1954
I n d . E n g . Chem. Res. 1990, 29, 1954-1962
Analysis by X-ray Photoelectron Spectroscopy/Depth Profiling of Thin, Gasoline-Derived Deposit Films Spyros I. Tseregounis Fuels and Lubricants Department, General Motors Research Laboratories, Warren, Michigan 48090-9055
Fuel-derived deposits on injectors and elsewhere in engines can severely impair engine performance. A laboratory test procedure was developed t o produce thin deposit films from oxidized fuel on steel. T h e deposit films were analyzed by using X-ray photoelectron spectroscopy (XPS or ESCA) and depth profiling. The deposits were carbonaceous with lesser amounts of oxygen, sulfur, and nitrogen. The total sulfur concentration in the deposits was 5-10 times higher than the concentration of sulfur in the original gasoline. Ion bombardment preferentially removed oxygen from the deposit layer, revealing that sulfur in the deposits was in the form of oxygenated compounds (RS02R,RSO,OR, ROSO,OR, RS020S02R)and removal of oxygen converted them to lesser or non-oxygen-containing compounds (RSR, RSOR, RSSR, RSS02R). Fuel samples were spiked with two sulfur-containing chemicals, thioanisole and thianaphthene. Neither compound participated directly in the deposit formation process. Thinner deposit films were formed on polished and oxidized surfaces rather than on polished-only surfaces.
Introduction The formation of deposits during autoxidation of gasoline has drawn wide attention since 1985due to the adverse effects of such deposits on the performance of the multiport fuel-injection systems used in modern automobile engines. The deposits originate a t the injector tip area where the temperature reaches 100 OC during “hot-soak’’ (45-60-min period that immediately follows ignition turn-off). Benson and Yaccarino (1986) showed that the quality (stability) of the gasoline and the presence of detergent additives influence the formation of deposits. The injector deposits contain both organic and inorganic components (Taniguchi et al., 1986; Benson and Yaccarino, 1986). The carbon concentration is the highest, followed by varying amounts of oxygen, sulfur, and nitrogen. A number of other elements and metals may also be present, Zn, Pb, Na, and Si, to a lesser extent K, Cu, and Ca, and in some cases even Fe, W, Mg, P, and C1, but the relative concentrations of these elements vary considerably. A two-layer deposit structure has been observed: an inner varnish-like organic layer covered by an outer inorganic layer. Taniguchi et al. (1986) suggest that the outer layer contains mainly impurities that do not affect the deposit formation process. In any case, the inner deposit layer is thick enough by itself to cause injector failure (Kim et al., 1987). Taniguchi et al. (1986)reported that blending a base fuel with either cyclopentadiene or di-tert-butyl disulfide increases its tendency to deteriorate and form deposits. Kim et al. (1987) used a laboratory test to study the formation of deposits from oxidized fuels. They showed that certain preoxidized materials (polar, oxygen-containing hydrocarbon species) present in the original fuel influence deposit formation. These species either initiate or accelerate the oxidation-deposit formation process, or they are the deposit precursors. The soluble deposit precursors react to form insoluble material, which interacts with surfaces to form deposits. Taylor and Wallace (1968) studied the effect of sulfur species on deposit formation from aviation fuels on steel surfaces. They found that sulfides (through the formation of free radicals) had the largest effect on deposits; Le., they increased the tendency of the fuel to form deposits by about a factor of 20. Lauer et al. (1985) analyzed jet fueld deposits by using FTIR and suggested a two-stage deposit formation process: an early stage where the deposits 0888-5885/9012629- 1954$02.50/0
formed are rich in hydroperoxide and epoxide groups and a later stage where the deposits originate from highly oxidized hydrocarbon species and are rich in ketone and ester groups. Nayo and Lan (1986) proposed that fuel-derived deposits originate from soluble gum and that agitation of oxidation flasks may cause deposition of deposit films on the walls instead of deposit sediments. Hazel et al. (1986) in an Auger electron spectroscopy (AES)/depth-profiling study of deposits from aviation kerosene on aluminum surfaces found that segregation of Mg from the bulk of the aluminum to its surface oxide and the flow dynamics of the test reduced the formation of deposits. They also showed that optical reflectivity methods, used in the determination of deposit film thickness, may be misleading in certain cases. In this work, thin deposit films formed from oxidized gasoline on steel surfaces are analyzed by using X-ray photoelectron spectroscopy (XPS or ESCA) in combination with depth profiling. Chemical information about the deposits and ion-induced effects on the deposit layer are considered. Evaluation of ESCA/depth profiling as an analytical tool for studies of thin, fuel-deriveddeposit films is also considered.
