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Coadsorption of water and lithium on the Ru(001) surface
Surface Science 176 (1986) 165-182 North-Holland, Amsterdam
165
COADSORPTION OF WATER AND LITHIUM ON THE Ru(OO1) SURFACE S. SEMANCIK Chemical Process Metrology Division, National Bureau of Standard,
Gaithersburg,
MD 20899, USA
D.L. DOERING Department
of Physics,
University of Florida, Gainesville, FL 3261 I, USA
and T.E. MADEY Surface Science Division, National Bureau of Standards, Received
4 May 1986; accepted
for publication
Gaithersburg,
MD 20899, USA
15 May 1986
The interactions between water and lithium have been studied on the surface of a Ru(OO1) crystal using thermal desorption spectroscopy, electron stimulated desorption ion angular distributions (ESDIAD), Auger spectroscopy and LEED. The presence of Li was found to influence strongly the H+ ESD yield and the ESDIAD patterns from adsorbed water even at Li coverages of 0.02 monolayer; changes in the thermal desorption states for water were also observed at low Li coverages. For coadsorbed Li coverages above 0.05, ESDIAD measurements provided clear evidence of water decomposition, even at surface temperatures near 80 K; evidence for dissociation was also obtained from thermal desorption and Auger measurements. The present results are compared and contrasted to those reported previously for the H,O/Na/Ru(OOl) system.
1. Introduction A number of coadsorption experiments have been carried out during the last few years to study the influence of surface additives on a variety of chemisorption systems. These investigations often relate directly to fundamental processes encountered in electrochemistry and in catalytic promotion and poisoning. Since alkali metals serve as modifiers of surface reactivity in many practical situations, these elements have been of particular interest in coadsorption studies. Bonzel [l] has recently reviewed work in the area of alkalipromoted gas adsorption. Despite the extensive interest in coadsorption of alkalis with small molecules, only recently have studies of H,O coadsorbed with alkalis appeared [2], and only a few cases have been reported in detail.
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et al. / Condsorption of water and Li on Ru(O01)
Lithium has been coadsorbed with water on Ag(ll0) and Cu(l11) [3], while Na coadsorption has thus far been examined only on Ru(001) [4]. The interaction of K with water has been studied by several groups, on both Pt(lll) [5-71 and on Ru(001) [8]. C esium-water coadsorption has been investigated only on Ag(ll0) [9]. In this study, we have examined the interaction between lithium, the alkali having the lowest atomic number, Z, and smallest ionic radius, and Hz0 on Ru(001) *. The basal plane Ru crystal used is the same sample previously used in our study of Na-H,O coadsorption [4], therefore allowing direct comparisons to be made with those results. Although similarities exist for the interaction of H,O with lithium, sodium and potassium on Ru(OOl), the most significant difference concerns the minimum alkali coverage required for water dissociation to occur at 80 K. The onset value was found to be ai_, = 0.05, considerably lower than the onset value of flNa = 0.25 reported previously [4]. Although the minimum K coverage needed for dissociation of H,O on Ru(001) at 80 K has not been reported [8], Kiskinova, Bonzel et al. [5,6] have reported an onset coverage of 8, = 0.07 for dissociation of Hz0 on Pt(ll1) at 305 K; a similar onset value is seen for Hz0 dissociation at 100 K [6]. Due to the high reactivity of adsorbed Li, our experimental efforts and the results presented here deal primarily with the low Li coverage regime (0,, < 0.1). Thermal desorption spectroscopy (TDS), LEED and electron stimulated desorption ion angular distribution (ESDIAD) measurements were used to monitor binding characteristics as well as long range and short range order. The direct information about surface molecular structure obtained using ESDIAD was especially helpful in understanding the interactions between Li and water [lo]. 2. Experimental The vacuum system used in this study has a base pressure of - 1 x lo- ‘I’ Torr and was equipped with instrumentation for performing Auger electron spectroscopy (AES), LEED, ESDIAD and thermal desorption measurements. The system also contained a sputter ion gun and gas and alkali dosing capabilities. Details on these various components have been discussed previously [ll-131. The Ru crystal examined had been cut and mechanically polished to produce a surface whose normal was within lo of the [OOl] direction, as determined using Laue back reflection. The crystal was connected to the end of a precision manipulator using 0.020” Ta wire mounts; there it could be heated resistively to 1550 K and cooled to 80 K using a liquid nitrogen flow * Note that we employ the 3-digit notation to index hcp Ru rather than the redundant 4-digit notation.
S. Semancik et al. / Coadrorption of water and Li on Ru(OO1)
167
loop. A W-S%Re/W-26SRe thermocouple was spot-welded to the sample to monitor its temperature. Cleaning procedures used for the Ru crystal are described in ref. [ll]. Lithium was dosed onto the Ru surface by moving the sample into a Li beam generated by resistive heating of a SAES Getters Inc. * source with a current of 6.3 A. The dosing times under these conditions ran from 2 to 200 s (after a - 30 s induction period) to obtain Li coverages ranging from approximately 0.01 monolayer to several monolayers. Auger measurements indicated that .the contamination introduced during Li dosing (mainly background CO) was less than a few percent of a saturated layer, even for long exposures that produced multilayer Li. Water exposures were produced through a microcapillary array doser that allowed the chamber to maintain a vacuum in the lo-” Torr range while the local H,O pressure at the sample (which was moved into the H,O beam) was raised up to lo-’ to lo-* Torr. Lithium and water doses were generally carried out with the Ru sample between 80 and 100 K; however, some of the experimental measurements performed on the chemisorbed overlayer involved heating the sample after adsorption. For example, in TDS measurements typical heating rates of approximately 5.5 K/s for H,O and 28.5 K/s for Li were used. The LEED and ESDIAD results were recorded photographically from a phosphor screen mounted behind a set of hemispherical grids and a microchannel array image intensifier [ll]. The current level of the electron gun was adjusted down to 5 X lop9 A during ESDIAD measurements to ensure that beam damage effects were minimized.
3. Results Individual adsorption experiments for Li and H,O on Ru(001) were performed prior to studying overlayers of coadsorbed H,O and Li. Fig. 1 shows a series of thermal desorption spectra obtained upon heating the Ru crystal following successive doses of Li. The coverage value for a given Li dose was determined by comparing the integrated area of the 7 amu desorption curve it yielded to the desorption curve area produced by a saturated first layer of Li (0G’ = 0.5 [14]). This saturated condition was assumed to exist when the amount of adsorbed Li produced the first noticeable sign of a multilayer desorption feature near 530 K (the multilayer feature became sharp and continued to grow as the Li doses were increased further). For relatively high values, coverage estimates determined from Li TDS areas could be checked * Commercial materials are identified in order to specify experimental such identification in no way implies recommendation or endorsement of Standards.
procedure adequately; by the National Bureau
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et al. / &adsorption
of water and LI on Ru(O01)
r
lNor
Li on Ru(001)
I
100
1
300
I
I
700
/
I
1100
TEMPERATURE
I
1300 (K)
Fig. 1. TDS spectra (smoothed) for Li desorbing from Ru(OO1)
using the known coverages for observed LEED patterns. As the Li exposures were increased, the overlayer at 80 K went through a (2 x 2) structure (B,, = 0.25) and a (6 X fi)R30” structure (0,, = 0.33) before exhibiting a series of compressed incommensurate phases [15]. Although most of the results reported here related to low Li concentrations, no ordered LEED patterns were observed from adsorbed Li coverages less than 0.25. The absolute coverages for even the smallest Li doses are believed to be correct to within a factor of two. Coverages for water adsorbed on Ru(OO1) were obtained using LEED. ESDIAD and thermal desorption measurements, and the details of these calibrations have been described previously [ll]. In reporting the conditions used in coadsorption experiments with Li, water exposures are expressed here in terms of the equivalent coverage that would be produced on the clean
S. Semancik
et al. / Coadsorption of water and Li on Ru(001)
WATER
DESORPTION
H20/Li/Ru(OO
1)
I
150
FROM H,O,
200
Q0
169
Li/Ru(OOl)
=0.6
I
250 TEMPERATURE
300 (K)
Fig. 2. The effect of increasing amounts of predosed Li on the thermal desorption the Ru(OO1) surface. The water dose in each case was equivalent to that producing 0.6 on clean Ru(001).
