J. Phys. Chem. 1995, 99, 1498-1504
1498
Reaction of Trifluoromethyl Iodide on Ni( 100) K. B. Myli and V. H. Grassian" Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received: June 22, 1994; In Final Form: October 11, 1994@
We have studied the surface chemistry of trifluoromethyl iodide adsorbed on Ni( 100) under ultrahigh-vacuum conditions. Temperature-programmed desorption and reflection absorption infrared spectroscopy were used to detennine gas-phase products and species formed at the surface. Several reaction products were found from trifluoromethyl iodide dissociation on Ni(100). Iodine atoms and nickel fluoride, NiF2, desorb at high temperatures near 1000 and 800 K, respectively. At high coverages, carbon-containing species desorb from the surface as well. CF31 and CF3 desorb at 168/136 K (mono/multilayer) and 316 K, respectively. In the presence of background hydrogen, HF and C H ~ F are Z also detected in temperature-programmed desorption at 178 and 235 K, respectively. The infrared spectra of multilayer, monolayer, and submonolayer coverages of CF31on the surface and adsorbed CF, groups have been measured. The temperature-programmed desorption and infrared data show that both carbon-iodine and carbon-fluorine bonds are readily activated on nickel at low temperatures. It is estimated that approximately 90% of adsorbed CF31 decomposes on Ni(100).
Introduction The surface chemistry of fluorinated hydrocarbons has received much attention in recent years. Gellman and coworkers have used the inductive effect of fluorine substituents to probe the reactivity of alkyl and alkoxide groups adsorbed on Other studies have addressed the fundamental differences between C-H and C-F bond activation on metal surfa~es.4~~ Studies of fluorinated hydrocarbons are also of technological importance and relevant to the areas of lubrication and chemical vapor deposition (CVD). For example, perfluoropolyalkyl ethers can be used as high-temperature lubricants; however, problems related to fluid degradation are common with these lubricant^.^,^ Several studies have been initiated in an attempt to understand the processes which lead to degradation of fluorinated ethers on metal surfaces.8-12 The use of metal complexes with fluorinated acetylacetonate ligands as precursors for chemical vapor deposition has motivated studies of the decomposition of these ligands on metal surfaces to determine more about CVD mechanisms and causes of contamination in the deposited metal ~ v e r l a y e r . ' ~ , ' ~ CF31is an excellent molecule to study in order to learn more about the interaction between metal surfaces and fluorocarbon species. The generation of CF, fragments from the dissociation of carbon-iodine and carbon-fluorine bonds can lead to an understanding of the chemistry of these fragments on the surface. Several studies of CF31adsorption on metal surfaces have been reported in the literature. The weak C-I bond dissociates at low temperatures to yield atomic iodine and trifluoromethyl groups on Ru(OOl),15 Pt( 11l),4s5 Ni( 10O),l6 Ni( I1 1),17 and Ag(lll).18 Iodine desorbs as iodine atoms from Ru, Pt, Ni, and Ag. Carbon-fluorine bond dissociation yields CF2 and fluorine atoms on Pt and Ru surfaces, as well as on Ni surfaces (vide infra). Silver is the only metal studied thus far that does not promote carbon-fluorine bond cleavage. In the present study, we have investigated the surface chemistry of CF3I on Ni( 100). Temperature-programmed desorption (TPD) and reflectance absorption infrared spectroscopy (RAIRS) are used to identify gas-phase products and ~~~~
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absrracrs, January 1, 1995.
0022-365419512099-1498$09.00/0
characterize surface-bound intermediates. Our study shows that CF31 is very reactive on Ni(100). Carbon-halogen bond dissociation is evident by the evolution of atomic iodine and nickel fluoride, NiF2, in TPD. Iodine atoms and NiF2 are the only reaction products at low coverage. At higher coverages, carbon-containing fragments desorb from the surface as well. CF31desorbs at low temperatures near 170 and 136 K for the monolayer and multilayer, respectively, CF3 desorbs at 316 K, and, in the presence of background hydrogen, CH2F2 desorbs at 235 K. Using the kinetic data and known thermodynamic quantities, we have determined an energy diagram for this system.
