Langmuir 1994,10, 2640-2646
2640
Adsorption and Thermal Decomposition of Propyl Iodides on Ni(100) Surfaces Sariwan Tjandra and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521 Received December 13, 1993@ The thermal decomposition of 1-and 2-propyl iodides on Ni(100) was studied by using temperature programmed desorption (TDP),X-ray photoelectron (XPS), and static secondary ion mass (SSIMS) spectroscopies. Both compounds adsorb molecularly below 100 K but decompose between 100 and 180 K via the dissociation of the C-I bond, a reaction that requires an activation energy of only about 2 kcaymol and that yields propyl moieties and iodine atoms on the surface. At low coverages (1langmuir) the propyl groups generated this way undergo total decomposition to carbon and hydrogen (which desorbs at 350 K), but at higher coverages (3 langmuirs and above) significant amounts ofboth propylene and propane desorb from the surface as well. Isotope labeling experiments indicate that most of the propane and propylene detected in the gas phase form simultaneously above 150 K as the result of reductive and /3-hydride elimination steps, respectively, but some low temperature disproportionation is observed as well, a process most likely related to the surface chemistry of the propyl iodides themselves. No C-C coupling reactions take place on this nickel surface under the conditions of the experiments. 1. Introduction
2. Experimental Section
Recent studies have shown that alkyl halides, particularly the iodides, decompose readily on transition metals to yield alkyl species and iodine atoms on the s u ~ f a c e . l - ~ This observation has led to the use of such compounds as precursors for the preparation of alkyl intermediates as a way to explore their stability and reactivity in relation to their role in catalytic reactions such as Fischer-Tropsch synthesis and H-D exchange.8 As part of our continuing program on the study of the thermal reactions of alkyl iodides on transition-metal s ~ r f a c e s , ~ -in l ' this paper we report temperature programmed desorption (TPD), X-ray photoelectron (XPS), and static secondary ion mass (SSIMS) spectroscopy studies on the adsorption and decomposition of propyl iodides on Ni(100). Both 1-and 2-propyl iodides adsorb molecularly at 100 K, but the C-I bonds break below 180 K, and propyl species and iodine atoms form on the surface. At low coverages the propyl groups decompose to surface carbon and hydrogen atoms, but at high coverages significant amounts of propylene and propane desorb from this surface as well. We show that propylene is mostly formed by /3-hydride elimination from propyl groups, whereas propane is generated by recombination of propyl groups with surface hydrogen. @
Abstract published inAdvanceACSAbstracts, August 1,1994.
(1) Zaera, F. Acc. Chem. Res. 1992,25, 260. (2) Zhou, X.-L.;White, J. M. Surf. Sci. 1988, 194, 438. (3) Zhou, X.-L.;Solymosi,F.; Blass, P. M.; Cannon, K. C.; White, J. M. Su$. Sci. 1989,219, 294. (4) Chiang, C.-M.; Wentlaff, T. H.; Bent, B. E. J.Phys. Chem. 1992, 96, 1836. (5)Zhou, X.-L.;White, J. M. Catal. Lett. 1969,2, 375. (6)Jenks, C. J.; Chiang, C.-M.;Bent, B. E. J . A m . Chem. SOC.1991, 113, 6308. (7) Henderson, M. A,; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L325. (8) Zaera, F. J. Mol. Catal. 1994, 86, 221. (9)Tjandra, S.; Zaera, F. Langmuir 1992, 8, 2090. (10)Tjandra, S.; Zaera, F. J . Am. Chem. SOC.1992, 114, 10645. (11)Zaera, F. Surf. Sci. 1992,262, 335. (12) Zaera, F. Surf. Sci. 1989,219, 453. (13) Zaera, F. J . Am. Chem. SOC.1989,111, 8744. (14) Zaera, F.; Hoffman, H. J . Phys. Chem. 1991, 95, 6297. (15)Jenks, C. J.;Bent, B. E.; Bernstein,N.; Zaera, F. J.Am. Chen. SOC.1993, 115, 308. (16) Tjandra, S.; Zaera, F. J . Vac. Sci. Technol. A 1992, 10, 404. (17) Zaera, F.; H o h a n n , H.; Griffiths, P. R. J . Electron Spectrosc. Relat. Phenom. 1990, 54/55, 705.
