Langmuir 1995,11, 1201-1214
1201
Water Adsorption and Coadsorption with Potassium on Graphite(0001) D. V. Chakarov, L. Osterlund, and B. Kasemo" Department of Applied Physics, Chalmers University of Technology and Gijteborg University, S-412 96 Giiteborg, Sweden Received October 10, 1994. In Final Form: January 23, 1995@ Water and water coadsorbed with potassium on the basal plane of graphite were studied with thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS)in the temperature range 85-900 K. Water alone adsorbs nondissociatively on the clean graphite surface at 85 K, forming hydrogen bonded aggregates. Its structure depends both on the coverage and on substrate temperature. With increasing coverage at 85 K (0.5-1.0monolayer(ML)) the libration mode at -86 meV shows a rapid upward shift, indicating a phase transition from a 2D to a 3D structure. The transition can also be induced by annealing the low coverage structure. Water coadsorption with potassium is nonreactive or reactive,dependingon temperature and potassium coverage. The nonreactivecoadsorption at T8= 85 K occurs only below a critical potassium coverage of OK 5 0.3 ML. It is characterized by substantial symmetry changes of the adsorbed water molecules, compared to the pure water adsorption, and is attributed to formation of hydrated-ion species on the surface. The surface solvation number at the lowest Kcoverage is three to four Hz0 molecules per potassium atom. Kand HzO react at submonolayer coverages at 120-160 K to form surface KOH, KH, GO,, and volatile products. The surface species gradually transforms/decomposesat elevated temperatures (200-500 K) to first form potassium-oxygen complexes that then serve as precursors to graphite oxidation to COz at -750 K.
I. Introduction A large number of studies have addressed the interaction ofwater with clean and alkali metal (AM)coveraged metaZsurfaces.1!2Surprisingly few such studies have been made on well-characterized carbon su$aces,3 in spite of their importance in scienceand in technological(including biomedical) application^.^^^ Due to the high bond saturation of the carbon atoms in the basal plane of graphite, one might expect the chemical water-substrate interaction to be weaker, and the HzOHzO lateral interactions to be less perturbed by the substrate, compared to metal surfaces. Studies of HzO and AM-HzO interaction on graphite should provide valuable information for comparison with corresponding results on metal surfaces, addressing such questions as water binding strengths and orientation on clean and AM covered surfaces, respectively (including wettinglnonwetting properties), possible formation of hydrated alkali ions, the critical AM coverage (if any) for HzO dissociation, the reaction paths, and the nature of reaction complexes (solid and volatile)formed upon water dissociation. Alkali metal coadsorption with oxidizing molecules like 0 2 and HzO might also show similarities with alkali metal promoted oxidation of silicon,with a major differencebeing that final product CO or COZ is volatile, in contrast to Si02 formed in silicon oxidation. Technologically,watedwgraphite interactions are of interest, e.g., as model systems for carbon gasification reaction^.^-^ The catalytic effect of alkali compounds in such reactions is well established experimentally and is technologically e m p l ~ y e d but , ~ , ~the reaction mechanism(s) and the nature of the alkali metal complexes formed in Abstract published inAdvanceACSAbstracts, March 15,1995. (1)Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987,7,211. (2)Bonzel, H.P.; Pirug, G. In Coadsorption, Promoters and Poisons; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1993;Vol. 6. (3)Kelemen, S. R.; Freund, H.; Mims, C. A. J . Vac. Sci. Technol. 1984,A2,987. (4)McKee, D.W. Chem. Phys. Carbon 1981,16,1. (5)McKee, D. W. Fuel 1983,62,170. (6)Kelemen, S. R.; Freund, H.; Mims, C . A. J . Catal. 1986,97,228. ( 7 ) Papers presented at a symposium on Fundamentals of Catalytic Coal and Carbon Gasification, Fuel 1983;Vol. 62;140 pp. @
the course of reactions are still far from satisfactorily kno~n.A ~ central ~ ~ J ~issue is the identity of the chemical state(s) of the catalytically active alkali specie(s). Such identificationswould lead to a better mechanistic description of AM-catalyzed reactions. The combination of vibration spectroscopy (HREELS in our case) and TDS is a useful combination for such studies, providing identification of both surface species and volatile products. In this paper we present results from an experimental study in the temperature range 85-900 K of (i) water adsorption on the clean (0001) graphite surface at ultrahigh vacuum (UHV) conditions and low (85 K) temperature, (ii) coadsorption of HzO K at low temperature, (iii) thermal reactions between the two adsorbates on the surface, and (iv) adsorbatesubstrate reactions ultimately forming COZ. A previous TDS study from our laboratory,ll which partly dealt with the reaction kinetics of the same system, is now extended with new experimental data exploringlower temperatures and, most importantly, employing HREELS for spectroscopic identification of the surface complexes. The paper is organized as follows: section I1 briefly describes the apparatus and the experimental procedures. Section I11 presents the results from TDS and HREELS experiments. The discussion (section IV) concentrates on four topics: (i) water adsorption on clean graphite at 85 K, (ii)the low-temperature nonreactive coadsorption of K with HzO at low K coverages, i.e., the formation of hydrated K ions, and accompanying changes in water structure (molecule orientation); (iii) water dissociation and its products at higher K coverages; (iv) the sequence of thermally stimulated reactions occurring in the coadsorbed layer upon heating, ultimately leading to COZ formation. Section V summarizes the paper.
+
(8)Cabrera, A.L.;Heinemann, H.; Somorjai, G. J . Catal. 1982,75, 7.
(9)Moulijn, J.A.; Cerfontain,M. B.; Kapteijn, F. Fuel 1984,63,1043. (10)Mims, C. A.; Pabst, J. K. Fuel 1983,62,176. (11)Sjovall, P.; Kasemo, B. Surf. Sci. 1993,290,55.
0743-746319512411-1201$09.00/00 1995 American Chemical Society
Chakarou et al.
1202 Langmuir, Vol. 11, No. 4, 1995
11. Experimental Section The experiments were performed in a UHV system (base pressure 1 x 10-10 Torr) described in detail elsewhere.12 The sample, highly oriented pyrolytic graphite (HOPG, grade ZYA, Union Carbide Corp.), exposing the freshly cleaved basal plane, was resistively heated up to 1100 K and cooled by liquid nitrogen down to 85 K. A chromel-alumel thermocouple was used to measure the sample temperature. Potassium depositions were in all experiments done a t 85 K and a t the base pressure, using a well-degassed getter source (SAESgetters) encased in a collimatortube. Potassium coverage calibration was made by means of a quartz-crystal microbalance in combination with TDS, LEED, and work function measurements.12 All potassium coverages will be given in fractions of a monolayer (ML), where 1 ML is defined to be 5.4 x 1014 atoms*cm-2,and approximately correspondsto the surface density of a closed packed potassium layer. The H2O (distilled, deionized) was degassed by many freezepump-thaw cycles prior to use. A stainless steel gas line and reservoir were used for the water supply. The H2O vapor pressure in the reservoir was measured with a capacitance manometer. The gas in the line was exchanged before each dosing. Sample exposures to the water vapor were accomplished by feeding it through a quartz glass tube (8 mm diameter) directed at the sample surface and terminating 1-2 mm from the surface, in order to minimize water contamination of the chamber. The local H2O pressure a t the sample was typically -103 times larger than the H2O background pressure. Reproducible doses, using the exposure time and the water vapor pressure in the supply volume as parameters, were achieved keeping the leak valve opening and the distance between the doser and the front of the sample constant. Since the dose units are unique to the geometry of our system and not in a simple manner related to the absolute HzO coverage, they were calibrated by comparison with TDS peak areas obtained from background H2O exposures. The calibration was done assuming a vacuum gauge sensitivity for H2O of 1.1relative to N213and a coverage independent, constant sticking coefficient of unity at 85 K. One monolayer (ML) of water is taken to be equal to 1.15 x 1015 molecules.cm-2 corresponding to an exposure of 2.4 langmuirs (1 langmuir = 1 0 - 6 T o r r ~ - or ~ , 4.79 x l O I 4 H20 molecules.cm-2*s-1). Physically the coverage corresponds t o the density of a finite bilayer cluster, consistent with Bernal-Fowler-Pauling rules.' The TD spectra were obtained recording the mass spectrometer signal of desorbing products a t a constant heating rate of the sample of 2.5 K-s-l (if not otherwise stated). The quadrupole mass spectrometer (Balzers, MSQ3111, operating in the pulse counting mode, was positioned with its axis perpendicular to the sample surface and with a distance of 2 mm between the surface and the entrance aperture (2 mm diameter) of the cross beam ion source, a geometry that, due to the high pumping speed, especially for HzO and K, strongly discriminates the detection of desorbing products from the manipulator and sample supp o r t ~ .The ~ ~ mass spectrometer (MS) was multiplexed so that multiple desorption products (up to six) could be monitored (practically) simultaneously. The high-resolution electron energy loss spectroscopy (HREELS) measurements were performed with a LeyboldHeraus ELS22 spectrometer. The spectra were recorded in a single electron energy scan (typical recording time 40 min) with a primary energy of 5 eV, in specular reflection geometry and with an incident angle of 60". A typical elastic peak intensity of 50 kHz and a full width at a half maximum (fwhm) of -6 meV were routinely obtained for the clean graphite surface. The surface reflectivity drops -10 times when adsorbates are present on the surface. In order to prevent collection of misleading spectral features of the studied layers (due to radiation, contamination, or other postadsorption effects) a TD spectrum was as a rule measured aRer each HREELS scan and compared with one recorded immediately after adsorption. Usually the spectra were identical; otherwise the measurement was disregarded. (12) Chakarov, D. V.; Osterlund, L.; Kasemo, B. To be submitted for publication. (13)Balzers operating manual for ionisationvacuum gauge IMR132.