Experimental Section An ultrahigh-vacuum chamber (UHV) with a typical base pressure of 2 x 10-lo Torr is used in these experiment^.'^ The chamber is equipped with a quadrupole mass spectrometer (UTI1OOC; 0-300 amu mass range) for TPD and as a residual gas analyzer. The quadrupole mass spectrometer (QMS) is mounted on a linear translator and can be positioned to within a few millimeters of the crystal face. A glass cone shield placed over the ionizer region of the QMS serves a twofold purpose. First, molecules desorbing from the face of the crystal can be preferentially detected in TPDS2OSecond, the glass cone greatly reduces, by several orders of magnitude, the number of electrons incident on the sample. In addition, the experiments were done with the sample biased during dosing and TPD to remove the possibility of electron beam chemistry. A cylindrical mirror analyzer (CMA) is used for Auger electron spectroscopy. Both the Auger electron and mass spectrometers are interfaced to a 286-PC for data acquisition and analysis. Currently five mass signals can be monitored simultaneously in TPD. The UHV chamber is also equipped with three dosers and an ion sputter gun. The Ni crystal is attached to a copper block sample holder with tantalum wires. The sample holder is held by a precision translation stage and a differentially pumped rotary drive. The sample can be cooled to -100 K with liquid nitrogen and resistively heated. A chromel-alumel thermocouple spotwelded to the back of the crystal was used to measure the temperature to within f 3 K. A power supply interfaced to a 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 5, 1995 1499
Reaction of Trifluoromethyl Iodide on Ni( 100)
CF,I/ Ni(100)
50
31
25 18
I
0.75
I
100
200
300
400
500
10 0 75
I
600
600
800
Temperature (K) Figure 1. Molecular desorption of cF3I ( d e = 196) from Ni(100) after adsorption at 110 K as a function of exposure: 0.75, 1.0, 2.2, 3.7, and 5.0 langmuirs. There is almost no molecular desorption at low CF31 exposures ( < 2 . 2 langmuirs). At higher exposures, the monolayer desorbs with a desorption rate maximum near 170 K. The multilayer peak grows in at higher exposures and desorbs with a desorption rate maximum near 136 K.
second PC provided a linear temperature ramp from 100 to 1200 K. A heating rate of 2 IUS was used in TPD experiments. The RAIRS measurements are made with a Mattson FT-IR spectrometer (Galaxy 6021). The infrared beam exits the spectrometer and is reflected from a flat gold-coated mirror onto a 10 in. focal length 90" parabolic mirror. The light is focused through a BaF2 viewport onto the Ni(100) surface at an angle of 86" with respect to the surface normal. Light reflected from the Ni surface is directed onto a narrow-band MCT detector by two mirrors. The entire optical path outside the chamber is purged with dry nitrogen. The low-frequency cutoff of the detector is near 725 cm-'. The reported IR spectra have been baseline corrected. The sample is cleaned by 2 kV Ar ion bombardment to remove sulfur and oxygen impurities from the surface. Carbon is removed by heating the sample to 800 K in the presence of 1x Torr of oxygen. After an Auger spectrum is taken to ensure a clean surface, the crystal is dosed by back-filling the chamber with CF31 (PCR Inc.) through a high-precision leak valve. CF31exposures are expressed in langmuirs (1 langmuir =1x Torrs).
Results TPD of CF31 from Ni(100). Molecular desorption of CF31 ( d e = 196) was monitored after adsorption on Ni(100) at 110 K. Figure 1 shows the desorption data as a function of CF31 exposure. No CF31 desorption occurs at low exposures. As the CF31 exposure increases to 2.2 langmuirs, there is a peak with a desorption rate maximum near 170 K and a shoulder near 200 K. At higher exposures, the peak at 170 K shifts by approximately 8 deg to lower temperatures, and a new lowtemperature peak develops with a desorption rate maximum of 136 K. This low-temperature feature does not saturate with increasing exposure of CF31 and is assigned to multilayer desorption. The dissociation of CF31 is evident from the desorption of atomic iodine ( d e = 127) at high temperatures near 1000 K (see Figure 2). At the lowest coverages studied, only iodine desorption is detected in TPD. In Figure 2, iodine desorption
1000
1200
Temperature (K) Figure 2. TPD curves of iodine atoms ( d e = 127) desorption from Ni(100) as a function of CFJ exposure: 0.75, 1.0, 1.8, 2.5, 3.7, and 5.0 langmuirs.
