8832 J. Phys. Chem. B, Vol. 102, No. 44, 1998
Fukui et al.
of transition-state species, resulting in a lower preexponential
factor (2 × 1013(2 s-1). Considering that dissociation of C-H
and C-O bonds of methoxy occurs on the Mo atoms in a MoNC
row, effective motion of the methoxy for dissociation may be
a frustrated rotation along the MoNC row. Extra oxygen atoms
may restrict the diffusion and frustrated rotation of methoxy
even at the lower coverage, given to the one-dimensionality of
the MoNC row.
Conclusion
The results presented here indicate that methanol reaction
paths are dramatically sensitive to the coverage and arrangement
of oxygen modifiers and the surface structure. On Mo(112)-
(1 × 2)-O, methoxy species are stabilized by the oxygen
modifiers in the (1 × 2)-O phase, which selectively block the
Mo atoms with high coordination and form one-dimensional
reaction sites on the surface. This type of modification
contributes to the formation of H2CO on the surface, but with
50% selectivity. The extra oxygen species adsorbed on the (1
× 2)-O surface increase the selectivity to 88% and decrease
the activation energy for the rate-limiting C-H bond scission
of methoxy species. Selective catalytic oxidation of methanol
proceeds in a constant flow of O2 and CH3OH without
deactivation. Thus the present results suggest that we can
control the reaction paths by designing a reaction field with
two types of coadsorbed oxygen.
Figure 10. (a) Supposed arrangement of methoxy species and extra
oxygen atoms on Mo(112)-(1 × 2)-O (θO′ ≈ 0.21 and θCH O ≈ 0.07,
3
respectively). (b) Sectional view at A-A′ in (a), also showing the van
der Waals spheres for the extra oxygen atoms. Methoxy species are
drawn with the van der Waals sphere of each element by assuming the
same bond distances as those of methanol. Extra oxygen and methoxy
are located at bridge sites on the MoNC rows with a Mo-O bond at
0.21 nm.
Acknowledgment. This work has been supported by CREST
(Core Research for Evolutional Science and Technology) of
Japan Science and Technology Corporation (JST).
TPR of Figure 4b (θO ≈ 0.21 and θCH O ≈ 0.07, respectively).
3
This model suggests that C-O bond scission of the methoxy
was inhibited not through steric blocking but through electronic
modification of Mo atoms by the extra oxygen atoms. As noted
above, C(a) species accumulated on the Mo(112)-(1 × 2)-O
surface during the catalytic reaction with CH3OH feed alone
(Table 2), but, in contrast to extra oxygen species, C(a) did not
inhibit C-O bond scission of methoxy species. This result also
excludes the possibility of steric blocking as a major reason of
the inhibition. Effective trapping of hydrogen atoms by extra
oxygen atoms (step 13) may increase the selectivity of methoxy
dehydrogenation to formaldehyde if step 8 is the major path
for CH4 formation.
The extra oxygen decreases the activation energy of C-H
bond scission of the methoxy species, which is the rate-limiting
step of the selective methanol oxidation, and the extra oxygen
was desorbed as H2O during the catalytic reaction. Thus, the
selective catalytic oxidation of methanol may involve the direct
extraction of a hydrogen atom by an extra oxygen atom (step
14), which is similar to the mechanism proposed in oxidative
dehydrogenation of methanol on oxide surfaces.3 One-
dimensionality of the MoNC row may increase the probability
of direct interaction between methoxy and extra oxygen species.
TPD spectra of CO indicate that extra oxygen species reduce
the electron density of Mo atoms in MoNC rows. This
modification causes the decrease of the activation energy for
the methoxy dehydrogenation.
References and Notes
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On the other hand the extra oxygen atoms decrease the
preexponentilal factor of the reaction. A high preexponential
factor of 7 × 1015(1 s-1 was observed on the Mo(112)-(1 ×
2)-O surface without extra oxygen. A high preexponential factor
was also reported on CO desorption from Ru(001) and was
explained by high mobility of transition state species on the
surface.39 Methoxy species in the transition-state are probably
weakly bound to the surface and may diffuse freely along a
MoNC row. The extra oxygen atoms may restrict the diffusion