Study of Asphaltenes Adsorption on Metallic Surface Using XPS and

J. Phys. Chem. C 2008, 112, 18963–18972

18963

Study of Asphaltenes Adsorption on Metallic Surface Using XPS and TOF-SIMS Wael A. Abdallah* and Shawn D. Taylor DBR Technology Center, 9450 - 17 AVe, Edmonton, AB Canada T6N 1M9 ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: October 5, 2008

X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) were used to characterize an adsorbed layer of four different asphaltenes on a stainless steel surface. Surface composition indicated different atomic concentrations of C, S, N, and O with similar chemical nature of their functional group as aromatic and aliphatic hydrocarbon. The presence of nitrogen compounds as pyrrolic and pyridinic, sulfur as thiophenic and carbon as carboxylic and hydrogenated (C-O) were confirmed by both XPS high-resolution binding energies and the high sensitive ToF-SIMS analysis of the detected negative fragments of nitrogen and sulfur. Depth profiling showed a carbidic nature on surface after surface sputtering with no surface sulfur and nitrogen. The optimum sputtering rate needs to be identified to allow studying and characterizing the asphaltene metallic interface. 1. Introduction Heavy oil, enhanced oil recovery, and offshore petroleum production have gained great momentum in the past decade with more oil companies shifting and investing significantly in that direction compared to conventional oil resources.1 With the new direction, other production challenges follow such as flow assurance. The precipitation and deposition of organic and inorganic solids produced from the reservoir fluids have a tremendous negative economical impact to the petroleum industry.2 Therefore, flow assurance planning provides a compelling incentive for the petroleum industry to study solid precipitation and deposition. Asphaltenes in particular have received great attention from the industry and researchers due to their prominent negative effect in the production process formation damage,3 pipeline blockage,1 wettability alteration and further catalyst deactivation in the refining processes.4 Most of the investigations focused on asphaltene colloidal and molecular properties (chemical structure and elemental analysis), but little on their molecular interactions and adsorption behavior, which is important to understanding the chemistry of asphaltene inhibitors. Alteration of the production conditions due to temperature, pressure, or compositional changes can greatly affect the crude oil equilibrium and the asphaltene stability within the crude oils, where instability refers to the precipitation of asphaltenes from crude oil. Unstable asphaltenes raise concerns over the possibility of asphaltene adsorption within the reservoir rock and production surfaces. Current mitigation methods (injecting chemicals) are either to prevent further agglomeration within the flow or preventing further surface adsorption to form multilayer. In both cases, the target is weakening the molecular interactions within asphaltenes molecules and blocks it. Asphaltenes are defined as a solubility class material that is soluble in toluene but insoluble in alkanes such as n-heptane.6 Asphaltenes are polar, and because of that polarity, asphaltene has a tendency to adsorb on surfaces.5 It is important to understand that the structure and stability of asphaltenes are dependent on the origin of the oil, which makes each study more * To whom correspondence should be addressed. Phone: +1.780.577.1380. Fax: +1.780.450.1668. E-mail: [email protected].

Figure 1. Typical XPS survey spectra for SS01, SS02, SS03, and SS04.

specific to that oil in use. What factors that plays a role in asphaltene adsorption on pipelines surfaces (primary cause of blockage-formation damage) or inorganic surface (believed to be primary cause for surface wettability alteration)7,8 and what are the responsible groups in asphaltenes for its molecular interaction are still to be understood from a fundamental perspective. There are limited asphaltene adsorption studies in the open literature on metallic surfaces9,10 and inorganic surfaces.3,5,11,12 In summary, a Langmuir-type adsorption using quartz crystal balance (QCM) for studying the asphaltenes adsorption on gold surfaces indicated asphaltenes adsorb in large quantities and demonstrate the presence of aggregates on the surface.9 In a different approach, Masliyah et al., showed asphaltenes can form a monolayer at an air-water interface, indicating that asphaltenes act as surface-active molecules.13,14 In a different technique, a multilayer of asphaltenes was formed on glass surfaces using photothermal surface deformation spectroscopy.15 Marshall et al.16 characterized the adsorbed asphaltenes at the water-in-oil interface using electrospray ionization Fourier transform ion cyclotron resonance mass spectroscopy (ESI-FT-ICR MS) to gain insight into the asphaltene composition. One of the keys

10.1021/jp804483t CCC: $40.75  2008 American Chemical Society Published on Web 11/08/2008

18964 J. Phys. Chem. C, Vol. 112, No. 48, 2008

Abdallah and Taylor

Figure 2. High-resolution spectra of SS01 surface for N 1s, S 2p3/2, Si 2p, C 1s, O 1s, and Fe 2p3/2 with their peak fittings and indicating their fitted binding energies. The blue lines are the number of fitted peaks, while the red peak represents the overall fitted peak.

