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Adsorption and dissociation kinetics of alkanes on CaO(100)
Surface Science 605 (2011) 1534–1540
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Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Adsorption and dissociation kinetics of alkanes on CaO(100) A. Chakradhar, Y. Liu 1, J. Schmidt, E. Kadossov 2, U. Burghaus ⁎ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND, USA
a r t i c l e
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Article history: Received 8 March 2011 Accepted 17 May 2011 Available online 25 May 2011 Keywords: Surface chemistry Heterogeneous catalysis Gas-surface interactions Bond activation in alkanes Kinetics Thermal desorption spectroscopy Auger electron spectroscopy CaO
a b s t r a c t The adsorption kinetics of ethane, butane, pentane, and hexane on CaO(100) have been studied by multi-mass thermal desorption (TDS) spectroscopy. The sample cleanliness was checked by Auger electron spectroscopy. A molecular and dissociative adsorption pathway was evident for the alkanes, except for ethane, which does not undergo bond activation. Two TDS peaks appeared when recording the parent mass, which are assigned to different adsorption sites/configurations of the molecularly adsorbed alkanes. Bond activation leads to desorption of hydrogen and several alkane fragments assigned to methane and ethylene formation. Only one TDS feature is seen in this case. Formation of carbon residuals was absent. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bond activation of alkanes has been studied for decades, with a historic focus on metal single crystal surfaces such as Ir and Pt (see refs [1–8]). However, although the high chemical activity of some metal oxides in this respect has been known for quite some time from catalysis studies on powders [9–13], only a rather few surface science projects about the adsorption of alkanes on nonmetallic systems have been conducted. For MgO [14], ZnO [15], rutile TiO2 [16,17], silica [18,19], and graphitic systems [20–22], only molecular adsorption is seen. However, theoretical studies predict an increase in the catalytic activity along the group of, for example, the alkaline earth oxides [23,24]. Accordingly, a higher activity of CaO, as compared with MgO, is expected and is related to a lower Madelung potential, which leads to a more delocalized electron distribution of surface oxygen and hence to an eventually more efficient overlap with the orbitals of adsorbing molecules. Indeed, recently, for an alkaline earth metal oxide single crystal, CaO(100) (ref. [25]), bond activation of butane was evident. Later, bond cleavage on transition metal oxides, PdO thin films, was reported in a surface science study [26]. (A related topic may be the bond activation seen in ethene (ethylene) on O/Ni(111) [27]) At present, apparently the only metal oxide single crystals/thin films studied in more detail with surface science techniques that
⁎ Corresponding author. Tel: + 1 701 231 9742; fax: + 1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.ndsu.edu/chemistry/ (U. Burghaus). 1 Current address: School of Chemical Engineering and Technology, P.O. Box 796666, Tianjin University, Tianjin 300072, China. 2 Current address: Xplosafe, LLC., Stillwater, OK 74074, USA. 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.05.022
promote alkane dissociation are Pd and Ca oxides. Note that bond activation of alkanes was also present for anatase TiO2 thin films (, probably the first system studied in this regard [17]). However, here the alkanes decompose entirely, which makes a detailed characterization very cumbersome. Basically, two mechanisms have been considered for metal catalysts [1–8]. At low impact energies, a precursor-mediated bond activation, where the alkanes are trapped in the physisorption well before dissociation, dominates. At high impact energies a direct (impact-induced) bond breakage in alkanes is evident. It appears that basically the same mechanism explains the molecular beam scattering data gathered so far for the butane/CaO(100) system [25]. Molecular beam scattering on CaO(100) provided the first evidence for the occurrence of bond activation of n-/iso-butane on this metal oxide surface [25]. The selectivity of the bond activation could be tuned by changing the impact energy and gas temperature of the probe molecules. No evidence for the oxidation of the alkane or catalyst poisoning was present. A donor–acceptor mechanism has been proposed for alkane bond activation on transition metal oxide films such as PdO [26,28]. Utilizing multi-mass reactive TDS, C–H bond cleavage leading to adsorbed propyl/propoxy and hydrogen has been concluded for propane/PdO(101). C3H7 is subsequently oxidized without dehydrogenation. Besides molecular propane, only the desorption of CO2 and water were evident. Propyl appears to form already below 200 K via a precursor-mediated process, but the subsequent reactions were only evident above 400 K. Formation of dative bonds with coordinatively unsaturated Pd atoms has been suggested based on DFT (density functional theory) calculations. This weakens the C–H bonds via a donor–acceptor mechanism, leading to alkane bond activation on
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PdO. In addition, TDS of the parent mass of the alkane leads to two features, similarly to CaO. Whereas the lower temperature TDS peak is assigned to physisorbed alkanes, the higher temperature feature is discussed in conjunction with dative bonding and coordinatively unsaturated Pd atoms. According to ref. [26,28], surface defects appear not to affect directly the dissociation of alkanes on PdO(101), but coordinatively unsaturated (cus) cus-Pd/cus-O sites may act similarly to O-vacancy sites. On the other hand, an O-vacancy site based mechanism for methane decomposition on other PdO surfaces has been proposed [29]. In a prior study on identical CaO(100) surfaces [30], CO TDS was used to estimate the surface defect density. Two CO TDS peaks were present. The CO TDS feature which appeared at greater temperatures was assigned to adsorption on defect sites. If we assume that every defect site anchors just one CO molecule, the defect density (based on the integrated TDS intensities) would amount to 40%. It is known, however, that defects on metal oxides can bind more than one probe molecule [31,32]. Therefore, this estimate is a maximum estimate of the defect density. In this study, we present kinetics data characterizing the bond activation of small chain alkanes on CaO(100). Two TDS peaks were present for the parent mass of the alkane. Except for ethane, bond cleavage was seen for all alkanes considered (Table 1). 2. Experimental procedures The experiments were conducted in two standard ultra-high vacuum (UHV) chambers. The thermal desorption spectroscopy (TDS) setup has been described previously [15]. Briefly, a shielded mass spectrometer (QME) was used, the samples are mounted on a small (non-bulky) sample holder, and the QME to sample distance amounts to ~1 mm. These precautions generally prevent that the TDS data are heavily affected by contributions from the sample holder. In case of less-conducting samples, as studied here, the crystals are clamped on a small Ta foil. The sample temperature could be reduced within 3 min below 100 K, even for less conducting metal oxides. In doing so, He gas was bubbled through a liquid nitrogen containing dewar [33]. The reading of the thermocouple was calibrated with an accuracy of ±5 K by TDS measurements of condensed alkanes. However, due to the width of the monolayer TDS peaks, we estimate an error of ±20 K in determining monolayer TDS peak temperatures. The heating rate amounts to 1.5 K/s. This rather small value will minimize eventual temperature gradients along the sample. The exposures are given in Langmuir (1 L = 1 s gas exposure at 1 × 10 − 6 mbar). TDS data were collected for all masses up to the parent mass of the alkanes. Two different CaO(100) samples were studied (sample 1 from Goodfellow, UK and sample 2 from Crystal Gurus, USA). The surfaces
