Adsorption Kinetics of Alcohols on Single-Wall Carbon Nanotubes: An

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J. Phys. Chem. C 2008, 112, 10114–10124

Adsorption Kinetics of Alcohols on Single-Wall Carbon Nanotubes: An Ultrahigh Vacuum Surface Chemistry Study J. Goering, E. Kadossov, and U. Burghaus* Department of Chemistry, Biochemistry, and Molecular Biology, North Dakota State UniVersity, Fargo, North Dakota 58105 ReceiVed: February 26, 2008; ReVised Manuscript ReceiVed: April 23, 2008

Presented are thermal desorption spectroscopy (TDS) data of alcohol adsorption on single-wall carbon nanotubes (CNTs) supported on silica. The adsorption kinetics of methanol, ethanol, propanol, 2-propanol, butanol, pentanol, and hexanol have been studied. Multimass TDS confirms molecular adsorption/desorption with low coverage binding energies, E0, increasing linearly with the number of carbons, n. The coverage, Θ, dependence of the heat of adsorption, Ed(Θ), has been obtained by a Redhead analysis. The heat of sublimation increases linearly with n whereas the zeroth-order pre-exponential factor is approximately independent of n. In addition, TDS of alkanes (butane, pentane, hexane, trimethylpentane) as well as alcohol-alkane coadsorption data have been collected to characterize possible adsorption sites. Alkane TDS leads to fingerprint curves indicating three different adsorption sites (external, groove, internal) on CNTs, in agreement with earlier studies. For most of the alcohols only one monolayer TDS peak is seen indicating a dominance of lateral interactions. However, the coadsorption data provide evidence for alcohol adsorption on interior sites of the CNTs. 1. Introduction The use of carbon nanotubes (CNTs) as a new class of supports, for example, for fuel cell catalysts,1,2 Fischer-Tropsch synthesis,3 and desulfurization catalysis,4–8 has been demonstrated. Direct liquid fuel cells can be regarded as an alternative means of electricity production based on renewable energy sources. Although fuel cell systems with up to 100 MWh2,9 are operational most realistic applications concern low-power portable devices such as cell phones, lap tops, etc. Promising are fuel cells based on methanol10 (MeOH) or ethanol (EtOH), which are reasonably safe, renewable, and easily storable.2,11–14 Membranes for direct MeOH fuel cells have been developed (see e.g. ref 15). Crucial to the operation of fuel cells is the adsorption of the fuel on the electrode surface. More efficient catalysts are desirable. Their development can in principle be supported by mechanistic information obtained in basic studies about adsorption kinetics. Metals supported by activated carbon are commonly used as fuel cell catalysts.2,9 Therefore, it is not too surprising that carbon nanotube-based fuel cell prototypes already show superior performance, which my be related to the enhancement of surface area by the nanotubes.2,16–18 In addition, alcohols are the most frequently used solvents in a variety of technical applications. Furthermore, studying the interactions of hydrocarbons with carbon surfaces has implications for lubrication19 and self-assembly in nanoelectronics20 applications. Methanol adsorption also has been studied on a variety of systems due to the importance of methanol synthesis.21 Before studying the adsorption of probe molecules on metal-on-CNT systems, it appears pertinent to characterize first the adsorption kinetics on clean CNTs. The adsorption and decomposition of MeOH has been studied extensively on metal surfaces,22–24 alloys,25 and metal oxides,26–28 as well as theoretically.29 However, rather few surface chemistry * Corresponding author. E-mail: [email protected] and www. chem.ndsu.nodak.edu. Fax: 701 0-231-8831.

Figure 1. (I) Scanning electron microscopy characterization of the single-wall carbon nanotube sample; dark lines CNT on top of the first layer of CNTs (light lines) supported on silica. (II) Schematics indicating the different adsorption sites.

studies with methanol have been conducted on graphitic systems30–33 although the adsorption of liquid alcohols has been studied at ambient (high) pressure.34–39 We are not aware of

10.1021/jp801686u CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

Adsorption Kinetics of Alcohols on CNTs

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10115

Figure 2. Thermal desorption spectroscopy (TDS) data of alkanes on SWCNTs as a function of exposure. The pictograms have been obtained with the Gaussian program as well as the maximum C-C distance, which is given in angstroms. (The heating rate for all TDS experiments is 1.6 deg/s.)

