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Photoemission of Adsorbed Xenon, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption Studies of H2O on FeS2(100) J. M. Guevremont,† D. R. Strongin,*,† and M. A. A. Schoonen‡ Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 and Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11974-2100 Received July 3, 1997. In Final Form: October 7, 1997 The reaction of H2O with the (100) crystallographic plane of pyrite, FeS2, has been investigated in the vacuum environment with photoemission of adsorbed xenon (PAX), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). TPD data indicate that H2O desorbs from FeS2(100) in a broad range of temperatures (150-300 K). XPS data suggests that the vast majority of H2O that initially adsorbs on pyrite at 79 K desorbs from pyrite during thermal annealing to 300 K. PAX, a technique that is sensitive to the short range order of a surface, has been used to elucidate the types of sites that are available on FeS2(100) for the binding of adsorbate. Within the resolution of our PAX data, the surface of FeS2(100) consists of at least two types of sites. It is proposed that these two types of sites are associated with the stoichiometric surface and defect (i.e., sulfur-deficient or anion vacancy) sites. PAX further suggests that at low adsorbate coverage, H2O predominately resides on defect sites. As the coverage of H2O is increased, defect sites become saturated and additional adsorption occurs on the less reactive stoichiometric surface.
Introduction The mineral pyrite, FeS2, plays a role in processes ranging from the cycling of trace elements in marine sediment through sorption1 to the production of acidic contamination related to mining activities and the combustion of coal. An understanding of the surface reactivity of pyrite has relevance to all these processes. New methods, for example, are being explored to limit the rate of oxidation by sorbing “inactive” compounds, such as organic acids and phosphate, onto the pyrite surface.2,3 Concerning coal combustion, recent research has been focused on using surfactants that bind to pyrite to facilitate its removal from coal.2,4,5 Aside from these circumstances, which are concerned with the detrimental processes resulting from pyrite decomposition in certain environments, the surface of pyrite has been shown interest as a semiconducting material6,7 as well as a possible catalytic surface in the environment.8 The objective of research presented in this contribution is to investigate the chemisorption and reaction of H2O on * To whom correspondence should be addressed; Tel: (215) 204-7119, Fax: (215) 204-1532, E-mail: dstrongi@ nimbus.ocis.temple.edu. † Temple University. ‡ State University of New York. (1) Brown, J. R.; Bancroft, G. M.; Fyfe, W. S.; McLean, R. A. N. Environ. Sci. Technol. 1979, 13, 1142. (2) Lalvani, S. B.; DeNeve, B. A.; Weston, A. Corros. Sci. 1991, 47, 55. (3) Huang, X.; Evangelou, V. P. In Environmental Geochemistry of Sulfide Oxidation; Alpers, C. N., Blowes, D. W., Eds.; ACS Symp. Ser. 550: Washington, 1994; Chapter 34, p 562. (4) Ogunsola, O. M.; Osseo-Assare, K. Fuel 1987, 66, 467. (5) Olson, T. J.; Aplan, F. F. In Processing and Utilization of High Sulfur Coal; Chug, Y. P., Cauldle, R. D., Eds.; Elsevier: New York, 1987 p 71. (6) Ennaoui, A.; Fiechter, S.; Jaegermann, W.; Tributsch, H. J. Electrochem. Soc. 1986, 133, 97. (7) Chen, G.; Zen, J.-M.; Fan, F.-R.; Bard, A. J. J. Phys. Chem. 1991, 95, 3682. (8) Xu, Y.; Schoonen, M. A. A. Geochim. Cosmochim. Acta 1995, 59, 4605.
