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Surface Science 530 (2003) 37-54
S U R F A C E S C I E N C E
w w w . c l s c v i e r . c o m / l o c a t e / s u s c
Adsorption
of NH3 on oxygen
pre-treated
Ni(l I 1)
E . L a k s o n o , A . G a l t a y r i e S
* ,
C . A r g i l e , P . M a r c u s
Luboratoire da Pb,sico-Chirnie dcs Surfuces CNRS (UMR 7045), Uniuarsiti Piarra ct Muria C'urLc, Emlc Nutktnulc Suptricurc da Chitnic dc Puris, ll rua Piarrc ct Muric Ctrria. l--75005 Puri.t, l"runtt
R c c c i v e d l4 N o v e m b e r 2 0 0 2 ; a c c c p t c d fo r p u b l i c a t i o n 2 5 J a n u a r y 2 0 0 i
Abstract
T h e a d s o r p t i o n o f N H 3 o n o x y g e u p r e - t r e a t e d N i ( l I l ) s u r l a c e s h a s b e e n s t u d i e c l a t r o o m te m p e r a t u r e u s i n g X - r a y
photoelectron spectroscrpy (XPS). Oxygen pre-treatments have been performed at 650 K. This protocol leads to a
"two-phase domain" (O-adsorbed phase + NiO islands) over a large range of oxygen exposures. The investigation of the
surface reactivity towards NH3 shows that ammonia is adsorbed provided that the O-adsorbed phase is present; the
surface reactivity increases with the O-adsorbcd phase covcrage. Two N-adspccics havc bccu dctectcd from the N ls
core level peaks at 399.8+0.2 and 397.8f0.2 eV and assigned to molecular NH3 and dissociated NH2 species,
re-spectively. The molecular adsorptior.r results from direct impingement of the NH: molecules, whereas the dissociated
one results from the dissociation ola part of the preadsorbed molecular species. At saturation, the dissociated species is
the more abundant one (about 4/5 of the total N ls peak) whatever the initial coverage (>0) of the surface by the
O-adsorbed pl'rase. The XPS data indicate that this dissociation is correlated to the formation of OH from oxygen of
the adsorbed phase and hydrogen abstraction from the molecular ammonia. When the O-adsorbed phase is absent on
the surface, i.e. for clean Ni(l I l) or the complete NiO layer, none of these surface reactions with ammonia occurs,
under the same adsorption conditions.
@ 2003 Elsevier Science B.V. All rights reserved.
K e y w o r d s : C h e m i s o r p t i o n ; A n r m o u i a ; L o w i n d c x s i n g l c c r y s t l l s u r l a c c s ; N i c k e l ; N i c k c l o x i d c s t O x y g c n ; X - r a y p h o t o e l e c t r o n spectroscopy
l , I n t r o d u c t i o n
Over the past three decades, the adsorption and
decomposition of ammonia on metals, as well as on
metal oxide layers, has attracted much interest. Structural techniques as well as electronic and
v i b r a t i o n a l s p e c t l o s c o p i e s h a v e b c e n u s e d . S t u d i e s
of this system cover model surfaces (single crystals
of metals, or metal oxide laycrs on metal single
crystals) as well as more complex ones
(polycrystal-line metals, oxide layers on metaliic polycrystals),
o n w h i c h m o l e c u l a r o r d i s s o c i a t i v e a d s o r p t i o n o f
ammonia can occur over a wide range of
temper-atures. In all cases, understanding of the
mecha-nisms of ammonia interaction with the surfaces is
essential. There are important applications in more
complex processes, for example in heterogeneous
catalysis, where ammonia is either a reactant (for
' C o r r e s p o n d i n g
a u t h o r . T e l . : + 3 3 - l - 4 1 2 ' 7 6 ' 7 3 7 ; f a x : + 3 3 l -46340'15t.
E-mail addr ess : anouk-girltayrics(r)enscp jussicu.fr (A. Gal-tayries).
0039-6028/03/$ - see front matter O 2003 Elsevier Science B.V. All riehts reserved doi: I 0. I 0 I 6/50039-6028(03)00267-X
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E. Luksono at ul. I SurJutc Scicncc 530 (2003) 37 51
example the industrial synthesis of HNO3 by
am-monia oxidation over a platinum catalyst-known
as "Ostwald proces5"-sr the HCN synthesis, by
reaction of methane with ammonia and oxygen over a platinum-rhodium catalyst-known as "Andrussow process") or a product (for example
the industrial synthesis of NH3 from nitrogen and
hydrogen over an iron catalyst-known as "Ha-ber-Bosch proces5"-sr the undesirable produc-tion of ammonia in the automotive exhaust gas
converter) or even the catalyst (NH3-catalysed
se-quential surface reactions [1]). Another example is
the corrosion of metals in the presence of
atmo-spheric pollutants such as ammonia. Surface nit-ridation for semiconductor applications may also
be mentioned 12-71. Finally, NH3 is an often used
probe molecule for investigating the acidity of solid
surfaces [8].