detailed ultrahigh vacuum (UHV) studies with alcohols on CNTs.2,32,40 Furthermore, kinetic data for longer chain alcohols are very limited in the surface chemistry literature.41–47 UHV studies about gas-surface interactions with nanotubes are still scarce, see, e.g., refs 48- 52. In the monolayer adsorption regime and according to IR and high-resolution electron energy loss (HREELS) data on HOPG (highly oriented pyrolytic graphite) and metal oxides, MeOH is adsorbed perpendicular to the surface with the O-atom pointing toward the surface plane.26,53 Longer chain alcohols appear to adsorb flat on HOPG due to the effect of the nonpolar alkyl chain;34–39 however, the curvature-induced strain in CNTs

may lead to a different adsorption geometry. Despite the expected hydrogen bonding in dense multilayers, a shift of TDS peaks to lower desorption temperatures has been seen in the monolayer coverage range25,26,43 and was assigned to repulsive interactions26 between the alcohol molecules. No indications for bond activation has been seen for (oxygen free) HOPG,53 silver,54 NiO,26 and Pt-Sn25 single crystals. The MeOH coverage typically increases linearly with exposure consistent with a precursor-mediated adsorption dynamics.54 Several different adsorption phases of condensed methanol have been identified by infrared spectroscopy and thermal desorption spectroscopy (TDS).53–55 On HOPG three distinct TDS peaks

10116 J. Phys. Chem. C, Vol. 112, No. 27, 2008

Goering et al.

Figure 3. Results of multimass TDS of alcohols adsorbed on CNTs. The TDS peak intensities match the intensities obtained from mass scans of the gas phase, i.e., alcohols adsorb molecularly on SWCNTs.

2. Experimental Procedures

10-6 Torr); a heating rate of 1.6 deg/s has been used. The reading of the thermocouple has been calibrated by means of the condensation peaks of the alkanes. The single-walled CNTs (based on HiPco powders from Carbon Nanotechnologies Inc.) supported on silica were obtained by the procedure detailed in refs 52 and 62. Briefly, the CNT powder was dispersed in aqueous sodium dodecyl sulfate (SDS) and deposited by the drop-and-dry technique on the support. To clean off the SDS, the sample has been annealed at 600 K in a N2 stream for 30 min. It has been shown in prior studies63 that HiPco-produced CNTs have an average diameter of 13.6 Å. The alcohols (SigmaAldrich) have been cleaned by multiple freeze-pump-thaw cycles. Mass scans very similar to those in the NIST database have been obtained. In mass scans the intensity ration of water (m/e 18) to the most abundant fragment of the alcohols was below a few percent [alcohol (specified purity) ) water-to-most abundant mass ratio: MeOH (99.9% HPLC) ) 4%; EtOH (HPLC) ) 5%, PropOH (HPLC) ) 4%, ButOH (99.9% GC) ) 4%, PentOH (GC) ) 7%, HexOH (GC) ) 6%]. The maximum molecular size and dipole moments of the probe molecules have been calculated with the Gaussian64 software package, using the B3LYP/6-31G(d) method; the calculate dipole moments agree within an uncertainty of 10% with the data in ref 65.

The experiments have been conducted with a standard ultrahigh vacuum system including a shielded mass spectrometer. The TDS setup is identical with the one described in ref 61. All gas exposures are given in Langmuir (1 L ) 1 s at 1 ×

3. Data Presentation and Discussion 3.1. Sample Characterization. The CNT sample has been characterized by scanning electron microscopy (SEM) and

have been observed and were assigned to a physisorbed monolayer and different multilayer structures.53 In most studies, a layered structure has been proposed. A mixed crystalline/ amorphous layer (hereafter cr/c-phase) forms above a disordered buffer layer (b-phase) and a physisorbed monolayer (p-phase) building a sandwich structure. Furthermore, in X-ray diffraction studies two different crystalline phases of MeOH have been seen.56 The degree of crystallization depends on adsorption temperature. A disordered buffer layer is expected due to the lattice mismatch of the monolayer and crystalline phase.54 The multilayer appears to be dominated by hydrogen-bonded chains of MeOH molecules which are parallel to the surface.55 We present thermal desorption spectroscopy (TDS) data of a variety of primary and secondary alcohols adsorbed on openend single-wall carbon nanotubes (SWCNT) supported on silica. To characterize the sample, TDS data of alkanes have been collected which lead to “fingerprint” curves indicating the adsorption sites.57 In addition, alcohol and alkane coadsorption experiments have been conducted providing evidence for alcohol adsorption on internal adsorption sites of the CNTs. Some TDS data of MeOH/CNTs,58 butane/CNTs,52 thiopene/CNTs,8 alkanes/silica,59 and MeOH/silica60 have been presented recently.



