Infrared Spectroscopic Investigation of the Surface Reaction of

May 28, 2003 - Department of Chemistry, Bryn Mawr College, 101 North Merion Avenue,. Bryn Mawr, Pennsylvania 19010. Received November 5, 2002...
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Infrared Spectroscopic Investigation of the Surface Reaction of Phosphorus Trifluoride on γ-Alumina Xianlong Wang and Edward A. Wovchko* Department of Chemistry, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010 Received November 5, 2002. In Final Form: April 2, 2003 The adsorption and subsequent reaction of phosphorus trifluoride (PF3) on a γ-alumina (γ-Al2O3) surface was studied using high vacuum techniques and transmission infrared spectroscopy (FT-IR) over a temperature range of 150-800 K. Phosphorus trifluoride molecules adsorb on the alumina surface between 150 and 230 K, via an interaction with surface isolated hydroxyl groups, to form OH-FPF2 and/or (OH)-PF3 intermediate species. The O-H bond of the proposed intermediate is thermally activated, and at sample temperatures >230 K, the phosphorus center inserts into the O-H bond of the hydroxyl group, generating surface hypophosphoryl fluoride species of the form FHPO. Reaction of PF3 molecules with associated hydroxyl species occurs from extended thermal treatment at 310 K to yield additional hypophosphoryl fluoride species of the form FHPO2. Infrared band assignments are presented. The new species are stable on the aluminum oxide surface to a thermal treatment temperature of 800 K under evacuation. The hypophosphoryl fluoride species resist extensive oxidation when heated in O2 gas at 700 K.

1. Introduction High surface area aluminum oxide materials are commonly used as catalysts and catalyst supports for a variety of chemical processes, such as petroleum reforming, ethylene oxide synthesis, hydrodesulfurization, automotive exhaust and pollution control, and alkane hydrogenolysis.1-15 The surface hydroxyl groups, Al-OH, offer basic sites and play a key role during the selective adsorption of molecules and the dispersion of catalyst metals.16,17 Additionally, they may participate as a reagent in various surface hydrolysis reactions.18 Also common in the design of catalysts are organometallic compounds containing phosphine and phosphoryl halide ligands, such as PPh3, P(CH3)3, PH3, PCl3, and PF3.19-28 A number of important organic synthetic processes, such as hydroge* To whom correspondence may be addressed. (1) Neinska, Ya.; Penchev, V.; Kanazirev, V. Kinet. Katal. 1973, 14, 774. (2) Mukherjee, D. K.; Misra, J.; Chowdhury, R. L.; Sen, S. P. Proc. Indian Natl. Sci. Acad., Part A 1975, 41, 74. (3) Katsuno, T.; Matsuda, S.; Yoshinaga, M.; Saito, K. Jpn. Kokai Tokkyo Koho JP 2001279257, A2, 2001. (4) Isoda, T.; Kusakabe, K.; Morooka, S. Eco Industry 1998, 3, 18. (5) Matsumoto, H. Petrotech 1985, 8, 574. (6) Kim, J. T. Hwahak Kwa Kongop Ui Chinbo 1972, 12, 206. (7) Beecroft, T.; Miller, A. W. Rep. Prog. Appl. Chem. 1971, 55, 385. (8) Cai, S.; Huang, Y. Gongneng Cailiao 1998, 29, 225. (9) Skotak, M.; Lomot, D.; Karpinski, Z. Appl. Catal., A: General 2002, 229, 103. (10) Teschner, D.; Duprez, D.; Paal, Z. J. Mol. Catal. A: Chemical 2002, 179, 201. (11) Teschner, D.; Paal, Z. React. Kinet. Catal. Lett. 1999, 68, 25. (12) Bond, G. C.; Hooper, A. D Appl. Catal., A: General 2000, 191, 69. (13) Dixit, L. Asian J. Phys. 2000, 9, 468. (14) Boitiaux, J. P.; Deves, J. M.; Didillon, B.; Marcilly, C. R. Chem. Indu. 1995, 61, 79. (15) Davis, B. H.; Antos, G. J. Chem. Ind. 1995, 61, 113. (16) Ballinger, T. H.; Yates, Y. T., Jr. Langmuir 1991, 7, 3041. (17) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev. Sci. Eng. 1978, 17, 31. (18) Kuznetsova, A.; Wovchko, E. A.; Yates, J. T., Jr. Langmuir 1997, 13, 5322. (19) Herber, U.; Guerrero Sanchez, R.; Gevert, O.; Laubender, M.; Werner, H. New J. Chem. 2001, 25, 396. (20) Nixon, J. F.; Swain, J. R. J. Organomet. Chem. 1974, 72, C15. (21) Sakakura, T.; Tanaka, M. J. Chem. Soc., Chem. Commun. 1987, 758.