of H,O from a coverage of
Ru(001) surface. Note that the saturation H,O coverage on the clean Ru(001) surface has a surface atom density of 1.0 X 1015 molecules/cm2, corresponding to 4tzo = 0.66 monolayer; this is a “bilayer” of adsorbed H,O. The effects of interactions between coadsorbed water and Li are evident in the TDS results shown in fig. 2. The double-peaked spectrum in fig. 2a results from a water overlayer coverage of 0.6 on the clean Ru(001) surface, and it agrees with the published data [11,16-181. The A, feature in the spectrum is believed to develop when water desorbs from small H,O clusters, while the A, peak corresponds to desorption from the H,O bilayer [ll]. The spectra in figs. 2b-2f are produced when increasing amounts of Li are preadsorbed prior to dosing the surface with an amount of water equivalent to that used in the
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et al. / Coadsorption
of waterand Li on Ru(001)
experiment of fig. 2a. Even very small amounts of Li begin to alter the desorption curve shape in the region at 200 K and in the tail near 230 K. For a Li coverage of 0.03, a distinct new feature labeled B in fig. 2 becomes evident. A qualitatively similar feature developed in the TDS curves obtained for H,O-Na coadsorption on Ru(001) [4]. In that case, the feature was associated with water molecules which interacted with hydroxyl-like species that formed as H,O molecules began to dissociate. It seems likely that the B feature for H,O-Li has a similar origin; however, the preadsorbed Li coverages where this effect can be observed are considerably lower than for Na. As the preadsorbed Li coverage is increased, the intensity of desorption in the A z and A, temperature regions steadily drops off. The amount of B state desorption initially increases with Li coverage, then appears to pass through a maximum as the Li level is increased from near 0.1 to 0.17. Significant water desorption has also been reported at 565 K for H,O/K/Ru(OOl) [8] for higher potassium coverages, 8,2 0.33. Only two very small water desorption features were observed for the smaller lithium coverages used in this study, at - 550 and - 650 K. The integrated desorption flux in these features is about 5% of that which was recorded for the desorption occurring between 150 and 250 K. When the areas under the water desorption curves in fig. 2 are measured and plotted against the alkali coverage used in each TDS experiment, the solid curve shown in fig. 3a is obtained. The dashed curve included in this figure is for Na predosing, and was taken from fig. 2 of ref. [4]. In each case, the water doses corresponded to an effective coverage of 0.6 for clean Ru(001). The reduction in the amount of desorbed H,O is presumably connected to the loss of H,O to a decomposition channel, which opens up at Li coverages well below the 0.25 level indicated for the previous Na results. It is important to note that little or no dissociative reaction of H,O occurs when water is adsorbed on clean Ru(001) [11,17,18]. To further investigate this apparently high reactivity for adsorbed Li and water, measurements were done on coadsorbed layers formed with low doses of both Li and H,O. By using Li exposures yielding coverages < 0.1 and H,O exposures that produced a coverage of 0.1 (on clean Ru(001)). the interaction between Li and water was emphasized. Fig. 3b shows the results for interactions that occur when a temperature ramp from 80 to 300 K was applied to each of several coadsorbed overlayers. At these low H,O coverages. a drop in the total amount of desorbed water is noted even for 8ri -C 0.03, indicating that decomposition may occur as the surface is heated above 80 K, even at extremely low Li coverages. The amount of desorbed H,O dropped off steadily as the preadsorbed Li coverage was increased. Following each of the temperature ramps, an AES measurement of the residual oxygen level was made. The linear increase of the remnant oxygen as the Li increased (and the desorbed water amount dropped) further supports the interpretation that H,O molecules are lost to Li-assisted decomposition, which produces one oxygen
S. Semancik
a
et al. / Coacisorption of water and Li on Ru(001)
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171
= 0.6
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0.2
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’
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0.1
0
ALKALI I
I
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0.2
0.3
COVERAGE 1
I
0.5
(monolayer) I
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I
t1 B $ 0 2
0.1
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I
0.4
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/ H20/Li/Ru(OO
K -\
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eH*O = 0.1
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z fl
0.02
0 0
0.01
0.02
0.03
Li COVERAGE
0.04
0.05
0.06
O.Oi
(Monolayer)
Fig. 3. (a) Solid curve - area under water desorption curves of fig. 2 plotted against Li coverage; dashed curve - results from similar experiments for H,O/Na/Ru(OOl), from ref. [4]. Water exposures were the same as for fig. 2. (b) Area under water desorption curves and AES oxygen signal measured following TDS experiments, plotted versus Li predose coverage. These coadsorption experiments were conducted at lower coverages of both water and Li than the results in the solid curve of (a). The water dose used would produce a coverage of 0.1 on clean Ru(001).
172
SURFACE CONDITION
S. Semanc~k et al. / Coadsorption of water and Li on Ru(001)
eLi
;yso
0
0.02
0.05
O.,
0.1
0.1
a
b
Fig. 4. ESDIAD H’ emission obtained from 0.1 monolayer of water adsorbed onto (a) clean Ru(OOl), (b) Ru(001) predosed with eLI = 0.02, (c) Ru(001) predosed with /I,_, = 0.05. Doses and photographic recordings were all done at 80 K.
atom at the surface (which could still be part of a hydroxyl species after heating to 300 K) for each original adsorbed H,O molecule. Although it seems evident that low coverages of Li are capable of inducing Hz0 decomposition on Ru(OOl), the results presented in figs. 2 and 3 are based on TDS measurements in which substrate heating was used. The dissociation effects observed may therefore have been thermally activated during these measurements. The TDS results also do not provide any conclusive information on the nature of the reaction products left on the surface. Electron stimulated desorption ion angular distribution (ESDIAD) measurements provide direct information on adsorbate structure which can be used to help determine the identity of surface species. This capability was extremely useful in monitoring the effects of Li on adsorbed water, both as a function of the Li coverage and as a function of surface temperature. Fig. 4 shows the H+ ESDIAD patterns obtained by adsorbing approximately 0.1 monolayer of H,O onto the Ru(001) surface at 80 K, and onto surfaces that contained preadsorbed Li at levels of 0.02 and 0.05. (Note: No ESDIAD patterns were observed for Li alone on Ru(OOl).) For water on clean Ru(001) (fig. 4a) the “halo” of H+ emission is believed to arise from H,O monomers bonded with random azimuthal orientation via their oxygen atoms or from H,O hydrogen-bonded to the edges of small, disordered clusters [ll]. Figs. 4a and 4b illustrate that the Ht ESD emission from the adsorbed water was greatly increased by the presence of only - 0.02 monolayer of Li. In addition,
S. Semancik et al. / &adsorption
SURFACE CONDITION
of water and Li on Ru(OO1)
add BLi = 0.03
add
173
6‘i
Z
0.04
ESDIAD H * emission (Image) b
Fig. 5. ESDIAD H+ emission obtained from 80 K Ru(OO1) surface (a) dosed with 0.1 monolayer of water, (b) after adsorbing BLi = 0.03 onto the preadsorbed H,O, (c) after adsorbing an additional BLi = 0.04 onto the surface of (b).
the characteristic halo from the low coverage of water on Ru(001) was reduced in radius while the annulus was broadened. A similar pattern was found for H,O + Na on Ru(OO1) [4], and interpreted as due to a reorientation (“tilting”) of molecular H,O. A slightly higher Li coverage was used to obtain the result in fig. 4c. The central portion of the ESDIAD pattern is produced by intense H+ emission perpendicular to the surface, which we believe results from Li-induced decomposition of water to form a hydroxide species oriented normal to the surface. Such species have a high cross section for ESD and very small quantities can be detected easily [19]. Both ESDIAD and TDS results indicate that H,O dissociation can occur in the presence of Li, but quantitative differences concerning the amount of Li necessary for dissociation are observed using these two techniques. At 80 K, a coverage of BLi = 0.02 appears to reorient the H,O molecules, but apparently does not cause appreciable dissociation; this suggests that the TDS behavior observed for the very lowest Li coverages may have involved thermal activation: molecular H,O is stable at 80 K to a Li coverage higher than that required to dissociate H,O upon heating. Fig. 5 shows further ESDIAD results involving somewhat higher levels of adsorbed water. The Hz0 dose used in these experiments gives a coverage of = 0.3 and produces the hexagonal ESDIAD pattern shown in fig. 5a. This is due to ESD of H+ from oriented H,O clusters on the Ru surface [ll]. When Li is added following the adsorption of H,O, the H+ emission pattern changes to a broadened halo of relatively high intensity. When an additional dose of Li is added to the surface, a bright central core develops suggesting the formation of hydroxyl species. The ESDIAD emission patterns pictured in figs. 5b and
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S. Semancik
a.,55K
et al. / Condsorption of water and Li on Ru(OO/)
/:g$&
b.,85K
E S D I
e. 310K
f. 345K
A
D
Fig. 6. ESDIAD H+ patterns obtained from Ru(001) surface dosed to B,,, = 0.02 and 0,, = 0.1 and annealed briefly up to the indicated temperatures. Photographic recordings were done at 80 K.
5c are quite similar to those in figs. 4b and 4c obtained when water was adsorbed onto a surface containing preadsorbed Li. (The amount of Li needed to obtain these effects is somewhat higher in the results depicted in fig. 5, but that observation probably relates to the higher water coverage involved.) It appears that reorientation and reaction of water with Li in the coadsorption system is not significantly affected by the order of adsorption at low coverages. The clearest indication that the Li-assisted dissociation of H,O is thermally activated is provided by the ESDIAD patterns in fig. 6. The patterns were obtained from a surface having eL, = 0.02 and 0u,o = 0.1, which was heated in steps from 80 to 345 K. Up to at least 155 K, there is no obvious sign of normal emission, suggesting that a significant amount of H,O dissociation has not yet occurred for this low Li coverage. By 185 K, some normal emission becomes noticeable, and this central core gets more intense as the temperature is increased to 210 K, where the halo disappears. The emission intensity remains rather constant up to 310 K indicating stability of the hydroxyl species over that range. Between 310 and 345 K, the normal Hi emission is
S. Semancik et al. / Coadsorption of water and Li on Ru(OO1) EFFECT
1
OF Hz0
I
I
I
175
ON Li DESORPTION
I
I
I
I
Li Desorption
200
400600
800
1000
TEMPERATURE Fig. 7. TDS spectra
for Li desorbing
1200
13001350
(K)
from Ru(OO1) (a) dosed to tiL = 0.12, (b) dosed to Br, = 0.09 and BHzO = 0.6.
significantly reduced and by 360 K, ESDIAD indicates that the hydroxyl species yielding H+ has disappeared. To summarize, the patterns in fig. 6 illustrate thermally activated and Li-assisted dissociation of water, which produces hydroxyl species that are stable to > 300 K. Lithium remains on the surface to temperatures hundreds of degrees above the point (- 360 K) where ESDIAD Hf emission is last observed; this is demonstrated by the TDS trace in fig. 7b. It is not known if any significant amounts of Li are lost from the surface as some desorbing reaction product; species that might be expected to form (such as LiOH) were not detected in TDS measurements, however. Hydrogen desorption spectra were taken in an attempt to gain further insight into the nature of the surface complexes existing above room temperature, but high background desorption signals (probably from the sample holder) made meaningful interpretations of these results virtually impossible. Auger measurements indicate that remnant oxygen is present on the surface (see also fig. 3b) following the water decomposition reaction. As the surface temperature is increased, the chemical nature of the surface oxygen changes, probably from a hydroxyl-like species to adsorbed
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et (II. / Coadsorption of water and Li on Ru(OO1)
atomic oxygen at the higher temperatures. The TDS results for desorbing Li in fig. 7 show a comparison between the interaction of Li with: (a) clean Ru(001): (b) remnant oxygen from the H,O on Ru(001).
