J. Phys. Chem. B 2000, 104, 5603-5609
5603
Fourier Transform Infrared Study of 1,1-Dichlorotetrafluoroethane Adsorption on an Alumina-Supported Pd Catalyst Victor Yu. Borovkov,† Ferenc Lonyi,‡ Vladimir I. Kovalchuk, and Julie L. d’Itri* Department of Chemical Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 ReceiVed: February 18, 2000
An FTIR investigation of CF3CFCl2 adsorption on 5% Pd/γ-Al2O3 at room temperature shows that three types of surface species form. Two of the species are identified as trifluoroethylidyne (CF3Ct) and the third species is assigned to tetrafluoroethylidene (CF3CFd). All three species are on the metal sites, not on the γ-Al2O3. The surface of the reduced fresh catalyst favors the formation of trifluoroethylidyne species. However, CF3CFCl2 adsorption on a catalyst preexposed to the CFC + H2 mixture at 473 K for 1 h, followed by reduction with H2 at 573 K for 1 h, results in the predominant formation of CF3CFd. Both surface CF3CFd and CF3Ct species readily interact with H2 at room temperature to form CF3CFH2 and CF3CH3.
Introduction Supported Pd catalysts exhibit desirable catalytic properties for the selective cleavage of C-Cl bonds in CFCs.1-7 They are highly selective toward the fully dechlorinated hydrofluorocarbons in hydrodechlorination reactions.5,8 For example, CF2Cl2 was converted to CH2F2 on alumina-supported Pd under steady-state conditions with a selectivity in excess of 60%. For CF3CFCl2 and the same catalyst, the selectivity toward CF3CH2F exceeded 75%.9 Even higher selectivities have been reported for Pd supported on other materials such as carbon.10 Several factors appear to influence the activity and selectivity of Pd/Al2O3 in the hydrodechlorination of CFCs. An increase in particle size of the supported metallic Pd from 11 to 53 nm resulted in a 5-fold increase in the conversion rate of CF3CFCl2.11 A similar result was reported for CH2Cl2 hydrodechlorination catalyzed by Pd/Al2O3 with Pd particle sizes of 1.5 and 8 nm.4 In that case the reaction rate increased by a factor of 2.5. Modification of both the alumina support and the Pd metal by chlorine and fluorine deposition during the reaction might also affect the catalyst performance.4,5,9,11 In particular, the activity of the Pd/Al2O3 catalyst for the hydrodechlorination of CF3CFCl2 decreased by a factor of 2 during 15 h time on stream (TOS) while selectivity toward CF3CH2F increased from 45 to 76%.9 Analyses of product distributions have provided the majority of mechanistic information. The high selectivity of CF2Cl2 conversion into CF2H2 indicated the preferential removal of two chlorine atoms. As noble metals are known to favor homolytic bond cleavage, a difluorocarbene surface species was suggested as the reaction intermediate.3-5,12 Similarly, an adsorbed :CFCF3 carbene was proposed to be the active intermediate for the hydrodechlorination CF3CFCl2 to CF3CH2F.5,11,12 It was assumed that these surface carbenes were hydrogenated to form the hydrofluorocarbon.3-5,11,12 Although the macroscopic catalytic results strongly suggest that carbene-like species participated in the hydrodechlorination † Permanent address: N. D. Zelinsky Institute of Organic Chemistry, 117334 Moscow, Russia. ‡ Permanent address: Central Research Institute for Chemistry, H-1525 Budapest, P.O. Box 17, Hungary.
