A new aspect of catalysis at designed surfaces: The role of gas phase molecules in surface catalytic reactions

Article abstract of DOI:10.1016/S1381-1169(99)00080-1

The role of gas phase molecules in surface catalytic reactions as a new aspect of catalysis was investigated using Nb/SiO₂ and Co/Al₂O₃ catalysts prepared by using Nb and Co precursor complexes with surface OH groups of SiO₂ and Al₂O₃, respectively. The catalytic reaction mechanisms in a molecular scale were studied. The catalytic ethanol dehydrogenation proceeded through Nb ethoxide intermediates (7). The species (7) was stable, and was not decomposed to CH₃CHO and H₂ at 423-523 K in vacuum but rather converted to C₂H₄ and H₂O at 700 K. The unreactive Nb ethoxide (7) was transformed to reactive Nb ethoxide (8) under the catalytic reaction conditions, where weak ethanol coordination occurred on a vacant site of the species (8) and electronically activated the Nb ethoxide (8) to produce CH₃CHO and H₂. The presence or absence of ethanol molecules in the gas phase did not affect the switchover of the reaction path from dehydration (γ-CH bond break) to dehydrogenation (β-CH bond break) of the intermediate (X). These results showed that surface catalytic reaction pathways are made accessible only under the ambient gas.

Full text of DOI:10.1016/S1381-1169(99)00080-1

Ž
.
Journal of Molecular Catalysis A: Chemical 146 1999 65–76
www.elsevier.comrlocatermolcata
A new aspect of catalysis at designed surfaces: the role of gas
phase molecules in surface catalytic reactions
)
Aritomo Yamaguchi, Kiyotaka Asakura, Yasuhiro Iwasawa
Department of Chemistry, Graduate School of Science, The UniÕersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received in revised form 28 December 1998
Abstract
This paper attempts to extend our previous studies on the chemical design and characterization of attached metal atoms at
surfaces of a NbrSiO2 catalysts prepared by using a suitable metal-complex precursor and the elucidation of catalytic
reaction mechanisms in a molecular scale. The paper also reports a new aspect of catalysis on a CorAl2O3 catalyst which
includes surface catalytic reactions assisted by gas phase molecules undetectable at the surface. On the basis of this concept,
one may design new catalytic systems where the reaction intermediates, stable and unreactive in vacuum, can be activated or
controlled by gas phase molecules in situ under catalytic reaction conditions. q 1999 Elsevier Science B.V. All rights
reserved.
Keywords: Catalytic reaction mechanisms; Design of active structures; Characterization by EXAFS and FT-IR; NbrSiO2; CorAl2O3
1. Introduction
which may be advantages of homogeneous cata-
lysts. Homogeneous catalysts may be readily
heterogenized so that the advantages of homo-
geneous catalysts on the one hand, their high
specificity and tunability, and heterogeneous
catalysts on the other, their built-in ease of
separation of product from reactant, durability,
stability and easy handling may be jointly har-
nessed in a synergistic mode at the surface.
Synergistic phenomena at surfaces may be ex-
pected in a different manner from those in
solutions because two different single-site struc-
tures can be organized at different places of a
surface.
New and distinct materials and chemistry
prepared stepwise in a controllable manner by
using organic and inorganic metal complexes
and clusters as precursors provide an opportu-
nity for the development of efficient catalytic
molecularly-organized surfaces en route to the
ultimate catalyst technologies in the micro-
Ž
.
scopic atomicrmolecular , mesoscopic, and
Ž
.
w
x
macroscopic mean-field regions 1–6 . The
notable superiority of this kind of catalytic sys-
tems is in single-site reactions, high selectivity,
generality or wide applications, and tunability,
The longer term challenge to surface chem-
istry of oxide-supported metal catalysts is to
address important issues in activity and selectiv-
)
Corresponding author. Fax: q81-3-5800-6892; E-mail:
iwasawa@chem.s.u-tokyo.ac.jp
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 99 00080-1

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)
66
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
.
ity in catalytic reactions, in particular the princi-
ples of tuning of metal reactivity and activating
reaction intermediates at oxide surfaces. In the
development of new catalysts, new chemical
concepts regarding structure or composition are
conceived. The requirements and design of defi-
nite ensemble sizes represent important but as
yet few addressed challenge to the field. The
role of gas phase molecules in surface catalytic
reactions remained to be solved to understand
catalytic phenomena at surfaces under reaction
gory 1 metal atoms are attached to oxide sur-
faces via metal–oxygen bonding at metal–oxide
interfaces. The attached metal atoms are often
highly dispersed as monomers or distributed
forming ensemble structures such as dimers and
clusters by direct and oxygen-bonded metal–
metal bondings depending on the oxidation state
of attached metals, the acidityrbasicity of oxide
surfaces, the distribution of hydroxyl groups at
oxide surfaces, etc.; e.g., Mo monomerrSiO2 or
w x
w
x
Al2O3 8 , Nb monomerrSiO2 or TiO2 9,10 ,
w x
w x
conditions 7 . Recently molecular-level catalyst
chiral Rh monomerrSiO2 11 , titanocener
w x Ž . Ž .
