The specificity of surface oxygen in the activation of adsorbed water at metal surfaces

35

Surface Science 135 (1983) 35-51 North-Holland Publishing Company

THE SPECIFICITY OF SURFACE OXYGEN IN THE ACTIVATION ADSORBED WATER AT METAL SURFACES A.F. CARLEY, Depurtment Received

S. RASSIAS

of Chemistry, 8 March

* and M.W. ROBERTS

University College, Cardiff CFI IXL.

1983; accepted

OF

for publication

15 August

UK 1983

The role of surface oxygen in the activation of molecularly adsorbed water by Ni(210) and polycrystalline lead surfaces has been investigated by X-ray photoelectron spectroscopy. Activation has been shown to be dependent on whether the oxygen exists in a coordinatively unsaturated chemisorbed state or whether it is present as the anion of the oxide overlayer. Particular attention has been given to obtaining quantitative estimates of the concentrations of the surface species. The most active surfaces are those which have present oxygen chemisorbed at 77 K. Both the clean metals and an oxide overlayer (orthorhombic PbO) present at a lead surface are by comparison unreactive. Chemisorbed oxygen present at the surface of the PbO overlayer at 77 K interacts with species. At 150 K the molecularly molecularly adsorbed water to give, at 160 K, “ hydroxyl” adsorbed water desorbs and above 160 K dehydroxylation occurs. The Ni(210)-0 (77 K) surface, where the adsorbed oxygen 0 ‘- is the precursor state of surface oxidation, is the most reactive generating on exposure to water vapour a stable oxyhydroxide overlayer. The curve-fitted O(ls) spectrum of the overlayer at 297 K indicates the presence of three species (02-, 02- . H-OH and H,O); the water component is suggested to be intercalated within the surface oxide as in the bulk P-nickel oxyhydroxide structure. Comparisons are made with other metals: Ag(llO), Cu(lll), Pt(ll1) and Zn(0001). The concentration and stability of hydrogen-bonded surface complexes formed at low temperature are suggested to be key-factors in determining differences in the surface chemistry observed.

1. Introduction Activation of a wide range of adsorbates by chemisorbed oxygen is now well established [l-5] with a number of different metals, the evidence being obtained from chemical shifts associated with X-ray photoelectron spectra (XPS) in conjunction with the valence-level spectra (UPS) and also electron energy loss spectroscopy (EELS). In the case of the Cu(lll)-0 and Zn(OOOl)-0 + H,O systems activation is observed in the temperature range 150-170 K [1,2], while in the Ag(llO)-0 + H,O system activation occurs at 80 K [3,6]. Analogous conclusions were also drawn by Fisher and Sexton [7] on the basis of both EELS and UPS data for the Pt(lll)-0 + H,O system, and in all cases it was suggested that surface “ hydroxyls” were generated although * Present

address:

University

of Los Andes,

0039-6028/83/0000-0000/$03.00

Merida,

Venezuela.

0 1983 North-Holland

36

A. F. Carley et al. / Specificity

of sur/ace oxygen

differentiating between hydroxyl and hydrogen-bonded oxygen species is not easy by XPS alone [l]. In this paper we establish that in the activation of molecularly adsorbed water at the surface of a PbO overlayer, it is the weakly chemisorbed oxygen, present only in significant concentration at the surface of the oxide at low temperature, that is solely responsible for the activation. The lattice anions associated with the overlayer are inactive. These observations are compared with data for the Ni(210)-0 plus water system where it is established that chemisorbed oxygen present as the precursor of surface oxidation is the most reactive to molecularly adsorbed water. Where similarities between the two systems are observed they can be related to the incipient reactivity of coordinatively unsaturated chemisorbed oxygen. The particular advantages of obtaining spectroscopic data at very low temperatures (77 K) are highlighted in that the specific activities of particular surface oxygen species can be delineated. The conclusions are relevant to the mechanism of phenomena which may well occur at much higher temperatures and pressures, including selective oxidation catalysis, but under which conditions the species are transient, and, therefore, not observable spectroscopically. 2. Experimental The experiments were performed using a Vacuum Generators photoelectron spectrometer. Lead was deposited onto the stainless steel sample plate by evaporation from a basket wound from 0.3 mm molybdenum wire (Goodfellow Metals) and containing a charge of the spec-pure metal (Johnson Matthey). The lead was melted and degassed thoroughly prior to evaporation, which took place with the sample probe maintained at room temperature. During evaporation the pressure in the preparation chamber increased to = 2 X lo-’ Torr, but fell rapidly thereafter to about 10e9 Torr. The Ni(210) crystal (5N purity) was cut by spark erosion from a large crystal supplied by Metal Research Ltd. The crystal surface was shown by X-ray diffraction to be within f 1” of the correct orientation. It was electrochemically polished in a solution consisting of 360 ml of 2-butoxyethanol, 300 ml of glacial acetic acid and 60 ml of perchlorid acid. The nickel single crystal was made the anode, while a nickel foil was the cathode. A DC regulated power supply capable of delivering 0.3 A at 40 V was used. With fast stirring of the solution and a temperature of between 273 and 278 K polishing was complete in about 60 s. After mounting the crystal in the photoelectron spectrometer the crystal was chemically cleaned (cycles of oxygen of 773 K). Finally a clean and hydrogen at - 10e6 Torr and a temperature surface was generated by prolonged bombardment with argon ions (1 kV, 3.3 PA) at = 650 K. The Ni(210) surface consists of three crystallographically distinguishable nickel atoms; the outermost atoms (C,), second layer atoms (C,) and a third layer where the nickel coordination number is C,,. There are 7.2 X lOi atoms/cm2 in the top (C,) layer.

