Mo(1 0 0)

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Surface Science 602 (2008) 1497–1506 www.elsevier.com/locate/susc

Adsorption and reactions of dimethyl and diethyl ethers on Mo2C/Mo(1 0 0) A.P. Farkas, F. Solymosi * Reaction Kinetics Research Group, Chemical Research Centre of the Hungarian Academy of Sciences, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary Received 9 November 2007; accepted for publication 15 February 2008 Available online 6 March 2008

Abstract The adsorption, desorption and dissociation of dimethyl ether and diethyl ether on Mo2C/Mo(1 0 0) have been investigated by work function, thermal desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The adsorption of both molecules at 100 K caused a significant decrease in the work function of the Mo2C/Mo(1 0 0) surface. In the case of dimethyl ether almost 90% of the adsorbed monolayer desorbed intact with a Tp = 286 K. Another part decomposed to CO (Tp = 330 and 960 K) and H2 (Tp = 330 and 400 K). The desorption of diethyl ether at monolayer occurred with Tp = 256 and 340 K. Another fraction underwent decomposition as indicated by the release of CO (Tp = 336 and 436 K) and H2 (Tp = 400 K). In addition, the formation of ethylene (Tp = 342 K) and a very small amount of methane (Tp = 380 K) was also observed. HREEL spectra of both ethers confirmed their molecular adsorption at 100 K. From the spectral changes occurred upon increasing the exposures and in off-specular direction some conclusions were drawn on the bonding of the adsorbed molecules. Analysis of the HREEL spectra of the annealed layers suggested that in the primary steps the adsorbed ethers dissociate to methyl and methoxy (dimethyl ether), and to ethyl and ethoxy (diethyl ether) species, which react further to yield the desorption products. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Mo2C/Mo(1 0 0); Dimethyl ether; Diethyl ether; High resolution electron energy loss spectroscopy

1. Introduction Transition metal carbides, possessing electronic properties similar to those of precious metals, exhibit excellent catalytic behaviour in several important reactions [1,2]. One of the most important findings is that Mo2C combined with ZSM-5 is able to convert C1–C8 hydrocarbons into aromatics [3–8]. The aromatization of methane into benzene was commercialized by Japanese industry. Recently, we found that Mo2C/ZSM-5 effectively promotes the aromatization of ethanol [9] and methanol [10], too. However, when Mo2C was deposited on carbon support, the reaction pathway of these alcohols has been altered and their decomposition to H2 comes into prominence [11–13]. In or*

Corresponding author. Fax: +36 62 420 678. E-mail address: [email protected] (F. Solymosi).

0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.02.019

der to obtain a deeper insight into the activation and decomposition of these alcohols, their interaction with Mo2C/Mo(1 0 0) surface has been investigated by several spectroscopic methods (HREELS, TPD and work function measurements) under UHV conditions [14–17]. In the present work we perform similar studies on the adsorption, desorption and dissociation of dimethyl ether (CH3)2O (DME) and diethyl ether (C2H5)2O (DEE) on the same Mo2C/Mo(1 0 0) surface. These compounds are the primary products of the interaction of alcohols with pure and supported Mo2C catalysts. In order to establish the mechanism of the aromatization and decomposition of alcohols on Mo2C we have to know more on the adsorption and dissociation as well as on the reaction pathways of the intermediate products, DME and DEE. In addition, the steam reforming of DME itself seems to be a promising alternative to produce hydrogen. Another motivation is that

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A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

