Adsorption and Reaction of Methyl and Ethyl Iodide on Potassium

18502

J. Phys. Chem. C 2008, 112, 18502–18509

Adsorption and Reaction of Methyl and Ethyl Iodide on Potassium-Promoted Mo2C/ Mo(100) Surface ´ . Koo´s,‡ L. Bugyi,† and F. Solymosi*,†,‡ A. P. Farkas,† A Reaction Kinetics Research Group, Chemical Research Centre of the Hungarian Academy of Sciences Institute of Solid State and Radiochemistry and UniVersity of Szeged, P.O. Box 168, H-6701 Szeged, Hungary ReceiVed: June 10, 2008; ReVised Manuscript ReceiVed: July 25, 2008

X-ray photoelectron spectroscopy (XPS) studies revealed that potassium on Mo2C/Mo(100) induced cleavage of the C-I bond in adsorbed CH3I even at ∼100 K. The temperature of complete C-I bond breaking occurred 60-80 K lower compared to the clean surface. Preadsorbed potassium also influenced the reaction pathway of adsorbed CH3 formed. It decreased its self-hydrogenation into methane and facilitated the coupling reactions into ethane and ethylene below 200 K. High-resolution electron energy loss spectroscopy (HREELS) confirmed formation of adsorbed CH3 species and revealed its thermal stability. Potassium exerted a similar influence on the chemistry of C2H5I on Mo2C/Mo(100) surface. Rupture of the C-I bond also occurred more easily on K-covered surface. Ethyl species, the primary product of the dissociation, dehydrogenated into ethylene on one hand and hydrogenated into ethane on the other. Rupture of the C-C bond was not observed even at high potassium coverage. Illumination of coadsorbed layers promoted further dissociation of the C-I bond in both compounds at ∼100 K. 1. Introduction Mo2C on ZSM-5 is an effective catalyst for conversion of methane into aromatics.1-12 This behavior is in contrast to that of Pt metals, which under the same conditions catalyze complete decomposition of methane into carbon and hydrogen.13-16 In the explanation of the peculiar effect of Mo2C it was assumed that the CH3 or CH2 species formed in the activation of methane have a certain lifetime on the Mo2C surface to recombine into C2 compounds before their complete break up into carbon and hydrogen.3,5 Deeper insight in the reaction was provided by a study of the chemistry of hydrocarbon fragments prepared on Mo2C surface. Experiments performed in an ultra-high-vacuum (UHV) system revealed that the main route of the reaction of CH3 species produced by pyrolysis of azomethane as well as by dissociation of CH3I on Mo2C/Mo(100) surface is formation of CH4, H2, and surface carbon.17 Coupling into ethane was not observed, but evolution of ethene was registered above 400 K. Formation of ethylene from CH2 species, generated by thermal and photodissociation of CH2I2, also occurred to a limited extent on Mo2C/Mo(100) under UHV conditions.18 In agreement with this finding theoretical consideration on the reaction of adsorbed CH3 on Mo2C confirmed that recombination of CH3 into C2H6 is much less likely compared to its conversion into C2H4.19 These results suggest that Mo2C not only is capable of activating the methane but can also promote recombination of CHx fragments into olefins or hydrocarbons. In subsequent works it turned out that Mo2C on ZSM-5 can also catalyze or effectively promote aromatization of C2-C8 alkanes,20-27 ethanol,28 methanol,29 and dimethyl and diethyl ether,30 which required the study of the chemistry of their primary dissociation products.31-37 In a continuation of this research program we undertook the evaluation of the effects of potassium, the well* To whom correspondence should be addressed. Fax: +36-62-420-678. E-mail: [email protected]. † Chemical Research Centre of the Hungarian Academy of Sciences Institute of Solid State and Radiochemistry. ‡ University of Szeged.

