684
Langmuir 1985, 1, 684-691
Synthesis and Reactivity of Thin Oxide Films on the Mg(001) Surface Ramiro Martinezt and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received March 4, 1985 The adsorption and reaction of a series of Brernsted acids were examined on the oxidized Mg(001) surface. Continuous oxide films of one to two monolayers in thickness were formed by exposure of the clean Mg(001) surface to O2 at room temperature; these oxide layers were characterized by AES and LEED. Temperature-programmed desorption experiments indicated that Br~nstedacids interacted with these oxide films in three characteristic patterns. Acetic acid and acetylene were irreversibly adsorbed, and large amounts of surface carbon were observed after TPD experiments. Alcohols and water were reversibly dissociated into hydrogen and alkoxy species (hydroxylin the case of water). Upon heating, the dissociated components formed only the parent molecule via recombination above 300 K. Neither dehydration nor dehydrogenation products were observed following alcohol adsorption. Formaldehyde, propylene, and propyne were weakly adsorbed, desorbing from the surface below 260 K. The above results are in excellent agreement with previous studies of adsorption on bulk magnesium oxide powders and suggest that the chemical properties of the highest coordination ion pair sites for MgO may be reproduced on such oxide films. 1. Introduction Metal oxides are an essential component of the majority of heterogeneous catalysts, either as supports for highly dispersed metals or as the active component or its precursor in the mixed oxides utilized for oxidation and hydroprocessing. Even in a supporting role, the influence of the oxide on catalyst performance is often unclear, and an extensive literature has evolved in the area of metalsupport interaction^.'-^ In systems containing several oxide components of which one or more may participate directly in a surface reaction, both the identity of the active site and the influence of its environment are, with the exception of acidic zeolites, frequently difficult to define. One can find, for example, that for reactions such as the oxidation of propylene with bismuth molybdate catalysts, nearly every possible surface reaction has been ascribed to nearly every possible surface site in at least one s t ~ d y . ~ While the methods utilized to address such issues have included variation of bulk oxide structure and composition, surprisingly few studies have examined the surface reactivity of well-defined surfaces of single-crystal oxides. The reasons for the disparity between the number of studies of metal surfaces and that of oxide surfaces are twofold: first, the presence of surface defects appears to be of greater importance in determining the surface reactivity5 of oxides (which may be quite low for thermally stable planes) and, second, electrostatic charging of insulating or semiconducting oxides may complicate or preclude identification of surface intermediates by photoelectron spectroscopic techniques. The approach taken in this study to circumvent these difficulties is the utilization of thin oxide films grown on single-crystal metal surfaces as models of the behavior of bulk oxides. While a number of recent studies have examined the growth of thin (10-100 A) oxide films on metals, including Mg,GgA1,l@l2Sn,I3 Cr,14M o , ~ Fe,1s,19 ”~~ Ni,20Pb,21and Zn,22-24relatively few studies have been concerned with the surface reactivity of these oxide layers once formed. We report here results on the oxidation of the Mg(001) surface and on the activity of the oxide layer formed for dissociation of a series of Brmsted acids. These
* Author to whom correspondence should be addressed. ‘Present address: Dpto. de Ing. Quimica, Universidad Industrial de Santander, Apartado Aereo 678, Bucaramanga, Colombia. 0743-7463/85/2401-0684$01.50/0
results demonstrate that thin oxide films may indeed successfully model some of the chemical properties of bulk oxides. Several recent studiesH have examined the initial stages of oxidation of the Mg(001) surface. There appears to be general agreement that oxidation proceeds through a series of stages which involve (1)dissociative chemisorption and incorporation of oxygen atoms, (2) oxide layer formation via island growth and coalescence, and (3) thickening of the surface MgO layers. There is less agreement, however, as to the exact oxygen exposures which define each stage and as to the LEED patterns observed for the various stages of oxidation. It is reasonably clear that for oxygen exposures greater than 10-12 langmuirs (1langmuir = lo4 (1)Huizinga, T. Dissertation, Technische Hogeschool, Eindhoven, 1983. (2) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science (Washington,D.C.)1981,211, 1121. (3) Ryndin, Y. A.; Hicks, R. F.; Bell, A. T.; Yermakov, Y. I. J. Catal. 1981, 70, 287. (4) van der Baan, H. S. In “Chemistry and Chemical Engineering of Catalytic Processes”; Prins, R., Schuit, G. C. A., Eds.; Sijthoff and Noordhoff Alphen an den Rijn, 1980; p 523. (5) Heiland. G.: Luth. H. In “The Chemical Phvsics of Solid Surfaces and Heterogeneous Catalysis”; King, D. A., Woidruff, D. P., Eds.; Elsevier: Amsterdam, 1984; Vol. 3, p 219. (6) Namba, H.; Darville, J.; Gilles, J. M. Surf. Sci. 1981, 108, 446. (7) Flodstrom, S. A.; Martinsson, C. W. B. Surf. Sci. 1981, 111, 26. (8) Havden, B. E.; Schweizer. E.: Kotz, R.; Bradshaw. A. M. Surf. Sci. 1981, I l l ; 26. (9) Fueele. J. C. Surf. Sci. 1977. 69. 1981. (10) B%a; E. P.; Kle‘inman, L. i.Electron Spectrosc. Relat. Phenom. 