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Some aspects of the surface chemistry of coal, kerogen and bitumen as revealed by ESCA
Org. Geochem. Vol. 6, pp. 455-461, 1984
0146-6380/84 $03.00+00.00 Copyright © 1984 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
Some aspects of the surface chemistry of coal, kerogen and bitumen as revealed by ESCA DAVID T. CLARK* and ROSEMARY WILSONt
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K.
A b s t r a c t - - T h e application of ESCA (electron spectroscopy for chemical applications) to the study of the structure, bonding and reactivity of coals, kerogens and bitumen samples has been examined, with particular reference to torbanite, vitrinite, Kimmeridge kerogen, brown coal and Gilsonite pitch. The ESCA valence band spectra and Ct~ band profiles of a rank range of coal samples are discussed. Differences in Ch signal between naturally occurring heat altered coal and the unaltered coal are reported. The surface sensitivity of the ESCA experiment reveals enriched silicon and aluminium concentrations on the cleaved surfaces of grahamite bitumen, relative to bulk ESCA analyses. Surface oxidation of anthracite through atmospheric weathering is found to be amenable to study by ESCA and substantial oxidation of the coal occurs after an exposure period of one week. Artificiallyinduced oxidation by means of low power radiofrequency oxygen plasma treatments have been performed. In the case of Gilsonite pitch the distribution of oxidized carbon functionality is found to differ from that reported during the irradiation of Gilsonite in u.v. light (254 nm) in air.
Direct elemental analysis of all elements except hydrogen is accomplished by the measurement of absolute binding energies and relative intensities of the photoelectron peaks. The electronic environment of the constituent elements is reflected by the shift in binding energy of a given core level relative to that of a standard. In this way information on the oxidation state of elements and on functional group distribution for organic substances may be obtained. ESCA is unable to distinguish directly between aliphatic and aromatic carbon, or between c a r b o n in p r i m a r y , s e c o n d a r y or t e r t i a r y environments using the C~ photoionization signal alone. However, the indirect interrogation of c o n j u g a t e d systems is possible from detailed examination of Ir --+ 7r* shake-up satellites (Clark, 1976). Valence band spectra have been found to reflect subtle differences associated with purely carbon-containing organic polymer chains, for example (Pireaux et al., 1981). The main feature of the ESCA technique, which has been the underlying key to its popularity amongst scientists and technologists in a wide range of areas, is its surface sensitivity. This arises from the inherently short mean free paths (and hence sampling depths) for photoelectrons in the kinetic energy range 0-1500 eV associated with p h o t o i o n i z a t i o n with the commonly employed A#,~:~,,.,_ and MgK~,._, soft X-ray sources (Clark, 1978). Hence, employing MgK~._, X-radiation (hv = 1253.6 eV) with a typical sampling depth ~ 50 /~, allows the investigation of the outermost few monolayers of a solid.
INTRODUCTION
In previous articles (Wilson, 1981; Clark et al., 1983; Clark and Wilson, 1983a), we have applied and e v a l u a t e d the p o t e n t i a l of E S C A ( E l e c t r o n Spectroscopy for Chemical Applications), also known as XPS (X-ray Photoelectron Spectroscopy), in the characterization of materials of interest to the organic geochemist. The promise of ESCA, a technique which had received surprisingly little attention by either coal (Frost et al., 1978; Brown et al., 1981), kerogen or bitumen scientists, has prompted further studies in this area. It is the purpose of this paper to present an overview of these new topics of research. The material to be reported will illustrate the suitability of ESCA to study coal, kerogen and bitumen not only in their unaltered form but also after modification, e i t h e r n a t u r a l l y occurring, as effected through thermal alteration and by weathering processes, or artificially induced, as exemplified by plasma oxidation reactions. First it is useful to describe in brief the essentials and scope of the ESCA experiment. Accounts of ESCA investigations of the structure, bonding and reactivity of organic, biological and polymeric systems serve as a useful background to the following discussion (Clark 1973, 1976, 1978). The E S C A technique E S C A involves the measurement of binding energies of electrons ejected by interactions of a molecule with a monoenergetic beam of soft X-rays. *Present address: New Science Group, Imperial Chemical Industries pie, P.O. Box 11, The Heath, Runcorn, Cheshire WA7 4QE. U.K. tPresent address of author to whom correspondence should be addressed: Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, U.S.A.
EXPERIMENTAL
Samples The samples used in this study include torbanite (Bathgate, Lothian), vitrinite (Babbington, 455
456
DAVID T. CLARKand ROSEMARYWILSON
Nottingham), Kimmeridge kerogen (Kimmeridge Bay, Dorset, a type II kerogen) and brown coal (Miocene Age, Indonesia), and in addition naturally occurring bitumens, Gilsonite 'selects' (Eocene, Uinta Basin, Utah, U.S.A.), Grahamite (Sardis, Oklahoma, U.S.A.). Also used are coal samples from the Lower Kittanning seam in Ohio and Pennsylvania, U.S.A. from the Pennsylvania State University Coal Bank (PSOC Nos 308, 1017 and 1133). Full descriptions of these samples are given elsewhere (Clark et al., 1983; Clark and Wilson, 1983a). Samples of heat altered coal from the Togston open-cast site, adjacent to the Acklington Dyke, Northumberland, and anthracite (Pumpquart vein, Cynheidre Colliery, South Wales) were obtained from the National Coal Board of Great Britain. The sample of graphite was obtained from Union Carbide.
Sample preparation The samples were prepared for ESCA analysis by grinding a portion of the material to a fine powder using a pestle and mortar. The freshly powdered sample was mounted on the spectrometer probe tip by means of double-sided adhesive Scotch insulating tape as described previously (Clark et al., 1983).
Natural weathering A freshly prepared sample, as described above, was placed outdoors in a south facing aspect at 60°C to the horizontal, and allowed to weather for 1 week in Durham during the period 6-13 August 1983 under bright and dry conditions.
Plasma oxidation Samples, prepared for ESCA analysis, were exposed to low power inductively coupled radiofrequency oxygen plasma (0.4 and 10 W, 0.2 torr) for varying periods of time, using the experimental plasma rig and conditions reported previously (Clark and Wilson, 1983b).