100
120
140
160
180
200
Temperature (K)
Figure 1. Thermal desorption spectra (B = 2.5 K-s-l) of water adsorbed on clean graphite surface a t T = 85 K. The initial coverage varies between 0.2 and 4.0 ML. The insert shows the percentage of intercalated water as a function of initial coverage (see text). h
5
H20 on graphite (0001)
ea
T=85K
v
0
2
4
6 8 1 0 Exposure (L) Figure 2. Coverage of water adsorbed on graphite(0001) surface a t 85 K versus water exposure as estimated from the TD peak areas.
111. Results
3.1. Water Adsorption on the Clean Graphite Surface. 3.1.1. Thermal Desorption Spectra of HzO. Figure 1shows thermal desorption spectra of H2O on the clean graphite(0001) surface after exposure to H20 at 85 K (obtained by monitoring the mle = 18 ion signal of the MS). Water desorbs molecularly in a single peak with a common rise at low T for all coverages and with a peak maximum at 148 K at low coverage ( 6 H z O 0.25), which moves toward higher temperatures with increasing coverage. The peak area grows linearly with increasing exposure with no sign of saturation (Figure 2). A single desorption peak has been observed also on other surf a c e ~ ' ~and , ~ is ~ characteristic for a weak substrateadsorbate interaction. The observed temperature shift of the peak maximum and the shape of the main desorption peak (the leading edge rises exponentially and the high temperature edge has a sharp drop) are in accord with an approximatezeroth order desorption. The lack of saturation, the order ofthe desorption kinetics, and the temperature range of the TD (14) Hinch, B. J.; Dubois, L. H.
J. Chem. Phys. 1992, 96, 3262. (15)Bange, K.; Madey, T. E.; Sass, J. K. Surf. Sci. 1987,183, 334.
Water Adsorption
Langmuir, Vol. 11, No. 4, 1995 1203 T (K) 161
156
152
147
H20 on graphite (0001)
I 6
6,2
6,4
6,6
6,8
7
l o 3 (K.') Figure 3. Arrhenius plot for ice sublimation from clean graphite surface based on TDS data. The solid circles correspond to the data of Figure 1 and the open circles are additional measurements. The error bars in Figures 1-3 represent the uncertanity of a single measurement,mainly due to the uncertanity in the initial coverage.
1
T" x
maximum all support the assignment of this state to sublimationof ice. The Arrhenius plot (Figure 3, assuming strictly zeroth order kinetics) yields a straight line, with no deviation from linearity outside the experimental uncertainty. The derived desorption energy is 0.45 f 0.03 eV per molecule. The obtained sublimation energy is in good agreement with other reported values, obtained on other surfaced6and by other technique^.^' The value can be compared with the enthalpy of sublimation of H2O ice I, which at 0 K is 0.49 eV per mo1ecule.l' In addition to the ice sublimation peak there is a small but significant fraction of the adsorbed water molecules (less than 3% for one monolayer initial coverage) that desorbs in additional peaks at higher temperatures, around 180 K, as shown in Figure 1. The intensity of these peaks depends on the initial coverage, on the temperature of the substrate during H20 exposure, and on the residence time prior to desorption, i.e., the time between deposition and desorption. The inset of Figure 1shows how the amount of this more stable water depends on the initially deposited water coverage (as determined from the integrated peak areas (PA) for different initial coverages at 85 K, at a constant residence time of 500 8). Note in the inset of Figure 1 that there is a threshold coverage below which the high temperature peaks are not observed. Note also that the growth of these peaks is initially strongly nonlinear with increasing coverage. For larger doses the high temperature peaks seem to constitute a constant fraction (-10%) of the deposited amount. After a careful check for possible artifacts, and then observing the similarities in the appearance and behavior ofthese peaks with TDS peaks from intercalated potassium,12they are tentatively assigned to intercalated water. The kinetics of water intercalation into the graphite bulk will not be fruther discussed here, but is only mentioned as a quite surprising result. The total HzO coverage, represented by the integrated thermal desorption peak areas (PA) (including the intercalated water), as a function ofexposure is shown in Figure 2. The data can be fit by a single straight line with a correlation coefficient of 0.996, indicative of HzO adsorption into a condensed phase with a constant, coverage invariant, sticking coefficient. (16) Klaua, M.; Madey, T. E. Surf. Sci. 1984, 136, L42. (17)Eisenberg, D.; Kauzmann, W. The structure and properties of water; Oxford University Press: London, 1969.
0
100 150 200 250 Time (s) Figure 4. Dashed line: TD spectrum (p = 0.5 K-s-l) of 1.45 ML water adsorbed on clean graphite surface at 85 K. The solid line shows MS signal for the same initial coverage but with interruption ofthe temperature ramp at T = 141 Kfor 150 s. Note the almost constant rate of desorption down to a coverage corresponding to -1 ML. Initial oscillation of the signal is a result of a nonperfect temperature regulation.
Zl 0
50
I
4 '
1
I
I
I
I
I
1 5 0 200 250 300 Time (s) Figure 5. Isothermal desorption of water from graphite at T = 143 Kploted in semilogaritmic scale. Initial water coverages were 0.18(a),1.1 (b), and 2.4 (c) ML; ,& = 0.5 K-s-l, / 3 ~= 2.5
0
50
100
K*s-l. In order to explore more carefully for possible deviations from the apparent zero-order desorption kinetics, we performed additionalTD measurements,applying a slower heating rate of 0.5 K-s-l, thus resolving better the leading edges of the TD peaks. Furthermore, a few isothermal desorption rates were measured at different initial coverages: Some of the latter experiments are shown in Figures 4 and 5. The desorption rate at constant temperature is initially constant (within our nonperfect experimental accuracy) as expected for zero order kinetics. (The oscillatorybehavior between 50 and 100 s in Figures 4 and 5 is due to the heating circuit, which does not keep the temperature exactly constant, but makes it oscillate f 0 . 5 Kimmediately after interuption ofthe ramp.) Thus the leading edge ofthe desorption peaks in the TD spectra at different coverages is common for all peaks and the initial desorption rate at a fxed temperature is constant over a fairly large coverage range, which are both observations consistent with zeroth order kinetics. How-
Chakarov et al.
1204 Langmuir,Vol. 11, No. 4, 1995
H 2 0 on graphite (0001)
t :
1
!25 ~ 1 0 0
1 0 0 200 300 400 500
0
Energy Loss (mev) Figure 6. HREEL spectra for clean graphite surface (a),and after deposition of 0.18 monolayer ofwater (b),0.35 (c), 0.75 (d), and 2.5 ML (e) at 85 K. The elastic peak shown corresponds to spectrume. Primary electron energy 5 eV, specular scattering
(ei = er = 600).
ever, we also found (Figures 4 and 5) that the isothermal desorption rate after the initially constant and coverage independant range eventually decreases approximately exponentially, when the remaining amount of water on the surface approaches 1 ML. The latter observation identifies a second desorption regime indistinguishible in the TD spectra. The second regime is characteristic for coverages I1ML, showing first-order desorption kinetics in the isothermal runs, as indicated by the exponential decrease of the desorption rate as a function of time.l8 The latter is illustrated in Figure 5 showing a plot of the logarithm of the desorption rate vs time. The change in desorption kinetics is not the result of an experimental artifact, connected for instance with (i) pumping speed effects or (ii)with temperature anisotropies. The first of these points could be checked by the sudden drop of the signal when turning off the heating and cooling down: The mass spectrometer signal returned to the background level almost instantaneously. The proof of the second point is the obvious coverage dependence of the desorption kinetics. The difference between the ramped TDS results (apparent zeroth order kinetics over the whole coverage regime) and the isothermal kinetics at coverages 5 1ML is attributed to a combination of the different sensitivities of the two methods to the exact kinetics and the different annealing histories of the overlayer in the two cases (see Discussion, section N). 3.1.2. High-ResolutionElectron Energy Low Spectra of HzO. Figure 6 shows high-resolution electron energy loss spectra of water adsorbed at different coverages on the clean graphite surface at T = 85 K. The characteristicvibration spectrumof HzO in the gas phase consists ~~
~
(18)Opila, R.; Gomer, R. Surf. Sci. 1981,112, 1.