Figure 4. TDS data for a set of linear and branched alcohols on CNTs as a function of exposure, as indicated. The panel at the left depicts data for large exposures and the one on the right data for small exposures.

thermal desorption spectroscopy (TDS). The SEM figure (Figure 1, panel I) shows bright and dark lines which are SWCNTs

deposited on the silica support (bright lines) and CNTs in a second layer (darker lines) on top of the first layer of CNTs.52

10118 J. Phys. Chem. C, Vol. 112, No. 27, 2008 The ticks on the scale correspond to a distance of 3 µm. It is evident that the support is fully covered with little or no silica areas left bare. The lower panel in Figure 1 defines the label of different adsorption sites used hereafter. TDS curves of linear hexane, pentane, and butane are shown in Figure 2 as a function of exposure, χ(L). TDS data of, for example, n-pentane adsorbed on CNTs have been published before, see refs 57 and 66. On the basis of infrared spectroscopy data and the determination of filling factors,57,66 the structures could be assigned to the adsorption of the alkane on internal (A peak), groove (B peak), and external (C peak) sites (see Figure 1, panel II). Thus, the CNT sample used here is clean and the tube ends open. Reference data collected for a bare silica support do not show any of those features.59,60 The lining up of the low-temperature edges of the curves reveals the D peak as an adsorption site unspecific condensation structure. The fingerprint TDS curves obtained regularly for alkane adsorption on CNTs also can be used to identify adsorption sites for other molecules in coadsorption experiments.57,58 3.2. Multimass TDS of Alcohols on CNTs. Figure 3 summarizes the results of multimass TDS experiments conducted for several alcohols. Panels I and II depict TDS curves obtained for different m/e settings of the mass spectrometer after a constant exposure of methanol and propanol on the CNT sample. The shape of the TDS curves is approximately independent of the alcohol fragment and the peak intensities agree with those obtained from mass scans by backfilling the UHV chamber with the alcohol (see the insets). A comparison of TDS peak intensities obtained from the gas phase (by backfilling the vacuum chamber with the given alcohol) and those detected in TDS runs are also shown for a number of other alcohols in panels III to V. In all cases, the fragment pattern of the desorbing alcohols matches the patterns of the gas-phase species within the uncertainty of the experiments. Furthermore, besides a minor water desorption feature, the TDS peak temperatures are for a given alcohol independent of the m/e setting. Thus, negligible bond activation and molecular adsorption/desorption can be concluded by alcohol adsorption at low temperatures, which is consistent with studies conducted with MeOH on HOPG.53 The desorption of water at 160 K, independent of the alcohol, indicates a background adsorption due to the impurities of the alcohols and not the formation of water during the TDS experiment. Note that water desorption from “bucky paper” (thick multiwall CNTs layers) has been observed at about the same temperature.32 3.3. Adsorption Kinetics of Alcohols on CNTs. Figures 4 and 5 show sets of TDS curves for alcohol desorption from SWCNTs; curves for large exposures are shown in the left panel and those obtained at small exposures are shown in the right one. The mass spectrometer has been set to the most intense peak obtained in mass scans. (a) Monolayer Adsorption Regime. A broad TDS feature is observed at small exposures for all alcohols (p peaks) studied here (Figures 4 and 5, right panel). The small exposures related to the p peaks strongly suggest submonolayer coverages on the CNTs. For the smallest chain alcohols a clear separation in two distinct features is seen (p1 and p2 peaks). The peaks shift to smaller temperatures with increasing exposure (which is evident for all alcohols) is the result of lateral interactions, an overlap of desorption events from different adsorption sites on the CNTs, or a combination of both effects. Similar TDS peak shifts have been observed in prior studies about alcohol adsorption.26,43 Alcohols are polar molecules which adsorb upright at UHV conditions on a number of planar surfaces according to

