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An XPS study of the KCl surface oxidation in oxygen glow discharge
Applied
426
AN XPS STUDY OF THE KCI SURFACE IN OXYGEN GLOW DISCHARGE J. STOCH
Science 31 (198X) 426-436 North-Holland, Amsterdam
OXIDATION
and M. LADECKA
Institute of Crrtulym and Surface Chemistry, 30.239 Krcrkw, Poland Received
Surface
17 August
1987: accepted
Polrsh Acrrden~y of Sawc~e.v, N~rxqxmrnqeh
for publication
14 December
1987
The reaction between the surface of KC1 and oxygen in a glow discharge has been studied by X-ray photoemission spectroscopy (XPS). Oxygen glow discharge treatment resulted in the formation of a superoxide. which decomposed under vacuum at room temperature to KO, and finally to K,O. No evidence of KCIO, or KClO, formation has been found. Binding energies of some oxygen species in potassium oxides were determined. The possible role of potassium in K-doped silver catalysts of ethylene epoxidation is discussed.
1. Introduction For the understanding of the oxidation mechanism of hydrocarbons in the presence of oxide catalysts it is necessary to be able to identify the atomic or molecular oxygen forms present at the surface. X-ray spectroscopy offering the possibility of determining surface composition has become one of the few methods capable of identification of surface atomic structures [1,2]. In ref. [3] various oxygen species found in inorganic compounds have been described and their properties and possible role they play in catalytic processes have been discussed. Among many atomic and molecular oxygen species which at an oxide surface can transform according to the equilibrium O,(ads)
+ 0;
+ Of-
+ 2 O2
the superoxide 0; or peroxide 0, 2p ions seem to play an important part in the partial oxidation of hydrocarbons. Papers concerning XPS studies of 02- ions are numerous. Much less data have been reported for other oxygen species. Only the O- species have been studied [2,4-63 somewhat more thoroughly, although it is still difficult to distinguish between O- and OH- ions. In view of that we attempted to characterize different oxygen ions present in inorganic compounds. The 0; ions are of special interest to us because of their common presence in reactions 0169-4332/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. Stoch, M. Ladecka / KC1 surface oxidation in OGD
427
of catalytic oxidation of hydrocarbons [7,8]. We did not however succeed in obtaining reliable results from experiments on direct interaction between gaseous oxygen and an oxide. Therefore we decided to characterize the 0; ions by studying solid superoxides. We have chosen KO, because of its high stability. On the other hand, potassium is a very important promoter in transition metal catalysts, e.g. for selective oxidation of ethylene to ethylene oxide [8], as well as for the Fischer-Tropsch synthesis of hydrocarbons from CO and H, [9], so our studies could contribute to a better understanding of the role of potassium in the mechanism of catalytical processes. Surface studies of KO, are complicated by its high reactivity towards water. Thus a method of obtaining potassium oxide in situ in the spectrometer chamber through an exchange of chloride ions in KC1 by the oxygen ions from a glow discharge had to be applied.
2. Experimental Surfaces
of the samples
studied
were prepared
as follows:
KC1 (la) Deposited from water-alcohol solution in the form of a dense layer of very small crystals (lb) Heated in UHV at 670 K for 15 min (lc) Treated with oxygen glow discharge (OGD) at 570 K for 2 min (Id) After 1 h in UHV at room temperature (2a) Prepared as sample (la) and after OGD at 670 K (2b) After 1 h in UHV at room temperature (2~) Heated in UHV at 620 K for 17 h (3a) Deposited by sublimation in dry air (3b) Treated with OGD at room temperature (3~) After 1 h in UHV at room temperature (3d) Heated in UHV at 570 K for 0.5 h (3e) Heated in UHV at 670 K for 0.5 h KOH (4) Pellets melted on the sample holder; after cooling the top layer was scraped off; a very small amount of KC1 (< 10%) was added to produce a Cl(2p) reference level in the spectra (5) Deposited from ethyl-alcohol suspension and heated KCIO, (6) Deposited from water solution KCIO, (7) Deposited from water solution All compounds used were pure of anal. grade.
