Selective oxidation of ammonia over Ir(5 1 0). Comparison with Ir(1 1 0)

Article abstract of DOI:10.1016/S0039-6028(03)00085-2

The decomposition and oxidation of NH₃ have been studied on the Ir(5 1 0) surface in the temperature range from 300 K up to 800 K. Decomposition of NH₃ at room temperature and combination of N-atoms to form N₂ was observed. The products of ammonia oxidation were N₂ and H₂O. NO and N₂O were not observed. Inhibition of the reaction by N species seems to exist until a certain temperature (between 400 and 500 K, depending on the NH₃/O₂ ratio). Above those temperatures the reaction starts as manifested by a decrease in NH₃ and O₂ pressures and a simultaneous increase in H₂O and N₂ pressures. Reaction orders with respect to NH₃ and O₂ were found to have values between 0.5 and 0.7. A comparison with previous results obtained on the Ir(1 1 0) surface is made.

Full text of DOI:10.1016/S0039-6028(03)00085-2

Surface Science 532–535 (2003) 87–95
www.elsevier.com/locate/susc
Selective oxidation of ammonia over Ir(510). Comparison
with Ir(110)
*
ꢀ
Sonia A.C. Carabineiro, Bernard E. Nieuwenhuys
Surface Science and Heterogeneous Catalysis Group, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55,
Leiden CC 2333, The Netherlands
Abstract
The decomposition and oxidation of NH3 have been studied on the Ir(5 1 0) surface in the temperature range from
300 K up to 800 K. Decomposition of NH3 at room temperature and combination of N-atoms to form N2 was ob-
served. The products of ammonia oxidation were N2 and H2O. NO and N2O were not observed. Inhibition of the
reaction by N species seems to exist until a certain temperature (between 400 and 500 K, depending on the NH3/O2
ratio). Above those temperatures the reaction starts as manifested by a decrease in NH3 and O2 pressures and a si-
multaneous increase in H2O and N2 pressures. Reaction orders with respect to NH3 and O2 were found to have values
between 0.5 and 0.7. A comparison with previous results obtained on the Ir(1 1 0) surface is made.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Ammonia; Nitrogen oxides; Nitrogen molecule; Oxygen; Catalysis; Chemisorption; Surface chemical reaction; Thermal
desorption; Iridium; Auger electron spectroscopy; Low energy electron diffraction (LEED)
4NH3 þ 3O2 ! 2N2 þ 6H2O
4NH3 þ 4O2 ! 2N2O þ 6H2O
Reaction (1) is the so-called Ostwald process used
to produce nitric acid. Because of its industrial
importance it has been studied intensively in the
past. High temperatures (>800 K) are needed for
this reaction. Relatively few studies were focused
on reactions (2) and (3). The interest in reaction (2)
is becoming more important however due to the
environmental aspect [5].
ð2Þ
ð3Þ
1. Introduction
Ammonia is a harmful byproduct of catalytic
reactions occurring in automotive catalytic con-
verters. It is also present in tail gases from elec-
tricity and chemical plants. The oxidation of
ammonia proceeds via three main overall reactions
[1–4]:
4NH3 þ 5O2 ! 4NO þ 6H2O
ð1Þ
Most of the studies concerning ammonia oxi-
dation have been performed in connection with the
production of NO on Pt and Pt/Rh surfaces [2,3,6–
13]. Only a few studies were carried out on Ir. It
was found recently that this metal is more active
and more selective to nitrogen than other metals
[4,14–17].
* Corresponding author. Address: Gorlaeus Laboratories,
Faculty of Mathematics and Sciences, Leiden University,
Einsteinweg 55/P.O. Box 9502, Leiden 2300 RA, The Nether-
lands. Tel.: +31-71-527-4545; fax: +31-71-527-4451.
E-mail addresses: s.carabineiro@chem.leidenuniv.nl (S.A.C.
Carabineiro), b.nieuwe@chem.leidenuniv.nl (B.E. Nieuwen-
huys).
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00085-2

