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Benzene formation from acetylene on Pd(1 1 1): a high-resolution electron energy loss spectroscopy study
Surface Science 470 (2000) L39±L44
www.elsevier.nl/locate/susc
Surface Science Letters
Benzene formation from acetylene on Pd(1 1 1): a highresolution electron energy loss spectroscopy study I. Jungwirthov a a,b, L.L. Kesmodel a,* b
a Department of Physics, Indiana University, Swain Hall West 117, Bloomington, IN 47405, USA Department of Electronics and Vacuum Physics, Charles University, V Holesovi ck ach 2, 180 00 Praha 8, Czech Republic
Received 17 July 2000; accepted for publication 15 September 2000
Abstract High-resolution electron energy loss spectroscopy (HREELS) is employed to detect the formation of benzene following acetylene adsorption on the Pd(1 1 1) surface. At low exposures (0.25 L) the resulting spectra at 253 K are due to ethylidyne formation, but at higher exposures the presence of low coverages of benzene are indicated by the appearance of the strong benzene m4 (720 cmÿ1 ) band. A spectral subtraction method for HREELS is introduced which reveals an essentially complete benzene spectrum. The subtraction method proves favorable in removing spectral features which arise from the competing channel of vinylidene formation. The benzene coverage as a function of acetylene exposure is in close agreement with previous thermal desorption results and consistent with scanning probe microscopy studies. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Alkynes; Aromatics; Chemisorption; Palladium
Benzene formation following the adsorption of acetylene on Pd(1 1 1) at low temperatures was discovered independently by three dierent groups [1±3] nearly two decades ago. The studies employed thermal desorption spectroscopy and, in Refs. [1,2], the transformation of adsorbed C2 H2 overlayer to benzene was con®rmed by comparing the exposure [1] or temperature [2] dependence of UV spectroscopy data to UV spectra taken after direct benzene adsorption on Pd(1 1 1). Benzene desorption peaks observed in these experiments occur at 230 and 500 K. The cyclotrimerization activation energy, preexponential factor, and ini*
Corresponding author. Tel.: +1-812-855-0776; fax: +1-812855-5533. E-mail address: [email protected] (L.L. Kesmodel).
tial rate dependences on temperature were measured by laser-induced thermal desorption/Fourier transform mass spectroscopy [4,5]. Benzene formation was reported for temperatures exceeding 150 K. However, the initial rates remain low up to approximately 170 K. Scanning tunneling microscopy (STM) experiments have provided new insight into the acetylene to benzene transformation as it proceeds on the Pd(1 1 1) surface [6]. At low acetylene exposures (<0.5 L) 1 ordered, 2 2 acetylene domains form on the surface and no mutual chemical reactions of
1 Actual acetylene exposures used in Ref. [6] may have been approximately 2 smaller than indicated in the paper. The correction factor for acetylene pressure readings and shielding eect of the STM tip have not been accounted for [7].
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adsorbed species were observed. At larger exposures p (>0.5 p L) the 2 2 domains transform into 3 3R30° domains p and new p bright spots are observed along the 3 3R30° domain boundaries, interpreted as the reactively formed benzene. In the STM experiment the cyclotrimerization to benzene was observed at temperatures as low as 140 K. Some indications of benzene formation have been reported in previous high-resolution electron energy loss spectroscopy (HREELS) studies [8± 10]. However, this subject has been highly controversial and has remained rather inconclusive to date. Gates and Kesmodel [8] investigated thermally activated transformations of adsorbed acetylene. In these early studies they envisioned only C2 Hx class of species resulting from C2 H2 and the loss at 714 cmÿ1 in the 300 K spectrum was therefore assigned to residual acetylene. Later, Marchon [9] adsorbed acetylene and benzene at 150 K on Pd(1 1 1). He associated a peak at 690 cmÿ1 observed after C2 H2 exposure with the benzene m4 mode. Timbrell et al. [10] argued against association of a peak near 660 cmÿ1 to the benzene m4 mode because the loss at 660 cmÿ1 exhibited an impact energy dependence (negative ion resonant scattering) inconsistent with that of the benzene m4 loss. They found no direct vibrational evidence for benzene formation from C2 H2 on Pd(1 1 1). HREELS used in our work proved to be a powerful technique for investigating the full complexity of acetylene transformations on Pd(1 1 1) as a function of temperature and acetylene exposure. The method allows for a direct identi®cation of dierent species that form on the metal surface before the particular species have desorbed from the surface. In a recent study [11] we observed ethylidyne production, with vinylidene as an intermediate of the reaction, and a subsequent ethylidyne dehydrogenation to CCH species. HREELS data presented in this paper provide clear evidence of an additional reaction channel in which acetylene transforms directly into benzene. The acetylene exposures and temperatures at which benzene is detected on the Pd surface are consistent with the above STM and thermal desorption experiments. This work represents, to our knowledge, the ®rst vibrational spectroscopic evi-
