Dynamics and Energetics of Methane …
113
There may be a discussion here as to whether or not the 3f–O atoms can be regarded
as unsaturated sites. In the case of the PdO surface, they are no doubt classified as an
unsaturated site because the O atoms are in the tetrahedral coordination environment
and they have four bonding partners in the crystalline bulk structure. However, in the
case of the IrO 2 or RuO 2 surface, they might well be regarded as saturated because
their coordination pattern is not tetrahedral and they enjoy only three bonding partners
even in the bulk structure.
Similarly to the (101) surface of PdO, one can easily find a pair of cus metal and cus
O atoms in the proximity of each other both on the (110) and (100) surfaces of IrO 2
and RuO 2 . The 5f–Ir/Ru sites appear to act as an active site for methane activation.
A dative bond between the C–H bond of methane and the surface metal cus site will
be generated. However, there is a recent study that uses the low-energy ion scattering
spectroscopy (LEISS) techniques, temperature-programmed reaction spectroscopy
(TPRS) methods, and density functional theory (DFT) calculations [43], in which
the authors found that the cus 5f–Ir sites on the IrO 2 (100) surface are completely
terminated by oxygen atoms. The condition of being devoid of the cus Ir atoms on
the (100) surface makes it less reactive, so that it may be challenging to prepare a
catalytically active IrO 2 (100) surface. This observation can be corroborated by a
TPRS spectrum showing a significantly weak binding of H 2 O and CO molecules on
the surface [43]. Were it possible to generate an O-free, stoichiometric IrO 2 (100)
surface, the C–H bond of methane could be effectively activated [20].
When it comes to the RuO 2 (110) surface, two regions with (1 × 1) and c(2 ×
2) symmetries have been identified, and the (1 × 1) phase possesses a high catalytic
activity, yet the c(2 × 2) one does not [44]. The coexistence of the (1 × 1) and c(2 ×
2) phases on the RuO 2 (110) surface makes things intricate. Though a low-energyelectron diffraction inspection supported by a scanning tunneling microscopy (STM)
measurement pointed out that the c(2 × 2) phase is a reconstructed structure [45],
the surface structure of the c(2 × 2) phase has yet been unknown at the atomic level.
As far as the (1 × 1) phase of the (110) surface of RuO 2 goes, the surface structure
has been well-defined and is expected to be close to what is shown in Fig. 7 with the
help of LEED and DFT [41]. Its high catalytic activity toward CO is known [44].
3.3 Adsorption of Methane on a Metal Oxide
A modus operandi of methane activation on the surface of a transition metal oxide
is the following: First methane’s C-H bond forms a dative interaction with a cus
metal site on the surface, and second the neighboring cus O site abstracts the H atom
of the activated C–H bond [46]. In the literature, one can easily find a variety of
experimental and theoretical studies on methane activation on late transition metal
oxide surfaces, including PdO (101) [37, 47, 48], IrO 2 (110) [29, 29–51], and RuO 2
(110) surfaces [20, 52]. Among them, the study of the IrO 2 (110) surface [29] has
made a significant impact because it was experimentally revealed that the initial C–H
bond dissociation of methane highly facilely occurs on the surface at temperatures
113
There may be a discussion here as to whether or not the 3f–O atoms can be regarded
as unsaturated sites. In the case of the PdO surface, they are no doubt classified as an
unsaturated site because the O atoms are in the tetrahedral coordination environment
and they have four bonding partners in the crystalline bulk structure. However, in the
case of the IrO 2 or RuO 2 surface, they might well be regarded as saturated because
their coordination pattern is not tetrahedral and they enjoy only three bonding partners
even in the bulk structure.
Similarly to the (101) surface of PdO, one can easily find a pair of cus metal and cus
O atoms in the proximity of each other both on the (110) and (100) surfaces of IrO 2
and RuO 2 . The 5f–Ir/Ru sites appear to act as an active site for methane activation.
A dative bond between the C–H bond of methane and the surface metal cus site will
be generated. However, there is a recent study that uses the low-energy ion scattering
spectroscopy (LEISS) techniques, temperature-programmed reaction spectroscopy
(TPRS) methods, and density functional theory (DFT) calculations [43], in which
the authors found that the cus 5f–Ir sites on the IrO 2 (100) surface are completely
terminated by oxygen atoms. The condition of being devoid of the cus Ir atoms on
the (100) surface makes it less reactive, so that it may be challenging to prepare a
catalytically active IrO 2 (100) surface. This observation can be corroborated by a
TPRS spectrum showing a significantly weak binding of H 2 O and CO molecules on
the surface [43]. Were it possible to generate an O-free, stoichiometric IrO 2 (100)
surface, the C–H bond of methane could be effectively activated [20].
When it comes to the RuO 2 (110) surface, two regions with (1 × 1) and c(2 ×
2) symmetries have been identified, and the (1 × 1) phase possesses a high catalytic
activity, yet the c(2 × 2) one does not [44]. The coexistence of the (1 × 1) and c(2 ×
2) phases on the RuO 2 (110) surface makes things intricate. Though a low-energyelectron diffraction inspection supported by a scanning tunneling microscopy (STM)
measurement pointed out that the c(2 × 2) phase is a reconstructed structure [45],
the surface structure of the c(2 × 2) phase has yet been unknown at the atomic level.
As far as the (1 × 1) phase of the (110) surface of RuO 2 goes, the surface structure
has been well-defined and is expected to be close to what is shown in Fig. 7 with the
help of LEED and DFT [41]. Its high catalytic activity toward CO is known [44].
3.3 Adsorption of Methane on a Metal Oxide
A modus operandi of methane activation on the surface of a transition metal oxide
is the following: First methane’s C-H bond forms a dative interaction with a cus
metal site on the surface, and second the neighboring cus O site abstracts the H atom
of the activated C–H bond [46]. In the literature, one can easily find a variety of
experimental and theoretical studies on methane activation on late transition metal
oxide surfaces, including PdO (101) [37, 47, 48], IrO 2 (110) [29, 29–51], and RuO 2
(110) surfaces [20, 52]. Among them, the study of the IrO 2 (110) surface [29] has
made a significant impact because it was experimentally revealed that the initial C–H
bond dissociation of methane highly facilely occurs on the surface at temperatures