Decomposition of NO2 on Pt(1 1 0): Formation of a new oxygen adsorption state

Article abstract of DOI:10.1016/S0039-6028(02)01381-X

NO₂ decomposition on Pt(1 1 0) was studied by XPS and TDS at various temperatures. At room temperature, NO₂ decomposes to adsorbed NO and oxygen adatoms on Pt(1 1 0). Strong repulsive interactions exist between the adsorbed NO and the oxygen adatoms. Above 450 K, NO_ads desorbs completely from the Pt(1 1 0) surface and only adsorbed oxygen is left on the surface. O₂ thermal desorption spectra results reveal that, comparing with oxygen adsorption on Pt(1 1 0), NO₂ exposure above 350 K gives rise to an additional oxygen adsorption state on Pt(1 1 0) that desorbs at a lower temperature. The new oxygen adsorption state has an O1s binding energy of 529.2 eV. The formation of the new oxygen adsorption state may be associated with the local phase transition of the Pt(1 1 0) surface from (1 × 2) structure to (1 × 1) structure induced by the NO_ads that is formed from NO₂ decomposition.

Full text of DOI:10.1016/S0039-6028(02)01381-X

Surface Science 506 (2002) L287–L292
www.elsevier.com/locate/susc
Surface Science Letters
Decomposition of NO on Pt(110): formation of a
2
new oxygen adsorption state
Weixin Huang, Zhiquan Jiang, Jian Jiao, Dali Tan, Runsheng Zhai,
Xinhe Bao *
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, P.O. Box 110, 457 Zhongshan Road, Dalian 116023, China
Received 29 June 2001; accepted for publication 7 January 2002
Abstract
2 2
NO decomposition on Pt(1 1 0) was studied by XPS and TDS at various temperatures. At room temperature, NO
decomposes to adsorbed NO and oxygen adatoms on Pt(1 1 0). Strong repulsive interactions exist between the adsorbed
NO and the oxygen adatoms. Above 450 K, NOads desorbs completely from the Pt(1 1 0) surface and only adsorbed
oxygen is left on the surface. O
Pt(1 1 0), NO exposure above 350 K gives rise to an additional oxygen adsorption state on Pt(1 1 0) that desorbs at a
2
thermal desorption spectra results reveal that, comparing with oxygen adsorption on
2
lower temperature. The new oxygen adsorption state has an O1s binding energy of 529.2 eV. The formation of the new
oxygen adsorption state may be associated with the local phase transition of the Pt(1 1 0) surface from ð1 Â 2Þ structure
to ð1 Â 1Þ structure induced by the NOads that is formed from NO
2
decomposition. Ó 2002 Published by Elsevier
Science B.V.
Keywords: Oxygen; Nitrogen oxides; Platinum; X-ray photoelectron spectroscopy; Thermal desorption
The three-way converters utilized for the low-
ering of automobile exhaust emissions have Rh-,
Pd- and Pt-based catalysts as their key compo-
nents, and these catalyst systems have motivated a
large number of studies on NO reactions over well-
defined surface of Rh, Pd and Pt [1–3]. However,
in the presence of excess oxygen (‘‘lean-burn’’
condition), NO is believed to be oxidized first to
2 2
NO by oxygen and then the resulting NO reacts
with hydrocarbons on the surface of catalysts [4,5].
Therefore, there is an increasing need for a fun-
2
damental understanding of the NO chemistry on
the surfaces of Rh, Pd and Pt. Several studies on
the interaction of NO2 with Pt(1 0 0), Pt(1 1 1),
Pd(1 1 1) and Rh(1 1 1) have been reported [6–14].
The results showed that NO
on the surfaces of transition metals. When the
Pt(1 1 1) surface is exposed to NO at 100 K and
then heated, NO completely decomposes to ad-
2
readily decomposes
2
2
*
Corresponding author. Address: Dalian Institute of Chem-
ical Physics, Chinese Academy of Sciences, 161 Zhongshan
Road, Dalian 116011, China. Tel.: +86-411-4686637; fax: +86-
sorbed NO and oxygen adatoms at 285 K; at lar-
ger exposure, some of the NO will desorb [8,9].
2
The behavior of NO on Pd(1 1 1) is similar to that
2
411-4694447.
E-mail address: xhbao@ms.dicp.ac.cn (X. Bao).
on Pt(1 1 1) [10]. On Rh(1 1 1), NO decomposes at
2
0
039-6028/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0039-6028(02)01381-X