TABLE 1: Atomic Concentration of O 1s, N 1s, C 1s, S 2p, and Si 2p for SS01, SS02, SS03, and SS04 Surfaces at. %a element

SS01

SS02

SS03

SS04

O 1s C 1s N 1s S 2p

17.5 80.9 0.9 0.7

10.6 87.3 1.0 1.2

13.6 85.5 0.5 0.4

8.0 89.5 0.4 2.1

a

The atomic concentration is normalized after removing Fe 2p, Cr 2p, and Si 2p peak areas.

to understanding and, in turn, preventing asphaltene precipitation and deposition is the nature of molecular interactions, the type of bonds that asphaltenes form with a surface, and the chemical species involved in the surface interaction.

Most asphaltene characterization focuses on precipitated asphaltene molecules but not the adsorbed ones; therefore, we believe it is important to characterize such molecules and compare to the precipitated ones (aggregates). Therefore, as a first stage to the research, we are focused on characterizing the adsorbed asphaltene composition on stainless steels, which are one of the internal materials used in petroleum pipelines, are exposed to solid deposition. Different adsorbed asphaltene films on stainless steel are characterized by X-ray photoelectron spectroscopy (XPS) in conjunction with time-of-flight secondary ion mass spectroscopy (ToF-SIMS).17 2. Methodology The stainless steel specimens were prepared from AISI 316 foil (Good fellow) to a diameter of 10 mm and a thickness of

Study of Asphaltenes Adsorption on Metallic Surface

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18965

Figure 3. High-resolution XPS spectra of O 1s, C 1s, and Fe 2p of SS01 sample measured before and while ion sputtering. The black lines in all spectra show the original peak shape as measured, and the navy blue line is the peak after sputtering is completed. Each individual spectrum was fitted in a similar approach.

TABLE 2: Binding Energies of High-Resolution Spectra for All Surfaces and Elements Found on Surface as Peak Fitted SS01

SS02

SS03

SS04

position fwhm position fwhm position fwhm position fwhm element BE (eV) (eV) BE (eV) (eV) BE (eV) (eV) BE (eV) (eV)

bond assignments

refs

O 1s

532.2 530.0

2.2 1.2

532.7 530.3

1.5 1.5

532.9 530.3

1.5 1.5

532.2 530.0

2.2 1.2

C-O, O-H, or CdO metal (iron) oxide

18, 21

C 1s

284.9 286.1

1.2 2.2

285.1 286.2

1.2 1.3

285.2 286.4

1.2 1.2

285.1 286.5

1.2 1.0

C-H or C-C C-O

10, 18-21 10, 21

N 1s

398.6 400.3

1.3 1.5

398.7 400.3

1.1 1.7

398.4 400.4

1.1 2.0

398.6 400.3

1.1 1.5

pyridinic nature pyrrolic nature

10, 22, 23, 26 10, 22, 23, 26

S 2p3/2

164.2 165.6

1.3 1.0

164.3 165.6

1.3 1.0

164.2 165.4

1.1 1.3

164.2 165.6

1.3 1.2

thiophenic signatue sulfite or sulfonyl nature

10, 12, 18, 22-25 18

Si 2p

102.6 100.5

1.4 1.2

102.8 -

1.4 -

102.9 -

1.4 -

102.7 100.4

1.4 0.8

Si-O Si-C

Fe 2p3/2

711.0 713.7

2.4 2.1

710.8 713.2

2.2 3.0

711.0 713.6

2.3 2.3

710.5 711.9

1.8 3.1

iron(III) oxide either pyrite (FeS2) or satellite peak of Fe(II) 29

0.5 mm and polished to a grain size of ∼0.05 µm. The surface was cleaned by ultrasonication in dichloromethane (DCM) and then acetone to clean and remove any surface contamination resulted from polishing and handling. Four different types of

asphaltenes were used and abbreviated as SS01, SS02, SS03, and SS04. For each one of the asphaltenes, an asphaltene solution with a concentration of 5 × 10-4 g/cm3 was prepared by dissolving appropriate amounts of dried asphaltenes in