a natural sample of pyrite. Special emphasis is placed on understanding the role of short range order of the pyrite on this chemistry. Scanning tunneling microscopy (STM) studies have started to shed light on the microscopic structural properties of natural pyrite. While the bulk structure of pyrite can be looked at as a superposition of disulfur (S22- groups) and metal ions face-centered cubic sublattices, the surface structure is more variable. STM studies9-11 have shown that the surface of FeS2(100) consists of regions that are similar to what might be expected by an ideal truncation of the bulk and other regions having a short range order characterized by imperfections such as steps.9 While STM has characterized the geometric structure of pyrite, photoelectron spectroscopy studies have addressed the stoichiometry of pyrite, and these studies have suggested that monosulfide and polysulfide imperfections exist on FeS2.12-14 Research presented in this contribution builds upon this earlier research and starts to address the implications that these surface imperfections have on the binding of adsorbate. H2O has been used as the probe adsorbate, and photoemission of adsorbed Xe (PAX) has been used to characterize the binding sites of pyrite in addition to the role that these sites play in adsorbing H2O. X-ray photoelectron spectroscopy (XPS) and temperatureprogrammed desorption (TPD) have been used to characterize the atomic composition and desorption behavior of H2O. The basis of PAX is that the binding energy of the core or valence levels of Xe weakly bound on a surface, relative to the Fermi level of the substrate, depend on the local (9) Eggleston, C. M.; Hochella, M. F. Am. Mineral. 1992, 77, 221. (10) Siebert, D.; Stocker, W. Phys. Status Solidi A 1992, 134, K17. (11) Eggleston, C. M.; Ehrhardt, J.; Stumm, W. Am. Minerol. 1996, 81, 1036. (12) Bronold, M.; Tomm, Y.; Jaegermann, W. Surf. Sci. 1994, 314, L931. (13) Nesbitt, H. W.; Muir, I. J. Geochim. Cosmochim. Acta 1994, 58 (21), 4667. (14) Buckley, A. N.; Woods, R. Appl. Surf. Sci. 1987, 27, 437.
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work function of its adsorption site.15-17 It is experimentally observed in this study of H2O/FeS2(100) that within our instrumental resolution there are at least two general types of binding sites that adsorb Xe. These sites are thought to be associated with stoichiometric FeS2 and S-deficient Fe. These same sites play a role in the adsorption of H2O in the vacuum environment. Under the conditions used in this study where Xe cannot condense, it is experimentally observed that H2O bound on a particular site physically inhibits Xe adsorption. Hence, the strategy in this study is to adsorb H2O and to titrate the vacant sites with Xe. Results from this type of experiment suggest that H2O almost exclusively adsorbs on defect sites (surmised to be, at least in part, anion vacancy sites) at low coverage, and after these sites become saturated, H2O begins to populate the less reactive stoichiometric surface. XPS suggests that there is some dissociation of H2O on FeS2(100), but the majority desorbs from the surface by 300 K as evidenced by TPD. Experimental Methods An ultrahigh-vacuum (UHV) chamber evacuated by ion and turbomolecular pumps with a working base pressure of 2 × 10-10 Torr was used to carry out all the experiments described in this contribution. The apparatus was equipped with a cylindrical mirror analyzer and an X-ray source for PAX and XPS, a mass spectrometer to carry out TPD experiments, and low-energy electron diffraction (LEED) optics. The analyzer was equipped with a coaxially mounted electron gun that was used for Auger electron spectroscopy (AES). An ion gun was also present for sample cleaning. Pyrite used in this research was obtained from Logrono, Spain. The area of the square sample used in this study was approximately 1 cm on a side and 2 mm in width. It is emphasized that the sample used in this study was the “growth surface” and was not altered by chemical or mechanical treatment. Furthermore, while the structure of the surface was not characterized by microscopy, the sample exhibited macroscopic striations. Mounting of the sample was carried out by using ceramic cement to bind the back of the sample to a tantalum plate that was attached to a liquid nitrogen cryostat. With such a setup, the sample could be cooled to temperatures of 79 K. Furthermore, the sample could be resistively heated by passing current through