of bands due to the 0-H stretch (v,(OH) = 453 meV; v,(OH) = 466 meV) and to the intramolecular deformation mode (also called the scissoring mode NHOH) at 198 meV17). These vibrations here appear around -415 and -200 meV, respectively,illustrating nondissociative water adsorption. It is a common observation that the vibration frequencies of nondissociated HzO are fairly weakly perturbed, compared to the gas phase values, by interaction with surfaces.' On the graphite basal plane one might expect that the HzO-surface interaction is even weaker than that on metal surfaces. Formation of hydrogen-bonded clusters is therefore very likely on energetic grounds. The appearance oftwo additional bands at -25 and -100 meV, i.e., in the frequency range of collective HzO molecular modes,lg are indeed indicative of clustering (2D or 3D) even at submonolayer coverages (Figure 6, spectrum b). A further indication is the observed broadening of the v(0H) vibration (a direct result of hydrogen bondingYO and the relatively small intensity increase (compared to the collective modes)of the scissoringmode with increasing coverage.21 The vibrational spectrum of ice is far more complex than that of isolated molecules, because the strong intermolecular interactions perturb the three normal modes of the isolated molecule and because the free rotations (VR) and translations (VT)ofthe isolated molecule are "frustrated" by lockingthe molecule into the ice lattice. At high coverages the vibration spectrum for multilayer (ice)on graphite (Figure 6e)is identical with corresponding spectra observed on other surfaces.20 In contrast, the low coverage evolution of the spectra with increasing coverage provides substrate specific information about the waterwater and water-surface interactions. Figure 7b shows the intensity variation (integrated peak area) of the VR mode as a function of coverage at 85 K (solid symbols). The behavior is typical for fairly weak interaction with the surface (compare, e.g., with PdZ2)and increasing hydrogen bonding with increasing coverage. The quite rapid frequency shift from -85 to -100 meV as a function of coverage (Figure 7a) will be discussed later (section 4.1). Annealing of the low coverage structure to -135 K results in an upward frequency shift of YR as indicated by the open circles in Figure 7, while there is no measurable intensity change upon annealing (in all cases the intensities are measured relative to the elastic peak). It appears that the low V R frequency of -85 meV at t?HzO I0.5 ML corresponds to a metastable phase formed during adsorption, since an annealed layer (at 135 K or higher, but below the desorption temperature) of the same coverage has a significantly higher VR frequency (see the data points indicated by open circles in Figure 7). 3.2. Coadsorption of Water and Potassium at 85 K. 3.2.1. High-Resolution Electron Energy Loss Spectra of K HzO. All spectra reported for the coadsorption system in the low coverage regime were recorded by first adsorbing water and then potassium. No qualitative differences were, however, observed for the reverse order of deposition, at low coverages of both adsorbates. The former sequencewas chosen in order to minimize the amount of intercalated potassium.12 The water sticking probability is the same (unity or near unity) on the potassium precovered graphite (K coverages 50.3
+
(19)Hornig, D. F.; White, H. F.; Reding, F. P. Spectrochim. Acta 1968,12, 33g (20) Novak, A. Structure Bond (Berlin) 1974,18, 177. (21)Hauge, R. H.: Kaufman, J. W.: Marsave, J. L. J . Am. Chem. SOC.1980,y02, 6005. (22) Brosseau, R.; Brustein, M. R.; Ellis, T. H. Surf. Sci. 1993,280, 23.
Water Adsorption
Langmuir, Vol. 11, No. 4, 1995 1205
H 0 on graphite, vR(H20) 2
x 128
-
33
203 x 100-
0
,
179
;’
100 n
-% E
95
h
0 C
0 3
= 2 90
0
LL
io
3zt a .
85 I
I-
I
I
I
1 0 0 200 300 4 0 0 5 0 0
0
I
I
1
Energy Loss (mev) Figure 8. HREEL spectra (a) for 0.4 ML water coverage deposited on clean graphite surface at 85 K and (b)after dosing the preadsorbed water with 0.3 ML of potassium. The elastic peak shown corresponds to spectrum a. The solid lines drawn
through magnified data points are cubic splines. Experimental conditions are as in Figure 6.
V$H20)VJH20)
substrate:
I
0
W H )
&HOH)
0,5
1
I
I
1,5
2
4
I
1
1
graphite
+ K, subML
~
Coverage (ML) I
Figure 7. Panel a: Coveragedependenceof v~(Hz0) frequency for water adsorbed on clean graphite surface at 85 K. Panel b: Variation of the YR(H~O) peak area (PA)on clean graphite surface as a function of coverage. The PA are normalized to the elasticpeak and integratedover the whole YR(H~O) frequency region. The solid circles represent results obtained at 85 K the open circles showthe results of annealingexperiments(see text).
ML) as on the clean surface, within the experimental uncertainty (425%). In Figure 8 the HREEL spectrum for 0.4 ML of HzO adsorbed on clean graphite at T = 85 K (a) is compared with the spectrum for the same amount of water coadsorbed with 0.3 ML potassium (b). Coadsorption with K results in substantial shifts of the HzO vibration frequencies, compared to those for water adsorbed alone. In Table 1the HREELS observations are summarized for water adsorption alone and for coadsorption with potassium, respectively (IR data are a compilation from refs 19,23, and 24). A major result to emphasize already here is that water does not appear to dissociate at this particular potassium coverage (0.3 ML) and temperature (85 K). Noticeable differences in the HREEL spectra between HzO on the clean surface and HZ0 coadsorbed with potassium are as follows. (i)The OH stretch mode is raised by the (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1986. (24) Bertic, E.; Labbe, H. J.; Whalley, E. J. Chem. Phys. 1968,49, 775.
I
1
0
I
1
1
graphite, ML
1mm
I
TM**
l l !
I
Ice*
I
100
1
graphite, subML
200
300
I
I I 1
400
500
Vibrational Energy (mev) Figure 9. Vibrational frequencies of water in the ice; water adsorbed on transition metals (TM);clean graphite surface(at low (subML) and high coverages (ML),respectively) and for low water coverageon potassium precovered graphite (graphite + K, subML)). *The data for ice are a compilation from refs 23,19, and 24. **The vibrationaldata for the transition metals are taken from refs 25,26 and 27 for Pt, 28 and 29 for Ru, 25 for Rh, 30, 22, 31, and 32 for Pd, and 22, 33, and 34 for Ni. presence of potassium by more than 20 meV, from 415 meV on the clean surface, to a broad structure at -435 meV in the coadsorption case. (ii) The scissoring mode frequency also shifts upward. (iii)The frustrated translation mode decreases considerably in intensity, while (iv) the libration mode remains at the same intensity, but shifts down from 88 to 79 meV and broadens. In Figure 9 the HREELS results for the K HzO coadsorption system are compared with corresponding
+
1206 Langmuir, Vol. 11, No. 4, 1995
Chakarov et al.
Table 1. Energies of Vibrations in meV for HzO Adsorbed on Clean Graphite Surface and Coadsorbed with Potassium Measured in the Present Study HREELSa on graphite
415 (418) on Wgraphite 435 IRb solid 421.6 liquid gas
427.8 453.4
200 (-198) 203
25 (23) 100 (86) 33
79
399.2
200.9
23.2
64, 74.4, 101
448.2 465.7
203.2 197.8
I -a - .
z5 E
33
440 -
2
I.
0.4 ML x 500
"/.'
I
420[
90
2
1
I
LL
"'Q
-0
2CT 4 3 0 -
H 0 + K on graphite
6
c
-0-
C
a Data in parentheses represent the low coverage observations. IR data are a compilation from refs 19, 13, and 24.
x 256
450
0,.
0-0-d
r-----t
78t
Jk,
.c h. 4 -
+!