Abdallah and Taylor of 16 scans. Peak deconvolution of the high-resolution scans and atomic concentrations were calculated using the algorithm and sensitivity factors provided in the Kratos Vision 2 software. The static ToF-SIMS analyses for the asphaltene films were carried out in a ToF-SIMS IV instrument (Ion-TOF GmbH, Germany). A Gallium liquid metal ion gun was operated at 25 keV with a primary pulsed beam current of 2.5 pA. Imaging analyses were achieved by rastering the adsorbed asphaltene stainless steel surface with the primary ion beam and simultaneously collecting secondary mass spectra at each point to generate an image of ions of interest. Negative secondary ion mass spectra were then collected for each sample. The maximum mass resolution of the instrument is 9000 (m/∆m), nonetheless, the instrument resolution while analyzing our samples was poor. 3. Results and Discussion

Figure 4. (a) Survey scan of SS01 surfaces before and after ion sputtering. (b) XPS depth profile as a function of the sputtering time with a total sputtering time of 3105 s. The individual lines correspond to the deconvoluted peaks. All surfaces showed similar behavior for O concentration, as it goes up once you sputter the surface and then goes down. The inset magnifies only the N and S profiles.

toluene. The solution was left in a thermostatted ultrasonic bath for 30 min to ensure complete dissolution. This concentration allowed for the formation of a consistent thin film of adsorbed asphaltene on each stainless steel surface, as previously investigated.10 The stainless steel disks were placed inside the solutions of asphaltenes contained in a designed glass apparatus at room temperature, sealed to avoid the evaporation of the solvent and placed in a dark area in a nitrogen-filled glovebox. After 3 days, the disks were removed and dried using high purity nitrogen gas and then placed in a sample container. The XPS depth profile measurements were carried out on Axis-165 spectrometer (Kratos Analytical), which is equipped with mini-beam III ion gun. The base pressure in the analytical chamber was less than 1 × 10-10 Torr. The surface films were sputter by Ar+ beam incident at an angle of 45° in typical depth profiling mode with base pressure of 1 × 10-07 Torr. The beam current on the target was 1 µA with primary ion energy of 4 keV with scanned area 2 × 2 mm2. The composition of the etched surface was monitored in situ by XPS using Al (mono) X-ray source operated at 210 W. The scanning parameters were optimized to have good signal-to-noise ratio and to lower the acquisition time, which in turn would help to avoid decomposing the asphaltene film structure with long scanning time. The survey (wide) spectra were collected from 1100 to 0 eV with pass energy of 160 eV, a step size of 0.35 eV, and a dwell time of 150 ms with an average of three scans. High-resolution (narrow) spectra over the C 1s, S 2p, N 1s, O 1s, Si 2p, Fe 2p, Cr 2p, and Ni 2p regions were acquired with pass energy of 40 eV and a dwell time of 300 ms with an average of 8 scans for C1s and O 1s, while other high-resolution spectra had an average

XPS Analysis. Surface composition and identification of the chemical state are the most prominent advantages of the XPS technique for thin film studies, which is based on the analysis of the peak shape and chemical shift associated with each alloying element. The instrument resolution was controlled by the pass energy, step size, and dwell time to allow higher resolution without damaging the film. Data interpretation in general could be tedious, depending on the associated shape of the XPS peaks, which have features as a result of multiple splitting (coupling between unpaired electrons), satellite peaks, or excited-state effects. An initial survey scan was conducted for a blank cleaned stainless steel surface before and after submerging the surface in toluene in order to evaluate the carbon peak binding energy position. For both survey scans, the surface showed Si, C, O, Fe, Cr, Ni, and Mo peaks at different concentrations. Focusing on the C peak, the binding energy before and after submerging it in toluene was 285.3 eV. This binding energy reflects C-C or C-H bonds (peak is convoluted). Figure 1 shows the stainless steel survey scan with toluene submerging. Following the analysis of the blank surfaces, analysis was performed on the set of samples containing adsorbed asphaltene, namely SS01, SS02, SS03, and SS04. It is worth noting that several adsorption trials were conducted to optimize the asphaltene film thickness on the surface, where asphaltene concentration within the solution and adsorption times were the main variables to control the film thickness. XPS survey scans are the main technique used to check the surface film. These variables are discussed in more detail in an earlier work.10 Survey scan spectra for all surfaces (SS01, SS02, SS03, and SS04) were achieved using Kratos Analytical system with monochromatic Al KR radiation and automatic charge neutralization to monitor and identify the atomic concentration variation of surface elements. Figure 1 illustrates the typical survey scan spectra of all surfaces within a binding energy range of 1100-0 eV. Each spectrum shows the expected elements on surface such as carbon, oxygen, nitrogen, and sulfur in addition to O and C Auger features. Table 1 summarizes only the atomic concentration of C 1s, O 1s, N 1s, and S 2p after normalizing with relative sensitivity factor of 0.278, 0.780, 0.477, and 0.668, respectively (obtained from Kratos sensitivity tables). Typical elemental composition of asphaltenes as reported by Speight6 for numerous asphaltenes from the world sources shows carbon in the range of 78.0-88.7%, sulfur in the range of 0.3-10.3%, nitrogen in the range of 0.2-3.3%, while oxygen in the range of 0.3-4.9%. Comparing these bulk concentration ranges with the values for the adsorbed asphaltene reported in Table 1, it is clear that the tested asphaltenes have a higher oxygen concentration. While