dence for the cyclotrimerization reaction on Pd(1 1 1). We also introduce a spectral subtraction technique which proves useful in identifying the benzene species and which may have general applicability to HREELS studies of surface reactions. The experimental apparatus consisted of an ultrahigh vacuum chamber (base pressure 5 10ÿ11 Torr) equipped for ion sputtering, lowenergy electron diraction, Auger electron spectroscopy, quadrupole mass spectroscopy, and HREELS. The spectrometer was operated with a resolution (FWHM) of 6±7 meV (50±60 cmÿ1 ) and the typical elastic beam rate was 106 cps. Prior to each C2 H2 adsorption, the Pd(1 1 1) sample was cleaned by cycles of Ar sputtering and annealing. Residual carbon was removed by heating the sample in oxygen atmosphere followed by thermal desorption to 1100 K. An acetone stabilizer was removed from the acetylene by passing the mixture through a dry ice/acetone-cooled trap. The surface was exposed to acetylene by back®lling the chamber. Pressure readings were corrected for the ion gauge sensitivity; according to Varian ionization gauge sensitivity tables we used a factor 2. The acetylene was dosed at a low sample temperature (120 K). The temperature was then increased to Table 1 Comparison of benzene modes following 1 L exposure [13] and acetylene modes following 0.5 L exposure [11] on Pd(1 1 1) Mode
Lossa (cmÿ1 )
Benzene m(Pd±C) q(CH) (m4 ) q(CH) (m11 ) d(CH) (m10 ) d(CC) (m13 ) m(CH) (m1 )
470w 720s 810m 1100w 1410w 2990m
Acetylene m(Pd±C) q(CH)sym q(CH)asym d(CH)sym d(CH)asym not assigned m(CC) m(CH)
490w 675s 768m 872m 1030w 1113w 1355w 2985w
a Intensities of losses in specular direction: w weak, m medium, s strong.
CE N IE SC RS CE E A TT RF LE SU
I. JungwirthovaÂ, L.L. Kesmodel / Surface Science 470 (2000) L39±L44
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Fig. 1. Vibrational spectra of species resulting from low-temperature acetylene adsorption and subsequent heating to 253 K. Also shown a chemisorbed benzene spectrum [13]. (a) Specular direction and (b) 6° o specular direction.
the indicated value for a period of 5 min. The sample was then allowed to cool to 120 K, at which point the spectra were recorded. A normalization method described in detail in Refs. [11,12] was used to compare spectra presented in this work. Brie¯y, we obtained a spectrum of clean palladium, divided it by its elastic peak count rate and multiplied by 106 , i.e., the elastic peak count rate for clean palladium is set to 106 cps. Each spectrum for adsorbed acetylene was then multiplied by a constant to normalize the background to the clean palladium spectrum value. The normalization constants were obtained from intervals where no loss peaks are present; these intervals were always found between 1950 and 2700 cmÿ1 . The fundamental diculty in the measurement of acetylene to benzene conversion via electron energy-loss spectroscopy (EELS) stems from small dierences between the acetylene and benzene vibrational mode frequencies relative to the HREELS resolution. To illustrate this point
quantitatively we give, in Table 1, a list of bands observed at low temperature after acetylene [11] and benzene [13] 2 exposures. The relatively low benzene production (10% [6]), i.e., low benzene peak intensities, represents another challenge for the data analysis. An apparent way around the resolution problem is to search for a temperature interval in which acetylene species are either desorbed or transformed into other species while benzene is still present on the Pd surface. We found experimentally that the temperature 253 K falls into this interval, and corresponding HREELS spectra for several acetylene exposures are presented in Fig. 1 and compared to the benzene vibrational spectrum [13,14]. For the 0.25 L exposure no benzene losses are detected, consistent with the low exposure STM measurements [6] and
2 We note that for exposure 6 1 L at 150 K the benzene vibrational spectra are identical to the 300 K spectra [14].
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Fig. 2. Intensity of 726 cmÿ1 peak as a function of acetylene exposure. Inset: Benzene desorption yields from Pd(1 1 1) as a function of acetylene exposure at 170 K [15] (used with permission). The ®tting curves have the same functional form.