MCM-41 12 , V acac –Al C2 H5 ClrSiO2
preparation has become realistic on the basis of
modern physical techniques and accumulated
knowledge of oxide surfaces, and a clear exam-
ple of the surface catalytic reaction through
strongly adsorbed intermediates assisted by
weakly adsorbed molecules present only in the
presence of the gas phase molecules has been
uncovered on a designed NbrSiO2 catalyst
3
w
x
w
x
13 , Mo dimerrSiO2 or Al2O3 2,4,14,15 , Nb
w
x
w
x
dimerrSiO2 16 , Rh dimerrSiO2 17,18 , Ru6C
w
x
clusterrLa2O3 or MgO 19,20 , Nb2O5 mono-
w x
w
x
layerrSiO2 1 , ZrO2 monolayerrZSM-5 21 ,
w
x
etc. 1–6,8 . Active intermediates can be synthe-
sized at oxide surfaces via attaching a metal
complex with appropriate ligands to oxide sur-
faces, and subsequently chemical transformation
of the ligands of the attached complex to reac-
w
x
1,9,10 .
The use of molecularly-designed catalysts
Ž
.
seems to be prerequisite to examining the mech-
anism and concept of catalysis. There are sev-
eral approaches to chemical design of active
metal sites and ensembles on oxide surfaces by
metal–organic and –inorganic complexes and
clusters as shown in Table 1. It has been
demonstrated that stable and active structures at
catalyst surfaces can be prepared by reaction
and strong interaction of inorganic and organic
metal-complex precursors with oxides such as
SiO2, Al2O3, TiO2, MgO, La2O3, zeolites, etc.
tive intermediate species Category 2 . A suc-
cessful example of this method for surface de-
w
x
sign is Rh dimerrSiO2 17 , in which an acyl
ligand in a Rh dimer is produced by ligand-reac-
tions among methyl, m-methylene and carbonyl
groups in situ at the SiO2 surface. The obtained
Rh dimer-acyl species attached on SiO2 was
active for ethene hydroformylation with a high
selectivity of 88.9% under mild reaction condi-
tions at a reduced pressure and 413 K, in con-
trast to only 5.6% with a conventional RhrSiO2
catalyst. Precursor complexes may be supported
Ž
In the catalysts obtained by this method Cate-
Table 1
Typical approaches to chemical design of active metal sites and ensembles on oxide surfaces by metal–organic and –inorganic complexes
and clusters
Ž .
Ž .
1
Selective reactions of complexes and clusters with surface OH groups: a Ligand–OH reactions, b Redox reactions,
Ž .
c Acid–base reactions
2
3
4
5
6
7
8
Surface synthesis and surface transformation
Electrostatic or electronic interaction at interface
Ship-in-a bottle synthesis
Epitaxial growth of active structures
Ž
.
Promoters on, inside, near and far
Selective element-exchange at cation and anion sites
Ž
.
Others molecular imprinting, etc.

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)
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
67
on oxide surfaces by electrostatic interaction
0.302 nm and Pt–O s 0.360 nm. The
Ž
.
w
x
at interfaces Category 3 . An example is
PtMo6 rMgO catalyst was active for the dehy-
U
U
Ž
.
Ž
.
Ž
.
Au PPh3 NO3 on Fe OH or Ti OH , where
drogenation of propane, butane and isobutane to
the corresponding alkenes without significant
3
4
the asterisk denotes as-precipitated wet metal
Ž
.
w
x
hydroxides. The obtained Au PPh3 3NO3ras-
precipitated hydroxides treated by temperature-
programmed calcination were converted to
tremendously active Au catalysts for low tem-
deactivation 26,27 .
