3006
Langmuir 1991, 7, 3006-3011
Adsorption of H20 on Alkali-Metal-Covered Pt( 111) and Ru(001): A Systematic Comparison H. P. Bonzel,* G. Pirug, and C. Ritke Institut fur Grenzflachenforschung und Vakuumphysik, Forschungszentrum Julich, Postfach 1913, W-5170 Jiilich, Germany Received January 30, 1991.I n Final Form: June 7, 1991 The adsorption of H20 on Na-covered Pt(ll1) was studied at 110 and 320 K by recording changes in work function, A4. At a Na coverage below 0.10 the adsorption was molecular and fully reversible. A comparisonof work function changes due to HzO on Na/Pt(lll) and K/Pt(lll) shows general similarities, with large dipole moments of HzO, opposite in sign to HzO adsorbed on clean Pt(ll1). A further comparison of A 4 versus HzO coverage for alkali-coveredPt(ll1) and Ru(001) surfaces illustrates the alkali-metaland substrate specificity of HzO/alkali coadsorption. HzO dipole moments derived from A$ versus coverage are interpreted in terms of HzO reorientation compared to the adsorbed HzO state on the clean surface. These dipole momenta indicate increasing tilt of the HzO molecule from Na to K to Cs for Ru(001). Also, the amount of tilt is stronger, the weaker the HzO substrate interaction (for a particular alkali). A higher degree of alkali-inducedHzO reorientation generally correlates with a more difficult dissociation of HzO.
1. Introduction Despite the wealth of experimental data on alkali-metal/ molecule coadsorption on transition metals1p2little work has been done to elucidate the specific influence of the alkali metal or the substrate metal itself. This alkalimetal or substrate specificity can be studied by coadsorbing the same molecule with different alkali metals on the same substrate and with the same alkali metal on different substrates. A convenient moleculefor this purpose is H20,3 and work in this direction has been published for numerous different systems covering the alkali metals Li, Na, K, and Cs as promoters and a variety of transition-metal substrate^.^-'^ On the other hand, many of these investigations report different characteristic properties of these coadsorbates, and a systematic comparison is difficult. To facilitate such a comparison, we focus in this paper on (1)Bonzel H.P. Surf. Sci. Rep. 1987, 8, 43. (2)For references to previous work Bonzel, H. P., Bradshaw, A. M.; Ertl, G., Eds. Physics and Chemistry of Alkali Metal Adsorption; Materials Science Monograph; Elsevier: Amsterdam, 1989; Vol. 57. (3)Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 17,211. (4)Bange, K.; Grider, D.; Sass, J. K. Surf. Sci. 1983, 126, 437. (5)Doering, D. L.; Semancik, S.; Madey, T. E. Surf. Sci. 1983,133,49. (6) Thiel, P. A.; Hrbek, J.; de Paola, R. A.; Hoffmann, F. M. Chem. Phys. Lett. 1984, 108, 25. (7)Sass, J. K.; Bange, K.; Dohl, R.; Piltz, E.; Unwin, R. Ber. BunsenGes. Phys. Chem. 1984,88, 354. (8)Bonzel, H.P.;Pirug, G.; Winkler, A. Chem. Phys. Lett. 1985,116, 133. (9) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1985, 150, 319. (10)Lindgren, S. A.; Paul, J.; Walldbn, L. Surf. Sci. 1985, 155, 165. (11)Paul, J. Surf. Sci. 1985, 160, 599. (12)Semancik, S.;Doering, D. L.; Madey, T. E. J. Vac. Sci. Technol. 1985, A3, 1571;Surf. Sci. 1986, 176, 165. (13)Stuve, E. M.;Bange, K.; Sass, J. K. In Trends in Interfacial Electrochemistry; NATO AS1 Series; Fide Silva, A., Ed.; Reidel: Dordrecht, The Netherlands, 1986;p 255. Stuve, E. M.; Dohl-Oelze,R.; Bange, K.; Sass, J. K. J. Vac. Sci. Technol. 1986, A4, 1307. (14)Bonzel, H.P.; Pirug, G.; Miiller, J. E. Phys. Reu. Lett. 1987,58, 2138. (15)Sass, J. K.; Bange, K. In Electrochemical Surface Science; Soriage, M. p., Ed.; ACS SymposiumSeries 378;American Chemical Society: Washington DC, 1988;Chapter 4,p 54. Note: These authors correlate in their Figure 6 negative dipole moments with decreases in work function, oooosite .. to the convention of ea 1. (16)Clendening, W. il;Rodrig&, J. A.; Campbell, J. M.; Campbell, C. T. Surf. Sci. 1989, 216, 429. (17)Lackey, D.; Schott, J.; Straehler, B.; Sass, J. K. J. Chem. Phys. 1989, 91, 1365;Surf. Sci., in press. Sass, J. K.; Schott, J.; Lackey, D. J. Electroanal. Chem. 1990, 283, 441. (18)Blass, P. M.; Zhou, X. L.; White, J. M. J . Phys. Chem. 1990, 94, 3054. (19)Pirug, G.;Ritke, C.; Bonzel, H. P. Surf. Sci., in press.
