Langmuir 1989,5, 1155-1162
1155
Adsorption of Carbon Dioxide on Mica Surfaces Krishna G. Bhattacharyya Department of Chemistry, Gauhati University, Guwahati- 781014, Assam, India Received January 12, 1989. I n Final Form: April 20, 1989 Carbon dioxide is shown to adsorb on vacuum-cleaved and hydrogen atom bombarded air-cleaved mica surfaces. Two desorption peaks were obtained. Only the high-temperature peak was characterized and is shown to follow second-order desorption kinetics with the peak maximum varying between 372 and 395 K depending upon coverages. The saturation coverage was 9 X 10l2molecules cm-2 for the hydrogen atom bombarded air-cleaved surface and 4.8 X 10l2molecules cm-2for the vacuum-cleaved surface. A possible mechanism for the chemisorption of carbon dioxide on mica surfaces is proposed. The results for the vacuum-cleaved surface were verified with XPS measurements. An apparently abnormal C Is peak with a binding energy of 279.6 eV was observed when a mica specimen was cleaved in an atmosphere of carbon dioxide. This is explained by proposing the existence of patch electrical fields on the mica surface immediately after the cleaving. Introduction Natural muscovite mica having the ideal formula KA12Si~1010(OH)2 is conveniently described as a hydrous aluminosilicate with a composite layer structure. The structure has been to consist of three layers: two identical tetrahedral layers, each with the composition (Si,A1)20, and with the vertices pointing inward, are sandwiched with a layer of octahedrally coordinated metal atoms, mostly aluminum. This 2:l sheet has a net negative charge due to substitution of approximately one-fourth of the tetravalent silicons by trivalent aluminum in the tetrahedral layer, and the charge is balanced by potassium ions in muscovite mica. The hydroxyl groups lie at the common plane of the tetrahedral and octahedral layers. The edge-on view of the ideal muscovite structure is shown in Figure 1. Mica, which has excellent insulating properties, is obtained naturally in high-grade single-crystal form. It can be cleaved very easily in the (001) plane along the layer of potassium ions. Thermal stability of mica is also quite reasonable up to a temperature of 900 K. These properties have made mica an important substrate material for epitaxial vapor deposition studies, and numerous reports4-12 have appeared on this. No systematic effort has, however, been made to elucidate the chemical reactivity of the mica surface, which is generally regarded as inert because of the presence of the potassium ions on the surface. The mica layers are to occlude large amounts of hydrogen, carbon monoxide, nitrogen, carbon dioxide, oxygen, and water vapor. M e r c e P detected some gases, including carbon dioxide, desorbing from a mica surface (1)Deer, W.A.; Howie, R. A.; Zwman, J. In Rock-Forming Minerals; Long": London, 1962;Vol. 3. (2) Bragg, L.; Claringbull, G. F.; Taylor, W. H. In Crystal Structure of Minerab; Cornel1 University Press 1965; Chapter 13, p 253. (3)Baily, S. W.In Crystal Structure of Clay Minerals and Their X-Ray Identification; Brindley, G. W.; Brown, G., Eds.; Mineralogical Society: London, 1980, Chapter 1. (4)Jaeger, H.; Mercer, P. D.; Sherwood, R. G. Surf. Sci. 1967,6,309. (5)Allpress, J. G.;Sanders, J. V. Surf. Sci. 1967,7, 1. (6) Poppa, H.; Heinemann, K.; Elliot, A. G. J. Vac. Sci. Technol. 1971, 8,471. (7) Poppa, H.; Elliot, A. G. Surf. Sci. 1971,24, 149. (8)Elliot, A. G.J. Vac. Sci. Technol. 1974,11, 826. (9)Elliot, A. G.Surf. Sci. 1974,44, 337. (IO) Cruenbaum, E. In Epitaxial Growth; Matthews, J. W., Ed.; Academic Press: New York, 1975. (11)Poppa, H.; Lee, E. H. Thin Solid Films 1976,32, 223. (12)Lewis, B.; Anderson, J. C. In Nucleation and Growth of Thin Films; Academic Press: New York, 1978. (13)Metsik, M. S.;Zhidikhanov, R. A. Krystallografiya 1958, 3, 95. (14) Collins, R.H.; Turnbull, J. C. Vacuum 1961,11, 119. (15)Lawson, R. W. Vacuum 1962,12, 145. (16) Mercer, P. D. Vacuum 1967,17, 267.