The studies of the chemisorption of ammonia on
metal surfaces published up to the early eighties (1984) have been reviewed by Lambert and Bridge
[9]. The major part of the surface studies of NH:
adsorption in the eighties were focused on the
well-defined surfaces of transition or noble metals
[0-l5], at low temperature of adsorption between about 100 and 200 K. The question of non-disso-ciative versus dissonon-disso-ciative adsorption behaviour, and the structure of the surface ammonia
com-plexes were investigated by different electronic and
vibrational spectroscopies as well as by thermal
desorption measurements. As we will not focus our
work on the NH3 adsorption on metals, we only briefly summarise here that, depending on the metal, the NH3 ability to dissociate is strongly
decreasing from the left hand side to the right hand
side of the transition metal series. A more com-plete picture of the NH3 adsorption on vanous metals at the end of this period rvas given by T h o r n b u r g a n d M a d i x |61 in 1989. From experi-mental work, it has been shown that in the mo-lecuiar state, ammonia molecules bond with their nitrogen end nearest to the surface, and their C3"
symmetry axis normal to the surface. This bonding
has been interpreted as irnplying that nitrogen
donates its lone pair of electrons to tl.re surfacc to
form the bond to the metal. However, according to theoretical work of the time (late eighties), the
bonding is mostly electrostatic in nature, involving
the permanent dipole of ammonia and an induced
dipole in the surface. It was stated in this work [ 6]
t h a t t h e b i n d i n g s i t e o f a m m o n i a w a s n o t u n a m
-biguously established for any of the quoted single
crystal surfaces, and different electropositive sites
have been proposed implying both models of b o n d i n g : e l e c t r o n d o n a t i o n o f t h e n i t r o g e n l o n e
pair to the metal as well as electrostatic bonding
on the surface. Recently, an interesting biblio-graphic review of the previous research on NH3 adsorption on Ni(l I 0) has been published by Chrysostomou et al. [7]. The authors also raised the problem of the stability of molecularly-adsorbed ammonia when probing with photons or
electrons, as carly suggestions on the
decomposi-tion pathways of ammonia on Ni(l I 0) have been later questioned (see in Ref. [7]). In particular, Iaeger et al. [18] have provided clear evidence for the occurrence of photon-induced reduction of a m m o n i a o n N i ( 1 l 0 ) d u r i n g s u r f a c e c h a r a c t e r i
-sation. Nevertheless, surface analysis techniques
employing eiectron irradiation (LEED, Auger,
H R E E L S . . . ) a n d t e m p e r a t u r e - p r o g r a m m e d d e
-sorption (TPD or TD) have been found to be even more damaging to the integrity of the adsorbed ammonia than those based on photon irradia-tion. All logical dehydrogenated intermediates expected to form during the surface decomposi-tion of ammonia: NH2(ads), NH(ads), N(ads) have been proposed on Ni(l I 0). However, there
is some disagreement in the literature on the
ex-act mechanism of ammonia decomposition and
on the intermediates involved in the
decomposi-tion.
Despite the large number of works carried out
during the eighties, several fundamental questions
are still debated in the more recent surface science literature on the ammonia interaction with metals:
t h e i n t e r a c t i o n s o f a m m o n i a w i t h R h ( l I 1 ) [9]
have been studied at low pressure whereas the
at-tention was previously focused on the
decompo-s i t i o n o f a m m o n i a in m o d e r a t e p r e s s u r e c o n d i t i o n s
o n l y ; t h e i n t e r a c t i o n s o f N H r w i t h R u ( O 0 1 ) [20],
P t ( l I l ) [21,22), thc multilayer coverage on Ag(l I l) 1231, the intcractior.r of ammor.ria witl.r nickel single crystals studied by more recent
tech-niques as angle-resolved photoemission extended
E Luksono et ul. I Surface Science 530 (2003) 37-54
Ni(l I l) surfaces, as a function of the initial
oxy-gen exposure, investigated by XPS.
2. Experimental
For the XPS characterisations, Ni 2p, O 1s and
N ls core level spectra have been recorded with a VG ESCALAB Mk II X-ray photoelectron
spectrometer, with an un-monochromatised AlK"
anode, at a pass energy of 20 eV. The binding
energies were referenced to the Ni 2p372 line and
A u 4 f 7 p , s e t a t 8 5 2 . 8 a n d 8 4 . 0 e V , r e s p e c t i v e l y , a n d
given with an accuracy of 0.1 eV for intense spectra and 0,2 eV for less intense signals
corrc-sponding to low coverage adspecies (N ls, O 1s).
Clean and O-treated surfaces were also checked from contamination by recording a survey
spec-trum and the C ls and S 2p core level spectra. The
spectra of the surfaces prior to ammonia
adsorp-tion were characterised at electron take-off angles of 90o with respect to the sample surface. Con-nected to the UHV analysis chamber (base
pres-sure 3 x l0-r0 Torr) of the spectrometer is a UHV
preparation chamber (base pressure 5 x l0-r0 Torr) with heating and gas introduction facilities.
The Ni(l I l) sur'facc was clcancd by cyclcs of Ar+
sputtering and annealing, first in H1 and then rn
v a c u u m . T h i s s e q u e n c e w a s l o u n d t o b c t h e b e s t
compromise, in our experiments, to avoid both S
segregation due to prolonged annealing under v a c u u m a n d o x y g e n o r c a r b o n r e - c o n t a m i n a t i o n . After cleaning the surface, high purity oxygen and ammonia (from Air Liquide) were introdubed in the treatment chamber. Oxygen exposures were t y p i c a l l y p e r f o r m e d a t P o . : 1 x 1 0 - 6 m b a r ( 1 x
l0-7 mbar for the two lower 02 exposures). The
O-pre-treated Ni(l I l) samples have been exposed to
ammonia at room temperature, at I x l0-7 mbar. Ammonia was typically admittcd for periodb of 3 min in the preparation chamber, after which the gas was evacuated and the sample analysed by XPS in the analysis chamber. Exposures in the preparation chamber arc rcportcd in langmuirs (L) ( l L : l 0 6 T o r r ' s ) , N o t e t h a t t h e r e p o r t e d p r e s -sures and expo-sures, in the text or in the figure captions, are not correctcd for ion gaugc sensitiv-ity. The samplc tempcrature, duling oxygcrl
cx-posures, was measured with a pyrometer. To
analyse the individual contributions of the Ni 2p:tz,
O ls, and N ls core levels, peak decomposition was carried out with a computer program using
gaussian/lorentzian peak shapes, and a Shirley
background.
3. Results and discussion
3.1. Oxygen ilteraction on lVi(l I 1) at 650 K prior to NHj exposlLre
A t r o o m t e m p e r a t u r e , a n d u p t o 5 0 0 K , i t i s
usually accepted (see, for cxample [40] and refer-e n c refer-e s th refer-e r refer-e i n ) t h a t 0 3 i r . r t refer-e r a c t i o n w i t h N i ( l I l )
proceeds via three steps: (i) a rapid chemisorption
ending with a first plateau, (ii) a second increase in
oxygen uptake correlated with the nucleation and growth of NiO islands and then (iii) the lormation of a full oxide layer, leading to a second plateau. Up to the end of the first platcau, it is established
[41] that chemisorption occurs rvith little effects on
the core level spectra, which indicates that the surface atoms rctain thcir metallic character. At room temperature, this chemisorption stage gives rise to a p(2 x 2) structurc for a maximum cover-a g e o f l l 4 o f cover-a m o n o l cover-a y e r . A t c o v e r cover-a g e s _ cover-a b o v e - 0 . 2 8 ML, sonic authols fcpoft rr (V3 .o /l)R30" structure, and some do not (see, for example, [40]
and references therein). Raising the temperature
a b o v e -3 0 0 K c a u s e s t h e ( r , 4 / / 3 ) R 3 0 " s t r u c t u r e
(when observed) to convert to a split p(2 x 2) [a0].