0743-7463/91/2407-3006$02.50/0
properties that reflect the orientation of the adsorbed HzO molecules at low coverages of the alkali metal. Those properties are either angular distributions of emitted ions (electron-stimulated emission, ESDIAD5J2)or work function changes as a function of H2O c ~ v e r a g e . , ~ J ~ - ~ ~ J ~ J ~ The present paper will therefore primarily deal with the question of reorientation of H20 moleculescoadsorbed with alkali metals. The underlying idea is the dipole/ dipole interaction between the adsorbed alkali and HzO species, as described by effective medium theory20or by an ab initio calculation;14the stronger the dipole moment of the adsorbed alkali, the more tilting of the coadsorbed H2O molecule is expected t o o c ~ u r A. systematic ~ ~ ~ ~ ~ survey of published data together with new measurements for Na/HzO and K/HzO on Pt(ll1) will essentially confirm this expectation. In addition we shall address the general question of alkali-metal-induced H2O dissociation. We suggest that the local electric field in the vicinity of the alkali species may reach a value sufficient for HzO to dissociate. This critical field strength is then supposed to correlate with the critical alkali coverage found for HzO dissociation. We also discuss whether the tilting of HzO molecules due to alkali metal has any effect on HzO dissociation,3 and the formation of OH versus atomic oxygen.lg It turns out that dissociation is more substrate specific than alkali-metal specific. Alkali-metal-induced tilting of H20 may counteract the tendency for H2O dissociation. 2. Experimental Section The technical facilities and experimentalprocedures used for obtaining the experimentalresults of this paper were described in detail in previous publications.19~21 3. Alkali-Metal/HzO Coadsorption on Pt( 111)
In this section we report new data on the coadsorption of H20/Na and H20/K on Pt(ll1). The latter are related to a previous study of this system.QJ4Figure 1shows the change in work function versus Na coverage compared to a previous measurement for K22although the K coverage ~~
~~~
(20)Nsrskov, J. K.; Holloway, S.; Lang, N. D. J. Vac. Sci. Technol. 1985, A3, 1668. (21)Pirug, G.; Ritke, C.; Bonzel, H. P. Surf. Sci. 1991, 241, 289. (22)Kiskinova, M.;Pirug, G.; Bonzel, H. P. Surf. Sci. 1983, 133, 321.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 12, 1991 3007
Alkali-Metal-Covered P t ( l l 1 ) and Ru(001) H20 Adsorption I
OL o 0
P t l l l l ) +Na Ptlllll + K
I
1L
I
Pt 11111+ No + H20, T=320K
i
e
QNa=o.220
1 -5 "[
Pt1111l+K+H~O.T=320K
1
er = o 141
eK= o 129 QK
02
A'-
01
0'2
03
04
05
016
COVERAGE 0
Figure 1. Work function change due to adsorbed Na or K on
Pt(111) versus coverage.