DME, as an alternative fuel, can replace diesel oil, as its burning produces much less pollutants [18]. Therefore, the study of its surface and catalytic chemistry on the potential catalyst materials may provide useful data for the extensive use of this compound. There are only few papers dealing with the adsorption of DME on metal single crystal surfaces [19–24]. It adsorbs weakly and non-dissociatively at 100 K on most surfaces. On polycrystalline Pd DME undergoes a thermally activated decomposition only at 300 K resulting in a layer of chemisorbed CO [19]. A multilayer of DME desorbed from Cu(1 0 0) with a Tp=100 K and a monolayer with a Tp=150 K [20]. Surprisingly, no adsorption of DME was observed over Pt(1 1 1) at 100 K [19]. TPD studies revealed that DME desorbed from Al(1 1 1) without participating in any reaction [21]. DME exhibited very little reactivity on Rh(1 1 1): the adsorbed layer produced at 100 K desorbed with a Tp=150–220 K without detectable decomposition [22]. However, on exposing Rh(1 1 1) to DME at and above 250 K, a partial dehydrogenation occurred, presumably via the transient formation of CHx–O–CHx and CHO surface complexes, the decomposition of which gives the final products, CO and H2. On oxygen-dosed surface, however, methoxy species was clearly identified by HREEL spectroscopy [22]. The formation and stabilization of CH3O species was also established on potassium-dosed Rh(1 1 1) [23]. A recent work studied the orientation of DME on Ag(1 1 0) and Cu(1 1 0) single crystal surfaces with IRAS [24]. The chemistry of DEE has been investigated on Cu(1 0 0) [25], Ag(1 1 0) [26], Ru(0 0 1) [27] and Cu(1 1 1) [28,29] surfaces. Adsorbed DEE on Cu(1 0 0) has four different conformations, trans–trans, trans–gauche and two different gauche– gauche conformers [25]. The lowest energy conformer is the trans–trans and that is the reason why it is the dominant species at low temperature. Accordingly, DEE can be bonded in different modes. Meyers et al. [28] found that the absorptions of DEE on Cu(1 1 1) surface associated with the symmetric methyl (vs(CH3)) and methylene group (vs(CH2)) stretches in the multilayer are also present at 2870 cm1 in the spectrum of the monolayer. Consequently, the molecular plane is not parallel to the surface, since their transition dipole moment vectors lie in the molecular plane.

ular direction were taken the electron beam incidence angle, Hin, was decreased by 15° keeping all other parameters fixed. Count rates in the elastic peak were typically in the range of 1  104–1  105 counts-per-second (cps), and the spectral resolution was between 40 and 50 cm1 full-width at half maximum (FWHM). In the TPD measurements the heating rate was 5 K/s. The Mo(1 0 0) crystal used in this work was the product of Materials Research Corporation, purity 99.99%. Initially the sample was cleaned by cycled heating in oxygen. This was followed by cycled argon ion bombardment (typically 1–2 kV, 1  107 mbar Ar, 10 lA for 10–30 min), and annealing at 1270 K for several minutes. The preparation of Mo2C over Mo(1 0 0) surface has been described before [16]. The Mo(1 0 0) surface was exposed to 200 L of ethylene at 900 K and then flashed to 1200 K in UHV. The partial pressure of ethylene near the sample was about 107 mbar. The resulting surface showed the characteristic three-peaked line shape of carbidic carbon in AES at 255.6, 262.1 and 272.7 eV [16]. The Mo2C/Mo(1 0 0) sample pre-

2. Experimental The experiments were performed in a two-level UHV chamber with a routine base pressure of 5  1010 mbar produced by turbomolecular pump. The setup has been described in detail in our previous paper [16]. Briefly, the chamber was equipped with facilities for Auger electron spectroscopy (AES), high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). The HREEL spectra reported here were acquired with a primary beam energy of 6.5 eV. Angles of incidence and reflection were 60o with respect to the surface normal in the specular direction. When spectra in off-spec-

Fig. 1. Changes in the work function (DØ) of Mo2C/Mo(1 0 0) as a function of DEE and DME exposure at 100 K (A) and annealing temperature (B).

A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

pared in this way has been previously examined by XPS. The binding energies at 227.5–228.2 eV and 230.7– 231.05 eV for Mo(3d5/2) and Mo(3d3/2), respectively, and 283.0 eV for C(1s) correspond very well to the values of Mo2C [30–32]. Dimethyl ether was the product of GHC Gerling Holz & Co., with 99.9% purity, and diethyl ether was from Scharlau, 99.7%. Both compounds were cleaned by several freeze–pump–thaw cycles. Exposures of DME and DEE were not corrected for ionisation gauge sensitivities. According to the measurements of Bartmess and Georgiadis [33] the relative sensitivity of ion gauge for DME is 2.5 and that for DEE is 4.0. 3. Results 3.1. Adsorption of dimethyl ether (DME) 3.1.1. Work function measurements The adsorption of DME resulted in a decrease of the work function of Mo2C/Mo(1 0 0) at 100 K, which reached the final value at 8.0 L exposure. The extent of the change was 3.5 eV. Annealing the sample saturated with DME at 100 K caused a gradual increase in the work function. The