known promoter of catalytic reactions of hydrocarbons, on the stability and reaction pathways of hydrocarbon fragments, CH2 and C3H7 on Mo2C/Mo(100) surface.33,34 In the present paper we report the effect of potassium on the adsorption and dissociation of CH3I and C2H5I and on the chemistry of CH3 and C2H5 species on the same surface. We note here that adsorption and dissociation of various alkyl iodo compounds on metal single crystals have been extensively studied in the last two decades, and the main characteristics are summarized in several reviews.38-40 The results obtained on the reaction of CxHy fragments significantly contributed to a better understanding of the hydrocarbon reactions occurring on metal catalysts. 2. Experimental Section The experiments were performed in an UHV chamber with a routine base pressure of 5 × 10-10 mbar produced by turbomolecular, ion-getter, and titanium sublimation pumps. The chamber was equipped with facilities for Auger electron spectroscopy (AES), HREELS, and temperature-programmed desorption (TPD). The HREEL spectrometer (LK, ELS 3000) is situated in the lower level of the chamber and has a routine resolution of 30-50 cm-1. The UV light source was a focused 700 W Hg lamp. To eliminate the heating effect we used a water filter. The light passed through a high-purity silica window into the vacuum chamber. XPS measurements have been carried out in a Kratos XSAM 800 instrument at a base pressure of 10-9 mbar using Mg KR primary radiation (14 kV, 10 mA). To compensate for possible charging effects, binding energies (BE) were normalized to the Fermi level for Mo2C. The pass energy was set at 40 eV, and an energy step width of 50 meV and dwell time of 300 ms were used. Typically 10 scans were accumulated for each spectrum. Fitting and deconvolution of the spectra were made using the VISION software (Kratos). Pretreatments of the samples were performed in the preparation chambers attached to the UHV system. The Mo(100) crystal was mounted between two tantalum wires, which were connected via a copper block to a liquid

10.1021/jp805078d CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

Reaction of Methyl and Ethyl Iodide

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18503

Figure 1. Effects of annealing on the I(3d5/2) XPS signals of adsorbed CH3I on Mo2C/Mo(100) surface at 100 K. Exposition of CH3I was 4.0 L: ΘK ) (A) 0.0, (B) 0.35, and (C) 1.0 ML.

nitrogen reservoir. The sample was heated resistively from 100 to 1275 K; its temperature was monitored by a chromel-alumel thermocouple spot welded to the edge of the crystal and controlled with a feedback circuit to provide a linear heating rate of ca. 5 K/s. Alkyl iodide was dosed through a 0.1 mm diameter capillary that terminated 2 cm from the sample. The dosing temperature was 100 K unless otherwise noted. The Mo(100) crystal used in this work was the product of Materials Research Corp., purity 99.99%. Initially the sample was cleaned by heating in oxygen at 800 K. This was followed by cycles of argon-ion bombardment (typically 1-2 kV, 1 × 10-7 mbar Ar, 1000 K, 1 µA for 10-30 min) and annealing at 1270 K for several minutes. Mo2C was prepared by exposing Mo(100) surface to 200 L of ethylene at 900 K and then flashed to 1200 K in UHV.41 The partial pressure of ethylene near the sample was about 10-7 mbar. The resulting surface as checked by AES turned out to be carbidic, showing the characteristic three-lobe line shape of carbidic carbon in AES at 255.6, 262.1, and 272.7 eV.37 Alkyl iodides were the product of Fluka, 99%. They were cleaned by several freeze-pump-thaw cycles. A commercial SAES getter source situated 3 cm from the sample was used to deposit potassium metal onto the Mo2C surface. The getter was resistively heated. Deposition of the potassium was done at 250-300 K. The onset of potassium desorption from the second adlayer (Tp ) 355 K) was accepted as an indication of the completion of a monolayer denoted by ΘK ) 1.0 ML. The potassium coverage related to the underlying unit cell can be estimated to be 0.33 on the basis of similar thermal desorption and depolarization behavior of K on Pt metals and on the Mo2C/Mo(100) surface. 3. Results and Discussion 3.1. Characteristics of Pure and Potassium-Promoted Mo2C/Mo(100). Previous HREELS and near-edge X-ray absorption fine structure (NEXAFS) study of Fru¨hberger et al.42 revealed that the carbon modified Mo(100) with interstitial/ subsurface carbon atom remained active toward decomposition of ethylene and cyclopentene. Ion scattering spectroscopy (LEIS) in conjunction with XPS and AES showed that the LEIS peak areas of Mo and C in the topmost layer of polycrystalline Mo2C

