X-ray Photoelectron Spectroscopy, Photoelectron Energy Loss

1744

Energy & Fuels 2004, 18, 1744-1756

X-ray Photoelectron Spectroscopy, Photoelectron Energy Loss Spectroscopy, X-ray Excited Auger Electron Spectroscopy, and Time-of-Flight-Secondary Ion Mass Spectroscopy Studies of Asphaltenes from Doba-Chad Heavy Crude Hydrovisbreaking Faı¨c¸ al Larachi,*,† Soumaı¨ne Dehkissia,† Alain Adnot,† and Esteban Chornet‡ Department of Chemical Engineering, Laval University, Que´ bec, Canada G1K 7P4, and Chemical Engineering Department, University of Sherbrooke, Sherbrooke, Canada J1K 2R1 Received February 19, 2004

The sp2-to-sp3 carbon bonding character in asphaltenes that results from heavy crude visbreaking was evaluated by means of X-ray photoelectron spectroscopy (XPS), photoelectron energy loss spectroscopy (ESCALOSS), X-ray excited Auger spectroscopy (XAES), and time-offlight secondary ion mass spectrometry (TOF-SIMS). The asphaltenes were precipitated from virgin Doba-Chad heavy crude or from treated crudes after undergoing noncatalytic and (FeS and MoS2 mediated) catalytic hydrovisbreaking to reduce heavy crude viscosity for meeting pipeline transportation specifications. The sp2 and sp3 characters of the asphaltenes were calibrated using, respectively, highly oriented pyrolytic graphite (HOPG) and diamond. Composites of asphaltenes with polyethylene and polystyrene references were used to assess the shift from sp3- to sp2-dominated structures. It was postulated that the π aromatic character in the asphaltenes condensed polynuclear aromatic rings correlates with sp2 carbon, whereas the asphaltenes aliphatic character correlates with sp3 carbon. Percentages of sp2 sites ranged between 36% and 70% for all the asphaltenes samples with values from XPS being underestimated, with respect to XAES. The splitting between the principal nonloss C 1s line and the largest plasmon loss peak, and the evolution in shape of the π f π* plasmon satellite in the energy loss spectra were in qualitative agreement with XPS and XAES data. In addition to aromatic and aliphatic hydrocarbon ion fragments, the high sensitivity of TOF-SIMS allowed the detection of several hetero-elements (such as nickel, vanadium, sulfur, and nitrogen, in the organometallic-bearing asphaltenes, or iron, chlorine, sodium, potassium, and silicon, as foreign elements). Probing the surface composition through the ΣCHx+/C2+ ratio indicated that asphaltenes were bearing more aliphatic character than the carbon blacks or graphite, presumably because of the presence of hydrogen during hydrovisbreaking. However, surface probing via the C2H-/C2- ratio suggested that asphaltenes were less aromatic, as expected, than graphite while exhibiting an aromatic character close to that of some carbon blacks.

Introduction The pumping and transportation of virgin heavy crude oils from remote oil fields to the refining infrastructures are seldom practiced without implementing some forms of on-site chemical or physical alteration of the heavy oil viscosity to make it comply with pipelining specifications.1 Methods for reducing viscosity are numerous, whereas all advocate some form of treatment near the reservoir before transport via pipeline.2 Heavy oil hydrovisbreaking, whether catalytic or not, in conjunction with distillation and deasphalting, is one such * Author to whom correspondence should be addressed. Telephone: 1-418-656-3566. Fax: 1-418-656-5993. E-mail address: faical.larachi@ gch.ulaval.ca. † Laval University. ‡ University of Sherbrooke. (1) Lepage, J. F.; Chatila, S. G.; Davidson, M. Raffinage et Conversion des Produits Lourds du Pe´ trole; E Ä ditions Technip: Paris, 1990; p 190. (2) ] Speight, J. G. The Chemistry and Technology of Petroleum, 3rd Edition; Marcel Dekker: New York, 1999; p 909.

measure.1 It consists of contacting the heavy oil with hydrogen at high pressure and temperature to reduce crude oil viscosity through the break up and hydrogenation of the constituent bulky hydrocarbons composing the asphaltenic fraction and the polar and polyaromatics maltenic subfractions, besides reducing the level in objectionable heteroatoms. One important aspect in the treated crudes that is worth addressing is to warrant that the balance between the highest-molecular-weight asphaltenes and the surrounding maltenic compounds in the synthetic crude, to be pipelined, is not disturbed, which would otherwise result in a loss of stability. Eventually, problems such as sedimentation and plugging explained by an aggregation of asphaltenes may cause severe setbacks during transportation of upgraded feedstocks. Asphaltenes have been described to consist of graphitelike stacks of condensed polynuclear aromatic ring systems that carry aliphatic chains and embed several

10.1021/ef049951e CCC: $27.50 © 2004 American Chemical Society Published on Web 08/20/2004

Studies of Asphaltenes from Doba-Chad Heavy Crude

heteroatoms, such as that observed in the porphyrinvanadyl structure or other structures involving vanadium, nickel, iron, sulfur, nitrogen, and oxygen.2,3 Preventive measures to preclude asphaltenes aggregation rely on an understanding of the surface phenomena at the colloidal (or asphaltene particles) level. For instance, use of amphiphile additives to stabilize and impede asphaltenes aggregation through intercalation and steric hindrances is routinely used commercially.4 However, successful implementation of such additives is case dependent, with regard to the crude under consideration. The use of surface spectroscopy to investigate the surface outer layers of asphaltenes has rarely been practiced. Such techniques are extremely powerful, because of their ability to sense the chemical environment over a depth scale of a few nanometers, where the molecular interactions between asphaltenes and the surrounding crude oil molecules are indeed governed by the surface chemistry. O ¨ stlund et al.4 recently presented an X-ray photoelectron spectroscopy (XPS) study of the functional groups on the surface of asphaltenes subfractions from Cold Lake crude. The curve fitting of the high-resolution XPS C1s spectra of these asphaltenes revealed ketonic and nonketonic oxygen-bearing compounds (with a carboxylic binding energy (BE) shift), along with an aromatic signature. Speciation of organic sulfurs in petroleum asphaltenes using X-ray photoelectron (XP) S 2p spectra has been attempted by Kelemen et al.5 to differentiate the alkyl sulfide and the thiophenic sulfur signatures from their BE shifts. The contrast in XPS BE shifts between thiophenic and sulfidic sulfurs has been further increased by selective chemical/air oxidation of the alkyl sulfides into sulfones and sulfoxides in asphaltenes.6 According to a similar approach, Wilhelms et al.7 investigated the nitrogen chemistry in asphaltenes extracted from several oils from Australia/offshore, Egypt, North Sea, California, Java, and China. The N 1s high-resolution spectral envelopes were modeled to consist of mixtures of fivemembered ring pyrrolic nitrogen and six-membered ring pyridinic nitrogen. XPS studies of asphaltene subfractions fractionated from the precipitation of asphaltenes from coal and coal-derived hydrocarbons led to similar observations regarding the content in pyrrole and pyridine nitrogen.8,9 Asphaltenes from petroleum and pyrolytic bitumen, as well as asphaltenes-impregnated (3) Mullins, O. C.; Sheu, E. Y. Structures and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (4) O ¨ stlund, J. A.; Nyden, M.; Fogler, H. S.; Holmberg, K. Functional Groups in Fractionated Asphaltenes and the Adsorption of Amphiphilic Molecules. Colloid Surf. A 2004, 234, 95-102. (5) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Direct Determination and Quantification of Organic Sulphur by X-ray Photoelectron Spectroscopy (XPS) and Sulphur K-Edge Absorption Spectroscopy. Fuel Process Technol. 1990, 24, 425-429. (6) Ismail, K.; Snape, C. E. The Selective Oxidation of PhenolFormaldehyde Resins as a Model in the Speciation of Organic Sulfur Forms in Coals and Petroleum Heavy Oils via XPS and Sulfur K-Edge XANES. ACGC Chem. Res. Commun. 1998, 7, 8-20. (7) Wilhelms, A.; Patience, R. L.; Larter, S. R.; Jorgensen, S. Nitrogen Functionality Distributions in Asphaltenes Isolated from Several Oils from Different Source Rock Types. Geochim. Cosmochim. Acta 1992, 56, 3745-3750. (8) Wallace, S.; Bartle, K. D.; Perry, D. L. Quantification of Nitrogen Functional Groups in Coal and Coal Derived Products. Fuel 1989, 68, 1450-1455. (9) Bartle, K. D.; Perry, D. L.; Wallace, S. The Functionality of Nitrogen in Coal and Derived Liquids: An XPS Study. Fuel Process Technol. 1987, 15, 351-361.

