3610
Ind. Eng. Chem. Res. 1998, 37, 3610-3617
Adsorption of N,N′-Disalicylidene-1,2-propanediamine on 304 Stainless Steel Charles C. Chusuei,†,‡ Robert E. Morris,§ and John A. Schreifels*,‡ Department of Chemistry, George Mason University, Fairfax, Virginia 22030, and Navy Technology Center for Safety & Survivability, Code 6181, Naval Research Laboratory, Washington, D.C. 20375-5342
The behavior of the jet fuel metal deactivator additive (MDA), N,N′-disalicylidene-1,2propanediamine in some high-temperature tests, has raised questions concerning whether it would decompose on contact with hot metal surfaces in aircraft engines and fuel systems. The adsorption of this additive was studied on oxidized and oxide-free 304SS surfaces using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). XPS binding energy (BE) shifts indicated thermal decomposition on the surfaces. TPD revealed the growth of both physisorbed and chemisorbed states in accordance with the Stranski-Krastanov mechanism. MDA was only weakly chemisorbed onto the 304SS surface, with desorption energies of 47 kJ/mol on an oxidized surface and 33 kJ/mol on the oxide-free surface. These findings suggest that stainless steel would not likely contain a contiguous chemisorbed MDA multilayer that the adsorbed MDA could be removed from internal surfaces of aircraft fuel systems in regions of high-temperature and turbulent flow. Introduction In the presence of oxygen, thermally stressed hydrocarbons can undergo autoxidation to form insoluble reaction products. This process is catalyzed by dissolved copper ions.1 Hydrocarbon fuels can and do solubilize copper upon contact with copper-bearing surfaces. The usual sources of copper, aside from copper-sweetening procedures, are bronze pump bearings. However, in the U.S. Navy, many air-capable vessels employ coppernickel alloys in shipboard fuel handling systems for protection from saltwater corrosion. As a consequence, copper levels in excess of 1000 ppm have been found2 in fuels onboard aircraft carriers in fleet surveys. Although copper speciation in fuels has not been examined in depth, it is known that fuels with high acid numbers tend to take up greater amounts of copper than those with low acidity. Presumably, the copper is bound to polar constituents in the fuel, by either ion pairing or coordination with species such as carboxylic and sulfonic acids, or esters of oxygenated fuel components. Using a glass reactor, Kendall3 demonstrated that, in some fuels, as little as 5 ppb of soluble copper can enhance autoxidation. Measurable increases in jet fuel autoxidation have been measured after the addition of less than 2 ppb soluble copper.4 The most common approach for counteracting the catalytic effects of soluble copper involves the use of a metal deactivator additive (MDA). The active ingredient of the additive approved for use by military specification MIL-T-5624 and ASTM specification5 D1655 is N,N′-disalicylidene1,2-propanediamine (C17H18N2O2; CAS No. 94-91-7; m/e ) 282). This compound forms a soluble catalytically inactive tetradentate coordination complex with copper. * Corresponding author. E-mail:
[email protected]. Fax: (703) 993-1055. Phone: (703) 993-1082. † George Mason University. ‡ Current address: Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, TX 77842-3012. § Naval Research Laboratory.
Copper catalysis of autoxidation occurs via electrontransfer reactions to reduce fuel hydroperoxides to alkoxy radicals and hydroxide ions. Thus, complexation with MDA effectively blocks these electron-transfer reactions and offsets or deactivates the catalytic effect of copper. The effectiveness of the MDA in thermal stability testing by the Jet Fuel Thermal Oxidation Test (JFTOT) method6 has been confirmed using 304 stainless steel (304SS) JFTOT heater tubes by Morris et al.4,7 and Clark.8,9 Moreover, the presence of this additive in a fuel, which produces measurable amounts of thermal deposits, nearly always results in significant reductions of deposition in the JFTOT. Significant reductions in insoluble gum deposits on the JFTOT 304SS heater tubes were observed at the 260-310 °C (533-583 K) temperature range. The benefits of MDA have not been as clear in other thermal stability tests. During tests conducted10,11 with an injector feed-arm simulator (IFAR), initial reductions in deposition were reported from jet fuel after adding MDA. However, in long-term IFAR tests and in fullscale fuel atomizer tests,12 a point was reached where increased deposition was observed to occur at a rate similar to that observed when no additive was present. This behavior has raised the question about the capacity of MDA and similar compounds to exert an influence on the activity of a metal surface to promote thermal deposition from jet fuels. These findings led to the conclusion10 that the metal surface was being passivated by the formation of a contiguous layer of MDA. Removal of this MDA monolayer or its covering by subsequent layers of thermal deposits12 was thought to be responsible for the induction periods observed. The possible presence of MDA on the metal surface was verified with secondary ion mass spectrometry,11 which indicated that MDA was present and that it accumulated on the surface with increased temperature and solution concentration. This was later confirmed by Baker13 and Schreifels.14 Using X-ray photoelectron