scale has been corrected in the meantime.23 The work function change was determined from the low-energyedge of a UPS spectrum. The Na coverage was evaluated via LEED, which showed a 2 X 2 pattern for B = 0.25 and a ( 3 V X (3)lI2 R30° pattern for B = 0.33. The relative coverages of Na were measured by Na(1s) XPS intensity. Both functions of A$ versus coverage exhibit the decrease and minimum typical for alkali metahZ4 The initial dipole moments were determined according to
where n is the alkali coverage (cm-2), e the elementary charge, and p the dipole moment (D) including the image charge c o n t r i b u t i ~ n These . ~ ~ dipole moments are 7.2 and 15.0 D for Na and K, respectively. The value for K is smaller compared to the one previously reportedz2because of the coverage recalibration. Previous investigations of HzO/K coadsorption on Pt(111)have shown9J4that molecular HzO as well as HzO dissociation is observed depending on the K coverage. The distinction of the chemical state has been made directly by photoemission spectroscopy (UPS and XPS) and indirectly also by a combination of work function measurements and a n n e a l i ~ .A~ critical coverage of K was J~ determined below which H20 did not d i s s o ~ i a t e . ~In this coverage range a work function increase due to adsorbed H2O was interpreted in terms of a reoriented HzO mole~ule.'~Above the critical coverage UPS and XPS showed the formation of OH. No evidence of atomic oxygen was seen in this case. In relation to this study, work function changes due to HzO adsorption were recorded at 320 and 110 K for Na- and K-precovered Pt(111). Whereas the low-temperature data shall serve as a source of information with regard to the HzO orientation, the high-temperature data yield critical alkali coverages for HzO dissociation. The latter are summarized in Figure 2 for various alkali coverages. In all cases there is a quick increase in work function followed by a plateaulike region a t higher H20 exposures. The magnitude of work function change increases with rising alkali coverage. A plot of the near-steady-state A$ values versus alkali coverage is shown in Figure 3. The dependence is approximately linear. Extrapolation to A$ = 0 yields critical alkali coverages below which no or only a very small change in work function is observed. These critical coverages are 0.09 and 0.10 for K and Na, respectively. In another set of experiments we measured the intensity of the O(1s) photoelectron peak (23)Pirug, G.; Bonzel, H.P. Surf. Sci. 1988,194, 159. (24) Aruga, T.;Murata, Y. B o g . Surf. Sci. 1989, 31, 61.
= 0105
2
1
H20 EXPOSURE i10-6mbar SI
mbar s I
H20 EXPOSURE
Figure 2. Work function changes due to adsorbed HzO on Naor K-covered Pt(ll1) at 320 K: (a) for several Na coverages; (b) for several K coverages. I
14,
t
F'tl111I + AM+ HO , T = 320 K I
* u
I
/
1
A
,
0
1(Q' 02
01
03
QA,
Figure 3. Plot of work function changes after 1 X lo4 mbar of HzO exposure at 320 K on Na- and K-covered Pt(ll1) versus
alkali-metal coverage.
Pt 11111 + Na + Hfl T=3lOK l r l O ~ ' R ~ Hs,O
-v, k 2
/
:
-> Ew t-z -0 k
P
I
Y)
r
~
o / o / d o
,
,
,
after exposure of the Na-covered Pt(ll1) to HzO at 310 K. These data are presented in Figure 4. Extrapolation to zero O(1s) intensity yields a critical Na coverage for HzO dissociationof 0.10, in excellent agreement with Figure 3. Similar data had been previously collected for K/Pt(lll).14In that case we had determined the critical K coverage as 0.10. The low-temperature A$.J behavior for HzO on Nacovered Pt(ll1) is summarized in Figure 5 for a number of Na coverages that are mostly below the critical coverage of 0.10. Almost every Na coverage causes a different
.
Bonze1 et al.
3008 Langmuir, Vol. 7, No. 12, 1991 20,
1
16
-0L
I
I
-0.Eo
2.L I
I
Pti111i+No+H20 T=llOK
~
//
\,
1
"'
0.1E3
--z
t
:
HzO .e/------
,/ ,,.....
"
oo65
1.6-
0 012
- 0 3 L H20 EXPOSURE 110e6mbar SI
2
i
2.0.
