J . Phys. Chem. 1984, 88, 5228-5232
5228
to be smaller when basis 2 is used, the general trends are preserved. Dissociation Energies. In order to obtain the dissociation energies of I and I1 according to the processes represented in (a) and (b), calculations at the SCF and CI levels and using both basis sets were carried out for Cu, OH, OH-, I, and 11. Final results are collected in Table IV. From the results concerning (a) it can be seen that at the SCF level only about 60% of the dissociation energy is recovered, regardless of the basis set utilized. Thus, correlation plays a fundamental role on the energetics of such processes. Although the CI wave function is mainly dominated by the SCF determinant, the role of correlation is fundamental to have a proper dissociation energy. When dealing with polyatomic molecules a quantification of the different kinds of contributions to the energy correlation appears to be difficult to evaluate, due to the mixing of atomic orbitals with MO. However, a simple qualitative picture could be obtained if attention is focussed on the major coefficients in the CI expansion as well as on the composition of the MO involved in the excitations from which they are constructed. The most important contributions in I, apart from the S C F determinant, is mainly due to two diexcited determinants (coefficients 0.041 and 0.052). The first is constructed from a diexcitation of the HOMO to a higher MO, both being symmetric. This determinant accounts for radial correlation of s and p electrons of Cu and 0, respectively, and to a lesser extent includes part of the angular correlation of the Cu s electron. The second determinant is generated from a diexcitation involving antisymmetric MO, where the initially occupied orbital is the one which immediately follows the HOMO in decreasing order of energies. As both MOs are dominated by the d and pz orbitals of Cu and 0 its contribution to the correlation energy is mainly of radial character.
The most important contributions to the correlation energy of I1 comes from determinants generated from the mono- and diexcitations HOMO-LUMO (coefficients 0.026 and 0.067). As both, HOMO and LUMO are strongly dominated by the s and d orbitals of Cu the inclusion of such determinants accounts for a radial correlation on Cu. The next important contribution appears to be due to a determinant which is generated from a diexcitation of HOMO to a higher M O with strong contributions of p orbitals of Cu, and accounts for the angular correlation of Cu (coefficient 0.029). The participation of radial correlations of p electrons of oxygen is accounted for through two other determinants also generated from diexcitations from the H O M O (coefficient 0.03 1). So, the description here presented agrees with the works on Cu219and Ni229 in the sense that the d correlation is an important factor of stabilization for bonds involving transition-metal atoms. The good agreement for the dissociation energy, calculated with basis 2 at the C I level and the geometry optimized at the S C F level with basis 1, means also that the predicted geometries must be close to the correct ones. Thus, although geometries can be approximated by using a S C F description at the double {level, the correct description of the energetics must be done at the CI level with extended basis sets when possible.
Acknowledgment. We thank the Laboratoire de Physique Quantique de 1’ Universiti Paul Sabatier de Toulouse (France) for making available the computer programs, and particularly to Dr. J. C. Barthelat for his helpful discussions. Calculations were supported by the Centre de Calcul de la Universitat de Barcelona. Registry No. CuOH, 12125-21-2; Cu(OH)F, 91670-61-0. Newton, M. D.; Hay, P. J.; Martin, R. L., Bobrowicz, (29) Noell, J. 0.; F. W. J. Chem. Phys. 1980, 72, 2360.
Surface Ionization of Pyrldine Molecules on an Oxidized Rhenium Emitter. Ion Desorption in an Associated Form Toshihiro Fujii Division of Chemistry and Physics, National Institute for Environmental Studies, Tsukuba, Ibaraki 305, Japan (Received: April 26, 1984)
Surface ionization of pyridine molecules on an oxidized-Re emitter in no electric field has been investigated mass spectrometrically. The mass spectrum shows mass peaks of (M + H)’, (M+ CH$, and (M+ C2H5)+, suggesting that these ion species originate from reactions between adsorbed species. The mechanism has been studied by variation of the surface properties and temperature of the emitter and the amount of pyridine sample on the emitter. The formation of protonated species, which is predominant, has been interpreted in terms of Brernsted acid sites, the formation of proton-transfer complexes, and ion desorption. In conclusion, Brernsted surface sites are most likely to be the key to pyridinium ion emission from the hot surface.