1984, 33, 175 and references therein. (11) Czanderna, K. K.; Morrissey, K. J.; Carter, C. B.; Merrill, R. P. J. Catal. 1984, 89, 182. (12) Cocke, D. L.; Johnson, E. D.; Merrill, R. P. Cat. Rev.-Sci. Eng. 1984, 26, 163. (13) Powell, R.; Spicer, W. E. Surf. Sci. 1976, 55, 681. (14) Sakisaka, Y.; Kato, H.; Onichi, M. Surf. Sci. 1982, 120, 150. (15) Walker, B. W.; Stair, P. C. Surf. Sci. 1980, 81, L40. (16) Henrv. R. M.: Walker, B. W.: Stair. P. C. Proc. Conf. Chem. Uses Molybdenum, 4th 1982. (17) Walker, B. W.; Stair, B. C. Surf. Sci. 1981, 103, 315. (18) Kelemen, S. R.; Kaldor, A.; Dwyer, D. J. Surf. Sci. 1982, 121, 45. (19) Udovic, T. J.; Dumesic, J. A. J. Catal. 1984, 89, 303. (20) Wandelt, K. Surf. Sci. Rep. 1982, 2, 2 and references therein. (21) Isa, S. A.; Joyner, R. W.; Matloob, M. H.; Roberts, M. W. Appl. Surf. Sci. 1980, 5, 345. (22) Unertl, W. N.; Blakely, J. M. Surf. Sci. 1977, 69, 23. (23) Au, C. T.; Roberts, M. W.; Zhu, A. R. Surf. Sci. 1982,115, L117. (24) Au, C. T.; Roberts, M. W. Proc. Int. Congr. Catal., 8th 1984, IV-239.
0 1985 American Chemical Society
Synthesis and Reactivity of Thin Oxide Films
Langmuir, Vol. 1, No. 6, 1985 685
torr s) a continuous oxide phase of greater than monolayer thickness is formed; this oxide layer exhibits features in AES and ELS that are characteristic of bulk MgOSH The present study reports results on the surface reactivity of such oxide layers. 2. Experimental Procedure The experiments were carried out in a stainless steel vacuum chamber pumped with a 250 L/s ion pump and by a titanium sublimation pump. The system was equipped with a single-pass cylindrical mirror analyzer for AES, four-grid LEED optics, and a quadrupole mass spectrometer for monitoring gaseous products. torr were routinely achieved, Base pressures of less than 1.5 X typical background pressures between TPD experiments were about 8 X torr. A magnesium single crystal oriented to expose the (001) face was mechanically polished to a mirror finish with 1-pm diamond paste. No electrochemical polishing was performed. The magnesium sample was mounted in the vacuum chamber by means of a tungsten clip attached to a rotatable sample manipulator. Sample heating was provided by a planar tungsten filament suspended approximately 3 mm behind the sample; cooling was provided via conduction through a copper braid attached to the sample manipulator and to a flow-throughliquid nitrogen reservoir mounted on the manipulator flange. The sample temperature was monitored by means of a chromel-alumel thermocouple press fit to the edge of the sample. The Auger spectrum following installation of the sample in the vacuum chamber indicated that oxygen and carbon were present on the surface. Initial attempts to remove these contaminants by argon ion bombardment (Physical Electronics Model 04-161 sputter ion gun) were unsuccessful when the chamber was simply backfilled with argon to a pressure of 5 X torr. Analysis of the gas phase suggested that CO and C 0 2 built up in the chamber during sputtering and that adsorption of these species replenished the carbon and oxygen levels on the surface. Ion bombardment was therefore carried out while flowing argon into the chamber while pumping with sorption pumps attached to the chamber. The beam voltage was 1 kV, and the sample was biased to -100 V to improve the ion etching. This procedure was effective in removing all detectable C and 0 as determined by AES, although the mirror fiiish of the crystal was destroyed by ion bombardment, with the crystal exhibiting a somewhat frosted appearance. It was not possible to restore the mirrorlike appearance except by removal of the crystal from vacuum and mechanical polishing. Thermal treatment of the sample in vacuum did not change the optical appearance of the sample, although a sharp (1x1)LEED pattern was obtained by annealing the crystal to 420 K for 20 min. Annealing at higher temperatures (up to 470 K) produced no appreciable improvement of the optical or LEED characteristics over annealing at 420 K; the lower temperature was therefore chosen in order to minimize evaporation of magnesium from the sample. Gases were admitted to the chamber through a variable leak valve connected to a stainless steel dosing needle designed to produce a sharp beam of reactant to the front side of the crystal. Since oxidation of magnesium is quite rapid, the crystal was faced away from the dosing needle (by rotation of the offset manipulator); there was no line of sight between the doser and the front face of the sample during oxygen adsorption experiments.
3. Results 3.1. Growth and Characterization of Oxide Films. Auger electron spectroscopy was the principal method used to characterize the oxide layers produced in this study. The AES spectrum of clean magnesium and the changes in the spectrum by exposure to oxygen are well documented; the results obtained here are in good agreement with those reported previously. Figure 1 illustrates the changes in both the magnesium and oxygen signals as a function of oxygen exposure. As the oxygen exposure is increased, the O(KLL) peak at 510 eV increases monotonically, as expected. There is a parallel reduction of the intensity of the magnesium line at 4.4 eV, attributed to the L2,3VVtransition for metallic magne~ium.~ As the Mg (44
100
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Figure 1. Auger spectra from the Mg(OO1)surface with increasing oxygen exposure. Beam energy = 2 k e y modulation = 10 eV peak to peak: (a) clean surface; (b)total oxygen exposure = 8 langmuir; (c) total oxygen exposure = 18 langmuir.
