Identification of surface hydronium: Coadsorption of hydrogen fluoride and water on platinum (111)

Surface Science 182 (1987) 125-149 North-Holland, Amsterdam

125

IDENTIFICATION OF SURFACE HYDRONIUM: COADSORFTION OF HYDROGEN FLUORIDE AND WATER ON PLATINUM (111) Frederick

T. WAGNER

and Thomas

Physrcd Chemistr?, Department, MI 48090-9055. USA Received

26 June 1986; accepted

E. MOYLAN

Germ-d Motors Research L.&oratories,

for publication

6 October

Warren,

1986

The adsorption of HF and its coadsorption with water were studied on Pt(ll1) by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) as a step in the UHV modeling of the acidic aqueous electrolyte/electrode interface. Anhydrous HF adsorbs without dissociation. HF coadsorbed with water reacts to form several phases distinguishable by TPD. HREELS spectra show that the reaction forms the H,O’ ion. The stoichiometries of thermal desorption identify an acid monohydrate phase ([H,O+][F1) and fully hydrated phases with stoichiometries of HF.SH,O in the monolayer and HF.RH,O in the multilayer. To the as-yet-unknown extent that low-temperature measurements are relevant to normal aqueous electrochemistry, these results indicate that even such classically “non-specifically” adsorbed ions as Ht and F- interact sufficiently strongly with Pt surfaces to displace some water from their inner solvation shells. These data also show that Bronsted acid-base chemistry can be carried out and spectroscopically observed in low temperature monolayers in UHV, and point the way towards UHV studies of such pH-dependent phenomena as corrosion and electrocatalysis.

1. Introduction The aqueous-solid interface controls such technologically important processes as corrosion, colloid processing, and electrochemical energy storage and conversion. This interface is characterized by the electrochemical double layer, consisting of a layer of charge at the solid surface largely balanced by a layer of ions and neutral molecules on the solution side thought to be - 2-10 A thick. Direct in situ spectroscopic investigation of the double layer has proven difficult, though progress in this area has been made [l-3]. UHV surface analytical techniques, which have contributed so much to the understanding of the gas-solid interface, cannot be directly applied to an electrode under potential control in a bulk liquid electrolyte. Encouraging results have been reported for electrodes removed from the electrolyte [4,5] and transferred to UHV under clean conditions [6-81, but it has become clear that the species seen in UHV after emersion and evacuation of the electrode are not always 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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,/ Identification

ofsurface hydronium

those which had been present in the electrochemical environment. Thus, while the more stable electrosorbed layers can be studied by transfer techniques, improved understanding of more fragile species may depend upon our ability to model double-layer conditions entirely in UHV at the cryogenic temperatures needed to maintain water layers. The “temperature gap” between the -C 180 K needed to adsorb water in UHV and the 293 K of normal aqueous electrochemistry is of as great concern as the “pressure gap” plaguing those who apply UHV techniques to understand gas-solid heterogeneous catalysis. However, Stimming and coworkers [9], working in bulk frozen electrolytes, have demonstrated mechanistic continuity for several electrochemical reactions in aqueous electrolytes from room temperature down through liquid nitrogen temperature, raising hopes that aqueous layers adsorbed below 180 K in UHV may be relevant to the double layer formed in normal liquid aqueous chemistry. Wagner and Moylan [lo] have presented evidence that monolayer water on Pt(ll1) at 90 K has a mean spacing between oxygen atoms in adjacent molecules more characteristic of liquid water than of ice, though this may be characteristic only of this particular surface. The initial tendency of the electrochemist to reject all UHV cryogenic “ice” studies as irrelevant to normal aqueous electrochemistry should be tempered by the above observations, though any unqualified claims of relevance by the surface scientist are also still premature. The relationships between the concepts of surface science and those of electrochemistry have been discussed and clarified by Sass et al. [11,12], who have proposed modeling the double layer in UHV and have taken previous experimental steps in that direction. Modeling of the electrochemical double layer in UHV can be carried out at three basic levels of complexity and accuracy. The first is to study the adsorption of pure water on clean surfaces, since water or products derived from it will be the majority species at most aqueous interfaces. This type of study has now been carried out on a number of noble and non-noble metal surfaces: extensive bibliographies are available elsewhere [13,14]. In general, water adsorbs weakly and molecularly in hydrogen-bonded aggregates on noble metal surfaces; on non-noble surfaces partial dissociation yielding adsorbed hydrogen has been observed. While these studies are essential starting points, the essence of electrochemistry is the double layer containing ionic species; inclusion of these ions constitutes the second level of UHV modeling, the level addressed in this report. In particular, the ions governing pH, i.e., OHS and HjOC (or their more highly hydrated forms), play unique roles in corrosion and electrocatalysis; one would like to gain control of the effective “pH” of the model layer in UHV. Previous efforts to introduce ions into UHV-adsorbed aqueous layers have involved coadsorption of water with electropositive species (reducing agents) such as Na [15,16], K [17,60], and Li [12,61,62] or with electronegative species (oxidizing agents) such as 0, [l&19] and Br2 [12,15,20]. The former can

F. T. Wagner, T. E. Moyiun / Identificatron of surface hydronium

produce

effectively

basic layers via reactions

Na + excess H,O -+ Na&,, + OH&

127

such as

+ H.

While 0, and Br, would impart Usanovich-type acidity [21] to the surface, the relationship between this and the more restrictive Bronsted-Lowry acidity has not been clear on metal surfaces. We have needed a more direct means of producing in UHV the low-pH conditions most conducive to corrosion. Preferably we would do this without simultaneously oxidizing or reducing the surface. Toward these aims, we have coadsorbed water with anhydrous mineral acid vapors, starting in this paper with HF. This method of introducing ions has solidified the second level of in vacua modeling of the electrochemical interface. The third level will be to control the effective electrode potential in UHV, either with coadsorbates which shift the work function or through UHV implementation of Stimming’s frozen electrolyte work [9]. The latter will require UHV growth of electrolyte sandwiches. Techniques for growing such structures are demonstrated in this paper. HF was chosen as the first mineral acid to be studied for a number of “non-specifically adsorbed” electroreasons. Aqueous HF is the prototypical lyte on Pt [22]. Capacitance and other electrochemical measurements have indicated that the interactions between F- or H,O+ and electrodes of mercury, the most tractable metal for detailed electrochemical studies, are almost purely electrostatic, with little in the way of specific chemical complications [23]. These ions are thought to maintain their full solvation shells at the surface due to their low polarizabilities and strong solvation. Since this picture has been generally accepted for Pt as well, aqueous HF on Pt should represent the simplest form of electrochemical interface. HF, though a weak acid under normal aqueous conditions, becomes a stronger acid at concentrations above 5M [24], i.e., when solvated by itself rather than by water. At high HF/H,O molecular ratios one can thus drive the reaction with water to produce substantial concentrations of H,O+ and F- even at room temperature. HF, its aqueous solutions, and its crystalline hydrate have relatively simple vibrational spectra, facilitating identification of species by EELS. Although HF adsorption has not previously studied, adsorption of atomic fluorine generated in UHV by a solid-state electrochemical source has been previously studied in UHV on Pt(ll1) [25] and Pt(lOO) [26]; oxidation of Pt(l11) to PtF, was observed. HF has much less oxidizing power than atomic F; the gas phase reaction F+$H,-,HF has a standard enthalpy change of about -99 kcal/mol, and one would not expect PtF, formation with the acid. Due to the lack of previous UHV surface chemistry literature on anhydrous HF, results on simple HF adsorption are reported here in addition to the major thrust of HF + H,O coadsorption.