1
Pt 11111 t K T="°K
5
Figure 5. Work function changes due to H20 on Na/Pt(lll) at 110 K for several Na coverages. functional behavior of Aq5 versus Hz0 coverage. At the very lowest Na coverage Aq5 first decreases, then increases, goes through a maximum, and decreases again. There is some systematic behavior in that the maximum increases in height and moves to larger HzO exposures with increasing dNa. At higher &a there is no initial decrease in Aq5 but a steady increase with possibly different slopes until a broad maximum is reached. It is interesting to note that the maximum slope dq5/dn (n = Hz0 coverage) is similar for all ONa I0.10. Figure 5 also illustrates that once the critical ONa is surpassed, the initial Aq5 is much stronger than for the lower Na coverages. This feature must be due to the immediate formation of OH, even at 110 KS9 Figure 6 shows a corresponding plot for HzO/K on Pt(111) at 110 K. This is a series of new measuremenb utilizing the onset of secondary electron emission in UPS spectraz1rather than the retarding field technique.l* Thus the Aq5 data for HZO/Na and HzO/K on Pt(ll1) are obtained under identical experimental conditions. In contrast to the case of Na in Figure 5, the initial changes in work function for HzO/K are always positive and with similar slope. The maxima in Aq5 and subsequent decreases are systematically related to the K coverage, as has been pointed out and evaluated previ0us1y.l~ The meaning of these Aq5 versus Hz0 exposure curves is that there is an attractive K/HzO interaction such that K-affected sites are occupied first, followed by more distant sites, which lead to a Aq5 decrease.14 The decrease is found also for the clean Pt(ll1) surface as seen in Figures 5 and 6. Note in Figure 6 that the initial rise in Aq5 versus Hz0 coverage is slightly larger for OK I 0.10. This is presumably again because of H2O dissociation into OH9 although the effect is by no means as clear as for Na in Figure 5. 4. Comparison for Pt(ll1) and Ru(001) Data similar to those presented in section 3 have recently been obtained for Hz0 adsorption on Na-, K-, or Cs-covered Ru(OOl).19 Without wishing to repeat the results of that investigation we would like to compare them to equivalent data obtained for Pt(ll1). Hence we focus on A 4 versus
H20 EXPOSURE 110-6mbor.s I
Figure 6. Work function changes due to H20 on K/Pt(lll) at 110 K for several K coverages. Ru(0011+ K + HzO
I
0 HO ,
1 2 3 EXPOSURE (10-6mbar s i
Figure 7. Work function changes due to H20 on K/Ru(001) at 120 K for several K coverages.19
HzO exposure measurements at 120 K for these metal systems. In the three cases studied with Ru(001) the response of the work function change to adsorbed HzO in the presence of a preadsorbed alkali (Na, K, or Cs) was weaker than seen for Na or K on Pt(ll1) in Figures 5 and 6. In other words, for comparable coverages of HzO and alkali metal the observed Aq5 was either less positive or even still negative on Ru(001) as compared to Pt(ll1). Looking at the Ru(001) data, the magnitude in changes of dq5/dn at low HzO exposures followed the sequence Na K Cs.l9 In order to facilitate the comparison we show in Figure 7 the example of HzO/K on R U ( O O ~ )For . ~ ~very low OK the decrease in Aq5 is just a little less than for the clean surface. For OK = 0.034 Aq5 rises to -0.3 eV whereas on Pt(ll1) and for the same OK it has increased to over 0.9 eV (Figure 6). The maximum At$ reached near the critical K coverage on Ru(001) is 0.5 eV and on Pt(ll1) it is >1.8 eV. Similar drastic differences are present for Na/Ru
--
Langmuir, Vol. 7, No. 12, 1991 3009
Alkali-Metal-Covered P t ( l l 1 ) and Ru(001)H20 Adsorption
