Chemical Reactivity of Triphenyl Phosphorothionate (TPPT) with Iron

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Chemical Reactivity of Triphenyl Phosphorothionate (TPPT) with Iron: An ATR/FT-IR and XPS Investigation† Filippo Mangolini,‡ Antonella Rossi,‡,§ and Nicholas D. Spencer*,‡ Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland, and Dipartimento di Chimica Inorganica ed Analitica, UniVersita` degli Studi di Cagliari, INSTM unit-Cittadella UniVersitaria di Monserrato, I-09100 Cagliari, Italy ReceiVed: August 12, 2010; ReVised Manuscript ReceiVed: September 23, 2010

The surface reactivity of triphenyl phosphorothionate (TPPT) on air-oxidized iron surfaces has been studied by in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR/FT-IR) and ex situ X-ray photoelectron spectroscopy (XPS). The ATR/FT-IR spectra indicate that a reaction took place at 423 K on the iron-coated germanium ATR crystal with the formation of pyrophosphates, organophosphates, and sulfates. The oxidation of the base oil (poly-R-olefin, PAO) during thermal testing, leading to the production of oxygenated compounds that can adsorb onto the iron surface and react with it, resulted in the presence of carbonates and carboxylates in the film. As the heating time increased, the corrosion of the iron coating by the aggressive species formed during the thermooxidative aging of PAO provided the cations necessary for the formation of orthophosphate compounds. The subsequent ex situ XPS analysis, besides substantiating the formation of short-chain polyphosphates, organophosphates, and sulfates on the air-oxidized iron surface, indicates the presence of oxygenated organic compounds in the outermost part of the reaction layer. Modeling the ATR/FT-IR system allowed the correlation of the changes observed in the experimental ATR/FT-IR spectra with the reflectivity alterations caused by the formation of reaction products on the iron-coated germanium ATR crystal. On the basis of these findings, a mechanism for the reaction of TPPT at the oil/iron oxide interface has been proposed. 1. Introduction Lubricants for mechanical systems (e.g., internal combustion engines, gear boxes, gas turbines) are usually based on mineral or synthetic oils. To improve or to impart new and useful properties, additives for lubricating oils have been employed since the 1920s. Among them, chemical compounds that are able to adsorb and/or react with the sliding surfaces to produce easily removable, low-shear-strength tribochemical reaction layers that prevent direct contact between the sliding surfaces, are often employed to reduce wear in the boundary-lubrication regime, where sliding speeds are too low and/or loads too high for a full fluid lubricating film to be maintained.1-6 Zinc dialkyldithiophosphates (ZnDTPs), widely used in many different kinds of industrial lubricants, represent one of the most successful classes of antiwear additives ever synthesized.1,3,6-8 The layers formed on steel surfaces by ZnDTPs under purely thermal conditions and tribological conditions in the boundarylubrication regime were found to primarily consist of patchy glassy phosphate with a layered structure:1,7,8 The first layer above the metallic steel substrate is thought to be made of iron oxide/sulfides and is covered by two poly(thio)phosphate layers. The innermost one consists of short-chain iron/zinc poly(thio)phosphates and the outermost one of long-chain zinc poly(thio)phosphates. Iron-zinc oxides/sulfides are also present in the layers as inclusions. A soft, weakly bound alkyl phosphate film was found to be present on top of the layers. †

Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. Phone: +41-44-6325850. Fax: +41-44-633-1027. E-mail: [email protected]. ‡ ETH Zurich. § Universita` degli Studi di Cagliari.

Despite the remarkable effectiveness of ZnDTPs under a wide range of conditions,1,8 the large amounts of phosphorus, sulfur, and zinc in this class of additives, which are known to lead to blocking filters and catalyst degradation in exhaust aftertreatment systems for gasoline and diesel automobile engines,9 have brought the environmental consequences of using ZnDTPs into focus. Consequently, the permissible levels of sulfated ash, phosphorus, and sulfur (SAPS) in oil formulations have been progressively limited in modern engine-lubricant specifications.3 Among the different chemistries proposed as a replacement or supplement to ZnDTPs, ashless equivalents to ZnDTPs, such as thiophosphates and phosphates without any metal, have been recently investigated quite extensively.10 Najman et al. studied the thermal films and tribofilms formed by phenyl phosphates and triphenyl phosphorothionate (TPPT) on oxidized steel by X-ray absorption near-edge structure (XANES) spectroscopy.11,12 In the case of TPPT, a reaction with oxidized steel was found to occur at 423 K, forming an iron (II) polyphosphate layer containing a small amount of iron sulfate. The films generated under tribological conditions in the boundary-lubrication regime, on the other hand, consisted of short-chain iron (II) polyphosphate, iron sulfide, and sulfates. The amount of sulfates turned out to increase with sliding time. The authors explained these results in terms of a decomposition temperature for TPPT of around 423 K. Following adsorption, the molecule was proposed to undergo a thermooxidative reaction, which started with the breakage of the PdS bond and led to the formation of iron phosphate and sulfate.11 Dithiophosphates were also found to be more reactive and to form thicker films upon sliding than monothiophosphates.11 The reactivity of alkylated phosphorothionates (triphenyl phosphorothionates (TPPTs) substituted with alkyl chains of

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different lengths) with air-oxidized steel (thickness of the oxide layer ) 2.7 ( 0.2 nm) was recently investigated in our group.13,14 The phosphorothionate molecules, which turned out to have an activation temperature for the thermal decomposition of around 423 K, were found to form a thin reaction layer, consisting of short-chain polyphosphates and oxidized sulfur species in the non contact region of the air-oxidized steel after ca. 5 h, as indicated by the XPS results. On the basis of these findings, a reaction mechanism was proposed: after the scission of the PdS bond, the cleavage of the C-O or P-O could occur. The released sulfur was then oxidized. To gain an insight into the mechanism of thermal film and tribofilm formation on metal surfaces, the chemical reactivity of this class of molecules, that is, alkylated triphenyl phosphorothionates (TPPTs), in synthetic oil at high temperature (423 and 473 K) was also investigated in our group by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. The unsubstituted molecule (TPPT) turned out to be highly thermally stable in lubricant oil solution.15 Upon heating at 423 and 473 K for more than 6 h, the thermooxidative degradation of TPPT led to the breakage of the PdS bond with the formation of triphenyl phosphate (TPP). The oxidation of the base oil as a result of the prolonged heating also suggested that the TPPT molecule is not an effective oxidation inhibitor, as opposed to ZnDTPs. The presence of metallic iron and iron oxide particles in the oil solution during the heating experiments was shown to catalyze the reaction of TPPT to yield TPP, but not to affect the reaction pathway.16 XPS results showed that a reaction layer, consisting of carbon, oxygen, phosphorus, and iron, was formed on the 100Cr6 steel filings immersed for 72 h in TPPT solution heated at 473 K. The XPS sputtering depth profile also revealed the presence of sulfur, starting from a sputtering depth of 3.5 ( 0.1 nm (relative to a Si/SiO2 reference sample). Most studies dealing with antiwear additives have focused on the characterization of films formed under purely thermal conditions and tribological conditions in the boundary-lubrication regime in order to understand their mechanism of action. The analytical studies were mostly performed ex situ, that is, outside the tribometer and after the friction experiment.17 Although this approach gives the possibility of combining various complementary modern surface analytical techniques,1 the specimens tend to be characterized in a very different environment from that found within the tribosystem. The inevitable air exposure during the transfer of the sample from the tribometer to the analytical instrument, even when this is carried out as rapidly as possible, as well as the solvent washing needed before performing the analysis, might render the surface unrepresentative of the original surface material. The recognition of these limitations, together with the need to study the kinetics of thermal film and tribofilm formation and how they are influenced by different experimental parameters (e.g., temperature, load, sliding speed), have encouraged tribologists in the past decade to develop in situ techniques, which allow the investigation of these layers as they form, in their “natural state”.18,19 Among the various in situ approaches used for tribological interface studies,17,19 molecular spectroscopies (i.e., infrared and Raman spectroscopy) were found to provide a valuable insight into the chemistry taking place under steady-state conditions.20-29 Such techniques often require one of the rubbing surfaces to be transparent to the wavelength of radiation concerned. We have previously reported the development of an attenuated total reflection (ATR) FT-IR tribotester for the in situ chemical