1499

initial value was approached above 800 K. Results are plotted in Fig. 1A and B. 3.1.2. TPD studies Fig. 2A illustrates the TPD spectra obtained at different DME exposures at 95–100 K. The parent molecule (45 amu) desorbs with a peak at 286 K (denoted by b), which is independent of the exposure in the range of 0.5–4.0 L suggesting a first order desorption. At higher exposure a sharp peak also appeared at 106 K (denoted by a), which is very likely due to the condensed layer, as it cannot be saturated (Fig. 2C). The activation energy for the desorption processes was calculated using the Redhead formula. Conventionally, for simpler molecules, frequency factors for desorption are assumed to be in the range of 1012–1015 s1. As was shown, however, small errors of one or two orders of magnitude induce only negligible error in the calculated activation energies [34]. We obtained 27 kJ/mol for the desorption of the condensed layer and 72 kJ/mol for that of adsorbed DME assuming a frequency factor of 1  1013. The desorbed amount of DME in the b peak reached saturation at about 8 L exposure (Fig. 2C).

Fig. 2. TPD spectra of (A) DME (45 amu), (B) hydrogen (2 amu) and CO (28 amu) as a function of DME exposure on Mo2C/Mo(1 0 0) at 100 K. (C) Integrated area the b peak of DME, (D) TPD spectra following the adsorption at 300 K. The contribution of the ethylene to 28 amu has been subtracted from the CO TPD curves.

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Table 1 Characteristic TPD data determined following the adsorption of 8.0 L of DME and DEE on clean Mo2C/Mo(1 0 0) surface at 100 K Product

Tp (K)

Dimethyl ether adsorption DME 106 286 330, 400 H2 CO 330, (960) CH4 (Ta = 300 K) 372 Diethyl ether adsorption DEE 144 256, 340 H2 400 CO 336, 436, (992) C2H4 342 a

Ea (kJ mol1)

State

a

a b

27 72 83, 100 83 93

F b1, b2

36 64, 85 100 84, 109 86

Ea values were calculated with a preexponential factor of 1013 s1.

Besides the DME we identified the release of H2 in peaks with Tp = 330 and 400 K, and CO with Tp = 330 and 960 K (Fig. 2B). Note that the adsorption of H2 on Mo2C/Mo(1 0 0) from the background was experienced in all previous studies [16,17]. It desorbed in a broad peak between 300 and 600 K. The amount of this background hydrogen was subtracted from the measured TPD of H2 after DME adsorption. Due to the limited decomposition of DME the increase in H2 desorption was only 20–30%. Attempts to identify desorption of other possible products, like H2O, CH4 and C2H4 brought no positive result at this adsorption temperature. Taking into account the amounts of desorption products we came to the conclusion that less than 8% of adsorbed DME underwent decomposition. Following the adsorption of DME at 300 K we found the same products with slightly different peak temperatures and a small amount of methane with a Tp372 K (Fig. 2D). TPD data are collected in Table 1. 3.1.3. HREELS measurements On the HREEL spectrum of cleaned Mo2C/Mo(1 0 0) we observed losses at 255 and 571 cm1. Adsorption of DME at low exposure (0.5–4.0 L) on Mo2C/Mo(1 0 0) surface at 100 K produced new vibration losses at 460, 580, 895, 1065, 1156, 1248, 1465, 2817, 2905 and 2990 cm1 (Fig. 3). At higher exposures the 580 cm1 peak was no longer detectable. Increasing the exposure all the losses intensified and the 895 cm1 peak shifted to 906 cm1. With multilayer coverage the loss at 1065 cm1 shifted to 1106 cm1, but no new losses were identified even at 18 L of exposure. HREEL spectra have been also measured in the off-specular (15o) direction (not shown). The intensities of all losses due to adsorbed DME significantly attenuated. New features appeared at 1312 (1380) and 3060 cm1. On heating the sample containing a multilayer (8 L) of DME to 150 K led to a significant attenuation of all peaks without any changes in their positions. New weak spectral signals appeared in the spectrum at 355, 659, 755, 1051 and 1352 cm1 after annealing the sample to 252 K (Fig. 4).