0743-7463/89/2405-ll55$01.50/0
freshly cleaved under ultrahigh vacuum (UHV). The gas had a desorption maximum around 340 K, but its source was not definitely established. A mica surface cleaved in air always contains carbon contamination7J7 while a UHV-cleaved surface is free of such contamination. The source of this contamination is also uncertain. It was found that the UHV-cleaved surface picked up carbon on exposure to the laboratory atmosphere. On the other hand, it was shown7Js that the UHV-cleaved surface did not chemisorb carbonaceous gases. Poppa and Elliot7 ruled out any possibility of impurities from inside diffusing onto the mica surface. These works do not appear to be exhaustive, as the main objective was to examine the suitability of the mica surface as a substrate for epitaxial growth. As the obvious carbonaceous agent in air is carbon dioxide, it is thought that it must be responsible in some way for the carbon contamination of the air-cleaved mica surface, and this has been investigated in this work. Experimental Section Apparatus. Carbon dioxide adsorption was studied by temperature-programmeddesorption (TPD) in a glass UHV appar a t u ~ 'pumped ~ , ~ ~ by a series of liquid-nitrogen-trapped mercury diffusion pumps. The ultimate vacuum obtained was in the region of 2 X lo4 Torr after bakeout at 470 K, and the main constituents of the residual gas were masses 2 and 28. The system was equipped with Bayard-Alpert-type ion gauges and a Vacuum Generators Anavac-2 quadrupole mass spectrometer for measurement of total pressure and partial pressures of various gases. Elaborate gas inlet and dosing lines were also connected to the system.
All the ion gauges and the mass spectrometer were calibrated, and the pumping speed of the system with respect to carbon dioxide was determined by using standard procedure.lg The effective volume of the system was estimated to be only 2.98 L, and therefore, the system was extremely sensitive to very small pressure fluctuations compared to large stainless steel systems. The sample holder assembly and the glass cell, where all the adsorption-desorption experiments were carried out, are shown in Figure 2. The cell consists of a wide-bore glass tube of diameter 4.5 cm and total height 47 cm. The tube T2 (diameter 1.2 cm) was sealed concentrically to the cell and provided the rigid support to the sample holder. H is the cleaving hook, made of 1.5-mm tungsten rod sealed to a glass-encapsulated iron slug, M,. This is contained in a side arm and can be moved by external magnets. (17)Dowsett, M.G.;King, R. M.; Parker, E. H. C. J. Vac. Sci. Technol. 1977,14, 711. (18)Muller, K.;Chang, C. C. Surf. Sci. 1969,14, 39. (19)Bhattacharyya, K. G.,Ph.D. Thesis, University of London, 1984. (20) Bhattacharyya, K. G. In Challenges in Catalysis Science and Technology; Naidu, S. R., Banerjee, B. K., Eds.; PDIL: Sindri, 1987;Vol. 1, pp 91-104.
0 1989 American Chemical Society
1156 Langmuir, Vol. 5, No. 5, 1989
Bhattacharyya
0
,,
THERMOCOUPLE /CLEAVING
LOOP
Si, AI
2.258A
0,OH 2.1 2 0 A
AI
O,OH 2.25SA
Si,AI
0
0000
PORTION TO BE
Figure 1. Edge-on view of the muscovite structure.
CLEAVED
Figure 3. Mica sample for adsorption/desorption experiments in the glass system. TANTALUM CLIP
I
MICA SAMPLE
CLEAVING
TO BE CLEAVED
TOP VIEW
SIDE VIEW
Figure 4. Mica sample and holder for ESCA experiments.
lwl I
1
Figure 2. Cell and the sample holder assembly. After cleaving, the hook deposits the cleaved mica flake in the storage tube W. The heating filament, F, made from 0.3-mmdiameter tungsten wire is introduced through two glass-to-metal seals. The cell has two additional glass-to-metal seals for the thermocouple terminals. For introduction of a sample, the cell is broken above the hook arm and resealed. The sample holder was made from 3-mm-diameter glass rods joined together with a narrow opening between them to form a cage which could hold a piece of mica. This cage is connected to a small glass-encapwhich in turn is connected to a long glass sulated iron slug, M2, tube, TI,which can freely move inside the tube, TP. This whole assembly is held in place by the balancing weight of a large glass-encapsulatediron slug, MI, in the top side arm, the slug being connected to the holder with a sterling silver chain. The sample can be raised or lowered by moving the slug MI while it can be rotated to face the cleaving hook, the gas inlet, or the heating filament by moving the slug Mz. All the operations are magnetically controlled from outside. Desorption experiments were done by heating the mica radiatively with the aid of the tungsten filament. The temperature of the mica surface was controlled by a specially designed linear temperature programmer unit based on the circuit of Herz, Conrad, and Ku~pers.2~ The sample face was about 15 mm away from the filament, which required a current of 4 A at 14 V to give a final mica temperature of 600 K. The results from the glass system were verified by X-ray photoelectron spectroscopy (XPS) in a Vacuum Generators ESThe analysis chamber CALAB MK I1 all stainless steel 5y~tem.l~ of this instrument, containing the 150" hemispherical analyzer, (21) Hew, H.; Conrad, H.;Kuppers, J. J. Phys. E 1979, 12, 369.
