Dynamics and Energetics of Methane
on the Surfaces of Transition Metal
Oxides
Yuta Tsuji, Masashi Saito, and Kazunari Yoshizawa
Abstract In the very early part of this chapter, we review the Langmuir model of
adsorption to understand the dynamics of the competition between adsorption and
desorption of methane on a surface. Then, the kinetics model introduced is further
developed so that one can apply it to the dissociation dynamics of the C–H bond
of methane on surface whether it happens in a high-pressure limit or low-pressure
one. In the latter limit, an interesting concept of the apparent activation energy is
introduced and its correlation with the desorption barrier or the adsorption energy
is clarified. In order to reduce the apparent activation energy associated with the
dissociation of the C–H bond of methane on a surface, one needs to find a surface
which enjoys a substantial affinity to methane; such a candidate can be found in a class
of metal oxides. In such a situation, what is so-called trapping-mediated mechanism
can apply. The (101) surface of PdO and the (110) and (100) surfaces of IrO 2 and
RuO 2 have been experimentally and theoretically characterized as a good candidate
for such a reaction mechanism. We carry out a comprehensive survey of their surface
features, stressing an importance of possessing a coordinatively unsaturated (cus)
metal site as well as a cus O site nearby on the surface so that a H atom of methane
can be abstracted by the cus O site and the resultant CH 3 fragment can be significantly
stabilized by an interaction with the cus metal site. Taking the IrO 2 (110) surface as
an example, we conduct a thorough scrutiny on the electronic structure of methane
on the surface at the level of the extended Hückel method, which can provide us
with a qualitatively correct insight into orbital interaction. We make good use of a
band decomposition technique called crystal orbital overlap population (COOP) to
unveil some important orbital interactions between the C–H bond dissociated and
the surface cus Ir and O atoms.
Keywords Adsorption · Orbital interaction · Crystal orbital overlap population ·
Metal oxide · Dative bond
Y. Tsuji · M. Saito · K. Yoshizawa (B)
Institute for Materials Chemistry and Engineering and IRCCS, Kyushu University, Nishi-ku,
Fukuoka 819-0395, Japan
e-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
K. Yoshizawa (ed.), Direct Hydroxylation of Methane,
https://doi.org/10.1007/978-981-15-6986-9_6
101
on the Surfaces of Transition Metal
Oxides
Yuta Tsuji, Masashi Saito, and Kazunari Yoshizawa
Abstract In the very early part of this chapter, we review the Langmuir model of
adsorption to understand the dynamics of the competition between adsorption and
desorption of methane on a surface. Then, the kinetics model introduced is further
developed so that one can apply it to the dissociation dynamics of the C–H bond
of methane on surface whether it happens in a high-pressure limit or low-pressure
one. In the latter limit, an interesting concept of the apparent activation energy is
introduced and its correlation with the desorption barrier or the adsorption energy
is clarified. In order to reduce the apparent activation energy associated with the
dissociation of the C–H bond of methane on a surface, one needs to find a surface
which enjoys a substantial affinity to methane; such a candidate can be found in a class
of metal oxides. In such a situation, what is so-called trapping-mediated mechanism
can apply. The (101) surface of PdO and the (110) and (100) surfaces of IrO 2 and
RuO 2 have been experimentally and theoretically characterized as a good candidate
for such a reaction mechanism. We carry out a comprehensive survey of their surface
features, stressing an importance of possessing a coordinatively unsaturated (cus)
metal site as well as a cus O site nearby on the surface so that a H atom of methane
can be abstracted by the cus O site and the resultant CH 3 fragment can be significantly
stabilized by an interaction with the cus metal site. Taking the IrO 2 (110) surface as
an example, we conduct a thorough scrutiny on the electronic structure of methane
on the surface at the level of the extended Hückel method, which can provide us
with a qualitatively correct insight into orbital interaction. We make good use of a
band decomposition technique called crystal orbital overlap population (COOP) to
unveil some important orbital interactions between the C–H bond dissociated and
the surface cus Ir and O atoms.
Keywords Adsorption · Orbital interaction · Crystal orbital overlap population ·
Metal oxide · Dative bond
Y. Tsuji · M. Saito · K. Yoshizawa (B)
Institute for Materials Chemistry and Engineering and IRCCS, Kyushu University, Nishi-ku,
Fukuoka 819-0395, Japan
e-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
K. Yoshizawa (ed.), Direct Hydroxylation of Methane,
https://doi.org/10.1007/978-981-15-6986-9_6
101