Fig. 3. HREEL spectra of Mo2C/Mo(1 0 0) as a function of DME exposure at 100 K.

Fig. 4. Effects of annealing on the HREEL spectra of Mo2C/Mo(1 0 0) exposed to 8.0 L of DME.

A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

These losses have been also observed at 150 K for the sample dosed only with 1.0 L of DME. Another new feature is the development of a loss at 2032 cm1. Note that the vibration of Mo2C at 571 cm1 also appeared for the annealed sample. All these losses were no longer detectable at or above 300 K, when only very weak peaks at 892, 1160, 1450, 2032 and 2956 cm1 remained on the spectrum, besides the intense feature at 571 cm1. When Mo2C/ Mo(1 0 0) was exposed to high dose (8 L) of DME at 300 K we observed the same weak losses as measured in Fig. 4 at 300 K. 3.2. Adsorption of diethyl ether (DEE) 3.2.1. Work function measurements Similarly to DME the adsorption of DEE also caused a decrease of the work function of Mo2C/Mo(1 0 0) at 100 K. The extent of the change is 2.8 eV, somewhat smaller than in the previous case. Annealing the sample led to the increase of the work function, the initial value was reached at much lower temperature, 500 K, compared

1501

to the case of DME. Results are also plotted in Fig. 1A and B. 3.2.2. TPD studies TPD spectra obtained at different DEE exposures at 100 K presented in Fig. 5. At low exposures the parent molecule (59 amu) desorbs in peaks with Tp380 and 290 K (b2 and b1). Both peaks shifted to lower temperatures, 340 and 256 K at higher exposures. At the same time we obtained a sharp peak at 144 K (a), which is very likely due to the condensed layer, as it cannot be saturated. Plotting the integrated areas of desorbed DEE above 200 K as a function of DEE exposure a saturation was attained at 8 L (see inset in Fig. 5A). As a result of the decomposition of chemisorbed DEE we measured desorption of H2 with Tp = 400 K, ethylene (amu 27) with Tp = 342 K and CO with Tp = 336 and 436 K (Fig. 5B–D). In this case, two or three times more hydrogen was desorbed compared to the background adsorption. A high temperature peak for CO, Tp992 K, was also observed. A very small amount of methane was also formed with Tp = 370 K

Fig. 5. TPD spectra of (A) DEE (59 amu), (B) hydrogen (2 amu), (C) ethylene (27 amu) and (D) CO (28 amu) as a function of DEE exposure on Mo2C/ Mo(1 0 0) at 100 K. In the inset of (A) the integrated areas of a and b peaks are plotted. The contribution of the ethylene to 28 amu has been subtracted from the CO TPD curves.

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(not shown). Search for other compounds, like H2O, CH3CHO and C2H6, brought no positive result. Note that the amount of the products was considerably larger than that measured in the case of DME. From the quantitative analyses of TPD curves it was found that about 30–35% of the adsorbed DEE at monolayer coverage decomposed. A small fraction of DEE underwent complete dissociation to surface carbon. Characteristic data of TPD measurements are given in Table 1. When the adsorption of the DEE was performed at 300 K, we found the desorption of the same products (hydrogen, ethylene, CO) but the peak temperatures were shifted to somewhat higher values. 3.2.3. HREELS measurements Low exposure of DEE (0.5–1.0 L) on Mo2C/Mo(1 0 0) surface at 100 K gave new vibrational losses at 490, 788, 835, 910, 1007, 1085, 1190, 1390, 1464, 2895 and 2986 cm1 (Fig 6). Similarly to the case of DME adsorption the 571 cm1 peak disappeared at higher exposure (above 4 L) and the 490 cm1 loss shifted to 449 cm1. At the same time the shoulders or weak peaks at 788, 910, 1007, 1190, 1329 cm1 disappeared above 4.0 L exposure and transformed into broad and intense losses peaking at 835, 940, 1150, 1285, 1390 and 1464 cm1. Note that while the 2895 cm1 loss was only a weak shoulder of the 2986 cm1 peak at lower exposures, it can be clearly distinguished at high exposures. Some measurements have been performed in off-specular direction (15°). The intensity of the peaks at 835, 1087, 1150, 1461 and 2895 cm1 dramat-