was routinely maintained at presaures in the region of Torr. The preparation chamber, used for cleaning, annealing, and other sample preparation techniques as well as for gas exposure experiments, was at a slightly higher pressure in the 10-B-lO-s-Torr region (1Torr = 133.3 N m-2). Sample Preparation. Natural muscovite mica, grade 5, of highest purity and supplied by Startin and Company (London, England) was used throughout these experiments. For the glass system, a sample was prepared from a 28 mm X 23 mm piece, less than 0.25 mm thick and cut carefully with a sharp blade to avoid opening of the edges. A 5-mm-wide section was cut away from the long edges on both sides of the sample 90 that the thicker middle portion could be cleaved. This is shown schematically below in Figure 3. Loops of very fine silver wire were inserted through holes made at the top of the middle portion on both sides of the sample, and these were used for pulling apart the two front pieces during cleaving. A chromel-alumel thermocouple was fitted at the middle of the sample by pushing it inside the layers. Samples for the ESCALAl3 were made by cutting away 1-cm squares from freshly air-cleaved mica sheets of thickness less than 0.25 mm. On one side of each sample, the corners were partially cut away to leave a raised octagonal area in the middle (Figure 4) which could be cleaved with the help of a small loop of fine silver wire fiied to it. The sample was mounted on a nickel stub with four small tantalum clips spobwelded to the top plate of the stub. For cleaving, the stub holder was locked into position in one of the high-precision sample manipulators, either in the analysis chamber or in the preparation chamber, and a wabble stick was used to lift off the octagonal front face with the aid of the loop. Only one cleaving could be done in this way, and whenever a freshly UHV-cleaved surface was required, a new sample was introduced. On heating the air-cleaved mica for the first time under UHV at 570 K a large evolution of gas was detected including water vapor, nitrogen, carbon monoxide, carbon dioxide, methane, hydrogen,oxygen, and some hydrocarbon fragments. Water vapor could not be eliminated completely even after repeated outgassings. It is likely that water vapor remaining inside the mica layers continues to evolve through a very slow diffusion process while new water molecules are generated by a slow dehydroxylation.
Langmuir, Vol. 5, No. 5, 1989 1157
Adsorption of CO, on Mica Surfaces
I Time/sec Temp/K
1
0 298
I
20 342
I
40 386
I\
*.
60 430
Figure 5. Typical desorption spectra for carbon dioxide adsorbed on cleaned, air-cleaved mica (a) and vacuum-cleaved mica (b) surfaces at room temperature. Exposure = 4.9 x 10l6 molecules cm-2; heating rate = 2.2 K s-l.
The air-cleaved mica surface containing a large amount of carbonaceous impurity was cleaned at least partially by bombardment with hydrogen atoms by using the tungsten filament to atomize hydrogen at a pressure of 2 X lo4 Torr. The residual gas during bombardment contained a large amount of water vapor along with appreciable quantities of hydrocdrbon fragments around masses 16,28, and 40. Subsequent XPS analysis showed almost total removal of carbon contamination from the mica surface. Carbon dioxide gas used in the experiments was generated from cardice by a double freeze-pump-thaw process. All exposures were done at room temperature, and before each desorption run the cell was pumped to the base pressure in order to remove any weakly held form of carbon dioxide. In the ESCALAB, the exposure was carried out at the preparation chamber, the chamber was pumped down to the base pressure, and the sample was transferred to the analysis chamber for XPS measurements.
Results and Discussion The air-cleaved mica surface showed no detectable affinity toward carbon dioxide. The surface showed some activity after prolonged outgassing between 520 and 570 K, but the TPD peaks were too small to be analyzed. Sufficient activity was shown only after the sample was bombarded for 1 h with hydrogen atoms. During bombardment, the mica temperature reached a value as high as 470 K. A surface cleaved in UHV was found to be almost as active as the hydrogen atom bombarded surface. The TPD data were obtained by monitoring the partial pressure of carbon dioxide (mass 44) simultaneously with time and temperature. The typical desorption spectra are shown in Figure 5. The spectra for air-cleaved and vacuum-cleaved surfaces were similar, each showing four distinct peaks: A, B, C, and D. The mica temperature started to rise only after an initial delay of 5-10 s, and because two peaks, A and B, appeared before the mica temperature showed any rise, they were attributed to the tungsten filament which rapidly became white hot. The peak C appeared as soon as the mica temperature began to rise, and this could be resolved from the filament peaks only when the sample was kept at a right angle to the filament, in which case the temperature of mica rose very slowly. Peak C might be due to a weakly held form of
carbon dioxide which desorbed just above the room temperature. In an attempt to further investigate peak C, the sample was cooled to 180 K by circulating liquid nitrogen through an aluminum coil wrapped round the middle of the cell and was followed by exposure to carbon dioxide. The desorption spectra were completely masked by a slow evolution of carbon dioxide from the cooler glass walls. The sample could not be cooled without cooling the cell in the present system, and hence a quantitative characterization of peak C was abandoned. Peak D was assigned to desorption of the gas from the mica surface. This was verified by a series of blank experiments in which the mica sample was removed from the cell leaving the holder, the thermocouple, and the filament in their normal positions. The blank experiments revealed only two sharp peaks identical to peaks A and B, thus showing definitely that the peaks C and D were not due to the filament, the holder, the thermocouple wires, or the glass walls. Desorption kinetics corresponding to peak D were determined by using the formula da = NoBnu, exp -dt RET)
(
--
due to where n is the order of the desorption process, a the surface coverage in molecules cm-2, No the number of adsorption sites per cm2,B the fractional surface coverage (a/No),Y , the preexponential factor, and E the activation energy of desorption. The temperature of the mica surface was varied linearly during the desorption process with a heating rate P. Assuming coverage-independent activation energy of desorption and negligible readsorption, eq 1 can be solved to obtain the expressions
p
E = v i ex"(-&)
for n = 1
R Tp2
(2)
where Tp is the temperature corresponding to the desorption peak maximum and a. is the initial surface coverage. The desorption of carbon dioxide from mica surface corresponding to peak D was found to follow second-order kinetics. This was proved by the symmetrical nature of the peak and variation of Tpwith coverage, both of which are true only for a second-order desorption process. Also, the second-order expression (3) may be rewritten as
Hence, for a second-order desorption process with constant activation energy, a plot of In (aoT 2, versus 1/ Tpshould yield a straight line with slope A/R and intercept In (/3ENO/Rv2).This was also found to be true with respect to peak D. Figure 6 shows the shift in the desorption peak maximum with coverage for a hydrogen atom bombarded, air-cleaved muscovite surface. The vacuum-cleaved surface produced similar data. The plots of In ( a o T ~versus ) l/Tp are shown in Figure 7. Variation of Tp with coverage is summarized in Table I along with the values of the activation energy and the preexponential factor found re(22) Redhead, P.A. Vacuum 1962,12, 203.