Table 1 Kinetics parameters. Estimated TDS peak temperatures and binding energies for low (0.1 L) and great (N 10 L) exposures. (Data for benzene, which adsorbs molecularly, are added.) The binding energies given in the right column take the variation of the prefactor with chain length of the alkanes into account. The prefactors from ref. [14]. were used. For the binding energies in the center column a constant prefactor of 1 × 1015/s was assumed. Probe molecule
Ethane iso-butane n-butane n-pentane n-hexane Benzene Estimated errors
Peak temperature (K), Θlow → Θhigh
Binding energy (kJ/mol), Θlow → Θhigh ν = 1 × 1015/s
Binding energy (kJ/mol), Θlow → Θhigh ν from ref. [14]
α
β
α
β
α
β
223 215–194 224–182 247–230 279–210 323–290 ± 20
168 150–140 156–133 200–197 183–177 248–224
66.4 64.0–57.6 66.8–53.9 73.8–68.6 83.7–62.5 97.2–87.0 ±6
49.7 44.2–41.2 46.0–39.1 59.4–58.5 54.2–52.4 74.1–66.8
66.0 66.9–60.2 69.8–56.4 78.1–72.5 89.0–66.5 – ±6
49.3 46.2–43.0 48.1–40.8 62.9–61.9 57.7–55.8 –
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were prepared by the vendors by cleaving in air, with one of them mechanically polished. Since CaO is hygroscopic, the samples (~10 × 10 × 2 mm) were received immersed in petroleum. Before mounting in UHV the crystals were rinsed in acetone and ethanol. Following literature procedures [30], sample 1 was cleaned by annealing in UHV at 800 K for a total of 160 min. This sample also was used in our prior study; see ref. [25]. The other sample was cleaned by 9 cycles of oxygen annealing (at 1 × 10 -5 mbar for 15 min) and Ar + sputtering (with 0.2 μA sample current at 1000 eV for 20 min, sample 2), a procedure common for cleaning oxide surfaces. Irrespective of the different cleaning procedures, qualitatively very similar results were obtained for both single crystals. Auger electron spectroscopy (AES) was used to check the cleanliness of the samples. Because the decomposition of alkanes was seen before for only a few metal oxides [17,26,27], extensive blind experiments were conducted assuring perfectly the correctness of the presented TDS data. To the best of our knowledge, none of the TDS data is heavily affected by sample holder effects. The following discussion is basically trivial and includes facts well-known by research groups applying the TDS technique. However, it may be of some interest for colleagues not familiar with the details of kinetics experiments. For the blind experiments the CaO sample was removed from the sample holder. Alkane TDS data were then collected for the bare sample holder initially consisting of a Ta foil spot welded on Mo pins. Molecular adsorption of alkanes is obviously seen on the Ta foil. However, only one TDS peak appeared in the monolayer range and it was located at a different temperature than the signal from the CaO samples. In addition, it turned out that the initially used Mo pins generated a background signal. In other words, alkane bond activation was initially seen on the Mo sample holder pins. As a result, in addition to the parent mass initially also a small signal at m/e = 16 was detected. This background signal (for m/e = 16) did amount to about 50% of the (m/e = 16) signal from the CaO samples when placing only the Mo pins directly in front of the mass spectrometer. With the sample mounted, this background will be smaller due to the measuring geometry and the shielded mass spectrometer used. In particular, for the same reasons, CaO TDS data of the dominant masses (e.g. parent mass) will not be heavily affected by the sample holder pins. In addition, bond breaking of the alkanes on the Mo sample holder pins would not affect qualitative conclusions. However, the Mo pins may lead to quantitatively wrong results when detecting certain fragmentation masses of the alkanes. Therefore, to perfectly avoid this complication, the Mo pins were replaced by tantalum, which dropped the TDS background signal (m/e = 16) from the bare sample holder below the detection limit. In other words, no alkane bond activation was seen when using a Ta sample holder. The blind experiments are briefly described below, together with the data. We include this discussion here since Mo is fairly often used as a component of sample holders (due to good spot welding properties and thermal expansion coefficients consistent with ceramics). We note again, that all the precautions and test experiments conducted in this study will insure that the TDS data are not heavily affected by sample holder effects.
3. Results and discussion 3.1. Sample cleaning Fig. 1 depicts AES spectra of an as-received and cleaned CaO(100) sample (sample 2). AES data for the first sample can be found in ref. [30]. Carbon contaminations were initially evident (see C AES peak at 278 eV) and could be removed by the UHV annealing procedure alone. In addition, it is known from the literature (see refs [34,36]) that Ca(OH)2 or CaCO3 layers form on CaO, which passivate the surface [30]. In a prior study it was evident that UHV annealing also removed the passivating carbonate layer [30]. Besides a charging related peak shift
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Fig. 1. Auger electron spectroscopy (AES) of an as-received and cleaned CaO(100) surface. (2000 eV, Vpp = 10 V, sample 2).
(~20 eV), the AES spectra finally obtained agreed well with reference data [37] (see the Ca AES line at 300 eV and O AES peak at 520 eV). 3.2. Molecular adsorption pathway Typical examples of alkane TDS data are depicted in Figs. 2–4. Shown are TDS curves of the parent mass of n/iso-butane (Fig. 2, sample 1), pentane (Fig. 3, sample 2), and n-hexane (Fig. 4, sample 2), which were collected as a function of exposure, χ. The parent mass must relate to molecular adsorption/desorption of the alkanes. Thus, a molecular adsorption pathway exists. However, the exact bonding type is unknown at present. The desorption temperatures would appear to be somewhat large for a pure physisorption. For PdO thin films, a system where similarly large binding energies were present, a dative bond formation has been proposed. At the lowest exposures, one TDS peak is visible (α peak). With increasing exposure, a second structure grows in intensity (β peak), and at very large exposures a low-temperature TDS peak appears (c peak). In addition, for butane and pentane, a second low-temperature TDS feature (d peak) is clearly evident. The c peak is assigned to multilayer formation of condensed alkanes. The d peak detected just before the onset of the condensation peak is assigned to bilayer formation. Note that the c and d peaks shift to greater temperatures with increasing exposure, whereas the α and β peaks shift in the opposite direction. The peaks shifts of the c and d peaks are consistent with 0th order kinetics. Although the c and d peaks are typical for condensed alkanes (multilayer and bilayer) [14–16,21,22], the α and β TDS peaks are rather uncommon for alkane adsorption kinetics, except that similar TDS data is reported for PdO [26,28]. Commonly, only one feature is seen in the monolayer range [1–8]. Both structures shift to lower temperatures with increasing exposure, which is consistent with repulsive lateral interactions and/or overlapping features resulting from different adsorption sites and/or configurations. Unfortunately, these different effects cannot be easily separated. In the simplest case, attractive lateral interactions would be expected, although the opposite has also been reported for metal oxides and was explained by substrate-induced polarization effects (see discussion in ref. [38]). Different adsorption sites as the reason for the rather small peak shifts appear unlikely, considering that two monolayer TDS features are seen. Alkanes commonly adsorb flat on surfaces, but they can be found in different adsorption configurations. Therefore, we relate the peak shifts to repulsive lateral interactions and the α and β TDS peaks to
Fig. 2. Thermal desorption spectroscopy (TDS) curves of m/e = 43 for n-butane and isobutane as a function of exposure, χ(L), in Langmuir (sample 1). (c: condensation; d: bilayer; α, and β are assigned to different adsorption sites/configurations.)