In the domain wherc the NiO formation occurs,
the Ni core level spectra reflect the occurrence of
the oxide nucleation and passive oxide film for-mation. The mcchanisms currently accepted for the oxidation involve the nucleation and growth o f o x i d e i s l a n d s w i t h i n t h c O - a d s o r b e d p h a s e .
The growth is cxpcctcd to occur at tl.re pcrimeter of
the oxide nuclei with oxygen supplied by surface diffusion [40,42]. It has been proposed that, up to 473 K [40], the hexagonal, kinetically favoured NiO(l I l) structurc grows rapidly to coalescencc c o v e r i n g t h e e n t i r e N i ( l I l ) s u r f a c e . A t h i g h e r
t e m p e r a t u r e ( T > 4 1 3 K ) , N i O ( 1 0 0 ) s t a r t s t o f o r m
i r r c v c r s i b l y , i n d i c a t i n g th a t t h i s i s t h e t h c r m o d y
E. Luksono at ul. I Surfucc Stience 530 (2003) 37-54
hydroxylated NiO islands were the minor compo-nent.
As indicated above, in the frequent controversy between OH(a) and O(a) or O- binding energies,
the assignment of binding energy of the O(a)
spe-cies remains questionable. We performed another
type of test to check the possibility of an OH contribution. A clean Ni(l I 1) sample was treated
in 02 at room temperature, and subsequently
ex-posed to water vapour at room temperature. The
O 1s region clearly showed a significant increase of
the high binding energy contribution located at
531.5 A 0.2 eV that we attributed to the presence of
hydroxyl groups on the surface.
ln our series of experiments it comes out that. after oxidation at 650 K, the O 1s core level peaks p r e s e n t a n h y d r o x y l c o n t r i b u t i o n a t 5 3 1 . 5 f 0 . 2 e V , corresponding to about 10ol' of the total O ls peak area. According to a previou5 work [46], this surface hydroxylation likely corresponds to the NiO(1 I 1) grains, which are the minor components
of the NiO film at high temperature, as NiO(100)
starts to form irreversibly above 473 K [40]. it has been also noted that for the lower 02 exposures ( c o r r e s p o n d i n g t o v a l u e s o f t h e X P S i n t e n s i t y l a t i o 1 ( O ) r " r , r / 1 ( N i ) r " r " r b c l o w 0 . 0 4 ) , t h e s u r f a c c h y -d r o x y l a t i o n is m o r c i n r p o r t a n t . T h i s u n c x p c c t c -d
r e s u l t m a y b e r e l a t e d to t h e e x p e r i n r e n t a l c o n d i
-tions: the lowest e xposures wcre pcrformcd at
P o r : 7 x l O - i m b a r ( i n s t e a d o f I x 1 0 - 6 ) s o t h a t
the partial pressure of residual watcr was
propor-tionally higher.
The data obtained for various oxygen exposurcs are presented in Fig. l; the XPS intensity ratio
1(O)t"t"l//(Ni),o,u,, obtained from the integration
o f t h e O l s a n d N i 2 p 3 7 2 c o r e le v e l s , i s p l o t t c d a s a
function of the O: exposurc expressed in langmuirs
( L ) . I n F i g . I , a f i r s t p l a t e a u (c o r r c s p o n d i n g t o t h e
o x y g e n a d s o r p t i o n p h a s e ) i s n o t e v i d e n c e d , a s
c o m p a r e d w i t h w h a t i s r c p o r t c d i n t h c l i t c r a t u l c
[a0]. This is not rclatcd rvith thc tcrnpqraturc of a d s o r p t i o n : li t e r a t u r e d a t a s l i o r v t h a t t h c h i g h e r the temperature of adsorption, the longer is the first plateau in the oxygen uptake curve. This might be attributed to a lack of data at the lowest
oxygen exposures, but it is more likely correlated
w i t h o u r e x p e r i m c n t a l p r o t o c o l : o x y g e n e x p o s u r e s
a n d s a m p l e h e a t i n g in t h e p r e p a r a t i o n c h a m b e r ,
0 200 400 600 800 1c00 1200 1400
02 exposure (L)
F i g . l . O l s i n t e n s i t y (n o r m a l i s e d w i t h r e s p e c t t o t h e N i 2 p r 7 z core level intensity) as a function of oxygen exposure (expressed i n L a n g m u i r ) a t 6 5 0 K .
a n d X P S m e a s u r e m e n t s i n t h e a n a l y s i s c h a m b e r .
Indeed, the different surface compositions w€re "frozen" in the preparation chamber, by stopping the sample heating, but the temperature decrease
was not instantaneous. Simultaneously, oxygen was
pumped away. However the pressure decrease was
gradual, so the true 02 exposul'e is always slightly
h i g h e r th a n i n d i c a t e d . C o n s e q u e n t l y , n o t a l l o f t h e
exposure was performed at constant temperature
a n d p r e s s u r e , s o o L l r r c s u l t s c a n n o t b e d i r e c t l y
c o m p a r e d w i t h t h e l i t e r a t u r e d a t a a t I < 5 0 0 K . Moreover, a change in the surface layer structure during the sample cooling cannot be excluded: it has been shown that thin oxide films, formed on
N i ( l I l ) a t 5 0 0 K , d e c o r n p o s e r a p i d l y u n d e r a n
-nealing at 550 K [47].
