Sorption of Athabasca Vacuum Residue Constituents on Synthetic

Mar 22, 2010 - †Department of Chemical and Materials Engineering, University of Alberta ... ‡Grant MacEwan University, Edmonton, Alberta, T5J 2P2,...
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Energy Fuels 2010, 24, 2500–2513 Published on Web 03/22/2010

: DOI:10.1021/ef901297e

Sorption of Athabasca Vacuum Residue Constituents on Synthetic Mineral and Process Equipment Surfaces from Mixtures with Pentane Cheng Xing,*,† R. W. Hilts,†,‡ and J. M. Shaw†,§ ‡

† Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6, Canada, and Grant MacEwan University, Edmonton, Alberta, T5J 2P2, Canada §Now working at Enbridge, Edmonton, Alberta, Canada.

Received November 4, 2009. Revised Manuscript Received February 16, 2010

Deposition of organic material on mineral and process equipment surfaces poses production, transport, and refining challenges for the petroleum industry. For high-asphaltene content hydrocarbon resources, such as bitumen, deposits are frequently assumed to be asphaltene rich. In this work, deposits formed from Athabasca vacuum residue (AVR) comprising 32 wt % asphaltenes þ pentane mixtures on acidic (FeS, SiO2) and basic (Fe2O3/ FeOOH/FeO, Ni/NiO/NiOH) substrates are analyzed using X-ray photoelectron spectroscopy. Control experiments with pure compounds are used to confirm experimental protocols and to address substrate contamination, which interferes with deposit composition measurements, if the organic deposit is thin or surface coverage is partial. Substrate properties are found to affect both deposit thickness and deposit composition. On basic substrates, deposits are thinner and are enriched in sulfur relative to AVR. On acidic substrates, deposits are thicker and are sulfur deficient relative to AVR, even though asphaltenes, which are rich in sulfur, sorb more strongly on acidic substrates in the absence of competition from other species. Deposit composition was also found to be invariant with the composition and phase behavior of the AVR þ pentane mixtures. These results were not expected.

formation arises throughout the hydrocarbon resource production to the refining chain.11-15 Vanadium, nickel, nitrogen, oxygen, and mineral matter are concentrated in the asphaltene fraction,16-18 and deposits rich in these species foul catalysts and pose other refining problems.2,3,16,17,19 Athabasca bitumen also comprises 1-2 wt % naphthenic acids.20 These acids pose corrosion problems21,22 and form metal naphthenates during processing. Deposition of metal naphthenates is also a

1. Introduction Heavy oils and bitumen play important roles in the North American energy sector.1 Their thermophysical properties differ markedly from conventional crudes,2,3 and they normally contain significant amounts of asphaltenes, vanadium, nickel, heteroatoms, and inorganic fine solids, depending on the method of production.4 Their hydrogen to carbon ratio is also low. Consequently, extensive refining is required to generate marketable products.5-7 Asphaltenes present well-known challenges globally due to their tendency to form aggregates in hydrocarbon liquids.8-11 Adherent asphaltene rich deposit

(12) Escobedo, J.; Mansoori, G. A. In Asphaltene and other heavy organic particle deposition during transfer and production operations, Proceedings of the 1995 SPE Annual Technical Conference, Richardson, TX, 1995, The Society of Petroleum Engineers: Richardson, TX, 1995; pp 343-358. (13) Lichaa, P. M. Asphaltene deposition problem in Venezuela crudes-usage of asphaltenes in emulsion stability. Can. Pet. Techonol. J. 1977, 15. (14) Escobedo, J.; Mansoori, G. A. In Heavy Organic Deposition and Plugging of Wells (Analysis of Mexico’s Experience), Proceedings of the II LAPEC, Richardson, TX, 1992; Society of Petroleum Engineers: Richardson, TX, 1992. (15) Escobedo, J.; Mansoori, G. A.; Balderas-Joers, C.; CarranzaBecerra, L. J.; Mendez-Garcia, M. A. In Heavy organic deposition during oil production from a hot deep reservoir: A field experience, Proceedings of the 5th Latin American and Caribbean Petroleum Engineering Conference and Exhibition, 1997; The Society of Petroleum Engineers: Richardson, TX, 1997. (16) Reynolds, J. G. Metals and heteroatoms in heavy oils. In Petroleum Chemistry and Refining; Speight, J. G., Ed. Taylor & Francis Press: Washington, DC, 1999. (17) Reynolds, J. G. Effects of asphaltene precipitation on the size of Vanadium-, Nickel- and Sulfur-containing compounds in heavy crude oils and residua. In Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science Press: New York, 1994. (18) Chung, K. H.; Xu, C. M.; Hu, Y. X.; Wang, R. N. Supercritical fluid extraction reveals resid properties. Oil Gas J. 1997, 95 (3), 66–69. (19) Yen, T. F. Genesis and Degradation of Petroleum Hydrocarbons in Marine Environments. ACS Symp. Ser. 1975, 18, 231–266. (20) Strausz, O. P. AOSTRA/University Reports for Industry Agreement, 1979. (21) Gutzeit, J. Naphthenic Acid Corrosion in Oil Refineries. Mater. Perform. 1977, 16 (10), 24–35. (22) Slavcheva, E.; Shone, B.; Turnbull, A. Review of naphthenic acid corrosion in oil refining. Br. Corros. J. 1999, 34 (2), 125–131.

*To whom correspondence should be addressed. E-mail: jmshaw@ ualberta.ca. (1) National Energy Board. Canada’s Oil Sands. Opportunities and Challenges to 2015: An Update; National Energy Board: Calgary, Alberta, Canada, June 2006. (2) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (4) Speight, J. G.; Francisco, M. A. Studies in Petroleum CompositionChanges in the Nature of Chemical-Constituents During Crude-Oil Distillation. Rev. Inst. Fr. Pet. 1990, 45 (6), 733–740. (5) Ali, V. A. The Impact of Phase Behaviour on Coke Formation in Delayed Cokers. M.S. Thesis, University of Toronto, Toronto, Canada, 2002. (6) Gray, M. R. Consistency of Asphaltene Chemical Structure with Pyrolysis and Coking Behaviour, Energy Fuels 2003, 17, 1556-1559. (7) McFarlane, R. A. Evaluation of New Co-Volume Mixing Rules for the Peng-Robinson Equation of State. M.S. Thesis, University of Alberta, Edmonton, Canada, 2007. (8) Lian, H. J.; Lin, J. R.; Yen, T. F. Peptization Studies of Asphaltene and Solubility Parameter Spectra. Fuel 1994, 73 (3), 423–428. (9) Sheu, E. Y.; Detar, M. M.; Storm, D. A.; Decanio, S. J. Aggregation and Kinetics of Asphaltenes in Organic Solvents. Fuel 1992, 71 (3), 299–302. (10) Speight, J. G. The chemical and physical structure of petroleum: effects on recovery operations. J. Pet. Sci. Eng. 1999, 22 (1-3), 3–15. (11) Mansoori, G. A. Asphaltene, resin, and wax deposition from petroleum fluids: Mechanisms and modeling. Arabian J. Sci. Eng. 1996, 21 (4B), 707–723. r 2010 American Chemical Society

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challenging issue related to the production and refining of acidic crude oils.23-27 Athabasca vacuum residue (AVR) þ pentane mixtures deposit thin, adherent, and sticky films on stainless steel surfaces.28 For example, mixtures comprising as little as 5% AVR generate adherent and sticky deposits if the mixture is cooled below ∼423 K. This behavior is of significant interest to both refiners and producers of heavy oil as the conditions where deposition occurs overlap both production and refining conditions for Athabasca bitumen, e.g., diluent injection based production and paraffinic deasphalting processes for bitumen. The phase behavior of AVR þ pentane mixtures is complex and, in addition to possible solid behaviors at low temperatures, up to three liquid phases may be present simultaneously.29 As the relative amounts of each phase varies significantly with composition, the phase in direct contact with surfaces, and the dispersed phase(s), is expected to vary. For example, a low-density pentane-rich liquid phase, L1, dominates by volume at low AVR mass fractions. An AVR constituent rich liquid phase, L2, appears at ∼5 wt % AVR but comprises a small volume fraction of the total liquid even at 40 wt % AVR. At ∼45 wt % AVR, an even smaller volume fraction of a third more dense liquid phase, L3, appears. From ∼60 to 100 wt % AVR, only the L2 phase is present. Deposition is reversible and appears to be worst in the low-density liquid þ vapor (L1 V) and the low-density liquid þ medium density liquid þ vapor (L1L2 V) regions present at low AVR mass fraction and least problematic in the L1L2L3 V four phase region (∼45 wt % AVR). Given the diversity of phase and deposit behaviors observed, differences in deposit thickness and composition with global composition are readily anticipated. From work in cognate fields, one cannot assume that deposits are solely or even principally asphaltenes.30,31 From prior work with asphaltene þ solvent mixtures, we are aware that substrates are likely to saturate quickly at low