L288
W. Huang et al. / Surface Science 506 (2002) L287–L292
a temperature 6 150 K and also produces ad-
sorbed NO and oxygen adatoms, while the ma-
jority of the formed NO dissociates further at
temperatures between 300 and 400 K [13]. Fur-
thermore, when dosing at temperatures high en-
ough for the resulting NO to desorb from the
metal surface, NO can be an effective source for
2
achieving high oxygen coverage on the metal single
crystal surfaces [11,12,15–19]. In this letter, we
report our results for the decomposition of NO
on Pt(1 1 0), in which a new oxygen adsorption
2
state forms when NO
atures.
2
is exposed at high temper-
Experiments were performed in a stainless ul-
tra-high vacuum chamber equipped with facilities
for XPS, LEED, AES and TDS experiments. The
base pressure was 1:0 Â 10À10 mbar. The Pt(1 1 0)
sample was fixed on the sample holder by two Ta
filaments, which could heat the sample to 1200 K.
The sample temperature was measured by the
chromel/alumel thermocouple spot-welded on the
backside of the sample. The Pt(1 1 0) sample was
þ
cleaned by repeated cycles of oxidation, Ar
sputtering and annealing until AES and XPS could
detect no contaminants and LEED gave a sharp
Pt(1 0 0)-(1 Â 2) pattern. Unmonochromated Mg
Ka radiation was employed during XPS experi-
ments and the transmission energy was 50 eV.
Fig. 1. NO (a) and O
2
(b) TDS from 6 L NO
2
/Pt(1 1 0) at
À1
various dosing temperatures. Heating rate was 8 K s
.
For small and large exposures of NO
Pt(1 1 0) at room temperature, no desorption sig-
2
on
nal of NO could be detected during the thermal
2
desorption experiments. Thus, at this tempera-
2
ture, NO appears to completely decompose to
adsorbed NO and oxygen adatoms on Pt(1 1 0),
which is also verified by XPS results. Fig. 1 shows
the NO and oxygen thermal desorption profiles
2
after 6 L of NO exposure on the Pt(1 1 0) at var-
ious temperatures. At room temperature, two de-
sorption peaks of NO are centered at 375 and 435
K. Comparing with the results of NO adsorption
on Pt(1 1 0) [20], these two peaks can be ascribed
to the desorption of NO from terminal sites (a-
NOads) and bridge sites (b-NOads) on the Pt(1 1 0),
respectively. However, due to the coexistence of
oxygen adatoms on the surface, the thermal de-
Fig. 2. NO-TDS from NO-saturated Pt(1 1 0) and NO
rated Pt(1 1 0) at room temperature. Heating rate was 8 K s
2
-satu-
À1
.
sorption characteristics of NO from NO /Pt(1 1 0)
are quite different from that from NO/Pt(1 1 0).
Fig. 2 presents NO-TDS profiles obtained from
2
2
NO-saturated Pt(1 1 0) and NO -saturated Pt-
(1 1 0) at room temperature. It can be clearly seen
that in contrast to the case of NO/Pt(1 1 0), when

