Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
379
calculated energy barriers of NH 3 , NH 2 , and NH dehydrogenation are in the range
of 29–67 kJ mol
–1 , indicating a high activity of the surface towards dehydrogenation and that the surface reactivity is attributed to the presence of a low-coordinated
surface lattice site.
The activation of gas-phase oxygen was addressed by modelling of the isotopic
oxygen exchange reaction (
18 O 2 +
16 O 2 → 2
16 O
18 O) over cobalt spinel (100) surface in time- and temperature-resolved catalytic experiments [352]. Overall process was divided into four elementary steps, and the corresponding reaction barriers
(both electronic and entropic contributions) were calculated by means of periodic
GGA+U and atomistic thermodynamics. The mobility of lattice oxygen (crucial in
MvK mechanism) was also studied [349, 354], showing that DFT+U calculated
vacancy formation energies conjoined with first-principles thermodynamics are able
to reproduce results of TPD-O 2 experiment over cobalt spinel [83]. In this context,
it was also reported that oxygen vacancy formation is generally much easier on
NiCo 2 O 4 (001) than on Co 3 O 4 (001) surfaces, suggesting that the former oxide may
be a better catalyst for oxidation reactions based on the MvK mechanism [375].
Low-cost catalytic decomposition of N 2 O into N 2 and O 2 is a subject of intensive investigations due to its harmful impact on the environment [256, 376], and the
kinetic of N 2 O decomposition over oxide material containing transition metal ions
has been extensively investigated to find the correlation between the electronic structure of the catalysts (characterised essentially by their electron donor properties) and
their reactivity [377–379]. The most promising performance was observed so far for
cobalt spinel-based catalysts [308, 380]. The energetics of the postulated elementary
steps (N 2 O adsorption, N 2 O activation through dissociative electron or oxygen atom
transfer, adoxygen surface diffusion and recombination and dioxygen desorption)
was calculated using a cluster approach [310]. It was concluded the N 2 O activation
and the formation of dioxygen are energetically most demanding steps, whereas the
barrier for the oxygen surface diffusion was distinctly smaller. The effect of potassium promotion on deN 2 O activity of various 3d electron spinels (Mn 3 O 4 , Fe 3 O 4 ,
Co 3 O 4 ) [381] was investigated in terms of periodic DFT-rPBE molecular modelling
of a surface dipole model (K
δ+ –O
δ−
surf ) whose formation weakened the interfacial
potential and increased the Fermi energy leading to the decrease of the calculated
spinel work function by 0.45 eV (very close to the experimental ΔΔ = 0.5 eV).
To better understand the hydrogenation mechanism, Lu et al. [382] modelled the
reaction steps using DFT+U+D. They have demonstrated that H 2 dissociation is a
complicated two-step process, followed by the surface diffusion of H adatom. The
calculations reveal that the presence of the surface oxygen vacancy facilitates the
stepwise hydrogenation of ethylene (drop of activation energy from 1.19 to 0.47 eV)
due to the weaker bond strength of formed OH group.
Spinel-type oxides are promising also in the ORR process because they have
excellent electrical conductivity originating from the electron hopping between different valence states of metals in octahedral sites, and in this way, they provide active
metal centres for O 2 adsorption and activation in ORR.
Recently, Si et al. [383] performed DFT-PW91 calculations to determine the structure and energy of O 2 , HOO*, O*, HO*, HOOH, H 2 OO, and CH 3 OH molecules
379
calculated energy barriers of NH 3 , NH 2 , and NH dehydrogenation are in the range
of 29–67 kJ mol
–1 , indicating a high activity of the surface towards dehydrogenation and that the surface reactivity is attributed to the presence of a low-coordinated
surface lattice site.
The activation of gas-phase oxygen was addressed by modelling of the isotopic
oxygen exchange reaction (
18 O 2 +
16 O 2 → 2
16 O
18 O) over cobalt spinel (100) surface in time- and temperature-resolved catalytic experiments [352]. Overall process was divided into four elementary steps, and the corresponding reaction barriers
(both electronic and entropic contributions) were calculated by means of periodic
GGA+U and atomistic thermodynamics. The mobility of lattice oxygen (crucial in
MvK mechanism) was also studied [349, 354], showing that DFT+U calculated
vacancy formation energies conjoined with first-principles thermodynamics are able
to reproduce results of TPD-O 2 experiment over cobalt spinel [83]. In this context,
it was also reported that oxygen vacancy formation is generally much easier on
NiCo 2 O 4 (001) than on Co 3 O 4 (001) surfaces, suggesting that the former oxide may
be a better catalyst for oxidation reactions based on the MvK mechanism [375].
Low-cost catalytic decomposition of N 2 O into N 2 and O 2 is a subject of intensive investigations due to its harmful impact on the environment [256, 376], and the
kinetic of N 2 O decomposition over oxide material containing transition metal ions
has been extensively investigated to find the correlation between the electronic structure of the catalysts (characterised essentially by their electron donor properties) and
their reactivity [377–379]. The most promising performance was observed so far for
cobalt spinel-based catalysts [308, 380]. The energetics of the postulated elementary
steps (N 2 O adsorption, N 2 O activation through dissociative electron or oxygen atom
transfer, adoxygen surface diffusion and recombination and dioxygen desorption)
was calculated using a cluster approach [310]. It was concluded the N 2 O activation
and the formation of dioxygen are energetically most demanding steps, whereas the
barrier for the oxygen surface diffusion was distinctly smaller. The effect of potassium promotion on deN 2 O activity of various 3d electron spinels (Mn 3 O 4 , Fe 3 O 4 ,
Co 3 O 4 ) [381] was investigated in terms of periodic DFT-rPBE molecular modelling
of a surface dipole model (K
δ+ –O
δ−
surf ) whose formation weakened the interfacial
potential and increased the Fermi energy leading to the decrease of the calculated
spinel work function by 0.45 eV (very close to the experimental ΔΔ = 0.5 eV).
To better understand the hydrogenation mechanism, Lu et al. [382] modelled the
reaction steps using DFT+U+D. They have demonstrated that H 2 dissociation is a
complicated two-step process, followed by the surface diffusion of H adatom. The
calculations reveal that the presence of the surface oxygen vacancy facilitates the
stepwise hydrogenation of ethylene (drop of activation energy from 1.19 to 0.47 eV)
due to the weaker bond strength of formed OH group.
Spinel-type oxides are promising also in the ORR process because they have
excellent electrical conductivity originating from the electron hopping between different valence states of metals in octahedral sites, and in this way, they provide active
metal centres for O 2 adsorption and activation in ORR.
Recently, Si et al. [383] performed DFT-PW91 calculations to determine the structure and energy of O 2 , HOO*, O*, HO*, HOOH, H 2 OO, and CH 3 OH molecules