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Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3611
spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) of MDA on the surface, Schreifels et al. found that it appeared to be present at a coverage that was significantly less than a monolayer under test conditions. XPS analysis revealed a very weak N 1s signal and a thick carbonaceous layer on the surface after adsorption. However, exact coverages could not be determined since MDA was sparsely distributed on the surface. Temperature-programmed desorption (TPD) experiments15 confirmed this on both oxidized and oxide-free 304SS surfaces. His findings also revealed that on increasing exposure a multilayer of MDA would develop on sparsely distributed sites containing monolayer coverages. This study extends Gwynn’s work by focusing on the desorption kinetics of chemi- and physisorbed MDA and possible dissociative desorption mechanisms from the chemisorbed state from 304SS at elevated temperatures. Experimental Section Experiments were performed in an ion-pumped stainless steel ultrahigh-vacuum (UHV) chamber with a base pressure of ca. 1 × 10-10 Torr and an operating system pressure of ∼5 × 10-10 Torr. The apparatus was equipped with a double-pass cylindrical mirror analyzer that allowed for XPS, an ion sputter gun, and a UTI100C quadrupole mass spectrometer (QMS) for the TPD experiments. Al KR X-rays with a photon energy of 1486.6 eV were used. The QMS was housed in a tube with an electrically isolated Al mesh screen that was biased at -100 V to deflect electrons away from the sample, thus preventing unwanted electron beam effects. When narrow XPS scans for the individual elements were performed, blank scans were taken and subtracted from the XPS signal to correct for XPS intensities attributed to background gases. XPS intensities of the C 1s, N 1s, and O 1s signals due to background gases constituted less than 10% of the total signal. The pass energy of the analyzer was 50 eV for narrow and 100 eV for broad scans. A narrow scan of the N 1s region of a multilayer of MDA produced a peak with a full-width at half-maximum (fwhm) of 2.0 eV. XPS binding energies were referenced16 to the Cu 2p3/2 level at 932.67 eV. After Cu 2p3/2 was referenced to, the Fe 2p3/2 level was found to have an XPS binding energy (BE) of 706.9 eV, which was in agreement with the literature17-19 and was therefore used as an internal calibration standard. The following levels were scanned: Fe 2p3/2, Cr 2p3/2, N 1s, O 1s, and the C 1s. Fe and Cr comprised almost all the XPS signal from the 304SS. XPS signals from other metals, such as Ni, were negligible. At the conclusion of a TPD experiment, the sample was again exposed to MDA under identical conditions so that the XPS data could be acquired. XPS examination of the oxide overlayer indicated that the BE of the O 1s line at 530.0 eV is consistent with reported values for oxygen in Cr2O3,20-22 and Fe3O4.18,23 Analysis of XPS binding energies from the observed Fe 2p3/2 and Cr 2p3/2 peaks revealed two oxidation states for each metal: one for the metallic state and the other for their respective metal oxides. Fe and Cr gave rise to elemental states at binding energy peak centers of 706.9 and 573.9 eV, respectively.24,25 The Fe 2p3/2 line appearing at 710.1 eV indicated the presence of FeO19,23 and/or Fe2O3,26 while the Cr 2p3/2 line appearing at 576.8 eV denoted Cr2O3.23,24,27-29 The ratios of the relative
amounts of metal to oxide based on XPS peak areas for the Fe and Cr oxide were 7.3 and 2.8, respectively. The sample holder was mounted onto a Huntington Mechanical Laboratories PM-600 xyz manipulator and a liquid-nitrogen reservoir so that the substrate temperature could be quickly cycled between 150 and 1100 K.30 Heating was provided by passing current through two tungsten wires spot welded to the back of the sample and to two nickel rods. These Ni rods were in thermal contact with and electrically isolated from a liquid-nitrogen reservoir for cooling. A chromel-constantan thermocouple was spot-welded to the back of the sample to monitor temperature. The 304 stainless steel substrate was 12 × 10 × 1 mm, was polished to a 0.05-µm finish, and was cleaned by heating to 1100 K for 300 s in UHV. Surface cleanliness of the oxide-free surfaces was verified by XPS broad scans.31 The N,N′-disalicylidene-1,2-propanediamine (MDA) was obtained from Phaltz and Bauer and used as received. The purity of the MDA was verified by examination of the mass spectrum obtained from the QMS. Dosing was accomplished by placing the MDA in a heated (348 K) Swagelok glassto-metal seal vial attached to a 304SS 1/4-in.