Figure 5. High-resolution XPS spectra of O 1s, C 1s, and Fe 2p of SS02 sample measured before and while ion sputtering. The black lines in all spectra show the original peak shape as measured, and the navy blue line is the peak after the sputtering completed. Each individual spectrum was fitted in a similar approach.

this may be related to the preferential adsorption of higher oxygen content asphaltenes at the surface, it is also quite likely that some level of oxidation of the asphaltene samples had occurred prior to analysis. Other elements were also detected including Fe 2p and Cr 2p, which are part of stainless steel composition. This suggests the asphaltene film on all surfaces does not cover completely the substrate surface. Si 2p was also detected; the silicon presence on surface is due to some traces in the stainless steel surface as shown in Figure 1, but mostly due to inherent content within asphaltenes composition used in this study as very small sand and clay particles are difficult to fully separate from asphaltenes. We were surprised by the high concentration (∼1.5%) in at least one of the samples. This may be an indication of the difficulty in separating the very fine inorganic material from the asphaltene during extraction from the original oil. This may also be an indication of high concentration of clays that are surface active in that specific oil sample. In comparing the four different asphaltene samples, SS01 showed the highest atomic concentration of oxygen and the lowest of carbon, while SS04 showed the lowest atomic concentration of oxygen and highest concentration of carbon. This is probably due to the different nature of asphaltenes in use and could also relate to their absorption capacity for atmospheric oxygen. Atomic concentrations of both nitrogen

and sulfur were relatively low as expected. However, it was observed that the nitrogen and sulfur concentrations were relatively similar for SS01, SS02, and SS03, the SS04 had a higher concentration of sulfur. This is due to the different type of asphaltenes in use for the study. A high-resolution spectrum for each element detected on the surface was obtained to investigate the nature of these elements. In SS01 (Figure 2), the oxygen peak was deconvoluted in two subpeaks with binding energies of 532.2 ( 0.1 and 530.0 ( 0.1 eV with full width at half-maximum (fwhm) of 2.2 and 1.2 eV, respectively. The two subpeaks most probably corresponded to the different metal oxides and therefore identify a two-layer structure. The high-resolution spectrum of carbon peak was also fitted into two subpeaks, a main peak at 284.9 ( 0.1 eV (fwhm of 1.2 eV) and a shoulder peak at 286.1 ( 0.1 eV (fwhm of 2.2 eV), which translate to carbons with different atoms bonded. The nitrogen peak was expected to have low intensity peak since it presents in low concentration on the surface was also deconvoluted into two subpeaks with binding energies of 400.3 ( 0.1 (fwhm of 1.5 eV) and 398.6 ( 0.1 eV (fwhm of 1.3 eV), which corresponds to different nitrogen nature present within the asphaltene film. Similarly, the sulfur peak was fitted into two peaks with binding energies of 165.6 ( 0.1 (fwhm of 1.0 eV) and 164.2 ( 0.1 eV (fwhm of 1.3 eV). The high-resolution silicon peak was curve fitted into two peaks with binding


Abdallah and Taylor

Figure 6. High-resolution XPS spectra of O 1s, C 1s, and Fe 2p of the SS03 sample measured before and while ion sputtering. The black lines in all spectra show the original peak shape as measured, and the navy blue line is the peak after the sputtering completed. Each individual spectrum was fitted in a similar approach.

energies at 102.6 ( 0.1 (fwhm of 1.4 eV) and 100.5 ( 0.1 eV (fwhm of 1.2 eV). In the case of Fe 2p high-resolution spectrum, the identification of its binding energy is difficult since it exists in several valence states as well as mixed valence with chemical shifts too close to each other to allow separation (e.g., Fe2O3 and Fe3O4). Nonetheless, the Fe 2p3/2 peak was fitted into two peaks with binding energies of 711.0 eV ( 0.1 (fwhm of 2.4 eV) and 713.7 ( 0.1 eV (fwhm of 2.1 eV), while the sharp peak at 706.6 eV as shown in Figure 3 indicates iron metal. Table 2 summarizes the binding energies of our findings for all four surfaces SS01, SS02, SS03, and SS04 with the fwhm. SS02 high-resolution surface analysis was similar to the results for the SS01 surface. The C 1s peak was curve fitted into two subpeaks 285.1 ( 0.1 (fwhm of 1.2 eV) and 286.2 ( 01 eV (fwhm of 1.3 eV). The N 1s peak showed a resolved binding energies of 400.3 ( 0.1 (fwhm of 1.7 eV) and 398.7 ( 0.1 eV (fwhm of 1.1 eV). The S 2p high-resolution peak showed a clear shoulder peak, and therefore, it was fitted with binding energies of 164.3 ( 0.1 (fwhm of 1.3 eV) and 165.6 ( 0.1 eV (fwhm of 1.0 eV). In contrast to the SS01 surface, the Si 2p high-resolution peak at SS02 surface demonstrated only one peak at a binding energy of 102.8 ( 0.1 eV (fwhm of 1.4 eV). The O 1s high-resolution spectrum demonstrated two main peaks, which was fitted to 532.7 ( 0.1 (fwhm of 1.7 eV) and