thermal desorption experiments [15]. For higher acetylene exposures a well pronounced peak develops at a frequency 726 6 cmÿ1 which we associate with the benzene m4 or q(CH) loss. The intensity of this peak as a function of acetylene exposure is plotted in the main graph of Fig. 2 and is compared with the previously measured [15] thermal desorption yield of benzene (see the inset). The similarity of the two experimental curves provides additional support for our assignment of the 726 cmÿ1 peak as the benzene m4 mode. Despite the absence of acetylene on the Pd surface at 253 K, the only benzene peak clearly seen in higher exposure curves of Fig. 1 is the m4 loss. Other benzene vibrational modes are masked by losses of species formed in a parallel reaction channel. Namely, acetylene also transforms into vinylidene which is a precursor for ethylidyne formation [11]. Fig. 1 shows several indications for the masked benzene peaks when comparing the higher acetylene exposure traces with the benzene spectrum and the 0.25 L curve with exclusively ethylidyne vibrational modes. For example a broad peak near frequency 3000 cmÿ1 can be viewed as a convolution of the ethylidyne (symmetric 2864 cmÿ1 and antisymmetric 2930 cmÿ1 ) and benzene (2990 cmÿ1 ) m(CH) modes. However, since the vinylidene m(CH2 ) loss is at nearly identical frequency (2995 cmÿ1 ) the assignment of the
higher frequency part of the broad peak to benzene requires more elaborate data analysis, as described in the following paragraph. To highlight the masked benzene peaks we subtracted lower exposure spectra from those taken after higher acetylene coverages. Subtraction results of 3.5 and 0.5 L exposures is presented in Fig. 3. Positive values correspond to a higher coverage of the particular species for higher acetylene exposure, and vice versa. From the acetylene exposure dependence of the benzene m4 loss at 726 cmÿ1 (Fig. 2) we expect the benzene spectrum to show up in the positive part of the subtraction curve. Indeed several benzene peaks, highlighted by vertical dotted lines in Fig. 3, are revealed by the subtraction which provides clear evidence for the presence of benzene on the Pd surface at 253 K. Two eects came into play here that allowed the extraction of benzene peaks from HREELS spectra using the subtraction method. First, the ethylidyne coverage at 253 K is lower for 3.5 L acetylene
Fig. 3. Spectrum resulting from subtraction of 0.5 L exposure spectrum from 3.5 L exposure spectrum: (a) both at 253 K and 6° o specular direction and (b) also shown a chemisorbed benzene spectrum at 6° o specular direction [13].