NiF2/Ni(100)
400
600
800
1000
Temperature (K) Figure 3. TPD curves of nickel fluoride, NiF2 ( d e = 96), from Ni(100) as a function of cF3I exposure: 0.5, 1.5, 2.0, 2.5, 2.7, 3.0, 3.5, and 5.0 langmuirs.
traces are shown for several CF31 exposures (0.75-5.0 langmuirs). As the CF31exposure is increased, the temperature of the desorption rate maximum (Tmax)decreases, and the area under the desorption trace increases. At a cF3I exposure of 0.75 langmuir, TmaXis 1035 K. The I+ signal saturates at a CF3I exposure of approximately 5 langmuirs with a Tmax near 980 K. The high-coverage desorption curves show an increasing background signal due to the desorption of CF31from the crystal support wires. (I+ is formed from the fragmentation of CF3I in the QMS ionizer.) Molecular iodine, 1 2 ( d e = 254), was not observed in TPD. Although NiI2 ( d e = 308) could not be detected with o u r QMS, the expected mass fragments, Ni+ ( d e = 58) and NiI+ ( d e = 185), were not present in TPD. Thus, the TPD data show that iodine desorbs from Ni(100) as atomic iodine and not as the nickel iodide, Nib. The Ni( 100) surface activates carbon-fluorine bonds to form nickel fluoride. Figure 3 shows the desorption of NiF2 ( d e = 96) as a function of exposure. At exposures above 1.5 langmuirs, NiF2 desorbs into the gas phase near 790 K. The temperature maximum of the NiF2 peak shifts to higher temperatures with CF31 exposure. At a CF31 exposure of 5.0
1500 J. Phys. Chem., Vol. 99, No. 5, 1995
Myli and Grassian
CF, iNi(100) 170
x5 HF' (m/e
=
20)
200
100
200
300
400
Figure 4. Ions detected in TPD after multilayer adsorption of CFJ on Ni(100) at 110 K; HF' ( d e = 20), CH2F2' ( d e = 52), CF' ( d e = 31), CF2' ( d e = 50), and CF3' ( d e = 69).
600
Temperature (K)
500
Temperature (K)
500
400
300
Figure 5. TPD curves for the CF2' ion channel as a function of CFJ exposure: 1.8, 2.8, 3.3, and 6.4 langmuirs. CF2' is the most intense fragment of the CF3 radical. The CF3 radical desorbs from Ni(100) at CF3I exposures near 3.3 langmuirs and above.
1086
langmuirs, the desorption rate maximum is 810 K. The integrated area of the NiF2 TPD peak levels off near a CF31 exposure of 3 langmuirs. At exposures near 3 langmuirs, CF3 radicals begin to desorb from the surface (vide infra). Several carbon-containing products were also detected in TPD. Included in Figure 4 are the TPD curves for CH2F2 ( d e = 52), CF3 ( d e = 69), CF2 ( d e = 50), and CF ( d e = 31) after adsorbing multilayers (5 langmuirs) of CF31 on Ni(100) at 110 K. The low-temperature peak near 136 K and the shoulder near 168 K in the TPD trace for CF3+, CF2+, and CF+ are attributed to multilayer and monolayer desorption of CF31. These ions follow the expected fragmentation pattem for gasphase cF3I. The next higher temperature peak present in TPD for reaction products that contain carbon is near 235 K in the CH2F2+, CF', and CF2+ channels. The CF+ is the most intense and the CH*F2+ is next in intensity, followed by the CF2+ peak which is of very weak intensity. This ion intensity pattem follows the trend ZCF+> ZCH~F>+> ZCF~- and is consistent with that for C H Z F ~CHzFz . ~ ~ forms at high CF3I exposures near 5 langmuirs. The source of hydrogen in CHzF2 is most likely from background hydrogen in the vacuum chamber. Coadsorption studies of D2 and CF3I lead to the formation of CDzF2. There is one other TPD peak present in the CF3+, CF2+, and CF+ ion channels. The desorption rate maximum of this peak is near 316 K. The ion intensity pattem for this peak is consistent with that for the CF3 radical, where the CF2+ intensity is greater than the intensity of the CF3+ parent i ~ n . ~ Des.'~ orption of CF3 radicals from CF31dissociation on metal surfaces has been observed for Pt(l1 1),4-5Ru(OO1),l5 and Ag( 11l ) . I x Figure 5 shows the desorption curves for CF3 as a function of cF3I exposure. CF2+ ion channel is shown because it is the most intense fragment of CF3. A peak appears in the CF2+ ion channel at a CF31exposure of 3.3 langmuirs. The intensity of this peak grows slightly with increasing coverage, and the temperature of the desorption rate maximum does not shift with increasing coverage. HF ( d e = 20) desorbs with desorption rate maximum near 178 K (Figure 4). The source of hydrogen is again presumed to be from background hydrogen present in the vacuum chamber. The detection of fluorine atoms ( d e = 19) was problematic in this study. Instead of distinct peaks in TPD,
I
1196
1
d
IlliS0
10.75
5.75 4.25 2.75 1.25
1400
I
I
1
I
I
1300
1200
1100
I000
900
800
Wavenumbers
Figure 6. FT-IR spectra of CFJ adsorbed on Ni( 100) at 100 K as a function of exposure: 1.25, 2.75, 4.25, 5.75, and 10.75 langmuirs. A total of 1000 scans were collected for each spectrum at an instrument resolution of 4 cm-I.