A. F. Carley et al. / Speci/icity

of surface

oxygen

37

Oxygen gas (BOC 99.99%) was admitted to the analyser chamber from a separately pumped gas handling line via a liquid nitrogen-cooled trap. Water vapour (distilled and deionised) was subjected to several freeze-pump-thaw cycles prior to admission. Data were acquired using an Apple II microcomputer and processed by software developed in this laboratory. Peak areas were computed from the smoothed, background-stripped spectra. Binding energies are referenced to the Pb(4f,,,) peak for the clean metal at 137.0 eV, this being the mean of three published values [8,17], and to the clean Ni(2p,,,) peak at 852.5 eV [9]. Composite spectra could be curve-fitted to gaussian components using a least-squares optimization program. Peak intensities were converted into adatom concentrations using a modified form [lo] of the Madey-Yates-Erickson equation [ll]. We have used this approach extensively and consistently obtained reliable concentration values for a variety of adsorbate-substrate systems. A further convincing demonstration of the validity of the procedure was obtained from a study of the physisorption of xenon on Ni(210) at 77 K. By monitoring the intensity of Xe(3d,,,) signal as a function of xenon pressure, an adsorption isotherm was constructed. In the pressure range 5 X lo-’ to 5 X lop6 Torr the isotherm was flat, and we take this uptake as corresponding to a monolayer of Xe adatoms. The monolayer concentration was calculated from the area ratio of the Xe(3d,,,) and Ni(2pJz) peaks using both the theoretical photoionisation cross-sections of Scofield [12] and the experimental data of Evans et al. [13]; the calculated concentrations were 7.2 X 1014 and 5.6 X lOI cme2 respectively. These values may be compared with the Xe-monolayer concentrations reported for Xe physisorption on a range of metal single crystal surfaces, obtained from LEED data [14]. In general a hexagonal close-packed adlayer was observed, corresponding to monolayer concentrations in the range 5.5 X 1014 to 6.0 X 1014 cmp2. As we have noted previously [15], the experimental cross-section data of Evans et al. [13] often give the more reliable values of surface concentration.

3. Results 3.1. Clean lead metal Fig. 1 shows the O(ls) spectra obtained when a freshly evaporated lead surface was exposed to water vapour at 77 K and subsequently warmed slowly to room temperature. The small peak at 529.5 eV binding energy (fig. la) due to traces of oxygen present on the original surface corresponds to a concentration of = 0.5 X lOi cm-‘, i.e. less than 5% of a monolayer. After exposure (15 L; 1 langmuir = lop6 Torr s) to water vapour at 77 K, a peak at 533.8 eV developed (fig. la), indicative of physisorbed water. The substrate was then

38

Fig. 1. C~(ls) spectra From a “cfean” lead surface exposed to then warmed to 140 K (curve b), 148 K (curve c) and 151 K 10 min to record.