0743-7463/94/2410-2640$04.50/0
The ultrahigh vacuum (UHV) chamber used in these studies has a base pressure below 1 x 10-10 Torr and is equipped with instrumentation for temperature programmed desorption(TPD), X-ray photoelectron (XPS),static secondary ion mass (SSIMS), Auger electron (AES),and ion scattering (ISS)spectroscopies.9J2 Thermal desorption spectra were obtained by recordingthe mass spectrometry signal of up to ten different molecular fragments simultaneously in a single experiment using an interfaced computer and later deconvolving the cracking pattern of the appropriate compounds from the raw data. A heating rate of approximately 10 Ws was used in the TPD runs. XPS spectra were taken by using a hemispherical electron energy analyzer set to a resolution of about 1.2 eV full width at half maximum and calibrated using binding energy values for the Pt 4f7/2 and Cu 2p3/2 signals of 70.9 and 932.4 eV, respectively.12 The C-I bond scission kinetics was measured by following the changes in the iodine X P S signal intensity at 620.5 eV as a function of time while heating the surface to a constant preset temperature.9J6 Finally,the SSIMSspectra were taken with ion current densities of about 20 nA/cm2 to prevent significant surface damage, and the secondary ions were filtered with a Bessel box tuned to a kinetic energy of 5.0 k 3.5 eV and then analyzed by using the mass quadrupole and counting electronics together with an interfaced computer. The nickel single crystal was cut and polished in the (100) orientation using standard procedures, and mounted in a manipulator capable of on axis rotation and translation in all three dimensions. The crystal could be cooled down to 90 K in less than 5 min by using a liquid nitrogen reservoir, and heated resistively up to 1500 K. The temperature was monitored by a chromel-alumel thermocouple spot-welded to the back of the crystal. Cleaning of the sample was done by oxygen treatment and argon ion sputtering cycles followed by annealing to about 1300Kuntilno impurities were detected by either Auger electron or X-ray photoelectron spectroscopies. The normal 1- and 2-propyl iodides (99% and 98% purity, respectively) were obtained from Alfa Products, and the deuterated propyl iodides(CH~CDZCHZI, CH~CHZCDZI, CD~CDZCDZI, CD3CHICD3, and CD3CDICD3) were purchased from MSD Isotopes. Because of the ease with which these compounds decompose, they were periodically subjected to several freezepump-thaw cycles, and were analyzed by mass spectrometry before use. Surface exposures, done by backfillingofthe vacuum Torrs); chamber,are reported in langmuirs (1langmuir = 1x the partial pressures were not corrected for differences in ion gauge sensitivities.
0 1994 American Chemical Society
Langmuir, Vol. 10, No. 8, 1994 2641
Propyl Iodides on Ni(lO0) Surfaces C3H7i / Nif100)TPD c) C3H8
b) C3H6
L i-^i-0
ikl' c
400 800 0 Temperature' I K
400
800
Figure 1. Hydrogen (a),propylene (b),propane (c), and propyl iodide (d) TPD spectra from C3H7I adsorbed on Ni(100) as a function of initial exposure. Dosing was done at 90 K, and heating rates of about 10 Ws were used.
3. Results The thermal chemistry of both 1-and 2-propyl iodides on Ni(100)surfaces was first characterized by temperature programmed desorption (TPD) spectroscopy. The only desorbing products detected in either case where hydrogen, propylene, propane, and the original molecule; no other gaseous hydrocarbons are produced during the thermal decomposition of those compounds on the nickel surface. The dependence of the TPD spectra for each of the four species on initial coverage is summarized for 1-C3H71in Figure 1. A single peak is observed at 350 K in the hydrogen desorption traces after a 1-langmuirC3H7I dose, the same as when a comparable amount of hydrogen is adsorbed directly on the clean nickel surface,l8but as the propyl iodide exposure is increased to 3 langmuirs or above, that initial peak splits into two new features about 320 and 380 K (Figure la). Above 5 langmuirs most of the hydrogen desorption shifts to even higher temperatures (the main peak is centered around 390 K after a 8-langmuir C3H7I exposure), and a shoulder also develops a t about 435 K. These observations suggest that while some of the hydrogen desorption may be limited by recombination of hydrogen atoms on the surface a t all coverages (the reaction responsible for the low temperature peak in the TPD), molecular decomposition may be somewhat inhibited a t higher coverages, and one or more hydrocarbon intermediates may form a t reasonably high temperatures. Figure l b displays the propylene TPD spectra obtained for 1-C3H71a s a function of initial exposure. After low propyl iodide doses no propylene desorption is detected at all, only after exposures of 3 langmuirs or more is significant olefin production observed. Also, this desorption occurs initially in two stages (about 180 and 250 K), but the peak a t 250 K disappears as the surface coverage increases at the expense of the growth of two new features around 115 and 150 K. In order to compare the desorption kinetics of the propylene produced by C3Hd decomposition to that of propylene adsorbed alone on Ni(1001, molecular TPD spectra were also obtained for propylene adsorbed on a clean nickel surface (Figure 2). It was found that the thermal activation of C3H6 a t low coverages (0.5langmuir exposure) leads only to hydrogen desorption around 345 K (data not shown), the same as when dosing equivalent amounts of hydrogen or propyl iodide on the clean surface, and that as the propylene exposure is increased the Hz TPD main peak grows and shifts to about 350 K while additional shoulders develop at about 310 and 385 K. The propylene TPD spectra for this system first display only one main peak about 230 K (1langmuir), but then a second feature grows about 160 K (1.5 langmuirs), and finally a third one develops around 110 K as multilayer condensation takes place (2.0 langmuirs and above). The main observation of interest for the study presented in this paper is that the TPD traces (18) Habenschaden, E.;Kupper, J. Surf. Sci. 1984, 138, L147.
