Langmuir 1991, 7, 1426-1431
1426
Acid-Base Properties of Model MgO Surfaces X. D. Pengt and M. A. Barteau' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received December 19, 1990. I n Final Form: February 14, 1991 The acid-base properties of three model MgO surfaces, a MgO thin film surface formed on the Mg(0001) surface by oxidation and sputtered and annealed MgO(100) single crystal planes, have been studied by adsorption experiments under ultrahigh vacuum conditions. The nature and coverage of the species derived from a variety of molecules, including formic and acetic acids, methanol, ethanol, acetylene, and water, on these surfaces have been examined by X-ray photoelectron spectroscopy at different temperatures. All of the above molecules underwent heterolytic dissociation on these MgO model surfaces. Ion scattering spectroscopy showed that the conjugate base anions are bound on surface Mg2+ sites. Three classes of molecules can be distinguished by their different surface coverages and their behavior in displacement/blocking experiments. Coverage measurements demonstrate that MgO thin film and sputtered MgO(100) surfaces possess higher populations of defect sites than the annealed MgO(100) surface and are thus better models for the surfaces exposed by MgO powders. Bransted acids has been detected by spectroscopic methods such as IR, UV-visible, NMR, and XPS. The One of the best-known chemical properties of magnedissociation on MgO of Bransted acids with strengths sium oxide is its strong basic character. Because of ranging from those of carboxylic acids and alcohols to alkextensive electron transfer from magnesium to oxygen enes, hydrogen, and ammonia has been shown to be similar upon MgO formation, the electron-rich oxygen anions on to acid-base reactions in s o l ~ t i o n . ~The * ~ propensity of MgO surfaces act as strongly basic, electron-donatingsites, Bransted acid molecules toward dissociation on MgO, as while the electron-deficient magnesium cations act as weak, determined by their ability to react with surface sites of electron-accepting sites. Numerous catalytic reactions different basicities (monitored with UV-visible, IR and over MgO, including isomerization of alkenes and unsatESR)3 and by titration/displacement experiments (monurated compounds containing heteroatoms, amination and itored with IR)? has been found to parallel the acidity of hydrogenation of conjugated dienes, and H-D exchange these reagents in aqueous solution, rather than in the gas between CH4 and Dz, have been shown to be basephase. A "solvation" effect provided by MgO surfaces, In general, the first step in these reactions stabilizing adsorbed conjugate base anions, has been involves the heterolytic dissociation of molecules through pr~posed.~~~ an acid-base interaction with the MgO surface, forming An intriguing aspect of the basic properties of MgO arises protons adsorbed on oxygen anions and conjugate base from the surface inhomogeneity of this material. Three anions adsorbed on magnesium cations. In other words, types of surface sites on MgO have been identified directly these base-catalyzed reactions on MgO, and other basic by photoluminescence6 and UV-visible3 spectroscopies. oxides such as CaO and BaO,' are characterized by anionic The different sites have been assigned to surface Mg-0 intermediates. pairs with different coordination numbers: 5-foldThe acid-base properties of MgO surfaces have been coordinated sites (on the extended MgO(100) plane), characterized by a variety of adsorption experiments.'