-diameter tube and sublimed into the chamber and onto the 304SS sample. Oxide overlayers were produced by positioning the sample surface 1 cm away from the front of a 1/4in.-diameter stainless steel transfer tube and exposing it to a stream of oxygen (MG Industries, extra dry 99.8%) using a leak valve at 3.0 × 10-8 Torr for 60 s while the sample was cooled to 203 K. The substrate was then flashed to 900 K for 1 s. The resulting oxide surface was checked with XPS to ensure that the same type of overlayer was produced for each experiment. Two methods were used to dose the surface with MDA and to vary the amount of coverage: (1) annealed and (2) dosed. In the annealed experiments, a multilayer of MDA was adsorbed at 100 K onto the 304SS and the substrate was heated to 373, 398, 473, 523, 573, and 633 K to obtain various coverages on the surface. In the dosed experiments, different coverages were obtained by exposing the surface to MDA for predetermined times ranging from 0 to 90 s at 100 K. TPD experiments were undertaken in order to (1) establish the order of desorption from the 304SS surface and (2) obtain the first-order activation energy of desorption (Ed) of the chemisorbed state on the two surfaces. The desorption energy (Ed) of the chemisorbed state was determined using the Redhead method,32 which is based on the relationship between the temperature at which the TPD desorption rate is at a maximum (Tm) and the heating rate (rh). Linear heating ramps were used for all TPD experiments. Both oxidized and oxide-free 304SS surfaces were exposed to MDA at various exposure times (0-30 s) to determine the adsorption at different exposure times. The QMS was tuned to a single m/e value to monitor the desorption of molecular MDA in these experiments. Results and Discussion TPD of Molecular MDA. The TPD of m/e ) 148 was found to have the same TPD shape and desorption temperature as the m/e ) 282 parent ion but had a stronger signal-to-noise ratio and hence was a more suitable mass to use to monitor molecular MDA desorption. A plot of QMS intensity of m/e ) 148, the base
3612 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
Figure 1. TPD of 1.0 ML MDA on oxide and oxide-free 304 stainless steel, showing R and β states.
peak of MDA as a function of temperature, was obtained at each coverage. The TPD spectrum of MDA, shown in Figure 1, reveals two binding states: a physisorbed (R) state, which desorbed at 318 K (45 °C), and a chemisorbed (β) state, which desorbed at 400 K (127 °C). The β state initially grew, with increasing coverage, followed by the appearance of the R state. When the chemisorbed β state became saturated, the physisorbed R state continued to grow. This simultaneous growth of the R and β states is consistent with a Stranski-Krastanov (SK) growth mechanism,33 a type of Brunauer, Emmett, and Teller (BET) growth. In the SK growth mechanism, three-dimensional islands form on the surface on top of an initial chemisorbed thin film and may do so before a complete monolayer is formed. Similar TPD spectra of SK growth of Ag on a Si(111) surface were reported by Kern.34 In their TPD spectra, physi- and chemisorbed states grew on the surface simultaneously, with the chemisorbed state appearing first and reaching saturation. Similar to the R state of adsorbed MDA, their physisorbed state, which denoted three-dimensional crystallites, continued to grow after saturation of the chemisorbed state. In this study, the desorption kinetics and Ed of the chemisorbed β state of MDA were studied. Estimation of Coverage. In the earlier work of Gwynn,15 the 1.0 monolayer (ML) coverage was defined as the point where the chemisorbed state reached saturation and no physisorbed material was detected. The same method of determining the coverage was used for the annealed experiments. Accordingly, in the annealed experiments the absolute XPS intensity of the C 1s peak was used for calibration and taken where the multilayer was heated to 298 K (25 °C) to remove the physisorbed R state, while leaving the saturated β state intact. Other coverages were assumed to be proportional to the C 1s signal intensity. However, this definition of the 1.0 ML coverage is not correct for a SK growth mechanism, where the R state began to grow before the β state reached saturation which, as stated earlier, is the growth mode in the dosed experiments. In determination of the coverage in the dosed experiments, the β state reached saturation when
