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Adsorption kinetics of thiophene on single-walled carbon nanotubes (CNTs)
Chemical Physics Letters 447 (2007) 121–126 www.elsevier.com/locate/cplett
Adsorption kinetics of thiophene on single-walled carbon nanotubes (CNTs) J. Goering, U. Burghaus
*
Department of Chemistry, Biochemistry, and Molecular Biology, North Dakota State University, Fargo, USA Received 7 August 2007; in final form 10 September 2007 Available online 14 September 2007
Abstract Metal supported carbon nanotubes (CNTs) appear promising for desulphurization catalysis. Therefore a characterization of the adsorption kinetics of small sulfur containing compounds on clean CNTs is important in order to develop a mechanistic understanding of more complex systems. Presented are thermal desorption spectroscopy (TDS) data of thiophene ðC4 H4 SÞ adsorption on CNTs supported on silica. According to multi-mass TDS, molecular adsorption/desorption is present with a binding energy of (60 ± 2) kJ/mol at small coverages. Alkane–thiophene co-adsorption TDS reveals that C4 H4 S molecules adsorb on interior sites of the CNTs. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction An increasingly large number of ultra-high vacuum (UHV) surface chemistry studies are devoted to characterize fundamental properties of gas-nanotube (NT) interactions which are motivated by possible applications in heterogeneous catalysis [1–7]. Studies on carbon nanotube (CNT) powders already revealed their potential for fuel cells [5] and the Fischer–Tropsch synthesis [8] as expected from the properties of traditionally used graphitic catalysts [5,9]. Another application has recently been explored: namely the hydrodesulfurization (HDS) of sulfur containing compounds catalyzed by metal supported CNTs [10– 12] as well as by clean inorganic MoS2 =WS2 nanotubes [13,14]. In addition, preferential sorption of sulfur compounds by CNTs has been considered [15]. According to studies on single crystal surfaces [16,17] defect sites as well as according to projects on sulfided Mo clusters, [18] the rim of the metal nanoparticles are pivotal for HDS catalysts. Similarly the HDS activity of hollow MoS2 nanoparticles was likened to an interplay of defect and confinement *
Corresponding author. Fax: +1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.chem.ndsu.nodak.edu
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.015
effects [13]. In another recent study, an enhancement for methanation activity in MoS2 nanotube powder catalysts has been reported [14] and was assigned to confinement effects. Furthermore, inorganic nanotube powders [19–21] often consist of a mixture of nanoparticles and nanotubes which makes it difficult to isolate effects. Therefore, it is not certain at this point if the curvature of the clean nanotube surface, confinement effects, and/or defects in the tubular structure dominate the catalytic reactivity of NT for HDS. However, the final result of this controversy will affect the synthesis strategies for optimizing the catalytic activity of NT for HDS. To characterize confinement effects and gas-NT interactions, it appears useful to study first an expected less reactive, clean (without metal clusters deposited) NT system with a small intrinsic defect density such as CNTs, in order to compile reference data for metal supported NTs. In HDS model studies, thiophene is the probe molecules of choice since it is among the simplest sulfur containing compounds present in natural petroleum. From studies on metal (see e.g. Ref. [22]) or metal oxide (see e.g. Ref. [23]) surfaces an initially flat adsorption of thiophene has been concluded and tilted molecules forming a compressed monolayer at larger coverages are frequently observed. Besides an amorphous phase, five different crystalline phases are documented for condensed films [24,25]
