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Anal. Chem. 1990, 62, 2225-2232
Selective Laser-Induced Resonant Two-Photon Ionization and Fragmentation of Substituted Nitrobenzenes at Atmospheric Pressure Jianzhong Zhu, David Lustig, Irit Sofer, and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109 Resonant two-photon ionization (R2PI) at 1 atm in helium is studied as a selective and specific method for softly ionizing or fragmenting substituted nitrobenzenes. RPPI at 266 nm in 1 atm He is shown to produce relatively soft ionization where either the molecular ion, Me', is observed or a specific fragment ion such as (M - NO)' or (M - OH)'. As the laser power is increased, additional fragments may be produced; however, ions due to fragmentation of the aromatic ring are rarely observed even at the highest laser powers used (>loo W/cm2). The ions produced by RPPI are shown to depend on the laser photon energy, and it is demonstrated that by selecting specific frequencies of light, specific ions can be enhanced in the mass spectrum of a given molecule. It is also shown that under the same conditions distinctly unique ions are produced for the para, meta, and ortho isomers of p-nitroaniiine, thus allowing easy discriminatlon among the isomers. I n addition, the effect of the atmospheric pressure buffer gas in producing relatively soft ionization Is demonstrated by performing a comparative study under vacuum conditions where extensive fragmentation is generally obtained. Also the ion products obtained under atmospherlc pressure conditions are shown to be uniquely different than those obtained by conventional methods such as electron impact or chemical ionization under vacuum conditions.
INTRODUCTION Resonance-enhanced multiphoton ionization (REMPI) has been shown to be a technique with unique properties for chemical analysis (1-14). In particular, relatively efficient soft ionization with production of the molecular ion can be produced for detection and identification of many aromatic compounds in a mass spectrometer (1,6, 7). This is generally performed in a two-photon process, known as resonant twophoton ionization (RBPI), in which one photon excites a molecule to a real intermediate electronic state and absorption of a second photon results in ionization. The sum of the energy of the two photons can be chosen to be sufficient to produce ionization without inducing fragmentation. In addition, extensive fragmentation can be produced either by increasing the energy of the laser radiation or by increasing the power density so that additional photons are absorbed by the molecular ion, M'+ ( I , 4 ) . Furthermore, since ionization depends upon absorption of the first photon, air, i.e. Oz and Nz, and other small molecules, COz, Ar, Xe, etc., are not ionized by laser radiation in the near-UV region. The laser light specifically targets organic molecules with absorbing organic chromophores for ionization. Although laser RBPI has been described as such a versatile ionization technique, soft ionization is still difficult to achieve for some very fragile compounds, which tend to predissociate upon absorption of UV light. In recent work, RBPI was performed under atmospheric pressure conditions, where RBPI was shown to be a direct and specific source for atmospheric pressure ionization in an ion mobility spectrometer (14) or
mass spectrometer (15). More important, it was demonstrated that under atmospheric pressure conditions soft ionization can be produced even under very high laser power density (>lo9W/cmz) where extensive fragmentation would normally result under vacuum conditions (15). Thus, laser RBPI under atmospheric pressure has the potential of being a soft ionization source for even very fragile molecules. In this work, we investigate the RBPI process in atmospheric pressure in He for aromatic molecules with -NOz containing groups. This group of molecules presents a real challenge for the soft ionization capabilities of RZPI since they tend to predissociate upon absorption of UV light. In addition, many of the molecules studied to date by RBPI have been strongly absorbing aromatic or polynuclear aromatic molecules with electron-releasing groups such as NHz, OH, OCH3, C1, or CH3 (26) or other strongly absorbing chromophores such as N or S heterocycles or various indole derivatives (17). In the case where electron-releasing groups are present, the electron