Fig. 7. Effects of annealing on the HREEL spectra of Mo2C/Mo(1 0 0) exposed to 8.0 L of DEE.

ically lowered, and the 1461 cm1 peak shifted to 1450 cm1. On heating the sample containing a multilayer (8 L) of adsorbed DEE the intensity of the losses gradually attenuated, and all became very weak at and above 300 K without any changes in their positions. A new loss developed at 2070 cm1 above 247 K. An important feature is that the losses of pure Mo2C at 255, 419 and 571 cm1 reappeared above 247 K (Fig. 7). When the adsorbed layer prepared only by low exposure (2 L) of DEE at 100 K was annealed, we observed only a gradual attenuation of all losses above 200 K without any observable shifts. A weak CO loss was seen first at 247 K. After room temperature adsorption we obtained only very weak losses, the locations of which corresponded to those observed before at 300 K.

4. Discussion 4.1. Adsorption of dimethyl ether

Fig. 6. HREEL spectra of Mo2C/Mo(1 0 0) as a function of DEE exposure at 100 K.

The adsorption of DME on Mo2C/Mo(1 0 0) at 100 K produced several intense vibrational losses in the HREEL spectra (Fig. 3). In Table 2 the characteristic vibrations of gaseous and adsorbed DME were collected together with those of methoxy and methyl species. Losses identified on Mo2C/Mo(1 0 0) at 100 K using a large exposure correspond very well to the characteristic vibrations of molecu-

A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

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Table 2 Characteristic vibrations of dimethyl ether and its possible dissociation products Vibrational mode

m(C–H) ma(CH3)/A1 m(CH3) 2d(CH3) ms(CH3)/A1 d(CH3), d(C–H)/B1 da(CH3) c(CH3)/A1 ds(CH3) q(CH3), qC–H/B2 ca(CO) ma(COC)/B1 m(CO) ms(COC)/A1 cs(CO) q(CH3), qCY2 d(VJCH3) d(COC) m(VJCH3)

(CH3)2O IR gas-phase [22]

(CH3)2O Rh(1 1 1) [22]

2996, 2925

(CH3)2O Cu(1 1 0) [24]

(CH3)2O multilayer Mo2C/Mo(1 0 0) Ta = 100 K (present work)

2914, 2902

2990, 2905

2801 1462

2817 1465

(CH3)2O Pd(1 1 1) [35]

CH3O(a) Rh(1 1 1) [36]

3015

2935

CH3(a) Rh(1 1 1) [37,38]

CH3O(a) C/Mo(1 0 0) [15]

CH3(a) Mo2C/Mo(1 1 1) [39]

2920

2936

2930

2950 2887 2817 1470–1456

1465

1430

1450

1441 1350

1244, 1179

1175

1248

1370 1150

1185 1171

1156

1094

1106

901

906

1140

1130

1005

1015

1180

1102 1021

885 760 645 450

460

larly adsorbed DME, which are only slightly different from the gas-phase values. For surface science studies DME represents a challenging compound as, being a symmetric molecule, its activation and dissociation on metal surfaces are very difficult. Sexton and coworkers [20] proposed that ethers are adsorbed with one hydrocarbon chain parallel and interacting with the metal surface and one extended away from it. This could be the reason that a major part of adsorbed DME at 100 K desorbs intact from the surface and a minor part, likely having an extra bond to the surface may decompose. The chemistry of DME on clean Mo2C/Mo(1 0 0) exhibits somewhat higher reactivity as compared to metal surfaces [19–23]. As regards its bonding to solid surfaces we can say the following. The adsorbed DME molecule is assumed to have C2v geometry similar to the free DME molecule itself. Increasing its exposure we experienced wavenumber shifts of the losses at 895 and 1065 cm1 due to vs(COC) and va(COC) to 906 and 1106 cm1, respectively. This occurred both in specular and off-specular direction. Following the suggestion of Itoh et al. [24,29], these shifts indicate an alteration of the coordination site from the bridging or three-fold site to an atop coordination site. The fact that the A1 and B1 (B2) modes are present even at low coverage suggests that the adsorbate tilts its axis away from the perpendicular orientation. This is in contrast to the adsorbed DME on Cu(1 1 0), where this orientation occurred only at high coverage [24]. The decomposition of adsorbed DME very likely proceeds on the metal-terminated carbide surface, which was assumed to play an important role in the rupture of the C–I bond in the adsorbed CxHy–I compounds [40]. In the case of carbon-terminated surface, which is less reactive,

