Supplementary Information
Title of the manuscript : Reduction of N2 by supported tungsten clusters gives a model
of the process by nitrogense
Author list : Junichi Murakami (corresponding author) and Wataru Yamaguchi
1. Fixation of size-selected tungsten clusters on HOPG
Figure S1 shows the results of the BE measurements of the W4p3/2 electrons of W2 to W6
deposited by the method described in the paper. As seen in the figure, the BE is the
largest for W2 and becomes smaller with cluster size in the following order: BE(W2) >
BE(W3) ~BE(W4) > BE(W5) ~ BE(W6) for the chemisorbed water amount studied. This
gives evidence that the W clusters are fixed on the HOPG surface. It is notable that the
binding energy change with cluster size resembles those found for the supported cluster
systems Ptn/SiO251 and Pdn/TiO252.
Figure S1 | Changes in the W4p3/2 electron binding energy with the amount of
chemisorbed water for clusters W2 to W6 deposited on an HOPG surface at 296 K. The
number of deposited tungsten atoms was adjusted to 4.5x1013 for all the cluster species.
The data points were obtained by repeated XPS measurements of the W4p3/2, O1s and
C1s peaks after the deposition of the clusters. As seen in the Figure, a few water
molecules are already adsorbed on the clusters from the 1st measurement and more
water molecules, coming from the ambient, are adsorbed during the measurement. The
peak energies and the areas of the peaks were deduced from the data using the program
XPSPEAK ver. 4.1. The area and the energy of the C1s peak was used as an intensity
and an energy standard respectively. The lines in the graph are the results of curve
fitting of the data points.
2. Adsorption state of water on the tungsten clusters
XPS measurements shows H2O is adsorbed by the tungsten clusters at room
temperature in two distinct manners with different O1s binding energies (BEs). One
species has an O1s BE of ~532.8 eV which is very close to that of H2O physisorbed on a
bare HOPG surface, suggesting the water molecule is also a physisorbed one. The other
species has much lower BE (~531.0 eV). An increase in the amount of the species with
the lower BE was found to correlate with an increase in the BE of the W core level
electrons as shown for W4p3/2 in Fig. S1. This suggests there is a substantial charge flow
from W to O. These observations suggest the species with the lower BE is a chemisorbed
(dissociatively adsorbed) water on the cluster. The 1st principles calculation shows the
chemisorption is energetically much more favorable than molecular adsorption. It is
illustrated for a single water molecule adsorption on a bare W5 in Figure S2. The figure
shows total energies and corresponding geometries of low-lying W5-H2O isomers, with
the origin of the total energy being at the calculated global minimum. As seen in the
figure, the chemisorbed states are distributed on an energy region which is much lower
than, and well separated from that for molecular adsorption states. The calculated
global minimum belongs to the H + OH mode of adsorption, which probably corresponds
to the experimentally observed water species with the BE of 531.0 eV.
Figure S2 | Geometries and total energies of low-lying W5-H2O isomers. The origin of
the total energy is set at the calculated global minimum. The calculations were carried
out by a density functional theory method using M06 hybrid meta generalized gradient
approximation functional53,54. The LANL2DZ effective core potential basis set55 was
employed for W atoms, and the 6-311G(d) basis set56,57 for H and O. The computations
were executed by using the GAMESS code58,59. For the most stable configuration (H +
OH)/W5, the electron population of the hydrogen released from H2O is 1.185294 (net
charge: -0.185294) by the Mulliken population analysis60-63 and 1.126417 (net charge:
-0.126417) by the Löwdin population analysis64. Thus the hydrogen on W5 originated
from the dissociative adsorption of H2O can be safely regarded as a neutral H atom.
3. Thermal desorption from N2H4 adsorbed onW5 (N2H4/W5) at room temperature
Fig. S3 | Thermal desorption spectrum for N2H4/W5. The result shows a species with
m/e=17 starts to desorb near 300 K and has a distinct broad peak at~ 370 K, which is
much larger than the corresponding hump for m/e=18. Since the background for m/e=18
is almost twice that for m/e=17, the result conclusively shows the peak for m/e=17 is not
due to
16OH+
cracked from H216O+ but
14NH3+.
Desorption of a species with m/e=32
(possible species: 16O2+ or 14N2H4+) at ~350 K was not observed for (14N2+H216O)/W5. This
strongly suggests the species with m/e=32 observed for
14N2H4/W5
is not
16O2+
but
14N2H4+.
4. Stabilization of the N2 bridge adsorption by water adsorption
In our previous studies, we have suggested that a precursor for the low-temperature N2
reaction is molecularly adsorbed N2 on Wn with a W-N-N-W bridge structure65. DFT
calculations concerned with isolated, bare clusters showed that the bridge-type
adsorption states are energetically favorable (i. e., have lower energy than isolated N2
and Wn) for Wn (n ≥ 4), but not for W2 and W3
66,67.
This indicates that N2 may not be
activated and hence does not react on W2 and W3.
In the present study, however, reduction of N2 was observed on W2 and W3. This is
possibly attributed to an effect of water adsorption on the clusters. DFT calculations
suggest dissociative adsorption of H2O stabilizes the bridge adsorption states of N2 on
W2 and W3. Figure S4 shows how the N2 bridge adsorption states are stabilized on
water-adsorbed Wn (n = 2,3,4). The calculated N-N bond length in the bridge state for
each species is much larger than that of a free N2 (1.09 angstroms), suggesting that N2
is activated in these states.
Figure S4 | Bridge adsorption states of N2 on Wn (n = 2, 3, 4) with or without water
coadsorption. Blue letters denote N-N bond lengths in angstroms and red or black
letters the total energy changes in units of eV by N2 adsorption. The computational
method is the same as that used in the Supplementary Information 2. The origin of the
total energy E is set at the sum of the energies of a free N2 and a bare or a
water-preadsorbed Wn separated from each other. Thus negative E means the
adsorption state is energetically favorable.
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