ESCA analysis The ESCA spectra were recorded using either AEI ES200B or Kratos ES300 instrumentation operating at typically 12 kV and 15 mA using MgK,~,_, X-radiation and with a base pressure of ~ 5 × 10-s torr. Under the experimental conditions employed for the magnesium anode, the gold 4f7/2level (at 84 eV binding energy) used for calibration purposes had a full width at half maximum height (FWHM) of ~ 1.15 eV. The ES300 has a higher energy photon source (TiK~ ,, hv = 4510 eV) which samples the outermost 130fi~ of a solid (compared with - 50/~ for the M g K ~ X-rays), thereby allowing a means of nondestructive depth profiling (Clark et al., 1982). The titanium anode was operated at 13.5 kV and 18 mA. A complete ESCA analysis took in the region of 2 h to complete; the core level spectra of the elements
carbon, oxygen, nitrogen and sulphur were recorded in typically 1 h, depending on the intensity of the respective photoionization signals. Radiation damage to the sample from long-term exposure to the X-ray beam was not evident. Deconvolution and area ratios were determined, in the case of the ES200B spectra, using a Dupont 310 analogue curve resolver as described by Clark (1973); the ES300 spectrometer is equipped with a Kratos DS300 data acquisition/manipulation system. The accuracy of measurement is of the order of - 5%.
RESULTS AND DISCUSSION
Core level spectra The nature of the information levels available in the ESCA experiment applied to geochemical materials is illustrated in Fig. 1 which shows the core level signals for CL~, Ot~, N~ and S2p levels for Gilsonite 'selects', Kimmeridge kerogen, vitrinite and brown coal. Visual inspection reveals the differences in surface chemistry in respect of nitrogen, oxygen and sulphur functionalities. Perhaps the most striking feature arises from the comparatively high concentration of sulphur in Kimmeridge kerogen. Here ESCA distinguishes between sulphur present in at least two oxidation states, as identified by their absolute binding energies as sulphur in - 2 and +6 oxidation states, the latter component occurring at higher binding energy (Wagner etal., 1979). This contrasts with the case for vitrinite concentrate where the sulphur level is much lower and exists almost exclusively in an unoxidized form as organic sulphide. Sulphur was not detected in the ESCA spectrum of brown coal. Deconvolution of the Ct~ photoionization envelopes reveal component peaks to the high binding energy side of the hydrocarbon component at 285 eV binding energy. These peaks are characteristic of functionalized carbon, and from extensive characterization of simple model compounds (Clark, 1978; Clark and Harrison, 1981) the following assignment can be made: C-N 286.0 eV, C-O 286.6 eV, C=O 288.0 eV and COOH 289.2 eV. There may also be a contribution to this high binding energy shoulder arising from shake-up satellites associated with conjugated structure (Clark. 1976). Relative area ratios may be used to quantify surface chemistry (Clark, 1973). Whilst the Ot~ core level signal will consist of contributions from both the organic and mineral phases (Wagner et al., 1979) it is possible to estimate ESCA organic oxygen concentrations. This may be achieved by subtracting mineral oxygen away from the total oxygen, taking the mineral oxygen to be bound with silicon and aluminium as SiO2 and A/~203, respectively (Clark and Wilson, 1983a). The limitations of this approach have been discussed elsewhere (Wilson, 1984).
Surface chemistry of coal, kerogen and bitumen
457
Gilsonite 'selects'
L
xlO
xloo
xlO
Kimmeridgekerogen xlO
/C 3
Vi-trinite xlO
~
00
Brown coat
x I00
291 289 285 C~s
537 533 529
O,s
•
403
" 399
:395
N,~ Binding energy(eV}
168
164
160
S2p
Fig. 1. Ct~, Or,, Nt, and S2p core level signals for Gilsonite 'selects', Kimmeridge kerogen, vitrinite and brown coal.
The ability of ESCA to monitor demineralization procedures, such as are commonly employed in kerogen isolation from sediments (Durand and Ni~aise, 1980), is illustrated in Fig. 2 which shows wide scan (1200 eV) spectra of torbanite, before and after wet chemical demineralization. The untreated torbanite spectrum is dominated by the OL~ photoelectron signal. Contributions from the silicon and aluminium component of the mineral matrix are clearly evident. This is in contrast to the spectrum for demineralized torbanite in which the Cu peak predominates over a greatly reduced Ot~ component, corresponding almost exclusively to organic oxygen: the silicon and aluminium are not visible at the sensitivity at which the spectrum was recorded. The surface sensitivity of ESCA is such that the fracture surface composition of cleaved Grahamite bitumen has been studied. Hence low levels of silicon (Si2p 104.8 eV) and aluminium (A(2p 76.3 eV) were clearly discernible whereas the ESCA spectrum of the powdered Grahamite detected no mineral content. The elemental composition of the fracture
surface, as determined by ESCA, is Ct0o N1.3 05.5 S0.6 Sio.3 A~0.3, and this shows an expected enrichment of surface (mineral) oxygen over the composition of Cl(x) N1.3 04.6 50.6 for the powdered (bulk) material.
Valence band signals Little attention has been paid to the study of ESCA valence band spectra, despite their ability to depict directly the bonding within molecules (Pireaux et al., 1981). The valence band region (0-50 eV binding energy) has been shown to be of particular value for distinguishing between skeletally isomeric polymer systems which exhibit similar overall core level band profiles (e.g. Clark, 1978). The valence energy levels are able to provide a 'fingerprint' for a given polymeric structure which may then be compared with a p p r o p r i a t e m o d e l c o m p o u n d s . This methodology has been applied to the study of a rank range of coals, from brown coal to anthracite. The relevant spectra are displayed in Fig. 3: the valence band spectrum of graphite is included for the sake of
458
DAVID T. CLARKand ROSEMARYWILSON
015
I TORBANITE
DEMINERALISED
TORBANITE
2O0
400
60O
80O
1000
1200
KINETIC ENERGY(eV)
Fig. 2. ESCA wide scan spectra of untreated and demineralized torbanite.