at 770 K for 0.5 h
The photoelectron spectra were obtained with an ESCA-3 Vacuum Generators spectrometer using nonmonochromatized Al Ka,,z radiation as the X-ray excitation source. During XPS measurements the vacuum was better than 10 ’ Pa. Binding energies (BE) were measured for O(ls). C(ls), Cl(2p). and K(2p) bands. As the KC1 spectra obtained showed too low carbon contents for calibration, a specially prepared surface with a high contamination level (after 4 days in open air and overnight in ethanol vapour) was used. Assuming the C(ls) BE for this sample to be 285.0 eV the K(2p3,?) peak appeared at 293.6 eV. In the course of the experiments on KC1 the K(2p) band was a single doublet showing neither other contribution from doublets nor broadening. The measured value of the Cl(2p) binding energy remained very stable at 199.15 i 0.09 eV (n = 12). For the compounds studied the C(ls) line position varied with pretreatment becoming unsuitable for calibration. Therefore the Cl(2p) line has been used as a reference level. The KOH spectra were calibrated against the Cl(2p) line from the KC1 added. The surface of KOH contained also a small amount of carbonate. The Table 1 Measured Surface
binding
energies
assignments
and oxygen Binding
contents
data Oxygen overlayer
energies (eV)
Formula
Treatment
K(~P, /z )
CUP)
O(ls)
KCI
(la) (lb) (lc) (ld) (2a) (2b) (2c) (3a) (3b) (3c) (3d)
293.6 ‘I) 293.6 I” 293.6 “’
199.2 199.0 199.2
293.6 ,”
199.1
293.6 I” 293.6 ‘I’ 293.6 ‘)
199.1 199.1 199.1
293.6 .‘I
199.3
(3e) KJO, KClO,
(4) (5) (6)
293.6 *) 293.5 293.6 293.7
KCIO,
(7)
292.6
199.3 199.2 ,” _ 206.9 199.2 *’ 208.1 199.2 ,‘)
531.7 531.6 532.1 531.5 533.2 532.0 531.7 528.8 532.8 531.x 528.6 532.1 528.6 531.6 531.9 532.6
KCI
KCI
KOH
‘) Assumed to be calibration h’ Determination of O/K,, spectra.
standard. ratio was impossible
thickness
(l/K,>, ratio (A)
0.8 0.5 10.4 x.4 13.3 7.6 1.4 0.7 7.0 5.5 I .7
0.5
1.2
0.4
1.4 1.2 2.47 1.64
532.5
because
of a very complex
character
of the
J. Stoch, M. Ladecka
C(ls) line position of carbonate at a reference for spectra calibration Because the KClO, and KClO, position, the chloride Cl(2p) line their spectra.
/ KCI surface oxidation in OGD
429
289.6 eV as measured at KOH was used as of the K&O, sample. samples contained KC1 from their decomwas used as a standard for calibration of
K(2pl
6 3d,e 3b.c
300 Fig. from later and
BE[eV]
1. K(2p) XPS spectra of the potassium compounds studied. KC1 surface: untreated, deposited water solution (la) or sublimated (3a) and treated with OGD at room temperature (3b). 1 h (3c), after UHV heating at 570 K (3d) and at 670 K (3e). Spectra of KOH (4), KClO, (6), KCIO, (7), surfaces as prepared. Denotations in the figure are the same as used in the description of surface preparation (section 2).
J. Stoch, M. Ludecko
430
/ KCI surfucr o.uidution VI OGD
In order to investigate an oxygen overlayer on the KC1 surface the samples were subjected in the preparation chamber to oxygen in a glow discharge at different temperatures. It has been observed that under such conditions the
C1(2p)
7 6
3d,e 3 b,c
la 3a I
I
I
195
I
1
I
I
I
200
,
,
,
,
1
,
,
,
205
Fig. 2. Cl(2p) XPS spectra of the potassium deposited from water solution (la) or sublimated (3b), 1 h later (3~). after UHV heating at 570 K KCIO, (7) surfaces as prepared. The low binding Ku~,~ satellite contributions. Denotations in the
,
1
210
,
,
,
,
BE [eV]
compounds studied. KCI surface: untreated. (3a) and treated with OGD at room temperature (3d) and 670 K (3e). Spectra of KCIO, (6) and energy peaks of the spectra 6 and 7 contain AI figure are the same as used in the description of
surface preparations
(section
2).
J. Stoch, M. Ladecko / KC1 surface oxidation in OGD
431
overlayer of an oxygen compound is formed. Upon subsequent heating decomposition of the primarily produced compounds has been observed. The XPS spectra were recorded of all samples as prepared and for KC1
3e
3d
3c 3b la 3a 525
530
Fig. 3. O(ls) XPS spectra of the potassium deposited from water solution (la) or by temperature (3b), 1 h later (3c), after UHV KOH (4) KClO, (6) and KClO, (7) surfaces as used in the description
BE [eV] compounds studied. Oxygen contamination of KC1 sublimation (3a). KC1 treated with OGD at room heating at 570 K (3d) and at 670 K (3e). Spectra of as prepared. Denotations in the figure are the same of surface preparations (section 2).
after each step of experimental treatment. The results are given in table 1 and figs. l-3. Quantitative analysis was carried out according to Seah [lo]. The intensities (peak areas) of K(2p), O(ls), Cl(2p). and C(ls) levels were considered. Scofield’s calculated [ll] subshell ionization cross-sections were used, but for the K(2p) and Cl(2p) bands experimental corrections were derived from measured peak intensities for KClO, and KClO,. The mean free paths were calculated according to Penn [12]. Estimation of the thickness of an oxygen overlayer requires the intensity of the O(ls) peak from an infinitely thick oxygen layer. To that end the measured intensity value of the O(ls) band from Li,O was used.