the R/β TPD peak area ratio at m/e ) 148 was 1.27. This coverage was interpreted to be equivalent to 2.27 monolayers and was used to calibrate the flux of MDA to the sample from the doser. An exposure of 75 s produced the equivalent of 1.00 mL of MDA on the surface. In the TPD studies, where the surface was dosed with MDA, coverages were reported in terms of exposure times for simplicity. Kinetic Order of Desorption. When TPD spectra are interpreted, the Arrhenius expression known as the Polanyi-Wigner equation can be used to describe the rate of gas evolution from a surface.35 It has been shown36 that TPD peaks of different desorption orders exhibit different symmetries. A TPD spectrum of a firstorder desorption mechanism generally will have an asymmetrical Gaussian shape, whereas second-order desorption will result in a symmetrical Gaussian line shape. Also, in first-order desorption, Tm (the temperature at which the desorption rate is a maximum) is coverage-independent, but for other orders of desorption, there is a coverage dependence37 on Tm. The TPD peak symmetry of the β state of the m/e ) 148 ion, on both oxidized and oxide-free surfaces, had an asymmetrical line shape and was thus consistent with first-order desorption kinetics (Figure 2). On the oxide-free 304SS surface, there was no variation in the Tm position as a function of coverage, which is indicative of first-order behavior. The shift in Tm on the oxidized surface, however, suggests some deviation from firstorder desorption behavior and suggests the possibility of second-order desorption kinetics. However, it is also possible that this shift may have been the result of a change in the nature of the chemical bonding of the β state as more MDA accumulated onto the surface. Changes in bonding would lead to changes in Ed and hence a shift in the Tm of the peak. A representative curve fit is shown in Figure 3 for a 58-s exposure of MDA on oxide-free 304SS with fixed centers for the R and β states. The peak center used for the β peak was obtained from the 0.42 ML MDA coverage for the oxide-free surface and the 0.48 ML coverage for the oxidized surface. The peak center for the R state was obtained from the multilayer MDA. Overall, the correlations of the fits for both oxidized and oxide-free surfaces were close to unity. Thus, there was reasonable agreement with first-order desorption of the β state on both surfaces. As shown in Table 1, the fwhm of the peaks representing the chemisorbed β state on both the oxide-free and oxidized surfaces did not change appreciably with increased coverage, thus exhibiting first-order desorption behavior. The oxide overlayer dramatically affected the 304SS surface properties and hence affected MDA surface interactions. For instance, in comparing TPD stackplots of m/e ) 148 on the two surfaces, the intensity increase in the peak background signal was much slower at higher temperatures of the spectra on the oxide-free surface (Figure 2). Note that, even at a dosing time of 31 s (0.4 ML), some background intensity was detected at temperatures as high as ∼500 K on the oxidized 304SS but was absent on the oxide-free 304SS. Desorption Energy of Chemisorbed MDA. A series of TPD experiments were taken with MDA dosed to the surfaces at a fixed coverage of 0.7 monolayer (ML) at predetermined heating rates. As shown in Figure 4, the Ed of the β state was 47 ( 5 kJ/mol on the oxidized surface and 33 ( 7 kJ/mol on the oxide-free surface.
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3613
Figure 3. TPD spectrum of MDA on an oxide-free 304 stainless steel surface, showing the chemisorbed (R) and physisorbed (β) states. The fitted peaks are also shown. Table 1. TPD Desorption Peak Characteristics for the Physisorbed (r) State and the Chemisorbed (β) State from Both Oxide-Free and Oxidized 304 Stainless Steel oxide-free 304SS
oxidized 304SS
fwhma
fwhma
dosing time, s
R
β
dosing time, s
R
β
63 57 52 47
37.1 53.0 55.5 55.6
82.8 60.2 59.3 64.3
64 58 57 36
37.2 53.3 55.7 56.1
161.2 110.7 133.2 115.1
a
Full width at half-maximum height, in Kelvin.
Figure 2. TPD adsorption uptake curves of MDA on oxidized and oxide-free 304 stainless steel.
Although these Ed values suggest that chemisorbed MDA is more strongly bound to the oxidized surface than the oxide-free 304SS, within the errors of measurement, the difference is not significant. The correlation coefficient (r2) for the least-squares fit of the Redhead equation for the oxide-free surface (0.83) was lower than that for the oxidized surface (0.96). The poorer fit may be due to the differences in interactions between MDA and the oxidized and oxide-free metal surfaces. Oxide formation on Fe/Cr alloys consists of multilayered structures in which mostly iron oxide exists at the outer gas-oxide interface and chromium oxide in the inner region, closer to the bulk substrate.38,39 The calculated Ed values for chemisorbed MDA are considerably weaker compared to other gases. For example, the Ed for O2 bound to polycrystalline Fe is reported40 to be between 297 and 569 kJ/mol. For O2 bound on polycrystalline Cr the Ed is 728 kJ/mol. CO2 bound to polycrystalline Fe is reported to have an Ed of 280 and 481 kJ/mol if it were bound to polycrystalline
Figure 4. Redhead activation energy plots for the chemisorbed MDA from oxide-free and oxidized 304 stainless steel surfaces.
Cr. However, the magnitudes of the Ed values for chemisorbed MDA, which contains imine functionalities, are comparable with desorption energies of chemisorbed NH3 on Cr41 and Fe,42 with Ed of 7.6 and 30 kJ/mol, respectively. Hydrazine adsorbed on Fe was reported43 to have an Ed of 46 kJ/mol, which is close to the calculated Ed values for the β state of MDA on 304SS.