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with crystallization temperatures between 90 and 235 K. At UHV condition a crystalline phase has been observed by IR (infrared) spectroscopy for adsorption temperatures of 125 K and exposures above 20 Langmuir (L) of thiophene [26]. We present thiophene TDS for a CNT monolayer supported on silica. In addition, thiophene and n-pentane coadsorption experiments have been conducted which provide strong evidence for confinement effects. 2. Experimental procedures The experiments have been conducted with a standard UHV system including a shielded mass spectrometer [27]. All gas exposures are given in Langmuir ð1L ¼ 1 sec at 1 106 torrÞ; a heating rate of 1.7 K/sec has been used. The reading of the thermocouple has been calibrated with an accuracy of 5 K by means of the known condensation temperatures of alkanes [28]. The single-walled CNTs (high pressure CO disproportionation (HiPCo) powders from Carbon Nanotechnologies Inc.) were dispersed on a 10 10 mm silica wafer by the drop-and-dry technique; the sample has been annealed in a tube furnace at 600 K in N2 for 30 min as well as flashed in UHV to 400 K in order to clean off solvent (SDS – sodium dodecyl sulfate) residuals. The sample morphology has been characterized by scanning electron microscopy and n-pentane TDS [28]. Thiophene and pentane (HPLC grade from Sigma-Aldrich) have been further cleaned by multiple freeze-pump-thaw cycles. Mass spectra similar to those in the NIST database have been obtained. 3. Data presentation and discussion Fig. 1 summarizes multi-mass TDS data which reveal molecular adsorption/desorption of thiophene on CNTs. In the case of a bond activation, the desorption of H2 S, H2 , and small chain alkanes (methane, ethane, propane, butane) are expected which would give rise to mass spectrometer readings at m=e ¼ 16; 27; 29; 30; 41; 43; 53; 54 which have not been observed within the sensitivity limits of the experiments. The multi-mass TDS signal is consistent with mass scans obtained by backfilling the UHV chamber with thiophene (Fig. 1I). Furthermore, the TDS peaks of thiophene fragments are all centered at about the same temperature of 170 K (Fig. 1II). A minor water TDS signals (smaller than 2% of the most abundant thiophene fragment) has been observed at 200 K indicating a background adsorption. Fig. 2I depicts a set of thiophene TDS data for small exposures, v, and Fig. 2II for large exposures. In the former case, a single TDS peak (a-peak) shifting from 232 K to 190 K with increasing exposure has been observed which is consistent with repulsive lateral interactions or with desorption of thiophene from kinetically distinct adsorption sites on the CNTs. Assuming a standard 1st order pre-exponential factor ðm ¼ 1 1013 = secÞ leads to a bind-
Fig. 1. (I) Comparison of thiophene fragmentation patter as obtained by TDS and by backfilling ð5 108 mbarÞ the UHV chamber, indicating molecular adsorption. (II) TDS spectra of the most intense thiophene fragment masses.
ing energy of E0d ¼ ð60 2Þ kJ/mol in the limit of zero coverage, applying a Redhead analysis of the TDS peak position. This binding energy is in agreement with results (58 kJ/mol) obtained from equilibrium vapor-phase isotherms for C4 H4 S adsorption on CNTs powders [15]. For defect free MoS2 single crystals, even smaller values of 40 kJ/mol have been reported, [29] and for Au(1 1 1) binding energies in the range of (46–75) kJ/mol, depending on the adsorption site, were determined [22]. Increasing the exposure of C4 H4 S leads to the detection of a low temperature peak (c-peak at 150 K) shifting to larger temperatures with coverage such that the leading edges line up. This TDS peak could not be saturated. Therefore, the c-peak is assigned to the formation of an amorphous thiophene film following most likely 0th order kinetics. A leading edge analysis gives a binding energy of ð33 4Þ kJ/mol and a 0th order pre-exponential of 10ð21:71:6Þ = sec. Assigning the TDS curve detected at the
J. Goering, U. Burghaus / Chemical Physics Letters 447 (2007) 121–126
Fig. 2. TDS of thiophene as a function of exposure, v, for (I) large and (II) small exposures. (III) Coverage, H, dependent heat of adsorption, Ed , as extracted from the TDS peak positions assuming a pre-exponential of m ¼ 1 1013 =s. The curve can be parameterized by Ed ¼ 52:6 11Hþ 9 expðH=0:01Þ in kJ/mol.
onset of the c-peak (labeled by a star in Fig. 2II) to saturation (‘1 ML’) of the CNTs sample allows for a coverage calibration by integrating the set of TDS curves.1 Using 1 For a similarly prepared CNT sample [6] an enhancement of the surface area by internal adsorption sites (as compared with a planar catalyst) by a factor of 40 has been estimated. A precise determination of the total surface area (absolute coverage) is, however, challenging.
