Langmuir 1992,8, 2090-2097
2090
Methyl Iodide Thermal Reactions When Chemisorbed on Ni( 100) Surfaces Sariwan Tjandra and Francisco Zaera* Department of Chemistry, University of California at Riverside, Riverside, California 92521 Received March 9,1992. In Final Form: June 25,1992 The thermal reactivity of methyl iodide on Ni(100)surfaceahas been studied by usingthermal programmed desorption (TPD),X-ray photoelectron (XPS), and static secondary ion mass (SSIMS)spectroscopies. TPD results indicate that only methane and hydrogen desorb at coverages below saturation, but physisorbed methyl iodide was also observed above saturation. Both I 3d and C 1s XPS spectra suggest that methyl iodide adsorption below 100K is mostly molecular and that the carbon-iodine bond breaks between 120and 160 K. The activation energy for this bond scission step was determined to be about 3.6 kcaVmol by isothermal XPS experiments, and SSIMS studies show that the surface species resulting from that reaction are methyl groups. Those methyl moieties then hydrogenate below 300 K to form methane via a complex mechanism that involves a direct reductive elimination step.
Introduction Recent studies in our laboratory and in a few others have focused on the adsorption and subsequent thermal decomposition of alkyl halides on a variety of metal surfaces.'-l6 One property of those halides, particularly of alkyl iodides, is that they dissociate easily upon thermal activation on metal surfaces to generate alkyl fragments. There is a great deal of interest in carrying out detailed investigations on the reactivity of those hydrocarbon moieties because of their direct relation with more complex catalytic reactions such as those involved in oil refining and in methanati~n.l'-~O In the present article we report results from thermal programmed desorption (TPD), X-ray photoelectron (XPS), and static secondary ion mass (SSIMS)spectroscopic studies on the adsorption and thermal decomposition of methyl iodide on (100)oriented nickel surfaces. The main purpose of this research was to measure the kinetics of the breaking of the C-I bond in methyl iodide and to characterize the surface species that result from that reaction. We determined that the initial bond scission occurs below 160 K and yields methyl moieties on the surface. Those methyl fragments then either incorporate a hydrogen atom and form methane or dehydrogenate to carbon and hydrogen on the surface. ~
~~
(1)Zaera, F. Surf. Sci. 1989,219,453. (2)Zaera, F. J. Am. Chem. SOC.1989, 111, 8744. (3)Zhou, X.-L.; White, J. M. Surf. Sci. 1991,241,244. (4)Zhou, X.-L.; White, J. M. Surf. Sci. 1991,241,259. (5)Zhou, X.-L.; White, J. M. Surf. Sci. 1991,241,270. (6)Benziger, J. B.; Madix, R. J. J. Catal. 1980,65,49. (7) Chen, J. G.; Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J.Am. Chem. SOC.1987,109,1726. (8)Zhou, X.-L.; White, J. M. Chem. Phys. Lett. 1987,142,376. (9)Zhou, X.-L.; White, J. M. Surf. Sci. 1988,194,438. (10)Magrini, K. A,; Gebhard, S. C.; Koel, B. E.; Falconer, J. L. Surf. Sci. 1991,248, 93. (11)Solymoai, F.; Berk6, A.; RBvBez, K. Surf. Sci. 1990,240,50. (12)Marsh,E.P.;Tabares,F. L.;Schneider,M.R.;Gilton,T. L.;Meier, W.; Cowin, J. P. J. Chem. Phys. 1990,92,2004. (13)Zhou, X.-L.; Yoon, C.; White, J. M. Surf. Sci. 1988,206,379. (14)Nuzzo, R. G.; Dubois, L. H.J. Am. Chem. Soc. 1986,108,2881. (15)Jenka, C. J.; Chiang, C.-M.; Bent, B. E. J. Am. Chem. SOC.1991, I13,6308. (16)Zaera, F. Ace. Chem. Res. 1992,25,260. (17)Biloen, P.;Sachtler, W. M. H. Adu. Catal. 1981,30,165. (18)Bell, A. T. Catal. Rev.-Sci. Eng. 1981,23,203. (19)Somorjai, G.A. Catal. Rev.-Sci. Eng. 1981,23,189. (20)Paal, Z., Menon, P. G., Eds., Hydrogen Effectsin Catalysis;Marcel Dekker: New York, 1988.