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of surface hydronium

Anhydrous HF does, in fact, adsorb intact without reaction on Pt(ll1). However, it reacts with coadsorbed water to form several phases whose stoichiometries elucidate the surface and bulk hydration states of the H’ and F- ions.

2. Experimental Experiments were carried out in a standard UHV chamber equipped for high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) which has been described previously [18]. The Pt(ll1) crystal was oriented within 0.5” using Laue X-ray backdiffraction, cut with a wire saw and SIC slurry, and polished by standard metallographic techniques ending with 0.25 pm diamond paste. It was then subjected to two cycles of annealing in 1 atm of flowing oxygen at 1200 K followed by cleaning in 40% aqueous HF to remove oxidizable impurities such as Si. After bakeout and a number of sputter-anneal cycles in UHV, a surface clean to AES and EELS producing a bright, sharp LEED pattern was achieved. This surface reproduced the water adsorption and water/oxygen coadsorption TPD and HREELS results which have been previously reported [18,27,28]. Anhydrous HF, obtained in a steel cylinder from Matheson, was subjected to freeze-pump-thaw cycles to remove He and any other more volatile impurities. HF was admitted into UHV through a standard leak valve and a capillary array doser. Post-experiment disassembly of the vacuum equipment showed no visible degradation of the UHV components, though some corrosion was noted in the dosing manifold lines exposed to atmosphere pressures of HF and other acid vapors. The exposure scale for dosing was calculated using the uncorrected background pressure rise in an ion gauge (typically Ap = 3 x 10e8 Pa) multiplied by a lOO-fold intensification factor for the doser. The intensification factor was determined by comparing the thermal desorption area obtained from dosing CO or H,O via the capillary array beam to the area obtained from dosing via a non-directed overall pressure of the same gas. The Pt(ll1) crystal was cooled to around 100 K during dosing and EELS spectroscopy. The crystal could be heated resistively via two 0.020 inch Ta wires spot-welded across its back. Water was admitted into UHV from the vapor above a freeze-pump-thawed sample of the ultrapure liquid (Harleco no. 64112) through a second leak valve/capillary array assembly. TPD data were taken at 5 or 10 K/s constant heating rate with a multiplexed quadrupole mass spectrometer encased in a nozzle assembly [29] with a 6 mm aperture to discriminate against molecules desorbing from structures other than the crystal plane of interest. Temperatures were de-

F. T. Wagner, T. E. Moylan / Identification

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129

termined by a 0.003 inch Ni-Cr/Ni-Al thermocouple spot-welded to the side of the crystal with an estimated precision of k 1 K and accuracy within 5 K; identical water desorption curves have been measured in several different apparatuses. Special multiplexing software was written to allow closely-spaced peaks to be accurately measured. EELS spectra were taken with a 60° incident angle (from the surface normal), specular takeoff angle, and a beam energy of 3.6 eV.

3. Results and analysis 3. I. Thermal desorption 3.1.1. Desorption of anhydrous HF Fig. 1 shows M/e = 20 (HF) desorption spectra for various exposures of HF on Pt(ll1) at 90 K. For low exposures (O-9 L) a single monolayer peak grew in at 120 K, shifting to higher temperatures with higher coverages, stabilizing at 130 K at 5 L. Above 9 L a second peak appeared at 126 K which did not saturate at higher exposures, identifying it as being due to multilayer of HF” will refer to the condensed HF. In this paper the term “monolayer amount of HF required for the onset of the multilayer desorption peak on

Y

HF/Pt

(Ill) 5K/s

_i

L a-

3

200

150

-3

250

T (K)

Fig. 1. HF thermal desorption after dosing anhydrous HF on Pt(111) at 100 K. Langmuir doses and monolayer coverages (from integrating TDS signal as described in text) are: (a) 0.02 L, 0.002 ML: (,b) 0.2 L, 0.03 ML; (c) 0.6 L, 0.05 ML; (d) 0.9 L, 0.10 ML; (e) 2.2 L, 0.21 ML; (f) 4.5 L. 0.49 ML; (g) 9 L, 1.3 ML; (h) 12 L, 1.5 ML; (i) 16 L, 2.5 ML; (j) 22 L, 4.0 ML.

130

F. T. Wugner,

T. E. Mqylun

/ Identificumn

of surface h_ydronium

clean Pt(ll1). The water monolayer is similarly defined. As previously reported [27,28], water desorption showed similar behavior to HF, though at - 50 K higher temperatures. Integrated thermal desorption areas proved quite reproducible and were the basis for quantitative comparison, rather than doses in langmuirs which proved somewhat less reproducible. No LEED spots other than those due to the substrate were seen for any coverage of HF, and the diffuse background increased with HF coverage, indicating an adsorbed state lacking long-range order. No M/e = 19 (F) nor M/e = 2 (HZ) signals not attributable to ionization fragmentation were observed, nor was M/e = 38 (F2) observed. The simple form of the HF desorption and the lack of desorption peaks for dissociation products indicates that HF was molecularly adsorbed. A small M/e = 18 (H,O) desorption peak, corresponding to about 1 H,0/60 HF, was seen for all coverages. This was probably due to an H,O impurity produced by reaction of HF with oxides on the gas manifold walls. No (HF). (n = 2, 3, 4, 6) PtF,, or PtF, signals were seen, even at temperatures up to 1300 K. Desorption of F, PtF,, and PtF, have been reported for atomic F adsorbed on Pt(ll1) [25] and/or Pt(lOO) [26], but F is a much stronger oxidant than HF. No F Auger signal was seen for adsorbed HF at 90 K. The - 10 pm diameter, 1 PA, 3 keV electron beam apparently rapidly desorbed HF, as it does water, on Pt(ll1). An interpretation of the energetics of HF desorption will be given in section 4.4.

DoseClangmulrs)

Fig. 2. Integrated

HF thermal deaorption area versus dose of anhydrous No other F-containing products were observed.

HF on Pt(ll1)

at 100 K.