IS L water

vapour

at 77 K (curve a). took about

(curved). Each spectrum

Fig. 2. O(ls) spectra observed when lead was exposed to 300 L oxygen at 297 K (curve a) followed by 15 L water vapour at 77 K (curve b), then warmed 143 K (curve c) and 160 K (CUFVSd).

allowed to warm slowly, and the variation with temperature of the concentration of adsorbed water molecules was monitored (fig. 4a). The concentration decreases rapidly to zero from an initial value of = 55 x lOi4 ~rn‘.~, in the temperature range 140-145 K. If we define the desorption temperature ( 7; f as the temperature at which the surface concentration has decreased to half of its original magnitude, then T,= 142 K. At 151 K the O(ls) spectrum is essentialiy that typical of the “clean” surface (fig. Id). 3.2, Lead surface pm-oxidised at 295 X The exposure of a clean lead surface to oxygen at 295 K (300 L at 2 X 10v6 Torr) results in a narrow O(ls) peak (FWHM = 1.6 eV) centred at 529.4 eV (fig. 2a). Adsorption of water vapour at 77 K results in the o(ls) spectrum

A. F Cdey

39

et 01. / Specijiciy of surface oxygen

shown in fig. 2b. The peak due to physically adsorbed water lies at 533.9 eV (cf. fig. la). The deep “valley” between the two O(ls) peaks (fig. 2b) suggests that there are no surface species present with binding energies intermediate between those of the two distinct components. By subtracting the lower binding energy O(ls) peak (fig. 2a) from the O(ls) spectra observed when the water-exposed surface is warmed slowly to 295 K, the intensity of the Ofls) feature characteristic of the adsorbed water may be obtained and hence the concentration of admolecules u calculated. Fig. 4b shows the behaviour of u with increasing temperature yielding a desorption temperature Td of 142 K, which is identical with the value obtained for the clean surface. The O(ls) profile at 160 K (fig. 2d) is identical with that recorded before exposure to water vapour (cf. fig. 2a). 3.3. Lead surface pre-exposed

lo oxygen at 77 K

In contrast with the observations of the oxidation of lead at 295 K, oxidation at 77 K (150 L, 1 x low6 Torr) generates an obviously asymmetric O(ls) profile with a very distinct shoulder at - 531 eV (fig. 3a). A curve fit analysis of the spectrum, assuming gaussian lineshapes, shows that the minor

1

530 b

e INI

”

‘7

535

b e [eVl

Fig. 3.0(k) spectra for a lead surface exposed to 150 L oxygen at 77 K (curve a) Followed by 15 L water vapour at 77 K (curve b), then warmed slowly to room temperature (curves c-g).

40

er al. / Specifictt_y of surface oxygen

A. F. Carley

component (r) is centred at 530.7 eV, and has a FWHM of 3.1 eV. The adsorption of water vapour at 77 K (fig. 3b) results in a shift of the 530.7 eV peak to 531.0 eV and a narrowing of its FWHM to 1.6 eV, the concentration of the species corresponding to this O(ls) peak being 1.1 X 1014 cm-‘. The binding energy of the peak corresponding to the physisorbed water is 533.6 eV. Warming the surface to 147 K (fig. 3c) leads to an increase of the intensity at about 531 eV and a slight decrease in the intensity of the peak at 533.6 eV. At 150 K the molecular water peak is significantly diminished (fig. 3d) whereas there remains substantial intensity at about 531 eV which is still present when

(10'5

/

5

n-2

IlO'

.. .

d .

H20

d 5

‘/.

~._

.?G_ . .

H20

l‘, l

I

i

3

3

.

2

2

i

1

1

: 100

I,

T IK)

i

T--.1-

--T

100

200

(bl

_

TIK)

~1 200

. ..

,

100

100

TlKl

T(K)

200

300

200

300

Fig. 4. Variation of surface concentration a of the various oxygen containing species with temperature T: (a) Pb-H,O (77 K) (cf. fig. 1); (b) Pb-0 (297 K)-H,O (77 K) (cf. fig. 2); (c), (d) Pb-0 (77 K)-H,O (77 K) (cf. fig. 3). The assignment of the various species is discussed in the text. The time taken for the sample to warm from 100 to 160 K was about 80 min.