tantalum wires spot-welded to the tantalum plate. An earlier publication details the method used in our laboratory to clean the growth surface of FeS2(100) sample for UHV studies.18 In short the surface is exposed to 200 eV He+ bombardment and anneal cycles. Annealing of the sample after bombardment was achieved by heating the sample to 550 K. After cleaning, a LEED pattern consistent with “ideally” truncated FeS2(100) was obtained. A diffuse background, however, suggests that there are regions of disorder existing with the ordered domains. TPD data presented in this contribution were obtained by adsorbing H2O on FeS2(100) at temperatures near 100 K and then heating the sample at a rate of 8 K/s. Species desorbing from the surface were monitored with a multiplexed quadrupole mass spectrometer. Water used in this study was triply distilled and was further purified before experiments by several freezepump-thaw cycles. This reactant was admitted into the apparatus through a UHV variable leak valve, and exposures [given in langmuirs (10-6 Torr‚s)] quoted in this paper are uncorrected for the sensitivity of the ionization gauge. Unmonochromatized Mg KR radiation (1253.6 eV) was used to acquire all the PAX and XPS data. An analyzer pass energy of 50 eV was used to acquire Xe 3d5/2 data for PAX, and a pass energy of 25 eV was used to obtain O 1s data. It is important to mention that all PAX data presented in this study were obtained while the sample was at a temperature of 79 K. At (15) Ku¨ppers, J.; Wandelt, K.; Ertl, G. Phys. Rev. Lett. 1979, 43, 928. (16) Wandelt, K.; Hulse, J.; Ku¨ppers, J. Surf. Sci. 1981, 104, 212. (17) Jablonski, A.; Wandelt, K. Surf. Interface Anal. 1991, 17, 611. (18) Chaturvedi, S.; Katz, R.; Guevremont, J.; Schoonen, M. A. A.; Strongin, D. R. Am. Mineral. 1996, 81, 261.
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Figure 1. TPD of H2RO/FeS2(100). such a temperature, Xe will not condense, so a background pressure of Xe needs to be maintained in the apparatus to establish a steady-state concentration of adsorbed Xe. This steady-state concentration is dependent on the Xe pressure, and this is discussed below.
Results TPD and XPS of H2O/FeS2(100). Figure 1 presents TPD data for H2O/FeS2(100). After an exposure of 0.1 langmuir, the peak temperature, Tp, for H2O desorption is at 185 K. A decrease in Tp is experimentally observed as the exposure is increased to 2 langmuirs, at which point Tp is 170 K. Exposure of FeS2(100) to 10 langmuirs of H2O results in a shift of Tp to a value of 150 K, typical of H2O desorption from a condensed layer. Each desorption trace has a broad tail in the temperature range 200-300 K that we will attribute, after presentation of PAX studies, to the desorption of H2O from defect sites on FeS2(100). No other desorption species, such as H2, were observed in any TPD experiment. Figure 2 exhibits XPS O 1s spectra of FeS2(100) that has been exposed to 10 langmuirs of H2O and heated to various temperatures. Various spectral changes occur in these XPS spectra as the temperature is increased. Heating the surface from 100 to 180 K results in a loss of a feature centered at -533.8 eV and the appearance of an O 1s peak at -532.8 eV. The -533.8 eV feature is assigned to water in a condensed layer on FeS2(100). This contention is supported by noting that heating to 180 K leads to a significant reduction in the O 1s photoelectron intensity, consistent with the desorption of a weakly bound water multilayer as evidenced by TPD data. The O 1s peak centered at -532.8 eV in the 180 K XPS spectrum is tentatively assigned to the O 1s binding energy of molecularly adsorbed H2O. Further heating to 200 K results in a reduction in the O 1s spectral intensity, although the binding energy of the peak maximum shows no noticeable shift. By 400 K any adsorbed H2O has desorbed from the pyrite surface, and the associated O 1s spectrum shows a relatively small feature peaked near -531.8 eV that we suspect is adsorbed OH or atomic oxygen. There also is spectral weight at -530.1 eV that is believed to be associated with atomic oxygen. With regard to this latter feature, note that the clean O 1s spectrum exhibits a smaller spectral feature near -530 eV that is believed to be strongly bound residual oxygen that cannot be removed by our cleaning procedure. On the basis of integrated peak areas from the 180 and 400 K data, we can state, however, that 5% of the initial H2O adsorbed at 180 K dissociates upon heating to 400 K.