comparison. Each valence level takes the form of an unresolved band profile since there are a large number of occupied levels compressed into a narrow range. Even so, there is a clear rank-dependent trend in the coal valence band traces. The Oz~ band centred at - 28 eV binding energy diminishes in intensity with increasing rank. The structure to the lower binding energy side of the O2~ signal will reflect the environments of the carbon valence electrons. Clearly evident from the form of the valence band spectra displayed in Fig. 3 is the progressive extension of the occupied levels towards the Fermi edge in going from brown coal to graphite. Ct~ photoionization signals It is of interest to note that the line-width of the individual photoionization peaks of the C~ core level envelopes show a narrowing as coal rank increases. Hence the F W H M of brown coal is ~ 2.1 eV, whilst for the 84.5%C (dmmf) coal and for anthracite the corresponding values are - 1.8 and ~ 1.5 eV, respectively. The graphite Ct~ core level peak has a F W H M of -- 1.2 eV. (These values were obtained for spectra recorded on the ES300 spectrometer with a MgK~,., X-ray source.) There are also concomitant decreases in sample charging during analysis, a feature associated with insulating materials (Clark, 1978), and in the absolute binding energy of the predominant Cls signal. These factors may be attributable to the increase in electrical conductivity
VALENCEBAND
~Brovn ~ / ~
Coal
~oe0.2 % c dmmf
~ = 90.7% C
~,, Anthracite ~ firophife /,0 20 0 BE. (eV} Fig. 3. Valence band spectra for a rank range of coals and for graphite.
of coal with increasing coal rank (Van Krevelen, 1961). These trends also exist for the ESCA spectra of naturally occurring heat altered coal. Figure 4 shows the C~ core level signals of coal located in the proximity of the Acklington Dyke, Northumberland, U.K. (Robson, 1980) and a coal sample from the
459
Surface chemistry of coal, kerogen and bitumen
,
ACKLINSTONDYKECOAL
ANTHRACITE
Cls
CIs
,
,
,
,
,
,
,
,
293
289 285 281 B.E. (eV) Fig. 4. The Cb core level spectra of Acklington Dyke heat altered coal (FWHM - 1.3 eV) and its unaltered counterpart (FWHM - 1.8 eV).
same seam which was unaffected by the igneous intrusion.
Surface oxidation The oxidation of geochemical materials, and of coals in particular, has attracted much interest from both geochemists, as an aid to structural elucidation (Vitorovic, 1980), and coal technologists, concerned with the deteriorative effect of low level oxidation in the industrial properties of coal, e.g. coking power (Van Krevelen, 1961). The ability of ESCA to detect low level surface functionalization where bulk analytical techniques would detect no change, relative to the bulk, lends the technique to coal oxidation studies.
Natural weathering Figure 5 shows the MgK,~_, CL~, envelopes of anthracite coal before and after atmospheric weathering for I week. There is a noticeable increase in oxidized carbon content in the surface regions of the coal on weathering. TiK~L2spectra show that this level of oxidation extends through the 130 sampling depth of the harder X-ray source. A low level SiEp peak (~ 1 at. %) is observed in the ESCA spectrum for the weathered coal arising from extraneous atmospheric contamination. Deconvolution of the CI~ signal reveals (2-0 (286.4 eV), C = O (288.0 eV) and C ~ O (289.3 eV) com"O ponents, their respective intensities (relative to the total Cls peak intensity) being 15.3, 6.4 and 5.3%. Wilson (1984) has discussed the ability of ESCA to distinguish carbon-oxygen functional groups in more detail.
"Fresh"
II
C: 0 =100:5
"Weathered" I week
/
E: 0 =100:18
293 289 285 281 BE. (eV) Fig. 5. The effect of weathering on the Ct~ core level spectrum of a Welsh anthracite.
Plasma oxidation The use of plasma chemistry in the field of organic geochemistry has focused on two main areas: first, the low temperature ashing of carbonaceous materials and the subsequent analysis of the mineral residue (Miller et al., 1979); and second, the plasma p y r o l y s i s a n d g a s i f i c a t i o n of fossil fuels (Venugopalan et al., 1980). Here we report in brief the changes in surface chemistry of a variety of carbonaceous materials subjected to oxygen plasma treatments. A detailed account is given elsewhere (Wilson, 1981). The CL~, OLd, N/s and S2p c o r e level spectra of Gilsonite 'selects' exposed to oxygen plasma (0.4 W, 0.2 torr) as a function of exposure time are displayed in Fig. 6. Progressive uptake of oxygen is evident as treatment time increases, with Little change in nitrogen level or functionality (Fig. 7). The low level of sulphur shows signs of oxidation, the S2p band broadening to higher binding energy. Deconvolution of the Cls envelopes reveals the formation of C-O, O C=O, C ~ O and O-(7-O functional groups in a different distribution to that found for the air oxidation of Gilsonite 'selects' initiated by u.v. light (254 nm) (Clark and Wilson, 1983a). The effects of oxygen plasma treatment are limited to the outermost few monolayers of solid (Wilson, 1981). The reactivity of other geochemical materials towards oxygen plasma has been examined. All materials show oxidation of the Cls and S2t, peaks: this oxidation is more extensive at higher power (Fig. 8). The trend to emerge is that the greater the proportion of unoxidized carbon in the untreated material, the greater will be the change in oxygen level, relative to
460
DAVID T. CLARKand ROSEMARYWILSON
¢~ ," k~eC~' 25 ~
,~
~\~e
o . . . . . 289 285
,, 281
, , , , • 5 3 7 5,33 5 2 9
Cis
, , , , , 404 400 396
Ois
, , ,. . . . . 1"/'2 168 164
Nis ( x l O )
160
SZp ( x l O 0 )
Binding energy (eV) Fig. 6. Core level spectra of OilsonJte 'selects' vs time of exposure to an oxygen plasma (0.4 W, 0.2 torr). GILSONITE Oxygen Plasma [0.4W, 0-2tort ] Atom %
20
10 ~ . .•
.
(~)
~
~
•
1'0
1'
NIs
,
2'0
2'5
Time (secs) Oxygen
Fig. 7. Oxygen and nitrogen surface concentrations of gilsonite 'selects' as a function of oxygen plasma (0.4 W, 0.2 tort) exposure time.
the oxygen level in the u n t r e a t e d sample, on exposure to oxygen plasma. This is quite reasonable since there will be more available sites at which reaction could occur for a material such as Gilsonite, with predominantly hydrocarbon character, than for brown coal or a material already containing a substantial proportion of oxidized carbon prior to oxygen plasma treatment.
plasma treatment of geochemical m a t e r i a l s
tntensity Cls 285eV B E (untreated) 90-
80-
G
O K
•
W
70-
60-
50-
1S I n t e n s i t y
G
Gilsonit e
WG
Weathered gilsonde
'selects"
K
Kirnmeridge kerogen
V
VitrJnite
BC
BrOwn
DT
DerninerQlised t o r b o n i t e
T
Untreated
o •
0.4W 5sec lOW 5sec
cool torbQnlte
rQbo t tre~3ted
Fig. 8. A measure of the oxygen uptake of a series of geochemical materials exposed to oxygen plasma (0.2 torr, 5 s).