0
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Figure 2. Auger signal intensities as a function of oxygen exposure at 298 K.
eV) peak decreases in intensity, the magnesium peak at 33 eV increases; this peak has been previously assigned to the Mg(L)O(L)O(L)transition for oxidized magnesium. These observations are consistent with previous reports. The variation with oxygen exposure of the peak-to-peak intensity for these three principal transitions is shown in Figure 2. The decrease in intensity of the Mg(4.4 eV) peak is essentially the mirror image of the increase in the O(510 eV) peak. The Mg(33 eV) peak, however, appears to pass through a minimum before increasing in parallel with the oxygen peak as expected. This behavior is also in agreement with previously reported data7 and results from t h e overlap of a metallic Mg(L2,3VV)peak (which decreases with oxygen exposure) and the principal oxidic peak, which increases with oxygen e~posure.~ Since the O(KLL) signal is easily resolved owing to the absence of such overlap, the O(510 eV) peak-to-peak amplitude was utilized as the principal measure of oxygen uptake. As has also been noted the oxygen uptake profile exhibited three distinct regimes. At low exposures (0-3 langmuirs) the oxygen uptake with exposure was
686 Langmuir, Val. 1, No, 6, 1985
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Figure 3. Oxygen uptake on the Mg(001) surface as a function of adsorption temperature and surface order: (a) clean surface annealed to 420 K, oxygen adsorbed at 298 K; (b) clean surface annealed to 420 K, oxygen adsorbed at 200 K; (c) clean surface not annealed following sputtering, oxygen adsorbed at 298 K.
roughly linear. As the exposure was increased beyond 3 langmuirs, the rate of uptake increased by about a factor of 4 as compared with the low-exposure regime, resulting in a sharp upturn in the oxygen concentration as a function of exposure. For oxygen exposures greater than about 10 langmuirs the rate of oxygen uptake again decreased, and the uptake profile was relatively flat above 15 langmuirs. These three regimes correspond roughly to the incorporation, monolayer completion, and film thickening regimes proposed by other workers; the shape of the oxygen uptake profile is in excellent agreement with that reported by Flodstrom and Martinsson and with subsequent results obtained on Mg(001) surfaces exhibiting a mirrorlike appearance. Several quantitative changes in the oxygen uptake profile were noted with variations in the exposure sequence, sample temperature, and surface roughness. These may account for quantitative differences, particularly in definition of the boundaries between adsorption regimes, between previous studies. It was observed, for example, that the position of the oxygen uptake profile in Figure 1 relative to the exposure scale was somewhat dependent on the number of successive exposures. The number of exposures in Figure 2 was 22; as this number was increased the uptake curve shifted to the left of that in Figure 2, and the opposite occurred when the number was decreased. This variation was due to the experimental technique: the oxygen pressure transients between the base pressure and the steady-state dosing pressure were not accounted for in determining the cumulative exposure; the importance of such transients is maximized for a large number of small exposures. It was also observed that the oxygen uptake profile was dependent upon the temperature of the magnesium surface. Figure 3 displays uptake profiles for adsorption at 200 and 298 K, the number of exposures is the same for each curve. Although the qualitative shape of the LWO profiles is the same, the rate of oxygen uptake at 200 K was faster then that at 298 K in both the first and secmd regimes. This result suggests that the rates of oxygen incorporation and of oxide monolayer formation are limited by the rate of oxygen adsorption rather than by oxygen incorporation or island coalescence. Although it is not unusual for the sticking coefficient to decrease with increasing temperature, oxygen incorporation and island coalescence would be expected to be activated processes and the oxygen uptake rate would increase with increasing temperature if these steps were rate-limiting. In contrast to the relatively small effects of temperature and exposure sequence, increasing the surface disorder by ion bom-
bardment produced dramatic qualitative changes in the shape of the uptake profile and in the rate of oxygen adsorption. Figure 3 compares the uptake profiles for a surface that was annealed at 420 K following ion bombardment and for a surface that was not annealed following bombardment. For the latter surface the slow uptake characteristic of the oxygen incorporation regime is absent; uptake is rapid even at the lowest exposures and there is no inflection point in the uptake profile. These results are consistent with the explanation of oxygen incorporation: the rate of oxygen adsorption is increased when the surface is disordered; this disorder may be produced by oxygen incorporation beneath an initially ordered surface. The effect of oxygen exposure on the LEED patterns obtained on the Mg(001) surface corresponds most closely to the observations reported by Flodstrom and Martinsson.' For exposures of the clean surface to up to 4 langmuirs of O2at 298 K, the original sharp diffraction features of the hexagonal (1x1) pattern became more diffuse. At around 4 langmuirs of O2the apparent width of the diffraction features reached a maximum, and increasing exposures between 4 and 1 2 langmuirs resulted in fading of this diffuse pattern. For exposures greater than 12 langmuirs no diffraction pattern could be resolved from the diffuse background. No diffraction