Energy & Fuels, Vol. 18, No. 6, 2004 1745

pyrolytic carbon blacks obtained from tire pyrolysis, have been thoroughly investigated by Darmstadt et al.,10 using XPS and the static secondary ion mass spectrometry (s-SIMS) technique. C 1s, O 1s, and S 2p highresolution spectra unveiled the complex chemical composition and functionalities of pyrolytic and petroleum bitumen asphaltenes and impregnated asphaltenes, whereas SIMS analysis of aliphatic charged fragments provided insights on the condensation level of polyaromatic rings. This study was motivated by the discovery in Chad of an important heavy oil reserve that has been estimated at one billion barrels, is capable of producing 225 mbpd at peak operation, and is currently exploited by an international consortium of petroleum companies. Expected revenues for Chad are estimated to 8.5 billions U.S. dollars during the 30 years of production life. Despite this important heavy oil reserve, major obstacles to developing this resource have been the remote and landlocked geography of Chad. A proposed pipeline measuring 1050 km in length and 76 cm in diameter has been constructed and has recently begun operation for transporting the heavy oil from the Doba oilfield (Chad) to the Atlantic shore (Cameroon) for transport to international markets. This paper is one in a series of three11,12 that address the spectroscopic surface characterization of the asphaltenes precipitated from virgin heavy crude and treated crudes resulting from the catalytic and noncatalytic hydrovisbreaking, as well as thermal visbreaking of Doba heavy crude. To the best of our knowledge, several alternative yet powerful spectroscopies such as photoelectron energy loss spectroscopy (ESCALOSS), X-ray excited Auger spectroscopy (XAES), and time-offlight secondary ion mass spectrometry (TOF-SIMS), have not been specifically applied previously to investigate the chemical functionalities in asphaltenes. Therefore, in this study, it was judged opportune to evaluate the potential of these techniques in the characterization of a different brand of asphaltenes. Hence, for this purpose, it was assumed that the π-aromatic character in the condensed polynuclear aromatic rings of the asphaltenes could be inferred from an evaluation of sp2 carbons, whereas the asphaltenes aliphatic character could be correlated with sp3 carbon. Percentages of sp2 sites were evaluated from XPS and XAES, as well as being qualitatively evaluated from the loss splitting between the principal C 1s line and the largest plasmon loss peak in the energy loss spectra (ESCALOSS). Finally, the use of TOF-SIMS and analysis of the ΣCHx+/C2+ and C2H-/C2- intensity ratios allowed the surface to be probed, in terms of the polycondensation level and the proportion in aliphatic components. Experimental Section Materials. The asphaltenes were precipitated from a series of virgin and treated heavy crude oils. The raw crude origi(10) Darmstadt, H.; Chaala, A.; Roy, C.; Kaliaguine, S. SIMS and ESCA Characterization of Bitumen Reinforced with Pyrolytic Carbon Black. Fuel 1996, 75, 125-132. (11) Dehkissia, S.; Larachi, F.; Rodrigue, D.; Chornet, E. Characterization of Doba-Chad Heavy Crude Oil in Relation with the Feasibility of Pipeline Transportation. Fuel 2004, 83, 2157-2168. (12) Dehkissia, S.; Larachi, F.; Rodrigue, D.; Chornet, E. Lowering the Viscosity of Doba-Chad Heavy Crude Oil for Pipeline Transportation: The Hydrovisbreaking Approach. Energy Fuels 2004, 18, 11561168.

1746

Energy & Fuels, Vol. 18, No. 6, 2004

Larachi et al.

Table 1. (Hydro)Visbreaking Reaction Conditions Yielding the Analyzed Asphaltenes sample run R02 R03

temp, T (°C) No Catalyst 440 with H2 440 no H2

time (min)

log R0

25 25

6.3 6.3

With Catalyst MoS2 catalyst R08 FeS catalyst R15

430

with H2

25

6.2

430

with H2

25

6.2

nated from the Doba oilfield, which is located in southeastern Chad. The treated crudes were those obtained from the catalytic and noncatalytic hydrovisbreaking, and thermal visbreaking, which were explored as upgrading routes for mitigating crude oil viscosity for pipeline transportation. Powdered iron sulfide (FeS, CAS 1317-37-9; 110 mesh, density at 20 °C ) 4.74 g/cm3, melting point ) 1195 °C, purity ) 99%, Alfa-Aesar Company) and molybdenum sulfide (MoS2, CAS 1317-33-5; 325 mesh, density at 20 °C ) 4.80 g/cm3, melting point ) 1185 °C, purity ) 99%, Alfa-Aesar Company), which were used as hydrovisbreaking catalysts, were mixed before reaction in the hydrocarbon feedstock. The catalytic/ noncatalytic (hydro)visbreaking tests were conducted in a fourbaffle stainless-steel 300-mL Parr autoclave reactor agitated with a magnetically driven six-bladed pitched turbine impeller. Gaseous hydrogen (H2) was delivered from 6000 psi gas cylinders, and the H2 pressure in the reactor headspace was maintained at 13.8MPa by means of a pressure regulator. Hydrogen was sparged into the autoclave reactor in semibatch mode (open flow, with respect to gas, and batch, with respect to the crude). Hydrogen was metered at a mass-flow-controlled specific flow rate per unit mass of crude that was equal to 0.2 LSTP per minute per gram. Table 1 summarizes the reaction time and temperature conditions explored in this work to yield the asphaltenes. Toluene was used to isolate coke from the virgin crude oil, whereas CH2Cl2 solvent was used to separate the fraction of coke from the treated Doba crude. Being more volatile than toluene, CH2Cl2 was more convenient to avoid loss of light products generated by the (hydro)visbreaking reaction. Treated and virgin crude samples of 20 g were mixed, respectively, with CH2Cl2 and toluene in a ratio 1:50 v/v, providing rapid separation between fractions that were and were not soluble in dichloromethane or toluene. The insoluble fractions (i.e., coke) were separated on 934 AH Whatman fiberglass filters (Fisher Scientific Company, Catalog No. 09873K) with 1.5-µm pores. After solvent evaporation, the solubles that contributed to the filtrate were mixed with either n-pentane, n-hexane, or n-heptane. As can be seen from Figure 1a, the fraction of the resulting insolubles that represent the asphaltenes was the highest with n-pentane as the precipitant solvent. As expected, the longer the paraffinic solvent chain, the lower the asphaltenes recovery yield.2 Furthermore, the maximum asphaltenes yield that was recovered corresponded to a volume ratio of n-C5 solvent to crude oil at least equal to 30 v/v (see Figure 1b), which is consistent with the recommendation of Andersen and Speight.13 In addition, sedimentation times of at least 8 h (Figure 1c) were required for the asphaltene particles to agglomerate into particles of a filterable size; such times were also required because of the diffusion-controlled nature of the process. The feedstock, because it is heavier, also needed time for the hydrocarbon (solvent) to penetrate. Afterward, the precipitated asphaltenes were filtered off on a medium (1015 µm) fritted glass filter (Fisher Scientific Company, Catalog No. 10-358-22L). Mixtures of asphaltenes with high-density polyethylene (PE, 100% sp3 additive) and with polystyrene (PS, 75% sp2 additive) (13) Andersen, S.; Speight, J. G. Petroleum Resins: Separation, Character, and Role in Petroleum. Pet. Sci. Technol. 2001, 1&2, 1-34.