Surface chemistry of coal, kerogen and bitumen In contrast, plasma leads functionality carbonaceous
the reductive nature of the hydrogen to a decrease in oxidative carbon in the s u r f a c e r e g i o n s of t h e s e materials (Wilson, 1981).
CONCLUSIONS This paper has demonstrated the capability of E S C A to probe the surface chemistries of a wide range of geochemical materials. E S C A may provide a wealth of information on the structure, bonding and reactivity of these materials as studied in their solid state, with minimal sample preparation. The surface s e n s i t i v i t y of this s e m i q u a n t i t a t i v e a n a l y t i c a l technique is of special value in the characterization of surface specific reactions. This has been exemplified by studies of surface oxidation reactions, both naturally occurring through weathering processes and artificially i n d u c e d . E S C A may p r o v i d e information, not only on the extent of reaction, but also on the chemical bonding at the reacted surface.
Acknowledgements--The authors wish to thank Dr A. G. Douglas and Dr P. F. V. Williams (University of Newcastle-upon-Tyne), Dr P. H. Given (Pennsylvania State University, PA, U.S.A.), Dr J. M. Hunt (Woods Hole Oceanographic Institution, Woods Hole, MA, U.S.A.), Mr G. J. Liposits, Dr A. Mills and Mr K. Philpot (National Coal Board, U.K.) and Morris Ashby Ltd for their generous gift of materials. Thanks are due to Dr J. M. Jones and Dr P. M. R. Smith (University of Newcastle-upon-Tyne) and to Dr J. M. E. Quirke (Florida International University, Miami, U.S.A.) for their help and advice. The SERC are to be acknowledged for provision of equipment, and for the award of a research studentship to one author (R.W.).
REFERENCES
Brown J. R., Kronberg B. I. and Fyfe W. S. (1981) Semiquantitative ESCA examination of coal and coal ash surfaces. Fuel 60, 439-446. Clark D. T. (1973) Chemical aspects of ESCA. In Electron Emission Spectroscopy (Edited by Dekeyser W. and Riedel D.), pp. 373-507. D. Riedel, Dordrecht. Clark D. T. (1976) Some chemical applications of ESCA. In Molecular Spectroscopy (Edited by West A. R.), pp. 339-382. Heyden, Philadelphia. Clark D. T. (1978) ESCA applied to organic and polymeric systems. In Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy (Edited by Briggs D.), pp. 211-247. Heyden, Philadelphia. Clark D. T. and Harrison A. (1981) ESCA applied to polymers. XXXI. A theoretical investigation of molecular core binding and relaxation energies in a series of prototype systems for nitrogen and oxygen
461
functionalities in polymers. J. Polym. Sci. Polym. Chem. Educ. 19, 1945-1955. Clark D. T. and Wilson R. (1983a) ESCA applied to aspects of coal surface chemistry. Analytical Methods for Coals, Cokes and Carbons Conference, London, UK, 28-29 April 1983. Fuel 62, 1034-1040. Clark D. T. and Wilson R. (1983b) Selected surface modification of polymers by means of hydrogen and oxygen plasmas. J. Polym. Sci. Polym. Chem. Educ. 21, 837-853. Clark D. T., Abu-Shbak M. M. and Brennan W. J. (1982) Electron mean free paths as a function of kinetic energy: a substrate overlayer investigation of polyparaxaylylene films on gold using a Ti Ks X-ray source. J. Electron Spectrosc. Rel. Phenom. 28, 11-21. Clark D. T., Wilson R. and Quirke J. M. E. (19831 An evaluation of the potential of ESCA (electron spectroscopy for chemical applications) (and other spectroscopic techniques) in the surface and bulk characterization of kerogens, brown coal and gilsonite. Chem. Geol. 39, 215-239. Durand B. and Ni~aise G. (1980) Procedures for kerogen isolation. In Kerogen: Insoluble Organic Matter ,t?orn Sedimentary Rocks (Edited by Durand B.), pp. 35-54. Editions Technip, Paris. Frost D. C., Wallbank B. and Leeder W. R. (1978) X-ray photoelectron spectroscopy of coal and coal related problems. In Analytical Methods for Coal and Coal Products (Edited by Karr C. Jr), Vol. 1. pp. 349-379. Academic Press, London. Miller R. N., Yarzab R. F. and Given P. H. (1979) Determination of the mineral matter contents of coals by low-temperature ashing. Fuel 58, 4-1(I. Pireaux J. J.. Riga J., Caudano R. and Verbist J. (19811 Electronic structure of polymers. X-ray photoelectron valence band spectra. In Photon, Electron and Ion Probes of Polymer Structure and Properties (Edited by Dwight D. W., Fabish T. J. and Thomas H. R.), pp. 169-2111. ACS Symposium Series. No. 162. Washington. Robson D. A. (ed.) (19801 The Geology of North East England.The Natural History Society of Northumbria, J. and P. Bealls Ltd, Newcastle-upon-Tyne. Van Krevelen D. W. (1961) Coal. Elsevier, Amsterdam. Venugopalan M., Roychowdhury U. K., Chan K. and Pool M. L. (1980) Plasma chemistry of fossil fuels. In Topics in Current Chemistry (Edited by Veprek S. and Venugopalan M.), Vol. 90, pp. 1-57. Springer-Verlag, Berlin, Vitorovic D. (198(/) Structure elucidation of kerogen by chemical methods. In Kerogen: Insoluble Organic Matter from Sedimentary' Rocks (Edited by Durand B.), pp. 301-338. Technip, Paris. Wagner C. D., Riggs W. M., Davis L. E., Moulder J. F. and Muillenberg G. E. (1979) Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corporation, Physical Electronics Division. Minnesota. Wilson R. (1981) ESCA applied to a study of some materials of geochemical interest. M.Sc. Thesis. University of Durham, U.K. Wilson R. (1984) Surface and bulk characterisation of selected coals, bitumens and polymers as revealed by ESCA and other analytical techniques., Ph.D. Thesis, University of Durham, U.K.