and Na/Pt but also between the different alkali metals on the same substrate. We believe that these differences in A@ versus HzO coverage are linked to changes in HzO orientation on the surface. Our aim is to evaluate this effect in more detail. The first mention of a change in H2O orientation due to a coadsorbed alkali metal was by Doering et al.5 In the course of investigating the coadsorption of Na and H20 on Ru(001) by ESDIAD they found a characteristic change in the H+ ion emission pattern and attributed this to a slight tilting of the adsorbed HzO molecules. The cause for the tilt was sought in the dipole/dipole interaction between the coadsorbed specie^.^ Norskov et a L 2 0 discussed this reorientation of HzO in the presence of an alkali species in terms of electrostatic theory. Kiskinova et al.9 found a strong work function increase due to adsorbed HzO on K-covered Pt(l l l ) , which they attributed to either a reorientation of H2O or a charge transfer to HZO. Stuve et al.13 studied the coadsorption of Cs and HzOon Ag(ll0) and also observed an increase in A 4 at low Cs and HzO coverages. They attributed this effect to the reorientation of Hz0 molecules because of dipole/dipole interaction. Finally, an ab initio cluster calculation for K/H20 interaction on Pt(ll1) by Bonze1 et showed unambiguously that the large dipole moments of adsorbed K and HzO caused a strong reorientation of the HzO molecule such that the hydrogen atoms are actually pointing toward the Pt surface. In this case H2O no longer bonded to the surface via the oxygen lone pair orbital^.^ Charge-transfer effects for H20 or H20/K onPt(ll1)were found to be of minor im~0rtance.l~ The experimental results for Na/H20 on Ru(OO05and the theoretical results for K/HzO on Pt(ll1)together with experimental A 4 measurements9J4at the time represented an apparent discrepancy because in the first case H2O tilting (due to Na) of perhaps 20° relative to the normal was envisioned whereas in the latter case an upside-down HzO molecule next to K was obtained. Also, the ESDIAD pattern for HzO/Na on Ru(001) suggested a tilt of HzO in its plane5 whereby the calculation for HzO/K on Pt(111)indicated a reorientation in a plane perpendicular to the HzO plane. The question was raised whether these differences could be due to the alkali-metal and substrate specificity of alkali/HzO coad~orption.'~A careful comparison of measured work function changes versus coverage should shed light on this problem. Returning to the work function changes A$ versus H20 coverage in Figures 5-7 for Pt(ll1) and Ru(001), we see drastic differences. It is also known that HzO adsorption on clean Pt(ll1)and Ru(001) is different: The adsorption energy of HzO is larger on Ru(001) than on Pt(lll), as seen from a comparison of thermal desorption data25v26 and initial dipole moments. The latter as obtained from Figures 5 and 6 are 0.75 and 1.13 D for Hz0 on Pt(ll1) and Ru(001), respectively. Depending on the strength of the HzO/substrate interaction energy we qualitatively distinguish two cases: (a) Interaction of HzO/alkali is comparable or weaker than HzO/substrate bonding. Here we expect competitive HzO adsorption in clean substrate and alkali-modified sites. (b) Interaction of HzO/alkali is stronger than HzO/substrate bonding. Here we expect the sequential occupation of alkali-modified and clean substrate sites. The consequences for A@(n) follow accordingly. Assuming a reorientation of HzO adsorbed in alkali-modified (25) Doering, D.; Madey, T. E. Surf. Sci. 1982, 123, 305. (26) Fisher, G. B.; Gland, J. L. Surf. Sci. 1980, 94, 446. (27) Hrbek, J. Surf. Sci. 1985, 164, 139. (28) Rangelov, G.; Surnev, L. Surf. Sci. 1987, 185, 457.
Table I. Survey of HzO/Alkali Coadsorption Data for Pt(ll1) and R U ( O O ~ ) ~ ~ ~~
~
av H20 dipole_moment system Na/Ru(001) K/Ru(001) Cs/Ru(001) Na/Pt(lll) K/Pt(lll) a
Ocrlt
Abmsl, eV
0.05 0.05 0.08 0.10 0.10
-0.1 0.5 -0.9 21.2 2.0
P,
D
-0.2 -0.4 -0.9 -0.53 -1.14
~~
alkali dipole moment NAM,
D
6.0 13.8 21.5O 7.1 15.0
From refs 27 and 28.