Mangolini et al. characterization of boundary lubricant layers formed by antiwear and extreme-pressure additives between iron and steel surfaces.23-25,30 ATR spectroscopy allowed the analysis of the oil/ metal interface from “underneath”, through a thin metallic film, which represents one surface of the tribopair. By periodically acquiring ATR/FT-IR spectra, changes in the lubricant chemistry and the growth of reaction films could be investigated as a function of different parameters, such as sliding speed, time, normal load, or temperature. In the case of pure ZnDTP, no thermal or tribochemical reactions with iron were detected at temperatures up to 353 K after 38 or 90 h, respectively.23,25,30 At 423 K, a rearrangement of the molecule was observed both under purely thermal conditions and following tribological tests: in the former case, the formation of P-O-P bonds, characteristic of polyphosphates, was found, whereas in the latter, a phosphate film was formed on iron. The comparison of these results with those obtained using an uncoated germanium ATR crystal allowed the authors to highlight the role of iron in the surface chemistry of ZnDTP: iron seemed not to be involved in the thermal reaction, but to affect the formation of tribofilms. The tribochemical reaction, in fact, was found to occur only on iron surfaces. The same authors also investigated the effect of blending ZnDTP with a synthetic oil, that is, poly-R-olefin (PAO):24,25 the rate of thermal decomposition of the ZnDTP blend turned out to be lower than that of the pure additive. The surface chemistry of tributyl thiophosphate (TBT) on airoxidized iron was investigated by Rossi et al. combining ATR/ FT-IR tribometry with XPS and temperature-programmed reaction spectroscopy (TPRS).29 Both thermal films and tribofilms were found to consist of iron polyphosphate and sulfate. Under tribological conditions in the boundary-lubrication regime, however, the amount of sulfate and polyphosphate in the film was higher. The comparison of these results with those obtained using tributyl phosphate (TBP) allowed the authors to clarify the role of sulfur in this class of compounds: it facilitates the formation of long-chain polyphosphates and lowers the temperature of the chemical and tribochemical reactions by around 50 K. A reaction mechanism between TBT and the airoxidized iron surface was proposed on the basis of these findings: after the breakage of the PdS bond to give tributyl phosphite, butoxy species can be produced through the scission of the P-O bond, in accordance with refs 1 and 31-35. Molecular spectroscopies have also been the analytical techniques of choice for the investigation of lubricant properties within the rubbing interface, that is, in contact. Cann has recently reviewed the development and application of in contact infrared and Raman spectroscopies:20,21 information about the chemical composition36-38 and the conformation of the lubricant film21,39 as well as about the pressure distribution40 could be gained by carrying out the analysis in the contact region. Singer et al. developed an in situ Raman tribometer for investigating the effect of the third bodies formed in the contact by boric acid,41 Pb-Mo-S,42 diamond-like carbon (DLC),43-46 molybdenum disulfide (MoS2),47,48 and nanocomposite materials49 on their friction and wear behavior. Chemical and microstructural transformations occurring in solid lubricants could be followed during tribological tests and correlated with frictional changes.41,42 At the same time, the intensity of the Raman signals could be used to quantify and monitor the interfacial film thickness, allowing the buildup and subsequent removal of the transfer film to be associated with friction instabilities.45,46

Chemical Reactivity of TPPT with Iron In the present work, ATR/FT-IR spectroscopy has been used for the in situ investigation of the reactivity of triphenyl phosphorothionate (TPPT) on air-oxidized iron. Understanding the decomposition pathways leading to the growth of reaction layers on metal surfaces at high temperature may shed light on the mechanism of tribofilm formation. ATR/FT-IR spectroscopy, providing information about the dissolved (i.e., reactants and products in the bulk liquid) and adsorbed (such as adsorbed intermediates) species as well as about the catalyst itself, has been demonstrated to be a powerful tool for probing heterogeneous catalytic solid-liquid interfaces in situ.50-53 In this work, in situ ATR/FT-IR spectroscopy has been combined with ex situ XPS in order to obtain further insight into the chemical changes occurring at the metal/oil interface during heating at 423 K. The transmission FT-IR and NMR analyses of the oil solution at the end the heating tests also provided complementary information about the degradation products of TPPT present in the bulk liquid. A reaction mechanism for TPPT at the oil/iron oxide interface has been proposed. 2. Experimental Section 2.1. Materials. Purified TPPT (Irgalube TPPT, Ciba Specialty Chemicals, Basel, Switzerland) and commercial poly-Rolefin (PAO, Durasyn 166, Tunap Industrie GmbH & Co., Mississauga, Canada) were used as a lubricant additive and base oil, respectively. The purification of TPPT was carried out by liquid chromatography. The characterization of the purified additive by microelemental analysis, Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR) spectroscopy are described in ref 15. For the heating experiments, a 0.044 mol dm-3 (1.82 wt %) solution of TPPT in PAO was prepared. For dissolving the crystalline TPPT in the base oil, the oil solution was sonicated six times for 10 min. The temperature of the lubricant solution was always maintained below 318 K during the ultrasonic bath treatment. Trapezoidal ATR crystals of monocrystalline germanium (Greaseby Specac, Portmann Instrument AG, Biel-Benken, Switzerland) with an angle of incidence of 45° and dimensions of 72 × 10 × 6 mm (number of reflections, calculated according to ref 54, equal to 12) were used for acquiring ATR/FT-IR spectra. In this work, the germanium ATR crystals were coated with iron by means of magnetron sputtering at the Paul Scherrer Institut (Villigen, Switzerland). The planar magnetron sputtering target (ISO 9001 certified, target type PK 75) had a purity higher than 99.9%. The argon pressure during the sputtering process was 0.24 Pa, and the sputtering rate was measured to be 10 Å/min. To remove the iron coating after each experiment, the germanium ATR crystals were cleaned in a 6 M HCl solution (HCl fuming 37%, puriss p.a., Fluka, Buchs, Switzerland) first with a soaked tissue paper and afterward by immersion in the HCl solution for at least 20 min. The crystals were then flushed with Milli-Q water and ethanol (puriss p.a., Fluka, Buchs, Switzerland) prior to being dried under a nitrogen (N5) stream. Before depositing a new iron coating on the germanium ATR crystal, a contamination check was performed by X-ray photoelectron spectroscopy (XPS). 2.2. Methods. 2.2.1. Heating Experiments. The heating experiments were performed using a heatable (up to 473 K) top plate for the benchmark-ATR (Portmann Instruments AG, Biel-Benken, Switzerland) with a temperature controller (FCS23A, Shinko Technos Co., LTD, Japan). The temperature was measured with a PT-100 resistance element.

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1341 TABLE 1: Transmission FT-IR and ATR/FT-IR Experimental Conditions detector beamsplitter spectral range (cm-1) resolution (cm-1) number of scans scan velocity (cm/s) acquisition time (s) gain control