Goering et al. spectroscopy data.26,53 Therefore, repulsive lateral interactions are expected.26 Although the p1 and p2 peaks seen for MeOH and EtOH overlap, which makes it difficult to determine exactly the peak position of the smaller structure, it is evident that the p2 peak shift is larger than the p1 peak shift. An almost identical effect has been seen by Godman et al. for MeOH and EtOH adsorption on a NiO surface and was attributed to tilted MeOH molecules (p1 peak) which would lead to smaller repulsive lateral interactions as compared with more upright adsorbed molecules (p2 peak).26 However, for CNTs and concerning the TDS data obtained for the alkanes (Figure 2) the alcohols should occupy kinetically distinct adsorption sites on the CNTs. Therefore, most likely a combination of both effect lateral interactions and different adsorption sites is responsible for the observed peak shifts. Interestingly, for longer chain alcohols only one TDS feature is seen in the monolayer coverage range (p peak, Figure 4, panels III/IV and Figure 5, panles I-III). In studies conducted at ambient pressures and with liquid alcohols, flat adsorption has been concluded particularly for longer chain alcohols.34–39 Therefore, we may speculate that the disappearance of the double peak (p1 and p2, Figure 4) seen for MeOH and EtOH with increasing length of the alkyl chain may indicate a transition from tilted to flat adsorbed alcohols. (b) Multilayer Adsorption Regime. At large exposures further TDS peaks emerge (see the left panel in Figures 4 and 5). The c peak can simply be identified as a condensation feature obeying zeroth-order kinetics: (1) This peak grows in intensity without saturation being observed; (2) the low-temperature edges line up perfectly as can be seen clearly for most of the alcohols studied here; (3) the peak position shifts to larger temperatures with increasing exposure; and (4) the peak appears solely after saturation of the monolayer TDS features. Therefore, this structure is a simple condensation TDS peak. It is assigned to an amorphous alcohol layer formed at large exposures.54 For other systems slight deviations from zeroth-order kinetics have been determined53 which was accompanied by a non layer by layer growth of the alcohol films, i.e., the multilayer TDS feature had been observed before saturation of the monolayer structures.26,53 This is not the case here. The b and cr peaks grow in sympathy with the c peak and are not observed at small exposures strongly suggesting that these features are also related to condensed alcohol films. Furthermore, the low-temperature edges of the cr peak appear to line up. Decreasing the heating rate shifts the b, cr, and c peaks to smaller desorption temperatures ensuring that those peaks are not related to desorption from the sample holder or readsorption effects. (Note that these structures were not present for the clean silica wafer studied in the same UHV system.60) Increasing the adsorption temperature from 105 to 130 K in MeOH TDS experiments leads to an intensity increase in the cr peak with respect to the c peak that has an onset of desorption at 145 K. Furthermore, these peaks are most distinct for the smaller chain alcohols. Therefore, it appears plausible to assign the cr peak to crystalline MeOH, which is part of a mixed amorphous-crystalline MeOH film, as suggested in prior studies.53 Crystallization will be favored at larger adsorption temperatures and may become more difficult the larger the molecules are, as is indeed observed. We tentatively assign the small shoulder (b peak) to the disordered buffer layer between the monolayer and mixed amorphous/crystalline layer.54 An alternative explanation would assign the cr and b peaks to different crystalline phases which have been observed (in X-ray and neutron diffraction studies67) for MeOH and EtOH but are so far not reported for longer chain alcohols. However, it appears



Figure 5. TDS data for linear and longer chain alcohols on CNTs as a function of exposure.

somewhat unlikely that these different crystalline phases can be separated by TDS. 3.4. Size Effects in the Adsorption Kinetics. At small exposures size effects in the desorption kinetics are evident. With increasing size of the alcohols, the total width of the monolayer TDS peaks decreases and the p1 peak is only detectable for the smallest alcohols. Indeed, it appears plausible that effects of different adsorption sites on the CNTs are more distinct in TDS for smaller molecules. Unfortunately, no clear assignment to different adsorption sites (Figure 1, panel II) is possible for the alcohols (Figures 4 and 5) as it was for the alkanes (Figure 2) studied on the same sample. It appears that the lateral interactions of the alcohols affect the shape of the TDS curves more distinctly than possible differences in binding