sites ( A 4 > 0) we anticipate in case a a mixture of adsorbing H2O molecules with A 4 > 0 and A 4 < 0, a t least at low alkali coverages such that the net A@ measured should be less negatiue than for the clean substrate surface. In case b, on the other hand, H2O adsorption should initially cause A$ > 0 followed by A 4 I0 a t higher HzO coverages. Complications to this simple rule come through the alkali-coverage dependence. Clearly, at higher dAM where clean metal sites are less available, the effects of competitive adsorption will diminish. Then the effects due to HzO/alkali interaction will become more visible as long as the alkali coverage stays below the critical eAM for HzO dissociation. Based on this qualitative discussion of HzO/alkali interaction and A 4 versus Hz0 coverage behavior, we propose the following evaluation of the A 4 data. We assume that the maximum work function change observed for HzO/alkali coadsorption for 8AM < is a measure of the reorientation of H20 molecules relative to HzO on the alkali-free surface. The average slope of A 4 versus H20 coverage is then taken as a measure of the average dipole moment of alkali-modified H2O. This quantity is calculated from a straight line connecting the origin and a point on the measured curve near 1 X lo+ mbar of H20 exposure.lg For the alkalis on Ru(001) this evaluation was done for 8AM very close to ecrit.lg For Na on Pt(lll), however, we deviated from this rule because the slope d(Ad)/dn varied so much with HzO exposure and alkali coverage. Here the average dipole moment was calculated for a linear interpolation between origin and the 2 X lo+ mbar exposure point, 8Na = 0.105. A summary of critical alkali coverages, A$" values, and average H20 dipole moments is given in Table I. In the dipole/dipole interaction picture the reorientation of HzO is induced by the dipole moment of the adsorbed alkali species. The latter were obtained from measured work function changes versus alkali coverage and are included in Table I. We expect a proportionality between the average dipole moment of HzO and that of the alkali. Figure 8 illustrates this expected dependence for Pt(ll1) and Ru(001). The corresponding values for H2O on the clean crystals are also included. Both sets of data in Figure 8 show a steady increase from Na to K to Cs. The absolute values of the evaluated HzO dipole moment and of the measured Admar are larger for Pt(ll1) than for the Ru(001) substrate. These data are a direct illustration of the alkali and of the substrate specificity of HzO/alkali coadsorption. The effects seen are in good qualitative agreement with the general arguments given above. For example, the amount of Hz0 reorientation increases from Na to K to Cs according to the increasing dipole moment of the coadsorbed alkali species, in accordance with a dipole/dipole interaction scheme. Also, the degree of H20 reorientation is stronger on Pt(ll1) than Ru(001) because H20 is more weakly adsorbed on clean Pt(ll1) and therefore shows less resistance toward rotation. The maximum effect of rotation is observed for
Bonze1 et al.
3010 Langmuir, Vol. 7, No. 12, 1991
2oF---=-
Table 11. Summary of &O/Alkali Coadsorption Systems and €I20 Dissociation
system
Na/Cu(llO) csjcu(iio) Cs/Cu(llO) Na/Cu(111) Li/Ag(llO) Na/Ag(llO) Cs/Ag(llO) K/Ag(lll)
Figure 8. Plot of the (negative) average H20 dipole moment (a) and of the maximum work function change (b) for alkali-covered Pt(ll1) and Ru(001) surfaces at 110-120 K.
Cs/Ru(001) and K / P t ( l l l ) . The results in Figure 8 also reconcile qualitatively the older observations of reoriented HzO on Na covered Ru(OO~)~ and K-covered Pt(lll).14 H2O on Na/Ru is less rotated than on K/Pt but a quantitative assessment of this difference cannot be offered. 5. Comparison with Other Systems and Final Discussion of Molecular HzO
Recently Sass and Bangels published a comparison of H~O/alkalicoadsorption data for Ag(110). They plotted the initial dipole moment of HzO versus the alkali ion radius for Li, Na, and Cs and obtained a dependence similar to that in Figure 8. They interpreted these data in terms of a gradual reorientation of Hz0 from the oxygen lone pair bonded configuration on the clean surface to the nearly opposite configuration in the case of Cs. The initial dipole moments for H20/Li and HZO/Na were positive, which indicates a gradually less negative change in work function with HzO ~0verage.l~ Such behavior may be expected for small dipole moments of the alkali, such as Li and Na. A parameter entering into this comparison is the alkali coverage, which Sass and Bange chose to be the very low value of 0.01.15 Under that condition there are many free Ag sites for H2O adsorption. The initial dipole moment of HzO is then likely to be dominated by HzO in pure Ag sites and therefore not necessarily characterisic of the alkali-modified, reoriented H2O. 6. Tilting of Alkali-Coadsorbed HzO and Dissociation Alkali-metal-induced dissociation of Hz0 has been reported for a number of well-studied systems. Table I1 is a summary of all systems studied to date. There are a number of interesting features with H2O dissociation on alkali-promoted transition-metal surfaces. First, in most cases a critical alkali coverage, fi'crit, below which no H20 dissociation is observed, has been reported. Second, the dissociation products can be hydroxyl groups (together with the partially ionic alkali and possibly water of hydration) and oxygen species. Third, the dissociation of H2O into OH and atomic oxygen for fi' > &it may be temperature dependent. Fourth, the reorientation of HzO molecules may have an effect on dissociation behavior. It would be desirable to come to an understanding of the relationship between these characteristic features of HzO dissociation but at this time we can only offer some qualitative arguments.
