N+NO reaction on Rh(111) surfaces studied with fast near-edge X-ray absorption fine structure spectroscopy: Role of NO dimer as an extrinsic precursor

Article abstract of DOI:10.1021/jp0665933

We studied the mechanism of the N+NO reaction on Rh(111) surfaces by means of fast near-edge X-ray absorption fine structure spectroscopy. This reaction is important as a basis of NO_x reduction reactions on platinum-group metal surfaces. Atomic nitrogen layers on Rh(111) were titrated with NO at various temperatures. N₂O is exclusively formed and desorbs into the gas phase below 350 K. The consumption rate of atomic nitrogen exhibits strange temperature dependence between 100 and 350 K; the reaction proceeds slower with increasing temperature. Reaction kinetics analyses and isotope-controlled experiments have revealed that the surface N atoms do not react with chemisorbed NO molecules but with NO dimers weakly bound on top of the chemisorbed layer, which play a role as an extrinsic precursor. The present results may support the possibility that NO dimers participate in various NO-related synthetic, biochemical, and surface reactions as an intermediate.

Full text of DOI:10.1021/jp0665933

25578
2006, 110, 25578-25581
Published on Web 11/30/2006
N+NO Reaction on Rh(111) Surfaces Studied with Fast Near-Edge X-ray Absorption Fine
Structure Spectroscopy: Role of NO Dimer as an Extrinsic Precursor
Ikuyo Nakai,†,§ Hiroshi Kondoh,*,† Toru Shimada,†,| Masanari Nagasaka,† Reona Yokota,†,
Kenta Amemiya,†,# Hideo Orita,‡ and Toshiaki Ohta†,)
Department of Chemistry, School of Science, The UniVersity of Tokyo, Tokyo 113-0033, Japan,
National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan,
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, National Institute for Materials Science,
Tsukuba, Ibaraki 305-0047, Japan, Sekisui Chemical CO., Ltd., Tsukuba, Ibaraki 300-4292 Japan, High Energy
Accelerator Research Organization, and SR Center, Ritsumeikan UniVersity, 1-1-1 Noji-Higashi, Kusatsu,
Shiga 525-8577, Japan
ReceiVed: October 7, 2006; In Final Form: NoVember 13, 2006
We studied the mechanism of the N+NO reaction on Rh(111) surfaces by means of fast near-edge X-ray
absorption fine structure spectroscopy. This reaction is important as a basis of NOx reduction reactions on
platinum-group metal surfaces. Atomic nitrogen layers on Rh(111) were titrated with NO at various
temperatures. N2O is exclusively formed and desorbs into the gas phase below 350 K. The consumption rate
of atomic nitrogen exhibits strange temperature dependence between 100 and 350 K; the reaction proceeds
slower with increasing temperature. Reaction kinetics analyses and isotope-controlled experiments have revealed
that the surface N atoms do not react with chemisorbed NO molecules but with NO dimers weakly bound on
top of the chemisorbed layer, which play a role as an extrinsic precursor. The present results may support the
possibility that NO dimers participate in various NO-related synthetic, biochemical, and surface reactions as
an intermediate.
The adsorption behavior of NO and the NO+CO reaction
on Rh(111) surfaces have been extensively studied as model
processes of the NOx reduction reaction.1,2 NO molecules
dissociatively adsorb on Rh(111) above 350 K. The NO+CO
reaction proceeds in a steady state only above 350 K since the
NO dissociation is necessary for the progress of the reaction.1,2
One of the major products in the whole catalytic NO+CO
reaction is N2. Zaera et al. proved that the N2 molecules are
not formed by association of atomic N but via formation and
decomposition of N2O on Rh(111).2 Though the reaction
mechanism has been investigated at high temperatures where
NO dissociation proceeds in a steady state, this reaction is
expected to occur even at lower temperatures. In order to
understand the N+NO reaction comprehensively, it is necessary
to investigate it over a wide temperature range including low
temperatures where no dissociation of NO takes place. For this
purpose, the mechanistic study of N+NO reaction on artificially
prepared N-covered Rh surfaces is a promising approach. So
far the number of surface science studies on the N+NO reaction
using N-precovered Rh surfaces has been quite limited.3
Here we studied the N+NO reaction on N-precovered Rh-
(111) surfaces by monitoring with fast near-edge X-ray absorp-
tion fine structure (NEXAFS) spectroscopy. It was revealed that
transiently trapped NO dimer (NO)2 in an extrinsic precursor
state plays an important role in the reaction below 350 K.