The present study reports the role of gas
phase molecules in surface catalytic reactions as
a new aspect of catalysis, using designed cata-
lysts, NbrSiO2 and CorAl2O3 with definite,
active structures which are prepared by taking
advantage of selective reactions of Nb and Co
precursor complexes with surface OH groups of
SiO2 and Al2O3, respectively, in a key step for
the catalyst preparation. The present prepara-
tions are regarded as a combination of the cate-
gories 1 and 2 in Table 1. The phenomena
found on the designed catalysts were not clearly
observed on traditional NbrSiO2 and Cor
Al2O3 catalysts with heterogeneous surface
structures prepared by a conventional impregna-
tion method.
w
x
perature CO oxidation 22–25 . It has been re-
ported that ZrO2 monolayers can be formed
at the external surface of ZSM-5 by using
Ž
.
Zr OC2 H5 4 as a precursor. The ZrO2 mono-
layerrZSM-5 catalyst with Zr–Zrs0.369 nm
and Zr–Os0.206 nm showed a high activity
for methanol conversion to isopentane with a
selectivity of 88.9% 21 . It seems that active
tetragonal ZrO2 111 plane epitaxially grew on
a ZSM-5 001 surface Category 5 . It is known
that promoters placed on metal surfaces or close
to metal particles can dramatically change cat-
w
x
Ž
.
Ž
.
Ž
.
Ž
.
alytic performance of the metal Category 6 . A
new approach to promoter effect has been re-
ported with Ru6CrLa2O3 or MgO catalysts
w
Ž
.
xy
which were prepared using Ru6C CO 16Me
2. Experimental
Ž
.
with m6-carbide interstitial carbido carbon in-
side an octahedral Ru6 framework as a precur-
sor. The attached Ru6C clusters exhibited the
specific catalysis for CO conversion to oxy-
genated compounds such as methanol, dimethyl
ether and formaldehyde, whereas conventional
impregnated Ru catalysts mainly produced
methane and hydrocarbons. The interstitial car-
bido carbon plays a decisive role in the control
of selectivity to oxygenates in COrH2 condi-
tions, where the interstitial carbon as central
spring makes the cluster framework flexible to
expand and shrink depending on the CO pres-
sure and stabilizes the cluster framework under
2.1. Catalyst preparation
Ž .
The attached Nb monomer catalyst 3 in Fig.
1 was prepared via the surface complex 1
which was obtained by reaction between Nb h -
C3H5 4 and surface OH groups of SiO2 at
Ž .
3
Ž
.
233–273 K, followed by treatments with H2
w
x
Ž
.
and O2 9,10 . SiO2 Aerosil 200 was pretreated
at 673 K in situ before use as a support for
3
Ž
.
Ž .
Nb h -C3H5 4. The surface complex 1 , which
was characterized by FT-IR, ESR and EXAFS,
was the first to be highly active for propene
w
x
Ž .
metathesis 28 . The complex 1 was succes-
w
x
the reaction conditions 19,20 . Metal ions may
be supported at oxide surfaces by replacing the
cation sites of the oxide top layer Category 7 .
sively reduced with H2 and oxidized with O2 at
q
673 K to obtain the Nb5 state 3 . The Nb-
Ž .
Ž
.
Ž .
carbide catalyst 2 , which was characterized by
O2 titration and extended X-ray absorption fine
structure EXAFS , was found to be active for
selective conversion of COrH2 to C2–C4 hy-
drocarbons in the Fischer–Tropsch synthesis
w
x
An example is PtMo6 rMgO which is prepared
8y
w
x
Ž
.
using PtMo6O24
as a precursor. It has been
demonstrated that Pt cations are located at the
top layer substituting Mg cations of the MgO
surface, showing Pt–Os0.202 nm, Pt–Mgs
5
q
w
x
Ž .
29 . The Nb
state in the catalyst 3 was

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)
68
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
2, with the release of one carbonyl ligand per
Co atom. The s™s U band of Co2 CO re-
Ž
.
8
lated to the Co–Co bonding disappeared and
was replaced by a new absorption peak at 413
nm with appearance of peaks at 270 and 595
nm, which were similar to the spectrum of the
Ž
.
Al2O3-supported Co4 CO
cluster. The aver-
12
age Co–C bond distance was observed at 0.197
nm, which is comparable to that in a Co-tetra-
Ž .
mer rather than a Co-dimer. The species 4
with a Co–Co bond distance of 0.254 nm larger
Ž
.
than 0.249 nm for C3v–Co4 CO was very
8
sensitive to O2 to be oxidized to a new species
w
x
Ž .
Co–O rAl2O3 5 , accompanied with disap-
4
pearance of the direct Co–Co bond and appear-
ance of a new Co–O bond at 0.205 nm which is
Fig. 1. Preparation steps for NbrSiO2.
confirmed by the XPS Nb 3d binding energies.
The X-ray absorption near-edge structure
Ž
.