325

345

adsorption and reactions may occur on the carbon-deficient sites. The irreversibly adsorbed DME may be bonded to this site by its O atom. The extensive lowering of the work function of Mo2C/Mo(1 0 0) (3.5 eV) following the adsorption of DME (Fig. 1A) suggests a strong electronic interaction between DME and Mo2C/Mo(1 0 0) surface, which very likely consists of an electron donation from the oxygen lone pair to the carbide surface. The extent of Da is larger than observed on Rh(1 1 1) surface, where it was only 1.2 eV [22]. In harmony with this DME desorbed from Rh(1 1 1) without detectable dissociation. By means of TPD measurements we can distinguish three adsorption states of adsorbed DME (Fig. 2). The condensed layer desorbing with an activation energy of 27 kJ mol1, a more strongly adsorbed molecule desorbing with an activation energy of 72 kJ mol1 and an irreversibly bonded DME, which instead of desorption decomposes at higher temperatures to H2 and CO. According to the calculation this DME fraction is only 8% of all the adsorbed molecules. 4.2. Decomposition of adsorbed dimethyl ether TPD measurements revealed that the strongly and irreversibly bonded DME decomposed at higher temperatures according to the overall reaction ðCH3 Þ2 OðaÞ ! COðgÞ þ 3H2ðgÞ þ CðsurfÞ

ð1Þ

When DME was adsorbed at 300 K the formation of CH4 was also detected. Comparing the peak temperatures of the desorption of these compounds with those observed after their adsorption on Mo2C [40,41], we can state that their releases are desorption limited processes.

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The question to be answered is the mode of the dissociation of DME. It may occur by the step-wise removal of H atoms via the transient formation of (CHx–O–CHx)(a), which would decompose to CO, H2 and surface C. Alternatively, we may assume the cleavage of one C–O bond as a primary step ðCH3 Þ2 OðaÞ ! CH3ðaÞ þ CH3 OðaÞ

ð2Þ

which leads to adsorbed methoxy and methyl intermediates. These surface species decompose further according to the elementary steps CH3ðaÞ ¼ CH2ðaÞ þ HðaÞ

ð3Þ

CH2ðaÞ ¼ C þ H2ðgÞ

ð4Þ

CH3ðaÞ þ H ¼ CH4ðgÞ CH3 OðaÞ ! COðaÞ þ 3HðaÞ

ð5Þ ð6Þ

COðaÞ ! CðaÞ þ OðaÞ CðaÞ þ OðaÞ ! COðgÞ

ð7Þ ð8Þ

In order to confirm the occurrence of the primary process (Eq. (2)), we made a great effort to identify either adsorbed CH3 or CH3O. This is not easy at all taking into account the similarities of their vibrational spectra with the parent compound. The main losses of adsorbed CH3 and CH3O species determined on various surfaces are also collected in Table 2. Comparing their vibration with those of DME, we conclude that the spectral features at 345, 645 and 1021 cm1 are characteristic only of adsorbed CH3O, and the peaks at 760 and 1350 cm1 of adsorbed CH3. We detected these spectral features at slightly different wavenumbers when we annealed the adsorbed layer to 250 K after 8.0 L of DME exposure (Fig. 4). These peaks were also observed after heating the 1.0 L adsorbed layer to 150 K (not shown). At low coverage we identified only weak signals of these species at 730 and 1350 cm1 (Fig. 3). The fact that we measured a high temperature (960 K) TPD peak for CO and at the same time no CO loss feature remained on the HREEL spectra above 420 K confirms the occurrence of the dissociation of CO (step 7), and the recombination of C and O (step 8) at higher temperatures. Note that the dissociation of CO on Mo2C/Mo(1 0 0) occurs at 300–350 K, and the recombination process around 960 K [41]. 4.3. Adsorption of diethyl ether Following the adsorption of DEE on Mo2C/Mo(1 0 0) at 100 K we obtained very intense losses in the HREEL spectrum, which are presented with their possible assignment in Table 3. Results obtained on other surfaces are also shown. The chemistry of DEE on Mo2C/Mo(1 0 0) surface is very similar to that of DME. The difference is that the alkyl group is longer, which may affect its reactivity. In the description of the bonding of DEE on this sample, we take into account the results and considerations of previous detailed spectroscopic works