Experimental Section The experimental measurements reported here were carried out in a stainlesssteel ultrahigh vacuum (UHV)belljar evacuated with a turbomolecular pump to pressures below 1X 10-lOTorr.1921 Thie vacuum chamber contains instrumentation for thermal programmed desorption (TPD), X-ray photoelectron ( X P S ) , static secondary ion mass (SSIMS),Auger electron (AES),and ion scattering (ISS) spectroscopies. The quadrupole mass spectrometer used for TPD is capable of detecting mames in the 1-800 amu range and has its ionizer located inside an enclosed compartment with 7 mm diameter apertures in ita front and back for gas sampling and exit to the quadrupole rods, respectively. The sample was positioned within 1 mm of the front aperture, a procedure that resulted in an enhancement in sensitivity of about a factor of five for desorptionfrom the front face of the crystal and additional diacrimination against desorption from the edges and back of the crystal and from the supporting wires. The mass spectrometer was interfaced to a computer in order to acquiredesorption data for up to 10different masses simultaneously in a single desorption experiment. A heating rate of about 10 K/s was used in the TPD runs. XPS spectrawere taken using an aluminum anode X-ray source and a hemispherical electron energyanalyzer (50 mm radius) set at a constant pass energy of 50 eV. The resolution of our instrument in this configuration is about 1.2 eV full width at half maximum, and the absolute energy scale, which was calibrated against values of 70.9 and 932.4 eV for the binding energies of Pt 4fTp and Cu 2 ~ 3 1 2electrons, respectively, is accurate within 0.1 eV. The iodine XPS data reported here were obtained after subtracting the corresponding spectra from the clean surface. The kineticsof the C-I bond cleavagewas studiedby measuring the rates for methyl iodide decomposition isothermally using XPS. The temporal changes in XPS signalintensity at 620.2 eV were followed as a function of time while keeping the surface at a given preset temperature. The noise on each individual kinetic run amounted to about 50 counts/s (countsper second)per point, which corresponds to a coverage close to 5 % of saturation. SSIMSexperimentswere carried out by utilizinga 1-keVargon ion beam of about 0.2 mm diameter as the primary excitation source. The angle of incidence was set at 60° to the surface normal, ion current densities as low as 20 nA/cm2 were used, and the beam was continuously rastered in order to minimize any surface damage during data acquisition.22 The secondary ions were filtered using a Bessel box tuned to a kinetic energy of 5.0 3.5 eV and subsequently analyzed by means of a quadrupole mass spectrometer using counting electronics and an interfaced computer.
*
(21)Zaera, F.J. Vue. Sci. Technol. A 1989, 7,640. (22)Tjandra, S.;Zaera, F. Langmuir 1991, 7, 1432.
0743-7463/92/2408-2090$03.00/0 0 1992 American Chemical Society
Letters
Langmuir, Vol. 8, No. 9, 1992 2091
CH31/Ni(lOO) TPD
a. CHd
b.,H2
CH4
I
4.0
Expll 7.0 5.0 4.0
-
& C.
3.0
.
2.5 I
."
1.o
d 400
800 0
400
81
Temperature / K Figure 1. Methyl iodide (a), hydrogen (b), and methane (c) TPD spectra from CH31adsorbed on Ni(100) as a function of initial exposure. Dosing was done at 90 K, and heating rates around 10 K/s were used. The nickel single crystal was cut and polished in the (100) orientation by using standard procedures and mounted in a manipulator so it could be cooled to liquid nitrogen temperatures and resistively heated above 1500 K, temperatures were measured using a chromel-alumelthermocouple spot-weldedonto the edge of the crystal. Cleaning of the surfaceby cycles of oxygen treatment, ion sputtering,and annealingwere done prior to each experiment until no impurities were detected by using either Auger electron or X-ray photoelectron spectroscopies. CH31 (minimum purity of 99%) and CDJ (99.5% D) were obtained from Alfa Products and Cambridge Isotope Laboratories, respectively, and were subjectedto several freeze-pumpthaw cycles before introducing them into the vacuum chamber. The purity of both compounds was checked periodically by mass spectrometry. Ultrahigh purity oxygen and argon gases were supplied by Matheson and used as received. In this paper gas doses are reported in langmuirs (1langmuir = 1X 10" Torr-s);the pressure readings were not corrected for ion gauge sensitivities.
Results Figure 1showsTPD spectra obtained from methyl iodide adsorbed on Ni(100) as a function of initial exposure. Only hydrogen, methane, and methyl iodide were found to desorb from the surface; the formation of other hydrocarbons was never detected in this system. Figure l a indicates that almost no molecular desorption takes place after low methyl iodide doses, only a small signal can be seen around 180K at 3 langmuirs, so the heat of adsorption of this first layer must be at least 10 kcal/m01.~~ However, a peak does grow in the spectra around 145 K after exposures above 3 langmuirs due to the desorption of molecules The physisorbed on the surface, the same as on Pt(lll).24 sublimation energy of this condensed methyl iodide layer was estimated to be about 6.0 f 1.0 kcal/mol by analysis of the leading edge of the TPD peak. The Hz TPD traces obtained for the same system are shown in Figure lb. A single peak is seen at low coverages, initially centered at 370 K (0.5 langmuir) but shifting to lower temperatures with increasing initial exposures until reaching a value of about 310 K at saturation. This behavior is typical of second-order processes and implies that the limiting step in this case is the recombination of hydrogen atoms on the surface. A second desorption (23) Redhead, P. A. Vacuum 1962,12,203. (24) Zaera, F.; Hoffmann,H. J . Phys. Chem. 1991, 95, 6297.