~~'~ 4-fold-coordinated sites (on the edges between the (100) Formation of surface hydroxyl groups (Le., protons bound planes), and 3-fold-coordinated sites (on kinks and coron 02-sites) and adsorbed conjugate base anions from ners). Sites of lower coordination have been reported to exhibit stronger basicity, and are thus capable of reacting * To whom correspondence should be addressed. with weaker acids, as verified by UV-visible and photot Current address: Department of Chemistry, Northwestern luminescence experiments with MgO powder^^*^ and X-ray University, Evanston, IL 60208. photoelectron spectroscopy experiments with different (1)Hattori, H. In Adsorption and Catalysis on Oxide Surfaces;Che, MgO single crystal planes.6 M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985,p 319. (2)Baird, M. J.; Lunsford, J. H. J. Catal. 1972,26,440. The acid-base properties of magnesium oxide have been (3)Stone, F.S.;Garrone, E.; Zecchina, A. Mater. Chem. Phys. 1985, investigated in this laboratory with various surface spec13, 331. troscopies, including X-ray and ultraviolet photoelectron (4)Spitz, R. N.;Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A. W. J. Phys. Chem. 1986,90,4067. spectroscopies (XPS and UPS) and ion scattering spec(5) Coluccia, S.;Tench, A. J. In Proceedings of the 7th International troscopy (ISS). Two systems, MgO thin films formed on Congress on Catalysis, Tokyo,1980; Elsevier: Amsterdam, 1981;p 1160. the Mg(0001) single crystal plane by oxygen adsorption (6)Onishi, H.;Egawa, C.;Aruga, T.;Iwaaawa, Y. Surf. Sei. 1987,191, 479. and the MgO(100) surface with and without ion bom(7)Kagel, R. 0.;Greenler, R. G. J. Chem. Phya. 1968,49,1638. bardment, have been used as model MgO surfaces. (8)Kondo, J.;Sakata,Y.; Maruya, K.4; Tamaru,K.;Onishi, T. Appl. Comparison of the reactivity of different MgO surfaces Surf. Scr. 1987,28,475. (9)Malinowski, S.; Szczepanska, S.; Bielanski, A.; Sloczynski, J. J. can provide information on the site requirements of Catal. 1966,4, 324. different reactions and permit the interpretation of the (10)Malinowski, S.;Szczepanska, S.; Sloczynski,J. J. Catal. 1967,7, reactivity of polycrystalline materials in terms of the 67. (11)Martinez, R.;Bartaau, M. A. Langmuir 1985,1,684. surface structures exposed. The conductive nature of the (12) Liang, S.H. C.; Gay, I. D. Langmuir 1985,1, 593. thin film samples simplified the use of techniques such as (13)Liang, S.H. C.; Gay, I. D. J. Catal. 1986,101,293. UPS and ISS, which can be disrupted by electrostatic (14)Tench, A.J.; Giles, D.; Kibblewhite, J. F. J. Trans.Faraday SOC. 1971,67,854. charging effects, to characterize the properties of the solid
Introduction
~~
0743-7463/91/2407-1426$02.50/0 0 1991 American Chemical Society
Acid-Base Properties of Model MgO Surfaces
surface and adsorbates. The preparation and characterization of MgO thin film surfaces have been described elsewhere.15J6 In brief, the thin oxide films are continuous, consisting of magnesium oxide with one-to-one stoichiometry, with thicknesses of around 20 A. It has been shown that the thin surface cannot be represented either by the extended MgO(100) plane or by randomly or regularly azimuthally oriented Mg0(100)-like patches.'6 Probe reagents utilized in this study included formic and acetic acids, methanol, ethanol, acetylene, and water. Characterization by XPS and UPS of the intermediates derived from these molecules on the thin film surface has been reported previously.15J7J8 The current report presents the results of adsorption experiments on the MgO(100) surface, as well as further results from the thin film surface including the measurement of surface coverages,the effect of preadsorbed water on the adsorption of other molecules, and the identification of adsorption sites.