asphaltene concentrations and that surface contamination is a significant experimental artifact.32-34 Key findings from recent asphaltene þ solvent studies show that the extent to which asphaltenes sorb on mineral substrates is determined primarily by the acid-base characteristics of the mineral surface. Time and temperature of contact are only important variables if the concentration of asphaltenes is low.35,36 Asphaltenes appear to bind to minerals primarily by forming dipole-dipole bonds with surface hydroxyl groups.37 The largest adsorption values have been observed for highly acidic substrates, such as kaolinite (Al2Si2O5(OH)4) and hydrophilic hydroxylated quartz (SiO(OH)2).35,38 By contrast, basic or neutral substrates display a much lower affinity for asphaltenes.35 Variations of 20-fold in average deposit thickness have been observed, as shown in Table 1, where deposit thickness values range from 0.4 to 7.6 nm. This range corresponds to partial coverage to at most a few molecular layers as the nominal leading dimensions of asphaltene molecules exceed 1 nm. If asphaltene aggregates as opposed to asphaltene molecules are sorbing on these substrates, then the observed coverages are partial to at most two layers as the minimum leading dimensions of asphaltene aggregates are thought to exceed ∼3 nm.39 In the absence of competition, asphaltene sorption appears to be quite variable and frequently yields only partial coverage of substrate surfaces. In reservoir, transport, and refining applications, surface sorption is competitive. A study, performed by Henry and Fuhr40 who ultracentrifuged Athabasca oilsand samples without adding solvent, showed that the bitumen adhering to the sand grains was asphaltene deficient relative to the supernatant bitumen. This important finding contradicts normal expectations and has been overlooked in the literature. Four substrates, two acidic and two basic, were chosen for the present study. The metal sulfide selected, iron(II) sulfide, is a synthetic mimic for mackinawite (FeS). The hydrated, disordered surface of this mineral is strongly acidic overall owing to the presence of highly acidic monocoordinated and weakly acidic tricoordinated sulfur centers on the surface.41

(23) Brandal, O.; Sjoblom, J. Interfacial behavior of naphthenic acids and multivalent cations in systems with oil and water. II: Formation and stability of metal naphthenate films at oil-water interfaces. J. Dispersion Sci. Technol. 2005, 26 (1), 53–58. (24) Rousseau, G.; Zhou, H.; Hurtevent, C. Calcium Carbonate and Naphthenate Mixed Scale in Deep-offshore Fields. SPE Oilfield Scale Symposium, Aberdeen, U.K., 2001; Vol. SPE 68307. (25) Vindstad, J. E.; Bye, A. S.; Grande, K. V.; Hustad, B. M.; Hustvedt, E.; Nergard, B. Fighting naphthenate deposition at the Statoil-operated heidrun field. In SPE 5th International Symposium on Oilfield Scale, Aberdeen, U.K., 2003; Vol. SPE 80375. (26) Brandal, O.; Hanneseth, A. M.; Hemmingsen, P. V.; Sjoblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Isolation and characterization of naphthenic acids from a metal naphthenate deposit: Molecular properties at oil-water and air-water interfaces. J. Dispersion Sci. Technol. 2006, 27 (3), 295–305. (27) Poggesi, G.; Hurtevent, C.; Buchart, D. Multifunctional Chemicals for West African Deep Offshore Fields. SPE Oilfield Scale Symposium, Aberdeen, U.K., 2002; Vol. SPE 74649. (28) Xiang-Yang, Z.; Shaw, J. M. Dispersed Phases and Dispersed Phase Deposition Issues Arising in Asphaltene Rich Hydrocarbon Fluids. Pet. Sci. Technol. 2004, 22 (7/8), 759–771. (29) Zou, X.; Zhang, X.; Shaw, J. M. The Phase behaviour of Athabasca Bottoms þ n-Alkane Mixtures. SPE Prod. Oper. 2007, 22 (2), 7. (30) Wu, X. Investigating the Stability Mechanism of Water-inDiluted Bitumen Emulsions through Isolation and Characterization of the Stabilizing Materials at the Interface. Energy Fuels 2003, 17 (1), 179–190. (31) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A. Compositional Characterization of Bitumen/Water Emulsion Films by Negative- and Positive-Ion Electrospray Ionization and Field Desorption/Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2007, 21 (2), 963–972. (32) Xie, K.; Karan, K. Kinetics and Thermodynamics of Asphaltene Adsorption on Metal Surfaces: A Preliminary Study. Energy Fuels 2005, 19 (4), 1252–1260.

(33) Taylor, S. E. The electrodeposition of asphaltenes and implications for asphaltene structure and stability in crude and residual oils. Fuel 1998, 77 (8), 821–828. (34) Akbarzadeh, K.; Alboudwarej, H.; Svrcek, W. Y.; Yarranton, H. W. A generalized regular solution model for asphaltene precipitation from n-alkane diluted heavy oils and bitumens. Fluid Phase Equilib. 2005, 232 (1-2), 159–170. (35) Dudasova, D.; Simon, S.; Hemmingsen, P. V.; Sj€ oblom, J. Study of asphaltenes adsorption onto different minerals and clays: Part 1. Experimental adsorption with UV depletion detection. Colloids Surf., A 2008, 317 (1-3), 1–9. (36) Mendoza de la Cruz, J. L.; Castellanos-Ramı´ rez, I. V.; OrtizTapia, A.; Buenrostro-Gonzales, E.; Duran-Valencia, C. A.; LopezRamirez, S. Study of monolayer to multilayer adsorption of asphaltenes on reservoir rock minerals. Colloids Surf., A 2009, 340, 149–154. (37) Bantignies, J.-L.; Cartier dit Moulin, C.; Dexpert, H. Asphaltene adsorption on kaolinite characterized by infrared and X-ray absorption spectroscopies. J. Pet. Sci. Eng. 1998, 20 (3-4), 233–237.  Berkesi, O.; Dekany, I. Asphaltene (38) Pernyeszi, T.; Patzk o, A.; adsorption on clays and crude oil reservoir rocks. Colloids Surf., A 1998, 137 (1-3), 373–384. (39) Chainelli, R. R.; Siadati, M.; Mehta, A.; Pople, J.Ortega, L. C.; Chiang, L. Y. Self-assembly of asphaltene aggregates: synchrotron, simulation and chemical modelling techniques applied to problems in structure and reactivity of asphaltenes. In Asphaltenes, Heavy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; p 375. (40) Henry, D.; Fuhr, B. Preparation of bitumen form oil sand by ultracentrifugation. Fuel 1992, 71 (December), 4. (41) Wolthers, M.; Charlet, L.; van Der Linde, P. R.; Rickard, D.; van Der Weijden, C. H. Surface chemistry of disordered mackinawite (FeS). Geochim. Cosmochim. Acta 2005, 69 (14), 3469–3481.

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Table 1. Asphaltene Sorption on Mineral Substrates35,38 mineral surface kaolinite (Al2Si2O5(OH)4) BaSO4 CaCO3 FeS Fe3O4 SiO2 (hydroxylated) SiO2 SiO2(hydroxylated) kaolinite (Al2Si2O5(OH)4) illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] a

surface nature

asphaltene source

acidic basic basic acidic acidic acidic neutral acidic acidic mildly acidic

Gulf of Mexico, W. Africa, North Sea, Brazil

1:1 toluene-n-heptane

Algyo oil field, Hungary

toluene

solvent

asphaltene sorbed (mg/m2)

deposit thickness, nma

1.9-2.8 0.8-1.4 1.0-1.6 1.0-3.2 1.4-1.9 1.8-3.8 0.3-0.7 8.8 1.9 0.6

1.6-2.4 0.7-1.2 0.9-1.4 0.9-2.8 1.2-1.6 1.6-3.3 0.3-0.6 7.6 1.6 0.5

Estimated assuming asphaltene density is 1160 kg/m3.