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the integrated TDS areas yields the coverage dependent heat of adsorption, Ed ðHÞ, (by means of the Redhead equation) which is shown in Fig. 2III. Ed ðHÞ decreases initially rapidly from 60 to 53 kJ/mol within a very small coverage interval of 0.04 ML. This indicates initial adsorption of C4 H4 S on high binding energy sites, the effect of repulsive lateral interactions, or a change in the adsorption geometry (flat vs. tilted thiophene). The small coverage associated with this initial decrease in Ed ðHÞ suggests adsorption on defect sites along the CNT surface as the most favorable explanation. For coverages above 0.04 ML, Ed decreases linearly which would be typical for repulsive lateral interactions or consistent with a superposition of TDS structures associated with kinetically distinct adsorption sites. At exposures above 20 L where the c-peak also started to grow, two weak TDS features are seen at 184 K (b-peak) and 197 K (cr-peak). The leading edges of the cr-peak appear to line up and increasing the adsorption temperature (100–130 K) increases the cr-peak intensity with respect to the c-peak. TDS features similar to the cr-peak have been obtained for condensed methanol (MeOH) films [30–32] and have been assigned to a crystalline phase on top of a disordered buffer layer compensating for the difference in the lattice match of the monolayer and crystalline film. Mixed amorphous/crystalline MeOH films formed. Although these TDS features have apparently not been observed before for thiophene, a crystalline phase also has been identified for C4 H4 S by IR spectroscopy [26]. Therefore, we assign tentatively the cr-peak to a crystalline thiophene adsorption phase embedded in an amorphous film (c-peak) and the b-peak to a buffer layer on top of the saturated surface (cf., Fig. 12 in Ref. [30]). Note that, the intensity of the cr/b-peaks are too small to assign these features to adsorption on interior or groove sites on the CNTs. In addition, these peaks are clearly related with a condensed phase of thiophene. The maximum molecular size of thiophene (as deter˚ . Theremined by Gaussian calculations) amounts to 4.6 A fore, adsorption on interior sites of the CNTs with an ˚ is expected. Unfortuaveraged diameter [28,33] of 13.6 A nately, no distinct monolayer TDS structures which could easily be assigned to CNT specific adsorption sites have been observed. Therefore, co-adsorption experiments with alkanes [2,34] have been conducted since, for example, n˚ ) adsorption leads to characteristic TDS pentane (6.9 A peaks which can be assigned to adsorption on external (C-peak), groove (B-peak), and internal (A-peak) adsorption sites. A typical n-pentane TDS curve (which is consistent with published data [28]) is shown in Fig. 3I together with fits of gauss functions, just to indicate the contributions of the desorption rate from the kinetically distinct adsorption sites. The D-peak is an adsorption site unspecific condensation peak. This peak assignment is based on IR spectroscopy results and on determining filling factors, see Ref. [28]. Site blocking experiments have often been used to help identifying possible adsorption sites [2,27].
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Fig. 3. (I) N-pentane (50 L) TDS reference curve (solid spheres) indicating possible adsorption sites on the CNTs. The solid lines are the result of a fit with gauss functions. (II and III) N-pentane–thiophene co-adsorption experiments for 2 L n-pentane and v Langmuir thiophene. The simultaneously detected n-pentane and thiophene TDS curves are shown in panel II and III, respectively.
Fig. 3II/III depict the result of thiophene-pentane coadsorption TDS where a constant exposure of n-pentane
(2 L) and different amounts of thiophene (as indicated) have been exposed. Fig. 3II shows the pentane TDS curves and Fig. 3III the simultaneously detected C4 H4 S desorption rate. The starting curve (2 L n-pentane, 0 L thiophene) of this set of experiments is dominated by the adsorption of pentane on A and B sites (Fig. 3II). Increasing the C4 H4 S exposure, v, leads first to an increase in the C-peak TDS intensity which is accompanied with decreasing A/B-peaks and finally to the appearance of the D-peak at large v. (The curves in Fig. 3II are plotted over each other to allow for an intensity comparison; the total pentane TDS area is approx. conserved since always the same amount of pentane has been exposed.) Thus, the alkane and sulfur compound compete about identical adsorption sites including interior sites (A-peak) of the CNTs. As panel III in Fig. 3 depicts, the thiophene TDS intensity simply increases, reproducing the results already shown in Fig. 2. In order to facilitate a more quantitative discussion, the pentane-thiophene TDS data are reproduced in Fig. 4I, including fits with gauss functions. The curves are offset to allow following the filling sequence of the sites more clearly. Fig. 4II shows the TDS peak intensities as estimated from the fits. The thiophene molecules are initially replacing part of the interior and groove site adsorbed pentane, i.e., the A and B alkane TDS peak intensities decrease (Fig. 4II). However, no complete site blocking is seen, i.e., a mixed pentane–thiophene phase forms, as expected, since the binding energies of those two molecules do not differ dramatically. Interestingly, although the A sites are the highest binding energy sites for pentane, pentane adsorbed on groove sites appear to be replaced by thiophene efficiently as well; see the intensity decrease of the B-peak in Fig. 4II. This may indicate that groove sites are sterically favored over internal sites for the adsorption of C4 H4 S. We may speculate about upright thiophene adsorption with the sulfur atom towards the CNTs and the ring perpendicular with respect to the main CNT axis. In sympathy with the decrease of the A and B pentane peaks, the C peak intensity initially increases. Thus, the pentane