Figure 1. Effect of (a) precipitant solvent, (b) solvent-to-crude oil volume ratio, and (c) settling time on the asphaltene yields. from Styron Dow Plastics were analyzed to examine any shift in asphaltenes surface properties from sp3 (aliphatic) to sp2 (aromatic) dominance. Both PS and PE were mixed with asphaltenes and melted using 50:50 wt % proportions in a petroleum water determination apparatus,11 using, respectively, toluene and dichloromethane as solvents. After melting, CH2Cl2 solvent was evaporated at room temperature by air blowing, whereas toluene was evaporated under vacuum using a model R-200 Bu¨chi evaporator. In addition, 100% sp3 diamond powder (6 µm, from Warren/ Amplex Superabrasives) and 100% sp2 highly oriented pyrolytic graphite (HOPG, from SPI Supplies) were spectroscopi-

Studies of Asphaltenes from Doba-Chad Heavy Crude cally evaluated to fingerprint the sp2 and sp3 characters of the asphaltenes. Characterization Techniques. C 1s X-ray photoelectron (XPS), C 1s X-ray photoelectron energy loss (ESCALOSS),14 and C KLL X-ray excited Auger (XAES) spectra of the asphaltenes samples were acquired at room temperature using a VG Scientific ESCALAB Mark II system and nonmonochromated Mg KR (hν ) 1253.6 eV) X-ray radiation. The source, operated at 300 W, sensed an ∼5-mm-diameter spot on the wafers. The kinetic energies of photoelectrons were measured using a hemispherical electrostatic analyzer working in the constant pass-energy mode. The background pressure in the analyzing chamber was kept below 7.5 × 10-8 Torr. XP survey (0-1150 eV) and C 1s high-resolution spectra were acquired, respectively, at pass energies of 50 and 20 eV over ca. 6 min accumulation times. Correspondingly, the nominal resolutions were 1.8 and 1.1 eV. The XAES spectra were obtained at an increment of 0.1 eV with a pass energy of 50 eV over acquisition times of 250 min. The first derivative spectra dN/dKE versus electron kinetic energy KE were obtained through numerical differentiation using a SavitzkyGolay method. The ESCALOSS spectra near the C 1s region included the principal loss peaks to evaluate the C 1s loss splitting energy (EL).14 They were acquired at steps of 0.2 eV over a pass energy of 50 eV, at a dwell time of 50 ms and after 350 scans. It was verified that no alteration of the samples by the X-ray irradiation occurred during the acquisition periods. To correct for charging effects in the XPS, XAES, and ESCALOSS data, the samples were referenced with respect to the principal component of the C 1s at BE ) 284.5 eV for HOPG. Subsequently, this position was slightly readjusted for each sample, using corrected BEs yielded from the XPS curve fitting of the C 1s core-level spectra. For all acquired spectra, the uncertainty in peak position was (0.3 eV. Except for the HOPG sample, all samples exhibited some to significant intrinsic insulation. Because of the X-ray source used, being non-monochromated (as in almost all spectra recorded on polymers or isolating surfaces), the effects of differential charging were unavoidable, because the spectrometer was not equipped with a flood gun for charge compensation. Hence, the differential charging would impact the XP C 1s spectral envelope and, therefore, should be taken into account in the peak line fitting by adding an extra “ghost” component to the genuine C 1s peak in the low-BE region.15,16 XPS data reduction of the measured C 1s high-resolution spectral envelopes was performed by curve-fitting synthetic peak components, using a modified version of the Hughes and Sexton software.17 The raw experimental data were used with no preliminary smoothing. Symmetric Gaussian-Lorentzian product functions were used to approximate the line shapes of the fitting components. Peak identification was performed after Shirley background subtraction. Quantification of the (dominant) C and (trace) O and N atomic number fractions on the sample surfaces was obtained from integration of the C 1s, O 1s, and N 1s core-level spectra with appropriate corrections including photoionization cross-sections. The static TOF-SIMS of asphaltenes samples were recorded on an ION TOF-SIMS IV instrument using the following experimental conditions. The source of primary ions consisted of 69Ga+ with a target current of 2.5 pA and pulsed at a (14) Barr, T. L. Modern ESCA: The Principles and Practice of X-ray Photoelectron Spectroscopy; CRC Press: Boca Raton, FL, 1994; p 358. (15) Gengenbach, T. R.; Chatelier, R. C.; Griesser, H. J. Characterization of the Aging of Plasma-Deposited Polymer Films: Global Analysis of X-ray Photoelectron Spectroscopy Data. Surf. Interface Anal. 1996, 24, 271-281. (16) Larachi, F.; Pierre, J.; Adnot, A.; Bernis, A. Ce 3d XPS study of Composite CexMn1-xO2-y Wet Oxidation Catalysts. Appl. Surf. Sci. 2002, 195, 236-250. (17) Hughes, A. E.; Sexton, B. A. Curve Fitting XPS Spectra. J. Electron. Spectrosc. Relat. Phenom. 1988, 46, 31-42.

Energy & Fuels, Vol. 18, No. 6, 2004 1747 Table 2. Mass Fraction Distribution of the Untreated and Treated Doba Crude Oil for Different Treatments sample run untreated R02 R03 R08 R15

gas

coke

13.9 13.0 8.3 11.1

0.1 2.4 4.0 2.4 1.4

Mass Fraction asphaltenes 1.8 6.2 7.9 5.5 5.4

maltenes 97.4 77.4 75.2 83.7 82.1

repetition rate of 100 kHz at a pulse width of 27.5 ns. The instrument resolution was better than 8400 for 29Si. The beam was rastered on a 12 µm × 12 µm surface area in positive polarity and 100 µm × 100 µm in negative polarity. An O2 flux at 10-6 mbar and the flood gun at 2.5 A were applied to compensate for charge effects on the isolating samples. Secondary ions in the mass range 0 e m/z e 300 were collected during 100 s time spans corresponding to a maximum total primary ion dose per spectrum after analysis of 6 × 109 ions/ cm2, which is much less than ca. 5 × 1012 ions/cm2, suggested to fulfill analyses under static conditions.

Results and Discussion (Un)treated Heavy Crude Separation. First, the extent of the various (hydro)visbreaking conversion routes of the Doba crude oil was assessed, in terms of mass fractional partition of gases, coke, maltenes, and asphaltenes. Table 2 summarizes the yields in the product distributions, expressed per 100 g of crude petroleum. The averaged yields for the untreated crude were 97.4% maltenes (n-pentane solubles), 1.8% asphaltenes (n-pentane insolubles), and 0.1% coke (tolueneinsoluble). The highest yield in asphaltene was achieved during heavy oil visbreaking in the absence of hydrogen (sample run R03). The role of hydrogen in limiting the accumulation of asphaltenes and coke1 during hydrovisbreaking treatment at the same temperature and pressure is worth noting, from a comparison of the results of sample run R02 versus those of sample run R03 (see Table 2). In addition, reducing the hydrovisbreaking reaction temperature from 440 °C to 430 °C and dispersing FeS or MoS2 catalysts yielded lesser asphaltenes than that under the noncatalytic conditions. Note that, despite a higher coke yield with MoS2 and a higher gas yield with FeS, comparable yields in asphaltenes were obtained, presumably because of differences in the catalytic activities and penetration of the powdered catalysts in the hydrocarbon matrix. X-ray Photoelectron Spectroscopy Analyses. Representative X-ray photoelectron (XP) survey spectra for the asphaltenes precipitated from the treated crudes (sample runs R02, R03, R08), and mixtures of asphaltenes with polyethylene (PE) and polystyrene (PS) polymers, along with the survey spectra of HOPG and diamond, are illustrated in Figure 2. The atomic surface elemental concentrations for all investigated samples are summarized in Table 3. Three signatures arise in the survey spectra in the order of decreasing importance from carbon, to oxygen, then to nitrogen. The presence of other trace elements (such as sulfur, nickel, vanadium, iron, sodium, potassium, chlorine, and fluorine) will be assessed later, using the more highly sensitive TOF-SIMS technique. In addition to the main and systematic peak of carbon in the 285 eV region, the oxygen peak in the 535 eV region is particularly prominent for the diamond sample. Such relatively high

1748


Larachi et al.