3. Results and discussion Our experiments were aimed at the study of oxygen interaction with potassium. Therefore, surface studies of KOH, K,CO,, KClO,. and KClO, stable compounds were performed to provide necessary information on core level binding energies. K-O stoichiometry and experimental coefficients for quantitative calculations. Potassium oxides are very active in reaction with Hz0 and show low thermal stability. Thus it is impossible to obtain them in a pure state in open air. That is why we produced potassium oxides in situ in the spectrometer chamber by exposing to oxygen plasma such a stable and hydrophobic compound as KCl. In fact, the spectra of pure KC1 showed a very low (-c 3 at%) oxygen content. In the KC1 samples prepared through crystallization from aqueous solution the binding energy of O(ls) from this oxygen contamination was 531.6 eV, which corresponds well to the 531.6 eV in KOH. In the case of KC1 deposited by sublimation the O(ls) peak was found at 528.8 eV, while the O/K,,, ratio was about 0.4. This binding energy was assigned to K,O. These results indicate that during crystallization a small amount of KC1 reacts with water KC1 + H,O 2 KOH + HCl and the chemical equilibrium is slightly shifted to the right because of evaporation of HCl perhaps during drying of the deposit. We found about 3 at% of oxygen in the surface layer. Having detected K,O in the sublimated deposit we have to assume that KC1 vapour reacts with oxygen 2 KCl+:
O,+K,O+Cl,,
or with water 2 KCl+2H,O+2
KOH+2
HCl.
J. Stoch, M. Ludecka / KCI surface oxidation in OGD
followed
433
by dehydration
2 KOH + K,O + H,O. In this case we found also about 3 at% of oxygen in the surface layer. Although our KC1 samples were prepared in open air, the surfaces did not contain any measurable amounts of carbonates. The KOH samples however were considerably contaminated with carbonates; regardless of neither another peak contribution nor any broadness being observed in the K(2p) and O(ls) bands. Thus we conclude that the binding energies of these bands for KOH and K,CO, are the same within the error limits (k 0.1 eV>. For potassium compounds, determination of the chemical state of chlorine from XPS binding energies is simple because their measured values distinctly differ (see table 1). We assumed the BE of K(2p,,,) to be 293.6 eV from our measurements of the highly carbon-contaminated KC1 sample. The same value has been found by Bonzel et al. [13]. So the Cl(2p) band in KC1 appeared at 199.2 eV. Our values remain in good agreement with the literature data [14-171. Because of the low polarizability of the Kt and Cl- ions we expect that these values should not be distinctly shifted when KC1 is present in other matrixes. For KClO, and KClO, chlorate and perchlorate Cl(2p) peaks appeared at 206.9 and 208.7 eV respectively, while that of chloride was fixed at 199.2 eV. The values of the O(ls) core level binding energy in KClO, and KClO, were found to be 532.6 and 532.5 eV respectively. See figs. 2 and 3. After the oxygen glow discharge treatment the KC1 surface composition was changed. The intensity of the Cl(2p) peak decreased. The O(ls) band increased considerably and was shifted towards higher binding energies as shown in figs. 2 and 3. The highest measured binding energy of the O(ls) level was 533.1 eV. It has been observed that at room temperature under our experimental conditions the intensity of the O(ls) peak decreased with time and its BE value was shifted back to lower energies (see fig. 3 and table 1). This fact illustrates the low vacuum stability of the produced oxygen compound. With heating in vacuum its decomposition proceeds even faster. Various degrees of decomposition can be seen in the O(ls) band in fig. 3(3a-3e). In a glow discharge in oxygen various oxygen species are present, for example highly reactive 03, 0; and O- [18-201. Even though the oxygen used was dried in a liquid nitrogen trap one cannot preclude the possible presence of very small amounts of H,O or OH- from water traces, which can always be found in the spectrometer chamber. However, since the vast majority of species are oxygen ions, we expected an exchange of lattice Cl- ions with oxygen to be the main process. The BE of the O(ls) peak from plasma oxidized surface was in the range 532.8-533.1 eV. This value is considerably higher than the 531.6 eV measured for KOH. On the other hand, the decreasing intensity and associated shift
towards a value as low as 528.6 eV exclude KOH as a principal component of the oxygen overlayer. Shim and Wittig [21] studied the interaction between NaCl crystals and oxygen afterglow. Under the conditions of their experiment an oxygen compound was obtained, which by means of IR measurements was reported to be NaClO, formed in the possible process: 0 + O2 + NaCl
--)
NaClO,.