energies on different adsorption sites on the SWCNTs. Consistent with this conclusion is the experimental fact that nonpolar alkanes and nonpolar CCl4 (see ref 68) show distinct TDS features when adsorbed on nonpolar CNTs but polar alcohols and polar thiophene (see ref 8) do not. Note that, for example, n-butane and 1-propanol are approximately of the same total size (5.5 Å vs 5.4 Å for the largest C-C distance as determined from Gaussian calculations). Increasing the chain length in alcohols decreases their dipole moment (from 1.7 debye for MeOH to 1.5 debye for HexOH from Gaussian calculations). However, even for the longest chain alcohols studied here the dipole moments are large, i.e., the dipole-dipole interaction energies are significant as compared with the binding energies (see section 4).

164,74 17543

139 (disordered),53157 (ordered)53

155,25 160,32 16543

40.1,73 47,43 57.6 ( 0.744 229 222 243 19.8 17.9 20.0 19.8 19.6 185 179 187 200 215 1-propanol (1.5; 5.4) 2-propanol (1.6; 4.3) 1-butanol (1.5; 6.6) 1-pentanol (1.5; 7.9) 1-hexanol (1.5; 9.2)

38.4 31.2 41.9 44.2 45.8

164 ethanol (1.6; 4.1)

34.3

19.7

206

7.6 × 1016 (ref 44)

135,72 ∼140,32 145,25 191 20.0 30.9 153 methanol (1.7; 2.4)

p1 ) 55.3, p2 ) 47.3 p1 ) 58.8, p2 ) 50.8 58.1 57.3 61.2 63.7 67.6 p1 ) 207, p2 ) 184 p1 ) 227, p2 ) 197 224 221 236 245 260

molecules (dipole moment (debye); size (Å)) E0 (kJ/mol ( 2) Tpeak (K ( 5)

monolayer regime

31 f 40 (disordered)53

Tpeak (K) νdes (1/s) Tpeak (K ( 5)

Ec (kJ/mol ( 2)

νdeslog (1/s ( 0.4)

Tpeak (K ( 5)

Ec (kJ/mol)

literature data for the condensation regime cr peak c peak

condensation regime this study

The TDS peak position of the monolayer structure increases with the size of the alcohols as expected from their molecular structure and a physisorption. For example, a shift of EtOH monolayer TDS peaks of about 40 K to larger desorption temperatures as compared with MeOH has been observed for a planar catalyst.26 Similarly, here a shift by 16 K is observed (see Table 1). Interestingly, the TDS peak positions for the linear 1-propanol are larger by ∼10 K than for the branched 2-propanol (Figure 4, panels III/IV, right columns) as might be expected from their molecular structure. Similarly the position of the condensation TDS peaks (c, b, cr peaks, Figure 4 and 5, left panel) shifts to greater temperatures with increasing size of the alcohols, as expected (cf. Figure 2). Also in this case, the desorption temperatures are for the branched alcohol slightly smaller than for the linear counterpart (Figure 4, panels III/IV, left columns). 3.5. Coadsorption Experiments and Adsorption of Alcohols in CNTs. Figures 6 and 7 summarize TDS results of coadsorption experiments which allow concluding the adsorption of alcohols on interior adsorption sites of the SWCNTs although no distinct features are seen in the monolayer TDS curves of the alcohols. The maximum size (largest C-C distance) of the alcohols studied here ranges from 2.4 to 9.2 Å (determined by Gaussian calculations, see Figures 2 and 3 and Table 1), which compares with an estimated diameter63 of the CNTs of 13.6 Å, i.e., confinement effects are indeed expected. (a) Small Exposures. Coadsorption experiments for small exposures of the alkanes are shown in Figure 6, panels I/II, for MeOH and in Figure 6, panels III/IV, for propanol. At small exposures mostly the A peak corresponding to internal adsorption sites is initially populated by the alkanes. Thus TDS for small exposures allows separating more clearly the filling sequence of adsorption sites on the CNTs. The exposure of the alkane has been kept constant and the one of the alcohols has been varied, as indicated. The upper row shows the TDS curves printed one over the other, which allows for a comparison of the peak intensities. In the lower row, the data are offset to demonstrate the filling sequence of the different adsorption sites on the CNTs. Unfortunately, the fragmentation pattern of larger alcohols overlaps with those from alkanes, i.e., the largest alcohol that can be studies with this procedure (without data deconvolution) is propanol. As is evident from Figure 6, panel I, MeOH replaces pentane from internal sites to groove sites since the A peak intensity decreases and the B peak intensity increases with increasing MeOH exposure. Increasing the amount of exposed MeOH further results in the appearance of the TDS peaks related to external sites (C peak) and finally the condensation peak of the alkane appears. Note that the total n-pentane TDS area is conserved since always the same amount of pentane has been dosed. Thus, the filling sequence of the alkane follows the order of the binding energy of the different adsorption sites, starting with sites corresponding to the largest heat of adsorption, as expected and clearly visible in Figure 6, panel II. Essentially the same filling sequence is seen for propanol (Figure 6, panels III/IV). Thus, MeOH and propanol occupy interior sites of the CNTs which will be important for applications in catalysis since the total surface area of the catalyst is enhanced. This suggests that MeOH and propanol adsorb on the same sites as n-pentane with the same sequence of binding energies. Note that the adsorption order does not affect the filling sequence since diffusion processes are fast as compared with the TDS experiments, as has been shown before.57,66,68