of CFCs, direct spectroscopic evidence is lacking. In the present study, the chemisorption of CF3CFCl2 on a 5% Pd/γ-Al2O3 catalyst was investigated with FTIR spectroscopy. The nature of the adsorbed species was identified. Their reactivity with hydrogen was also determined. Experimental Section A 5% Pd/Al2O3 catalyst was prepared by impregnation of powdered γ-Al2O3 (Vista-B, 300 m2/g, average pore diameter, 55 Å) with an aqueous solution of Pd(NH4)Cl2‚H2O (Alfa, 99.9% purity). The material was dried at ambient temperature for 10 h and then at 373 K for 20 h. The catalyst sample was oxidized with flowing O2 (60 mL/min) while being heated from ambient temperature to 773 K at a rate of 4 K/min and held at 773 K for 1 h. Then it was cooled to ambient temperature in flowing O2 and reduced in H2 (90 mL/min) at 573 K for 1 h. Subsequently, the fraction of Pd atoms exposed was determined by O2-H2 titration13 to be 44% (2.0 nm, equivalent particle size).14 Infrared spectra were recorded with a Research Series II FTIR spectrometer (Mattson, Inc.) equipped with a liquid N2 cooled MCT detector. The resolution was 4 cm-1 and 112 scans were accumulated per spectrum. The IR cell was similar to that described elsewhere.15 It was equipped with glass stopcocks connected to gas inlet/outlet ports. An additional glass sidearm cooled with liquid N2 was used to remove the gaseous CFCs rapidly from the cell volume by condensation. Self-supporting catalyst wafers (15-30 mg/cm2 thick) were prepared from the powder by pressing at 830 atm for 3 min. The wafers were pretreated in situ by heating to 773 K at 4 K/min in a 10% O2/He mixture (60 mL/min) and held at 773 K for 1 h before cooling to room temperature in the flowing 10% O2/He mixture and evacuating the cell for 0.5 h to a pressure of 10-5 Torr to remove all traces of O2. Then, the samples were heated in flowing H2 (80 mL/min) to 573 K at 5 K/min and held at this temperature for 1 h before evacuating at 573 K to a pressure of 10-5 Torr. Under these evacuation conditions the chemisorbed hydrogen is completely removed from the sample.13 The CF3CFCl2 was adsorbed at room temperature by two different methods. With one method, continuous adsorption, the
10.1021/jp000646d CCC: $19.00 © 2000 American Chemical Society Published on Web 05/17/2000
5604 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Figure 1. FTIR spectra of surface species resulting from the exposure of 1.75 Torr of CF3CFCl2 at room temperature to the freshly reduced 5% Pd/Al2O3 for 2 (spectrum A) and 5 (spectrum B) min followed by the removal of gaseous and physisorbed CFC by condensation in the liquid N2 cooled sidearm.
CFC was adsorbed at a pressure of 1-2 Torr. At fixed time intervals, as specified in the figure legends, the gaseous and physisorbed CFC was rapidly condensed in the liquid N2 cooled sidearm and the FTIR spectrum of chemisorbed species was collected. The complete removal of the physisorbed CFC from the catalyst surface was verified by the absence of an absorption band at 1280-1290 cm-1, a signature band of the symmetrical stretching vibration of the CF3 group of CF3CFCl2.16 The entire procedure, including the removal of gaseous and physisorbed CFC from the IR cell and recording the spectrum of chemisorbed species, took approximately 1.5 min. With the other method, pulse adsorption, the catalyst wafer was exposed to 1-2 Torr of CFC for 15 s followed by continuous evacuation of the gas phase and physically adsorbed CFC. During evacuation, the spectra of chemisorbed CFC were collected at discrete time intervals, as specified in the figure legends. The interaction of H2 with the chemisorbed species was investigated by monitoring the temporal behavior of the band intensities associated with the chemisorbed species after 1-2 Torr of H2 was introduced into the IR cell at room temperature. Results Continuous exposure of the reduced fresh 5% Pd/γ-Al2O3 catalyst to 1.75 Torr of CF3CFCl2 followed by evacuation resulted in an IR spectrum with four absorption bands at 1065, 1160, 1188, and 1228 cm-1 (Figure 1). The band intensity increased by ∼50% when the contact time was increased from 2 to 5 min. When 1.5 Torr of H2 was added at room temperature, all bands disappeared in less than 3 min (not shown). Treating the catalyst used in the previous experiment in vacuum at 573 K for 1 h changed both the number and the position of the adsorption bands formed during subsequent exposure to the CF3CFCl2. After evacuation (3 min) of a 1.5 Torr pulse of the CFC, the IR spectrum consisted of five absorption bands: 1070, 1165, 1188, 1220, and 1233 cm-1 (Figure 2, spectrum A). Another band at 1190-1200 cm-1 was also visible as a shoulder of the 1220 cm-1 band. Evacuation for 25 min decreased the intensities of the bands at 1070 and 1233 cm-1. The intensity of the 1190-1200 cm-1
Borovkov
Figure 2. FTIR spectra of surface species resulting from the exposure of a 1.5 Torr pulse of CF3CFCl2 at room temperature to the catalyst wafer used in the experiment depicted in Figure 1 followed by evacuation of preadsorbed CFC for 3 (A), 5 (B), 10 (C), 15 (D), and 25 (E) min. Before exposure to the CFC pulse, the wafer was heated in a vacuum at 573 K for 1 h.