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I. JungwirthovaÂ, L.L. Kesmodel / Surface Science 470 (2000) L39±L44
exposure than for the 0.5 L exposure [11] and hence ethylidyne vibrational modes appear as negative peaks on the subtraction curve. Second, vinylidene peaks whose intensity at 253 K is similar for 0.5 and 3.5 L acetylene exposures [11] nearly cancel. To con®rm the second point we subtracted the 0.25 L trace, with no vinylidene features present, from the 3.5 L exposure data. In this case both benzene and the nearby vinylidene peaks appear in the positive part of the subtraction curve (not shown here) and as expected the benzene losses remained masked, except for the 726 cmÿ1 loss. Employing the same subtraction method we attempted to con®rm the presence of benzene on the Pd surface at lower temperatures at which the surface coverage is still dominated by the adsorbed acetylene. The lowest experimental temperature was 183 K. Note that at this temperature the initial rate of benzene conversion obtained from the laser-induced thermal desorption experiment [4] is high enough to allow for benzene detection after ®ve minutes of the reaction time used in our experiment. Measured traces analogous to those presented in Fig. 1 show no signs of benzene vibrational modes at 183 K (see Fig. 4). To decide whether benzene losses are masked completely by acetylene or some other species, or whether no benzene has formed on the surface up to this temperature, we subtracted the 183 K spectra taken after 3.5 and 0.5 L acetylene exposures. The resulting curve, also shown in Fig. 4, reveals a strong loss at 720 cmÿ1 which con®rms the presence of benzene on the Pd surface at 183 K. Note that other benzene peaks on the subtraction curve overlap with vinylidene losses. Since the vinylidene coverage at 183 K is signi®cantly larger for the 3.5 L acetylene exposure than for the 0.5 L exposure [11] vinylidene losses also appear in the positive part of the subtraction curve. This eect explains the enhanced intensities at 2990 and 1420 cmÿ1 on the subtracted trace compared to the benzene spectrum, a shift of the 1100 cmÿ1 benzene peak to the frequency 1150 cmÿ1 , and an extra strong peak at 880 cmÿ1 on the subtraction curve, assigned to the qw vinylidene mode. The presence of benzene on the Pd(1 1 1) surface following acetylene exposures 0.5±3.5 L was con®rmed for temperatures ranging from 183 to 300
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Fig. 4. Comparison of spectrum obtained: (a) after 3.5 L acetylene exposure at 183 K, (b) spectrum resulting from subtraction of 0.5 L exposure spectrum from 3.5 L exposure spectrum, both at 183 K, and (c) chemisorbed benzene spectrum [13]. Dotted lines represent observed peak positions of chemisorbed benzene [13]. Inset: Intensity of 726 cmÿ1 peak resulted from subtraction of 0.5 L exposure spectra from 3.5 L exposure spectra at indicated temperature.
K. On the subtraction spectra we identi®ed the main benzene loss (726 6 cmÿ1 ) whose intensity as a function of temperature is plotted in the inset of Fig. 4. In several cases, including the 253 K measurement discussed above, we also observed other benzene vibrational modes. Due to the small initial rate for benzene formation at very low temperatures our experimental technique did not permit study of the acetylene to benzene transformation at temperatures below 183 K. Above 300 K, the benzene m4 mode at 720 cmÿ1 is completely masked by a very strong CH bending mode due to CCH formation [11]. 3 The subtraction 3 The CCH species form on the surface as a result of hydrocarbon temperature activated dehydrogenation. A broad hydrogen thermal desorption peak was observed between 300 and 500 K [2].
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method in this case yields complex spectra which are not well reproducible and the benzene losses, if present, cannot be discerned. Acknowledgements This work was supported by the NSF grant no. INT-9807837. References [1] W. Sesselmann, B. Woratschek, G. Ertl, J. K uppers, H. Haberland, Surf. Sci. 130 (1983) 245. [2] W.T. Tysoe, G.L. Nyberg, R.M. Lambert, Surf. Sci. 135 (1983) 128. [3] T.M. Gentle, E.L. Muetterties, J. Phys. Chem. 87 (1983) 2469.
[4] I.M. Abdelrehim, N.A. Thornburg, J.T. Sloan, T.E. Caldwell, D.P. Land, J. Am. Chem. Soc. 117 (1995) 9509. [5] I.M. Abdelrehim, T.E. Caldwell, D.P. Land, J. Phys. Chem. 100 (1996) 10265. [6] T.V.W. Janssens, S. V olkening, T. Zambelli, J. Wintterlin, J. Phys. Chem. B 102 (1998) 6521. [7] J. Wintterlin, private communication. [8] J.A. Gates, L.L. Kesmodel, Surf. Sci. 124 (1983) 68. [9] B. Marchon, Surf. Sci. 162 (1985) 382. [10] P.Y. Timbrell, A.J. Gellman, R.M. Lambert, R.F. Willis, Surf. Sci. 206 (1988) 339. [11] I. Jungwirthov a, L.L. Kesmodel, J. Phys. Chem. (in press). [12] C. Gregoire, L.M. Yu, F. Bodino, M. Tronc, J.J. Pireaux, J. Elect. Spectros. Rel. Phenom. 98±99 (1999) 67. [13] G.D. Waddill, L.L. Kesmodel, Phys. Rev. B 31 (1985) 4940. [14] G.D. Waddill, Ph.D. Thesis, Indiana University, 1987. [15] R.M. Ormerod, R.M. Lambert, J. Phys. Chem. 96 (1992) 8111.