the result of the interaction of π electrons in the aromatic rings with terminal hydroxyl groups on the surface of the substrate, along with classical van der Waals forces.48 PAHs are also expected to bind to the sorbed organic contaminants, albeit with relatively weak van der Waals forces alone. Sorbed PAH molecules are expected to have their molecular planes aligned parallel to the substrate surface. This type of planar sorption geometry, which has been observed for acenes on rutile (TiO2),49 maximizes the van der Waals forces between the sorbed PAH and the substrate. Whether deposits on these substrates arise due to adsorption or absorption remains an open question as both asphaltene adsorption and absorption have been reported.38,50-53 In the absence of molecular level markers, elemental composition, accessible using XPS measurements, is the primary determinant for which oil constituents are sorbed on a specific surface. XPS also offers the possibility of subspeciation of elements, such as sulfur into thiophenic and sulfide forms. For example, the sulfur content of supercritical fluid extracts from Athabasca bitumen vacuum residue has been studied systematically.54 The authors suggest that sulfur is present in all fractions in both sulfide and thiophenic forms. The total sulfur content ranges from ∼3.5 wt % for the lightest fractions to ∼6.5 wt % for the heaviest fraction (34% of the bitumen). Thiophenic sulfur comprises a minimum of ∼65% of the sulfur present in the lightest fractions and comprises ∼80% of the sulfur present in the heaviest fraction, which comprises ∼88 wt % heptane asphaltenes. The sulfur speciation error is not reported, but from the scatter in the data it is ∼5% or more for individual measurements. It would appear that deposits that are essentially asphaltenes can in principle be

Synthetic silica was chosen as an artificial quartz. Solid silica rapidly reacts with water vapor to give a surface bristling with polar hydroxyl groups. The high surface acidity of hydrolyzed silica is due primarily to a combination of its small dielectric constant and its large bond strength to bond length ratio (s/r).42 Synthetic nickel(II) oxide, or imitation bunsenite (NiO), also hydrolyzes in moist air to give a substrate that is extensively covered by hydroxyl groups.43-45 Hydrated nickel monoxide is mildly basic owing to covalent bonding between Ni2þ ions, which impart additional stability to the oxide, thereby lowering its basicity to a level below that expected from its optical basicity value.44 Hydroxylated iron(II) oxide which mimics the oxide films on native iron samples is generated by exposing hematite (Fe2O3) to air containing water vapor. An X-ray photoelectron spectroscopy (XPS) study performed on a hydroxylated iron(III) oxide surface suggests that the film possesses a bilaminate structure composed of an upper layer containing hematite (Fe2O3) and goethite (FeO(OH)) and a lower layer containing (wustite) FeO and Fe3O4 (magnetite).46 Again, the upper surface has a substantial hydroxyl population. Hydroxylated iron oxide films possess an isoelectric point of approximately 10 for iron(III) oxide.46,47 Thus, hydroxylated, hydrolyzed iron(III) surfaces are strongly basic. By contrast, hydrolyzed iron(II) oxide is expected to be less basic because of covalent bonding between the divalent iron centers.44 As metal oxides and sulfides also comprise wetted surfaces of process equipment and catalysts, experimental outcomes with these substrates are equally applicable to down hole and surface processing applications. The structure of molecules present in Athabasca vacuum residue is poorly defined, but polynuclear aromatics are the principal constituents. The sorption of polynuclear aromatics (PAHs), such as pyrene, on hydroxylated oxide substrates is

(48) Ara ujo, R. S.; Azevedo, D. C. S.; Cavalcante, C. L., Jr.; JimenezL opez, A.; Rodrı´ guez-Castell on, E. Adsorption of polycyclic aromatic hydrocarbons (PAHs) from isooctane solutions by mesoporous molecular sieves: Influence of the surface acidity. Microporous Mesoporous Mater. 2008, 108 (1-3), 213–222. (49) Reiss, S.; Krumm, H.; Niklewski, A.; Staemmler, V.; W€ oll, C. The adsorption of acenes on rutile TiO[sub 2](110): A multi-technique investigation. J. Chem. Phys. 2002, 116 (17), 7704. (50) Gonzalez, M. F.; Stull, C. S.; Lopez-Linares, F.; Pereira-Almao, P. Comparing asphaltene adsorption with model heavy molecules over macroporous solid surfaces. Energy Fuels 2007, 21 (1), 234–241. (51) Crocker, M. E.; Marchin, L. M. Wettability and Adsorption Characteristics of Crude-Oil Asphaltene and Polar Fractions. J. Pet. Technol. 1988, 40 (4), 470–474. (52) Aske, N. Characterisation of Crude Oil Components, Asphaltene Aggregation and Emulsion Stability by means of Near Infrared Spectroscopy and Multivariate Analysis. Ph.D. Thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2002. (53) Mullins, O. C.; Sheu, E. Y. Structures and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (54) Zhao, S.; Xu, Z.; Xu, C.; Chung, K. H.; Wang, R. Systematic characterization of petroleum residua based on SFEF. Fuel 2005, 84 (6), 635–645.

(42) Sahai, N. Is Silica Really an Anomalous Oxide? Surface Acidity and Aqueous Hydrolysis Revisited. Environ. Sci. Technol. 2002, 36 (3), 445–452. (43) Ageeva, Y.; Gorichev, I.; Izotov, A.; Pichugina, N.; Martynova, T. Calculation and comparison of the acid-base equilibrium constants for nickel(II) oxide determined by various methods. Theor. Found. Chem. Eng. 2007, 41 (6), 859–867. (44) Duffy, J. A. Acid & Base Reactions of Transition Metal Oxides in the Solid State. J. Am. Ceram. Soc. 1997, 80 (6), 1416–1420. (45) Pereira, P.; Lee, S. H.; Somorjai, G. A.; Heinemann, H. The conversion of methane to ethylene and ethane with near total selectivity by low temperature (< 610° C) oxydehydrogenation over a calciumnickel-potassium oxide catalyst. Catal. Lett. 1990, 6 (3), 255–262. (46) Kurbatov, G.; Darque-Ceretti, E.; Aucouturier, M. Characterization of hydroxylated oxide film on iron surfaces and its acid-base properties using XPS. Surf. Interface Anal. 1992, 18 (12), 811–820. (47) Simmons, G. W.; Beard, B. C. Characterization of acid-base properties of the hydrated oxides on iron and titanium metal surfaces. J. Phys. Chem. 1987, 91 (5), 1143–1148.

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distinguished from those that are essentially maltenes from differences in sulfur subspeciation, as well as total sulfur compositions. However, there is significant overlap, among the binding energies for the stronger 2p3/2 peaks for thiophenic, sulfidic, and thiol sulfur and weaker 2p1/2 peaks (appearing at slightly higher binding energies) in the 162.5-166.5 eV range. The details of molecular structures appear to be important. NIST has compiled a large database of XPS data.55 Peak assignment is uncertain. For example, the 2p3/2 peak for C16H10S, where the sulfur is thiophenic, is reported to be 164 eV, while for ;C6H4S;, a polymeric material where S is nominally sulfidic, the peak is at 163.8 eV. The 2p3/2 peak for sulfur in FeS2 is in the 162-163 eV range and can interfere with measurements of the 2p3/2 peak for sulfur in numerous compounds including either sulfide or thiol. Compound specific peak smearing and peak shifting also occurs when surface sorption occurs, even on gold.56 This latter effect, which broadens peaks toward lower binding energies further complicates peak assignment and peak identification on substrates. Thus it is unclear whether sulfur subspeciation is feasible in the present work. Adherent deposits observed for AVR þ pentane mixtures may arise from asphaltene or asphaltene-rich deposits because asphaltene precipitation has been shown to be largely reversible,57-61 and asphaltenes62 and asphaltene-rich deposits undergo transitions to a liquid in a relevant temperature interval.59,63-66 In this exploratory experimental work, deposits formed from AVR þ pentane mixtures are investigated using well-defined acidic (FeS,41 SiO242,67,68) and basic (Fe2O3/ FeOOH/FeO46,47,68 and Ni/NiO/NiOH43,44) substrates. The impacts of surface properties, mixture phase behavior, and mixture composition on deposit thickness and deposit composition are evaluated using XPS. Competition among vacuum residue

constituents as functions of mixture and substrate composition are subjects of particular interest. Deposition experiments with model compounds are used to validate the experimental approaches and analytical methods employed. 2. Experimental Section 2.1. Materials. The sources and purities of pure compounds used are listed in Table 2. The composition of AVR, a 525 °C boiling fraction of Athabasca bitumen, supplied by CANMET, is given elsewhere.69,70 The carbon and sulfur wt % for AVR and AVR pentane asphaltenes used here are 82% and 7% and 81% and 7.5%, respectively. The sulfur values are higher than reported above, but such variation is not unexpected when working with industrial samples and different experimental techniques. Silicon Valley Microelectronics provided silicon wafers with 100 mm diameters and 525 μm thicknesses that were prepolished on one face and etched on the other face. Fe2O3 and NiO sputtering targets were obtained from Kurt J. Lesker. 2.2. Naphthenic Acid and Metal Naphthenates. Naphthenic acid is a generic term used to include aromatic and acyclic acid components in crude oils.71 Naphthenic acid homologues can be represented by a general formula CnH2nþzO2, where n indicates the carbon number and z specifies a homologous series. For example, z is equal to 0 for saturated aliphatic carboxylic acids. Naphthenic acids in crude oils comprise C5-C50 compounds containing 0-6 fused rings, most of which are saturated, in which the carboxylic acid group is attached to a ring through a short side chain.72,73 Naphthenic acid (C11H8O2) and metal naphthenates (M(C11H7O2)2, where M = Ni or Zn) were chosen for this study. Zinc and nickel TPP complexes (TPP = tetraphenylporphine dianion), viz., Zn(C44H28N4) = Zn(II)(TPP) and Ni(C44H28N4) = Ni(II)(TPP), were also selected for investigation as these constitute an important class of basic compounds found in bitumen and in reservoir rock.74 2.3. Substrate Construction. Under exposure to oxygen and water vapor, silicon substrates naturally oxidize. Thin silicon dioxide (SiO2) layers are formed.75 Piranha solution, a 3:1 mixture of concentrated sulfuric acid and 30% hydrogen peroxide, is used to remove organic residues from silica. Piranha solution also hydroxylates the silica surface, thereby making it more polar and hence more hydrophilic.76-78 Piranha solution