0146-6380/84 $03.00+00.00 Copyright © 1984 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
Some aspects of the surface chemistry of coal, kerogen and bitumen as revealed by ESCA DAVID T. CLARK* and ROSEMARY WILSONt
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K.
A b s t r a c t - - T h e application of ESCA (electron spectroscopy for chemical applications) to the study of the structure, bonding and reactivity of coals, kerogens and bitumen samples has been examined, with particular reference to torbanite, vitrinite, Kimmeridge kerogen, brown coal and Gilsonite pitch. The ESCA valence band spectra and Ct~ band profiles of a rank range of coal samples are discussed. Differences in Ch signal between naturally occurring heat altered coal and the unaltered coal are reported. The surface sensitivity of the ESCA experiment reveals enriched silicon and aluminium concentrations on the cleaved surfaces of grahamite bitumen, relative to bulk ESCA analyses. Surface oxidation of anthracite through atmospheric weathering is found to be amenable to study by ESCA and substantial oxidation of the coal occurs after an exposure period of one week. Artificiallyinduced oxidation by means of low power radiofrequency oxygen plasma treatments have been performed. In the case of Gilsonite pitch the distribution of oxidized carbon functionality is found to differ from that reported during the irradiation of Gilsonite in u.v. light (254 nm) in air.
Direct elemental analysis of all elements except hydrogen is accomplished by the measurement of absolute binding energies and relative intensities of the photoelectron peaks. The electronic environment of the constituent elements is reflected by the shift in binding energy of a given core level relative to that of a standard. In this way information on the oxidation state of elements and on functional group distribution for organic substances may be obtained. ESCA is unable to distinguish directly between aliphatic and aromatic carbon, or between c a r b o n in p r i m a r y , s e c o n d a r y or t e r t i a r y environments using the C~ photoionization signal alone. However, the indirect interrogation of c o n j u g a t e d systems is possible from detailed examination of Ir --+ 7r* shake-up satellites (Clark, 1976). Valence band spectra have been found to reflect subtle differences associated with purely carbon-containing organic polymer chains, for example (Pireaux et al., 1981). The main feature of the ESCA technique, which has been the underlying key to its popularity amongst scientists and technologists in a wide range of areas, is its surface sensitivity. This arises from the inherently short mean free paths (and hence sampling depths) for photoelectrons in the kinetic energy range 0-1500 eV associated with p h o t o i o n i z a t i o n with the commonly employed A#,~:~,,.,_ and MgK~,._, soft X-ray sources (Clark, 1978). Hence, employing MgK~._, X-radiation (hv = 1253.6 eV) with a typical sampling depth ~ 50 /~, allows the investigation of the outermost few monolayers of a solid.
INTRODUCTION
In previous articles (Wilson, 1981; Clark et al., 1983; Clark and Wilson, 1983a), we have applied and e v a l u a t e d the p o t e n t i a l of E S C A ( E l e c t r o n Spectroscopy for Chemical Applications), also known as XPS (X-ray Photoelectron Spectroscopy), in the characterization of materials of interest to the organic geochemist. The promise of ESCA, a technique which had received surprisingly little attention by either coal (Frost et al., 1978; Brown et al., 1981), kerogen or bitumen scientists, has prompted further studies in this area. It is the purpose of this paper to present an overview of these new topics of research. The material to be reported will illustrate the suitability of ESCA to study coal, kerogen and bitumen not only in their unaltered form but also after modification, e i t h e r n a t u r a l l y occurring, as effected through thermal alteration and by weathering processes, or artificially induced, as exemplified by plasma oxidation reactions. First it is useful to describe in brief the essentials and scope of the ESCA experiment. Accounts of ESCA investigations of the structure, bonding and reactivity of organic, biological and polymeric systems serve as a useful background to the following discussion (Clark 1973, 1976, 1978). The E S C A technique E S C A involves the measurement of binding energies of electrons ejected by interactions of a molecule with a monoenergetic beam of soft X-rays. *Present address: New Science Group, Imperial Chemical Industries pie, P.O. Box 11, The Heath, Runcorn, Cheshire WA7 4QE. U.K. tPresent address of author to whom correspondence should be addressed: Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, U.S.A.
EXPERIMENTAL
Samples The samples used in this study include torbanite (Bathgate, Lothian), vitrinite (Babbington, 455
456
DAVID T. CLARKand ROSEMARYWILSON
Nottingham), Kimmeridge kerogen (Kimmeridge Bay, Dorset, a type II kerogen) and brown coal (Miocene Age, Indonesia), and in addition naturally occurring bitumens, Gilsonite 'selects' (Eocene, Uinta Basin, Utah, U.S.A.), Grahamite (Sardis, Oklahoma, U.S.A.). Also used are coal samples from the Lower Kittanning seam in Ohio and Pennsylvania, U.S.A. from the Pennsylvania State University Coal Bank (PSOC Nos 308, 1017 and 1133). Full descriptions of these samples are given elsewhere (Clark et al., 1983; Clark and Wilson, 1983a). Samples of heat altered coal from the Togston open-cast site, adjacent to the Acklington Dyke, Northumberland, and anthracite (Pumpquart vein, Cynheidre Colliery, South Wales) were obtained from the National Coal Board of Great Britain. The sample of graphite was obtained from Union Carbide.
Sample preparation The samples were prepared for ESCA analysis by grinding a portion of the material to a fine powder using a pestle and mortar. The freshly powdered sample was mounted on the spectrometer probe tip by means of double-sided adhesive Scotch insulating tape as described previously (Clark et al., 1983).
Natural weathering A freshly prepared sample, as described above, was placed outdoors in a south facing aspect at 60°C to the horizontal, and allowed to weather for 1 week in Durham during the period 6-13 August 1983 under bright and dry conditions.
Plasma oxidation Samples, prepared for ESCA analysis, were exposed to low power inductively coupled radiofrequency oxygen plasma (0.4 and 10 W, 0.2 torr) for varying periods of time, using the experimental plasma rig and conditions reported previously (Clark and Wilson, 1983b).