prepared by carburization of Mo(100) with ethylene are almost identical.43 Accordingly, in contrast to Mo2C single crystals we cannot discuss the C- or Mo-terminated surface. The adsorption and surface behavior of potassium on Mo2C/ Mo(100) has been examined previously in great detail.44 Potassium adatoms exhibited similar features to those determined for metal single-crystal surfaces.39,40 The steep work function decrease, max 3.5 eV up to ΘK ≈ 0.2 ML, suggests a considerable charge transfer from potassium to Mo2C at lower coverages while leveling of WF the gradual neutralization at and above one monolayer. As a consequence, the potassium adlayer is mainly ionic at low and metallic at high coverages. At low K coverage, potassium desorbed with a Tp ) 950 K. This peak shifted to 525 K for monolayer and ∼354 K for multilayer coverage. The potassium adlayer gave intense K(2p3/2) and K(2p1/2) peaks in the XPS: at ΘK ) 1.0 ML their positions were at 293.5 and 296.1 eV, and above monolayer these values shifted to higher binding energies to 294.2 and 296.8 eV, respectively. 3.2. Adsorption and Dissociation of CH3I. 3.2.1. XPS Measurements. XPS is a sensitive tool in the study of the dissociation of iodo compounds as the binding energy of I(3d5/2) in the atomically adsorbed state is about 1.0-1.5 eV lower than that for molecularly adsorbed iodo compounds.17,18,38-40 Adsorption of CH3I on a clean Mo2C/Mo(100) sample at 100-110 K gave a binding energy of I(3d5/2) at 619.7 eV corresponding to the molecularly adsorbed CH3I (Figure 1A). The position of the peak was practically independent of the coverage. In the region of the binding energy of C(1s), a new C(1s) signal developed at 284.85 eV. No appreciable changes occurred in the position of the C(1s) peak of Mo2C at 282.4 eV (not shown). Upon heating the adsorbed layer the first spectral change occurred at 170 K when a weak signal appeared at 618.3 eV. This became stronger at higher temperatures, when the binding energy at 619.7 eV lost intensity. The latter peak was present up to ∼240 K. On exposing CH3I to Mo2C/Mo(100) containing 0.35 ML of potassium at 115 K the binding energy of I(3d5/2) appeared at 620.5 eV (Figure 1B). A small shoulder, however, can be detected at 618.5 eV, suggesting the occurrence of dissociation of CH3I to some extent. On annealing the adsorbed

18504 J. Phys. Chem. C, Vol. 112, No. 47, 2008

Farkas et al.

Figure 2. TPD spectra of (A) methyl iodide (15 amu), (B) hydrogen (2 amu), (C) methane (16 amu), (D) ethene (27 amu), (E) ethane (30 amu), and (F) potassium (39 amu) after exposing the clean and K-covered Mo2C/Mo(100) surface to 8.0 L CH3I at 100 K. In F, curve x shows the desorption of potassium (ΘK ≈1.5 ML) adsorbed on clean Mo2C/Mo(100).

layer this shoulder became stronger, whereas the 620.5 eV peak gradually attenuated. Above 170 K it disappeared, and the peak at 618.5 eV dominated the spectra. Similar features have been observed when CH3I was adsorbed on Mo2C containing a monolayer of potassium (Figure 1C). In this case the 618.5-619.2 eV peak was initially stronger, and the high-energy peak at 621.0 eV disappeared at somewhat lower temperature. In summary, we conclude that potassium additive promotes cleavage of the C-I bond in the adsorbed CH3I

