Langmuir 1995,11, 2572-2575
2572
Coadsorption of Dioxygen and Water on the Ni(ll0) Surface: Role of 0--Type Species in the Dissociation of Water G. U. Kulkarni, C. N. R. Rae,? and M. W. Roberts” Department of Chemistry, University of Wales, CardiffCFl 3TB, U.K. Received October 4, 1994@ While the adsorption of diox gen at a clean Ni(ll0) surface gives rise to two O(1s)features at 531 and 530 eV assigned to O-(a) and Ol-(a) type species respectively, coadsorptionof dioxygen and water mixtures result in the additional formation of hydroxyl species characterized by an O(1s) peak at 532.3 eV. The latter is attributed to the oxygen induced dissociation of water via a low energy pathway involving the 0-(a)-type species. The proportions of the O-(a) and the hydroxyl species are greater for small OZ/HzO ratios and lower temperatures (120 K). With increase in temperature, the relative surface concentrations of the O-(a) and the hydroxyl species decrease while there is an increase in the concentration of the oxidic 02-(a) species. Thus, the surface concentrations of both the hydroxyl and the 02-(a) species depend critically on the presence of 0- type species. Above 300K the surface chemistry in the main involves the conversion of 0- to 02-species via the hydroxyl species.
Introduction Previous photoelectron spectroscopic studies from this laboratory have established that a number of distinct oxygen species can exist a t metal surfaces which can be characterized both by their O(1s) binding energies and through their chemical reactivities.’ Oxygen chemisorbed a t nickel surfaces a t low temperatures is exceptionally active in H-abstraction from molecularly adsorbed water, and this was attributed2 to a n 0--like (0”) species, i.e. one which had not fully developed the charge associated with the chemisorbed oxide overlayer CY-.This prompted us to develop the molecular probe or chemical trapping approach3which successfully established the existence of oxygen transients 0zs- and Od- in the dynamics of the dissociative chemisorption of dioxygen a t metal surfaces. The transients participate as intermediates in the formation of the oxide 02--like overlayer
O,(g) =Z O,(g)
-
O;-(S)
-
20’-(s)
-
202-(a)
and may rapidly convert to 02-species, exist as a highly reactive metastable Os-(,) species a t low surface coverage or as a defective species within a nonstoichiometric oxide overlayer. However, the identification of such intermediates on nickel by XPS has not been straightforward, and in particular the assignment of the high binding energy O(1s) feature at 531 eV has been debated exten~ively.~ The contribution of the latter to the O(1s) intensity has been shown to be variable depending on the surface structure, t e m p e r a t ~ r e ,and ~ exposure,6 and for a “perfect NiO + Permanent address: CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore-560012, India. Abstract published in Advance A C S Abstracts, June 15,1995. (l)Au, C. T.;Carley, A. F.; Pashuski, A,; Read, S.; Roberts, M. W.; Zeini-Isfahan, A. In Adsorption on Ordered Surfaces ofIonic Solids and Thin Films; Springer Series in Surface Science 33; Springer-Verlag: Berlin and Heidelberg, Germany, 1994; p 241. (2) Carley, A. F.; Rassias, S.; Roberts, M. W. Surf: Sci. 1983,135,35. (3) Au, C. T; Roberts, M. W. Nature 1986,319,6050; J. Chem. SOC. Faraday Trans. 1987,83,2047. Carley, A. F.; Yan, Song; Roberts, M. W. J. Chem. SOC.,Faraday Trans. 1990,86, 2701. (4)Brundle, C. R.; Broughton, J. Q.In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1990; Vol. 3, p 131. (5) Rajumon, M. K.; Prabhakaran, K.; Rao, C. N. R.; Surf. Sci. Lett. 1990,233, L237. (6) Carley, A. F.; Chalker, P. R.; Roberts, M. W. Proc. R. SOC.London 1985, A399, 167. @
crystal” is negligible-the 531 eV feature being assigned either to a defective 0--like oxygen species6 or surface hydroxyl^.^ Of note also is the observation that nickel surface on exposure to oxygen under high flux conditions generates a highly symmetrical narrow O(1s) peak whereas exposure under low flux conditions-but to the same total exposure-generates a n asymmetric O(1s) peak.4 The reason for this is not clear. We explore here how studies of the coadsorption of dioxygen and water i.e. mixtures of various compositions a t Ni( 110)surfaces can provide information on the identity and the chemical reactivity of different chemisorbed oxygen species-in this case we regard water as the probe molecule. Recent analogous studies’ of Cu(ll1) and Zn(0001) surfaces have shown that extensive hydroxylation occurs when dioxygen-water mixtures were coadsorbed and this was attributed