A. F. Carfey et al. / ~pee~~ieir?~ uf surfueeoxygen

41

the molecular species has desorbed completely at 160 K (fig. 3e). Increasing the temperature further leads to a decrease in the 531 eV feature (fig. 3f), although there remains an obvious shoulder even at 297 K (fig. 3g). The variation of or with temperature for each of the three O(ls) components is plotted in figs. 4c and 4d. The value of Tdfor the molecularly adsorbed water is 150 K, which is significantly higher than in the two cases considered previously. The surface concentration-temperature plot calculated for the 531.0 eV peak exhibits a maximum in the value of cr at = 160 K which is twice its initial value at 77 K. At 295 K, u has decreased to 0.8 x 1014cm-‘, which is somewhat lower than its value at 77 K. The intensity of the 529.4 eV component is invariant in the temperature range 77-150 K, increases slightly in the range 150-200 K and is constant thereafter (fig. 4d). In fig. 5 we compile O(ls) difference spectra corresponding to the raw data of fig. 3. These are computed by subtracting out the contribution of the surface oxide component at 529.4 eV (using an “oxide O(ls) spectrum” obtained after exposure of the clean metal to oxygen at 297 K, when the high binding energy shoulder is absent) from the spectra shown in fig. 3. The difference spectra have been subjected to 13 point quadratic smoothing to improve the signal-tonoise ratio. These data should illustrate more clearly the spectral changes resulting from any interaction between the physically adsorbed water and the surface oxygen species responsible for the O(ls) shoulder marked with an arrow in fig. 3a. In view of our previous studies we assign the shifted O(ls) peaks as follows: O’- (oxide) species, 529.5 eV; H,O(a), 533.6 eV; O’-(a) and OH(a), 531.0 eV.

I

5250

5300

5350 E,,(eV)

Fig. 5. O(ls) difference

spectra

for some of the data of fig. 3 (see text)

42

It should be also emphasised, as we have done previously [l], that strong as may well occur at low hydrogen bonding interaction, O”‘- - . . H-OH, temperature between molecularly adsorbed water and surface oxygen species, cannot be distinguished from OH species by XPS alone.

When the clean Ni(210) surface is exposed to water vapour at 77 K and the adlayer warmed to room temperature the surface “oxygen” concentration, as reflected by the total O(ls) intensity, varies as in fig. 6a. During desorption, when the O(ls) component at - 533 eV due to H,O(a) decreases rapidly above 140 K, a second component emerges with an O(ls) binding energy of 531.5 eV, The latter we attribute to the generation of OH(a) species and the relative proportions of these two species, H,O(a) and OH(a), are shown in fig. 6b. A surface concentration of approximately 0.2 X lOI cm-’ of each is present at 297 K. In contrast to atomically clean lead the clean Ni(210) surface shows some, albeit small, reactivity to water vapour resulting in its dissociation.

T LKI

T IK)

:~~~~o ll,.::i alto TIKI

T(Kj

Fig. 6. Variation of oxygen surface concentration u with temperature for the Ni(ZlO)-H,O ((a) and (b)) and for the Ni(210)-0 (297 K)-Hz0 (77 K) ((c) and (d)). An explanation assignment of the various species is given in the text.

(‘77 K) of the

A. I? Carley et al. / Specificity of surJace oxygen

Id)

lb)

530

560

535

b e IeV) Fig. 7. O(k) spectra observed followed by 10 L water vapour

d

for a Ni(210) surface exposed to 10 L oxygen at 77 K (curve a) at 77 K (curve b), then warmed to room temperature (curves c-e).

30

uo'5m21 20 (RI

10 100

200

300

T IKI

d

3.0

(b)

110'5m-2) 20 10

Fig. 8. Variation of oxygen surface concentration o with temperature for the different species observed in the Ni(ZlO)-0 (77 K)-H,O (77 K) system: (a) total oxygen concentration; (b) concentrations of the different species identified (see text).

44

A. F. Carley

et al. / Speclficify

3.5. Ni(210) surface pre-exposed

oJ surface

oxygen

to oxygen at 297 K

After exposing the Ni(210) surface to oxygen at 297 K the crystal was cooled to 77 K and exposed to water vapour. The adlayer was then warmed to 297 K and the O(ls) intensity monitored. Fig. 6c shows the variation of total surface oxygen concentration as a function of temperature, while fig. 6d shows the concentrations of the individual species, chemisorbed oxygen, surface hydroxyl species and molecularly adsorbed water. The assignments of the latter were deduced from the shifted O(ls) components at 530, 531.5 and 533 eV respectively. There is an obvious maximum in the surface hydroxyl concentration at a temperature of about 180 K. Clearly Ni(210)-0 (297 K) shows some analogous features to that of the

I’\

_Jj,: I

525

“,

_..“..‘,....?L-._ .

I.,

530

.