C2H4 is about 1-3% of that of CH4. We found only slight desorption of ethane from the clean surface. These features are consistent with those observed before for Mo2C/Mo(111).31 All these results suggest that a fraction of the CH3 species formed in the primary dissociation process (eq 1) is decomposed very likely according to the following equations

CH3I(a) ) CH3(a) + I(a)

(1)

resulting very likely in adsorbed CH3 and I. Complete C-I dissociation in CH3I can be achieved at 170-190 K. Regarding the bonding of CH3I to pure Mo2C/Mo(100) we took into account the results obtained on metal surfaces.38-40 Similar to adsorption of other alkyl iodides,31-37 we propose that the CH3I molecule bonds via iodine to the Mo of the top layer of Mo2C. Formation of the strong Mo-I bond is supported by the high desorption temperature of iodine (Tp ) 950-1050 K) from the Mo2C/Mo(100) surface.33 The promoter effect of potassium can be explained by the extended electron donation from the K + Mo2C/Mo(100) system to the adsorbed CH3I to form a partially negatively charged species that dissociates more easily. At high ΘK values, when potassium is mainly in metallic form, we can count on a strong interaction between potassium and iodine, which also facilitates rupture of the C-I bond. Formation of a surface ionic compound, K-I, is reflected by alteration of the desorption of potassium at high coverage (Figure 2F). 3.2.2. TPD Measurements. Molecularly adsorbed CH3I desorbs from the clean Mo2C/Mo(100) with a peak temperature of 140 K (Figure 2). As this peak cannot be saturated, it is attributed to a condensed phase. Regarding formation of other compounds, we detected desorption of H2 (Tp ≈ 410 K), CH4 (Tp ) 280 K), and C2H4 (Tp ) 244 and 478 K). The amount of

CH3(a) ) CH2(a) + H(a)

(2)

CH2(a) ) CH(a) + H(a)

(3)

CH(a) ) C + H(a)

(4)

and another fraction of CH3 is hydrogenated to methane

CH3(a) + H(a) ) CH4(a)

(5)

which is desorbed with Tp ) 280 K. At the same time the coupling of CH2(a) species

2CH2(a) ) C2H4(a)

(6)

also occurs. Unlike previous studies on Mo2C/Mo(111),31 we measured a small amount of ethane formation

2CH3(a) ) C2H6(a)

(7)

which we attribute to K contamination of Mo2C/Mo(100). Preadsorbed potassium induced formation of a new weak state for adsorbed CH3I characterized by Tp ) 170 K. Desorption of hydrogen shifted from Tp ) 410 K (clean surface) to Tp ) 424 K (1.5 ML of potassium) accompanied by a TD peak area increase of 2. A more significant change occurred in the desorption of methane, particularly at high K coverages: a new desorption peak appeared at 360-380 K. If we assume that methane is the product of the hydrogenation of adsorbed CH3, the shift in the Tp value to higher temperature may be a consequence of the stabilization of adsorbed CH3 by potassium. In the case of Rh(111) we obtained spectroscopic evidence for



TABLE 1: Vibrational Frequencies of CH3I and CH3 (cm-1) vibrational modes

CH3I

νa(CH3) νs(CH3) δa(CH3) δs(CH3) F(CH3) ν(C-I)