Goering et al.

TABLE 1: Binding Energies of the Alcohols on Single-Walled Carbon Nanotubes As Compared with Reference Data on Single-Crystal Surfaces




Figure 6. Alkane-alcohol coadsorption TDS data (for small exposures) revealing the filling sequence of kinetically distinct adsorption sites on the SWCNTs by the alcohols: (I, II) pentane-MeOH and (III, IV) propanol-trimethylpentane.

Interestingly, the A peak position shifts with increasing alcohol adsorption (decreasing occupation of interior sites by the alkane) to smaller desorption temperatures, which is consistent with the TDS peak shifts detected for alkane TDS in Figure 2. (b) Large Exposures. Although it is not the object of this project to determine the complex alkane-alcohol phase diagram, some coadsorption experiments have been conducted for large exposures to complement the data sets. Data for large but constant pentane exposures (40 L) are shown in Figure 7. In this case, all available adsorption sites start to fill up with the pentane, i.e., the A-D peaks are already seen in pentane TDS without coexposure of MeOH. The MeOH coexposure has been varied; TDS data for small MeOH coexposures are depicted in Figure 7, panel I, and for large MeOH exposures in Figure 7, panel II. Apparently complicated mixed MeOH-pentane phases form: at least two different MeOH exposure regimes have to be distinguished: At small MeOH exposures (Figure 7, panel I) the A, B, and C TDS peak intensity decreases and the D peak intensity increases with increasing MeOH exposure. Thus, MeOH is replacing pentane from the A-C sites (a mixed pentane-MeOH phase forms) and is pushing pentane mostly into condensation sites as the lowest binding energy sites. Note again that at large pentane exposure all adsorption sites are initially covered with pentane. The total pentane TDS area is conserved. Interestingly, the C peak intensity (external sites) drops faster with MeOH

exposure than the A and B peak intensities. The most plausible explanation is that the more linear pentane molecule adsorbs preferentially along the one-dimensional groove sites (B peak) in the MeOH-pentane coadsorption phase. The adsorption of the more spherical MeOH molecules on groove sites appears to be sterically hindered when competing about these sites with the more linear pentane molecules. Therefore MeOH adsorbs in the mixed layer preferentially on external sites (C peak) leading to a preferential decrease in the pentane C peak intensity. Since the total coverage (MeOH + pentane) increases, a dense and mixed MeOH-pentane phase is formed. For example, it is know from thermodynamics studies69 of mixed fluid phase diagrams that clusters form. At large MeOH coexposures (Figure 7, panel II) the B and D peak intensity increases; the A peak intensity decreases whereas the C peak intensity is little affected. The total pentane TDS peak area is conserved (within an accuracy of 10%) since always the same amount of pentane has been exposed. Again it appears that MeOH adsorbs most efficiently on external sites in a dense MeOH-pentane coadsorption phase. However, Figure 7 indicates that basically the same filling sequence of sites (A f B f C) is present at large pentane exposures as the one seen at small pentane exposures (Figure 6). However, with one exception, the linear groove sites are preferentially occupied by the linear pentane molecules and MeOH appears to adsorb most efficiently at C (external) sites. In contrast and according to a prior study, thiophene8 also appears to adsorb efficiently