/
v)
5
CI
2 -
C
-
0 -
x(diss. products)
4
.-e0 /
(c) 0
0
100 200 300 400 Energy Loss (mev)
Figure 10. HREEL spectra for 0.25 ML HzO on graphite for different potassium coverages. The spectra are taken at 85 K, that was also the deposition temperature. In all cases the first coadsorbate deposited on the graphite surface was water. The bars in low loss energy region indicate peak positions as averaged from several measurements.
results for bulk i ~ e ~and~ water , ~ ~ adsorbed , ~ ~at low temperatures on transitional metals (TM)(Pt;25-27 Ru;zs,zg Rh;25 Pd;22930-32 and Ni22,33*34). The most pronounced variation in the vibration frequencies for the different systems is observed for the v~(H20) mode at 85-100 meV, which is sensitive to,e.g., the water coordination number.17 For high water coverages the libration mode at -100 meV dominates the spectrum and lies at the same (25) Wagner, F. T.; Moylan, T. E. Surf. Sci. 1987, 191, 121. (26)Sexton, B. A. Surf. Sci. 1980, 94, 435. (27) Ibach, H.; Lehwald, S. Surf Sci. 1980, 91, 187. (28)Thiel, P. A.; Hoffmann, F. M.; Weinberg, W. H. J.Chem. Phys. 1981. ~ . , 7.5. -_ -201. -_ (29) Thiel, P. A.; DePaola, R. A.; Hoffmann, F. M. J. Chem. Phys. 1984, 80,5326. (30)Zhu, X.; White, J. M.; Wolf, M.; Hasselbrink, E.; Ertl, G. J.Phys. Chem. 1991,95, 8393. - 7
0,2
0,4
0,6
0,8
1
K coverage (ML) Figure 11. A summary of the HREEL observations for potassium-induced water dissociation. Water coverages vary between 0.25 and 0.4 ML. Xdiss. products) shows the appearance and growth of K-0 and K-H vibrations at 33-55 and 106-126 meV, respectively. The dashed lines are drawn to guide the eye; see text.
frequency as the most intense libration mode of bulk ice. These results are all consistent with nondissociated HzO coadsorbed with a low coverage of K, as will be discussed in section IV. In contrast, H20 was found to dissociate at larger (OK L 0.3 ML) K-coverages. The threshold K-coverage for water dissociation at T = 85 K was explored by HREELS in the HzO coverage range 0.25-0.75 ML and K coverage range 0.2-0.6 ML. The spectra in Figure 10 were recorded with a constant water coverage of 0.25 ML and varying K coverages (0-0.6 ML). Each spectrum corresponds to a freshly prepared HzO
+
(31)Brosseau, R.; Ellis, T. H.; Morin, M.; Wang, H. J. Electron Spectrosc. Relat. Phenom. 1990, 54, 659. (32)Nyberg, C.; Tengstll, C. G. J.Chem. Phys. 1984, 80, 3463. (33) Olle', L.; Salmero'n, M.; Baro', A. M. J. VUC.Sci. Technol. 1985, A3, 1866. (34)Hock, M.; Seip, U.;Bassignana, I.; Wagemann, K.; Kiippers, J. Surf. Sei. 1986, 177, L978.
Langmuir, Vol. 11, No. 4, 1995 1207
Water Adsorption
A
100
200
300
400
500
600
Temperature (K)
80
180
280
380
480
580
Temperature (K)
Figure 12. TD spectrafor potassium (panelA) and water (panel B) deposited on graphite at 85 K in the single (a) and coadsorption cases (b). The coverages are 0.4 ML HzO and 0.6 ML K. For the assignment of the water peaks, see Table 2.
Figure 13. Panel A TD spectra of water (0.4 ML) coadsorbed with different amounts of potassium: (a)0, (b) 0.2, (c) 0.35, (d) 0.75, (e) 1.5, and (f) 2.1 ML. Panel B shows the molecular hydrogen signal recorded simultaneously.
Kcolayer and was recorded in a single, 40 min long energy scan. (The spectrum obtained at each K coverage is thus the result of an individual H2O K coadsorption experiment rather than one in a sequence of accumulated depositions. This minimized the exposure time to the background residual gases and the electron beam induced effects on the colayer.) The transition from nondissociative to dissociative coadsorption as the K coverage is increased is evidenced by several features in the vibrational spectra: the disappearance of the HzO collective modes; the energy shift of the OH stretch vibration (415 454 meV); the frequency shift and intensity variation of the v~(H20) mode (88 78 meV), and, most convincing,the appearance and intensity growth of K-0 and K-H associated vibrations at 33-55 and 106-126 meV, respectively. The development of these features with increasing f3K is graphically displayed in Figure 11, for a constant water coverage of 0.25 ML. Analyzing these data, we conclude that water dissociation (at 8 ~ = ~ 0.25 0 ML) does not occur for potassium coverages below 0.25-0.3 monolayer and that dissociation occurs above this coverage and seems to be complete around 0.4 ML K coverage. 3.2.2. Thermal Desorption Spectra, K HzO. When adsorbed alone at low coverages, both potassium and water desorb in a single TD peak (Figure 12, panels A and B, spectra a, respectively). In contrast, coadsorption of K and HzO results in several new peaks (Figure 12, panels A and B, spectra b). The detected total amount of desorbing potassium for a given coverage (as judged by integrating the TDS peak areas in the spectra), is the same in the single and coadsorption cases. The water mass balance is more difficult t o ascertain (i)due to HzO
dissociation (above the critical potassium coverage, see above), with an accompanying desorption of dihydrogen, and (ii)due to potassium-oxygen complex formation and eventual C02 desorption (see below). A general observation is that potassiumalwaysdesorbs as K atoms (noKOH, KH, or KO, (xly = l/2 - 2) were detected in the gas phase mass spectrometrically) and secondly that no atomic or molecular oxygen leaves the surface. Water can desorb as H20, and/or as H2 C02, the latter after reaction with the carbon substrate. The relative amounts of HzO and H2 desorption depend on the initial K and H2O coverages. The results demonstrate, for the water dissociation case, an overall reaction route according to
+
-
-
+
+
-
2H20(*,+ C(S) 2H2(g, + C02(g)
(1)
However, several intermediate reaction complexes are involved before C02cg, is formed, as illustrated below by both the TD and HREEL spectra. The water peaks in the TD spectra are labeled a to E following their appearance and prevalence in the spectra with increasing potassiumconcentration in the coadsorbed layer (Figures 12 and 13). The origin and assignments of the peaks are summarized in Table 2 and will be discussed in section IV. Note the observed clear correlation, on the temperature axis, between the potassium, hydrogen, and water TD signals, respectively. The mutual stabilization and eventual reaction between HzO and K, resulting in generally higher desorption temperatures,compared to the single adsorbates, are wellknown from studies of the same coadsorption system on metal surfaces.2 An interesting point to note for the present system is the additional appearance of H2O and
Chakarov et al.
1208 Langmuir, Vol. 11, No. 4, 1995 Table 2. Temperatures for Maxima in Thermal Desorption Spectra from Water and Potassium Coadsorbed on Graphite and Their Assignment TD maximum, maximum approximately,b intensity peak" K for OK desorption ouassignment a 150 0 unaffected water 180 0.3 K-stabilized water P 135 0.3 rearrangement in K-hydride Y ion 6 220 0.5 formation of KOH t 480 1.0 decomposition of KOH "The peak due to intercalation observed for considered. Depends on OK.