To investigate the role of oxygen in the
interac-tion of N H3 with Ni( I I I ), it appeared necessary to
characterize the NiO growth mode in our
experi-m e n t a l p r o t o c o l , u s i n g o u r X P S d a t a . F o r t h i s
p u r p o s e , w e h a v c c o n s i d e r c d t w o m a i n h y p o t h e s e s :
c i t h c r t h c o x i d c f o r m a t i o n p r o c c c c l s v i a a " l a y c r
-b y - l a y e r " g r o w t h m o d c o r i t c o r r e s p o n d s t o a
" t w o - p h a s c d o m a i n " ( n u c l c a t i o n a n d g r o w t h o f
N i O i s l a n d a m o n g th e O - a d s o r b e d p h a s e ) , a s in t h e
k i n e t i c c o n d i t i o n s b e t w c e n 3 0 0 a n d 5 0 0 K . I n t h i s
approach, the Ni2p312 core levels have been
de-composed systematically into the metallic and Ni2*
(in NiO) features, and the oxidic contribution at
529.8 eV has bcen extracted from the O 1s core level
1 a n
1 6 0
E t . + U
r l t n
E roo
o5 q n z
Y ^ ^
: .+u 20
[image:4.595.319.531.120.271.2]NO marn p€ak 854 4 eV
Nio satelhle Ni
"o"l'lt]ft n'rr=u.u Nodouoer
I
,urrarrpeak b b r r e v
i 5 6 i l i e s o l ' v ,ta ,8s3oev
E, Luksono at ul. I Surfucc Sciantc 530 (2003) 37-54
o I 3 U
' *
Z 1An
=
CJ
=
xE n O J U
o :
o
o t c u p
o
-=
o p
x E n
O J U
:
n
€
o>
o
,n
d-><
- "layer by layer" model . experimental results
4 6 8 1 0 1 2 1 4
l(O)oxide / l(Ni)total (x100)
O - e d c n r h o d n h 2 c a F . '_ _ _ _ _ a n \/ e r a g e
0 . 8 0 . 6 0 4 0 . 2
1 6 1 8
- "two-phase domain" model
a o Y n a r i m a n i 2 l r p a r ' l l a
0 2 4 6 8 1 0 1 2 1 4 1 6 1 8
(b) I(O)oxide / t(Ni)total (x100)
Fig. 3. (a) ONi2* XPS intensity ratio, obtained from the Ols
c o r e l e v e l a n d t h e N i 2 p r r z c o r e l e v e l f o r N i 2 + i n N i O , a s a f u n c t i o n o f t h e t o t a l O / N i X P S i n t e n s i t y r a t i o , o b t a i n c d f r o m t h e O l s a n d N i 2 p 1 7 2 c o r e l e v e l s . P o i n t s : e x p e r i m e n t a l r e s u l t s ; f u l l l i n e : la y e r - b y - l a y e r n r o d e l . ( b t O , N i ' z - X P S i n t e n s i t y r a t i o , o b t a i n e d fr o m t h c O l s c o r e l e v e l a n d t h c N i 2 p 3 7 2 c o r e le v e l fo r N i 2 + in N i O , a s a f u n c t i o n o f t h e t o t a l O A I i X P S i n t e n s i t y r a t i o , o b t a i n e d f r o m t h e O l s a n d N i 2 p : , : c o r e l e v e l s . P o i n t s : e x p e r -i m e n t a l r e s u l t s ; f u l l l i n e : t w o - p h a s e d o m a i n m o d e l .
more recent equations [49,50], have been tested: they induce changes in the calculated thickness of the nickel oxide layer but the shape of the curves (Fig. 3a and b) remains unchanged. The main reason for the small deviation of some experi-mental points most probably comes from the too simplistic hypothesis of the isomorphic growth of
the oxide islands in the very early stages of the NiO
growth. We conclude that the two-phase domain
model described well the growth of the NiO oxide
layer, in our experimental conditions, and this
re-sult will be used for the following NH3 adsorption s t u d i e s .
(a)
535 533 531 529 527 525
(b) Binding EnergY (eV)
Fig. 2. (a) Ni 2p3p and (b) O ls core level peaks decomposition into the metallic and oxide fNiO) features, after 450 L of ex-posure of 02 with Ni(l I l) at 650 K.
1 6 . 2 A f o r O i n t h e ( 1 0 O ) - o r i e n t e d o x i d e
( r . f i o ( ' 0 0 ) ) . I t w a s a s s u m e d th a t t h e a t t e n u a t i o n
of the nickel signal through an O-adsorbed
mono-Iayer can be negiected, and thus exp (-d" f 2fl, sin B) = L For both models, the following ra-tios have been used:
I ( o ) I i a .
: n , , 7
/ ( N i ) n i , i "
: o R r s
4Ni;;,0"
/(Ni)r",,r
a n d
1(o)lXlll"to'ln"*
: o o4s
/(o)Iio.
Details on the calculations are rcportcd in the Appendix. In Fig. 3a and b, calculated and
ex-perimental values of' /(O)"",0"/1(Ni)o^,,1. have been
superimposed. One can easily observe that the "layer-by-layer" model does not fit the data, whereas the "two-phase domain" model provides
[image:5.595.72.285.93.400.2] [image:5.595.313.526.116.417.2]E. Laksono et al. I Surfucc Sticnce 530 (2003) 37-54
3.2. NHr adsorption
In the present work, adsorption of ammonia on
clean and oxygen pre-treated Ni(l 1 1) surfaces was
performed at room temperature, in the
prepara-tion chamber of the spectronteter. A typical
con-trolled exposure of immonia lasts 3 min at the desired pressure. At the end of the treatment, ammonia is pumped down to a pressure of l0-8 mbar, and the sample is transferred to the analysis
chamber for XPS measurements. The sample can
then be re-exposed to ammonia under the same conditions.
In the absence of oxygen on the Ni(l l l)
sur-face, no adsorption of NH3 was observed by XPS,
in agreement with previous data [39]. After
inter-action of ammonia with the oxygen pre-treated
Ni(l I l) samples, two N-adspecies have been
de-tected in the N 1s core level region: one located at
3 9 9 . 8 + 0 . 2 e Y a n d t h e o t h e r o n e a t 3 9 7 . 8 * 0 . 2 e V .