there was a F+ background that decreased in intensity during the course of the TPD experiment. The presence of a high fluorine ion background has previously been attributed to efficient electron-stimulated desorption of fluorine from the mass spectrometer grids.I5 Other ion fragments that were monitored but did not show any desorption features include NiF4+, NiF3+, F2+, IF+, C2F6+, C2F5+, C2F4+, C2F3+, CHF3+, and CH3Ff. The C2F4 channel ( d e = 100) also corresponds to the nickel isotope, 62NiF2,and showed a peak near 800 K that mirrored the 5xNiF2peak with the expected mass ratio for the two isotopes.23 RAIRS of CF31 Adsorbed on Ni(100). Figure 6 shows the IR spectra of CF31 adsorbed on Ni(100) at approximately 100 K as a function of exposure. The spectral region extending from 800 to 1400 cm-' is shown in Figure 6 and consists mainly
J. Phys. Chem., Vol. 99, No. 5, 1995 1501
Reaction of Trifluoromethyl Iodide on Ni( 100)
T
1070
\ 0.000s
T
-
370 K
A
A
b
b
/I
T = 280 K
e
I\ I
0
r b
a n E
e
I
I
I
I
I
I
I
l
1600
1500
1400
1300
1200
1'lOO
1000
SO0
1
1600
Figure 7. FT-IR spectra of CF3I on Ni(100) as a function of temperature. Initially, an exposure of 5.0 langmuirs was adsorbed on the surface at T = 100 K. The sample was then warmed to the following temperatures: 190, 275, and 370 K. The sample was cooled back to 100 K before each spectrum was recorded.
of bands associated with the C-F stretching motion. Each spectrum represents 1000 scans collected with an instrument resolution of 4 cm-'. A background spectrum was recorded under identical conditions but for a clean surface. At the lowest exposure, 1.25 langmuirs, there are several broad features observed in the spectrum near 1037, 1069, and 1190 cm-I. As the exposure is increased to 2.75 langmuirs, these bands grow in intensity. Upon further exposure, to 4.25 langmuirs and above, a significant increase in intensity results with two prominent bands near 1086 and 1196 cm-' and a weaker band near 1024 cm-I. These three bands are assigned to multilayer CF31 absorptions. Figure 7 shows the infrared spectrum after warming multilayer exposures to various temperatures. The same sample was used for successive heatings, and a spectrum was recorded after cooling the sample back to 100 K. The infrared spectrum after heating to 190 K shows three absorption bands in the C-F stretching region. Two very broad bands near 1032 and 1104 cm-I and a narrower band near 1067 cm-'. Heating the sample to 275 K results in the disappearance of the two broad peaks, while the peak at 1067 cm-' remains but shifts by 2 cm-' to higher frequencies. This peak disappears completely after heating the sample to 370 K. Figure 8 shows the IR spectra, in the region extending from approximately 700 to 1600 cm-', of CF3I multilayers (10.75 langmuirs) and after heating to 280 K. In this spectral region, an additional feature besides the three bands at 1196, 1086, and 1024 cm-' is observed in the cF3I multilayer spectrum at 742 cm-I. After heating, the IR spectrum shows two peaks at 1070 and 770 cm-I with full width at half-maxima of 14 and 11 cm-I, respectively. The peaks in the 700-800 cm-] region are near the cutoff of the BaF2 windows and the MCT detector. The 742 cm-I peak of the multilayer spectrum is sufficiently intense enough to be easily observed while the 770 cm-' peak is barely detectable. Auger Electron Spectroscopy. In an attempt to estimate the amount of decomposition of CF31on Ni( loo), the peak-to-
1200
1400
1000
800
Wavenumbers
Wavenumbers
Figure 8. FT-IR spectra of multilayers of CFJ (10.75 langmuirs) adsorbed on Ni(100) at 100 K and after heating to 280 K.