0.5 0
400 Temperature / K
800
Figure 2. Molecular TPD spectra for propylene adsorbed on Ni(100) as a function of initial exposure.
-2 0.4
s . P)
8
8 0.2
8
0.0 0
2
4 6 8 Exposure / L
1
0
Figure 3. Hydrogen, propylene, and propane absolute TPD yields as a function of initial exposure from 1-propyl iodide adsorbed on Ni(100).
for 1.5 langmuirs of propylene and 3 langmuirs of propyl iodide are quite similar, indicating that in both cases the kinetics is most likely limited by the desorption of the olefin. Figure ICsummarizes the coverage dependence of the propane TPD spectra for 1-C3H71on Ni(100) surfaces. As in the case of propylene, no propane is detected a t low C3H7I doses, only above 4 langmuirs does a sharp peak start to grow a t about 110 K along with a second broad feature around 170 K. As the exposure is increased further, the 110 K peak decreases steadily in size, while the high temperature broad feature becomes larger and sharper. Finally, Figure Id displays TPD spectra for C3H7I molecular desorption, which starts only around 8 langmuirs of C3H71and is first seen a s a sharp peak at about 180 K and then a s a second peak that grows indefinitely a t 150 K starting at 10 langmuirs, after monolayer saturation. The second peak is due to multilayer desorption; a leading edge analysis of that feature gives a value for the sublimation energy of condensedpropyl iodide of about 11kcaymol, close to that reported in 1 i t e r a t ~ r e . l ~ The dependence of the hydrogen, propylene, and propane desorption yields on the propyl iodide exposures is summarized in Figure 3. The hydrogen TPD signal was calibrated by comparing with that from saturation Hz on a clean surface, which correspond to one monolayer of atomic hydrogen (that is, one hydrogen atom per surface (19) CRC Handbook of Chemistry and Physics, 66th ed.; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press: Boca Raton, FL, 1985.
Tjandra and Zaera
2642 Langmuir, Vol. 10, No. 8, 1994 CH3CH2CD21/Ni(100) TPD
v
o
400
aoo o
400
aoo o
400
800 dl CHJCD-CHZ
-o 400 Temperature I K
aoo
Figure 4. Hz (a), HD (b), Dz (c), CHsCH=CDz (d), CH3CH2CDzH (e), CH3CH2CD3 (0, and CH3CHzCDzI (g) TPD spectra from CH~CH~CDZI adsorbed on Ni(100) as a function of initial exposure.
o
400
aoo
IO
Figure 5. Hz (a),HD (b),D2 (c),CH3CD=CH2 (d),CHsCDzCH3 (0, and CH3CDHCH3 (g) TPD spectra from (e), CH~CD~CHZD CH3CDzCHzI dosed on Ni(100)as a function of initial coverage.