PAX, XPS, and TPD Studies of H2O on FeS2(100)
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Figure 2. O 1s data for H2O/FeS2(100). Data are obtained by condensing a H2O multilayer on FeS2(100) and then heating momentarily to the indicated temperature. Figure 4. (a) PAX data obtained from FeS2(100) after exposure to various amounts of H2O. (b) Difference spectra obtained by individually subtracting the different exposure spectrum from the clean surface spectrum.
Figure 3. (a) PAX data obtained after FeS2(100) is exposed to 15 langmuirs of H2O and then heated in stepwise fashion to the indicated temperatures. (b) Difference spectra obtained by individually subtracting the different temperature spectra from the clean surface spectrum.
PAX of H2O/FeS2(100). Figure 3a exhibits PAX data obtained from clean FeS2(100), FeS2(100) exposed to 15 langmuirs of H2O at 80 K, and H2O/FeS2(100) heated to 150 and 200 K. Each spectrum is obtained by maintaining a Xe background pressure of 1 × 10-5 Torr. Data obtained at 80 K shows no noticeable spectral weight, and we propose here that the adsorbed H2O is blocking surface
sites that could otherwise provide binding sites for Xe. This statement is supported by the 200 K spectrum that shows significant Xe adsorption. TPD results show that the majority of H2O has desorbed from FeS2(100) by 200 K, and this loss of H2O then results in vacant surface sites for the adsorption of Xe. Also shown in the figure is a clean spectrum that is obtained by exposing atomically clean FeS2(100) to a background Xe pressure of 1 × 10-5 Torr. A similar spectrum is obtained by heating H2O/ FeS2(100) to 400 K (data not shown). Notice, however, that the Xe 3d5/2 peak maximum is at -669.7 eV in the 200 K spectrum and resides at -669.5 eV for the clean spectrum. Furthermore, the Xe 3d5/2 spectral intensity of the clean spectrum is greater than that of the 200 K spectrum, due to the presence of site blocking of Xe by H2O on FeS2(100) at 200 K. This latter point is emphasized by a difference spectrum presented in Figure 3b. Difference spectra shown in Figure 3b are obtained by individually subtracting the 80 and 200 K spectra from the spectrum associated with clean FeS2(100). We assert at this point that the spectral weight exhibited by the difference spectrum is associated with surface sites that bind H2O and are therefore unable to adsorb Xe. The 200 K spectrum shows a feature that is peaked at -669 eV, while the 80 K spectrum is peaked at -669.5 eV, the same as the clean spectrum. The 80 K difference spectrum, for example, is indicating that H2O is blocking all the surface sites that could otherwise bind Xe. It is proposed now that the -669 eV feature of the 200 K difference spectrum is associated with the adsorption of H2O in a distinct binding site on FeS2(100). Support for this proposition is obtained by examination of Figure 4a, which presents PAX data that are obtained after FeS2(100) is exposed to various doses of H2O at a temperature of 79 K. After each H2O exposure, a background Xe pressure of 1 × 10-5 Torr is introduced
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into the UHV chamber and PAX data are obtained. The PAX data show that the Xe 3d5/2 intensity decreases as the H2O exposure is increased, consistent with adsorbed H2O blocking surface sites for Xe adsorption. Difference spectra are presented in Figure 4b that are obtained by individually subtracting the 1 and 2 langmuirs spectral data from the clean surface data. Most revealing is the 1 langmuir spectrum that exhibits a feature with a binding energy maximum at -669 eV. This binding energy is similar to that which characterizes the 200 K difference spectrum of the preceding figure. This similarity suggests that the H2O that remains on FeS2(100) after a H2O multilayer is heated to 200 K resides on a binding site that is indistinguishable from the one that H2O resides on after atomically clean FeS2(100) is exposed to 1 langmuir of H2O. PAX data also suggest that there is another significant binding site, but one that binds H2O more weakly. This contention is partly inferred by further analysis of Figure 4a. Of note is the experimental observation that the 2 and 4 langmuir spectra are similar in that