The NO dimer has been assumed to be an essential interme-
diate in various NO-related synthetic, biochemical, and surface
reactions.4-7 Since the NO monomer is relatively inactive, it is
important to understand the role of the dimer in the complicated
reaction mechanisms. Recently, the contribution of the NO dimer
to NO-induced oxidation and nitrosation reactions was proposed
by molecular orbital calculations.4 However, NO dimers have
not been directly detected under reaction conditions. Spectro-
scopic observation of the NO dimer as a reaction intermediate
will help the understanding of the mechanisms of NO-related
reactions.
The experiments were carried out at BL-7A of the Photon
Factory (Tsukuba, Japan). The real-time monitoring of the
reaction was conducted with the fast NEXAFS on the basis of
the energy dispersive technique.8
The atomic nitrogen covered surface was prepared on a clean
Rh(111) surface following the previously reported procedure:
first molecularly adsorbed NO is thermally dissociated into
atomic N and O, and then the atomic O was removed by dosing
H2.9 The partially N-covered surfaces were exposed to NO gas
at a constant pressure of 5.0 × 10-9 Torr at constant surface
temperatures ranging from 70 to 480 K. During the NO
exposure, N K-edge NEXAFS spectra were obtained every 13 s.
* Corresponding author. Tel&Fax: 81-3-5841-4418; E-mail: kondo@
chem.s.u-tokyo.ac.jp
† Department of Chemistry, School of Science, The University of Tokyo.
‡ National Institute of Advanced Industrial Science and Technology
(AIST).
§ Institute for Molecular Science.
| National Institute for Materials Science.
Sekisui Chemical Co., Ltd.
# High Energy Accelerator Research Organization.
) Ritsumeikan University.
10.1021/jp0665933 CCC: $33.50 © 2006 American Chemical Society

Letters
J. Phys. Chem. B, Vol. 110, No. 51, 2006 25579
Arrhenius plot (inset of Figure 1) to be a negative value of
-0.04 eV. Figures 2a and 2b show time evolution of adsorbate
coverages and an example of NEXAFS spectra, respectively,
taken at 120 K. Atomic N and molecular NO were observed
on the surface during the reaction in this temperature range.
When the gas-phase species were monitored with a quadrupole
mass spectrometer (QMS) during the NEXAFS measurement,
only N2O was observed accompanying consumption of the
surface atomic N. Decomposition of N2O into N2 and O, which
was observed in the reaction above 350 K, was not observed at
all. These results indicate that the total reaction in the temper-
ature range of 100-350 K can be regarded as
N(a) + NO(a) f N2O(g)
(1)
Figure 1. Temperature dependence of the initial reaction rate (V0) at
the NO pressure of 5 × 10-9 Torr. The reaction rates are normalized
with the initial N coverage (θ0) to exclude the effect of variety in the
initial coverage of N (in the range of 0.18-0.22 ML). The solid line
is guide for eyes. The inset is the Arrhenius plot for the reaction rate
in the temperature range from 100 to 200 K.
Negative activation energies have been so far observed for
surface reactions where weakly adsorbed species, whose de-
sorption rate is faster than its reaction rate, are involved.10,11 In
Figure 2a, the consumption of N exhibits a delayed start at
around 300 s where NO is accumulated up to almost the
completion of the N+NO monolayer on the surface (induction
period). This result suggests that the reactive NO species is an
extrinsic-precursor, which is weakly bound on top of the already-
formed chemisorbed monolayer.12 Since the desorption and
reaction rate of the NO molecules trapped in the extrinsic-
precursor state is faster than the adsorption rate, its residence
time may be too short to be observed with NEXAFS.
At sufficiently low temperatures, however, the desorption and
reaction rate of the NO precursor should become slower than
the adsorption rate. Furthermore, if the desorption rate is slower
than the reaction rate, the temperature dependence of the total
reaction rate (the consumption rate of atomic N) is expected to
turn positive and the trapped precursors are expected to be
observed on the surface during the reaction. The temperature
dependence of the reaction rate is actually positive below
100 K (Figure 1). As shown in Figures 2c and 2d, a new species
besides atomic N and molecular NO is observed on the surface
below 100 K. The new species is attributed to NO dimer (NO)2,
since the NEXAFS spectrum of the new species is almost the
same as that of NO dimer.13 The formation of the NO dimer
and the consumption of atomic N start simultaneously at around
400 s, though the start of N consumption is not clear due to the
low consumption rate. These results suggest a possibility that
the NO dimer formed on top of the chemisorbed layer acts as
an extrinsic precursor. When the NO exposure was stopped
during the reaction at 3170 s in Figure 2c, a part of the NO
dimers decomposed and desorbed into the gas phase. However,
the consumption of atomic N still continued by the reaction of
remaining NO dimer and atomic N even after the stop of NO
exposure.