3614 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 Table 2. XPS Peak Centers and Widths from Constituent Elements of MDA Dosed on Oxide-Free 304 Stainless Steel coverage
element
atom %
center (eV)
C/N
C/O
5.0 ML
N O C N O C N O C N O C N O C
7.5 9.8 71.2 6.2 8.4 56.5 5.6 6.0 50.1 4.5 9.1 44.1 2.3 4.4 25.1
399.2 533.2 285.4 399.6 530.0, 533.1 285.5 399.6 530.0, 533.0 285.3 399.6 530.0, 532.9 285.3 400.1 532.3 285.0
9.5
9.2
9.2
6.8
9.0
8.3
9.9
4.9
10.8
5.7
2.5 ML 2.3 ML 1.6 ML 0.77 ML
Dissociation of MDA on the Stainless Steel Surface. The TPD experiments provided some initial evidence that MDA molecules were dissociating on the 304SS surface. As shown in Figure 2, no evidence for the m/e ) 148 mass fragment was detected at low MDA exposures to the oxidized (31 s) or oxide-free (21 s) surfaces. This could be due to thermal decomposition and/or conversion into other species during the TPD experiment. XPS experiments were undertaken to determine if it could provide additional evidence of its decomposition. A multilayer of MDA was adsorbed onto the 304SS surface, and XPS scans were taken. The averaged values for the peak centers and widths of signals from the constituent elements from a set of nine experiments were as follows. The N 1s, O 1s, and C 1s peaks had centers of 399.1 ( 0.1, 532.9 ( 0.1, and 285.2 ( 0.1 eV, respectively. These values were in agreement with XPS binding energies reported44 of 399.0 eV for N 1s and 532.8 eV for O 1s of a compound similar to MDA, with an n-propyl bridge between the two salicylidene groups. No BE was reported for the C 1s signal in their paper. By use of XPS area and elemental sensitivity factors,45 atomic percentages of multilayer MDA elements obtained in this laboratory were 9.1 ( 0.4% N, 8.9 ( 0.1% O, and 80.0 ( 0.5% C, which led to a C/N ratio of 8.8 and a C/O ratio of 9.0. These ratios were in good agreement with the theoretical value of C/N ) C/O ) 8.5 of MDA. In the annealed experiments, heating was used to vary the coverage, while exposure times at low temperature were used to control coverage in the dosed experiments. Therefore, comparison of XPS binding energies between the annealed and dosed experiments at equivalent coverages would reveal thermally induced changes in the chemical state of adsorbed MDA molecules. Atomic mole fractions obtained from the XPS intensities and atomic sensitivity factors were also used to observe any trends in C/N and C/O as a function of coverage to look for evidence of thermal decomposition. MDA Adsorbed on Oxide-Free 304SS. Tables 2 and 3 summarize the XPS data in the dosed and annealed experiments, respectively. Coverages ranged from 0.77 to 5.0 ML in the dosed experiments and from 0.25 to 1.0 ML in the annealed experiments. In the dosed experiments, the trend of the BE shift of the N 1s peak was inconclusive. Peak centers decreased from 400.1 eV at 0.77 ML to 399.2 eV at 5.0 ML (Table 2). In addition, the C/N ratio decreased from 10.8 at 0.77 ML to 9.5 at 5.0 ML, but a gradual change was not observed. There appeared to be an increase in the relative amount
Table 3. XPS Peak Centers and Widths of MDA Annealed on Oxide-Free 304 Stainless Steel temp, K
coverage
element
atom %
center (eV)
C/N
C/O
298
1.0 ML
5.5
9.1
473
0.34 ML
6.4
4.4
523
0.21 ML
398.8 532.8 285.1 399.4 532.3 285.0 399.3 532.6 284.8 399.0 532.1 284.6
8.4
0.54 ML
4.8 5.9 49.9 6.7 4.0 36.7 4.4 6.7 28.0 4.5 4.8 19.6
10.5
373
N O C N O C N O C N O C
4.4
4.1
of nitrogen as more MDA was dosed onto the surface. The O 1s peak had a relatively broad envelope in the dosed oxide-free experiments and was thus fit with two peaks to represent multiple states. One peak center, denoting the metal oxide, was at a BE of 530.0 eV. At higher coverages, the other center increased from 532.3 eV at 0.77 ML to 533.2 eV at 5.0 ML. The C 1s BE increased from 285.0 eV at 0.77 ML to 285.4 eV at 5.0 ML, and the C/O increased from 5.7 at 0.77 ML to 9.2 at 5.0 ML. In the annealed experiments, the BE position of the N 1s signal remained relatively constant as coverage was varied (Table 3). Peak centers for the N 1s peak were at 399.0 eV from 0.25 ML and at 398.8 eV from 1.0 ML. The C/N ratio dramatically changed from 4.4 at 0.25 ML to 10.5 at 1.0 ML. Although the nitrogen intensity increased at higher coverages, there was a disproportionate increase in the relative amount of carbon compared to nitrogen at higher coverages. The O 1s peak showed no intensity for the 530.0 eV state, which may have either desorbed off the surface or converted to the higher BE state when the substrate was heated. Peak centers shifted from 532.1 eV at 0.25 ML to 532.8 eV at 1.0 ML. The C/O increased from 4.1 at 0.25 ML to 8.4 at 1.0 ML. The trend in the C/O ratio was the same as that observed in the dosed experiments. The XPS C 1s BE increased at higher coverages. Peak