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Electron Capture Induced Reactions in CF, Clusters Johannes Lotter and Eugen Illenberger* Institut f u r Physikalische und Theoretische Chemie. der Freien Universitat Berlin, Takustrasse 3, 0-1000 Berlin, FRG (Received: May 4 , 1990; In Final Form: July 6,1990)
Electron capture by van der Waals aggregates of CF4 and the subsequent decomposition into negatively charged product ions have been studied in a beam experiment. Apart from the fragments known from dissociative electron attachment to the isolated compound (Fand CF,-), a variety of larger ionic complexes such as M i , M,.F, and M,CF), n 2 1 (M = CF4), are observed. Their formation probabilities as a function of electron energy indicate that the initial step of electron capture proceeds via formation of an individual CF4- within the target cluster which then decomposes. Within the many products the monomeric anion CF4- in its relaxed configuration is generated. In that case, a relaxation energy of more than 3.5 eV has to be distributed among the target aggregate. CF4- has not been observed in the gas phase before. It is likely that it represents a metastable species with respect to autodetachment (negative adiabatic electron affinity of CF4). A time-of-flight analysis reveals that all ionic products are formed with low translational energy, including F and CF,-. The latter ions are known to arise with high kinetic energy in dissociative attachment to isolated CF4.
Introduction We have applied electron attachment spectroscopy (EAS) to CF4clusters in order to study the behavior of the basic quantities such as attachment energies, product ions, and fragmentation dynamics when proceeding from the isolated compound to van der Waals aggregates. In EAS the formation probability of negative ions (parent or fragment ions) is recorded mass spectrometrically as a function of the incident electron energy.' In previous experiments it has been shown that isolated perfluoromethane captures electrons within two very broad and overlapping resonances located between 4.5 and 10 eV.2-5 The lower resonance (peak maximum near 6.8 eV) has been interpreted as the electronic ground state of CF4-,'*, which immediately decomposes into the complementary channels CF4-
-
F
F-
+ CF,
+ CF,-
(lb)
Time-of-flight (TOF)measurements of the ionic products revealed' that the decomposition of CF4- is associated with remarkably high translational energy imparted to the products. In the case of reaction la 30% of the available total excess energy arises as kinetic energy of the two fragments while in channel 1b it is 65%. The energetic threshold for F formation is 2.26 eV,6 and for CF3- formation it is 3.9 eV.' The resonance of higher energy (peak maximum near 7.6 eV), in contrast, is exclusively coupled with F formation. This electronic state is likely to be described as a "core excited resonance" which ultimately decomposes into three fragments like F + CF2 F or F C F F2 with comparatively low kinetic energy.' In the present contribution, we apply electron attachment spectroscopy and TOF analysis of ionic products to study the formation and decomposition of negative ions in CF4 aggregates in a supersonic beam experiment. In a previous Letter' we have demonstrated that 7-eV electron impact to clusters of tetrafluoromethane results (among various other products) in the formation of an ion with a stoichiometric composition of CF4-. This compound is not accessible in electron
+
+
+
( I ) Oster, T.; Kiihn, A.; Illenberger, E. Int. J . Mass Specrrom. fon Processef 1989, 89, 1. (2) Harland, P. W.; Franklin, J. L. J . Chem. Phys. 1974, 61, 1621. (3) Illenberger, E. Chem. Phys. Lett. 1981, 80, 153. (4) Spyrou, S.M.; Sauers, 1.; Christophorou, L. G. J. Chem. Phys. 1983, 78, 7200.
( 5 ) Hunter, S.R.;Christophorou, L. G. J. Chem. Phys. 1984,80,6150. (6) McMillen, D. F.; Golden, D. M. Annu. Reu. Phys. Chem. 1982, 33, 493. (7) Lotter, J.; KOhn, A.; Illenberger, E. Chem. Phys. Lett. 1989, 157, 171.