530.3 ( 0.1 eV (fwhm of 1.4 eV). The iron Fe 2p3/2 peak was fitted into 713.2 ( 0.1 (fwhm of 3.0 eV) and 710.8 ( 0.1 eV (fwhm of 2.2 eV). Similar high-resolution analysis of SS03 and SS04 was followed, and the results are summarized in Table 2. In general, the spectra of the C 1s feature for all surfaces do not indicate any chemical shift that could be attributed to charging effects.10 Furthermore, several preliminary scans with the charge neutralizer on and off were conducted to investigate charging effect on sample surface and indicated charging has no effect on the XPS spectra for these particular surfaces. All surfaces showed a main C 1s peak with a binding energy range of 284.9-285.2 ( 0.1 eV that attributes to a systematic carbon peak with C-H or C-C bond.10,18-21 The subpeak at a higher biding energy range of 286.1-286.5 ( 0.1 eV is assigned to carbon with oxygen atom bonding.10,21 This has been reported ¨ stlund et al. on asphaltene using as well by previous work of O XPS.2 The high-resolution spectrum of O 1s as shown in Figure 2 for SS01 and summarized in Table 2 showed two main peaks, the main peak is the range of 532.2-532.9 ( 0.1 eV. This peak indicates either an O-C bond or O-H bond18 and possibly CdO as reported by Beguin et al. at 532.5 eV,21 which agrees with the C 1s peak assignment of C-O bond. The other high resolved subpeak of O 1s indicates a binding energy range of



Figure 7. High-resolution XPS spectra of O 1s, C 1s, and Fe 2p of SS04 sample measured before and while ion sputtering. The black lines in all spectra show the original peak shape as measured, and the navy blue line is the peak after the sputtering completed. Each individual spectrum was fitted in a similar approach.

530.0-530.3 ( 0.1 eV, which is assigned to a metal (iron) oxide film resulting from surface oxidation. The individual S 2p3/2 photoemission feature for all surfaces was fitted with two main peaks as summarized in Table 2 and shown in Figure 2 for SS01, a binding energy at 164.2 ( 0.1 eV that represents thiophenic signature.10,12,18,22,23 The thiophenic nature of sulfur compounds in asphaltenes is consistent with the work of Gorbaty et al.24 where they examined and quantified the sulfur compounds using model compounds, sulfur compound mixtures and three different asphaltene samples using X-ray absorption near-edge structure (XANES). This is also consistent with the findings of Cramer et al.,25 where they studied the chemical speciation of sulfur in number of asphaltene samples using XANES and reported the thiophenic form of sulfur appears to be dominant with some fraction of sulfur exist in the sulfidic form. The other subpeak binding energy at 165.6 ( 0.1 corresponds most properly to sulfite nature, but it could also indicate a sulfonyl functional group.18 The N 1s peak was fitted within two main peaks for all highresolution spectra of N 1s photoemission feature as demonstrated for SS01 in Figure 2, the first at a range of 398.4-398.7 ( 0.1 eV which correspond to (C-N) pyridinic nature,10,22,23,26 and the second subpeak at ∼400.3 ( 0.1 eV which indicates a (C-N) pyrrolic nature.10,22,23,26 The nitrogen binding energy and