density in the aromatic center is increased, resulting in highly efficient excitation and also a shift of the So SI transition and the ionization potential to lower energy relative to benzene (16-19). However, -NOz is a strong electron-withdrawing group and shifts the absorption and the ionization potential (IP) to higher energy relative to benzene (18,19). In addition, the electron withdrawal from the aromatic ring has been shown, in electron impact studies, to result in a significant decrease in the ionization efficiency relative to groups that produce electron release (20). Thus there are several key issues to be explored in this study. The first question is whether -NOz containing aromatic molecules can be ionized softly via RZPI at the high laser frequency and power required to induce relatively efficient ionization under atmospheric pressure in these species. The use of the relatively high frequency radiation required to produce efficient RBPI in these molecules, because of the high IP, is a critical factor since the energy involved is sufficient to break weak bonds. In addition, the increased laser power required to induce efficient ionization in these species with strong electron-withdrawing groups will also enhance further fragmentation. The second issue is, if indeed fragmentation does result, can the extent of fragmentation be controlled by the laser radiation and by the collisional effects of the background gas under the atmospheric pressure conditions. In particular, we explore the effects of a light background gas with a low collisional effectiveness constant (21),i.e. helium, on the laser-induced fragmentation at atmospheric pressure and compare the results to those obtained in pure Nzand under vacuum at lo4 Torr. A light background gas might be expected to enhance fragmentation relative to that obtained with Nz, air, Ar, etc., while still moderating the fragmentation patterns obtained, by the effect of collisions relative to that obtained for RSPI under vacuum conditions. Most significant, though, we will explore the use of different frequencies of laser radiation to produce specific energy selected fragments based upon excitation of a well-defined narrow bandwidth excitation source as compared to a broad band source such as 70-eV electron impact (EI) ionization. Further, we will explore the
0003-2700/90/0362-2225$02.50/00 1990 American Chemical Society
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+3 b
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Flgure 1.
Schematic of the experimental setup.
use of specific frequencies of light t o selectively control the fragmentation obtained in combination with the effects of collisional relaxation induced by the background gas. This methodology will be studied for its capabilities in isomer analysis of substituted nitrobenzenes and for detection and identification of these compounds.
EXPERIMENTAL SECTION The experimental setup consists of a SpectraEL atmospheric pressure ionization quadrupole mass spectrometer with an API cell and ionization source as shown in Figure 1. The API cell consists of a stainless steel cell 1.5 in. in diameter by 2.6 in. long, in which ionization is produced. The ions are then biased to drift under the influence of an electric field toward a 30-wm orifice and into the mass spectrometer. There are several l/g in. inlet stainless steel tubes for the atmospheric pressure support gas to flow into and out of the cell. There are also several Cajon ports for a discharge source, sample probe, and quartz windows to allow the UV laser ionization beam to pass through the cell. In an alternate configuration, an inlet port is available for the interface of a vacuum-UV lamp source to the API cell. The API source was attached to the mass spectrometer via a Teflon screw seal and Teflon seals were used as needed in the Cajon fittings and end plate to minimize air leakage into the cell. The end plate was biased a t typically +350 V in order to force the ions to drift toward the aperture. The sides of the cell were separately biased at +150 V to prevent ions from being lost to the cell walls, while the aperture remained at ground potential. A constant flow of background gas was maintained through the cell at 400-500 cm3/min while the cell was kept at 100-230 "C using cartridge heaters in order to keep the walls free from background contamination between samples. The flow