Low-temperature formation of nitrous oxide from dinitrogen, mediated by supported tungsten nanoclusters

Article abstract of DOI:10.1021/ja071860j

N₂ was found to react with H₂O on supported tungsten nanoclusters (W_n) at a temperature as low as 140 K, leading to the formation of N₂O. This shows the ability of W_n to effectively activate notoriously inert N₂ at low temperatures while it remains a molecule. The N₂ activation scheme may be a hint for developing novel heterogeneous catalysts for nitrogen reaction. Copyright

Full text of DOI:10.1021/ja071860j

Published on Web 04/26/2007
Low-Temperature Formation of Nitrous Oxide from Dinitrogen, Mediated by
Supported Tungsten Nanoclusters
†
,‡
Wataru Yamaguchi and Junichi Murakami*
Materials Research Institute for Sustainable DeVelopment, National Institute of AdVanced Industrial Science and
Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan, and Nanotechnology
Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), Central 4,
1
-1-1 Higashi, Tsukuba 305-8562, Japan
Received March 16, 2007; E-mail: j.murakami@aist.go.jp
It is now realized that metal clusters have unique catalytic
properties that do not exist for bulk surfaces of corresponding
metals. This is well illustrated by gold clusters: they catalyze
1
various chemical reactions while the bulk gold is almost inert. In
nature, nanoclusters, which usually contain a few atoms of
transition-metal elements, can be found at the reaction centers of
metalloenzymes. The enzymes catalyze difficult chemical reactions
under mild conditions, for which the clusters are known to play
2
key roles. This leads us to expect that, if nanoclusters of a few
transition-metal atoms are prepared and supported on a surface,
they may also mediate chemical reactions under mild conditions.
The present study explores such a possibility by focusing on a
reaction of dinitrogen (N
on a graphite surface. We show that the tungsten nanoclusters can
activate and mediate reaction of almost inert N in molecular form
to yield N O at a temperature as low as 140 K, which has never
been observed on metal surfaces.
In the present study, size-selected, supported W
clusters were prepared and exposed to N , and adsorption/reaction
of N on the clusters was investigated by X-ray photoelectron
2 n
) on tungsten nanoclusters (W ) supported
Figure 1. N 1s X-ray photoelectron spectra measured at 140 K. (a) For
exposure of W to N . The peak at ∼400.0 eV is assigned to a molecular
5
2
adsorption state of N2, which is a precursor to dissociation (see ref 4). (b)
For exposure to N2 with ∼10 ppm H2O. The spectrum quite resembles the
one obtained for exposure to N2O shown in the inset in terms of the overall
shape and the peak energy positions.
2
2
n
(n ) 2-6)
2
2
spectroscopy (XPS) and thermal desorption spectroscopy (TDS)
in ultrahigh vacuum. Figure 1a shows an XPS spectrum for the N
1
s binding energy (BE) region, for exposure of W
5
supported on
+
an Ar bombarded graphite (highly oriented pyrolytic graphite,
HOPG) surface, to N at 140 K. As seen in the figure, we observed
2
Figure 2. Optimized geometries for side-on N2 adsorption on (a) W4, (b)
W5, and (c) W6 as determined by DFT calculations. Light gray and dark
gray spheres denote W and N atoms, respectively. The values below each
cluster are the length of the N-N bond and the frequency of the N-N
stretching vibration.
3
a single peak located at ∼400.0 eV, which was absent for a bare
HOPG surface exposed to the gas and was assigned to a molecular
4
adsorption of N
molecular N
2
.
The spectrum totally differs from that for
2
adsorption on a bulk polycrystalline tungsten surface
at the same temperature, which has a satellite line (unscreened
state) in addition to the main line (screened state). The dual-peak
cm- (2445 cm-1 for free N
1
).8 This indicates that N
is activated
2
2
4
in the adsorption state.
When water was mixed in the N
hand, we observed a completely different XPS spectrum: two peaks
2
gas (∼10 ppm), on the other
structure originates from the interaction of the 2π* antibonding
5
orbital of N
line in the N
interaction is absent or not feasible for the N
suggests that N is adsorbed on W taking a highly tilted or a side-
on geometry,6 which was also inferred from the fact that the
adsorption state on W is a molecular precursor to the dissociation
2
with the 5d orbital of W. The absence of the satellite
with almost equal intensity at ∼402.6 and ∼406.6 eV showed up
2
XPS spectrum on W
5
indicates that such an N
2
-W
(
Figure 1b). It turned out the XPS spectrum was quite similar to
2
/W system. This
5
9
that of the N
was indeed N
an XPS spectrum obtained for N
the inset of Figure 1b). We further confirmed this by TDS using
2
O molecule adsorbed on a W surface. That the species
O was evidenced by comparing the spectrum with
O, intentionally fed to W (see
2
5
2
,7
2
5
5
4
state of N
To obtain insight into the nature of the adsorbed N
performed density functional theory (DFT) calculations of the side-
on adsorption states on an isolated W . The optimized geometry
calculated for the side-on adsorption state on W is shown in Figure
b. In this configuration, the N-N bond is elongated to 1.27 Å
2
.