features in addition to the original hexagonal pattern were observed for any oxygen exposure. Both Namba et a1.6 and Flodstrom and Martinsson7 divided the LEED behavior into exposure regimes (0-3, 3-10, and >10 langmuirs) which correspond to the three regimes in the oxygen uptake curve determined by AES. As observed in this study the first regime is characterized by a (1x1) pattern: Namba et al. reported that the diffraction spots broaden with increasing exposure, as was observed here, while Flodstrom reported fading of these spots with no apparent broadening. In the second regime (3-10 langmuirs) Flodstrom observed a diffuse (1x1) pattern which was attributed to epitaxial growth of MgO(111)domains, while Namba et al. observed additional diffraction features which were also attributed to epitaxial MgO(ll1). (The (111)plane of the rock salt MgO structure and the (001) plane of the hcp structure of bulk Mg both possess hexagonal symmetry.) As noted by Flodstrom, the discrepancies between the various studies can most likely be accounted for by the sensitivity of epitaxial oxide growth to the preparation of the Mg(001) surface. The principal conclusion, however, is unaffected: the growth of the first MgO layer occurs by island formation; the first oxide layer is complete for oxygen exposures of approximately 10 langmuirs. In order to obtain a reproducible homogeneous oxide layer for the surface reactivity studies described below, the surface oxide layer was prepared by using a fixed sequence of oxygen exposures until the AES peak ratio O(510 eV)/Mg(33 eV) was 2.3. A t this point the total exposure was 15.1 f 1.8 langmuirs and the number of successive exposures was 16 f 2. On the basis of the characterization of the oxidation of the Mg(001)surface in this and previous studies, the oxide layer produced by this procedure would be expected to be of the order of 1-2 monolayers thick. 3.2. Adsorption of CO and COz on the Clean and Oxidized Mg(001) Surfaces. In order to demonstrate that the oxide layer formed above completely covered the surface, the adsorption of CO and C02were examined on the clean and oxidized surfaces. When the clean surface was exposed to either of these molecules at 298 K, rapid uptake and dissociation occurred, resulting in new AES signals for O(510 eV) and C(273 eV) as well as a decrease
Langmuir, Vol. 1, No. 6, 1985 687
Synthesis and Reactivity of Thin Oxide Films a
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Figure 5. TPD spectra for methanol for increasing initial coverages on the oxidized Mg(001) surface. Methanol exposure: (a) 75 mtorr s; (b) 210 mtorr s; (c) 450 mtorr s; (d) 1050 mtorr s; (e) 1500 mtorr s; (f) 5100 mtorr s. (Note: the above exposures are based on the pressure of methanol in the dosing manifold, and reflect only the relative exposures at the surface.) 200
300 350 400 TEMPERATURE ( K 1
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Figure 4. Temperature-programmeddesorption spectra following adsorption of alcohols and water on the oxidized Mg(001)surface. The heating rate was 16 K/s.
in the Mg(44 eV) peak similar to that observed for exposure of the surface to dioxygen. Exposure of the clean Mg(001) surface to 20 langmuirs of either CO or C02was sufficient to produce an O(510 eV) signal of comparable intensity to that observed following a 20-langmuir exposure of 02,In addition the C(273 eV) signal following a 20langmuir exposure to CO or COz had a peak-to-peak height approximately one-fourth of that of the O(510 eV) peak. Thus the rate of dissociative adsorption of CO and C02 on the clean Mg(001) surface is comparable to that for 02. In contrast, when the clean surface was oxidized according to the standard oxygen exposure sequence described above, neither CO nor COP were adsorbed for surface temperatures between 200 and 300 K. Following exposures of up to 20 langmuirs of CO or 300 langmuirs of C02, there was no evidence for (1)carbon in the AES spectrum, (2) an increase of the O(510 eV) signal in AES, or (3) CO or COz desorption when the surface was subsequently heated to 420 K. Thus it was concluded that CO and C02 do not adsorb on the oxidized surface and that the surface treated by the above procedure was indeed completely covered by the oxide layer. 3.3. Adsorption of Organic Species on the Oxidized Mg(001) Surface. 3.3.1. Alcohol and Water Adsorption. The adsorption and temperature-programmed desorption of methanol, ethanol, isopropyl alcohol, and water were examined on the oxidized Mg(001) surface. All of these species exhibited quite similar adsorption and desorption behavior. Further, adsorption appeared to be completely reversible in all cases: there was no detectable increase in the carbon or oxygen Auger signals after adsorption and thermal desorption of these reactants; detectable levels of carbon were present only after ca. 60 adsorption/ desorption cycles with the alcohols. TPD spectra following adsorption of alcohols or water at 195 K always exhibited two desorption peaks for satu-