XANES spectrum and diffuse reflectance
UvrVis spectrum indicated a four-coordinated
structure. The Nb–O, Nb5O and Nb–Si inter-
Ž .
atomic distances in the catalyst 3 were deter-
mined to be 0.193, 0.168 and 0.326 nm, respec-
tively, by EXAFS. No Nb–Nb bonding was
observed by EXAFS, demonstrating that the
q
Nb5 ions in the catalyst 3 are distributed as
Ž .
w
x
monomers 10 . The arrows in bonding scheme
Ž
.
of Fig. 1 species 2 and 3 cannot be specified
as a definite kind of bonding feature unlike the
others; a candidate may be m-oxo coordination
from the surface.
A CorAl2O3 catalyst was prepared in a simi-
lar manner to that reported in the literature
w
x
Ž
.
Ž
30–33 . Co2 CO was attached on Al2O3 De-
8
.
gussa Alon C by a chemical vapor deposition
method. Al2O3 was pretreated for 2 h at 573 K
Ž
.
8
before use as a support for Co2 CO . Support-
Ž
.
ing Co2 CO on the pretreated Al2O3 led to
8
Ž
.
Ž .
4 as shown in Fig.
12
the formation of Co4 CO
Fig. 2. Preparation steps for CorAl2O3.

(
)
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
69
shorter than the Co–O bond in CoO crystal
cell with a catalyst wafer were subtracted by the
spectra for the gas phase in a reference cell
without catalyst wafer which was also combined
in the closed circulating system. Thus the spec-
tra were measured in situ under the catalytic
reactions and the NO adsorption processes.
Ž
.
Ž .
0.2133 nm . The species 5 has not fully been
characterized, but the Co atoms are suggested to
be situated in a bivalent state with trigonally
distorted tetrahedral symmetry by diffuse re-
w
x
flectance UvrVis spectra 30,31 . On heating to
Ž .
513 K under vacuum, species 5 reacted with
surface OH groups of Al2O3, as indicated by
Ž
.
the decrease in the intensity of the n OH band
3. Results and discussion
q
centered at 3550 cmy1, to form a Co2 rAl2O3
Ž .
catalyst 6 accompanied with the formation of
3.1. Surface catalytic reactions assisted by gas
phase molecules
H2O. The diffuse reflectance UvrVis spectrum
Ž .
of the catalyst 6 exhibited peaks at 657, 600
q
and 525 nm, indicating that the Co2 ions are
A simplified form of the usual mechanism
for heterogeneous catalytic reactions is shown
situated in a trigonally distorted Td position.
Ž .
Ž .
Ž .
The bonding scheme of 6 in Fig. 2 is simply
in Fig. 3 a , where the reaction intermediate X
q
illustrated as a bivalent Co2 structure with two
is transformed to the product P by surface
Ž .
Co–O bonds at 0.197 nm characterized by EX-
AFS. The Td symmetry may be consistent with
unimolecular decomposition; that is, a stoichio-
metric reaction step proceeds without aid of
other molecules. In contrast to the simple expec-
tation of no special role of additional gas phase
molecules in a catalytic mechanism, we have
found evidence that the reaction intermediate of
an important catalytic reaction can be pro-
Ž
Ž
. .
the formation of Co-dinitrosyls Co NO by
2
coordination of two NO molecules as discussed
hereinafter.
2.2. Catalytic reactions
w x
foundly influenced by the ambient gas 7 . In
the catalytic dehydrogenation reaction of ethanol
Catalytic NO–CO reactions were carried out
Ž
in a closed circulating system dead volume:
on the Nb monomer catalyst, the intermediate
.
Ž .
X was stable and did not decompose to a
205 ml equipped with a gas chromatograph
Shimadzu GC-8A . A 2 m column of 5A
Ž
.
Ž .
product P up to 600 K, whereas in the coexis-
X
Ž
.
Molecular Sieve and a 2 m column of Unibeads
C were used for analyses of N2, NO and CO,
and N2O and CO2, respectively. The reactions
of NO alone were also carried out in the same
system. The detail of procedure for the catalytic
ethanol dehydrogenation on NbrSiO2 is re-
tence of another reactant molecule A the in-
w
x
ported in the literature 10 .
2.3. FT-IR measurements
FT-IR spectra were measured in an IR cell
with two CaF2 windows which were combined
in a closed circulating system, on a JEOL KIR
7000 spectrometer. The spectra were recorded
Fig. 3. Surface catalytic reactions assisted by gas phase molecules;
Ž .
Ž
.
Ž .
a
Surface catalytic reaction A™P via intermediate X; b
1
as difference spectra with a 2 cmy resolution
Ž
.