Experimental Sections All experiments were conducted in a VG Scientific ESCALAB UHV system described previou~ly.~~J7 The magnesium anode, operated at 600 W, was chosen from the twin-anode X-ray source for XPS experiments in order to avoid interference from Auger transitions in the C(1s) region. The chamber pressure was maintained in the vicinity of 1 X 10-lO Torr by a diffusion pump and a titanium sublimination pump. This low background pressure permitted performance of adsorption experiments without being subject to the influenceof backgroundwater, which, as shown below, could alter the adsorption patterns of some molecules. In general, we have found it to be easier to create and to maintain clean surfaces of reactive metals, such as magnesium, in this apparatus than in the ion-pumped systems used previously."Jg The MgO thin films used in this study were formed on the Mg(0001) plane. The magnesium single crystal (99.999%,Metallschmelz Gesellschaft) was oriented to expose the (0001) face, cut into a 10 X 8 X 1.5 mm disk, polished with diamond paste t o a mirrorlike finish, and attached to the specimen manipulator with a tantalum wire. The tantalum wire was connected to a liquid nitrogen reservoir to provide cooling; heating was accomplished by radiation from a tungsten filament located behind the sample. The clean Mg(0001) surface was prepared by argon ion bombardment and annealing at ca. 500 K. The MgO thin film was formed on the clean Mg(0001) surface by oxidation with dioxygen at about 300 K. The detailed procedures for thin film preparation have been given elsewhere, along with characterization information.llJ6 The MgO(100)sample (Atomergic)was cut into 8 X 8 X 1.5 mm disk and mounted in the same fashion as the metal sample; in this case, both heating and cooling were provided via the tantalum wire. Temperature was measured in both cases with a chromel-alumel thermocouple. The thermocouple was wedged between the crystal and the tantalum clip in the case of the metal sample or attached to the back face of the sample with high temperature cement (AremcoUltra-Temp 516) in the case of the MgO single crystal. The clean and well-ordered MgO(100) surface, which exhibited a square LEED pattern, was prepared by argon ion bombardment followed by annealing at up to 900K for several hours. We have found no evidence in this or in previous studies18sms21 for contamination of the surface by migration of components from the cement during cleaning or annealing procedures. A typical adsorption experiment was initiated by introduction of a gaseous reagent onto the sample surface at either low temperature or room temperature. Gas samples were introduced (15) Peng, X. D.; Barteau, M. A. Surf. Sci. 1990,233,283. (16) Peng, X. D.; Barteau, M. A. Appl. Surf. Sci. 1990,44, 87. (17) Peng, X. D.; Barteau, M. A. Surf. Sci. 1989, 224, 327. (18) Peng, X. D.; Barteau, M. A. Langmuir 1989,5, 1051. (19) Peng, X. D.; Edwards, D. S.; Barteau, M. A. Surf. Sci. 1988,195, 103. (20) Kim, K. S.; Barteau, M. A. Surf. Sci. 1989, 223, 13. (21) Kim, K. S.; Barteau, M. A. J. Catal. 1990, 125, 353.
Langmuir, Vol. 7,No. 7, 1991 1427 into the ultrahigh vacuum (UHV) chamber through two dosing lines from a gas manifold. The chamber pressure usually rose to the 10" Torr range during adsorption. The properties of the adsorbed layer as a function of temperature were determined by monitoring the surface with spectroscopic techniques while the sample was at the adsorption temperature or at elevated temperatures. XPS was the primary technique used in this study. The binding energies from the MgO thin film sample were referenced to the Mg(2p) peak from the bulk metal phase at 49.5 eV, while those from the MgO(100)surface were aligned with the Mg(2p) peak from magnesium oxide at 50.8 eV. As previously demonstrated," this procedure produced good agreement of the Mg2+(2p),O(ls), and C(1s) levels for experiments carried out on the two samples.