Figure 7. Pentane-MeOH coadsorption TDS data for large exposures.

on groove sites in coadsorption phases with alkanes. We may speculate about upright thiophene adsorption in groove sites with the sulfur atom toward the CNTs. Thus, coadsorption experiments can provide some insights about possible adsorption sites even when no distinct TDS features are present. 4. Determination of Kinetics Parameters 4.1. Monolayer Coverage Range of the Alcohols and Size Effects. To obtain a more quantitative description of the kinetics, the TDS peak positions in Figures 4 and 5 have been used to obtain Ed, the heat of adsorption, by means of the Redhead equation (assuming a pre-exponential factor of ν ) 1 × 1013/ s). Integrating the TDS curves yields the initial coverage for each of the TDS experiments. The (monolayer) saturation coverage corresponds to the TDS curve (labeled by a star in Figures 4 and 5) detected just before the onset of condensation (onset of the c peak in Figures 4 and 5). Combining all data, the coverage-dependent heat of adsorption, Ed(Θ), can be obtained and is depicted in Figure 8, panel I, for the different alcohols studied here. The Ed(Θ) curves for the p1 and p2 peaks of MeOH and EtOH (Figure 4, panels I/II) follow approximately the same trend but are offset by ∼8 kJ/mol, respectively, with the larger binding energy for the p1 peak. To avoid cluttering the figure only the data for p2 are shown. Figure 8, panel II, depicts the heat of adsorption in the limit of zero coverage, E0, as a function of the molecular size (see also Table 1). The overall decrease in Ed with Θ (Figure 8, panel I) is expected either for the effect of lateral interactions (consistent

Goering et al.

Figure 8. (I) Coverage-dependent binding energies of the alcohols adsorbed on SWCNTs, assuming a pre-exponential factor of 1 × 1013/s. (The curves for the p1 and p2 peaks of MeOH and EtOH follow approximately the same trend but are offset by ∼8 kJ/mol, respectively, with the larger binding energy for the p1 peak. To avoid cluttering the figure only the data for p2 are shown.) (II) Binding energies in the limit of small exposures (Θ f 0 ML) as a function of the chain lengths of the alcohols. (The reference data are from. ref 52.) All data have been obtained from the TDS peak positions, using a Redhead analysis, see the discussion.

with the shift of the TDS peaks to lower temperatures with increasing exposure) or with adsorption on kinetically distinct adsorption sites. Unfortunately, no distinct TDS features are seen within the monolayer adsorption regime; however, a dominance of lateral interactions appears plausible (see section 3.4). Indeed, the dipole-dipole interaction energies, Ed-d, are significant as compared with the binding energies, assuming a standard distance dependence (Ed-d ) -2µ2/r3). For example, at a binding distance of r ) 2.4 Å, which equals the dimensions of the graphite unit cell, Edd is of the order of the binding energy for MeOH and amounts to ∼40% of E0 for hexanol. Thus, differences in the dipole moments can affect the adsorption kinetics via lateral interaction. Studying CNT samples of different crystal structures may reveal whether adsorbate-induced modifications of the electronic structure are additionally present. These experiments are on the way. The initial (Θ < 0.1 ML) decrease in Ed is related to saturating defect sites on the surface of the CNTs rather than with saturating internal adsorption sites since a coverage much larger than 10% of the saturation coverage would be expected in the latter case. Intrinsic defects can consist of the tube ends



TABLE 2: TDS Peak Temperatures, Tpeak, and Binding Energies, E, Estimated from the Onset of the A (internal), B (grooves), and C (external) TDS Features of the Alkanes Adsorbed on SWCNTsa A

n-butane n-pentane trimethylpentane n-hexane a

molecular size (max) (Å) 5.5 6.9 6.8 by 4.2 by 3.3 8.0

B

Tpeak (K)

E (kJ/mol)

176 189 229 211

45 49 59 55

C

Tpeak (K)

E (kJ/mol)

Tpeak (K)

E (kJ/mol)

128 154 183 172

33 39 47 44

116 136 167 160

29 35 43 41

Pre-exponential ) 1 × 1013/s, heating rate ) 1.6 deg/s.