Description of the Experiment Formation of Thin Deposit Films. The experimental setup for the formation of thin deposit films is shown in Figure 1. It consisted of a wide-mouth, 250-mL, roundbottom flask and flask head. The flask head had standard ground-glass joints allowing for the mounting of a stirring rod bearing, a condenser, and a thermometer. The stirring rod was a hollow cylinder with a side hole to allow for gas introduction into the flask during rod rotation. The rest of the equipment included a coupon holder attached to the end of the stirring rod, a stirrer, a temperature bath, and an air flowmeter. Deposit films were formed on 440C steel coupons (see Figure 1 for coupon dimensions). The 440C steel composition in mass percent units is Fe, 78.0-80.0; Cr, 16.018.0; and C, 0.95-1.2. Traces may or may not include Mn, Si, Mo, P, and S at maximum mass percent concentrations of 1.0, 1.0, 0.75, 0.04, and 0.03, respectively. The surface of the coupons was finely polished with 6-pm diamond paste on nylon cloth, using a lapping oil as a lubricant, and was cleaned with acetone in an ultrasonic bath. To investigate the effect of surface oxidation on deposit for$2 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1955 Table I. ESCA System Parameters For Data Acquisition A1 KO;120 W of power (10 mA, 12 kA) X-ray detector 2.9 kV (electron multiplier) constant AE mode, 100 pass energy (except otherwise CHA noted) pressure 12.0 X lo+ mbar
Stirring Rod (hollow. lOmm 0 D
For Sputtering ion gun" low rates: ion energy = 3 keV; ion current density Jp = pA mm-2; partial Ar pressure in ion gun 6.4 X -1.0 X lo4 mbar; sputter rate through a Ta205film 1.2-1.4 A/min high rates: ion energy = 4 keV; ion current density j , = 6.6 X pA mm-*; partial Ar pressure in ion gun -2.0 X lo-' mbar; sputter rate through a Ta205film 10-11 A/min ion beam scanned over a 10 X 10 mm2 area pressure (2.0-4.0) X 10" mbar
,A"
1
'
-4"
i' Metal Coupons 14x10x1.1 mm3
Figure 1. Experimental setup for the generation of fuel-derived thin deposit films. Coupon arrangement and dimensions as shown.