A l l N l s c o r e le v c l s p c c t r a h a v c b e c n s y s t c m a t i c a l l y
decomposed into a combination of these two
fea-tures. For a low NH3 exposure (13.5 L), if the XPS
analysis is performed just after the exposure to ammonia, the N ls core level presents a marn feature at high binding energy (HBE: 399.8 eV). If the same analysis is repeated after one night in
UHV, the ratio betWeen the surface adspecics is
significantly changed in favour of the low binding
energy adspecies (LBE: 397.8 eV). The situation is
different for cumulated NH3 exposures from 40.5
L up to about 180 L: the N 1s core level presents a
main feature at LBE and the XPS spectra remain
unchanged with time in the analysis chamber.
To focus on the initial stages ol' adsorption for low ammonia exposures, the O-pre-treated
N i ( l I l ) s a m p l e s w e r e e x p o s e d t o N H r f o r 1 3 , 5 L
in the preparation chamber, then the N ls signal was measured as a function of time in UHV, during about 500 min. The same procedure was r e p e a t e d f o u r t i m e s . F i g . 4 s h o w s a t y p i c a l s e r i c s o f N ls spectra obtained as a function of time in
U H V a f t e r e x p o s u r e t o N H 3 ( X P S r a t i o 1 ( O ) , " , " , /
/(Ni)roor :0.022). Each N ls core lcvel peak has
been decomposed into a combination of the LBE
and HBE components. Fig. 5 shows the results obtained as a function of time, flor the two
ad-species and for the total N ls signal. After the first
405 403 401 399 397 395
Binding Energy (eV)
F i g . 4 . C h a n g e s in t h e N l s s p e c t r u m a s a f u n c t i o n o f t i m e i n U H V , a f t e r t h e f i r s t N H 3 e x p o s u r e o l ' 1 3 . 5 L a t r o o m t e m p e r -a t u r e . T h e N l s p e -a k d e c o m p o s i t i o n i n t o t h e H B E a n d L B E c o m p o n e n t s is s h o w n . T h e N i ( l I l ) s u b s t r a t e w a s p r e - t r e a t e d b y 0 2 a t 6 5 0 K , 3 0 s e c o n d s a t I x I 0 - ? m b a r . F r o m t h e f i r s t (lower part) to the fourth spectrum, as well as from the fifth to t h e l a s t s p e c t r u m (u p p e r p a r t ) , t h e t i m e b e t w e e n tw o s p e c t r a is 4 5 m i n . B e t w e e n t h e f o u r t h a n d f i l t h s p e c t r a , t h i s t i m e i s 9 0 m i n . T h e t o t a l t i m e s c a l e is - 9 h .
exposure to ammonia, the low BE signal increases
with time whereas the high BE signal decreases.
Moreover, the total NH3 uptake dccreases with
time, suggesting a partial desorption of the high
B E a d s p e c i e s . A f t e r t h c s e c o n d a m m o n i a e x p o s u r e ,
tl.tc observations ale simrlar, with a less pro-nounced increase in the iow BE signal. After the third dose, the surface distribution between the H B E a n d L B E N l s f c a t u r e s re m a i n s a l m o s t u n -changed with respect to the last N ls spectrum recorded after the second dose. However, the total
N ls area has slightly increased compared to the
second dose. No lurther chanse is observed after
-A E G c c
U)
[image:6.595.330.499.105.410.2]4 8 E. Luksono et al. I Surface Science 530 (2003) 37-54
u )
C
=
-i E>\
' i ; .
r
',;
nX
tn
. E
=
L
.;;
-. c
a
><
q ? )
Binding
energy
(eV)
540
536
532
528
Binding
energy
(eV)
intensity ratios of these sut'faces after the
men-tioned treatments.
After annealing under UHV, the Ni 2p372 core
level peak exhibits a main feature corresponding to
532
528
Binding
energy
(eV)
540
536
532
s2B
Binding
energy
(eV)
metallic nickel, larger than before annealing, and
the O ls core level peak area is less intense, with a
shoulder at 531.5 eV, less pronounced compared to the spectrum beiore annealing. From these ob-U)
--ct
tJ)
c
I-V,
o-X
a=
A
VI
-a
X
F i g . 9 . C h a n g e s in t h e O l s s p e c t r u m a s a f u n c t i o n o f t i m e i n U H V o b t a i n e d a f t e r ( a ) t h e f i r s t a m m o n i a e x p o s u r e (1 3 . 5 L ) ' ( b ) t h e s . " o n d a 1 n m o n i a e x p o s u r e ( 1 3 . 5 L ) , ( c ) t h e t h i r d a m m o n i a e x p o s u r e ( 1 3 . 5 L ) , ( d ) t h e f o u r t h a m m o n i a e x p o s u r e ( 1 3 . 5 L ) . B e t w e e n t h e f i r s t ( b o t t o m ) s p e c t r u m a n d t h e l a s t ( t o p ) s p e c t r u m , t h e t i m e s c a l e i s : - 6 h f o r t h e f i r s t e x p o s u r e ( a ) , - 8 h f o r t h e s e c o n d e x p o s u r e ( b ) , - 7 h for the third exposure (c) and -8 h for the fourth exposure (d).
(d)
0H I
z t / / t
"^-*
- r ; / ../ / [
/ / /
. r / |*n-' ,/ / /
- / . 1 /
^ N
[image:7.595.96.484.84.558.2]E. Laksono er ul. I Surface Science 530 (2003) 37-54
Table I
XPS intensity ratios obtained from levels after 02 interaction at 650 annealing at 650 K
Thin film I Thin film 2
/(o),,,,,r//(Ni),.,,,r /(o)r,,,,r//(Ni),,,,.,r
B e f o r e a n n e a l i n g 0 . I 5 A f t c r a n n e a l i n g 0 . 1 2
servations on the core levels, different hypotheses
c a n b e c o n s i d e r c d in v o l v i n g a d c c r e a s e i n t l i c
quantity of nickel oxide zrndior a decrease in the
o x i d e s u r f a c e c o v e r a g e : (i ) c i t h e r a d c c r e a s e i n
the oxide islands tl.rickness, the oxide covcragc
remaining constant; or (ii) a decrease in the oxide
coverage, the oxide islands thickness being
con-stant; or (iii) a decrease in the covcrage of oxide
with an increase of the oxide island thickness; or
(iv) both a decrease in the oxide coverage and the
oxide thickness.