transmission FT-IR

ATR/FT-IR

DTGS KBr 4000-400 2 64 0.6329 136 1

MCT/A KBr 4000-600 4 1024 2.5317 568 1

The thermal tests were carried out by pouring ca. 1.5 cm3 of oil solution on the iron-coated germanium ATR crystal and then heating the assembly to the desired temperature, that is, 423 K. During the experiment, the solution was open to air. The relative humidity (RH) was measured as being between 20 and 40%. ATR/FT-IR spectra were collected at 298-303 K both before and during thermal testing. Cooling the system to 298-303 K before the acquisition of ATR/FT-IR spectra was necessary due to the strong reduction in the transmittance of the germanium ATR element at elevated temperatures. As pointed out by Piras,30 germanium becomes opaque at wavenumbers below 1600 cm-1 at 423 K. This effect is even more pronounced when the crystal is coated with iron: the transmittance of the iron-coated germanium crystal is almost zero over the whole spectral range at 423 K. 2.2.2. Fourier Transform Infrared Spectroscopy (FT-IR). Transmission FT-IR and ATR/FT-IR spectra were acquired with a Nicolet 5700 Fourier Transform Infrared spectrometer (Thermo Electron Corporation, Madison, WI, U.S.A.) equipped with a Greasby-Specac advanced overhead (specaflow) ATR system P/N 1401 series. The experimental conditions are listed in Table 1. As for the transmission FT-IR analysis of the oil solutions used for the heating experiments, sampling was performed by placing one drop of solution onto a KBr pellet. 2.2.2.1. Data Processing. The spectra were processed with OMNIC software (V7.2, Thermo Electron Corporation, Madison, WI, U.S.A.). In the case of the transmission FT-IR spectra, a single-beam spectrum of the KBr pellet was acquired before each measurement as a background. Normalization was performed with respect to the methyl asymmetric deformation band, overlapped by the methylene scissor vibration band of PAO at 1466 cm-1.55 For the ATR/FT-IR spectra, a background correction was always applied to the experimental spectra using the singlebeam spectrum of the iron-coated germanium ATR crystal collected before each experiment. All the ATR/FT-IR spectra presented in this work are reported without any baseline and ATR correction, that is, as-acquired. 2.2.3. Nuclear Magnetic Resonance (NMR) Spectroscopy. The NMR spectra were recorded in CDCl3 (99.8 atom % D, Armar Chemicals, Do¨ttingen, Switzerland) at 300 K using a Bruker Avance 500 NMR spectrometer operating at 500.1 (1H), 125.8 (13C), and 202.5 (31P) MHz. The chemical shifts, given as dimensionless δ values, were referred to TMS (1H and 13C) and H3PO4 (85%) (31P) following the IUPAC recommendation.56 2.2.4. X-ray Photoelectron Spectroscopy (XPS). The surface chemistry of the iron-coated germanium ATR crystal before and after thermal testing in the presence of lubricant oil solutions was investigated by means of X-ray photoelectron spectroscopy (XPS). The purified TPPT, pressed into a pellet, was analyzed with a PHI Quantera SXM (ULVAC-PHI, Chanhassen, MN, U.S.A.) spectrometer, whereas the surface chemistry of the ironcoated germanium ATR crystal before and after the heating

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experiments was characterized using a Theta Probe (Thermo Fisher Scientific, East Grinstead, U.K.). The X-ray source of the PHI Quantera SXM is a focused and scanned monochromatic Al KR with beam diameters between 9 and 200 µm. The emitted electrons are collected and retarded with a Gauze lens system at an emission angle (EA) of 45°. After passing the hemispherical analyzer, the electrons are detected in a 32-channel detector. The system is also equipped with a high-performance, floating-column ion gun and an electron neutralizer for charge compensation. The points to be analyzed were chosen using optical microscopic images. XPS spectra were collected with a beam size of 200 µm with a power of 50 W in the constant-analyzer-energy (CAE) mode with a pass energy of 26 eV and a step size of 0.1 eV (full width at half-maximum (fwhm) of the peak height for Ag 3d5/2 ) 0.55 eV). Survey spectra were acquired with a pass energy of 280 eV and a step size of 1 eV. The Theta Probe is equipped with an Al/Mg KR twin source as well as with a monochromatic Al KR source with a beam diameter varying between 15 and 400 µm. The electrons emitted from the sample surface are collected with a radian lens having an acceptance angle of 60° (ranging from 23° to 83° emission angle) and, after passing the hemispherical analyzer, are detected by a two-dimensional detector with 112 energy channels and 96 angular channels. The average emission angle is 53°. The system is also equipped with an argon ion gun and a combined low-energy electron/ion flood gun for charge compensation. Angle-resolved XPS (ARXPS) analyses were carried out at 16 emission angles using a monochromatic Al KR source with a beam diameter of 400 µm and a power of 100 W in the constantanalyzer-energy (CAE) mode. The pass energy and the step size were, respectively, 100 and 0.1 eV (full width at half-maximum (fwhm) of the peak height for Ag 3d5/2 ) 0.84 eV). Survey spectra were acquired with a pass energy of 300 eV and a step size of 1 eV. The residual pressure in the analysis chamber was always below 5 × 10-7 Pa. The spectrometers were calibrated according to ISO 15472:2001 with an accuracy of (0.1 eV. The highresolution spectra were processed using CasaXPS software (v2.3.15, Casa Software Ltd., Wilmslow, Cheshire, U.K.). An iterated Shirley-Sherwood background subtraction was applied before peak fitting using a linear least-squares algorithm. Minor charging was corrected by referencing to aliphatic carbon at 285.0 eV. The quantitative evaluation of XPS data was performed on the basis of the integrated intensity (i.e., the peak area in cps · eV obtained from the original spectra after background subtraction and curve synthesis) using a first-principles model and applying the equations of Powell.57 The apparent atomic concentration was calculated as

Xj )

Iij /Sij

∑ Iij/Sij

(1)

j

where Iij and Sij are the area and the sensitivity factor of the peak i of the element j, respectively. The sensitivity factors were calculated from the Scofield photoionization cross-section,58 the angular asymmetry factor,59 the spectrometer transmission function, and the inelastic mean free path (IMFP) corrected for the emission angle, assuming the sample to be homogeneous. The inelastic mean free path, that is, the mean distance traveled by electrons with a given kinetic energy (KE) between

inelastic collisions in a material M, was calculated using the equation proposed by Seah and Dench60

Λi,M )

(

)

A + B · √KE cos θ KE2

(nm)

(2)

where the values of A and B were, respectively, 31 and 0.087 in the case of the XPS analysis of purified TPPT and of the reaction layers formed in the presence of lubricant additives and 641 and 0.096 in the case of the XPS analysis of the asreceived iron-coated germanium ATR crystal. According to ref 60, these values are valid for organic and inorganic compounds, respectively. Although the formulas given by Seah and Dench,60 which were used in the present work, were not found to adequately represent the shape of the IMFP versus energy curves and the IMFP dependence on material parameters (such as number of valence electrons, density, atomic/molecular mass, band-gap energy),61-63 they satisfactorily provide a first estimation of the IMFP. 2.2.5. Variable-Angle Spectroscopic Ellipsometry (VASE). The thickness of the iron coating on top of the germanium ATR crystal was measured using a variable-angle spectroscopic ellipsometer (VASE, M-2000 FTM, LOT Oriel GmbH, Germany). Measurements were performed under ambient conditions at three different incident angles (65, 70, and 75°) over a spectral range of 400-700 nm. Before the analysis, the iron-coated germanium ATR crystal was flushed with ethanol (puriss p.a., Fluka, Buchs, Switzerland) and dried under a nitrogen (N5) stream. Spectroscopic scans were taken every 5 mm along the sample for a total of 10 spots. The beam diameter is estimated to be below 1 mm. Data processing was performed using WVASE32 software (J.A. Wollman Co., Inc., Lincoln, NE, U.S.A.) applying a multilayer model, which consisted of a germanium substrate (assumed to be semi-infinite), a metallic iron layer, and an iron oxide (hematite, R-Fe2O3) layer. While the optical constants (refractive index n, extinction coefficient k) of germanium, metallic iron, and iron oxide were kept fixed during data evaluation, the thickness of the metallic iron and iron oxide layers were determined by fitting Ψ(λ) and ∆(λ). Figure S.6 (Supporting Information) reports the optical constants of the three layers as a function of the wavelength. 2.2.6. Profilometry. A Sensofar PLu Neox (Sensofar-Tech, SL., Terrassa, Spain) 3D optical profiler was used for measuring the roughness of the as-received (i.e., not coated) and iron-coated germanium ATR crystal. Data acquisition and data processing were carried out with SensoSCAN software (v.3.1.1.1, SensofarTech, SL., Terrassa, Spain) and SensoMAP software (v.5.1.1., Digital Surf, Besancon, France), respectively. All the measurements were performed using the interferometric method (phase shifting interferometry, PSI) with a 10× objective (vertical resolution < 0.1 nm64). The surface roughness parameters were evaluated following ISO/TS 16610-31 and ASME B46.1: a robust Gauss (RG) filter with a cutoff wavelength of 80 µm was applied. 3. Results 3.1. Characterization of Reference Materials. The characterization of reference materials, that is, purified triphenyl phosphorothionate (TPPT) and the “as received” iron-coated germanium ATR crystal before and after heating at 423 K for 18 h in air, is reported in the Supporting Information.

Chemical Reactivity of TPPT with Iron

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Figure 1. ATR spectra collected during a thermal test at 423 K in the presence of PAO on a Ge ATR crystal coated with iron (10 nm). Left: whole spectral region. Right: fingerprint region.

