112
Y. Tsuji et al.
(cus O atoms), and fourfold-coordinated (4f) saturated Pd and O atoms, each of which
runs parallel to the a-axis. Methane is anticipated to form a dative bond with a cus
Pd atom (3f-Pd) on the (101) plane [27, 37]. Here is the active site of its C–H bond
dissociation. The H atom of methane can be abstracted by the cus O atom nearby.
The existence of a pair of cus metal and cus O atoms in the vicinity of each other is
of significant importance for a facile C–H bond breaking. Here, we realize that we
are facing a contradiction between stability and reactivity. The catalytically active
surface is characterized by a high density of the cus sites, and this in turn makes
the chance of exposing of such a surface lower. We may need to make a note here,
however, that recent advances in UHV experiments have made it possible to produce
a high-quality PdO (101) thin film by oxidizing Pd (111) surface with an oxygen
atom beam [38]; thence, such a surface is indeed available.
3.2.2 IrO 2 and RuO 2 Surfaces
There are two dominant surfaces for IrO 2 and RuO 2 : (110) and (100) facets. Generally, they can be generated by oxidizing the surface of pure metal Ir or Ru. The
oxidation of the Ir (111) surface at an oxygen pressure of about 100 mbar and higher
temperatures of 775 K or above results in (110)- and (100)-oriented domains of rutile
IrO 2 [37]. Their surface characteristics have been well investigated by utilizing in situ
surface X-ray diffraction technique. Also, one could generate a high-quality IrO 2
(110) plane by oxidizing Ir (100) surface [29]. By changing oxygen exposure during
oxidation, a corundum Ir 2 O 3 (001) domain appears [40].
The RuO 2 (110) surface can be grown epitaxially on the Ru(0001) plane during
the oxidation of Ru, while the growth of the RuO 2 (100) facet can be observed on the
Ru
1010
plane [41]. In contrast with Ir, lattice matching determines the selectivity
of either (110) or (100) surfaces in the case of Ru. It is also possible to yield both
(110) and (100) facets of IrO 2 or RuO 2 on the (001) surface of SrTiO 3 or MgO
substrates on the basis of pulsed laser desorption method [42].
Properties of those surfaces have been thoroughly scrutinized by employing lowenergy electron diffraction (LEED) techniques as well as density functional theory
(DFT) calculations. DFT-calculated surface energies of the RuO 2 (110) and (100)
facets are 71 and 87 meV/Å
2 [41], respectively, implying that the (110) surface is
more stable than the (100) one. Generally, this is also the case with the IrO 2 surface
though the oxygen pressure affects the relative surface energy more or less [39].
On the (110) plane, one can spot alternating rows of threefold-coordinated
trigonal-planar O atoms (3f–O), twofold-coordinated bridging O atoms (br–O), and
fivefold-coordinated square-pyramidal Ir or Ru atoms (5f–Ir/Ru). As for the (100)
surface, alternating rows of br–O atoms and 5f–Ir/Ru atoms can be seen. In both
surfaces, the alternating rows run parallel to the a-axis. The difference in the surface
energy or stability between the two facets can be traced back to the density of the coordinatively unsaturated (cus) sites, namely the 5f–Ir/Ru and br–O sites. The density
of the cus sites on the (100) surface is higher than that of the (110) surface.
Y. Tsuji et al.
(cus O atoms), and fourfold-coordinated (4f) saturated Pd and O atoms, each of which
runs parallel to the a-axis. Methane is anticipated to form a dative bond with a cus
Pd atom (3f-Pd) on the (101) plane [27, 37]. Here is the active site of its C–H bond
dissociation. The H atom of methane can be abstracted by the cus O atom nearby.
The existence of a pair of cus metal and cus O atoms in the vicinity of each other is
of significant importance for a facile C–H bond breaking. Here, we realize that we
are facing a contradiction between stability and reactivity. The catalytically active
surface is characterized by a high density of the cus sites, and this in turn makes
the chance of exposing of such a surface lower. We may need to make a note here,
however, that recent advances in UHV experiments have made it possible to produce
a high-quality PdO (101) thin film by oxidizing Pd (111) surface with an oxygen
atom beam [38]; thence, such a surface is indeed available.
3.2.2 IrO 2 and RuO 2 Surfaces
There are two dominant surfaces for IrO 2 and RuO 2 : (110) and (100) facets. Generally, they can be generated by oxidizing the surface of pure metal Ir or Ru. The
oxidation of the Ir (111) surface at an oxygen pressure of about 100 mbar and higher
temperatures of 775 K or above results in (110)- and (100)-oriented domains of rutile
IrO 2 [37]. Their surface characteristics have been well investigated by utilizing in situ
surface X-ray diffraction technique. Also, one could generate a high-quality IrO 2
(110) plane by oxidizing Ir (100) surface [29]. By changing oxygen exposure during
oxidation, a corundum Ir 2 O 3 (001) domain appears [40].
The RuO 2 (110) surface can be grown epitaxially on the Ru(0001) plane during
the oxidation of Ru, while the growth of the RuO 2 (100) facet can be observed on the
Ru
1010
plane [41]. In contrast with Ir, lattice matching determines the selectivity
of either (110) or (100) surfaces in the case of Ru. It is also possible to yield both
(110) and (100) facets of IrO 2 or RuO 2 on the (001) surface of SrTiO 3 or MgO
substrates on the basis of pulsed laser desorption method [42].
Properties of those surfaces have been thoroughly scrutinized by employing lowenergy electron diffraction (LEED) techniques as well as density functional theory
(DFT) calculations. DFT-calculated surface energies of the RuO 2 (110) and (100)
facets are 71 and 87 meV/Å
2 [41], respectively, implying that the (110) surface is
more stable than the (100) one. Generally, this is also the case with the IrO 2 surface
though the oxygen pressure affects the relative surface energy more or less [39].
On the (110) plane, one can spot alternating rows of threefold-coordinated
trigonal-planar O atoms (3f–O), twofold-coordinated bridging O atoms (br–O), and
fivefold-coordinated square-pyramidal Ir or Ru atoms (5f–Ir/Ru). As for the (100)
surface, alternating rows of br–O atoms and 5f–Ir/Ru atoms can be seen. In both
surfaces, the alternating rows run parallel to the a-axis. The difference in the surface
energy or stability between the two facets can be traced back to the density of the coordinatively unsaturated (cus) sites, namely the 5f–Ir/Ru and br–O sites. The density
of the cus sites on the (100) surface is higher than that of the (110) surface.