4. Discussion It is generally accepted that adsorption of an alkali metal atom on a metallic surface involves significant charge transfer from the alkali to the substrate and results in the formation of a relatively large surface dipole. This process is responsible for the large work function changes that have been recorded for Li adsorption [3] and for other cases of alkali adsorption [20]. In addition, the repulsive adatom-adatom interactions between the dipoles in an alkali overlayer ensure that island growth or two-dimensional cluster formation do not occur at low coverages; thus, for example, for 13< 0.2 no ordered LEED patterns are seen for adsorbed Li [15] or Na [13] on Ru(001). (At higher coverages (0 > 0.25) for these adsorbates on Ru(OOl), a series of ordered LEED patterns can develop [15,21].) Although the qualitative properties of many alkali overlayers on different metallic substrates are similar, differences have been observed as well. For example, Sass et al. [3] have reported that Li dissolves into Ag; we observed no such effects for Li on Ru(OO1). The differences discovered in the orientational ordering behavior of Na [21] and Li [15] on Ru(001) are probably related to the different electronegativities and ionic radii of these two alkalis. When considering the ways in which surface alkalis can modify reactivity, the picture is again one of similar general behavior among different alkali/substrate systems, but with clear differences in kinetic detail. This scheme seems to apply in the case of alkali modification of water adsorption on ruthenium; the lithium-H,0 interaction reported here, for example, appears to be stronger than the Na-H,O interaction. 4.1. Influence
of Li on the structure of udsorbed molecular
water
When H,O is adsorbed onto a clean Ru(001) surface at 80 K, a dim halo-like Ht ESDIAD pattern is observed at low H,O coverages. It is believed that H,O is adsorbed oxygen-end down via the lone-pair orbital(s), with H atoms pointed away from the surface [ll]. The halo ESDIAD pattern may be due in part to isolated H,O monomers which are either randomly oriented in an azimuthal sense, or freely rotating; azimuthally disordered clusters of H,O may also contribute to the halo. When Li is coadsorbed with H,O. the intensity of the halo annulus is greatly increased. This behavior is very similar to that observed when water is coadsorbed with 0.1 monolayer of Na on Ru(001) (ref. [4], fig. 4).
S. Semancik
et al. / Gadsorption
of water and Li on Ru(OO1)
171
We suggest that the increase of Ht intensity in the ESDIAD halo and the broadening of the halo annulus are both a consequence of reorientation of adsorbed H,O molecules in the presence of Li. Lang et al. [22] have studied the electrostatic interaction between an adsorbed atom (e.g., Li) and an adsorbed molecule. For molecules with permanent dipole moments (e.g., H,O), their calculations indicate that the molecule can “turn around” to minimize energy in the electrostatic field of the adsorbed atom. Independent evidence for reorientation of H,O and NH, by both electronegative and electropositive additives has been seen in other ESDIAD studies [10,23,24]. For H,O on clean Ru(OOl), the O-H bond angle is close to the “cut-off” angle for ion desorption [25]. That is, the desorbing ions have a high probability of recapture due to the image interaction between the H’ ion and the conducting substrate. If the H,O molecule is tilted so that one of the O-H bonds is inclined with its polar angle closer to the surface normal, the Hf ion desorption probability increases dramatically. Variable amounts of molecular tilt for the adsorbed H,O resulting from a range of Li-H,O separations could explain, in part, the broadening of the halo; large amplitude bending or vibrational modes of adsorbed H,O will also contribute to its width. It does not seem necessary to invoke geometrical changes within H,O molecules to explain the ESDIAD halo; the slight Li-induced changes in the H,O binding energy to Ru are not consistent with a major intramolecular structural change. The interaction between molecular water and surface Li which produces the enhanced H+ ESD halo emission from H,O also gives rise to a new desorption state in the TDS measurements. This state, which is best observed by comparing spectra b and c to spectrum a in fig. 2, occurs just below 200 K, and fills in the valley between features A, and AZ. The development of this state with added Li is consistent with the kind of downward shift in temperature of a desorption feature for a polar molecule (like H,O or NH,) caused by an electropositive coadsorbate. Such behavior has been predicted by Norskov et al. [26] and by Stair [27], and has been observed experimentally, e.g., by Benndorf and Madey [23]. 4.2. Lithium
induced decomposition
of adsorbed water
On clean Ru(OOl), water adsorbs and desorbs reversibly, and little or no decomposition of water is observed during TDS experiments Ill]. The behavior is qualitatively different when water is coadsorbed with Li or Na. The interactions between water and the alkalis create a new upshifted desorption state (state B at 230 K for Li and 240 K for Na), as evidenced in the TDS spectra in fig. 2 and in fig. 1 of ref. [4]. These features are believed to be connected to surface water that partially bonds to surface hydroxyl-like species that themselves produce central core ESD emission - the ESDIAD signature that H,O decomposition has occurred. The B-type state formation in
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et al. / Coudsorption
of waterand
Li on Ru(OO1)
the range of 230 to 240 K represents a temperature upshifted H,O desorption feature due to an interaction with an electronegative additive (OH), again in agreement with predictions and previous observations [4,23,26,27]. Our results do not permit a definitive statement concerning the nature and structure of the water decomposition products. Possible products include H and OH bound to Ru, as neighbors to Li atoms; other possible products include LiOH and LiH. However, bulk LiH reacts vigorously with H,O to form LiOH and H,, so that with an excess of surface H,O it is more likely that any stable surface complex would be hydroxide rather than hydride. In fact, in EELS studies of the H,O/K/Ru(OOl) system, Thiel et al. [8] reported the formation of KOH, and it seems likely that the species formed in our present and previous experiments would be LiOH and NaOH [28]. Moreover, for Li (as well as Na), the ESDIAD data indicate that H+ desorbs along directions about the normal to the surface; we identify this emission as due to an OH species, perhaps bonded as LiOH. Although LiOH may have been present at the surface, none was observed during thermal desorption measurements. As mentioned above, OH may also have been bonded directly to Ru near the Li atom. In this case, the OH dipole might be expected to “tilt” due to interaction with Li, giving rise to a halo of ESDIAD emission like that in fig. 4a. The halo intensity visible in addition to the normal emission in figs. 5 and 6 is assumed to be due to residual reoriented H,O; no halo emission is seen after heating to desorb molecular water, leaving hydroxyl species on the surface. Therefore, we conclude that dissociation to form hydroxide occurs at the surface, with OH bonded primarily with its bond directed along the surface normal. The TDS results show no evidence for an OH recombination process to form H,O in the temperature region near 300 K. This is in contrast to the water formation from hydroxyl recombination which occurs on Cu(ll0) [29] and Ni(ll0) [30]. The small amount of water which desorbs at temperatures near 600 K might be connected to the decomposition reaction: 2LiOH + Liz0 + H,O. The bulk LiOH dissociation occurs rapidly at 1197 K [31], but a lower reaction temperature could be expected on a surface. Despite the qualitative similarity of the effects for H,O/Li and H20/Na coadsorption layers on Ru(OOl), key quantitative differences do exist. For example, the relative alkali coverages producing the ESDIAD and TDS results discussed above are significantly lower for Li than Na. The stronger reactivity of Li than Na is evidenced most clearly by considering the alkali onset coverages necessary for inducing H,O dissociation on Ru at 80 K. ESDIAD results (and remnant AES oxygen levels) indicate this value to be - 0.05 for Li and - 0.25 for Na. The repulsive interactions between Li adatoms also imply that the observed dissociation would not be occurring at local regions of high coverage (islands). In work on Pt(lll), potassium at a level of OK = 0.07 was required to produce H,O dissociation, both at 305 and 100 K [5,6]. It is
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119