different adsorption sites/configurations, including possibly the effect of surface defects. In a prior study on sample 1, CO TDS was used to estimate the (maximum) density of defects which did amount to 40% [30]. Due to the strong overlap of the α and β TDS peaks, no attempt was made to fit the data. The TDS peak temperatures increase with chain length, as expected (Tab. 1). The TDS peak temperatures are within the range of 170–280 K. Therefore, assuming a pre-exponential (independent of chain length) of 1 × 10 15/s (see ref. [14]), for first order kinetics, the binding energies are within the range of 50–84 kJ/ mol. These binding energies on CaO are larger than for, e.g., MgO, (see ref. [14]) but are comparable to PdO [26,27,14]. On PdO thin films, for example, desorption temperatures, even for methane, up to 150 K have been reported. The apparent discrepancy of CaO and PdO to MgO may be expected since, on the latter surface, alkanes adsorb molecularly, whereas on PdO and CaO bond activation is also evident. The width of the TDS peaks seen for MgO is smaller than for CaO. First of all, the width of ultra-low temperature TDS peaks is always smaller than the one of TDS features detected at greater temperatures. This simply reflects the variation in desorption rates with temperature, as predicted by Arrhenius type kinetics. In addition, lateral interactions can broaden TDS peaks significantly, as well as different
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Fig. 4. Thermal desorption spectroscopy (TDS) curves of m/e = 57 for hexane as a function of exposure, χ (sample 2). The inset depicts TDS curves of the bare sample holder (Ta plate mounted on Ta pins). Fig. 3. Thermal desorption spectroscopy (TDS) curves of m/e = 43 for n-pentane as a function of exposure, χ (sample 2).
3.3. Dissociative adsorption pathway – alkane bond activation
adsorption configurations of the alkane. Furthermore, intrinsic defects can affect the electronic structure of a surface and, hence, the adsorption kinetics. Regarding the blind experiments mentioned in the experimental section, note that all TDS features, were present independent of the metal used for the sample holder pins. For the data in Fig. 3, a sample holder with Mo pins was used, whereas the data in Fig. 4 were collected with the Ta pins sample holder. The c, d, β, and α peaks are detected in both cases. Therefore, in particular the β, and α peaks are intrinsic features of the sample. In addition, in prior experiments with alkanes, using the same sample holder (Ta foil, Mo pins) and vacuum system, only one monolayer TDS feature was present. For example, see e.g. ref. [18]. about alkane adsorption on silica, or ref. [16]. about alkane adsorption on TiO2 nanotubes. The inset of Fig. 4 shows TDS data from the bare sample holder (Ta plate, Ta pins) corresponding to molecularly adsorbed hexane. As evident only one monolayer TDS feature is present and a condensation peak. In addition, the peak positions of the monolayer TDS feature differ from those recorded for the CaO sample. It is certainly clear that when mounting a metal plate, instead of a CaO sample, in front of the mass spectrometer, distinct molecular/adsorption desorption also will be seen in blind TDS experiments. Therefore, a shielded mass spectrometer in close proximity of the sample is used for the actual TDS experiments.
Experimentally, the simplest possibility in distinguishing molecular and dissociative adsorption is to look for possible desorption of reaction products, i.e., to record TDS traces for m/e ratios besides the parent and fragmentation masses of the gaseous species. If the TDS mass pattern does not match with the expected fragmentation pattern of the gaseous probe molecule, bond activation has taken place. The intensities may deviate from the fragmentation pattern, and/or new fragment masses, not observed for the gaseous species, may be present. The latter is a simpler means of identifying a dissociative adsorption pathway, since even a qualitative data analysis reveals unique bond activation. Typical examples of multi-mass TDS experiments of this kind are shown in Figs. 5–7 for ethane, n-butane, and n-hexane. Although the sample was not cold enough to observe the condensation of ethane, adsorption in the monolayer range was evident. The top panels show the mass spectra of the gaseous alkanes measured with our mass spectrometer. The center panels depict the results of multi-mass TDS experiments where a constant exposure (of 4 L) was used. Shown are the integrated intensities. (The intensities are normalized with respect to the signal at m/e = 43.) Finally, the bottom panels display the difference spectra (TDS minus gas phase intensities, respectively). The mass scans and TDS data are not corrected for sensitivity factors that depend on a given mass (see below). For the sake of comparison, the
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Fig. 6. Multi-mass TDS experiments of n-butane. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure). Fig. 5. Multi-mass TDS experiments of ethane, i.e., intensities vs. m/e setting of the mass spectrometer. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure).
inset of Fig. 7 depicts gas phase spectra of methane (measured with our mass spec) and ethylene (from the NIST database). For ethane only a molecular adsorption pathway is evident, since the mass scans for the gaseous molecules match those of the multimass TDS data. In other words, the difference spectra (bottom panel) consist of close to zero intensity peaks. This result is perhaps expected, since the bond activation in methane on metal surfaces, for example, requires a much larger activation energy than bond-breaking in longer chain alkanes [5,39].
For alkanes larger than ethane, however, bond activation is evident (see Figs. 6 and 7), since the multi-mass TDS data deviate significantly from the data of the gaseous molecules. First, the relative intensities of the fragmentation pattern of the gaseous molecules do not match those obtained by TDS. Second, new masses that are not present at all in the fragmentation pattern of the gaseous molecules appear in the multi-mass TDS scans. For example, no signal at m/e = 16 is present for gaseous butane and hexane, but it appears in their multi-mass TDS data. Fig. 8 depicts the signal detected at m/e = 16 when dosing varying amounts of n-butane and n-hexane on the surface. Unlike the parent mass (and most dominate fragmentation masses), interestingly, only
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Fig. 8. TDS data for m/e = 16 when dosing A) n-butane and B) n-hexane. The exposures have been varied. The inset in B shows results of blind experiments, as described in the text.
holder, as shown in the inset of Fig. 8B. (Note that the intensity scales for all panels in Fig. 8 are identical.) 3.4. Proposed mechanism
Fig. 7. Multi-mass TDS experiments of n-hexane. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure). The insets depict the mass scans of gaseous methane and ethylene.