1158 Langmuir, Vol. 5, No. 5, 1989
Bhattacharyya Table I. Variation of Peak Maximum Temperature with Surface Coverage for Adsorption of Carbon Dioxide on
Mica activation coverage, molecules
cm-2
10
30
50
70
408
452
Ti m e / s e c 320
364
8.82 X 8.36 X 7.00 x 4.88 X 3.50 X 3.15 X 2.35 X 1.84 X
10l2 10l2 10'2 10l2 10l2 10l2 10l2 10l2
4.52 X 4.41 X 4.07 X 3.72 X 3.51 X 3.31 X 2.77 X 2.14 X 1.58 X
10l2 10" 10l2
Temp/K
Figure 6. Shift of desorption peak maximum with exposure for room temperature adsorption of carbon dioxide on cleaned, air-cleaved mica surface. Exposures for a, b, c, and d, respectively, 1.0 X were 4.9 x 10l6, 2.2 x and 3.4 x W5molecules cm-2. Heating rate for desorption was 2.2 K s-l.
temp of energy of peak max desorption E , preexponential kJ mol-' factor v , s-l Tp, K Air-Cleaved Mica Surface 372.8 373.5 375.5 380.5 1.3 X 10'2" 384.0 93.7 385.5 1.98 X 389.0 394.4
Vacuum-Cleaved Mica Surface 381.0 382.0 382.5 1.12 x 10'2' 10l2 383.3 94.4 10l2 384.0 1.02 x 10126 385.0 10" 387.5 10l2 390.5 10l2 395.0 10l2
"These values are calculated from eq 3. *Thesevalues are obtained from the intercepts from the plots of In ( u o T t )versus l / T v
'
41.3
4 0.9
7
40.5
t6 25 3
261
257 ( 11 T~ ) /
265
269
K -1
Figure 7. Plot of In (uoTp2)versus l/Tp for room temperature adsorption of carbon dioxide on vacuum-cleaved (a) and cleaned, air-cleaved (b) mica surfaces.
1 0
spectively from the slopes and the intercepts of these plots. The exposures and the coverages were obtained by using standard pro~edure.'~The plots of coverage versus exposure for air-cleaved and vacuum-cleaved surfaces are shown in Figure 8, from which the saturation coverage of carbon dioxide on the air-cleaved, hydrogen atom bombarded surface (9 X 1OI2 molecules cm-2) appears to be twice as large as that on the vacuum-cleaved surface (4.8 X 1OI2 molecuIes cm-2). The coverages are, however, very small when compared to those on active metal surfaces, indicating that the number of active sites on the mica surfaces is small. That these small coverages could be measured shows the extreme sensitivity of the glass adsorption-desorption system. The initial sticking probability values obtained from the initial slopes of plots of Figure 8 are (i) 1.3 X for the for the airvacuum-cleaved surface and (ii) 3.8 X cleaved surface. These values are very small, pointing to a very slow uptake of carbon dioxide by both types of mica surfaces, although the vacuum-cleaved surface had a comparatively higher sticking probability. The amount ad-
I
I
I
I
I
2
4
6
8
10
Exposure110 16molecules ~
m
'
I
J
12
14
~
Figure 8. Coverage versus exposure plots for room temperature adsorption of carbon dioxide on cleaned, air-cleaved (a) and vacuum-cleaved (b) mica surfaces. sorbed also did not vary when the exposure was carried out at different temperatures; thus no activated adsorption occurs. According to B a r ~ - e reach , ~ ~ oxygen hexagon in the mica surface has an area of about 24 A2. Since the potassium ions sit in these hexagons, each square centimeter of mica surface is capable of holding about 4.2 X 1014potassium ions. Cleavage divides the potassium ions equally between the cleaved surfaces, and hence, each of the surfaces holds about 2.1 x 1014cm-2 potassium ions. If these ions are the adsorption sites on the mica surface, then only 1in 23 sites is occupied by carbon dioxide in the hydrogen atom bombarded air-cleaved surface, whereas the occupation for the (23) Barrer, R. M. In Zeolites and Clay Minerals as Sorbents and Molecular Sieues; Academic Press: New York, 1978; p 414.