Results and Discussion The adsorption of several Bransted acids, including HCOOH, CHBCOOH,CH30H, C~HSOH, C2H2, and HzO, on MgO thin film surfaces has been reported p r e v i ~ u s l y . ~ ~ J ~ XPS and UPS spectra demonstrate that these molecules undergo heterolytic dissociationon MgO thin film surfaces, forming the corresponding conjugate base anions, i.e., carboxylates, alkoxides, acetylides, and hydroxides, respectively. We report here other results from the MgO thin film surface, including identification of the adsorption sites for conjugate base anions, effects of preadsorbed HzO on the adsorption of these acids, and coverage measurements of different adsorbates on the surface. As noted previously, the assumption of heterolytic dissociation of Bransted acids on MgO surfaces and the assignment of the corresponding adsorption sites have been based on the observation of the conjugate base anions by UV and IR and of the proton adsorbed on lattice oxygen by IR spectro~copy.~ The following experiments with ion scattering spectroscopy (ISS) provide direct evidence of the binding of conjugate base anions to surface Mg2+sites. ISS is a technique sensitive only to the topmost layer of solid surface.22 A surface component can be identified by its characteristic peak on the scale of the ratio of the energies of the ions scattered from the surface (E2)to their initial energy (El). The peak intensity is proportional to the concentration of the surface component. Figure 1 displays ISS spectra from the clean MgO thin film surface and thin film surfaces dosed with HzO, DCOOD, and CD3COOD. These spectra are all on the same intensity scale. The clean surface exhibited an oxygen peak at E2/E1 = 0.44 and a magnesium peak at E2/E1 = 0.56. It can be seen that the magnesium peak was attenuated considerably upon adsorption of HzO, DCOOD, or CD&OOD, while much smaller changes in the absolute intensity in the vicinity of the oxygen peak occurred. This observation was found to hold for different sample positions (rotations about the axis of the sample manipulator; the manipulator axis is normal to the axis of the energy analyzer9, as shown in Figure 2. The attenuation can be ascribed to the blockage of Mg2+sites by the conjugate base anions, Le., OH-, HCOO-, and CH&OO-. The magnesium signal was attenuated more than the oxygen signal due to the larger sizes of the conjugate base anions than of the protons. The greater attenuation of the magnesium signal by carboxylic acids than by water may reflect the larger size of the carboxylates and/ or their higher coverage. Coverages of surface species derived from dissociative adsorption of HCOOH, CHaCOOH, CH30H, CZHSOH, CzHz, and H20 were determined by XPS as summarized in Table I. Adsorption was carried out at 150 K to saturation levels for all molecules. The coverage was measured after the sample was heated to 300 K following (22) Taglauer, E.; Heiland, W. Appl. Phys. 1976,9,261.
1428 Langmuir, Vol. 7, No. 7,1991
Peng and Battsau C(18) XPS
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adsorption; only dissociatively adsorbed species remained on the surface at this temperature, i.e., carboxylates from carboxylic acids, alkoxides from alcohols, acetylides from acetylene, and hydroxyls from water. The areas of the corresponding C(ls) peaks were used to estimate coverages. For ethoxides, acetates, and acetylides the raw C(1s) peak
areas were divided by 2 in order to base the coverage on the number of adsorbed intermediates, instead of the number of carbon atoms. The absolute scale of coverage was determined from the O(1s) spectrum of the HCOOHdosed surface. The details of this calculation are given in the Appendix. The coverage of surface hydroxyls on the water-dosed MgO thin film surface was calculated from the corresponding O(1s) spectrum (see Appendix). Three groups of Bronsted acids can be distinguished by the coverages of their corresponding dissociated species as listed in Table I. The first group, including formic and acetic acids, has the highest coverage, about one monolayer. Methanol, ethanol, and water make up the second group with a coverage around one-half monolayer. Acetylene is characteristic of a third group with the smallest coverage, about one-sixth monolayer. The coverage ratio of formic acid to methanol in this study is consistent with that reported on the MgO(ll1) surfacee6 Adsorption of formic acid, acetic acid, methanol, acetylene, and water was also performed on the (100) surface of a bulk MgO single crystal. Two different surface conditons were used: the annealedsurface (620-900 K, several hours) and the sputtered surface (Ar+,5 X lW7Torr, 2-5 keV, 10-20 min). The annealed surface exhibited a sharp LEED pattern with square symmetry. TPD and XPS results for the carboxylic acids and water have been reported previ0usly.~6J3 In brief, formic and acetic acids adsorbed dissociatively on both annealed and sputtered surfaces at 170 K as well as at room temperature, forming surface formate and acetate species. Both carboxylates underwentdehydration at 520 K.23 Water also dissociated on Mg0(100), forming surface hydroxyl groups. Its adsorption, however, was favored by low temperature and surface defect structures resulting from ion bombardment.15 Adsorption of CHsOH was conducted at both low temperature and room temperature and on both annealed and sputtered MgO(100) surfaces. Figure 3 depicts the C(ls) spectra of the MgO(100) surface dosed with CHsOH at 180 K and then heated to several intermittent temperatures. The surface exhibited a single C(1s) peak at 287.6 eV from 180to 300 K, indicative of the formation of surface (23) Peng, X.D.;Barteau, M.A. Catal. Lett. 1990, 7,396.