they both exhibit a similar binding energy of -670 eV, and as expected the intensity of the 4 langmuir spectrum is reduced due to the greater amount of adsorbed H2O. We now propose that the constancy of the binding energy of this Xe feature and its reduction with H2O exposure are due to the population of a second H2O binding site. In support of this assignment we return to Figure 3a and note that the 150 K spectrum also exhibits a Xe feature with a peak maximum at -670 eV. Heating to 150 K then partially results in the depopulation of sites on pyrite that weakly adsorb H2O, relative to those sites that adsorb Xe with a 3d5/2 binding energy of -669 eV. Our assignment of the -670 eV Xe 3d5/2 feature to a specific set of sites that weakly chemisorb H2O is inferred by additional PAX data. Figure 5a exhibits Xe 3d5/2 spectral data as a function of Xe background pressure at a sample temperature of 79 K. As expected the Xe spectral weight increases as the background pressure is increased from 1 × 10-7 to 3 × 10-5 Torr. It is emphasized that data for higher background pressures of Xe were not obtained, so we cannot comment on whether 3 × 10-5 Torr of Xe results in a saturation coverage of adsorbed Xe. More important is the experimental observation that there is a noticeable increase in high binding energy spectral weight as the Xe pressure increases from 1 × 10-7 to 3 × 10-5 Torr. Difference spectra presented in Figure 5b are obtained by individually subtracting spectra at the indicated pressures from the 3 × 10-5 spectrum. A revealing aspect of these data is that the 5 × 10-6 and 1 × 10-5 Torr difference spectra are similar in width and binding energy (-669.9 eV). We infer from these data that increasing the Xe pressure from 5 × 10-6 to 3 × 10-5 Torr results in the population of a specific binding site with Xe. This site characterized by Xe with a binding energy of -669.9 eV is proposed to be the same site that weakly adsorbs H2O and depopulates upon heating a H2O multilayer to 200 K. The slight difference in binding energy is presumably due to electrostatic differences between atomically clean and H2O-covered FeS2(100). The enhanced spectral width of the 1 × 10-7 and 1 × 10-6 Torr difference spectra relative to the 5 × 10-6 and 1 × 10-5 Torr difference spectra is due to the presence of a second binding site that populates and then saturates at a pressure between 1 × 10-6 and 5 × 10-6 Torr. This second site is believed to be the same site that binds H2O up to temperatures near 300 K. Within our experimental resolution, by using the peak parameters (3d5/2 binding
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Figure 5. (a) PAX for atomically clean FeS2(100) as a function of the Xe background pressure. These data were obtained at a sample temperature of 79 K. (b) Difference spectra obtained by individually subtracting spectra obtained at 1 × 10-5 Torr and below from the 3 × 10-5 Torr Xe/FeS2(100).
Figure 6. Example of the fitting procedure for Xe/FeS2(100) obtained with a background pressure of 5 × 10-6 Torr. Schematic of relevant energy levels for understanding the PAX spectrum. EA and EB refer to the -670 and -669 eV binding energy (relative to EF) features, respectively. It is postulated that the local work function of the site (ΦA) associated with the -670 eV feature is lower than the work function (ΦB) associated with the -669 eV feature. E, which is invariant, is the binding energy of the Xe 3d5/2 level relative to the vacuum level, EV.
energy and full width at half-maximum) of the -670 and -669 eV feature from the 2 langmuir spectrum of Figure 4a and the 200 K difference spectrum of Figure 3b, respectively, each spectrum of Figure 5a can be fitted with two gaussian peaks. A sample fit of the 5 × 10-6 Torr spectrum is shown in Figure 6. The spectral weight at high binding energy that is not well fit by the two gaussian peaks is presumably due to some oxygen contamination
PAX, XPS, and TPD Studies of H2O on FeS2(100)
Figure 7. (a) PAX of clean FeS2(100), after exposure to 25 langmuirs of H2S at 79 K, and after heating to 600 K. The inset to the figure shows TPD of 10 langmuirs of H2S/FeS2(100). (b) Difference spectrum obtained by subtracting the 600 K spectrum from the clean spectrum.