2
% 0
-
CHd/Ni(lOO) Desorption Yields
feature also developsabove 1.5langmuirs and peaks around 375 K a t saturation, a temperature high enough to suggest that desorption in this case may be related to the irreversible decomposition of hydrocarbon fragments that form on the surface after the C-I bond in CH3I breaks. Finally, a small third peak was sometimes observed above 400 K which we associate with the dehydrogenation of either methyl iodide or methyl moieties on surface defect sites. Methane formation from chemisorbed methyl iodide was also observed even after exposures as low as 0.5 langmuir (Figure IC).At low coverages the TPD spectra display a small peak with maxima that start a t about 255 K and broaden and shift to lower temperatures with increasing doses. This initial peak shows an exponential leading edge characteristic of a zero-order process with an apparent activation energy of about 2.5 f 0.5 kcaltmol. A second, more symmetric peak develops around 230 K for exposures above 2 langmuirs, and a low temperature tail also grows in after even higher doses. These TPD spectra are clearly the result of a complex nonelementary mechanism which we will discuss in more detail in the next section. The TPD data reported above were used to calculate yields for Hz and CHI desorption as a function of methyl iodide exposure (Figure 2). The hydrogen signal was calibrated by comparison with saturation coverages obtained by dosing a clean surface with H2 (which corre-
2092 Langmuir, Vol. 8, No. 9, 1992
Letters
CD3I/Ni(l00)
CDsVNi(l00) I 3dw XPS VS T
I 3d XPS at 160 K
-
u)
. w,
ExD/L
P
3 A
XI 6 2 1
]I:$(;;,
A 5L
lo
Expll 10.0 4.5
z
619.9
3.5 3.0 2.5
2.0
i
1.5 1.o
si0
'
620
'
630
Binding Energy I eV Figure 3. Iodine 3d XPS as a function of initial exposure from CHJ initially dosed on Ni(100)at 90 K and subsequently annealed to 160 K to desorb any condensed molecules. The inset shows the changes in I 3ds/z XPS peak areas with dose.
sponds to one monolayer of atomic hydrogen25), and methane coverages were then calculated by using a mass balance argument and by assuming a constant sticking The total coefficientfor methyl iodide up to ~aturation.2~ HPdesorptionyield increases with exposuresuntil reaching amaximum at about 2 langmuirs CH31and then decreases again for larger doses, and the contribution from the high temperature peaks to the total signal increases with increasing coverages until reaching a value close to 85% at saturation. In addition, the metal surface is always exposed to a small amount of background hydrogen gas, which in our experiments leads to the adsorption of about 0.05 of a monolayer of atomic hydrogen before any methyl iodide dosing is performed. The yield for CHI formation is negligible at low coverages, but increases suddenly around 2 langmuirs of CH31, until reaching a maximum of about 0.16 monolayers above 4 langmuirs (about 85% of the total methyl iodide adsorbed). Finally, the saturation coverage for methyl iodide was estimated to be slightly below 0.20 monolayer, and the sticking coefficient about 0.05 monolayer/L. We should clarify that since our pressure readings were not corrected by the ion gauge sensitivities, the latter value is not absolute and could be an underestimation of the real sticking coefficient. Figure 3 displays I 3d XPS results from CHJ on Ni(lOO), first adsorbed at 90 K and then flashed to 160 K to desorb any condensed overlayer. The iodine 3d3p and 3d5p peaks are centered at 631.3 and 619.8 eV, respectively, in all spectra regardless of the initial methyl iodide exposure; those values are typical for metal iodides.26A plot of the I 3d5p peak area as a function of initial dose shows that the iodine surface coverage increases linearly with increasing doses up to 3 langmuirs and then saturates (Figure 3,inset), a result thatjustifies the constant sticking coefficient hypothesis used above for the calibration of the TPD signals. Figure 4 displays I 3d5p XPS spectra as a function of annealing temperature for a 3 langmuir methyl iodide dose (left frame). Neither the peak areas nor their shapes are altered significantly in this case by heating of the sample, but the binding energy, which is (26) Chrietmann, K.; Schober, 0.;Ertl, G.; Neumann, M. J. Chem. Phys. 1974,60,4528. (26) Wagner, C. D., Riggs, W. M., Davis, L. E., Madder, J. F., Muilenberg, G. E., Ede. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Praire, MN, 1978.