centers increased from 284.6 eV at 0.25 ML to 285.1 eV at 1.0 ML. Comparison of the dosed and annealed experiments indicates that the relative increase in the amount of nitrogen from low to high coverage was greater when higher MDA was dosed onto the surface as compared to the relatively low coverage in the annealed experiments. The BE shift was not significant due to the fact that the coverage range for the dosed experiments was much greater than for the annealed experiments. The XPS signals for O 1s in the annealed and dosed experiments were similar, showing a slight increase in BE at higher coverage. Thermal effects upon the O 1s peak on oxide-free 304SS were not obvious with the exception of the disappearance of the 530.0 eV state in the dosed experiments. The C 1s XPS peaks in both experiments were very similar, with no discernible differences. MDA Adsorbed on Oxidized 304SS. The XPS measurements of MDA adsorbed on the oxidized 304SS surface are summarized in Tables 4 and 5. MDA coverage ranged from 0.98 to 4.3 ML in the dosed experiments and 0.21 to 1.0 ML in the annealed experiments. In the dosed experiments (Table 4), the N 1s peak position remained constant as coverage increased. The BE position at 0.98 ML was 399.0 eV
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3615 Table 4. XPS Peak Centers and Widths from Constituent Elements of MDA Dosed on Oxidized 304 Stainless Steel coverage
element
atom %
center (eV)
C/N
4.3 ML
N O C N O C N O C N O C N O C
6.8 29.4 59.4 3.2 55.8 33.9 3.0 55.8 31.4 2.4 62.8 23.2 1.0 57.5 4.4
398.9 532.7, 530.0 284.9 398.8 532.4, 530.0 284.5 398.8 532.3, 530.0 284.5 398.7 532.3, 530.0 284.1 399.0 532.3, 530.0 284.1
8.8
2.0
10.7
0.62
10.5
0.58
9.6
0.44
8.5
0.078
2.3 ML 1.8 ML 1.5 ML 0.98 ML
C/O
Table 5. XPS Peak Centers and Widths from Constituent Elements of MDA after Annealing on Oxidized 304 Stainless Steel temp, K coverage element atom % 298
1.0 ML
373
0.53 ML
523
0.47 ML
573
0.43 ML
633
0.21 ML
N O C N O C N O C N O C N O C
3.5 58.1 28.2 2.5 70.5 16.1 2.0 72.4 13.6 2.0 76.6 6.4 1.6 78.7
center (eV)
C/N
C/O
398.8 532.8, 530.0 285.1 399.4 532.8, 530.0 285.0 399.3 532.8, 530.0 284.8 399.0 532.9, 530.0 284.6 399.3 532.9, 530.0 284.8
8.1
0.47
6.3
0.22
6.8
0.18
6.5
0.16
4.3
0.081
and that at 4.3 ML was 398.9 eV, within the precision of the measurement. In the dosed experiments, at the lowest coverages, XPS examination revealed the metal oxide at 530.0 eV and another peak at 532.3 eV. The latter peak could have been from either a change in the O 1s chemical state in the multilayer or a decrease in the BE due to extra-atomic relaxation effects. Since the degree of the BE shift is related to the polarizability of the valence shells of the analyte atoms and the polarizability46 of C > N > O, the degree of these shifts should have followed this trend. However, the BE shifts of the C 1s, N 1s, and O 1s peaks from 0.98 to 4.3 ML shifted by +0.4, -0.1, and +0.8 eV, respectively (Table 4). Thus, the O 1s state at 532.3 eV cannot be fully explained by extraatomic relaxation alone. At higher coverages, however, the BE shifts did follow trends governed by extra-atomic relaxation. From 4.3 ML to multilayer coverage (>5 ML), the C 1s, N 1s, and O 1s peaks shifted by +0.3, +0.2, and +0.2 eV, respectively. It should be noted that when the O 1s peak center at 1.0 ML was fixed at 532.8 eV, the intensity of the BE position of the O 1s signal from the multilayer was 5% of the entire O 1s peak envelope. Thus, there may have been some extraatomic relaxation occurring from 532.7 eV at 4.3 ML to 532.4 eV at 2.3 ML (Table 4). Large XPS core level shifts as a result of extra-atomic relaxation would imply that there was no chemical change taking place. One possible explanation for a third chemical state in the O 1s spectrum is the presence of adsorbed hydroxyl groups on the surface, which may come from dissociation of hydroxyl groups from MDA. A study by
Russell et al.47 showed evidence of OH and CO bond scission from phenol decomposition after interaction with a Ni(110) surface between 250 and 300 K. Decomposition of the hydroxyl groups from MDA may have occurred in a similar manner on the 304SS surface, resulting in the observed 532.3-eV BE position. Adsorbed hydroxyl oxygen on an iron oxide surface, FeO(OH), has been reported23 to have a BE peak center in that general vicinity, at 531.8 eV. In subsequent TPD experiments undertaken in another study in this laboratory,48 no intensity was detected above background intensity at m/e ) 2 and 18, where water from surface hydroxyls and free hydrogen would be found. The C 1s BE increased by 0.8 eV as MDA coverage varied from 0.98 to 4.3 ML (Table 4). The lower BE at low coverage may have been a result of extra-atomic relaxation, implying that no chemical change took place. The degree of the BE shift was