initial proply iodide, which sets a lower limit for the nickel atom).18 The yields for propylene and propane, on efficiency of reactions other than total decomposition a t the other hand, were calculated by using both independent 95%. mass spectrometry calibration experiments and a mass Only three types of hydrocarbons are produced from balance argument based on a n approximately constant the thermal decomposition of CH3CHzCDzI on this nickel sticking coefficient for propyl iodide up to s a t u r a t i ~ n . ~ ~surface, ~~ namely, CHBCH=CDz and CH~CH~CDZH, which Figure 3 shows that the hydrogen desorption yield first desorb in significant amounts, and CH3CHzCD3,which is increases with increasing propyl iodide exposure, until seen only a t high coverages and in very small quantities. reaching a maximum a t about 3 langmuirs of C3H71, and The propylene-1,I-Clz(CH3CHECD~)is produced only after then decreases at higher exposures. Both propylene and doses above 1langmuir, at which point the TPD spectra propane desorption are insignificant below 2 langmuirs, display roughly the same peak shapes and temperatures but then the propylene yield increases until maximizing as those for normal propylene desorption from C3H71 a t about 4.5 langmuirs of C3H71,at which point it decreases (Figure 4d). Also, this is the only olefin produced in this slightly, while the production of propane increases steadily system, so the dehydrogenation ofpropyl iodide must occur with exposures up to saturation. The maximum yield for by the loss ofboth the iodine atom and a normal hydrogen, a result that rules out the possibility of any elimination propylene is about 2 times that for propane. Finally, the steps from the a position. The desorption of the main saturation coverage for propyl iodide is estimated from hydrogenated product that forms from CH~CH~CDZI these data to be a little over 0.3 monolayer. decomposition, CH3CHzCD2H,also behaves in a similar In order to learn more about the mechanism for the fashion to that of C3H8 from C3H7I (Figure 4e), but, in formation of the products that result from the adsorption addition, a small amount of C H ~ C H Z C is D ~produced a t and decompositionof 1-propyliodide on Ni(100), additional high coverages (Figure 40. Molecular CH3CHzCD21 TPD experiments were done using partially deuterated desorbs above 8 langmuirs (Figure 4g). propyl iodides. Besides molecular desorption, CH3CHzThe TPD results for the decomposition of CH~CDZCHZI CD2I decomposition only produces Hz, HD, Dz, CH3on Ni(100), although a bit more complex to interpret, are CH=CD2, and CH3CHzCDzH; Figure 4 summarizes the consistent with the data from the other compounds. Hz, coverage dependence ofthe TPD spectra for those species. HD, D2, CH&D=CHZ, CH3CDzCH3, CH~CD~CHZD, and Hydrogen desorption behaves in almost the same way as CHsCDHCH3 are all produced in this case. The coverage in the case of normal propyl iodide, namely, it starts with dependence of the hydrogen desorption (Hz,HD, and Dz), a single peak about 350 K after a 1 langmuir dose and shown in Figure 5a-c, resembles that seen for CH3CH2then first splits into two and subsequently shifts to higher CDzI, except that more HD and Dz desorb a t high coverages temperatures with increasing coverages (Figure 4a); the (above 5 langmuirs) in this case, most likely because total hydrogen yield doubles in going from 1to 3 langmuirs propylene formation from CH3CDzCH21involves the loss and then decreases back to about the initial amount after of a deuterium atom a t the p position. The propylene a 9 langmuir dose. The HD peak for 1langmuir of CH3(CH&D=CH2) TPD traces also exhibit approximately the CHzCDzI is centered at 360 K (Figure 4b), a temperature same shapes and trends as those for CH&H=CHz from slightly higher than that for HZ(perhaps because of an l-c~H.11 and for CH&H=CDz from CH~CHZCD~I, namely, isotope effect), and grows first with increasing CH3CH2a small peak develops a t 250 K around 3 langmuirs CDzI exposure but then drops drastically at higher together with a lower temperature shoulder which disapcoverages (above 3 langmuirs), and only a small amount pears a t the expense of the formation of new features of D2 desorbs in these experiments and only a t low around 120 and 150 K a t hgher coverages (Figure 5d). No coverages (Figure 4c);no Dz is detected a t all a t saturation. proplyene with any other isotopic composition was seen The combined areas under the HD and Dz TPD peaks a t in these experiments, a result that establishes that the saturation correspond to no more than about 5% of the dehydrogenation of propyl iodide occurs through the elimination of a deuterium from the p position exclusively. (20)Tjandra, S.; Zaera, F. Surf. Sci. 1993,289, 255. The propane produced by CH~CD~CHZI decomposition is
Propyl Iodides on Ni(lO0) Surfaces
Langmuir, Vol. 10,No. 8, 1994 2643
I
CH3CHICH31NillOO) TPD C)
61 CD~CHICDJ/N~(~OO) TPD
C3He
6L CD3CDlCD3/Ni(lOOt TPD
Lo
I
Eap/L
P-L
j
0
7.0 5.0
:::1
400 800 0 Temperature I K
400
81
Figure 6. HZ(a),CH&H=CHz (b), and CH3CH2CH3 (c) TPD spectra from 2-propyl iodide on Ni(100)as a function of initial exposure.