peak height ratios of the Auger signal for C(272 eV)/ Ni(848 eV) and I(511 eV)/Ni(848 eV) were measured as a function of temperature. The beam current was kept as low as possible in order to minimize electron beam effects and still get a good signal. Initially, multilayers of CF31were adsorbed on the surface, the sample was then warmed to 140 K to remove the multilayer, and an Auger electron spectrum was recorded. The sample was then warmed to 210 K to desorb the monolayer, and another spectrum was recorded. Another Auger spectrum was recorded after warming to 375 K from 210 K. The C(272 eV)/Ni(848 e v j ratio remains almost constant upon warming to 375 K. This suggests that most of the carbon remains on the surface and that only a small amount desorbs as CF3 and CH2F2. The I(511 eV)/Ni(848 eV) ratio decreased by approximately 10% after desorbing off the monolayer; therefore, 90% of the monolayer dissociates on the surface.
Discussion Surface Reactions of CF31 on Ni(100). The desorption of atomic iodine near 1000 K provides definitive evidence for carbon-iodine bond dissociation on the surface, CF,I(a)
-
CF3(a)
+ I(a)
(1)
The desorption kinetics for atomic iodine are coverage dependent. At low coverages, the activation energy for iodine desorption is 60 kcal/moLZ1 As discussed above, it is estimated that approximately 90% of the CF31monolayer dissociates on the surface. The formation and desorption of NiF2, and in the presence of hydrogen, CH2F2, and HF clearly show that carbon-fluorine bonds are activated on the nickel surface, CF,(a)
-
CF2(a)
+ F(a)
(2)
The CF2 radical must be accommodated on the surface in order for it to react with hydrogen atoms to form CH2F2 which desorbs at 235 K. Fluorine atoms desorb as NiF2 near 800 K or HF at 178 K. The desorption of HF at 178 K indicates that C-F bond activation is occumng at very low temperatures. One
1502 J. Phys. Chem., Vol. 99, No. 5, 1995
Myli and Grassian
TABLE 1: Vibrational Assignment of Adsorbed CFd (in cm-9 mode (symmetry) CF3Ug)" CF3I(a)-Ni( 100) multilayer C-F symmetric stretch (a]) 1076 1086 143 742 C-Fj symmetric deformation (a]) n.0. C-X stretch (al) 284 C-F asymmetric stretch (e) 1185 1196 n.0. C-F3 asymmetric deformation (e) 539 C-X bend (e) 260 n.0. a Reference 25. n.0. = not observed. ~~
~
~
might expect that C-F bond activation is a high-energy process and would be unlikely to occur at such low temperatures. However, it should be noted that in the gas phase the bond dissociation energy of the second C-F bond in gas-phase CF4, Le., for the process CF3 CF2 F, is only 80 kcal/mol as compared to 139 kcal/mol for dissociation of the first C-F bond.24 Recently, an interesting mechanism has been proposed for the formation of CF2 on Pt(ll1). Zhu et al. have suggested that one channel for CF31 decomposition is through the formation of CF2 IF.4 While our data show no evidence for IF formation, we cannot completely rule out such a process. Because no carbon-containing products desorb from the surface at low coverages, all of the CF, fragments must dissociate on the surface (eq 3).
-
+
+
CF,(a)
-
C(a)
+ xF(a)
(3)
At high coverages, there is little change in the peak-to-peak ratio of C(272 eV)/Ni(848 eV) before and after desorption of CHzF2 and CF3. Therefore, the Auger data suggest that even at high coverages a large fraction of the CF3 and CF2 fragments dissociates on Ni(100). A small amount of CF3 desorbs at exposures above 3 langmuirs. The CF3 desorption rate maximum does not shift with increased coverage. This temperature dependence of the desorption rate with coverage indicates that CF3 desorbs directly into the gas phase and is not formed from the recombination reaction of CF, groups and fluorine atoms. As discussed below, the IR data support this mechanism, Le., direct desorption of adsorbed CF3. CF31 desorbs molecularly from Ni(100) as well at high exposures. Multilayer desorption occurs near 136 K and monolayer desorption near 170 K. The IR data support the idea that CF31 desorption is from molecularly adsorbed CF31 and not from the recombination of CF3 and atomic iodine (vide infra). The desorption of HF and CH2F2 implies that hydrogen is playing a role in the surface chemistry of CF31on Ni(100). The fact that CF2 groups are easily hydrogenated on Ni(100) may explain why CF2 fragments desorb from the surface at lower temperatures than CF3. In a recent study by Liu et al. of CF31 on Pt( 11l), it was shown that some adsorbed CF3 could be easily hydrogenated at low temperature^.^ The exact role of adsorbed hydrogen on the chemistry of fluorocarbon fragments is not well understood and is worthy of further investigation. Identification of Adsorbed CF, Fragments. Infrared spectroscopy in conjunction with the TPD data can be used to identify surface-bound species. The infrared spectra obtained at multilayer coverages can be assigned by comparison to gasphase cF3I absorptions and are given in Table 1. The two intense peaks in the multilayer spectrum at 1086 and 1196 cm-I are assigned to the symmetric and asymmetric C-F stretch, vs(C-F) and v,(C-F), respectively, of CF31. The band at 742 cm-' is assigned to the CF3 symmetric deformation mode. The vibrational frequencies of the multilayer absorption bands in Table 1 agree well with the gas-phase values.25 The weaker
~~
~
~~
~~~
CF3I(a)-Ni( 100) monolayer 1037 n.o.b n.0.