mation, some of the coupons were heated in quiescent air a t 220 "C for 2 h. The properties of the test fuel (which was specifically blended for deposit studies) are described in detail elsewhere (fuel batch 4 in Kim et al. (1987)). The fuel was stripped of its lower than 100 "C boiling point components by heating it in a nitrogen atmosphere, a procedure similar to that described by Kim et al. (1987). For the formation of deposit films, two coupons were placed vertically on the coupon holder (Figure 1) and 200 mL of the fuel was poured into the flask. The flask was lowered to the temperature bath, the stirring motor was turned on, and the whole assembly, including the rod, the holder, and the coupons, was rotated a t a constant speed of 150 rpm. Hydrocarbon-free air was introduced continuously through the stirring rod into the fuel, at a constant flow rate of 30 scc/min. The temperature of the fuel was maintained a t 99-100 "C. Tap water (22-24 "C)was passed through the condenser a t a rate of 450-500 mL/ min. After 24 h, the experiment was terminated and the coupons were removed and stored in a desiccator for later analysis. At that time, the fuel had undergone slight oxidation and a thin, tan deposit film had formed on the metal surface. In addition to the test mentioned above, two more experiments were conducted. For these tests, the fuel was spiked with a sulfide (thioanisole) and a thiophene (thianaphthene) as part of an ongoing project to investigate the effects of sulfur-containing compounds on the depositforming tendency of the fuel. Film Analysis. Depth profiling of the thin deposit films was accomplished by XPS and ion sputtering with Ar+ ions. A Leybold-Heraeus, Model LH-12, XPS/AES electron spectrometer was used for the analysis. The energy analyzer is a concentric hemispherical analyzer (CHA) and operates a t either constant resolution (AI3mode) or constant relative resolution ( M / E mode). Sputtering was accomplished with a scanning ion gun using ultra-highpurity Ar. Table I lists the system parameters during data acquisition and sputtering. The calibration of the binding energy scale was based on the Au 4f,,, peak a t 83.7 eV. The binding energy of the peak positions has an accuracy
"The values of jpare evaluated from ion gun calibration data supplied by the manufacturer. The values reported here have a 20% uncertainty margin.
of f0.3 eV. During sputtering and subsequent data acquisition, we made certain that the area of the sample that was analyzed by ESCA was positioned entirely within the rastered area of the ion beam. More details about the calibration and the characteristics of our XPS/AES system are given by Tseregounis (1987). The operation of the electron energy analyzer and the acquisition of the ESCA spectra, in the form of intensity (counts per second, cps) versus kinetic (or binding) energy of the electrons (electronvolts, eV), was controlled by a Hewlett-Packard (Model HP1000-A600) computer. A software package (developed by Leybold-Heraeus, DS100, Version 86B) was used for curve smoothing, background subtraction, and curve fitting. The quantification of our data was performed by using the sensitivity factors published by Wagner et al. (1981). The sensitivity factor, SFi, is the quantity by which the intensity, litof an XPS peak of an element, i, has to be normalized in order to yield the atomic concentration of this element on a homogeneous surface containing n elements:
k=l
The sensitivity factors reported by Wagner et al. (1981) were evaluated for instruments with a detection efficiency proportional to Ek-*(Ek is the kinetic energy of the photoelectrons). In this present work, quantitative data were obtained with the analyzer working at constant aE mode, for which a similar detection efficiency has been suggested by Noller et al. (1974). Therefore, the uncertainty of our quantitative results should be of the order of f20% similar to the uncertainty of the sensitivity factors (Wagner et al., 1981). The acquired ESCA spectra from each sample were smoothed by using a second-degreefitted polynomial. The number of points was chosen such as to provide a smoothing interval equal to approximately 0.7 times the energy value (in eV) of the full width a t half maximum (FWHM) of the C 1s peak, as suggested by Proctor and Sherwood (1980). For quantitative analysis, background subtraction was performed based on the method described by Shirley (1972). The background integration limits were carefully chosen, and the area under each peak was evaluated and used in computing the relative atomic concentrations of the major elements present on the sample surface.
1956 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 Felp
1400
1000
600
200
Binding Energy (eV) Figure 2. ESCA spectrum of a polished 440C steel surface after sputtering for 2 min a t low rates (Table I). The spectrum was obtained at constant relative resolution (hE/E) analyzer mode. 290 t
A 0 C Fe 0 Cr
Fe I
.o
............... ...............
$....0.................... %*V 3
K
"
20
"
......