To confirm the validity of the extrinsic-precursor-mediated
reaction model, we checked contribution of the precursor species
to the reaction with an isotope-controlled experiment. First, the
surface was completely precovered with 14N and chemisorbed
14NO. Then it was exposed to 15NO gas while monitoring the
mass signals of the desorbing N2O with the QMS. The result is
shown in Figure 3. 14N2O does not appear but 14N15NO is
exclusively observed, which evidences that the extrinsic-
precursor species in the second layer directly reacts with atomic
N in the first layer. Note that no intermixing between the first-
and second-layer NO species occurs under this condition.
Chemisorbed NO does not react with N probably due to a strong
N-NO repulsion in the first layer.14
Figure 2. Time evolution of the coverages of surface adsorbates during
reactions at 120 K (a) and 70 K (c) and example of corresponding
N-K NEXAFS spectra and curve-fitting analysis results at 120 K (b)
and 70 K (d). The red, green and blue lines in the spectra show the
fitting component for atomic N, chemisorbed NO and physisorbed NO
dimer (NO)2, respectively, and the gray lines show the sums of them.
For the experimental run at 70 K(c), the NO exposure was stopped at
3170 s. The small structure around 400 eV in (b) is due to inhomo-
geneous sensitivity of the imaging detector, which can be neglected in
the curve-fitting analyses.
Consumption of atomic N by the reaction was observed during
the NO exposure. Time evolution of coverages of surface species
was obtained by the curve fitting analysis for each spectrum
(examples of curve fitting are presented in Figures 2 b and d).
The errors in the estimated coverages were below 5%. The
reaction rate was estimated from the slope of the consumption
curve of atomic N.
The reaction rate (N consumption rate, namely) at the initial
stage of the NO exposure was measured as a function of
temperature as shown in Figure 1. The reaction-rate change is
classified into three temperature regions: (1) below 100 K, (2)
100-350 K, and (3) above 350 K. The reaction above 350 K,
which proceeds faster with increasing temperature, was revealed
to be identical process with the one in the steady-state NO+CO
reaction2 where N2 is formed via dissociation of transiently
formed N2O. The detail of the reaction above 350 K will be
presented elsewhere. In the temperature range from 100 to
350 K, the reaction proceeds slower with increasing temperature,
which is opposite to the cases of usual elementary reaction steps.
The activation energy was estimated from the slope of the
From these experimental results, the reaction formula (1) is
rewritten as follows:

25580 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Letters
Figure 3. Time evolution of mass signal intensity of m/e ) 45 (15N14-
NO) and m/e ) 44 (14N2O) when a Rh(111) surface completely covered
with chemisorbed 14N and 14NO was exposed to 15NO gas (T ) 120 K,
PNO ) 1 × 10-6 Torr). The small structure in the m/e ) 45 curve at
around 5 s is ascribed to a drop of NO pressure.
Figure 4. (a) Experimentally obtained energy diagram for the reaction.
The errors for Ed, Er, and Ea′ was 0.05, 0.02, and 0.01 eV, respectively.
(b) Proposed reaction model based on the density functional theory
(DFT) calculations.15
2NO(g) a (NO)2 (precursor)
(2-1)
atures, but the extrinsic-precursor-mediated N2O formation may
not be negligible, even under realistic conditions at the higher
temperatures due to high NO pressures.
(NO)2 (precursor) + N(a) f N2O(g) + NO(a) (2-2)
Negative activation energies have been sometimes interpreted
in terms of the difference between the real activation energy of
the reaction (Er) and that of the desorption of the weakly bound
reactant (Ed): Er - Ed.10,11 If Er is smaller than Ed, a negative
apparent activation energy is observed. Figure 4a shows the
energy diagram obtained from the NEXAFS experiments. Ed
was estimated from the rate of decomposition-desorption
process of the NO dimer on top of a pure NO monolayer, and
Er was obtained from the consumption rate of atomic N in the
reaction below 100 K. The estimated energies are consistent
with one another within their errors, supporting the proposed
reaction mechanism (2-1 and 2-2). Surface reactions where
extrinsic precursors directly participate are very rare.11
Although the microscopic reaction mechanism including NO
dimers cannot be deduced from the present experimental results,
we tentatively propose a reaction model based on the density
functional theory calculations15 as illustrated in Figure 4b. The
reaction proceeds via formation of a new N-N bond between
an N atom and an NO dimer (ii). According to a recent
theoretical study, NO dimer is electrophilic and easily forms a
chemical bond with electron-rich species.4 Then, one of the NO
units of the complex (ii) replaces the N atom bound to the
surface concomitantly with elongation of the N-N bond of the
dimer (iii). Finally, the N-N bond dissociates to release an N2O
molecule with an NO molecule remaining on the surface (iv).