Surface catalytic reaction A™P promoted by reactant molecule
X
or coexisting molecule A, where enhancement of the reaction rate
and about 50 s time acquisition by a double-
beam method, where the spectra for a sample
for the formation of P or switchover of the reaction path from P
X
formation to Q formation is caused by A.

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70
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
Ž .
termediate X is activated to decompose to a
catalytic dehydrogenation reaction is assisted by
the ambient ethanol, where the reaction path of
adsorbed ethanol is switched from dehydration
to dehydrogenation by the ambient ethanol. In
other words, adsorbed ethanol prefers the dehy-
dration to form ethene and water by the g-hy-
drogen abstraction, while in the presence of the
ambient ethanol the b-hydrogen abstraction from
adsorbed ethanol to form acetaldehyde and hy-
Ž
.
different product Q even at low temperatures
like 473 K, where the rate of the intermediate
decomposition was enhanced and the reaction
path was switched over as illustrated in Fig.
Ž . w x
3 b 7 . In this article two examples of surface
catalytic reactions assisted by gas phase
molecules are reported below, describing the
previous finding of catalytic ethanol dehydro-
genation on Nb monomers attached on SiO2,
promoted by weak coordination of additional
ethanol that can be present only in the presence
of gas phase ethanol, and presenting a new
feature of catalytic NO–CO reaction at a Cor
Al2O3 catalyst surface assisted by gas phase
CO molecules undetectable at the surface.
w
x
drogen dominates as shown in Fig. 4 7,10 .
To examine how and why the surface ethanol
reaction is assisted by the gas phase ethanol, the
following experiments were conducted. Ethanol
vapor was first admitted onto the Nb monomer
Ž .
Ž .
Ž .
catalyst 3 to form C2 H5O a and OH a at
373 K, followed by evacuation, and then the
system was maintained at 523 K for 10 min,
where neither hydrogen nor acetaldehyde were
evolved because the adsorbed species were sta-
ble up to 600 K in vacuum. Then, various
electron-donating compounds such as triethyl
amine, pyridine, THF, diethyl ether, etc. were
introduced to the surfaces preadsorbed with the
same amount of ethanol, which led to a stoi-
chiometric evolution of H2 and CH3CHO. As
3.2. Dehydrogenation of ethanol on NbrSiO2
and role of the gas phase ethanol molecules
Ž .
The SiO2-attached Nb catalyst 3 exhibited a
high activity and selectivity for the dehydro-
genation of ethanol. Acetaldehyde and hydrogen
were stoichiometrically produced during the cat-
alytic reaction. Ethanol dissociatively adsorbs
Ž
Ž ..
on Nb to form ethoxyl C2 H5O a and hy-
Ž
Ž ..
droxyl OH a as proved by FT-IR spectra.
Ž .
Ž .
While C2 H5O a and OH a are only the ad-
sorbed species under the catalytic reaction con-
ditions, these adsorbed species are stable in
vacuum. To gain insight into the reactivity of
adsorbed ethanol, a temperature-programmed
Ž
.
desorption TPD spectrum for the adsorbed
species formed during the catalytic ethanol de-
hydrogenation reaction was measured. The TPD
revealed that the reaction of adsorbed ethanol in
vacuum was only possible at higher tempera-
tures than 600 K, with a TPD peak at 700 K,
Ž
and the products were ethene and water dehy-
.
drated products . When the adsorbed species are
exposed to the gas phase ethanol at 423–523 K,
the dehydrogenation reaction to form acetalde-
hyde and hydrogen begins at these mild temper-
atures. Note that the behavior of adsorbed
ethanol is entirely different in vacuum and in
Fig. 4. Switchover of the reaction path by weak ethanol coordina-
tion.
w
x
the presence of ambient ethanol 9,10 . The

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)
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
71
the electron donating molecules have no ex-
tractable hydrogen, it is evident that both H2
and CH3CHO were produced from the pread-
sorbed ethanol by the assistance of the post-
dosed molecules. The logarithm of the initial
rates of the CH3CHO or H2 formation from the
preadsorbed ethanol was plotted against the
donor number of the postadsorbed molecules
which associatively adsorb on the Nb atom,
where the donor number is defined as the loga-
rithm of the equilibrium constant for the forma-
tion of electron donor–acceptor complexes be-
tween the nucleophilic molecules and SbCl5 and
regarded as the electron donor strength of the
postadsorbed molecules. The linear relationship
was observed, which indicates that the electron
donor–acceptor interaction between the postad-
sorbed molecule and the coordinatively unsatu-
rated Nb d0 ion is a key issue for the dehydro-
3.3. Catalytic NO–CO reaction on CorAl2O3
and role of the gas phase CO molecules
Slow N2O formation from NO alone in a
closed circulating system was observed above
Ž .