passed in front of the orifice and exited at the opposite end of the cell in order to prevent sample and dust from reaching and blocking the orifice. The orifice was heated 10-50 "C higher than the cell to further prevent clogging. In a later phase of this experiment a dual-orifice differentially pumped interface was developed to overcome clogging problems that resulted from the use of a 30-pm orifice. This interface used a 350-pm sampling orifice, followed by a 520-pm skimmer orifice inlet to the mass spectrometer. The region between the two orifices is pumped by a 650 L/s mechanical pump to a pressure of -5 Torr. The pressure in the mass spectrometer under these conditions is -2 X 10" Torr for He sampled from atmospheric pressure. The ions are focused into the mass spectrometer using an electrostatic lens with a voltage of +75 V between the orifices and potentials of +15 and +lo0 V are placed on each of the skimmers, respectively. The use of this interface with large orifces provided an enhancement in sensitivity of 10-20 times over that of the single 30-pm orifice to the mass spectrometer. The laser ionization source was the fourth harmonic (266 nm) or the fifth harmonic (212.8 nm) of a Quanta-Ray DCR-3 Nd:YAG laser. The fifth harmonic was generated by mixing the fundamental (1.06 Mm) and fourth harmonic of the Nd:YAG laser in an angle tuned BBO crystal (7 mm cube), The laser was usually
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softly focused within the API cell using a quartz lens (10 cm focal length) to produce a power density that could be varied between lo6 and IO8 W/cm2 in a 1 mm diameter spot. Alternatively, vacuum-UV discharge lamps (HNU Systems, Inc.) could be used to produce direct ionization in helium at 1 atm. A Xe discharge lamp produced two lines with energies of 8.4 and 9.5 eV, where 95% of the energy was in the former line. The strongest line in a Kr lamp was at 10.0 eV (80%) and a second weaker line at 10.6 eV (20%) was also generated. An alternate ionization source used in these studies was an atmospheric pressure glow discharge in helium. This source has been described elsewhere (22) and involves using a tungsten tip to generate a glow discharge operating at -1 kV (80-300 wA) in He. Ionization of the analyte occurs via charge exchange from He to water clusters followed by proton transfer to the trace analyte to produce MH'. The sample was introduced into the API cell using either diffusion tubes or a direct sample insertion probe. Diffusion tubes (Vici Metronics, Santa Clara, CA) with an aperture of 0.5 cm and 7.62 cm in length were used to create samples in concentrations on the order of several parts per million by flowing He over the diffusion tube enclosed in a stainless steel housing and through 1/8 in. tubing into the API cell. Provided the carrier gas was flowng more slowly than the drift gas ( l o % as well as peaks at (M - NOz)+and (M - NO2- H)+. It is not clear why M'+ is not observed under vacuum at 266 nm since the IP of o-NA is given as 8.27 eV (PI) (23). In addition, a host of lower molecular weight fragments are observed including clusters of CIH,+, C2Hn+,C3Hn+,C4Hn+,and CSH,+ fragments. Figure 4C shows the laser-induced REMPI mass spectrum of m-nitroaniline (m-NA) in 1atm He at 266 nm. The major peaks observed are M'+ and (M + NO)+. In contrast to the case of o-NA, only a minor (M - OH)+ peak is observed for m-NA. In addition, for laser REMPI of m-NA (M - NO)+ is only a minor peak while M + is a major peak. This contrasts to the laser REMPI/MS of p-NA where no M'+ is observed and (M - NO)+ is the major peak. In the E1 mass spectrum of m-NA, M'+ and (M - NOz)+are the dominant high mass ions. p-Nitrophenol ( p - N P ) :Laser REMPI. For comparison, several other substituted p-nitrobenzene compounds were studied by MPI. Figure 5 shows a laser-inducedREMPI mass spectrum of p-nitrophenol (p-NP) at 266 nm in 1atm of He at an estimated concentration of 150 ppb. Once again, there is one dominant ion peak, which corresponds to (M - OH)+. No molecular ion was observed under the conditions of mild focusing used in this experiment. The IP of p-NP is listed as -9.38 eV by photoelectron spectroscopy (29) and 9.52 eV (18) by E1 so that the effect of longer wavelength on the ionization process could not be reasonably tested here by using a two-photon process. A low IP value of 8.84 eV is also given by Brown (30) using