ration coverages of these adsorbates. Figure 4 illustrates this behavior: for all four of these reactants the desorption spectrum contains a peak near 240 K, designated CY, and one near 300 K, designated @. No evidence for any decomposition products (including aldehydes, hydrocarbons, water, and hydrogen) during either adsorption or temperature-programmed desorption was observed for the alcohols; thus the parent molecule appears to have been the only product in the above cases. Coverage variation experiments (Figure 5) demonstrated that the two peaks fill sequentially with increasing exposure, with the @ peak filling first, as expected. As is also evident in Figure 5, the /3 peak exhibited a coverage-dependent peak temperature, shifting to lower temperatures as the initial coverage was increased. Further, for exposure of the oxidized Mg(001) surface to alcohols or water at temperatures below 240 K, the magnitude of the @ peak was independent of the adsorption temperature; i.e., the @-stateof these adsorbates exhibited saturation behavior. In contrast, the a-state did not shift with increasing exposure for a fixed adsorption temperature; the magnitude of this peak was, however, dependent on the surface temperature during adsorption. Several aspects of the @-statesuggest that this desorption peak results from a surface recombination reaction rather than from desorption of molecularly adsorbed species. The symmetry and shift of this peak with coverage can be explained either by simple second-order kinetics or by lateral interactions which result in a coverage dependence of the activation energy for a first-order reaction. The similarity of the desorption spectra for methanol, ethanol, isopropyl alcohol, and water argues against lateral interactions, since one would expect the large differences in the relative sizes of these molecules to produce correspondingly large changes in through-space repulsive interactions, if these in fact occur. As shown in Figure 6, the shift of peak temperature with coverage may be fit quantitatively by a second-order rate law with no interactions. Figure 6 consists of a plot of In (002'~') vs. ~ / T M for the CH30H @-state,where Bo is the initial coverage and TM is the peak temperature; such a plot should produce a straight line of slope E / R . The activation energy for the
Martinez and Barteau
688 Langmuir, Vol. 1, No. 6, 1985
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TEMPERATURE ( K )
CH30H @ peak in Figure 3 is 19.1 kcal/mol, the correFigure 7. TPD spectra for acetylene and acetic acid following adsorption on the oxidized Mg(001) surface at 200 K. sponding second-order preexponential is 4 X cm-2 based on the assumption that the saturation coverage of the @ state is 6.5 X 1014molecules/cm2 (one-half the density of Mg in the MgO(ll1) plane). Regardless of the exact coverage of the @-stateassumed, it is clear that the preexponential factor for this process is of the order of magnitude expected for a second-order reaction. Quite similar behavior has been reported for methanol adsorption and desorption on the Cu(110) s u r f a ~ e . ~ ~ ~ ~ ~ Methanol desorbs from that surface in a two-peak spectrum without decomposition; UPS and XPS studies%have conclusively demonstrated that the higher temperature peak (at 275 K) results from the reaction of adsorbed methoxy (CH30) species and hydrogen atoms. Thus KINETIC ENERGY ( e V I methanol is adsorbed in both dissociated and molecular states on the Cu(ll0) surface, the energetics for recomIO 20 30 bination of the dissociated fragments to reform methanol C 2 H 2 EXPOSURE ( L ) is favored over hydrogen atom recombination, and the Figure 8. Carbon buildup with exposure of the oxidized surface initial dissociation does not lead to dehydrogenation of to C2HZ. methanol to other products in TPD experiments. These observations are directly analogous to those for alcohol exhibited an irreversible decomposition pathway leading adsorption and desorption on the oxidized Mg(001) surto carburization (and further oxidation in the case of acetic face. acid) of the oxidized Mg(001) surface. Neither of these The kinetics of HzOdesorption from this surface are also species appeared to undergo reversible dissociative adquite similar to those observed on a number of metal and sorption as was the case for the alcohols and water. oxygen-containingmetal surfaces. It has been shown that Following exposure of the oxidized Mg(001) surface to surface hydroxyl species may be formed by dissociation acetylene at 200 K, small amounts of acetylene desorbed of water on both the clean and oxidized Fe(ll0) surfacesn at 218 K as shown in Figure 7. No dependence of the and in the presence of surface oxygen on the Ag(l10),28-30 acetylene desorption temperature with coverage was obCU(~~O and) ,Ni(l10)32 ~~ surfaces. On all of these surfaces served, and the comparison between the desorbed product water is evolved in TPD experiments by reaction of surface distribution and the experimental cracking pattern for hydroxyls either with each other or with adsorbed hyacetylene showed no evidence for desorbing products other drogen atoms; the peak temperatures for HzO production than acetylene. Neither water nor hydrogen nor other by these pathways are between 300 and 340 K in all cases. hydrocarbons were detected in the TPD experiments. Thus the kinetics of HzO formation from dissociated Likewise no volatile products were observed during acetfragments on the oxidized Mg(001) surface are in good ylene adsorption, though the evolution of small amounts agreement with those on other oxidized metal surfaces. of Hz or HzO, which could not be resolved from the higher 3.3.2. Acetylene and Acetic Acid Adsorption. The background levels during acetylene exposure, cannot be adsorption of acetylene and acetic acid was markedly ruled out. different from that of the alcohols; the former reactants In spite of the absence of volatile reaction products, the adsorption of acetylene on the oxidized Mg(001) surface produced a sharp and irreversible increase in the level of (25) Wachs, I. E.; Madix, R. J. J. Catal. 1978,53, 208. (26) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. carbon present on the surface as detected by AES following (27) Dwyer, D. J.; Kelemen, S. R.; Kaldor, A. J.Chem. Phys. 1982, 76, heating of the surface to 400 K to desorb molecular 1832. acetylene. Figure 8 shows the carbon buildup as a function (28) Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980,92, 528. (29) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1981, I l l , 11. of cumulative acetylene exposure for a series of adsorption (30) Barteau, M. A.; Madix, R. J. Surf. Sci. 1984, 140, 108. and desorption experiments. Following a 30-langmuir (31) Spitzer, A.; Luth, H. Surf. Sci. 1982, 120, 376. cumulative exposure to CzHz the ratio of C(273 eV) to (32) Benndorf, C.; Nobl, C.; Rusenberg, M.; Thieme, G. Surf. Sci. 1981, 111,87. O(510 eV) peak-to-peak heights was 0.4. This ratio is I