F. T. Wagner, T. E. Moylan / Identification of surface hydromum

131

Fig. 2 shows the integrated M/e = 20 (HF) thermal desorption area as a function of exposure. The upward concavity of this curve plus the lack of other desorbed products or residual surface F indicate that the sticking coefficient increased with coverage. In contrast, similar data for water gave an essentially linear plot. Comparing the HF dosing scale with that for water (s = 1) indicates an absolute sticking coefficient of - 0.1 for the monolayer and 0.3 for the multilayer, subject to an unknown ion gauge sensitivity factor (here assumed equal for HF and H,O). HF has a higher sticking coefficient on condensed HF than on Pt. In view of the very weak HF-Pt interactions (section 4.4) and strong HF-HF interactions this result is not surprising. The sticking coefficient of HF on monolayer H,O was intermediate between that for the clean surface and that for multilayer HF. 3.1.2. Desorption of coadsorbed H,O and HF Figs. 3 and 4 show M/e = 18 (H,O) and M/e = 20 (HF) desorption from layers produced by sequential dosing at 100 K; no other desorbing species were seen. First a nearly constant dose of water giving a small amount of the multilayer desorption (- 1.25 monolayers) was administered. Then a variable dose of HF was added, followed by thermal desorption. Reversal of the dosing order produced qualitatively similar results, suggesting rapid interdiffusion of the coadsorbates. As shown in fig. 3, water monolayer desorption remained at

100

200

150

250

T (K)

Fig. 3. HF and H,O thermal desorption after coadsorption 1.8 monolayers HF. H,O desorption is identical in absence peak were also seen for the same HF/H*O ratio when monolayer.

of 1.4 monolayers H,O followed by of HF. All peaks except 162 K Hz0 the total coverage was less than 1

132

F.T. Wagner, T.E. Moylun / Identificntion of surface hydronium

100

150

200

250

T (Kl

Fig. 4. HF thermal desorption signals, normalized to H,O coverage. for HF/H,O ratios of: (a) 0.12, (b) 0.15, (c) 0.20, (d) 0.42, (e) 0.56, (f) 0.75, (g) 1.28. Water coverage ranged from 1.1 to 1.4 monolayers. Similar results were seen for water coverages around 0.25, 0.5, and 0.7 monolayers.

176-181 K. and multilayer desorption at 160-170 K, very close to the clean-surface temperatures. Fig. 4 shows that low coverages (< 0.13 monolayer) of HF desorbed with the water monolayer at 180 K, i.e., - 50 K higher than they would have in the absence of water. For intermediate coverages ( > 0.15 monolayer) an additional HF peak at 162 K was seen. While some of this was due to HF desorption along with the small amount of multilayer water, the 162 K/180 K HF ratio grew much larger than the multilayer/ monolayer water ratio, indicating an independent origin for at least part of this peak. This was shown unambiguously for submonolayer water doses. Still larger HF coverages (> 0.7 monolayer) produced a third peak at 136 K, slightly higher than the 130 K HF desorption from the water-free surface. The integrated areas of the three desorption peaks and their sum are shown as a function of HF monolayer coverage (both normalized to water monolayer coverage) in fig. 5. In separating the contributions due to the three peaks it was assumed that the 180 K peak maintained its low-coverage (i.e., isolated) shape and that the 136 K peak was symmetric. Data for water coverages around 0.25, 0.5, and 1.25 are plotted together, as the data for these three ranges superimposed well. Because of the two possible origins of the HF desorbing around 162 K for the 1.25 monolayer water data, these points were corrected by shifting HF intensity calculated as (multilayer water/monolayer water) x (180 K HF) from the 162 K to the 180 K peak. This small correction

F. T. Wagner, T. E. Moylan / Identification of surface hydronium

H,O 105 I”

0

.2

monolayers/HF

monolayer

3

2

1.5

1

.a

’

1

I

I

1

.4

.6

HF monolayersllml

133

.6

1.0

1.2

H,O

Fig. 5. Hz0 and HF coadsorption on Pt(ll1) at 100 K. Integrated intensities of each of the HF desorption peaks, and their sum, all normalized to the water coverage, versus initial HF/H,O monolayer ratio. Data for water coverages ranging from 0.25-1.4 monolayers.

did not change any results, but increased the self-consistency of the data. Fig. 5 shows that each higher-temperature HF peak saturated as the next lower temperature peak started to grow in with increasing HF/H,O ratio. This suggests reaction of H,O and HF to produce several different phases, from which desorbed the more volatile component, HF, in a form of fractional distillation. Vibrational spectroscopy supporting this interpretation will be presented in the next section. The critical stoichiometries of these phases (in monolayers H,O/monolayers HF) could be determined from the break points of the lines in fig. 5. The 162 K peak grew in, and the 180 K intensity flattened, at H,O/HF monolayer ratios of 6.5 and 5.7, respectively, with the former point somewhat more accurately determined. This establishes the minimum amount of water needed to stabilize HF on the surface to 180 K. The 136 K peak grew in, and the 162 K peak intensity flattened out at an H,O,/HF monolayer ratio of 1.4, establishing the stoichiometry of the 162 K

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F. T. Wrrgner, T.6

Moylun / Identificatron

of surface hydronium

phase. The slight nonzero slopes after saturation in fig. 5 are believed due to error in the deconvolution of the desorption peaks. The H,O/HF monolayer ratio for fully stabilized HF was also studied for water coverages around 3-4 monolayers, using a similar analysis. The corrected 162 K peak grew in, and the corrected 180 K intensity flattened, at H,O/HF monolayer ratios of 9.2 and 9.5, respectively. These numbers are significantly higher than those for the monolayer film, showing more complete hydration in the bulk than in the surface phase. All stoichiometries have so far been quoted in experimental monolayer units, rather than absolute molecular ratios. In the next section vibrational spectra will be presented which identify products of the HF + H,O reaction. A chemical interpretation of the thermal desorption peak multiplicity and a somewhat tentative calculation of absolute molecular ratios will then be given in section 4.

3.2. Vibrational spectroscopy

of coadsorbed HF and H,O

Fig. 6 shows EELS spectra for a monolayer of water (a); a monolayer of water plus increasing amounts of HF up to one monolayer (b-e); a monolayer of HF plus l/4 monolayer of water (f); and a monolayer of HF (g) showing, in order, the effects of increasing HF/H,O ratio on the surface. Clear evidence for HF + H,O reaction was found in a peak around 1150 cm-’ for mixtures, a peak which was absent for both pure substances. This section will consider in detail the vibrational spectra first of the pure adsorbed substances and then of their reaction products.