OK =
0 is not
K peaks with lower desorption temperature (-135 K), compared to the single adsorption systems. We tentatively suggest that this low temperature desorption is caused by an exothermic rearrangement of water-potassium complexes at the observed desorption temperature, providing the energy source for the low temperature desorption. Desorption of hydrated ions is an alternative explanation that cannot be excluded. (See section W.) Considering for the moment only the a and L,3 peaks in the water TD spectra (Figures 12 and 131, and assuming that water stabilization is due to a direct interaction between HzO and coadsorbed potassium, the surface solvation number (SSN),35 i.e. the number of water molecules associated with a single K atodion, can be calculated. For a water coverage of 0.4 ML and potassium coverages between 0.2 and 0.35 ML, the SSNis calculated to be between 3.4 and 2.6, respectively. Thus the SSN for purely nondissociative coadsorption of HzO with K is 3.4. Since this is a noninteger number the emerging picture is that solvated K-species are surrounded by an average number of HzO molecules of 3.4, but where each K-(HzO), complex is H-bonded to neighboring K-(HzO), complexes. The strong dependence of the water desorption and decomposition kinetics on K-coverage is demonstrated in Figure 13,displaying the water (mle = 18)and dihydrogen (mle = 2) MS signals. The arrows in panel B (labeled a-4 mark the positions of the water peaks. Subtracting the small cracking fraction for water from the simultaneously recorded mle = 2 signal (the ratios between the various crackingproducts of water is constant for the fluxes measured here) allows us to deduce if there is any molecular hydrogen desorption from the surface. Analyzing the TDS results in this way, and comparing them with the HREELS data, we find that the water dissociation is indeed accompanied by significant Hz release at T > 120K and up to 180-220 K, in parallel with the formation of KOH, i.e., the net reaction is
There are, however, also parallel reactions forming potassium oxide complexes (see below). It is worth noting that there is, in contrast, no dihydrogen or OH but only HzO desorption during the KOH decomposition at -480 K (see the 6 peak position in Figure 13, panel A). 3.3. Thermally Induced Reactionsin the K H20 Coadsorption Layer. 3.3.1. Thermal ReactionsAs Observed by HREELS. The evolution of the K HzO coadsorbed layer, as the temperature is progressively raised, is shown in Figure 14 in terms of its HREELS signatures. Each spectrum was recorded after annealing the sample to the indicated temperature for 2 min in vacuum, followed by fast cooling down to 85 K, where the
spectra were recorded. The spectrum labeled 85 K is the same as in Figure 8b. The spectrum recorded after 2 min of annealing at 120 K exhibits, in comparison with the 85 K spectrum, a new broad loss feature at 224 meV. In addition new peaks have appeared at 41, 51, and 105 meV and the water libration mode, VR(HZO), has increased in frequency from 79 to 85 meV. We conclude that molecular water is still present on the surface a t 120K, indicated by the presence of the libration mode, the scissoring mode, and the broad v(OH)mode at 439 meV. The molecular water is, however, now accompanied by the presence of reaction products formed by reaction of K with HzO; potassium hydroxide, nonstoichiometric potassium oxides, and potassium hydride have all been formed in the temperature range 85120K. These reactions are accompanied by release to the gas phase of Hz, HzO,and K (see Figure 13). Guided by a previous study in our laboratory of the K 0 2 coadsorption system, we tentatively assign the new peak a t 41 meV to the K-0 stretch mode in The peak at 51 meV is assigned to the K-0 stretch mode in KOH and the peak a t -105 meV to the K-H stretch mode in KH. The latter two assignments are based on the available information about vibrational energies of KOH in the gas, liquid, and solid on data for KH compounds,23and on results from a HREELS study of KOWRU(OO~).~~ Thus, HzO dissociation is evidenced both by KOH, KO,,, and KH formation and by release of Hz. The shift of the libration mode from 79 to 85 meV with increasing temperature (2' 5 120 K, still before complete dissociation) is in the opposite direction compared to the observed shift that foretells HzO dissociation in the experiments with varying potassium coverage (compare Figure 10). The temperature induced shift is attributed to an increased amount of hydrogen bonding and consequently to a higher degree of ordering in the adsorbed layer, probably as a result of the higher mobility of the HzO molecules with increasing temperature. Here, we are assuming that the v~(H20)frequency is influenced both by the strength of the potassium-water bond and by water-water hydrogen bonding. A similar upward frequency shift of v~(Hz0) was also observed in the case of single water adsorption, after the system had been shortly annealed to -135 K (see Figure 7a). The possible relevance of these changes with respect to the average HzO coordination will be discussed in section W . Annealing to 160 K causes the libration mode, Y R , and the vibration mode at -105 meV, associated with KH species, to disappear almost completely. The absence of the VR mode indicates that no hydrogen bonded HzO molecules are present a t this temperature. The disappearance ofthe KH species is accompanied by hydrogen desorption, and taken as evidence for the reaction(s)
+
and/or
+ +
~
~~
~
135) Kizhakevariam, N.; Stuve, E. M.; Dohl-Oelze, R. J.Chem.Phys. 1991, 94, 670.
(3)
-
since, based on thermodynamic data, the simple decomposition reaction 2KH(,) +Hz(~) 2Qa)is judged to be too endothermic to occur below 200 K.
+
~
~~
~~
(36) Chakarov, D. V.; Sjovall, P.; Kasemo, B. Surf. Sci. 1993,2871 288, 278. (37) Paul, J.; Robert, R.; dePaola, R. A.; Hoffmann, F. M. In Physics and Chemistry of Alkali Metal Adsorption; Bonze], H., Bradshaw, A, M., Ertl, G., Eds.; Elsevier: Amsterdam, 1989.
Langmuir, Vol. 11, No. 4, 1995 1209
Water Adsorption
H 0 + K on graphite 2
I
x t28
27
i
\
256.
K
...
-00
27
i Y
L
400 K
\
12
739
\
+
12
300 H
0
100 200 300 400 500
0 50 Energy Loss (mev)
100
150
Figure 14. HREEL spectra for coadsorbedwater and potassium on graphite surface after annealing for 2 min in vacuum at the indicated temperatures. The initial water and potassium coverages at 85 K are 0.4 and 0.35 ML, respectively. All spectra are recorded at 85 K. In addition the 4OH) mode narrows, a typical behavior accompanying formation of KOH.38 This observation, together with the peaks at 41,58, and 126 meV, suggests that after annealing to 160 K there is mainly KOH on the surface plus a mixture of nonstoichiometric (KO,)potassium oxides. At further elevated temperatures (300-500 K) both KOH and KOy decompose to form the thermodynamically more stable KzO. This decompositionof KOH is accompanied by HzOrelease. The characteristic K-0 stretch mode of KzO is associated with the energy loss at 39 meV.39 Simultaneouslyan energy loss peak evolves at -27 meV, which has been previously observed in K 0 2 coadsorption studies36and assigned to be a precursor to COZformation. 3.3.2. Thermal Reactions As Observed by TDS. The TD spectra recorded for K, HzO, Hz, and C02 upon H2O layer with a linear heating the coadsorbed K temperature ramp of 2.5 K0s-l are shown in Figure 15. These species are the only detected desorbing products. Qualitatively, the obtained TD spectra support the EELS observations, although there cannot be an exact coverage and temperature correspondence between Figures 14and 15, since the EEL experiments were performed by annealing the sample a t different constant temperatures for 2 min,while the TD spectra were obtained with a linear temperature ramp. The main observation above room temperature is decompsition of KOH, partly to water, that desorbs, the partly to potassium-oxygen complexes, that remain adsorbed on the surface. Upon further increasing the temperature the above mentioned potassium-oxygencarbon complex represented by the 27-meV HREELS peak, which eventually decomposes with formation of COZand K desorption. The COz desorption is illustrated in Figure 16, for different potassium-water concentrations in the initial colayer. (Spectra a of both panels show that there, as
+
+
(38)Thiel, P.A.;Hrbek, J.;DePaola, R. A.; Hoffmann, F. M. Chem. Phys. Lett. 1984, 108, 25. (39)Andrews, L.In Cryochemistry;Moskovits, M., Ozin, G. A., Eds.; Wiley-Interscience: New York, 1976.
f
H20 + K on graphite
5 kHz
I 4
3
400 600 800 Temperature (K) Figure 15. TDSof coadsorbedwater and potassium on graphite at 85 K. The initial coveragesare 1.8and 1.0 ML, respectively. The molecular hydrogen, mle = 2 , water, mle = 18, carbon dioxide,mle = 44,and potassium,mle = 39 signals are recorded simultaneouslyduring the temperature programmed reaction withp = 2.5 K-s-l. Note the scalefactorfor the hydrogensignal. The hatched areas mark simultaneous K and COZdesorption during the decompositionof potassium-oxygen-carbon complexes. 200
expected, is no COZ formation when either of the coadsorbates is missing.) An overview of the dependence of the CO2 yield on the H20 and K coverages is shown in Figure 17. There are marked similarities between C02 formation/ HzO and K 02 coadsorption, desorption upon K respectively, on graphite as shown in Figure 18. This will be further discussed in section IV.
+
+
IV. Discussion 4.1. H80 on the Clean Graphite Surface. It is obvious from the combined HREELS and TDS results that the water-on-graphite system isjust as complex as wateron-metalsystems. It is further clear that there are strong dynamical rearrangements in the deposited HzO overlayers as the system is heated from the deposition temperature (85 K) through the thermal desorption range
Chakarov et al.