Langmuir, Vol. 7, No. 7, 1991 1429
Acid-Base Properties of Model MgO Surfaces
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Figure 4. Adsorption of CHsOH on the (a) annealed and (b) sputtered MgO(100) surfaces at 300 K, (c).surfacein (b) heated to 400 K; (d) surface in (c) heated to 500 K.
Figure 5. C(1s) spectra of the CzHz-dosed sputtered MgO(100) surface at (a) 200, (b) 300, (c) 400,and (d) 500 K.
meth0~ides.l~ This peak disappeared when the sample was heated to 350 K (Figure 3). The results shown in Figure 3were obtained from a MgO(100) surface annealed at 620 K for 1h prior to the adsorption experiment. The sputtered MgO(100) surfaces produced a similar temperature dependence; however, the initial coverage of surface methoxides on the sputtered surface was approximately 1.2 times that on the annealed surface. For methanol adsorption conducted at room temperature, the annealed and the sputtered MgO surfaces behaved differently. As shown in Figure 4,methanol adsorbed on the sputtered MgO(100) surface a t room temperature to form surface methoxides, but very little adsorption occurred on the annealed MgO(100) surface at room temperature. The peak on the sputtered surface disappeared as the sample was heated to between 400 and 500 K. Adsorption temperature and surface pretreatment (annealing or sputtering) also appeared to be crucial in acetylene adsorption on the MgO(100) surface. The annealed MgO(100)surface did not exhibit activity toward CzHz adsorption even at 170 K, the lowest temperature achievable in this study. On the sputtered MgO(100) surface, C2H2 adsorption occurred only if the sample was cooled to low temperature, e.g., 200 K (Figure 5). The CzHa-dosed surface exhibited a broad C(1s) peak at 285.2 eV, corresponding to surface a~ety1ides.l~ Note that the acetylide exhibited unusual stability a t MgO, remaining detectable on the surface up to 500 K (the highest temperature used in this experiment), despite ita low surface coverage and high demand on adsorption conditions. The coverages of dissociatively adsorbed species derived from formic acid, acetic acid, methanol, acetylene, and water on MgO(100) surfaces a t room temperature are shown in Table I. Two seta of resulta for each MgO(100) surface correspond to adsorption conducted at 170 and 300 K, respectively. Similar to the thin film case, coverages of carbon-containing species were estimated from the corresponding C(ls) peaks, and the absolute scale was determined from the O(1s) spectra of HCOOH-dosed surfaces (see Appendix). Coverages of hydroxyl species from H2O dissociation on MgO(100) were determined previously.15 Three classes of Brransted acid molecules can be
discerned from the results in Table I: (1) carboxylic acids; (2) alcohols and water; (3)acetylene. These distinctions are apparent in the results for adsorption conducted at low temperature. On all three surfaces, carboxylic acids exhibited the largest coverage, acetylene the smallest, and alcohols and water fell in between, with the coverage of water somewhat larger than that of the alcohols. The classification can be further illustrated by the results for adsorption experiments conducted a t room temperature on sputtered and annealed MgO(100) surfaces. While carboxylic acids adsorbed on both surfaces at 300 K, water and methanol adsorbed on the sputtered MgO(100) surface but not on the annealed MgO(100) surface. Little adsorption of acetylene occurred on any MgO surface at room temperature. The ranking of the saturation coverages of these three classes of molecules parallels their acidities in aqueous solution, indicating that molecules of greater acid strength dissociate on MgO more easily. The relative acidities of these reactant classes are further illustrated by titration/displacement experiments. These were conducted on the oxidized Mg(0001) surface and are analogousto those previously reported for MgO powder^.^ Figure 6 demonstrates the displacement of water by formic acid on the MgO thin film surface. The clean MgO thin film surface was dosed with water at room temperature to saturation (ca. 100 langmuirs). The water-dosed surface exhibited a characteristic O(1s) peak at 533.1 eV (Figure 6b), indicative of the formation of surface hydroxyl groups.15 This surface was then exposed to formic acid at room temperature. Upon exposure,the original O(1s)peak was replaced by a larger O(ls) peak at 533.4 eV (Figure 64. The new peak can be assigned to surface formate species derived from formic acid, since it agrees well in both the peak position and peak area with the O(1s) spectrum from the clean MgO surface dosed with formic acid at room temperature shown in Figure 6d. The formation of surface formates was also evidenced by the appearance of the formate C(ls) peak a t 290.2 eV upon adsorption.'" This experiment demonstrates that surface hydroxyl groups can be displaced completely by formic acid on the MgO thin film surface. Similar experiments were conducted for methanol and acetylene. The resulta are summarized as follows.