one TDS feature appears for this fragment mass. Taking measured sensitivity factors of the mass spectrometer into account, the ratio of signals at m/e = 16 to m/e = 57 of, e.g., hexane, amounts to about 0.15, approximately independent of exposure. Thus, the desorption signals of the fragmentation masses are fairly large as compared with the most dominant mass. However, conversion rates cannot be easily determined using TDS data. Importantly, a signal at m/e = 16 was below the detection limit of the mass spectrometer when removing the sample and conducting TDS experiments on the bare sample
The deviations seen in the mass pattern and gas phase spectra (determined with the same mass spectrometer) are similar for all the longer chain alkanes studied. Most striking is the appearance of TDS peaks at m/e = 16 to 13, which could correspond to methane (see the inset of Fig. 7, which shows the mass spectra of gaseous methane). The m/e = 16 and 28 ratios do not match with CO. Likewise, strong contributions from background adsorption of CO2 can be ruled out. The peak at m/e = 28 could result from fragmentation into either ethane or ethylene. Hydrogen desorption was seen in the TDS experiments, and the signal for m/e = 30, characteristic of ethane, was small. Therefore, besides methane, it is likely that mostly ethylene is formed via hydrogen abstraction. This would lead to the following scheme for the initial steps in, for example, n-butane bond activation: Surface O + CH3 CH2 −CH2 CH3
ð1Þ
→Surface OH + •CH2 −CH2 CH2 −CH3 •CH2 −CH2 −CH2 −CH3 →CH2 = CH2 + •CH2 −CH3
ð2Þ
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Eq. (1) describes the result of the hydrogen abstraction and Eq. (2) depicts the C-C bond activation. A similar mechanism has been proposed in previous catalysis studies on n-hexane/MoO2 (see ref. [40]). This mechanism suggests that the bond activation involves lattice oxygen. For example, the formation of carbonates on CaO by CO2 involving lattice oxygen is an established mechanism for CaO powders [41] and single crystals [30,42]. Reaction/decomposition products such as propane, propylene, and butene might also form. Indeed, features at m/e = 29, 39, and 41 are present. However, we can rule out a number of other possibilities. AES did not detect carbon residuals on the surface, and catalyst deactivation was absent. Thus, the alkanes do not undergo complete bond activation, seen in the anatase TiO2 system [17] or often observed for metal catalysts (see ref. [7]). No distinct signal was present for m/ e = 18, which would be expected for an oxidation of the alkanes, as seen for the PdO system [13,26]. An obvious but difficult to address question is the effect of defects. The CaO single crystals used have certainly had a large, but not precisely known intrinsic density of defects. In a prior study (on sample 1), CO TDS was used to estimate a 40% (maximum) density of defects [30]. Therefore, it appears plausible that defects influence adsorption kinetics. However, definite proof would require a study of a nearly defect-free surface first. Unfortunately, this type of system is presently not available for CaO. Crystalline CaO thin films, for example, could not have been made so far, to the best of our knowledge. Similarly to BaO, CaO clusters form rather than crystalline thin films. 4. Summary The adsorption kinetics of small chain alkanes has been studied by TDS and AES on CaO(100) single crystals. Two reaction pathways are evident: molecular adsorption and bond activation for the linear alkanes larger than ethane. This is a rare example for the bond activation of alkanes by a metal oxide single crystal. Only gaseous reaction/decomposition products were seen, and oxidation of the alkanes can be ruled out. It appears that mostly methane and ethylene are formed via hydrogen abstraction. The molecular adsorption pathway in the monolayer range leads to the detection of two features in TDS data that are assigned to different adsorption sites/configurations. Acknowledgements Discussions with M. Komarneni and A. Sand at NDSU are acknowledged. Financial support was provided by the Department of Energy's EPSCoR program under Award DE-FG02-06ER46292, “Performance Impacts of Impurities in Clean Coal Systems Equipped with Carbon Capture Technologies” as well as from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, U.S. Department of Energy (Project DEFG02-08ER15987). References [1] C.B. Mullins, W.H. Weinberg, Kinetics and dynamics of alkane activation on transition metal surfaces, Springer Series in Surface Science, vol. 34, 1994, ch. 7. [2] W.H. Weinberg, Y.K. Sun, Science 253 (1991) 542. [3] J.F. Weaver, A.F. Carlsson, R.J. Madix, Surf. Sci. Rep. 50 (2003) 107. [4] F. Zaera, Catal. Today 81 (2003) 149. [5] L.B.F. Juurlink, D.R. Killelea, A.L. Utz, Prog. Surf. Sci. 84 (2009) 69. [6] A.M. Wodtke, D. Matsiev, D.J. Auerbach, Prog. Surf. Sci. 83 (2008). [7] J. Ding, U. Burghaus, W.H. Weinberg, Surf. Sci. 446 (2000) 46. [8] J.H. Larsen, I. Chorkendorff, Surf. Sci. Rep. 35 (1999) 163. [9] J.N. Finch , D.L. Ripley, Catalytic cracking of alkanes, (1976) US Patent 3985641. [10] K.K. Pant, D. Kunzru, Chem. Eng. Journal 87 (2002) 219. [11] M.C. Paganini, M. Chiesa, P. Martino, E. Giamello, J. Phys. Chem. B 106 (2002) 12531. [12] O.B. Koper, E.A. Wovchko, J.A. Glass, J.T. Yates, K.J. Klabunde, Langmuir 11 (1995) 2054. [13] J.G. McCarty, Catalysis Today 26 (1995) 283. [14] S.L. Tait, Z. Dohnalek, C.T. Campbell, B.D. Kay, J. Chem. Phys. 122 (2005) 164708. [15] J. Wang, B. Hokkanen, U. Burghaus, Surf. Sci. 577 (2005) 158. [16] B. Hokkanen, S. Funk, U. Burghaus, A. Ghicov, P. Schmuki, Surf. Sci. 601 (2007) 4620. [17] J. Goering, E. Kadossov, U. Burghaus, Z.Q. Yu, S. Thevuthasan, L.V. Saraf, Catal. Lett. 116 (2007) 9. [18] S. Funk, T. Nurkic, U. Burghaus, Appl. Surf. Sci. 253 (2007) 4860. [19] S. Funk, J. Goering, U. Burghaus, Appl. Surf. Sci. 254 (2008) 5271. [20] K.R. Paserba, A.J. Gellman, Phys. Rev. Lett. 86 (2001) 4338. [21] E. Kadossov, J. Goering, U. Burghaus, Surf. Sci. 601 (2007) 3421. [22] S. Funk, B. Hokkanen, T. Nurkig, U. Burghaus, B. White, S. O'Brien, N. Turro, J. Phys. Chem. C 111 (2007) 8043. [23] M.B. Jensen, L.G.M. Pettersson, O. Swang, U. Olsbye, J. Phys. Chem. B 109 (2005) 16774. [24] G. Pacchioni, J.M. Ricart, F. Illas, J. Am. Chem. Soc. 116 (1994) 10152. [25] E. Kadossov, U. Burghaus, Chem. Commun. (2008) 4073. [26] J.F. Weaver, S.P. Devarajan, C. Hakanoglu, J. Phys. Chem. C 113 (2009) 9773. [27] M.P.A. Lorenz, T. Fuhrmann, R. Streber, A. Bayer, F. Bebensee, K. Gotterbarm, M. Kinne, B. Tränkenschuh, J.F. Zhu, C. Papp, R. Denecke, H.