mostly in the form of either C H ~ C D Z C or H ~CH~CDZCHZD, and therefore must form via the recombination of the propyl species (CH~CDZCHZ) with either H or D atoms adsorbed on the surface (Figure 5e,fj; the desorption of both those molecules follow basically the same behavior as that of the propane (CH~CHZCDZH and C H ~ C H Z C D ~ ) generated from CH~CHZCDZI, except that in the former case the relative yield for normal hydrogen to deuterium incorporation (CH~CDZCH~ to CH~CDZCHZD formation) is slightly lower. An additional small amount of CH3CDHCH3 desorption is also seen a t high temperatures (220 K) and high coverages (larger than 3 langmuirs), presumably as the result of the hydrogenation of proplyene (CH3CD=CHz)with normal hydrogen (Figure 5g). Similar results were obtained for fully deuterated 1-propyl iodide (data not shown). The TPD experiments for 2-propyl iodide on the same Ni(100) surface are shown in Figure 6. The HZ TPD spectrum for 1langmuir of CH3CHICH3 displays a large peak at 350 K and a small shoulder a t about 420 K(Figure 6a). A second shoulder develops around 320 K at 3 langmuirs, the main peak shifts to 385 K a t 5 langmuirs, and the low temperature shoulder (320 K) disappears while the feature a t 440 K grows to about half the size of the main peak a t saturation. The overall hydrogen yield increases by 30% in going from 1to 5 langmuirs and then decreases back to its original size at 8 langmuirs. Propylene desorption (Figure 6b) starts around 3 langmuirs and exhibits the same behavior as that seen for CH~CHZCHZI, that is, three peaks develop sequentially around 250,170, and 110 K, propylene desorption is again dominated by the kinetics of its desorption. The propane TPD spectra also behave in a similar way to those from 1-propyl iodide (Figure 6c), one main peak developing around 115 K above 5 langmuirs and small shoulders growing around 130-200 K a t higher coverages, until the peak a t 115 Kdecreases in size a t 9 langmuirs to the point of completely disappearing after very large doses. Molecular desorption occurs around 175 Kand after exposurs above 7 langmuirs, slightly below those required for CH3CHzCHzI monolayer saturation. Analogous results were obtianed for CD3CHICD3 and for CD3CDICD3. Figure 7 displays typical TPD spectra obtained for 6 langmuir doses in each case, which corresponds to coverages close to saturation. Hz,HD, and DZare all produced from both compounds, in the case of the fully deuterated compound because the normal hydrogen originates from adsorption from the background gases, which can never be avoided. Normal hydrogen desorbs a t slightly lower temperatures than either HD or Dz, which, since the latter originate from decomposition of the surface moieties, suggests that such decomposition takes place a t least in part at high temperatures, above
CDjCDHCD,
400
800 Temperature I K
Figure 7. TPD spectra from 6 langmuirsof CD3CHICD3 (left) and CD3CDICD3 (right) thermal decomposition on Ni(100). 7L C3H71/Ni(100)
?::::fq
I 3d5/2 XPS vs T
1500
p 619.6
619.4 1w
n
61 2
200 TIK
300
TIK
620 Binding Energy I eV
t 28
Figure 8. Iodine 3dwz XPS spectra from 2 langmuirs of 1-propyl iodide dosed on Ni(100)at 90 K after annealingto the indicated temperatures. The inset summarizes the correspondingchanges in peak maxima position.
350 K. The propylene that desorbs from the labeled 2-propyl iodide decompositonhas in both cases the isotopic compositionexpected for the product of/?-elimination,and is seen in the TPD traces in three peaks around 110,170, and 250 K as in the case of the normal 2-propyl iodide. Most ofthe propane detected have the isotopic composition expected from reductive elimination of the 2-propyl groups with either H or D, but small amounts of the hydrogenated olefins (CD~CHZCDZH and CD3CDHCDzH respectively) are seen as well. The chemistry of 1-propyl iodide on Ni(100) was also studied by using XPS and SSIMS. Figure 8 displays I 3 d ~ zXPS data for 7 langmuirs of CH~CHZCHZI dosed initially a t 90 K and then annealed to the indicated temperatures. While neither the shape nor the area of this peak change significantly by heating of the sample, its binding energy shifts from 620.0 eV a t 100 K to 619.5 eV a t 180 K, a drift associated with the dissociation of the C-I b ~ n d The . ~ ~isothermal ~ ~ rate of this C-I bond scission, which was measured by followingthe changes in the XPS signal intensity a t 620.5 eV as a function of time, increases a t higher temperature as expected (Figure 91, but the kinetics of this step is complex and cannot be easily explained by a simple rate law. Initial rates were estimated from the slope of the kinetic traces right after
2644 Langmuir, Vol. 10,No.8,1994
Tjandra and Zaera
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3L C3H71/Ni(100) C-l Bond Scission Kinetics
3L C3H71/Ni(l00) SSIMS
1
T = 90K
I
40
80 amu
120
160
Figure 11. SSIMS spectra from 3 langmuirs of 1-propyliodide dosed on Ni(100) at 90 K.