nation and hydroformylation utilize organometallic catalysts with phosphine ligands.29-32 An aspect of our research focuses on developing photoassisted heterogeneous synthetic routes using a γ-Al2O3 supported Rh(PF3)2 catalyst. The work is an extension of previous work concerning photochemical bond activation in molecules using supported Rh(CO) species.33-38 As a preliminary step in our development of a supported Rh(PF3)2 catalyst, we examined the adsorption and subsequent chemical behavior of PF3 with the γ-alumina surface. Imai39 demonstrated that aluminum oxide, which had been exposed PF3 and then calcined, was a good heterogeneous catalyst for the oligomerization of olefinic hydrocarbons. However, neither the details of the interaction between PF3 and the alumina surface nor the specific nature of the (22) Rosini, G. P.; Zhu, K.; Goldman, A. S. J. Organomet. Chem. 1995, 504, 115. (23) Margl, P.; Ziegler, T.; Blochl, P. E. J. Am. Chem. Soc. 1995, 117, 12625. (24) Koga, N.; Morokuma, K. J. Am. Chem. Soc. 1993, 115, 6883. (25) Scott, S. L.; Szpakowicz, M.; Mills, A.; Sanitni, C. C. J. Am. Chem. Soc. 1998, 120, 1883. (26) Davies, S. Organotransition Metal Chemistry: Applications to Organic Synthesis; Pergamon Press: Oxford, 1982. (27) Omae, I. Applications of Organometallic Compounds; John Wiley & Sons: Chichester, 1998; Chapters 16 and 18. (28) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic Molecules; University Science Books: Mill Valley, 1994; Chapters 2 and 3. (29) Genet, J. P. Pure Appl. Chem. 2002, 74, 77. (30) Langer, F.; Puentener, K.; Stuermer, R.; Knochel, P. Tetrahedron: Asymmetry 1997, 8, 715. (31) Nindakova, L. O.; Shmidt, F. K.; Reshetnikova, O. M.; Dmitrieva, T. V.; Saraev, V. V. Koord. Khim. 1990, 16, 534. (32) Benyei, A.; Joo, F. J. Mol. Catal. 1990, 58, 151. (33) Wovchko, E. A.; Yates, J. T., Jr. J. Am. Chem. Soc. 1995, 117, 12557. (34) Wovchko, E. A.; Yates, J. T., Jr. J. Am. Chem. Soc. 1998, 120, 7544. (35) Wovchko, E. A.; Yates, J. T., Jr. J. Am. Chem. Soc. 1998, 120, 10523. (36) Wovchko, E. A.; Yates, J. T., Jr. J. Am. Chem. Soc. 1996, 118, 10250. (37) Wovchko, E. A.; Zubkov, T. S.; Yates, J. T., Jr. J. Phys. Chem. B 1998, 102, 10535. (38) Wovchko, E. A.; Yates, J. T., Jr. Langmuir 1999, 15, 3506. (39) Imai, T. U.S. Patent 4,465,885, 1984.

10.1021/la026802k CCC: $25.00 © 2003 American Chemical Society Published on Web 05/28/2003