"
....................... '
30 " "
"
540
530
............. 0
'3-""""
F....s.--
O f . ' '10' '
280
"
---.-v '"'.A: '
40 ~
'
520
Binding Energy (eV) Figure 4. Carbon and oxygen Is peaks from deposit sample on a polished coupon as a function of sputtering time. The time marked with an asterisk indicates sputtering a t high rates (Table I). The unmarked time indicates sputtering at low rates (Table I). Soutter Time
=I
-4-
50
Sputter Time (min)
A
lP n
E
'g E
Figure 3. Depth profiles of polished (filled points, solid lines) and polished/oxidized (open points, dashed lines) 440C steel surfaces.
Results and Analysis Steel Surfaces. Figure 2 shows a survey spectrum of a polished steel surface acquired with the analyzer working a t constant relative resolution ( U / E mode). This spectrum indicates a typical chromium-steel surface covered by an oxide layer. All the peaks are identified in Figure 2 with some close-up details mentioned below. The small C 1s peak at 283.4 eV of binding energy (BE) is the sum of some contaminant carbon and the carbidic carbon in the steel. The Fe 2pSl2 peak is a doublet due to the presence of the oxide (mainly Fe2+)at 709.6 eV of BE and the iron metal at 707.0 eV of BE (Wagner et al., 1979; McIntyre and Zetaruk, 1977). The Cr 2p doublet a t 544.4 eV (2p3/J and 583.9 eV (2pIl2)also shows the presence of Cr metal and its oxide. The 0 1s peak a t 530.2 eV shows that the oxygen on the surface is in the form of metal oxides (Wagner et al., 1979). Traces of Ar (due to ion sputtering) and Mo (present in the steel) are also noticed. Figure 3 shows the depth profiles of the two metal coupons having polished and polished/oxidized surfaces. A contaminant layer, rich in carbon, which is removed very quickly with sputtering, is present on both surfaces (slightly thicker on the polished surface, as we will see later). Beneath this layer, the metal surface is a mixture of iron and chromium oxides. The oxide film is much thicker on the polished/oxidized surface than on the polished one. The ratios of the atomic fraction of Fe divided by that of Cr in the oxide layer (after about 2-5 min of sputtering) of the two surfaces, polished and polished/oxidized, are
100
'5
165
155
-.
410
400
Binding Energy (eV) Figure 5. Sulfur 2p and nitrogen 1s peaks in the deposit film. See Figure 4 for details.
approximately 1.9-2.3 and 3.2-3.6, respectively. This is in agreement with the results of Asami and Hashimoto (19791, who have shown that polished chromium/iron alloy surfaces appear to be enriched in Cr. On the other hand, heated Cr/steel surfaces are richer in iron oxide than just polished surfaces as indicated by Coad and Cunningham (1974). The relative Fe/Cr ratio of the bulk metal is 4-5. The data in Figure 3 show that the Fe/Cr ratio approaches its bulk value after long sputtering periods. Accordingly, the surfaces of both coupons are richer in Cr than in the bulk of the metal, with the polished surface being even richer in Cr than in the polished/oxidized surface. Deposit Films. Figures 4-6 show ESCA/depth-profiling results for the six major elements, carbon, oxygen, nitrogen, sulfur, iron, and chromium, in the deposit film formed on the polished metal surface. Figure 7 shows the relative concentration of the atomic elements as function
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1957
I
Fe
2p3/2
2p3/2
I
n
Cr
1 C - 300
0
50
100
150
200
Sputter Time (min)
590
580
570
Binding Energy (eV)
Figure 6. Iron and chromium 2p ESCA peaks from the metal substrate. See Figure 4 for details.