Although formation of potassium chlorates cannot be totally excluded. no experimental evidence was found for their presence. The Cl(2p) peak at 206.0-209.0 eV was not observed. In an independent experiment we found KClO, to be stable in vacuum at room temperature. Even assuming its presence and decomposition, the only stable end product would be KC1 but not an oxide, while in our experiment a stable oxide at 528.6 eV was formed as the end product. Also carbonate could not be considered because of its high stability at room temperature. On the other hand, an increase in the C(ls) band at 289 eV has not been observed. Therefore, relying on this experimental evidence, we conclude that under oxygen glow discharge treatment a higher potassium oxide is formed. Different potassium oxides are known. The existence of superoxide KOz is well documented, though disuperoxide KO, can also be obtained [22]. The O/K,, ratio in our experiment with glow discharge was changing in time. The XPS measurement was carried out after about 1 h after exposure of the sample to oxygen plasma. Therefore, the highest value observed (O/K,,, = 2.4) indicates that its initial value was higher. Taking that into account we can suppose that in direct contact with OGD the KC1 surface is oxidized to KOd which under vacuum at room temperature decomposes quickly to KOz and afterwards to K,O, and K,O. The presence of K,O as the end product is ratio of 0.4 and BE as low as 528.6 eV. confirmed by the estimated O/K,, For the samples deposited from water solution we were not able to distinguish between possible KOH and K,Oz as the end product after vacuum thermal decomposition. From all our measurements we have determined binding energies for potassium oxides which are given in table 2. The values for O’- and 0, agree well with the data found by Ayyoob and Hegde [23,24]. Our results can be useful for the interpretation of XPS spectra in studies of the chemical state of oxygen in oxides. We have shown that 0, species can be formed at an oxide surface and they can be identified and studied by the XPS method. Moreover. the ability of potassium to form easily superoxides with gaseous oxygen was demonstrated. This fact is of some importance for studies of hydrocarbons oxidation catalysts containing potassium. As it was suggested [25] ethylene can be oxidized to ethylene oxide at dioxygen molecules housed in meshes of the oxygen-silver surface 2D struc-
J. Stoch, h4. Ladeckn Table 2 Measured
binding
energies
/ KCI surface oxidation WI OGD
of O(ls) in potassium
oxides and A BE a)
Compounds
Species
Binding
K2O
02..
528.6 531.6-532.0 532.8 z 533.0
KGH,
W202)
(O;-),
KC’2
0;
KG,
(0,))
‘) ABE = the difference ion.
between
OH-
the electronic
435
energies (eV)
state of oxygen
ABE 0 3.0-3.4 4.2 > 4.4
in an oxide and the state of 02-
ture, e.g. Ag(lOO)c(2 x 2)0. The role of this structure is to keep oxygen molecules vertically, i.e., to create the ability of oxygen molecules to react with ethylene molecules for epoxidation. Thus a substitution of oxygen with chlorine atoms strengthens the structure and, as it is well known, increases the selectivity. But the selectivity can also be enhanced by promoting Ag surface with potassium. It has recently been shown [24] that dioxygen species are stabilized on potassium-dosed Ag sufaces. When chlorine is adsorbed prior to oxygen, only molecular dioxygen is bonded at the surface [24]. This result agrees well with the above-mentioned model and leads to the conclusion that the role of potassium is to supply the reacting system with dioxygen active species being one of the substrates of epoxidation.
References 111T.A. Carlson, PI I31 [41 [51 161 171 PI [91 UOI [Ill [121 P31 P41 [I51 [I61 1171
in: Electron Spectroscopy, Ed. D.A. Shirley (North-Holland, Amsterdam, 1972) p. 53. CT. Au, A.F. Carley and M.W. Roberts, Phil. Trans. Roy. Sot. (London) A318 (1986) 61. J. Haber and A. Bielahski. Catalysis Rev.-Sci. Eng. 19 (1979) 1. M.W. Roberts and R.St.C. Smart, J. Chem. Sot. Faraday Trans. I, 80 (1984) 2957. A.F. Carlay, P.R. Chalker and M.W. Roberts, Proc. Roy. Sot. (London) A399 (1985) 167. C.T. Au, A.F. Carley and M.W. Roberts, Intern. Rev. Phys. Chem. 5 (1986) 57. P.A. Kilty and W.M.H. Sachtler, Catalysis Rev. 10 (1974) 1. D.J. Hucknall, Selective Oxidation of Hydrocarbons (Academic Press, New York, 1974). R.B. Anderson, in: Catalysis, Vol. 4, Ed. P.H. Emmett (Reinhold, New York, 1956) p. 29. M.P. Seah, Surface Interface Anal. 2 (1980) 222. J.H. Scofield, J. Electron Spectrosc. Related Phenomena 8 (1976) 129. D.R. Penn, J. Electron Spectrosc. Related Phenomena 9 (1976) 29. H.P. Bonzel, G. BrodCn and H.J. Krebs, Appl. Surface Sci. 16 (1983) 373. V.V. Nemoshkalenko, V.G. Aloshin and A.I. Senkevich, Dopovidi Akad. Nauk USSR A3 (1976) 252. W.E. Morgan, V. Waser and J.R. Stec, J. Am. Sot. 95 (1973) 751. V.I. Nefedov, Ya.V. Salyn, G. Leonhardt and R. Scheibe, J. Eectron Spectrosc. Related Phenomena 10 (1977) 121. W.E. Moddeman, J.R. Blackbum, G. Kumar and K.A. Morgan, J. Electron Spectrosc. Proc. Intern. Conf. 1972 (1972).