3.2. I. Vibrational spectra of water on Pt(l1 I) The spectrum for pure water (fig. 6a) was as reported previously [10,18] with dominant peaks for librations (670 cm-‘), the H-O-H scissor (1610 cm ‘), and the OH stretch (3440 cm-‘). It indicates predominantly molecular adsorption with extensive hydrogen bonding. Water features unique to Pt(ll1) include an unusually sharp high-frequency OH stretch at 3440 cm-‘, a predominant sharp librational peak at 670 cm-‘, and additional peaks around 1000 and 1950 cm-’ not seen on other surfaces. A detailed interpretation of these features has been given elsewhere [lo]. The low frequency libration and additional peaks remained evident with increasing HF dose through spectrum c, showing that the unique interactions of water with Pt(ll1) were not destroyed by low coverages of the H,O + HF reaction products. This result leaves open the possibility that these spectral features may be relevant to unusually cyclic voltammetry previously reported on Pt(ll1) in dilute aqueous HF [30].

F. T. Wagner, T. E. Moylan / Identification of surface hvdronium Loss Energy 1200 2000

400

(l/cm)

2600

770

‘0

___~ 100

/~ 200 300 Loss Energy (mV)

135

3600 .___-

400

~A_

500

Fig. 6. EELS spectra for H,O and HF adsorption and coadsorption on Pt(lll) at 100 K, HF/H,O ratio increasing upwards. (a) 1 monolayer H,O ( X 100); (b) 1 ML H,O + 0.05 ML HF ( x 100); (c) 1 ML H,O + 0.12 ML HF ( x 100): (d) 1 ML H,O - 0.5 ML HF (X 100); (e) 1 ML H,Omkl ML HF (x100); (f) 1 ML HF+0.25 ML H,O (x100); (g) 1 ML HF (x16.5) and ( X 110). Coverages are based on gas doses.

3.2.2. Vibrational spectra of adsorbed anhydrous HF The EELS spectrum for monolayer HF (fig. 6g) was dominated by a single librational peak at 770 cm-’ and its first overtone at 1550 cm-‘. Considering the weak bonding of HF indicated by thermal desorption and the - 300 cm-’ frequency for the stronger Pt-Cl bond [31,32], this frequency is much too high for an M-F stretch. The fact that the EELS spectrum for multilayer HF gave the same peaks as the monolayer and submonolayer coverages also indicates that no modes with predominantly metal-adsorbate stretch character appear. In light of the high dipole moment of the HF molecule (1.83 debye) [33], a high EELS intensity for librational modes is reasonable. Much as in the case

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T.EL Mcylan

/ Identification

ofsurface hydronium

for water, the adsorbed state on Pt(ll1) yielded a sharper, simpler librational spectrum than has been reported for the bulk solid phases. Bulk HF ice prepared at 148 K (i.e., near the melting point) has given IR absorption ascribed to librations over the broad frequency range of 550-1130 cm--’ [34]. Rapid deposition at 88 K, thought to give a less crystalline deposit, accentuated the absorption at 792 cm--’ [34], a frequency similar to that observed for the monolayer on Pt(ll1). The lack of any overlayer LEED pattern for any coverage of HF on Pt(lll) showed that the surface phase responsible for the 770 cm- ’ peak was also disordered. The high intensity of the 770 cm- ’ peak indicates a strong dynamic dipole moment normal to the surface. If the adsorbed phase consists of the zigzag linear chains known to comprise bulk HF ice [35], these chains should zig out of the surface, rather than parallel to it. As will be seen, this conclusion is also consistent with the estimated molecular density of the HF monolayer. Several weaker EELS peaks were also observed. The second harmonic of the libration was seen at 2340 cm-‘, with a fundamental:lst:2nd harmonic intensity ratio of 200:15:1. A slightly larger peak was seen at 3110 cm-‘, the proper frequency for the 3rd harmonic; but an intensity - 15-fold larger than that suggested by the intensity progression of the first three peaks indicated a different origin. It was paired with another peak of roughly equal intensity at 3370 cm- ‘, for a mean frequency of 3240 cm-l. This is near the 3273 cm ’ [lo] or 3277 cm-’ [ll] F-H stretch reported for the IR of an isolated HF molecule in DF ice. In bulk isotopically pure HF ice, coupling between FH oscillators splits the stretch into two major components at 3060 and 3414 cm-’ [36]. This major splitting in the bulk isotopically pure material was ascribed to coupling between adjacent FH oscillators, which could also occur in the adsorbed state. The mean FH stretching frequencies for adsorbed HF and HF ice, which are - 900 cm-- ’ below those for the gaseous HF monomer, are indicative of the strong hydrogen bonding in these condensed phases. Two additional small EELS peaks were seen for the HF monolayer at 3870 and 4090 cm- ‘; these are combination bands of the FH stretching peaks and the very intense libration. The vibrational spectra of adsorbed anhydrous HF are thus consistent with molecular adsorption and strong hydrogen bonding between molecules. 3.2.3. Vibrational spectra of H,O + HF reaction products Spectra b-e in fig. 6 show the effects of adding submonolayer HF to a monolayer of H,O on Pt(ll1) at 100 K. A major new feature grew in at 1150 cm ’ in spectrum b, shifting downwards in frequency with increased HF the OH stretch (at exposure, reaching 1100 cm ’ in spectrum e. Concurrently, 3440 cm-’ for pure water) broadened and shifted to lower frequency. The H-O-H scissor at 1610 cm-’ decreased in intensity and was replaced by a peak around 1740 cm-’ in spectrum e. The sharp, low frequency libration at

137

F. T. Wagner, T. E. Moylan / Idenrificahon of surface hydromum Table 1 Vibrational and HF

frequencies

for H30+

Symmetric bend (cm

Compound

[H,O+ I[ClO,_1 (H,0+][NOm3] [H@+ l[F- I Liquid aqueous strong H,O + HF on Pt(ll1)

670

in solid acid hydrates,

acids

1175 1135 1048 1200 1080-1150

‘)

aqueous

solutions,

and coadsorbed

Asymmetric bend (cm- ‘)

OH stretch (cm-‘)