that accumulates at high Xe background pressures. Repeated exposure of FeS2(100) to H2O without cleaning also results in an enhancement of this feature. Figure 6 also shows schematically the relevant energy level considerations for the PAX experiment (details are given in the associated figure caption). It is proposed that the binding site associated with the low binding energy feature has a higher work function than the weaker binding site, and this point is briefly discussed later in terms of the possible chemical compositions of each site. Insight into the chemical nature of the binding sites associated with the Xe 3d5/2 photoelectron spectrum is obtained in the following experiment that is concerned with PAX of FeS2(100) that has been exposed to H2S at 80 K and then annealed to 600 K. As mentioned in the Introduction, prior photoemission research of FeS2(100) has suggested that the surface of this material contains monosulfide or sulfur-deficient sites. If Xe adsorbing on such a site is responsible for one of the features comprising the Xe 3d5/2 spectrum, it is postulated that addition of sulfur to the mineral may be expected to decrease the concentration of that site. Furthermore, it might be expected that this sulfur addition would lead to the creation of a new site that resembled stoichiometric FeS2. Figure 7a exhibits data for FeS2(100) that has been exposed to H2S and then heated to 600 K. The inset to Figure 7a shows TPD data for 10 langmuirs of H2S/FeS2(100). TPD shows that a significant amount of the H2S adsorbed at 150 K desorbs with a Tp of 200 K. There is perhaps some H2 evolution from FeS2(100) near 400 K upon heating the surface after H2S exposure, and this experimental observation suggests that some H2S dissociates on FeS2. (Recent results in our laboratory are more conclusive and show that H2S does dissociate on FeS2(100), but the resulting hydrogen does not desorb.19) The 25 langmuir H2S PAX spectrum exhibited in Figure 7a shows a decreased intensity relative to the clean spectrum, con-
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sistent with adsorbed H2S at 79 K blocking sites for Xe adsorption. PAX data obtained after heating H2S/ FeS2(100) to 600 K shows an increased intensity relative to the 25 langmuir H2S spectrum due to H2S desorption from FeS2(100). However, there is a significant difference between the 600 K and clean spectrum, and this is emphasized by the difference spectrum exhibited in Figure 7b. Inspection of the difference of the clean and 600 K spectrum suggests that after adsorbing H2S on pyrite and heating to 600 K the results are an increase of Xe spectral weight near -670 eV and a reduction in spectral weight near -669 eV. We propose that these changes in the Xe spectrum are due to the dissociation of H2S, which transforms sulfur-deficient sites on FeS2(100) to sites more similar to those expected to be on stoichiometric FeS2. A reaction such as FeS + H2S f FeS2 + H2 may be operative. The small amount of H2 desorbing during TPD also may be suggestive of a scenario where some H remains on the FeS2(100) surface. This possibility and further details of the interaction of H2S with FeS2(100) are presently being investigated in our laboratory. It also is mentioned that experiments in our laboratory have exposed FeS2(100) to atomic hydrogen and have subsequently removed S from the surface in the form of H2S. In this circumstance, where sulfur vacancies are increased, PAX shows an increase in the -669 eV feature and a decrease in the -670 eV feature.20 Defect sites removed by H2S decomposition can be recovered by 200 eV He+ bombardment. This result suggests to us that only a weakly bound sulfur product is formed after H2S decomposition and that this S-product is distinct from pyrite lattice sulfur. It is important to mention that He+ bombardment, beyond what is needed to remove the weakly bound S-product, leads to no further noticeable change of the surface structure, as judged by PAX. This experimental observation emphasizes that He+ bombardment is a largely nondestructive method of cleaning FeS2(100). Finally, we surmise that the initial He+ bombardment that is used to clean FeS2(100) after introduction into the experimental chamber is not the reason for the sulfur-deficient sites associated with the -669 eV Xe feature. Instead, it is believed that these sulfur-deficient sites are a property of the natural pyrite surface. Discussion Within the instrumental resolution used in this study, PAX data suggest that the surface of FeS2(100) can be largely thought of in terms of two types of binding sites. On the basis of the H2S experiments, we propose that one of these binding sites is sulfur-deficient. Furthermore, these