88
S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
2. Experimental
The adsorption of ammonia has also been
studied on several metal surfaces including Pt
[13,18–24] and other metals [17,24–34]. Ammonia
can adsorb molecularly [20,24,27,29–33] or disso-
ciatively [18,22,27,28]. The adsorption on a clean
surface results in a bond to the metal via the N
atom with H pointing away from the surface. It
has been reported that, in the presence of pread-
sorbed oxygen, NH3 interacts with adsorbed O via
a hydrogen bond, leading to local azimuthal or-
dering in the absence of long range order [29].
The present work is part of a collaborative
project in which ammonia oxidation is studied on
several noble metal catalysts, both polycrystalline
[4,5,35] and in the form of single crystal surfaces. In
addition, theoretical calculations are performed.
The ultimate purpose is to understand the selec-
tivity and the specific differences in behaviour of
the various noble metal surfaces. An earlier paper
dealt with this reaction on the Ir(1 1 0) surface
[36]. This surface was then selected because its
‘‘N-chemistry’’ has been documented in detail in
our earlier papers [37–39]. In the present work the
reaction was studied on Ir(5 1 0), a surface with
(1 0 0) terraces and (1 1 0)-like steps (Fig. 1).
Reactions were performed in a UHV system
equipped with facilities for LEED, AES and a
differentially pumped quadrupole mass spectro-
meter. Further details of experimental procedures
are described elsewhere [36].
3. Results and discussion
3.1. Adsorption of NH3 on clean surface
Fig. 2a shows the results of TDS experiments
carried out after NH3 exposure at 300 K. Spectra
obtained at higher exposures are shifted to higher
values for better comparison. A minor broad peak
of mass 17 is observed around 430 K, slightly in-
creasing with increasing exposure. Fig. 2b shows
one main peak for mass 28 approximately at 575 K
whose intensity increases with increasing exposure.
The intensity of mass 28 desorption peak is larger
by one order of magnitude than that of mass 17.
These results show that desorption of molecular
ammonia is a minor process only and the small
Fig. 1. The structure of the stepped face centred cubic Ir(5 1 0) surface (a). This surface consists of (1 0 0) terraces (b) and (1 1 0)-like
steps (c). Pictures adapted from the NIST surface structure database.

S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
89
Fig. 2. TDS experiments carried out after NH3 exposure at 300 K, for mass 17 (a) and mass 28 (b), at a heating rate of 25 K/s.
broad peak of mass 17 can also be attributed to
desorption from the support. A small desorption
peak of mass 2 (H2) was observed at lower tem-
peratures (not shown), at the same temperatures as
found in earlier H2 adsorption/desorption studies
carried out on the same surface [40]. These results
show evidence of decomposition of ammonia at or
near room temperature and combination of N-
atoms at around 500 K, to form N2. Similar results
were also reported by other authors using other
metal surfaces [18,27]. Desorption of masses 14,
15, 16 was not observed (except of fragmentation
products of NH3 and N2), nor of masses 30 (NO)
and 44 (N2O). Similar results were obtained with
the Ir(1 1 0) surface. However, N2 desorption was
found at the significantly higher temperature of
640 K [36].
3.2. Effect of oxygen precoverage on NH3 adsorp-
tion
Fig. 3a and b shows the TD spectra obtained
after exposing the sample to 5 L NH3 at 300 K on
a surface precovered with oxygen. There is a large
decrease in the peaks of both N-containing masses
with increasing O-precoverage. This decrease in

90
S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
(a)
no O2
1L O2
5L O2
10L O2
15L O2
(b)
no O2
1L O2
5L O2
10L O2
15L O2
300
350
400
450
500
550
600
650
700
750
800
850
Temperature (K)
Fig. 3. TDS experiments carried out after 5 L NH3 exposure at 300 K, on a surface precovered with oxygen, for mass 17 (a) and mass
28 (b), at a heating rate of 25 K/s.
N-containing products is explained by the ex-
pected blocking of adsorption sites by O.
it should be noted that the NH3 decomposes at the
lowest temperature used for this study, that is, 300
K. Hence, it might be that preadsorbed O can affect
the decomposition of NH3 at lower temperatures.
Similar results were obtained with the Ir(1 1 0)
surface [36].
A small desorption amount of mass 18 (H2O)
was also observed, increasing with increasing O-
precoverage. No desorption of mass 30 (NO) was
observed, in contrast with the results obtained by
other authors on a Pt(1 1 1) surface [13]. It should
be noted that formation of NO is more likely to
occur at higher temperatures [3,4,12,41].
3.3. Heating and cooling cycles
No beneficial effect of Oads on NH3 decompo-
sition was found on Ir, in contrast with was pro-
posed for Pt(1 0 0) [24,42] or Ir(1 0 0) [17]. However,
Fig. 4 shows the results of the heating and
cooling cycles from 300 to 800 K performed in a
flow of NH3 at a pressure of 1 Â 10À6 mbar, in the