[1X] A.7‘. Bell, Ind. Eng. Chem.
[ 191 CC. Morgan. in: Handbook [20] [21] [22] [23] [24] [25]
Fundamentals of Vacuum
10 (1971) 373. Physics, Vol. 2(l). Ed. A.H. Beck (MacMillan.
York. 1965). F.K. Kaufman. Advan. Chem. 80 (1969) 29. D. Shim and C. Wittig. Appl. Opt. 15 (1976) 1896. L. Andrew, J. Chem. Phys. 54 (1971) 4935. M. Ayyoob and MS. Hegde. Surface Sci. 133 (1983) 516. M. Ayyoob and MS Hegde, J. Catalysis 97 (1986) 516. L. Matachowski and J. Stoch. J. Mol. Catalysis 26 (1984) 1X7.
Ne&
426
AN XPS STUDY OF THE KCI SURFACE IN OXYGEN GLOW DISCHARGE J. STOCH
Science 31 (198X) 426-436 North-Holland, Amsterdam
OXIDATION
and M. LADECKA
Institute of Crrtulym and Surface Chemistry, 30.239 Krcrkw, Poland Received
Surface
17 August
1987: accepted
Polrsh Acrrden~y of Sawc~e.v, N~rxqxmrnqeh
for publication
14 December
1987
The reaction between the surface of KC1 and oxygen in a glow discharge has been studied by X-ray photoemission spectroscopy (XPS). Oxygen glow discharge treatment resulted in the formation of a superoxide. which decomposed under vacuum at room temperature to KO, and finally to K,O. No evidence of KCIO, or KClO, formation has been found. Binding energies of some oxygen species in potassium oxides were determined. The possible role of potassium in K-doped silver catalysts of ethylene epoxidation is discussed.
1. Introduction For the understanding of the oxidation mechanism of hydrocarbons in the presence of oxide catalysts it is necessary to be able to identify the atomic or molecular oxygen forms present at the surface. X-ray spectroscopy offering the possibility of determining surface composition has become one of the few methods capable of identification of surface atomic structures [1,2]. In ref. [3] various oxygen species found in inorganic compounds have been described and their properties and possible role they play in catalytic processes have been discussed. Among many atomic and molecular oxygen species which at an oxide surface can transform according to the equilibrium O,(ads)
+ 0;
+ Of-
+ 2 O2
the superoxide 0; or peroxide 0, 2p ions seem to play an important part in the partial oxidation of hydrocarbons. Papers concerning XPS studies of 02- ions are numerous. Much less data have been reported for other oxygen species. Only the O- species have been studied [2,4-63 somewhat more thoroughly, although it is still difficult to distinguish between O- and OH- ions. In view of that we attempted to characterize different oxygen ions present in inorganic compounds. The 0; ions are of special interest to us because of their common presence in reactions 0169-4332/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. Stoch, M. Ladecka / KC1 surface oxidation in OGD
427
of catalytic oxidation of hydrocarbons [7,8]. We did not however succeed in obtaining reliable results from experiments on direct interaction between gaseous oxygen and an oxide. Therefore we decided to characterize the 0; ions by studying solid superoxides. We have chosen KO, because of its high stability. On the other hand, potassium is a very important promoter in transition metal catalysts, e.g. for selective oxidation of ethylene to ethylene oxide [8], as well as for the Fischer-Tropsch synthesis of hydrocarbons from CO and H, [9], so our studies could contribute to a better understanding of the role of potassium in the mechanism of catalytical processes. Surface studies of KO, are complicated by its high reactivity towards water. Thus a method of obtaining potassium oxide in situ in the spectrometer chamber through an exchange of chloride ions in KC1 by the oxygen ions from a glow discharge had to be applied.