sites bind H2O on FeS2(100) to a temperature near 300 K. It is emphasized that prior studies (see Introduction) have observed such features as steps with STM. It is unresolved in the present study whether such sites contribute to the PAX spectrum in the spectral region that we have assigned to, at least in part, sulfur-deficient sites. Analysis of PAX data of atomically clean FeS2(100) at a background Xe pressure of 3 × 10-5 Torr shows that the area of the feature associated with the sulfur-deficient site is a factor of 0.2 less than that of the -670 eV feature. We suspect, however, that this value is at best an upper limit, since a background pressure of 3 × 10-5 Torr may (19) J. Guevremont, D. R. Strongin, and M. A. A. Schoonen, submitted to Am. Mineral., in press. (20) Guevremont, J.; Strongin, D. R.; Schoonen, M. A. A. Surf. Sci. 1997, 391, 109.
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not lead to a saturation coverage of Xe. It is difficult to convert the area ratios into relative site concentrations, since prior research has shown that there need not be a 1:1 correspondence between Xe adsorption and site concentration.21,22 Every defect or sulfur-deficient site, for example, may bind more Xe than does the site responsible for the -670 eV Xe feature. On the basis of the relative areas of the Xe 3d5/2 features, however, we infer that this latter site is more abundant than the defect site. We postulate that the -670 eV feature in the PAX spectrum is due to Xe adsorbing on the stoichiometric FeS2 surface. It is emphasized that the atomic structure of FeS2(100) is largely undetermined and that this tentative assignment is based on the assumption that the stoichiometric surface might be expected to be the dominant surface phase. This assumption is consistent with our results that show that H2S decomposition on FeS2(100) leads to an increase in these types of sites and prior research using surface-sensitive photoemission that suggests that disulfide is the major species at the surface of FeS2(100).12 We also mention that PAX experiments have been carried out in our laboratory for FeS2(100) that has received no cleaning in UHV. A typical AES spectrum of such an “as-introduced” sample has been published elsewhere and shows that, without cleaning, C and O are the dominant impurities.23 Presented elsewhere,20 PAX data of such a sample exhibits the -670 eV Xe feature, but the -669 eV feature is absent. It was inferred in the study that this result suggested that binding sites associated with the -670 eV feature were relatively unreactive. Consistent with this contention is the result that this site starts to become depopulated after H2O/ FeS2(100) is heated to 150 K (Figure 3a). Furthermore, prior STM studies carried out at ambient conditions show atomic resolution of regions on FeS2(100) that appear to be stoichiometric surfaces.9,10 This STM result would also imply that the stoichiometric surface is rather unreactive, consistent with our observations. Further support for our assignment of the -670 and -669 eV features to stoichiometric and sulfur-deficient sites comes from prior research. Prior research by Bronold et al.24 has suggested that S in the outermost layer of FeS2(100) experiences an increased electrostatic potential relative to S in the inner layers. The cause of this phenomenon is proposed to be the loss of Fe coordination at the surface and subsequent charge flow to this Fe from the inner layers. It might be that further S loss at the defect may enhance this effect, consistent with the association of this site with the lower binding energy -669 eV feature in the PAX spectrum of FeS2(100). It also is mentioned that prior PAX research of TiO225 has shown that Xe atoms associated with oxygen-deficient sites exhibit a lower binding energy than do Xe atoms adsorbed on stoichiometric TiO2, analogous to our experimental observations for a metal sulfide. TPD and XPS results taken together suggest that the majority of H2O desorbs from FeS2(100) upon heating to 300 K. On the basis of the O 1s XPS data, it is estimated that 5% of the H2O monolayer at 180 K dissociates upon heating to 400 K. It is noted that at 180 K it is postulated (21) Malafsky, G. P.; Fu, S. S.; Hsu, D. S. J. Vac. Sci. Technol., A 1992, 10 (6), 3472. (22) Malafsky, G. P. Surf. Sci. 1994, 306, L539. (23) Chaturvedi, S.; Katz, R.; Guevremont, J.; Schoonen, M. A. A.; Strongin, D. R. Am. Minerol. 1995, 81, 261. (24) Bronold, M.; Bu¨ker, K.; Kubala, S.; Pettenkofer, C.; Tributsch, H. Phys. Status Solidi A 1993, 135, 231. (25) Dolle, P.; Markert, K.; Heichler, W.; Armstrong, N. R.; Wandelt, K.; Kim, K. S.; Flato, R. A. J. Vac. Sci. Technol., A 1986, 4 (3), 1465.