EI. However, when laser-induced REMPI was studied at 213 nm in 1 atm He (Figure 5B), a strong M + ion was clearly observed. In addition, strong ion peaks were observed a t (M - OH)+, (M - NO)', and (M + NO)+. This once again illustrates the ability to selectively control fragmentation based upon wavelength. It should be noted that the discharge source showed p-NP without the presence of significant decomposition. In addition, p-NP was studied by laser evaporation followed by laser-induced REMPI at 266 and 212.8 nm under vacuum conditions (lo* Torr). The dominant ion peak observed at 266 nm under vacuum is m/z 30, which is probably NO+. In addition, several peaks due to cleavage of the benzene ring are observed. However, no significant molecular ion or (M - OH)+ peak is observed a t 266 or 212.8 nm under vacuum. In this case, it is possible that decomposition may be occurring in the desorption process, although care was taken to avoid thermal effects by using a glycerol matrix to act as a heat sink (31). In the E1 mass spectrum at 70 eV the base peak is (M - NO)+, although strong M'+ and (M - NOz)+peaks are also
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Figure 5. Laser-induced REMPI mass spectra under atmospheric pressure conditions in He of p-nitrophenol at (A) 266 nm and (b) 213
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observed (32). The results obtained by laser-induced REMPI for p-NP under atmospheric pressure conditions are not consistent with those normally observed in EI. This is expected since laser energy of a specific frequency produces very specific fragments. Nevertheless, a unique, simple spectrum results with which this species can be identified. p-Nitroanisole (p-NAS): Laser REMPI. The laserinduced REMPI spectrum of p-NAS was also studied at 266 nm at atmospheric pressure as shown in Figure 6A. The dominant ion here is (M - 30)'. This could be either (M NO)+ or (M - CHzO)+,the latter being the main fragmentation pathway for anisoles. However, high-resolution electron impact studies have shown that the main component is (M NO)+ (-97%) for p-NAS (30). This could conceivably be different under atmospheric pressure conditions, but we are unable to discriminate between the two species within the resolution of our instrument. In addition, there are two smaller peaks corresponding to (M - 15)+ and (M - 17)'. These are probably (M - CH3)+and (M - OH)+, the former being a common fragmentation product in anisoles. Another peak corresponding to (M + 17)' was also observed. No (M - NOz)+ ion was observed at 266 nm. A possible small molecular ion peak at m / z 153 appears to be present. If the laser frequency was tuned to 282 nm, only a (M - NO)+ ion peak was observed at the laser power available. No molecular ion was observed even a t this lower energy. In comparison, a photoionization spectrum taken using a 10.0-eV lamp in helium (Figure 6C) was almost exactly the same as the laserinduced REMPI spectrum at 266 nm. The appearance potential for (M - NO)+ from p-NAS is given in the literature as 10.03 eV obtained by E1 (30). This is comparable to the UV lamp energy but significantly higher than that obtained from two 266-nm photons. In contrast, a strong molecular
hfass l a m u )
Figure 6. Atmospheric pressure ionization mass spectra of p-nitroanisole at 50 'C in helium obtained by (A) laser-induced REMPI at 206 nm, (B) laser-induced REMPI at 213 nm, and (C) vacuum-UV photoionization at 10.0 eV.
ion is observed in the E1 mass spectrum (32)and also a smaller peak at M - 30. At 213 nm, the laser-induced REMPI mass spectrum in 1 atm He (see Figure 6B) also has a dominant (M - NO)+ ion, as at 266 nm, as well as smaller peaks a t (M - 17)' and (M - 15)'. However, a significant ion peak is also observed a t (M - NOz)+at the increased laser frequency, as expected, since the two-photon energy of the fifth harmonic exceeds the literature appearance potential of 11.63 eV (30). Under vacuum conditions in the reflectron TOF device, laser-induced REMPI of p-NAS at 266 nm produces numerous peaks including those at (M - 17) and (M - 30) (Figure 7a). However, the dominant ion peak occurs at mlz 30. No molecular ion is observed in the spectrum. The difference in mass spectra between that obtained under vacuum and that in 1 atm He appears to be very much due to the effects of collisional deactivation under atmospheric pressure conditions. However, significantly