Synthesis and Reactivity of Thin Oxide Films
Langmuir, Vol. 1, No. 6, 1985 689
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approximately that expected for monolayer coverage of surface carbon on the basis of Auger sensitivity ratios reported at the same primary beam energy (2 kV) for similar i n ~ t r u m e n t a t i o n . The ~ ~ chemical nature of this carbon layer could not be determined from AES. The peak is shown inset in Figure 8; the peak shape is strictly characteristic of neither carbidic nor graphitic carbon. Classical graphitic line shapes were obtained for the adventitious carbon present when the crystal was first inserted in the vacuum chamber; thus it is unlikely that the carbon from acetylene is a graphitic species which is poorly resolved owing to instrumental limitations. It cannot be determined, on the basis of the present data, whether the carbon-carbon bond of acetylene remains intact following adsorption and thermal treatment; there is no evidence that any of the acetylenic hydrogens remain following this treatment. Quite similar behavior was observed for adsorption and decomposition of acetic acid on the oxidized Mg(001) surface. As shown in Figure 7 only a single, low-temperature desorption peak was observed following acetic acid adsorption; no evidence for decomposition products, including acetaldehyde, carbon monoxide, carbon dioxide, hydrogen, water, and ketene, was found. Like acetylene, acetic acid adsorption produced an irreversible increase in the level of surface carbon; an increase in the O(510 eV) AES signal was observed as well. Figure 9 illustrates the changes in the surface carbon and oxygen levels with cumulative exposure to CH3COOH; a cumulative exposure of approximately 40 langmuirs was sufficient to increase the O(510 eV) peak-to-peak height by 50%. The C(273 eV) line shape following acetic acid decomposition was quite similar to that produced by acetylene decomposition. While it is possible that one or more layers of magnesium acetate may be formed by acetic acid adsorption, the absence of apparent electron-beam effects in the Auger spectra suggest that such species are not, in fact, present. In any case the irreversible deposition of carbon by acetylene and acetic acid demonstrates that these species dissociate on the oxidized Mg(001) surface. 3.3.3. Adsorption of Propyne, Propylene, and Formaldehyde. A third characteristic pattern of adsorption/ desorption behavior was observed for a number of other organic species. These exhibited only weak molecular adsorption on the oxidized Mg(001) surface, as illustrated in Figure 10. No evidence for decomposition products during TPD, nor for reversible dissociative ad(33) "Handbook of Auger Electron Spectroscopy";Physical Electronics Industries: Eden Prairie, 1976.
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400
Figure 10. TPD spectra for propyne, propylene, and formaldehyde following adsorption on the oxidized Mg(001)surface at 200 K.
sorption (as for the alcohols), nor for irreversible deposition of carbon and oxygen was observed. There is some indication that formaldehyde interacts slightly more strongly with the surface than the other molecularly adsorbed species in this study. This effect may be due to interactions of both the carbon and oxygen centers in this molecule with Lewis base and Lewis acid sites, respectively, on the surface. Such interactions of carbonyl compounds have previously been reported for other surfaces containing basic oxygen ~ p e c i e s . ~ ~ . ~ ~ Perhaps more striking is the dramatic difference in the reactivity of the two alkynes, acetylene and propyne. On the basis of a small amount of carbon buildup after ca. 20 adsorption/desorption cycles with propyne, the relative rate of carbon deposition from propyne may be estimated to be approximately 3 orders of magnitude lower than that for acetylene. Possible explanations for this large difference in reactivity between such closely related molecules are discussed below. 4. Discussion The surface and catalytic chemistry of magnesium oxide is commonly described as that of a solid base;%thus typical reactions include dissociative adsorption of Brernsted acids, as well as base-catalyzed dehydrogenation and isomerization reactions. While this qualitative characterization is undoubtedly a useful one, it should not be considered all encompassing, nor does it offer any insight into the quantitative "strength" of MgO as a base. For example, MgO has been reported to produce dehydration3' (typically an acid-catalyzed reaction) as well as dehydrogenation of alcohols. Likewise the formation of a variety of anion radical complexes of CO has been reported on MgO;38it is not clear that this activity is related to the basic properties of the solid. Recent experiments by Garrone and Stone39have suggested that the dissociation of Brernsted (34) Barteau, M. A.; Bowker, M.; Madix, R. J. Surf. Sci. 1980,94,303. (35) Wachs, I. E.;Madix, R. J. Surf. Sci. 1979, 84, 375. (36) Tanabe, K. "Solid Acids and Bases"; Academic Press: New York, 1970. (37) Parrott, S.L.;Rogers, J. W., Jr.; White, J. M. Appl. Surf. Sci. 1978, 1, 443. (38) Klabunde, K.J.; Kaba, R. A.; Morris, R. M. ACS Adu. Chem. Ser. 1979, No. 173, 140 and references therein.