620 Blndlng Energy / *V
625
619.7 90
190 240 Temperature / K
140
Figure 4. Left frame: Iodine 3d5,z XPS for a 3 langmuir CHsI dose on Ni(100) after annealing to the indicated temperatures. Right frame: Iodine 3d5I2 XPS peak position as a function of annealingtemperaturefor 1 langmuir (top),3 langmuir (middle), and 5 langmuir (bottom) initial doses of methyl iodide on Ni(100).
quite sensitive to the chemical environment surrounding the iodine atom, shifts from 620.2 eV at low temperatures (when CH3I is in its molecular form) to 619.8 eV after heating to 160K (a value that correspondsto iodinedirectly adsorbed on the surface'). The right frame of Figure 4 summarizes the changes in peak position as a function of annealing temperature for different initial coverages. In addition to the changes related to the dissociation of the C-I bond discussed above, the I 3d5p binding energy also shifts slightly at 90 K as the methyl iodide exposure is increased, perhaps due to screening effeds, or maybe because a small fraction of adsorbed molecules dissociate upon adsorption on the surface (see below). Finally, all I XPS peaks disappear completely after annealing above 850 K, indicating that the iodine a t o m either desorb or diffuse into the crystal bulk at those temperatures. Desorption of atomic iodine has been reported from CHaI and we suspect that the adsorbed on Pt(ll1) surf~ces,2~ same process may occur on nickel as well; the temperatures at which iodine disappears in the latter system would correspond to a nickel-iodine bond energy of roughly 60 kcal/mol. The rate of the C-I bond breaking reaction was measured isothermally by following the changes in XPS signal intensity at 620.2 eV with time while keeping the sample at a fixed temperature (Figure No significant changes were seen over a period of several minutes when heating the sample to 100 K, but at temperatures of 120 K or above the shift in peak position that takes place as methyl iodide is dissociated induces a reduction of the overall intensity of this XPS signal with time. It is clear that the kinetics of this step is not simple, since at temperatures below 140K a rapid decrease in signal intensity is followed by leveling off before all the methyl iodide has been dissociated only about 70% of the initial molecules decomposeat 125K, while 95 % of completion or more can be reached a t 150 K (in each run the sample was heated to 200 K after 2 min to determine the signal level that corresponded to 1009% dissociation). Nevertheless, initial rate constants were extracted from data like that shown in Figure 5 by using an analysis that takes into conaideration the contributions from both undissociated CHsI and iodine surface atoms to the total XPS signal and that assumes firsborder behavior (see Appendix); the signal ~
~~
~~
~~~
(27) Jo, S. K.; White,J. M. Submitted for publication in Surf. Sci. (28) Tjandra, S.; Zaera, F. J. Vac. Sci. Technol. A 1992,10, 404.
Letters
Langmuir, Vol. 8,No. 9,1992 2093
CDd/Ni(lOO) SSIMS
4L CHsl/Ni(lOO) C-l Bond Scission Kinetics
INi+
I_j
Clean Ni(lO0)
500
500 cps
TIK
3 L CD31 I Ni(100) T = 120K
100 120
125 130 140
3 L CD31 I Ni( 100) T = 200K
150
0
100
200
3
Time I s Figure 5. Isothermal kinetic runs for the C-I bond breaking reaction: the XPS signal intensity at 620.2 eV binding energy was followed as a function of time at the indicated temperatures for 3 langmuirsof methyl iodide adsorbed on Ni(100). The sample was also heated to 200 K in each run after 2 min to determine the signal level that corresponds to total dissociation.
C 1s XPS on Ni(l00) 3L CD31 120
j I . . References
TIK
.
.
Figure 6. Left frame: Carbon l e XPS for a 3 langmuir CDJ dose on Ni(100) after annealing to the indicated temperatures. Right frame: reference C 1sXPS spectra for a saturation coverage of CO and for a CD3I layer condensed at 90 K. intensity does decrease in an exponential fashion at the beginning of each run as expected for such rate law. The apparent activation energy for the bond scission reaction was then estimated using that data, and a value of about 3.5 1.0 kcal/mol was obtained.% This number may be an underestimation of the actual energy barrier because in our experiments a finite amount of time (5-10 s) is needed to stabilize the crystal temperature and that tends to make the rate constants measured at high temperatures seem smaller than what they really are, but in any case we can safely quote an upper limit no larger than 5 kcal/mol for the activation energy of this reaction. The left panel in Figure 6 shows C 1s XPS spectra from 3 langmuirs of CD3I dosed on Ni(100) after annealing to the indicated temperatures; spectra for a layer of CO at saturation coverage and for condensed methyl iodide are shown in the right frame for reference. The XPS trace obtained at 90 K shows a broad feature that contains contributions from two peaks, a large one centered at
NiCD3+ . I .