2. Experimental Surfaces
of the samples
studied
were prepared
as follows:
KC1 (la) Deposited from water-alcohol solution in the form of a dense layer of very small crystals (lb) Heated in UHV at 670 K for 15 min (lc) Treated with oxygen glow discharge (OGD) at 570 K for 2 min (Id) After 1 h in UHV at room temperature (2a) Prepared as sample (la) and after OGD at 670 K (2b) After 1 h in UHV at room temperature (2~) Heated in UHV at 620 K for 17 h (3a) Deposited by sublimation in dry air (3b) Treated with OGD at room temperature (3~) After 1 h in UHV at room temperature (3d) Heated in UHV at 570 K for 0.5 h (3e) Heated in UHV at 670 K for 0.5 h KOH (4) Pellets melted on the sample holder; after cooling the top layer was scraped off; a very small amount of KC1 (< 10%) was added to produce a Cl(2p) reference level in the spectra (5) Deposited from ethyl-alcohol suspension and heated KCIO, (6) Deposited from water solution KCIO, (7) Deposited from water solution All compounds used were pure of anal. grade.
at 770 K for 0.5 h
The photoelectron spectra were obtained with an ESCA-3 Vacuum Generators spectrometer using nonmonochromatized Al Ka,,z radiation as the X-ray excitation source. During XPS measurements the vacuum was better than 10 ’ Pa. Binding energies (BE) were measured for O(ls). C(ls), Cl(2p). and K(2p) bands. As the KC1 spectra obtained showed too low carbon contents for calibration, a specially prepared surface with a high contamination level (after 4 days in open air and overnight in ethanol vapour) was used. Assuming the C(ls) BE for this sample to be 285.0 eV the K(2p3,?) peak appeared at 293.6 eV. In the course of the experiments on KC1 the K(2p) band was a single doublet showing neither other contribution from doublets nor broadening. The measured value of the Cl(2p) binding energy remained very stable at 199.15 i 0.09 eV (n = 12). For the compounds studied the C(ls) line position varied with pretreatment becoming unsuitable for calibration. Therefore the Cl(2p) line has been used as a reference level. The KOH spectra were calibrated against the Cl(2p) line from the KC1 added. The surface of KOH contained also a small amount of carbonate. The Table 1 Measured Surface
binding
energies
assignments
and oxygen Binding
contents
data Oxygen overlayer
energies (eV)
Formula
Treatment
K(~P, /z )
CUP)
O(ls)
KCI
(la) (lb) (lc) (ld) (2a) (2b) (2c) (3a) (3b) (3c) (3d)
293.6 ‘I) 293.6 I” 293.6 “’
199.2 199.0 199.2
293.6 ,”
199.1
293.6 I” 293.6 ‘I’ 293.6 ‘)
199.1 199.1 199.1
293.6 .‘I
199.3
(3e) KJO, KClO,
(4) (5) (6)
293.6 *) 293.5 293.6 293.7
KCIO,
(7)
292.6
199.3 199.2 ,” _ 206.9 199.2 *’ 208.1 199.2 ,‘)
531.7 531.6 532.1 531.5 533.2 532.0 531.7 528.8 532.8 531.x 528.6 532.1 528.6 531.6 531.9 532.6
KCI
KCI
KOH
‘) Assumed to be calibration h’ Determination of O/K,, spectra.
standard. ratio was impossible
thickness
(l/K,>, ratio (A)
0.8 0.5 10.4 x.4 13.3 7.6 1.4 0.7 7.0 5.5 I .7
0.5
1.2
0.4
1.4 1.2 2.47 1.64
532.5
because
of a very complex
character
of the
J. Stoch, M. Ladecka
C(ls) line position of carbonate at a reference for spectra calibration Because the KClO, and KClO, position, the chloride Cl(2p) line their spectra.
/ KCI surface oxidation in OGD
429
289.6 eV as measured at KOH was used as of the K&O, sample. samples contained KC1 from their decomwas used as a standard for calibration of
K(2pl
6 3d,e 3b.c
300 Fig. from later and
BE[eV]
1. K(2p) XPS spectra of the potassium compounds studied. KC1 surface: untreated, deposited water solution (la) or sublimated (3a) and treated with OGD at room temperature (3b). 1 h (3c), after UHV heating at 570 K (3d) and at 670 K (3e). Spectra of KOH (4), KClO, (6), KCIO, (7), surfaces as prepared. Denotations in the figure are the same as used in the description of surface preparation (section 2).
J. Stoch, M. Ludecko
430
/ KCI surfucr o.uidution VI OGD
In order to investigate an oxygen overlayer on the KC1 surface the samples were subjected in the preparation chamber to oxygen in a glow discharge at different temperatures. It has been observed that under such conditions the
C1(2p)
7 6
3d,e 3 b,c
la 3a I
I
I
195
I
1
I
I
I
200
,
,
,
,
1
,
,
,
205
Fig. 2. Cl(2p) XPS spectra of the potassium deposited from water solution (la) or sublimated (3b), 1 h later (3~). after UHV heating at 570 K KCIO, (7) surfaces as prepared. The low binding Ku~,~ satellite contributions. Denotations in the
,
1
210
,
,
,
,
BE [eV]
compounds studied. KCI surface: untreated. (3a) and treated with OGD at room temperature (3d) and 670 K (3e). Spectra of KCIO, (6) and energy peaks of the spectra 6 and 7 contain AI figure are the same as used in the description of
surface preparations
(section
2).