1,

535

‘,

(al

: *

be IeVl _I!&..I’..““““.. .. .._ 530

. ... ,,. ‘.,.... 535

be teV1

Fig. 9. Examples curve-fits for the O(ls) spectra: (a) Ni(210)-0 (297 K)-H,O (77 K); spectrum recorded during warming, T= 219 K. (b), (c) Ni(210)-0 (77 K)-H,O (77 K); spectra recorded during warming, T = 136 K (b) and T = 293 K (c) of figs. 7c and 7~. (d) fi-NiO’OH; spectrum recorded at 297 K from ref. [21]).

A. F. Carley et al. / Specificity oj sur/ace oxygen

45

clean Ni(210) surface. A point, however, to note is that hydroxyl formation is observed at a lower temperature ( - 138 K) than with the clean Ni(210) surface (- 155 K), and reflects the involvement of the chemisorbed oxygen. 3.6. Ni(210) surface pre-exposed

to oxygen at 77 K

Fig. 7 shows the O(ls) spectra observed when a Ni(210) surface was exposed to oxygen at 77 K and then to water vapour at the same temperature. Spectrum 7a is clearly composite and a 2-peak curve-fit indicates a broad high binding energy feature corresponding to an adatom concentration of 0.75 X 1015 cm *. The intensity of this component increases on adsorption of water vapour at 77 K (fig. 7b), and we conclude that substantial hydroxylation or hydrogen bonding occurs even at 77 K. The subsequent evolution of the O(ls) profile as the temperature is raised is shown in figs. 7c, 7d and 7e. There are two important points to note: the large FWHM of the O(ls) peak generated at 77 K, and the relative stability of the species responsible for the O(ls) peak in the temperature range 77 to 293 K. Fig. 8a shows the variation with temperature of the total oxygen uptake calculated from the integrated O(ls) intensity data. Fig. 8b shows the concentrations of the individual species O(a), “OH(a)” and H,O(a), calculated from the curve-fitted O(ls) profiles in the temperature range 77 to 297 K. Typical curve-fitted O(ls) spectra are shown in fig. 9. It is obvious, even from a comparison on figs. 6, 7 and 8, that the surface pre-exposed to oxygen at 77 K is the most reactive to water vapour, most of the “hydroxyl” species generated at low temperature still being present at 297 K, together with a significant concentration of adsorbed “water molecules”.

4. Discussion 4.1. Clean lead surface and lead surface pre-exposed In both these cases the interaction consists simply of the physisorption of rapid desorption which is characterised assume that the desorption is complete we may apply the Frenkel equation in AH=

to oxygen at 297 K

between the surface and water vapour water molecules at 77 K followed by a by a T, temperature of 142 K. If we within t - 15 min at T, = 142 K, then the form:

RT, log,(t/t,),

with t, = lo-l3 s and estimate the heat of adsorption AH of molecularly adsorbed water to be - 40 kJ mol-‘. It should be noted that AH is not very sensitive to the value of t which is only known approximately (15 + 2 min). The fact that the surface oxidised at 297 K exhibits an identical T, value to

46

A. F. Cathy

et al. / Specrficrty of surface oxygen

that observed with the clean surface suggests that there is no significant interaction, such as hydrogen-bonding, between the physisorbed water molecules and the oxide anions. We shall see later that an increase in AH of only 2-3 kJ mol-’ would be reflected in a detectable increase in the value of Td. This observation is not entirely unexpected since work function data [16] suggest that penetration of the lead surface by oxygen adatoms occurs readily; furthermore there is good evidence from LEED and X-ray diffraction data for the surface oxide formed being orthorhombic PbO, both on single crystal and polycrystalline surfaces [17,18]. The O(ls) peak at 529.5 eV binding energy is thus characteristic of the oxide, which develops by an island growth or nucleation mechanism [17,19], the 02- ions lying below the plane of the surface lead atoms. 4.2. Lead surface pre-exposed

