J. Phys. Chem. 1995,99, 16059-16066
16059
Growth and Characterization of Sulfuric Acid under Ultrahigh Vacuum Elizabeth D. Guldan, Lies1 R. Schindler, and Jeffrey T. Roberts* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 Received: May 30, 1995; In Final Form: August 8, 1 9 9 P
The growth and characterization of sulfuric acid (H2S04) films in ultrahigh vacuum (UHV) is described. The films were synthesized in situ by co-condensing SO3 and H20 onto a single crystal metal surface at 100 K and carefully annealing the mixtures above 160 K. Films were typically 30-50 monolayers thick and were characterized using Fourier transform infrared reflection absorption spectroscopy (FTIRAS),temperatureprogrammed desorption (TPD),and X-ray photoelectron spectroscopy ( X P S ) . The FTIRAS, TPD, and XPS spectra show that solid homogeneous mixtures of varying water content can be prepared, from approximately 10 mol % H2S04 to nearly pure H2S04. The nearly pure films can be prepared in crystalline and amorphous forms, depending on the annealing procedure employed. Some surface chemical properties of the pure H2S04 films were investigated. 2-Propanol is readily adsorbed and absorbed by H2S04, with extensive dehydration to water and propene occurring at relatively low temperatures (94 mol % H2S04. Mode assignments are summarized in Table 2. Note in particular that the 1200-1400 cm-I stretching region has modes attributed to the symmetric and asymmetric S=O double-bond stretching motions, as well as to the S-0-H asymmetric bending vibration. Although H20 and SO3 evolution is nearly complete by 240 K, changes continue to occur in the IR spectra of the films with increased annealing. In Figure 3 we compare the spectra of two films: the first was slowly heated to 260 K and cooled to 100 K, and the second was heated to 260 K, annealed at 240 K for 2 min, and cooled to 100 K. Differences between the first and second spectra (Figure 3a and b, respectively) are significant. In particular, the OH stretching region is na?rrower in the annealed film, with what appears to be a distinct narrow peak overlaying a broader absorption band. The relative intensities of the S=O stretching modes also change, although
3500
3000 2500 2000 frequency / cm"
1500
1000
Figure 4. FTIR spectrum of an HzO/SO3/H20 film on W(100) after rapid heating (6 Kss-') to 260 K. The spectrum was collected at 100 K.
the changes are quite subtle. The compositions of the films in Figure 3a and b must be identical, and we therefore attribute the spectral changes to the annealing-induced crystallization of H2S04, although crystallization may not be complete. Indeed, IR spectra of crystalline H2S04 have been published that bear striking similarity to Figure 3b.I7 Rapid heating of an H20 SO3 mixture leads to the formation of an undesired side product. The IR spectrum that results from rapidly heating an HzO/SO~/H~O film from 100 to 240 K at 6 K-s-' is shown in Figure 4. Although vibrational modes associated with H2S04 are readily apparent, there is also an intense absorption at 1520 cm-'. The feature is not observed in the spectra presented in Figure 2, nor is it reported in previous vibrational studies of the H20 H2SO4 or H2S04 so3 systems. The band is tentatively assigned to the S=O stretch in (SO& or in a polymeric form of SO3. Previous studies of solid SO3 have reported an intense vibrational mode at 1510 cm-l in trimeric S03.9 Whatever its identity, the product can be partially converted to H2S04 by re-exposing the film to water at 100 K and then slowly heating to above 160 K. However, conversion to H2SO4 is never quantitative. Furthermore, in contrast to unreacted SO3, which can be removed from a film by annealing at 180 K, the polymer persists until H2S04 itself begins to sublime, just above 300 K. The putative so3 polymer
+
+
+
Growth and Characterization of Sulfuric Acid
J. Phys. Chem., Vol. 99, No. 43, 1995 16063
d
b
( d e 41 corrected)
isopropanol ( d e 41 corrected) .O
535
530 '
1;s
'
165
tt
80
70
Figure 5. O(ls), S(2p), and Pt(4f) X-ray photoelectron spectra of a nearly pure H2SO4 film, approximately 20 ML thick, deposited on Pt( 1 1 1).