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catalytically active surface sites was reported. Several studies involved modifying an Al2O3 surface with PCl3 for olefin isomerization and polymerization, and the use of Al2O3 materials for PCl3 capture were found, but they lack detailed information regarding the surface reaction.40-43 The nature of the adsorption of PCl3 on silica was studied by Morrow et al.44 with IR spectroscopy and 31P magic angle spinning NMR techniques. The physically adsorbed PCl3 species dissociated to yield SiOPCl2 chemisorbed species and HCl gas after several hours at room temperature. Paul et al.45,46 conducted experiments which examined the decomposition and oxidation of PH3, as catalyzed by MoO3 supported on Al2O3. Lu et al.47,48 studied a similar reaction with supported Rh(CO)2. While excellent accounts of the decomposition and oxidation of PH3 on catalyst surface species were provided, no obvious reaction of PH3 with the alumina surface was mentioned. To our knowledge, there is no detailed report of the surface chemical reaction between PF3 and γ-Al2O3. Using infrared spectroscopy, high vacuum techniques, and low temperatures, we have been able to detect the adsorption of PF3 onto high area aluminum oxide and the formation of a transient intermediate. A reaction with surface hydroxyl species was readily observed during sequential heating. Near room temperature, PF3 was oxidized to form surface FHPO and FHPO2 species, which were stable upon evacuation and oxidation up to 700 K. This study provides necessary background information to be considered during the preparation of PF3-based Rh/Al2O3 photocatalysts. 2. Experimental Section Experiments were conducted in a special, wide temperature range, transmission infrared cell similar in design to that described in detail previously.49 The cell is a stainless steel cube with six conflat flange ports. KBr windows, sealed with differentially pumped double O-rings, were used on two ports for infrared beam transmission. A viewport was used for external visualization. Degussa aluminum oxide C (γ-Al2O3), having a surface area of 104 m2/g, was used in these experiments. Powdered samples were pressed, under 1200 psi pressure for 30 s, into the closely spaced square openings (0.048 mm2) of a thin tungsten grid (0.025 mm), using a hydraulic press.50 Excess sample was scraped from the grid using a steel blade. Net mass of deposited samples was 3-4 mg having an optical density of 2.0-2.7 mg cm-2. The powdered grid was rigidly secured by nickel clamps and attached to electrical feedthroughs mounted on the bottom of a reentrant Dewar which enters the cell. The grid and sample temperature was varied by cooling with a liquid nitrogen filled Dewar and by electrical heating with a dc power supply, regulated by a LabView-based digital temperature control program. A chromel/alumel thermocouple (0.08 mm diameter) was spotwelded to the top center of the grid to measure the sample temperature. The temperature could be held constant and maintained to (1 K in the range 100-800 K in these experiments. The cell was connected via stainless steel bellows tubing to a (40) Krzywicki, A.; Marczewski, M. J. Chem. Soc., Faraday Trans. 1 1980, 76, 311. (41) Krzywicki, A.; Marczewski, M.; Modzelewski, R.; Pelszik, K.; Malinowski, S. React. Kinet. Catal. Lett. 1980, 13, 1. (42) Klendworth, D. D. U.S. Patent 4788171, 1988. (43) Caunt, A. D.; Gavens, P. D.; McMeeking, J. Eur. Pat. Appl. EP32308, 1981. (44) Morrow B. A.; Lang, S. J. Langmuir 1994, 10, 756. (45) Paul, D. K.; Rao, L.-F.; Yates, J. T., Jr. J. Phys. Chem. 1992, 96, 3446. (46) Rao, L.-F.; Yates, J. T., Jr. J. Phys. Chem. 1993, 97, 5341. (47) Lu, G.; Crowell, J. E. J. Phys. Chem. 1990, 94, 5644. (48) Lu, G.; Darwell, J. E.; Crowell, J. E. J. Phys. Chem. 1990, 94, 8326. (49) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321. (50) Ballinger, T. H.; Wong, J. C. S.; Yates, J. T., Jr. Langmuir 1992, 8, 1676.

Wang and Wovchko Pyrex high vacuum and gas delivery system. The system was pumped by a three-stage oil diffusion pump and backed by a mechanical pump. Prior to experiments, base pressures of 1 × 10-6 Torr, as measured with a cold cathode gauge, were achieved after approximately 12 h of baking. The system was equipped with a KJLC RGA 100F quadrupole mass spectrometer for gas analysis and leak checking. Gas pressures were measured with 0-1 Torr and 0-1000 Torr MKS 626A Baratron capacitance manometers. Transmission IR spectra of the Al2O3 samples were collected using a pure nitrogen gas purged Nicolet Nexus 470 FT-IR spectrometer equipped with a liquid nitrogen cooled HgCdTe wide band detector. It was controlled by E.Z. Omnic E.S.P. 5.2 software. Spectra were measured at 2 cm-1 resolution. To optimize the signal-to-noise ratio, 500 scans were averaged per spectrum. Background spectra were measured by translating the cell laterally so the IR beam could pass through a sample-free portion of the grid. Absorbance spectra of the sample were obtained by ratioing single-beam spectra of the sample to the sample-free single-beam spectra. Spectra have been further processed using MicroCal Origin graphing software. The PF3 (>98%) gas was purchased from Ozark-Mahoning and used without additional purification. Oxygen gas (99.9%) was extracted from a gas cylinder and used without further purification. Liquid deuterium oxide (99.96%) was obtained from MSD Isotopes, transferred to a vial, attached to the gas handling system, and purified by evacuation. Freshly prepared samples were generally heated in a vacuum at 475 K for 12 h. For lowtemperature experiments, samples were first cooled to 100 K. Then PF3 was slowly introduced to the cell and sample. Due to the condensation of PF3 on the sample, the temperature increased to approximately 150 K. This temperature was maintained. During thermal treatments, samples were warmed to the desired temperature for a specified time. Then the sample was subsequently cooled to 150 K for infrared spectroscopic analysis. Some spectra were measured at 150 K to eliminate spectroscopic thermal effects. Other spectra were measured at 300 or 310 K.