to oxygen at 77 K

The O(ls) peak at 530.7 eV observed after the exposure of the clean surface to oxygen at 77 K (fig. 3a), but not present after oxidation at 297 K, we assign to oxygen species chemisorbed at the surface of the oxide. The major O(ls) component, due to PbO, is at a binding energy of 529.5 eV (fig. 3a). The nature of the chemisorbed oxygen species cannot be unequivocally identified from the O(ls) data, but the large value of the FWHM of the 530.7 eV peak (see also fig. 5a) and its sensitivity to subsequent water adsorption (see later discussion) suggest that this feature consists of Osp(a) species with a range of surface dipoles giving rise to the “flat-top” to the O(ls) profile (fig. 5a). This peak sharpens as a result of hydrogen-bonding on adsorption of water (fig. Sb). Similar data were reported for the Cu(lll)-0 + H,O system [l] but at a somewhat higher temperature (- 150 K). We also exposed a lead surface oxidised at 297 K to oxygen at 77 K and investigated the reactivity of this surface to water vapour. The behaviour was identical to that of the surface oxidised entirely at 77 K, confirming that the high binding energy O(ls) peak was due to oxygen species chemisorbed on the oxide and further that it was this species that was solely responsible for the reactivity of the surface towards water vapour. Evidence for the presence of oxygen adatoms is also provided by work function data [16]. The work function decrease (0.3 eV) observed upon exposure to oxygen at 77 K was interpreted as oxygen adatom penetration below the lead surface during oxide growth. However, on warming to room temperature the work function decreases further to a value 0.6 eV below the clean surface, suggesting that at 77 K there are present chemisorbed oxygen adatoms with negative dipoles pointing away from the surface. These are only weakly chemisorbed and absent at 297 K. The intensity of the O(ls) peak at 531.0 eV increases steadily as we approach r, (fig. 4c) reaching a maximum value at - 160 K which corresponds

A. F. Carley et al. / Specificity

of surface oxygen

47

to twice the initial concentration of chemisorbed oxygen. The most obvious explanation of this observation is hydroxylation, the small FWHM value of the H,O(a)

+ O’-(a)

+ OS-

. . . H-OH

-+ OH”-(a)

+ OH’-(a),

531.0 eV peak suggesting that the two oxygen atoms in the hydrogen-bonded complex are equivalent so that we may regard the adlayer as consisting of incipient surface hydroxyl groups OH(ad). As a general rule we observe that the maximum in the hydroxyl concentration-temperature plot occurs at or close to the Td value for molecularly adsorbed water, e.g. see fig. 4c. As the desorption temperature is approached the adsorbed water molecules will become mobile leading to the possibility of a rearrangement within the adlayer. This rearrangement we associate with a change from weak hydrogen-bonding involving water clusters [20] to strong The “excess” water hydrogen-bonding, that is to O”- . . . H-OH formation. molecules, desorb at a higher temperature (150 K) than they do from the oxide surface (142 K), the magnitude of the increase in Td suggesting that the increase in AH (i.e. over and above the heat of adsorption of physically adsorbed water) is about 3 kJ mall’. As the temperature is raised to 297 K, dehydroxylation, accompanied by an increase in the oxide O(ls) peak at 529.5 eV (fig. 4d), occurs. At 297 K the concentration of “hydroxyls” remaining on the surface is close to the initial concentration of the weakly chemisorbed oxygen present at 77 K (fig. 4~). 4.3. Ni(210) surface pre-oxidised

at 297 K

For low exposures of oxygen the adlayer present at a nickel surface at 297 K can be regarded as consisting of chemisorbed oxygen adatoms. Following the adsorption of water vapour at 77 K and subsequent heating to - 180 K nearly all of the oxygen species (fig. 6b) are hydroxylated; they are, however, regenerated on warming to 297 K. In both this and the lead system there is present at 297 K an adlayer containing almost as many hydroxyls as there were chemisorbed oxygen adatoms initially present. Thus the chemisorbed oxygen present at a Ni(210) surface at 297 K behaves just like the weakly chemisorbed oxygen species present on a lead surface oxidised at 77 K. Clearly, what is common is the presence of easily accessible, negatively charged oxygen species, and it is these that interact with adsorbed water molecules. 4.4. Ni(210)

surface pre-oxidised

at 77 K

The most significant aspect of the interaction of water with nickel is the high activity of the chemisorbed oxygen formed at 77 K, both the raw and the curve-fitted data (figs. 7 and 9) indicating appreciable “hydroxyl” formation