Figure 2. Typical X-ray photoelectron (XP) survey spectra for highly oriented pyrolytic graphite (HOPG), diamond, R02, R03, and R08 asphaltenes, and the PE/R15 asphaltenes mixture. Table 3. C, O, and N Atomic Concentrations, Shift (D), and Loss Splitting (EL) atomic concentration (%) sample

C

O

N

D (eV)

EL (eV)

AS untreated crude AS (R02) AS (R03) AS (R08) AS (R015) AS + PE (50:50 wt %) AS + PS (50:50 wt %) HOPG diamond powder

98.0 97.0 96.6 97.6 97.6 98.5 97.2 99.0 91.1

1.5 1.6 2.1 1.7 1.4 1.3 1.8 0.9 8.7

0.5 1.4 1.3 0.7 1.0 0.2 1.0 0.1 0.2

17.0 16.5 16.8 16.2 16.2 15.2 16.8 19.0 13.0

24.4 ( 1 24.1 ( 1 24.9 ( 1 25.0 ( 1 22.6 ( 1 25.4 ( 1 30.1 ( 1 35.0 ( 1

surface oxygen concentration is plausibly ascribed to surface contaminants forming at some stages during production/handling of the diamond powder. Similar atypically high surface oxygen contaminations have been reported to form in the deposition of tetrahedral amorphous (diamond-like sp3 bonding) carbon films by pulsed unfiltered cathodic vacuum arc process.18 Amorphous hydrogen-free carbon (a-C) films deposited on Si(001) by radio-frequency (RF) magnetron sputtering also incorporated very high levels of oxygen impurities (∼10%).19 Note that the commercial diamond and HOPG samples were analyzed as received, without prior preparative treatments. The contributions of oxygen in the other samples were in the range of 0.9%-2.1% (see Table 3). For the asphaltenic samples, some of this oxygen originated from the surface oxidation that occurred in solvent evaporation and air blowing during sample preparation and also during the hydrovisbreaking tests, because the residual air sequestered in the autoclave headspace during heavy oil charging was not (18) Panwar, O. S.; Aparna, Y.; Shivaprasad, S. M.; Khan, M. A.; Satyanarayana, B. S.; Bhattacharyya, R. XPS and XAES Studies of As Grown and Nitrogen Incorporated Tetrahedral Amorphous Carbon Films Deposited by Pulsed Unfiltered Cathodic Vacuum Arc Process. Appl. Surf. Sci. 2004, 221, 392-401. (19) Patsalas, P.; Handrea, M.; Logothetidis, S.; Gioti, M.; Kennou, S.; Kautek, W. A Complementary Study of Bonding and Electronic Structure of Amorphous Carbon Films by Electron Spectroscopy and Optical Techniques. Diamond Relat. Mater. 2001, 10, 960-964.

purged. Finally, the nitrogen signature near 400 eV contributed between 0.2% and 1.4% and corresponded to a N/C ratio of ∼1%. This ratio was compatible with the bulk N/C ratio from elemental analysis of the asphaltenes extracted from the untreated heavy crude oil.11 Elemental analysis also yielded a bulk S/C ratio of 0.001. On the other hand, according to XPS, sulfur and other heavy metals went undetected at the surface, suggesting that these hetero-elements were below the instrumental detection limits. The high-resolution C 1s peaks in Figure 3 were obtained from the untreated-crude asphaltene sample and the treated-crude asphaltenes from catalytic and noncatalytic hydrovisbreaking (samples R02, R08, and R15, respectively), and mixtures of R15 asphaltenes with PE and PS. BE shifts reminiscent of sp3 and sp2 carbons were set based on diamond (285.1 ( 0.1 eV, full width at half maximum (fwhm) of 1.7 eV) and HOPG (284.5 ( 0.1 eV, fwhm ) 1.2 eV) XP C 1s spectra, respectively (Figure 4). However, without attempting to curve-fit the corresponding C 1s regions, diamond was assumed to bear 100% sp3 character and HOPG, a 100% sp2 character.20,21 The BE shifts for diamond and graphite were similar to those reported by Me´rel et al.20 The fwhm for graphite was generally in agreement with literature findings22-24 whereas that corresponding to diamond seemed to be larger than the reported values.20,22-25 It is postulated in this study that the availability of condensed polynuclear aromatic ring systems within the petroleum-derived asphaltenes confers a π aromatic character, which correlates with the availability of sp2 carbon, as inferred from CdC π-conjugation in HOPG. Hence, the larger the sp2/(sp2 + sp3) percentage, the more aromatic the asphaltenes. Such an analogy between asphaltenes and HOPG is made because the association between asphaltenes is reminiscent of graphitelike stacks.2 In contrast, the presence of aliphatic (C-C and C-H bonded) components is associated with an sp3 carbon, as approximated from the C-C bonding in diamond. Figure 3 reveals that the XP core-level C 1s spectra for the asphaltenes are rather difficult to interpret. The lack of suggestive shoulders emerging from the main C 1s lobe hardly supports their decomposition into an unequivocal set of individually resolved features. However, the fwhm of the C 1s region was observed to be ∼2.1 eV for the asphaltenes-containing samples. This width notably exceeds the HOPG/diamond fwhms to hypothesize that Doba asphaltenes are likely contrib(20) Me´rel, P.; Tabbal, M.; Chaker, M.; Moisa, S.; Margot, J. Direct Evaluation of the sp3 Content in Diamond-Like-Carbon Films. Appl. Surf. Sci. 1998, 136, 105-110. (21) Turgeon, S.; Paynter, R. W. On the Determination of Carbon sp2/sp3 Ratios in Polystyrene-Polyethylene Copolymers by Photoelectron Spectroscopy. Thin Solid Films 2001, 394, 44-48. (22) Lascovich, J. C.; Giorgi, R.; Scaglione, S. Evaluation of the sp2/ sp3 Ratio in Amorphous Carbon Structure by XPS and XAES. Appl. Surf. Sci. 1991, 47, 17-21. (23) Lascovich, J. C.; Scaglione, S. Comparison among XAES, PELS and XPS Techniques for Evaluation of sp2 Percentage in a:C-H. Appl. Surf. Sci. 1994, 78, 17-23. (24) Filik, J.; May, P. W.; Pearce, S. R. J.; Wild, R. K.; Hallam, K. R. XPS and Laser Raman Analysis of Hydrogenated Amorphous Carbon Films. Diamond Relat. Mater. 2003, 12, 974-978. (25) Bertrand, P.; Weng, L. T. Carbon Black Surface Characterization by TOF-SIMS and XPS. Rubber Chem. Technol. 1999, 72, 384397.



Figure 3. C 1s photoemission spectra for six asphaltenes samples: (a) native asphaltenes from heavy crude, (b) R02 asphaltenes, (c) R08 asphaltenes, (d) R15 asphaltenes, (e) R15 asphaltenes mixed with polyethylene (PE), and (f) R15 asphaltenes mixed with polystyrene (PS). Plotted on the figures are the probable individual peak contributions corresponding to C 1s lines. Empty circles represent the measured spectrum and the solid line envelope, the actual fit using the sum of all the contributions. (Absolute) residuals between measured and curve-fitted counts are shown in the bottom.