Introduction In the study of organic surface ionization mass spectrometry’ using a hot oxidized-Re-filament emitter, an interesting ionization phenomenon was observed incidentally. After the flux of pyridine molecules is adsorbed on pretreated metal at room temperature, abundant positive ions emit momentarily in an associated form, predominantly, as protonated pyridine molecules, by the rapid heating of the emitter. It appeared to be different from the phenomenon of Saha-Langmuir model surface i ~ n i z a t i o n ~of- ~
intact organic species with low ionization potential on hot metal surfaces, since an association process was involved and all the observed ion species indicated that an association reaction on the emitter surface was induced during the heating process. More than a decade ago, Zandberg et al. studied the formation of protonated ions in thermal ionization for some organic compounds such as pyridine and attempted to consider the ionization p r o c e s ~ . ~The experiments were carried out in a conventional (3) Zandberg, E. Ya. Sou. Phys.-Tech. Phys. (Eng. Transl.) 1975, 19,
1133. (1) Fujii, T. Int. J. Mass Spectrom. Ion Phys. 1984, 57, 63. (2) Ionov, N. I. In “Progress in Surface Science”: Pergamon Press: New York, 1972; Vol. 1.
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(4) Zandberg, E. Ya., Rasulev, U. Kh.Rum. Chem. Rev. 1982, 51, 819. ( 5 ) Zandberg, E. Ya.; Rasulev, U. Kh.Sov. Phys.-Doklady (Engl. Trawl.) 1970, 14, 769.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5229
Surface Ionization of Pyridine Molecules ion gauge 1
reservoir I
analyser system
b.V.
El source capacitance manometer
emitter on
-J WIL
surface ionization source assembly
Figure 2. Manner of heating the emitter in a pulse mode. The pulse
heating operation induces reaction in the adsorbed layer on the emitter.
L.V.
Figure 1. Scheme of experimental setup: (L.V.) leak valve, (EI) electron ionization, (QMS) quadrupole mass spectrometer, (FE) flow element, (C.V.) controlled valve. The design of the surface ionization source assembly is such that the emitter filament can be placed in the center of the electron ionization ion source chamber when the emitter probe (see Figure 2) is inserted.
mass spectrometer with the thermal ionization source of the oxidized-W emitter in the continuous heating mode. They pointed out in their investigation that the species (M H)' are formed on the surface of the emitter via Saha-Langmuir model surface ionization as a result of the association of the specific kinds of adsorbed molecules and the subsequent decomposition of the associated species. It has been well established in the surface science field that the chemisorbed pyridine ion can exist on the surface6-8 of various catalytic materials under certain conditions. According to Basila et al.,7 an infrared spectroscopic study of pyridine chemisorption on silica-alumina has shown that H20 has the effect of converting a proton of the Lewis acid sites to Brcansted sites. Subsequently the proton-transfer reaction to the adsorbed pyridine molecule leads to the formation of pyridine ion. On the other hand Block et aL9 have investigated desorption of carbonium ions from zeolite surfaces mass spectrometrically. It has been revealed that the reactive system (Le., triphenylmethyl compounds on zeolite surfaces) forms ionic surface compounds without external fields and ions are desorbed thermally or commensurably by the aid of low external fields. The observation of this ion-desorption phenomenon, together with the existence of pyridinium ion, allows one to speculate on a possible alternative to Zandberg's model for the surface ionization mechanism of the pyridine molecule, that is, proton-transfer-complex formation followed by thermal ion desorption. In this study a number of experiments have been performed for the surface ionization of pyridine molecules on an oxidized-Re emitter. The primary requirements for ion formation, quantitative analysis of ion intensity using labeled compounds, the change of the ion response with the nature of the emitter, and the extent of emitter heating (thermal dependency) in the pulsed and continuous modes were investigated. All the observations of these experiments are consistent with the postulated mechanism that pyridinium ions are produced by a second-order reaction between protons and pyridine on the surface and then desorbed (evaporated) thermally.