0
50
100 150 200 Sputter Time (min)
250
300
Figure 7. Depth profile of thin deposit film on the polished 440C steel surface.
of sputter time for the deposit film on the polished metal surface. The BE of the C 1s peak decreases with sputter time. This phenomenon depends on the preferential sputtering of oxygen from the surface (Hofmann, 1984) as we will see later. The BE of the C 1s peak at 285.8 eV (no sputtering) shifts very quickly to about 285 eV where it remains relatively unchanged, until it shifts again to a lower BE after long sputtering periods, particularly after sputtering at higher rates. This occurs because, after the initial removal of oxygen, a steady state is reached where in the altered surface layer the concentration of each atom in the surface remains the same. After long times, the carbidic form of carbon present in the substrate metal causes the peak to shift even further to a lower BE (283 eV). In Figure 7, we see that the concentration of carbon increases sharply at the beginning of the sputtering process (due mainly to removal of oxygen), reaches a peak fraction of 85-87% after about 10-20 min, and decreases slowly for longer sputter times (due to the depletion of the deposit film and the appearance of the substrate metal). The 0 1s peak, in Figure 4, shifts to lower binding energies during initial sputtering, but not as much as the C 1s peak. After about 10-20 min, the oxygen of the metal oxides widens and shifts the 0 1s peak further. A twocurve, Gaussian fit of the spectra a t 35 and 50 min results in two distinct peaks a t 532 and 530.2 eV with a FWHM of 2.9 and 2.0 eV, respectively. The first peak is due to
250
Figure 8. Depth profile of thin deposit film on the polished/oxidized 440C surface.
oxygen in the deposit film. The second is due to the iron and chromium oxides in the substrate. In Figure 7, the oxygen concentration is reduced dramatically during the first 2-5 min of sputtering, to a minimum of 0.07-0.08 atomic fraction. This is indicative of the preferential removal of oxygen with ion etching. It could also imply that the concentration of 0 in the contaminant layer is higher than that in the deposit layer. Hydroxyl impurities, mainly water, with high oxygen content, may adsorb on the deposit film and sputter off quickly during the first few seconds of ion etching. The oxygen concentration increases after 10-20 min of sputtering (due to the appearance of the metal oxides) and decreases again later, after 70-80 minutes, because of the removal of the metal oxides. The changes in sulfur 2p and nitrogen 1s peaks during sputtering are shown in Figure 5. Before sputtering, the sulfur 2p (a nonresolved doublet) peak is located at a BE of 169.3 eV. After as little as 2 min of etching, a second peak appears at 163.9 eV. As the sputtering continues, the first peak decreases in intensity while the second increases. This behavior is related to the preferential sputtering of oxygen and will be examined in more detail later. Figure 7 indicates that the atomic concentration profile of sulfur follows pretty much that of carbon. The S concentration increases slightly at early sputter times and decreases later as the deposit film is sputtered away. Prior to sputtering, the nitrogen signal shows two peaks at about 402.3 and 400.3 eV. With as little as 2 min of etching, the nitrogen peak at 402.3 eV decreases substantially and a new peak at lower BE (399.3 eV) appears. For longer sputter times, the peak a t approximately 399.3 eV dominates. The nitrogen profile in Figure 7 shows that the nitrogen concentration decreases steadily with sputtering and eventually disappears when the deposit film is removed. The iron and chromium 2p lines are shown in Figure 6. As the deposit film is depleted during ion etching, iron oxide appears as indicated by the Fe 2~312.line a t 710 eV of BE. The shape of this line is a convolution of different peaks under the envelope, which correspond to the Fe2+ and Fe3+ states and their multiplet splitting effects (McIntyre and Zetaruk, 1977). As sputtering proceeds, the 2~312line for the Fea state a t 707 eV becomes visible. The Cr 2p3,, line is also composed of two characteristic peaks, one a t 576.3 (Cr203)and one at 574.3 (chromium metal) (Wagner et al., 1979). Figure 8 depicts the depth profile of the deposit layer formed on a surface that was oxidized before being exposed to the fuel. During the first 20 min of sputtering, the