Langmuir, Vol. 5, No. 5, 1989 1159
Adsorption of C02 on Mica Surfaces
(b) I1 then dissociates into carbonate I11 and a proton which migrates along the layer of oxygen atoms
I
\
\
\
Max Count Rate
=
\ \
1696
\
\ \
1 \
o/
,: ,
0
/o\si/co'
\
\
\ob
AI
/OH
/
si+
0
\ /\
\ o b 'oo/
protonmigration-
si
0 '
carbonate (III) I
I
282
I
I
1
2ea ?E8 284 ind ding Energy ( correctedVeV
280
Figure 9. Change in C 1s peak intensity after the vacuum-cleaved surface was exposed to carbon dioxide: (a) C 1s peak before exposure; (b) C 1s peak after exposure to 600 langmuirs of CO,; (c)C 1s peak after heating to 670 K. Grazing emission, CAE = 20 eV, step size = 0.05 eV, 10 s X 25 scans. vacuum-cleaved surface is only one in 44 sites. The air-cleaved surface had gone through a number of heating cycles during outgassing, and it was also bombarded with hydrogen atoms. It is therefore likely that this surface suffered some potassium depletion compared to the vacuum-cleaved surface. XPS measurements showed this to be true. Thus, if the potassium ions are the adsorption sites, the vacuum-cleaved surface is expected to have a higher saturation coverage of carbon dioxide than the air-cleaved surface. The reverse experimental evidence points to some other adsorption sites, possibly below the potassium layer. Loss of some potassium from the air-cleaved surface evidently makes more sites accessible to carbon dioxide, and consequently, this surface has a higher coverage. Second-order desorption kinetics implies that two surface species are involved in the rate-determining step, the concentration of each species being proportional to the total reversible uptake of carbon dioxide. The two species come together randomly during desorption. Several possibilities exist, e.g., (i) the carbon dioxide may be dissociatively adsorbed: CO,(g) + CO(ads) + O(ads)
At least one of the adsorbed species must be mobile and capable of diffusing across the surface. However, since the conceivable sites of adsorption on the mica surface are likely to be far apart, the diffusion is unlikely to be operative. (ii) The mobile species may be an electron or more likely a positive hole, when the rate-determining step is likely to be COz-(ads) + (e+)
-
\ob
Si
'Od \oo/ 0 ' hydroxylatedtetrahedral layer (I)
0/
V (d) Desorption occurs when the carbonate and the hydroxyl species come together again. Mechanism iii above seems the most probable one. Infrared measurements by Bertsch and Habgood%on carbon dioxide adsorption on alkali and alkaline-earth X-zeolites revealed several carbonate species. According to their scheme, the carbon dioxide attacks a lattice oxygen bonded to a silicon or an aluminum atom, weakening the Si-0 or Al-0 bond, but the position of the surface oxygen remains more or less unchanged. They have shown that the carbonate formation is most likely with a surface oxygen adjacent to a positive ion, e.g., an exchangeable Na+ or K+ ion whose positive field extending out would stabilize the carbonate. Works of several other a ~ t h o r have s ~ ~sup~ ~ ~ ported this scheme. Formation of both bicarbonate and carbonate species is also shown.,' Morterra et a1.28detected two surface bicarbonates by reacting carbon dioxide with y-alumina, the concentrations of the species depending on the thermal pretreatment. In a recent review,29 the formation of bicarbonate and carbonate complexes by reaction of carbon dioxide with M-0 and M-OH bonds s ~ ~ also has been described. Lercher and ~ o - w o r k e r have studied the adsorption of carbon dioxide on alumina, magnesia, and the mixed oxides with infrared spectroscopy and have proposed the following two species: 0. Ky/..o
I
0
-
/o\sI/HC03 AI/o\si/o\s(o\
0/ \oo/
site
COz(g)
(iii) The mobile species may also be a proton. In this case, the mechanism will be as follows: (a) carbon dioxide attacks initially a t hydroxyl sites (I) on the mica surface to form a bicarbonate species (11) AI
lv (c) Protons become stabilized at a quadrivalent aluminum
\ob
\oo/
bicattmate (II)
\o
surface bicahonate
I
M
' 0
surface monodentate carbonate Alvero and ~ o - w o r k e r son , ~ ~the other hand, have shown (24) Bertsch, L.; Habgood, H. W. J. Phys. Chem. 1963,67,1621. (25) Ward, J. W.; Habgood, H. W. J. Phys. Chem. 1966, 70, 1178. (26) Angell, C. L.; Howell, M. V. Can. J . Chem. 1969,47, 9831. (27) Lerot, L.; Poncelet, G.;Debru, M. L.; Fripiat, J. J. J. Catal. 1975, 37, 396. (28) Morterra, C.; Zecchina, A.; Coluccia, S.; Chiorino, A. J. Chem. SOC.,Faraday Trans. 1 1977, 73, 1544. (29) Palmer, D. A.; van Eldik, R. Chem. Reu. 1983,83, 651. (30) Lercher, J. A.; Colombier, C.; Noller, H. J. Chem. SOC.,Faraday Trans. 1 1984,80,949. (31) Alvaro, R.; Bernal, A.; Carrizosa, I.; Odrizola, J. A.; Trillo, J. M. Appl. Catal. 1986,25, 207.