peak assignments is consistent with what have been reported on asphaltene analysis using different techniques. Mitra-Kirtley et al. used X-ray absorption near-edge structure (SANES) spectroscopy to study the asphaltenes and found most nitrogen present in aromatic forms (pyrrolic and pyridinic) with a very small amount as saturated amine.26 Wilhelms et al. used XPS on seven different asphaltene samples and concluded fivemembered ring pyrrolic nitrogen and six-membered ring pyridinic nitrogen are the main types present, with pyrrolic nitrogen dominating in all samples.28 The Si 2p resolved peak indicates two peaks at 102.6-102.9 ( 0.1 and 100.4-100.5 ( 0.1 eV, which correspond to Si-O and Si-C bonds, respectively. This indicates potential clay content in the asphaltene, which are typically associated with very-low energy depositional environments such as marine deposits. The last peak to observe on the surface is the Fe 2p peak. Two peaks were fitted for Fe 2p3/2, the first fitted at a binding energy range of 710.5-711.0 ( 0.1 eV, which correspond to iron(III) oxide and the second at a range of 713.2-713.7 ( 0.1 eV, which assigned according to what observed in literature to pyrite (FeS2) or satellite peak of Fe(II).29 XPS Depth Profile. The high-resolution spectra of SS01 sample before sputtering and while sputtering are shown in Figure 3 for main elements present on the surface including O


Figure 8. ToF-SIMS negative-ion spectra for SS01, SS02, SS03, and SS04 surfaces showing peak assignments for SS01 fragments.

1s, C 1s, and Fe 2p for explanatory purposes. Both nitrogen and sulfur maintained their peak position while sputtering and their concentration were reduced to noise level when the substrate bulk was reached as shown in Figure 4b. It is worth noting, while sputtering, sulfur, and nitrogen concentrations went up before going to noise level, which could indicate the film/ stainless steel interface where potentially, we suspect both to have higher concentration due to sulfur/nitrogen interaction with the surface. The carbon 1s line was used for calibration of the binding energy scale, being positioned at 284.9 ( 0.1 eV with subpeak at 286.1 ( 0.1 eV. While sputtering, the carbon peak gradually shifted to lower binding energy and its surface atomic concentration was reduced (13.2 at. % compared to 76.0 at. % before sputtering). After sputtering for 3105 s, the position of C 1s high resolution peaks shifted to lower binding energies at 282.8 and 284.4 ( 0.1 eV, respectively (fwhm is shown in Figure 3). This indicates the nature of atomic carbon present at the surface has changed to carbidic form (Fe3C) which is a component used in steel manufacturing. The O 1s line was initially composed of two peaks, one at 532.2 ( 0.1 eV binding energy and the other at 530.0 ( 0.1 eV, where both are identified as O-C bond or O-H bond and metal oxide with oxygen in second valence state O2-, respectively. After surface etching, the atomic concentration of O 1s increased and shifted to lower binding energy with a shoulder peak at higher binding energy. This was opposite to the original surface before sputtering. After the surface sputtering was completed, the atomic surface concentration of O 1s was reduced to half its original value. It was also observed that the peak at higher binding energy dissipated and a resolved peak at 530 ( 0.1 eV emerged, which was assigned to metal oxide. A subpeak was also fitted to a binding energy of 531.5 ( 0.1 eV, this peak most probably being from the OH- group. This indicate the