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impact to isolated CF4 and, to our knowledge, has not been observed before. We will now present explicit ion yield curves of the various products. In addition, we will investigate the energy distribution following electron attachment to CF4 clusters. We have chosen the system CF4 since in the isolated case the temporary negative parent ion in its electronic ground state decomposes with high excess translational energy. The system may thus represent a prototype case for the study of energy transfer on proceeding from the isolated molecule to the molecular aggregates. Experimental Section Ion Formation. Negative ion formation following low-energy electron (0-10 eV) impact to CF4 clusters has been studied mass spectrometrically in a crossed-beam experiment. The new experimental arrangement has previously been described in detaiL8 Briefly, CF4 aggregates are generated by adiabatic expansion of CF4 seeded in Ar ( 1 :IO) through an 80-pm nozzle. The molecular beam (consisting of a distribution of clusters including monomers) is crossed at right angles with an electron beam (Figure I). The electrons are produced by emission from a hairpin tungsten filament and aligned by a weak homogeneous magnetic field (50-IO0 G ) generated by a pair of Helmholtz coils mounted outside the vacuum system. The electrons are transmitted through a series of electrodes to the reaction volume, which is defined by the crossing of the electron beam with the molecular beam. Since in the present configuration no electron monochromator is used, the beam possesses a poor energy resolution (fwhm = 0.7 eV) as estimated from the well-known resonance in SF6 yielding SF6-.9-'' Negative ions arising from the interaction of the electron beam with the cluster beam are extracted by a small electric field and analyzed by a commercial quadrupole mass filter. Although the present experimental configuration does not allow experiments on size selected clusters, we can indirectly witness the transition from the isolated molecule to molecular aggregates of increasing average size by changing the expansion conditions of the supersonic beam. TOF Analysis of Product Ion. The initial kinetic energy of product ions is determined by recording their flight times from the reaction volume through the mass filter to the detector. For that purpose, the electron beam is pulsed at one of the electrodes before the reaction volume (gate width < I ps). , Ions generated with low translational energy ( E ; 5 0.2 eV) exhibit one peak in the TOF spectrum while ions with higher ( 8 ) Kiihn, A.; Illenberger, E. J . Chem. Phys. 1990, 93, 357. (9) Fenzlaff. M.; Gerhard, R.; Illenberger, E. J . Chem. Phys. 1988, 88, 149. (10) Chutjian, A.; Alajajian, H. Phys. Reo. A 1985, 3 / , 2885. ( I 1 ) Orient, 0. J.; Chutjian, A . Phys. Reu. A 1986, 34, 1841.
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The Journal of Physical Chemistry, Vol. 94, No. 26, 1990
I
Expanslon Chamber
Lotter and Illenberger
Maln Chamber
elecyon beam
Figure 1. Schematic representation of the supersonic beam apparatus for the study of electron attachment to clusters.
I
M = CF,
__3
4
5
6 7 8 9 10 Electron Energy [ev] d
Figure 4. Ion yield curves of MY, M2.F, M 2 C F 3 - , and M
Figure 11. Suggested potential energy curves for CF, and CF, in their electronic ground states.