BINDING ENERGY (eV)
Peng and Barteau
1430 Langmuir, Vol. 7, No. 7, 1991 ql.) xP8
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Figure 6. Displacement of surface hydroxyls by formic acid on the MgO thin film surface: (a) clean MgO thin film surface; (b) surface in (a) dosed with HzO at 300 K for 100 langmuirs, saturated; (c) surface in (b) dosed with HCOOH at 300 K, saturated; (d) clean MgO thin film surface dosed with HCOOH at 300 K, saturated.
(1)It was observed that methanol could adsorb on clean MgO thin film surfaces at room temperature." However, preexposure of the clean MgO thin film surface to water at room temperature (100 langmuirs, saturated) blocked the adsorption of methanol completely, as evidenced by the absence of any detectable C(1s) signals. (2) The adsorption of C2H2 was very sensitive to water contamination. It has been reported previously that C2H2 could adsorb on MgO film surfaces at low temperature, forming surface acetylides." Such adsorption, however, was not observed if the background water partial pressure was high, for example, due to the preceding water adsorption experiments. In this case, the surface sites responsible for acetylene adsorption might have been occupied by water, preventing adsorption of C2H2. The present results provide further insights into the relative strengths of adsorbed Bransted acids and the site requirements for their dissociation on MgO. As noted above, the qualitative acidity scale obtained on thin film and single crystal MgO samples is in agreement with that in previous powder studies.3*4 Previous authors have attempted to connect the acidity of probe molecules with the nature of the surface sites required to dissociate them. Stone and co-~orkers,312~ for example, concluded that &fold coordinated Mg-0 sites pairs possess sufficient basicity to dissociate acids of greater or equal strength to water and alcohols. Acetylene was proposed to require 4-fold coordinated Mg-0 sites, while very weak acids such as ammonia or ethylene require 3-fold coordinated site pairs. A previous reportll from this laboratory based on qualitative adsorption behavior of Brolnsted acids on MgO thin film samples provided less dramatic distinctions between these molecules. As in the present work, carboxylic acids, alcohols,water, and acetylene were all found to dissociate below 200 K; olefins did not. Subsequent single crystal studies have provided greater discrimination (24) Henrich, V. E.Rep. h o g . Phys. 1986,48,1481.