are not available from the literature although the adsorption kinetics of some alkanes on CNTs has been studied before.66 Fitting the TDS curves in Figure 2 would need to include too many free parameters. However, Table 2 depicts an estimate of the binding energy of the different adsorption sites based on the peak position detected for the onset of the TDS features. 5. Summary

Figure 9. Heat of condensation of the alcohols obtained by a leading edge analysis. The pre-exponential is independent of chain length.

or holes in the tubular structure of the CNTs. Although some CNTs are in a second layer (Figure 1, panel I), adsorption on interstitials is only expected for very small adsorbates70,71 (He, Ne, hydrogen). E0 increases linearly with the number of carbon atoms of the alcohols, which may be expected for a physisorption. Interestingly, E0 of 2-propanol is smaller than that for 1-propanol, consistent with their molecular structure. Table 1 summarizes additionally the kinetics parameters. 4.2. Condensation of the Alcohols. The heat of condensation, Ec, and the pre-exponential have been determined for the c peak (Figures 3 and 4) by using a leading-edge analysis. Whereas the pre-exponential factor is approximately independent of the molecular size (Table 1), Ec increases with chain length of the alcohols (Figure 9), as expected for the condensation of molecules. Again, the linear alcohol has a larger binding energy than the branched alcohol. Ec agrees reasonably well with the enthalpy of vaporization obtained from gas-phase thermodynamics data.65Although the reference database for longer chain alcohols is rather small (Table 1), the scattering of the kinetics parameters obtained in different studies is amazingly large. However, the condensation of alcohols is complicated by the formation of different amorphous/crystalline phases. The formation of those phases will depend on the very details of the measuring parameters such as the heating rate and adsorption temperatures which may explain, besides differences in the calibration of the temperature reading, the discrepancies in different studies. The formation of the crystalline phase may also depend on the density of defects of the support. 4.3. Kinetics Parameters for the Alkanes. Extracting kinetics parameters for the adsorption of alkanes on CNTs is complicated by the overlap of numerous TDS peaks (Figure 2). Therefore, it is not too surprising that kinetics parameters

The adsorption kinetics of a set of alcohols and alkanes on a monolayer of single-wall carbon nanotubes (SWCNTs from HiPco powders from Carbon Nanotechnologies Inc.) has been studied by thermal desorption spectroscopy (TDS) at ultrahigh vacuum conditions: The nonpolar alkanes show distinct CNT-induced features in TDS which can be assigned to different adsorption sites consistent with prior studies (Table 2). Kinetics parameters are given in Table 2. The binding strength decreases as internal > grooves > external consistent with the curvature-induced strain of the CNTs. The binding energies of the alkanes increase for all adsorption sites with increasing chain length indicating a mostly electrostatic interaction. The alcohols adsorb/desorb molecularly as is evident from multimass TDS. No adsorption site specific TDS peaks were present for the alcohols as perhaps expected for the interaction of polar molecules with a nonpolar substrate. Alkanes and alcohols of approximately the same geometrical size show very distinct differences in the kinetics. The dipole-dipole interaction energies are indeed large as compared with the binding energies of the alcohols. Thus, the kinetics is dominated by the effect of lateral interactions. Adsorbate-induced modifications appear unlikely but more experiments on different types of CNTs are required to address this question. As shown in Figure 8 and Table 1, the heat of adsorption at small coverage increases linearly with the chain length. Linear alcohols bind more strongly on SWCNTs than branched alcohols, see Table 1 and Figure 8. The heat of condensation and the pre-exponential have been determined for the alcohols. Alcohol-alkane coadsorption experiments reveal adsorption of the alcohols (at least from MeOH to propanol) on internal sites of the CNTs. The data strongly suggest that the same filling sequence of sites (internal f grooves f external f codensation) as those seen for the alkanes is also obeyed for the alcohols. In mixed alkane-alcohol coadsorption phases the more spherical alcohols preferentially occupy external sites while the “linear” alkanes occupy the linear groove sites. Acknowledgment. We are grateful for the support and the gift of the CNTs/silica sample from B. White, S. O’Brien, and

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JP801686U

Adsorption Kinetics of Alcohols on Single-Wall Carbon Nanotubes: An

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