Ref

1577 1680 1705 1750 1750

3285 2780 3150 2900 3230

[371 [381 [391 [401 This work

H,O

cm-’ and the anomalous feature at 1950 cm-’ characteristic of pure water on Pt(lll) survived HF addition up to HF/H,O around 0.1 but disappeared with more HF. When the HF/H,O ratio was further increased in spectrum f the intensity of the major H,O + HF peak around 1100 cm-’ decreased, and libration and FH stretch peaks similar to those for HF alone on the surface grew in. The peak at about 1100 cm-’ lies in the frequency range observed in the IR for the symmetric bend of the H,O+ ion in frozen acid hydrates HA. H,O (table 1) shown by diffraction experiments [41] to have the structure of an ionic crystal [H,O+][A-1. H,O+ in liquid aqueous strong acids gives this bend around 1200 cm-’ [40]. NMR experiments [42,43] have confirmed a pyramidal structure for H30+ analogous to that for the isoelectronic NH, molecule. The IR of acid hydrates and aqueous acids also gave an H,O+ asymmetric bend near the H-O-H scissor mode of H,O at 1610 cm-l (table l), seen for the coadsorbed state at 1740 cm-‘. The fact that the asymmetric/symmetric intensity ratio was lower in the monolayer EELS spectra than in the IR of bulk phases suggests that H,O+ has C,, symmetry with the rotation axis normal to the surface. H,O+ in bulk phases gives an OH stretch frequency lower than that for liquid water (table 1). The OH stretch for coadsorbed HF and H,O did in fact fall at lower frequency than that for water alone. H,O+ in the hydrogen halide monohydrates has also yielded an IR librational peak at 740-810 cm-’ [39]. This was seen for the coadsorbed state as the peak at 800-850 cm-’ in spectra b-e. An additional peak at 400 cm-‘, visible in spectra d-f and more prevalent for multilayer H,O + HF, appears from its intensity/coverage relationship to be the coadsorption analog of the hindered translational mode of pure adsorbed water at 260 cm-‘. The intensities of the - 1100 cm-’ peak and of the other H,O+ features maximized, and the intensity of H,O or HF features minimized, at dosing ratios in a range equivalent to 0.5-l HF monolayers per H,O monolayer. Overall, the unique features of the coadsorption EELS spectra gave a good match with the known vibrational properties of H30+ in both crystalline hydrates and aqueous solutions, demonstrating that the Brdnsted-Lowry acid-base reaction

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/ Identifmttion

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HF + H,O + H,O+ + F’ occurs even on the monolayer scale. More detailed discussion, considering stoichiometries of higher hydrates and the extent of ionization, will be given in section 4. The previous paragraph demonstrated the production of an H30+ species from H,O and HF. Mass and charge balance require that F- also be present. At low HF/H,O ratios the F- should be solvated by water. The vibrational spectrum of such a species would be that of water perturbed by the presence of an ion; ion-solvent modes are not readily observed for monovalent ions and lie in the 200-500 cm-’ range (where they would be masked by librations) when visible for di- and tri-valent ions [44]. Such subtle effects would be hard to see in low concentration and, in fact, no vibrational features readily attributable to F- ions were seen for HF/H,O < 1. Conventional wisdom from aqueous solutions indicates that at higher HF/H,O ratios F- would be solvated by HF, forming HFZp ions or their higher HF adducts [24]. However, Giguere [45] has suggested that the data indicating HF*- formation in concentrated solutions can be better explained by strongly H-bonded H,O+ . . . Fion pairs. Vibrational spectra of HFZe are available from KHF, and NaHF, as crystalline solids [46,47] or concentrated aqueous solutions [47] or, subject to some controversy, from concentrated aqueous HF [47]. The HFZp ion is characterized by a v3 asymmetric stretch around 1500 cm-‘, a v2 bending mode at 1000-1200 cm- ‘, and a vi symmetric stretch (non-IR active) around 600 cm-’ [46]. The first two might be expected to be seen by EELS. The v2 bend would overlap with the H,O+ symmetric bend. The vj peak would lie just below the HOH bend of water. Intensity in this region may be visible in spectra e and f, serving to flatten out this region above the baseline. However, the 2nd harmonic of the molecular HF libration also contributes intensity in this frequency range at still higher HF/H,O ratios, so observations in this frequency range cannot conclusively distinguish possible species. While F- or HF; cannot be unambiguously identified in the EELS data, neither are the data inconsistent with their presence. Vibrational peaks due to anions will be apparent in future reports on water coadsorption with other mineral acids [32]. The strong evidence for H,O’ formation, even when HF and H,O are codeposited to thicknesses greater than a monolayer of each, is an indirect indication that F- in some solvation state is formed.

4. Discussion In this section coordinated interpretation of the HREELS and TDS results will identify the different hydration states of HF on Pt(ll1). The surface and near-surface hydration numbers of H+ and F- ions will be compared with the solvation numbers of these ions in room-temperature aqueous electrolytes, the structure of the hydrated species will be speculated upon, and the possible

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electrochemical significance of these results will be noted. Interpretive details of the energetics and structure of anhydrous adsorbed HF will be filled in, and the prospects for in vacua modeling of the electrochemical double layer will be commented upon. 4.1. The ionization

of adsorbed hydrous HF

Section 3.2.3 presented EELS evidence for H,O+ (and by inference, FP) production from coadsorbed HF and H,O.‘ This result is by no means surprising for nearly equimolar coadsorption (figs. 6d and 6e) since the interactions with the surface are weak and the H,O+-containing monohydrate is a well-established bulk phase. However, during coadsorption at 100 K even HF/H,O monolayer ratios as low as 0.05 (fig. 6b) produced a readily-detectable H,O+ EELS peak at 1150 cm-‘. At room temperature, dilute aqueous HF is a weak acid with a dissociation constant K, of 6.71 X lop4 [48]; though it becomes somewhat stronger at concentrations above 5M [24]. The reported acid strength for 3.7M HF [24] (which is equivalent to the HF/H,O ratio for fig. 6b) would yield a surface HsO+ density of 5 X 10” H,O’/cm’, below what one would expect to be able to see by EELS in the presence of a water monolayer of 1 X 10 l5 H,O/cm* [27]. The EELS data, in fact, suggest nearly complete ionization of HF even at low HF/H,O ratios. While this result would at first appear to run counter to expectations for the aqueous HF system, it can be understood when the reasons behind the weak-acid behavior of HF at room temperature are considered. Myers [49] has proposed that the low ionization of HF compared to other hydrohalic acids is due in large part to a greater negative entropy of hydration for the fluoride ion than for other halides. The F- ion restricts the translation of more water molecules, and to a greater extent, than do other halides. In the monolayer of water adsorbed at 100 K the mobility of water molecules is already restricted by the presence of the surface and the low temperature, so solvation of F- does not involve a major entropy penalty. The extensive ionization of coadsorbed HF simplifies analysis of the data, but provides a warning that entropy effects may complicate comparison of normal aqueous chemistry and UHV aqueous chemistry below 180 K. 4.2. The hydration

states of ionized HF on Pt(lll)

Dilute HF coadsorbed with water did not desorb until the water left the surface, i.e., 50 K above the temperature observed for HF alone. This result shows that the higher vapor pressure (and more weakly adsorbed) HF component reacted with water to form species with effective HF partial pressures equal to or less than the partial pressure of water. This reaction was thermally reversible, since we observed no species other than HF and H,O desorbing.