different results are observed a t 213 nm under vacuum, where a relatively strong Me+ion peak is observed in addition to strong peaks a t mlz 39 (C3H3+)and 30 (NO+) (Figure 7B). This result once again illustrates that fragmentation can be selectively controlled based upon the wavelength of excitation. Nitrotoluenes: Laser REMPI. Of great interest are the nitrotoluenes because of their relationship to explosive compounds. The laser-induced REMPI mass spectrum of pnitrotoluene (p-NT) was studied at 266 and 213 nm in 1 atm at 50 "C. At 266 nm the dominant ion is (M - NO)+. However, as one focuses the laser and increases the laser power density, significant peaks are also observed at (M - 17)+,(M + 17)+,and (M + 30)+,corresponding to (M - OH)+, (M + OH)+and (M + NO)', respectively. In addition, a small peak that may correspond to the molecular ion a t m / z 137 is ob-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 20, OCTOBER 15, 1990
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Figure 7. Laser-induced REMPI mass spectra of p-nitroanisole under vacuum conditions at (A) 266 nm and (6) 213 nm. served. The IP of p-NT is given as 9.50 eV (by EI) (30); however, ionization is observed at 266 nm probably due to hot band population. Very similar results are also observed for o-NT. At 213 nm, a spectrum very similar to that produced by the focused 266-nm laser beam was observed where the main peak was (M - NO)+ accompanied by smaller peaks at (M - OH)+,M + , and (M + OH)+,as shown in Figure 8A. No small fragments due to fragmentation of the benzene ring are observed in the laser-induced REMPI mass spectra at atmospheric pressure. Photoionization at 10.0 eV was also attempted in 1atm He, but no significant signal was observed. At present there is no apparent reason for this lack of signal. Laser-induced REMPI at 266 nm was also performed for p-NT under vacuum and a strong peak at m / z 105 was observed, which probably corresponds to (M - NO)+ minus several hydrogens. No molecular ion was observed. REMPI was also performed at 213 nm and very small signals were observed at (M - NO)+ and (M - NOz)+;however, the dominant ion peak was due to NOz+. The appearance potential for formation of (M - NOz)+is given in the literature as 11.80 eV and the sum of two 212.8-nm photons is close to the required energy. The E1 mass spectrum of p-NT shows a molecular ion peak and a strong peak due to (M - NOz)+,while no (M - NO)+ ion is observed (32). It should be noted that photodissociation of p-NT using a 2.541-eV photon laser beam in previous work resulted in loss of NOz (33). At 266 nm in REMPI this is not observed as a fragmentation route, thus once again illustrating the selective nature of photon-induced ionization for producing specific ion products. The laser-induced REMPI spectra of DNT and TNT were also studied at atmospheric pressure. The ionization potential of 1,3,5-TNT is given as 10.59 eV (23). Neither of these compounds produced an observable signal at 266 nm. However, at 213 nm a spectrum of 2,6-DNT was obtained with a dominant ion at (M - NO)+ (see Figure 8B). No signal was observed for 2,4-DNT or 2,4,6-TNT. The E1 spectrum of 2,6-DNT provides a strong (M - OH)+ ion which is characteristic of nitrotoluenes with o-N02groups present as opposed to the (M - NO)+ product obtained in laser REMPI under atmospheric pressure conditions. Once again, this illustrates the differences obtained when radiation of a specific energy is used as an ionization source compared to EI, especially under atmospheric pressure conditions.
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Figure 8. Laser-induced REMPI mass spectra under atmospheric pressure conditions in H e at 213 nm (A) p-nitrotoiuene and (B) 2,6dinitrotoiuene. For comparison, the 10.0-eV lamp was used to explore direct photoionization of DNT and TNT. Since 20% of the lamp energy is in the 10.6-eV line, it was expected that some signal would be observed. However, no signal was observed for DNT or TNT. In addition, an attempt was made to ionize these compounds under vacuum using 212.8 nm. No signal was observed for 2,6-DNT or 2,4,6-TNT. A signal was observed for 2,4-DNT, but the only significant peak was a strong low mass ion at m / z 39. The lack of large mass ions from DNT under vacuum conditions clearly illustrates the relatively soft ionization capabilities of REMPI under atmospheric pressure conditions.
