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Y. Tsuji et al.
the desorption barrier of methane from the surface, or the adsorption energy. This
is the apparent activation energy. As the surface–methane interaction increases, the
apparent activation energy decreases, and even becomes a negative value. Thus, it
would be possible to envisage the C–H bond dissociation reaction of methane under
the room temperature despite a large bond dissociation energy of methane, exceeding
about 100 kcal/mol.
In this context, it is natural that we should seek for a surface which has a significant
affinity to methane though a high symmetry of T d of methane makes it a non-polar
molecule so that an electrostatic interaction between methane and a surface cannot
be anticipated. However, as far as a class of metal oxides goes, coordinatively unsaturated (cus) metal sites accompanied by a neighboring cus O site appear on their
specific surfaces. A dative bond or a coordination bond can be formed between
methane and the surface cus metal site as if methane itself served as a kind of ligand
as can be seen in coordination chemistry.
The PdO (101) and IrO 2 /RuO 2 (110) and (100) surfaces meet the requirements
for the catalytic activity of the C–H bond dissociation mentioned above. Especially,
the IrO 2 (110) surface shows a remarkable activity, which is exemplified by a low
temperature activation of methane thereon. A recent experimental work has reported
that the reaction happens at the temperature as low as 150 K [29]. Such a low
temperature activation would pave a new way of upgrading methane as an important
feedstock for the chemical industry because one could have a control over the reaction
there, deterring unfavorable side reactions.
This is why we focus on the activation of the C–H bond of methane on the
IrO 2 (110) surface in the latter part of this chapter. A periodic-boundary slab-model
calculation is suitable for simulating the surface reaction. However, such a periodicity perforce obscures a discrete nature of the methane molecule on the surface,
whose electronic structure should be better described by using the molecular orbital
rather than the crystal orbital. Nevertheless, with the help of a band decomposition
technique named crystal orbital overlap population (COOP) and some small clustermodel computations, we have gained a clear picture of how methane is adsorbed
and activated on the IrO 2 surface in terms of the orbital interaction. Two important
features should be noted: First, there are bonding-type orbital interactions between
methane’s HOMOs and cus Ir
s d orbitals, reinforcing the affinity of the surface to
methane. Second, in the transition state of the abstraction of the H atom by the cus
O atom, the H atom is stabilized by an in-phase orbital interaction among H
s 1s
orbital, O
s 2p orbital, C
s 2p orbital, and Ir
s 5d yz orbital, resulting in lowering of
the activation energy of the C–H bond dissociation reaction.
Enhancing the affinity of the surface to methane is a rational strategy to create a
new heterogeneous catalyst that can effectively attract methane and break its robust
C–H bond. Thus, theoreticians are now active in finding a surface satisfying such a
requirement in order to identify a catalyst that is expected to be more active than the
IrO 2 surface. One such example is the (110) surface of β-PtO 2 [51], where a good
energy level alignment between methane’s HOMO and the unoccupied 5d z2 orbital
of the surface cus Pt atom is achieved. Even if a Pt atom is doped on the rutile-type
Y. Tsuji et al.
the desorption barrier of methane from the surface, or the adsorption energy. This
is the apparent activation energy. As the surface–methane interaction increases, the
apparent activation energy decreases, and even becomes a negative value. Thus, it
would be possible to envisage the C–H bond dissociation reaction of methane under
the room temperature despite a large bond dissociation energy of methane, exceeding
about 100 kcal/mol.
In this context, it is natural that we should seek for a surface which has a significant
affinity to methane though a high symmetry of T d of methane makes it a non-polar
molecule so that an electrostatic interaction between methane and a surface cannot
be anticipated. However, as far as a class of metal oxides goes, coordinatively unsaturated (cus) metal sites accompanied by a neighboring cus O site appear on their
specific surfaces. A dative bond or a coordination bond can be formed between
methane and the surface cus metal site as if methane itself served as a kind of ligand
as can be seen in coordination chemistry.
The PdO (101) and IrO 2 /RuO 2 (110) and (100) surfaces meet the requirements
for the catalytic activity of the C–H bond dissociation mentioned above. Especially,
the IrO 2 (110) surface shows a remarkable activity, which is exemplified by a low
temperature activation of methane thereon. A recent experimental work has reported
that the reaction happens at the temperature as low as 150 K [29]. Such a low
temperature activation would pave a new way of upgrading methane as an important
feedstock for the chemical industry because one could have a control over the reaction
there, deterring unfavorable side reactions.
This is why we focus on the activation of the C–H bond of methane on the
IrO 2 (110) surface in the latter part of this chapter. A periodic-boundary slab-model
calculation is suitable for simulating the surface reaction. However, such a periodicity perforce obscures a discrete nature of the methane molecule on the surface,
whose electronic structure should be better described by using the molecular orbital
rather than the crystal orbital. Nevertheless, with the help of a band decomposition
technique named crystal orbital overlap population (COOP) and some small clustermodel computations, we have gained a clear picture of how methane is adsorbed
and activated on the IrO 2 surface in terms of the orbital interaction. Two important
features should be noted: First, there are bonding-type orbital interactions between
methane’s HOMOs and cus Ir
s d orbitals, reinforcing the affinity of the surface to
methane. Second, in the transition state of the abstraction of the H atom by the cus
O atom, the H atom is stabilized by an in-phase orbital interaction among H
s 1s
orbital, O
s 2p orbital, C
s 2p orbital, and Ir
s 5d yz orbital, resulting in lowering of
the activation energy of the C–H bond dissociation reaction.
Enhancing the affinity of the surface to methane is a rational strategy to create a
new heterogeneous catalyst that can effectively attract methane and break its robust
C–H bond. Thus, theoreticians are now active in finding a surface satisfying such a
requirement in order to identify a catalyst that is expected to be more active than the
IrO 2 surface. One such example is the (110) surface of β-PtO 2 [51], where a good
energy level alignment between methane’s HOMO and the unoccupied 5d z2 orbital
of the surface cus Pt atom is achieved. Even if a Pt atom is doped on the rutile-type