3060 2933 1436 1252 882 533

(g)47

CH3I/Rh (111)48

CH3I/Pt (111)49

CH3I/Mo2C/Mo (111)31

CH3/Pt (111)a,49

CH3/Pt (111)b,50

CH3/Rh (111)a,48

CH3/Rh (111)a,43

CH3/K/Rh (111)b,43

3044

3025

3065

2918

2930

1395 1220 875 480

1443 1245 900 554

2950 2770 1410 1180 820 495

2920

1430 1230 900 522

2925 2775 1425 1165 790 520

1350 1185 760

1353 1141

1320 1185

a Values were obtained after dissociation of adsorbed CH3I. azomethane according to the method developed by Stair et al.43

b

Values were obtained after adsorption of CH3 produced by pyrolysis of

this phenomenon: preadsorbed K enhanced the stability region of CH3 by more than 100 K.45 The primary reason of the stabilization is the strong interaction between K and CH3

K(a) + CH3(a) ) K - CH3(a)

(8)

K-CH3 is known to be a stable compound; even its structure has been determined.46 Formation of ethylene has been altered also in the presence of potassium: the high-temperature peak for ethylene desorption was strongly suppressed and larger peaks intensified at lower temperatures, Tp ≈ 244 and 170 K. Whereas on clean Mo2C desorption of ethane was minimal, on K-dosed surface ethane also formed together with ethylene. The capability of K/Mo2C to promote the coupling of CHx species basically differs from that of K/Rh(111) surface, where this process did not occur at all. This difference is probably reflected in their divergent catalytic properties. Whereas on Mo2C/ZSM-5 methane can be converted into benzene, on Rh/ZSM-5 it decomposes to hydrogen and carbon under the same experimental conditions.5 In the case of metal single crystals it was observed that not only potassium influences desorption of adsorbed species but bonding of potassium is also affected by the coadsorbed compounds, indicating a strong interaction between them.39 This phenomenon also occurred on Mo2C surface, particularly at high K coverage. While potassium at and above monolayer mainly desorbs in a peak with Tp) 354 K, in the presence of adsorbed CH3I this peak temperature is shifted to 648 K. In other words, desorption of potassium coincides with that of iodine formed in the dissociation of CH3I. Some selected TPD curves for the effects of potassium coverage are displayed in Figure 2F. 3.2.3. HREELS Measurements. Following adsorption of a large quantity of CH3I (4.0 L) on clean and K-dosed Mo2C surface at 100 K we found losses at 520-560, 880, 1230, 1430, and 3050 cm-1. These features correspond very well to the vibrations of molecularly adsorbed CH3I listed in Table 1, which also contain the vibrational frequencies of adsorbed CH3 determined on various surfaces. The peak at 520-560 cm-1 belongs to the ν (C-I) vibration.35 When we used low exposure (0.4 L) of CH3I, vibration losses were obtained at 510, 650, 880, 990, 1230, 1410, 2950, and 3050 cm-1 (Figure 3A). Additional peaks at 1630 and 2350 cm-1 are very likely due to some adsorbed water and CO2, respectively. With an increase of the K coverage attenuation of the former losses were experienced. An exception was the vibration loss at 650 cm-1, the intensity of which increased to ΘK ) 0.33 and then markedly decreased. This peak belongs to the surface phonon mode of KI ionic compound.51 Its appearance can be also considered as a strong indication for dissociation of alkyl iodide induced by potassium. Development of the above losses with an increase of CH3I exposure at ΘK ) 1.0 is presented in Figure 3B. Whereas at low exposures the intensities of the losses at ∼2920 and 3050 cm-1 are almost equal, at high exposure (4 L) the

Figure 3. Effects of potassium coverage on the HREEL spectra of Mo2C/Mo(100) exposed to 0.4 L CH3I at 100 K (A), and effects of CH3I exposure on the HREEL spectra of Mo2C/Mo(100) at ΘK ) 1.0 ML (B).

intensity of the loss at 3050 cm-1 is around twice that at ∼2950 cm-1. Another difference: the peak at 1360 cm-1 was a dominant spectral feature up to 1 L CH3I exposure; it was hardly seen at the highest exposure. On annealing the adsorbed layer up to 200 K or higher temperatures (ΘK ) 1.0 ML) the vibrational losses at 520, 890, 1250, 1430, and 3050 cm-1 characteristic for the molecularly adsorbed CH3I disappeared or markedly decreased in intensity. At the same time spectral features at 650, 1250-1260, 1380, and 2930 cm-1 underwent only a slight change up to 250 K


Figure 4. Effects of annealing on the HREEL spectra of Mo2C/ Mo(100) (ΘK ) 1.0 ML) exposed to 10.0 L CH3I (A), and effect of illumination on the HREEL spectra of adsorbed CH3I on K-free and K-dosed Mo2C/Mo(100) at 100 K. Exposure of CH3I was 1.0 L (B).

