Adsorption and Decomposition of Ethene and Propene on Co (0001

Article pubs.acs.org/JPCC

Adsorption and Decomposition of Ethene and Propene on Co(0001): The Surface Chemistry of Fischer−Tropsch Chain Growth Intermediates C. J. Weststrate,*,†,‡ Ionel M. Ciobîcă,‡ Jan van de Loosdrecht,¶ and J. W. Niemantsverdriet†,§ †

SynCat@DIFFER, Syngaschem BV, P.O. Box 6336, 5600 HH Eindhoven, The Netherlands Sasol Technology Netherlands B.V., Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ¶ Sasol, Group Technology, 1 Klasie Havenga Street, Sasolburg 1947, South Africa § SynCat@Beijing, Synfuels China Technology Co. Ltd, 1 Leyuan 2 South Street, Section C, Yanqi Economic Development Area, Beijing 101407, China ‡

S Supporting Information *

ABSTRACT: Experiments that provide insight into the elementary reaction steps of CxHy adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer−Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C2Hx and C3Hx adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of COad strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogenlean adsorbates such as alkynes and alkylidynes will have long residence times on the surface and are therefore feasible intermediates for chain growth. The comparatively lower desorption barrier for propene relative to ethene can to a large extent be attributed to the higher stability of the molecule in the gas phase, where hyperconjugation of the double bond with σ bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C2Hx production rate that is typically observed in cobalt-catalyzed FTS.

1. INTRODUCTION The interaction of small hydrocarbon molecules with singlecrystal metal surfaces has often been studied to obtain molecular level insights into catalytic processes.1 Metallic cobalt is used as a catalyst in low-temperature Fischer−Tropsch synthesis (LT-FTS), where long-chain hydrocarbons are produced from synthesis gas, a mixture of H2 and CO. The chemistry of hydrocarbonaceous surface intermediates plays a central role in determining the overall efficiency of this complex process. Only a few studies exist in which the surface chemistry of CxHy species on surfaces of cobalt is discussed.2−8 Simple molecules such as ethene and propene can be used as model compounds to investigate the relative stabilities of the © XXXX American Chemical Society

various possible C2Hx and C3Hx intermediates and give information on their hydrogenation and dehydrogenation. The C2Hx intermediates form the simplest class of CxHy species, and the nature of the various C2Hx intermediates can be determined with relative ease, in particular because reference data is available for various metal surfaces.1 However, in cobaltcatalyzed FTS, C2 products stand out in two significant ways from the ideal Anderson−Schulz−Flory (ASF) distribution. First, the observed C2 production rate is smaller than expected Received: September 27, 2016 Revised: December 5, 2016 Published: December 7, 2016 A

DOI: 10.1021/acs.jpcc.6b09760 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

wires in thermal contact with a liquid nitrogen reservoir allowing temperatures of around 80 K. The sample was heated by passing a direct current through the tungsten support wires, while the temperature was measured by a k-type thermocouple spotwelded to the back of the sample. At the SuperESCA beamline, the sample was spotwelded to a tungsten rod in thermal contact with a liquid nitrogen-cooled reservoir and heated radiatively by a filament placed at the backside of the sample. In all experiments the sample temperature was kept below 650 K, significantly lower than the temperature where the hexagonal close-packed−face-centered cubic (hcp−fcc) phase transition takes place. The sample was cleaned by various cycles of sputtering at 630 K (20−30 min, 1−1.5 kV Ar+) followed by a 20−30 min anneal in vacuum at 630 K. Sample cleanliness was checked either by XPS (at the synchrotron beamlines) or by hydrogen desorption experiments, which is sensitive to the presence of surface defects and contaminants such as C and O. CO temperature-programmed desorption (TPD) experiments were used to indirectly detect C, O, and S contaminations.15,16 To compare the sticking behavior of ethene and propene, the pressure readings from the ion gauge were corrected in multiple ways: the ion gauge sensitivity factors of ethene and propene as reported by Tamm et al.17 were used to obtain the “true” pressure. In addition, a correction factor of 1.22 was used to correct for the comparatively lower arrival rate of propene at a given pressure because of its higher mass.18 The shielding effect induced by the close proximity of the Kelvin probe to the surface during the “continuous” measurements, where the Kelvin probe is vibrating in close proximity (1−2 mm) to the surface, was corrected for by comparing with a stepwise experiment, where the work function was measured after a specific dose with the probe far from the surface. The term “corrected” exposure is used hereafter to emphasize that several corrections have been applied to the pressure reading from the Bayard−Alpert gauge. During a typical adsorption experiment, reactant pressures in the 10−9 mbar regime were used so that a significant number of data points could be recorded during buildup of the adsorbate layer. Theory calculations were performed on an fcc-Co(111) surface (iso-structural to the experimentally employed hcp(0001) surface), using the approach described in detail by Ciobı̂că et al.19 Different from the approach described there, zero-point energy corrections were applied as the C−H vibrations have a significant impact on the relative stabilities.20 A (3 × 3) unit cell was selected because the corresponding coverage of 0.11 ML is comparable to the typical CxHy coverages encountered in the experiments. For comparison between the various hydrocarbon adsorbates with the same carbon number but with different hydrogen content the appropriate number of adsorbed hydrogen atoms was added, adsorbed at infinite distance in a (3 × 3) unit cell.

from the ideal ASF product distribution, and second, the olefin content of the C2 fraction is much lower than that of other light products.9 We therefore also look at propene to explicitly investigate how the addition of a methyl group to the C2Hx adsorbate affects its reactivity. In the present study we use a combination of experimental and theoretical approaches to study the interaction of ethylene and propylene with the close-packed surface of cobalt. In our experiments we use various spectroscopic tools to determine the nature and concentration of reaction intermediates as well as adsorption and activation energies. Density functional theory (DFT) calculations were performed in a complementary manner. In particular, the adsorbate geometries derived for the various species aids the interpretation of the spectroscopic data. In addition, DFT was also used for a detailed comparison between C2Hx and C3Hx adsorbates.