www.elsevier.nl/locate/susc
Surface Science Letters
Benzene formation from acetylene on Pd(1 1 1): a highresolution electron energy loss spectroscopy study I. Jungwirthov a a,b, L.L. Kesmodel a,* b
a Department of Physics, Indiana University, Swain Hall West 117, Bloomington, IN 47405, USA Department of Electronics and Vacuum Physics, Charles University, V Holesovi ck ach 2, 180 00 Praha 8, Czech Republic
Received 17 July 2000; accepted for publication 15 September 2000
Abstract High-resolution electron energy loss spectroscopy (HREELS) is employed to detect the formation of benzene following acetylene adsorption on the Pd(1 1 1) surface. At low exposures (0.25 L) the resulting spectra at 253 K are due to ethylidyne formation, but at higher exposures the presence of low coverages of benzene are indicated by the appearance of the strong benzene m4 (720 cmÿ1 ) band. A spectral subtraction method for HREELS is introduced which reveals an essentially complete benzene spectrum. The subtraction method proves favorable in removing spectral features which arise from the competing channel of vinylidene formation. The benzene coverage as a function of acetylene exposure is in close agreement with previous thermal desorption results and consistent with scanning probe microscopy studies. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Electron energy loss spectroscopy (EELS); Alkynes; Aromatics; Chemisorption; Palladium
Benzene formation following the adsorption of acetylene on Pd(1 1 1) at low temperatures was discovered independently by three dierent groups [1±3] nearly two decades ago. The studies employed thermal desorption spectroscopy and, in Refs. [1,2], the transformation of adsorbed C2 H2 overlayer to benzene was con®rmed by comparing the exposure [1] or temperature [2] dependence of UV spectroscopy data to UV spectra taken after direct benzene adsorption on Pd(1 1 1). Benzene desorption peaks observed in these experiments occur at 230 and 500 K. The cyclotrimerization activation energy, preexponential factor, and ini*
Corresponding author. Tel.: +1-812-855-0776; fax: +1-812855-5533. E-mail address: [email protected] (L.L. Kesmodel).
tial rate dependences on temperature were measured by laser-induced thermal desorption/Fourier transform mass spectroscopy [4,5]. Benzene formation was reported for temperatures exceeding 150 K. However, the initial rates remain low up to approximately 170 K. Scanning tunneling microscopy (STM) experiments have provided new insight into the acetylene to benzene transformation as it proceeds on the Pd(1 1 1) surface [6]. At low acetylene exposures (<0.5 L) 1 ordered, 2 2 acetylene domains form on the surface and no mutual chemical reactions of
1 Actual acetylene exposures used in Ref. [6] may have been approximately 2 smaller than indicated in the paper. The correction factor for acetylene pressure readings and shielding eect of the STM tip have not been accounted for [7].
0039-6028/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 0 8 4 2 - 6
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adsorbed species were observed. At larger exposures p (>0.5 p L) the 2 2 domains transform into 3 3R30° domains p and new p bright spots are observed along the 3 3R30° domain boundaries, interpreted as the reactively formed benzene. In the STM experiment the cyclotrimerization to benzene was observed at temperatures as low as 140 K. Some indications of benzene formation have been reported in previous high-resolution electron energy loss spectroscopy (HREELS) studies [8± 10]. However, this subject has been highly controversial and has remained rather inconclusive to date. Gates and Kesmodel [8] investigated thermally activated transformations of adsorbed acetylene. In these early studies they envisioned only C2 Hx class of species resulting from C2 H2 and the loss at 714 cmÿ1 in the 300 K spectrum was therefore assigned to residual acetylene. Later, Marchon [9] adsorbed acetylene and benzene at 150 K on Pd(1 1 1). He associated a peak at 690 cmÿ1 observed after C2 H2 exposure with the benzene m4 mode. Timbrell et al. [10] argued against association of a peak near 660 cmÿ1 to the benzene m4 mode because the loss at 660 cmÿ1 exhibited an impact energy dependence (negative ion resonant scattering) inconsistent with that of the benzene m4 loss. They found no direct vibrational evidence for benzene formation from C2 H2 on Pd(1 1 1). HREELS used in our work proved to be a powerful technique for investigating the full complexity of acetylene transformations on Pd(1 1 1) as a function of temperature and acetylene exposure. The method allows for a direct identi®cation of dierent species that form on the metal surface before the particular species have desorbed from the surface. In a recent study [11] we observed ethylidyne production, with vinylidene as an intermediate of the reaction, and a subsequent ethylidyne dehydrogenation to CCH species. HREELS data presented in this paper provide clear evidence of an additional reaction channel in which acetylene transforms directly into benzene. The acetylene exposures and temperatures at which benzene is detected on the Pd surface are consistent with the above STM and thermal desorption experiments. This work represents, to our knowledge, the ®rst vibrational spectroscopic evi-