molecules displaced from interior and groove sites are forced to C (external) sites. At even larger exposures, the external sites are occupied by thiophene (C pentane peak intensity decreases) pushing pentane to adsorption site unspecific condensation sites (D peak intensity increases further). Thus, the filling sequence ðA and B ! C ! DÞ basically follows the binding energy sequence (for alkanes) of the different adsorption sites. This suggests that thiophene adsorbs on the same sites as n-pentane with the same sequence of the binding energies. Due to the relatively small difference in the binding energies no complete site blocking is seen, i.e., mixed alkane-thiophene co-adsorption phases form. Similar experiments have recently been conducted with methanol and n-pentane [34]. In this case, the rather linear n-pentane molecule could not as efficiently be replaced by the more spherical methanol from the linear groove sites. Instead external sites were predominantly decorated by
J. Goering, U. Burghaus / Chemical Physics Letters 447 (2007) 121–126
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for other linear and branched alcohols from methanol to hexanol [35]). Indeed, the dipole-dipole interaction energies, Ed-d , are significant as compared with the binding energies, assuming a standard distance dependence ðEd-d ¼ 2l2 =r3 Þ. For example, at a binding distance of ˚ which equals the dimensions of the graphite unit r ¼ 2:4A, cell, Ed-d amounts to 10% of the binding energy of thiophene and is even larger for MeOH. Thus, differences in the dipole moments can affect the adsorption kinetics via lateral interaction. For example, thiophene appears to adsorb efficiently on groove sites while methanol does not. We may speculate that strong intermolecular interactions in dense methanol layers are a possible explanation which is less applicable for thiophene because of its smaller dipole moment. However, a sophisticated analysis should also include indirect interactions such as a modification of the electronic structure of the CNTs caused by the adsorbates. Unfortunately, the experimental and theoretical data base is still small. In summary, the following information has been collected:
Fig. 4. (I) Reprinted n-pentane data (solid spheres) from Fig. 3II, however, including fits to gauss functions (solid lines). The curves have been offset to visualize the filling sequence of sites. (II) Intensity variations in the n-pentane TDS peaks with thiophene exposure, as obtained from the fits. The solid lies are included as a guide for the eye.
the alcohol, especially at larger total exposures of both molecules [35]. In addition to differences in the molecular size of methanol, pentane, and thiophene, the dipole moments, l, differ significantly which may also qualitatively explain observed trends. The non-polar alkanes (from butane to pentane) [28] show the most distinct CNT induced TDS features while interacting with the non-polar graphitic sample. (Note that also the non-polar CCl4 shows distinct CNT induced features [28]). Whereas polar methanol (2.16 debye – from Gaussian) and thiophene (0.83 debye) do not show any distinct TDS features in the monolayer range. (Similar results have been obtained
Thiophene adsorbes molecularly. At large exposures indications for a crystalline phase are evident. Thiophene adsorbed on internal sites of the CNTs, which will be important for catalysis applications. Kinetics parameters have been obtained such as the binding energies in the monolayer and multilayer coverage range including the coverage dependence for small exposures and the pre-exponential factor for the condensed phase. This information will be important to gain a mechanistic understanding of surface reactions. Polar thiophene as well as alcohols [35] do not show distinct CNT induced TDS features which have been seen for non-polar alkanes and CCl4 . The latter holds true independently of the size and molecular structure (linear vs. branched) of the probe molecules since alkanes [28,6] from butane up to pentane (as well as CCl4 ) [28] and alcohols [34,35] from methanol up to hexanol (as well as thiophene) show distinct differences in their kinetics. Possible explanations such as differences in the dipole moments of the probe molecules have been discussed. Acknowledgement Financial support by the DoE Office of Science under Award DE-FG02-06ER46292 (ND State Grant) is acknowledged. References [1] O. Byl, J.C. Liu, Y. Wang, W.L. Yim, J.K. Johnson, J.T. Yates, J. Am. Chem. Soc. 128 (2006) 12090. [2] P. Kondratyuk, J.T. Yates, Chem. Phys. Lett. 410 (2005) 324. [3] H. Ulbricht, G. Moos, T. Hertel, Surface Sci. 532–535 (2003) 852.
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[4] M. Muris, N. Dupont-Pavlovsky, M. Beinfait, P. Zeppenfeld, Surface Sci. 492 (2001) 67. [5] A.L. Dicks, J. Power Sources 156 (2006) 128. [6] S. Funk, B. Hokkanen, T. Nurkig, U. Burghaus, B. White, S. O’Brien, N. Turro, J. Phys. Chem. C 111 (2007) 8043. [7] S. Funk, B. Hokkanen, U. Burghaus, A. Ghicov, P. Schmuki, Nano Lett. 7 (2007) 1091. [8] M.C. Bahome, L.L. Jewell, D. Hildebrandt, D. Glasser, N.J. Coville, Appl. Catal., A: General 287 (2005) 60. [9] X. Wu, V. Schwartz, S.H. Overbury, T.R. Armstrong, Energy & Fuels 19 (2005) 1774. [10] K. Dong, X. Ma, H. Zhang, G. Lin, J. Natural Gas Chem. 15 (2006) 28. [11] X. Li, D. Ma, L. Chen, X. Bao, Catal. Lett. 116 (2007) 63. [12] X.C. Song, Y.F. Zheng, Y. Zhao, H.Y. Yin, Mater. Lett. 60 (2006) 2346. [13] N.A. Dhas, K.S. Suslick, J. Am. Chem. Soc. 127 (2005) 2368. [14] J. Chen, S.L. Li, Q. Xu, K. Tanaka, Chem. Commun. (2002) 1722. [15] D. Crespo, R.T. Yang, Ind. Eng. Chem. Res. 45 (2006) 5524. [16] R. Prins, V.H.J. De Beer, G.A. Somorjai, Catal. Rev. Sci. Eng. 31 (1989) 1. [17] H. Topsoe, B.S. Clausen, F.E. MassothHydrotreating Catalysts, Science and Technology, vol. 11, Springer, Berlin, 1996. [18] J. Kibsgaard, J.V. Lauritsen, E. Laegsgaard, B.S. Clausen, H. Topsoe, F. Besenbacher, J. Am. Chem. Soc. 128 (2006) 13950.