48

A. F. Carley et al. / Speclficlty

of surJme ox.vgen

even at 77 K. These species are in the main stable up to 300 K, but with a maximum concentration at about 190 K (fig. 8). The molecular water component of the adlayer decreases slowly in the temperature range 160-220 K (in contrast to the other two nickel surfaces) and at 297 K the following concentrations are present: H,O(a), 0.4 X 1015 cm-2; OH(a), 1.2 x 1015 cmm2; and O’-(a), 0.8 X 1015 cm- 2. Clearly the unusual feature of this model is the apparent stability of molecularly adsorbed water at room temperature. However, recent work in this laboratory [21] has shown that the O(ls) spectrum of /3-NiO . OH exhibits a broad O(ls) peak with a binding energy of about 531 eV (fig. 9d). This peak has been shown to have three components assigned as oxide (02-), hydroxide and water, the water being intercalated within the nickel oxide lattice. Interaction with 02- species through hydrogen bonding results in the near equivalence of the oxygen species observed in the O(ls) spectrum. We, therefore, suggest that with the Ni(210)-0 (77 K) surface an overlayer of an oxyhydroxide is formed on warming to 297 K, the active species in its formation being the chemisorbed oxygen (see scheme below) where O”- is a dissociative state of oxygen which at 77 K we regard as oxygen adatoms that have not developed the ionicity associated with the Ni’+-02bond; above 77 K (e.g. 297 K) transformation has occurred as indicated by work function and photoelectron energy distribution data [22,23]; O,(g)

+ O*-(a)

+ O’-(a),

(1)

O’-(a)

+ 02-

(oxide),

(2)

O’-(a)

+ H,O(a)

O’-(a).

+ Ofi- . . . H-O-H

. . H-OH(a)

+ OH(a)

+ OH(a)

(H-bonded

water),

(3)

(hydroxyls).

(4)

There is evidence for oxygen adsorption (8 < 0.25) * on polycrystalline nickel at 297 K and even higher temperature ( - 400 K) to be in a stable chemisorbed state, usually considered to be 02-(a). Only for values of 6 > 0.25 at 297 K is there evidence for the formation of oxide. The evidence for this conclusion is long-standing coming from photoemission [22] and work-function data [23]; more recently it has been substantiated by XPS (O(ls) and Ni(2p,,,) data studies [24]). At 77 K oxygen is in the chemisorbed state (eq. (1)) although there has been some discussion as to whether more than one type of species (including a molecular species) exists on nickel at this temperature [23,25]. At high coverage (0 = 1.0) the oxygen species adsorbed at 77 K are incorporated easily (eq. (2)) into the sub-surface on warming the adlayer to 297 K (see above scheme). As with lead, it is the presence of O”- species that activates the O-H bonds in the molecularly adsorbed water. There is, of course, good experimental evidence for chemisorbed oxygen present at 77 K, having appreciable * 0 defined with reference to monolayer

determined

by physical

adsorption

[23].

A. F. Carley et al. / Specificiry of surface oxygen

49

ionicity [23] so that its participation in hydrogen bonding is to be expected [4b]. For polycrystalline nickel the surface potential (A+) associated with the chemisorbed oxygen adlayer at 77 K is - 1.6 eV [23].

5. General comments It is evident from this work that the oxygen species most active in water activation are those which can be considered to be in a chemisorbed state, coordinatively unsaturated and accessible to adsorbed water molecules. In the case of an overlayer of orthorhombic PbO, which itself is inactive, these oxygen species are weakly adsorbed, and only present in significant concentration at the surface at low temperature. Benndorf et al. [26] have shown recently that oxygen preadsorbed on Ni(ll0) activates molecularly adsorbed water in the temperature range of 150-300 K, and suggested that a hydrogen bonded Os-(a) . . . H-OH adlayer is formed. This is similar to earlier views [1,4] on the activation of adsorbates by O(a); these authors [26], however, rule out the formation of surface hydroxyls at 297 K on the grounds that there are at least two different oxygen species with O(ls) binding energy values of 530.8 and 532.8 eV. No information on the concentrations of these two species are reported by Benndorf et al. [26]. Although there are obvious similarities with our present data (fig. 9) we have proposed the formation of a surface structure analogous to P-nickel oxyhydroxide, the total concentration of surface species present at the Ni(210) surface at 297 K (fig. 8) being nearly four times the xenon monolayer. Bulk nickel oxyhydroxide thermally decomposes at > 400 K to generate the oxide; during decomposition all three species can be delineated [21] in the carefuly fitted O(ls) profile. The O(ls) components are at 529.5, 531.0 and 532.5 eV with characteristic FWHM values; these are identical to the O(ls) peaks observed in the Ni(210)-0 (77 K) + water system. There are obvious similarities between our data and the observations of Norton et al. [27] who concluded that oxide overlayers present on nickel surfaces react slowly with water vapour at room temperature “to give a stable chemisorbed entity”. During exposure to water vapour over a period of 14 days Norton et al. reported that the O(ls) spectrum which initially had a peak, characteristic of the oxide, at a binding energy of 529.7 eV, developed a second peak at 531 eV. With further exposure to water vapour the two O(ls) peaks merged to give a single peak at 531 eV with two distinct shoulders at 529.7 and 532 eV. This general behaviour (fig. 10 of ref. [27]) has a strong resemblance to our observations with Ni(210)-0 (77 K) at low temperature (fig. 9) and the spectrum after 14 days exposure to water vapour at 295 K [27] is virtually identical with the O(ls) spectrum of /3-NiO . OH (fig. 9). The particularly interesting aspect of the Ni(210)-0 (77 K) surface is the high reactivity of the chemisorbed oxygen resulting in the formation of the