1958 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 Table 11. Relative Atomic Concentrations of Fe, S , and C in the Deposit Films Produced by the Fuel Blended with Two S u l f u r Compounds" elemental atomic fractions, % fuel + 2.5% by vol fuel + 2.5% by vol fuelConly thioanisoled thianaphthenee sputter time,* min C s (XlO) Fe C s (X10) Fe C s (X10) Fe 25 0 70 70 30 73 24 29 tr 86 33 20 86 90 27 50 71 25 3 65 25 6 74 23 2 16 8 33 13 100 46 16 51 17 6 n 13 19 8 150 32 20 36 10 10 "Both compounds were acquired from Aldrich Chemical Co. and were used as received. *Sputtering at low rates (Table I). cInterpolation of data points in Figure 7. dThioanisole (phenyl methyl sulfide), 99% purity, bp 188 "C
OSCH3
CThianaphthene (benzo[b]thiophene),97% purity, bp 221-220 "C
composition of the deposit film is similar to that described in Figure 7. For longer sputter times, the main difference is the higher oxygen concentration in the substrate metal. Comparing Figures 7 and 8, it can be seen that the C, S, and N fractions in Figure 8 decrease somewhat faster than in Figure 7 and also that iron metal is uncovered somewhat earlier. These observations indicate that oxidation of the steel surface lowers slightly the rate of deposit formation and accumulation. The relative positions of the XPS peaks in the deposit layer on the polished/oxidized surface were similar to those observed on the polished surface (Figures 4-6), implying that the chemical character of the deposit layer in the two surfaces was the same. The Fe/Cr ratio in Figures 7 and 8 is about 2.9-3.5, indicating that the surface of the polished coupon is enriched in Fe during the deposit formation test (due to heating as we saw earlier). Figure 9 shows the ESCA spectrum of the deposit layer on a polished surface after 2 min of sputtering. In addition to the peaks of the major elements found in the deposit layer (C, 0, S, N), small peaks that are due to traces of other atoms are also observed. Specifically, peaks due to the Zn 2p line (1022 and 1045 eV), the Na 1s line (1072 eV), the Cu 2p3,* line (934 eV), the Si 2p line (102 eV), and the Auger transitions for Na(KLL) and Zn(LMM) a t 497 and 498 eV, respectively, are observed. During depth profiling, both the Na and Zn signals disappear with the appearance of the metal substrate, indicating that these metals are present in the deposit layer. Similarly, Cu and Si were also found to be present only in the deposit layer. Si and Na may have resulted from clay or sodium salt impurities in the gasoline. Zn and Cu could have originated from containers used for fuel storage, but their origin may be more subtle. Sulfur Compounda. Table I1 shows data from tests run with the fuel spiked with the two sulfur-containing chemicals. The relative concentration of sulfur in the deposit film has not been affected significantly by the presence of either of the sulfur chemicals. On the other hand, addition of thioanisole results in the formation of a thinner deposit layer than that from the base fuel; blending the fuel with thianaphthene yields a slightly thicker deposit film. In addition to the above, the peak positions of the different atomic species in the fuel blends behaved similarly to those for the pure fuel as shown in Figures 4-6, implying that these two compounds did not affect directly the deposit formation mechanism.
26001
U
Id0
1200
860 600 Binding Energy (eV)
1000
400
260
Figure 9. ESCA spectrum of the deposit film on the polished steel surface after 2 min of sputtering at low rates (Table I). The spectrum was obtained at constant relative resolution (AEIE)analyzer mode.