Abdallah and Taylor surface was oxidized before asphaltene adsorption, but also inline with sputtering time where high concentration of sulfur and nitrogen were observed as shown in Figure 4b. The last peak shown in Figure 3 is the Fe 2p3/2 peak, and as expected, the atomic concentration increased tremendously. The Fe 2p3/2 peak was fitted into a binding energies of 706.8 ( 0.01 and 708.3 ( 0.1 eV that represent iron metal and iron oxide, respectively. Figure 4a shows a comparison of a survey scan spectrum of SS01 surface before and after ion sputtering. It clearly indicates the growth of Ni 2p and Cr 2p, which represents the stainless steel bulk composition. Figure 4b shows the atomic concentration profile of Ni 2p, Fe 2p, Cr 2p, O 1s, N 1s, C 1s, S 2p, and Si 2p as a function of sputtering time. In the case of the SS02 sample, the C 1s line shifted to lower binding energy, as noticed previously on SS01 sample. The fitted binding energies (Figure 5) were at 284.2 ( 0.1 eV with subpeak at 284.8 ( 0.1 eV, which indicated the carbidic nature of the current surface (Fe3C). The O 1s line changed its atomic nature to lower binding energy during the sputtering process. The O 1s peak was fitted into two peaks with binding energies of 531.3 and 530.0 ( 0.1 eV with the first binding energy being identified as OH- bond and the other as metal oxide. After surface etching was completed, the atomic concentration was reduced. Although the identification of the iron compounds is more difficult because the Fe 2p3/2 line could be deconvoluted into three separate curves, the Fe 2p3/2 peak was only fitted into two main peaks: 706.8 ( 0.1 eV for Fe metal and an oxidized Fe at ∼708.7 ( 0.1 eV, which can be related both to Fe2- or Fe3-. Similar to SS01 sample, both the nitrogen and sulfur concentrations were reduced to noise level when reach substrate bulk with similar observation of intermediate spike in their concentration during sputtering. Similar analysis was preformed for SS03 and SS04; the results are summarized in Figures 6 and 7, which do not differ greatly from SS01 and SS02. When surface sputtering was completed, data indicated iron metal and oxides were present in the surface, both nitrogen and sulfur were reduced to noise level, and oxygen and carbon has similar chemical nature as explained for SS01 sample. ToF-SIMS. Combining both XPS results with ToF-SIMS spectrum provided a great advantage to the surface characterization and thin film analysis. SIMS has the advantage to recognize molecules through their characteristic fragmentation patterns and its high sensitivity for elements. Nonetheless, the relationship in SIMS spectrum and the chemical state of the surface is not straightforward as in XPS.17 Therefore, the main purpose of the ToF-SIMS spectra collected in this study was to complement the XPS chemical state analysis for the adsorbed asphaltene film and surface composition. XPS O 1s spectrum could give the information that OH and oxide (iron oxide) were present on the surface. It could also give information from the C 1s spectrum that hydrocarbon and carbides were present, but only ToF-SIMS could be used to identify the particular hydroxide or hydrocarbons. ToF-SIMS spectra were obtained for SS01, SS02, SS03, and SS04 surfaces after asphaltene adsorption using ION-TOF-SIMS IV (CAMECA). Only high-intensity negative fragments for all elements (m/z ) 1-500) were recorded coming off the surface. Figure 8 shows only m/z up to 75, as no further fragment signals were detected. The possible heteroatom-containing negative ions detected were O- (m/z ) 16), OH- (m/z ) 17), N- (m/z ) 14), CN(m/z ) 26), S- (m/z ) 32), SO- (m/z ) 48), and SiO2- (m/z ) 60). The presence of oxygen and nitrogen confirms the XPS



Figure 9. ToF-SIMS images of the asphaltenes film SS01 before surface sputtering.

survey spectra assessment. On the other hand, the detected sulfur could be the resultant of native thiocompounds that are present in the asphaltenes. Moreover, there were some strong peaks that associated with pure hydrocarbon anions observed at m/z ) 13, 24, 25, 26, 36, and 37, which are good indicators for the chemically bonded hydrogen, although the peak at m/z ) 24 is possibly aliased with the CN- peak. Larachi et al. investigated asphaltenes from heavy oil using different spectroscopic techniques including ToF-SIMS.30 They used the C2H-/C2- ratio to assess the extent of condensed polynuclear aromatic ring systems in asphaltenes and obtained a ratio of 1.2, which was higher than 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). This confirmed the studied asphaltenes were composed of heterocyclic aromatic rings. In this study, the ratio of C2H-/C2- was 1.27, 0.98, 0.75, and 0.77 for SS01, SS02, SS03, and SS04, respectively, which also confirms the presence of nitrogen heterocyclic compound in the asphaltene adsorbed film in all surfaces. The results from the ToF-SIMS analysis evidently parallel to those conclusions of the XPS experiments. To investigate the surface elemental/molecular distribution of asphaltene film, Figure 9 shows the ToF-SIMS spatial images of the asphaltene films before surface sputtering of SS01. The images show masses of 12 (carbon), 14 (nitrogen), 16 (oxygen), 28 (silicon), 32 (sulfur), 62 (dimethylsulfide), 67 (pyrrole), 68 (furan), 79 (pyridine), and 84 (thiophene). Although, the intensity of surface oxygen is dominating, its presence was localized compared to carbon, which was widely distributed around the surface. The images also indicate sulfur was spread on the surface but with less intensity. All other monitored masses showed weak intensity on the surface. 4. Conclusions In this work, four different asphaltenes were adsorbed on prepared stainless steel surfaces and then surface characterized using XPS with depth profiling and ToF-SIMS. XPS Surface analysis of adsorbed films indicated the presence of carbon, oxygen, nitrogen and sulfur with different atomic percent, but with similar functional groups. Nitrogen presents on the asphaltene film as pyrrolic and pyridinic heterocyclic compounds, while sulfur present mainly in form of thiophenic with the presence of sulfite functional groups. The carbon peaks on surface attributes to a systematic carbon peak with C-H or C-C bond and C-O bond, where the later bond was confirmed based on surface oxygen analysis. The presence of C, O, S, N, and Si was confirmed by the ToF-SIMS analysis. ToF-SIMS also indicated the nitrogen was present in pyrrolic nature while sulfur