-P. Steinrück, J. Chem. Phys. 133 (2010) 014706. [28] J.F. Weaver, C. Hakanoglu, J.M. Hawkins, A. Asthagiri, J. Chem. Phys. 132 (2010) 024709. [29] M. Blanco-Rey, S.J. Jenkins, J. Chem. Phys. 130 (2009) 014705. [30] E. Kadossov, U. Burghaus, J. Phys. Chem. C 112 (2008) 7390. [31] M.A. Henderson, Surf. Sci. 343 (1995) L1156. [32] M.A. Henderson, W.S. Epling, C.L. Perkins, C.H. Peden, U. Diebold, J. Phys. Chem. B 103 (1999) 5328. [33] J.T. Yates, Experimental Innovations in Surface Science: a Guide to Practical Laboratory Methods and Instruments, AIP Press Springer, New York, 1988. [34] H.N. Waltenburg, P.J. Moller, Sci. Scie. 439 (1999) 139. [35] C.S. Doyle, T. Kendelewicz, X. Carrier, G.E. Brown, Surf. Rev. Lett. 6 (1999) 1247. [36] F. Voigts, F. Bebensee, S. Dahle, K. Volgmann, W. Maus-Friedrichs, Surf. Sci. 603 (2009) 40. [37] C.L. Hedberg, Handbook of Auger Electron Spectroscopy, Handbook of Auger Electron Spectroscopy, (Published by Physical Electronics, Inc.). [38] E. Kadossov, U. Burghaus, Surf. Sci. 603 (2009) 2494. [39] Z. Dohnalek, R.S. Smith, B.D. Kay, J. Phys. Chem. B 106 (2002) 8360. [40] J.H. Song, P. Chen, S.H. Kim, G.A. Somorjai, R.J. Gartside, F.M. Dautzenberg, J. Mol. Catal. A: Chemical 184 (2002) 197. [41] D. Cazoria-Amoros, J.P. Joly, A. Linares-Salano, A. Marcilla-Gomis, C.S.M. Lecea, J. Phys. Chem. 95 (1991) 6611. [42] D. Ochs, B. Braun, W. Maus-Friedrichs, V. Kempter, Surf. Sci. 417 (1998) 406.
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Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Adsorption and dissociation kinetics of alkanes on CaO(100) A. Chakradhar, Y. Liu 1, J. Schmidt, E. Kadossov 2, U. Burghaus ⁎ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND, USA
a r t i c l e
i n f o
Article history: Received 8 March 2011 Accepted 17 May 2011 Available online 25 May 2011 Keywords: Surface chemistry Heterogeneous catalysis Gas-surface interactions Bond activation in alkanes Kinetics Thermal desorption spectroscopy Auger electron spectroscopy CaO
a b s t r a c t The adsorption kinetics of ethane, butane, pentane, and hexane on CaO(100) have been studied by multi-mass thermal desorption (TDS) spectroscopy. The sample cleanliness was checked by Auger electron spectroscopy. A molecular and dissociative adsorption pathway was evident for the alkanes, except for ethane, which does not undergo bond activation. Two TDS peaks appeared when recording the parent mass, which are assigned to different adsorption sites/configurations of the molecularly adsorbed alkanes. Bond activation leads to desorption of hydrogen and several alkane fragments assigned to methane and ethylene formation. Only one TDS feature is seen in this case. Formation of carbon residuals was absent. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bond activation of alkanes has been studied for decades, with a historic focus on metal single crystal surfaces such as Ir and Pt (see refs [1–8]). However, although the high chemical activity of some metal oxides in this respect has been known for quite some time from catalysis studies on powders [9–13], only a rather few surface science projects about the adsorption of alkanes on nonmetallic systems have been conducted. For MgO [14], ZnO [15], rutile TiO2 [16,17], silica [18,19], and graphitic systems [20–22], only molecular adsorption is seen. However, theoretical studies predict an increase in the catalytic activity along the group of, for example, the alkaline earth oxides [23,24]. Accordingly, a higher activity of CaO, as compared with MgO, is expected and is related to a lower Madelung potential, which leads to a more delocalized electron distribution of surface oxygen and hence to an eventually more efficient overlap with the orbitals of adsorbing molecules. Indeed, recently, for an alkaline earth metal oxide single crystal, CaO(100) (ref. [25]), bond activation of butane was evident. Later, bond cleavage on transition metal oxides, PdO thin films, was reported in a surface science study [26]. (A related topic may be the bond activation seen in ethene (ethylene) on O/Ni(111) [27]) At present, apparently the only metal oxide single crystals/thin films studied in more detail with surface science techniques that
⁎ Corresponding author. Tel: + 1 701 231 9742; fax: + 1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.ndsu.edu/chemistry/ (U. Burghaus). 1 Current address: School of Chemical Engineering and Technology, P.O. Box 796666, Tianjin University, Tianjin 300072, China. 2 Current address: Xplosafe, LLC., Stillwater, OK 74074, USA. 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.05.022
promote alkane dissociation are Pd and Ca oxides. Note that bond activation of alkanes was also present for anatase TiO2 thin films (, probably the first system studied in this regard [17]). However, here the alkanes decompose entirely, which makes a detailed characterization very cumbersome. Basically, two mechanisms have been considered for metal catalysts [1–8]. At low impact energies, a precursor-mediated bond activation, where the alkanes are trapped in the physisorption well before dissociation, dominates. At high impact energies a direct (impact-induced) bond breakage in alkanes is evident. It appears that basically the same mechanism explains the molecular beam scattering data gathered so far for the butane/CaO(100) system [25]. Molecular beam scattering on CaO(100) provided the first evidence for the occurrence of bond activation of n-/iso-butane on this metal oxide surface [25]. The selectivity of the bond activation could be tuned by changing the impact energy and gas temperature of the probe molecules. No evidence for the oxidation of the alkane or catalyst poisoning was present. A donor–acceptor mechanism has been proposed for alkane bond activation on transition metal oxide films such as PdO [26,28]. Utilizing multi-mass reactive TDS, C–H bond cleavage leading to adsorbed propyl/propoxy and hydrogen has been concluded for propane/PdO(101). C3H7 is subsequently oxidized without dehydrogenation. Besides molecular propane, only the desorption of CO2 and water were evident. Propyl appears to form already below 200 K via a precursor-mediated process, but the subsequent reactions were only evident above 400 K. Formation of dative bonds with coordinatively unsaturated Pd atoms has been suggested based on DFT (density functional theory) calculations. This weakens the C–H bonds via a donor–acceptor mechanism, leading to alkane bond activation on