50

A. F. Carley et al. / Specifrclty of surface oxygen

stable oxyhydroxide overlayer. If we refer to the reaction scheme outlined above then it is obvious that it is the relative rates of oxygen incorporation (oxidation), hydrogen-bond formation, and hydroxylation and also the stability of the hydrogen-bonded complex that determine whether or not the oxyhydroxide is likely to form. Our data with Ni(210)-0 (77 K) suggest that these processes (eqs. (1) (2) and (3)) occur readily at temperatures below about 150 K, the temperature at which molecularly adsorbed water would be expected to desorb. There are obvious analogies with the Ag(llO)-0 surface at 77 K [3,6] where chemisorbed oxygen interacts with molecularly adsorbed water, but in contrast to Ni(210)-O(77 K) the hydrogen-bonded complex on silver is unstable and decomposes above 200 K. It may also be significant that the Ni(210) surface has an open-structure which is “rough” on the atomic scale. Certainly with the close packed Cu(lll)-0 surface the hydrogen-bonded complex is only stable [l] over a comparatively small range of temperature, 150-170 K, with evidence for only a small surface hydroxy concentration present at 297 K. The subtleties of metal-water interaction and the significance of surface activators (such as chemisorbed oxygen) have become, during the last three or four years, more obvious. The data have wide implications not only for surface chemistry and catalysis but also for developing detailed models of the chemistry of electrode surfaces.

Acknowledgement We are grateful

to the Science Research

Council

for its support

of this work.

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[ll] T.E. Madey, J.T. Yates and N.E. Erickson, Surface Sci. 43 (1974) 526. [12] J.H. Scofield, J. Electron Spectrosc. Related Phenomena 8 (1976) 129. [13] S. Evans, R.G. Pritchard and J.M. Thomas. J. Electron Spectrosc. Related Phenomena 14 (1978) 345. 1141 M.A. Chesters and J. Pritchard, Surface Sci. 29 (1971) 460; J. Kuppers, F. Nitschke, K. Wandelt and G. Ertl, Surface Sci. 87 (1979). 295; R.H. Roberts and J. Pritchard, Surface Sci. 54 (1976)687. (151 A.F. Carley, S. Rassias and M.W. Roberts, J. Chem. Sot. Res. 5 (1979) 208. [16] J.M. Saleh, B.R. Wells and M.W. Roberts, Trans. Faraday Sot. 60 (1964) 1865. [17] R.W. Joyner, K. Kishi and M.W. Roberts, Proc. Roy. Sot. (London) A358 (1977) 223. 118) J.R. Anderson and V.B. Tare, J. Phys. Chem. 68 (1964) 1482; J.M. Eldridge and D.W. Dong, Surface Sci. 40 (1973) 512. [19] D. Chadwick and A.B. Christie, J. Chem. Sot. Faraday II, 76 (1980) 267. [20] K. Kretzschmar, J.K. Sass, A.M. Bradshaw and S. Holloway, Surface Sci. 115 (1982) 183. [21] L.M. Moroney, R. St.C. Smart and M.W. Roberts, J. Chem. Sot. Faraday I, to be published. [22] C.M. Quinn and M.W. Roberts, Trans. Faraday Sot. 61 (1965) 1775. [23] C.M. Quinn and M.W. Roberts, Trans. Faraday Sot. 60 (1964) 899; M.W. Roberts and B.R. Wells, Disc. Faraday Sot. 41 (1966) 162. 1241 Reviewed by K. Wandelt, Surface Sci. Rept. 2 (1982) 1. [25] A.M. Horgan and D.A. King, Surface Sci. 23 (1970) 259. [26] C. Benndorf, C. Nob1 and F. Thieme, Surface Sci. 121 (1982) 249. [27] P.R. Norton, R.L. Tapping and J.W. Goodale, Surface Sci. 65 (1977) 13.