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We now turn to the inspection of rate-determining steps in
the above-depicted reaction schemes. Actually, Fukuda et al.44
previously suggested that the desorption of surface hydroxyl
groups as water molecule was the rate-determining step of the
catalytic dehydration reaction of formic acid on alumina and
silica and similar postulate has been suggested for the catalytic
dehydration of formic acid on TiO2 and Cr2O3. In the cases of
the reaction of formic acid over Fe3O4 and MnO, however, the
breaking of the C-O bond of the formate has been suggested
to be rate-determining.45,46 In the present study, however, no
significant change in the amount of surface oxygen was detected
on the AES signals as examined after each experiment and thus
the reduction of the surface during the studied catalytic reaction
must have been negligible. This observation may suggest that
the oxygen defect sites, if produced, were recovered by the
decomposition of formic acid. In any event, both of hitherto-
depicted rate-determining steps can be eliminated from the
present reactions. Well-designed experiments which enable
distinctive examination of the two reaction pathways are
required for further details.
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The catalytic decomposition of formic acid on the NiO(111)
layers grown on Ni(111) surface was studied using IRAS and
gas analysis by QMS and under steady-state conditions. The
IRA spectra of the adsorbed formate suggested the presence of
two types of formate species, bidentate with vertical to the
surface and monodentate formates in contrast to the surface
under UHV where only the tilted-bidentate formate was
observed. The formic acid underwent decomposition through
two parallel reaction pathways: dehydrogenation to produce H2
and CO2 started at 373 K and dehydration to produce CO and
H2O occurred above 423 K with the activation energies of 22
( 2 and 16 ( 2 kJ/mol, respectively. The order of the both
reactions was derived as 0.5 with respect to the pressure of
formic acid. Pressure dependent features of the IRA spectra
suggested strongly that the monodentate formate is the inter-
mediate of the investigated reactions on the NiO(111) surface.
The activation energies of 58 ( 3 and 49 ( 3 kJ/mol were
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dehydrogenation and dehydration, respectively. The change of
the orientation of the bidentate formate from the one found in
UHV conditions was attributed to the change of the surface
structure from oxygen-terminated (2×2) structure in UHV to
the nickel-terminated (1×1) structure due to the production of
the hydroxyl groups during the decomposition of formic acid.
The catalytic reaction rate as measured by QMS agreed with
the rate of decomposition of monodentate formate measured
by IRAS to indicate considerably lower reactivity of the
bidentate formate the presence of which was also identified by
IRAS.
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Acknowledgment. The authors are indebted to Dr. J. N.
Kondo for her helpful comments and suggestions. One of the
authors (A.B.) gratefully acknowledges the Ministry of Educa-
tion, Science and Culture (MONBUSHO) of Japan for awarding
a Japanese Government (Monbusho) Scholarship to study at
Tokyo Institute of Technology. This work was supported by
the Grant-in-aid on Priority-Area-Research “Photoreaction
Dynamics” from the Ministry of Education, Science and Culture,
Japan (no. 06239110).
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