1160 Langmuir, Vol. 5, No. 5, 1989
Bhattacharyya
the formation of a formate species on interaction of carbon dioxide with a lutetia-alumina catalyst in the presence of hydrogen. On the basis of these observations, it is quite reasonable to assume that carbon dioxide forms bicarbonate and carbonate species on mica surface. The TPD study is, however, not suitable to probe for such species. The mica surface could not be freed of adsorbed water even by repeated heating cycles, and it is possible that some of the water exists as hydroxonium ions replacing a few potassium ions. During dehydration, a surface hydroxyl may be formed:
-
/o\si/oP;yo\ Si
0 ’
\oo/
\oo’
‘00’
0 ‘
-Hzo
I
Such surface hydroxylation is well-established in alumina, silica, silica-alumina, and zeolites. With the air-cleaved mica, hydroxylation was also possible during the hydrogen atom bombardment, as is the case with the zeolite surface, which undergoes hydroxylation during reduction with hydrogen.32 The hydrogen atom bombarded surface of mica was expected to have more hydroxyl groups than the relatively untreatel vacuum-cleaved surface. This eaplains why the air-cleaved and hydrogen atom bombarded mica surface takes up twice as much carbon dioxide. The validity of the second-order desorption kinetics rests on the ability of the proton in I11 to migrate across the basal oxygen layer of the mica. Proton mobility in zeolite is well-known, and it is one of the reasons for their high catalytic activity. According to Ward,% the interaction of the hydroxyl group with an adjacent aluminum atom leads to proton mobility: AI
o/ \oo/
\o
-
Si
o/ \oo’
\o
-
The proton can attach itself with another surface oxygen, reverting aluminium to the trivalent state. Magnetic resonance m e ~ s u r e m e n t sshow ~ ~ the protons on a zeolite surface to be actually mobile, and the proton jump frequency is shown to be of the order of (2-10) X lo4 s-l a t 200 “C with very small activation energy in the range 5-10 kcal/mol.3s A similar situation can be reasonably assumed in respect to the mica surface. The formation of trivalent silicon with a positive charge as in I11 is also a well-known postulation in zeolite chemistry. The tetrahedral layer in mica has three silicons to one aluminium on the average, and the distribution of aluminium atoms is also known to be random. Mica also has a very high Al/Si ratio compared to any zeolite. The random presence of aluminium in mica is thus likely to promote proton mobility. (32) Haynee, H.W.,Jr. Catal. Reu.-Sci. Eng. 1978, 17, 290. (33) Ward, J. W.In Zeolite Chemistry and Catalysis; Rabo,J. A., Ed., ACS Monogaph 171; American Chemical Society: Washington, D.C., 1976; p 118. (34) Fripiat, J. J. Catal. Rev. 1971, 5 , 269. (35) Freude, D.;Oehme, W.; Schmiedel, H.; Staudte, B. J. Catal. 1974, 32, 137.
Finally, the electrical field of the potassium ions is likely to favor the formation of the Carbonate species as discussed above; therefore, the conversion of the bicarbonate to the carbonate form takes place easily.
XPS Observations Mica, being an insulator, charges up during the XPS measurements, and hence, it is not possible to determine the absolute binding energies of the different levels of constituents. The XPS data in the present work were calibrated with respect to the K 2p312 level, as the potassium energy levels are shown to be insensitive to chemical shift.% The energy of the reference level was determined as 293.8 eV by evaporating platinum on to the mica surface and then estimating the apparent shift of the Pt 4f5 and 4f,,, levels due to charging. The binding energies oithese platmum levels were determined from platinum deposited on a nickel single crystal to be 71.1 and 74.3 eV. On the above basis, the C 1s peak on the air-cleaved mica was found to have a binding energy in the range 285.3-285.9 eV from a large number of measurements. The C 1s peak due to hydrocarbon contamination is generally assigned a binding energy in the range 284.4-284.8 eV by various authors. Therefore, it is reasonable to assume that the C 1s peak in mica has some contribution from carbon species other than the common hydrocarbon contaminants. The vacuum-cleaved mica surface showed a small C 1s peak whose binding energy after correction for charge shift was 285.7 f 0.3 eV. This may be due to the air-cleaved corners and edges of the sample. On exposure to carbon dioxide, the intensity of this peak increased: one typical result after exposure to 600 langmuirs (1langmuir = lo* Toms) of the gas is shown in Figure 9. Although the increase in peak area was about 3 times, only a part of the gain could be removed by heating to a temperature of 670 K. A mica sample, cleaved in vacuum and subsequently exposed to air, showed a C 1s peak as big as the one from the air-cleaved surface. However, an equally big peak could not be generated by exposing the vacuum-cleaved surface to carbon dioxide. Assuming that water vapor in air might be a responsible factor, the vacuum-cleaved surface was exposed to a mixture of carbon dioxide and water vapor, which did not generate the desired result. The ESCALAB did not have an arrangement for hydrogen atom bombardment, and therefore, the XPS study of the bombarded surface was not possible. The XPS data on the vacuum-cleaved surface have shown that the C 1s peak, appearing a t 285.7 f 0.3 eV, must be identified with the carbon dioxide species studied in the desorption experiments in the glass system. The data also show that only a part of the carbon is removable by heating up to a temperature of 670 K and that although there is a single peak for C 1s some adsorbed species are more strongly held than others. The protons involved in the second-order mechanism of desorption kinetics may be very strongly adsorbed in some of the basic sites of the mica surface so that they will not become availabile for causing desorption of carbon dioxide. This will explain a residue of irreversibly adsorbed carbonate species on the surface. Considerable variations exist in the values of binding energy reported for carbonyl carbon atom. Kantschewa and c o - w ~ r k e r sfor , ~ ~example, observed two C 1s peaks a t 285.7 f Q.2 and 291 eV in their study of a K&O3/7A 1 2 0 3 system. The F i t peak was resistant to heating while (36) Kantmhewa, M.; Albano, E. V.; Ertl, G.; Knozinger, H. Appl. Catal. 1983, 8, 71.