TABLE 3: Comparison of O(1s) and S(2p) Photoelectron Yields Obtained for SO2 and H2S04 photoelectron yieldarbitrary units
O( 1s)
WP)
0:s molar ratio
147 000 1 340000
40 800 173 000
2" 4.3
species
so2 H2S04 a
I
1
binding energy / eV
By definition.
is also formed during the heating of an SO3 layer deposited on the metal substrate, of an SO3 layer on ice, or of an H20 layer deposited on SO3: the successful synthesis of H2SO4 requires that SO3 be sandwiched between two H20 layers. The stoichiometry and the S(2p) and O(1s) binding energies of an H2SO4 film were determined using X-ray photoelectron spectroscopy (XPS). Figure 5 shows the O(ls), S(2p), and Pt(4f) regions of the photoelectron spectrum of an approximately 20 ML pure H2SO4 film deposited on Pt( 111). The film was annealed at 260 K, by which temperature all free H20 had evolved into the gas phase, before analysis at 100 K. Binding energies were calibrated against the Pt(4f) photoemission peaks. The O( 1s) binding energy is 532.7 eV, compared to 534.2 and 532.8 eV in condensed H20 and S02, respectively. The S(2p312) binding energy is 169.2 eV, a value that is indicative of highly oxidized sulfur. These binding energies are in good agreement with those reported some time ago for films much thicker than those investigated here: 532.3 eV for the O(1s) core level and 169.4 eV for the S(2p312) level.'* The stoichiometry of an annealed film is close to that expected for pure H2SO4, namely 4 equiv of 0 per equiv of S. (The 0:s ratio of the azeotrope is 4.1 :1.) In Table 3, we compare the relative O( 1s) and S(2p) photoelectron yields of an annealed H2SO4 film to those in condensed S02, for which the 0:s molar ratio is by definition 2:l. A linear background subtraction method was used to determine the photoelectron yields. From these measurements we estimate that the 0:s ratio in the H2SO4 film is 4.3:l. The H2SO4 films were susceptible to damage during irradiation by the X-ray source. The O(1s) and S(2p) spectra gradually broadened with increasing irradiation. Furthermore, TPD analysis immediately after an XPS experiment resulted in
200
250
300
350
temperature / K
Figure 6. Temperature-programmed desorption of 2-propanol (9 x Pa*s) on a 60 ML H2SO4 film. The four products observed were H2S04 ( d e 98), H20 ( d e 18), propene ( d e 41), and 2-propanol ( d e 41). The propene desorption spectrum was corrected for cracking of 2-propanol, and the 2-propanol spectrum was corrected for cracking of propene, as described in the text. The heating rate was 6 K0s-I.
significant H20 evolution at 260 K and SO3 evolution at 180 K, whereas no desorption occurs at these temperatures during TPD of an annealed H2SO4 film. Beam damage is often observed during XPS analysis of covalently bound materials like H2S04.19 Damage may be photochemically derived but is more likely the result of electron-induced chemistry, for the secondary electron current generated by X-ray irradiation is significant. In any case, beam damage does not affect the validity of the film stoichiometry analysis, because irradiation at 100 K results only in decomposition, not desorption, of the film. Moreover, the S(2p) and O( 1s) binding energies reported above are likely to be quite accurate. The spectra in Figure 5 were obtained after a short (%3 min) exposure to the X-ray beam, before significant broadening of the spectra occurred. 2. Temperature-Programmed Desorption of 2-Propano1, Propene, and HCI. Four gas phase products result from the interaction of 2-propanol (CH3CHOHCH3) with the sulfuric acid azeotrope: water at 300 K, sulfuric acid at 320 K, 2-propanol at 160 and 280 K, and the dehydration product propene at 310 K. In Figure 6 are shown TPD spectra resulting from the adsorption of 2-propanol (9 x Pa-s) on a %60 ML thick H2S04 film deposited on Pt( 111). Water and sulfuric acid were detected as their molecular ions, d e 18 and d e 98, respectively. 2-Propanol and propene were detected as d e 27 (C2H3+) and d e 41 (C3H5+). Propene and 2-propanol both fragment to d e 41 and 27 in the mass spectrometer. However, the line shapes of the d e 41 and 27 traces during TPD of 2-propanol from H2SO4 are different, an observation that implies the desorption of more than one substance. Specifically, the d e 41:27 ion ratio we measure for desorption of pure 2-propanol from platinum is 0.70, while the ratio during TPD from H2S04 is 0.65 near 160 K and 1.0 near 310 K. Given the close correspondence between the fragmentation pattern of the 160 K product and that of pure 2-propanol, as well as the fact that a condensed 2-propanol multilayer sublimes at 160 K, the lowtemperature state is assigned to the sublimation of condensed
Guldan et al.