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PdO. In addition, TDS of the parent mass of the alkane leads to two features, similarly to CaO. Whereas the lower temperature TDS peak is assigned to physisorbed alkanes, the higher temperature feature is discussed in conjunction with dative bonding and coordinatively unsaturated Pd atoms. According to ref. [26,28], surface defects appear not to affect directly the dissociation of alkanes on PdO(101), but coordinatively unsaturated (cus) cus-Pd/cus-O sites may act similarly to O-vacancy sites. On the other hand, an O-vacancy site based mechanism for methane decomposition on other PdO surfaces has been proposed [29]. In a prior study on identical CaO(100) surfaces [30], CO TDS was used to estimate the surface defect density. Two CO TDS peaks were present. The CO TDS feature which appeared at greater temperatures was assigned to adsorption on defect sites. If we assume that every defect site anchors just one CO molecule, the defect density (based on the integrated TDS intensities) would amount to 40%. It is known, however, that defects on metal oxides can bind more than one probe molecule [31,32]. Therefore, this estimate is a maximum estimate of the defect density. In this study, we present kinetics data characterizing the bond activation of small chain alkanes on CaO(100). Two TDS peaks were present for the parent mass of the alkane. Except for ethane, bond cleavage was seen for all alkanes considered (Table 1). 2. Experimental procedures The experiments were conducted in two standard ultra-high vacuum (UHV) chambers. The thermal desorption spectroscopy (TDS) setup has been described previously [15]. Briefly, a shielded mass spectrometer (QME) was used, the samples are mounted on a small (non-bulky) sample holder, and the QME to sample distance amounts to ~1 mm. These precautions generally prevent that the TDS data are heavily affected by contributions from the sample holder. In case of less-conducting samples, as studied here, the crystals are clamped on a small Ta foil. The sample temperature could be reduced within 3 min below 100 K, even for less conducting metal oxides. In doing so, He gas was bubbled through a liquid nitrogen containing dewar [33]. The reading of the thermocouple was calibrated with an accuracy of ±5 K by TDS measurements of condensed alkanes. However, due to the width of the monolayer TDS peaks, we estimate an error of ±20 K in determining monolayer TDS peak temperatures. The heating rate amounts to 1.5 K/s. This rather small value will minimize eventual temperature gradients along the sample. The exposures are given in Langmuir (1 L = 1 s gas exposure at 1 × 10 − 6 mbar). TDS data were collected for all masses up to the parent mass of the alkanes. Two different CaO(100) samples were studied (sample 1 from Goodfellow, UK and sample 2 from Crystal Gurus, USA). The surfaces
Table 1 Kinetics parameters. Estimated TDS peak temperatures and binding energies for low (0.1 L) and great (N 10 L) exposures. (Data for benzene, which adsorbs molecularly, are added.) The binding energies given in the right column take the variation of the prefactor with chain length of the alkanes into account. The prefactors from ref. [14]. were used. For the binding energies in the center column a constant prefactor of 1 × 1015/s was assumed. Probe molecule
Ethane iso-butane n-butane n-pentane n-hexane Benzene Estimated errors
Peak temperature (K), Θlow → Θhigh
Binding energy (kJ/mol), Θlow → Θhigh ν = 1 × 1015/s
Binding energy (kJ/mol), Θlow → Θhigh ν from ref. [14]
α
β
α
β
α
β
223 215–194 224–182 247–230 279–210 323–290 ± 20
168 150–140 156–133 200–197 183–177 248–224
66.4 64.0–57.6 66.8–53.9 73.8–68.6 83.7–62.5 97.2–87.0 ±6
49.7 44.2–41.2 46.0–39.1 59.4–58.5 54.2–52.4 74.1–66.8
66.0 66.9–60.2 69.8–56.4 78.1–72.5 89.0–66.5 – ±6
49.3 46.2–43.0 48.1–40.8 62.9–61.9 57.7–55.8 –
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were prepared by the vendors by cleaving in air, with one of them mechanically polished. Since CaO is hygroscopic, the samples (~10 × 10 × 2 mm) were received immersed in petroleum. Before mounting in UHV the crystals were rinsed in acetone and ethanol. Following literature procedures [30], sample 1 was cleaned by annealing in UHV at 800 K for a total of 160 min. This sample also was used in our prior study; see ref. [25]. The other sample was cleaned by 9 cycles of oxygen annealing (at 1 × 10 -5 mbar for 15 min) and Ar + sputtering (with 0.2 μA sample current at 1000 eV for 20 min, sample 2), a procedure common for cleaning oxide surfaces. Irrespective of the different cleaning procedures, qualitatively very similar results were obtained for both single crystals. Auger electron spectroscopy (AES) was used to check the cleanliness of the samples. Because the decomposition of alkanes was seen before for only a few metal oxides [17,26,27], extensive blind experiments were conducted assuring perfectly the correctness of the presented TDS data. To the best of our knowledge, none of the TDS data is heavily affected by sample holder effects. The following discussion is basically trivial and includes facts well-known by research groups applying the TDS technique. However, it may be of some interest for colleagues not familiar with the details of kinetics experiments. For the blind experiments the CaO sample was removed from the sample holder. Alkane TDS data were then collected for the bare sample holder initially consisting of a Ta foil spot welded on Mo pins. Molecular adsorption of alkanes is obviously seen on the Ta foil. However, only one TDS peak appeared in the monolayer range and it was located at a different temperature than the signal from the CaO samples. In addition, it turned out that the initially used Mo pins generated a background signal. In other words, alkane bond activation was initially seen on the Mo sample holder pins. As a result, in addition to the parent mass initially also a small signal at m/e = 16 was detected. This background signal (for m/e = 16) did amount to about 50% of the (m/e = 16) signal from the CaO samples when placing only the Mo pins directly in front of the mass spectrometer. With the sample mounted, this background will be smaller due to the measuring geometry and the shielded mass spectrometer used. In particular, for the same reasons, CaO TDS data of the dominant masses (e.g. parent mass) will not be heavily affected by the sample holder pins. In addition, bond breaking of the alkanes on the Mo sample holder pins would not affect qualitative conclusions. However, the Mo pins may lead to quantitatively wrong results when detecting certain fragmentation masses of the alkanes. Therefore, to perfectly avoid this complication, the Mo pins were replaced by tantalum, which dropped the TDS background signal (m/e = 16) from the bare sample holder below the detection limit. In other words, no alkane bond activation was seen when using a Ta sample holder. The blind experiments are briefly described below, together with the data. We include this discussion here since Mo is fairly often used as a component of sample holders (due to good spot welding properties and thermal expansion coefficients consistent with ceramics). We note again, that all the precautions and test experiments conducted in this study will insure that the TDS data are not heavily affected by sample holder effects.