1190 n.0. n.0.
band at 1024 cm-' is assigned to a combination band of CF31 corresponding to the C-I stretch plus the CF3 symmetric deformation mode.25 The intensity of this band is probably increased by a Fermi resonance interaction with the C-F symmetric fundamental mode at 1086 cm-I. The bands in the IR spectra recorded at T = 100 K for low exposures, Le., before the formation of the multilayer, can be assigned to two species, CF31 and CF3. The bands near 1037 and 1190 cm-' are assigned to the symmetric and asymmetric stretch of molecularly adsorbed CF3I (Table 1). The nearly 40 cm-' decrease in frequency of the symmetric stretch for CF3I adsorbed in the first layer suggests there is an interaction of at least one of the fluorine atoms and the nickel surface. The C-F-Ni interaction will cause a decrease in the force constant of the C-F stretch. The other peak in the low-temperature spectrum near 1069 cm-' is assigned to the symmetric stretch of CF3 groups. This assignment implies that C-I bond dissociation occurs at temperatures as low as 100 K. As discussed below, further evidence in support of the CF3 assignment comes from spectra taken after heating the sample to higher temperatures. The IR spectrum in the C-F stretching region recorded after warming multilayers of CF31 to 190 K consists of three bands near 1032, 1067, and 1104 cm-I. The feature near 1067 cm-' is assigned to CF3 groups which are also present at lower temperatures. The other two bands appear to belong to the same adsorbed species because they both exhibit similar temperature behavior and disappear after heating to 275 K. Concomitant with the loss of these two peaks in the IR spectrum is the desorption of CH2F2 into the gas phase. Therefore, we attribute the bands at 1032 and 1104 cm-I to adsorbed CF2. The width of the bands assigned to CF2 groups is large, 30-40 cm-I. It is not clear as to the cause of such behavior; however, one possibility is that there may be more than one species contributing to these features. Although most of the monolayer has desorbed by 190 K, there is a shoulder that extends out to 220 K; therefore, there is potential overlap between the bands assigned to CF2 with some adsorbed CF3I. The proposed band assignments for adsorbed CF2 groups are listed in Table 2. Also listed in Table 2 are the frequencies for gas-phase CF2,26RU(CF~)(CO)~(PP~&,~' and C O ~ ( C F ~ ) ( C O ) ~ . ~ * There is good agreement between CF2 ligand frequencies in organometallic model compounds with the bands observed for adsorbed CF2. The similarity between adsorbed CF2 to the dinuclear cobalt complex is especially good and may suggest that CF2 is bonded to two Ni atoms on the surface. The appearance of both the symmetric and asymmetric C-F stretch suggests that the symmetry of adsorbed CF2 is reduced from c2v.
The peak assigned to CF3 at 1067 cm-I remains but shifts to slightly higher frequency after heating the surface to 275 K. As discussed in the Results section, the feature disappears upon warming to 370 K concomitant with the desorption CF3 into the gas phase. Accordingly, the 275 K spectrum in Figure 7 is assigned to CF3 groups adsorbed on the surface (see Table 2).
J. Phys. Chem., Vol. 99, No. 5, 1995 1503
Reaction of Trifluoromethyl Iodide on Ni( 100)
TABLE 2: Vibrational Assignment of Adsorbed CF, Groups ~
CFz mode (symmetry) C-F symmetric stretch (al) CF2 bend (a) C-F asymmetric stretch (b2) CF3 mode (symmetry) C-F symmetric stretch (a,) CF3 symmetric deformation (al) C-F asymmetric stretch (e) CF3 asymmetric deformation (e)
CF2(g)" 1225 667 1114
Ru(CFZ)(CO)Z(PW~~ 1083 n.r/ 980
COZ(CFZ)(CO)~~ 1079 698 1038
CFZ-Ni( 1104 n.o.e 1032
CFdg)f 1090 70 1 1260 n.0.