16064 J, Phys. Chem., Vol. 99, No. 43, 1995 2-propanol from H2SO4. Part of the d e 41 and 27 signals near 310 K must be associated with the evolution of a reaction product, which we assign as propene (vide infra). Because the d e 27 and 41 spectra in Figure 6 were acquired during a single experiment, the 2-propanol contribution can be subtracted from the d e 41 spectrum. The corrected spectrum, which represents the desorption of the propene reaction product, is shown in Figure 6. The mass spectra of 2-propanol and its reaction product overlap, and it is impossible to unambiguously attribute the excess d e 41 signal to propene. Signals that cannot be associated with 2-propanol alone are also observed for the ions d e 42, 39, and 40, all of which are part of the mass spectrum of propene. The product cannot be propane because there is no significant production of d e 44,the propane molecular ion, nor can it be propionaldehyde (CH3CH2CHO) or acetone [(CH3)2CO]. The primary reaction product is therefore assigned as the dehydration product propene, formed from the dehydration of 2-propanol by sulfuric acid: CH3CHOHCH,
%SO,
+ H,O
CH2CHCH3
(3)
Further evidence for eq 3 is found in the H20 TPD spectrum. Most of the d e 18 signal in Figure 6 is associated with the gas phase dissociation of H2SO4 or with the fragmentation of H2S04 in the mass spectrometer. However, only that part of the d e 18 signal which is coincident with d e 98 can be derived from H2SO4. Water evolution precedes H2S04 sublimation in Figure 6, although no H20 desorbs at this temperature during TPD of a carefully annealed film. The d e 18 signal at =300 K (indicated by the shaded portion of the spectrum) is associated with the water of 2-propanol dehydration. Because propene is formed during the reaction of 2-propanol with H2S04, the interaction of propene with H2S04 was briefly investigated. Propene is neither absorbed nor adsorbed significantly by pure H2S04 at 100 K. A negligible d e 41 signal is observed in the thermal desorption spectra of a H2S04 film which was exposed to a very large amount of propene (2.4 x Pa-s, =70 langmuirs); the desorption yield was far less than a monolayer. Attempts to adsorb HC1 on a pure, 50 ML H2S04 film at 100 K were unsuccessful. Even for HC1 exposures as high as 2x P a s , no evolution of HC1 or C12 was observed during subsequent temperature-programmed desorption. Auger analysis of the metal substrate after exposure of an H2S04 film to HCl followed by evaporation of the film showed that no HC1 had adsorbed to the metal substrate. We conclude that HC1 does not adsorb on the sulfuric acid azeotrope at 100 K. Discussion
1. General Significance. The central conclusion of this work is that sulfuric acid, despite its exceedingly low vapor pressure at room temperature, can be introduced into an ultrahigh vacuum environment by way of in situ synthesis from so3 and H20. Film thickness can be controlled through the initial H20/ S03/H20 layer. The H2S04 water content can be varied over a reasonably broad range via the annealing temperature. Much thicker sulfuric acid films have been generated in a similar fashion previously, but at higher temperatures and pressures, never under UHV.20 Although the motivation for this work was to gain insight into the gas-HzSO4 interface, the same methodology could be applied to the study of the metal-H~SO4 interface. This opens the possibility of applying UHV-based surface science techniques to the study of certain electrochemical and corrosion
2. Characterization of HzS04 under UHY. The formation of H2S04 from H20 and SO3 takes place via three steps: (i) the mixing and reaction of an H20/S03/H20 “sandwich” to form a solid H20 H2S04 solution (