Langmuir, Vol. 5, No. 5, 1989 1161
Adsorption of COP on Mica Surfaces
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the second peak disappeared above 520 K. The 291-eV peak was assigned to carbonate carbon while the lower binding energy peak remained unspecified. Barteau and Madix3' studied the adsorption of carbon dioxide on preadsorbed oxygen on a silver surface and reported a C 1s peak a t 287.7 eV for an adsorbed =C03 species and another C 1s peak a t 287.2 eV for an adsorbed -C02 species. Bonzel and Krebs,ss on the other hand, reported a carbonate C 1s peak a t 290.7 eV. In the present case, the potassium 2 ~ 3 1 2peak a t 293.8 eV was very broad, and any small C 1s peak in the region of 291 eV was almost impossible to detect. Binding energy shifts of the C 1s level in a large number of compounds have been found30 to be predominantly influenced by the nearest neighbors and not by the state of hybridization of the carbon atom. XPS data for adsorption of carbon dioxide on zeolites, silica-aluminas, or clayminerals are not available, and a definite assignment of the C 1s peak on mica is therefore not possible. However, the carbonate complex certainly cannot be ruled out. When the muscovite sample was cleaved in Torr of pressure of carbon dioxide, the XPS data immediately afterwards showed (in the grazing emission mode) two very prominent C 1s peaks a t 285.8 and 279.6 eV and a small C 1s peak a t 289.5 eV. The wide-scan photoelectron spectrum is shown in Figure 10. The 279.6-eV peak was very much attenuated in the normal emission spectra, showing that this is definitely a surface species. On heating to 570 K, the peaks a t 285.8 and 279.6 eV came down in intensity by almost equal amounts, while the peak at 289.5 eV disappeared. When the sample was reexposed to 600 langmuirs of carbon dioxide all three peaks showed en(37) Barteau, M. A.; Madix, R. J. J. Electron Spectroscc. Relat. Phenom. 1983, 31, 101. (38) Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1981, 109, L527. (39) Gelius, U.; Hedin, P. F.;Hedman,J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970,2, 70.
hancement, but the lowest binding energy peak increased much more. All these data are shown in Figure 11. On the other hand, after about 24 h in the analysis chamber of the ESCALAB at a pressure of 5 X 10-l' Torr, the peaks a t 279.6 and 289.5 eV disappeared, and only the peak at 285.8 eV remained. The appearance of two main carbon peaks cannot be attributed simply to peak splitting due to gross surface charging, because no other peak showed such splitting. The 279.6-eV C 1s peak is apparently due to a negatively charged carbon species. Carbidic carbon gives peaks with low binding energies, but no C 1s peak with binding energy as low as 279.6 eV is known. Hafnium carbide,40for example, yields a C 1s peak of 281.0 eV (referenced to the adventitious C 1s peak of 285.0 eV) while titanium carbideq1is reported to give a C 1s peak a t 281.2 eV. No data have been available for C 1s binding energies in ionic carbides and acetylides (C22-)of alkali and alkaline-earth metals. XPS measurements on silicon carbide in the course of the present investigation indicated two C 1s peaks with a binding energy difference of 4 eV; i.e., if the adventitious carbon is assigned a binding energy of 284.6 eV, then that for the carbidic carbon would be 280.6 eV. A carbide or acetylide of potassium formed by decomposition of carbon dioxide on some local excess of potassium ions immediately after cleaving may be considered responsible for the low binding energy C 1s peak. However, alkali metal carbides and acetylides are known to be relatively unstable. Also, if carbon dioxide decomposes on adsorption, a suitable location for the oxygen atoms must be found, bearing in mind that part of the adsorption process is reversible. The lack of such a dissociative adsorption of carbon dioxide on the vacuum-cleaved mica surface also points to inadequacy of this mechanism. Both 279.6- and 285.8-eV C 1s peaks diminish by almost the same amount on heating, showing that carbon dioxide at both the states has approximately the same adsorption energy. A degree of interchangeability also appears to exist between the two states. On reexposure to carbon dioxide, (40) Ramqvist, L.; Hamrin, K.; Johaneson, G.; Fahlman, A.; Nordling, C . J. Phys. Chem. Solids 1969,30,1835. (41) Tanaka, K.; Miyahara, K.; Toyoshima,I. J . Phys. Chem. 1984,88, 3504.