Na/Pt(L11) K/Pt(lll) Li/Ru(001) Na/Ru(001) Na/Ru(001) K/Ru(001) Cs/Ru(001) This work.
Hz0 dissoc product
critical alkali coverage &it
ref
?
OH,0
0.22 0.10 0.46
?
OH ? ? ?
4 16
17 10,11 7, 15 15 13, 15 18
(>0.16)
OH OH OH OH OH OH,0 OH,0 OH, 0
0.10 0.10 0.05 0.25 0.05 0.05 0.08
a
14, a 12 5 19 6, 19 19
Inspecting Table 11,we note that HzO dissociation has been deduced for most systems, with the exception of Na/ Cu(110),4Li/Ag(llO), Na/Ag(llO), and Cs/Ag(l10).15The situation for Ag is surprising in the sense that apparently HzO dissociates on K/Ag(lll) at all K coverages18but no dissociation has been mentioned for alkali-covered Ag(l10).13J5 For all other systems the formation of.OH and, in some cases, of atomic oxygen is stated, together with the result of a critical alkali coverage for dissociation. The dissociation products have been detected spectroscopicauy6,9110,11,14,16,18,19 or have been inferred more indirectly.5J2 The discrepancy in fi'crit for Na/Ru(001) of O.2ij5and 0.0519 may be explained by this difference; in the latter case XPS, UPS,and A 4 measurements were used to identify the presence of molecular or dissociated H2O. Thus we prefer the value of 0.05 for this system. What causes HzOto dissociate on an alkali-covered metal surface? It is obviously not the mere presence of an alkali species even if it is situated next to a HzO molecule in the case of attractive i n t e r a c t i ~ n . ~This J ~ ~is~also ~ borne out by the theoretical calculation for H20/K coadsorption on Pt(ll1) in the limit of low K ~0verage.l~The alkali coverage has to reach a certain value for H2O to dissociate. Charge transfer between HzO and the alkali appears to be of minor importance in the coadsorbed state14and hence the filling of antibonding states cannot be primarily responsible for dissociation. Another reason for HzO dissociation has to be thought of. A possibility seems to be field-induced dissociation29~30 due to the high local electric field in the immediate vicinity of adsorbed alkalimetal species. For example, for HzO and K coadsorbed on a metal surface we can estimate the local electric field at the H20 adsorption site on the basis of a simple Coulomb law.29>31We assume a hexagonal net of positive charges ei and image charges -ei representing the adsorbed alkali species:31
-E(-- ei(r- ri)
E(r) = 1
1
ei(r- Ri)
(2)
Ir- ri 1 3 Ir - ~ ~ 1 ~ The vectors ri and Ri describe the locations of charges and image charges, respectively, and r the absorption site of 4 ~ i3
(29) Kreuzer, H. J., In Chemistry and Physics of Solid Surfaces VIII; Vanselow,R., Howe, R., Eds.; SpringerSeries in Surface Science;Springer: Berlin, 1990; Vol. 22, p 133. (30)Block, J. H., In Chemistry and Physics of Solid Surfaces IV; Vanselow, R., Howe, R., Eds.; Springer Series in Chemical Physics; Springer: Berlin, 1982; Vol. 20, p 407. (31)Uram, K. J.; Ng, L.; Yates, J. T., Jr. Surf. Sci. 1986,177, 253.