CONCLUSION In conclusion, laser-induced REMPI a t 266 nm in atmospheric pressure produces relatively soft ionization in substituted nitrobenzenes. Although the molecular ion is generally not observed, a rather simple mass spectrum is observed in which either (M - NO)+ or (M - OH)+ is the dominant product ion. Even under the highest laser power densities used in this work in He (P 2 X 10s W/cm2), small mass fragqents due to fragmentation of the benzene ring were rarely observed or were of relatively low abundance. It is also believed from studies using vacuum-UV photoionization lamps that the fragmentation observed follows ionization rather than predissociation followed by ionization. In addition, laser-induced REMPI in 1atm He at 213 nm generally produced ions similar to those obtained a t 266 nm, but often with the appearance of additional ions of higher appearance potential, i.e. (M NOz)+. In comparison with laser-induced REMPI at 266 nm under vacuum conditions, the use of the atmospheric pressure background gas at elevated temperature often provided simple mass spectra. In particular, the mass spectra produced at
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Anal. Chem. 1990, 62,2232-2236
atmospheric pressure are modified by the effect of the collisional background as compared to the mass spectra obtained under vacuum. The mass spectra produced under vacuum often resulted in extensive fragmentation with mainly low mass ions dominating the mass spectrum at the power levels required to observe higher mass ions such as (M - NO)+ or (M - NOz)+. The molecular ion was rarely observed under vacuum conditions, although it was sometimes observed at 213 nm. This latter observation is as yet not understood. Most significant, the spectra produced by laser-induced REMPI are distinctly different from those observed in EI. The use of a specific wavelength of light allows the selective production of specific ions which can be varied as a function of the input wavelength.
ACKNOWLEDGMENT We thank Abe Berger of HNU Systems, Inc. for helpful suggestions made during the course of this work. We also thank HNU Systems, Inc. for the generous gift of VUV lamps. LITERATURE CITED (1) Lubman, D. M. Mass Spectrom. Rev. 1988, 7, 535-554, 559-592. (2) Dietz. T. G.; Duncan, M. A.; Liverman, M. G.: Smalley, R. E. Chem. Phys. Len. 1980, 7 0 , 246. (3) Klimcak, C.; Wessel. J. Anal. Chem. 1980, 5 2 , 1283. (4) Zandee. L.; Bernstein, R . B. J . Chem. Phys. 1979, 7 0 , 2574. (5) Seaver. M.; Hudgens, J. W.; DeCorpo, J. J. Int. J . Mass Spectrom. Ion Processes 1980, 34, 159. (6) Rettner. C. T.; Brophy, J. H. Chem. Phys. 1981, 25, 53. (7) Boesl, U.; Neusser, H. J.: Schlag, E. W. Chem. Phys. 1981, 55, 193. (8) Sin, C. H.: Tembreull, R.: Lubman, D. M. Anal. Chem. 1984, 5 6 , 2776. (9) Irion, M. P.; Bowers, W. D.; Hunter, R. L.; Rowland, F. S.; McIver, R. T., Jr. Chem. Phys. Len. 1982. 93, 375. (10) Sack, T. M.; McCrery. D. A.; Gross, M. L. Anal. Chem. 1985, 5 7 , 1290. (11) Rhodes, G.; Opsal, R. 8.: Meek, J. T.; Reilly, J. P. Anal. Chem. 1983, 5 5 , 280.