0
40
80
amu
Ni2CD3+ Y
-
120
r
160
Figure 7. Positive SSIMS spectra for a Ni(100) surface clean (top) and after dosing it with 3 langmuirs of deuteriated methyl iodide at 90 K and annealing for 120 K (middle) and to 200 K (bottom). around 284.5eV and ashoulder about 283.0eV. The main peak (the one at 284.5 eV) must corresponded to undissociatedmethyl iodide since it is seen, within experimental error, at the same position as that of the multilayer; this indicates that methyl iodide adsorption at low temperatures is mostly molecular. The shoulder observed in that spectrum at lower binding energies may be the product of some surface decomposition, but it only amounts to about 30% of saturation at the most. The high binding energy signal disappears by 160 K, leaving behind a small peak around 283.5 eV (presumably associated with methyl groups on the surface), and almost no signal is seen after annealing to 200 K because the surface carbon containing speciesdesorb as methane (there is a small feature at 284.5 eV in the 200 K C 1s XPS spectrum which could be due to CO contamination). Finally, an attempt to calibrate the C 1s XPS signal by comparison with that of a CO saturation overlayer yielded an absolute coverage of 0.25 monolayer for methyl iodide saturation. This value is slightly higher than that estimated from the TPD data, but the discrepancy can be explained at least in part by the fact that the signal intensity in the case of the carbon monoxide layer needs to be corrected to account for the attenuation induced by the oxygen atoms. Figure 7 shows positive SSIMS spectra for a Ni(100) surface both clean and after dosing 3 langmuirs of deuteriated methyl iodide at 90 K and then annealing to 120 and to 200 K. The main peaks seen at 58 and 60 amu in all spectra correspond to nickel ions; they were used 88 reference for relative intensity measurements. In addition, the peaks at 27, 39, and 40 amu are due to aluminum, potassium, and calcium impurities which originate from a surface sputtering process inside the Bessel box, those at 116,118, and 120 amu correspond to nickel clusters (Niz+),and the ones at 59 and 61 amu are due to NiH+.Z2 After methyl iodide is chemisorbed (either normal or fully deuteriated) the spectra exhibit few additional peaks
2094 Langmuir, Vol. 8, No. 9,1992 'Oo0-
.
3L CDd/Ni(lOO) XPS and SSIMS vs T
between 15and 20 amu and between 70 and 80 amu which are associated with the formation of CX3+ and NiCX3+ ionic fragments, respectively (X stands for H or D depending on the starting material). In the case of CD31 the yields for both CD3+ and NiCD3+ signals decrease gradually when going from 120 to 200 K (Figure 8), and analogous behavior is observed for normal methyl iodide (not shown). This loss of signal with increasing temperature is explained both by molecular desorption from the surface and by formation of methane. C 1s XPS peak areas are displayed in the same figure for comparison; the agreement between the results from both techniques is quite good. The temperature at which carbon disappears according to this figure is somewhat lower than that suggested by the TPD data, but this is due to the different procedures used to obtain both sets of experiments; while TPD is a dynamical technique where the temperature changes at a rate of 10 K/s, both XPS and SSIMS spectra were taken after annealing to the indicated temperatures for about 100 s.
Discussion The combined results from thermal desorption, X-ray photoelectron, and secondary ion mass spectroscopy experiments have provided a fairly complete picture of the reactions that methyl iodide undergoes on Ni(100) single crystal surfaces. Below 100K methyl iodide chemisorbs in a molecular fashion with the iodine atom directly attached to the surface. Molecular adsorption is indicated by the fact that the I 3d binding energies in the CH31 monolayer are close to those seen for a physisorbed layer, which means that the C-I bond remains intact upon adsorption at low temperatures (Figure 4). A small XPS peak shift was nevertheless observed at 90 K as the surface coverage was increased, but we assume that those changes are due to subtle changes in the iodine electronic configuration upon adsorption; this thesis would support the idea that chemisorption occurs through the iodine end of the molecule. A similar adsorption geometry has been proposed on Pt(ll1); both RAIRS and high resolution electron energy loss spectroscopy (HREELS) results have shown that while the vibrational frequencies of the methyl group do not change significantly upon methyl iodide adsorption, the carbon-iodine stretching frequency de-
Letters