+
Experimental Section The experiments were performed by using a Finnigan 3300 mass spectrometer with an ohmically heated filament emitter (surface ionization source assembly) placed in the manufacturer's E1 ion source chamber (Figure 1). The details of this assembly have been described elsewhere.' The measurements were made without ( 6 ) Parry, E. P. J . Cuzuf. 1963, 2, 371. (7) Basila, M. R.; Kantner, T. R.; Rhee, K. H. J. Phys. Chem. 1964,68, 3197. (8) Mapes, J. E.; Eischens, R. P. J . Phys. Chem. 1954, 58, 1059. (9) Block, J. H.; Zei, M. S. Surf. Sci. 1971, 27, 419.
any applied voltage between the emitter filament and the E1 ion source chamber. The experimental procedure was the same as that of the previous study' on organic surface ionization mass spectrometry except for the manner of heating the emitter and recording the spectrum. Filament heating was done by use of a dc power supply (0-40 V, 0-8 A, Nippon Stabilizer, Tokyo) which operated in the pulse or continuous mode. The pulse heating operation (Figure 2) was made possible by an on-off controller, which could vary both pulse plateau current and pulse width at any level. The typical operating pulses were 10 s long in a minute cycle with the plateau current level at 1.4 A. Once the emitter is initiated at a preset emitter temperature for the pulse heating operation, ion emission of an entire sample occurs in a time interval much shorter than that required to scan the entire mass range. Therefore, a full-scan spectrum was recorded in the signal accumulation mode with the combination of a multichannel analyzer and an ion counting system.1° The intensity of a specified ion was observed by the usual single-ion detection method and drawn on a beam-oscillograph (technique for recording single-ion abundance profiles). The constant admission of pyridine gas was accomplished by using a pressure gauge (capacitance manometer, MKS 315 BHS-10) and a molecular flow element (MKS, FE 1.0) that can be used as a control unit (170-44A).11 The sample gas pressure was measured by ionization gauge 1 (VG-1, Wakaida), which was calibrated beforehand for all the compounds used with another capacitance manometer (315 BHS-1, MKS). The additional gas was admitted to the mass spectrometer from a reservoir via a variable leak valve (series 203, Granville-Phillips). The ultimate vacuum in the analyzer tube measured by ion gauge 1 after bakeout was typically in the 10-8-torr range. The oxidized-Re emitter filament was prepared and used in the presence of O2at a controlled pressure of 2 X torr which was obtained by adjusting the Granville-Phillips variable leak valve (series 203) unless otherwise noted. The nonlabeled pyridine material and the labeled compounds of CEA products were purchased from a chemical company (Nakarai, Tsukuba) and used without further purification. The emitter temperature was measured with an optical pyrometer. But the temperature in the region where no luminescence is given was characterized only by the heating currents. Results and Discussion Mass Spectrum. The surface ionization (SI) mass spectrum of pyridine is shown in Figure 3, both in the continuous heating mode and in the pulse heating mode. The measurements have been done under a sample pressure of 3 X torr. The lower spectrum, taken with the emitter current in the continuous heating mode at 0.8 A (475 "C), exhibits the peak of the protonated pyridine molecule of m / e 80 as well as a few background peaks. These ions have their origin in the residual organic gases in the mass analyzer; it has been well-known'* that pyrolysis products (10) Fujii, T. Anal. Chim. Acta 1977, 92, 117. (11) Kiesling, R. A,; Sullivan, J. J.; Santeler, D. J. J . VUC.Sci. Technol. 1978, 15, 771.
5230 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
Fujii
4
(M+1
I
1
/
ii' /
0'
N?
'1
I
I
1
1
1
Partial Pressure of Pyridine-d, (X 1O-8torr) Figure 4. Intensity of the protonated pyridine ions as a function of sample pressure. Experimental conditions: Re emitter surface at 520 O C in the presence of 2 X torr of 02.