3. Results 3.1. Low-Temperature PF3 Reaction with the Alumina Surface, 150-310 K. Figure 1 provides selected infrared spectra measured during the low-temperature adsorption and slow surface reaction of PF3 on 475 K activated γ-Al2O3. The upper portion of Figure 1 includes original absorbance spectra in the OsH, PsH, and PdO stretching frequency regions. The lower portion of Figure 1 shows infrared difference spectra, where the 150 K alumina sample (spectrum a), prior to PF3 exposure, has been subtracted. Unfortunately, bands in the P-F stretching region, expected below 1000 cm-1, are not clearly observed due to the strong infrared absorption by the Al2O3 bulk material. Spectra b-e illustrate the effect of introducing 5.4 × 1019 PF3 molecules into the cell, subjecting them to a sequence of sample flash warmings to the indicated temperature, and holding at that temperature for the specified times, followed by rapid coolings to 150 K and spectral recording. As seen in the difference spectra of Figure 1, the 3743 and 3700 cm-1 bands, attributed to isolated hydroxyl groups on the alumina surface,16,17 decreased in intensity throughout the experiment. However, the broad band centered at 3370 cm-1, assigned to associated hydroxyl groups, grew throughout the thermal treatment sequence up to 310 K (spectrum e). In the low-frequency, PdO stretching region (1400-1000 cm-1), a new feature developed at 1309 cm-1 along with a weak band at 1246 cm-1. Additional overlapping features were detected near 1113 cm-1. After 5 min at 150 K, and 10 min warming at 230 K, the intensity of these bands increased only slightly, comparable to the minimal changes observed in the O-H stretching region (spectra b and c in Figure 1). No

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Figure 1. Infrared spectra measured in the OsH, PsH, and PdO stretching regions during the adsorption of a 5.4 × 1019 PF3 molecules exposure on 475 K activated Al2O3 at 150 K, followed by sequential heating to the specified temperature for the indicated time. The arrows indicate the direction of the absorbance changes. Upper portion presents absorbance spectra. Lower portion presents difference spectra where spectrum a has been subtracted.

Figure 2. Continuation of infrared spectra measured in the OsH, PsH, and PdO stretching regions during the reaction of PF3 with 475 K activated Al2O3 at 310 K for the indicated time. The arrows indicate the direction of the absorbance changes. Dashed spectrum a reproduces the initial sample spectrum a measured at 150 K in Figure 1. Upper portion presents absorbance spectra. Lower portion presents difference spectra where spectrum a has been subtracted.

appreciable band development was observed in the P-H stretching region during this period.

However, upon warming the sample to 270 K for 10 min and 310 K for 10 min more, dramatic changes in each