[20,25,28]. Similarly to the results of Meyers et al. [28] we also identified a feature at 2895 cm1 in the HREEL spectra of adsorbed DEE on Mo2C/Mo(1 0 0) even at lower coverage (Fig. 6), which was also present when we collected electrons in the off-specular direction (15°). This suggests that the molecular plane is not parallel to the surface, since its transition dipole moment vector lies in the molecular plane. Furthermore Cook and coworkers [25] proposed that the ma(COC) stretch is not expected to be observed if the C2 axis is perpendicular to the surface, since the dynamic dipole is parallel to the surface. However, if it is perfectly parallel to the surface then the asymmetric COC stretch should dominate the RAIRS spectrum (specular spectra). In the HREEL spectra presented in Fig. 6 we observed a loss at 1150 cm1 in the specular direction at high coverages. This loss decreased in intensity in the off-specular direction suggesting that there are some molecules in which the C2 axis is slightly tilted away from perpendicular position. However, this spectral feature and the absence of the 1451 cm1 in plane mode from the spectrum (which was present in the off-specular direction) and the loss at 1285 cm1 indicate that a large number of molecules (or side chains) is in flat lying position on the surface. Similar to DME, the strongest bond is formed with the adsorption of DEE with its O atom connecting to the metal-terminated carbide surface of Mo2C. A weaker van der Waals or dispersion forces could occur between the carbon chain and the surface, when one side of molecule is lying parallel to the surface. The adsorption of DEE on Mo2C also caused a lowering of its work function, the extent of which was somewhat less, 2.8 eV, than in the previous case. This result can be explained again by an electron donation from the oxygen lone pair to the carbide. TPD studies disclosed that DEE forms an irreversible adsorbed layer, which desorbed in the temperature range of 200–450 K with the variation of the peak temperature, and a condensed layer desorbing with an activation energy of 36 kJ/mole. 4.4. Decomposition of DEE On heating the adsorbed layer, the strongly and irreversibly bonded DEE decomposed to give H2, CO, C2H4 and a small quantity of CH4. Interestingly, the amount of H2 and CO was much larger than in the case of the DME decomposition. In this case we cannot describe the decomposition merely by the following reaction ðC2 H5 Þ2 OðaÞ ¼ COðgÞ þ 5H2ðgÞ þ 3CðsurfÞ

ð9Þ

as the formation of ethylene clearly occurred. Accordingly, we may assume here, too, that the primary step of the decomposition of adsorbed DEE is the rupture of one of the C–O bonds ðC2 H5 Þ2 OðaÞ ! C2 H5 OðaÞ þ C2 H5ðaÞ to give ethoxy and ethyl groups.

ð10Þ

A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

1505

Table 3 Characteristic vibrations of diethyl ether and its possible products Vibrational modes

Trans–trans (C2H5)2O IR solution [28]

(C2H5)2O monolayer Cu(1 1 1) [28]

ma(CH2) ma(CH3) ms(CH3) ms(CH2) + mCH3 + 2  d(CH3) m(CO) d(CH2) da(CH3) in plane da(CH3) out of plane ds(CH3) + xCY2 s(CH2) q(CH2 + CH3) q(CH2) + m(CO) + m(CC) + bends xCY2 m(CO), ma(COC) q(CH3), m(CC) m(CC + CO) + q(CH3) s(CH2) m(CC), ms(CCO), q(CH3) q(CH3 + CH2) q(CH2) d(CCO) ms(MC)