J. Stoch, M. Ladecko / KC1 surface oxidation in OGD
431
overlayer of an oxygen compound is formed. Upon subsequent heating decomposition of the primarily produced compounds has been observed. The XPS spectra were recorded of all samples as prepared and for KC1
3e
3d
3c 3b la 3a 525
530
Fig. 3. O(ls) XPS spectra of the potassium deposited from water solution (la) or by temperature (3b), 1 h later (3c), after UHV KOH (4) KClO, (6) and KClO, (7) surfaces as used in the description
BE [eV] compounds studied. Oxygen contamination of KC1 sublimation (3a). KC1 treated with OGD at room heating at 570 K (3d) and at 670 K (3e). Spectra of as prepared. Denotations in the figure are the same of surface preparations (section 2).
after each step of experimental treatment. The results are given in table 1 and figs. l-3. Quantitative analysis was carried out according to Seah [lo]. The intensities (peak areas) of K(2p), O(ls), Cl(2p). and C(ls) levels were considered. Scofield’s calculated [ll] subshell ionization cross-sections were used, but for the K(2p) and Cl(2p) bands experimental corrections were derived from measured peak intensities for KClO, and KClO,. The mean free paths were calculated according to Penn [12]. Estimation of the thickness of an oxygen overlayer requires the intensity of the O(ls) peak from an infinitely thick oxygen layer. To that end the measured intensity value of the O(ls) band from Li,O was used.
3. Results and discussion Our experiments were aimed at the study of oxygen interaction with potassium. Therefore, surface studies of KOH, K,CO,, KClO,. and KClO, stable compounds were performed to provide necessary information on core level binding energies. K-O stoichiometry and experimental coefficients for quantitative calculations. Potassium oxides are very active in reaction with Hz0 and show low thermal stability. Thus it is impossible to obtain them in a pure state in open air. That is why we produced potassium oxides in situ in the spectrometer chamber by exposing to oxygen plasma such a stable and hydrophobic compound as KCl. In fact, the spectra of pure KC1 showed a very low (-c 3 at%) oxygen content. In the KC1 samples prepared through crystallization from aqueous solution the binding energy of O(ls) from this oxygen contamination was 531.6 eV, which corresponds well to the 531.6 eV in KOH. In the case of KC1 deposited by sublimation the O(ls) peak was found at 528.8 eV, while the O/K,,, ratio was about 0.4. This binding energy was assigned to K,O. These results indicate that during crystallization a small amount of KC1 reacts with water KC1 + H,O 2 KOH + HCl and the chemical equilibrium is slightly shifted to the right because of evaporation of HCl perhaps during drying of the deposit. We found about 3 at% of oxygen in the surface layer. Having detected K,O in the sublimated deposit we have to assume that KC1 vapour reacts with oxygen 2 KCl+:
O,+K,O+Cl,,
or with water 2 KCl+2H,O+2
KOH+2
HCl.
J. Stoch, M. Ludecka / KCI surface oxidation in OGD
followed
433
by dehydration
2 KOH + K,O + H,O. In this case we found also about 3 at% of oxygen in the surface layer. Although our KC1 samples were prepared in open air, the surfaces did not contain any measurable amounts of carbonates. The KOH samples however were considerably contaminated with carbonates; regardless of neither another peak contribution nor any broadness being observed in the K(2p) and O(ls) bands. Thus we conclude that the binding energies of these bands for KOH and K,CO, are the same within the error limits (k 0.1 eV>. For potassium compounds, determination of the chemical state of chlorine from XPS binding energies is simple because their measured values distinctly differ (see table 1). We assumed the BE of K(2p,,,) to be 293.6 eV from our measurements of the highly carbon-contaminated KC1 sample. The same value has been found by Bonzel et al. [13]. So the Cl(2p) band in KC1 appeared at 199.2 eV. Our values remain in good agreement with the literature data [14-171. Because of the low polarizability of the Kt and Cl- ions we expect that these values should not be distinctly shifted when KC1 is present in other matrixes. For KClO, and KClO, chlorate and perchlorate Cl(2p) peaks appeared at 206.9 and 208.7 eV respectively, while that of chloride was fixed at 199.2 eV. The values of the O(ls) core level binding energy in KClO, and KClO, were found to be 532.6 and 532.5 eV respectively. See figs. 2 and 3. After the oxygen glow discharge treatment the KC1 surface composition was changed. The intensity of the Cl(2p) peak decreased. The O(ls) band increased considerably and was shifted towards higher binding energies as shown in figs. 2 and 3. The highest measured binding energy of the O(ls) level was 533.1 eV. It has been observed that at room temperature under our experimental conditions the intensity of the O(ls) peak decreased with time and its BE value was shifted back to lower energies (see fig. 3 and table 1). This fact illustrates the low vacuum stability of the produced oxygen compound. With heating in vacuum its decomposition proceeds even faster. Various degrees of decomposition can be seen in the O(ls) band in fig. 3(3a-3e). In a glow discharge in oxygen various oxygen species are present, for example highly reactive 03, 0; and O- [18-201. Even though the oxygen used was dried in a liquid nitrogen trap one cannot preclude the possible presence of very small amounts of H,O or OH- from water traces, which can always be found in the spectrometer chamber. However, since the vast majority of species are oxygen ions, we expected an exchange of lattice Cl- ions with oxygen to be the main process. The BE of the O(ls) peak from plasma oxidized surface was in the range 532.8-533.1 eV. This value is considerably higher than the 531.6 eV measured for KOH. On the other hand, the decreasing intensity and associated shift
towards a value as low as 528.6 eV exclude KOH as a principal component of the oxygen overlayer. Shim and Wittig [21] studied the interaction between NaCl crystals and oxygen afterglow. Under the conditions of their experiment an oxygen compound was obtained, which by means of IR measurements was reported to be NaClO, formed in the possible process: 0 + O2 + NaCl
--)
NaClO,.