350 K on the CorAl2O3 catalyst 6 . The N2O
formation at 463 K is still slow as shown in Fig.
Ž .
5 a , and the amount of N2O formed in the first
15 min of reaction was only ca. 2% of total Co
quantity located at the catalyst surface. Hence,
the observed rate of N2O formation in the initial
stage of reaction is not an apparent rate which
has been affected by the oxygen atoms left on
the catalyst surface. If CO alone was admitted
to the fresh catalyst, no CO2 was produced,
indicating that there existed no reactive oxygen
Ž .
species on the fresh CorAl2O3 catalyst 6 .
The catalytic reaction of NO in the coexis-
tence of CO to form N2O and CO2 by the
equation 2NOqCO™N2OqCO2, proceeded
rapidly compared to the reaction of NO alone as
w
x
genation of the preadsorbed ethanol 10 .
Ž
The postadsorbed ethanol species 8 in Fig.
Ž .
Ž .
shown in Fig. 5 a and b . The initial rate of
.
4
is weakly adsorbed on the Nb ethoxide
Ž
.
monomer species 7 in Fig. 4 , which can be
present only in the presence of gas phase ethanol,
and it was kinetically characterized. Assuming
the mechanism shown in Fig. 4, the rate con-
stant k3 and the equilibrium constant for weak
adsorption of ethanol, krkX, were determined to
1
4
be 1.5 mmol miny and 9.0=10y Pay1, re-
w
x
spectively 10 . The weakly adsorbed ethanol is
in equilibrium with the gas phase ethanol, and
easily desorbs from the surface in vacuum.
The dehydrogenation mechanism reveals the
switchover of the reaction path from dehydra-
Ž
.
Ž
tion g-CH bond break to dehydrogenation b-
.
CH bond break by whether or not ethanol
molecules are present in the gas phase. In the
presence of additional gas phase molecules, the
catalytic reaction is able to proceed.
We have found another example of surface
catalytic reactions assisted by gas phase mo-
lecules, where gas phase molecules participate
as promoters in adsorption processes, but are
undetectable at the catalyst surface, as described
in Section 3.3.
Ž . Ž .
Fig. 5. NO reaction on the attached CorAl2O3 catalyst 6 a and
catalytic NO–CO reactions on the attached CorAl2O3 catalyst
Ž . Ž . Ž .
b and an impregnation CorAl2O3 catalyst c ; Reaction
6
Ž .
Ž .
Ž .
temp.: 463 K; a : NOs1.3 kPa; b and c : NOs1.3 kPaqCO
Ž .
Ž .
Ž .
s1.3 kPa; a and b : Cat.s0.50 g; Co loadings2.2 wt.%; c :
Cat.s0.50 g; Co loadings3.0 wt.%.

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A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
the NO–CO reaction was 23 times larger than
that for the NO reaction at 463 K. The activity
Ž .
of the CorAl2O3 catalyst 6 was much higher
than that of a conventional impregnation
CorAl2O3 catalyst as shown in Fig. 5. The
products of the catalytic NO–CO reaction in the
temperature range 353–463 K were N2O and
CO2. In the more elevated temperatures N2O
was further converted to N2, where N2 and CO2
were main products by the equations 2NOq
CO ™ N2O q CO2 and N2O q CO ™ N2 q
CO2. In this article the first reaction step, 2NO
qCO™N2OqCO2, has been studied from
the mechanistic interest.
The surface catalytic NO–CO reaction was
monitored by IR spectroscopy and the gas phase
products were analyzed by gas chromatography.
Note that CO molecules which promote the
surface NO conversion were volumetrically and
spectroscopically undetectable as adsorbates on
the CorAl2O3 catalyst surface as shown in Fig.
Ž .
6 at 298
Fig. 7. FT-IR spectra for NO adsorbed on CorAl2O3
K; NOs1.25 kPa; A–D: spectra at 2 min, 5 min, 30 min and 120
min after NO exposure, respectively; E–G: spectra after evacua-
tion for 5 min, 30 min and 120 min after spectrum C, respectively.
w
x
6 34 . Fig. 7 shows the IR spectra of adsorbed
NO molecules under the ambient NO and in
1
vacuum. The two peaks at 1831 and 1776 cmy
1
are assigned to the dinitrosyl geminal NO
antisymmetric stretching peak at 1776 cmy
Ž
.