ESCA analysis The ESCA spectra were recorded using either AEI ES200B or Kratos ES300 instrumentation operating at typically 12 kV and 15 mA using MgK,~,_, X-radiation and with a base pressure of ~ 5 × 10-s torr. Under the experimental conditions employed for the magnesium anode, the gold 4f7/2level (at 84 eV binding energy) used for calibration purposes had a full width at half maximum height (FWHM) of ~ 1.15 eV. The ES300 has a higher energy photon source (TiK~ ,, hv = 4510 eV) which samples the outermost 130fi~ of a solid (compared with - 50/~ for the M g K ~ X-rays), thereby allowing a means of nondestructive depth profiling (Clark et al., 1982). The titanium anode was operated at 13.5 kV and 18 mA. A complete ESCA analysis took in the region of 2 h to complete; the core level spectra of the elements
carbon, oxygen, nitrogen and sulphur were recorded in typically 1 h, depending on the intensity of the respective photoionization signals. Radiation damage to the sample from long-term exposure to the X-ray beam was not evident. Deconvolution and area ratios were determined, in the case of the ES200B spectra, using a Dupont 310 analogue curve resolver as described by Clark (1973); the ES300 spectrometer is equipped with a Kratos DS300 data acquisition/manipulation system. The accuracy of measurement is of the order of - 5%.
RESULTS AND DISCUSSION
Core level spectra The nature of the information levels available in the ESCA experiment applied to geochemical materials is illustrated in Fig. 1 which shows the core level signals for CL~, Ot~, N~ and S2p levels for Gilsonite 'selects', Kimmeridge kerogen, vitrinite and brown coal. Visual inspection reveals the differences in surface chemistry in respect of nitrogen, oxygen and sulphur functionalities. Perhaps the most striking feature arises from the comparatively high concentration of sulphur in Kimmeridge kerogen. Here ESCA distinguishes between sulphur present in at least two oxidation states, as identified by their absolute binding energies as sulphur in - 2 and +6 oxidation states, the latter component occurring at higher binding energy (Wagner etal., 1979). This contrasts with the case for vitrinite concentrate where the sulphur level is much lower and exists almost exclusively in an unoxidized form as organic sulphide. Sulphur was not detected in the ESCA spectrum of brown coal. Deconvolution of the Ct~ photoionization envelopes reveal component peaks to the high binding energy side of the hydrocarbon component at 285 eV binding energy. These peaks are characteristic of functionalized carbon, and from extensive characterization of simple model compounds (Clark, 1978; Clark and Harrison, 1981) the following assignment can be made: C-N 286.0 eV, C-O 286.6 eV, C=O 288.0 eV and COOH 289.2 eV. There may also be a contribution to this high binding energy shoulder arising from shake-up satellites associated with conjugated structure (Clark. 1976). Relative area ratios may be used to quantify surface chemistry (Clark, 1973). Whilst the Ot~ core level signal will consist of contributions from both the organic and mineral phases (Wagner et al., 1979) it is possible to estimate ESCA organic oxygen concentrations. This may be achieved by subtracting mineral oxygen away from the total oxygen, taking the mineral oxygen to be bound with silicon and aluminium as SiO2 and A/~203, respectively (Clark and Wilson, 1983a). The limitations of this approach have been discussed elsewhere (Wilson, 1984).
Surface chemistry of coal, kerogen and bitumen
457
Gilsonite 'selects'
L
xlO
xloo
xlO
Kimmeridgekerogen xlO
/C 3
Vi-trinite xlO
~
00
Brown coat
x I00
291 289 285 C~s
537 533 529
O,s
•
403
" 399
:395
N,~ Binding energy(eV}
168
164
160
S2p
Fig. 1. Ct~, Or,, Nt, and S2p core level signals for Gilsonite 'selects', Kimmeridge kerogen, vitrinite and brown coal.
The ability of ESCA to monitor demineralization procedures, such as are commonly employed in kerogen isolation from sediments (Durand and Ni~aise, 1980), is illustrated in Fig. 2 which shows wide scan (1200 eV) spectra of torbanite, before and after wet chemical demineralization. The untreated torbanite spectrum is dominated by the OL~ photoelectron signal. Contributions from the silicon and aluminium component of the mineral matrix are clearly evident. This is in contrast to the spectrum for demineralized torbanite in which the Cu peak predominates over a greatly reduced Ot~ component, corresponding almost exclusively to organic oxygen: the silicon and aluminium are not visible at the sensitivity at which the spectrum was recorded. The surface sensitivity of ESCA is such that the fracture surface composition of cleaved Grahamite bitumen has been studied. Hence low levels of silicon (Si2p 104.8 eV) and aluminium (A(2p 76.3 eV) were clearly discernible whereas the ESCA spectrum of the powdered Grahamite detected no mineral content. The elemental composition of the fracture
surface, as determined by ESCA, is Ct0o N1.3 05.5 S0.6 Sio.3 A~0.3, and this shows an expected enrichment of surface (mineral) oxygen over the composition of Cl(x) N1.3 04.6 50.6 for the powdered (bulk) material.
Valence band signals Little attention has been paid to the study of ESCA valence band spectra, despite their ability to depict directly the bonding within molecules (Pireaux et al., 1981). The valence band region (0-50 eV binding energy) has been shown to be of particular value for distinguishing between skeletally isomeric polymer systems which exhibit similar overall core level band profiles (e.g. Clark, 1978). The valence energy levels are able to provide a 'fingerprint' for a given polymeric structure which may then be compared with a p p r o p r i a t e m o d e l c o m p o u n d s . This methodology has been applied to the study of a rank range of coals, from brown coal to anthracite. The relevant spectra are displayed in Fig. 3: the valence band spectrum of graphite is included for the sake of
458
DAVID T. CLARKand ROSEMARYWILSON
015
I TORBANITE
DEMINERALISED
TORBANITE
2O0
400
60O
80O
1000
1200
KINETIC ENERGY(eV)
Fig. 2. ESCA wide scan spectra of untreated and demineralized torbanite.