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EELS showed essentially complete ionization of dilute HF to form H,O+. Hydration of the H,O+ and F- ions sufficient to break up ion pairs would produce a system with the observed negligible HF partial pressure below 180 K. The eventual HF desorption around 180 K is precipitated by the loss from the surface of the water needed to hydrate the ions. These ions then revert to a less-hydrated phase in which they are incompletely screened from one another by water. They then readily combine to yield HF and water, which leave the surface concurrently. The ~~rnurn amount of water needed to produce the fully hydrated ionized phase was shown in section 3.1.2 to be 6.5 H,O/HF (in monolayer units) for a monolayer or less of water on the surface, and 9.2 H,O/HF (monolayer units} for the average over a monolayer plus two or three additional layers of water. These directly obtainable ratios in monolayer units need not be the same as the desired molecular ratios. While the surface density of the water monolayer has been established by XPS as (10.1 + 1) X 1014 molecules/cm* [27] (or 0.75 H,O/surface Pt), the density of the HF monolayer was not known. The current lack of XPS in the HF-exposed chamber prevented determining the HF monolayer density with equal precision to that for water, but consideration of the EELS spectra and the TPD stoichiometry of the 162 K HF desorption peak allows a less direct deter~nation of the HF monolayer density, and thereby of the hydration ratios in units of molecules. The 162 K thermal desorption peak started growing for HF/H,O monolayer ratios above 0.15 and saturated above an HF/H,O monolayer ratio of 0.7 (i.e., below an H,O/HF monolayer ratio of 1.4). The EELS around this saturation ratio showed no definite peaks due to excess H,O or HF, suggesting complete reaction to form a phase such as [H,O+][F-] with H,O+ but no HF or H,O local units. The observed monolayer ratio (and a first-order assumption that the monolayer densities of these two similar hydrogen-bonded molecules should be close) would suggest a saturation stoichiometry of either HF. H,O or HF - 2H,O. While the former is a well-established bulk solid phase of exact stoichiometry [39], the latter is unknown, as freezing curves have shown that no water-rich solid phases form [50]. The EELS, thermal desorption stoichiometry, and mere presence of a saturation phase of well-defined stoichiometry make it most likely that the stoichiometry of the phase corresponding to the saturated 162 K peak is HF * H,O, with true composition [H,O+]]F-1. If this is accepted, the saturation monolayer ratio of 0.7 for this phase, combined with the known surface density of the water monolayer of [27], gives the surface density of the (10.1 + 1) x 1o14 molecules/cm* anhydrous HF monolayer as 1.4 X 1015 molecules/cm*, or 0.9 HF/surface Pt atom. This is in good agreement with the 2/3 power of the 3D density of hulk crystalline HF ice, 1.36 x 10’” molecules/cm*. No single flat plane of the HF ice structure [35] has a 2D density this high, but corrugated “planes” with alternate fluorines in the plane and slightly above it do have densities this large. The inferred monolayer density thus agrees with the conclusion drawn

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from EELS data (section 3.2.2) that the H-bonded HF chains zigzag out of the plane proximal to the surface rather than lying entirely within it. Thermal desorption of the [H,O+][F-] phase proceeds by fractional distillation. Adjacent protons and fluoride ions in the hydrogen-bonded ion pairs combine to exit the surface as HF around 162 K, leaving water behind. This water more completely hydrates some of the remaining [H,O+][F-1, forming H,O&, + F&. Once only this fully hydrated surface form remains, HF desorption ceases, giving the desorption minimum around 170 K. When the surface temperature reaches - 180 K, water desorbs. As the ions lose their hydration “spheres”, the H,O+ and F- become directly hydrogen-bonded, the HF vapor pressure rises, and both components desorb together. For initial HF/H,O molecular ratios greater than 1 both the monohydrate and molecular HF were present on the surface. The molecular HF desorbs at due to solvation 136 K, - 5 K higher than the clean surface temperature interactions with the monohydrate (possibly including HF,- formation). Once the excess molecular HF leaves the surface, only the monohydrate remains. Its desorption proceeds as outlined above. The estimation of the surface density of the HF monolayer allows the minimum amount of water needed to stabilize HF to 180 K (i.e., the number of water molecules required for full solvation of both H+ and F-) to be expressed directly as a molecular ratio: 5 molecules water per molecule HF for water coverages below 1.4 monolayers or 7 molecules water per HF molecule for water coverages of 3 or 4 monolayers. If the latter is assumed to arise from a weighted average of one monolayer and 2.5 “bulk” layers, the molecular ratio for the bulk layers alone would be 8. We estimate that these numbers are accurate to -+_1, based on uncertainties induced by experimental scatter and by ambiguities in the separation of thermal desorption peaks. The critical solvation ratios given above can be thought of as the sum of the solvation numbers of the H+ and F- ions in the monolayer (5 water molecules) and multilayer (8 water molecules) adsorbed phases. It is of interest to compare these sums with that of the solvation numbers previously measured in room temperature, bulk aqueous solution. Though published bulk solvation numbers [51-531 exhibit considerable scatter (e.g., for H+ from 0.3 to lo), and all of the methods used require assumptions, consensus appears to have been reached on probable ranges for the strongly-bound inner solvation sphere. The solvation number of bulk aqueous H+ (substract 1 for H,O+) has been determined as 5 from the entropy of hydration, 3 from compressibility, 4 from density, and 4 from heat capacity, for a most-probable value of 4 [52]. This corresponds to H904+, an easily-visualized complex of a pyramidal H,O+ with one water H-bonded to each of its H atoms (fig. 6 in ref. [53]). It should be noted that even these bulk measurements do not correspond to a complete space-filling “sphere” or octahedron of inner shell water. The solvation number of bulk aqueous F- has been determined as 5 from the entropy of

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hvdronrum

solvation, 2 from compressibility, and 4.3 from density, for a most-probable range of 4 + 1 [52]. The sum of the bulk aqueous solvation numbers of H+ and F- thus lies in the range of 7-9, in good agreement with the 8 water molecules/HF observed for the multilayer adsorbed state at 100 K. The 5 water molecules/HF observed for the monolayer adsorbed state indicates the loss of - 1.5 water molecules/ion in the layer next to the surface. This shows that, although these monolayer ions remain extensively hydrated, the surface interacts with and contributes to the hydration of even these prototypical “non-specifically adsorbed” ions, at least under our experimental conditions. Recently reported differences in the cyclic voltammograms of single-crystal gold in aqueous HF versus aqueous HClO, [54] indicate specific ion-surface adsorbed” acids under normal interactions for these two “ non-specifically electrochemical conditions as well. This agreement and the equivalent solvation numbers observed for the multilayer adsorbed state and the bulk aqueous solution support a preliminary conclusion that our low-temperature vacuum experiments are relevant to real electrochemical conditions, Among a number of other in vacua determinations of surface ionic hydration numbers previously reported [6,12,15,20,55,59-621, several provide particularly interesting contrasts to the current results. Sass and coworkers have studied the coadsorption of Br, and H,O on copper surfaces [12,15,20]. Br stabilizes water on the surface to the extent of 1 H,O/Br at low coverages, dropping to zero water at the Br saturation coverage. Since the probable range for bulk aqueous solvation number of Br- is 1 t_ 1 [52], it is difficult to discern whether the surface modifies the bulk solvation. The thermal desorption and LEED results indicate that metal-Br interactions governed the system, while in the HF/H,O system interactions between adsorbate species dominate metal-adsorbate bonding. Frank et al. [6] have reported that Ca2+ maintains in vacuum at room temperature a solvation shell of about 5 water molecules when Pt(ll1) is emersed from an aqueous Ca2+ solution, compared with 7.5-10.5 water molecules from mobility and 10 from entropy measurements in bulk aqueous solutions [51]. The ions (or a hydroxide phase) actually picked up water from vacuum background gases [55], as did the Ba and Mg systems. Since significant water remained at room temperature, one might expect these ions to be fully hydrated at temperatures where free solvent also remains on the surface. Thus the highly polarizable Br and the very hard Ca2+ appear somewhat nearer the extremes of specific and non-specific adsorption, respectively, than are H,O+ and F--. 4.3. The structure of hydrated ionized HF Neither anhydrous HF nor HF coadsorbed with water gave ordered LEED patterns. Thus no direct structural information is available. However, TPD results indicated that the surface density of water (free or incorporated into