3. Results and discussion 3.1. Sample cleaning Fig. 1 depicts AES spectra of an as-received and cleaned CaO(100) sample (sample 2). AES data for the first sample can be found in ref. [30]. Carbon contaminations were initially evident (see C AES peak at 278 eV) and could be removed by the UHV annealing procedure alone. In addition, it is known from the literature (see refs [34,36]) that Ca(OH)2 or CaCO3 layers form on CaO, which passivate the surface [30]. In a prior study it was evident that UHV annealing also removed the passivating carbonate layer [30]. Besides a charging related peak shift
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Fig. 1. Auger electron spectroscopy (AES) of an as-received and cleaned CaO(100) surface. (2000 eV, Vpp = 10 V, sample 2).
(~20 eV), the AES spectra finally obtained agreed well with reference data [37] (see the Ca AES line at 300 eV and O AES peak at 520 eV). 3.2. Molecular adsorption pathway Typical examples of alkane TDS data are depicted in Figs. 2–4. Shown are TDS curves of the parent mass of n/iso-butane (Fig. 2, sample 1), pentane (Fig. 3, sample 2), and n-hexane (Fig. 4, sample 2), which were collected as a function of exposure, χ. The parent mass must relate to molecular adsorption/desorption of the alkanes. Thus, a molecular adsorption pathway exists. However, the exact bonding type is unknown at present. The desorption temperatures would appear to be somewhat large for a pure physisorption. For PdO thin films, a system where similarly large binding energies were present, a dative bond formation has been proposed. At the lowest exposures, one TDS peak is visible (α peak). With increasing exposure, a second structure grows in intensity (β peak), and at very large exposures a low-temperature TDS peak appears (c peak). In addition, for butane and pentane, a second low-temperature TDS feature (d peak) is clearly evident. The c peak is assigned to multilayer formation of condensed alkanes. The d peak detected just before the onset of the condensation peak is assigned to bilayer formation. Note that the c and d peaks shift to greater temperatures with increasing exposure, whereas the α and β peaks shift in the opposite direction. The peaks shifts of the c and d peaks are consistent with 0th order kinetics. Although the c and d peaks are typical for condensed alkanes (multilayer and bilayer) [14–16,21,22], the α and β TDS peaks are rather uncommon for alkane adsorption kinetics, except that similar TDS data is reported for PdO [26,28]. Commonly, only one feature is seen in the monolayer range [1–8]. Both structures shift to lower temperatures with increasing exposure, which is consistent with repulsive lateral interactions and/or overlapping features resulting from different adsorption sites and/or configurations. Unfortunately, these different effects cannot be easily separated. In the simplest case, attractive lateral interactions would be expected, although the opposite has also been reported for metal oxides and was explained by substrate-induced polarization effects (see discussion in ref. [38]). Different adsorption sites as the reason for the rather small peak shifts appear unlikely, considering that two monolayer TDS features are seen. Alkanes commonly adsorb flat on surfaces, but they can be found in different adsorption configurations. Therefore, we relate the peak shifts to repulsive lateral interactions and the α and β TDS peaks to
Fig. 2. Thermal desorption spectroscopy (TDS) curves of m/e = 43 for n-butane and isobutane as a function of exposure, χ(L), in Langmuir (sample 1). (c: condensation; d: bilayer; α, and β are assigned to different adsorption sites/configurations.)
different adsorption sites/configurations, including possibly the effect of surface defects. In a prior study on sample 1, CO TDS was used to estimate the (maximum) density of defects which did amount to 40% [30]. Due to the strong overlap of the α and β TDS peaks, no attempt was made to fit the data. The TDS peak temperatures increase with chain length, as expected (Tab. 1). The TDS peak temperatures are within the range of 170–280 K. Therefore, assuming a pre-exponential (independent of chain length) of 1 × 10 15/s (see ref. [14]), for first order kinetics, the binding energies are within the range of 50–84 kJ/ mol. These binding energies on CaO are larger than for, e.g., MgO, (see ref. [14]) but are comparable to PdO [26,27,14]. On PdO thin films, for example, desorption temperatures, even for methane, up to 150 K have been reported. The apparent discrepancy of CaO and PdO to MgO may be expected since, on the latter surface, alkanes adsorb molecularly, whereas on PdO and CaO bond activation is also evident. The width of the TDS peaks seen for MgO is smaller than for CaO. First of all, the width of ultra-low temperature TDS peaks is always smaller than the one of TDS features detected at greater temperatures. This simply reflects the variation in desorption rates with temperature, as predicted by Arrhenius type kinetics. In addition, lateral interactions can broaden TDS peaks significantly, as well as different
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Fig. 4. Thermal desorption spectroscopy (TDS) curves of m/e = 57 for hexane as a function of exposure, χ (sample 2). The inset depicts TDS curves of the bare sample holder (Ta plate mounted on Ta pins). Fig. 3. Thermal desorption spectroscopy (TDS) curves of m/e = 43 for n-pentane as a function of exposure, χ (sample 2).
3.3. Dissociative adsorption pathway – alkane bond activation
adsorption configurations of the alkane. Furthermore, intrinsic defects can affect the electronic structure of a surface and, hence, the adsorption kinetics. Regarding the blind experiments mentioned in the experimental section, note that all TDS features, were present independent of the metal used for the sample holder pins. For the data in Fig. 3, a sample holder with Mo pins was used, whereas the data in Fig. 4 were collected with the Ta pins sample holder. The c, d, β, and α peaks are detected in both cases. Therefore, in particular the β, and α peaks are intrinsic features of the sample. In addition, in prior experiments with alkanes, using the same sample holder (Ta foil, Mo pins) and vacuum system, only one monolayer TDS feature was present. For example, see e.g. ref. [18]. about alkane adsorption on silica, or ref. [16]. about alkane adsorption on TiO2 nanotubes. The inset of Fig. 4 shows TDS data from the bare sample holder (Ta plate, Ta pins) corresponding to molecularly adsorbed hexane. As evident only one monolayer TDS feature is present and a condensation peak. In addition, the peak positions of the monolayer TDS feature differ from those recorded for the CaO sample. It is certainly clear that when mounting a metal plate, instead of a CaO sample, in front of the mass spectrometer, distinct molecular/adsorption desorption also will be seen in blind TDS experiments. Therefore, a shielded mass spectrometer in close proximity of the sample is used for the actual TDS experiments.