0
90
180
270
Z 0
Time I s
Figure 9. Time evolution of the XPS signal intensity at 620.5 eV binding energy (I 3d5,~)for 3 langmuirs of 1-propyl iodide
on Ni(100) while heating to the indicated temperatures. The inset displaysan Arrhenius plot of the temperature dependence of the initial rate constant for the C-I bond scission reaction as calculated from the data shown in this figure. 7L C3H$/Ni( 100)
amu
;bl
C 1s XPS vs T
I
90
150
T/K
210
T/K
200 180
281 285 289 Binding Energy I eV
Figure 10. Carbon 1sXPS spectra from 7 langmuirs of 1-propyl iodide dosed on Ni(100)at 90 K after annealing t o the indicated
temperatures. The inset shows the correspondingchanges in carbon peak area.
the temperature jump a t the 40 s mark and plotted in an Arrhenius fashion in the inset of Figure 9; the slope of the linear fit through the points corresponds to an apparent activation energy of 2 f 2 k ~ a l / m o l . ~The J ~ iodine XPS signal disappears completely after annealing above 850 K, presumably because the iodine desorbs as atoms from the s ~ r f a c e . ~ ? ~ ~ Figure 10 shows the C 1s XPS spectra for 1-C3H71 as a function of annealing temperature. The main carbon peak is centered around 284.7 eV a t 90 K but shifts to lower values as the sample is heated to 160K and decreases in size until disappearing around 200 K. The value for the binding energy obtained a t low temperatures is most likely associated with the presence of molecular propyl iodide on the surface since the C-I bond is still intact ~
~~
~~~~
(21) Jo, S. K.; White, J . M . Surf. Sci. 1992,261,111. (22) Tjandra,S.; Zaera, F . Langmuir 1991,7, 1432.
Figure 12. 90- 110 amu SSIMS spectra for a Ni(100)surface right after dosing 3 langmuirs of C3H7I at 90 K (a) and after annealing (a) to 140 K (b).
under those circumstances,but the shifted signal observed a t 160 K is probably due to a new species, presumably propyl groups. The general decrease in signal intensity with increasing annealing temperature is explained by the formation of both propane and propylene which, according to the TPD results, desorb below 200 K. The shoulder seen in all spectra in the low binding energy side of the main peak may be due to the different environment surrounding the three carbon atoms in the propyl moieties. Figure 11shows a SSIMS spectrum for 3 langmuirs of 1-C3H7I on Ni(100) a t 90 K. The large peaks a t 58 and 60 amu are due to Ni+, and the small signals a t 116,118, and 120 amu to Ni,+; those peaks are also observed in the spectra of the clean surface.22 The additional peaks in the 12-15 amu (CH,+), 24-28 amu (C2HX+), 40-45 amu (C3HX+), and 70-104 (NiC,H,+) amu regions, on the other hand, are all associated with molecular propyl iodide. Figure 12 shows detailed spectra in the 90-110 amu region for the same 3 langmuirs of C3H71both right after dosing a t 90 K (top), and after annealing that surface to 140 K (bottom). The 102 and 103 amu peaks in these spectra, which are associated with the NiC&t6+and NiC3H7+ions, respectively, display different relative intensities depending on the annealing temperature (Figure 13):the NiC3H7+ peak a t 103 amu originates from the ionization of both propyl iodide and propyl fragments on the surface, and decreases steadily in intensity with increasing temperature because of both the desorption and the conversion of the adsorbed propyl iodide. The peak a t 102 amu (NiC3&+), on the other hand, grows first until reaching a maximum at about 140 K, and then almost disappears. The latter behavior is most likely associated with the formation and subsequent decomposition of propylene on the surface. 4. Discussion The combined results from the TPD, XPS, and SSIMS experiments reported here can be used to elucidate the
Propyl Iodides on Ni(100) Surfaces
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Langmuir, Vol. 10, No. 8, 1994 2645
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some of the C-I bond scission at even lower temperatures, most likely involvingthe direct dissociation of propyl iodide 20 SSlMS Signal Ratlos into propyl free radical^.