(55) NIST X-ray Photoelectron Spectroscopy Database, version 3.5; National Institute of Standards and Technology: Gaithersburg, MD, 2003; http://srdata.nist.gov/xps/. (56) Castner, D. G.; Hinds, K.; Grainger, D. W. X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12 (21), 5083–5086. (57) Andersen, S. I.; Stenby, E. H. Thermodynamics of asphaltene precipitation and dissolution investigation of temperature and solvent effects. Fuel Sci. Technol. Int. 1996, 14 (1-2), 261–287. (58) Mohamed, R. S.; Loh, W.; Ramos, A. C. S.; Delgado, C. C.; Almeida, V. R. Reversibility and inhibition of asphaltene precipitation in Brazilian crude oils. Pet. Sci. Technol. 1999, 17 (7-8), 877–896. (59) Kokal, S. L.; Najman, J.; Sayegh, S. G.; George, A. E. Measurement and Correlation of Asphaltene Precipitation from Heavy Oils by Gas Injection. J. Can. Pet. Technol. 1992, 31 (4), 24–30. (60) Buckley, J. S. Sill North American Chemical Congress; 1997. (61) Hammami, A.; Changyen, D.; Nighswander, J. A.; Stange, E. An Experimental Study of the Effect of Paraffinic Solvents on the Onset and Bulk Precipitation of Asphaltenes. Fuel Sci. Technol. Int. 1995, 13 (9), 1167–1184. (62) Fulem, M.; Becerra, M.; Hasan, M. D. A.; Zhao, B.; Shaw, J. M. Phase behaviour of Maya crude oil based on calorimetry and rheometry. Fluid Phase Equilib. 2008, 272 (1-2), 32–41. (63) Zou, X. Y.; Shaw, J. M. The Phase Behavior of Athabasca Bitumen Vacuum Bottoms þ Alkane Solvent Systems. In Heavy Organics Deposition (HOD-2002), Puerto Vallarta, Mexico, 2002. (64) Chung, F. T.-H.; Sarathi, P.; Jones, R. Modeling of Asphaltene and Wax Precipitation; NIPER-498, 1991. (65) Hischberg, A.; de Jong, L. N. J.; Schipper, B. A.; Meijers, J. G. Influence of temperature and pressure on asphaltene precipitation. Soc. Pet. Eng. 1984, 283. (66) Storm, D. A.; Sheu, E. Y. Colloidal nature of petroleum asphaltenes. In Asphaltenes and Asphalts, 1; Elsevier Science Press: New York, 1994. (67) Casamassima, M.; Darque-Ceretti, E.; Etcheberry, A.; Aucouturier, M. Acid-base behavior of aluminum and silicon oxides-a combination of two approaches: XPS and Lewis acido-basicity; rest potential and Br€ onsted acido-basicity. Appl. Surf. Sci. 1991, 52 (3), 205–213. (68) Parks, G. A. The Isoelectric Points of Solid Oxides, Solid Hydroxides, and Aqueous Hydroxo Complex Systems. Chem. Rev. 1965, 65 (2), 177–198.

(69) Zhao, B.; Shaw, J. M. Composition and Size Distribution of Coherent Nanostructures in Athabasca Bitumen and Maya Crude Oil. Energy Fuels 2007, 21 (5), 2795–2804. (70) Zou, X.-Y.; Dukhedin-Lalla, L.; Zhang, X.; Shaw, J. M. Selective Rejection of Inorganic Fine Solids, Heavy Metals, and Sulfur from Heavy Oils/Bitumen Using Alkane Solvents. Ind. Eng. Chem. Res. 2004, 43 (22), 7103–7112. (71) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14 (1), 217–223. (72) Robbins, W. K. Challenges in the characterization of naphthenic acids in petroleum. Abstr. Pap. Am. Chem. Soc. 1998, 215, U119–U119. (73) Seifert, W. K.; Teeter, R. M. Identification of Polycyclic Napthenic, Monoaromatic and Diaromatic Crude Oil Carboxylic Acids. Anal. Chem. 1970, 42 (2), 180. (74) Ocampo, R.; Bauder, C.; Callot, H. J.; Albrecht, P. Porphyrins from Messel oil shale (Eocene, Germany): Structure elucidation, geochemical and biological significance, and distribution as a function of depth. Geochim. Cosmochim. Acta 1992, 56 (2), 745–761. (75) Jaeger, R. C. Introduction to Microelectronic Fabrication; Prentice Hall: Upper Saddle River, NJ, 2002. (76) Leewis, C. M.; Kessels, W. M. M.; van de Sanden, M. C. M.; Niemantsverdriet, J. W. On the H-exchange of ammonia and silica hydroxyls in the presence of Rh nanoparticles. Appl. Surf. Sci. 2007, 253 (7), 3600–3607. (77) Altavilla, C.; Ciliberto, E.; La Delfa, S.; Panarello, S.; Scandurra, A. The cleaning of early glasses: investigation about the reactivity of different chemical treatments on the surface of ancient glasses. Appl. Phys. A: Mater. Sci. Process. 2008, 92 (1), 251–255. (78) Dugas, V.; Chevalier, Y. Surface hydroxylation and silane grafting on fumed and thermal silica. J. Colloid Interface Sci. 2003, 264 (2), 354–361.

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chemical hydrogen nitrogen carbon disulfide pyrene (C16H10) naphthenic acid (C11H8O2) zinc(II) naphthenate ∼(Zn(C10H19O2)2 pentane tetrahydrofuran nickel(II) tetraphenylporphine (nickel (TPP)) Ni(C44H28N4) zinc(II) tetraphenylporphine (zinc (TPP)) Zn(C44H28N4)

grade, purity

supplier

research, 99.999% research, 99.998% research, 99.8þ% research, 98% technical, 90% 46 wt % þ mineral spirits (diethylene glycol, monoethyl ether, paraffinic hydrocarbons) research, 99% research, 99.9þ% research, 99.9% research, 99.9%

Praxair Praxair Aldrich Lancaster Aldrich Aldrich

2.5. Substrate and Deposit Analysis. While in situ competitive sorption measurement techniques are beginning to emerge,82 ex situ XPS measurements suffice for this application because of the low volatility of the materials depositing and their adherence to the surfaces. XPS has a variety of applications, including determination of the elemental composition of substrates (110 nm deep usually), identification of elements that contaminate a surface, assessment of the chemical or electronic state of each element on a surface, appraisal of the uniformity of elemental composition across the top of a surface, and estimation of the thickness of one or more thin layers (1-8 nm) within the top 10 nm of a surface. It is used in this work to obtain average deposit thickness, average deposit composition, and the chemical states of elements. The measurements were performed on an AXIS ULTRA spectrometer (Kratos Analytical) at the Alberta Centre for Surface Science and Engineering. The base pressure in the analytical chamber was lower than 4  10-8 Pa, and the working pressure was less than 3  10 -7 Pa. The resolution function of the instrument for an Al-mono source in the hybrid lens mode is 0.4 eV on the basis of the Co Fermi edge, 0.55 eV for Ag 3d, and 0.70 eV for Au 4f peaks. Monochromated Al KR X-rays (hν = 1486.6 eV) were used at a power of 210 W. Fixed analyzer transmission mode was applied. The analysis spot was 700 μm  400 μm. Charge neutralization was required during the measurements. All survey scans spanned from 1100 to 0 eV binding energy and were collected with an analyzer pass energy of 160 eV with a step of 0.3 eV. For the high-resolution spectra, the pass energy was 20 eV with a step of 0.1-0.15 eV. Raw bond energy values were scaled linearly with respect to carbon 1s (284.8 eV) for each elemental analysis. Adjustments ranged from þ1.7 to -0.4 eV. Such calibration is essential for elemental speciation. 2.6. Average Deposit Thickness. The inelastic mean free path (IMFP), a measure of the average distance traveled by electrons through a solid before they scatter inelastically, is defined by eq 1: PðdÞ ¼ expð-d=λÞ ð1Þ

