaction still takes
place. The prerequisite step for the reduction of NO by ammonia catalysed by ceria
is the adsorption which is facilitated by the surface energy rise due to the presence of
defects like kinks, steps, or vacancies. This was confirmed by the DFT+U calculations
by Yang et al. [162] and shown that the adsorption energy of NO on reduced ceria is
ca. 10 times higher, then on stoichiometric ceria. The DFT+U calculations [163] of
NO on reduced CeO 2 (110) revealed the preference of NO to adsorb on the surface
oxygen vacancy. The subsequent NO 2 formation–dissociation cycles lead to the NO
diffusion on the ceria surface. Two NO molecules adsorbed at neighbour O vacancies
can form a N 2 O 2 dimer, eventually liberating N 2 , what was confirmed by Nolan [164]
who studied computationally also the CO adsorption. The adsorption of NO on nonstoichiometric ceria easily leads to the NO dissociation due to the strong interaction
between NO and ceria, what was confirmed by Luo et al. [165] who found that during
the thermoprogrammed desorption of NO the new species: N 2 , N 2 O, and O 2 were
detected.
The NO 2 adsorption on (100), (110), and (111) surfaces of defected ceria leads
to the embedding of single O into the vacancy, to bending of the O–N–O and to
elongation of N−O vac bond (by ca. 10%) with accompanying partial reoxidation of
Ce
3+ , as modelled computationally by Nolan et al. [166]. The adsorption energies
were found: −2.40 eV for the (111) surface,−2.25 eV for the (110) surface, and
−2.32 eV for the (100) surface.
One of the main motivations of the carbon dioxide reduction by hydrogen is the
facilitation of the remote product transport using the conventional pipeline system,
what is virtually impossible in the case of hydrogen. Such process is economically
efficient with source of cheap hydrogen other than the fossil hydrocarbons, e.g. from
the biomass, or, perspectively, from water electrolysis. The CO 2 reduction process is
also interesting from the fundamental point of view, as a way to study the utilisation
of CO 2 yielding other products of industrial importance.
It is commonly assumed that the CO 2 methanation follows two steps: the first one
is the reduction of CO 2 to CO and further conversion to alkanes along the FischerTropsch process. The CO 2 reduction process was modelled computationally by, e.g.,
Sharma et al. [167] (Ru-doped ceria), who studied their systems with LDA+U(5.5)
approach and concluded that LDA+U gives better agreement with experiment than
GGA+U [25, 26] as for the unit cell parameter and the energetics of the reaction
Ce 2 O 3 + 1 /2O 2 → 2 CeO 2 . Sharma et al. [167] conclude that ruthenium atoms tend
to stay at (or migrate to, if allowed, i.e. if well equilibrated) the surface region and the
catalytic CO 2 methanation over Ru-doped ceria does not include the Fischer-Tropsch
step. The authors also point out the application of the Sabatier principle (they call
it the “moderation principle”), namely the substitutional doping by the replacement
of the host cations by the lower valency ones (e.g. La-doped ceria) weakens the
W. Piskorz and F. Zasada
2 CO → C + CO 2 ) and through a COH intermediate. Nucleation and subsequent
oligomerisation of carbon deactivate the catalyst.
In the review by Tang et al. [161], there are discussed three important factors
governing initial steps of the NH 3 -SCR process, namely the interaction of process
reactants—NO, O 2 , and NH 3 —with ceria. The presence of O 2 relevantly increases
the efficiency of NO reduction, although in the absence of O 2 the reaction still takes
place. The prerequisite step for the reduction of NO by ammonia catalysed by ceria
is the adsorption which is facilitated by the surface energy rise due to the presence of
defects like kinks, steps, or vacancies. This was confirmed by the DFT+U calculations
by Yang et al. [162] and shown that the adsorption energy of NO on reduced ceria is
ca. 10 times higher, then on stoichiometric ceria. The DFT+U calculations [163] of
NO on reduced CeO 2 (110) revealed the preference of NO to adsorb on the surface
oxygen vacancy. The subsequent NO 2 formation–dissociation cycles lead to the NO
diffusion on the ceria surface. Two NO molecules adsorbed at neighbour O vacancies
can form a N 2 O 2 dimer, eventually liberating N 2 , what was confirmed by Nolan [164]
who studied computationally also the CO adsorption. The adsorption of NO on nonstoichiometric ceria easily leads to the NO dissociation due to the strong interaction
between NO and ceria, what was confirmed by Luo et al. [165] who found that during
the thermoprogrammed desorption of NO the new species: N 2 , N 2 O, and O 2 were
detected.
The NO 2 adsorption on (100), (110), and (111) surfaces of defected ceria leads
to the embedding of single O into the vacancy, to bending of the O–N–O and to
elongation of N−O vac bond (by ca. 10%) with accompanying partial reoxidation of
Ce
3+ , as modelled computationally by Nolan et al. [166]. The adsorption energies
were found: −2.40 eV for the (111) surface,−2.25 eV for the (110) surface, and
−2.32 eV for the (100) surface.
One of the main motivations of the carbon dioxide reduction by hydrogen is the
facilitation of the remote product transport using the conventional pipeline system,
what is virtually impossible in the case of hydrogen. Such process is economically
efficient with source of cheap hydrogen other than the fossil hydrocarbons, e.g. from
the biomass, or, perspectively, from water electrolysis. The CO 2 reduction process is
also interesting from the fundamental point of view, as a way to study the utilisation
of CO 2 yielding other products of industrial importance.
It is commonly assumed that the CO 2 methanation follows two steps: the first one
is the reduction of CO 2 to CO and further conversion to alkanes along the FischerTropsch process. The CO 2 reduction process was modelled computationally by, e.g.,
Sharma et al. [167] (Ru-doped ceria), who studied their systems with LDA+U(5.5)
approach and concluded that LDA+U gives better agreement with experiment than
GGA+U [25, 26] as for the unit cell parameter and the energetics of the reaction
Ce 2 O 3 + 1 /2O 2 → 2 CeO 2 . Sharma et al. [167] conclude that ruthenium atoms tend
to stay at (or migrate to, if allowed, i.e. if well equilibrated) the surface region and the
catalytic CO 2 methanation over Ru-doped ceria does not include the Fischer-Tropsch
step. The authors also point out the application of the Sabatier principle (they call
it the “moderation principle”), namely the substitutional doping by the replacement
of the host cations by the lower valency ones (e.g. La-doped ceria) weakens the