Figure 4. Measured C 1s photoemission spectra for HOPG and diamond.

uted by both sp2 and sp3 carbon types. To distinguish, at least qualitatively, between the two carbon signatures, the XP C 1s spectra from the asphaltenes/PE and

asphaltenes/PS polymer mixtures were also analyzed. As a matter of fact, PE is free from sp2 carbon (i.e., 100% sp3), whereas PS is dominated by an sp2 character (75% sp2, according to bulk stoichiometry). It is thus anticipated that the addition of PE would attenuate the upfield π and π + σ plasmon energy losses, whereas, in contrast, the addition of PS would exacerbate the plasmon contributions. This aspect of the asphaltenes C 1s spectroscopy will be discussed later in greater detail in the section titled “Energy Loss Spectroscopy (ESCALOSS) Analyses”. As mentioned previously, a phenomenological feature was required downfield to account for differential charging (dc peaks; see Figure 3). Hence, besides the sp2, sp3, and dc lines contributing to the main C 1s lobe, a broad flat peak embracing assignments inherent to the C-O, CdO, and COO functionalities and to the π f π* shakeup peak was added in the 286-293 eV BE region. Note that the presence of a shakeup near 291 eV could be evidence for an aromatic character of the asphaltenes. To save cumbersome curve-fitting efforts, no attempt was made to distinguish between the components in the 286-293 eV cluster. However, because of the low oxygen concentration prevailing at the surface of the asphaltene samples (see Table 3), the majority of the cluster peak is likely contributed by π-plasmon satellite. As an

1750


Figure 5. (a) XP C 1s spectra, expanded to show the details of peak reconstruction in the region of the CO/shake-up region. (b) Normal quantile-quantile plot for the residuals in the C 1s curve fitting. Plots refer to the R02 asphaltenes sample.

illustration, the CO/shake-up cluster peak was expanded for detail in Figure 5a for the R02 asphaltenes sample. To reduce the number of degrees of freedom in curve fitting, additional constraints were applied, including restrictions on BE positions and on the fwhm of the fitted components. The positions and the fwhm were constrained to within (0.5 eV and (0.3 eV, respectively. The average fwhm for the fitted sp2 and sp3 lines were equal and near 1.6 eV; for the CO/ shakeup, it was 1.8 eV; and 1.5 eV for the dc peak. As illustrated in Figure 5b, the validity of the proposed four-peak fit was a likely statistical solution, as justified by an analysis of the residuals,26 using the normal standard quantile-quantile plots of residuals between measured and calculated counts. As can be observed for sample run R02, the residual plot exhibited high fidelity to a straight line, indicating normally (26) Mason, R. L.; Gunst, R. F.; Hess, J. L. Statistical Design and Analysis of Experiments with Applications to Engineering and Science; Wiley: New York, 1989; p 530.

Larachi et al.

distributed residuals around a zero mean. Another goodness-of-fit indicator is given by the value of the probability function Q(χ2|ν) of χ2 (i.e., sum of squared residuals normalized by the number of degrees of freedom, ν). The 95% confidence limits of the χ2 distribution correspond to Q values of 0.005 and 0.995. Perfect mathematical fits yield Q ) 0.5 when χ2 f 1, whereas any model with Q values outside the range 0.005-0.995 fails statistically and should be rejected. Table 4 summarizes the curve-fitting results obtained for the untreated crude asphaltenes sample, the R02, R08, and R015 asphaltenes samples, and the R015 asphaltenes/PE & PS mixtures, in terms of Q values, peak positions, assignments, relative contributions, and sp2 percentages. All the fits were statistically acceptable, as judged from the Q values and from the distribution of (absolute) residuals between measured and fitted counts in Figure 3. Strictly speaking, however, such fits are not proof that the obtained solutions bear physical likeliness, because of the reasons outlined previously. Even by embedding prior knowledge in the form of fwhm and peak position constraints in the curve fitting, there is no guarantee that the solutions yielded from the fits are unique. This issue will be re-discussed further in the next section, in conjunction with the XAES data. Table 4 indicates that the noncatalytic hydrovisbreaking test of Doba crude seems to yield asphaltenes whose surface region is less aromatic than that of the asphaltenes extracted from the untreated heavy crude. However, the other catalytic tests yielded asphaltenes that exhibit larger degrees of aromaticity (sample runs R08 and R15; see Table 4). Mixing PE with R15 asphaltenes created a significant decrease in the degree of aromaticity, as expected from the sp2-free PE polymer. Conversely, mixing PS with R15 asphaltenes markedly increased the aromatic character of the mixture. However, the lower sp2 percentage (61% vs 70%) could be ascribed to (i) a mismatch between PS bulk and surface sp2 concentrations, (ii) possible segregation between PS and R15 asphaltenes during sample preparation, yielding different bulk and surface weight composition, or (iii) a curve-fitting solution that is simply not unique. Photoelectron Energy Loss Spectroscopy (ESCALOSS) Analyses. Another perhaps less-popular yet powerful alternative to evaluate the bonding character in carbonaceous materials is based on the analysis of the photoelectron energy loss (or ESCALOSS) spectra ∼20-30 eV upfield from the main C 1s lobe.14 As will be discussed, the dichotomy between sp2 and sp3 carbons is not as subject to arbitrariness as XPS curve fitting to assess the various changes in the chemistry of the carbonaceous asphaltenes qualitatively. Figure 6 shows the energy-loss regions corresponding to photoemission of the C 1s transition for graphite, diamond, virgin and treated asphaltenes, and PE/PS (R15) asphaltenes composites. Note that the spectral data were rescaled beforehand to fall within 0-1 intervals. Apart from the π f π* shakeup ca. 6 eV upfield of the 285 eV feature that appears for all the samples but diamond and PE/asphaltenes, the splitting between the principal (nonloss) C 1s line and the largest loss peak is ascribed to photoelectron energy loss due to π + σ plasmon loss excitations in aromatic-bearing compounds



Table 4. Binding Energy, Assignment, and Relative Intensity of C 1s Components sample asphaltene untreated crude Q ) 0.488 asphaltene R02 Q ) 0.248 asphaltene R08 Q ) 0.132 asphaltene R15 Q ) 0.168 AS + PE (50:50 wt %) R15 Q ) 0.175 AS + PS (50:50 wt %) R15 Q ) 0.228

a

peak position (eV)

assignment

contribution to total carbon content (%)

sp2/(sp2 + sp3) (%)

41

282.8 284.6 285.2 288.3

dca sp2 sp3 π f π* shakeup C-O, CdO, COO

1.2 39.8 57.8

283.2 284.5 285.3 288.9


5.3 33.2 59.3

282.2 284.7 285.3 289.4


0.8 46.9 48.6

282.2 284.8 285.5 289.7


0.5 66.5 27.9

283.0 284.4 285.5 286.7

dca

4.2 42.6 50.2

281.7 284.6 285.5 291.2

sp2 sp3 π f π* shake up C-O, CdO, COO dca sp2 sp3 π f π* shake up C-O, CdO, COO

1.2

36

2.2

49

3.7

70

5.1

46

3.0 0.7 59.2 37.2

61

2.9

HOPG

sp2

100.0

diamond powder

sp3

100.0

Differential charging.

or to non-π delocalized electrons, such as those in diamond.14,23 For graphite, the loss splitting EL, which occurs ∼30 eV upfield of the C 1s line, was interpreted as being due to the aromatic π structure experiencing quasi-total collective plasmonlike loss behavior, as in the case of free electron plasmon.14 For nonaromatic hydrocarbon polymers, such as polypropylene, EL is reported to shift down to ∼20 eV,14 which is often commensurably correlated with the reduction in the degree of delocalized conjugation (or π character) or

Figure 6. Energy loss spectroscopy (ESCALOSS) spectra showing the plasmon loss features associated to C 1s transitions in the treated crude asphaltenes, asphaltene composites, HOPG, and diamond. Calculation of C 1s loss splitting (EL) is also illustrated.