3.2. Thermal TestssPure PAO. 3.2.1. ATR/FT-IR Spectroscopy. The ATR/FT-IR spectra acquired during thermal tests of PAO on iron-coated germanium ATR crystals at 423 K are reported in Figure 1. The spectrum collected before thermal testing (0 h) showed the characteristic IR absorption bands of PAO at 2953 cm-1 (νasCH3), 2917 cm-1 (νasCH2), 2870 cm-1 (νsCH3), 2849 cm-1 (νsCH2), 1454 cm-1 (δasCH3, δCH2), and 1375 cm-1 (δsCH3).55,65,66 Upon heating at 423 K for 3 h, new bands were detected in the fingerprint region and could be assigned to the characteristic vibrations of carboxylate salts and carbonate complexes (monodentate and bidentate):65,67-71 in the 1600-1500 cm-1 region, where the νasCO2- vibration of carboxylate salts as well as the νasCO2- and νC-O vibrations of monodentate and bidentate carbonate complexes, respectively, are found, three peaks appeared at 1590, 1551, and 1512 cm-1, whereas two bands were detected at 1264 and 1077 cm-1 and assigned to the νsCO2-(bidentate) and νasCO2-(bidentate)/νC-O(monodentate) vibrations, respectively, in carbonate complexes. The spectrum also showed the appearance of two strong absorption bands at 881 and 823 cm-1, which correspond to the bending vibration of the carbonate anion (δC-O). The intensity of the peaks appearing in the fingerprint region was found to increase upon heating at 423 K. Moreover, after 13 h, a new weak peak, which became more intense after 18 h, was detected at 1712 cm-1 and assigned to the CdO stretching vibration (νCdO).55,65,66 A progressive variation of the baseline of the ATR/FT-IR spectra acquired during thermal testing was also observed. 3.2.2. X-ray Photoelectron Spectroscopy (XPS). Ex situ XPS analyses of the iron-coated germanium ATR crystal were performed after in situ ATR/FT-IR thermal testing at 423 K for 18 h. The survey spectrum (Figure S.7a, Supporting Information) showed an intense oxygen peak at 532 eV (O 1s), a carbon peak at 285 eV (C 1s), and weak iron signals at 711 eV (Fe 2p3/2), 94 eV (Fe 3s), and 56 eV (Fe 3p).72 Figures 2a-c show the high-resolution C 1s, O 1s, and Fe 2p3/2 spectra of the iron-coated germanium ATR crystal after heating at 423 K for 18 h in the presence of pure PAO. The XP spectra acquisition was carried out in standard lens mode (Std) and at different emission angles (EAs). Because of the dependence of the sampling depth with the EA, the measurements performed at higher emission angles are more representative of the composition of the outermost layers compared to the data acquired at lower EAs.73 However, because of elastic scattering

of electrons at emission angles higher than 60°, only angles up to 60° should be considered for quantitative analysis.73 The carbon 1s signal was a convolution of four components at 285.0 ( 0.1 eV, originating from the aliphatic contamination,72,74 and at 286.5 ( 0.1, 287.6 ( 0.1, and 289.1 ( 0.1 eV due to carbon bound to oxygen.74 The high-resolution spectrum of the oxygen 1s signal was found to contain contributions at 529.9 ( 0.1, 531.3 ( 0.1, 532.4 ( 0.1, 533.6 ( 0.1, and 534.7 ( 0.2 eV. The component at low binding energy (529.9 ( 0.1 eV) can be assigned to iron oxide,75-78 whereas the contributions at 531.3 ( 0.1, 532.4 ( 0.1, and 533.6 ( 0.1 eV can be assigned to iron oxyhydroxide,72,74-76,78 carbonate/carbonyl oxygen in esters/ketones (-CO-O-/C-CO-C),72,74-80 and bridging oxygen in ester compounds (-CO-O-),74 respectively. The component at the highest binding energy (534.7 ( 0.2 eV) might be due to oxygen-containing compounds adsorbed on the sample surface. The high-resolution spectrum of the iron 2p3/2 signal turned out to contain only two contributions: one at 710.3 ( 0.1 eV, assigned to iron (III) oxide and iron carbonate, and one at 712.1 ( 0.1 eV, assigned to iron oxyhydroxide.76-78,81,82 The peak-fitting parameters for the high-resolution spectra acquired in standard lens mode are summarized in Table S.4 (Supporting Information). Figures 2d-f report the apparent atomic concentrations, calculated assuming a homogeneous compound within the whole analyzed depth, of the iron-coated germanium ATR crystal heated at 423 K for 18 h in the presence of pure PAO as a function of the emission angle (EA). The apparent concentration of aliphatic carbon was found to increase with the EA, indicating the presence of a carbonaceous layer on top of the sample. The oxide apparent concentration (both of iron and of oxygen) exhibited a negative slope, which supports that this species is at the bottom of the analyzed system. The apparent concentration of all the other signals was found to slowly decrease with the EA. 3.2.3. Analysis of the Oil Solution at the End of the Thermal Test. After the heating experiments, the oil solutions were removed from the iron-coated germanium ATR crystal surface and analyzed by transmission Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. 3.2.3.1. Transmission Fourier Transform Infrared Spectroscopy (FT-IR). The transmission FT-IR spectrum of pure PAO heated at 423 K for 18 h on the iron-coated germanium ATR

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Figure 2. High-resolution XP spectra (C 1s (a), O 1s (b), Fe2p3/2 (c)) of the thermal film formed at 423 K on a Ge ATR crystal coated with iron (10 nm) in the presence of PAO. Spectra acquired in standard lens mode (Std) and at different emission angles. The apparent atomic concentration is reported in (d)-(f) as a function of the emission angle. Because of elastic scattering of electrons at emission angles higher than 60°, only angles up to 60° should be considered for quantitative analysis.73

crystal is shown in Figure 3a together with the spectrum of the unheated solution. In the fingerprint region, the FT-IR spectrum of unheated PAO is dominated by the methyl asymmetric deformation band (δasCH3), overlapped with the methylene scissor vibration band (δCH2), at 1466 cm-1 and by the methyl symmetric deformation band (δsCH3) at 1375 cm-1.55,65,66 Smaller bands occurred at 891 and 722 cm-1 due to the methyl and methylene rocking vibration,55,65,66 respectively. Upon heating at 423 K for 18 h on the iron-coated germanium ATR crystal, a new strong peak appeared at 1718 cm-1 and exhibited a shoulder at higher wavenumbers (around 1781 cm-1). This absorption band can be assigned to the CdO stretching vibration.55,65,66 In the 850-1300 cm-1 region, where the C-O stretching vibration is found,55,65,66 several new, but weak, peaks were detected together with an increase of the baseline. 3.2.3.2. Nuclear Magnetic Resonance (NMR) Spectroscopy. The 1H NMR spectrum of pure PAO heated at 423 K for 18 h on the iron-coated germanium ATR crystal is shown in Figure 3b together with the spectrum of the unheated solution. The intense peaks between 2 and 0 ppm correspond to the aliphatic functional groups of the base oil (PAO).83,84 Upon

heating at 423 K for 18 h the on iron-coated germanium ATR crystal, new, but weak, peaks were detected in the 6-2 ppm region, which can be assigned to the hydrogen of a methyl or methylene group bound to a carbonyl or ester group.85 In the 13 C spectrum (Figure S.8, Supporting Information), no peaks that can be assigned to these species were found due to the small magnetic moment and low natural abundance of carbon13.86 3.3. Thermal TestssSolution of TPPT in PAO. 3.3.1. ATR/ FT-IR Spectroscopy. The ATR/FT-IR spectra acquired during thermal tests of a 0.044 mol dm-3 solution of TPPT in PAO on iron-coated germanium ATR crystals at 423 K are reported in Figure 4. The spectrum of the unheated solution (0 h) showed the characteristic peaks of TPPT (1185 cm-1 νC-O-(P), 931 cm-1 νP-O-(C)), and PAO (2953 cm-1 νasCH3, 2917 cm-1 νasCH2, 2870 cm-1 νsCH3, 2849 cm-1 νsCH2, 1454 cm-1 δasCH3/δCH2, 1375 cm-1 δsCH3).15,55,65,66 In the 3150-3000 cm-1 region, where the peaks due to the stretching vibrations of the ring CH bonds are found,55,65 no absorption bands were detected. Moreover, compared with the transmission FT-IR spectrum of a 0.044 mol dm-3 solution of TPPT in PAO,15 the νC-O-(P)



Figure 3. Transmission FT-IR (a) and 1H NMR (b) spectra of PAO before (1) and after (2) heating at 423 K for 18 h in the presence of a Ge ATR crystal coated with iron (10 nm).