1210 Langmuir, Vol. 11, No. 4, 1995
The (incomplete)picture that emerges from the results is the following: At 85 K surface diffusion of water molecules is expected to be quite fast40 and during the annealing or thermal desorption experiments the overlayer will continuously rearrange.41 At the deposition temperature the ice layer initially grows with a large fraction of fairly low coordinated H20 molecules, since the collective mode Y R has a significantlylower frequency below 0.5 ML than that of well-annealed, low coverage layers or thicker layers (Figure 7a). At the same time the collective modes, YR,YT,are very intense even at the lowest coverages, demonstrating the presence of H-bonded networks. This shows that below 0.5 ML coverages,there 500 700 900 500 700 900 is a large fraction of low coordinated H2O molecules, Temperature (K) compared to the situation in 3D crystalline ice. This can Figure 16. TDS traces of C02 desorption as a function of initial in principle be due to (i) 2D clusters, (ii) very small 3D water and potassium coverages at 85 K. Panel A shows the clusters with large surface area, or (iii) large 3D clusters spectra for constant K coverage of 0.5 ML and different H2O with a high degree of disorder (defects). Their relative coverages: (a)0, (b) 0.2, (c) 0.5, (d) 0.75, (e) 1.1,and (0 1.8ML. likelihoods are discussed below. Panel B represents the dependence of C02 formation for constant water coverage of 1.5 ML and different K coverages: (a)0, (b) In the range 0.5-1.0 ML the YRfrequency rapidly shifts 0.35,(c) 0.5, (d) 0.75, (e) 1.0, and (0 1.5 ML. up to about 100 meV where it remains a t even larger coverages, demonstratingthat in this coveragerange most .ofthe water molecules become highly coordinated, pointing to formation of large 3D aggregates. First we address the question whether the low coverage regime, with a constant low frequency, consists of 3D aggregates (small or large) or 2D islands, with naturally low coordinated H20. Large 3D clusters with so much disorder or defects that the frequency is shifted down due to low coordination of H20 is unlikely. First, one would expect such disorder to continue to exist above 0.5 ML, which is contrary to what the rapid Upshift OfYR at 0.5 ML suggests (Figure 7). Second, 3D ice growth at much lower temperatures on other surfaces shows a high YRfrequency, -100 meV. Thus there is no inherent growth mechanism creating low-coordinated water due to the low temperature. The possibility of small 3D clusters with so large a surface area that there is a large fraction of lowcoordinated H20 also does not seem to agree, even qualitatively, with the rapid rise of YR at 0.5 M L If there Figure 17. Coverage dependence of CO2 formation at -800 K were just a successive growth of the size of 3D islands based on TD measurements similar to those shown in Figure 16. from very low up to high coverages, the frequency change should be much smoother than that observed. This argument was supported by a simple kinetic calculation, 1 O2 + K on graphite I H 2 0 + K on graphite assuming that all H2O molecules landing on the bare graphite surface are incorporated in 2D islands and low coordinated, while molecules landing on occupied sites build a second layer and create high-coordinated H20. There is of coursethe possibility that the structure growing up to 0.5 ML undergoes a coalescence transition to much larger 3D clusters just at 0.5-1 ML due to the addition of more water. However, if we look at absolute numbers I . . . . . . I R we find that to obtain predominantlylow-coordinated H2O 300 500 700 900 200 400 600 800 molecules in 3D clusters (say half-spherical), a t 0.5 ML, Temperature (K) would require an extremely high nucleation density ( Figure 18. A comparison between CO2 formation at -800 K 1013cm-2)to make the aredvolumeratio sufficiently large in the course of the 0 2 + K and H2O + K reaction on graphite. to account for predominantly low-coordinated H20 (the The hatched areas mark the simultaneous K and CO2 desorption during the decomposition of potassium-oxygen-carbon comquoted density of nucleation sites on the graphite basal plexes. plane is -lo8 cm-2 based on its defect density, ref 42). Although we cannot completely exclude the last alterof temperatures (140-180 K). The latter is evident, for native above, we regard it as quite improbable and are . example, by the significant changes in both the frequency consequently led to suggest 2D-islands as the most likely and intensity of the collective modes, YR, YT, as the ice structure a t 10.5 ML. This explains easily the low Y R layer is annealed to 135 K, and also by the difference in frequency, since the H20 molecules in 2D islands have a thermal desorption kinetics as desorption is performed by either isothermal desorptionruns or by a linear T-ramp. (40) Andersson, S.; Nyberg, C.; Tengstal, C. G. Chem. Phys. Lett. It is therefore not surprising that these two types of 1984,104,305. desorption experimentsa t first sight appear contradictory; (41) Payne, S. H.; Kreuzer, H. J. Surf. Sci. 1988,205,153. Wu, K. they actually represent different structural (and energetic) J.; Peterson, L. D.; Kevan, S. D. J . Chem. Phys. 1989,91, 7964. (42) Chu, X.; Schmidt, L. D. Surf. Sci. 1992,268, 325. arrangements, depending on their annealing history.
I
I
Water Adsorption
Langmuir, Vol.11,No.4,1995 1211
quite low coordination number. Upon addition of more HzO above 0.5 ML the frequency changes so rapidly upward that it suggests a phase transition from 2D to 3D structures. Slight annealing converts the 2D structure below 0.5 ML to a more 3D-like structure, as evidenced by the upward shift of (open circles, Figure 7). In order to elucidate the processes responsible for the observed pseudo-ero-orderdesorption kinetics in TDS(also observed for submonolayer coverages of H20 on Ag(ll0) and Cu(111)15y41943) the HREELS observations and the possible growth modes of the water islands should be considered. The growthmode of a film is usually classified according to the morphology and the wetting properties of the substrate.44An initial 2D growth may a t first sight appear to be in conflict with the otten presumed hydrophobic nature of the graphite s u r f a ~ e .However, ~~~~ inspecting the literature we found no solid support for that assumption. We therefore measured the contact angle (in air)ofwater droplets (< 1mm) on the basal plane of HOPG under two conditions: (i) immediately after exposing a fresh surface by cleavage, and (ii)on the same surface after 15 min of waiting time. In the first case the contact angle was 37" (usually considered to be in the wetting regime). In the latter case it had increased dramatically to 74", indicating a fairly hydrophobic surface. The reason for the increase in contact angle is almost certainly slow contamination from (hydrophobic) impurities in the air. Since some time elapsed (less than 1min) before the water drop was deposited on the freshly cleaved surface, it is likely that it was somewhat contaminated as well and that the contact angle on a really clean surface is probably lower than 37". Thus the graphite surface appears to be qualified as a wetting or nearly wetting surface, making 2D growth at low coverage a t least plausible. It remains, however, questionable how fair it is to apply these results to the UHV conditions.*' The annealing results (Figure 7a) indicate that whatever the low coverage structural arrangement is, it is not very stable, since mild annealing shifts the frequency in the direction signaling the fonnation of 3D structures. Note also that the observation of collective modes at low coverages reveals that water molecules are not adsorbed with their molecular axis parallel to the surface normal, since otherwise these modes should be dipole forbidden (it is assumed that nondipolar scattering gives only a small contribution to the observed loss intensity9 Thus the low coverage situation seems to be best described as a metastable, kinetically stabilized phase. We now turn to the thermal desorption data, keeping in mind that they reflect an ice structure, which has really spent some time at T 1 140 K, and therefore, due to annealing, is quite different from the structure seen in HREELS. The isothermal desorption runs involve an interruption of the temperature ramp and thus simultaneous desorption and annealing, at constant temperature, of the system for a few minutes in vacuum. Such a treatment results in significant changes in the subsequent thermal desorption kinetics when the TD ramp is resumed, in comparison with a continuous ramp started at 85 K (Figure 5). For example, the fraction of intercalated water increases due to the annealing at constant temperature and the TD maximum of the main peak shifts (43)Hinch, B. J.;Dubois, L. H. Chem. Phys. Lett. 1991,181,10. (44)Venables, J. A.; Spiller, G. D. T.; Handbucken, M. Rep. Prog. Phys. 1984,47,399. (45)Herwig, K.W.;Trouw, F. R. Phys. Rev. Lett. 1992,69,89. (46)Kiselev, A. V.; Ebvaleva, N. V. Izv.Akad. Nauk SSSR, Ser. Khim. 1969,1959,989. (47)Li,D.; Neumann, A. W. J.Colloid Interface Sci. 1992,148,190. Mills, D. L. Electron Energy Loss Spectroscopy and (48)Ibach, H.; Surface Vibrations; Academic Press: New York, 1982. ~
to a higher temperature. The average upward shift is -3", when compared with the uninterrupted TD ramp for the same amount of water (see spectrum c in Figure 5). Obviously there are considerable annealing effects resulb ing in rearrangement and intercalation of the adsorbed water. As we will see this is also reflected in vibrational features of the ice overlayers after annealing. The qualitative features of the isothermal desorption traces are easy to understand Initially when the coverage is high the desorption rate is constant, reflecting that the whole surface is ice covered. When about one monolayer remains the desorption rate begins to decline, since an increasing fraction of the surface is no longer covered by ice. Also, the ramped TD spectra for multilayer deposits, obtained with a linear T vs tramp have the straightforward interpretation that the zero-order kinetics is due to multilayer coverage. The peculiar observation differing from the isothermal desorption results is that the zeroorder kinetics seem to hold even at submonolayer coverages. This may a t least partially be due to the lower sensitivity of TDS to the detailed kinetics, compared to isothermal desorption data. Remembering that the ramped TD runs reflect a different ice structure, as discussed above, it may, however, also be a significant observation. Taking into account the predominance of low coordinated water at low coverages, one may speculate that this is the regime when water molecules cluster predominantly in a 2D net, or forming "flat" clusters.49 Subsequently, with increasing coverage, 3D-clusters are formed. For this reason, a critical coverage and critical temperature may be anticipated for the nucleation of the initial (low 7')clusters giving rise to both a coverage and temperature dependent phase transition that can also be interpreted as a wetting-nonwetting transition. Zero-order kinetics below one monolayer could thus indicate a two-phase system of the annealed structure where the origin of the zero-order kinetics is that one phase feeds particles to a second phase, from which desorption exclusively occurs.41 More detailed isothermal and ramped data are needed to motivate a more extensive discussion of the €I20desorption kinetics. 4.2. Coadsorption of HzO K. 4.2.1. Nondissociative Coadsorption at Low Temperature and Low K-Coverage. Water coadsorbed with a sufficiently low coverage of potassium (OK 5 0.3 ML) does not dissociate at 85 K as evidenced by the HREEL spectrum in Figure 8. The spectrum shows typical energy losses for molecular water, but with significant vibration energy and intensity differences compared to H20 adsorption alone. It is a common observation that water is stabilized on surfaces by coadsorption with foreign species.2 Water stabilization by chemical interaction with K atoms is expectedto influence both the bonding and the orientation of the H2O molecules on the surface. When coadsorbed with water K atomdions interrupt the hydrogen bonded network of water molecules,decreasing the average H2OH2O coordination, which is reflected by the lowering of the YR frequency (see section 111). At OK .c 0.3 ML coverages, with no water present, the potassium atoms on graphite are partially ionized and do not cluster but stay well separated from each other.12 It is then a reasonable picture that water molecules bind preferably with their oxygen atoms toward these individual potassiumatoms, changing simultaneouslythe H2O orientation relative to the surface Such reori-
+
(49)Doering, D.; Madey, T. E. Surf. Sei. 1982,123, 305. (50)Bonzel, H. P.; Pirug, G.; Muller, J. E. Phys. Rev. Lett. 1987,58, 2138.