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of the designated regions of the infrared spectrum occurred (Figure 1, spectra d and e). Most notable was the emergence of a 2480 cm-1 infrared feature in the PsH stretching region. In the PdO stretching region, the 1309, 1246, and 1113 cm-1 features continued to develop. Interestingly, the band at 1246 cm-1 has markedly increased in intensity at 310 K, relative to the other features. In the O-H stretching region, the isolated hydroxyl bands at 3743 and 3700 cm-1 continued to decrease while a gain in intensity was seen for the associated hydroxyl feature centered at 3370 cm-1. 3.2. Room-Temperature PF3 Reaction with the Alumina Surface at 310 K. The PF3 thermal reaction with Al2O3 was continued at 310 K, under evacuation, for 130 min. Selected infrared spectra recorded during the reaction are presented in Figure 2. Absorbance spectra are shown in the upper portion of Figure 2. Infrared difference spectra are provided in the lower portion of Figure 2, where the initial spectrum measured at 150 K prior to PF3 exposure from Figure 1 (spectrum a), has been subtracted. Virtually no change in intensity occurred with the 3743 and 3700 cm-1 isolated hydroxyl bands. However, an obvious decrease in the broad, 3370 cm-1 associated hydroxyl infrared feature was seen. Following 130 min at 310 K, strong developments in the P-H infrared stretching region occurred. Spectral band comparison reveals a major increase of the 2480 cm-1 band and a splitting of these bands to 2492 and 2470 cm-1, respectively (spectrum g). In the PdO stretching region, the 1309 cm-1 dominant feature in low-temperature experiments has disappeared. A poorly resolved overlap of growing bands and newly developing bands were observed after 130 min at 310 K in this region. The infrared band at 1246 cm-1 became most intense. The absorption band 1113 cm-1 also increased appreciably. A new shoulder developed at 1142 cm-1, and a feature centered at 1029 cm-1 emerged. 3.3. Room-Temperature PF3 Reaction with Dehydroxylated γ-Al2O3. To investigate the role of hydroxyl groups in the reaction with PF3, a γ-Al2O3 sample was heated to 1200 K for 90 min under evacuation prior to PF3 exposure. Results from the experiment are presented in Figure 3. For comparison, Figure 3A includes infrared spectra measured during a 10 min, 300 K, PF3 reaction with 475 K activated alumina. The spectroscopic results were similar to those seen in Figure 2. High-temperature activation of Al2O3 under vacuum removes most surface hydroxyl groups, including isolated hydroxyl groups, and exposes Al3+ Lewis acid sites.16 Some isolated hydroxyl species remained on our sample after the 1200 K thermal treatment, as weak bands were observed in the Al-OH stretching region of Figure 3B. Our sample spectroscopically resembled a 1000 K activated γ-Al2O3 sample reported in the work of Ballinger et al.16 A possible explanation is the readsorption of residual water vapor present in our vacuum system onto the cooled, activated sample. Remaining infrared bands near 3700 cm-1 on the dehydroxylated sample are assigned to residual isolated hydroxyl species. After exposure to 5.4 × 1019 PF3 molecules (introduced into the cell) at 300 K for 10 min, these two bands disappeared. A new broad band was produced near 3500 cm-1, and it was accompanied by the appearance of a weak P-H infrared stretching band centered at 2480 cm-1 and the 1246 and 1113 cm-1 bands in the PdO stretching region. Qualitatively, the PF3 reaction on the dehydroxylated sample spectroscopically resembled the reaction on the hydroxylated sample. There were, however, marked quantitative differences in infrared band intensities. Band intensities

Wang and Wovchko

Figure 3. Influence of the extent of hydroxylation on the reaction of PF3 with Al2O3 at 300 K. Part A shows infrared spectra measured before and after a 5.4 × 1019 PF3 molecules exposure on 475 K activated Al2O3. Part B shows analogous infrared spectra measured before and after a 5.4 × 1019 PF3 molecules exposure on 1200 K activated Al2O3. (Note: Parts A and B are plotted on the same absorbance scale. Spectra have been offset.)

in the PsH and PdO stretching regions were considerably lower for the 1200 K activated Al2O3 sample. 3.4. PF3 Reaction with Deuterated γ-Al2O3. To confirm infrared band assignments and to further investigate the role of the hydroxyl groups, a PF3 reaction was conducted on a partially deuterated γ-Al2O3 sample. A fresh, 475 K activated alumina sample was exposed to 2.4 Torr D2O vapor at 300 K for 10 min, followed by evacuation and cooling to 150 K. An infrared spectrum measured after the D2O exposure is provided in Figure 4 (spectrum a). Deuterium atom exchange with hydrogen atoms and/ or OD species exchange with OH species occurred with both isolated and associated hydroxyl groups as evidenced by the formation of new infrared bands at 2762, 2718, and 2550 cm-1.51 These bands were produced at the expense of O-H infrared bands, which decreased in intensity (not shown in Figure 4). Into the cell containing the partially deuterated Al2O3 sample, 1.9 × 1019 PF3 molecules were introduced at 150 K (spectra b-d). The upper portion of Figure 4 presents infrared spectra measured in the OsH, OsD/PsH, PsD, and PdO stretching regions. Infrared difference spectra are provided in the lower portion of Figure 4, where the initial spectrum measured at 150 K, prior to PF3 exposure (spectrum a), has been subtracted. All of the isolated surface OH group infrared bands at 3743 and 3700 cm-1 and surface OD group bands at 2762 and 2718 cm-1 decreased in intensity slightly, similarly to the 150 K PF3 exposure on nondeuterated Al2O3. Associated OH infrared (51) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967; p 161.