Figure 4. ATR spectra collected during a thermal test at 423 K in the presence of a 0.044 mol dm-3 solution of TPPT in PAO on a Ge ATR crystal coated with iron (10 nm). Left: whole spectral region. Right: fingerprint region.

and νP-O-(C) peaks appearing in the ATR/FT-IR spectrum shifted toward lower wavenumbers. Upon heating at 423 K for 3 h, new bands were detected in the fingerprint region. In the 1600-1500 cm-1 region, where the νasCO2- vibration of carboxylate salts as well as the νasCO2and νC-O vibrations of, respectively, monodentate and bidentate carbonate complexes occur,55,67-71 three peaks appeared at 1588, 1551, and 1512 cm-1. An absorption band was also detected at 1263 cm-1 and assigned to the νasCO2-(bidentate) vibration in carbonate complexes.55,67-71 A broad peak appeared at 1087 cm-1 and exhibited a shoulder on the lower-wavenumber side, which increased in intensity with the heating time and became a well-defined peak at 1022 cm-1 after 13 h of heating at 423 K. These two bands at 1087 and 1022 cm-1 can be assigned to the stretching vibration of PO32-/SO42-(νasPO32-/ νasSO42-) and of PO32-/C-O-P (νasPO43-/νasC-O-P) groups, respectively.55,87,88 A further contribution to the peak at 1087 cm-1 might come from the characteristic νsCO2-(bidentate)/ νC-O(monodentate) vibrations of carbonate complexes.67-71 The appearance of an absorption band at 939 cm-1, assigned to the νasP-O-P, suggests the formation of pyrophosphate groups

at the iron/oil solution interface.55,87 The ATR/FT-IR spectra were also characterized by the presence of two strong peaks at 881 and 822 cm-1, which correspond to the bending vibration of the carbonate anion (δC-O).67-71 The intensity of the bands detected in the fingerprint region was found to increase upon heating at 423 K. At the same time, a decrease in absorbance of the characteristic C-O-(P) stretching vibration of TPPT and a progressive change of the baseline of the ATR/FT-IR spectra acquired during thermal testing were observed. 3.3.2. X-ray Photoelectron Spectroscopy (XPS). XPS analysis of the iron-coated germanium ATR crystal was subsequently performed ex situ, after thermal testing for 18 h at 423 K in the presence of a 0.044 mol dm-3 solution of TPPT in PAO. The survey spectrum (Figure S.7b, Supporting Information) showed the presence of the characteristic peaks of carbon (C 1s at 285 eV), oxygen (O 1s at 532 eV), phosphorus (P 2p at 134 eV, P 2s at 192 eV), sulfur (S 2p at 169 eV, S 2s at 233 eV), and iron (Fe 2p3/2 at 713 eV, Fe 3s at 97 eV, and Fe 3p at 57 eV).72

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Figure 5. High-resolution XP spectra (C 1s (a), O 1s (b), Fe2p3/2 (c), P 2p (d), S 2p (e)) of the thermal film formed at 423 K on a Ge ATR crystal coated with iron (10 nm) in the presence of a 0.044 mol dm-3 solution of TPPT in PAO. Spectra acquired in standard lens mode (Std) and at different emission angles.

Figure 5 reports the high-resolution XP spectra (C 1s, O1s, Fe 2p3/2, P 2p, and S 2p) acquired in standard lens mode (Std) and at different emission angles (EAs). The carbon 1s signal was fitted with four components: the most intense one at 285.0 ( 0.1 eV, originating from the aliphatic carbon,72,74 and three high-binding-energy ones at 286.4 ( 0.1, 287.6 ( 0.1, and 289.2 ( 0.1 eV due to carbon bound to oxygen.72,74 The oxygen 1s signal consisted of four peaks at 531.7 ( 0.1, 532.4 ( 0.1, 533.5 ( 0.1, and 534.7 ( 0.2 eV. The component at low binding energy (531.7 ( 0.1 eV) can be assigned to the oxygen in phosphates (PO43-), sulfates (SO42-), and to the terminating oxygen in polyphosphates (the so-called nonbridging oxygen, NBO).82,89-92 The two peaks at 532.4 ( 0.1 and 533.5 ( 0.1 eV can be assigned to carboxnate/carbonyl oxygen in esters/ketones (-CO-O-/C-CO-C)72,74-80 and bridging oxygen in ester compounds (-CO-O-),74 respectively. A contribution to the peak at 533.5 ( 0.1 eV may derive from the oxygen that links phosphate groups in polyphosphate chains (the so-called bridging oxygen, BO)89-93 and from the oxygen in C-O-P groups.29,94 Also, in the case of the iron-coated germanium ATR crystal heated for 18 h at 423 K in the presence of a 0.044 mol dm-3 solution of TPPT in PAO, a component at

534.7 ( 0.2 eV is revealed by the curve-synthesis procedure and might be due to adsorbed oxygen-containing organic species. The phosphorus and sulfur 2p spectra were the convolution of the 2p3/2 and 2p1/2 components due to spin-orbit splitting. Curve synthesis was performed constraining the integrated intensity ratio of these two signals to 0.5 and their energy separation to 0.85 eV in the case of phosphorus and 1.25 eV in the case of sulfur. As for phosphorus, the binding energy of the 2p3/2 component was found to be 133.6 ( 0.1 eV, a typical value for phosphate/pyrophosphate.13,25,90,93 The sulfur 2p spectrum exhibited a high-binding-energy 2p3/2 signal at 168.9 ( 0.1 eV, assigned to sulfates.13,81,82 The high-resolution spectrum of the iron 2p3/2 signal turned out to contain three components: the main one, assigned to iron (II) phosphate,91,95 was found at 712.1 ( 0.1 eV (including its shake-up satellite calculated to be 5.5 eV higher in binding energy and 8% of the main peak intensity), whereas minor components were detected at 709.9 ( 0.1 eV (including its shake-up satellite calculated to be 5.5 eV higher in binding energy and 8% of the main peak intensity) and 714.1 ( 0.1 eV. The former might be induced by the X-ray degradation of iron phosphate96 and might contain a contribution coming from


Figure 6. Apparent atomic concentration of carbon (a), oxygen (b), iron (c), and phosphorus and sulfur (d) for the thermal film formed at 423 K on a Ge ATR crystal coated with iron (10 nm) in the presence of a 0.044 mol dm-3 solution of TPPT in PAO. The [P]/[S] ratio is also reported in (d). Because of elastic scattering of electrons at emission angles higher than 60°, only angles up to 60° should be considered for quantitative analysis.73

iron carbonate, whereas the latter can be assigned to iron (III) phosphate and iron (III) sulfate.82,91,97,98 The peak-fitting parameters for the high-resolution spectra acquired in standard lens mode are summarized in Table S.5 (Supporting Information). Figure 6 shows the apparent atomic concentration, calculated assuming a homogeneous compound within the whole analyzed depth, of the iron-coated germanium ATR crystal heated at 423 K for 18 h in the presence of a 0.044 mol dm-3 solution of TPPT in PAO as a function of the emission angle (EA). The apparent concentration of aliphatic carbon turned out to increase with the emission angle, suggesting the presence of a carbonaceous layer on top of the crystal. Whereas the signals assigned to carbonate and oxygen-containing organic compounds had a constant apparent concentration, all the contributions due to phosphates and sulfates (in the oxygen, phosphorus, sulfur, and iron XP spectra) exhibited a negative slope, which indicates the presence of these species at the bottom of the analyzed surface. To gain an insight into the depth distribution of phosphates and sulfates in the reaction layer formed on the ironcoated germanium ATR crystal, the [P]/[S] ratio was calculated and plotted in Figure 6d. The phosphorus-to-sulfur ratio was found to be constant upon increasing the emission angle up to 60°, but quickly decreased passing from 60° to 80°.


Figure 7. (a) Transmission FT-IR spectra of a 0.044 mol dm-3 solution of TPPT in PAO heated at 423 K for 18 h in the presence of a Ge ATR crystal coated with iron (10 nm). The degradation index (DI) for the characteristic vibrations of TPPT in PAO are compared in (b), whereas the area of the νCdO band appearing in the FT-IR spectra upon heating at 423 K for 18 h is shown in (c).