Chakarov et al.
1212 Langmuir, Vol. 11, No. 4, 1995
entation would account for the observed intensity changes, of v ~ ( H ~in0 the ) EEL spectrum (Figure 81, compared with pure HzO adsorption. This picture also suggestsformation of hydrated ion-like species51(a kind of hydration shell around each alkali atom) due to the electrostatic interaction between water and potassium via the permanent dipole moment of 1.84 D for water molecules and an adsorption induced dipole moment of 6.3 D for the potassium atoms (at low K coverages).lZJ7 Thus, the coadsorbate system shows dipole-induced water stabilization resulting in an induced surface hydrophilicity. In section I11we calculated the number of HzO molecules affected by one alkali atom to be about 3.4 at low potassium coverages, i.e., on the average 3.4 HzO molecules in the first hydration shell around the adsorbed K 'ion". A noninteger value either may be due to a possible underestimation of SSN, (see below) or may reflect that the hydrated ions are incorporated into a H-bonded network, with a noninteger average number of HzO molecules per K atom. In the calculation of SSN, following Stuve et a1.,5I we considered only the water desorption at -180 K, neglecting the small amount desorbing a t lower temperature (-135 K),which may cause a slight underestimation of SSN. The total number of water molecules in the hydration shell must be partly restricted by steric effects52 and will also depend sensitively on the amount of charge transfer per atom from K to graphite (i.e. on OK) and on temperature.12 The formation of hydrated ions on the surface suggests that such ions might desorb as whole entities. Consequently we searched for possible desorbing hydrated ions by locking the MS at mle = 39 18n, n = 1 , 2 , 3 , ... mass numbers. No such signal was observed. This is, however, not surprising and does not exclude the possibility of desorbinghydrated ions. Their instability probably makes them rapidly dissociate into, e.g., K+ and HzO or K and HzO+ after electron impact ionization in the MS. The latter possibility actually draws our attention to the simultaneous desorption of water and potassium at -135 K(be1ow the desorption temperature of pure water), which could be due to desorbing and decomposing hydrated ion complexes. This explanation is quite speculative, as is the alternativeone mentioned in section 111,an exothermic HzO to KOH, and to other rearrangement of K complexes, providing the energy source for desorption. 4.2.2. Water Dissociation. The reactivity of alkali metals with respect to HzO at atmospheric pressure and room temperature is known to be high, producing alkali metal hydroxides and HZwith a large exotermicity (102 kcal.g-'.mol-l for P).The observation of nonreactive coadsorption at 85 K (sections 3.2.1 and 4.2.1) might therefore, at first glance, appear surprising, but can be understood taking into account the relatively low K coverage,with ionic, dispersed K monomers on the surface rather than metallic potassium. Our coverage-dependent measurements (Figure 10)reveal the existence of a critical potassium coverage, above which HzOdissociation takes place, suggesting that water dissociation requires interaction with more than one K atom (see below). Similar observations for the existence of a critical alkali metal coverage for water dissociation have been made earlier on metal surfaces.53 A possible mechanism for the water dissociation with increasing potassium coverage is related to the work function change due to the charge redistribution within
+
+
(51) Stuve, E. M.; Bange, K.; Sass, J. K. In Trends in Interfacial Electrochemistry; Silva, A. F., Eds.;Riedel: Dordrecht, 1986;Vol. C179; 255 pp. (52) Bornemann, T.; et al. Surf. Sci. 1991,254, 105. (53) Bonzel, H.P.; Pirug, G.; Ritke, C. Langmuzr 1991, 7, 3006.
the K ~ v e r l a y e r .Increasing ~ ~ ~ ~ ~ the potassium coverage leads to a successive lowering of the work function and as a result there is successively more charge transfer to the HzOantibondingorbitals, thus destabilizingthe molecule. If this mechanism was responsible for HzO dissociation, one would expect a softening of the O-H stretch mode with increasing OK at constant 6 H z 0 , in the predissociation region. The HREEL spectra in contrast reveal an opposite tendency: v(OH) shifts from 415 to 435 meV, Figure 8. Model calculations for HzO/Kon Pt(lll)54 demonstrate, contrary to the suggestion above, that the mere presence of an alkali species next to an adsorbed water molecule is not sufficient to cause dissociation in the limit of low K coverage. The calculations show that charge transfer between HzO and the adsorbed alkali is of minor importance, such that the filling of antibonding states cannot in this case be primarily responsible for dissociation. It has also been suggested that field-induced dissociation due to the high electric field in the vicinity of an adsorbed alkali species could be a possible mechanism.53This model assumes that a water molecule is situated between adsorbed alkali species. On the graphite surface there is no reason to place a HzO molecule between two alkali atoms (dipoles) in contrast to the case of a metal surface, where the water-substrate bond is stronger and may be capable to complete with alkali metal-water bonds. One rather expects that water binds to the alkali metal and adjusts to the alkali metal rearrangements with increasing alkali coverage. According to the latter picture the K(H20),-K(H20), distance becomes successively shorter, as the potassium coverage increases, thus increasing the steric probability for H-H overlap and eventually for concerted KOH and HZformation. Thermodynamicallysuch a mechanism for water dissociation is also favored by the energy gain resulting from the H H association reaction (4.48 eV per molecule). (Note that the oxygen-oxygen nearestneighbour distance in ice varies from 2.74 to 2.76 A between 90 and 270 Kand the distance ofclosest approach of two non-hydrogen bonded molecules in gas phase is
+
4.52
The estimated critical potassium coverage for dissociation (0.3 ML) coincides with the coverage where the pure potassium overlayer on graphite undergoes a phase transition from a dispersed, ionic phase to a metallic phase.lZ At this particular coverage the K-K distance in the new metallic phase suddenly (due to the phase transition) becomes shorter than twice the t ical H bonding distance (O-H-0) for water in ice (2.76T I1. This structural change and the associated change in electron structure from an ionic to metallic phase may explain the correlation between the critical K coverage for HzO dissociation and the critical coverage for the 2DK phase transition; i.e., it appears that the metallic K phase is a requirement for HzO dissociation to occur. In summary we regard the K-condensation mechanism (with the accompanying decrease of the alkali metal adatom distance),maybe cooperatingwiththe antibonding fillingmechanism (caused by a charge redistribution from the alkali metal-graphite complex to HzO), as a likely explanation for the critical coverage for HzO dissociation. 4.3. HzO K. Thermal Reactions. We now turn to the evolution of the K-Hz0 coadsorption system with increasing surface temperature starting at 85 K. At 120 K some molecular water is still present on the surface. Simultaneouslythere are already products resulting from reactions between K and HzO. The data suggest that there
+
(54) Muller, J. E. In Coadsorption, Promoters and Poisons;King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1993; Vol. 6, p 29.