(Figure 4A). Taking into account the previous results obtained on different surfaces (Table 1), the losses at 1380 and 2930-2950 cm-1 can be clearly attributed to vibration of CH3 species formed in the dissociation process (eq 1). An important feature of the spectra is detection of the vibration loss at 650 cm-1 and its strengthening up to 250 K. This vibration became very weak above 600 K, when the coincidence desorption of K and I begins. All these results confirm the conclusions drawn from the XPS and TPD measurements, namely, that the presence of potassium markedly promotes dissociation of CH3I. In the following experiments we examined the effect of illumination on the behavior of adsorbed CH3I on K-free and K-dosed Mo2C at ∼100 K. Spectra are presented in Figure 4B. The illumination exerted a well-appreciable influence on the HREEL spectra in both cases as indicated by the appearance or intensification of the vibration losses at 650 (on K-dosed surface), 1380, and 2930-2950 cm-1 and the weakening of loss feature at ∼3050 cm-1. All these results support the occurrence of photoinduced cleavage of C-I bond and formation of adsorbed CH3 and I even at ∼100 K. 3.3. Adsorption and Dissociation of C2H5I. 3.3.1. XPS Measurements. On exposing Mo2C/Mo(100) to C2H5I at ∼90 K, the binding energy of I(3d5/2) appeared at 620.0 eV, which

Farkas et al. is characteristic for I in C2H5I (Figure 5A). This value did not change with the variation of exposure, indicating that C2H5I does not dissociate at this temperature on pure Mo2C. A different picture was observed for K-dosed sample. At low exposures, 0.125-1.0 L, the binding energy of I(3d5/2), registered at 619.5-619.1 eV, corresponds to the atomically bonded I, suggesting the occurrence of dissociation of C2H5I (Figure 5B). Above this exposure a new peak appeared at 620.4 eV, which grows with exposure. In the C(1s) region adsorption of ethyl iodide at a low exposure yields a broad peak centered at 284.5 eV, which shifted to higher binding energy with an increase of the exposure. At higher exposures it was centered at 285.2 eV with a full width at half-maximum (fwhm) of 2.6 eV. The large fwhm value suggests that this peak is the result of two overlapping peaks at 285.7 and 285.0 eV corresponding to each of the carbon atoms in the molecule. Selected XPS spectra of adsorbed layers warmed to different temperatures are displayed in Figure 5A and 5C. In the case of K-free surface (Figure 5A) no appreciable changes occurred in the XPS spectra below 130 K. Above this temperature a significant decrease in the intensities of both the I(3d5/2) and the C(1s) signals was observed as a result of the desorption of weakly adsorbed C2H5I. A weak shoulder of the binding energy of I(3d5/2) at 618.4 eV can be resolved at 130 K, the intensity of which exceeded that of the peak at 619.8-620.0 eV above 170-180 K. The low binding energy of I(3d5/2) appeared even at 110 K in the spectrum of Mo2C/Mo(100) at ΘK ) 0.35, and its intensity was commensurable with that of 620.5 eV (Figure 5C). The low-energy peak dominated the spectrum above 140 K. The high binding energy due to molecularly adsorbed C2H5I disappeared or became extremely weak above 200 K. Fewer changes were experienced in the position of the C(1s) signal. At 152 K it centered at 284.8 eV, and at 202 K it was at 284.3 eV. The intensity of the peak radically decreased when the adsorbed layer was heated above 355 K, and it was eliminated completely only above 480-500 K. Following the previous discussion (section 3.1), all these results suggest that rupture of the C-I bond in the adsorbed C2H5I on Mo2C/Mo(100)