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R. Tenne, Nature Nanotechnol. 1 (2006) 103. C.N.R. Rao, M. Nath, Dalton Trans. 1 (2003) 1. M. Remskar, Adv. Mater. 16 (2004) 1497. L. Gang Liu, J.A. Rodriguez, J. Dvorak, J. Hrbek, T. Jirsak, Surface Sci. 505 (2002) 295. M.N. Hedhili, B.V. Yakshinskiy, T.W. Schlereth, T. Gouder, T.E. Madey, Surface Sci. 574 (2005) 17.
Adsorption kinetics of thiophene on single-walled carbon nanotubes (CNTs) J. Goering, U. Burghaus
*
Department of Chemistry, Biochemistry, and Molecular Biology, North Dakota State University, Fargo, USA Received 7 August 2007; in final form 10 September 2007 Available online 14 September 2007
Abstract Metal supported carbon nanotubes (CNTs) appear promising for desulphurization catalysis. Therefore a characterization of the adsorption kinetics of small sulfur containing compounds on clean CNTs is important in order to develop a mechanistic understanding of more complex systems. Presented are thermal desorption spectroscopy (TDS) data of thiophene ðC4 H4 SÞ adsorption on CNTs supported on silica. According to multi-mass TDS, molecular adsorption/desorption is present with a binding energy of (60 ± 2) kJ/mol at small coverages. Alkane–thiophene co-adsorption TDS reveals that C4 H4 S molecules adsorb on interior sites of the CNTs. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction An increasingly large number of ultra-high vacuum (UHV) surface chemistry studies are devoted to characterize fundamental properties of gas-nanotube (NT) interactions which are motivated by possible applications in heterogeneous catalysis [1–7]. Studies on carbon nanotube (CNT) powders already revealed their potential for fuel cells [5] and the Fischer–Tropsch synthesis [8] as expected from the properties of traditionally used graphitic catalysts [5,9]. Another application has recently been explored: namely the hydrodesulfurization (HDS) of sulfur containing compounds catalyzed by metal supported CNTs [10– 12] as well as by clean inorganic MoS2 =WS2 nanotubes [13,14]. In addition, preferential sorption of sulfur compounds by CNTs has been considered [15]. According to studies on single crystal surfaces [16,17] defect sites as well as according to projects on sulfided Mo clusters, [18] the rim of the metal nanoparticles are pivotal for HDS catalysts. Similarly the HDS activity of hollow MoS2 nanoparticles was likened to an interplay of defect and confinement *
Corresponding author. Fax: +1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.chem.ndsu.nodak.edu
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.015
effects [13]. In another recent study, an enhancement for methanation activity in MoS2 nanotube powder catalysts has been reported [14] and was assigned to confinement effects. Furthermore, inorganic nanotube powders [19–21] often consist of a mixture of nanoparticles and nanotubes which makes it difficult to isolate effects. Therefore, it is not certain at this point if the curvature of the clean nanotube surface, confinement effects, and/or defects in the tubular structure dominate the catalytic reactivity of NT for HDS. However, the final result of this controversy will affect the synthesis strategies for optimizing the catalytic activity of NT for HDS. To characterize confinement effects and gas-NT interactions, it appears useful to study first an expected less reactive, clean (without metal clusters deposited) NT system with a small intrinsic defect density such as CNTs, in order to compile reference data for metal supported NTs. In HDS model studies, thiophene is the probe molecules of choice since it is among the simplest sulfur containing compounds present in natural petroleum. From studies on metal (see e.g. Ref. [22]) or metal oxide (see e.g. Ref. [23]) surfaces an initially flat adsorption of thiophene has been concluded and tilted molecules forming a compressed monolayer at larger coverages are frequently observed. Besides an amorphous phase, five different crystalline phases are documented for condensed films [24,25]