dence for the cyclotrimerization reaction on Pd(1 1 1). We also introduce a spectral subtraction technique which proves useful in identifying the benzene species and which may have general applicability to HREELS studies of surface reactions. The experimental apparatus consisted of an ultrahigh vacuum chamber (base pressure 5 10ÿ11 Torr) equipped for ion sputtering, lowenergy electron diraction, Auger electron spectroscopy, quadrupole mass spectroscopy, and HREELS. The spectrometer was operated with a resolution (FWHM) of 6±7 meV (50±60 cmÿ1 ) and the typical elastic beam rate was 106 cps. Prior to each C2 H2 adsorption, the Pd(1 1 1) sample was cleaned by cycles of Ar sputtering and annealing. Residual carbon was removed by heating the sample in oxygen atmosphere followed by thermal desorption to 1100 K. An acetone stabilizer was removed from the acetylene by passing the mixture through a dry ice/acetone-cooled trap. The surface was exposed to acetylene by back®lling the chamber. Pressure readings were corrected for the ion gauge sensitivity; according to Varian ionization gauge sensitivity tables we used a factor 2. The acetylene was dosed at a low sample temperature (120 K). The temperature was then increased to Table 1 Comparison of benzene modes following 1 L exposure [13] and acetylene modes following 0.5 L exposure [11] on Pd(1 1 1) Mode
Lossa (cmÿ1 )
Benzene m(Pd±C) q(CH) (m4 ) q(CH) (m11 ) d(CH) (m10 ) d(CC) (m13 ) m(CH) (m1 )
470w 720s 810m 1100w 1410w 2990m
Acetylene m(Pd±C) q(CH)sym q(CH)asym d(CH)sym d(CH)asym not assigned m(CC) m(CH)
490w 675s 768m 872m 1030w 1113w 1355w 2985w
a Intensities of losses in specular direction: w weak, m medium, s strong.
CE N IE SC RS CE E A TT RF LE SU
I. JungwirthovaÂ, L.L. Kesmodel / Surface Science 470 (2000) L39±L44
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Fig. 1. Vibrational spectra of species resulting from low-temperature acetylene adsorption and subsequent heating to 253 K. Also shown a chemisorbed benzene spectrum [13]. (a) Specular direction and (b) 6° o specular direction.
the indicated value for a period of 5 min. The sample was then allowed to cool to 120 K, at which point the spectra were recorded. A normalization method described in detail in Refs. [11,12] was used to compare spectra presented in this work. Brie¯y, we obtained a spectrum of clean palladium, divided it by its elastic peak count rate and multiplied by 106 , i.e., the elastic peak count rate for clean palladium is set to 106 cps. Each spectrum for adsorbed acetylene was then multiplied by a constant to normalize the background to the clean palladium spectrum value. The normalization constants were obtained from intervals where no loss peaks are present; these intervals were always found between 1950 and 2700 cmÿ1 . The fundamental diculty in the measurement of acetylene to benzene conversion via electron energy-loss spectroscopy (EELS) stems from small dierences between the acetylene and benzene vibrational mode frequencies relative to the HREELS resolution. To illustrate this point
quantitatively we give, in Table 1, a list of bands observed at low temperature after acetylene [11] and benzene [13] 2 exposures. The relatively low benzene production (10% [6]), i.e., low benzene peak intensities, represents another challenge for the data analysis. An apparent way around the resolution problem is to search for a temperature interval in which acetylene species are either desorbed or transformed into other species while benzene is still present on the Pd surface. We found experimentally that the temperature 253 K falls into this interval, and corresponding HREELS spectra for several acetylene exposures are presented in Fig. 1 and compared to the benzene vibrational spectrum [13,14]. For the 0.25 L exposure no benzene losses are detected, consistent with the low exposure STM measurements [6] and
2 We note that for exposure 6 1 L at 150 K the benzene vibrational spectra are identical to the 300 K spectra [14].
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Fig. 2. Intensity of 726 cmÿ1 peak as a function of acetylene exposure. Inset: Benzene desorption yields from Pd(1 1 1) as a function of acetylene exposure at 170 K [15] (used with permission). The ®tting curves have the same functional form.
