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Y. Tsuji et al.
great importance so that the sufficient energy is transferred to the reaction coordinate,
allowing the molecule to surmount the activation barrier. Generally, this mechanism
plays an important role when the barrier associated with the C–H bond rapture is
substantially large and the adsorption energy of methane to the surface is negligibly
small. Thus, it can mainly be applied to the kinetics of methane reaction on pure
metal surfaces.
On the other hand, in the trapping-mediated mechanism is first generated the
molecularly adsorbed state of methane, i.e., the precursor state. This state can be
viewed as part of surface, so it has the same temperature as the surface. It follows that
the probability of the C–H bond dissociation in this mechanism depends significantly
on the surface temperature. The kinetics competition between the C–H bond breaking
reaction and desorption of methane from the surface determines the net reaction
rate. Since the reaction begins with the precursor state, it has to have an access to
low-barrier paths for dissociation during its lifetime on the surface.
Whether methane on a surface takes the direct mechanism or the trappingmediated mechanism for its C–H bond dissociation is contingent upon the nature
of the molecule–surface interaction. As a general rule of thumb, if the adsorption
energy is small and the activation barrier is high, the direct mechanism applies, while
if there is a strong molecule–surface interaction and the barrier height is relatively
low, then the trapping-mediated mechanism applies. The above-discussed dichotomy
is reminiscent of the well-known distinction between the activated and non-activated
adsorptions [23].
The difference in energetics between the two mechanisms above is simply depicted
in Fig. 4. In the direct mechanism, the desorption barrier is so small that the apparent
activation energy, E app , can be regarded as almost the same as that of the true activation energy, E a , or the intrinsic activation energy, while the trapping-mediated
mechanism is characterized by a relatively deep initial potential well and the resultant high desorption barrier. Therefore, the deeper the initial potential well becomes,
the smaller the apparent activation energy is, and even we have a situation where the
apparent activation energy becomes negative due to a substantially large adsorption
energy.
Since in the direct mechanism the molecule has to have a substantially large
kinetic energy for the reaction, one needs to provide a large thermal energy with
the gaseous molecule. However, in such a high-temperature experiment, it would be
very difficult to have a control over the subsequent reactions or to deter unfavorable
side reactions. This is why, in this chapter, we would like to cling to the trappingmediated mechanism rather than the direct mechanism. Here, we stop taking a look
at the kinetics of methane on the surface, and move on to scrutiny of the energetics
of methane on the surface, including the strength of the adsorption and the extent to
which the C–H bond of methane is activated in the molecularly adsorbed state.
Y. Tsuji et al.
great importance so that the sufficient energy is transferred to the reaction coordinate,
allowing the molecule to surmount the activation barrier. Generally, this mechanism
plays an important role when the barrier associated with the C–H bond rapture is
substantially large and the adsorption energy of methane to the surface is negligibly
small. Thus, it can mainly be applied to the kinetics of methane reaction on pure
metal surfaces.
On the other hand, in the trapping-mediated mechanism is first generated the
molecularly adsorbed state of methane, i.e., the precursor state. This state can be
viewed as part of surface, so it has the same temperature as the surface. It follows that
the probability of the C–H bond dissociation in this mechanism depends significantly
on the surface temperature. The kinetics competition between the C–H bond breaking
reaction and desorption of methane from the surface determines the net reaction
rate. Since the reaction begins with the precursor state, it has to have an access to
low-barrier paths for dissociation during its lifetime on the surface.
Whether methane on a surface takes the direct mechanism or the trappingmediated mechanism for its C–H bond dissociation is contingent upon the nature
of the molecule–surface interaction. As a general rule of thumb, if the adsorption
energy is small and the activation barrier is high, the direct mechanism applies, while
if there is a strong molecule–surface interaction and the barrier height is relatively
low, then the trapping-mediated mechanism applies. The above-discussed dichotomy
is reminiscent of the well-known distinction between the activated and non-activated
adsorptions [23].
The difference in energetics between the two mechanisms above is simply depicted
in Fig. 4. In the direct mechanism, the desorption barrier is so small that the apparent
activation energy, E app , can be regarded as almost the same as that of the true activation energy, E a , or the intrinsic activation energy, while the trapping-mediated
mechanism is characterized by a relatively deep initial potential well and the resultant high desorption barrier. Therefore, the deeper the initial potential well becomes,
the smaller the apparent activation energy is, and even we have a situation where the
apparent activation energy becomes negative due to a substantially large adsorption
energy.
Since in the direct mechanism the molecule has to have a substantially large
kinetic energy for the reaction, one needs to provide a large thermal energy with
the gaseous molecule. However, in such a high-temperature experiment, it would be
very difficult to have a control over the subsequent reactions or to deter unfavorable
side reactions. This is why, in this chapter, we would like to cling to the trappingmediated mechanism rather than the direct mechanism. Here, we stop taking a look
at the kinetics of methane on the surface, and move on to scrutiny of the energetics
of methane on the surface, including the strength of the adsorption and the extent to
which the C–H bond of methane is activated in the molecularly adsorbed state.