W. Huang et al. / Surface Science 506 (2002) L287–L292
L289
Pt(1 1 0) is exposed to NO
2
, most NO desorbs as
terminal-bonded NO and the desorption temper-
ature of bridge-bonded NO is about 40 K lower
than that on the NO/Pt(1 1 0). The desorption
temperature of terminal-bonded NO is also about
2
0 K lower than that on the NO/Pt(1 1 0). The
results indicate an actual existence of the strong
repulsive interactions between the oxygen adatoms
and the adsorbed NO, especially the bridge-bon-
ded NO, which causes more NOads to desorb as
terminal-bonded NO and lowers the activation
energy for the desorption of bridge-bonded NO.
Desorption and decomposition are two competi-
tive processes when NOads on Pt(1 1 0) is heated.
Thus, due to the decrease in desorption activation
Fig. 3. O
500 K. Heating rate was 8 K s
2
-TDS from Pt(1 1 0) as a function of NO
À1
2
exposure at
.
energy, adsorbed NO from the NO /Pt(1 1 0) will
2
prefer to desorb rather than decompose. When
NO
sulting NO may desorb from the Pt(1 1 0). There-
2
is dosed at an elevated temperature, the re-
resulted from NO
the bulk of Pt(1 1 0) when the NO
For NO exposures between 1.0 and 3.5 L, the O
TDS exhibited similar characteristics as a direct
exposure of O to Pt(1 1 0). The new desorption
peak around 660 K (a-Oads) appears at 4 L NO
exposure and its peak area increases with the in-
crease in exposure of NO
2
decomposition dissolves into
fore, the amount of desorbed NO from NO
Pt(1 1 0) decreases with the increasing of the NO
2
/
2
exposure is low.
2
2
2
-
dosing temperature. After dosing at 450 and 500
K, respectively, no NO desorption signals could be
detected and only oxygen adatoms were left on the
Pt(1 1 0) surface.
2
2
At an elevated temperature, NO
2
can decom-
2
.
pose on the vacant Pt sites left by NO desorption
and produce oxygen adatoms. Thus the coverage
of oxygen adatoms on the Pt(1 1 0) increases with
Two possibilities can account for the new oxy-
gen desorption peak in the TDS. One is the kinetic
effect, in which the repulsive interaction between
oxygen adatoms with a similar chemical state at
high oxygen coverage results in the appearance of
the new desorption peak. The other is that dis-
tinctly different multiple chemical states of oxygen
may exist, each giving rise to a separate peak in the
desorption profile. It has been reported that the
2
the increasing of the NO dosing temperature,
as shown in Fig. 1. When NO is dosed at room
2
temperature, the oxygen TDS appears to be com-
parable to that of O
2
/Pt(1 1 0) (mainly b
21,22]. When the dosing temperature is increased,
1
-Oads)
[
due to the partial desorption of adsorbed NO, the
amount of oxygen desorbed increases dramati-
cally. Moreover, a new desorption peak, labeled as
dosing of NO
2
at high temperatures produces
high coverages of oxygen adatoms on Pt(1 1 1) and
Ag(1 1 0) [15,18], in which new oxygen desorption
peaks appear in the TDS. On the Pt(1 1 1) surface,
the observed multiple oxygen in TPD is interpreted
as arising largely from kinetic effects, because
HREELS, UPS and work function measurement
results evidence only atomic oxygen on the surface
[15]. However, on the Ag(1 1 0) surface, the two
a-Oads in Fig. 1, appears around 660 K. When NO
is dosed at temperatures at which the resulting NO
2
is totally desorbed from the surface, i.e. 450 and
5
00 K respectively, the peak areas of the O
from the Pt(1 1 0) surface are identical.
A series of O -TDS curves after various NO
2
-TDS
2
2
exposures on Pt(1 1 0) at 500 K is shown in Fig. 3.
When the exposure is below 1 L, no oxygen de-
sorption signal could be detected, although AES
verified the presence of oxygen on the surface after
the desorption cycle. This implies that the oxygen
2
oxygen desorption peaks resulting from NO ex-
posure are attributed to two different oxygen spe-
cies on the surface [18]. To make clear where the
a-Oads comes from in our case, experiments on the