40
.-*
m/e
Figure 3. SI mass spectra of pyridine in a continous mode (upper spectrum) and in a pulse mode (lower spectrum). The star-marked peaks
originate from residual gases. of residual organic gases with low ionization potential can be surface ionized. The pyridine spectrum is completely devoid of the molecular ion and any fragment ions. This result agrees with those of Zandberg5 and Chatfield" on the spectrum of pyridine on oxidized-W ( W - 0 ) and oxidized-Re (Re-0) surfaces, respectively. Measurements with perdeuterated pyridine revealed that the associated hydrogen atom originates from the pyridine molecule as well as from the residual gases in the analyzer. Other ion species, mle 94 and 113, in addition to the abundant protonated species, have been observed under the condition that the pulsed current passed through the emitter filament for a 10-s period in a minute repetition cycle, causing the surface to reach the preset temperature of 1200 "C, and the ion counting-MCA recording mode is employed (lower spectrum of Figure 3). The peak having mass number m l e 94 corresponds to a pyridine molecule ion associated with CH,. This assignment has been made by the additional admission of perdeuterated acetic acid (CD3COOD), which is supposed to be the source of CH3 radical due to thermal decomposition on the hot emitter surface. Presumably the m / e 113 peak is (M CZH5)'. Comparison of the results in the pulse mode with those in the continuous mode indicates that the emission of ion species requires sample adsorption from the gas phase on the emitter surface and surface reaction between adsorbed species and the heating of the emitter up to the target temperature. The reason for the observation of (M + H)+ ions from the continuously heated emitter surface is explained by the assumption that the lifetime of the pyridine molecules on the heating surface is long enough to form
+
(12) Palmer, G. H. J . Nucl. Energy 1958, 7, 1. (13) Chatfield, D. A. 30th Annual Conference Mass Spectrometry and Allied Topics, Honolulu, 60 (1982).
the protonated ions through the surface reaction, but not enough for the formation of (M + CH3)+ and (M + C2H9+;in other words, the probability of the surface association reaction competes with the evaporation and dissociation of the adsorbed molecules. The pulse heating operation provides these surface processes with more favorable conditions (higher reaction probability) and consequently gives rise to the observation of the associated ions with CH3 and C2H5 radicals as well as the protonated ions, since the higher density of the adsorbed molecules is given after the adsorption step during the no-heating state of the emitter filament. The highest peak of m l e 60 in the spectrum under the pulsed heating mode was temporarily assigned as the protonated ion species of trimethylamine molecules which stubbornly remained at less than 1 X lo-* torr with the result of the previous experiment.' Ion Intensity us. Sample Amount. The relationship of the protonated-ion intensity ( I ) to the amount of sample (P,torr) was studied by using pyridine-d5. The result is shown in Figure 4. The experimental conditions are described in the caption. Experimental data give a straight line at a constant temperature of 550 OC, if the square root of I (C5D5ND)is plotted against the sample partial pressure. It indicates that the process is bimolecular; Z 0: p2, This result is in agreement with Zandberg's. Since the process is second order, the (M H)+ ions are formed by reaction either between two pyridine molecules or between a pyridine molecule and its dissociated species on the hot surface. In order to know which reaction actually takes place, molecules of water (a well-known proton donor) have been admitted additionally on the surface. D 2 0 and nonlabeled pyridine were introduced to the analyzer. Figure 5 shows the results, indicating that the intensity of C5H5ND+ion is linear over the D 2 0 sample pressure. Hence, it can be concluded that (M H)+ ions are generated through the reaction process between a pyridine molecule and a dissociated hydrogen species on the surface. The next question concerns whether the counterpart of the pyridine molecule is atomic hydrogen or proton. This will be discussed later. The pulse-heating experiment allows the sample to be adsorbed on the emitter surface during the off-state of the emitter filament; the amount of sample on the surface could be controlled by varying the adsorption time between the recurring filament's on-states. Figure 6 shows the relative production of CSDsNDf ions as a function of the adsorption time. The ion intensity was obtained
+
+
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5231
Surface Ionization of Pyridine Molecules
'
O
C
0 Filament Temperature ("C)
Figure 7. Emitter filament temperature dependence of the resulting ion current of (M + H)+ for pyridine in a continuous heating mode. Sample pressure: 2 x ioF7torr.