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Figure 4. Infrared spectra measured in the OsH, OsD/PsH, PsD, and PdO stretching regions during the adsorption of a 5.4 × 1019 PF3 molecules exposure on a partially deuterated Al2O3 at 150 K, followed by sequential heating to the specified temperature for the indicated time. Upper portion presents absorbance spectra. Lower portion presents difference spectra where spectrum a has been subtracted. (Note: Spectra have been smoothed and offset.)

bands centered at 3370 cm-1 and associated OD infrared bands centered at 2550 cm-1 increased considerably (spectrum b-a). In the PdO stretching region, the expected formation of 1309, 1246, and 1113 cm-1 bands were detected. When the sample was warmed to 300 K for 10 min, two additional infrared features developed (spectrum c and c-a of Figure 4). One was the expected 2480 cm-1 band in the P-H stretching region, and the other was an anticipated band in the P-D stretching region centered at 1807 cm-1. At the same time, associated hydroxyl features in both the O-H and O-D stretching regions decreased. The intensity of the band at 1309 cm-1 decreased while bands at 1246 and 1113 cm-1 formed, as expected. Thermal treatment at 400 K for 10 min assisted the reaction, causing an enhancement of the 2480 and 1807 cm-1 PsH and PsD features. Furthermore, strong overlapping bands continued to develop in the PdO stretching region. 3.5. Thermal Stability and Oxidation Resistance of Surface Species. The stability of new surface species produced in the reaction of PF3 with Al2O3 was investigated by thermally treating an exposed sample to high temperatures under evacuation and in the presence of O2 gas. Infrared spectra are shown in Figure 5. The characteristic 2480 cm-1 band in the P-H stretching region, and the 1246, 1142, and 1113 cm-1 bands in the PdO stretching region were observed following a thorough alumina surface reaction with PF3 at 300 K. An additional band was observed at 1029 cm-1 in accordance with the weak feature observed in Figure 2 at extended thermal treatment. Upon evacuation and heating at 800 K for 120 min, essentially no depletion of the 2480 cm-1 P-H mode and the 1142 and 1113 cm-1 PdO modes occurred. A notable intensity

decrease in the 1246 cm-1 PdO feature was observed along with the notable intensity increase of the 1029 cm-1 PdO band. When the sample was heated at 700 K in 3.2 Torr O2 gas for 60 min, no change in the 2480 cm-1 PsH and 1142 and 1113 cm-1 PdO bands was observed. However additional loss of the 1246 cm-1 band occurred, while the 1029 cm-1 band gained intensity. 4. Discussion 4.1. PF3 Adsorption on Al2O3 and Surface Reaction. Spectroscopic Assignment of Surface Species. The chemical reaction of PF3 with alumina samples was investigated through a series of infrared spectroscopic and thermal treatment experiments designed to detect new surface species. Our work shows that PF3 reacts readily with the Al2O3 sample at room temperature (Figure 2), and the reaction products are very stable to thermal treatment and high-temperature oxidation (Figure 5). Cryogenic temperatures were employed to control the extent of interaction of PF3 (Figure 1). At atmospheric pressure, the melting point and boiling point of PF3 are 122 and 172 K, respectively.52 It is expected that PF3 exists as a condensed state on the Al2O3 surface in equilibrium with its vapor at the final sub-milliTorr pressures used in this experiment at 150 K. Again, PF3 infrared stretching modes, expected near 835-720 cm-1, occur below 1000 cm-1 and are unobservable due to the overwhelming Al2O3 bulk mode infrared absorption.53 At these conditions, PF3 molecules were observed to interact (52) Linde, D. R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2003. (53) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1990; pp 363-371, 403.