thermal desorption experiments [15]. For higher acetylene exposures a well pronounced peak develops at a frequency 726 6 cmÿ1 which we associate with the benzene m4 or q(CH) loss. The intensity of this peak as a function of acetylene exposure is plotted in the main graph of Fig. 2 and is compared with the previously measured [15] thermal desorption yield of benzene (see the inset). The similarity of the two experimental curves provides additional support for our assignment of the 726 cmÿ1 peak as the benzene m4 mode. Despite the absence of acetylene on the Pd surface at 253 K, the only benzene peak clearly seen in higher exposure curves of Fig. 1 is the m4 loss. Other benzene vibrational modes are masked by losses of species formed in a parallel reaction channel. Namely, acetylene also transforms into vinylidene which is a precursor for ethylidyne formation [11]. Fig. 1 shows several indications for the masked benzene peaks when comparing the higher acetylene exposure traces with the benzene spectrum and the 0.25 L curve with exclusively ethylidyne vibrational modes. For example a broad peak near frequency 3000 cmÿ1 can be viewed as a convolution of the ethylidyne (symmetric 2864 cmÿ1 and antisymmetric 2930 cmÿ1 ) and benzene (2990 cmÿ1 ) m(CH) modes. However, since the vinylidene m(CH2 ) loss is at nearly identical frequency (2995 cmÿ1 ) the assignment of the
higher frequency part of the broad peak to benzene requires more elaborate data analysis, as described in the following paragraph. To highlight the masked benzene peaks we subtracted lower exposure spectra from those taken after higher acetylene coverages. Subtraction results of 3.5 and 0.5 L exposures is presented in Fig. 3. Positive values correspond to a higher coverage of the particular species for higher acetylene exposure, and vice versa. From the acetylene exposure dependence of the benzene m4 loss at 726 cmÿ1 (Fig. 2) we expect the benzene spectrum to show up in the positive part of the subtraction curve. Indeed several benzene peaks, highlighted by vertical dotted lines in Fig. 3, are revealed by the subtraction which provides clear evidence for the presence of benzene on the Pd surface at 253 K. Two eects came into play here that allowed the extraction of benzene peaks from HREELS spectra using the subtraction method. First, the ethylidyne coverage at 253 K is lower for 3.5 L acetylene
Fig. 3. Spectrum resulting from subtraction of 0.5 L exposure spectrum from 3.5 L exposure spectrum: (a) both at 253 K and 6° o specular direction and (b) also shown a chemisorbed benzene spectrum at 6° o specular direction [13].
CE N IE SC RS CE E A TT RF LE SU
I. JungwirthovaÂ, L.L. Kesmodel / Surface Science 470 (2000) L39±L44
exposure than for the 0.5 L exposure [11] and hence ethylidyne vibrational modes appear as negative peaks on the subtraction curve. Second, vinylidene peaks whose intensity at 253 K is similar for 0.5 and 3.5 L acetylene exposures [11] nearly cancel. To con®rm the second point we subtracted the 0.25 L trace, with no vinylidene features present, from the 3.5 L exposure data. In this case both benzene and the nearby vinylidene peaks appear in the positive part of the subtraction curve (not shown here) and as expected the benzene losses remained masked, except for the 726 cmÿ1 loss. Employing the same subtraction method we attempted to con®rm the presence of benzene on the Pd surface at lower temperatures at which the surface coverage is still dominated by the adsorbed acetylene. The lowest experimental temperature was 183 K. Note that at this temperature the initial rate of benzene conversion obtained from the laser-induced thermal desorption experiment [4] is high enough to allow for benzene detection after ®ve minutes of the reaction time used in our experiment. Measured traces analogous to those presented in Fig. 1 show