1
5
N
2
. The TDS spectrum, shown at the second line from the top of
Figure 3, is for exposure of W at 140 K. In the
to the wet 15
TDS measurement, in addition to 15
, desorption of a species with
a mass of m/e ) 46 was observed with a distinct peak at 143 K, as
2
, we
5
N
2
N
2
n
5
seen in the figure. For the combination of 15
is the only molecular species having the mass of 46 amu.
The observation that only the wet N , not dry one, yields N
indicates that N O forms by the reaction of N
nitrogen in N O, there are two possible sources, that is, the N
molecule itself or the N atom resulting from dissociation of N . As
16
15
16
N
2
and H
2
O,
N
2
O
2
8
2
(cf. the calculated value for free N ) 1.10 Å), and the vibrational
2
2
O
frequency of the N-N stretching mode is greatly reduced to 1239
1
0
2
2 2
with H O. For
†
2
2
Materials Research Institute for Sustainable Development.
Nanotechnology Research Institute.
‡
2
6102
9
J. AM. CHEM. SOC. 2007, 129, 6102-6103
10.1021/ja071860j CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
5
geometry at low temperature are commonly seen. Another type
of molecular adsorption, which possibly has a side-on geometry,
5
is also known for the (111) and the (100) surface. For the (100)
surface, the N-N stretching vibrational frequency is as low as 1452
-
1 15
cm
,
suggesting that the N
2
molecule is activated. The present
study indicates that N
2
O formation from such N
2
molecules on the
bulk surfaces may be possible, although no experimental studies
to explore the point have been reported.
In summary, we have shown that supported tungsten nanoclusters
can mediate the formation of N
without dissociation is involved in the reaction. The DFT calculation
has shown that W (n ) 4-6) activates N in molecular form, and
this should be the key that enables the nanoclusters to mediate such
a low-temperature reaction.
2 2 2
O from N at 140 K, in which N
n
2
Figure 3. Thermal desorption of a species with a mass number of 46 from
1
5
Wn exposed to wet N2 at 140 K. For W5, a desorption peak is observed
1
5
16
15
16
at 143 K. With the ingredients of N2 and H2 O, N2 O is the only
possible product with the mass number of 46. For Wn (n ) 2-6), the
desorption peak was observed for n g 4.
Acknowledgment. This work was supported by the “Support
of Young Researchers with a Term” program of the Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of
Japan.
mentioned above, a single peak located at ∼400.0 eV in the XPS
7
2
spectrum (Figure 1a) is due to a molecular adsorption state of N .
On the other hand, a peak at ∼397.6 eV, a fingerprint for N atom
9
Supporting Information Available: Details of deposition and
fixation of nanoclusters on a graphite surface, TDS measurements, DFT
calculation, and so on. This material is available free of charge via the
Internet at http://pubs.acs.org.
adsorption, is absent in the XPS spectrum, indicating that N
2
does
not dissociate on the nanocluster at 140 K. Since N
at 140 K in the presence of water, it is not likely that the N atom
is the source of nitrogen in the N O formation. We therefore
in molecular form is involved in the N
2
O forms also
2
conclude that N
formation.
2
2
O
References
(
1) (a) Hutchings, G. J. Catal. Today 2005, 100, 55-61. (b) Haruta, M. Gold
Bull. 2004, 37, 27-36. (c) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.;
Freund, H.-J. Gold Bull. 2004, 37, 72-124. (d) Yoon, B.; H a¨ kkinen, H.;
Landman, U.; W o¨ rz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz,
U. Science 2005, 307, 403-407.
The present observation of N
suggests that the N molecule, possibly taking the adsorption
geometry shown in Figure 2, is indeed an activated species and
responsible for the reaction. Then the question is if a N activation
and the reaction are unique for W or is it feasible for W clusters
with other sizes. To answer the question, we investigated the cluster
size dependence of the N O formation/desorption also for W (n
2-4, 6). It was found, as shown in Figure 3, that W and W
O desorption peaks with substantial intensity, but W and
O gas, on the
O from all the cluster
species. These observations indicate that the formation of the
activated N and the subsequent reaction are cluster size depend-
ent: they are feasible for W , W , and W but not for the smaller
clusters W and W
To elucidate whether adsorption states similar to that for W
Figure 2b are responsible for activating N on W or W , N
adsorption geometries for W and W and also for W and W were
examined by the DFT calculation. The result was that the side-on
adsorption geometry, similar to the one for W , is a stable one also
for W (Figure 2a) and W (Figure 2c) but not for W and W
These results further suggest that N is activated in the side-on
adsorption geometry also on W and W and the adsorption state
is responsible for the N O formation.