the "reflection principle", which states that the Franck-Condon factors as a function of electron energy simply reflect the shape of the Gaussian wave function of the ground vibrational state. The argument is based on the fact that the continuum wave function for nuclear motion can be approximated by a delta function 6(R-RT),with R, the classical turning point.'9*20 For more complex reactions, in particular for reactions in aggregates involving intramolecular energy transfer, P(X-)generally depends on energy, which will modify the Gaussian peak shape. It is plausible that the CF4- formed at the lower energy side of the resonance has a higher chance to be stabilized, since less relaxation energy has to be distributed among the target aggregate. This results in an ion yield curve with an asymmetric profile with the maximum shifted toward lower energies as observed in Figure 4c. Although the potential energy surface of CF4- is strongly repulsive in the Franck-Condon region (as evident from the width of the resonance and the translational energy of the products in the isolated case), the observation of CF, implies that its potential energy surface must possess a minimum below the energy of the lowest dissociation channel, F + CF3, which lies 2.26 eV above the neutral ground state of CF,. Formation of CF4- at the high-energy end of the resonance (-9-10 eV) implies that a relaxation energy of up to 7-8 eV has to be distributed among the target aggregate! This can occur only by substantial evaporation of the initial cluster. It is likely that CF4- represents a weakly bound F C F 3 adduct with one bond significantly weakened rather than a tetrahedral CF4-. It is, however, not clear whether CF, possesses a positive (adiabatic) electron affinity or not. The situation in Figure 1 1 illustrates CF4- in a thermodynamic unstable state (negative adiabatic electron affinity). Such a metastable ion may exist long enough to be detected by mass spectrometric techniques since autodetachment is a Franck-Condon forbidden transition. Returning to the stagnation pressure dependent count rate (Figure 6), we see that with increasing size of the ionic product the maximum of its formation probability is shifted to higher pressures with the maximum becoming increasingly flat. Since increasing stagnation pressure generally results in a neutral cluster distribution of increasing average size, Figure 6 then suggests that small product ions (like M- and M F ) are predominantly formed from small target clusters. The overproportional increase of the CF3- signal between 1.5 and 2.5 bar is simply explained by the fact that these ions are increasingly formed with low kinetic energy and, hence, higher detection probability. On the other hand, the observation that the CF3- signal becomes constant for pressures above 3 bar indicates that the formation probability of this ion decreases with cluster size. It should be remembered that F recorded at 8 eV shows a linear pressure dependence (Figure 6). At that energy the temporary parent ion CF; yields thermal F. (19) O'Malley, T. F. Phys. Reu. 1966, 1-70, 14. (20) Taylor, H . S. In Aduances in Chemical Physics; Prigogine. I., Rice, S. A,, Eds.; Interscience Publishers: New York, 1970; Vol. XVII.
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The present experiments indicate that even those products that appear with high translational energy in the isolated system (F, CF3-) are formed thermally when CF4- is generated within an aggregate. Multiple-scattering events in the ionized complex, strongly controlled by short-range attractive interactions between the additional charge and the induced dipoles, prevent the ejection of fragment ions with high kinetic energy. Conclusion
Electron capture by CF, clusters yields a variety of larger negatively charged products such as Mn-, M,-F, and M,.CF3-, n = I , 2 , ... . The ion yield curves of these products suggest that the initial step of electron capture proceeds via formation of CF4within the cluster. While the isolated temporary ion immediately
decomposes into the complementary channels F + CF3 and F + CF3- with high translational excess energy release, intramolecular interaction in the aggregates results in the formation of various product ions, among them the monomeric anion CF4- in its relaxed configuration, not accessible in electron impact to isolated CF4. It is suggested that CF4- represents a weakly bound ion molecule adduct CF3.F in a metastable state with respect to autodetachment. The TOF spectra indicate that all ionic products formed in electron capture by CF4 clusters (including F and CF3-) appear with low kinetic energy. Acknowledgment. We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (through Sonderforschungsbereich 337) and the Fonds der Chemischen Industrie.
Influence of Oxidatlon of Pd/AI20, for the Reactions of CO with NH, and Formamide Decomposition D.K. Pault and S . D.Worley* Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: May 14, 1990)
The reaction of CO with NH3 and the decomposition reactions for HNCO and formamide over Pd/AI2O3 films have been investigated by infrared spectroscopy. The formation of isocyanate species (NCO) was observed during all of these reactions over preoxidized Pd/AI2O3, but not over prereduced surfaces. These NCO species were formed on Pd but spilled over to the A1203support. Palladium became reduced during the course of the reaction of CO with NH,. A mechanism is proposed in which the reduction of Pd6+ and the formation of a Pd formamide species are key features.