also worth noting that Thiel et al. [8] used EELS to study KOH formation from H,O and K on Ru(001); they reported that H,O dissociation occurs at /3x = 0.33 at 80 K. However, their results do not demonstrate conclusively whether or not water decomposed below room temperature when coadsorbed with potassium at coverages significantly below 0.33. Dissociation can apparently be thermally activated for both the Li and Na cases on Ru(OOl), since decomposition was observed to occur at lower alkali coverages when the crystal was heated. (Note that observable hydroxyl concentrations were produced below B,, = 0.25, at eNa = 0.1 at 230 K (fig. 7c of ref. [4]) [32].) Since the (2 x 2) LEED pattern that forms for Li is less sharp than the (2 x 2) for Na on Ru(OOl), and develops for Li only at the coldest available temperatures (- 80 K), it would appear that Li is somewhat more mobile than adsorbed Na; however, it should be noted that Li, Na and H,O all have at least some mobility even at 80 K since all can form ordered LEED patterns between coverages of 0.25 and 0.33. Fig. 3 shows that Li-induced dissociation of Hz0 occurs, even at very low Li coverages, when a heating ramp to 300 K is applied to the Ru crystal. The drop off in the amount of water that desorbs (fig. 3b) as the level of preadsorbed Li is increased is linear with a slope between one and two. When plotted against the Li preadsorption coverage, the amount of oxygen build up from H,O dissociation found after’ the heatings increases with a slope greater than one. These changes both suggest that, at least for low coverages of water and Li, a linear proportionality exists between the number of water molecules and the number of reacting Li atoms. Experiments done with Na instead of Li included an induction period to greater than 19,~ = 0.2 in which no significant oxygen build up or decrease in water desorption occurred [33] (dashed curve in fig. 3a). (At the higher Na exposures, a nearly 1 : 1 ratio exists between the number of Na atoms and the number of decomposed water molecules [4].) 4.3. Why does a critical coverage of Li exist for H,O decomposition? In considering the coadsorption of H,O and alkali atoms on metal surfaces, it is clear that H,O will invariably dissociate to form hydroxyl products when the substrate temperature and the alkali coverage are high enough. The key questions are: why is there a critical coverage for alkali-induced dissociation of H,O at 80 K, and why do the critical coverages vary so strikingly for different alkali/metal systems (Li/Ru, Na/Ru, K/Pt)? In particular, why is Li so much more reactive than Na/Ru or K/Pt at low coverages? The answer must involve a combination of factors including the degree of charge transfer as a function of alkali coverage, the electronic structure and relative atomic sizes of the adsorbed alkalis, the local electrostatic fields near the adsorbed alkali, and the local alkali and H,O binding sites. It is difficult, however, to isolate the key factor(s) controlling the reactivity of Li. Doering et al. [4] and Kiskinova
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ofwuter and Li on Ru(001)
et al. [5] have suggested that H,O dissociation induced by coadsorbed alkalis may be related to occupancy of the alkali valence s-orbitals: the more ionic alkali at low coverages (low s-orbital occupancy) is less reactive than the more metallic alkali at high coverages (high s-orbital occupancy). This charge-transfer argument was based largely on the well-known coverage dependence of the work function for alkalis adsorbed on transition metals. In a recent paper, however, Woratschek et al. [34] have measured directly the occupancy of the potassium 4s-orbital as a function of coverage on Cu(ll0) using Penning ionization to quantify the charge transfer, and they find that significant partial s-orbital occupancy occurs even at low coverage. Although similar measurements are not available for Li on Ru(OOl), we have no basis for suggesting that the s-orbital occupancy will vary strongly in the critical coverage range, tYLi= 0.02 to 0.05, and do not believe this is the major factor in the Li-coverage dependence of H 2O dissociation. In general, lithium is a less reactive metal than Na or K. This inherent unreactivity is offset in solution, however, by the exothermic hydration of the very small Lit ion. In fact, the enthalpy of formation of (HzO)Lif is 34 kcal/mole, considerably greater than the enthalpies of formation of (H zO)Naf (24 kcal/mole) or (H,O)K+ (16.9 kcal/mole) [35]. The Li+ ion is exceptionally small, and has, therefore, an exceptionally high charge-radius ratio. This leads to anomalous behavior of a number of Li compounds in relation to the other alkalis. For example, LiOH decomposes at red heat to Li,O + HzO, whereas the other hydroxides sublime unchanged [36]. It is tempting to suggest that the higher reactivity of adsorbed Li (relative to K and Na) is related to its small size and partial ionic character, but the calculations of Lang et al. [20] do not appear to support such a picture. These authors have used an effective medium theory to compute, self-consistently, the induced electrostatic potential for Li, Na and K at their calculated equilibrium distances outside a jellium surface. Their results indicate surprisingly small differences between the electrostatic potential contours for adsorbed Li and Na: it is not obvious from the calculations why the reactivity for these two alkalis should be significantly different at low coverages. Moreover, if a critical electrostatic field is necessary for dissociation, there should be little difference between the critical coverages of Li and Na. Perhaps substrate crystallographic effects not considered in the jellium calculations have an influence on the relative reactivities. Since the results indicate that Li is so reactive toward H,O, it is unclear why molecular water should be stable at 80 K for the very low range of Li coverages (up to B,i = 0.05). One possibility is that substrate steps and defects are playing a role in the surface chemistry. Perhaps the low-coverage Li is trapped at point defects or step/kink sites where its electronic structure differs from Li adsorbed on the planar terraces.
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181
5. Conclusions Several techniques have been used to obtain a consistent picture of the interaction of water with Li on Ru(001). ESDIAD, TDS and AES measurements all indicate that decomposition of water occurs at 80 K when Li is preadsorbed to levels greater than - 0.05 monolayer. Upon heating, H,O dissociation can be produced by even lower levels of adsorbed Li. Previous work has shown, that similar effects occur with Na on Ru(001) only for coverages 2 0.25 monolayer (> 0.1 upon heating). ESDIAD patterns observed following water-alkali reactions show emission normal to the crystal surface, and suggest that the hydroxyl products have an upright orientation.
Acknowledgements The Department of Energy, Division of Basis Energy Sciences and the University of Florida, Division of Sponsored research (Grant No. RDAl7983-84) are gratefully acknowledged for partially supporting this work.
References [l] H.P. Bonzel, J. Vacuum Sci. Technol. A2 (1984) 866. [2] See the discussion on electropositive additives in the review of water adsorption by P.A. Thiel and T.E. Madey, Surface Sci. Rept., to be published. [3] J.K. Sass, K. Bange, R. Dohl, E. Piltz and R. Unwin, Ber. Bunsenges. Phys. Chem. 88 (1984) 354. [4] D.L. Doering, S. Semancik and T.E. Madey, Surface Sci. 133 (1983) 49. [5] M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 150 (1985) 319. [6] G. Pirug, A. Winkler, M. Kiskinova and H.P. Bonzel, Symposium on Surface Science, Obertraun, Austria, 1985, p. 95; H. Bonzel, private communication. [7] H.P. Bonzel, G. Pirug and A. Winkler, Chem. Phys. Letters 16 (1985) 133. [8] P.A. Thiel, J. Hrbek, R.A. DePaola and F.M. Hoffmann, Chem. Phys. Letters 108 (1984) 25. [9] E.M. Stuve, K. Bange and J.K. Sass, in: Proc. AS1 Study Institute of Interfacial Electrochemistry, Ed. A.F. DeSilva, Oporto, 1984, in press. [lo] T.E. Madey, C. Benndorf, N.D. Shinn, Z. MiSkoviC and J. VukaniC, Desorption Induced by Electronic Transitions, Springer Series in Surface Sciences, Vol. 4, Eds. W. Brenig and D. Menzel (Springer, New York, 1985) p. 104. [ll] D.L. Doering and T.E. Madey, Surface Sci. 123 (1982) 305. [12] T.E. Madey and J.T. Yates, Jr., Surface Sci. 63 (1977) 203. [13] D.L. Doering and S. Semancik, Surface Sci. 129 (1983) 177. [14] As discussed in ref. [15] the theoretical saturation coverage is 0.53, but the maximum coverage experimentally observed in the monolayer case was 0.46. [15] D.L. Doering and S. Semancik, Surface Sci. 175 (1986) L730. [16] T.E. Madey and J.T. Yates, Jr., Chem. Phys. Letters 51 (1977) 77. [17] P.A. Thiel, F.M. Hoffmann and W.H. Weinberg, J. Chem. Phys. 75 (1981) 5556.
182
[1X] [19] [20] [21] [22] [23] [24] [25] [26] [27] [2X]
[29] [30] [31] [32]
[33] [34] [35] [36]
S. Semancik
et ul. / Coadsorption
of wuter and LI on Ru(001)
P.A. Thiel, R.A. DePaola and F.M. Hoffmann, J. Chem. Phys. 80 (19X4) 5326. R. Stockbauer, D.M. Hanson. S.A. Flodstrom and T.E. Madey. Phys. Rev. B26 (1982) 1885. See, e.g., R.L. Gerlach and T.N. Rhodin. Surface Sci. 19 (1970) 403. D.L. Doering and S. Semancik, Phys. Rev. Letters 53 (1984) 66. N.D. Lang, S. Holloway and J.K. Norskov, Surface Sci. 150 (1985) 24. C. Benndorf and T.E. Madey, Chem. Phys. Letters 101 (1983) 59. K. Bange, T.E. Madey and J.K. Sass, Surface Sci. 162 (1985) 252. Z. MiSkoviC, J. Vukanic and T.E. Madey, Surface Sci. 141 (1984) 285: 169 (1986) 405. J. Norskov, S. Holloway and N.D. Lang. Surface Sci. 137 (1984) 65; J. Vacuum Sci. Technol. A3 (19X5) 1668. P.C. Stair, J. Am. Chem. Sot. 104 (1982) 4044. It is interesting to note that water adsorption onto a Li substrate has been suggested to yield a H,O-Li complex. rather than a reaction product like LiOH. See: W. McLean, J.A. Schultz. L.G. Petersen and R.C. Jarnagin, Surface Sci. 83 (1979) 354. In the case of water interaction with 30-110 A thick K films. H.P. Bonzel, G. Pirug and A. Winkler report H,O adsorption dissociation and formation of KOH followed by dissolution and hydration (Surface Sci. 175 (1986) 287). K.Bange. D.E. Grider. T.E. Madey and K.J. Sass. Surface Sci. 137 (1984) 38. C. Benndorf, C. Nob1 and T.E. Madey, Surface Sci. 138 (1984) 292. CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton. FL. 1978). The effects of heating on water dissociation were evident for the case of Li using both ESDIAD (fig. 6) and TDS (fig. 3b). In the case of Na, there is an indication that only small amounts of water might dissociate between B,, = 0.1 and 0.25 and that ESDIAD is more sensitive to such small amounts of H,O dissociation than is TDS (see fig. 7c of ref. [4] and fig. 3a of this paper). The local maxima in the Na and Li curves in fig. 3a appear to be real, but the effect is not understood. B. Woratschek, W. Sesselmann, J. Kippers. G. Ertl and H. Haberland. Phys. Rev. Letters 55 (1985) 1231. A.W. Castleman. P.M. Holland, D.M. Lindsay and K.I. Peterson, J. Am. Chem. Sot. 100 (1978) 6039. J.E. Huheey, Inorganic Chemistry (Harper and Row, New York, 1978) p. 706.