shoulder remained constant during the 25 min evacuation, while the intensity of the band at 1188 cm-1 increased slightly (Figure 2, spectrum E). The adsorption of CF3CFCl2 after the surface species formed on the reduced fresh sample were decomposed at 573 K in a vacuum resulted in a 5 cm-1 shift to higher frequencies of the bands at 1065 and 1160 cm-1 shown in Figure 1. In addition, the band at 1228 cm-1 associated with the reduced fresh sample (Figure 1) was more clearly resolved into two bands at 1220 and 1233 cm-1 (Figure 2). Exposing the wafer used in the experiments described above to flowing H2 (80 mL/min) at 573 K for 30 min and evacuating at 573 K for 1 h did not regenerate the catalyst. This was established by exposing the wafer to CF3CFCl2 (1.6 Torr) at room temperature for 3 min after exposure to H2.17 Four bands in the range of 1100-1400 cm-1 were present: 1165, 1188, 1218, and 1232 cm-1 (Figure 3). These bands were in the same position as those shown in Figure 2. The band at 1232 cm-1 was the most intense, while the 1218 cm-1 band was a shoulder of the 1232 cm-1 band. As the exposure time increased, the intensity of the bands at 1165, 1188, and 1218 cm-1 gradually increased. The intensity of the 1188 cm-1 band increased only during the first 25 min and was constant thereafter. However, the intensities of the 1165 and 1218 cm-1 bands continued to increase, both at the same rate, even after more than 1 h exposure. The intensity of the 1232 cm-1 band remained essentially constant with increasing contact time. The changes in the band intensities of adsorbed CF3CFCl2 as a function of evacuation time of the adsorbed CFC (Figure 2) or exposure time (Figure 3) indicate that at least three types of adsorbed species were on the surface of the Pd/Al2O3 catalyst (Table 1). One species was characterized by a band at 1232 cm-1 that remained constant with longer exposure time. The second was characterized by a band at 1188 cm-1 that only increased in intensity for the first 25 min. The two bands at 1165 and 1218 cm-1 that continuously increased characterize the third species. When 2.5 Torr of CF3CFCl2 and 6.0 Torr of H2 was reacted at 473 K with the same wafer for 1 h under static conditions,
FTIR Study of 1,1-Dichlorotetrafluoroethane
Figure 3. FTIR spectra of surface species resulting from the exposure of 1.6 Torr of CF3CFCl2 at room temperature to the catalyst wafer used in the experiment depicted in Figure 2 for 3 (A), 5 (B), 10 (C), 25 (D), 60 (E), and 120 (F) min. Before exposure to the CFC, the wafer was reduced by H2 at 573 K for 0.5 h followed by evacuation for 1 h at the same temperature.