C r - - C
2 ,
8-
/
0
sistent with the fact that pyridine has a proton affinity as high as 250 kcal/mol in the gas phase. Besides, the validity of this mechanism can be given from the experimental results of Basila et al? that pyridine is successfully used for determining the relative numbers of Lewis and Brernsted acid sites by making use of the differences in the infrared spectra of the pyridinium ion and coordinately bonded pyridine. However, the explanation which is based upon the interaction of the pyridine with the neutral hydrogen atom cannot be ruled out completely. Ionization reactions, such as the Saha-Langmuir model surface ionization mechanism of the hydrogenated complex, could be involved. Thermal Dependency of Ion Intensity. A plot of the protonated-ion signal ( i ) against the emitter temperature ( T ) is shown for pyridine-d5 in the continuous heating mode (Figure 7). As can be seen in Figure 7, the relative intensity increases initially with the emitter temperature and then decreases after reaching a maximum, which occurs at a temperature of around 550 "C. An ionization threshold temperature (To)was observed around To = 400 OC, which is less than T (475 " C ) for the appearance of Na+ and K+ ions from Na and K impurity attoms in rhenium. To of the present study is ca. 260 OC higher than To= 140 OC of Zandberg's r e ~ u l t . This ~ discrepancy, which is too big to attribute to the accuracy of the emitter temperature, cannot be explained. For the interpretation of these temperature characteristics, the temperature variation of the ion formation and ion desorption rate should be taken into account. The rate law for the desorption process, the energy of which is AE, is described as follows: V = Voexp(-AE/kr)
(14) Basila, M. R.; Kantner, T. R. J . Chem. Phys. 1967, 71,467.
where V represents the rate of desorption at T K and Vois the preexponential factor. The initial intensity increase with the temperature may be interpreted partly by the expression of this equation. The temperature dependency is complicated for the ion formation by the proposed mechanism that the (M + H)+ ions are formed through the adsorption process of the proton. The dissociative chemisorption of the proton is the essential step, which must be the activated step,. However, molecular association such as the proton-transfer-complexformation is reduced by the thermal energy at higher surface temperature. The variation of the lifetime of the pyridine and proton on the surface with the temperature is also rate determining for the ion formation. All these parameters
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J. Phys. Chem. 1984, 88, 5232-5235
explain qualitatively the shape of the i ( T ) curve in a complex manner. Recently Rollgen et aI.l5 reported the production of R4N+ ions from quaternary ammonium chloride salts. The mechanism of this phenomenon is that desorption of intact ionic species takes place simply by heating salt deposits. The energy ( A E ) of this desorption process will be mainly related to the Coulombic interaction of the R4N+ion with Cl-. Therefore, the similar behavior of temperature dependency can be expected for the pyridinium ion, since the AE of the protonated-ion desorption may be also due to the electrostatic interaction force. For the objective of comparing temperature variation of both cases the experiment was conducted by using the present surface ionization filament assembly as the emitter filament on which tetramethylammonium chloride was slightly loaded. The threshold temperature at which the signal of (CH3)4N+could be obtained was ca. 400 OC,which was identical with that of the protonated pyridine ions. Model for Surface Ionization of Pyridine Molecule. Protontransfer-complex formation followed by ion desorption is the most probable mechanism for the base peak ions of pyridine molecules formed on the hot oxidized-Re emitter surface. This model is (15) Stofl, R.; Rollgen, F. W. J . Chem. Soc., Chem. Commun. 1980,789.
consistent with all the experimental observations so far. The present ionization phenomena could be termed, temporarily, chemical surface ionization, because ionic molecule reactions involving chemical surface interaction are responsible. Further experiments would be desirable to find out other kinds of organics which are ionized by the present experimental setup. Because this study leads to a better understanding of the associated ion species taken with the recent nonvolatile compound mass spectrometry techniques, such as, in-beam electron impact ionization,16 in-beam chemical ionization," activated field desorption emitter chemical ionization,I8 and ioni~ation,'~ I believe these techniques owe some portion of their success to the present phenomenon observed in conjunction with direct ionic desorption.