Although formation of potassium chlorates cannot be totally excluded. no experimental evidence was found for their presence. The Cl(2p) peak at 206.0-209.0 eV was not observed. In an independent experiment we found KClO, to be stable in vacuum at room temperature. Even assuming its presence and decomposition, the only stable end product would be KC1 but not an oxide, while in our experiment a stable oxide at 528.6 eV was formed as the end product. Also carbonate could not be considered because of its high stability at room temperature. On the other hand, an increase in the C(ls) band at 289 eV has not been observed. Therefore, relying on this experimental evidence, we conclude that under oxygen glow discharge treatment a higher potassium oxide is formed. Different potassium oxides are known. The existence of superoxide KOz is well documented, though disuperoxide KO, can also be obtained [22]. The O/K,, ratio in our experiment with glow discharge was changing in time. The XPS measurement was carried out after about 1 h after exposure of the sample to oxygen plasma. Therefore, the highest value observed (O/K,,, = 2.4) indicates that its initial value was higher. Taking that into account we can suppose that in direct contact with OGD the KC1 surface is oxidized to KOd which under vacuum at room temperature decomposes quickly to KOz and afterwards to K,O, and K,O. The presence of K,O as the end product is ratio of 0.4 and BE as low as 528.6 eV. confirmed by the estimated O/K,, For the samples deposited from water solution we were not able to distinguish between possible KOH and K,Oz as the end product after vacuum thermal decomposition. From all our measurements we have determined binding energies for potassium oxides which are given in table 2. The values for O’- and 0, agree well with the data found by Ayyoob and Hegde [23,24]. Our results can be useful for the interpretation of XPS spectra in studies of the chemical state of oxygen in oxides. We have shown that 0, species can be formed at an oxide surface and they can be identified and studied by the XPS method. Moreover. the ability of potassium to form easily superoxides with gaseous oxygen was demonstrated. This fact is of some importance for studies of hydrocarbons oxidation catalysts containing potassium. As it was suggested [25] ethylene can be oxidized to ethylene oxide at dioxygen molecules housed in meshes of the oxygen-silver surface 2D struc-
J. Stoch, h4. Ladeckn Table 2 Measured
binding
energies
/ KCI surface oxidation WI OGD
of O(ls) in potassium
oxides and A BE a)
Compounds
Species
Binding
K2O
02..
528.6 531.6-532.0 532.8 z 533.0
KGH,
W202)
(O;-),
KC’2
0;
KG,
(0,))
‘) ABE = the difference ion.
between
OH-
the electronic
435
energies (eV)
state of oxygen
ABE 0 3.0-3.4 4.2 > 4.4
in an oxide and the state of 02-
ture, e.g. Ag(lOO)c(2 x 2)0. The role of this structure is to keep oxygen molecules vertically, i.e., to create the ability of oxygen molecules to react with ethylene molecules for epoxidation. Thus a substitution of oxygen with chlorine atoms strengthens the structure and, as it is well known, increases the selectivity. But the selectivity can also be enhanced by promoting Ag surface with potassium. It has recently been shown [24] that dioxygen species are stabilized on potassium-dosed Ag sufaces. When chlorine is adsorbed prior to oxygen, only molecular dioxygen is bonded at the surface [24]. This result agrees well with the above-mentioned model and leads to the conclusion that the role of potassium is to supply the reacting system with dioxygen active species being one of the substrates of epoxidation.
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Ne&