TABLE 1: Spectral Characteristics of Various Surface Species Resulting from the Adsorption of Gaseous CF3CFCl3 on a Pd/Al2O3 Wafer at Room Temperature species
band position, cm-1
1 2 3
1232, 1200-1190, 1077-1065 1190-1188 1228-1218, 1165-1160
CF3CFH2, CF3CH3, CO, and CO2 were formed. Removal of these products by evacuation at 573 K for 1 h followed by reduction of the wafer with flowing H2 (mL/min) at 573 K for 30 min, evacuation at 573 K for 1 h, and subsequent exposure of the wafer to a 1.8 Torr pulse of CF3CFCl2 for 15 s at room temperature followed by evacuation for 2 min resulted in spectrum A of Figure 4. The 1077 and 1232 cm-1 bands had the highest intensity. Evacuating for a longer time decreased the intensity of both 1077 and 1232 cm-1 bands and increased the intensity of the 1188 cm-1 absorption band. During the same evacuation period, the intensities of 1160 and 1218 cm-1 bands remained essentially constant. As the 1077 and 1232 cm-1 band intensities decreased, so did the absorption of bands in the range of 1190-1200 cm-1, suggesting that one surface species was characterized by the three absorption bands at 1077, ∼11901200, and 1232 cm-1 (Table 1). Whereas all absorption bands disappeared upon admission of H2 at room temperature after exposure to CFC for only 5 min (Figure 1), a different behavior was observed when the reduced fresh catalyst wafer was preexposed to the CFC + H2 mixture at 473 K (Figure 5). The initial spectrum consisted of bands at 1075, 1188, and 1232 and shoulders at 1165 and 1220 cm-1 (Figure 5, spectrum A). After H2 was admitted, all bands except the shoulder at 1220 cm-1 disappeared and new absorption bands appeared at 1200 and 1290 cm-1. The bands present after H2 exposure readily disappeared during evacuation at room temperature. Increasing the temperature of the wafer with adsorbed CFC to 373 K in static vacuum yielded the same result as continuous evacuation at ambient temperature. In this case, the IR spectrum of the gas phase in the cell was the same as that of gaseous
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5605
Figure 4. FTIR spectra of surface species resulting from the exposure of a 1.8 Torr pulse of CF3CFCl2 at room temperature to the catalyst wafer used in the experiment depicted in Figure 3 followed by evacuation of the wafer for 2 (A), 7 (B), 15 (C), 30 (D), and 60 (E) min. Before exposure to the CFC pulse, the wafer was exposed to 2.5 Torr of CF3CFCl2 and 6.0 Torr of H2 at 473 K for 1 h, evacuated at 573 K for 1 h, and reduced by H2 at 573 K for 0.5 h followed by evacuation for 1 h at the same temperature.
Figure 5. FTIR spectra of surface species resulting from the exposure of a 2.0 Torr pulse of CF3CFCl2 at room temperature to the catalyst wafer used in the experiment depicted in Figure 4 (spectrum A) followed by the exposure of 1.5 Torr of H2 for 10 min at the same temperature (spectrum B). Spectrum C represents surface species that resulted from the adsorption of CF3CFH2 at an equilibrium pressure of ∼3 mTorr on the same wafer after its evacuation for 30 min at room temperature. Before exposure to the CFC pulse, the wafer was reduced by H2 at 573 K for 0.5 h followed by evacuation for 1 h at the same temperature.
CF3CH2F. Subsequent adsorption of CF3CH2F at an equilibrium pressure of 1-3 mTorr on the evacuated sample resulted in a spectrum similar to that obtained after admission of H2 (spectra B and C in Figure 5). The thermal stability of the surface species that resulted from CF3CFCl2 adsorption is illustrated by Figure 6. Adsorption of 1.7 Torr of CF3CFCl2 on the reduced fresh 5% Pd/Al2O3 at
5606 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Figure 6. FTIR spectra of surface species resulting from the exposure of 1.7 Torr of CF3CFCl2 to the freshly reduced 5% Pd/Al2O3 for 17 h followed by sequential evacuation for 30 min at 298 K (A). Spectra B-D were recorded after the temperature of the wafer was increased in static vacuum to 373, 403, and 433 K, respectively.