to the high reactivity of chemisorbed oxygen O”-like species present a t low coverage or a t the periphery of “oxide”islands. With the development of the oxidic 02--like species, activity for H-abstraction leading to surface hydroxylation decreased rapidly. Similar behavior has been reported8 for the coadsorption of ammonia-dioxygen mixtures where imide formation a t Cu(l10)surfaces is both facile and extensive provided the oxygen surface coverage is maintained close to zero. The overall surface reaction, oxydehydrogenation of ammonia, is accompanied by the desorption of water. Our aim in this paper is to examine quantitatively how such potentially reactive 0- species assigned to the 531 eV feature in the O(1s)spectrum participate in the reaction with water when the latter is coadsorbed with dioxygen a t a Ni(ll0) surface. We have analyzed the O(ls) spectra to obtain quantitative information on the dependence of the relative surface concentrations of the different oxygen species on both the composition of the 02-H20 mixture and the temperature. We also examine the interaction of water with a Ni(ll0)-0 surface, i.e. where oxygen has been preadsorbed, by monitoring the changes in the O(ls) spectrum. There have been a number of previous studies18 of water adsorption by Ni( 110) but none of dioxygenwater mixtures. (7) Carley, A. F.; Davies, P. R.; Roberts, M. W.; Shukla, N.; Song, Y.; Thomas, K. K. Appl. Surf: Sci. 1994, 81, 265. (8)Afsin, B.; Davies, P. R.; Pashusky, A.; Roberts, M. W.; Vincent, D.; Surf: Sci. 1993,284, 109.
0743-7463/95/2411-2572$09.00/0 0 1995 American Chemical Society
Coadsorption of Dioxygen and Water
Langmuir, Vol. 11, No. 7, 1995 2573
l20K
. I-.'-. . ....................... .. . .. . .. . .. . . . ... . . /
.?
o'i a I
525
530
535
...
540
.......
(eV)
Figure 1. O(ls)spectra for aNi(ll0)surfaceexposed to oxygen (20 L)at 120 K (a) as a sequence of consecutive doses and (b)
as a single "highflux" dose.
The present work has enabled us to unambiguously identify the 531 eVfeature with 0- species and to establish that 0- species indeed provide a low energy pathway to dehydrogenation giving rise to hydroxyl species. The proportions of the O-(a) and the hydroxyl species decrease with increase in both the 02/HzO ratio and temperature; this is accompanied by an increase in the proportion of the oxidic species 02-(a).
Experimental Section Core-level O(1s) spectra were recorded with a VG Scientific X-ray photoelectron spectrometer,at a pass energy of 50 eV. The binding energies were referenced to a Ni(2p~iz)value of 852.6 eV. Vibrational electron energy loss (VEEL) spectra were recorded in the specular mode with an electron beam energy of 3 eV employing a VSW spectrometeroperating at a resolutionof -60 em-'. Both spectrometersare incorporated intoa single ultrahigh vacuum chamber with a base pressure of 5 x Torr enabling both types of spectra to be collected simultaneously. The Ni( 110)crystal surface was cleaned by repeatedly&+ ion etching and heating to 1100 K by electron bombardment until the surfacewas devoid of carbon. The crystallinityof the surface was frequently checked using a LEED system attached to the chamber. Mixtures of oxygen (purity,99.999; supplied by P. J. Mason)and water were prepared using a gas-handlingmanifold fitted to the vacuum chamber and admitted close to the crystal; the gas pressures used were Torr. Exposures are given in Langmuir units (1 L = Torr s). The concentration of surface species were calculated using the procedure described by Carley and Roberts9 Data acquisition was achieved with commercial software (SPECTRA)and analyzed using software developed in-house. Results and Discussion Adsorption of Oxygen at Ni(ll0) Surface. We discuss first the results of the XPS studies of the adsorption of oxygen a t Ni(ll0) a t 120K under two different conditions: (a) as a series of consecutive exposures at -lo+ Torr and (b)as a single high flux oxygen exposure at Torr (Figure 1). In both cases the total exposure is -20 L. There is clearly a distinction to be made between the two spectra; in addition to the two O(1s) peaks a t about 530 and 531 eV (spectrum a) there is asymmetry to the high binding energy side. On the other hand only a single narrow O(1s) peak a t -530 eV (529.6 eV) is observed in spectrum b. We assign the two major features a t 530 and (9) Carley, A. F.; Roberts, M. W. Proc. R. Soc. London 1978,A363, 403. (10)Carley, A.F.;Grubb, S. R.; Roberts, M. W.; J.Chem. SOC., Chem. Commun. 1984,459;Uhlenbrock, St.; Scharfschwerdt, Chr.;Neumann, M.; Illing, G.; Freund, H-J. J.Phys. Condens. Matter 1992,4 , 7973.