Table 3. Sputtering System Parameters thin film sputtering techniques argon pressure argon flow rate sputter power shutter delay

nickel(II) oxide

iron(III) oxide

dc sputtering 4 mtorr 10 SCCM 150 W 120 s

rf sputtering 4 mtorr 10 SCCM 49 W 3300 s

Table 4. Deposition Temperature for AVR þ Pentane Mixtures AVR, wt fraction

T, K

0.00 0.05 0.15 0.30 0.45 0.60 0.80 1.00

298 443 398 373 333 353 373 413

Fisher Aldrich Aldrich Aldrich

can be explosive, and thus great care must be exercised when preparing or working with it.79 Iron(III) oxide and nickel substrates were deposited on the Si substrate using an ATC ORION 5 UHV sputtering system (AJA International Inc.).80,81 For Ni deposition, a direct current (dc) gun was chosen for sputtering; while in the case of iron(III) oxide, a radio frequency (rf) gun was used as an insulating target cannot be sputtered by a dc gun. A film of native nickel(II) oxide formed on top of the metallic nickel shortly after it was deposited. Sputtering parameters are provided in Table 3. Iron(II) sulfide substrates were prepared by sulfidation of iron(III) oxide substrates. Sulfidation was realized by reacting iron(III) oxide coated wafer segments in a 15 mL batch reactor with 15 μL of CS2 in the presence of H2 at a pressure of 101.325 kPa for 150 min in a sand bath operated at 523 K. 2.4. Deposition Experiments. Deposition experiments were designed to mimic the conditions and time frames of the pentane þ AVR phase equilibrium measurements. A total of 5 g of specific AVR þ pentane mixtures were prepared in a 15 mL batch reactor. A wafer segment was added. The reactor was then heated to a temperature just above the deposition temperature, which varied with composition, Table 4, for a time period t1 and then quickly cooled to room temperature by immersion in a water bath. Wafer segments were then removed from the reactors and immersed in pentane at room temperature for a time period t2. Washing the organic deposit to remove bulk liquid from the surface is a key step as illustrated in Figure 1, as much of the deposit does not adhere. Washed wafer segments were dried and subjected to XPS analysis. Values for time periods t1 and t2 are shown in Table 5. While these time variables were not expected to play a significant role, they were varied as a precautionary measure and for completeness.

where P(d) is the probability that an electron with an energy E travels a distance, d, through a material and λ is the IMFP for the electron. λ is dependent on the initial kinetic energy of an electron and the nature of the solid. However, most elements show very similar IMFP versus energy relationships. The probability decays exponentially with distance and is less than 5% for normalized distances d/λ > 3. The average thickness of deposits is obtained from the ratio of the intensity signal from the substrate, Io, and the coated substrate, I, assuming the element is not present in the deposit: t ¼ 3λ logðIo =IÞ ð2Þ For example, consider the XPS analysis of an iron(III) oxide substrate using monochromatic incident Al KR X-rays with a kinetic energy of 1486.6 eV. In the Fe region of the XPS spectrum, the ejected electrons possess a kinetic energy of

(79) Park, J.; Henn-Lecordier, L. http://www.enma.umd.edu/LAMP/ Sop/Piranha_SOP.htm. (80) Mattox, D. M. The Foundation of Vacuum Coating Technology; William Andrew: Norwich, NY, 2003. (81) Ohring, M. The Materials Science of Thin Films 2nd ed.; Academic Press: San Diego, CA, 2002.

(82) Yang, Z.; Li, Q.; Hua, R.; Gray, M.; Chou, K. C. Competitive adsorption of toluene and n-alkanes at binary solution/silica interfaces. J. Phys. Chem. C 2009, 113, 20355–20359.

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Figure 1. (a) Liquid film þ deposit on an iron(III) oxide substrate; (b) surface deposit following a pentane wash at room temperature. Table 5. Deposition Experimental Matrix for AVR þ Pentane Mixtures

Table 6. XPS Parameters for the Substrates Examined under Al KR X-ray Irradiation

iron(III) oxide surface 40 nm

137 nm

t2 = 30 min t1 = 2 h t1 = 2h 30 min t1 = 3 h t1 = 3h 30 min t1 = 4 h t1 = 4h 30 min

t1 = 2 h t1 = 2h 30 min t1 = 3 h t1 = 3h 30 min t1 = 4 h t1 = 4h 30 min t1 = 5 h t2 = 0 min t1 = 3h 30 min t2 = 0 min t2 = 30 min t2 = 30 min t2 = 1 h t2 = 1 h t2 = 1h 30 min t2 = 1h 30 min t2 = 2 h t2 = 2 h t2 = 2h 30 min t2 = 2h 30 min

nickel(II) oxide and silica substrates t1 = 1 h t1 = 1h 30 min t1 = 2 h t1 = 2h 30 min t1 = 3 h t1 = 3h 30 min t1 = 4 h t1 = 4h 30 min t1 = 5 h

photoelectron peak/substrate

kinetic energy of ejected electrons (eV)

λ(IMFP) (nm)

Fe 2p/iron(III) oxide Ni 2p/nickel(II) oxide Fe 2p/iron(II) sulfide Si 2p/silica

777 634 777 1387

1.65 1.25 1.63 3.75

and AVR pentane asphaltenes are ∼11.7 and ∼10.8, respectively. Sulfur subspeciation is a variable of secondary interest.

3. Results and Discussion 3.1. Substrate Compositions and Surface Contaminants. Substrate composition and contamination are of critical importance in this work. Raw data for substrate compositions, summarized in Table 7, clearly indicate nitrogen, carbon, and sulfur contamination of the substrate substrates. Oxygen contamination from constituents in the air, i.e., O2, CO2, and H2O, was also anticipated. All of the substrates are contaminated. Parts a and b of Figure 2 show high-resolution XPS spectrograms for elemental oxygen and carbon, respectively, in a hydrolyzed, hydroxylated iron(III) oxide substrate. Peak fitting and line shape analyses were performed on each peak. The oxygen 1s ensemble is composed largely of two superimposed peaks. The more intense peak, which possesses a binding energy of 530.1 eV can be assigned to oxygen in the metal oxide (O2-)46 and represents 65.4% of the oxygen. The smaller signal at 531.6 eV corresponds to oxygen in the surface hydroxyls (O;H).46 Much weaker signals arising from C;O and CdO moieties and the O;H bonds of absorbed water are buried well beneath the two dominant peaks.46 The carbon 1s region contains three overlapping-peaks. The largest peak at 285.1 eV, which constitutes 72.7% of the area of the C 1s signal, indicates the presence of C;C and C;H bonds.84 The second largest peak at 286.0 eV (18.2% of the total signal area) is assigned to C;O type bonding,84 while the shoulder centered at 288.8 eV (9.1% of the total signal area) corresponds to carbon atoms in carboxylate groups (OdC;O).85 These forms of carbon all come from airborne contaminants. The C 1s

776.6 eV, which is equal to the difference between the binding energy for the Fe 2p electron (710.0 eV) and the kinetic energy for the impinging Al KR X-rays. Analysis of this data with the Quases IMFP-TPP2 M program,83 yields a λ value of 1.65 nm for iron(III) oxide. Equation 2 was applied to calculate the average thickness of deposits. This approach is valid as long as the thickness is less than ∼3λ (5 nm for this case). The IMFP (λ) values for all of the substrates investigated in this study are summarized in Table 6. 2.7. Deposit Composition. Elements present and their electronic states can be detected by XPS for all elements except hydrogen and helium, and detailed compositions for the sputtered inorganic substrates, contaminants, and organic deposits were obtained. Signals arising from contaminants on the substrates were subtracted on an element specific basis, using eq 1, once the average deposit thickness was established. The thicker the deposit, the less correction was required. A key assumption in this work is that the contaminants on a surface are unaffected by subsequent processing. This assumption is readily validated using pure compounds. For pure compounds, the elemental composition is known and direct comparison with measured elemental surface compositions can be performed. As surface contamination by carbon monoxide, carbon dioxide, water, etc. is expected, we also elected to compare carbon to sulfur ratios for deposits from AVRþpentane mixtures, with carbon to sulfur ratios for AVR constituents. The mass fractions of these elements are large and this ratio is the most robust comparative measure available. The bulk carbon to sulfur ratios for AVR

(84) Strein, E.; Allred, D. Eliminating carbon contamination on oxidized Si surfaces using a VUV excimer lamp. Thin Solid Films 2008, 517 (3), 1011–1015. (85) Rodriguez, N. M.; Anderson, P. E.; Wootsch, A.; Wild, U.; Schl€ ogl, R.; Paal, Z. XPS, EM, and Catalytic Studies of the Accumulation of Carbon on Pt Black. J. Catal. 2001, 197 (2), 365–377.