Mn(CF3)(C0)5g 1063 700 1045 555
W%)(PPh3)zIh 1085 n.r. 1022 n.r.
CF3-Ni( 1OO)d 1069 770 n.0. n.0.
Reference 26. Reference 27. Reference 28. This work. e n.0. = not obseved, n.r. = not reported. f Reference 29. g Reference 30. * Reference 31.
The asymmetric stretch, normally very strong in the gas phase,29 must either be of very weak or zero intensity due to the orientation of CF3 on the surface. Therefore, the IR data suggest that CF3 is of higher symmetry than CF31, which is C, on the surface. (Both AI and E modes are observed.) The local symmetry of the CF3-surface complex is of C3,,, indicating that the molecule is bonded on a top or bridge site. In addition, we may have been able to detect the band associated with the symmetric CF3 deformation of adsorbed CF3, at 770 cm-I. The identification of this peak is somewhat tentative because the frequency is close to the instrumental cutoff. The vibrational frequencies for CF3 adsorbed on Ni( 100) are close to the frequencies observed for this species adsorbed on Pt( 111). The vibrational spectrum of CF3 adsorbed on Pt( 111) has been measured by electron energy loss s p e c t r o ~ c o p yThe .~~~ symmetric C-F stretch at 1060 cm-' for CF3 adsorbed on Pt(ll1) is -10 cm-l lower than that reported here for'CF3 adsorbed on Ni(100). The symmetric deformation mode at 745 cm-' for CF3 adsorbed on Pt( 111) is 25 cm-' lower than that found for CF3 adsorbed on Ni(100). The IR spectrum of adsorbed CF3 is similar to that of CF3 bonded in transition metal complexes. The frequencies of the vibrational modes for CF3 groups adsorbed on Ni(100) and Pt( 11l), the CF3 radical in the gas phase,29and the CF3 ligand in two organometalliccomplexes, Mn(CF3)(C0)530and Pt(CF3)(PPh3)21,31are compared in Table 2. The frequency of the symmetric stretch for adsorbed CF3 is somewhere between the frequency for this motion found in the Mn and Pt complexes. Reaction Energetics. An energy level diagram for the surface reactions of CF31on Ni( 100) is shown in Figure 9. The energy levels were determined from the high coverage TPD data and known thermodynamic quantities.23$24*32-34 Desorption energies were calculated assuming that the desorption process was nonactivated. Also, a preexponential factor of 1 x l O I 3 was assumed.35 From this analysis, the desorption energy for CF3 was determined to be 20 kcavmol. The desorption energy for atomic iodine has been determined previously to be near 60 kcal/mol.2' Because the desorption of CF2 does not occur directly, there is uncertainty in the location of the energy level for CF2. For this reason, this level on the energy level diagram is represented by a thicker line. Although activation barriers have not been measured for carbon-iodine and carbon-fluorine bond dissociation, it appears that these processes occur at low temperatures. The IR data suggest that CF3 groups are present on the surface at 100 K, and desorption o f HF at 170 K provides evidence for lowtemperature C-F bond dissociation. Therefore, although the kinetic data are complex, it appears that some C-I bond dissociation occurs upon adsorption and additional chemistry takes place near 170 K, coincident with the desorption of HF and CF31. In addition, it should be noted that the dependence
,I
Y
I
I
CF, (a) + F (a) + I (a)
Figure 9. Energy level diagram for the surface reactions of CF3I on Ni( 100). Reaction energetics were determined from high coverage kinetic data and known thermodynamic quantities.
of exposure on the presence of various species suggests that the bond dissociation kinetics are coverage dependent.
Conclusions The main conclusions derived from this work are summarized below. 1. CF3I adsorbed on Ni(100) dissociates to form several reactions products including I, NiF2, and CF3. CHzF2 and HF also form in the presence of background hydrogen. It is estimated that 90% of the CF31 monolayer dissociates on Ni( 100). 2. Both carbon-iodine and carbon-fluorine bonds are activated on the nickel surface at temperatures below 200 K. 3. The infrared spectra of adsorbed CF3 and CF2 groups have been assigned by direct comparison to model organometallic compounds. The agreement between the surface and the molecular species is quite good. 4. The reaction energetics for this system have been determined from the kinetic data along with known thermodynamic quantities.