Acknowledgment. I am grateful to Drs. H. Soma and Y . Shiraishi for helpful discussions of this work. Registry No. Re, 7440-15-5; pyridine, 110-86-1. (16) Ohashi, M.; Tsujimoto, K.; Yasuda, A. Chem. Letf. 1976, 439. (17) Baldwin, M. A.; McLafferty, F. W. Org. Muss Specrrom. 1973, 7, 1353. (18) Hunt, D. F.; Shabanwitz, J.; Botz, F. K.; Brent, D. Anal. Chem. 1977, 49, 1160. (19) Hansen, G.; Munson, B. Anal. Chem. 1978, 50, 1139.
Catalytic Reactions on Well-Characterized Vanadium Oxide Catalysts. 3. Oxidation of Hydrogen Kenji Mori,? Morimichi Miura, Akira Miyamoto,* and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa- ku, Nagoya 464, Japan, and Kinuura Research Department, JGC Corporation, Sunosaki-cho, Handa, Aichi 475, Japan (Received: May 8, 1984)
Catalytic activities of unsupported and supported vanadium oxides for H2oxidation were investigated in relation to the catalyst structures. From results of reaction rates and steady-state catalyst structures at various O2concentrations, the active oxygen species was found to be the surface V = O species. The specific activity of the surface V=O species, or the turnover frequency, on unsupported V2O5 catalyst did not change with the treatment of the catalyst. Neither fusion nor reduction-oxidation treatment affected the turnover frequency. This indicates that Hz oxidation on vanadium oxide catalyst is a structure-insensitive reaction. It was also found that the turnover frequency for V2O5/TiOZand V205/A1203is constant and independent of the kind of support or V 2 0 5 content. In other words, the rate of H2 oxidation on the vanadium catalyst is determined only by the number of surface V=O species for all catalysts. These behaviors in Hzoxidation are discussed in comparison with those in CO or ethylene oxidation.
Introduction Supported metal oxide catalysts exhibit interesting catalysis depending on the kind of support and on the composition of the catalysts. However, the activity and selectivity on the supported metal oxide catalyst have not been well clarified in terms of the structure of the metal oxide on support. This seems to be due to the lack of a well-established method to determine the number of active sites on supported metal oxide catalysts. As for the supported vanadium oxide catalysts, we have previously established the rectangular pulse technique which allows the determination of the number of surface V=O species and the number of V,05 layers on support.' Furthermore, the structures of V2OS/TiO2 and V205/A1203catalysts have been determined by using various physicochemical measurements together with the rectangular pulse technique.2 Through investigation of the oxidation of C O and ethylene on well-characterized vanadium oxide catalysts, the structure-activity correlation has been found to change greatly JGC Corp.
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with the kind of reaction. The activity for the CO oxidation varies significantly with the surface structure of V2O5 and with the composition of the supported catalysts (the structure-sensitive r e a ~ t i o n ) . ~On the other hand, the activity for the ethylene oxidation is not affected by the surface structure of V2Os and the activity of the supported catalyst is mainly determined by the number of surface V=O species (the structure-insensitive react i ~ n ) . It~ seems therefore interesting to investigate the activity of well-characterizedvanadium oxide catalysts for various reactions (1) (a) Miyamoto, A,; Yamazaki, Y . ;Inomata, M.; Murakami, Y. J . Phys. Chem. 1981.85, 2366. (b) Inomata, M.; Miyamoto, A,; Murakami, Y. Ibid. 1981, 85, 2372. (2) (a) Inomata, M.; Mori, K.; Miyamoto, A,; Ui, T.; Murakami, Y . J . Phys. Chem. 1983, 87, 754. (b) Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. Ibid. 1983, 87, 761. (c) Murakami, Y.; Inomata, M.; Mori, K.; Ui, T.; Suzuki, K.; Miyamoto, A.; Hattori, T. "Proceedings of the 3rd International Symposium on the Preparation of Catalysts, 1982, Louvainla-Neuve"; Elsevier: New York, 1983; p 531. (3) Mori, K.; Miyamoto, A,; Murakami, Y . J . Phys. Chem. 1984,88, 2735. (4) Mori, K.; Miyamoto, A,; Murakami, Y. J. Phys. Chem. 1984,88, 2741.
0 1984 American Chemical Society