creases by 40-70 cm-1.29930 C 1s XPS experiments offer additional evidence for molecular adsorption; the main peak in the spectrum for the CH31monolayer at 90 K is centered around 284.5 eV, a value that we assign to a carbon-containing fragment still bonded to the iodine atom (undissociated methyl iodide) based both on spectra from a CH31multilayer (Figure 6) and on previous reports for methyl iodide adsorbedon Pt(lll)24 and Ag(lll).3l There is a small shoulder in the C 1sXPS spectrum at low binding energies which may be due to some dissociated carbon fragments (methyl groups), but those species amount to less than 30% of the total carbon present on the surface. According to the shifts in binding energy observed in the iodine XPS spectra (Figure 4), methyl iodide chemisorbed on nickel starts to decompose around 120 K via the cleavage of the C-I bond. This bond scission reaction requires only about 3.5 kcal/mol for its activation, a value that may seem unreasonably low at first but that is in agreement with numbers obtained for carbon-halogen bond breaking by magnesium in the presence of polar ~olvents.3~933 The kinetics of this first decomposition step is complex, because even though the initial rates appear to follow first-order behavior, the extent of dissociation that can be reached varies with surface temperature. Above 160K all C-I bonds break within few seconds,but between 120 and 140 K the initial rapid dissociation of a fraction of the adsorbed molecules is followed by a complete stop of that reaction after a 50-90% conversion; the decomposition of the remaining methyl iodide requires higher surface temperatures. This non-Ahrrenius behavior may be explained by some type of geometric constraints on the surface such as the formation of islands or the need of large ensemblesof surface atoms for the C-I bond scission to occur, but we do not have a good model to reproduce the kinetic behavior observed at the present time. The main products from the breaking of the C-I bond in methyl iodide adsorbed on Ni(100) are iodine atoms and methyl surface groups; after annealing to 160 K the C 1s XPS peak shifts from 284.5 to 283.5 eV, a value associated with alkyl surface group^,^^^^ and the SSIMS peaks for CX3+ and for NiCX3+ disappear only above 200 K, after methane desorption is complete. On the other hand, no significant peaks correspondingto either methylene ((3x2)or methylidyne (CX) species were detected with SSIMS under any of our experimental conditions. There are few other pieces of information that suggest that methyl moieties form exclusively below 200 K on Ni(100). First, isotopic labeling experiments with CD31have shown than no fully deuteriated methane is formed below 200 K, all low temperature methane formation occurs by direct reductive elimination of methyl groups with hydrogen adsorbed from background gases.34 The only possible way of generating deuterium atoms on the surface in this case is via the decompositionof CD3 moieties, and since the results cited above indicate that no surface deuterium is available for methyl deuteriation at low temperatures, that reaction must not take place below 200 K. Second, no H-D exchange was seen in coadsorption experiments with either D2 and CH31or H2 and CD4, even (29) Zaera, F.;Hoffmann, H.; Griffiths, P. R. J. Electron Spectrosc. Relat. Phenom. 1990,54/55,705. - (30) Henderson, M. A.; Mitchell, G. E.; Adnot, A. Surf. Sci. 1991,248, 279. (31) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M.Surf. Sei. 1989,219, 294. (32) Rogers, H. R.; Craig, L. H.; Fujiwara, Y.; Rogers, R. J.; Mitchell, H.L.;Whitesides, G. M. J . Am. Chem. SOC.1980,102, 217. (33) Rogers, H. R.; Deutch, J.; Whitesides, G. M. J. Am. Chem. SOC. 1980,102, 226. (34) Tjandra, S.; Zaera, F. To be submitted for publication.
Langmuir, Vol. 8,No. 9,1992 2095
Letters '6
CHd/Ni(lOO) CH4 TPD in Arrhenius Form
I
Rate Order Plot
Exp I L 4.0 3.0
2.5
1180 K 200 K
1.5
k-1.0 1
1
1
1
1
1
4
1
1
1
1
1
1
1
1
1
1
1
1
5
though CD2Hz can be formed from D2 plus CH& at temperatures as low as 140 K; if methylene groups were to form, they could be easily hydrogenated to form methane at thosetemperatures.34 Finally, in cases where the surface is saturated with CH31, the high percentage of hydrogen that desorbs a t high temperatures in TPD experiments suggests that the hydrocarbon fragments that remain adsorbed after methane formation have high H/C stoichiometric ratios such as those expected from methyl groups. The decomposition of some methyl groups is required at temperatures above 200 K in order to provide the hydrogen atoms needed for methyl hydrogenation: the residual carbon resulting from such reaction should have been detected by XPS, but the signal to noise ratio in our C 1s XPS spectra is too low to allow us to reach a definite conclusion concerning this point. The methyl surface moieties that form from the thermal activation of methyl iodide on Ni(100) are hydrogenated a t higher temperatures via a reductive elimination step (the insertion of a hydrogen atom) to produce methane molecules in a way similar to that seen on other s u r f a ~ e s . l ~ *The ~ & ~CHI ~ TPD spectra, plotted in an Ahrrenius form in Figure 9 for more clarity, suggest that this process is quite complex and may involve three or more different mechanisms. One single peak is observed at low coverages with a leading edge characteristic of zeroorder kinetics but with maxima that shift to lower temperatures with increasing initial coverage. This "pseudo" zero-order rate dependence is easily understood since the total methane yield in this case represents only 1020 % of the total methyl surface coverage; the concentrations of reactants are comparatively large and change little as the reaction proceeds. The more puzzling aspect of the results shown in these spectra is the fact that for doses below 2.5 langmuirs the absolute methane formation rate between 180 and 220 K increases proportionally to the square of the initial methyl iodide coverage (Figure 9, right (35)Zaera, F. Catal. Lett. 1991, 11, 95. (36)Zaera, F.Surf. Sci. 1992,262, 335. (37)Zaera, F. Langmuir 1991, 7,1998.