S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
91
the desorption temperature expected for nitrogen,
and oxygen desorbs at much higher temperatures
[40].
R = 0
Fig. 5 shows changes in pressure of O2 during
heating and cooling cycles. Increasing the O2/NH3
ratio results in a decrease in the temperature at
which the reaction starts, an observation consis-
tent with those shown in Fig. 4. These results again
suggest that the inhibitive species is N-species and
not O.
The observed products for this reaction were N2
and H2O. N2O was not observed in contrast with
the results on the Ir(1 0 0) surface [36]. Hence, the
selectivity to N2 is higher on Ir(5 1 0) than on
R = 1.2
R = 4.8
R = 1.2
R = 12
300
400
500
600
700
800
Temperature (K)
R = 3.6
Fig. 4. Heating performed in a flow of 1 Â 10À6 mbar NH3, for
several O2/NH3 ratios (R) at a heating rate of 20 K/min. Change
in pressure of NH3.
absence and presence of O2. These results dem-
onstrated that upon increasing the O2/NH3 ratio
the temperature at which reaction starts decreased.
AES experiments were also carried out during
heating and cooling cycles to check what was
present on the surface. However, due to equipment
limitations, that had to be done in the 10À8 mbar
range. Under those conditions no significant build-
up of N or O on the Ir surface was found. How-
ever, the strong dependence of the temperature at
which the reaction starts on the O2/NH3 ratio,
shown in Fig. 4, suggests that there is inhibition of
the reaction in the higher pressure regime until a
certain temperature (between 400 and 500 K, de-
pending on the O2/NH3 ratio). This can be caused
by adsorbed N- or O-species. However, it is more
likely to be N-caused, since the inhibition ends at
R = 6
R = 12
300
400
500
600
700
800
Temperature (K)
Fig. 5. Heating performed in a flow of 1 Â 10À6 mbar NH3, for
several a O2/NH3 ratios (R) at a heating rate of 20 K/min.
Change in pressure of O2.

92
S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
Ir(1 1 0). A sudden increase in the pressures of the
products H2O and N2 was observed at a certain
temperature (between 400 and 500 K, depending
on the O2/NH3 ratio). Fig. 6 which shows the
heating and cooling cycles for all the reactants and
products for a O2/NH3 ratio of 1, demonstrates
that the sharp increase in the H2O and N2 pres-
sures, at a temperature of 450 K, coincides with a
sharp decrease in the NH3 and O2 pressures. Ob-
viously the reaction starts at that temperature, for
that ratio and overcomes the inhibition present
until then, possible by N species, as described
earlier. A similar behaviour was observed for other
ratios. Similar results were obtained for the
Ir(1 1 0) surface. However, on this surface, the in-
hibition was present until much higher tempera-
tures (550–650 K) [36]. NO formation was not
detected on both Ir surfaces, in contrast with re-
sults obtained by other authors on different sur-
faces [3,5,9–11,13,43,44].
The NH3 conversion obtained varied, under our
experimental conditions, from 20% (for a ratio of
1.2) to 58% (for a ratio of 12). These values were
also higher than those obtained on Ir(1 1 0) [36],
under similar conditions. Hence, the Ir(5 1 0) sur-
face is much more reactive. Since this surface
consists of (1 1 0)-like steps and (1 0 0) terraces, this
enhanced reactivity may be due to the existence of
those (1 0 0) terraces. Studies performed by other
authors on Ir(1 0 0) [16,17] showed this surface is
very active indeed in dissociation of ammonia.
However further studies need to be carried out on
the oxidation reaction to examine the specific role
of the terraces and the steps.
It was not possible to check for eventual chan-
ges in the surface structure during the reactions by
LEED, since the pressure used (10À6 mbar range)
is too high for this technique to be used. The
original pattern characteristic of the (5 Â 1) surface
structure was however observed before and after
the reaction (in the latter case, after evacuation of
gases).
NH3
O2
N2
3.4. Kinetic experiments
Tables 1 and 2 show the values of reaction or-
ders with respect to NH3 and O2 obtained under
steady state conditions, for Ir(5 1 0) and Ir(1 1 0).
Details of the procedure used can be found else-
where [36]. Values vary from 0.36 to 0.70 and seem
to increase a little with increasing temperature, for
both surfaces. Similar values were obtained by
other authors on supported Pt [35]. However, for
supported Ir catalysts, at much higher pressures,
reaction orders close to zero have been reported
[35]. The positive order of reaction suggests there
is no significant inhibition of the reaction by N or
O species in that temperature range. However that
is only true above 400–500 K, depending on the
O2/NH3 ratio.
H2O
300
400
500
600
700
800
Temperature (K)
AES experiments performed in the end of
these experiments, after the system was evacuated,
showed that surface is not oxidised during the rate
Fig. 6. Heating and cooling cycles performed in a flow of
8:3 Â 10À7 mbar NH3, for a O2/NH3 ratio of 1 at a heating rate
of 20 K/min.