2 2 5
O formation involving N on W
2
2
(2) (a) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science Books: Herndon, VA, 1997. (b) Bertini, I., Gray, H.
B., Stiefel, E. I., Valentine, J. S., Eds. Biological Inorganic Chemistry:
Structure and ReactiVity; University Science Books: Herndon, VA, 2006.
5
(
c) Tolman, W. B., Ed. ActiVation of Small Molecules: Organometallic
2
n
and Bioinorganic PerspectiVes; Wiley-VCH: Weinheim, Germany, 2006.
3
)
give N
4
6
14
-2
(
3) The estimated densities of supported W
n
are ∼5/n × 10 cm , and that
11
of the defects for pinning the clusters is comparable to them.
4) Yamaguchi, W.; Murakami, J. Chem. Phys. Lett. 2003, 378, 521-525.
5) Raval, R.; Harrison, M. A.; King, D. A. Nitrogen Adsorption on Metals.
In Chemisorption Systems Part A: The Chemical Physics of Solid Surfaces
and Heterogeneous Catalysis; King D. A., Woodruff, D. P., Eds.;
Elsevier: Amsterdam, 1990; Vol. 3, pp 39-129.
2
2
(
(
W
3
do not. When these clusters were exposed to N
2
other hand, we observed desorption of N
2
2
(6) Grunze, M.; Golze, M.; Hirshwald, W.; Freund, H.-J.; Pulm, H.; Seip,
U.; Tsai, M. C.; Ertl, G.; K u¨ ppers, J. Phys. ReV. Lett. 1984, 53, 850-
4
5
6
8
53.
2
3
.
(
(
7) Rao, C. N. R.; Rao, G. R. Surf. Sci. Rep. 1991, 13, 221-263.
8) The experimental values for the bond length and the vibrational frequency
5
in
1
of a gas-phase N
2
are 1.09 Å and 2360 cm- , respectively. See: Herzberg,
2
4
6
2
G. Spectra of Diatomic Molecules; Van Nostrand Reinhold: New York,
1950.
4
6
2
3
(
9) Fuggle, J. C.; Menzel, D. Surf. Sci. 1979, 79, 1-25.
(
10) The counterpart of the reaction is O from H
2
O. At 140 K, XPS revealed
2
there were chemisorbed and physisorbed H O on the cluster. By TDS
18 18
5
using H
2
O, chemisorbed on W
5
, we found O is not involved in N
2
O.
2
4
6
2
3
.
We thus conclude the physisorbed H
2
O is the source for O in the N O
2
formation. The enhancement of water reactivity by dissociation of the
O-H bond in the hydrogen bonding in the physisorbed state has been
pointed out recently. See: (a) Johnson, M. A.; Stefanovich, E. V.; Truong,
T. N.; G u¨ nster, J.; Goodman, D. W. J. Phys. Chem. B 1999, 103, 3391-
4
6
2
3
398. (b) Kato, H. S.; Shiraki, S.; Nantoh, M.; Kawai, M. Surf. Sci. 2003,
For N
2
n
adsorption on small gas-phase W , molecular adsorption
5
44, L722-L728.
is favored,1
2-14
and the molecular state seems to act as a precursor
(
11) The difference in the N
2
O intensities among W
4
, W
5
, and W
6
is possibly
to dissociation.12 It was also shown for anionic clusters (W
-
attributed to the difference in the number of deposited clusters.
n
) that
are very small compared
to those for larger species.14 These findings are in line with the
(
12) Mitchell, S. A.; Rayner, D. M.; Bartlett, T.; Hackett, P. A. J. Chem. Phys.
-
-
2 2 3
adsorption energies of N for W and W
1
996, 104, 4012-4018.
(13) Holmgren, L.; Andersson, M.; Ros e´ n, A. J. Chem. Phys. 1998, 109, 3232-
3239.
size-dependent properties of supported W
study.
n
revealed in the present
(
14) Kim, Y. D.; Stolcic, D.; Fischer, M.; Gantef o¨ r, G. J. Chem. Phys. 2003,
119, 10307-10312.
15) Ho, W.; Willis, R. F.; Plummer, E. W. Surf. Sci. 1980, 95, 171.
(
2
For N on tungsten surfaces, the most stable, dissociative
adsorption and a weakly bound molecular adsorption with end-on
JA071860J
J. AM. CHEM. SOC.
9
VOL. 129, NO. 19, 2007 6103