was present as thiocompounds. Surface depth profiling indicated surface nitrogen and sulfur concentration were increased during sputtering before going to noise level. Their binding energy though did not shift compared to carbon and oxygen. Therefore, it would be an interest to investigate the depth required to maintain the maximum sulfur and nitrogen concentration (potentially the asphaltene/stainless steel interface region) and then investigate that film and its bonding mode/s with the metallic surface. Surface oxidation was obvious in the depth profiling measurements as high concentration of metal oxide was detected at what is potentially the interface. We should emphasis the possibility of asphaltene oxidation, therefore, it is important in future studies to have better oxidation control, not only in surface preparation, but also in asphaltene preparation since this may skew results away from the reality that most adsorption in reservoirs or production processes occurs in an oxygen free environment. Acknowledgment. The authors gratefully acknowledge the financial support provided by Alberta Ingenuity and are grateful to Alberta Centre for Surface Engineering and Science at the University of Alberta for providing access to make use of their instruments. References and Notes (1) Dehkissia, S.; Larachi, F.; Rodrigue, D.; Chornet, E. Fuel 2004, 183, 83. ¨ stlund, J. A.; Nyde´n, M.; Fogler, H. S.; Holmberg, K. Colloids (2) O Surf., A 2004, 234, 95. (3) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutie´rrez, L.; Ortega, P. Fuel 1995, 74, 595. (4) Trejo, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169. (5) Piro, G.; Canonico, L. B.; Galbariggi, G.; Bertero, L.; Carniani, C. SPE Production and Facilities 1996, SPE 30109, 156. (6) Speight, J. G. The Chemistry and Technology of Petroleum; Dekker: New York, 1999. (7) Al-Maamari, R. S. H.; Buckley, J. S. SPE ReserVoir EValuation and Engineering 2003, SPE 59292, 1. (8) Kim, S. T.; Boudh-Hir, M.-E.; Mansoori, G. A. 1990, SPE 20700, 799. (9) Sztukowski, D. M.; Jafari, M.; Alboudwarej, H.; Yarranton, H. W. J. Colloid Interface Sci. 2003, 265, 179. (10) Abdallah, W.; Taylor, S. Nucl. Instr. Methods Phys. Res. B 2007, 258, 213. (11) Pernyeszi, T.; Patzko´, A.; Berkesi, O.; De´ka´ny, I. Colloids Surf., A 1998, 137, 373. (12) Marczewski, A. W.; Szymula, M. Colloids Surf., A 2002, 208, 259. (13) Zhanh, L. Y.; Lopetinsky, R.; Xu, Z.; Masliyah, J. Energy Fuels 2005, 19, 1330. (14) Zhanh, L. Y.; Xu, Z.; Masliyah, J. Langmuir 2003, 19, 9730. (15) Acevedo, S.; Castillo, J.; Ferna`ndez, A.; Goncalves, S.; Ranaudo, M. A. Energy Fuels 1998, 12, 386. (16) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, Z. A. Energy Fuels 2007, 21, 963.

18972 J. Phys. Chem. C, Vol. 112, No. 48, 2008 (17) Bubert, H.; Jenett, H. Surface and Thin Film Analysis; Wiley-VCH: Germany, 2002. (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Inc.: Eden Prairie, MN, 1995. (19) Ignatova, V. A.; Berghe, S.V-Den.; Dyck, S. V.; Popok, V. N. Microsc. Microanal. 2006, 12, 432. (20) Gu-ping, O.; Wen-ming, G.; Shi-chao, J.; Fu-jia, Z. Appl. Surf. Sci. 2006, 252, 3417. (21) Delpeux, S.; Beguin, F.; Benoit, R.; Erre, R.; Manolova, N.; Rashkov, I. Eur. Polym. J. 1998, 34, 905. (22) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227. (23) Kelemen, S. R.; Siskin, M.; Gorbaty, M. L.; Ferrughelli, D. T.; Kwiatek, P. J.; Brown, L. D.; Eppig, C. P.; Kennedy, R. J. Energy Fuels 2007, 21, 927.

Abdallah and Taylor (24) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3183. (25) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53. (26) Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9. (27) Mitra-Kirtley, S.; Mullins, O. C.; ven Elp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 253. (28) Wilhelm, A.; Patience, E. L.; Larter, S. R.; Jorgensen, S. Geochim. Cosmochim. Acta 1992, 56, 3745. (29) de Donato, Ph.; Mustin, C.; Benoit, R.; Erre, R. Appl. Surf. Sci. 1993, 68, 81. (30) Larachi, F.; Dehkissia, S.; Adnot, A.; Chornet, E. Energy Fuels 2004, 18, 1744.

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Study of Asphaltenes Adsorption on Metallic Surface Using XPS and

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