S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
93
Table 1
Reaction orders with respect to NH3 at several temperatures,
for the Ir(5 1 0) and Ir(1 1 0) surfaces
formed mainly from NO dissociation and below
350 K, when NO dissociation is not possible, N2
comes from NH3 molecules that have been strip-
ped of hydrogen by oxygen adatoms. Hence as
NH3 molecules are stripped, they are converted to
Temperature (K)
Reaction order
Ir(5 1 0)
400
450
0.48 Æ 0.01
0.47 Æ 0.02
0.54 Æ 0.01
0.62 Æ 0.02
either N adatoms or NOads
:
NH3ads þ Oads ! NHads þ OHads þ Hads
NHads þ Oads ! Nads þ OHads
ð6Þ
ð7Þ
ð8Þ
500
550
Ir(1 1 0)
520
587
0.36 Æ 0.01
0.38 Æ 0.01
0.41 Æ 0.01
0.44 Æ 0.02
NHads þ 2Oads ! NOads þ OHads
Higher O coverages favor steps (7) and (8).
620
675
OHads þ Hads ! H2O
Above 350 K NOads can dissociate:
ð9Þ
ðgÞ
The O2 pressure was 5 Â 10À8 mbar and the NH3 pressure was
varied from 1 Â 10À8 mbar to 1 Â 10À5 mbar. Values for
Ir(1 1 0) were taken from Ref. [36].
NOads ! Nads þ Oads
ð10Þ
ð11Þ
2Nads ! N2ðgÞ
Table 2
Reaction orders with respect to O2 at several temperatures, for
the Ir(5 1 0) and Ir(1 1 0) surfaces
This step competes with NO desorption, so at
higher temperatures (above 0.2 ML), NO
formed [12,45]:
is
ðgÞ
Temperature (K)
Reaction order
Ir(5 1 0)
400
450
NOads ! NO
in preference to N2ðgÞ (steps (10) and (11)).
ð12Þ
ðgÞ
0.67 Æ 0.01
0.67 Æ 0.02
0.70 Æ 0.02
0.70 Æ 0.01
500
550
According to the same authors N2 production is
suppressed at high oxygen coverages that is, step
(10) will not occur. Other authors reported that
ammonia oxidation on silver is a two step reaction,
where ammonia is first oxidised to NO and NOads
further reacts with NHx to produce N2 [5]. In our
case no formation of gaseous NO was observed,
only N2.
It has been suggested that the higher activity
of Ir in ammonia oxidation and selectivity to N2
(compared to Pt) can be explained by its higher
activity to dissociate NO [4]. In our case it is
possible that there may be formation of NOads that
readily dissociates into N2.
Ir(1 1 0)
534
575
0.41 Æ 0.01
0.48 Æ 0.02
0.53 Æ 0.01
0.54 Æ 0.03
627
670
The NH3 pressure was kept constant at 4:2 Â 10À8 mbar and the
O2 pressure was varied from 1 Â 10À8 mbar to 1 Â 10À5 mbar.
Values for Ir(1 1 0) were taken from Ref. [36].
measurements, since no significant increase of
O-species was detected.
3.5. Reaction mechanism
Since Ir is more active than Pt, some authors
suggested that the reaction mechanism must pro-
ceed through a stepwise dehydrogenation of the
ammonia molecule [4]:
The following processes describe the ammonia
and oxygen adsorption:
NH3ðgÞ ! NH3ads
O2ðgÞ ! 2Oads
King and co-workers [12] suggested that there are
two routes for N2 production: above 350 K, N2 is
ð4Þ
ð5Þ
NH3ads þ Oads ! NH2ads þ OHads
NH2ads þ Oads ! NHads þ OHads
ð13Þ
ð14Þ
NHads þ Oads ! Nads þ OHads
ð15Þ