comparison. Each valence level takes the form of an unresolved band profile since there are a large number of occupied levels compressed into a narrow range. Even so, there is a clear rank-dependent trend in the coal valence band traces. The Oz~ band centred at - 28 eV binding energy diminishes in intensity with increasing rank. The structure to the lower binding energy side of the O2~ signal will reflect the environments of the carbon valence electrons. Clearly evident from the form of the valence band spectra displayed in Fig. 3 is the progressive extension of the occupied levels towards the Fermi edge in going from brown coal to graphite. Ct~ photoionization signals It is of interest to note that the line-width of the individual photoionization peaks of the C~ core level envelopes show a narrowing as coal rank increases. Hence the F W H M of brown coal is ~ 2.1 eV, whilst for the 84.5%C (dmmf) coal and for anthracite the corresponding values are - 1.8 and ~ 1.5 eV, respectively. The graphite Ct~ core level peak has a F W H M of -- 1.2 eV. (These values were obtained for spectra recorded on the ES300 spectrometer with a MgK~,., X-ray source.) There are also concomitant decreases in sample charging during analysis, a feature associated with insulating materials (Clark, 1978), and in the absolute binding energy of the predominant Cls signal. These factors may be attributable to the increase in electrical conductivity
VALENCEBAND
~Brovn ~ / ~
Coal
~oe0.2 % c dmmf
~ = 90.7% C
~,, Anthracite ~ firophife /,0 20 0 BE. (eV} Fig. 3. Valence band spectra for a rank range of coals and for graphite.
of coal with increasing coal rank (Van Krevelen, 1961). These trends also exist for the ESCA spectra of naturally occurring heat altered coal. Figure 4 shows the C~ core level signals of coal located in the proximity of the Acklington Dyke, Northumberland, U.K. (Robson, 1980) and a coal sample from the
459
Surface chemistry of coal, kerogen and bitumen
,
ACKLINSTONDYKECOAL
ANTHRACITE
Cls
CIs
,
,
,
,
,
,
,
,
293
289 285 281 B.E. (eV) Fig. 4. The Cb core level spectra of Acklington Dyke heat altered coal (FWHM - 1.3 eV) and its unaltered counterpart (FWHM - 1.8 eV).
same seam which was unaffected by the igneous intrusion.
Surface oxidation The oxidation of geochemical materials, and of coals in particular, has attracted much interest from both geochemists, as an aid to structural elucidation (Vitorovic, 1980), and coal technologists, concerned with the deteriorative effect of low level oxidation in the industrial properties of coal, e.g. coking power (Van Krevelen, 1961). The ability of ESCA to detect low level surface functionalization where bulk analytical techniques would detect no change, relative to the bulk, lends the technique to coal oxidation studies.
Natural weathering Figure 5 shows the MgK,~_, CL~, envelopes of anthracite coal before and after atmospheric weathering for I week. There is a noticeable increase in oxidized carbon content in the surface regions of the coal on weathering. TiK~L2spectra show that this level of oxidation extends through the 130 sampling depth of the harder X-ray source. A low level SiEp peak (~ 1 at. %) is observed in the ESCA spectrum for the weathered coal arising from extraneous atmospheric contamination. Deconvolution of the CI~ signal reveals (2-0 (286.4 eV), C = O (288.0 eV) and C ~ O (289.3 eV) com"O ponents, their respective intensities (relative to the total Cls peak intensity) being 15.3, 6.4 and 5.3%. Wilson (1984) has discussed the ability of ESCA to distinguish carbon-oxygen functional groups in more detail.
"Fresh"
II
C: 0 =100:5
"Weathered" I week
/
E: 0 =100:18
293 289 285 281 BE. (eV) Fig. 5. The effect of weathering on the Ct~ core level spectrum of a Welsh anthracite.
Plasma oxidation The use of plasma chemistry in the field of organic geochemistry has focused on two main areas: first, the low temperature ashing of carbonaceous materials and the subsequent analysis of the mineral residue (Miller et al., 1979); and second, the plasma p y r o l y s i s a n d g a s i f i c a t i o n of fossil fuels (Venugopalan et al., 1980). Here we report in brief the changes in surface chemistry of a variety of carbonaceous materials subjected to oxygen plasma treatments. A detailed account is given elsewhere (Wilson, 1981). The CL~, OLd, N/s and S2p c o r e level spectra of Gilsonite 'selects' exposed to oxygen plasma (0.4 W, 0.2 torr) as a function of exposure time are displayed in Fig. 6. Progressive uptake of oxygen is evident as treatment time increases, with Little change in nitrogen level or functionality (Fig. 7). The low level of sulphur shows signs of oxidation, the S2p band broadening to higher binding energy. Deconvolution of the Cls envelopes reveals the formation of C-O, O C=O, C ~ O and O-(7-O functional groups in a different distribution to that found for the air oxidation of Gilsonite 'selects' initiated by u.v. light (254 nm) (Clark and Wilson, 1983a). The effects of oxygen plasma treatment are limited to the outermost few monolayers of solid (Wilson, 1981). The reactivity of other geochemical materials towards oxygen plasma has been examined. All materials show oxidation of the Cls and S2t, peaks: this oxidation is more extensive at higher power (Fig. 8). The trend to emerge is that the greater the proportion of unoxidized carbon in the untreated material, the greater will be the change in oxygen level, relative to
460
DAVID T. CLARKand ROSEMARYWILSON
¢~ ," k~eC~' 25 ~
,~
~\~e
o . . . . . 289 285
,, 281
, , , , • 5 3 7 5,33 5 2 9
Cis
, , , , , 404 400 396
Ois
, , ,. . . . . 1"/'2 168 164
Nis ( x l O )
160
SZp ( x l O 0 )
Binding energy (eV) Fig. 6. Core level spectra of OilsonJte 'selects' vs time of exposure to an oxygen plasma (0.4 W, 0.2 torr). GILSONITE Oxygen Plasma [0.4W, 0-2tort ] Atom %
20
10 ~ . .•
.
(~)
~
~
•
1'0
1'
NIs
,
2'0
2'5
Time (secs) Oxygen
Fig. 7. Oxygen and nitrogen surface concentrations of gilsonite 'selects' as a function of oxygen plasma (0.4 W, 0.2 tort) exposure time.
the oxygen level in the u n t r e a t e d sample, on exposure to oxygen plasma. This is quite reasonable since there will be more available sites at which reaction could occur for a material such as Gilsonite, with predominantly hydrocarbon character, than for brown coal or a material already containing a substantial proportion of oxidized carbon prior to oxygen plasma treatment.
plasma treatment of geochemical m a t e r i a l s
tntensity Cls 285eV B E (untreated) 90-
80-
G
O K
•
W
70-
60-
50-
1S I n t e n s i t y
G
Gilsonit e
WG
Weathered gilsonde
'selects"
K
Kirnmeridge kerogen
V
VitrJnite
BC
BrOwn
DT
DerninerQlised t o r b o n i t e
T
Untreated
o •
0.4W 5sec lOW 5sec
cool torbQnlte
rQbo t tre~3ted
Fig. 8. A measure of the oxygen uptake of a series of geochemical materials exposed to oxygen plasma (0.2 torr, 5 s).