equivalently with the increase in the valence density fraction tied up in localized covalent bonding.14 Qualitative evolution of the plasmon losses from high-quality diamond films to graphitized surfaces was also proved by Chourasia et al.27 In the case that is of interest to us, the reduction in EL for asphaltenes, with respect to the HOPG EL value, can be thought of as the contribution of aliphatic (or sp3) carbon in the asphaltenes. Note from Figure 6 that the characteristic downfield shift of EL, along with attenuation of the loss peak from HOPG to the various asphaltenes, as well as occasional bursting of π f π* shake-up satellites, are all clear indications of coexisting sp2 and sp3 carbons. The proportions between these two bonding states in the asphaltenes are clearly modulated in terms of these three indicators when one compares the ESCALOSS spectra of R15 asphaltenes, and mixtures thereof with PE and PS: (i) squeezing of the loss splitting (EL ) 22.6 eV) and occlusion of π f π* shake-up satellite in the asphaltenes/PE system, with respect to the R15 asphaltenes (EL ) 25 eV) and asphaltenes/PS systems (EL ) 25.4 eV), and (ii) stretching of the EL and enhancement of π f π* shake-up satellite for the asphaltenes/PS system, with respect to the R15 asphaltenes ESCALOSS spectrum. Table 3 summarizes the characteristic loss splitting for the systems shown in Figure 6. Note that the EL value for HOPG (30.1 eV) is coherent with the value (27) Chourasia, A. R.; Chopra, D. R.; Sharma, S. C.; Green, M.; Dark, C. A.; Hyer, R. C. Characterization of Low-Pressure Deposited Diamond Films by X-ray Photoelectron Spectroscopy. Thin Solid Films 1990, 193/194, 1079-1086.

1752


reported by Barr14 but was 2-3.4 eV higher than the values measured by Lascovich and Scaglione23 and McFeely et al.28 For diamond, our value was ∼2 eV higher than that reported by Lau et al.29 and Barr.14 The EL data in Table 3 is an indirect support to the sp2/ sp3 curve fitting of the XP C 1s core spectra, because ESCALOSS predicts that the R08 and R15 asphaltenes exhibit greater degrees of aromaticity, with respect to the noncatalytically reformed asphaltenes (sample runs R02 and R03). Mixing PE with R15 asphaltenes created a reduction in the degree of aromaticity, as expected from the EL values. Conversely, mixing PS with the R15 asphaltenes markedly increased the aromatic character of the mixture. X-ray Auger Electron Spectroscopy Analyses. Rescaled spectra of the X-ray excited C KLL transitions and the corresponding derivatives, as a function of electron kinetic energy, for the HOPG and diamond references, as well as for the asphaltenes samples, are shown in Figure 7a and b, respectively. The peak shapes of the diamond and HOPG spectra are qualitatively similar to those recorded by other investigators.21,30,31 The XAES spectra were analyzed according to the broadly accepted D-parameter analysis22,23 to infer bonding states by identifying the different C atom arrangements, i.e., sp2 and sp3 proportions. This approach has been tremendously applied to estimate the sp2 and sp3 content in diamond-like carbon (DLC) films that consist, depending on the source precursor, of hydrogen-free amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H).18,20,22,32 However, to the best of our knowledge, this is the first time that XAES is applied to characterize sp2 and sp3 hybridized carbons in asphaltenes. The most informative energy features arising from the N(KE) and dN(KE)/dKE vs KE spectra occur in the 235280 eV region, where the shape of the differential derivative spectra unveils a refined structure. This structure is much richer for the asphaltenic samples and HOPG than for diamond. There is definitely a qualitative contrast between trigonally bonded graphite carbon and tetragonally bonded diamond carbon that emerges from the XAES spectra, in particular, the hump in the 250-260 eV region, which is indicative of π or sp2 bonding33 and the inflective rises near 275 and 285 eV, reminiscent of pπ electronic transitions.22 Note that the XAES spectra for the asphaltenic samples share a greater likelihood with the HOPG than with the dia(28) McFeely, F. R.; Kowalczyk, S. P.; Ley, L.; Cavell, R. G.; Pollak, R. A.; Shirley, D. A. X-ray Photoemission Study of Diamond, Graphite, and Glassy Carbon Valence Bands. Phys. Rev. B 1974, 9, 5268-5275. (29) Lau, W. M.; Huang, L. J.; Bello, I.; Yiu, Y. M.; Lee, S. T. Modification of Surface Band Bending of Diamond by Low Energy Argon and Carbon Ion Bombardment. J. Appl. Phys. 1994, 75, 33853391. (30) Fuchs, A.; Scherer, J.; Jung, K.; Ehrhardt, H. Determination of sp2/sp3 Carbon Bonding Ratio in a-C:H Including Irradiation Damage by Factor Analysis of Auger Electron Spectra. Thin Solid Films 1993, 232, 51-55. (31) Montero, I.; Galan, L.; Laurent, A.; Perrie`re, J.; Spousta, I. X-ray Photoelectron Spectroscopy and X-ray-Excited Auger Electron Spectroscopy Studies of the Initial Deposition of Hydrogenated Amorphous Carbon. Thin Solid Films 1993, 228, 72-75. (32) Sharma, R.; Panwar, O. S.; Kumar, S.; Sarangi, D.; Goullet, A.; Dixit, P. N.; Bhattacharyya, R. Effect of Substrate Bias on SE, XPS and XAES Studies of Diamond-Like Carbon Films Deposited by Saddle Fast Atom Beam Source. Appl. Surf. Sci. 2003, 220, 313-320. (33) Yamamoto, K.; Watanabe, T.; Wazumi, K.; Koga, Y.; Iijima, S. Carbon Films Deposited with Mass-Selected Carbon Ion Beams under Substrate Heating. Surf. Coat. Technol. 2003, 169-170, 328-331.

Larachi et al.

Figure 7. (a) Integral N(KE) XAS spectra and (b) XAES derivative dN(KE)/dKE for all the asphaltenes samples, as well as for HOPG and diamond. Calculation of the D-parameter value is also illustrated.

mond spectra. A plausible explanation could be ascribed to the aromatic rings (π bonding) in asphaltenes, because XPS suggested that 36%-70% of the surface carbon was trigonally bonded. Because of the distinctly most intense plasmon energy loss peak30 of electrons undergoing KLL transitions at ∼240 eV, the D-parameter was estimated as the KE width (in eV) between the successive maximum and minimum excursions of the derivative of the XAES spectra. Therefore, the corresponding main positive excursion was detected to occur in the vicinity of 255 eV and the main negative excursion near 270 eV. The D-parameter for HOPG and diamond were, respectively, 19 and 13 eV. The D-parameter of diamond was very similar to the literature values.20-22 For graphite, it was equal to that measured by Patsalas et al.;19 however, it was smaller than the literature values that were reported to evolve in the 20-23 eV range.20-22,31 The D-parameter values for all analyzed asphaltenic samples are gathered in Table 3. As can be seen, the C KLL peak broadening values for the asphaltene-containing samples are often closer to the 19 eV value than to the 13 eV



Figure 8. Parity plot of the sp2 content, as calculated from XAES and XPS for the asphaltene samples.

threshold, confirming the similarity of the bonding states between the trigonal structure of HOPG and the aromatic structure of the asphaltenes. Note that, for the asphaltenes/PE mixture, the D-parameter value shrinks to 15.2 eV, suggesting that the tetragonally bonded carbon in PE polymer has a tendency to attenuate the sp2 character of the sample. If a linear proportionality rule is assumed to hold between the D-parameter values and sp2 percentages throughout the 0%-100% scale, and taking, as a first approximation, diamond and HOPG as standards, the XAES sp2 contributions for the asphaltene-containing samples can be calculated as

sp2(%)|XAES )

