Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
377
are usually beyond the validity of density functional theory and the methods like GW
are used.
The fact that distinct surface planes of oxides differs in concentration and coordination of the exposed ions (active sites) is in line with the general observation
that the catalytic activity of the oxide catalysts depends on the grain size and surface morphological features which together influence the nature of the active sites
exposed by the spinel samples [339, 340]. The {110} facets of Co 3 O 4 nanocrystals
have higher catalytic activity for CO oxidation than the {100} and {111}, which is
reasoned by the high concentration of octahedral Co
3+ sites [341]. Contrary, for the
ORR reaction, the predominant {111} and {100} surfaces exhibit higher catalytic
activity attributed to the exposition of Co
2+ cations [342]. On the other hand, in the
case of methane combustion, the catalytic performance of the Co 3 O 4 is believed to
be related to the surface energy [343].
Owing to present state of computational chemistry, fairly accurate first-principles
modelling of surface structure and energetics made the theoretical predictions of
the resultant morphology for faceted nanocrystals possible [344]. By employing the
Wulff construction along with the ab initio thermodynamics, the influence of chemical environment can also be taken into account [345]. The mixed cobalt spinels were
studied computationally, and the predicted nanocrystal shapes were in good agreement with experiment [334, 346]. Such environmentally independent morphology
prediction may be treated as the first-order approximation of ambient conditions
and, via the first-principles thermodynamics, the inclusion of entropic corrections
and polarisable continuum (DFTsol) leads to more realistic shape modelling. Such
approach was recently employed for cobalt oxide which high catalytic performance
in redox processes attributed usually to the presence of reactive oxygen species (ROS)
and to lability of lattice oxygen. The latter can be released with formation of surface
oxygen vacancies even under mild conditions [347, 348].
In this context, the influence of oxygen rich and lean conditions on different terminations of cobalt spinel most abundant surfaces was studied [323, 349–351]. For
most abundant (100) surface, both reactive oxygen species [350, 352] and oxygen
vacancies [349, 353] were studied by means of DFT combined with ab initio thermodynamics revealing that three principal states of the spinel surface in function of temperature correspond to oxygen adsorption (T < 350−400
◦ C), bare surface region
(350−400
◦ C to 550−700
◦ C), and oxygen release region (T > 550−700
◦ C).
Recently, similar approach was employed for (111) surface, revealing that the
ROS species and vacancies temperature stabilities region are shifted towards higher
temperatures in this case [354]. As for (110) surface in contact with gaseous O 2 ,
Selloni et al. presented a comparative study of and using DFT+U calculations with
different U values, discussing influence of Hubbard parameter on structural parameters, electronic properties, and surface energetics [320]. Surface energy calculations
indicate that the (110)-A termination is more stable in a wide range of the oxygen
chemical potential [355], in agreement with surface science experiments [345, 356].
377
are usually beyond the validity of density functional theory and the methods like GW
are used.
The fact that distinct surface planes of oxides differs in concentration and coordination of the exposed ions (active sites) is in line with the general observation
that the catalytic activity of the oxide catalysts depends on the grain size and surface morphological features which together influence the nature of the active sites
exposed by the spinel samples [339, 340]. The {110} facets of Co 3 O 4 nanocrystals
have higher catalytic activity for CO oxidation than the {100} and {111}, which is
reasoned by the high concentration of octahedral Co
3+ sites [341]. Contrary, for the
ORR reaction, the predominant {111} and {100} surfaces exhibit higher catalytic
activity attributed to the exposition of Co
2+ cations [342]. On the other hand, in the
case of methane combustion, the catalytic performance of the Co 3 O 4 is believed to
be related to the surface energy [343].
Owing to present state of computational chemistry, fairly accurate first-principles
modelling of surface structure and energetics made the theoretical predictions of
the resultant morphology for faceted nanocrystals possible [344]. By employing the
Wulff construction along with the ab initio thermodynamics, the influence of chemical environment can also be taken into account [345]. The mixed cobalt spinels were
studied computationally, and the predicted nanocrystal shapes were in good agreement with experiment [334, 346]. Such environmentally independent morphology
prediction may be treated as the first-order approximation of ambient conditions
and, via the first-principles thermodynamics, the inclusion of entropic corrections
and polarisable continuum (DFTsol) leads to more realistic shape modelling. Such
approach was recently employed for cobalt oxide which high catalytic performance
in redox processes attributed usually to the presence of reactive oxygen species (ROS)
and to lability of lattice oxygen. The latter can be released with formation of surface
oxygen vacancies even under mild conditions [347, 348].
In this context, the influence of oxygen rich and lean conditions on different terminations of cobalt spinel most abundant surfaces was studied [323, 349–351]. For
most abundant (100) surface, both reactive oxygen species [350, 352] and oxygen
vacancies [349, 353] were studied by means of DFT combined with ab initio thermodynamics revealing that three principal states of the spinel surface in function of temperature correspond to oxygen adsorption (T < 350−400
◦ C), bare surface region
(350−400
◦ C to 550−700
◦ C), and oxygen release region (T > 550−700
◦ C).
Recently, similar approach was employed for (111) surface, revealing that the
ROS species and vacancies temperature stabilities region are shifted towards higher
temperatures in this case [354]. As for (110) surface in contact with gaseous O 2 ,
Selloni et al. presented a comparative study of and using DFT+U calculations with
different U values, discussing influence of Hubbard parameter on structural parameters, electronic properties, and surface energetics [320]. Surface energy calculations
indicate that the (110)-A termination is more stable in a wide range of the oxygen
chemical potential [355], in agreement with surface science experiments [345, 356].