165
COADSORPTION OF WATER AND LITHIUM ON THE Ru(OO1) SURFACE S. SEMANCIK Chemical Process Metrology Division, National Bureau of Standard,
Gaithersburg,
MD 20899, USA
D.L. DOERING Department
of Physics,
University of Florida, Gainesville, FL 3261 I, USA
and T.E. MADEY Surface Science Division, National Bureau of Standards, Received
4 May 1986; accepted
for publication
Gaithersburg,
MD 20899, USA
15 May 1986
The interactions between water and lithium have been studied on the surface of a Ru(OO1) crystal using thermal desorption spectroscopy, electron stimulated desorption ion angular distributions (ESDIAD), Auger spectroscopy and LEED. The presence of Li was found to influence strongly the H+ ESD yield and the ESDIAD patterns from adsorbed water even at Li coverages of 0.02 monolayer; changes in the thermal desorption states for water were also observed at low Li coverages. For coadsorbed Li coverages above 0.05, ESDIAD measurements provided clear evidence of water decomposition, even at surface temperatures near 80 K; evidence for dissociation was also obtained from thermal desorption and Auger measurements. The present results are compared and contrasted to those reported previously for the H,O/Na/Ru(OOl) system.
1. Introduction A number of coadsorption experiments have been carried out during the last few years to study the influence of surface additives on a variety of chemisorption systems. These investigations often relate directly to fundamental processes encountered in electrochemistry and in catalytic promotion and poisoning. Since alkali metals serve as modifiers of surface reactivity in many practical situations, these elements have been of particular interest in coadsorption studies. Bonzel [l] has recently reviewed work in the area of alkalipromoted gas adsorption. Despite the extensive interest in coadsorption of alkalis with small molecules, only recently have studies of H,O coadsorbed with alkalis appeared [2], and only a few cases have been reported in detail.
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Lithium has been coadsorbed with water on Ag(ll0) and Cu(l11) [3], while Na coadsorption has thus far been examined only on Ru(001) [4]. The interaction of K with water has been studied by several groups, on both Pt(lll) [5-71 and on Ru(001) [8]. C esium-water coadsorption has been investigated only on Ag(ll0) [9]. In this study, we have examined the interaction between lithium, the alkali having the lowest atomic number, Z, and smallest ionic radius, and Hz0 on Ru(001) *. The basal plane Ru crystal used is the same sample previously used in our study of Na-H,O coadsorption [4], therefore allowing direct comparisons to be made with those results. Although similarities exist for the interaction of H,O with lithium, sodium and potassium on Ru(OOl), the most significant difference concerns the minimum alkali coverage required for water dissociation to occur at 80 K. The onset value was found to be ai_, = 0.05, considerably lower than the onset value of flNa = 0.25 reported previously [4]. Although the minimum K coverage needed for dissociation of H,O on Ru(001) at 80 K has not been reported [8], Kiskinova, Bonzel et al. [5,6] have reported an onset coverage of 8, = 0.07 for dissociation of Hz0 on Pt(ll1) at 305 K; a similar onset value is seen for Hz0 dissociation at 100 K [6]. Due to the high reactivity of adsorbed Li, our experimental efforts and the results presented here deal primarily with the low Li coverage regime (0,, < 0.1). Thermal desorption spectroscopy (TDS), LEED and electron stimulated desorption ion angular distribution (ESDIAD) measurements were used to monitor binding characteristics as well as long range and short range order. The direct information about surface molecular structure obtained using ESDIAD was especially helpful in understanding the interactions between Li and water [lo]. 2. Experimental The vacuum system used in this study has a base pressure of - 1 x lo- ‘I’ Torr and was equipped with instrumentation for performing Auger electron spectroscopy (AES), LEED, ESDIAD and thermal desorption measurements. The system also contained a sputter ion gun and gas and alkali dosing capabilities. Details on these various components have been discussed previously [ll-131. The Ru crystal examined had been cut and mechanically polished to produce a surface whose normal was within lo of the [OOl] direction, as determined using Laue back reflection. The crystal was connected to the end of a precision manipulator using 0.020” Ta wire mounts; there it could be heated resistively to 1550 K and cooled to 80 K using a liquid nitrogen flow * Note that we employ the 3-digit notation to index hcp Ru rather than the redundant 4-digit notation.
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167
loop. A W-S%Re/W-26SRe thermocouple was spot-welded to the sample to monitor its temperature. Cleaning procedures used for the Ru crystal are described in ref. [ll]. Lithium was dosed onto the Ru surface by moving the sample into a Li beam generated by resistive heating of a SAES Getters Inc. * source with a current of 6.3 A. The dosing times under these conditions ran from 2 to 200 s (after a - 30 s induction period) to obtain Li coverages ranging from approximately 0.01 monolayer to several monolayers. Auger measurements indicated that .the contamination introduced during Li dosing (mainly background CO) was less than a few percent of a saturated layer, even for long exposures that produced multilayer Li. Water exposures were produced through a microcapillary array doser that allowed the chamber to maintain a vacuum in the lo-” Torr range while the local H,O pressure at the sample (which was moved into the H,O beam) was raised up to lo-’ to lo-* Torr. Lithium and water doses were generally carried out with the Ru sample between 80 and 100 K; however, some of the experimental measurements performed on the chemisorbed overlayer involved heating the sample after adsorption. For example, in TDS measurements typical heating rates of approximately 5.5 K/s for H,O and 28.5 K/s for Li were used. The LEED and ESDIAD results were recorded photographically from a phosphor screen mounted behind a set of hemispherical grids and a microchannel array image intensifier [ll]. The current level of the electron gun was adjusted down to 5 X lop9 A during ESDIAD measurements to ensure that beam damage effects were minimized.
3. Results Individual adsorption experiments for Li and H,O on Ru(001) were performed prior to studying overlayers of coadsorbed H,O and Li. Fig. 1 shows a series of thermal desorption spectra obtained upon heating the Ru crystal following successive doses of Li. The coverage value for a given Li dose was determined by comparing the integrated area of the 7 amu desorption curve it yielded to the desorption curve area produced by a saturated first layer of Li (0G’ = 0.5 [14]). This saturated condition was assumed to exist when the amount of adsorbed Li produced the first noticeable sign of a multilayer desorption feature near 530 K (the multilayer feature became sharp and continued to grow as the Li doses were increased further). For relatively high values, coverage estimates determined from Li TDS areas could be checked * Commercial materials are identified in order to specify experimental such identification in no way implies recommendation or endorsement of Standards.
procedure adequately; by the National Bureau
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et al. / &adsorption
of water and LI on Ru(O01)
r
lNor
Li on Ru(001)
I
100
1
300
I
I
700
/
I
1100
TEMPERATURE
I
1300 (K)
Fig. 1. TDS spectra (smoothed) for Li desorbing from Ru(OO1)
using the known coverages for observed LEED patterns. As the Li exposures were increased, the overlayer at 80 K went through a (2 x 2) structure (B,, = 0.25) and a (6 X fi)R30” structure (0,, = 0.33) before exhibiting a series of compressed incommensurate phases [15]. Although most of the results reported here related to low Li concentrations, no ordered LEED patterns were observed from adsorbed Li coverages less than 0.25. The absolute coverages for even the smallest Li doses are believed to be correct to within a factor of two. Coverages for water adsorbed on Ru(OO1) were obtained using LEED. ESDIAD and thermal desorption measurements, and the details of these calibrations have been described previously [ll]. In reporting the conditions used in coadsorption experiments with Li, water exposures are expressed here in terms of the equivalent coverage that would be produced on the clean
S. Semancik
et al. / Coadsorption of water and Li on Ru(001)
WATER
DESORPTION
H20/Li/Ru(OO
1)
I
150
FROM H,O,
200
Q0
169
Li/Ru(OOl)
=0.6
I
250 TEMPERATURE
300 (K)
Fig. 2. The effect of increasing amounts of predosed Li on the thermal desorption the Ru(OO1) surface. The water dose in each case was equivalent to that producing 0.6 on clean Ru(001).