Martinez and Barteau
690 Langmuir, Vol. 1, No. 6,1985 Table I. Comparison between Reactants That Block CO Cluster Formation and Their Dissociation in This Investigation
reactant CzH2 CH,COOH CHSOH C2H50H i-CSH70H H20
adsorption and blocking of CO cluster formation45
dissociation in this study
Yes Yes Yes Yes
Yes Yes yes Yes Yes yes
HCl, HZS C2H4 C3H6 C3H4
yes yes Yes no no no
HZ, Nz
no
cs2, coz
H2CO
no no no
acids on MgO powders may be correlated with the pK,’s of these acids in aqueous solution-a somewhat surprising result given the absence of solvent in the gas-solid adsorption experiments. Thus it is clear that the surface chemistry of magnesium oxide is more complex than is implied by its generic description as a solid base; one of the challenges is therefore to account for the site or sites on MgO surfaces that give rise to the various patterns of reactivity observed. Recent theoretical studies have helped to clarify the roles of various sites on MgO, particularly with regard to the adsorption of CO and the formation of CO anion radicals. Colbourn and Mackrodt40 have pointed out that the electron affinity of CO is negative by 41 kcal/m01;~~ thus one would expect to find electron donation from CO to the surface. Thus the formation of (C0)- species appears to be ruled out, although charged clusters such as (CO)$- and (C60,J2-have been observed on Mg0.42943Consistent with this expectation Colbourn and Mackrodt have determined, on the basis of ab initio calculations, that CO is weakly (8-15 kcal/mol) adsorbed on nondefective MgO(001) surfaces (5-coordinated cation sites), ledge sites (4-coordinated), and corner sites (3-coordinated). In all cases the extent of electron transfer was calculated to be small and in the direction of the magnesium cation rather than CO ligand. Stone and ~ o - w o r k e r shave ~~,~ reached ~ somewhat different conclusions in several experimental studies. They conclude that the 5-coordinated sites are sufficient to dissociate water and methanol, while 4- and 3-coordinated sites are required for dissociation of acetylene and alkenes, as well as for adsorption of CO. In contrast, Klabunde and co-workers%l&attribute CO radical formation to “reducing sites” which they suggest are defects (most likely cation vacancies) rather than low-coordinationsites resulting from simple termination of the crystalline lattice. In addition, these workers have reported that some organic adsorbates are capable of blocking formation of anion radicals from CO and nitrobenzene, while a number of others are not. Perhaps the most striking result here is the strong agreement between Klabunde’s classification of “blocking” vs. “nonblocking“reagents and the classification of species (39) Garrone, E.; Stone, F. S.Proc. Int. Congr. Catal., 8th 1984, III441. (40) Colboum, E. A.; Mackrodt, W. C. Surf. Sci. 1984, 143, 391. (41) Rempt, R. Phys. Rev. Lett. 1969, 22, 1034. (42) Zecchina, A,; Stone, F. S . J . Chem. SOC., Faraday Trans. 1 1978, 74, 2278. (43) Morris, R. M.; Klabunde, K. J. J. Am. Chem. SOC. 1983,105,2633. (44) Stone, F. S.; Zecchina, A. R o c . Int. Congr. Catal., 6th, 1976 1977, A-8. (45) Morris, R. M.; Klabunde, K. J. Inorg. Chem. 1983, 22, 682.
which are dissociatively vs. nondissociatively adsorbed on the oxide films in the present study. Table I summarizes this comparison; of those species capable of acting as Brernsted acids, those molecules common to the two studies can be classified as blocking/dissociated or nonblocking/undissociated. There are no Brernsted acids which would fall into the categories blocking/undissociated or nonblocking/dissociated. Indeed even the rather surprising differentiation between acetylene and propyne is common to both studies. Morris and Klab~nde:~noting that the blocking reagents are all Brernsted or Lewis acids of reasonable strength, have suggested that the formation of surface radicals involves first adsorption at Lewis base sites followed by migration to the reducing sites. The correlation between the blocking and dissociated species in the two studies strongly suggests that the principal base sites for bulk MgO are reproduced by the oxidized Mg surface in this study. In other words these thin oxide films are a quantitative model for at least a fraction of the basic sites on MgO powders; the correlation with the blocking studies is indicative that this fraction must include the vast majority of basic sites present on the bulk material. Consideration of the nature of the surface of both the bulk oxide and the oxidized metal suggests that the predominant sites in both cases are the five-coordinated Mg-O ion pairs. Stone39has stated that the likely ratio of populations of 5-/4-/3-coordinated sites is 9O:lOl. Likewise one would expect similarly small populations of highly coordinatively unsaturated sites to be formed by oxidation of the first few atomic layers of a metal single-crystal surface. Perhaps more surprising is the equivalence of base strength for these high-concentration, high-coordination sites; the localized nature of the MgO surface sites appears to be such that these sites are not influenced by more than a few atomic layers of the bulk solid. Although the reactivity of the oxidized Mg(001) surface appears to be well accounted for by acid-base models, the mechanistic details of these reactions have not been resolved. For example, it is difficult to account for the dramatic difference in reactivity of acetylene and propyne on the basis of equilibrium constants, whether in aqueous solution or in the gas phase: the enthalpies of heterolytic dissociation differ by less than 5 kcal/mol in the gas phase46and in solution (propyne being the weaker acid in both cases). Further, the gas-phase acidities of terminal alkynes are comparable to those of the alcohols (and considerably greater than that of water), as are their rates of reaction with basic oxygen atoms on group 1150 metal surfaces.47@ The absence of propyne dissociation is thus difficult to account for on the basis of an acid-base model, although the same barriers to dissociation would appear to be present on both the bulk oxide and oxidized metal surfaces. One notable difference between the surface reactivity of the oxidized Mg(001) surface and that of bulk MgO is the activity for alcohol decomposition. Foyt and White49 (46) Bartmess, J. E.; McIver, R. T., Jr. In “Gas Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2, p 87. (47) Barteau, M. A.; Madix, R. J. Surf. Sci. 1982, 115,355. (48) Vohs, J. M.; Carney, B. A.; Barteau, M. A. J.Am. Chem. SOC.,in press. (49) Foyt, D. C.; White, J. M. J . Catal. 1977, 47, 260. (50) In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
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Langmuir 1985,1, 691-696
691
conducted TPD studies for methanol on MgO powders and observed H2C0 and CHI as the principal organic products, these were attributed to decomposition of surface methoxy species. The absence of thermal decomposition of methoxy species in the present study suggests that this reaction may require sites other than those on which the initial proton abstraction takes place. This is not unreasonable since the decomposition of an anionic methoxy species to formaldehyde requires elimination of a hydride ion rather than a proton. Again it appears that a simple acid-base model is inadequate to describe the surface reactivity of magnesium oxide.