l " " l " " 1 "
525
530
1 " " l " " l "
535
525
530
535
(eV)
Figure 2. (a)Curve-fitted O(ls)spectra for a Ni(ll0) surface after exposure (5 L) to 02-HzO mixtures ofvaryingcompositions (1:lO; 1:l and 2 5 1 at 120 K) and (b) a 25:l mixture at 300 K (pressure < Torr). Inset shows multilayers of water formed on exposing the surface to pure water (5L) at 120 K. Residuals obtained after subtracting the sum of fitted components from the experimental data are also shown.
531 eV to 02--oxide like species and 0- species respectively i.e. there is no evidence for the presence ofthe metastable 0- species (or 0") under the high flux and highly exothermic conditions. These assignments and also the origin of the asymmetry in the spectrum a become clear when we consider the experimental data (see below) of coadsorption studies. Coadsorption of Water and Dioxygen at the Ni(110)Surface. In Figure 2, we show typical O(1s)spectra obtained on exposing a Ni(ll0) surface exposed to oxygenwater mixtures of different compositions a t 120 and 300 K. There are substantial changes in the O(1s)profile when the O&zO ratio is vaned which suggest the presence of four components each separated by about 1.2 eV. A nonlinear least-squares procedure was applied, floating the energy and fwhm values of the Gaussian components until a normalized sum of the squares of the deviations was obtained close to unity in each case. The energy values were accurate to f0.15 eV. The area (or intensity) associated with each component enabled the concentrations of the particular species to be c a l ~ u l a t e dsuch ;~ an analysis gave (Figure 2) four features centered a t 529.6, 531,532.3 and 533.4 eV. Ofthese, the 529.6 and the 531 and 0- species, eV components are due to the 02respectively. The 533.4 eV feature is due to molecularly adsorbed water (see inset of Figure 2). The 532.3 eV feature observed is, however, distinctly different from the 531 eV feature assigned to O-(a) and also from the 533.4 eV feature characteristic of molecularly adsorbed water. We assign the 532.3 eV feature to the hydroxyl species resulting from the dissociative chemisorption of water. A feature around 532.5 eV has been assignedll to water (11)Thiel, P.A.;Madey, T. E.Surf Sci. Rept. 1987,7,211. Benndorf, C.; Nobl, C.; Madey, C. Surf Sci. 1984,138,292.Benndorf, C.; Nobl, C.; Thieme, F. Surf Sci. 1982,121, 249.
2574 Langmuir, Vol. 11, No. 7, 1995
I
I
I
0
I
I 3000
1
2000
Kulkarni et al. that the only O(1s) component present has a binding energy of 531 eV. The temperature dependence of the two components is, however, different. The intensity of the 531 eV component is higher at lower temperatures, while the reverse is the case for the 529.6 eV component. The intensities of the 532.3 eV and 533.4 eV features are comparatively small (Figure 4) and their temperature dependence analogous to that of the 531 eV feature. On the basis of the data in Figures 2 and 4, we conclude that the proportion of the oxidic species characterized by the 529.6 eV feature increases with temperature, it is also more prominent with a n oxygen rich mixture (02:HzO = 25:l). The 531 eV feature associated with O-(a), on the other hand, is more prominent with small OZ/H20 ratios and at low temperature. The 532.3 eV feature due to hydroxyl species is also more prominent a t 120 K than a t 300 K.13 It is significant that the concentration of OH species a t 120 K (O(1s) -532.3 eV) is substantial (0.35 x 1015cm-2)for a very low fraction of oxygen in the mixture and almost invariant as ~ ( 0 2 -unity. ) This is compatible with the asymmetry in the O(1s)spectrum observed under conditions of sequential dosing of dioxygen (Figure la) where even trace amounts of water present in the system can result in hydroxyl formation.
I
I
4000
( cm-'
+
Figure 3. VEEL spectra for a Ni(ll0) surface: (a) 0 2 H2O mixture (l:l),120 K, (b)0 2 HzO mixture (25:1), 120 K.