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Wang and Wovchko Scheme 2. Low-Temperature PF3 Reaction with Isolated Al-OH Species

Figure 5. Thermal stability and thermal oxidation stability of FHPO surface species. Presented are infrared spectra measured following an extensive PF3/Al2O3 reaction at 300 K followed by evacuation, heating to 800 K for 120 min, and heating at 700 K in 3.2 Torr O2 gas. (Note: Spectra have been offset.) Scheme 1. Low-Temperature PF3 Adsorption on Isolated Al-OH Species

with isolated hydroxyl groups to form an intermediate species. Primary evidence was the consumption of the isolated hydroxyl groups, which lost infrared band intensity, to form associated hydroxyl groups, which grew in intensity (Figure 1). We propose two general types of surface intermediate species, and they are illustrated in Scheme 1. One type is an OH-FPF2 species, where a fluorine atom of PF3 hydrogen bonds to the hydrogen atom of an isolated OH species. The other type is (OH)-PF3 species, where the phosphorus atom of PF3 binds to the hydroxyl moiety to form a three-membered ring. Both species explain the loss in intensity of the isolated hydroxyl bands and the gain in intensity of associated hydroxyl bands. The interaction resulted in the formation of a new P-O bond, since infrared spectral bands developed in the PdO stretching region (Figure 1). The 1309 cm-1 feature was initially most intense. We tentatively assign this band to the (O-H)-P stretch of the (OH)-PF3 intermediate species. It may also be due to the F-H stretching mode of the OH-FPF2 species. In the present study, we do not have additional evidence to confirm the assignment. Future work may involve solid-state 31P NMR or diffuse reflectance infrared spectroscopy to more directly observe P-F species.

When the sample was sequentially warmed from 150 to 310 K (Figure 1), increased diffusion of condensed PF3 occurred, generating additional PF3 intermediates. Furthermore, the increase in thermal energy, by warming above 230 K, initiated a second stage of the reaction in which insertion of the phosphorus atom of the adsorbed PF3 intermediate species, into the O-H bond of isolated surface hydroxyl groups, occurred. The reaction subsequently generated surface hypophosphoryl fluoride species of the form FHPO. This reaction is illustrated in Scheme 2. Marked infrared spectroscopic changes occurred in both the PsH and PdO stretching regions (Figure 1), which support the formation of this species, as opposed to a surface phosphoric acid species of the form, FxPOH (x ) 1, 2). The O-H stretching modes of a POH species are expected to show broad bands at 2700-2550 and 23502100 cm-1 in the infrared spectrum.53 No bands were detected in these regions. The infrared feature observed at 2480 cm-1 is definitively assigned to P-H stretching modes of FHPO species. Our assignment is supported by the literature.44-48,54-56 Studies of PH3 decomposition and oxidation by supported MoO3 and Rh(CO)2 catalysts report infrared absorption bands near 2490 cm-1 assigned to the PsH stretching vibration of HsPdO surface species.45-48 Morrow’s infrared study of the adsorption of PCl3 and OPCl3 on silica reported two overlapping absorption bands centered near 2490 cm-1 and also assigned the mode to P-H stretching.44 The PF3 reaction on our partially deuterated γ-alumina surface (Figure 4) revealed an additional band at 1807 cm-1. It is assigned to the P-D stretching mode of FDPO species. The 2480/1807 frequency ratio of 1.37 agrees well with the frequency ratio of 1.39 estimated from a simple diatomic P-H/P-D reduced mass dependence model, which assumes equivalent force constants. This result further supports our assignment of the vibrational mode, and it supports the argument for phosphorus atom insertion into the O-H bond of surface hydroxyl groups and the formation of surface hypophosphoryl fluoride species. The low-frequency region between 1300 and 1000 cm-1 measured during the PF3 thermal reaction is spectroscopically more complicated than the other regions. Several distinguishable, overlapping features developed (Figure 1). The major features at 1246 and 1113 cm-1 are assigned to PdO stretching vibrations of FHPO species. Bands are expected to appear in this region based on infrared spectroscopic literature of various phosphine oxide compounds.53-58 One may suspect the production of F3PO species in the reaction. However, the PdO stretching band occurs at 1415 cm-1 for this molecule in the gas phase.59 No spectral features were found near that frequency in our experiments. (54) Hirao, T.; Saito, S.; Ozeki, H. J. Chem. Phys. 1996, 105, 3450. (55) Withnall, R.; Andrews, L. J. Phys. Chem. 1987, 91, 784. (56) Ahmad, I. K.; Ozeki, H.; Saito, S. J. Chem. Phys. 1999, 110, 912. (57) Lugez, C. L.; Irikura, K. K.; Jacoxb, M. E. J. Chem. Phys. 1998, 108, 8381. (58) Sari-Zizi, N. B.; Najib, H.; Sebihi, R.; Pracna, P. J. Mol. Spectrosc. 1998, 190, 15. (59) Shimanochi, T. J. Phys. Chem. Ref. Data 1977, 6, 993-1102.