room temperature for 17 h followed by evacuation at room temperature for 30 min led to the formation of bands at 1165, 1188, 1219, and 1232 cm-1, similar to those shown in Figure 3. However, increasing the wafer temperature to 373 K in a static vacuum resulted in the complete disappearance of the 1232 cm-1 band and a substantial decrease in the intensities of the bands at 1165 and 1219 cm-1 (spectrum B). The intensity of the 1188 cm-1 band increased slightly. At 433 K all the bands completely disappeared (spectrum D). After the adsorbed CFC was evacuated at 433 K or even at 573 K, the sample did not chemisorb CF3CFCl2. The capability of the sample to chemisorb CFC was completely restored after additional treatment in flowing H2 at 573 K for 0.5 h. Exposure of the reduced fresh catalyst to 1-2 Torr of CF3CFCl2 for e30 min resulted in intense bands at 1228 and 1160 cm-1 (Figure 1) or at 1226 and 1160 cm-1 (Figure 7) that characterized the most predominant species formed on the clean metal surface. Addition of H2 (1.25 Torr) decreased the intensity of all bands, and a new band emerged at 1285 cm-1 (Figure 7, spectra B-D). The 1285 cm-1 band can be considered, within experimental error, to be the 1290 cm-1 band (see Figure 5) that appeared after H2 exposure to the reduced fresh catalyst after CFC adsorption. After 15 min the spectrum of adsorbed species consisted of two low intensity bands at ∼1200 and 1285 cm-1 corresponding to the adsorbed CF3CH2F (Figure 7, spectrum D). The spectrum of the gas phase collected 20 min after introducing H2 into the IR cell (Figure 7, spectrum E) contained only bands characteristic of CF3CH3.18,19 This indicates that the interaction of H2 with one of the surface species exclusively produces CF3CH3 desorbing from the catalyst surface into the gas phase. A spectrum quite similar to spectrum A in Figure 7 was obtained by adsorbing of CF3CCl3 on the catalyst (Figure 8). This spectrum consisted of two absorption bands at 1165 and 1227 cm-1. The ratio of their intensities was close to that of the 1160 and 1228 cm-1 bands formed upon chemisorption of CF3CFCl2 (Figure 1). The introduction of H2 into the IR cell after CF3CCl3 adsorption also led to a significant decrease in
Borovkov
Figure 7. FTIR spectra of surface species resulting from the exposure of 1.5 Torr of CF3CFCl2 at room temperature to the freshly reduced 5% Pd/Al2O3 for 30 min (spectrum A) followed by exposure of the wafer to 1.25 Torr of H2 for 3 (spectrum B), 8 (spectrum C), and 15 (spectrum D) min. Spectrum E represents gaseous products desorbed from the catalyst surface.
Figure 8. FTIR spectra of surface species resulting from the exposure of 0.75 Torr of CF3CCl3 to the freshly reduced 5% Pd/Al2O3 for 20 min (spectrum A) followed by the exposure of the wafer to 1.0 Torr of H2 for 10 (spectrum B) and 30 (spectrum C) minutes. Spectrum D represents gaseous products desorbed from the catalyst surface.
the intensity of the 1165 and 1127 cm-1 bands (Figure 8) with the evolution of CF3CH3 into the gas phase. Unlike the thermal decomposition of chemisorbed species, their removal by interaction with H2 did not inhibit the subsequent chemisorption of CF3CFCl2. Discussion The present investigation provides direct evidence that three chemisorbed species form from CF3CFCl2 on a Pd/Al2O3 surface at room temperature. The signature vibrations of each are 10651077, 1190-1200, and 1232 cm-1 (species 1), 1188-1190 cm-1 (species 2), and 1160-1165 and 1218-1228 cm-1 (species 3).