220 K 240 K
A
0
I
1
6 010
I
0.5
I
I
1.o
I
1.5
frame). This behavior can be explained in part by the competition between reductive elimination and dehydrogenation of the methyl groups, since the fraction of methane produced increases significantly with increasing methyl coverage (from less than 30% at 0.5 langmuir to over 85% a t saturation), but such branching pathways alone cannot completely explain the second-order dependence observed; once again, geometrical considerations and surface heterogeneity need to be included in the final interpretation of the results. A second methane desorption peak develops around 230 K in the TPD spectra as the initial methyl iodide surface coverage is increased. This feature is quite symmetric and does not shift significantly with coverage. A leading edge analysis of those traces yields an apparent activation energy of about 4.0 f 1.0 kcal/mol, but a value of about 9.0 kcal/mol is obtained using the method of Chan et al.3 The same as for the lower temperature region of the spectra, the final shape of this peak may need to be explained by a combination of several elementary steps or by energetics that changes with coverage because of surface heterogeneity. Isotopic labeling experiments have clearly established that in this temperature regime the extra hydrogen required for methane formation originates from methyl decomposition and not from background ad~orption.3~ Also, the methyl groups incorporate hydrogen atoms from the surface and not directly from other methyl moieties. For one, the formation of methane a t these temperatures is quite a bit slower than the breaking of the C-I bond in methyl iodide; for a 3 langmuir CHJ dose, the rate of C-I bond scission a t 180 K is a t least 0.20 monolayer/s, while methane is produced at the rate of only 0.008 monolayer/s (Figures 5 and 9). In addition, a large isotope effect is observed when a mixture of CH31 and CD31 is used in the TPD experiments; such an effect would not have been expected if hydrogenation were to occur by direct interaction between methyl iodide molecules or between methyl moieties.34 (38)Chan, C.-M.; h i s , R.; Weinberg, W. H. Appl. Surf. Sci. 1978,1, 360.
Letters
2096 Langmuir, Vol. 8, No.9, 1992 Finally, a third methane formation mechanism opens up at temperatures at low as 100 K after very high methyl iodide exposures. The apparent activation ene'rgyin this regime changes with initial dose, and the methane produced is the result of incorporating normal hydrogen a t o m (not deuterium) into methyl moieties even in the experiments done using CD31, which means that the extra hydrogen must come from the background gases. One possible way for this low temperature methane formation reaction to occur is by a concerted mechanism between molecular methyl iodide and coadsorbed hydrogen that may operate at these high coverages; a 8N2 step of this type has indeed been observed in cases where high coverages of hydrogen are coadsorbed with methyl iodide.39 An alternative explanation is based on the fact that under high coverage conditionsthe adsorption energy for methyl iodide becomes low enough to favor the formation of gasphase methyl radicals. The heat of that reaction, AHrsact, is given by the following expression:
AHreact = -AHa&H31) + Dg,JC-I) + AHa& The heat of adsorption of methyl iodide at saturation, AHab(CH3I), is about -6 kcal/mol according to our TPD results, the carbon-iodine bond dissociation energy in the gas phase, Dga(C-1), has been reported to be 56.3 f 1.0 kcal/mol,&and the heat of adsorption of atomic iodine on Ni(100), AHah(I), is estimated to be on the order of -60 kcal/mol based on our I 3d XPS data, so the enthalpy of methyl radical formation comes out to be about 2 kcal/ mol, close to the value for the activation energy for methane formation obtained under high coverageconditions. Since our mBs8 spectrometer does not have ita ionizer in direct line-of-sightwith the sample,we presume that these methyl radicals may recombine with hydrogen somewhere in the front of the spectrometer, before arriving at the m w fiiter. Direct radical formation should be tested using a direct line-of-sight mass spectrometer. The values obtained here for the activation energy of methane formation from adsorbed methyl groups can also be used,together with reported data for methane activation on Ni(100),to calculate the carbon-nickel bond energy on that surface. Beebe et al. obtained an energy barrier for the dissociative adsorption of methane of about 6.4 kcal/ mol based on high pressure sticking coefficient experiments,4l and molecular beam results from Hamza and Madix yielded an estimate of about 9.0 kcal/mol for the same reaction.42 Using those numbers, we estimate the heat of methane (dissociative) adsorption to be 6.0 kcal/ mol or less, a result that, combined with values for the heat of adsorption of hydrogen (23.0 kcal/mo12s) and for the bond energy for the C-H bond in methane (104.0 kcal/ mala), leads us to conclude that the strength of the carbonnickel bond in methyl groups adsorbed on Ni(100) is approximately 57 kcal/mol. This number seems quite high when compared with values reported for platinum, copper, and iron surfaces (on the order of 30 kcal/mol in all three cases,3644+ but this is due to the unusually low energy (39) Tjandra, S.;Zaera, F. Submitted for publication in J.Am. Chem. SOC.