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with crystallization temperatures between 90 and 235 K. At UHV condition a crystalline phase has been observed by IR (infrared) spectroscopy for adsorption temperatures of 125 K and exposures above 20 Langmuir (L) of thiophene [26]. We present thiophene TDS for a CNT monolayer supported on silica. In addition, thiophene and n-pentane coadsorption experiments have been conducted which provide strong evidence for confinement effects. 2. Experimental procedures The experiments have been conducted with a standard UHV system including a shielded mass spectrometer [27]. All gas exposures are given in Langmuir ð1L ¼ 1 sec at 1 106 torrÞ; a heating rate of 1.7 K/sec has been used. The reading of the thermocouple has been calibrated with an accuracy of 5 K by means of the known condensation temperatures of alkanes [28]. The single-walled CNTs (high pressure CO disproportionation (HiPCo) powders from Carbon Nanotechnologies Inc.) were dispersed on a 10 10 mm silica wafer by the drop-and-dry technique; the sample has been annealed in a tube furnace at 600 K in N2 for 30 min as well as flashed in UHV to 400 K in order to clean off solvent (SDS – sodium dodecyl sulfate) residuals. The sample morphology has been characterized by scanning electron microscopy and n-pentane TDS [28]. Thiophene and pentane (HPLC grade from Sigma-Aldrich) have been further cleaned by multiple freeze-pump-thaw cycles. Mass spectra similar to those in the NIST database have been obtained. 3. Data presentation and discussion Fig. 1 summarizes multi-mass TDS data which reveal molecular adsorption/desorption of thiophene on CNTs. In the case of a bond activation, the desorption of H2 S, H2 , and small chain alkanes (methane, ethane, propane, butane) are expected which would give rise to mass spectrometer readings at m=e ¼ 16; 27; 29; 30; 41; 43; 53; 54 which have not been observed within the sensitivity limits of the experiments. The multi-mass TDS signal is consistent with mass scans obtained by backfilling the UHV chamber with thiophene (Fig. 1I). Furthermore, the TDS peaks of thiophene fragments are all centered at about the same temperature of 170 K (Fig. 1II). A minor water TDS signals (smaller than 2% of the most abundant thiophene fragment) has been observed at 200 K indicating a background adsorption. Fig. 2I depicts a set of thiophene TDS data for small exposures, v, and Fig. 2II for large exposures. In the former case, a single TDS peak (a-peak) shifting from 232 K to 190 K with increasing exposure has been observed which is consistent with repulsive lateral interactions or with desorption of thiophene from kinetically distinct adsorption sites on the CNTs. Assuming a standard 1st order pre-exponential factor ðm ¼ 1 1013 = secÞ leads to a bind-
Fig. 1. (I) Comparison of thiophene fragmentation patter as obtained by TDS and by backfilling ð5 108 mbarÞ the UHV chamber, indicating molecular adsorption. (II) TDS spectra of the most intense thiophene fragment masses.
ing energy of E0d ¼ ð60 2Þ kJ/mol in the limit of zero coverage, applying a Redhead analysis of the TDS peak position. This binding energy is in agreement with results (58 kJ/mol) obtained from equilibrium vapor-phase isotherms for C4 H4 S adsorption on CNTs powders [15]. For defect free MoS2 single crystals, even smaller values of 40 kJ/mol have been reported, [29] and for Au(1 1 1) binding energies in the range of (46–75) kJ/mol, depending on the adsorption site, were determined [22]. Increasing the exposure of C4 H4 S leads to the detection of a low temperature peak (c-peak at 150 K) shifting to larger temperatures with coverage such that the leading edges line up. This TDS peak could not be saturated. Therefore, the c-peak is assigned to the formation of an amorphous thiophene film following most likely 0th order kinetics. A leading edge analysis gives a binding energy of ð33 4Þ kJ/mol and a 0th order pre-exponential of 10ð21:71:6Þ = sec. Assigning the TDS curve detected at the
J. Goering, U. Burghaus / Chemical Physics Letters 447 (2007) 121–126
Fig. 2. TDS of thiophene as a function of exposure, v, for (I) large and (II) small exposures. (III) Coverage, H, dependent heat of adsorption, Ed , as extracted from the TDS peak positions assuming a pre-exponential of m ¼ 1 1013 =s. The curve can be parameterized by Ed ¼ 52:6 11Hþ 9 expðH=0:01Þ in kJ/mol.
onset of the c-peak (labeled by a star in Fig. 2II) to saturation (‘1 ML’) of the CNTs sample allows for a coverage calibration by integrating the set of TDS curves.1 Using 1 For a similarly prepared CNT sample [6] an enhancement of the surface area by internal adsorption sites (as compared with a planar catalyst) by a factor of 40 has been estimated. A precise determination of the total surface area (absolute coverage) is, however, challenging.