Alkali-Metal-Covered P t ( l l 1 ) and Ru(001) HzO Adsorption
a= 29A zo= 21 A xl= 2 8 A q= 06e
HzO. The geometry of coadsorbed HzO/K in a cut vertical to the surface is illustrated in Figure 9a. The alkali species are assumed to be hexagonally arranged in the x-y plane at a certain height a at all coverages OK. We find that the field components parallel to the surface, E, and Ey, decrease with increasing OK. Only the component vertical to the surface, E,, increases with OK, as shown in Figure 9b. In this calculation only the K species in a distance of I d + X I from the HzO site were included, with d being the K-K separation. The line shown in Figure 9b was obtained for the geometric parameters of HzO/K on Pt(lll).14The field E, at the HzO site for the critical coverages OK = 0.05 (onRu(001)) andO.10 (onPt(ll1))is0.107andO.l36V/A, respectively. Depending on the orientation of the adsorbed HzO relative toE, it is conceivablethat this field strength is sufficient for HzO d i s s o ~ i a t i o n . ~The ~ . ~optimum ~ configuration of HzO/K as well as the critical field for dissociation may become more transparent in a quantum mechanical c a l ~ u l a t i o n . ~ ~ ~ ~ ~ A related important question in this context is how to explain the possible alkali-metal and substrate dependence of the critical alkali coverage, Ocrit, for HzO dissociation. Here we consider two points: first, the dependence of H20 dissociation on the local electric field at the adsorption (32) Kreuzer, H. J.; Wang, L. C. J. Chem. Phys. 1990,93, 6065.
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site and the orientation of H20 relative to the metal surface; second, the dependence of HzO orientation on the alkali dipole moment and the HzO/substrate interaction, as discussed in section 4. Taking these points together and assuming more effective dissociation of HzO when adsorbed through its oxygen lone pair orbital and hence little reorientation, we expect dissociation more readily on Ru(001)and for alkali specieswith the lowest dipole momenta. This is in fact observed because the critical coverages for HzO dissociation are lowest for Li, Na, and K on Ru(001) (Table 11). On the other hand, large critical coverages are reported for Na and K on Pt(lll),Cs on Ru(001), Cs on C U ( ~ ~ O and ) , ~Cs ~ Jon~ Ag(l10).13 It appears that in all the latter cases except Cs/Ru(001) HzO is relatively weakly adsorbed on these substrates, and hence a large degree of HzO reorientation, as observed via the dipole moments of HzO coadsorbed with the alkali, counteracts easy H20 dissociation. A high degree of HzO rotation moves the HzO molecule into a position where its oxygen lone pair orbital is spatially removed frorm the metal surface, weakening the direct HzO/metal interaction and hence the possibility of charge exchange with the substrate. For Cs/Ru(001), on the other hand, the large dipole moment of Cs is likely to cause a large rotation of HzO despite its relatively strong bonding such that HzO/Cs becomes similar to the other cases mentioned above. From this general discussion we extract the suggestion that a higher degree of HzO reorientation due to coadsorbed alkali correlateswith a higher critical alkali coverage and a more difficultdissociation of HzO. This idea could be further substantiated by additional quantitative work on the important case of HZO/Li coadsorption. Alkali-enhanced dissociation of H20 always seems to generate OH gpecies, which are likely to be coadsorbed with alkali ionic species on the transition-metal surface. Partial hydration of these ionic adsorbates has been reported.3,6~8~13~18~19~33 The formation of OH is quite reasonable in view of the high enthalpies of formation of alkali hydroxide^.^^ The temperature dependence of HzO dissociation leads in a few cases to the appearance of oxygen as a secondary product, usually at T 1 200 K.I9 This latter product has been observed on Cu(ll0) and Ru(OOl)16.19 but not on Ag or Pt. This is also understandable in a thermodynamic picture because oxide formation enthalpies per oxygen atom of Cu and Ru are higher than of Ag and Pt.3v34
Acknowledgment. We are grateful to H. J. Kreuzer (Univ. of Halifax) for a discussion concerning the fieldinduced dissociation of adsorbed HzO. Registry No. HzO, 7732-18-5; Pt, 7440-06-4; Ru, 7440-18-8; Na, 7440-23-5; K, 7440-09-7. (33) Bonzel, H. P.; Pirug, G.; Winkler, A. Surf. Sci. 1986, 175, 287. (34) CRC Handbook of Chemistry and Physics,64th ed.; CRC Press: Boca Raton, FL, 1984.
