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Y. Tsuji et al.
1 Introduction
In recent years, unconventional natural-gas resources, such as methane hydrate [1]
and shale gas [2], have attracted much attention. Along with the progress of the
drilling technology, methane output is expected to increase significantly in the future.
Therefore, the development of effective utilization methods of methane as a chemical
raw material to transform it to value-added products is of significant importance [3].
To this end, one needs to activate the C–H bond of methane as the first step; however,
such an activation scheme is challenging because methane’s C–H bonds are robust,
as exemplified by a large bond dissociation energy of 104 kcal/mol. Nevertheless,
human beings have achieved a variety of ways to activate methane, be it homogeneous
[4] or heterogeneous catalysts [5], as one could see in other chapters of this book.
Among them is the metal oxide catalyst. Detailed insights into methane activation on
oxide surfaces are expected to allow us to design a new catalyst or improve existing
catalytic processes. In this chapter, for starters, we will make a comprehensive survey
of methane activation on various well-defined metal oxide surfaces and later we will
take a closer look at how actually methane is activated on the surface, taking the
surface of a certain metal oxide, IrO 2 , as an example.
2 Kinetics of Methane on Surface
2.1 Langmuir Model
It would be instructive to begin with a basic surface chemistry so that the reader could
better understand the dynamics of the adsorption of methane on the surface. We will
begin with a simple yet effective model named after Irving Langmuir [6], wherein
many things are assumed; for example, the surface consists of a series of identical
sites, there is no interaction between the adsorbate molecules whether they are on
the surface or in the gas phase, a randomly moving adsorbate molecule plunges to
the surface, the adsorbate is trapped by the surface with a certain probability, the
adsorbate does not affect the adsorption abilities of the other sites, and the adsorbate
cannot be adsorbed at the already occupied site but will be scattered. These assumptions seem to be too simplified or extreme; however, they are mostly fulfilled in the
case of the adsorption of methane on a clean surface [7].
Figure 1 shows schematic representations of the Langmuir model describing the
adsorption and desorption dynamics of a molecule, say, methane, represented by a
shaded ball. When the total area of the surface is S, that of the occupied sites S ad ,
and that of the unoccupied sites S 0 , or S − S ad , the desorption rate can be equal to
k des S ad , where k des is the rate constant for desorption. As for the adsorption rate, since
it depends on the pressure of the adsorbate, P, as well as the area of the unoccupied
sites, it reads k ad S 0 P, where k ad is the adsorption rate constant. Since S 0 = S − S ad ,
Y. Tsuji et al.
1 Introduction
In recent years, unconventional natural-gas resources, such as methane hydrate [1]
and shale gas [2], have attracted much attention. Along with the progress of the
drilling technology, methane output is expected to increase significantly in the future.
Therefore, the development of effective utilization methods of methane as a chemical
raw material to transform it to value-added products is of significant importance [3].
To this end, one needs to activate the C–H bond of methane as the first step; however,
such an activation scheme is challenging because methane’s C–H bonds are robust,
as exemplified by a large bond dissociation energy of 104 kcal/mol. Nevertheless,
human beings have achieved a variety of ways to activate methane, be it homogeneous
[4] or heterogeneous catalysts [5], as one could see in other chapters of this book.
Among them is the metal oxide catalyst. Detailed insights into methane activation on
oxide surfaces are expected to allow us to design a new catalyst or improve existing
catalytic processes. In this chapter, for starters, we will make a comprehensive survey
of methane activation on various well-defined metal oxide surfaces and later we will
take a closer look at how actually methane is activated on the surface, taking the
surface of a certain metal oxide, IrO 2 , as an example.
2 Kinetics of Methane on Surface
2.1 Langmuir Model
It would be instructive to begin with a basic surface chemistry so that the reader could
better understand the dynamics of the adsorption of methane on the surface. We will
begin with a simple yet effective model named after Irving Langmuir [6], wherein
many things are assumed; for example, the surface consists of a series of identical
sites, there is no interaction between the adsorbate molecules whether they are on
the surface or in the gas phase, a randomly moving adsorbate molecule plunges to
the surface, the adsorbate is trapped by the surface with a certain probability, the
adsorbate does not affect the adsorption abilities of the other sites, and the adsorbate
cannot be adsorbed at the already occupied site but will be scattered. These assumptions seem to be too simplified or extreme; however, they are mostly fulfilled in the
case of the adsorption of methane on a clean surface [7].
Figure 1 shows schematic representations of the Langmuir model describing the
adsorption and desorption dynamics of a molecule, say, methane, represented by a
shaded ball. When the total area of the surface is S, that of the occupied sites S ad ,
and that of the unoccupied sites S 0 , or S − S ad , the desorption rate can be equal to
k des S ad , where k des is the rate constant for desorption. As for the adsorption rate, since
it depends on the pressure of the adsorbate, P, as well as the area of the unoccupied
sites, it reads k ad S 0 P, where k ad is the adsorption rate constant. Since S 0 = S − S ad ,