in agreement with the relative polarizabilities of the elements in MDA. The shift of the C 1s peak to lower BE was greater than those for the O 1s and the N 1s peaks as coverage varied from 4.3 to 0.98 ML. In the annealed experiments, the N 1s BE peak center was found at 399.2 eV, which is close to the BE of the N 1s signal from multilayer MDA (Table 5). No trend was observed in the O 1s XPS binding energies, with the two peak centers at 530.0 and 532.8 eV being indicative of metal oxide and undissociated MDA, respectively. The C 1s BE increased by 0.3 eV as coverage increased from 0.4 to 1.0 ML, with the BE peak center consistently appearing near the C 1s BE position of the multilayer. Even when the substrate was heated to 633 K (Table 5), the C 1s peak center was at 284.8 eV. One possible cause for the C 1s BE shift could be decomposition of an aromatic ring of the MDA during the annealing process. Benzene adsorbed on transitionmetal surfaces generally has been shown18 to undergo dissociative desorption upon heating (>350 K). However, on stainless steel, our multimass TPD experiments have shown that a benzenoid species from MDA will desorb at ∼650 K with the aromatic ring intact. Since this temperature is higher than that employed in the annealed experiments, it is more likely that aromatic rings adsorbed on 304SS did not decompose in these experiments and the observed C 1s BE shift was a result of extra-atomic relaxation. Comparison of the XPS measurements from the annealed and dosed experiments reveals that the BE for N 1s increased by similar amounts in both procedures at higher coverages, although the BE peak centers in the annealed experiments were slightly higher than those in the dosed experiments. When the C/N trends between the two procedures were compared, there was no apparent difference in the relative amount of nitrogen, with a C/N ratio of 8.5 at 0.98 ML in the dosed experiment (Table 5) and 8.2 in the annealed experiment at 1.0 ML (Table 4). At a MDA coverage of 0.98 ML in the dosed experiment (Table 4), O 1s peak centers were observed at 530.0 and 532.3 eV. If the 532.3-eV BE peak center was from a distinct chemical state, it may have desorbed when the substrate was annealed. In the dosed experiments, there was more intensity observed at the 530eV shoulder than in the annealed experiments. Some of this state could have either desorbed from the surface or been converted to the higher BE state when the substrate was heated, as observed on the oxide-free surface.
3616 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
The C 1s signal shifted to higher binding energies in both experiments at increased coverage, with a larger shift in the dosed than in the annealed experiments, on both the oxidized and oxide-free surfaces. Changes induced in the MDA after heating the substrate to high temperatures may account for the higher C 1s BE values. Interactions with neighboring desorbing species may have also decreased the electron density of adsorbed MDA. To prepare a coverage of 0.25 ML MDA on an oxidefree 304SS surface, the multilayer had to be heated to 532 K, whereas it was necessary to heat the substrate to 633 K when an oxide layer was present to obtain a similar coverage. Thus, the XPS measurements were consistent with the desorption energy calculations, which indicated that the MDA was more strongly bound to the oxidized 304 stainless steel surface. Examination of XPS BE positions indicated no apparent differences in the chemical states of MDA on both heated and unheated surfaces. The C/O ratio was higher in the annealed experiments (Table 3) than in the dosed experiments (Table 2), suggesting that there was less oxygen on the surface due to thermal decomposition. On the oxidized surface, the carbon and oxygen chemical states were thermally affected while the nitrogen remained relatively unchanged. At ∼1.0 ML the N 1s BE was at 398.8 eV on both annealed and dosed, oxidized surfaces (Tables 4 and 5). The O 1s peak not attributed to metal oxide was observed at 532.8 eV (Table 5) in the annealed experiment. In the dosed counterpart, however, it was seen at 532.3 eV. At the same coverage, the C 1s peak was observed at 285.1 eV (Table 5) in the annealed experiment but was at 284.1 eV (Table 4) in the dosed counterpart. Thus, under these conditions, the MDA was undergoing thermal decomposition on the oxidized surface. Conclusions The TPD adsorption uptake of the m/e ) 148 mass was consistent with first-order desorption kinetics for the chemisorbed (β) state. There was some deviation from first-order behavior on the oxidized 304SS surface, but the peak symmetry and peakfitting of its growth confirmed an overall first-order desorption behavior. The MDA was weakly chemically bound to the stainless steel surface, although the desorption energy on the oxide (47 ( 5 kJ/mol) was slightly higher than that on the oxide-free 304SS (33 ( 7 kJ/mol). The impact of the oxide overlayer on the desorption energy of chemisorbed MDA was confirmed by the XPS intensities of the C 1s peak in the annealed experiments. No signal was detected in the TPD by the mass spectrometer for molecular MDA at the lowest exposures to the surface, which suggests that the MDA was decomposing or undergoing some chemical changes. Comparison of XPS data between the annealed and dosed experiments showed evidence for thermal decomposition on both oxidized and oxide-free 304SS surfaces. A detailed multimass TPD investigation would detect the presence of possible non-MDA compounds that may have formed on the 304SS surfaces. When the stainless steel surface was exposed to the MDA, both physisorbed and chemisorbed states were developed in accordance with a Stranski-Krastanov mechanism. The physisorbed material desorbed from the surface at 318 K (45 °C) and the chemisorbed state at 400 K (127 °C). In commercial jet engines,49 surface
temperatures in heat exchangers, pumps, and fuel controls can range from 393 to 433 K (120 to 160 °C). In military jet aircraft,50 although fuel temperatures reaching the combustor are generally limited to 163 °C, the fuel temperature can be as high as 464 K (191 °C) in spray nozzles where the surface temperatures51 can reach 589 K (316 °C). Moreover, fuel in military jet afterburners has been measured to be as high as 700 K (427 °C) during shutdown. Therefore, it is unlikely that metal surfaces in such aircraft engine fuel systems would develop a contiguous layer capable of passivating the surface toward the formation of thermal deposits. In addition, the low desorption energy of the additive on both clean and oxide-bearing stainless steel surfaces suggests that the additive would not remain on the surface under conditions of high flow and high temperature. Acknowledgment The authors thank Dr. Noel Turner, of the Surface Chemistry Section, Code 6177 at the Naval Research Laboratory for his technical consultation. This work was supported by the Office of Naval Research and by Department of Chemistry, George Mason University, Fairfax, VA. Literature Cited (1) Smith, J. D. The Effect of Metals and Alloys on the Thermal Stability of Avtur 50. J. Aero. Eng. 1967, 33 (4), 19-22, 26. (2) Shertzer, R. H. Investigation of the Reduction of Thermal Stability of Fuel by Copper Contamination on Aircraft Carriers; Report NAPTC-PE-14; Naval Air Propulsion Test Center: Trenton, NJ, Jan 1973. (3) Kendall, D. R.; Earls, J. W. Thermal Stability of Aviation Kerosenes. Techniques to Characterize their Oxidation Properties. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 360-366. (4) Morris, R. E.; Turner, N. H. Influences Exerted by Metal Deactivator on the Thermal Stability of Aviation Fuel in the Presence of Copper. Fuel Sci. Technol. Int. 1990, 8(4), 327-350. (5) ASTM. Specification for Aviation Turbine Fuels; Annual Book of ASTM Standards ASTM: Philadelphia, 1987; Part 23, ASTM D1655-f2a. (6) ASTM. Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure); Annual Book of ASTM Standards; ASTM: Philadelphia, 1988; Vol. 5.03, ASTM D3241-85. (7) Morris, R. E.; Hazlett, R. N.; McIlviane, C. L., III. The Effect of Stabilizer Additives on the Thermal Stability of Jet Fuel. Ind. Eng. Chem. 1988, 27 (8), 1524-1528. (8) Clark, R. H. The Role of a Metal Deactivator in Improving the Thermal Stability of Aviation Kerosines. In Proceedings of 3rd International Conference on Stability and Handling of Liquid Fuels; Hiley, R. W., Penfold, R. E., Pedley, J. F., Eds.; The Institute of Petroleum: London, 1989; pp 283-293. (9) Clark, R. H.; Stevenson, P. A. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed-Arms: Results from a Half-Scale Rig. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (4), 1302-1314. (10) Kendall, D. R.; Houlbrouk, G.; Clark, R. H.; Bullock, S. P.; Lewis, C. The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed-Arms. Presented at the 30th International Gas Turbine Congress, Tokyo, Japan, Oct 1987. (11) Clark, R. H.; DeLargy, K. M.; Heins, R. J. The Role of a Metal Deactivator Additive in Improving the Thermal Stability of Aviation Kerosines: Additive Adsorption Studies. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1990, 35 (4), 1223-1232. (12) Moses, C. A. Effect of a Metal Deactivator Fuel Additive on Fuel Deposition in Fuel Atomizers at High Temperature. Southwest Research Institute Report No. 281, Contract DAAK7092-C-0059, Aug 1992. (13) Baker, C.; David, P.; Finley, R.; Hall, D.; Swatridge, R. Characterization and Quantification of Deposits from Thermally
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Received for review February 4, 1998 Revised manuscript received May 19, 1998 Accepted June 8, 1998 IE980076S
