378
W. Piskorz and F. Zasada
Catalytic Activity
Due to their unique properties in redox catalysis, spinels are often regarded as lowcost and stable substitutes of noble metals.
As in any TM oxides, the defects in the spinel-type oxides are crucial in the
electrocatalysis and photocatalysis activity [357]. It is well accepted that creation of
oxygen vacancies can improve the electron conductivity of oxides [358] and that such
defects are involved in catalytic activity in MvK mechanism [359]. In this context, it
is beneficial that for many spinel-type oxides the concentration of oxygen vacancy
may be controlled easily by O 2 partial pressure or the synthesis temperature [360,
361]. Non-intrinsic defects are often caused by external dopants such as substitutions
of A and B sites with different cations, thus the cation-doped spinel oxides can
be expressed as A 1−x A
x B 2-y B
y O 4 (0 x /y 1). The possibility of feasible nonintrinsic defect introduction provides the powerful tool for improvements of the
catalyst performance by bulk modification (doping with alien cations) [362, 363] or
by tuning the surface properties of the catalyst with alkali promoters [364, 365].
To address the phenomenon of CO oxidation at temperature as low as −77
◦ C,
Wang et al. [366] studied computationally the commonly exposed faces of Co 3 O 4 and
concluded that the MvK mechanism was preferred and Co
3+ was the active site. Furthermore, the high reactivity of low-coordination oxygen ions was attributed to their
weak binding to the substrate lattice [367]. The MvK mechanism was also assessed
computationally (DFT, PBE functional) in the CO oxidation by N 2 O over Co 3 O 4
(110) surface [368]. To explain spectacular activity of Ni-doped cobalt spinel in CH 4
combustion, Hu et al. investigated a complete catalytic cycle for methane combustion on the Co 3 O 4 (110) surface and compared it with that on the Co 3 O 4 (100) surface on the basis of first-principles calculations [369]. The conditions of Langmuir–
Hinshelwood and MvK mechanisms in CH 4 oxidation over cobalt spinel were shown
computationally, and the continuous Co partial reduction adapts to the catalyst redox
state to the current thermodynamic conditions [359]. It was shown [370] that in the
methane-to-methanol process, the surface Co–O pairs were the active sites, where
the two ions provide a synergistic effect for the first C–H bond activation to yield
surface Co–CH 3 and O–H species. Methanol oxidation on cobalt spinel (110) and
(100) surfaces was studied by means of spin-polarised density functional theory with
the GGA+U framework [371], and the results indicate that CH 3 OH can adsorb to
surface lattice oxygen atom (O 2f /O 3f ) to form Co–O bond directly, and the adsorption of CH 3 OH and its decomposition products on (110)-B is more stable than on
(111)-B, which means CH 3 OH prefers Co
3+ rather than Co
2+ .
Adsorption and oxidation of NH 3 were studied on ZnFe 2 O 4 [372] and Co 3 O 4 [373,
374] spinels. In the first case, it was shown (PW91 functional) that NH 3 molecule
preferred to adsorb on the surface Zn atom over the spinel ZnFe 2 O 4 (110) surface
with adsorption energy of 203.125 kJ/mol. Such stable configuration of H 3 N@Zn
corresponded to high activation degree of NH 3 with pronounced electron transfer (–0.157 e) and two N–H bonds elongated significantly. For cobalt spinel, the
successive dehydrogenation of ammonia on (110)-B termination was studied by
means of spin-polarised DFT within the PBE+U (U eff = 3.3 eV) framework and the
W. Piskorz and F. Zasada
Catalytic Activity
Due to their unique properties in redox catalysis, spinels are often regarded as lowcost and stable substitutes of noble metals.
As in any TM oxides, the defects in the spinel-type oxides are crucial in the
electrocatalysis and photocatalysis activity [357]. It is well accepted that creation of
oxygen vacancies can improve the electron conductivity of oxides [358] and that such
defects are involved in catalytic activity in MvK mechanism [359]. In this context, it
is beneficial that for many spinel-type oxides the concentration of oxygen vacancy
may be controlled easily by O 2 partial pressure or the synthesis temperature [360,
361]. Non-intrinsic defects are often caused by external dopants such as substitutions
of A and B sites with different cations, thus the cation-doped spinel oxides can
be expressed as A 1−x A
x B 2-y B
y O 4 (0 x /y 1). The possibility of feasible nonintrinsic defect introduction provides the powerful tool for improvements of the
catalyst performance by bulk modification (doping with alien cations) [362, 363] or
by tuning the surface properties of the catalyst with alkali promoters [364, 365].
To address the phenomenon of CO oxidation at temperature as low as −77
◦ C,
Wang et al. [366] studied computationally the commonly exposed faces of Co 3 O 4 and
concluded that the MvK mechanism was preferred and Co
3+ was the active site. Furthermore, the high reactivity of low-coordination oxygen ions was attributed to their
weak binding to the substrate lattice [367]. The MvK mechanism was also assessed
computationally (DFT, PBE functional) in the CO oxidation by N 2 O over Co 3 O 4
(110) surface [368]. To explain spectacular activity of Ni-doped cobalt spinel in CH 4
combustion, Hu et al. investigated a complete catalytic cycle for methane combustion on the Co 3 O 4 (110) surface and compared it with that on the Co 3 O 4 (100) surface on the basis of first-principles calculations [369]. The conditions of Langmuir–
Hinshelwood and MvK mechanisms in CH 4 oxidation over cobalt spinel were shown
computationally, and the continuous Co partial reduction adapts to the catalyst redox
state to the current thermodynamic conditions [359]. It was shown [370] that in the
methane-to-methanol process, the surface Co–O pairs were the active sites, where
the two ions provide a synergistic effect for the first C–H bond activation to yield
surface Co–CH 3 and O–H species. Methanol oxidation on cobalt spinel (110) and
(100) surfaces was studied by means of spin-polarised density functional theory with
the GGA+U framework [371], and the results indicate that CH 3 OH can adsorb to
surface lattice oxygen atom (O 2f /O 3f ) to form Co–O bond directly, and the adsorption of CH 3 OH and its decomposition products on (110)-B is more stable than on
(111)-B, which means CH 3 OH prefers Co
3+ rather than Co
2+ .
Adsorption and oxidation of NH 3 were studied on ZnFe 2 O 4 [372] and Co 3 O 4 [373,
374] spinels. In the first case, it was shown (PW91 functional) that NH 3 molecule
preferred to adsorb on the surface Zn atom over the spinel ZnFe 2 O 4 (110) surface
with adsorption energy of 203.125 kJ/mol. Such stable configuration of H 3 N@Zn
corresponded to high activation degree of NH 3 with pronounced electron transfer (–0.157 e) and two N–H bonds elongated significantly. For cobalt spinel, the
successive dehydrogenation of ammonia on (110)-B termination was studied by
means of spin-polarised DFT within the PBE+U (U eff = 3.3 eV) framework and the