Ž
Ž
. .
species adsorbed on a Co atom Co NO , as
changed with exposure time and also by evacua-
2
Ž .
characterized by the isotope peaks at 1772 and
tion as shown in Fig. 7. The bond angle u
between the two NO ligands on a Co atom
Ž15
.
1
1701 cmy for Co NO species and 1828
2
Ž14
.Ž15
.
Ž
.
1
and 1735 cmy for Co NO NO species.
angle: NO–Co–NO has been demonstrated to
be related to the ratio of the antisymmetric
stretching peak intensity Ias to the symmetric
The intensity and relative intensity of the sym-
1
metric stretching peak at 1831 cmy and the
Ž
Ž
.
.
stretching peak intensity Is for the dinitrosyl
2Ž
. Ž
groups by the equation, IasrIs stan ur2 Fig.
.
w
x
Ž .
8
35 . The bond angles u were calculated
using this equation and plotted against exposure
Ž .
time in Fig. 9 a , where the u was calculated by
using the symmetric and antisymmetric stretch-
ing peaks after deconvolution. It is derived that
Ž
.
the amount of adsorbed NO nNO is propor-
2Ž
.
2Ž
.
tional to Iasrsin ur2 or Isrcos ur2 , though
nNO is not absolute value for the amount of
adsorbed NO. The calculated values of nNO are
Ž .
plotted against time in Fig. 9 b . The change in
nNO coincided with the change in the amount of
adsorbed NO determined gravimetrically in a
Ž .
Fig. 6. Amounts of NO and CO adsorbed on CorAl2O3 6 at 298
Ž .
Ž .
separate experiment. Fig. 9 a and b reveals
K.

(
)
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
73
Ž .
Ž .
induced by modes a and b , respectively.
Fig. 8. Symmetric a and antisymmetric b stretching modes of the dinitrosyl on Co atom. mz and mx represent dynamic dipole moments
Ž . Ž .
Ž
that there are two kinds of dinitrosyls species A
for species A at room temperature was esti-
mated to be 808.
.
and species B ; the dinitrosyl with a small bond
Ž
.
angle species A is rapidly formed upon NO
adsorption and easily desorbed by evacuation at
room temperature, while the dinitrosyl with a
large bond angle species B is slowly formed
and not desorbed by evacuation at room temper-
ature. We estimate the ratio of species A to
species B to be 1: 1.9 at room temperature. As
the bond angle u of species B was 1078 and the
bond angle for a mixture of species A and
species B at 120 min of Fig. 9 was 978, the u
Fig. 10 shows the effects of CO on the bond
angle and amount of adsorbed NO at 323, 373,
423, 448 and 463 K. The amount of the dinitro-
syls decreased with increasing temperature,
while the bond angle u increased with an in-
crease of temperature. When CO was intro-
duced to the system, the bond angle u of the
surface dinitrosyls increased. The degree of the
angle expanding was different at temperatures
and larger at higher temperatures, and at 323 K
Ž
.
Ž
.
and
Fig. 9. Time profiles of the bond angle and amount of dinitrosyls
Fig. 10. Time profiles and CO effects of the bond angle
u
Ž .
Ž .
on CorAl2O3 6 at 298 K; NO of 1.25 kPa was exposed at time
amounts of dinitrosyls on CorAl2O3 6 ; NO: 1.3 kPa; CO: 1.3
kPa; `: 323 K; I: 373 K; ^: 423 K; \: 448 K; e: 463 K.
zero and evacuated at 120 min.

(
)
74
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
q
almost no effect was observed. It was also
found that the amount of the dinitrosyls in-
creased by CO admission. Again, the degree of
the increase was larger at higher temperatures.
At 463 K, the amount of the dinitrosyls was
increased by the presence of the gas phase CO
about twice as compared to the amount in the
case of NO alone. The enhancement of the NO
adsorption by the gas phase CO was not ob-
served at 323 K. Thus, the effects of the ambi-
ent CO were large at the temperatures where the
catalytic NO–CO reactions proceeded signifi-
cantly, whereas the bond angle and amount of
the dinitrosyls did not change by the presence of
CO at 323 K, where the catalytic NO–CO
reaction was negligible.
involve a local structural change of the Co2
–
Al2O3 interface by CO impinging under the
catalytic NO–CO reaction conditions.
4. Summary
The catalytic ethanol dehydrogenation pro-
Ž .
ceeds through Nb ethoxide intermediates 7 .
Ž .
However, the species 7 is stable and is not
decomposed to CH3CHO and H2 at 423–523 K
in vacuum. Instead, the species 7 is converted
to C2 H4 and H2O at so high temperatures as
700 K. On the other hand, the unreactive Nb-
Ž .