of H,O from a coverage of
Ru(001) surface. Note that the saturation H,O coverage on the clean Ru(001) surface has a surface atom density of 1.0 X 1015 molecules/cm2, corresponding to 4tzo = 0.66 monolayer; this is a “bilayer” of adsorbed H,O. The effects of interactions between coadsorbed water and Li are evident in the TDS results shown in fig. 2. The double-peaked spectrum in fig. 2a results from a water overlayer coverage of 0.6 on the clean Ru(001) surface, and it agrees with the published data [11,16-181. The A, feature in the spectrum is believed to develop when water desorbs from small H,O clusters, while the A, peak corresponds to desorption from the H,O bilayer [ll]. The spectra in figs. 2b-2f are produced when increasing amounts of Li are preadsorbed prior to dosing the surface with an amount of water equivalent to that used in the
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et al. / Coadsorption
of waterand Li on Ru(001)
experiment of fig. 2a. Even very small amounts of Li begin to alter the desorption curve shape in the region at 200 K and in the tail near 230 K. For a Li coverage of 0.03, a distinct new feature labeled B in fig. 2 becomes evident. A qualitatively similar feature developed in the TDS curves obtained for H,O-Na coadsorption on Ru(001) [4]. In that case, the feature was associated with water molecules which interacted with hydroxyl-like species that formed as H,O molecules began to dissociate. It seems likely that the B feature for H,O-Li has a similar origin; however, the preadsorbed Li coverages where this effect can be observed are considerably lower than for Na. As the preadsorbed Li coverage is increased, the intensity of desorption in the A z and A, temperature regions steadily drops off. The amount of B state desorption initially increases with Li coverage, then appears to pass through a maximum as the Li level is increased from near 0.1 to 0.17. Significant water desorption has also been reported at 565 K for H,O/K/Ru(OOl) [8] for higher potassium coverages, 8,2 0.33. Only two very small water desorption features were observed for the smaller lithium coverages used in this study, at - 550 and - 650 K. The integrated desorption flux in these features is about 5% of that which was recorded for the desorption occurring between 150 and 250 K. When the areas under the water desorption curves in fig. 2 are measured and plotted against the alkali coverage used in each TDS experiment, the solid curve shown in fig. 3a is obtained. The dashed curve included in this figure is for Na predosing, and was taken from fig. 2 of ref. [4]. In each case, the water doses corresponded to an effective coverage of 0.6 for clean Ru(001). The reduction in the amount of desorbed H,O is presumably connected to the loss of H,O to a decomposition channel, which opens up at Li coverages well below the 0.25 level indicated for the previous Na results. It is important to note that little or no dissociative reaction of H,O occurs when water is adsorbed on clean Ru(001) [11,17,18]. To further investigate this apparently high reactivity for adsorbed Li and water, measurements were done on coadsorbed layers formed with low doses of both Li and H,O. By using Li exposures yielding coverages < 0.1 and H,O exposures that produced a coverage of 0.1 (on clean Ru(001)). the interaction between Li and water was emphasized. Fig. 3b shows the results for interactions that occur when a temperature ramp from 80 to 300 K was applied to each of several coadsorbed overlayers. At these low H,O coverages. a drop in the total amount of desorbed water is noted even for 8ri -C 0.03, indicating that decomposition may occur as the surface is heated above 80 K, even at extremely low Li coverages. The amount of desorbed H,O dropped off steadily as the preadsorbed Li coverage was increased. Following each of the temperature ramps, an AES measurement of the residual oxygen level was made. The linear increase of the remnant oxygen as the Li increased (and the desorbed water amount dropped) further supports the interpretation that H,O molecules are lost to Li-assisted decomposition, which produces one oxygen
S. Semancik
a
et al. / Coacisorption of water and Li on Ru(001)
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0.04
0.05
0.06
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(Monolayer)
Fig. 3. (a) Solid curve - area under water desorption curves of fig. 2 plotted against Li coverage; dashed curve - results from similar experiments for H,O/Na/Ru(OOl), from ref. [4]. Water exposures were the same as for fig. 2. (b) Area under water desorption curves and AES oxygen signal measured following TDS experiments, plotted versus Li predose coverage. These coadsorption experiments were conducted at lower coverages of both water and Li than the results in the solid curve of (a). The water dose used would produce a coverage of 0.1 on clean Ru(001).
172
SURFACE CONDITION
S. Semanc~k et al. / Coadsorption of water and Li on Ru(001)
eLi
;yso
0
0.02
0.05
O.,
0.1
0.1
a
b
Fig. 4. ESDIAD H’ emission obtained from 0.1 monolayer of water adsorbed onto (a) clean Ru(OOl), (b) Ru(001) predosed with eLI = 0.02, (c) Ru(001) predosed with /I,_, = 0.05. Doses and photographic recordings were all done at 80 K.
atom at the surface (which could still be part of a hydroxyl species after heating to 300 K) for each original adsorbed H,O molecule. Although it seems evident that low coverages of Li are capable of inducing Hz0 decomposition on Ru(OOl), the results presented in figs. 2 and 3 are based on TDS measurements in which substrate heating was used. The dissociation effects observed may therefore have been thermally activated during these measurements. The TDS results also do not provide any conclusive information on the nature of the reaction products left on the surface. Electron stimulated desorption ion angular distribution (ESDIAD) measurements provide direct information on adsorbate structure which can be used to help determine the identity of surface species. This capability was extremely useful in monitoring the effects of Li on adsorbed water, both as a function of the Li coverage and as a function of surface temperature. Fig. 4 shows the H+ ESDIAD patterns obtained by adsorbing approximately 0.1 monolayer of H,O onto the Ru(001) surface at 80 K, and onto surfaces that contained preadsorbed Li at levels of 0.02 and 0.05. (Note: No ESDIAD patterns were observed for Li alone on Ru(OOl).) For water on clean Ru(001) (fig. 4a) the “halo” of H+ emission is believed to arise from H,O monomers bonded with random azimuthal orientation via their oxygen atoms or from H,O hydrogen-bonded to the edges of small, disordered clusters [ll]. Figs. 4a and 4b illustrate that the Ht ESD emission from the adsorbed water was greatly increased by the presence of only - 0.02 monolayer of Li. In addition,
S. Semancik et al. / &adsorption
SURFACE CONDITION
of water and Li on Ru(OO1)
add BLi = 0.03
add
173
6‘i
Z
0.04
ESDIAD H * emission (Image) b
Fig. 5. ESDIAD H+ emission obtained from 80 K Ru(OO1) surface (a) dosed with 0.1 monolayer of water, (b) after adsorbing BLi = 0.03 onto the preadsorbed H,O, (c) after adsorbing an additional BLi = 0.04 onto the surface of (b).
the characteristic halo from the low coverage of water on Ru(001) was reduced in radius while the annulus was broadened. A similar pattern was found for H,O + Na on Ru(OO1) [4], and interpreted as due to a reorientation (“tilting”) of molecular H,O. A slightly higher Li coverage was used to obtain the result in fig. 4c. The central portion of the ESDIAD pattern is produced by intense H+ emission perpendicular to the surface, which we believe results from Li-induced decomposition of water to form a hydroxide species oriented normal to the surface. Such species have a high cross section for ESD and very small quantities can be detected easily [19]. Both ESDIAD and TDS results indicate that H,O dissociation can occur in the presence of Li, but quantitative differences concerning the amount of Li necessary for dissociation are observed using these two techniques. At 80 K, a coverage of BLi = 0.02 appears to reorient the H,O molecules, but apparently does not cause appreciable dissociation; this suggests that the TDS behavior observed for the very lowest Li coverages may have involved thermal activation: molecular H,O is stable at 80 K to a Li coverage higher than that required to dissociate H,O upon heating. Fig. 5 shows further ESDIAD results involving somewhat higher levels of adsorbed water. The Hz0 dose used in these experiments gives a coverage of = 0.3 and produces the hexagonal ESDIAD pattern shown in fig. 5a. This is due to ESD of H+ from oriented H,O clusters on the Ru surface [ll]. When Li is added following the adsorption of H,O, the H+ emission pattern changes to a broadened halo of relatively high intensity. When an additional dose of Li is added to the surface, a bright central core develops suggesting the formation of hydroxyl species. The ESDIAD emission patterns pictured in figs. 5b and
174
S. Semancik
a.,55K
et al. / Condsorption of water and Li on Ru(OO/)
/:g$&
b.,85K
E S D I
e. 310K
f. 345K
A
D
Fig. 6. ESDIAD H+ patterns obtained from Ru(001) surface dosed to B,,, = 0.02 and 0,, = 0.1 and annealed briefly up to the indicated temperatures. Photographic recordings were done at 80 K.
5c are quite similar to those in figs. 4b and 4c obtained when water was adsorbed onto a surface containing preadsorbed Li. (The amount of Li needed to obtain these effects is somewhat higher in the results depicted in fig. 5, but that observation probably relates to the higher water coverage involved.) It appears that reorientation and reaction of water with Li in the coadsorption system is not significantly affected by the order of adsorption at low coverages. The clearest indication that the Li-assisted dissociation of H,O is thermally activated is provided by the ESDIAD patterns in fig. 6. The patterns were obtained from a surface having eL, = 0.02 and 0u,o = 0.1, which was heated in steps from 80 to 345 K. Up to at least 155 K, there is no obvious sign of normal emission, suggesting that a significant amount of H,O dissociation has not yet occurred for this low Li coverage. By 185 K, some normal emission becomes noticeable, and this central core gets more intense as the temperature is increased to 210 K, where the halo disappears. The emission intensity remains rather constant up to 310 K indicating stability of the hydroxyl species over that range. Between 310 and 345 K, the normal Hi emission is
S. Semancik et al. / Coadsorption of water and Li on Ru(OO1) EFFECT
1
OF Hz0
I
I
I
175
ON Li DESORPTION
I
I
I
I
Li Desorption
200
400600
800
1000
TEMPERATURE Fig. 7. TDS spectra
for Li desorbing
1200
13001350
(K)
from Ru(OO1) (a) dosed to tiL = 0.12, (b) dosed to Br, = 0.09 and BHzO = 0.6.
significantly reduced and by 360 K, ESDIAD indicates that the hydroxyl species yielding H+ has disappeared. To summarize, the patterns in fig. 6 illustrate thermally activated and Li-assisted dissociation of water, which produces hydroxyl species that are stable to > 300 K. Lithium remains on the surface to temperatures hundreds of degrees above the point (- 360 K) where ESDIAD Hf emission is last observed; this is demonstrated by the TDS trace in fig. 7b. It is not known if any significant amounts of Li are lost from the surface as some desorbing reaction product; species that might be expected to form (such as LiOH) were not detected in TDS measurements, however. Hydrogen desorption spectra were taken in an attempt to gain further insight into the nature of the surface complexes existing above room temperature, but high background desorption signals (probably from the sample holder) made meaningful interpretations of these results virtually impossible. Auger measurements indicate that remnant oxygen is present on the surface (see also fig. 3b) following the water decomposition reaction. As the surface temperature is increased, the chemical nature of the surface oxygen changes, probably from a hydroxyl-like species to adsorbed
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atomic oxygen at the higher temperatures. The TDS results for desorbing Li in fig. 7 show a comparison between the interaction of Li with: (a) clean Ru(001): (b) remnant oxygen from the H,O on Ru(001).