Surface chemistry of coal, kerogen and bitumen In contrast, plasma leads functionality carbonaceous
the reductive nature of the hydrogen to a decrease in oxidative carbon in the s u r f a c e r e g i o n s of t h e s e materials (Wilson, 1981).
CONCLUSIONS This paper has demonstrated the capability of E S C A to probe the surface chemistries of a wide range of geochemical materials. E S C A may provide a wealth of information on the structure, bonding and reactivity of these materials as studied in their solid state, with minimal sample preparation. The surface s e n s i t i v i t y of this s e m i q u a n t i t a t i v e a n a l y t i c a l technique is of special value in the characterization of surface specific reactions. This has been exemplified by studies of surface oxidation reactions, both naturally occurring through weathering processes and artificially i n d u c e d . E S C A may p r o v i d e information, not only on the extent of reaction, but also on the chemical bonding at the reacted surface.
Acknowledgements--The authors wish to thank Dr A. G. Douglas and Dr P. F. V. Williams (University of Newcastle-upon-Tyne), Dr P. H. Given (Pennsylvania State University, PA, U.S.A.), Dr J. M. Hunt (Woods Hole Oceanographic Institution, Woods Hole, MA, U.S.A.), Mr G. J. Liposits, Dr A. Mills and Mr K. Philpot (National Coal Board, U.K.) and Morris Ashby Ltd for their generous gift of materials. Thanks are due to Dr J. M. Jones and Dr P. M. R. Smith (University of Newcastle-upon-Tyne) and to Dr J. M. E. Quirke (Florida International University, Miami, U.S.A.) for their help and advice. The SERC are to be acknowledged for provision of equipment, and for the award of a research studentship to one author (R.W.).
REFERENCES
Brown J. R., Kronberg B. I. and Fyfe W. S. (1981) Semiquantitative ESCA examination of coal and coal ash surfaces. Fuel 60, 439-446. Clark D. T. (1973) Chemical aspects of ESCA. In Electron Emission Spectroscopy (Edited by Dekeyser W. and Riedel D.), pp. 373-507. D. Riedel, Dordrecht. Clark D. T. (1976) Some chemical applications of ESCA. In Molecular Spectroscopy (Edited by West A. R.), pp. 339-382. Heyden, Philadelphia. Clark D. T. (1978) ESCA applied to organic and polymeric systems. In Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy (Edited by Briggs D.), pp. 211-247. Heyden, Philadelphia. Clark D. T. and Harrison A. (1981) ESCA applied to polymers. XXXI. A theoretical investigation of molecular core binding and relaxation energies in a series of prototype systems for nitrogen and oxygen
461
functionalities in polymers. J. Polym. Sci. Polym. Chem. Educ. 19, 1945-1955. Clark D. T. and Wilson R. (1983a) ESCA applied to aspects of coal surface chemistry. Analytical Methods for Coals, Cokes and Carbons Conference, London, UK, 28-29 April 1983. Fuel 62, 1034-1040. Clark D. T. and Wilson R. (1983b) Selected surface modification of polymers by means of hydrogen and oxygen plasmas. J. Polym. Sci. Polym. Chem. Educ. 21, 837-853. Clark D. T., Abu-Shbak M. M. and Brennan W. J. (1982) Electron mean free paths as a function of kinetic energy: a substrate overlayer investigation of polyparaxaylylene films on gold using a Ti Ks X-ray source. J. Electron Spectrosc. Rel. Phenom. 28, 11-21. Clark D. T., Wilson R. and Quirke J. M. E. (19831 An evaluation of the potential of ESCA (electron spectroscopy for chemical applications) (and other spectroscopic techniques) in the surface and bulk characterization of kerogens, brown coal and gilsonite. Chem. Geol. 39, 215-239. Durand B. and Ni~aise G. (1980) Procedures for kerogen isolation. In Kerogen: Insoluble Organic Matter ,t?orn Sedimentary Rocks (Edited by Durand B.), pp. 35-54. Editions Technip, Paris. Frost D. C., Wallbank B. and Leeder W. R. (1978) X-ray photoelectron spectroscopy of coal and coal related problems. In Analytical Methods for Coal and Coal Products (Edited by Karr C. Jr), Vol. 1. pp. 349-379. Academic Press, London. Miller R. N., Yarzab R. F. and Given P. H. (1979) Determination of the mineral matter contents of coals by low-temperature ashing. Fuel 58, 4-1(I. Pireaux J. J.. Riga J., Caudano R. and Verbist J. (19811 Electronic structure of polymers. X-ray photoelectron valence band spectra. In Photon, Electron and Ion Probes of Polymer Structure and Properties (Edited by Dwight D. W., Fabish T. J. and Thomas H. R.), pp. 169-2111. ACS Symposium Series. No. 162. Washington. Robson D. A. (ed.) (19801 The Geology of North East England.The Natural History Society of Northumbria, J. and P. Bealls Ltd, Newcastle-upon-Tyne. Van Krevelen D. W. (1961) Coal. Elsevier, Amsterdam. Venugopalan M., Roychowdhury U. K., Chan K. and Pool M. L. (1980) Plasma chemistry of fossil fuels. In Topics in Current Chemistry (Edited by Veprek S. and Venugopalan M.), Vol. 90, pp. 1-57. Springer-Verlag, Berlin, Vitorovic D. (198(/) Structure elucidation of kerogen by chemical methods. In Kerogen: Insoluble Organic Matter from Sedimentary' Rocks (Edited by Durand B.), pp. 301-338. Technip, Paris. Wagner C. D., Riggs W. M., Davis L. E., Moulder J. F. and Muillenberg G. E. (1979) Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corporation, Physical Electronics Division. Minnesota. Wilson R. (1981) ESCA applied to a study of some materials of geochemical interest. M.Sc. Thesis. University of Durham, U.K. Wilson R. (1984) Surface and bulk characterisation of selected coals, bitumens and polymers as revealed by ESCA and other analytical techniques., Ph.D. Thesis, University of Durham, U.K.