7 is transformed to reactive Nb-
ethoxide 8 under the catalytic reaction condi-
Ž .
ethoxide
Ž .
q
The property of the Co2 sites for adsorption
tions, where weak ethanol coordination can be
Ž .
is different from that of Mo sites prepared from
present on a vacant site of the species 8 and
Ž
.
Ž
. Ž
.
Ž .
Mo CO where Mo NO CO has been pro-
electronically activate the Nb ethoxide 8 to
produce CH3CHO and H2. The dehydrogena-
tion mechanism reveals the switchover of the
reaction path from dehydration g-CH bond
2
w
6 x
posed 36 . As CO is not observed at the sur-
face, the conformation change to the spacious
dinitrosyls and the increase of the amount of
adsorbed NO might be collision-induced events.
Ž
.
Ž
.
break to dehydrogenation b-CH bond break
Ž .
Ž .
The catalytic reaction rate Õ was expressed by
Õsk NO CO
of the intermediate X by whether or not ethanol
molecules are present in the gas phase. The
catalytic ethanol dehydrogenation reaction on
the NbrSiO2 catalyst is able to proceed only in
1.6
0.95
w
x w x
in the pressure range 0.26–
4.0 kPa. The catalytic reaction rate is nearly
proportional to CO pressure, which might also
provide a picture of CO collision on the Co-di-
nitrosyl. However, N2 and Ar showed no effect
on the adsorbed NO species at 463 K, which
would exclude a possibility of the u extension
and the nNO increase by a physical collision
energy. Other explanation is necessary for the u
extension and the nNO increase; for example,
the CO molecules may have a short resident
the presence of additional gas phase molecules
X
Ž
.
A .
The enhancement of the rate of the interme-
diate decomposition and the switchover of the
reaction path were also found in water–gas shift
w
reactions on MgO, ZnO and RhrCeO2 5,37–
x
39 . These examples presented a new aspect of
catalysis, namely, that surface catalytic reaction
pathways are made accessible only under the
q
time on the Co2 sites to have interaction with
w x
the dinitrosyls, which depends on the surface
temperature. As the amount of adsorbed CO is
undetectable volumetrically and spectroscopi-
cally, the formation of the spacious dinitrosyl
species must be irreversible and the backward
ambient gas 7 . It has also been reported that
the reactivity of the formate ions on NirSiO2
catalyst was markedly increased by the presence
of formic acid in the ambient gas compared to
that at the same coverage under vacuum though
Ž
.
w x
change of a spacious conformation 1078 to a
the mechanism is not clear 40 .
Ž
.
small conformation 808 may be at least much
slower than the impinging rate of CO. We do
not know the exact reason for the observed
events at present, but this phenomenon may
The catalytic NO–CO reaction on the
Ž .
CorAl2O3 catalyst 6 proceeds through the
dinitrosyls as reaction intermediates X which
conformation and amount are affected by the
Ž .

(
)
A. Yamaguchi et al.rJournal of Molecular Catalysis A: Chemical 146 1999 65–76
75
gas phase CO molecules. There is no CO mo-
lecules at the catalyst surface during the cat-
alytic reactions. The reactivity of the dinitrosyls
many catalytic systems. The concept of surface
catalytic reactions assisted by weak or unde-
tectable coordination may provide an approach
to design new catalytic systems through activa-
tion or control of inactive and stable surface
sites in situ under catalytic reaction conditions.
q
on the trigonally distorted tetrahedral Co2 ions
attached on Al2O3 is promoted 23 times by the
gas phase CO molecules. In this reaction mech-
X
Ž
.
anism, impinging CO molecules A are specta-
tors at the catalyst surface, but they change the
conformation and amount of the active species
Acknowledgements
Ž .
X , and also behave as reactant. It is to be
noted that molecules undetectable at surface can
give profound effects on the surface structure of
reaction sites and promote remarkably the reac-
tivity of surface species.
The authors thank Dr. M. Nelson, Mr. S.
Takahara, and Ms. A. Kameyama for her data
acquisitions and helpful discussion. A part of
Ž
this work has been supported by CREST Core
The following examples may be categorized
in a different class of surface catalyticrstoichio-
metric reactions assisted by gas phase molecules,
where the phenomena involve strong adsorption
of gas phase molecules. Au and Roberts ob-
served that the surface oxygen transient gener-
ated during the dissociative chemisorption of
the coadsorbed promoter molecules such as O2,
Research for Evolutional Science and Technol-
.
Ž
.
ogy of the Japan Science and Technology JST .
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