^^,^^ The thermal conversion of the propyl groups that form on the surface after the scission of the C-I bond follow different pathways depending on the initial propyl iodide coverage. At low coverages all propyl groups decompose directly to carbon and hydrogen at low temperatures, and the hydrogen atoms then recombine and desorb around 350 K. As the propyl iodide coverage is increased,however, the relative yield for hydrogen desorption decreases, and the original H2 TPD desorption peak splits into two (at "go i i o ido i i o 2io about 320 and 360 K). This last observation indicates Temperature I K that the low temperature complete dehydrogenation steps Figure 13. SSIMS NiC3H6+ and NiC3H7+ signal intensities, predominant at low coverages now compete with the normalized to the Ni+ signal, for 3 langmuirs of C3H,I dosed formation of a series of carbonaceous species; the interon Ni(100) as a function of annealing temperatures. mediates produced by the new reactions remain on the surface until temperatures high enough so their dehythermal chemistry of propyl iodides on Ni(100) surfaces. drogenation becomes the limiting step in the H2 TPD. In First, given that the I 3d binding energy for the C3H71 fact, the low yields for D2 and HD from C H ~ C H Z C D ~ I monolayer a t 100 K is close to that seen for a condensed decomposition a t high coverages (equivalent to about 5% physisorbed layer, it can be concluded that the adsorption of the initial deuterium in the propyl iodide) indicate that of propyl iodide a t low temperatures is most likely the nondissociative pathways which lead to the formation This conclusion is consistent with results molecular. of propane or propylene dominate the high coverage reported previously for both CH3I and C Z H ~on I the same chemistry (the D atoms originate from propyl decomposis u b ~ t r a t eas, ~well ~ ~as ~ for those of other alkyl halides on tion exclusively). 2-Propyl iodide follows reactivity trends other metals.' Prior studies with HREELS,M R S , and similar to those of l-propyl iodide, that is, only hydrogen XPS have also indicated that the molecularly adsorbed is produced a t low coverages (its yield increases with alkyl iodides usually bond through the iodine atom4,7J7 coveragesup to 5 langmuirs), and hydrocarbon production and that the hydrocarbon chain reorients as the coverage becomes dominant near saturation. is increased; we believe that the same holds true for propyl Further understanding of the mechanism for propylene iodide on Ni(100). In this respect it is interesting to note and propane formation can be obtained by analysis of the that no deuterium adsorbs on nickel surfaces dosed with results from the isotope labeling experiments. For one, as little as 2 langmuirs of C3H71,even though that exposure since CH3CD2CH21decomposition yields CH3CD=CH2 as corresponds to only about 20% of ~ a t u r a t i o na, n~ obser~ well as significant amounts of HD and DZand CH3CHzvation that may be explained by a n adsorption geometry CD2I activation produces only CH&H=CDz and no a t low coverages where the propyl groups lie flat on the significant deuterium, it must be concludedthat propylene surface, blocking a large area and therefore preventing formation occurs by elimination of a hydrogen from the any subsequent hydrogen adsorption; rearrangement after P position of the propyl groups. The propylene produced higher doses set the hydrocarbon chains perpendicular to via this P-hydride elimination desorbs at temperatures the surface and open new adsorption sites for additional which depend on the initial propyl iodide coverage: for 3 propyl iodide molecules (until saturation is reached). langmuirs of 1-C&I the desorption peaks are centered At coverages below 80% of saturation, heating of the a t around 180 and 250 K, as for propylene adsorbed on propyl iodide covered Ni(100) surfaces only induces the a clean surface, but at higher initial exposure the original scission of the C-I bond; only after doses of 8 langmuirs peaks shift to 150 and 180 K and a third small feature or more molecular desorption competes with this decomgrows at about 115 K. Additional propylene forms a t low position reaction (saturation is reached a t about 10 temperatures near saturation, but that may be the result langmuir). The dissociation of at least some of the C-I of dehydrogenation of propyl free radicals formed by direct bonds occurs a t temperatures as low as 110K, as indicated propyl iodide d i ~ s o c i a t i o n .The ~ ~ ~isopropyl ~ groups genby the shift in the 13d binding energy from 620.0 to 619.5 erated from 2-propyl iodide decomposition undergo P-hyeV. The initial apparent activation energy for this step dride elimination as well and yield propylene, the same was estimated to be about 2 kcal/mol, slightly lower than as l-propyl iodide. The fact that the detection ofpropylene that for either methylg or ethylz0iodides (which require is limited in both cases by its desorption sets an upper 3-4 kcal/mol), but since the kinetics observed in the propyl limit for the P-hydride elimination activation energy a t case do not fit any simple rate law, it is not clear what the about 10 kcal/mol,20while the fact that the deuterium (D2 mechanism for this reaction may be, especially since more and HD) desorption from CH3CHzCD21 decompositon than one reaction pathway may be involved in the C-I occurs a t high temperatures indicates that a-dehydrobond breaking process. The main dissociation reaction genation reactions need to overcome a much higher starts around 140-180 K and yields iodine atoms and activation barrier, on the order of 22 kcaVmol or more. propyl groups on the surface: both the shifts in the C 1s The lack of any C3H4D2 desorption from propyl-2,2-dz XPS peak after heating to 160 K and the NiC3H7 + SSIMS iodide indicates that /?