within this first group than suggested by Stone. For example, Onishi et determined the coverages of formic acid, methanol, water, and methyl formate on the MgO(100) and -(111)surfaces from the areas of the corresponding XPS C(1s) and O(1s) peaks. While there was no difference observed in the coverage of formic acid on the two surfaces, the coverages of methanol, water, and methyl formate decreased to ca. one-third of their values on the (111)face in going to the (100) face. The higher coverages on the (111)surface were attributed to the dissociative adsorption of these molecules on the edges of the facets, i.e., on Mg-O pairs of 4-fold coordination or less. Theoretically, the perfect MgO(100) plane consists of fivecoordinated Mg-0 pairs only. However, the (111)plane of MgO is thermodynamically unstable, and the reconstruction of this plane results in surface irregularities. In fact, the MgO(ll1) surface was found by LEED to be faceted, with multicentered valley-sites consisting of the (loo), (OlO), and (001) planes! i.e., coordinatively unsaturated sites. The observed small coverages of probe molecules on the MgO(100) surface were explained by residual irregularities on the surface. The present results on MgO single crystal and thin film samples parallel those of Onishi. The adsorption of carboxylic acids is quite insensitive to the characteristics of the surface, and the 5-fold-coordinated Mg-0 pairs present on the sputtered and annealed MgO(100) surfaces, the thin oxide film surface, and the (100) and (111)surfaces in the Onishi et al. work appear to be sufficient to dissociate these strong acids. However, these two studies of model surfaces reveal a distinction of the behavior of alcohols (and their homologue, water) from that of carboxylic acids. Both we and Onishi et al. find less uptake of alcohols and water on more highly ordered surfaces, which presumably expose a lower fraction of site pairs with less than 5-fold coordination. Thus, it may be concluded that adsorption of these weaker acids is enhanced by more highly coordinatively unsaturated (more basic) surface sites. The saturation coverages (ca. 0.5 monolayer) obtained for RO and HO species at low temperature even on the annealed MgO(100) surface clearly exceed the likely population of sites with coordination numbers less than 5. This observation, as well as the negative influence of the adsorption temperature on alcohol and water dissociation, suggests that high coordination sites may become populated with these species by activation of the corresponding acids a t site of lower coordination number, followed by migration of the conjugate bases. "Spillover" phenomena of this sort would invalidate attempts to find specific titrants for different site types on the surface. Of the molecules examined in this study, acetylene appears to belong in a class by itself. Unlike the alcohols and water, for which low temperature or greater surface disorder were sufficient to produce high coverages, acetylene required both low temperature and a roughened surface in order to adsorb to measurable levels. This suggests either that acetylene requires sites of lower coordination number than the alcohols or that, if the same sites are utilized for dissociation of both, surface acetylides are much less mobile than the alkoxides. The convolution of these two phenomena suggests that quantitative surface site populations determined by adsorption on polycrystalline oxide materials may be of limited reliability. In summary, the three model MgO surfaces in the present work behaved differently with respect to the dissociation of Bronsted acid molecules. MgO thin film and sputtered MgO(100) surfaces exhibited stronger interactions with these molecules than the annealed MgO(100)surface. The
Acid-Base Roperties of Model MgO Surfaces coverages on the first two surfaces were consistently larger than those on the annealed MgO(100) surface under the same conditions. The results that distinguish theannealed MgO(100) from other two surfaces most are the absence of adsorption of water and methanol a t room temperature and of acetylene even at low temperature. The lower reactivity of the annealed MgO(100) surface may result from its higher population of 5-fold-coordinated Mg-0 pairs. This is supported in a qualitative fashion by the fact that adsorption could be enhanced by ion bombardment of the MgO(100) surface. Defect sites, most likely, low-coordinated MgO pairs, can be generated by ion b~mbardment.?~ The adsorption study by Onishi and coworkerssusingMgO(lOO)andMgO(lll),ahighlydefective surface, also showed that the dissociation of Brransted acid molecules on MgO is favored by defect structures. Similar results from MgO thin film and sputtered MgO(100) surfaces suggest that the thin film surface was also richer in defect sites than the annealed MgO(100)surface. Since a high population of defects on MgO powders is expected, MgO thin film and sputtered MgO(100) surfaces would appear to be better models of polycrystalline MgO powders than is the perfect MgO(100) plane.