Discussion Figure 10 shows scanning electron microscopy (SEM) pictures of the deposits on the steel coupons before and after sputtering. The deposits have the form of clusters, which are made of spherical particles fused together, bound to the metal surface. The morphology of these deposits is similar to that observed after oxidation of similar fuels in a different test (Kim et al., 1987). The main difference is that the size of the individual spherical particles in these deposits is small (less than 1 pm). This result can be attributed to coupon rotation during fuel oxidation (in the previous work (Kim et al. (1987)) the coupons remained stationary), which prevented the formation and adherence of large deposit particles to the surface. The morphology of the deposits (Figure 10) is responsible for the noisy XPS spectra and the low sensitivity in depth resolution. The deposits may be viewed as surfaces with very high roughness rather than uniform films. The effect of surface roughness on XPS analysis has been pointed out by other investigators (Le., Ebel et al., 1973; Hofmann, 1980). During XPS/depth profiling, surface roughness may worsen the depth resolution due to redeposition, resputtering and shadowing effects (Hofmann, 1980). Consequently, the relatively low depth resolution
Ind. Eng. Chem. Res., Vol. 29, No. 9,1990 1959
Compound HSNa/K NaSC (S) NR2 S=C (SSR) NR:
+sso s--3 KSC (S) OR RSH RSC (S) SR RSR RSSR ClSSCl RSSC (S) NR2
s=c=s RSS (0) R RSS03RSS02R 12NSNR2 RSC I '8 RSNH2
Sthiophene RSOR ROSSOR -3CSS!2R RS02 RS (0) R +SR3 RS (0) SR . _
--
-:$SO3RS (0) OR RS02R
-so3s+S (OR) R2 RS03ROS (0) OR RSO-SR RSO~NR~/H~
SOC I
Figure 10. SEM pictures of the deposits on a polished 440C steel surface, before sputtering (top picture) and after sputtering (bottom picture) for 190 min at low rates followed by 65 min at high rates (Table I).
seen in this work (Figures 7 and 8) is primarily attributed to high surface roughness and secondarily to other effects [preferential sputtering, elemental mixing (known as "knock on" and cascade mixing), atomic transport etc. (Hofmann, 1980)l. Extensive sputtering removes the deposit clusters from the coupon surface except for a few islands of eroded deposits. Figure 10 shows asperities on the metal surface induced by the sputtering process. This phenomenon, which is due to preferential sputtering of certain species from a surface and to higher sputter yields along grain boundaries, has been observed in surfaces of various alloys (Olson et al., 1984). The atomic fraction of carbon and sulfur in the deposits is 0.8-0.9 and 0.02-0.03, respectively. The ratio of hydrogen (not detectable by ESCA) to carbon in hydrocarbons of the gasoline range is of the order of 1.7-2.1, and it is expected to be much less in the deposits. Even if we assume that the hydrogen-to-carbon ratio in the deposits is of the order of 1.5, simple calculations will show that the sulfur atomic concentration in the deposit films in this study is about 5-10 times higher than the sulfur concentration in the original (unstripped) gasoline. Therefore, the deposit film is highly enriched in sulfur, and the sulfur-containing constituents in the gasoline may play an important role in the deposit-formation process. It was shown earlier that the sulfur 2p peak shifts immediately to lower binding energies with sputtering. In Figure 11, we have reproduced Figure 3 from Lindberg et
+s (0)R 3 RS0,OR RSOZC I
- so4-S03SR
-so3s02RS020S02R RS02SCF3 ROS020R RS02F
SOF,
Figure 11. Sulfur 2p binding energies from various sulfur compounds. Reprinted with permission from Lindberg et al. (1970). Copyright 1970 Swedish Academy of Sciences. The shaded areas show the BE of the S 2p line in our spectra, including a *0.3 eV uncertainty.
al. (1970), which lists the BE of the ESCA S 2p line for various sulfur-containing compounds. The shaded areas correspond to the position of the S 2p peak, which was observed in the spectra of this investigation before and after sputtering (Figure 5). Figure 11shows that the S 2p line with the higher binding energy is indicative of sulfur sulfonic primarily bound to oxygen, i.e., sulfates, acids and .derivatives, RSO,; sulfonic esters or similar species, RS020R, RS020S02R,ROS020R, etc. On the other hand, the lower BE of the S 2p line corresponds to sulfur that is primarily bound to other sulfur or alkyl or aryl carbon, i.e., thiols, RSH; sulfides or disulfides, RSR, RSSR, and various derivatives; or even sulfur bound to nitrogen, R2NSNR2and RSNH2. The large shift of the BE of the S 2p line with sputtering is attributed to preferential removal of oxygen from the deposits. The sulfur compounds in the deposit layer are species where the sulfur is bound primarily to oxygen.