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Table 1. Spectroscopic Assignments of the Various Surface Species Detected during the Adsorption and Reaction of PF3 with Hydroxylated Al2O3 in This Work species Al-OH (isolated) Al-OH (associated) Al-OD (isolated) Al-OD (associated) FHPO FHPO2 FDPO OH-FPF2 or (OH)-PF3

chemical group

ν(obsd), cm-1

reference

O-H O-H O-D O-D P-H PdO P-H O-P-O P-D PdO O-H-F or (OH)-P

3743, 3700 3370 2762, 2718 2550 2480 1246, 1113 2492, 2470 1142, 1029 1807 1246, 1113 1309

16, 17 16, 17 51 51 44-48, 53-56 53-58 44-48, 53-56 53 this work 53-58 this work

Scheme 3. Elevated Temperature PF3 Reaction with Associated Al-OH Species

The infrared band assignments to the various surface species detected in this work are organized in Table 1. 4.2. PF3 Reaction on Al2O3 at Elevated Temperatures. Reaction with Associated Hydroxyl Groups and Thermal Stability of Surface Species. By increase of the sample temperature to 310 K for extended periods of time, PF3 molecules continued to react with the Al2O3 surface. At this temperature, associated hydroxyl groups were able to participate in the reaction to form additional FHPO species. Evidence is the depletion of the 3370 cm-1 associated hydroxyl infrared band (Figure 2). Furthermore, dramatic increases and shifting in a number of infrared bands occurred in the PsH and PdO stretching regions. Careful examination of the low-frequency end of the PdO stretching region reveals the formation of an infrared band at 1029 cm-1. Additional development occurs near 1142 cm-1. We assign the 1142 and the 1029 cm-1 modes to the respective asymmetric and symmetric stretches of the PO2 moiety of a proposed FHPO2 surface species. This species forms from the reaction of a PF3 molecule with two hydroxyl species. Bands of phosphorous acids of the form, R(H)PO2- display infrared bands at 1190-1100 and 1075-1000 cm-1.53 The reaction described above is depicted in Scheme 3. During this reaction, we believe that HF molecules are generated and either are released from the surface or react with Al2O3 to make Al-F and O-H species. However, we do not have infrared spectroscopic or other experimental evidence to verify the formation of HF60 or the likely Al-F precursors to AlF3. Finally, it is important to note that the 1309 cm-1 feature disappears during the extended room-temperature PF3 reaction under vacuum. This indicates that the proposed PF3 intermediate species have either desorbed from the surface or (more likely) proceeded toward the oxidized product. Additional support for the formation of FHPO2 surface species is the enhancement of the 1029 cm-1 feature during high-temperature thermal treatment and high-temperature oxidation (Figure 5). Reaction of surface hypophosphoryl fluoride species with neighboring associated hydroxyl groups more readily occurs at elevated temperatures. Furthermore, it is reasonable that the presence of (60) HF gas was detected in the vacuum system by mass spectrometry. However, the spectrometer was not situated in close proximity to the sample for accurate analysis of desorbed HF.

O2 gas may further assist the hypophosphoryl fluoride species oxidation to generate dioxo species. However, the surfaces of FHPO and FHPO2 are extremely stable to thermal and oxidative destruction. The development of two overlapping bands at 2492 and 2470 cm-1 from the 2480 cm-1 band in the P-H stretching region indicates that at least two distinctly different surface sites exist for anchoring the FHPO and FHPO2 species. The heterogeneous nature of the powdered γ-Al2O3 surface makes this a reasonable explanation. It is possible that the two overlapping features arise from asymmetric and symmetric PH2 stretching modes of a surface hypophosphite species of the form H2PO2. Infrared spectra of inorganic hypophosphite salts, such as Ca(H2PO2)2 and Fe(H2PO2)3, do not have clearly resolved PH2 infrared bands.61 Furthermore, the P-H band frequency is centered at 2370-2390 cm-1, which is considerably lower than the P-H stretching frequency of the species observed in this work. The PH2 moiety in monoalkyl phosphines such as n-C8H17PH2 and C6H5PH2, display a single, lower frequency, unresolved infrared band.62 Gas phase and Ar matrix H2PF and H2PO do display asymmetric and symmetric PH2 stretching modes; however, they are separated by