(83) Tougard, S. QUASES IMFP-TPP2M Software Package, 2002.

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Table 7. Substrate Surface Compositions (Mass Percent)

Fe Ni Si O N C S

iron(III) oxide (40 nm thick film)

iron(III) oxide (137 nm thick film)

iron(II) sulfide

nickel(II) oxide

silica

57.4 ( 3.4 0 0 27.5 ( 2.0 0.23 ( 0.12 11.8 ( 1.4 0.7 ( 0.6

55.7 0 0 25.6 0.07 18.7 0.00

40.2 ( 7.8 0 0 17.2 ( 2.1 0.4 ( 0.3 24.8 ( 6.4 17.0 ( 1.3

0 69.2 ( 1.2 0 17.9 ( 0.3 0.06 ( 0.06 12.6 ( 1.0 0.04 ( 0.1

0 0 64.1 ( 0.6 25.7 ( 0.6 0.4 ( 0.2 9.5 ( 0.5 0.2 ( 0.5

Figure 3. High-resolution XPS spectra for iron(II) sulfide (a) O 1s and (b) C 1s.

groups.35 The relatively small peak at 288.8 eV is assigned to the carbon atoms in surface carbonate (CO32-) and hydrogen carbonate (HCO3-) groups.90,91 Carbon dioxide in air reacts with water on the outer layer of iron oxide to give carbonic acid (H2CO3), which then inserts into the O;H bonds of surface hydroxyls to give the observed metal carbonates and metal hydrogen carbonates. The O 1s envelope for the iron(II) sulfide surface is composed of three subpeaks (Figure 3a). The peaks at 529.8 eV (35.8%) and 531.6 eV (52%) are attributed to subsurface iron(III) oxyhydroxide (FeO(OH)).92 The third peak at 533.1 eV (12.1%) corresponds to water and/or oxygencontaining organics.46 The C 1s high-resolution spectrum, Figure 3b, is quite similar to the iron(III) oxide spectrum. The major carbon contamination is, once again, from

Figure 2. High-resolution XPS spectra for iron(III) oxide (a) O 1s and (b) C 1s.

hydrocarbon peak at 285.1 eV is attributed to the carbon in chains (;CH2;CH2;) of sorbed paraffins86,87 (e.g., C4C20 n-alkanes). Like the work reported by Furstenau at al.,88 the present XPS study shows that very light alkenes and alkanes such as ethylene and methane do not sorb appreciably on silica or hydroxylated metal oxide substrates at 300 K. The C 1s signal at 286.0 eV is a composite peak produced by carbon atoms in polar, oxygenated organic species, such as acetone, ethanol, dialkyl ethers, and alkyl aldehydes.84,89 These species adhere firmly to the oxide through the formation of strong dipole-dipole bonds with surface hydroxyl (86) Gaspar, A. B.; Perez, C. A. C.; Dieguez, L. C. Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS. Appl. Surf. Sci. 2005, 252 (4), 939–949. (87) Pa al, Z. Carbon accumulation on Pt black from hydrocarbons of different structure. React. Kinet. Catal. Lett. 2004, 82 (2), 381–386. (88) Furstenau, R. P.; Langell, M. A. Adsorption of ethylene on stoichiometric and reduced NiO(100). Surf. Sci. 1985, 159 (1), 108–132. (89) Wielant, J.; Hauffman, T.; Blajiev, O.; Hausbrand, R.; Terryn, H. Influence of the Iron Oxide Acid-Base Properties on the Chemisorption of Model Epoxy Compounds Studied by XPS. J. Phys. Chem. C 2007, 111 (35), 13177–13184.

(90) Ismail, H. M.; Cadenhead, D. A.; Zaki, M. I. Surface Reactivity of Iron Oxide Pigmentary Powders toward Atmospheric Components: XPS, FESEM, and Gravimetry of CO and CO2 Adsorption. J. Colloid Interface Sci. 1997, 194 (2), 482–488. (91) Baltrusaitis, J.; Grassian, V. H. Surface Reactions of Carbon Dioxide at the Adsorbed Water Iron Oxide Interface. J. Phys. Chem. B 2005, 109 (25), 12227–12230. (92) Mansour, A. N.; Brizzolara, R. A. Characterization of the Surface of alpha-FeOOH Powder by XPS. Surf. Sci. Spectra 1996, 4 (4), 357–362.

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Figure 4. High-resolution XPS spectra for nickel(II) oxide (a) O 1s and (b) C 1s.

polymethylene (B.E. 284.7 eV (70.0%)). The other two peaks at 285.8 (20.5%) and 288.4 eV (9.5%), once more, are attributed to oxygenated organic species and carbonate/ hydrogen carbonate, respectively. The O 1s envelope for the Ni(II) oxide substrate, Figure 4a, is composed of three superimposed peaks. The peak at 529.6 eV corresponds to O;Ni bonds and represents (31.4%) of the total oxygen on the surface. The other two subpeaks at 531.3 and 532.0 eV are assigned to O;C and O;H bonds in oxygenated organics (vide supra). The majority of the carbon contamination, Figure 4b, is in the form of polymethylene (vide supra), but some oxygen-bearing organics and carbonates are also present on the surface. The Ni 2p spectrum shows a broad envelope of peaks at ∼856 eV, corresponding to surface NiO, Ni(OH)2, and NiO(OH).45,93 Carbon associated with oxygen constitutes

3.98 wt %. Thus 5.67 wt % of oxygen is involved in oxygen carbon bonding, while 5.62 wt % oxygen is in the form of O;Ni bonds. So, there is about 6.60 wt % oxygen on the surface. The atomic ratio of oxygen to nickel is 0.3, indicating either a thin nickel oxide layer above a Ni substrate or a combined substrate comprising Ni/NiO/NiO(OH)/ Ni(OH)2. The oxygen 1s spectrum for the silica substrate, Figure 5a, has a single peak at 525.5 eV, which is due to superimposed signals for surface Si;O and Si;OH groups.67,78 The carbon 1s spectrum, Figure 5b, is very similar to that of the other substrates. Polymethylene, oxygenated organics, and carbonate contaminants are, once again, present on the surface. The N 1s portion of the silica spectrum, Figure 5c, exhibits a weak, two band structure at ∼401 eV. This signal is probably that of a hydrogen-bonded N2-OH complex.94 According

(93) Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S. New interpretations of XPS spectra of nickel metal and oxides. Surf. Sci. 2006, 600 (9), 1771–1779.

(94) Beebe, T. P., Jr.; Yates, J. T., Jr. Competitive physisorption: The interaction of CO and N2 with silica surfaces as studied by ir spectroscopy. Surf. Sci. 1985, 159 (2-3), 369–380.

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Figure 5. High-resolution XPS spectra for silica (a) O 1s and (b) C 1s.

to its atomic concentration, this bound dinitrogen species covers less than 1% of the surface of the silica. The atomic ratio of oxygen to silicon in the substrate is 0.72, which is less than the expected 1:2 ratio for SiO2. Again, this is a reflection of the thinness of the natural oxide layer. Typically, silicon wafers that have been exposed to air have a native oxide layer that is only ∼2 nm thick.84 Reflectance versus angle of incidence calculations show that the organic contamination on the substrates is between 0.1 and 0.2 nm thick on average. Ultraclean iron(III) oxide and silica reference substrates were prepared inside the XPS instrument by argon ion beam ablation. However, carbon contamination (mostly turbopump oil (paraffin oil)) on substrates became measurable as soon as the ion beam was shut off. Within 30 min, the thickness rose to ∼0.5 nm. The C 1s to Fe 2p and C 1s to Si 2p ratios for the

two test substrates were both well below 0.01 after sputtering, indicating that the surface organic contamination layer was removed. For all four substrates, prior to cleaning, the atomic concentration for carbon was 30% to 40%. Thus, the substrates investigated in this study were at least 30% to 40% covered with an organic layer with an average thickness of ∼0.2 nm and an average elemental composition of ∼CH2O0.1. The carboxyl groups within this layer are expected to behave as basic sites. The sorbed, saturated hydrocarbons and the polar organics form intermolecular bonds with oxide and hydroxyl centers on the surface of the substrate. Thus, the net effect of the sorption of organics is a decrease in the number of “free” hydroxyl and oxide groups on the surface, which leads to a partial chemical passivation of surfaces, i.e., they lower the overall acidity or basicity by effectively blocking chemically 2508