+
hydrogen bonded to chemisorbed oxygen, but we prefer to assign this to hydroxyl species, since there is substantial evidence for the dissociation of water at transition metal surfaces, especially in the presence of surface oxygen. Support for this assignment also comes from the VEEL spectrum which shows two losses a t 3400 and 3600 cm-l besides a feature due to a bending mode a t -1600 cm-l (Figure 3a). It is known12 that the hydroxyl species produced by the dissociation of H2O are associated with a higher stretching frequency in the 3600-3700 cm-l region (as seen in Figure 3b) compared to hydrogen bonded water molecules. The latter show a stretching frequency in the 3300-3500 cm-l range and a scissor mode at about 1600 cm-l. In order to understand changes in the O(1s) profile arising from the variation in the concentrations of the various oxygen species with the OZ/H20 ratio (Figure 21, we have plotted (Figure 4) their concentrations against the mole-fraction of oxygen,~ ( 0 2 ) . Clearly, the intensities of the 531 and 529.6 eV features in the O(1s) spectrum, increase with ~ ( 0 2 1 but , a t small values of ~ ( 0 2 it) is clear
In order to explain these observations, we propose the following transformation where the O-(a) species provide a low energy pathway for the dissociative chemisorption of HzO.
-
O(1s): 530
E .
I 0.8 -
$m
Ob
-
%
0.4
-
02
-
d
c
x Do
"
0
025
0 .5
0.15
5 . 5
.
0.6
-
0.4
-
Ln
0
1
(1)
-
(2)
I
0 (Is) : 53 I eV
0
0 25
0 .s
0 .l5
1
1
0(Is):53 2 0.8
-
+ O,(g) Ni2+(s)+ 20-(a) H,O(a) + O-(a) 2HO* *Ni
Ni
m n
P x
0 '
'
0
0 25
0 .5
0.75
1
I
x (OJ
Figure 4. Variation in the surface concentration of the oxygen species characterized by the 530,531,532, and 533 eV features ) two temperatures300 K (0) and 120 K(0). ~ ( 0 2 =) F([021/([02] respectively,with the mole fractionof oxygen in the mixture,~ ( 0 2 at + [HzOl)).
Coadsorption of Dioxygen and Water H,O(a)
+ 20-(a)
20H(a)
-
-
2HO- *Ni
H,O(g)
Langmuir, Vol. 11, No. 7, 1995 2575
+ 02-(a)
+ 02-(a)
(bl
(3)
(4)
We should of course recall that the following steps are already well recognized in the oxidation of nickel?
-
+ 1/20,(g) Ni2+ + '/,O,(g) Ni2+ + O-(a)
Ni
+ Ni3+ + O-(a)
Ni2+(s) 02-(a)
-
+
Ni3+ 02-(a)
(5)
A
(6)
0--type species can therefore be formed a t both low (eq 1) and high surface oxygen regimes (eq 61, i.e. a t both metal-oxide and oxide-gas interfaces.'j Both reactions 2 and 3 represent the role of 0--type species in providing a low energy pathway for the formation ofhydroxyl species and is much more facile than the formation of hydroxyl species by the dissociation of H20 a t the Ni(ll0) surface. At higher temperatures (2300 K), the essential reaction would therefore be the transformation of O-(a) to 02-(a) uia reactions 2-4; this is observed experimentally. In Figure 5a, we show the O(1s) spectrum when a Ni( 110) surface was exposed to a 1:l mixture ofoxygen and water (200 L) a t 220 K. The most prominent feature in the O(ls) spectrum is a t 531 eV due to O-(a). When the surface warmed to 300 K, the intensity of the 530 eV peak due to the 02-(a)species increases with a simultaneous decrease in the intensity of the 531 eV feature. At 370 K, the concentration of the 02-(a) species increases further, thereby demonstrating that the conversion of 0- to 02is favored by a n increase in temperature. We have also confirmed this transformation by investigating the adsorption of water at 300 K, but in the presence of a small coverage of presorbed oxygen (Figure 5b). When this surface was exposed to water vapor the intensity of the 531 eV feature decreases but this is accompanied by a n increase in the 530 eV feature, indicating the conversion of O-(a) to 0 V a ) species via surface hydroxyls (eqs 2-4). We also draw attention to the similarity between the O(1s)profiles for the coadsorption studies a t 120K(Figure 2) and the sequential dosing of dioxygen at the same temperature (Figure la). It seems reasonable to suggest that in the latter case the presence of the reactive 0--like species are responsible for the asymmetry toward high binding energy through the formation of hydroxyl species with ambient hydrogen or water present a t very low concentrations in the ultra-high vacuum system.