Conclusions The adsorption behavior of Brransted acids on MgO thin film and MgO(100) surfaces is similar to that on MgO powders reported in the literature. All molecules studied, including formic and acetic acids, methanol, ethanol, acetylene, and water, underwent dissociation on these model MgO surfaces through an acid-base mechanism. The adsorption of the resultant conjugate base anions on surface Mg2+sites was demonstrated by ISS experiments. The acid-base character of the interaction of Brransted acid molecules with MgO was further illustrated by the titration/displacement behavior of molecules with different acid strengths, and by their different surface coverages. Quantitative coverage measurements also demonstrate that MgO thin film and sputtered MgO(100) surfaces exhibit greater capacity for dissociation of weak acids than does the annealed MgO(100) surface, owing to their higher population of low-coordinated sites, and are thus better models of MgO powders. Acknowledgment. We gratefully acknowledge the support of the National ScienceFoundation (Grants CBT 84-51055 and CBT 87-14416) for this research. The assistance of Robert Andrews in determining adsorbate coverages is also recognized. Appendix Determinationof the AbsoluteCoveragesof Formic Acid and Water on Model MgO Surfaces at Room Temperature. The absolute coverage of surface formate species can be estimated by a method similar to that (26) Gamone, E.;Stone, F. S. Roceedings-ZnterMtion.al Congress on Catalysis, 8th; Ele.evier: Amsterdam, 1985; Vol. 3, p 441.
Langmuir, Vol. 7, No.7, 1991 1431 reported previously.16 This method employs the ratio of the area of the O(1s) emission from the surface formate species to that from surface MgO layers. The formates on MgO thin film surfaces have a characteristic O(1s) peak at 533.4 eV, 2.4 eV above that from the MgO thin film. The ratio of the formate peak to the MgO peak at room temperature was found, averaged over three spectra, to be 0.49 f 0.02 at a photoelectron emergence angle of 65" relative to the surface plane. It was observed that the attenuation of the O(1s) peak from the MgO layers at 531.0 eV upon formic acid adsorption was negligible. By neglecting attenuation effects and assuming that formate ions were present only on the topmost layer, one can at estimate the O(1s) signal intensity of the formates, Ifm, a given oxygen concentration, C1,and the O(1s) intensity of the MgO layers, lox, respectively, by CJI and CNJ,, (n = 1to m),where n represents the nth layer of the film, m stands for the total number of layers in the thin film, and N,, is the concentration of oxygen in layer n and is a constant for a lattice with a well-ordered layer structure. The term In,the signal intensity from the nth layer with unit oxygen concentration, is defined by exp[-(n - l)a/(X sin e)], where X is the mean free path of the O(1s) photoelectron, a is the distance between two adjacent crystal planes, the 8 is the emergence angle of the electrons. This approach assumes a layered structure of the solid and requires knowledge of the total layer number, m,both of which remain uncertain here. However, one can consider two extreme cases, Le., assume that the thin film as either a nonpolar structure (consists of the MgO(100) planes) or a polar structure (consists of the MgO(ll1) planes). For a film 20 A thick, the total layer number is 9-10 for the nonpolar structure and 6-7 for the polar structure. If one assumes that there exists one monolayer coverage of formate species, i.e., one formate ion for each surface Mg2+ ion, then C1= 2 (there are two oxygen atoms in each formate) andN,, = 1. Substituting these values into the above formulas, the ratios of the O(1s) intensity from formate ions to that from MgO layers can be estimated to be 0.44 for the nonpolar structure and 0.61 for the polar structure. The experimental result, 0.49, falls within the range, indicating that a monolayer of formate species on the topmost layer of the formic acid dosed MgO thin film surface a t room temperature is a reasonable assumption. The absolute coverage scale based on C(1s) peak area can then be established from the area of the formate C(ls) peak. A similar method can be used to estimate the coverage on the MgO(100) surface. In this case, m,the total number of MgO layers, is essentially infinity. For the absolute coverage of surface hydroxyl groups derived from water adsorption, aslight change in the above method needs to be made. The change is based on the observation that the intensity of the O(1s) peak from MgO layers was attenuated upon water adsorption,16as opposed to the negligible attenuation upon formic acid adsorption. Therefore, IoH/I, in this case is estimated by CII~/ENJ,,, where n ranges from 2 to m. The value of m is infinity for the MgO(100) surface and 8 for the thin film surface.