94
S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
tained by ammonia oxidation on the Ir(5 1 0)
surface are N2 and H2O. Inhibition of the reaction
presumably by N species was observed until 400–
500 K, depending on the NH3/O2 ratios. No for-
mation of NO nor N2O was observed. Ir(5 1 0)
surface is more reactive and more selective to N2
than Ir(1 1 0).
NH3ads þ OHads ! NH2ads þ H2Oads
ð16Þ
NH2ads þ OHads ! NHads þ H2Oads
NHads þ OHads ! Nads þ H2Oads
Then N2 can be formed as in reaction (11). An-
other alternative for N2 formation according to
the same authors [4], would be from NHx ads (x ¼ 1
or 2) via a N2Hy (y ¼ 2–4):
ð17Þ
ð18Þ
Acknowledgements
NHx ads þ NHy ads ! N2Hxþy ads
N2Hxþy ads þ ðx þ yÞOHads ! N2ðgÞ þ ðx þ yÞH2O
ð20Þ
These authors found that most of the nitrogen on
the surface was NH and concluded that the last
dehydrogenation step (18) (NH with OH to N and
water) is the rate determining step [4].
Alternatively, it might also be that under our
experimental conditions, N2 and H2O are formed
via
ð19Þ
This work has been performed under the aus-
pices of NIOK, The Netherlands Institute for
Catalysis Research, lab. report no. UL 01-2-03 and
of NRSC-Catalysis. The project was supported by
the NRSC-Catalysis and partly by INTAS (99-
01882).
References
[1] J.K. Dixon, J.E. Longfield, in: P.W. Emmel (Ed.), Catal-
ysis VII, 1960, p. 281.
NH3ads ! Nads þ 3Hads
(via NHx with x ¼ 1 and 2), followed by
2Nads ! N2ðgÞ
and
ð21Þ
ð22Þ
ð23Þ
[2] R.W. McCabe, T. Pignet, L.D. Schmidt, J. Cat. 32 (1974)
114.
[3] J.L. Gland, V.N. Korchak, J. Cat. 53 (1978) 9.
[4] A.C.M. van den Broek, J. van Grondelle, R.A. van Santen,
J. Cat. 185 (1999) 297.
[5] L. Gang, B.G. Anderson, J. van Grondelle, R.A. van
Santen, W.J.H. van Gennip, J.W. Niemantsverdriet, P.J.
Kooyman, A. Knoester, H.H. Brongersma, J. Cat. 206
(2002) 60.
Oads þ 2Hads ! H2O
ðgÞ
The present results cannot discriminate between
the various mechanisms. Recent XPS results [46]
showed three component peaks (possibly due to
adsorbed NH3, NH2, NH and/or N species). Since
dissociation of NH3 takes place at room temper-
ature, shown by formation of H2, it is possible that
the main species on the surface is NH, since other
authors reported that NH is the main species
present on the catalyst [4].
[6] T. Pignet, L.D. Schmidt, Chem. Eng. Sci. 129 (1974) 1123.
[7] R.T.K. Baker, R.B. Thomas, J.H.F. Norton, Plat. Met.
Rev. 18 (1974) 130.
[8] T. Pignet, L.D. Schmidt, J. Cat. 40 (1975) 212.
[9] J.L. Gland, G.C. Woodard, V.N. Korchak, J. Cat. 61
(1980) 543.
[10] M. Asscher, W.L. Guthriet, T.-H. Lin, G.A. Somorjai, J.
Phys. Chem. 88 (1984) 3233.
[11] W.D. Mieher, W. Ho, Surf. Sci. 322 (1995) 151.
[12] J.M. Bradley, A. Hopkinson, D.A. King, J. Phys. Chem. 99
(1995) 17032.
[13] D.S.Y. Hsu, D.W. Squire, M.C. Lin, J. Chem. Phys. 89
(1998) 2861.
4. Conclusions
[14] V.V. Gorodetskii, V.A. Sobyanin, in: Proc. 7th Int. Cong.
Catal., Tokio, 1980, p. 566.
TDS results show there is decomposition of
ammonia with formation of N2 and H2. There is a
large decrease in the desorption peaks of NH3 and
N2 with increasing O2 precoverage. This decrease
in N-containing products is explained by the ex-
pected blocking of sites by O. The products ob-
[15] G. Papapolymerou, V. Bontozoglou, J. Mol. Cat. 120
(1997) 165.
[16] A.K. Santra, B.K. Min, C.W. Yi, K. Luo, T.V. Choudh-
ary, D.W. Goodman, J. Cat. Lett. 77 (2001) 1.
[17] A.K. Santra, B.K. Min, C.W. Yi, K. Luo, T.V. Choudh-
ary, D.W. Goodman, J. Phys. Chem. B 106 (2002) 340.
[18] J.L. Grand, Surf. Sci. 71 (1978) 327.