3.3.3. Analysis of the Oil Solution at the End of the Thermal Test. As was the case for the tests carried out with PAO, the oil solutions (PAO + TPPT 0.044 mol dm-3) were removed from the iron-coated germanium ATR crystal after the heating experiments and analyzed by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. 3.3.3.1. Transmission Fourier Transform Infrared Spectroscopy (FT-IR). The transmission FT-IR spectrum of a 0.044 mol dm-3 solution of TPPT in PAO heated at 423 K for 18 h on the iron-coated germanium ATR crystal is shown in Figure 7a together with the spectrum of the unheated solution. In the fingerprint region, the FT-IR spectrum of the unheated solution showed the characteristic IR peaks of the TPPT (1190 cm-1 νC-O-(P), 940 cm-1 νP-O-(C), 803 cm-1 PdS(I)) and PAO (1466 cm-1 δasCH3/δCH2, 1375 cm-1 δsCH3, 891 cm-1 FCH3, 722 cm-1 FCH2) molecules.15,55,65,66 Upon heating at 423 K for 18 h on an iron-coated germanium ATR crystal, the characteristic νP-O-(C) and PdS(I) vibrations of TPPT were found to slightly decrease in intensity. A shoulder on the high-wavenumber side of the P-O-(C) stretching vibration band was also observed. In the 1550-1800 cm-1 region, where the CdO stretching vibration is found,55,65,66 a new peak with a maximum at 1721 cm-1 was detected and showed a shoulder at higher wavenumbers. Correspondingly,

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Figure 8. NMR spectra (1H (a), 31P (b)) of a 0.044 mol dm-3 solution of TPPT in PAO before (1) and after (2) heating at 423 K for 18 h in the presence of a Ge ATR crystal coated with iron (10 nm).

in the 850-1300 cm-1 region, where the C-O stretching vibration occurs,55,65,66 a slight increase of the baseline was observed. To gain an insight into the degradation of the TPPT molecule in oil solution, the degradation index (DI) has been defined as

DI )

At,i · 100 A0,i

(3)

where At,i is the absorbance of the ith vibration after t heating hours, whereas A0,i is the absorbance of the ith vibration for the unheated solution.15 Figure 7b reports the degradation index of three characteristic vibrations of TPPT (νPH1593 cm-1, νP-O-(C), and PdS(I)). Upon heating 0.044 mol dm-3 solutions of TPPT in PAO at 423 K for 18 h on the iron-coated germanium ATR crystal, the DI values decreased in the following order: νPH1593 cm-1 > νP-O-(C) > PdS(I). As for the characteristic νC-O-(P) vibration of the TPPT molecule, its DI (not shown) turned out to be comparable with that of the νPH1593 cm-1 peak. However, as was pointed out in our previous studies,15,16 because of the superposition of the C-O stretching vibration band of the ester compounds (at ca. 1175 cm-1) produced by the oxidation of the base oil, the DI values of the νC-O-(P) band of TPPT are not representative of the decrease in absorbance of the latter. To investigate the influence of TPPT on the oxidation of the base oil, the area of the CdO stretching vibration, after linear baseline subtraction, was calculated and plotted in Figure 7c. Compared with pure PAO, the presence of TPPT in the solution turned out to strongly reduce the oxidation of the base oil at 423 K. 3.3.3.2. Nuclear Magnetic Resonance (NMR) Spectroscopy. The NMR spectra (1H and 31P) of a 0.044 mol dm-3 solution of TPPT in PAO heated at 423 K for 18 h on the iron-coated germanium ATR crystal is shown in Figure 8 together with the spectra of the unheated solution. In the case of the unheated solution (0 h), the characteristic signals of the TPPT molecule were detected:15,83,86,99-101 in the 1 H spectrum, two multiplets, assigned to monosubstituted benzene, were found at 7.40 and 7.28 ppm, whereas in the 31P spectrum, a singlet was observed at 54.4 ppm.

4. Discussion 4.1. Analysis of the Oil Solution After Thermal Testing. The analysis of the oil solutions by transmission FT-IR and NMR spectroscopy at the end of the heating experiments provided valuable insights into the reactions taking place in the lubricants heated at high temperature in the presence of the airoxidized iron surface as well as into the chemical species produced at the metal/oil interface and dissolved in the oil itself. Hydrocarbons are known to be susceptible to thermooxidative degradation. The mechanism of hydrocarbon oxidation at high temperature has been thoroughly investigated:102-104 it is driven by an autocatalytic process involving a four-step free-radical reaction (initiation of the radical chain reaction, chain propagation, chain branching, and termination of the radical chain reaction), which leads to the formation of a complex mixture of oxygenated compounds, for example, hydroperoxides, alkyl peroxides, alcohols, carboxylic acids, peroxy acids, esters, ketones, aldehydes, and lactones. In addition, transition-metal ions having two valence states, such as Fe2+/3+, reduce the activation energy for the decomposition of the alkyl hydroperoxide molecules formed during the autoxidation of hydrocarbons at high temperatures.102-104 As already observed in our previous studies concerning the thermal degradation of TPPT in synthetic oil at high temperatures,15,16 in all the experiments carried out in the present work, the transmission FT-IR spectra, showing the appearance of a broad band in the 1850-1650 cm-1 region together with an increase in the baseline in the 1300-850 cm-1 region, and the 1H NMR spectra, where new peaks between 6 and 2 ppm were detected, clearly indicate that the base oil (PAO) was oxidized upon heating at 423 K for 18 h on the iron-coated germanium ATR crystal. Even if the broad absorption band appearing in the 1850-1650 cm-1 region of the FT-IR spectra is usually found to be a complex envelope of several overlapping peaks corresponding to different carbonyl compounds,55,65,66,84,105,106 the frequency of the peak maximum and of the shoulder characterizing this band could be useful for identifying the principal species produced during the thermooxidative aging of base oils. In the case of pure PAO and of the solution of TPPT in PAO heated at 423 K for 18 h on the iron-coated germanium ATR crystal, the peak maxima of the νCdO band were detected at 1718 and 1721 cm-1, suggesting the formation of ester compounds, ketones, and carboxylic acids during the heating experiments.55,65,66,84,105,106 The presence of a shoulder on the high-wavenumber side of the νCdO band (in the case of pure PAO, a well-defined band was even detected at 1781 cm-1) might correspond to the simultaneous production of five-membered-ring lactones and to small amounts of peroxy esters.55,65,66,84

Chemical Reactivity of TPPT with Iron To gain insight into the impact of TPPT on the oxidation of the base oil, the area of the carbonyl peak appearing in the FTIR spectra was calculated (after linear baseline subtraction). Compared with the experiments performed with pure PAO, blending the base oil with TPPT resulted in a significant reduction of the area of the carbonyl peak at 423 K, which indicates that TPPT has a strong antioxidant effect under the experimental conditions employed in the present work. This finding is in agreement with the results outlined in our previous study of the thermal degradation of TPPT in oil solution in the presence of metallic and oxidized steel/iron particles.15,16 The NMR and FT-IR spectra of the solution of TPPT in PAO also provided further information about the degradation of the additive during the heating experiments. The 31P NMR spectrum showed the appearance of a new peak at -16.4 ppm, which corresponds well to the characteristic 31P NMR signal of triphenyl phosphate (TPP). The exchange of the sulfur atom in the phosphorothionate molecule with an electron-withdrawing oxygen to give TPP upon heating at 423 K, which was already reported by Teichmann and Hilgetag107,108 as well as in our previous studies of TPPT solutions heated at high temperatures,15,16 explains the detection of a shoulder on the high-wavenumber side of the P-O-(C) stretching vibration band. The scission of the PdS bond upon heating at 423 K on the iron-coated germanium ATR crystal is also suggested by the degradation index (DI) values of the corresponding absorption band in the transmission FT-IR spectra (Figure 7b), which were found to be always lower than the values of the Ph and P-O-(C) stretching vibration peaks. As already pointed out in refs 15 and 16, no definitive statement about the reaction step following the PdS bond breakage, that is, the cleavage of the P-O bond to form aryloxy groups or the scission of the C-O bond to give aryl group, can be done on the basis of the transmission FT-IR spectra of the lubricant solution because the superposition of the C-O stretching vibration band of the ester compounds (at ca. 1175 cm-1), produced during the oxidation of the base oil, with the νC-O-(P) band results in DI values that are not representative of the decrease in absorbance of the latter. These findings indicate that the mechanism of the degradation reaction of the TPPT molecule in PAO heated on the iron-coated germanium ATR crystal does not differ from that found in our previous studies, where the TPPT solutions were heated at 423 K with and without metallic and oxidized steel/iron particles.15,16 4.2. Composition of the Thermal Films. The ATR/FT-IR spectra collected during thermal testing at 423 K in the presence of pure PAO indicated the occurrence of a surface reaction on the air-oxidized iron-coated germanium ATR crystal to give carbonate complexes and carboxylate salts. The ex situ XPS analysis of the specimen at the end of the heating experiments substantiates the formation of these species. The production of iron complexes upon heating hydrocarbons at elevated temperatures in the presence of an iron surface has already been reported in the literature.109-116 Cadvar and Ludema investigated the lubrication of steel by additive-free mineral oils and found by in situ ellipsometry the formation of a 5-10 nm thick layer consisting of organo-iron compounds on top of the oxide/carbide substrate.109-111 Hsu, on the other hand, analyzed oil samples used to lubricate sliding contacts by gel permeation chromatography and observed the presence of organo-iron species with molecular weights up to 100 000, which made the author hypothesize that the metal-catalyzed oil oxidation leads to the formation of polymers through condensation reactions on the metal surface.112,113