(40) Kerr, J. A. Chem. Rev. 1966,66,465. (41) Beebe, T. P., Jr.; Goodman, D. W.; Kay, B. D.; Yaks, J. T., Jr. J. Chem. Phys. 1987,87, 2305. (42) Hamza, A. V.; Madix, R. J. Surf. Sci. 1987, 179, 25. (43) CRC Handbook of Chemistry and Physics, 55th ed.; CRC Press: Cleveland, OH, 1974. (44)Zaera, F. J. Chem. Phys. 1990,94,8350. (45) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. Submitted for publication in J. Phys. Chem. (46) Burke, M. L.; Madix, R. J. J. Am. Chem. SOC.1992, 114, 2780.
reported for the activation of methane on nickel. Perhaps this behavior needs to be studied in more detail in the future. The adsorption and decomposition of methyl iodide on Ni(100) have also been characterized by Zhou and White by using TPD, SSIMS, and AES.899 Their experimental observationsare in general agreement with those reported here, even though they quote the use of higher doses in order to obtain surface coverages comparable to ours. The authors of that study, however, suggest that methyl iodide adsorption is nearly completely dissociative even at temperatures below 100 K. Our XPS data have shown that this is clearly not the case, we were able to prove that significant dissociation starts only around 120 K and that it requires about 3.5 kcal/mol to be activated. We believe that our interpretation of the chemistry of this system is better supported by the data because XPS is a more appropriate technique for detecting the occurrence of that first C-I bond breaking step than SSIMS (which they used to propose their mechanism). The adsorptions of methyl and other alkyl halides have been studied on a few metal surfaces.l6 Chen et al., for example, have shown that methyl iodide adsorbs both molecularly and dissociatively on Al(111)at low temperatures' and that the adsorbed CHsI decomposes between 250 and 450 K to form adsorbed CH groups and iodine atoms on that surface. Methyl halides chemisorbed on (100) oriented polycrystalline W dissociate completely to produce H2, surface carbon, and halogens,13 while the thermal decomposition of C H a r and of CH31 on both magnesium and copper surfacescanyield ethane and other polymerization products.14Js Nevertheless, despite the fact that the thermal activation of adsorbed alkyl halides may lead to the formation of different surface intermediates and of several types of products depending on the nature of the substrate, the dissociation of the carbonhalogen bond occurs readily at low temperatures in most cases, presumably requiring low activationenergiessimilar to those seen here for nickel.
Conclusions We have studied the adsorption and decomposition of methyl iodide on Ni(100) by TPD, XPS, and SSIMS. We found that below 100K methyl iodide adsorbs molecularly and that the scission of the C-I bond starts around 120 K. The energy required to activate that step is only 3.5 kcal/mol, and complete dissociation is reached by 160 K. The onlyreaction products that desorb from these surfaces are hydrogen and methane. At low coverages the production of hydrogen is dominant, but at higher coverages methane formation takes over. Finally, the desorption of methane follows quite complex kinetics but is attributed to the direct incorporation of hydrogen atoms into the methyl species that form on the surface after the C-I bond is dissociated.
Acknowledgment. Financial support for this research was provided by a grant from the National Science Foundation (CHE-9012560). Appendix The XPS signal intensity at 620.2 eV may contain contributions from both dissociated and undissociated methyl iodide. Lets defiie the following sensitivityfactors
Langmuir, Vol. 8, No. 9, 1992 2097
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where S(t) stands for the XPS total signal intensity at time t after background subtraction, and (Y and j3 refer to undissociated adsorbed methyl iodide and to surface iodine after dissociation respectively (times t = 0 and -). From these, S(t) at any time is given by
S ( t ) = a eCH,I(t)
+ 0q t )
where eCH,I(t) and OI(t) represent the surface coverages of methyl iodide and atomic iodine as a function of time, respectively. If the bond scission reaction is first order in methyl iodide coverage, the time evolution of the surface coverages should follow exponential decay laws of the form