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Table 8. Deposit Thickness (nm) for Compounds of Iron (III) Oxide and Silica reagent

iron(III) oxide (basic)

silica (acidic)

not detected not detected 0.05 ( 0.01 not detected

not detected 0.19 ( 0.05 0.18 ( 0.07 0.17 ( 0.07

0.09 ( 0.02

1.02 ( 0.15

0.05 ( 0.01

0.34 ( 0.08

not detected

0.12 ( 0.02

pentane pyrene (15 wt %) þ pentane naphthenic acid naphthenic acid (15 wt %) þ pentane zinc(II) naphthenate (15 wt %) þ pentane zinc(II)(TPP) (15 wt %) þ pentane nickel(II)(TPP) (15 wt %) þ pentane

Table 10. Deposit Composition (Mass Percent) of Analyzed Material Postexposure to Zinc(II) (TPP) (15 wt %) þ Pentane

Zn Fe Si O N C S

Fe Si O N C S

0.00 ∼0 0.16 ( 0.09 ∼0 0.74 ( 0.73

silica

3.0 ( 0.9 0.0 0.0 -2.75 ( 0.01 0.74 ( 0.08 10.6 ( 1.2

3.5 ( 0.10 0.0 0.0 -3.1 ( 0.1 1.7 ( 0.1 20.4 ( 4.1

10.l

8.6 81.3

Table 11. Deposit Composition (Mass Percent) of Analyzed Material Postexposure to Zinc(IiI) Naphthenate (15 wt %) þ Pentane

Table 9. Deposit Composition (Mass Percent) of Analyzed Material Post Exposure to Pentane þ Pyrene iron(III) oxide (basic)

iron(III) oxide

zinc(II) (TPP) composition (H-free basis)

Zn Fe Si O N C S

silica (acidic) 0.00 0.00 1.4 ( 2.4 0.4 ( 0.2 11.9 ( 3.8

iron(III) oxide

silica

5.9 ( 0.7 0.0 0.0 -0.57 ( 0.26 -0.15 ( 0.09 11.9 ( 4.0

8.6 ( 1.0 0.00 0.00 9.5 ( 1.8 0.07 ( 0.15 34.9 ( 4.5

zinc(II) naphthenate composition (H-free basis) 18 17 65

deposit on the substrates, and how thick do deposited layers need to be before quantitative measures of deposited layer elemental composition become feasible? Early work with methacrylates98 suggests that deposits need to be greater than 0.6 nm thick to make quantitative composition determinations. The composition of the organic deposit is obtained by subtracting the contribution made by the organic contaminants from the combined XPS signal for the organic deposit and the contaminants. Their impact diminishes as deposits become thicker, eq 2. For pyrene sorption, only carbon should be observed as hydrogen is inaccessible by XPS analysis. The deposited pyrene layer on silica is thin, 0.19 ( 0.05 nm, and while nitrogen and oxygen appear to be detected, Table 9, the errors are large because the pyrene, on a substrate and contaminant free basis, comprises only 13.5 wt % of the analyzed material. Deposit composition data is at best qualitative for this case. For zinc(II) (TPP) and zinc(II) naphthenate þ pentane mixtures, the deposited layers on silica are thicker, at 0.34 ( 0.08 and 1.02 ( 0.15 nm, respectively. Composition results are shown in Tables 10 and 11. The results for zinc(II)(TPP) are semiquantitative, if the oxygen interference is ignored. The Zn(II)(TPP) comprises ∼25% of the analyzed material. Results for zinc(II) naphthanate are fully quantitative. The zinc(II) naphthenate layer is thick and comprises more than half of the analyzed material. 3.3. Deposition of Athabasca Bitumen Constituents. 3.3.1. Overview. The experiments with pure compounds and binary mixtures show that deposition measurements are sensitive. Average deposit thicknesses of ∼0.1 nm are detectable, and deposit compositions become reliable, if the deposit thickness exceeds ∼0.5 nm. Approximately 200 deposition experiments were performed with AVR þ pentane mixtures, as exposure and wash times and mixture and substrate composition variables were explored. A number of these variables were expected to be insignificant, and this facilitated lumping

active sites on the surfaces.95,96 These contaminants do not interfere with deposit thickness calculations, but their impact must be subtracted from the deposit composition measurements. 3.2. Pure Compound Deposition. 3.2.1. Deposit Thickness. Deposition experiments with model compounds were performed at room temperature with t1 = 3.5 h and t2 = 30 min. Average deposit thicknesses are shown in Table 8. Reported values are the average of two measurements obtained at different positions on the same substrates and are based on the attenuation of the Fe 2p 777 eV band, the Si 2p 1387 eV band, and the Ni 2p 634 eV bands, respectively, using eq 2. “Not detected” in this context means that the postexposure substrate composition is the same as the corresponding substrate composition, within experimental error. This is a key point as it indicates that substrate contaminants are not removed or are reintroduced during processing and handling. This is illustrated for pyrene (15 wt %) þ pentane on iron(III) oxide in Table 9. Similar observations were made for other cases cited in Table 8. Further, as carbon-carbon bond lengths are ∼0.12 to ∼0.15 nm and the leading dimensions of small molecules such as benzene are ∼0.37 nm by ∼0.7 nm, surface coverage after washing is clearly partial except for the zinc(II) naphthenate and possibly the Zn(II)(TPP) deposits on silica. The deposit thicknesses are all less than or equal to the inelastic mean free paths (λ values) for organic films composed of compounds such as PAHs, napthanates, and porphines, which range from 2 to 4 nm with Al KR X-rays serving as the impinging photons.97 3.2.2. Deposit Composition. Two questions arise with respect to elemental composition analysis, namely, are the initial substrate contaminants adherent even if other materials (95) Crowe, L. L.; Tolbert, L. M. Silica Passivation Efficiency Monitored By a Surface-Bound Fluorescent Dye. Langmuir 2008, 24 (16), 8541–8546. (96) Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z. Ambipolar, High Performance, Acene-Based Organic Thin Film Transistors. J. Am. Chem. Soc. 2008, 130 (19), 6064–6065. (97) Peter, J. C. Estimation of inelastic mean free paths for polymers and other organic materials: use of quantitative structure-property relationships. Surf. Interface Anal. 2001, 31 (1), 23–34.

(98) Roberts, R. F.; L. A., D.; Pryde, C. A.; Buchanan, D. N. E.; Hobbins, N. D. Mean free path for inelastic scattering of 1.2 kev electrons in thin poly(methylmethacrylate) films. Surf. Interface Anal. 1980, 2 (1), 5–10.

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Figure 6. Deposit thickness (a) and C/S ratio (b) for iron(III) oxide. t1 is varied. t2 = 30 min and AVR composition = 15 wt %.

with respect to the key outcomes, deposit thickness and composition, rendering them as robust as possible. For example, Figures 6 and 7 illustrate that substrate thickness, exposure time, t1, wash time, t2, have no measurable effect on deposit thickness and composition, expressed as C/S ratios, on iron oxide substrates for 15 wt % AVR þ pentane mixtures over the range of values tested. Similar results were obtained for other cases. Clearly, 1 h of high-temperature exposure followed by a pentane rinse at room temperature are sufficient to saturate the substrates over the range of compositions employed and to remove nonadherent deposits. 3.3.2. Deposit Thickness. The lumped average deposit thicknesses and compositions with respect to these variables at 15 wt % AVR are shown in Table 12. A second set of lumped average deposit thicknesses and compositions for fixed exposure and washing times but where mixture composition was varied are also reported in Table 12. These sets of values agree with one another. Deposit thicknesses are large relative to the measurement threshold for all cases and are highest on the more strongly acidic silica surface, lower on less acidic iron(II) sulfide surface, lower on the basic iron(III) oxide surface, and lowest on the mildly basic nickel(II) oxide surface. Deposit thickness ranges from values consistent with partial coverage by molecules (nickel(II) oxide) to values equivalent to at most two or three molecular layers on average. These results are consistent with deposition results reported for sorption of asphaltenes alone, from asphaltene þ solvent mixtures, on natural mineral substrates, Table 1, both with respect to the average absolute and relative thicknesses of the deposits on acidic, basic, and neutral substrates. For convenience, we have cross converted the two deposition measures (deposit thickness and deposit