2953 2981

2930, 2961 2976

2872

1492 1454 1453 1381, 1371, 1354 1278 1170 1144

2870

1442 1280 1170 1157

(C2H5)2O multilayer Mo2C/ Mo(1 0 0) Ta = 100 K (present work)

C2H5O(a) Rh(1 1 1) [43]

C2H5O(a) Cu(1 0 0) [16]

2910

2990

2970 2860

1420

1405 1390

1450

C2H5(a) Rh(1 1 1) [42]

1112 1044

2910

2969 2930

1430

1447 1373

2895

1464 1390 1285

1150

1070

940 850

835

1446

1380

1384

1230

1030

1310 1033 953/978

955

1075

880

870

873

876

510

470

713 443 332

487

823 449 395

We made great efforts to prove the formation of these transitional products, ethyl and ethoxy surface species, by HREEL spectroscopy. The vibration losses of adsorbed C2H5 was determined by the photodissociation of adsorbed C2H5I on Rh(1 1 1) at 100 K [42]. We obtained the following major losses: 2910–2920, 1150–1185 and 850 cm1. The most intense ones have been also detected following the partial dissociation of C2H5I on Mo2C/Mo(1 0 0) [44]. Other spectral features attributed to C2H5(a) were ascertained at 955 cm1 on Rh(1 1 1) [43] and 732 cm1 on Ag(1 1 0) [26]. Relevant vibration data are also collected in Table 3. On the basis of these data the appearance of the loss features at 788, 910 and 1190 cm1 at low coverage (0.5–4.0 L, Fig. 6) may indicate the formation of ethyl species in the dissociation process of DEE. These bands were also observed after heating the adsorbed layer to 155 K (Fig. 7). The vibration characteristics of ethoxy groups were obtained following the dissociation of ethanol on Mo2C [12,16] and the coupling of adsorbed O and C2H5 on metal surfaces [43] (Table 3). Accordingly the losses at 490 and 1085 cm1 detected at low coverage (Fig. 6) and during annealing the adsorbed layer (Fig. 7) can be considered as signals of the formation of C2H5O in the primary step of the dissociation of DEE. Ethoxy species formed may decompose through its conversion to acetaldehyde C2 H5 OðaÞ ¼ CH3 CHOðaÞ þ HðaÞ

C2H5O(a) Mo2C/ Mo(1 0 0) [16]

940

848 823

C2H5(a) Mo2C/ Mo(1 1 1) [44]

2941 2986

1150 1130 1047 923

Ethyl/ ethoxide Ag(1 1 0) [26]

ð11Þ

The fact that we did not detect acetaldehyde indicates that it decomposes further to yield CH4 and CO CH3 CHOðaÞ ¼ CH4ðgÞ þ COðgÞ

ð12Þ

As regards the reaction pathways of ethyl species we found earlier that a fraction of adsorbed C2H5 undergoes dissociation on Mo2C/Mo(1 00) above 180–200 K to ethylene C2 H5ðaÞ ¼ C2 H4ðaÞ þ HðaÞ

ð13Þ

which is desorbed with Tp = 230–270 K [44]. Taking into account the peak temperature of the desorption of the products adsorbed separately on Mo2C/Mo(1 0 0) surface [39,40,44] we can conclude that the formation of C2H4 is a reaction limited process, whereas that of CO and hydrogen is a desorption limited reaction. 5. Conclusions

(i) Both the DME and DEE adsorb molecularly on clean Mo2C/Mo(1 0 0) surface at 100 K causing a significant lowering (3.5 and 2.8 eV, respectively) of the work function of the carbide. (ii) By means of TPD we can distinguish condensed, chemisorbed and irreversibly bonded layers. The major part of the adsorbed molecules desorbs intact, and a smaller fraction decomposes to give H2, CO and CH4 (DME) and H2, CO, C2H4 and CH4 (DEE).

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A.P. Farkas, F. Solymosi / Surface Science 602 (2008) 1497–1506

(iii) HREEL spectroscopic studies revealed that the primary step in the dissociation of both molecules is very likely the rupture of one of the C–O bonds to yield CH3(a), CH3O(a), C2H5(a) and C2H5O(a) species, respectively.

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