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H30t) was not significantly modified by formation of the fully hydrated HF phase. This was shown by experiments in which just above or just below a monolayer of water was dosed with little enough HF to ensure that only the 180 K HF desorption peak would be seen. If HF displaced water from the monolayer, a multilayer water desorption peak should have appeared for water doses just below a monolayer. No such peak appeared. If HF coadsorption increased the water density in the monolayer, the height of the multilayer peak for a larger-than-monolayer dose should have been decreased by HF coadsorption. It was not. This technique would have been sensitive to changes of density above - 10%. While addition of HF to form the fully hydrated state disrupts the imperfect fi X fi-R30” long-range order of the pure water monolayer, it leaves the surface density, and thus probably the short-range features of the adsorbed water structure, intact. The structure of monolayer pure water adsorbed at 100 K is thought to be derived from the corrugated basal plane of ice [56,57], in which the 0 atoms form an edge-bonded mesh of 6 member rings puckered with alternate 0 atoms near to and away from the surface in the manner of the chair form of cyclohexane. This structure preserves part of the three-dimensionality and predominantly tetrahedral coordination of bulk liquid water (the structural relationships between ice, liquid water, and adsorbed water will be discussed in another report [lo]). In the monolayer each 0 atom is hydrogen-bonded to 3 adjacent 0 atoms and has 2 H atoms near it, either in the H-bonds or in free bonds angling away from the surface. One can conceptually introduce H,O+ into this structure simply by moving another proton close to an 0 atom. The orientation of such an H,O+ would be correct to produce the enhanced symmetric/asymmetric bend intensity ratio seen in EELS. Grouping the H,O+ with the three adjacent waters in the monolayer structure, one gets an H904+ unit with structure similar to that proposed for aqueous solutions. F- can fit into this structure either within the hexagons or by displacing a bent-out 0 atom, which could then form a strongly-bound water on top of the fluoride, completing a hydration tetrahedron for the F-. The electrochemist, thinking of “hydration spheres”, is inclined to reject the “two-dimensional” coadsorbed layers studied here as irrelevant to electrochemistry. However, the hydration numbers of 4 for H+ and F- in aqueous solution suggest tetrahedral primary solvation, not complete “spheres”, and such tetrahedral coordination is consistent with the probable structure of the water monolayer. 4.4. The nature of adsorbed anhydrous

HF

The thermal desorption of HF on Pt(ll1) resembled that of many other weakly-bound molecular species in that separate saturating monolayer and non-saturating multilayer peaks were observed at very closely-spaced low temperatures. The EELS spectra confirmed molecular adsorption with strong

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of surfacehydronium

hydrogen bonding within the adsorbed layer. In bulk HF ice H-bonding produces zigzag linear chains with an F-F distance of 2.49 A and an F-F-F angle of 120.1” [35]. With a 10% stretch of the F-F distance, the chains could zigzag parallel to the Pt(lll) plane with every F in registry with the substrate. However, such an arrangement would equate the two types of F-F distances (with and without an intermediate hydrogen) and would occur only if the M-F interactions were stronger than hydrogen bonding within the HF layer. In fact, the strength of the EELS librational peak and the high (1.4 X 10’5/cm2) packing density of the HF monolayer inferred from the coadsorption experiments indicate that the zigzagging does not occur within the surface plane, but rather out of it, with alternate F atoms next to and away from the surface. Since LEED of adsorbed anhydrous HF showed only diffuse background in addition to substrate spots, the only further structural comments which can be made is that the layers lack long-range order. The HF monolayer peak shifted to higher temperatures with higher coverage in a manner analogous to that for water on Pt(ll1). A number of models have been proposed for this type of behavior, including zeroth-order [28] and a first-order desorption with a slight increase in activation energy with increasing coverage [58]. The leading edges of the desorption for various submonolayer coverages did not exactly align as would be expected for a zeroth-order process; rather, the desorption peaks for lower coverage lay entirely within those for higher. While the modified first-order model fit our HF data over a limited coverage range, it gave poor agreement at low coverage. Excellent fit of peak temperatures, heights, and half-widths at all coverages was obtained with a model assuming constant activation energy and preexponential and a l/4-order dependence on coverage. While l/2-order desorption can be explained as desorption from island edges, the mechanistic implications of a better fit for l/4-order than for 0 or l/2-order are not clear, but are probably tied to the mobility of HF on the surface. The best fit of the model, assuming a 1Ol3 s-l preexponential, was found for an E, of 7.8 kcal/mol and is shown in fig. 7 for comparison with the submonolayer data in fig. 1. The activation energy is much larger than the 2.73 kcal/mol sum of the equilibrium heats of fusion and vaporization [33], unlike the case for water, where agreement within 20% is seen between TPD-derived heats of desorption and the bulk heat of sublimation. The equilibrium vapor of HF, however, is highly non-ideal and contains a large proportion of oligomers even well above room temperature. Equilibrium vaporization thus entails a net breaking of only a fraction of the 2 H-bonds shared by each HF unit in the chains. In the desorption of HF from Pt(l11) only monomers were detected, requiring one full H-bond to be broken per HF unit released; the 7.8 kcal/mol E, for desorption is close to the 6 kcal/mol H-bond strength in condensed phases of HF [33]. While vaporization in our experiments occurred at pressures in the free molecular flow regime, preventing combination of monomers leaving the surface, the equi-

F. T. Wagner, T. E. Moylan / Identification

100

ofsurfacehydronium

200

150

143

250

T (K) Fig. 7. Calculated desorption curves for anhydrous HF, using the indicated rate equation for coverages of 0.002 (two points just above zero line at 121 K), 0.03, 0.05, 0.1, 0.2, 0.5, and 1 monolayers.