(12) Dobson, R . L. M.; D'Sllva, A. P.;Weeks, S. J.; Fassel, V. A. Anal. Chem. 1986, 58. 2129. Tembreull, R.; Lubman, D. M. Anal. Chem. 1987, 59, 1082. Lubman. D. M.; Kronick, M. N. Anal. Chem. 1982, 5 4 , 1546. Kolaitis, L.; Lubman, D. M. Anal. Chem. 1988, 58, 1993. Tembreull, R.; Sin, C. H.; Li, P.;Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 5 7 , 1166. (17) Tembreull, R.; Sin, C. H.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 5 7 , 2911. (18) Crable, G. F.; Kearns, G. L. J . Phys. Chem. 1962, 66, 436. (19) Jaffe, H. H. Chem. Rev. 1953, 53, 191. (20) Crable, G. F.; Kearns, G. L.; Norris, M. S. Anal. Chem. 1960, 32, 13. (21) Oman. R. A.; Bogan, A.; Weiser, C. H.; Li, C. H. Grumman Research Report RE-166, Grumman Aircraft Engineering Co., 1963. Lubman, D. M. Appl. Spectrosc., in (22) Sofer, I.; Lee, H. S.; Antos, W.; press. (23) Potapov, V. K.; Kardash, I . E.; Sorokin, V. V.; Sokolov, S. A,; Evlasheva, T. I. Khim. Vys. Energ. 1972, 6 , 392. (24) Altshuller, A. P.; Cohen, J. R. Anal. Chem. 1960, 32, 802. (25) Lubman. D. M.; Naaman, R.; Zare, R. N. J , Chem. Phys. 1980, 72, 3034. (26) Brown, P. Org. Mass Spectrom. 1970, 4 , 519. (27) Forst, W.Theory of Unlmolecular Reactions; Academic Press: New York, 1973. (28) McLafferty, F. W. Interpretatlon of Mass Spectra ; University Science Books: Mill Valley, CA, 1980. (29) Kobayashi, T.; Nagakura, S. J . Electron Spectrosc. Relat. Phenom. 1975, 6 , 421. (30) Brown, P. Org. Mass Spectrom. 1970, 4 , 533. (31) Li, L.; Lubman, D. M. Appl. Specfrosc. 1989, 43,543. (32) McLafferty, F. W.; Stauffer. D. 8. The WileylNBS Registry of Mass Spectral Data ; John Wiiey & Sons: New York. (33) Mukhtan, E. S.; Griffiths, I. W.; Harris, F. M.; Beynon, J. H. Org. Mass Spectrom. 1980, 15, 51. (13) (14) (15) (16)
RECEIVED for review April 2, 1990. Accepted June 29, 1990. We gratefully acknowledge support of this work from the Army Research Office under Grant DAAL 03-88-K-0191 for the atmospheric pressure work and from the National Science Foundation under Grant NSF CHE 8720401 for the work under vacuum conditions. David M. Lubman is an Alfred P. Sloan Foundation Research Fellow.
Ion Scattering and Electron Spectroscopic Study of Catalysts Prepared by Adsorption of Molybdate on Alumina Francis M. Mulcahy,' Marwan Houalla, and David M. Hercules*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
A setles of Mo/AI,O, catalysts was prepared by using an equlllbrlum adsorption method. The dkperslon of the molybdenum phase and coverage of the alumlna surface by Mo specles as a functlon of Mo loading were monitored by Ion scatterlng spectroscopy ( ISS) and X-ray photoelectron spectroscopy (XPS, ESCA). By modeWng changes In the ISS Mo/AI lntenstty ratio as a functlon of molybdenum coverage, It Is shown that, for Mo/AI,O, catalysts prepared by equlllbrlum adsorption, coverage of the alumlna surface should be complete at a loadlng of 47 X 10'' Mo atoms/cm2 for calcined catalysts and 55 X 10'' Mo atoms/cm2 for drled catalysts. These values agree wlth those obtalned from modellng of the ISS AVO lntenslty ratlo as a function of Mo loadlng.
INTRODUCTION Hydrodesulfurization (HDS) catalysts generally consist of University of Pittsburgh at Bradford, Bradford, PA 16701.
a Mo- or W-active phase supported on high-surface-area alumina and promoted by Co or Ni. Most often, molybdenum and tungsten phases are deposited by incipient wetness impregnation from ammonium heptamolybdate and ammonium metatungstate solutions, followed by drying and calcination. Wang and Hall ( I ) have shown that a more uniform active phase repartition can be achieved by using an equilibrium adsorption method to prepare the catalysts. This method involves adsorbing molybdate or tungstate anions on alumina from a relatively large volume of dilute solution. The amount of molybdate or tungstate which adsorbs can be controlled by adjusting the pH of the impregnating solution. The adsorption process itself is thought to be caused by the interaction of three factors: the isoelectric point (IEP) of the support, the pH of the impregnating solution, and the charge of the ion being adsorbed (1,2). At pH values below the IEP of the alumina support, the surface of the support becomes positively charged due to protonation of surface hydroxyls, favoring the adsorption of anions from solution. If the pH of the impregnating solution is above the IEP of the support,
0003-2700/90/0362-2232$02.50/0 C 1990 American Chemical Society