C2H5I(a) ) C2H5(a) + I(a)

(9)

is effectively promoted by potassium. 3.3.2. TPD Measurements. Following 8 L of C2H5I exposure to the K-free surface a single desorption peak developed at Tp ) 142, representing desorption of a multilayer of C2H5I. The position of the peak remained practically unaltered with C2H5I exposure. In the presence of potassium another small peak appeared at 176 K, indicating a slight stabilization of C2H5I on the surface. Besides desorption of C2H5I, formation of C2H4 and C2H6 was also observed. The desorption traces at m/z ) 27 were corrected by the molecular fragment intensities of C2H6 and C2H5I. On clean Mo2C/Mo(100) the peak temperatures for ethene desorption were 270 and 454 K and 248 K for ethane desorption (Figure 6). The values somewhat shifted to lower temperatures on K-dosed Mo2C. In addition, release of hydrogen was also observed in the same temperature range as in the case of CH3I adsorption. It is important to mention that there was no sign of formation of CH4, indicating the lack of cleavage of the C-C bond. The coupling of alkyl groups was also not established. 3.3.3. HREELS Measurements. Exposing a K-free and K-covered (1 ML) Mo2C/Mo(100) surface to 8 L C2H5I resulted in formation of multilayers (Figure 7A). Losses appeared at 490, 750, 990, 1045, 1230, 1450, and 2970-2985 cm-1, in good harmony with the characteristic vibrational modes of gaseous



Figure 5. Effects of annealing on the I(3d5/2) XPS signals of adsorbed C2H5I on Mo2C/Mo(100) surface at 100 K: ΘK ) (A) 0.0 and (C) 0.35 ML; exposition of C2H5I was 4.0 L. Effects of C2H5I exposure at 100 K (ΘK ) 1.0 ML) (B).

Figure 6. TPD spectra of ethene (27 amu) (A) and ethane (30 amu) (B) after exposing the clean and K-covered Mo2C/Mo(100) surfaces to 8.0 L C2H5I at 100 K.

and molecularly adsorbed C2H5I (Table 2). At lower exposure the HREEL spectra, particularly for K-dosed Mo2C, were somewhat different, suggesting the occurrence of a reaction in the adsorbed layer (Figure 7B). At 0.5 L exposure losses and shoulders appeared at 550, 890, 970, 1100, 1230, 1370, 1450, 2850, and 2985 cm-1. Note that the position of the deformation modes, δ(CH3), of adsorbed C2H5I and C2H5 are almost the same. The 2850 cm-1 vibration, which developed only when we can count on the presence of adsorbed ethyl, is very likely due to the methyl asymmetric deformation overtone [2δa(CH3)] of C2H5 species. Its appearance also provides an indication of the dissociation of C2H5I. Taking into account the previous assignments (Table 2) the peaks at 890, 970, 1100, and 2850 cm-1 are attributed to vibration of ethyl groups, supporting the conclusion drawn from XPS measurements, namely, that potassium promotes rupture of the C-I bond even at 100 K. When exposure is increased to 1.0 L, weak loss features can be identified at 1650 and 3050 cm-1, which are signs of formation of π-bonded ethylene (Table 2). A further increase in the exposure resulted in the spectrum characteristic of molecularly adsorbed ethyl iodide.50-53 Heating the coadsorbed layer containing a multilayer of C2H5I (ΘK ) 1.0 ML) to 152 K caused attenuation of all peaks

Figure 7. (A) HREEL spectra of adsorbed C2H5I on clean and K-dosed Mo2C/Mo(100) at 100 K. Exposure of C2H5I was 8.0 L. (B) Effects of C2H5I exposure on the HREEL spectra of Mo2C/Mo(100) (ΘK ) 1.0 ML) at 100 K.

resulting from molecular desorption of C2H5I (Figure 8A). Scission of the C-I bond in the chemisorbed state is indicated


Farkas et al.