1162
Langmuir 1989, 5 , 1162-1169
after heating, there was a transfer of intensity from the 285.8-eV peak to the 279.6-eV peak, whereas after 24 h in the UHV chamber, the 279.6-eV peak disappears and the whole of its intensity is transferred to the high-energy peak. It is thus possible that both these peaks belong to the same state of adsorption existing in different electrical environments. The act of cleaving may also be a significant factor in the formation of the 279.6-eV peak since this appears only when the sample is cleaved in an atmosphere of carbon dioxide. When mica is cleaved, the potassium ions become equally divided between the two cleaved faces to maintain electrical neutrality. However, there may be some regions rich in potassium and some others deficient in potassium in the cleaved face immediately after cleaving. The nonuniform distribution of potassium ions is likely to produce patch electrical fields on the surface. If the excess negative charge on a potassium-deficient patch is sufficient to lower the electrical potential by a few volts, this would provide an adequate explanation of the anomalously low binding energy found for the C 1s peak. This also explains the slow disappearance of this peak as the potassium ions rearrange to give a more uniform distribution after some time. The situation may be as follows: 0
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Although the normal carbonate species is formed, there is no compensating charge in the vicinity of this, and the carbon atom will be in a highly negative potential, which in turn will give a C 1s peak with very low binding energy. Further work will be necessary to study this abnormal species of carbon.
Conclusion In this work, clean mica surface has been shown to be reactive toward carbon dioxide. The normal chemical inertness of the surface must be due to the presence of carbonaceous overlayers of the air-cleaved surface. The number of adsorption sites on the clean surface is very small, and these sites, located in the tetrahedral layer, appear to be generated in a way identical with the generation of acid sites in zeolites, silica-aluminas, and other clay-minerals. Potassium depletion from the mica surface on heating also increases the activity. The measured uptake of carbon dioxide is very small to be detected in large, conventional stainless steel systems by volumetric measurement. The X P S data indicate that there is also a stronger, irreversible type of adsorption of carbon dioxide by the mica surface. Acknowledgment. I am grateful to Dr. D. 0. Hayward of the Department of Chemistry, Imperial College, London, for his guidance and constant encouragement during the experimental part of this work, which was actually carried out in his laboratory. Thanks are also due to Dr. M. Morris and Dr. A. 0. Taylor of the same department for their valued assistance in the experiments and in the interpretation of the data. Registry No. COz, 124-38-9.
Colloidal Iron Sulfate Layer Formation and Breakdown as a Source of Current Oscillations 0. Teschke,* D. M. Soares, and M. U. Kleinke Instituto de Fisica, Universidade Estadual de Campinas, 13081 Campinas, SP, Brazil Received January 3, 1989. I n Final Form: April 21, 1989 A new polarization device with a positive resistance and reflecting only the working electrode interface was developed. With this device, current oscillations were detected at both ends of the limiting current plateau for iron electrodes in 1M H2S04solutions, for both anodic and cathodic scans. The current oscillation potentials are coincident with the observed formation and breakdown of a colloidal salt film that precipitates adjacent to the electrode surface. Different morphologies of the precipitated iron sulfate are obtained in freeze-dried samples, as a function of the electrode polarization. The colloidal iron sulfate coating displays two apparently contradictory behaviors: it insulates the metallic surface and it generates turbulence which leads to its breakdown and to electrode exposure.
Introduction This paper describes current oscillations associated with the formation and breakdown of colloidal salt films that precipitate adjacent to the electrode surface. They are observed a t both ends of the limiting current plateau for iron electrodes in sulfuric acid solutions. The oscillations are detected by using a new polarization device that reflects only the working electrode-solution interface. Most cases of oscillations in electrochemical systems are observed during the anodic dissolution of certain metals. Oscillations are directly connected with the characteristic 0743-7463/89/2405-ll62$01.50/0
instability of passivating films, under specific conditions of potential and electrolyte composition. Franck’ obtained oscillations for iron in 1 N H2S04. Podesta et aL2 have studied the effects of the electrolyte convection on oscillations in the iron-sulfuric acid system. More recently, Russell and Newman3 observed sustained current oscillations for an iron rotating disk electrode in sulfuric acid. (1)Franck, U.F.Z. Elektrochem. 1958,62, 649. (2) Podesta, J. J.; Piatti, R. C. V.; Arvia, A. J. J. Electrochem. SOC. 1979, 126, 1363. (3) Russell, P.; Newman, J. J. Electrochem. SOC. 1986, 133, 2093.
0 1989 American Chemical Society