As regards the surface reactivity towards
am-monia, it was observed in both cases that the
ad-sorption was greatly enhanced compared to the
surface before annealing. For Thin Film l,
corre-s p o n d i n g t o 1 ( O ) r o r o , / / ( N i ) , o , , r : 0.i2 (see T a b l e l) ,
the XPS intensity ratio after ammonia adsorption
b e c o m e s / ( N ) / / ( N i ) r " r u r : 9 X l0-4 * I x 10-a and
for thin film 2, the ratio is 1(N)/1(Ni),o,u, : 14 x
l 0 4 + I x l 0 4 . B a s e d o n o u r p r e v i o u s o b s e r v a
-tion of the absence of reactivity of nickel oxide in
our conditions (Fig. 7), this new series of experi-ments confirms the enhanced reactivity of the
O-adsorbed phase on the surface of the sample. In the
particular case of Thin Film 1, in spite of a small
decrease of the XPS ratio ^/(O),o,u,/./(Ni),o,", (from
0 . 1 5 t o 0 . 1 2 ) , th e a m m o n i a a d s o r p t i o n e n h a n c e
-ment suggests a decrease in the covelage of oxide
with an increase of the oxide island thickness
(hy-pothesis (iii)): not only the NiO(l00) phase de-composed into the O-adsorbed phase but also a local thickening of the oxide islands occurred, as
already observed with Ni substrates (1 0 0)-oriented
l,s2l.
At this point, to summarise the results, we have
observed that NHj reacts with the Ni(l I l) surface
provided some O-adsorbed phase is present; once adsorbed, molecular NH3 reacts u'ith oxygen to
form dissociated ammonia species and hydroxyl
groups as a function of time, according to the following reaction:
N H 3 ( a d s ) + . r O ( a d s ) - N H r - * ( a d s ) + x O H ( a d s )
x : \ , 2 ( s c h e m e I )
The value of x will be discussed below.
3.3. Discussion
W e { i r s t c o m m e n t o n t h e b i n d i n g e n e r g i e s o f t h e
N - a d s o r b e d s p c c i e s . I n t h i s ' " v o r k , tw o d i f f e r e n t
v a l u e s h a v e b e e n m e a s u r c d : t h e L B E a t 3 9 7 . 8 + 0 . 2
e V , a n d t h e H B E a t 3 9 9 . 8 * 0 . 2 e V . S o m e s e l e c t e d
v a l u e s o f N 1 s B E , f r o m t h e l i t e r a t u r e , a r e r e p o r t e d
i n T a b l e 2 .
To focus more specifically on the comparison with previous works on ammonia adsorption at room temperature on oxygen pre-treated nickel single crystals, Grunze et al. [29] have already faced the difficulty of unambiguously identifying the nature of a NH3 adsorption complex on an oxygen pre-covered Ni(l 0 0) surface. They ob-served a broad peak centred at 399.6 eV, but did not attempt to decompose it and assigned it to N H 3 o r N H : . H O . I n t h e w o r k o f K u l k a r n i e t a l .
[ 3 3 ] o n o x y g e n p r e - t r e a t e d N i ( l 0 0 ) a n d N i ( l l 0 ) ,
the broad N ls spectra obtained after ammonia exposure at 300 K, centred at 398.0 eV, were
T a b l e 2
C o m p a r i s o n o f N l s B E f o r a d s o r b e d n i t r o g e n s p e c i e s o n d r f -f e r e n t m e t a l s a n d o x i d e s
A d s o r b e d n i t r o g c n s p e c i c s N l s B E ( e V ) R e f e r e n c c
B N o n N i ( 1 0 0 ) NI-lr on Ni(l I 0) N H 3 o n N i ( l 0 0 )
N H r o n A e ( l I l ) N H r o n C r : O r / C r ( l l 0 ) N H 2 o n d i f f e r e n t m e t a l s N H 2 o n C r : O r / C r ( l l 0 ) N H o n d i f f e r e n t m e t a l s N H , ( . r : 1 , 2 ) o n N i ( l 0 0 ) N H o n N i ( 1 0 0 )
N H o n N i ( l I 0 ) N o n d i f f e r e n t m e t a l s N o n N i ( I 0 0 ) t h e O I s a n d N i 2 p 3 7 2 c o r e
K, before and after UHV
0 . r 3 0 . 0 5
3 9 8 . 5 t 5 3 l 400.9 [ ]l
400.5 t33l
[image:8.595.308.525.495.674.2]E. Luksorto et ul. I Surface Sciance 530 (2003) 37-54
minority of the O-adsorbed atoms are in states 3a (or 3b) and 3c. The majority of oxygen is in state 3d. Moreover, the extrapolation of the data of Fig. 5 to t = 0 indicates that about 200h of the molec-ular adsorbed ammonia has desorbed in UHV conditions. In state 2, the atomic O/NH3 ratio can be roughly estimated to 4ll, which means thar, even before the ammonia desorption and conver-sion stage, all the equivalent fcc hollow sites are not occupied by molecular ammonia. The possible
electronic or steric effects hindering the adsorption
of ammonia in some fcc sites are not elucidated.
It is to be noted that the first hypotheses (states
I and 2) are the samc as those put forward by Netzer and Madey [27], with adsorption in hollow
sites. With the same sites for the O-adsorbed phase
(state 1), another possibility would be to assign the molecular ammonia of state 2 to top sites, except the"ones that are nearest to the adsorbed oxygen.
This configuration leads to the same alternative for
the NH2 sites (states 3a and 3b in Fig. l0). Theo-retical calculations might help in distinguishing
which hypothesis is more likely.
4. Conclusions
The general features that can be derived from the above results are as follows.