Conclusions That the perfect oxide overlayer (NiO) is unreactive to water was first established some time ago and confirmed more recently by further studies by Henrichl'j who was also "surprised that the reduced surface exhibits only very weak dissociative adsorption of water". Henrich,"j however, drew attention to the increased activity for water dissociation if the defective oxide was first exposed to oxygen. He suggested that "some form of adsorbed atomic (12) Hock, M.; Seip, U.; Bassignana, I.; Wagemann, K.; Kuppers, J. Surf Sci. 1986,177,L978. (13) Systematic investigations of the coadsorption of oxygen and ammonia on nickel surfaces carried out by us have yielded analogous results. The uptake of oxygen as well as of ammonia (total concentration of the nitrogenic species) increase with the proportion of oxygen in the 02-NH3 mixtures just as in the case of 02-HzO mixtures. The relative concentration of 0- decreases with the increase in the Oz/NHs ratio and The is accompanied by an increase in the concentration of 02-. dissociation of NH3, giving NH, (n = 1,2) and atomic nitrogen is seen to depend on the concentration of 0- species on the surface. (Kulkarni, G. U.; Rao, C. N. R.; Roberts, M. W. J. Phys. Chem. 1995,99, 3310.)
,;
/ '?!........... \,,. : . . ,
(7)
.I
.....
,
I
525
~
~
. . . . . . . . . . . . . . .. . . . ... ..:. .
.......... . . . . . . . . . . . . . .. . .
. ........ . " .
. I .
I
'
'
530
1
"
r
'
l
'
'
535
'
I " " I " " I "
525
530
535
(eV )
Figure 5. (a)O(1s) spectra ofthe Ni( 110)surface exposed (200 L) to a (1:l)oxygen-water mixture at 220 K and subsequently warmed t o 300 and 370 K. (b) O(ls)spectra for Ni(ll0) exposed (2 L, 400s at 5 x Torr) to oxygen and then water vapor (5000 L) at 300K.
oxygen that is different from the lattice 02-ions" is involved. Henrich's conclusionl'j that surface defects are involved in the process of hydroxylation a t NiO(100) surfaces is analogous to both our earlier low temperature including Ni(210) surfaces and also to those of Freund et a1.l' The present investigation is however the first where dioxygen-water mixtures are coadsorbed a t nickel surfaces and the experiments were designed to explore whether reactive oxygen species, that is different from 02-(see refs 1,2, and 161, were present during the formation of the NiO overlayer. The significance of 0--type species-also referred to e l ~ e w h e r e ' ~ as ~ , ~06--in ~' providing a pathway for the H-abstraction from water has been established. These are present a t low surface coverage and have a characteristic O(ls) binding energy of 531 eV; analogous observations were also reported for the Ni(210)-0 system.'j These species also account for the asymmetry of the O(1s)feature under low flux oxidation conditions, the highly reactive 0--type species scavenging hydrogen or water in the system to form OH(a) or H2O(a). On the other hand, 02--type species inhibit H-abstraction. Analogous chemistry has been observed' for water interaction with Cu(ll1) and Zn(0001) surfaces and also for Habstraction from ammonia a t Ni( 100) and Ni( 110) surf a c e ~ In . ~the ~ case ofthe Cu(110)-0-ammonia system,l* the active Oh- species are suggested to be present either as isolated adatoms or at the end of copper-oxygen chains. This was established through a Monte Carlo simulation of the surface topography of the Cu(ll0)-0 system observed by scanning tunnelling microscopy.15
Acknowledgment. We are grateful to Unilever plc and especially Dr. A. S. Ganguly for support of this project. LA9407790 (14)Carley, A. F.; Davies, P. R.; Roberts, M. W.; Vincent, D. Top. Catal. 1994,I , 35. (15) Jensen, F.; Besenbacher, F.; Laesgaard, E.; Stensgaard,I. Phys. Rev. 1990, B41, 10233. (16)Henrich,V. E. InAdsorption on Ordered Surfaces ofIonic Solids and ThinFilms; FreundH.-J.,Umbach, E.,Eds.; SpringerVerlag: Berlin 1993;p 125. (17) Freund, H.-J., Khulenbeck H.; Neumann, M. In ref 16, p 136. (18)Nobl, C.; Benndorf, C.; Madey, T. E.; Surf Sci. 1986,157, 29. Griffiths, K.; Memmert, U.; Harrington, D. A.; Bushby, Callen, B. W.; S. J.; Norton, P. R.; Surf Sci., 1990,230,159.