Experimentally, the simplest possibility in distinguishing molecular and dissociative adsorption is to look for possible desorption of reaction products, i.e., to record TDS traces for m/e ratios besides the parent and fragmentation masses of the gaseous species. If the TDS mass pattern does not match with the expected fragmentation pattern of the gaseous probe molecule, bond activation has taken place. The intensities may deviate from the fragmentation pattern, and/or new fragment masses, not observed for the gaseous species, may be present. The latter is a simpler means of identifying a dissociative adsorption pathway, since even a qualitative data analysis reveals unique bond activation. Typical examples of multi-mass TDS experiments of this kind are shown in Figs. 5–7 for ethane, n-butane, and n-hexane. Although the sample was not cold enough to observe the condensation of ethane, adsorption in the monolayer range was evident. The top panels show the mass spectra of the gaseous alkanes measured with our mass spectrometer. The center panels depict the results of multi-mass TDS experiments where a constant exposure (of 4 L) was used. Shown are the integrated intensities. (The intensities are normalized with respect to the signal at m/e = 43.) Finally, the bottom panels display the difference spectra (TDS minus gas phase intensities, respectively). The mass scans and TDS data are not corrected for sensitivity factors that depend on a given mass (see below). For the sake of comparison, the
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Fig. 6. Multi-mass TDS experiments of n-butane. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure). Fig. 5. Multi-mass TDS experiments of ethane, i.e., intensities vs. m/e setting of the mass spectrometer. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure).
inset of Fig. 7 depicts gas phase spectra of methane (measured with our mass spec) and ethylene (from the NIST database). For ethane only a molecular adsorption pathway is evident, since the mass scans for the gaseous molecules match those of the multimass TDS data. In other words, the difference spectra (bottom panel) consist of close to zero intensity peaks. This result is perhaps expected, since the bond activation in methane on metal surfaces, for example, requires a much larger activation energy than bond-breaking in longer chain alkanes [5,39].
For alkanes larger than ethane, however, bond activation is evident (see Figs. 6 and 7), since the multi-mass TDS data deviate significantly from the data of the gaseous molecules. First, the relative intensities of the fragmentation pattern of the gaseous molecules do not match those obtained by TDS. Second, new masses that are not present at all in the fragmentation pattern of the gaseous molecules appear in the multi-mass TDS scans. For example, no signal at m/e = 16 is present for gaseous butane and hexane, but it appears in their multi-mass TDS data. Fig. 8 depicts the signal detected at m/e = 16 when dosing varying amounts of n-butane and n-hexane on the surface. Unlike the parent mass (and most dominate fragmentation masses), interestingly, only
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Fig. 8. TDS data for m/e = 16 when dosing A) n-butane and B) n-hexane. The exposures have been varied. The inset in B shows results of blind experiments, as described in the text.
holder, as shown in the inset of Fig. 8B. (Note that the intensity scales for all panels in Fig. 8 are identical.) 3.4. Proposed mechanism
Fig. 7. Multi-mass TDS experiments of n-hexane. Top panel: gaseous molecules; middle panel: integrated TDS peak intensity; bottom panel: difference spectra. (4 L exposure). The insets depict the mass scans of gaseous methane and ethylene.
one TDS feature appears for this fragment mass. Taking measured sensitivity factors of the mass spectrometer into account, the ratio of signals at m/e = 16 to m/e = 57 of, e.g., hexane, amounts to about 0.15, approximately independent of exposure. Thus, the desorption signals of the fragmentation masses are fairly large as compared with the most dominant mass. However, conversion rates cannot be easily determined using TDS data. Importantly, a signal at m/e = 16 was below the detection limit of the mass spectrometer when removing the sample and conducting TDS experiments on the bare sample
The deviations seen in the mass pattern and gas phase spectra (determined with the same mass spectrometer) are similar for all the longer chain alkanes studied. Most striking is the appearance of TDS peaks at m/e = 16 to 13, which could correspond to methane (see the inset of Fig. 7, which shows the mass spectra of gaseous methane). The m/e = 16 and 28 ratios do not match with CO. Likewise, strong contributions from background adsorption of CO2 can be ruled out. The peak at m/e = 28 could result from fragmentation into either ethane or ethylene. Hydrogen desorption was seen in the TDS experiments, and the signal for m/e = 30, characteristic of ethane, was small. Therefore, besides methane, it is likely that mostly ethylene is formed via hydrogen abstraction. This would lead to the following scheme for the initial steps in, for example, n-butane bond activation: Surface O + CH3 CH2 −CH2 CH3
ð1Þ
→Surface OH + •CH2 −CH2 CH2 −CH3 •CH2 −CH2 −CH2 −CH3 →CH2 = CH2 + •CH2 −CH3
ð2Þ
1540
A. Chakradhar et al. / Surface Science 605 (2011) 1534–1540
Eq. (1) describes the result of the hydrogen abstraction and Eq. (2) depicts the C-C bond activation. A similar mechanism has been proposed in previous catalysis studies on n-hexane/MoO2 (see ref. [40]). This mechanism suggests that the bond activation involves lattice oxygen. For example, the formation of carbonates on CaO by CO2 involving lattice oxygen is an established mechanism for CaO powders [41] and single crystals [30,42]. Reaction/decomposition products such as propane, propylene, and butene might also form. Indeed, features at m/e = 29, 39, and 41 are present. However, we can rule out a number of other possibilities. AES did not detect carbon residuals on the surface, and catalyst deactivation was absent. Thus, the alkanes do not undergo complete bond activation, seen in the anatase TiO2 system [17] or often observed for metal catalysts (see ref. [7]). No distinct signal was present for m/ e = 18, which would be expected for an oxidation of the alkanes, as seen for the PdO system [13,26]. An obvious but difficult to address question is the effect of defects. The CaO single crystals used have certainly had a large, but not precisely known intrinsic density of defects. In a prior study (on sample 1), CO TDS was used to estimate a 40% (maximum) density of defects [30]. Therefore, it appears plausible that defects influence adsorption kinetics. However, definite proof would require a study of a nearly defect-free surface first. Unfortunately, this type of system is presently not available for CaO. Crystalline CaO thin films, for example, could not have been made so far, to the best of our knowledge. Similarly to BaO, CaO clusters form rather than crystalline thin films. 4. Summary The adsorption kinetics of small chain alkanes has been studied by TDS and AES on CaO(100) single crystals. Two reaction pathways are evident: molecular adsorption and bond activation for the linear alkanes larger than ethane. This is a rare example for the bond activation of alkanes by a metal oxide single crystal. Only gaseous reaction/decomposition products were seen, and oxidation of the alkanes can be ruled out. It appears that mostly methane and ethylene are formed via hydrogen abstraction. The molecular adsorption pathway in the monolayer range leads to the detection of two features in TDS data that are assigned to different adsorption sites/configurations. Acknowledgements Discussions with M. Komarneni and A. Sand at NDSU are acknowledged. Financial support was provided by the Department of Energy's EPSCoR program under Award DE-FG02-06ER46292, “Performance Impacts of Impurities in Clean Coal Systems Equipped with Carbon Capture Technologies” as well as from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy
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