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that any condensed H2O has left the surface and the remaining H2O is directly bound to FeS2(100). We suspect that the fraction of H2O that dissociates does so on the sulfur-deficient sites, based on PAX data that suggests that these sites bind H2O more strongly than the stoichiometric surface. It is worth mentioning, however, that the broadness of the temperature range over which H2O desorbs (see TPD) suggests that the sulfur-deficient sites may be variable in structure. Hence, the feature that we associate with sulfur-deficient sites in the PAX spectrum may have contributions from Xe residing at more than one type of sulfur-deficient site. Assignment of the O 1s features after H2O dissociation to specific surface species is difficult and somewhat arbitrary. It is tempting to assign the O 1s feature at -531.8 to adsorbed OH, but there is some uncertainty in such an assignment. Assignment of the OH in this way is consistent with earlier XPS research of H2O/FeS2(100) [cleaved sample], where O 1s features between -531.3 and -532.0 are assigned to OH.13,26 Unfortunately, this O 1s region is also where O bound as sulfur oxides exists,14,27 and this possibility cannot be firmly ruled out. Assignment of the O 1s feature at -530.1 to atomic oxygen is less ambiguous than the OH assignment, since O adsorbed on Fe surfaces typically exhibits such an O 1s value,27 in contrast to O bound to S. More definitive assignments will have to wait until complimentary techniques such as vibrational spectroscopy are carried out on this system. Summary The interaction of H2O with atomically clean FeS2(100) has been investigated. PAX data suggests that the binding sites on clean FeS2(100) can be broadly classified as being associated with stoichiometric FeS2(100) and a sulfurdeficient surface. These latter sites bind H2O more strongly than the former. The sites associated with the stoichiometric surface begin to become depopulated upon heating a H2O multilayer to 150 K, while temperatures near 300 K are needed to desorb H2O from sulfur-deficient sites. XPS data suggests that the majority of H2O that adsorbs at 100 K on both sites desorbs molecularly upon heating. A small fraction, estimated to be 5% of a monolayer, decomposes upon the thermal treatment. XPS results suggest that both OH and atomic O result from the decomposition, but additional experiments are needed to confirm these tentative conclusions. It is postulated that H2O dissociation occurs on the more reactive sulfurdeficient sites rather than on the unreactive stoichiometric surface. Acknowledgment. D.R.S. and M.A.A.S. greatly appreciate support from the Department of Energy, Basic Energy Sciences from Grants DEFG0296ER14644 and DEFG029ER14633, respectively. D.R.S. also appreciates support from the National Science Foundation (DMR 9258544) that was used to initiate the mineral work. The authors also gratefully acknowledge Richard Katz for acquiring the XPS data. LA970722C (26) Knipe, S. W.; Mycroft, J. R.; Pratt, A. R.; Nesbitt, H. W.; Bancroft, G. M. Geochim. Cosmochim. Acta 1995, 59, 1079. (27) Furuyama, M.; Kishi, K.; Ikeda, S. J. Electron Spectros. Relat. Phenom. 1978, 13, 59.