Physisorbed NO monomers were not detected in the present
NEXAFS observations.16 It should be noted that we cannot
completely exclude the possibility that a small amount of NO
monomers physisorbed on top of the chemisorbed monolayer
and/or supplied by dissociation of dimers react with the atomic
N. However, we assume that the NO monomers do not
contribute primarily to the reaction; if they can react with N
atoms, the reaction should immediately start to occur without
any induction period after the NO exposure is started. That the
reactivity of the NO dimer is higher than the monomer is
presumably due to its electrophilicity4 and increased van der
Waals interactions caused by the larger molecular weight.
It was revealed that in the N+NO reaction on Rh(111) at
70-350 K, the NO dimer in the extrinsic precursor state directly
reacts with atomic N to form N2O and NO. The practical
catalytic NO reduction on Rh surfaces, however, proceeds at
higher temperatures above 350 K. In the present study, the role
of the extrinsic precursor was prominent at the lower temper-
In conclusion, we found from the real-time monitoring of
the N+NO reaction on Rh(111) with the fast NEXAFS
technique that an extrinsic precursor formed on the chemisorbed
N+NO layer reacts with atomic N to form N2O. The precursor
is identified as an NO dimer species with an N-N bond. The
direct observation of the NO dimer contributing to the reaction
may support the possibility that NO dimers are involved as a
reaction precursor in many NO-related reaction systems.
Acknowledgment. The present work is supported by the
Grant-in-Aid for Scientific Research (KAKENHI) in Priority
Area “Molecular Nano Dynamics” from MEXT. The present
work has been performed under the approval of the Photon
Factory Program Advisory Committee (PF PAC No.2001S2-
003 and 2004G-320).
Supporting Information Available: Comparison between
an N-K NEXAFS spectrum of the precursor (Figure S1) and
that of NO dimer (Figure S2). This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
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Letters
J. Phys. Chem. B, Vol. 110, No. 51, 2006 25581
(13) (a) Pe´rez-Jigato, M.; Termath, V.; Gardner, P.; Handy, N. C.; King,
calculations without symmetry restriction. Transition state search was
performed with synchronous transition methods to estimate activation.17,18
We have found that the reaction scheme is energetically reasonable also
for the DFT calculations, although the absolute adsorption energies are
overestimated within the tendency of the state-of-the-art DFT method. The
negative activation energy is also confirmed.
(16) The N-K NEXAFS of the physisorbed NO monomer is expected
to be similar to that of isolated NO in the gas phase. It exhibits a sharp
N1s f π* peak at around 399 eV (Kosugi, N.; Adachi J.; Shigemasa E.;
Yagisita. A. J. Chem. Phys. 1992, 97, 8842). Such a peak is not observed
in the present spectra.
(17) East, A. L. L. J. Chem. Phys. 1998, 109, 2185.
(18) Orita, H.; Nakamura, I.; Fujitani, T. Surf. Sci. 2004, 571, 102.
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Information.
(14) DFT calculations15 indicated that the chemisorbed NO molecules
cannot react with the atomic N due to a high activation barrier. They react
with the N atoms above 350 K as their diffusion becomes active at the
high temperatures.
(15) We conducted DFT calculations to check this reaction model
qualitatively because NO dimer is one of the molecules that are not treated
explicitly within the state-of-the-art DFT methods.17 The detailed calculation
methods were described in the previous works.18,19 The DFT calculations
were performed with the program package DMol3 in Materials Studio
(version 2.2) of Accelrys Inc. A (2 × 2) surface unit cell of a slab of four
layers’ thickness (including 16 Rh atoms) was used. The slab was repeated
periodically with a 30 Å of vacuum region between the slabs. The adsorbates
and the two top layers of metal surfaces were allowed to relax in all the