no signs of benzene vibrational modes at 183 K (see Fig. 4). To decide whether benzene losses are masked completely by acetylene or some other species, or whether no benzene has formed on the surface up to this temperature, we subtracted the 183 K spectra taken after 3.5 and 0.5 L acetylene exposures. The resulting curve, also shown in Fig. 4, reveals a strong loss at 720 cmÿ1 which con®rms the presence of benzene on the Pd surface at 183 K. Note that other benzene peaks on the subtraction curve overlap with vinylidene losses. Since the vinylidene coverage at 183 K is signi®cantly larger for the 3.5 L acetylene exposure than for the 0.5 L exposure [11] vinylidene losses also appear in the positive part of the subtraction curve. This eect explains the enhanced intensities at 2990 and 1420 cmÿ1 on the subtracted trace compared to the benzene spectrum, a shift of the 1100 cmÿ1 benzene peak to the frequency 1150 cmÿ1 , and an extra strong peak at 880 cmÿ1 on the subtraction curve, assigned to the qw vinylidene mode. The presence of benzene on the Pd(1 1 1) surface following acetylene exposures 0.5±3.5 L was con®rmed for temperatures ranging from 183 to 300
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Fig. 4. Comparison of spectrum obtained: (a) after 3.5 L acetylene exposure at 183 K, (b) spectrum resulting from subtraction of 0.5 L exposure spectrum from 3.5 L exposure spectrum, both at 183 K, and (c) chemisorbed benzene spectrum [13]. Dotted lines represent observed peak positions of chemisorbed benzene [13]. Inset: Intensity of 726 cmÿ1 peak resulted from subtraction of 0.5 L exposure spectra from 3.5 L exposure spectra at indicated temperature.
K. On the subtraction spectra we identi®ed the main benzene loss (726 6 cmÿ1 ) whose intensity as a function of temperature is plotted in the inset of Fig. 4. In several cases, including the 253 K measurement discussed above, we also observed other benzene vibrational modes. Due to the small initial rate for benzene formation at very low temperatures our experimental technique did not permit study of the acetylene to benzene transformation at temperatures below 183 K. Above 300 K, the benzene m4 mode at 720 cmÿ1 is completely masked by a very strong CH bending mode due to CCH formation [11]. 3 The subtraction 3 The CCH species form on the surface as a result of hydrocarbon temperature activated dehydrogenation. A broad hydrogen thermal desorption peak was observed between 300 and 500 K [2].
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method in this case yields complex spectra which are not well reproducible and the benzene losses, if present, cannot be discerned. Acknowledgements This work was supported by the NSF grant no. INT-9807837. References [1] W. Sesselmann, B. Woratschek, G. Ertl, J. K uppers, H. Haberland, Surf. Sci. 130 (1983) 245. [2] W.T. Tysoe, G.L. Nyberg, R.M. Lambert, Surf. Sci. 135 (1983) 128. [3] T.M. Gentle, E.L. Muetterties, J. Phys. Chem. 87 (1983) 2469.
[4] I.M. Abdelrehim, N.A. Thornburg, J.T. Sloan, T.E. Caldwell, D.P. Land, J. Am. Chem. Soc. 117 (1995) 9509. [5] I.M. Abdelrehim, T.E. Caldwell, D.P. Land, J. Phys. Chem. 100 (1996) 10265. [6] T.V.W. Janssens, S. V olkening, T. Zambelli, J. Wintterlin, J. Phys. Chem. B 102 (1998) 6521. [7] J. Wintterlin, private communication. [8] J.A. Gates, L.L. Kesmodel, Surf. Sci. 124 (1983) 68. [9] B. Marchon, Surf. Sci. 162 (1985) 382. [10] P.Y. Timbrell, A.J. Gellman, R.M. Lambert, R.F. Willis, Surf. Sci. 206 (1988) 339. [11] I. Jungwirthov a, L.L. Kesmodel, J. Phys. Chem. (in press). [12] C. Gregoire, L.M. Yu, F. Bodino, M. Tronc, J.J. Pireaux, J. Elect. Spectros. Rel. Phenom. 98±99 (1999) 67. [13] G.D. Waddill, L.L. Kesmodel, Phys. Rev. B 31 (1985) 4940. [14] G.D. Waddill, Ph.D. Thesis, Indiana University, 1987. [15] R.M. Ormerod, R.M. Lambert, J. Phys. Chem. 96 (1992) 8111.