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the integrated TDS areas yields the coverage dependent heat of adsorption, Ed ðHÞ, (by means of the Redhead equation) which is shown in Fig. 2III. Ed ðHÞ decreases initially rapidly from 60 to 53 kJ/mol within a very small coverage interval of 0.04 ML. This indicates initial adsorption of C4 H4 S on high binding energy sites, the effect of repulsive lateral interactions, or a change in the adsorption geometry (flat vs. tilted thiophene). The small coverage associated with this initial decrease in Ed ðHÞ suggests adsorption on defect sites along the CNT surface as the most favorable explanation. For coverages above 0.04 ML, Ed decreases linearly which would be typical for repulsive lateral interactions or consistent with a superposition of TDS structures associated with kinetically distinct adsorption sites. At exposures above 20 L where the c-peak also started to grow, two weak TDS features are seen at 184 K (b-peak) and 197 K (cr-peak). The leading edges of the cr-peak appear to line up and increasing the adsorption temperature (100–130 K) increases the cr-peak intensity with respect to the c-peak. TDS features similar to the cr-peak have been obtained for condensed methanol (MeOH) films [30–32] and have been assigned to a crystalline phase on top of a disordered buffer layer compensating for the difference in the lattice match of the monolayer and crystalline film. Mixed amorphous/crystalline MeOH films formed. Although these TDS features have apparently not been observed before for thiophene, a crystalline phase also has been identified for C4 H4 S by IR spectroscopy [26]. Therefore, we assign tentatively the cr-peak to a crystalline thiophene adsorption phase embedded in an amorphous film (c-peak) and the b-peak to a buffer layer on top of the saturated surface (cf., Fig. 12 in Ref. [30]). Note that, the intensity of the cr/b-peaks are too small to assign these features to adsorption on interior or groove sites on the CNTs. In addition, these peaks are clearly related with a condensed phase of thiophene. The maximum molecular size of thiophene (as deter˚ . Theremined by Gaussian calculations) amounts to 4.6 A fore, adsorption on interior sites of the CNTs with an ˚ is expected. Unfortuaveraged diameter [28,33] of 13.6 A nately, no distinct monolayer TDS structures which could easily be assigned to CNT specific adsorption sites have been observed. Therefore, co-adsorption experiments with alkanes [2,34] have been conducted since, for example, n˚ ) adsorption leads to characteristic TDS pentane (6.9 A peaks which can be assigned to adsorption on external (C-peak), groove (B-peak), and internal (A-peak) adsorption sites. A typical n-pentane TDS curve (which is consistent with published data [28]) is shown in Fig. 3I together with fits of gauss functions, just to indicate the contributions of the desorption rate from the kinetically distinct adsorption sites. The D-peak is an adsorption site unspecific condensation peak. This peak assignment is based on IR spectroscopy results and on determining filling factors, see Ref. [28]. Site blocking experiments have often been used to help identifying possible adsorption sites [2,27].
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Fig. 3. (I) N-pentane (50 L) TDS reference curve (solid spheres) indicating possible adsorption sites on the CNTs. The solid lines are the result of a fit with gauss functions. (II and III) N-pentane–thiophene co-adsorption experiments for 2 L n-pentane and v Langmuir thiophene. The simultaneously detected n-pentane and thiophene TDS curves are shown in panel II and III, respectively.
Fig. 3II/III depict the result of thiophene-pentane coadsorption TDS where a constant exposure of n-pentane
(2 L) and different amounts of thiophene (as indicated) have been exposed. Fig. 3II shows the pentane TDS curves and Fig. 3III the simultaneously detected C4 H4 S desorption rate. The starting curve (2 L n-pentane, 0 L thiophene) of this set of experiments is dominated by the adsorption of pentane on A and B sites (Fig. 3II). Increasing the C4 H4 S exposure, v, leads first to an increase in the C-peak TDS intensity which is accompanied with decreasing A/B-peaks and finally to the appearance of the D-peak at large v. (The curves in Fig. 3II are plotted over each other to allow for an intensity comparison; the total pentane TDS area is approx. conserved since always the same amount of pentane has been exposed.) Thus, the alkane and sulfur compound compete about identical adsorption sites including interior sites (A-peak) of the CNTs. As panel III in Fig. 3 depicts, the thiophene TDS intensity simply increases, reproducing the results already shown in Fig. 2. In order to facilitate a more quantitative discussion, the pentane-thiophene TDS data are reproduced in Fig. 4I, including fits with gauss functions. The curves are offset to allow following the filling sequence of the sites more clearly. Fig. 4II shows the TDS peak intensities as estimated from the fits. The thiophene molecules are initially replacing part of the interior and groove site adsorbed pentane, i.e., the A and B alkane TDS peak intensities decrease (Fig. 4II). However, no complete site blocking is seen, i.e., a mixed pentane–thiophene phase forms, as expected, since the binding