As regards the interaction of oxygen with N i ( l I l ) a t 6 5 0 K , i t w a s s h o w n t h a t , u n d e r o u r experimental conditions, the surface oxidation proceeds via a "two-phase domain": O-adsorbed phase and NiO islands coexist in a large range of oxygen exposure.
As regards the surface reactivity towards NH3, a t r o o m t c m p e r a t u f c a n d a t I x l 0 - 7 m b a r . t h c
conclusions are the following:
(i) There is no adsorption on the clean Ni(l I l) surface.
(ii) The surface reactivity is strongly correlated
with the presence of O-adsorbed phase coverage:
the higher the 0o6, coverage, the higher the
a m o u n t o f a d s o r b e d a m m o n i a .
(iii) A continuous nickel oxide thin layer is not reactive in our conditions.
(iv) Two N-adspecies have been detected from
t h e N 1 s c o r e le v e l p e a k , a t 3 9 9 . 8 + 0 . 2 e V ( h i g h B E
feature) and at 397.8 + 0.2 eY (low BE feature), and assigned to molecular NH3 and dissociated
NH,(,=r.z) species, respectively.
( v ) F o r l o w a m m o n i a e x p o s u r e s , t h e d i s s o c i a t e d
ammonia adspecies is formed from a fraction of
t h e m o l e c u l a r a d s p e c i e s , a n d c o n c o m i t a n t l y , t h e
h y d r o x y l c o m p o n e n t in t h e O l s c o r e l e v e l p e a k
increases, while the main feature at 529.9 eV
de-c r de-c a s e s . T h i s s i m u l t a n e o u s tr a n s f o r m a t i o n h a s evidenced the hydrogen abstraction of ammonia by adsorbed oxygen to product OH and NH,. A quantitative treatment of the XPS data gives the
following stoichiometry :
N H 3 ( a d s ) + O ( a d s ) - N H z ( a d s ) + O H ( a d s ) .
(vi) From our experimental results, it appears that the kinetics of desorption of ammonia is faster than the kinetics of dissociation.
(vii) The total amount of adsorbed ammonia at saturation is about five times smaller than the
amount of O-adsorbed species. At equilibrium, the
ratio betwecn molecular NH3 and NH2 adspecies i s l ; 4 .
(viii) The behaviour of thc two nitrogen
adspe-cies suggests that two different adsorption sites are
required.
Appendix
Calculatiott of the XPS intensity ralios for the "
tvo-phase domain" and the "layer-by-layer" models
( l ) The 1(Ni );id" // (N i )T.,,r and / (o ) il"::llvd';h"," /
1(O)I,0" XPS ratios are respectively expressed by:
1 ( N i ) ; i d . _
INnil
-1(Ni)::id:r'
[r
- '-n(-
N.-r-)]
,(NmF-c,xP,Jl
and
/ 1 n \ m o n o l a y c r ; 1 6 1 m o n o l a l c r ' \ " , i a d s o r b c d p h a s c _ ' \ " / a d s o r b c d p h a s c
-'-e( ,*-h)
I(O)Ld"
r(
. [ ,
E. Luksono ar ul. I Surfuca Scianca 530 (2003) 37,54 ) J
,r(Ni):lidr:v.' _ ^ ,.1.7
.r(Ni)il:jiv.,
which leads to
ffi#:08r5 and
' / ^ \ m o n o l a v e r , 1 ( u . ) " d r o r b . i o h . , r .
a n O - = U . + U )
/ / o ) o n e r r ) e r
- \ - / o \ t d e
w h e r e :
r i s t h e t h i c k n e s s
o f o n e N i O ( I 0 0 ) l a y e r
( : 2 . 0 8 6 A ) a n d /' i s t h e th i c k n e s s
o f o n e
N i ( l I 1 )
I a y e r
( : 2 . 0 3 6 A).
A"suming that the photoelectron
emission
by
one atom (cross section) is independent
of its
chemical
environment,
the
/(Ni):ffifv" .._r 1(o)i:l:i:'d";h.,.
- s r r u
/(Ni)a:l?v"
r(o)::io'j"'
ratios are equivalent to the ratios of the number of atoms in one layer for each compound. For Ni (l I l)-oriented, NiO (1 0 0)-oriented and a p(2 x 2) O-adsorbed ohase. we obtain
[ 3 ] A . A . S a r a n i n , O . L . T a r a s o v a , V . G . K o t ) j a r , E . A . K h r a m t s -o v a , V . G . L i f s h i t s , S u r f . S c i . 3 3 1 - 3 3 3 ( 1 9 9 5 ) 4 5 8 .
[ 4 ] C . B a t e r , M . S a n d e r s , J , H . C r a i g J r . , S u r f . S c i . a 5 l ( 2 0 0 0 )
226.
[ 5 ] V . M . B e r m u d e z , T h i n S o l i d F i l m s 3 4 7 ( 1 9 9 9 ) 1 9 5 . [6] V.M. Bermudez, Chem. Phys. Lett. 317 (2000) 290 [7] H.L. Siew, M.H. Qiao, C.H. Chew, K.F. Mok, L. Chan,
C . Q . X u , A p p l . S u r f . S c i . l 7 l ( 2 0 0 1 ) 9 5 .
[8] M. Kaltchev, W.T. Tysoe, Suri. Sci. 430 (1999) 29. [ 9 ] R . M . L a m b e r t , M . E . B r i d g e , i n : D . A . K i n g , W . P . W o o d
-ruff (Eds.), The Chemical Physics of Solid Surfaces and H e t e r o g e n e o u s C a t a l y s i s , v o l . 3 B , E l s e v i e r , A m s t e r d a m , 1 9 8 4 , p p . 5 9 - 1 0 5 .
[ 0 ] C . W . S e a b u r y , T , N . R h o d i n , R . J . P u r t e l l , R . P . M e r i l l , S u r [ . S c i . 9 3 ( 1 9 8 0 ) I l t
I l ] C . R . B r u n d l e , J . V a c . S c i . T e c h n o l . l 3 ( 1 9 7 6 ) 3 0 1 . [ 2 ] K . J a c o b i , E . S . J c n s e n , T . N . R h o d i n . R . P . M c r r i l l , S u r f .