3. Acetic acid and acetylene dissociate irreversibly on these oxide films; alcohols and water are reversibly dissociated; propyne, propylene, and formaldehyde exhibit only weak molecular adsorption. The pattern for dissociative adsorption is consistent with previous observations on blocking of anion radical formation by Br~nstedacids on MgO powders. 4. The chemical characteristics of the majority 5-coordinated MgO site pairs appear to be equivalent for bulk MgO powders and for oxide layers formed by oxidation of the bulk metal. Thin oxide films thus represent promising and tractable models for study of the surface reactivity of bulk oxides.
V. Conclusions 1. Thin (one-two atomic layers) oxide layers may be formed on the Mg(001) surface by exposures of less than 20 langmuirs at 300 K. The uptake of oxygen with increasing exposure is sensitive to both temperature and the extent of surface disorder; the oxygen uptake profiles of this study are in agreement with previous results on clean, ordered surfaces. 2. The oxide layers formed are continuous and are inactive for adsorption of CO and C02at temperatures down to 200 K.
Acknowledgment. We gratefully acknowledge the Research Corporation and the National Science Foundation (Grant CPE8311912) for support of the various stages of this work. We wish to thank Professor Dietrich Menzel for providing the magnesium crystal. R.M. thanks the Latin American Scholarship Program of American Universities (LASPAU) for financial support. Registry No. Mg, 7439-95-4; MgO, 1309-48-4;CH3COOH, 64-19-7; CzH2, 74-86-2; HzO, 7732-18-5; H&O, 50-00-0; C3H6, 115-07-1;C3H4,74-99-7; CO, 630-08-0; COz, 124-38-9; MeOH, 67-56-1; EtOH, 64-17-5; i-PrOH, 67-63-0.
Kinetics of Coupled Primary- and Secondary-Minimum Coagulation in Colloidal Dispersions D.L. Feke* and N. D.Prabhu Department of Chemical Engineering, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106 Received March 22, 1985. In Final Form: July 8, 1985 Binary coagulation rates are often predicted on the basis of a steady-state assumption about the pair distribution function and particle fluxes. On the basis of estimates of characteristic times, this assumption is not always justified for slow coagulation processes. A new type of analysis, appropriate for the initial stages of slow coagulation, is presented. Coagulation is modeled in terms of the rates of rearrangement of particle distribution that occur after the interparticle potential is perturbed at the onset of coagulation. Asymptotic results of the model valid for short times are given. For a colloidal dispersion governed by a DLVO potential, the theory predicts an induction period for the filling of the primary minimum and a coupling between primary- and secondary-minimum coagulation. 1. Introduction Stability of colloidal dispersions is often explained and interpreted in terms of theories of interparticle forces. Coagulation rates are predicted to depend on the magnitude of the energy barriers that particles must overcome on their way to a collision. Colloidal systems that exhibit a high interparticle potential barrier are thought to be stable to coagulation while low potential barriers lead to rapid coagulation. However, for the intermediate case of moderate potential barriers, colloidal systems undergoing slow coagulation may exhibit a more complex behavior. Particularly, coagulation may occur in either the primary or secondary minimum, and the rates of coagulation into the two minima can be inherently coupled. In this paper we present an analytical treatment for the slow coagulation of monodisperse colloidal spheres. In order to better model the coupling between primary- and secondary-minimum coagulation, our theory does not rely on pseudo-steady assumptions about the coagulation 0743-7463/85/2401-0691$01.50/0
process as do most other analyses of coagulation phenomena. Rates of slow coagulation are presented in terms of a perturbation series for small changes in the interparticle potential. Our analysis is general and can incorporate DLVO theory to account for interparticle forces and can employ exact mobility functions to describe the hydrodynamic interactions between the particles. In addition, we present a simulation of a typical coagulation process to indicate the nature of the coupling between the two types of coagulation and also to exemplify the induction period for the filling of the primary minimum. 2. Background Coagulation processes can be viewed in terms of the time evolution of the spatial distribution of particles within a suspension. A model for the coagulation of dilute suspensions of monodisperse colloidal spheres is depicted in Figure 1. The analysis is developed in a Lagrangian (particle-centered) reference frame in which the origin 0 1985 American Chemical Society