TABLE 2: Vibrational Frequencies of C2H5I, C2H5, and C2H4 vibrational modes

C2H5I (g)52

C2H5I/Rh (111)53

C2H5I/Mo2C/Mo (111)18

C2H5/Rh (111)53

C2H5/Rh (111)54

νas(CH3) νs(CH3) νas(CH2) νs(CH2) 2δa(CH3) δa(CH3) δ(CH2) ω(CH2) ν(C-C) τ(CH2) F(CH3) F(CH2) ν(C-I) ν(M-C)

2968

2950

2960

2910-2930

2908

1452

1435

1430

1420

2850 1420

1199 949

1236 1000

1220 1025

1150-1180 940

1190 940

736 496

760 516

di-σ-C2H4/Mo2C/Mo (110)55

π-C2H4/Mo2C/Mo (110)55

3010 2935

3065 2975

1395 1180 1035 905

1335,1420 965 1595 1170

850 635 395

by the new losses at 990, 1100, 2850, and 2915 cm-1 corresponding to vibration of C2H5(a) species present up to ∼247 K. The appearance of the vibration losses at 890, 990, 1390, and 3030 cm-1 may belong to the di-σ-bonded ethylene (Table 2), which desorbs with Tp ≈ 252 K (Figure 6A). Similar to the case of CH3I, we also examined the effect of UV illumination on the chemistry of coadsorbed layer. Spectra

380

are presented in Figure 8B. Although at low exposure of C2H5I, 2.0 L, a partial dissociation already occurred in the dark, extended illumination clearly enhanced the extent of the rupture of the C-I bond, as indicated by the appearance of the loss features at 650, 950, 1120, 2850, and 2910 cm-1. The peaks at 1985 and 2376 cm-1 very likely belong to vibration of CO and CO2 adsorbed from the background gas. After prolonged irradiation by UV light, the losses appearing at 1630, 2970, and ∼3050 cm-1 can be related to formation of π-bonded ethylene. The assignment of 1630 cm-1 loss as δ(OH) can be excluded since there is no sign of a ν(OH) mode above 3200 cm-1. 4. Conclusions Preadsorbed potassium effectively promoted rupture of the C-I bond in the adsorbed CH3I on Mo2C/Mo(100). Complete C-I bond breaking was achieved at 60-80 K lower compared to the K-free surface. Formation of CH3 fragment was detected by HREEL spectroscopy. Potassium additive stabilized the adsorbed CH3, on one hand, and facilitated its coupling reactions into ethane and ethylene, on the other. The potassium and iodine arising from dissociation of CH3I desorbed with the same desorption peak temperatures (Tp ) 684 K), indicating their mutual stabilization through formation of a surface ionic compound. This species is characterized by a vibration at 650 cm-1 assigned as a surface phonon mode. Similar features were observed in the case of adsorption of C2H5I on K-promoted Mo2C/Mo(100). In this case, C2H5 species, the primary dissociation product, underwent dehydrogenation to yield ethylene and hydrogenation to give ethane. Rupture of the C-C bond was not observed. Acknowledgment. This work was supported by the OTKA under contract no. NI 69327. References and Notes

Figure 8. Effects of annealing on the HREEL spectra of K-dosed Mo2C/Mo(100) (ΘK ) 1.0 ML) exposed to 4.0 L C2H5I (A), and effects of illumination on the HREEL spectra of adsorbed C2H5I on Mo2C/Mo(100) at ΘK ) 1.0 ML at 100 K (B). Exposure of C2H5I was 2.0 L.

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JP805078D

Adsorption and Reaction of Methyl and Ethyl Iodide on Potassium

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