DASPH - DDiamond × 100 DHOPG - DDiamond

The parity plot in Figure 8 shows the XPS versus XAES determinations of sp2 content for the asphaltenecontaining samples. The scatter between the two techniques was within the +60% and -20% limits, with a slight trend toward the positively biased XAES sp2 percentage, with respect to that estimated from XPS. There is still a dispute in the literature in regard to whether XAES and XPS are able to correctly determine the surface sp2 contents in a-C and a-C:H materials. For instance, the DLC films obtained by magnetron sputtering using a graphite target revealed that XAES underestimated the sp2 content, with respect to XPS, spectroscopic ellipsometry, or X-ray reflectivity techniques.19 In contrast, the DLC films formed by pulsedlaser deposition using pyrolytic graphite as a precursor source yielded XAES sp2 estimates that were systematically higher than their XPS counterparts.20 The different kinetic energies between the electrons ejected from the C 1s orbital (∼950 eV) and carbon KLL Auger electrons (∼280 eV), corresponding to samples with different probing depths, could explain such discrepancies. The fact that the escape depth for Auger electrons is indeed smaller makes XAES more sensitive to depthwise surface compositional gradients, whereas XPS has a tendency to level off the compositional disparities, because of a larger averaging volume. In addition, some authors even questioned the validity of the choice of hydrogen-free graphite and diamond as standards for sp2 estimation in hydrogen-bearing materials such as

Figure 9. TOF-SIMS spectra for the R15 asphaltenes sample ((a) positive and (b) negative).

polymers, block co-polymers, or a-C:H materials.21 Asphaltenes are also very likely to fall into this category. However, because the bulk asphaltene H/C ratio was on the order of 15 H atoms per 100 C atoms11 and below the typical H/C ratios encountered in polymers (∼1)21 or in a-C:H (∼0.45),30,31 it is believed that hydrogenfree graphite and diamond are still acceptable proxies for calibrating the trigonal and tetrahedral bonding states in asphaltenes. However, at this stage, it is rather difficult to draw sharp lines among the various enumerated sources of mismatch. These sources, combined with the possible lack of fit in the XPS reconstruction of sp2/ sp3 contributions for the asphaltenes samples, as discussed previously, could be at the origin of the discrepancies observed in Figure 8. Further investigations may be required to delineate the contributions among these artifacts. Time-of-Flight-Secondary Ion Mass Spectroscopy Analyses. Examples of positive and negative TOF-SIMS spectra (m/z 0-300) are shown in Figure 9a and b, respectively, for the R15 asphaltenes. The perbin separation extended from 0.1 to 1.6 mass units, correspondingly from channel mass 1 (i.e., H) to 300. This mass resolution was not sufficient to discriminate several hydrocarbon isobar ions from the oxygenate hydrocarbon ions nor from some alkali-metal cations for which resolutions down to 0.04 mass units were re-

1754


Larachi et al.

Figure 11. TOF-SIMS signals for the R15 asphaltenes sample after C2+ and C2- normalization in the low mass unit range ((a) positive and (b) negative).

Figure 10. TOF-SIMS spectra for the R15 asphaltenes sample ((a) positive and (b) negative) with a logarithmic scale on relative intensity and a threshold at 10-4, showing several identified positive and negative fragments.

quired. It can be seen that the vertical logarithmic scale expressing the intensity range stretched over 5-6 orders of magnitude. Hence, the relative per-bin positive and negative intensities were computed (as peak area divided by total intensity, integrated over the mass spectrum) and plotted down to a threshold of 10-4 (which is typical spectrometer sensitivity) as shown in Figures 10a and b, respectively. The positive carbon clusters CxHy+, including the protons (m/z ) 1), were meaningfully detected up to 225 mass units, whereas meaningful detection of the negative fragments concerned the interval of 1-73 mass units. Although the spectra exhibited rather complex patterns, aromatic signatures can readily be identified from the positive spectrum in the region above ca. 70 mass units. Prominent peaks are visible near m/z ) 77, 86, 99, 113, 128, 133, 138, 155, and 188 mass units, around which the spectrum exhibits a saw-toothed, quasiperiodically damped pattern (Figure 10a). These peaks could be assigned to single-, double-, and triplecondensed aromatic cycle structures fragmented from the polycondensed aromatic rings that comprise the molecular edifice of the asphaltenes. This periodic pattern was akin to the series of fragments identified by Bertrand and Weng25 to form in the TOF-SIMS spectra of carbon blacks and model PS polymer. Moreover, the very low H content in these fragments confirms

that the aromatic structures in the asphaltenes are poorly hydrogenated25 and could be a posteriori justification of choosing HOPG as a 100% sp2 reference in XAES. Similar poor hydrogen content of the fragmented cations was also reported by Shi et al. in their TOFSIMS study of Illinois coal for the elimination of pyrite.34 The contributions of the open-chain hydrocarbon fragments, after normalizing with the intensity of the C2+ fragment, are illustrated in Figure 11a. Clusters of ions between m/z ) 12 and m/z ) 15 assigned to CHx+ (x ) 0-3) indicates the presence of aliphatic or sp3 compounds. Those with m/z ) 24 and m/z ) 40 were assigned to the following species: C2+ (m/z 24), C2Hx+ (x ) 1-3) m/z 25-27, C2H4+/CO+ (m/z ) 28), C2H5+/ CHO+ (m/z ) 29), C2H6+/CH2O+ (m/z ) 30), and C3H3+/ K+ (m/z ) 39). These fragments are coherent with those detected in other literature studies.35-39 The relative peak intensity ratios for the detected short-chain ions are summarized in Table 5. According to Albers and coworkers,40,41 the positively charged fragment ions such as CHx+ (x ) 1-3), indicate the presence of long-chain (34) Shi, Y.; Orr, E. C.; Shao, L.; Eyring, E. M. Pyrite Removal from Illinois #6 Coal by CrCl2 Reduction and Effect of Pyrite on the Coprocessing of Illinois #6 Coal with Waste Automotive Oil. Fuel Process Technol. 1999, 59, 79-94. (35) Weng, L. T.; Bertrand, P.; Stone-Masui, J. H.; Stone, W. E. E. Desorption of Emulsifiers from Polystyrene Latexes Studied by Various Surface Techniques: A Comparison between XPS, ISS, and Static SIMS. Langmuir 1997, 13, 2943-2952. (36) Hamerton, I.; Hay, J. N.; Howlin, B. J.; Jones, J. R.; Lu, S.-Y.; Webb, G. A.; Bader, M. G.; Brown, A. M.; Watts, J. F. ToF SIMS and XPS Studies of Carbon Fiber Surface during Electrolytic Oxidation in 17O/18O Enriched Aqueous Electrolytes. Chem. Mater. 1997, 9, 19721977. (37) Shard, A. G.; Davies, M. C.; Tendler, S. J. B.; Bennedetti, L.; Purbrick, M. D.; Paul, A. J.; Beamson, G. X-ray Photoelectron Spectroscopy and Time-of-Flight SIMS Investigations of Hyaluronic Acid Derivatives. Langmuir 1997, 13, 2808-2814. (38) Petrat, F. M.; Wolany, D.; Schede, B. C.; Wiedman, L.; Benninghoven, A. In Situ ToF-SIMS/XPS Investigation of Nitrogen Plasma-Modified Polystyrene Surfaces. Surf. Interface Anal. 1994, 21, 274-282. (39) Hamoudi, S.; Larachi, F.; Adnot, A.; Sayari, A. Characterization of Spent MnO2/CeO2 Wet Oxidation Catalyst by TPO-MS, XPS, and s-SIMS. J. Catal. 1999, 185, 333-344.