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1349 The ATR/FT-IR and XPS results outlined in the present study and obtained using pure PAO could then be explained considering the chemical species produced during the thermooxidative aging of the base oil. As already pointed out in the previous paragraph, the oxidation of hydrocarbons, which is catalyzed by transition-metal ions having two valence states (e.g., Fe2+/3+), results in the formation of oxygenated compounds, such as carboxylic acids. These polar species can adsorb onto the metal surface and react with it, forming metal complexes,112,113 as observed in the present work. In the case of the experiments performed with a 0.044 mol dm-3 solution of TPPT in PAO, the ATR/FT-IR spectra suggested that, upon heating at 423 K for 3 h, a reaction occurs on the air-oxidized iron surface that leads to the formation of pyrophosphates, organophosphates, and sulfates. A competitive reaction, that is, the oxidation of the base oil catalyzed by iron, was also found to take place and to result in the production of carbonate complexes and carboxylates. An increase in the absorbance of the peak detected at 1022 cm-1, assigned to the characteristic vibration of PO43-/C-O-P groups, with heating time was also observed. The corrosion of the iron coating caused by the aggressive species formed during the oxidation of the base oil (e.g., carboxylic acids) may provide the cations necessary for the formation of orthophosphate compounds. The ARXPS results, which corroborate the formation of shortchain iron polyphosphate, organophosphates, and iron sulfates, together with carbonates and carboxylates, also yielded an insight into the depth distribution of the species formed on the air-oxidized iron surface at high temperature: phosphates and sulfates seemed to be present in the innermost regions and to be covered by an oxygenated organic layer. The formation of short-chain iron phosphate and iron sulfate detected in the present study is in agreement with previous studies on TPPT.11,13,14,117 4.3. Modeling of the ATR/FT-IR System and Reflectivity Calculations. Reflectivity calculations were performed in order to determine the optical response of the overall ATR/FT-IR system to the formation, at the metal/oil interface, of a reaction layer, which is chemically and optically different from all the other layers. In the case of N parallel, optically isotropic layers, the reflectivity of plane-polarized light can be calculated starting from Maxwell’s equations and considering the continuity of the electric and magnetic fields across the boundaries.118 Abele`s and Hansen developed a matrix formalism, which allows the calculation of the overall reflectance of any combination of absorbing and nonabsorbing isotropic layers as well as of the mean-square electric field (MSEF, 〈E2〉) at any position within a certain layer of a N-isotropic-phase system.118-120 Unknown optical behavior could be clarified by numerically calculating the dependence of the reflectance on various parameters, such as angle of incidence, layer thickness, and optical constants.52,53,119 In the present work, two different models were considered (see schemes in the top part of Figure 9): In the first one, the growth of a layer consisting of a mixture of iron carbonate and oxyhydroxide on the iron-coated germanium ATR crystal was assumed. The thickness of such a film was calculated accounting for the conversion of metallic iron in the iron coating to iron carbonate/oxyhydroxide and taking into consideration the different densities of these materials (FFe ) 7.87 g cm-3, FFeCO3/FeOOH ) (FFeCO3 + FFeOOH)/2 ) 3.88 g cm-3).121 In the

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Figure 9. Calculated ATR absorbance for parallel, perpendicular, and isotropically polarized light in the case of (a) the growth of a reaction layer consisting of iron carbonate and oxyhydroxide on the iron-coated Ge ATR crystal (taking into account the different densities of iron and iron carbonate/oxyhydroxide) and (b) the growth of a reaction layer consisting of iron phosphate and iron sulfate on the iron-coated Ge ATR crystal.

second model, the growth of a reaction layer consisting of a mixture of iron phosphate and iron sulfate was simulated. In both models, the first (germanium ATR crystal) and last (PAO) layers were assumed to be semi-infinite, transparent (i.e., k ) 0), and with a refractive index of 4.0 and 1.46,121,122 respectively. Because of the strong variation of the optical properties of metals in the mid-infrared region, the dependence of the refractive index and extinction coefficient of iron on the wavelength had to be considered.123 As for the reaction layers, whose growth has been simulated in the present work, the average value of the refractive index of the two compounds121 constituting the phase was calculated. Figure 9 reports the employed optical properties and thickness values. Calculations were performed for parallel and perpendicular polarized light, and the results are given in absorbance (A ) -log(R/R0), where R and R0 are the reflectivity of the sample and of the reference, respectively. The reflectivity of the reference was obtained considering the presence of no reaction layer on top of the iron-coated Ge ATR crystal). Starting from the reflectivity of parallel (R//) and perpendicular (R⊥) polarized light, the reflectivity of isotropically polarized light was estimated as R ) mR// + (1 - m)R⊥, where m, which defines the polarization state of the spectrometer and should be, in principle, frequency-dependent, was assumed to be equal to 0.5 over the whole spectral range. Bu¨rgi et al. showed that the ATR signals, calculated assuming a fixed m value (0.5), were similar to those obtained considering a frequency-dependent m.53 Figure 9a shows the changes in the ATR/FT-IR spectra induced by the formation of a film consisting of iron carbonate and oxyhydroxide on the iron-coated germanium ATR crystal. It has to be emphasized that, in this model, the growth of such

a film implied a reduction of the thickness of the metallic iron. The calculated ATR signals significantly changed upon increasing the thickness of the film. In particular, the absorbance turned out to progressively decrease in the high-wavenumber spectral region and to increase at low wavenumbers as the thickness of the iron carbonate/oxyhydroxide layer increased. The response of the overall ATR system to the formation of a mixed iron phosphate/iron sulfate reaction layer on the ironcoated germanium ATR crystal is reported in Figure 9b. The reflectivity calculations predict an opposite effect for paralleland perpendicular polarized light upon increasing the thickness of the reaction film: while in the case of parallel-polarized light a progressive increase of the absorbance in particular at low wavenumbers, was observed, in the case of perpendicularpolarized light a decrease of the absorbance was found, especially at high wavenumbers. The resulting shape of the ATR signal for isotropically polarized light significantly varied with the thickness of the reaction layer. 4.4. Correlation of the Reflectivity Alterations in the ATR System with the Chemical Changes Occurring at the Lubricant/Iron Interface. Bu¨rgi and Baiker demonstrated that ATR/FT-IR spectroscopy is a powerful tool for probing heterogeneous catalytic solid-liquid interfaces in situ.50-53 Besides providing information about the dissolved (i.e., reactants and products in the bulk liquid) and adsorbed (such as adsorbed intermediates) species, variations of the state of metal films and supported metal particles on top of the ATR crystal as a consequence of an increase/decrease of the concentration of free electrons or of the changes in electronic transitions, could be simultaneously followed during the catalytic process.