mass per unit area) in Tables 1 and 12 using bulk densities for asphaltenes of 1160 ( 20 kg/m3 99 for data from the literature and densities for AVR (1043 (20 kg/m3 100) for the organic deposits found in this work. Nominal average thicknesses range from 0.6 to 3 nm for AVR and from 0.4 to 7.6 nm for asphaltenes. The largest values are on strongly acidic silica substrates and the smallest values on neutral (non-polar) substrates. 3.3.3. Impact of Substrate Composition on Deposit Composition. The carbon to sulfur ratio in the deposits is ∼11.3 for basic substrates and ∼12.5 for the acidic substrates. As the bulk carbon to sulfur ratios for AVR and AVR pentane asphaltenes are 11.7 and 10.8, respectively, the deposits on the basic substrates appear enriched in asphaltenes relative to AVR and the deposits on the acidic substrates appear depleted in asphaltenes relative to AVR. This result is consistent with the finding of Henry and Fuhr,40 who found that Athabasca bitumen adhering to oilsand (largely silica) was asphaltene deficient relative to bitumen in oilsand as a whole but is not definitive in and of itself. Sulfur subspeciation provides an additional qualitative constraint. From the carbon to sulfur ratios, the deposit on the basic substrates appears to be ∼55% asphaltenes, while the deposit on the acidic substrates appears to be ∼15% asphaltenes. If the findings of ref 54 are applied, the expected thiophenic sulfur content would be ∼72% and ∼67%, respectively, a difference falling within the margin of error of their bulk(99) Maham, Y.; Chodakowski, M. G.; Zhang, X.; Shaw, J. M. Asphaltene phase behavior: prediction at a crossroads. Fluid Phase Equilib. 2005, 227 (2), 177–182. (100) Cartlidge, C. R.; Dukhedin-Lalla, L.; Rahimi, P.; Shaw, J. M. Preliminary phase diagrams for ABVB þ n-dodecane þ hydrogen. Fluid Phase Equilib. 1996, 117 (1-2), 257–264.

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Figure 7. Deposit thickness (a) and C/S ratio (b) for iron(III) oxide. t2 is varied. t1 = 3.5 h and AVR composition = 15 wt %. Table 12. Organic Deposit Thickness and Composition from AVR þ Pentane Mixtures

substrate iron(III) oxide nickel(II) oxide iron(II) sulfide silica

t2 = 30 min, AVR wt % = 15%, t1 varied

t1 = 3.5 h, t2 = 30 min, variable composition

deposit

deposit

thickness, nm 1.1 ( 0.3 (23) 0.6 ( 0.2 (34) 1.4 ( 0.3 (16) 2.9 ( 0.6 (20)

b

2 a

(mg/m ) 1.1 0.6 1.5 3.0

C/S mass ratio 11.2 ( 0.4 (21) 11.4 ( 0.5 (24) 12.5 ( 0.5 (10) 12.6 ( 0.6 (23)

thickness, nm

(mg/m2)a

C/S mass ratio

1.4 ( 0.3 (30) 0.8 ( 0.2 (33)

1.5 0.8

11.3 ( 0.2 (27) 11.3 ( 0.3 (29)

3.3 ( 0.5 (32)

3.4

12.4 ( 0.5 (28)

Calculated assuming a density of 1043 kg/m3 for the organic deposit.100 b The values in parentheses are the number of repeat experiments performed with two measurements averaged per experiment. a

sample sulfur speciation measurements. Carefully calibrated XPS measurements performed with sorbed AVR, sorbed AVR pentane asphaltenes, and sorbed pentane AVR maltenes on iron oxide, a more challenging case, did not permit quantitative sulfur speciation. For example, 15 independent measurements from 10 experiments with 15 wt % AVR in pentane yielded three peaks, one in the range 162.7-163 eV (5% of the peak area), one in the range 163.8-164.1 eV (84% of the peak area), and one in the range 165.6-165.8 eV (11% of the peak area). Five independent measurements from three experiments with 5 wt % pentane asphaltenes in pentane and six independent measurements from three experiments with 10 wt % pentane asphaltenes in pentane yielded a single peak in the range 163.8-164.1 eV. Six sulfur speciation results for 10 wt % AVR maltenes obtained from three experiments yielded a peak in the range 163.7-163.9 eV, comprising 96%

of the peak area on average and a secondary peak in the range 165.3-165.7 eV, comprising the balance. As noted in the Introduction, sorption leads to peak shifting and to peak broadening. It should also be reiterated that the sorbed organic material comprises less than half of the material analyzed. This combination of factors renders secondary peak identification and quantification difficult. The differences in sulfur speciation among sorbed AVR, sorbed maltenes, and sorbed asphaltenes are not viewed as significant. Further, as the distribution of sulfur by mass and by species in the myriad of molecules comprising pentane asphaltenes or pentane maltenes is unknown, and the sorbed material comprises a small fraction of the samples in all cases, the carbon to sulfur ratios may only be interpreted as simply a variation in the tendency of sulfur containing species, regardless of origin or of the sulfur species type to sorb on acidic or basic surfaces. 2511

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3.3.4. Impact of AVR þ Pentane Mixture Composition on Deposit Thickness and Deposit Composition. Deposit thickness and composition were both anticipated to vary with mixture composition and phase behavior as the phase in contact with the surface and the dispersed phases vary with global composition. These effects, if present, are too subtle for the measures employed and are not evident in the results presented in Figures 8 and 9. For the experiments presented in these figures, only global composition is varied, other experimental conditions were fixed. In Figure 8, the deposit thicknesses on the various substrates are readily discriminated at all compositions but there are no clear trends with global composition on specific substrates. Phase behavior boundaries, also shown in Figure 8, confirm the insensitivity of deposit thickness vis- a-vis mixture composition. Deposit thickness is insensitive to whether the deposit arises where L1 (a pentane rich phase) or L2 (vacuum residue rich phase) is the dominant liquid phase or whether L3 (an asphaltene rich phase) is present as a dispersed phase. The impact of global composition on the carbon to sulfur ratio in the deposits on silica, nickel(II) oxide, and iron(III) oxide are shown in Figure 9a-c. With the possible exception of silica, Figure 9c, there are no evident trends with mixture composition. Again,

Figure 8. Impact of AVR þ pentane mixture composition and phase behavior on the thickness of organic deposits on substrates surfaces: b = iron(III) oxide, 2= nickel(II) oxide, 9 = silica, and ( = iron(II) sulfide. (The numbers of repetitions for each experiment appears above, below, or beside the symbol for the surface).

Figure 9. Impact of AVR þ pentane mixture composition on the C/S ratio for organic deposits on (a) iron(II) oxide, (b) nickel(II) oxide, (c) silica, and (d) the impact of phase behavior. b = nickel(II) oxide, 2= silica, and 9 = iron(III) oxide.

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Athabasca vacuum residue þ pentane mixtures appear to comprise a combination of asphaltenes þ other constituents in all cases. Substrate but not mixture composition was found to have a significant impact on deposit thickness and composition. On basic substrates, deposits were thinner and enriched in sulfur containing species relative to Athabasca vacuum residue, while deposits on acidic substrates were thinner and sulfur deficient. Lower sulfur content species in AVR are clearly more quickly or strongly sorbed on acidic substrates than high sulfur content species and less quickly or strongly on basic and neutral ones.

superposition of the phase diagram, Figure 9d, confirms the link between deposit composition and substrate composition. On silica, an acidic surface, carbon to sulfur ratios are consistently greater than those for AVR, while on iron(III) oxide and nickel(II) oxide, both basic substrates, carbon to sulfur ratios are consistently lower. Deposit composition is also insensitive to whether the deposit arises where L1 or L2 is the dominant liquid phase or whether L3 is present. These results suggest that species tending to form adherent deposits are present in all phases in sufficient concentration to saturate surfaces and that deposit thickness and deposit composition is primarily a function of surface composition.

Acknowledgment. The authors thank, Prof. K. Karan and Amit Rudrake at Queens University for their comments and encouragement, Prof. D. Mitlin and his students Colin Ophus and Christopher Harrower for help with sputtering, Prof. M. Gray for access to his laboratory and equipment, the staff at the Alberta Centre for Surface Engineering and Science (Dr. Dimitre Karpuzov, Dr. Anquang He, and Mr. Shihong Xu) for their hard work and patience, and Mr. Jordon French for his help in the laboratory. The authors gratefully acknowledge financial support from the scholarship program at the University of Alberta (scholarships for Cheng Xing) and the sponsors of the NSERC Industrial Research Chair in Petroleum Thermodynamics (Alberta Energy Research Institute, ConocoPhillips, Halliburton Energy Services Ltd., Imperial Oil Resources, KBR, NEXEN, Shell Canada Ltd., Total E&P Canada, and NSERC).

4. Conclusions Acidic and basic substrates were prepared and carefully analyzed. Substrate contamination was explored in detail ,and a clear understanding of substrate properties was achieved. Control experiments were performed with pure compounds and binary mixtures. Deposit thicknesses and elemental compositions obtained for model compound deposition were consistent with expectations and showed that the experimental techniques employed were sensitive enough to detect partial coverage of model compounds on acidic and basic substrates. Deposit elemental compositions were well-defined for deposits thicker than ∼0.5 nm. Adherent deposits from

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