Table 5. Normalized Time-of-Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS) Peak Ratios for Extracted Asphaltenes Sample from Sample Run R15a Σ(CHx+)

C2H+

C2H2+

C2H3+

C2H4+, CO+

C2H5+, CHO+

C2H6+, CH2O+

Σ(C3Hx+), for x ) 1-7

27.1

2.6

8.5

13.1

4.0

10.9

0.8

483

a

C-

CH-

CH2-

CH3-

C2H-

C2H2-

2.3

3.8

0.4

0.03

1.2

0.2

Normalized ratios given are relative to C2+ for positive ions and C2- for negative ions.

aliphatic surface compounds. The ratio of ΣCHx+ to C2+ observed for the R15 asphaltenes was equal to 27.1. This indicates that the long-chain aliphatic surface species are present in a lesser amount than those detected on the surface of the carbonaceous materials deposited over the manganese/cerium composite oxide catalysts during phenol wet oxidation, for which the mean value of this ratio was 100.39 However, the similarity of the R15 asphaltenes ΣCHx+/C2+ ratio to that of the pyrolytic asphaltenes (ΣCHx+/C2+ ) 23.3) is an indication that the amounts of aliphatic C-H groups in both asphaltenes were comparable.10 Interestingly, this ratio was only 3.8 for graphite and 1.6 for pyrolytic carbon black,10 suggesting that these structures are more deficient, in terms of surface sp3 carbon species. The C2Hx+ hydrocarbon fragments also exhibited high relative intensities. The oxygen-containing ions CHO+, along with C2H5+ (ratio of 10.9), gave more-intense signals than CH2O+/C2H6+ and CO+/C2H4+ (ratios of ca. 0.8 and 4, respectively). The aliasing between oxygenate and nonoxygenate ions makes exploitation of these fragments for identification illusory. Nonetheless, unlike the other hydrogen-rich C2H4+, C2H5+, and C2H4+ fragments, the C2H3+ is not aliased, so it can be used unambiguously to probe the surface structure of the asphaltenes in a way similar to CHx+ fragments. Its highest ratio (C2H3+/ C2+ ) 13.1) is further confirmation of the presence of surface C-H aliphatic groups. Moreover, oxygenated fragments of the ketonic, alcohol, and ether types, such as CHO+ (m/z ) 29), CH2O+ (m/z ) 30), CH3O+ (m/z ) 31), C2H3O+ (m/z ) 43), C2H5O+ (m/z ) 45), etc., were also detected (see Figure 10a). Their relative abundance from 0.1% to 2% indicates that these fragments are present at meaningful levels. An explanation could be that the oxygen beam used for neutralization could contribute to inflate the amount of oxygenated hydrocarbon cations through oxidation reactions. Another factor is the ambiguity involved in resolving the heavier hydrocarbon isobar ions such as CxHy (x g 3) for which intensities are counted near the same m/z bins. A last plausible factor could be ascribed to chemical or matrix effects that could considerably modify the emission coefficients of the secondary ions. Several inorganic cations such as Na+ (m/z ) 23), K+ (m/z ) 39.3), V+ (m/z ) 50.7), Ni+ (m/z ) 58.4), and Fe+ (m/z ) 56.3) were also detected. Detection of vanadium and nickel is indirect evidence of the presence of (40) Albers, P.; Deller, K.; Despeyroux, B. M.; Scha¨fer, A.; Seibold, K. XPS-SIMS Study on the Surface Chemistry of Commercially Available Activated Carbons Used as Catalyst Supports. J. Catal. 1992, 133, 467-478. (41) Albers, P.; Deller, K.; Despeyroux, B. M.; Prescher, G.; Scha¨fer, A.; Seibold, K. SIMS/XPS Investigations on Activated Carbon Catalyst Supports. J. Catal. 1994, 150, 368-375.

organometallic compounds such as, for example, the vanadyl-porphyrin structures in the asphaltenes.42 In addition, the detection of iron was attributed to the interference of the FeS catalyst with the isolated asphaltenes fraction during the separation process. Note that these metal contents were below the detection limits of XPS. Complementary chemical information can be acquired about the functional fragments that populate the surface of the asphaltenes, from an analysis of the negative ions spectra (see Figures 10b and 11b). The plausible heteroatom-containing negative ions detected with sufficient sensitivity (threshold of g10-4) were O- (m/z ) 16), OH- (m/z ) 17), CN- (m/z ) 26), S- (m/z ) 32), HS- (m/z ) 33), C2O- (m/z ) 40), and possibly SiO2(m/z ) 60). The presence of oxygen and nitrogen confirms the XPS survey spectra assessment. On the other hand, the detected sulfur could be the resultant of native thiocompounds that are present in the asphaltenes and the residual FeS catalyst used during the hydrovisbreaking of the Doba heavy crude. Note the peaks at m/z ) 19 and at m/z ) 35 and 37, which could be assigned to fluorine and chlorine impurities, respectively. Moreover, the strong peaks associated with the pure hydrocarbon anions observed at m/z ) 12-15 were attributed to CHx- (x ) 0-3) fragments. The nonaliased signals that stem from CHx- (x ) 1-2) are good indicators for the chemically bonded hydrogen in partially hydrogenated or hydrogen-rich surfaces.40 There were also peaks at m/z ) 24-26, which were assigned, respectively, to the C2-, C2H-, and C2H2- ions. Note that the latter peak is possibly aliased with the CN- peak. The significant peak at 36 mass units, between the Cland 37Cl- peaks, was attributed to C3-. The relative intensities of some characteristic negative ions are given in Table 5, using the C2- intensity for normalization (see also Figure 11b). Graphite or compounds that exhibit highly condensed aromatic rings are characterized by a decrease of their total hydrogen content, which is usually sensed using the relative intensity of the C2Hsignal. Low C2H- /C2- ratios were observed for hydrogendeficient graphitized surfaces.40,43 We will also assume that the C2H-/C2- ratio can roughly be used to assess the extent of condensed polynuclear aromatic ring systems in asphaltenes. In the current study, the value obtained for this ratio was 1.2, which was higher than (42) Scotti, R.; Montanari, L. Molecular Structure and Intermolecular Interaction of Asphaltenes by FT-IR, NMR, EPR. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998, 79-114. (43) Ashida, K.; Kanamori, K.; Watanabe, W. Surface Characterization of Various Graphites by X-ray Photoelectron, Secondary Ion Mass, and Raman Spectroscopies. J. Vac. Sci. Technol. A 1988, 6, 22322237.

1756


that of graphite (C2H-/C2- ) 0.28) but lower than that for pyrolytic carbon blacks (C2H-/C2- ) 1.63) or pyrolytic asphaltenes (C2H-/C2- ≈ 1.9).10 Furthermore, the SIMS study of a model 3,4-benzofluoranthene molecule, which consists of four condensed aromatic rings and one five-membered carbon ring, revealed a value of 2.2 for the C2H-/C2- ratio.10 Based on these observations, it can be claimed that SIMS analyses confirm that Doba asphaltenes, after hydrovisbreaking, are composed of aromatic rings, with a polycondensation level of >4 and possibly more severe than within pyrolytic asphaltenes. Conclusion In this work, a series of asphaltenes that resulted from the precipitation of heavy virgin and treated crudes that experienced hydrovisbreaking were analyzed, using four surface spectroscopic techniques, namely, X-ray photoelectron spectroscopy (XPS), photoelectron energy loss spectroscopy (ESCALOSS), X-ray excited Auger spectroscopy (XAES), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The purpose of using these techniques was to assess the sp2 and sp3 contributions in asphaltenes, which were believed to correlate,

Larachi et al.

respectively, with the π aromatic structure and the aliphatic structure. Analysis of the XPS C 1s highresolution spectra suggested that noncatalytic hydrovisbreaking yielded less-aromatic asphaltenes than those extracted from the virgin heavy crude but the catalytic routes yielded more-aromatic asphaltenes. Mixing asphaltenes with polyethylene (PE) resulted in a reduction in the degree of aromaticity, as expected from the sp2-free PE polymer. These trends were also confirmed by ESCALOSS and XAES analyses. The loss splitting of the asphaltenes was 5 eV less than that of highly oriented pyrolytic graphite (HOPG), suggesting the lesser-aromatic character of these latter, with respect to graphite. TOF-SIMS was able to detect heteroelements at tiny levels; such heteroelements went undetected using XPS. Several of these heteroelements, such sulfur, nitrogen, oxygen, nickel, and vanadium, are thought to belong to the asphaltenic structures. Moreover, analysis of the positive and negative ion fragments indicated that the investigated petroleum-derived asphaltenes were more aliphatic and less aromatic than graphite and exhibited polycondensation levels of >4. EF049951E

X-ray Photoelectron Spectroscopy, Photoelectron Energy Loss

Recommend Documents