4. Discussion It is generally accepted that adsorption of an alkali metal atom on a metallic surface involves significant charge transfer from the alkali to the substrate and results in the formation of a relatively large surface dipole. This process is responsible for the large work function changes that have been recorded for Li adsorption [3] and for other cases of alkali adsorption [20]. In addition, the repulsive adatom-adatom interactions between the dipoles in an alkali overlayer ensure that island growth or two-dimensional cluster formation do not occur at low coverages; thus, for example, for 13< 0.2 no ordered LEED patterns are seen for adsorbed Li [15] or Na [13] on Ru(001). (At higher coverages (0 > 0.25) for these adsorbates on Ru(OOl), a series of ordered LEED patterns can develop [15,21].) Although the qualitative properties of many alkali overlayers on different metallic substrates are similar, differences have been observed as well. For example, Sass et al. [3] have reported that Li dissolves into Ag; we observed no such effects for Li on Ru(OO1). The differences discovered in the orientational ordering behavior of Na [21] and Li [15] on Ru(001) are probably related to the different electronegativities and ionic radii of these two alkalis. When considering the ways in which surface alkalis can modify reactivity, the picture is again one of similar general behavior among different alkali/substrate systems, but with clear differences in kinetic detail. This scheme seems to apply in the case of alkali modification of water adsorption on ruthenium; the lithium-H,0 interaction reported here, for example, appears to be stronger than the Na-H,O interaction. 4.1. Influence
of Li on the structure of udsorbed molecular
water
When H,O is adsorbed onto a clean Ru(001) surface at 80 K, a dim halo-like Ht ESDIAD pattern is observed at low H,O coverages. It is believed that H,O is adsorbed oxygen-end down via the lone-pair orbital(s), with H atoms pointed away from the surface [ll]. The halo ESDIAD pattern may be due in part to isolated H,O monomers which are either randomly oriented in an azimuthal sense, or freely rotating; azimuthally disordered clusters of H,O may also contribute to the halo. When Li is coadsorbed with H,O. the intensity of the halo annulus is greatly increased. This behavior is very similar to that observed when water is coadsorbed with 0.1 monolayer of Na on Ru(001) (ref. [4], fig. 4).
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171
We suggest that the increase of Ht intensity in the ESDIAD halo and the broadening of the halo annulus are both a consequence of reorientation of adsorbed H,O molecules in the presence of Li. Lang et al. [22] have studied the electrostatic interaction between an adsorbed atom (e.g., Li) and an adsorbed molecule. For molecules with permanent dipole moments (e.g., H,O), their calculations indicate that the molecule can “turn around” to minimize energy in the electrostatic field of the adsorbed atom. Independent evidence for reorientation of H,O and NH, by both electronegative and electropositive additives has been seen in other ESDIAD studies [10,23,24]. For H,O on clean Ru(OOl), the O-H bond angle is close to the “cut-off” angle for ion desorption [25]. That is, the desorbing ions have a high probability of recapture due to the image interaction between the H’ ion and the conducting substrate. If the H,O molecule is tilted so that one of the O-H bonds is inclined with its polar angle closer to the surface normal, the Hf ion desorption probability increases dramatically. Variable amounts of molecular tilt for the adsorbed H,O resulting from a range of Li-H,O separations could explain, in part, the broadening of the halo; large amplitude bending or vibrational modes of adsorbed H,O will also contribute to its width. It does not seem necessary to invoke geometrical changes within H,O molecules to explain the ESDIAD halo; the slight Li-induced changes in the H,O binding energy to Ru are not consistent with a major intramolecular structural change. The interaction between molecular water and surface Li which produces the enhanced H+ ESD halo emission from H,O also gives rise to a new desorption state in the TDS measurements. This state, which is best observed by comparing spectra b and c to spectrum a in fig. 2, occurs just below 200 K, and fills in the valley between features A, and AZ. The development of this state with added Li is consistent with the kind of downward shift in temperature of a desorption feature for a polar molecule (like H,O or NH,) caused by an electropositive coadsorbate. Such behavior has been predicted by Norskov et al. [26] and by Stair [27], and has been observed experimentally, e.g., by Benndorf and Madey [23]. 4.2. Lithium
induced decomposition
of adsorbed water
On clean Ru(OOl), water adsorbs and desorbs reversibly, and little or no decomposition of water is observed during TDS experiments Ill]. The behavior is qualitatively different when water is coadsorbed with Li or Na. The interactions between water and the alkalis create a new upshifted desorption state (state B at 230 K for Li and 240 K for Na), as evidenced in the TDS spectra in fig. 2 and in fig. 1 of ref. [4]. These features are believed to be connected to surface water that partially bonds to surface hydroxyl-like species that themselves produce central core ESD emission - the ESDIAD signature that H,O decomposition has occurred. The B-type state formation in
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Li on Ru(OO1)
the range of 230 to 240 K represents a temperature upshifted H,O desorption feature due to an interaction with an electronegative additive (OH), again in agreement with predictions and previous observations [4,23,26,27]. Our results do not permit a definitive statement concerning the nature and structure of the water decomposition products. Possible products include H and OH bound to Ru, as neighbors to Li atoms; other possible products include LiOH and LiH. However, bulk LiH reacts vigorously with H,O to form LiOH and H,, so that with an excess of surface H,O it is more likely that any stable surface complex would be hydroxide rather than hydride. In fact, in EELS studies of the H,O/K/Ru(OOl) system, Thiel et al. [8] reported the formation of KOH, and it seems likely that the species formed in our present and previous experiments would be LiOH and NaOH [28]. Moreover, for Li (as well as Na), the ESDIAD data indicate that H+ desorbs along directions about the normal to the surface; we identify this emission as due to an OH species, perhaps bonded as LiOH. Although LiOH may have been present at the surface, none was observed during thermal desorption measurements. As mentioned above, OH may also have been bonded directly to Ru near the Li atom. In this case, the OH dipole might be expected to “tilt” due to interaction with Li, giving rise to a halo of ESDIAD emission like that in fig. 4a. The halo intensity visible in addition to the normal emission in figs. 5 and 6 is assumed to be due to residual reoriented H,O; no halo emission is seen after heating to desorb molecular water, leaving hydroxyl species on the surface. Therefore, we conclude that dissociation to form hydroxide occurs at the surface, with OH bonded primarily with its bond directed along the surface normal. The TDS results show no evidence for an OH recombination process to form H,O in the temperature region near 300 K. This is in contrast to the water formation from hydroxyl recombination which occurs on Cu(ll0) [29] and Ni(ll0) [30]. The small amount of water which desorbs at temperatures near 600 K might be connected to the decomposition reaction: 2LiOH + Liz0 + H,O. The bulk LiOH dissociation occurs rapidly at 1197 K [31], but a lower reaction temperature could be expected on a surface. Despite the qualitative similarity of the effects for H,O/Li and H20/Na coadsorption layers on Ru(OOl), key quantitative differences do exist. For example, the relative alkali coverages producing the ESDIAD and TDS results discussed above are significantly lower for Li than Na. The stronger reactivity of Li than Na is evidenced most clearly by considering the alkali onset coverages necessary for inducing H,O dissociation on Ru at 80 K. ESDIAD results (and remnant AES oxygen levels) indicate this value to be - 0.05 for Li and - 0.25 for Na. The repulsive interactions between Li adatoms also imply that the observed dissociation would not be occurring at local regions of high coverage (islands). In work on Pt(lll), potassium at a level of OK = 0.07 was required to produce H,O dissociation, both at 305 and 100 K [5,6]. It is
S. Semancik et al. / Coadrorption of water and Li on Ru(001)
119