SCHEME 1: Reaction Mechanism of TPPT in the Presence of PAO on an Air-Oxidized Iron Surface at 423 K

In the present work, ATR/FT-IR spectroscopy gave a valuable insight into the chemical changes occurring at the metal/oil upon heating at 423 K. During thermal testing (either in the presence of pure PAO or of a solution of TPPT in PAO), a progressive variation of the baseline of the ATR/FT-IR spectra was also observed (see Figure S.10 in the Supporting Information, where the ATR/FT-IR spectra are reported without any offset). The qualitative comparison with the reflectivity calculations reported in section 4.3 suggests that these baseline changes arise from the formation of a thin film at the metal/oil interface, which is chemically and optically different from the other layers. Any quantification of the layer thickness on the basis of the reflectivity calculations has been avoided due to the approximate optical constant values considered for the simulations as well as to the discrepancy between the models and the real systems. Although, in the case of the calculations, a stratified system having well-defined boundaries was always assumed, the reaction layers formed on the iron-coated Ge ATR crystal are likely to exhibit gradients in composition. 4.5. Proposed Reaction Mechanism. On the basis of the findings mentioned above, the reaction mechanism depicted in Scheme 1 is suggested. As reported in our previous study,15 upon heating at 423 K in synthetic oil, the TPPT molecule undergoes a nucleophilic attack at the phosphorus atom to cause PdS bond scission to give triphenyl phosphite. Volatile compounds might be also formed during this reaction step. A thermooxidative reaction then occurs in the oil, which leads to the oxidation of triphenyl phosphite to triphenyl phosphate. In the presence of an iron/iron oxide surface, TPPT can adsorb via the sulfur atom, in agreement with Heuberger et al.13 and Heuberger.14 As pointed out by Koyama et al.,124 a polar interaction between the OP-O-Ph atom and the nascent iron surface cannot be excluded. The sulfur-metal coordination induces a partial positive charge on the phosphorus atom, which makes the phosphorus atom itself more open to nucleophilic attack.108 The phosphoryl oxygen of TPP, produced by the thermooxidative reaction of TPPT in lubricant oil solution,15 can carry out a nucleophilic attack on the adsorbed phosphorothionate molecule,16 leading to the formation of pyrophosphates. At the same time, organophosphates might also be produced. The scission of the thiophosphoryl bond as a starting point for the decomposition of TPPT in synthetic oil not only might induce the production of volatile compounds but also

might result in a reaction of the released sulfur and the iron surface, in accordance with refs 13, 14, and 16. This, together with the breakage of the PdS bond of the TPPT molecules adsorbed on the iron surface, would lead to the formation of sulfides, which are then oxidized to sulfates. The oxidation of the base oil (PAO), which proceeds via a free-radical chain mechanism,3,4 induces the formation of polar species, which can adsorb onto the metal surface and react with it, forming metal complexes, as demonstrated by the experiments performed with additive-free PAO. The corrosion of the iron coating caused by the aggressive species (e.g., carboxylic acids) formed during the thermooxidative aging of the base oil also provides the cations necessary for the formation of orthophosphate compounds. 5. Conclusions ATR/FT-IR spectroscopy has been used in the present work for the in situ investigation of the surface reactivity of triphenyl phosphorothionate (TPPT), an ashless antiwear additive, on an air-oxidized iron surface. The ATR results support a rearrangement of the molecule upon heating at 423 K on an air-oxidized iron surface with the formation of pyrophosphates, organophosphates, and sulfates. At the same time, the oxidation of the base oil (poly-R-olefin, PAO) during thermal testing induces the production of oxygenated compounds, which can adsorb onto the iron surface and react with it to give carbonates and carboxylates. The corrosion of the iron coating by the aggressive species formed during the thermooxidative aging of PAO provides the cations necessary for the formation of orthophosphate compounds. The ex situ angle-resolved XPS analysis corroborates the formation of short-chain polyphosphates, organophosphates, and sulfates on the air-oxidized iron surface. In the outermost part of the reaction layer, oxygenated organic compounds were found to be present. Modeling the ATR/FT-IR system allows the correlation of the changes observed in the experimental ATR/FT-IR spectra with the reflectivity alterations caused by the formation of reaction products on the iron-coated germanium ATR crystal. Acknowledgment. The authors wish to express their gratitude to the ETH Research Commission for its support of this work. Dr. H. Camenzind (Ciba Speciality Chemicals, Basel, Switzerland) is

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thanked for supplying the pure additive. Mrs. D. Sutter kindly performed the NMR analysis. Mr. M. Horisberger kindly prepared the iron coatings by magnetron sputtering at PSI (Villigen, Switzerland). Dr. Nagaiyanallur V. Venkataraman is thanked for the helpful discussions about ATR/FT-IR spectroscopy. Supporting Information Available: The XP survey and detailed spectra together with the curve-fitting parameters are provided for purified TPPT and for an iron-coated germanium ATR crystal before and after thermal testing at 423 K for 18 h in the presence of pure PAO and of a 0.044 mol dm-3 solution of TPPT in PAO. The optical constants used for fitting the ellipsometry data, the 13C spectra of pure PAO and of a 0.044 mol dm-3 solution of TPPT in PAO before and after heating at 423 K for 18 h on a germanium ATR crystal coated with iron (10 nm), and the ATR/FT-IR spectra (without any offset) acquired during thermal testing at 423 K on an iron-coated germanium ATR crystal in the presence of pure PAO and of a 0.044 mol dm-3 solution of TPPT in PAO are also presented. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gellman, A. J.; Spencer, N. D. Surface chemistry in tribology. J. Eng. Tribol. 2002, 216, 443–461. (2) Hutchings, I. M. Tribology, Friction and Wear of Engineering Materials; Butterworth-Heinemann: Oxford, U.K., 1992; p 273. (3) Mang, T., Dresel, W., Eds. Lubricants and Lubrication, 2nd ed.; John Wiley & Sons: New York, 2007; p 850. (4) Mortier, R. M., Orszulik, S. T., Eds. Chemistry and Technology of Lubricants, 2nd ed.; Blakie Academic & Professional: London, 1997; p 378. (5) Pawlak, Z. In Tribochemistry of Lubricating Oils. Tribology and Interface Engineering; Briscoe, B. J., Ed.; Elsevier: Amsterdam, 2003; p 368. (6) Rudnick, L. R., Ed. Lubricant AdditiVes: Chemistry and Applications; Marcel Dekker, Inc.: New York, 2003. (7) Nicholls, M. A.; Do, T.; Norton, P. R.; Kasrai, M.; Bancroft, G. M. Review of the lubrication of metallic surfaces by zinc dialkyl-dithiophosphates. Tribol. Int. 2005, 38, 15–39. (8) Spikes, H. A. The history and mechanisms of ZDDP. Tribol. Lett. 2004, V17, 469–489. (9) Kubsh, J. Three-Way Catalyst Deactivation Associated with OilDerived Poisons. In Materials Aspects in AutomotiVe Catalytic ConVerters; Bode, H., Ed.; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2003; pp 215-222. (10) Spikes, H. A. Low- and zero-sulphated ash, phosphorus and sulphur anti-wear additives for engine oils. Lubr. Sci. 2008, 20, 103–136. (11) Najman, M. N.; Kasrai, M.; Bancroft, G. M. Chemistry of antiwear films from ashless thiophosphate oil additives. Tribol. Lett. 2004, 17, 217– 229. (12) Najman, M. N.; Kasrai, M.; Bancroft, G. M.; Miller, A. Study of the chemistry of films generated from phosphate ester additives on 52100 steel using X-ray absorption spectroscopy. Tribol. Lett. 2002, 13, 209– 218. (13) Heuberger, R.; Rossi, A.; Spencer, N. D. Reactivity of alkylated phosphorothionates with steel: A tribological and surface-analytical study. Lubr. Sci. 2008, 20, 79–102. (14) Heuberger, R. Combinatorial Study of the Tribochemistry of AntiWear Lubricant Additives. Ph.D. Thesis, no. 17207, ETH Zurich, Zurich, Switzerland, 2007. (15) Mangolini, F.; Rossi, A.; Spencer, N. D. Reactivity of triphenyl phosphorothionate in lubricant oil solution. Tribol. Lett. 2009, 35, 31–43. (16) Mangolini, F.; Rossi, A.; Spencer, N. D. Influence of metallic and oxidized iron/steel on the reactivity of triphenyl phosphorothionate in oil solution. Tribol. Int. 2010, in press. (17) Donnet, C. Problem-Solving Methods in Tribology with SurfaceSpecific Techniques. In Handbook of Surface and Interface Analysis: Methods and Problem-SolVing; Rivie`re, J. C., Myhra, S., Eds.; Marcel Dekker Inc.: New York, 1998; p 968. (18) Spikes, H. A. In situ methods for tribology research. Tribol. Lett. 2003, 14, 1. (19) Sawyer, W. G.; Wahl, K. J. Accessing inaccessible interfaces: In situ approaches to materials tribology. MRS Bull. 2008, 33, 1145–1150. (20) Cann, P. M. In-contact molecular spectroscopy of liquid lubricant films. MRS Bull. 2008, 33, 1151–1158.

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