FTIR Study of 1,1-Dichlorotetrafluoroethane None of the three species desorbed during evacuation as CFC molecules at elevated temperatures. However, they are readily hydrogenated. The chemisorbed species 1 and species 3 are hydrogenated at room temperature to form CF3CH2F and CF3CH3 (Figures 5 and 7). Both molecules formed during hydrogenation are easily removed from the catalyst surface by evacuation at room temperature and do not inhibit subsequent CFC chemisorption. These spectroscopic results agree well with a mechanism previously suggested by Karpinski et al. in which two or three of the carbon-halogen bonds on the R-carbon break to form a surface carbene or ethylidyne.11 Thus, the composition of hydrogenation products of chemisorbed species formed from CF3CFCl2, as well as the literature data,3-5,11,12 strongly suggest that species 1 and species 3 are tetrafluoroethylidene and trifluoroethylidyne, respectively. Recently, IR-spectroscopic evidence was provided for the formation of :CH2 carbene from CH2Cl2 on a Pd/SiO2 catalyst.20 Analysis of C-H stretching vibrations indicated that decomposition of adsorbed dichloromethane molecules at 233-243 K resulted in the formation of :CH2 species bonded to the metallic Pd surface. The homolitic cleavage of C-Cl bonds in a set of fluorinated 1,1-dichloroethanes on Pd(111) surface was suggested on the basis of a linear free energy relationship for dechlorination.21 The homolitic cleavage of the first C-Cl bond appears to be the limiting step of CFC chemisorption. The rapid elimination of the second chlorine atom was observed on a Cu(100) surface after scission of the first C-Cl bond in chlorinated ethanes and propanes containing two chlorine atoms bound to the same carbon.22 Consistent with these results for heterogeneous catalytic systems, both theoretical and experimental investigations of CF2Cl2 dissociation by thermal gas-phase C-Cl bond cleavage showed that the energy of the second C-Cl bond was decreased by ∼35 kcal/mol after the first C-Cl bond was broken (85 kcal/mol).23 The formation of an ethylidyne species (CH3Ct, analogue to CF3Ct) from ethylene on metallic surfaces is well documented.24-27 Ethylidynes can also be generated at low temperatures from saturated chlorinated compounds upon UV excitation, as shown for CH3CH2Cl adsorbed on Pt(111).28 The ethylidyne formation included the dissociation of the CsCl bond, dehydrogenation of the ethyl fragment to produce di-σbonded ethylene, and further dehydrogenation and isomerization of this ethylene complex. Similar reaction schemes for ethylidyne formation from CH3CH2I on Pt(111) and on Pt(100) surfaces were proposed by Zaera29 and by Kovacs and Solymosi.30 Direct evidence that the first step of ethylidyne formation proceeds via elimination of the hydrogen atom from the β-carbon atom of surface ethyl species to form a di-σcoordinated ethylene molecule was established through experiments using partially deuterated ethyl iodide.31 In our experiments, trifluoroethylidyne, characterized by absorption bands at 1160-1165 and 1218-1228 cm-1, was formed through the elimination of all three halogen atoms directly from the -CFCl2 group of CFC. This result parallels the formation of the chemisorbed species with the similar spectral signatures characteristic of dissociative CF3CCl3 adsorption. In the presence of H2, the surface species formed from CF3CCl3 adsorption is also selectively hydrogenated to CF3CH3 (Figure 8). The absence of chlorine-containing products supports the conclusion that the β-elimination step does not occur during ethylidyne formation from chemisorbed CF3CCl3.