S c i . 1 0 8 ( 1 9 8 1 ) 3 9 7 .
[ 3 ] M . G r u n z e , M . G o l z e , R . K . D r i s c o l l , P . A . D o w b e n , J . V a c . S c i . T e c h n o l . l 8 f l 9 8 l ) 6 l L
! 4 1 B . A . S c x t o n , G . E . M i t c h e l l , S u r f . S c i . 9 9 ( 1 9 8 0 ) 5 2 3 . l l 5 l G . B . F i s h e r , C h c m . P h y s . L c t t . 7 9 (1 9 8 1 ) 4 5 2 .
[6] D.M. Thornburg, R.J. Madix, Surf. Sci. 220 (1989) 268.
[ 7 ] D . C h r y s o s t o m o u , J . (t999) 34.
[8] R. Jaeger, J, Stohr, T 547.
F l o w e r s , F . Z a e r a , S u r ' f . S c i . 4 3 9
K e n d e l e w i c z , S u r f . S c i , 1 3 4 ( 1 9 8 3 )
[9] J.W. Niemantsverdriet, R.M. van Hardeveld, R.A. van Santen, Surf. Sci. 369 (1996) 23;
R.M. van Hardeveld, R.A. van Santen, J.W. Niemants-verdriet, J. Vac. Sci. Technol. A l5 (1997) 1558.
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340 fl999) 7.
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P h y s . R e v . B 4 8 ( 1 9 9 3 ) 4 7 6 0 ,
[ 2 5 ] K . - M . S c h i n d l c r , V . F r i t z s c h e , M . C . A s e n s i o , P . G a r d n e r , D . E . R i c k e n , A . W . R o b i n s o n , A . M . B r a d s h a w , D . P . WoodrufT, J.C. Conesa, A.R. Gonzalez-Elipe, Phys. Rev. B 46 0992\ 4836.
[26] I.C. Bassignana, K. Wagemann, J. Kiippers, G. Ertl, Surf. S c i . 1 7 5 ( 1 9 8 6 ) 2 2 .
l27l F.P. Netzer, T.E. Madey, Surf. Sci. ll9 (1982) 422. [28] T.E. Madey, C. Benndorf, Surf. Sci. I52-153 (1985)
5 8 7 .
[29] M. Grunze, P.A. Dowben, C.R. Brundle, Surf. Sci. 128 ( r 9 8 3 ) 3 i l .
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references therein.
[32] B. Afsin, P.R. Davies, A. Pashusky, M.W. Robcrts, D. V i n c e n t , S u r f . S c i . 2 8 4 ( 1 9 9 3 ) I 0 9 .
/ ( o ) I",l?l'.'o'i,,,,,
: 0.049
1(o)I'0.
(2) To obtain the ,r(O)fr,."/1(Nifiid"
ratio, we
have used the general
quantitative
formula:
I(M)ff
/ / p\""
oyDlnlflr$E)
op$ ).!r@E)
where: M, P are two different elements,
N is the
matrix formed by M and P, D is the atomic
density
of M or P in the matrix N, and o is the Scofield
cross
section
[51].
For the VG ESCALAB Mk ll spectrometer,
. i l - z , ' ^ r t ^ N - t , . - t
);;tf WE)lA:;I(KE) is constant,
which leads
to:
I(M);
ouDlt
7m- or4
w i t h D U i o : D N i o ( : 0 . 0 9 1 6 m o l / c n . r 3 ) , o 6 1 ,
2 . 9 3 a n d o N i 2 p r , r : 1 4 . 6 1 , t h e c a l c u l a t i o n l e a d s to :
1 ( O ) n i d . / 1 ( N i ) I i o . : 0'20 t 0'03, assuming a n u n
-certainty of 5o/u on the values of a mentioned by
Scofield [51]. Within this range, the best fit be-tween the experimental results and the theoretical
m o d e l is o b t a i n e d fo r / ( O ) f r , d . / 1 ( N i ) n i d . : 0 . 2 2 .
Rcfcrenccs
Ul J.W. Klaus, S.M. George, Surf. Sci. 447 (2000) 81.
54 E. Luksono et al. I Surface Science 530 (2003) 37-54
[33] G.U. Kulkarni, C.N.R. Rao, M.W. Roberts, J. Phys. [44] P. Finetti, M.J. Scantlebury, R. McGrath, F. Borgatti, M. Chem.99 (1995) 3310. Sambi, L. Zaratin, G. Granozzi, Surf. Sci.46l (2000) 240. [34] H.E. Dastoor, P, Gardner, D.A. King, Surf. Sci.289 (1993) [45] F. Rohr, K. Wirth, J. Libuda, D. Cappus, M. Batimer,
2 7 9 . H . - J . F r e u n d , S u r f . S c i . L c t t . 3 1 5 ( 1 9 9 5 ) L 9 1 7 .
[35] L. Ruan, I. Stensgaard, E. Legsgaard, F. Besenbacher, [46] N. Kitakatsu, V. Maurice, C. Ilinnen, P. Marcus, Surf. Sci.
Surf. Sci. 314 (1994) L873. 407 (1998) 36.
[ 3 6 ] M . - C . W u , C . M . T r u o n g , D . W . G o o d m a n , J . P h y s . C h e m . [ 4 7 ] N . K i t a k a t s u , V . M a u r i c e , P . M a r c u s , S u r l l S c i . 4 l l ( 1 9 9 8 ) 2 1 5 .
97 (t993) 4182.
[37] R.S. Mackay, K.H. Junker, J.M. White, J. Vac. Sci. [48] M.P. Seah, Auger and X-ray Photoelectron Spectroscopy, Technol. A 12 (1994) 2293. Practical Surface Analysis, vol. I, Wiley, Chichester, 1990, [38] H. Ma, Y. Berthier, P. Marcus, Appl. Surf. Sci. I53 (1999) pp. 201-255.
40. [49] P.J. Cumpson, M.P. Seah, Surf. Interf. Anal. 25 (1997) 430. [39] A. Galtayries, E. Laksono, C. Argile, P. Marcus, Surf. [50] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interf. Anal. 25
, Interf. Anal.30 (2000) 140. (1997)25.
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