Acknowledgment. Support of this work was provided by the National Science Foundation and the Carver Scientific Research Initiative Grant Program. References and Notes (1) Gellman, A. J.; Dai, Q. J . Am. Chem. SOC. 1993, 115, 714. (2) Forbes, J. G.; Gellman, A. J. J . Am. Chem. SOC. 1993, 115, 6277. (3) Dai, Q.; Gellman, A. J. J . Phys. Chem. 1993, 97, 10783.
1504 J. Phys. Chem., Vol. 99, No. 5, 1995 (4) Liu, Z.-M.; Zhou, X.-L.; Kiss, J.; White, J. M. Surf. Sei. 1993, 286, 233. ( 5 ) Armentrout, D. D.; Grassian, V. H. Langmuir 1994, 10, 2071. (6) Paciorek, K. J. L.; Kratzer, R. H.; Kaufman, J.; Nakahara, J. H. J . Appl. Polym. Sci. 1979, 24, 1397. (7) Chandler, W. L.; Lloyd, L. B.: Farrow, M. M.; Bumham, R. K.; Eyring, E. M. Corrosion 1980, 36, 152. (8) Walczak, M. M.; Thiel, P. A. Surf. Sci. 1989, 224, 425. (9) John P. J.; Liang, J. J . Vac. Sei. Technol. 1994, A I 2 , 199. (10) Napier, M. E.; Stair, P. C. J. Vac. Sci. Technol. 1991, A9, 649. (11) Dekoven, B. M.; Meyers, G. F. J . Vac. Sci. Technol. 1991, A9, 2570. (12) Mori, S . ; Morales, W. Trib. Trans. 1990, 33, 325. (13) Girolami, G. S.; Jeffries, P. M.; Dubois, L. H. J . Am. Chem. SOC. 1993, 115, 1015. (14) Parmeter, J. E. J . Phys. Chem. 1993, 97, 11530. (15) Dyer, J. S.; Thiel, P. A. Surf. Sci. 1990, 238, 169. (16) Jones. R. G.; Singh, N. K. Vacuum 1988, 38, 213. (17) Myli, K. B.; Grassian, V. H. Submitted to J . Phys. Chem. (18) Castro, M. E.: Pressley, L. A,; Kiss, J.; Pylant, E. D.; Jo, S. K.; Zhou, X.-L.; White. J. M. J . Phys. Chem. 1993, 97, 8476. (19) Myli, K. B.; Grassian. V. H. J . Phys. Chem. 1994, 98, 6237. (20) Feulner, P.; Menzel, D. J . Vuc. Sci. Technol. 1980, 17, 662. (21) Myli, K. B.; Grassian, V. H. Unpublished results. (22) Registn. of Mass Spectra Data; Stenhagen, E., Abrahamsson, S., McLafferty. F. W., Eds.; Wiley-Interscience: New York. 1974.
Myli and Grassian (23) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1991. (24) Benson, S. W. J . Phys. Chem. 1969, 73, 952. (25) McGee, P. R.; Cleveland, F. F.; Meister, A. G.; Decker, C. E.; Miller, S. I. J. Chem. Phys. 1953, 21, 242. (26) Jacox, M. E. J . Phys. Chem. Ref. Data 1984, 13, 945. (27) Clark, G. R.; Hoskins, S . V.; Jones, T. C.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1983, 719. (28) Seel, F.; Flaccos, R.-D. J . Fluorine Chem. 1978, 12, 81. (29) Carlson, G. A.; Pimentel, G. C. J . Chem. Phys. 1966, 44, 4053. (30) Cotton, F. A.: Wing, R. M. J . Organomet. Chem. 1967, 9, 511. (31) Johnson, M. P. Inorg. Chim. Acta 1969, 3 , 232. (32) Thermodynamic Properties of Individual Substances; Gurvich, L. V., Veyts, J. V., Alcock, C. B., Eds.; Hemisphere Publishing: New York, 1991. (33) Cottrell, T. L. The Strengths of Chemical Bonds; Buttersworth Scientific Publications: London, 1958. (34) Barin, I. In Thermochemical Data of Pure Substances; Egel, H. F., Dyllick-Brezinger, C., Eds.; VCH Publishing: Cambndge, 1989: Part 11.
(35) Redhead, P. A. Vacuum 1962, 12, 203. JP9415727