S.A.C. Carabineiro, B.E. Nieuwenhuys / Surface Science 532–535 (2003) 87–95
95
[19] G.B. Fisher, Chem. Phys. Lett. 79 (1981) 452.
[20] J.L. Gland, E.B. Kollin, Surf. Sci. 104 (1981) 478.
[21] N.R. Palmer, J.C. Campuzano, N.V. Richardson, D.A.
King, Vacuum 31 (1981) 679.
[34] A. Galtayries, E. Laksono, J.-M. Siffre, C. Argile, P.
Marcus, Surf. Interface Anal. 30 (2000) 140.
[35] A.C.M. van den Broek, Ph.D. Thesis, Technische Univer-
siteit Eindhoven, 1998.
[22] A.R. Cholach, V.A. Sobyanin, V.V. Gorodetskii, in: Proc.
Soviet–Japanese Seminar on Catalysis, Novosibrisk, 1983,
p. 177.
[36] S.A. Carabineiro, B.E. Nieuwenhuys, Surf. Sci. 505 (2002)
163.
[37] C.A. de Wolf, J.W. Bakker, P.T. Wouda, B.E. Nie-
uwenhuys, A. Baraldi, S. Lizzit, M. Kiskinova, J. Chem.
Phys. 113 (2000) 10717.
[23] J.M. Gohndrone, C.W. Olsen, A.L. Backman, T.R. Gow, E.
Yagasaki, R.I. Masel, J. Vac. Sci. Technol. A 7 (1989) 1986.
[24] J.M. Bradley, A. Hopkinson, D.A. King, Surf. Sci. 371
(1997) 225.
[38] C.A. de Wolf, B.E. Nieuwenhuys, M.M. Slinko, M.Yu.
Smirnov, Surf. Sci. 433–435 (1999) 63.
[25] L.R. Danielson, M.J. Dresser, E.E. Donaldson, J.T.
Dickinson, Surf. Sci. 71 (1978) 599.
[39] C.A. de Wolf, J.W. Bakker, P.T. Wouda, B.E. Nie-
uwenhuys, A. Baraldi, S. Lizzit, M. Kiskinova, J. Phys.
Chem. B 105 (2001) 4254.
[26] K. Yoshida, G.A. Somorjai, Surf. Sci. 75 (1978) 46.
[27] M. Grunze, M. Golze, R.K. Driscoll, P.A. Dowben, J. Vac.
Sci. Technol. 18 (1981) 611.
[40] B.E. Nieuwenhuys, G.A. Somorjai, Surf. Sci. 72 (1978) 8.
[41] Y. Li, J.N. Armor, Appl. Cat. B: Env. 13 (1997) 131.
[42] M.F.H. van Tol, J. Siera, P.D. Cobden, B.E. Nieuwenhuys,
Surf. Sci. 274 (1992) 63, and references therein.
[43] E. Molinari, F. Cramarossa, A. Pullo, L. Triolo, J. Cat. 4
(1965) 341.
[28] A.R. Cholach, V.A. Sobyanin, V.V. Gorodetskii, React.
Kinet. Catal. Lett. 18 (1981) 391.
[29] F.P. Netzer, T.E. Madey, Surf. Sci. 119 (1982) 422.
[30] J.L. Gland, B.A. Sexton, G.E. Mitchell, Surf. Sci. 115
(1982) 623.
[44] J. Zawadzki, Disc. Farad. Soc. 8 (1950) 140.
[45] M. Kim, S.J. Pratt, D.A. King, J. Am. Chem. Soc. 122
(2000) 2409.
[31] C. Egawa, S. Naito, K. Tamaru, Surf. Sci. 131 (1983) 49.
[32] V.V. Gorodetskii, M.Yu. Smirnov, A.R. Cholach, in: Proc.
10th Int. Cong. Catal., Budapest, 1992, p. 192.
[33] R.M. van Hardeveld, R.A. van Santen, J.W. Niemants-
verdriet, Surf. Sci. 369 (1996) 23.
[46] E.D.L. Rienks, C.J. Weststrate, J.W. Bakker, S.A.C.
Carabineiro, B.E. Nieuwenhuys, S. Lizzit, A. Baraldi,
unpublished results.