librium measurements were made at much higher pressures where monomers could combine to form the equilibrium distribution of oligomers. Consideration of the desorption and sublimation energies and of the relative dimensionalities of H-bonding in HF and in water indicates that Pt-adsorbate bonds are stronger for water than for HF. In condensed phases of water each molecule shares in 4 H-bonds, requiring a three-dimensional structure for the saturation of hydrogen bonding. Since each HF is involved in only 2 H-bonds, saturation of hydrogen bonding can occur in zigzag chains or rings which can fit into a single, albeit probably corrugated, layer. The observed similarity of the heat of desorption of monolayer water and the heat of sublimation of the bulk implies that the water-metal bond is strong enough to compensate for the H-bonding lost through disruption of the 3D H-bonding net by the surface, i.e., around 5 kcal/mol. Since the H-bonding in HF need not be disrupted by the surface, the HF-metal bond strength is estimated by direct subtraction of the sublimation energy to monomers (- 6 kcal/mol) from the heat of desorption, yielding - 1.8 kcal/mol. It thus appears that HF not only has a smaller net heat of desorption than water, but also has a smaller bond strength to the surface. This could explain the lower initial sticking coefficients observed for HF than for water. 4.5.Prospects for UHV modeling of the electrochemical This coadsorption work has demonstrated istry can be performed and spectroscopically layer or multilayer scale. We have observed

double layer

that Brijnsted acid-base chemfollowed in UHV on the monothermal stabilization of the more

146

F. T. Wugner,

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volatile HF stoichiometry

component indicates

in an hydration

/ Identrficution

of .surJwe h~~dromunt

aqueous phase whose multilayer critical numbers for the ions within the range

reported for inner shell solvation in standard aqueous solutions. This agreement may be fortuitous, as it is as yet uncertain whether the solvation properties of liquid water and of the adsorbed multilayer (which appears to be amorphous ice) should be equivalent. A more detailed comparison of monolayer and multilayer adsorbed water with crystalline ice, amorphous ice, and liquid water can be found in another report [lo]. Our observation of lower monolayer than multilayer solvation numbers for the “non-specifically adsorbed” F and H,O+ ions appears to correlate with the specific ion effects implied by Hamelin’s voltammetric differences between gold in aqueous HF and in HClO, [54], raising hopes for relevance which require further testing. If the results are relevant to standard aqueous electrochemistry, they perhaps require some rethinking of the standard models of ionic adsorption derived from data on mercury, as applied to noble metal surfaces, since surface modification of ionic hydration seems somewhat greater than previously thought. What is clear is that we can produce in UHV un, if not the, ionic aqueous environment with controlled composition and thickness. What we have grown so far should not be considered a typical double layer. We believe that our layers contain equal numbers of positive and negative ions, a condition characteristic of an electrode only when it is at the potential of zero charge (pzc). Our ionic concentrations to date (> 3M) have, for reasons of the need to establish the critical hydration stoichiometries, exceeded those expected for an electrode at the pzc in typical 0.3M HF electrolytes. To produce a full range of true double layers which can be studied by vacuum techniques we need to advance to the third level of modeling: the establishment of control over the effective electrode potential in UHV. Combination of our technique of growing electrolyte layers of desired thickness and composition with Sass’, Madey’s, and others’ coadsorption of oxidizing or reducing agents is one possible direction. In such experiments it is of critical importance to use sufficiently high water/coadsorbate ratios to allow full solvation of any ions produced; and water multilayers as well as monolayers should be studied. A more direct, though mechanically and spectroscopically challenging, approach is to do 3-electrode frozen electrolyte work [9] in UHV. The conceptual difficulty here lies in getting spectroscopic information out despite the thick electrolyte layer. This may require controlled evaporation of everything but a few electrolyte layers over part of the surface. Upcoming reports on the coadsorption of other mineral acids with water will show one way in which this might be done [32]. While demonstrated control of electrode potential in UHV aqueous systems remains a gleam in the eyes of electrochemical surface scientists, UHV-electrochemical transfer experiments are an established fact. The present thermally well-controlled experiments clearly confirm the opinions of those doing trans-

F. T. Wagner, T. E. Mqkm

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fer work that water, HF, and their ionic products will have left the Pt surface during emersion, transfer through dry argon, and evacuation at room temperature. This does not prove to be the case for all mineral acids [32]. Experiments of the type reported here can accurately identify the changes which occur in an electrolyte layer during the transfer process. Once this is known, one can more accurately think backwards from species detected in UHV after transfer to their electrochemical precursors. In some cases it may prove possible to rehydrate layers to their electrochemical state under cryogenic UHV conditions. Synergistic development of transfer experiments and cryogenic coadsorption experiments may thus answer some of the legitimate objections to the use of ex situ spectroscopies in electrochemical research, thereby unleashing the full power of vacuum surface science techniques onto the aqueous-metal interface. 5. Conclusions Anhydrous HF adsorbs molecularly on Pt(ll1) below 131 K to form hydrogen-bonded monolayer and multilayer phases. HF reacts with water on Pt(ll1) to form H30+ and F- ions, which in turn can react with water to form several different hydration states. When more than 5 water molecules per HF are present in the monolayer, the fully hydrated phase is formed, which does not desorb HF until water also starts leaving the surface at 180 K. In layers subsequent to the monolayer 8 water molecules are needed to stabilize one HF to 180 K, in good agreement with the sum of the primary solvation numbers of H+ and F- ions in aqueous solutions of 7-9 [51-531. The lower critical H,O/HF ratio for the monolayer shows that even these prototypical “non-specifically adsorbed” ions lose some waters of solvation at a surface. Coadsorption at a 1:l molecular stoichiometry produces the monohydrate ([H,O’J[F-1) phase, and with added HF layers containing both ions and undissociated HF are formed. Cryogenic coadsorption of water and acid vapors is a productive step towards the modeling of electrochemical interfaces in UHV, and thereby towards a molecular-scale understanding of the processes governing corrosion and electrocatalysis. References (11 A. Bewick and S. Pans, in: Advances in Infrared and Raman Spectroscopy, Eds. R.E. Hexter and R. Clarke (Heyden, London, 1984). [2] ht. Fleischmann, P. Graves, I. Hill, A. Oliver and J. Robinson, J. Electroanal. Chem. 150 (1983) 33. [3] D.M. Kolb. R. Kotz and D.L. Rath, Surface Sci. 101 (1980) 490. (41 D.M. Kolb and W.N. Hansen, Surface Sci. 79 (1979) 205. [S] W.N. Hansen, D.M. Kolb, D.L. Rath and R. Wille. J. Electroanal. Chem. 110 (1980) 369. [6] D.G. Frank, J.Y. Katekaru, S.D. Rosasco, G.N. Salaita, B.C. Schardt, M.P. Soriaga and A.T. Hubbard, Langmuir 1 (1985) 587, and references therein.

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of surfuce hydromunl

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