-hydrideelimination is preferred signal detected after annealing to 140 K can be taken as over y-hydride elimination as well. evidence for the presence of propyl species on the Ni(100) In regard to the hydrogenation reactions, the experisurface. The fact that the propane that forms in the TPD ments reported here show that propane molecules are is in the form of CH3CHzexperiments for CH~CHZCDZI CDzHalso suggests that the propyl groups (CH~CH~CDZ) produced at high propyl iodide coverages and mostly above 130 K. This propane formation occurs via the reductive are present on the surface a t temperatures lower than elimination of propyl gropus with surface hydrogen atoms, those where the reductive elimination of the propyl groups a reaction that in the case of CH~CDZCHZI produces takes place. A second minor channel is responsible for primarily CH3CD2CH3but also a smaller amount of CH3CDzCHzD;the CH3CDzCH3is produced by recombination (23) Zaera, F.; Tjandra, S. J.Phys. Chem. 1994, 98,3044. 1
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3L l-C3H7l/Ni(l00)
2646 Langmuir, Vol. 10, No. 8, 1994 of CH3CD2CH2 groups with surface hydrogen (from background adsorption), while CH3CD2CH2Dcomes from recombination with the D atoms produced by the #? elimination step discussed above. The small signal detected for CH3CDHCH3 in these experiments at high temperatures originates from hydrogenation of some of the propylene (CH~CD-CHZ)produced at lower temperatures. Similarly, CH~CHZCDZI produces only CH3CH2CDZH,CH3CHICH3 yields normal propane (C~HB), CD3CHICD3 produces CD~CHZCD~, CD3CHDCD3, and CD3CHZCDZH, and CD3CDICD3produces CD3CHDCD3, CD3CD2CD3, and CD3CDHCDzH. A small amount of propane forms in all cases a t low temperatures (about 110 K) around saturation, most likely as a result of a direct conversion of the chemisorbed propyl iodide into gas phase propyl free radicals.23 No couplingproduce such as hexane or 2,3-dimethylbutane ever forms on Ni(100) under the conditions of our experiments. The surface chemistry of propyl iodide has already been studied on a few other metals, specifically on Ag(lll), Al(lOO), and Cu(l10).6,24,25 According to Zhou and White the thermal activation of l-propyl iodide on Ag(ll1) leads to the cleavage of the C-I bond around 100 K and results in the formation of propyl fragments and iodine atoms, as in other surfaces. However, as opposed to the case of nickel, those propyl fragments then recombine around (24) Zhou, X.-L.; White, J. M. J.Phys. Chem. 1991,95,5575. (25)Bent, B. E.;Nuzzo, R.G.; Zegarski, B. R.;Dubois, L. H.J . Am. Chem. SOC.1991,113,1137.
Tjandra and Zaera 200 K to give hexane exclusively; neither propane nor propylene were produced in this surface.24 Propyl iodide was also shown to dissociate above 120 K on Al(100)and to form propyl species,25but the only hydrocarbon desorbing product from that surface is propylene,which comes off a t about 520 K, as on Ni(100), the mechanism for the formation of this olefin involves a @-hydrideelimination pathway. Finally, the thermal chemistry of l-propyl iodide on Cu(110)involves both &hydride elimination from propyl groups to generate propylene a t about 230 K and a small amount of C-C coupling to form hexane.6 It was found here that propyl iodide dissociates on Ni(100) between 100 and 180K to form propyl groups and that the resulting propyl groups readily decompose all the way to carbon and hydrogen a t low coverages (1langmuir) but mainly produce propane and propylene a t high coverages (3 langmuirs or higher). Propylene is formed via a @-hydride elimination step, while most of the propane is produced by the direct reductive elimination of propyl groups with surface hydrogen. A small amount of the alkane seen at high temperatures originates from olefin hydrogenation, but neither H-D exchange nor coupling reactions were seen in this system. The results reported here are also consistent with the surface chemistry observed for other alkyl groups on the same nickel s u r f a ~ e . ~ ~ ~ ~
Acknowledgment. Financial support for this research was provided by a grant from the National Science Foundation (CHE-9222164).