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5607 TABLE 2: CsF Stretching Frequencies (cm-1) in Some Fluorine-Substituted Compounds molecule CF3-CH3 CF3-CCl3 CF3CtN CF3CtCH CF3COCl CF3-CFH2 CF3-CFCl2 CH3-CFCl2
ν(C-F)
1104 1110 1096
νS(CF3)
νAS(CF3)
ref
1278 1255 1226 1254 1279 1294 1295
1230 1227 1212 1182 1191, 1234 1185, 1191 1232
19, 48 49 50 51 32 52 49 48
To assign the absorption bands in the spectra of chemisorbed CFCs, the frequencies of C-F bond stretching vibrations in gaseous fluorine-containing C2 compounds were analyzed (Table 2). These vibrations occurred in the range of 1000-1400 cm-1. For halogen-substituted ethanes the assignment of absorption bands to localized C-F vibrations as well as symmetric and asymmetric vibrations of CF3 groups is a matter of convention because the real normal vibrations of molecules corresponding to those bands can include an appreciable admixture of other bond vibrations.16 The information presented in Table 2 indicates that the stretching vibrational frequencies of isolated C-F bonds are localized in a range of 1095-1110 cm-1. As for the CF3 group vibrations, the variations in the frequency ranges of symmetric and asymmetric vibrational modes for different molecules approached 70 and 50 cm-1, respectively. The frequencies of the symmetric mode were always higher than those of the asymmetric mode, as Berney observed for CF3 groups attached to a carbon atom in the molecule.32 The characteristic feature of IR spectra of compounds adsorbed on metals is the attenuation of particular absorption bands33-37 due to the metal surface selection rule.38 According to this selection rule, the only infrared-active vibrational modes are those that have a component of their oscillating electrical dipole moment perpendicular to the metal surface. This rule has to be taken into account to interpret the IR spectra of species chemisorbed on supported metals because, according to theoretical considerations, the rule applies to curved surfaces of metallic particles with diameters larger than 20 Å.39 By considering the metal selection rule, the trifluoroethylidynes (CF3Ct) formed upon chemisorption of CF3CFCl2 or CF3CCl3 on metallic Pd particles (species 3) can be more definitively assigned. Similar to ethylidyne (CH3-Ct),40-42 the CsC bond is expected to be perpendicular to the metal surface:
For this molecular orientation, the symmetric CF3 stretching mode possesses an oscillating dipole moment normal to the metal surface, while the asymmetric CF3 stretching mode is nearly parallel to the metallic surface. Therefore, with flat metallic surfaces characteristic of single crystals, the asymmetric CF3 stretching mode is IR-inactive and only the symmetric CF3 mode can be observed. For supported metals, in particular for supported Pd, the surface roughness of the metallic particles is associated with deviation from the metal selection rule. Indeed, two absorption bands corresponding to both symmetric and asymmetric vibrations of CH3 groups have been observed for ethylidynes formed during C2H4 adsorption on Pd/Al2O325,43 and
5608 J. Phys. Chem. B, Vol. 104, No. 23, 2000 Pd/SiO2,20 whereas for ethylidynes on silica-supported Pt only one band of symmetric CH3 stretching vibrations was detected.41,42 Thus, according to the metal surface selection rule, the absorption band of the asymmetric CF3 group stretching vibration of trifluoroethlydines on the surface of supported Pd can be partially or completely suppressed. Hence, the intense band at 1218-1228 cm-1 is attributed to the symmetric CF3 group stretching vibration while the less intense band at 11601165 cm-1 is attributed to the corresponding asymmetric mode. The strong red shift of both the symmetric and asymmetric stretching vibrations of CF3 groups of species 3 (Table 1) relative to those for parent CF3CFCl2 or CF3CCl3 (Table 2) molecules has to be understood. As Table 2 indicates, large, low-frequency shifts of both symmetric and asymmetric stretching vibrations of CF3 groups are characteristic of the CF3CtN and CF3CtCH molecules in which the CF3 group is attached to the sp-hybridized carbon atom. It is possible that for chemisorbed species 3 the R-carbon has the same hybridization. The metal-carbon bond could form by the donation of electrons from the filled σ-orbital of R-carbon to the metal d-orbitals and by back-donation of metal electrons to empty p-orbitals of carbon. A similar mechanism of ethylidyne bonding to the metal for ethylene adsorption on Pt and Pt-Sn alloys supported on silica was established by quantum chemical calculations that employed density functional theory.40 Thus, the strong frequency decrease of CF3 group vibrational modes can be considered as additional evidence of trifluoroethylidyne formation upon chemisorption of CFCs. The spectrum of the chemisorbed species 1 consists of three bands centered at 1065-1077, ∼1190-1200, and 1232 cm-1. The low-frequency 1065-1077 cm-1 is assigned to the CsF vibration of the R-carbon of carbene. As this mode is IR-active, there is a nonzero component of the oscillating dipole moment normal to the metallic surface. For different substituted ethanes containing a single CsF bond, the variation of frequency of CsF vibration ranges within 1095-1110 cm-1 (Table 2). For the CF3CFd species the frequency is 20 cm-1 less than the lowest limit. This may indicate a strong modification of the electronic structure of the R-carbon of carbene that results in the weakening of the CsF bond. The possible orientation of the carbene-like species on a metallic Pd surface is shown below:
In this structure all C-F bond vibrations are IR-active. Similar to trifluoroethylidyne, the bands at 1230-1234 and 1190-1200 cm-1 are attributed to symmetric and asymmetric C-F bond stretching vibrations of the CF3 group. The red shift of both the symmetric and asymmetric CF3 vibrations indicates a strong modification of electronic structure of the R-carbon of carbene similar to that of ethylidyne. This is expected for fluorocarbenes in a singlet electronic state where the mechanism of metalcarbon bond formation also includes a direct electron pair donation from the occupied σ-orbital of the R-carbon to the metal and the back-donation of metal electrons to the empty p-orbital of carbon. As mentioned in the Results section, during evacuation of the catalyst wafer following CF3CFCl2 adsorption, the intensity of the 1188 cm-1 band increased while the intensities of the bands attributed to species 1 decreased. One possible explanation
Borovkov is that the carbene species slowly transforms into species 2 at room temperature. The band at 1188 cm-1 cannot be assigned to a -CF3 adsorbed species produced by C-C bond cleavage. Indeed, on single crystals surfaces such as Ag(111),35 Pt(111),44 Ni(100),45 and Ni(111),46 the spectrum of an adsorbed CF3 consisted of a single absorption band in the range 1050-1080 cm-1, corresponding to the symmetric vibration of CF3 fragment. Those frequencies were close to that of a symmetric mode of a gas-phase CF3 radical, which was equal to 1087 cm-1.47 Because the bond between the fluorine and R-carbon atoms in tetrafluoroethylidene weakens, these species may transform into ethylidynes by abstracting the F atom from the R-carbon. In this case it is unlikely that the spectral characteristics of the ethylidynes formed by this mechanism are significantly different from those of ethylidynes generated directly from CFCs. Therefore, the 1188 cm-1 band, the maximum position of which is 30 cm-1 lower than the frequency of the symmetric vibrational mode of CF3 group of trifluoroethylidynes, may be attributed to the asymmetric stretching vibration of the CF3 group. The absence of a band corresponding to the symmetric mode indicates that the orientation of the CsC bond of these species is parallel to the metallic surface. Perhaps these species are formed on the steps of the metallic surface from the carbene species that predominantly occupy the edges of the metal crystallites:
Species 2 are also hydrogenated, as indicated in Figure 7. However, their low concentration did not permit detection of the products of this reaction to confirm or reject the proposed ethylidyne structure. Conclusion (i) Adsorption of CF3CFCl2 on the alumina-supported Pd catalyst resulted in the formation of three species on the metal surface. They are characterized by absorption bands at 1232, 1200-1190, 1077-1065 cm-1 (species 1), 1190-1188 cm-1 (species 2), and 1228-1218, 1165-1160 cm-1 (species 3). (ii) Species 3 predominantly formed on the surface of reduced fresh catalyst, while species 1 was formed at the expense of species 3 after preexposure of reduced fresh catalyst to the CF3CFCl2 + H2 mixture at 473 K followed by high-temperature reduction with H2. (iii) All of the species readily hydrogenate at room temperature. CF3CFCl2 and CF3CH3 formed from species 1 and species 3, respectively. (iv) Bands at 1232, 1200-1190, and 1077-1065 cm-1 of species 1 are assigned to the symmetric and asymmetric vibrations of the CF3 group, the vibration of the CsF bond at the R-carbon of the adsorbed tetrafluoroethylidene (CF3CFd). The bands at 1228-1218 (species 3) are assigned to the symmetric vibration, while the bands at 1165-1160 (species 3) and 1190-1188 (species 2) are assigned to the asymmetric vibrations of the CF3 group of trifluoroethylidyne (CF3Cd) adsorbed on the steps (species 2) and terraces (species 3) of the Pd surface. The absence of the band of symmetric vibration of the CF3 group in the spectra of species 2 is likely due to the suppression of this band according to the metal surface selection rule. (v) Species 1 is slowly transformed into species 2.
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