also worth noting that Thiel et al. [8] used EELS to study KOH formation from H,O and K on Ru(001); they reported that H,O dissociation occurs at /3x = 0.33 at 80 K. However, their results do not demonstrate conclusively whether or not water decomposed below room temperature when coadsorbed with potassium at coverages significantly below 0.33. Dissociation can apparently be thermally activated for both the Li and Na cases on Ru(OOl), since decomposition was observed to occur at lower alkali coverages when the crystal was heated. (Note that observable hydroxyl concentrations were produced below B,, = 0.25, at eNa = 0.1 at 230 K (fig. 7c of ref. [4]) [32].) Since the (2 x 2) LEED pattern that forms for Li is less sharp than the (2 x 2) for Na on Ru(OOl), and develops for Li only at the coldest available temperatures (- 80 K), it would appear that Li is somewhat more mobile than adsorbed Na; however, it should be noted that Li, Na and H,O all have at least some mobility even at 80 K since all can form ordered LEED patterns between coverages of 0.25 and 0.33. Fig. 3 shows that Li-induced dissociation of Hz0 occurs, even at very low Li coverages, when a heating ramp to 300 K is applied to the Ru crystal. The drop off in the amount of water that desorbs (fig. 3b) as the level of preadsorbed Li is increased is linear with a slope between one and two. When plotted against the Li preadsorption coverage, the amount of oxygen build up from H,O dissociation found after’ the heatings increases with a slope greater than one. These changes both suggest that, at least for low coverages of water and Li, a linear proportionality exists between the number of water molecules and the number of reacting Li atoms. Experiments done with Na instead of Li included an induction period to greater than 19,~ = 0.2 in which no significant oxygen build up or decrease in water desorption occurred [33] (dashed curve in fig. 3a). (At the higher Na exposures, a nearly 1 : 1 ratio exists between the number of Na atoms and the number of decomposed water molecules [4].) 4.3. Why does a critical coverage of Li exist for H,O decomposition? In considering the coadsorption of H,O and alkali atoms on metal surfaces, it is clear that H,O will invariably dissociate to form hydroxyl products when the substrate temperature and the alkali coverage are high enough. The key questions are: why is there a critical coverage for alkali-induced dissociation of H,O at 80 K, and why do the critical coverages vary so strikingly for different alkali/metal systems (Li/Ru, Na/Ru, K/Pt)? In particular, why is Li so much more reactive than Na/Ru or K/Pt at low coverages? The answer must involve a combination of factors including the degree of charge transfer as a function of alkali coverage, the electronic structure and relative atomic sizes of the adsorbed alkalis, the local electrostatic fields near the adsorbed alkali, and the local alkali and H,O binding sites. It is difficult, however, to isolate the key factor(s) controlling the reactivity of Li. Doering et al. [4] and Kiskinova
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et a!. / Coadsorption
ofwuter and Li on Ru(001)
et al. [5] have suggested that H,O dissociation induced by coadsorbed alkalis may be related to occupancy of the alkali valence s-orbitals: the more ionic alkali at low coverages (low s-orbital occupancy) is less reactive than the more metallic alkali at high coverages (high s-orbital occupancy). This charge-transfer argument was based largely on the well-known coverage dependence of the work function for alkalis adsorbed on transition metals. In a recent paper, however, Woratschek et al. [34] have measured directly the occupancy of the potassium 4s-orbital as a function of coverage on Cu(ll0) using Penning ionization to quantify the charge transfer, and they find that significant partial s-orbital occupancy occurs even at low coverage. Although similar measurements are not available for Li on Ru(OOl), we have no basis for suggesting that the s-orbital occupancy will vary strongly in the critical coverage range, tYLi= 0.02 to 0.05, and do not believe this is the major factor in the Li-coverage dependence of H 2O dissociation. In general, lithium is a less reactive metal than Na or K. This inherent unreactivity is offset in solution, however, by the exothermic hydration of the very small Lit ion. In fact, the enthalpy of formation of (HzO)Lif is 34 kcal/mole, considerably greater than the enthalpies of formation of (H zO)Naf (24 kcal/mole) or (H,O)K+ (16.9 kcal/mole) [35]. The Li+ ion is exceptionally small, and has, therefore, an exceptionally high charge-radius ratio. This leads to anomalous behavior of a number of Li compounds in relation to the other alkalis. For example, LiOH decomposes at red heat to Li,O + HzO, whereas the other hydroxides sublime unchanged [36]. It is tempting to suggest that the higher reactivity of adsorbed Li (relative to K and Na) is related to its small size and partial ionic character, but the calculations of Lang et al. [20] do not appear to support such a picture. These authors have used an effective medium theory to compute, self-consistently, the induced electrostatic potential for Li, Na and K at their calculated equilibrium distances outside a jellium surface. Their results indicate surprisingly small differences between the electrostatic potential contours for adsorbed Li and Na: it is not obvious from the calculations why the reactivity for these two alkalis should be significantly different at low coverages. Moreover, if a critical electrostatic field is necessary for dissociation, there should be little difference between the critical coverages of Li and Na. Perhaps substrate crystallographic effects not considered in the jellium calculations have an influence on the relative reactivities. Since the results indicate that Li is so reactive toward H,O, it is unclear why molecular water should be stable at 80 K for the very low range of Li coverages (up to B,i = 0.05). One possibility is that substrate steps and defects are playing a role in the surface chemistry. Perhaps the low-coverage Li is trapped at point defects or step/kink sites where its electronic structure differs from Li adsorbed on the planar terraces.
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181
5. Conclusions Several techniques have been used to obtain a consistent picture of the interaction of water with Li on Ru(001). ESDIAD, TDS and AES measurements all indicate that decomposition of water occurs at 80 K when Li is preadsorbed to levels greater than - 0.05 monolayer. Upon heating, H,O dissociation can be produced by even lower levels of adsorbed Li. Previous work has shown, that similar effects occur with Na on Ru(001) only for coverages 2 0.25 monolayer (> 0.1 upon heating). ESDIAD patterns observed following water-alkali reactions show emission normal to the crystal surface, and suggest that the hydroxyl products have an upright orientation.
Acknowledgements The Department of Energy, Division of Basis Energy Sciences and the University of Florida, Division of Sponsored research (Grant No. RDAl7983-84) are gratefully acknowledged for partially supporting this work.
References [l] H.P. Bonzel, J. Vacuum Sci. Technol. A2 (1984) 866. [2] See the discussion on electropositive additives in the review of water adsorption by P.A. Thiel and T.E. Madey, Surface Sci. Rept., to be published. [3] J.K. Sass, K. Bange, R. Dohl, E. Piltz and R. Unwin, Ber. Bunsenges. Phys. Chem. 88 (1984) 354. [4] D.L. Doering, S. Semancik and T.E. Madey, Surface Sci. 133 (1983) 49. [5] M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 150 (1985) 319. [6] G. Pirug, A. Winkler, M. Kiskinova and H.P. Bonzel, Symposium on Surface Science, Obertraun, Austria, 1985, p. 95; H. Bonzel, private communication. [7] H.P. Bonzel, G. Pirug and A. Winkler, Chem. Phys. Letters 16 (1985) 133. [8] P.A. Thiel, J. Hrbek, R.A. DePaola and F.M. Hoffmann, Chem. Phys. Letters 108 (1984) 25. [9] E.M. Stuve, K. Bange and J.K. Sass, in: Proc. AS1 Study Institute of Interfacial Electrochemistry, Ed. A.F. DeSilva, Oporto, 1984, in press. [lo] T.E. Madey, C. Benndorf, N.D. Shinn, Z. MiSkoviC and J. VukaniC, Desorption Induced by Electronic Transitions, Springer Series in Surface Sciences, Vol. 4, Eds. W. Brenig and D. Menzel (Springer, New York, 1985) p. 104. [ll] D.L. Doering and T.E. Madey, Surface Sci. 123 (1982) 305. [12] T.E. Madey and J.T. Yates, Jr., Surface Sci. 63 (1977) 203. [13] D.L. Doering and S. Semancik, Surface Sci. 129 (1983) 177. [14] As discussed in ref. [15] the theoretical saturation coverage is 0.53, but the maximum coverage experimentally observed in the monolayer case was 0.46. [15] D.L. Doering and S. Semancik, Surface Sci. 175 (1986) L730. [16] T.E. Madey and J.T. Yates, Jr., Chem. Phys. Letters 51 (1977) 77. [17] P.A. Thiel, F.M. Hoffmann and W.H. Weinberg, J. Chem. Phys. 75 (1981) 5556.
182
[1X] [19] [20] [21] [22] [23] [24] [25] [26] [27] [2X]
[29] [30] [31] [32]
[33] [34] [35] [36]
S. Semancik
et ul. / Coadsorption
of wuter and LI on Ru(001)
P.A. Thiel, R.A. DePaola and F.M. Hoffmann, J. Chem. Phys. 80 (19X4) 5326. R. Stockbauer, D.M. Hanson. S.A. Flodstrom and T.E. Madey. Phys. Rev. B26 (1982) 1885. See, e.g., R.L. Gerlach and T.N. Rhodin. Surface Sci. 19 (1970) 403. D.L. Doering and S. Semancik, Phys. Rev. Letters 53 (1984) 66. N.D. Lang, S. Holloway and J.K. Norskov, Surface Sci. 150 (1985) 24. C. Benndorf and T.E. Madey, Chem. Phys. Letters 101 (1983) 59. K. Bange, T.E. Madey and J.K. Sass, Surface Sci. 162 (1985) 252. Z. MiSkoviC, J. Vukanic and T.E. Madey, Surface Sci. 141 (1984) 285: 169 (1986) 405. J. Norskov, S. Holloway and N.D. Lang. Surface Sci. 137 (1984) 65; J. Vacuum Sci. Technol. A3 (19X5) 1668. P.C. Stair, J. Am. Chem. Sot. 104 (1982) 4044. It is interesting to note that water adsorption onto a Li substrate has been suggested to yield a H,O-Li complex. rather than a reaction product like LiOH. See: W. McLean, J.A. Schultz. L.G. Petersen and R.C. Jarnagin, Surface Sci. 83 (1979) 354. In the case of water interaction with 30-110 A thick K films. H.P. Bonzel, G. Pirug and A. Winkler report H,O adsorption dissociation and formation of KOH followed by dissolution and hydration (Surface Sci. 175 (1986) 287). K.Bange. D.E. Grider. T.E. Madey and K.J. Sass. Surface Sci. 137 (1984) 38. C. Benndorf, C. Nob1 and T.E. Madey, Surface Sci. 138 (1984) 292. CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton. FL. 1978). The effects of heating on water dissociation were evident for the case of Li using both ESDIAD (fig. 6) and TDS (fig. 3b). In the case of Na, there is an indication that only small amounts of water might dissociate between B,, = 0.1 and 0.25 and that ESDIAD is more sensitive to such small amounts of H,O dissociation than is TDS (see fig. 7c of ref. [4] and fig. 3a of this paper). The local maxima in the Na and Li curves in fig. 3a appear to be real, but the effect is not understood. B. Woratschek, W. Sesselmann, J. Kippers. G. Ertl and H. Haberland. Phys. Rev. Letters 55 (1985) 1231. A.W. Castleman. P.M. Holland, D.M. Lindsay and K.I. Peterson, J. Am. Chem. Sot. 100 (1978) 6039. J.E. Huheey, Inorganic Chemistry (Harper and Row, New York, 1978) p. 706.