energies of those two molecules do not differ dramatically. Interestingly, although the A sites are the highest binding energy sites for pentane, pentane adsorbed on groove sites appear to be replaced by thiophene efficiently as well; see the intensity decrease of the B-peak in Fig. 4II. This may indicate that groove sites are sterically favored over internal sites for the adsorption of C4 H4 S. We may speculate about upright thiophene adsorption with the sulfur atom towards the CNTs and the ring perpendicular with respect to the main CNT axis. In sympathy with the decrease of the A and B pentane peaks, the C peak intensity initially increases. Thus, the pentane molecules displaced from interior and groove sites are forced to C (external) sites. At even larger exposures, the external sites are occupied by thiophene (C pentane peak intensity decreases) pushing pentane to adsorption site unspecific condensation sites (D peak intensity increases further). Thus, the filling sequence ðA and B ! C ! DÞ basically follows the binding energy sequence (for alkanes) of the different adsorption sites. This suggests that thiophene adsorbs on the same sites as n-pentane with the same sequence of the binding energies. Due to the relatively small difference in the binding energies no complete site blocking is seen, i.e., mixed alkane-thiophene co-adsorption phases form. Similar experiments have recently been conducted with methanol and n-pentane [34]. In this case, the rather linear n-pentane molecule could not as efficiently be replaced by the more spherical methanol from the linear groove sites. Instead external sites were predominantly decorated by
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for other linear and branched alcohols from methanol to hexanol [35]). Indeed, the dipole-dipole interaction energies, Ed-d , are significant as compared with the binding energies, assuming a standard distance dependence ðEd-d ¼ 2l2 =r3 Þ. For example, at a binding distance of ˚ which equals the dimensions of the graphite unit r ¼ 2:4A, cell, Ed-d amounts to 10% of the binding energy of thiophene and is even larger for MeOH. Thus, differences in the dipole moments can affect the adsorption kinetics via lateral interaction. For example, thiophene appears to adsorb efficiently on groove sites while methanol does not. We may speculate that strong intermolecular interactions in dense methanol layers are a possible explanation which is less applicable for thiophene because of its smaller dipole moment. However, a sophisticated analysis should also include indirect interactions such as a modification of the electronic structure of the CNTs caused by the adsorbates. Unfortunately, the experimental and theoretical data base is still small. In summary, the following information has been collected:
Fig. 4. (I) Reprinted n-pentane data (solid spheres) from Fig. 3II, however, including fits to gauss functions (solid lines). The curves have been offset to visualize the filling sequence of sites. (II) Intensity variations in the n-pentane TDS peaks with thiophene exposure, as obtained from the fits. The solid lies are included as a guide for the eye.
the alcohol, especially at larger total exposures of both molecules [35]. In addition to differences in the molecular size of methanol, pentane, and thiophene, the dipole moments, l, differ significantly which may also qualitatively explain observed trends. The non-polar alkanes (from butane to pentane) [28] show the most distinct CNT induced TDS features while interacting with the non-polar graphitic sample. (Note that also the non-polar CCl4 shows distinct CNT induced features [28]). Whereas polar methanol (2.16 debye – from Gaussian) and thiophene (0.83 debye) do not show any distinct TDS features in the monolayer range. (Similar results have been obtained
Thiophene adsorbes molecularly. At large exposures indications for a crystalline phase are evident. Thiophene adsorbed on internal sites of the CNTs, which will be important for catalysis applications. Kinetics parameters have been obtained such as the binding energies in the monolayer and multilayer coverage range including the coverage dependence for small exposures and the pre-exponential factor for the condensed phase. This information will be important to gain a mechanistic understanding of surface reactions. Polar thiophene as well as alcohols [35] do not show distinct CNT induced TDS features which have been seen for non-polar alkanes and CCl4 . The latter holds true independently of the size and molecular structure (linear vs. branched) of the probe molecules since alkanes [28,6] from butane up to pentane (as well as CCl4 ) [28] and alcohols [34,35] from methanol up to hexanol (as well as thiophene) show distinct differences in their kinetics. Possible explanations such as differences in the dipole moments of the probe molecules have been discussed. Acknowledgement Financial support by the DoE Office of Science under Award DE-FG02-06ER46292 (ND State Grant) is acknowledged. References [1] O. Byl, J.C. Liu, Y. Wang, W.L. Yim, J.K. Johnson, J.T. Yates, J. Am. Chem. Soc. 128 (2006) 12090. [2] P. Kondratyuk, J.T. Yates, Chem. Phys. Lett. 410 (2005) 324. [3] H. Ulbricht, G. Moos, T. Hertel, Surface Sci. 532–535 (2003) 852.
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