Langmuir, Vol. 11, No. 4, 1995 1213
Water Adsorption
is not a single water dissociation product. Both partial and full decomposition takes place, i.e. (4) and
-
H20(a)
H(a)+ OH(,)-
(5)
where OH(,)- and O(a)- are both bound to K. Hydrogen either forms H2, which desorbs, or H atoms that bind to potassium forming KH. The evidence for the first decompositionpathway is molecular hydrogen desorption at -220 K upon heating (shown in Figure 13A as &hydrogen peak) and the formation of potassium oxides (Figure 14, the HREELS peaks at -40 meV). For the second pathway, the evidence is the formation of KOH and KH compounds-as indicated by peaks a t 51, 447, and 105meVin the HREEL spectra (Figure14). Pathway 4 could of course proceed via 5 . The assignment of the peak at 51 meV to the K - 0 stretch in KOH is confirmed by the further evolution of the system with increasing annealing temperature and development of the narrow peak at 447 meV, corresponding to the O-H vibration in KOH (the spectrum for T = 160 K on Figure 14). It is worth noting that the observed energy of the K - 0 stretch in KOH corresponds to the gas phase suggesting formation of monomers and weak interaction of the KOH species with the graphite surface. This observation is also consistent with the low density of the initial K-coverage. The formation of potassium oxides and potassium hydroxide is completed at -200 K. To follow the decomposition of these compounds at higher temperatures by the HREEL spectra alone is complicated by two principal obstacles. First, the vibrational losses appearing at different annealing temperatures are likely to represent continuous transformations between different K O stoichiometries, starting with the above described mixture of different K O * and KOH. Thus there is considerable heterogeneous broadening of the HREELS features. Second, the characteristic K - 0 vibrations for all of these possible compounds lie in the low energy loss region, overlapping each other and with the tail of the intense elastic peak. There is also a scarcity of IR data in the above spectral region. The TD spectra shown in Figure 15 assist in the interpretation ofthe high-temperature EEL spectra. (Note, however, that there is not an exact coverage and temperature correspondencebetween Figures 14and 15, since the EELS experiments were performed by annealing the sample to the indicated temperature for 2 min while the TD spectra were obtained with a 2'-rise of 2.5 Kms-l. The reason that TD data are presented for higher coverages than for the EEL spectra is to better illustrate C02 desorption, which is much weaker for the low coverages correspondingto Figure 14.) The main observation above room temperature is decomposition of KOH, partly to water, which desorbs, and partly to K20, which remains on the surface, according to
2KOH(a) H2O(g)+ &OM
(6)
The reaction equilibrium (for bulk compounds) is toward the KOH side at 300 K and only above 700 K at the oxide side.ss EELS evidence for the process is the disappearance of the 4 O H ) vibration from the spectra and an increased (55) Schlogl, R. In Physics and chemisty of alkali metal adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds.; Elsevier: Amsterdam, 1989.
intensity of the K-0 stretch vibration at 39 meV. For a discussion of the latter, see ref 36. Upon further increase of the temperature,a potassiumoxygen-carbon complex is believed to form, that serves as a precursor for carbon dioxide formatioddesorption. One can speculate about formation and decomposition of potassiumcarbonate (KzCO3)species, a favorite candidate from both a thermodynamical and a stoichiometric point of view, but the only spectroscopic evidence for such a species so far is the weak peak at -27 meV, observed in the spectra obtained after annealing to 300-500 K No distinct C-0 bond is, surprisingly, seen in the HREEL spectra. The latter does not exclude a C - 0 bond formation, however, since it might lie parallel to the surface plane, and then be undetectable by dipole excitation. Alternatively the broad feature at -224 meV (Figure 14) may actually be due to a C - 0 bond, with a low cross section. The reduction of potassium oxides andor hydroxide to metallic K by chemical means requires fairly extreme conditions. This is exemplified by the reactions of carbon with the following potassium compounds
+ 2C - 2K + 3CO KOH + C -.K + CO + 1/2H2 %C03
(7)
which require temperatures above 1000 K.56 If we attribute the observed C02 formation and potassium desorption to either of these reactions, we must find a reason why the reaction temperature is much lower than 1000 K, as shown in Figure 15. A major difference between the reactions 7 and 8 and the present case is that the former are reactions between macroscopic 3D bulk compounds, while in the present case the reaction is between microscopic K2CO3 or KOH entities, maybe even monomers, siting on the graphite surface. The stability and reactivity of, e.g., a K2C03 monomer on the surface is expected to be quite different than for the 3D compound (lower stability and higher reactivity for the monomer). Furthermore, the energetics of reactions 7 and 8 will be different in the two cases since in one case they occur between two separatedbulk phases, maybe with gas phase transport between them, while in our case the reaction occurs between the surface complex and the substrate to which it is chemically attached. These differences make a much lower temperature for the gasification reaction on the graphite surface quite possible. The reaction temperature can be additionally lowered by the formation of graphite intercalation compounds, as explained by Fergusson et al. in ref 56. These authors showed that crystalline intercalation compounds of potassium into graphite disintegrate at lower temperatures than when the gasification reaction sets in. Assuming the K2C03 species as a precursor for CO2 formation, their decomposition may be described as
The double peak, observed in the TDS spectra of C02 (Figure 16), can thus be assigned t o a stepwise decomposition reaction, in which, additionally to the direct COz formation (91, an'oxygen atom reacts with the substrate according to
(10) (56) Fergusson, E.; SchliTgl, R.; Jones, W. Fuel 1984, 63, 1048.
Chakarov et al.
1214 Langmuir, Vol. 11,No.4, 1995 A similar process is observed in the course of potassium carbonate decomposition on c~(llO).~' As mentioned above, the same spectroscopic evidence as in this work (the peak at -27 meV) was obtained in a previous study of oxygen interaction with potassium precovered graphite.36 An additional argument, that supports the similarities found by HREELS, as well as the positions of the peak maxima, and their shape in "DS, are the ratios of KCOz, as measured from the correspondingpeak areas in the TDS spectra (Figure 18).They are close to each other for these two cases (0.75 and 0.85, respectively), suggesting similar stoichiometry of the precursors. A summary of the thermally induced surface reactions of water and potassium on graphite is presented in Table 3.
V. Conclusions Water molecules on graphite at low coverage and 85 K forms H-bonded networks with a low coordinationnumber, probably 2D islands. At coverages around 0.5-1 ML this structurerapidly converts to 3D (ice)aggregates with fully (4-fold) coordinated HzO molecules, as in ice I,or It,. Annealing of coverages below 0.5 ML also converts the 2D structure to a 3D structure. Coadsorption of HzO with K at 85 K and OK 5 0.3 ML is nondissociative, and forms hydrated-ion-likecomplexes with an average of 3.4 HzO molecules per potassium atom at the lower coverages. At larger K coverages, the coadsorption results in water dissociation forming KOH, KH, and KO, surface complexes. The critical K-coverage for HzO dissociation is 0.3 ML. With increasing temperature the potassium complexes undergo a number of reactive transformations, changing both the average and the local stoichiometry on the surface by release of Hz and (57) Thomsen, E. V.; Jwgensen, B.; Onsgaard, J. Surf. Sci. 1994, 304, 85.
Table 3. A Summary of Temperature-InducedReactions of Coadsorbed Water and Potassium on Graphitea temD (K) ~~
reactionb
+ &a)
~~
85-180
120-220
(HzO)(a,
H20(a)+ &a) H20(a1+ &a)
X + =(a)KH(.)
300-500
4
K(H20)n(a) n
- KOH(a,+ KH(a) KOy(a)+ H2(g) -.+
+ H2(g)
2 2.6-3.4
X/Y = '/~-2
ma,
+ KOH(,, -..KXOy(,)+ H20(a)
- -
&Oy(a) K2O(a) X ~ Y= '/2-2 [KOy(a)+ C(s) KzO(a)+ A*(a)
- -
X/Y
X/Y = '12-21'
400-500
KOH(,) &Oy(a)+ H2O(g) X/Y = '/2-2 [KOH(a, + C(s) &g) + CQa) + Hz(g)It
500-700
K20(a)+ C(s)
-
-
&g)
= '/2-2
+ A*(a)
A*(a) & g ~ + C O Z C ~ ) The temperatures shown are for potassium coverage below the critical for dissociation. (a), adsorbed; (g), gas; (SI, substrate; [-It, probable but not identified; X, see text, eq 3. A*, unidentified complexcontaining Kand 0 atoms and with a vibrational frequency of -27 meV. > 750
HzO and by conversion of KOH to KO, complexes. Eventually, these transformations result in a final precursor to carbon gasification. The nature of the precursor is yet unindentified. Its characeristics are (i)it contains potassium and oxygen, (ii) it has a vibrational mode of -27 meV, and (iii)it decomposes or reacts by simultaneous release of COZand K into the gas phase at -750 K. The main candidate for this precursor is a carbonate-like species.
Acknowledgment. This work is financially supported by the Swedish National Board for Industrial and Technical Development, Project 93-01036, and Swedish Research Council for Engineering Sciences, Grant 92-951. L4940789P