J
1960 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
Since most of the sulfur in the fuel is in the form of sulfides, thiophenes, and thiols (no oxygen-containing compounds), the oxygenated sulfur compounds in the deposits are produced during the oxidation of the fuel. Sputtering removes oxygen from the deposits, thus eliminating the S=O bonds is favor of the C-S bonds. Taniguchi et al. (1986),who analyzed deposits from field injectors, reported IR evidence of metal sulfates. Covitch et al. (1988), who studied deposits from the piston crown land area and ring grooves of diesel engines, reported that the deposits contain large concentrations of metal sulfates and that the strong adhesion of carbonaceous crown land deposits may be due to calcium sulfate crystals and an organic binder originating from oxidized lubricant. In the piston deposits, metals from oil additives and SOz (present as a combustion byproduct) may explain the sulfates and the high concentration of CaSO,. On the other hand, the gas atmosphere surrounding the injector tip during the hot soak period may contain blowby gases where the presence of sulfur (although not in high concentrations) could contribute to the formation of metal sulfates. In the tests described in this report, where the temperature is lower than that of the piston and there are no combustion products or blowby gases, it is hard to expect that any considerable concentration of metal sulfates will be present. In addition, recent data (not presented here) show no evidence of metals in deposits derived from a similar fuel. On the contrary, sulfur is present in about the same concentration and its 2p peak undergoes the same shift with sputtering. Therefore, sulfur in these deposit films is in organic form. The carbon and nitrogen Is peaks undergo a similar shift to lower BEs with sputtering, but not as dramatic as that of sulfur. Preferential sputtering of oxygen reduces the number of carbon or nitrogen atoms that are bound to oxygen in favor of atoms bound to less electronegative elements. A shift in the carbon 1s peak indicates the presence of carbon atoms bound to oxygen (C=O, C-0, O=C-0). The fact that the concentration of oxygen shows a minimum with sputter time (Figures 7 and 8), indicates that the oxygen concentration in the deposit film is less than the relative oxygen concentration in the metal oxide layer or that oxygen is more easily sputtered from the deposit molecules than from the metal oxide matrix. The N 1s peak positions for different nitrogen-containing compounds are reported in the literature (Wagner et al., 1979). The binding energy of the N 1s line in these current data (Figure 5 ) suggests that nitrogen in the deposits is not likely to be in the form of nitro compounds (NO2)for which a much larger BE (404-406 eV) has been reported. More likely, nitrogen is bound to a single oxygen atom (RNO, RNONR), and removal of oxygen with sputtering reduces these bonds to the amine type (NH,) or to the C-N, N-N, or N-H type, in which the BE of the N Is line is around 399-400 eV. We have seen that oxidation of the polished steel surface results in the formation of a layer rich in metal oxides that tend to suppress slightly the formation of deposits (Figures 7 and 8). Similar behavior has been observed by Tseregounis et al. (1987) in deposits from mineral oil base stocks, oxidized a t higher temperatures. Oxidation of the metal surface could reduce the number of active adsorption sites on the metal coupon and could slow the adsorption and accumulation of polar species and deposit precursors. This is also supported by the data shown in Figure 12 where the deconvoluted 0 1s peak is shown on polished and polished/oxidized steel surfaces, before and after 2 min of sputtering. The 0 1s peak at the higher BE is due to
20001
1_2pzksbd min sputering -surface
,'
,
,.4
-Tsputtwing
530
556
534
532
530
528
526
Binding Energy (eV)
Figure 12. Oxygen 1s peak on polished and polished/oxidized 440C steel surfaces before and after 2 min of sputtering at low rates (Table I). Table 111. Analysis of Fuel Samples for Metal Traces concentration, ~ g l g fuel sample Zn Na Si Cu pure fuel 3