L290
W. Huang et al. / Surface Science 506 (2002) L287–L292
Fig. 4. O
5
2
-TDS from 45 L O
00 K. Heating rate was 8 K s
2
/Pt(1 1 0) and 5 L NO
À1
2
/Pt(1 1 0) at
Fig. 5. O1s XPS spectra of oxygen adatoms on Pt(1 1 0) fol-
lowing 45 L oxygen dosing and 5 L NO
spectively.
.
2
dosing at 500 K, re-
thermal desorption of oxygen from O
and from NO /Pt(1 1 0) at 500 K were performed
as a comparison, and the results are shown in Fig.
. The integrated area of O -TDS from the O
2
/Pt(1 1 0)
peak with 529.2 eV of binding energy should be
originated from the formation of the a-Oads upon
NO adsorption on the Pt(1 1 0) at 500 K.
2
2
4
2
2
/
Pt(1 1 0) is a little larger than that from the NO2/
Pt(1 1 0), indicating that at 500 K more oxygen
adatoms are produced on the Pt(1 1 0) surface by a
Several new oxygen adsorption states, including
subsurface oxygen state, have been reported on the
Pt(1 1 0) surface under certain conditions [22–24].
Ertl and coworkers reported that oxygen adsorp-
tion on a faceted Pt(1 1 0) surface, which was
dosing of 45 L of oxygen than by a 5 L NO
dosing. Therefore, the repulsive interactions be-
tween oxygen adatoms should be stronger for the
2
prepared by keeping the Pt(1 1 0) sample at 460 K
À4
4
Pt(1 1 0) system. However, in the O
5 L O
2
/Pt(1 1 0) system than for the 5 L NO
2
/
-TDS from the
in a CO/O
gave rise to two high-temperature O
2
2
atmosphere in the 10 Torr range,
thermal de-
2
0
0
O /Pt(1 1 0) surface, no additional oxygen desorp-
2
sorption states (b -Oads and b -Oads) [22]. The au-
2
3
tion peak similar to the a-Oads desorbed from the
thors attributed these two new states to oxygen
adatoms adsorbed on the microfacets created
by catalytic CO oxidation [22]. Vishnevskii et al.
found that the Pt(1 1 0) surface underwent struc-
tural rearrangement during the oscillating catalytic
CO oxidation and the changes of the Pt(1 1 0)
surface resulted in the formation of two new states
NO /Pt(1 1 0) surface can be found. It implies that
2
2 2
the a-Oads existing in the O -TDS from the NO /
Pt(1 1 0) is largely due to the formation of a new
oxygen adsorption state different from the normal
1
oxygen adsorption state (b -Oads), rather than the
kinetic effects. The XPS results further confirm
that the chemical state of the a-Oads is different
of Oads (a
the TD maximum peak at 670 K as well as a O1s
1
1
-Oads and b-Oads) [23]. The a -Oads has
from that of the b -Oads. Fig. 5 presents the O1s
1
binding energy after 45 L oxygen and 5 L NO2
were dosed on the Pt(1 1 0) surface at 500 K. The
dosing of 45 L of oxygen on Pt(1 1 0) at 500 K gave
rise to an O1s binding energy of 530.4 eV, which
binding energy of 529.2 eV [23], similar to the a-
O
gen on the Pt(1 1 0) surface was achieved by dosing
ads in our case. The formation of subsurface oxy-
À5
oxygen (5 Â 10 mbar for 2 min) on a CO pre-
can be attributed to the b
dosed on Pt(1 1 0) at 500 K, an obvious shoulder
1
-Oads. After 5 L NO
2
was
covered Pt(1 1 0) surface, and was investigated in
detail by photoemission electron microscopy
(PEEM) [24]. One common point in the above
systems is that the formation of new oxygen ad-
sorption states is connected with a phase transition
peak with 529.2 eV binding energy emerged
alongside the 530.4 eV peak of the b -Oads. By
comparing with the TDS results, this shoulder
1

W. Huang et al. / Surface Science 506 (2002) L287–L292
L291
of the Pt(1 1 0) from the (1 Â 2) surface structure to
the (1 Â 1) surface structure induced by CO ad-
sorption. Similar to CO adsorption, above a cer-
tain coverage, NO adsorption can also induce the
phase transition of the Pt(1 1 0)-(1 Â 2) structure to
the Pt(1 1 0)-(1 Â 1) structure [25,26]. Therefore,
The formation of the a-Oads can be attributed to
the decomposition of NO on the Pt(1 1 0) do-
mains that have experienced phase transition from
the (1 Â 2) surface structure to the (1 Â 1) surface
structure induced by NO adsorption.
2
the appearing of the a-O might be concerned
ads
with the local phase transition of the Pt(1 1 0) in-
duced by NO. As shown in Fig. 1, only after part
of adsorbed NO has been desorbed from the sur-
face can the a-Oads appear in the TD profile. Ac-
cording to the view of Christmann and coworkers
Acknowledgements
This work was financially supported by the
National Natural Science Foundation of China.
[27], a-Oads belongs to subsurface oxygen, because
it desorbs at a temperature below the desorption
temperature of atomic oxygen (b -Oads) on the
Pt(1 1 0) surface.
The following mechanism is proposed to
1
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2
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1
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