I. Introduction The pioneering infrared studies of Unland1-3concerning the reaction of N O with CO over supported transition-metal catalysts to generate surface isocyanate species have led to considerable interest in the investigation of such species for the NO/CO reaction4 as well as, recently, for the N H 3 / C 0 system in these laboratorie~.~-~Fourier transform infrared spectroscopy has revealed the presence of RhNCO and RhOCN, simultaneously, during the reaction of CO with NH3 over preoxidized Rh/Si02? the migration of NCO from Pd to Si02 via the formation of HNCO gas,6 two mechanisms of formation of NCO on Ru/ AI2O3,’ and the effect of support material on NCO formation over M/X ( M = Rh, Ru, Pd; X = Al,03, S O 2 , Ti02).8 The present work addresses the changes in the oxidation state of Pd for Pd/AI2O3 during the course of the N H 3 / C 0 reaction and the decomposition of formamide. It will be demonstrated that Pd becomes reduced during the course of the reaction in which CO in the presence of NH3 is oxidized to isocyanate via the formation of a formamide intermediate. 11. Experimental Section
AI2O3-supported Pd catalyst samples were prepared9-” in a slurry containing PdCI2 solution (Johnson & Mathey) and A1203 (Degussa aluminum oxide C, 100 m2/g) in the appropriate ratio to produce 2.2 wt % Pd/AI2O3. The slurry was suspended in a liquid consisting of 9 parts of spectroscopic grade acetone and 1 part doubly distilled water and was sprayed with an atomizer onto a 2.0-cm-diameter CaF, disk maintained on a hot plate at approximately 360 K. The solvents evaporated rapidly, leaving a thin film of PdCl2.xH20/Al2O3adhered to the CaF2disk. A total sample weight of ( I .3-1.6) X 1 0-2 g was deposited in the preparations. yielding a final surface density (Pd/A1203)of (4.3-5.0) Author to whom correspondence should be addressed ‘Current address: Surface Science Center, Department of Chemistry, University o f Pittsburgh, Pittsburgh. PA 15260.
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X g/cm2 for the various samples. The catalyst sample was then mounted inside a specially designed infrared ~e11,~J’ which was subsequently evacuated by a bakable all-metal vacuum system employing a mechanical pump and a 20 L/s ion pump. The sample was outgassed under vacuum at ambient temperature for approximately IO h, heated at 513 K for 1 h at 1 X 10” Torr, and subjected to 5-, 5 - , lo-, and 20-min cycles of exposure to 80 Torr of H2 (or 0,) at 523 K, each cycle being terminated by Torr. The reduced or oxidized sample was evacuation to 1 X allowed to remain under vacuum (1 X 10” Torr) at 533 K for 1 h. Following evacuation overnight at 1 X 10” Torr at 298 K, the IR spectrum of the sample (background spectrum) was recorded. The sample was then exposed to reacting gases, and the IR spectra were monitored as a function of time and temperature during the reaction. Infrared spectra were obtained from a N2-purged IBM FTIR 44 spectrometer with a DTGS detector coupled with a data acquisition system. Measurement capability below the absorbance level was achieved with adequate signal-to-noise ratios and resolution by using the multiple-scan data accumulation feature. The spectra reported here are the differences between the samples before and after exposure to reactants. The IR cell was securely mounted throughout the course of data acquisition, thus minimizing artifacts in the subtraction technique. Smoothing
(1) Unland, M. L. J . Catal. 1973, 31, 459. (2) Unland, M. L. J . Phys. Chem. 1973, 77, 1952. (3) Unland, M. L. Science 1973, 179, 567. (4) See the numerous references quoted in ref 5-8. ( 5 ) Paul, D. K.; McKee, M. L.; Worley, S. D.; Hoffman, N . W.; Ash, D. H.; Gautney, J . J Phys. Chem. 1989, 93, 4598.
(6) Paul, D. K.: Worley, S. D.; Hoffman, N . W.; Ash, D. H.; Gautney, J. J . Catal. 1989, 120, 272. (7) Paul, D. K.; Worley, S . D.; Hoffman, N. W.; Ash, D. H.; Gautney, J . SurJ Sci. 1989, 223, 509. (8) Paul, D. K.; Worley, S. D.; Hoffman, N. W.; Ash, D. H.; Gautney, J. Chem. Phys. Leff. 1989, 160, 559. (9) Dai, C. H.; Worley, S. D. J . Phys. Chem. 1986. 90, 4219. (IO) Henderson, M. A.: Worley, S. D. J . Phys. Chem. 1985, 89, 1417. ( I 1 ) Dai, C. H.; Worley, S. D. Langmuir 1988, 4, 326.
0 1990 American Chemical Society