2. MATERIALS AND METHODS The synchrotron X-ray photoelectron spectroscopy (XPS) measurements for ethylene were performed at the I311 photoemission beamline of MAXlab, Sweden,10 whereas the propene experiments were performed at the SuperESCA beamline of ELETTRA, Italy.11 Spectra were recorded at normal emission, and binding energies are reported with respect to the Fermi edge. A photon energy of 380 eV was routinely used for the time-resolved C 1s measurements, whereas a photon energy of 321 eV was used to record the high-resolution C 1s spectra recorded here. These highresolution spectra were recorded at 90 K to minimize thermal broadening effects. C 1s spectra recorded with a photon energy of 800 eV were used for quantitative purposes, to minimize the impact of photoelectron diffraction on the signal intensity. The C 1s signal intensity of a CO-saturated surface at 330 K (1/3 ML)12,13 was used as a reference point to convert C 1s signal intensity to monolayer coverage, that is, number of adsorbates per cobalt surface atom. The thermal desorption and dynamic work function measurements were performed in a home-built vacuum system with a base pressure of 1 × 10−10 mbar, equipped with lowenergy electron diffraction (LEED) optics, a quadrupole mass spectrometer, a Kelvin probe (KP Technologies Ltd.), and a sputter gun for sample cleaning. The H2 desorption spectrum of a Had-saturated surface, with a known coverage of 0.5 ML, 14,15 was used to quantify H 2 produced by C x H y decomposition. Work function values are reported relative to the clean surface value measured prior to dosing of the reactants (ΔWF). The use of direct current heating induced small shifts in the work function. Through the use of reference measurements in which the clean surface was heated in vacuum, this contribution could be quantified and eliminated. Reflection absorption infrared spectroscopy (RAIRS) experiments where performed in a separate vacuum system. The upper chamber (2 × 10−10 mbar) is equipped with a mass spectrometer, LEED optics, X-ray source, electron analyzer, a sputter gun for sample cleaning, and a Kelvin probe (KP Technologies Ltd.). The lower chamber (p < 1 × 10−10mbar) is dedicated to RAIRS measurements. The spectra shown here were recorded at 130 K, averaged over 512 scans. A background spectrum, measured for the clean surface prior to dosing propene, was used to correct the spectra. The cobalt sample, supplied by Surface Preparation Laboratory, was cut and polished to within 0.1° of the desired (0001) direction. The sample was clamped between tungsten

3. RESULTS AND DISCUSSION 3.1. Alkene Adsorption on Clean and Had-Covered Co(0001). Adsorption of ethene and propene at 90 K was followed in situ by synchrotron XPS and work function measurements (XPS spectra provided in the Supporting Information). Figure 1a shows how the C 1s signal intensity increases as a function of alkene dose, for adsorption of ethene on clean and on 0.5 ML Had-covered Co(0001) as well as for adsorption of propene onto clean Co(0001). In all cases we find that the sticking coefficient for molecular adsorption is B


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does not affect the work function very much. In addition, the buildup of a second, physisorbed layer which occurs for exposures >1 L does not induce a significant work function change, as the ΔWF after 1 L exposure is found to be close to the saturation point already. The decreased ethene adsorption capacity brought about by preadsorbed Had is reflected by a smaller work function change at the saturation point, which is decreased in a similar manner for propene adsorption on Had. 3.1.1. Effect of Surface Temperature and COad on Alkene Adsorption. Ethene and propene adsorb dissociatively when the surface temperature is 300 K. This is evident from the XPS data shown in Figure 2a, where the growth of a peak at 283.3

Figure 1. Adsorption of ethene and propene at 90 K, in the absence and presence of 0.5 ML Had. (a) The C 1s signal intensity as a function of alkene dose, for ethene, propene, and ethene on 0.5 ML Had. The signal was normalized so that it corresponds to the number of alkene molecules per cobalt surface atom. In panel b, the work function change upon alkene adsorption is plotted, both for a “continuous” measurement (solid lines) and for a stepwise experiment (data points), as explained in Materials and Methods.

high. With the Co surface atom density of 1.86 × 1015 cm−2 for the close-packed surface a 1 L dose is equal to 0.2 ML, close to the coverage reached after a 1 L dose, i.e., the sticking coefficient is close to unity. The main difference between ethene and propene appears for doses beyond 1 L. For propene, the C 1s intensity continues to increase, which is attributed to the formation of a second, physisorbed propene layer. Because of its lower molecular weight the formation of a physisorbed ethene layer does not occur at 90 K, and the coverage reaches a saturation point at ∼0.21 ML instead. This saturation coverage value is somewhat lower than the ethene saturation coverage reported for Ni(111), for which a (2 × 2) LEED pattern corresponding to a coverage of 0.25 ML was reported.21 LEED measurements did not show the presence of an ordered pattern for ethene on cobalt, i.e., ethene appears to form a disordered layer with a density slightly lower than that on Ni(111). When hydrogen is present, the ethene sticking coefficient is unaffected, but the saturation point is found at a coverage of 0.11 ML ethene, around 50% of the saturation coverage found in the absence of preadsorbed hydrogen. Alkene adsorption causes a significant decrease of the work function. By comparing work function measurements during adsorption of ethene and propene (Figure 1b) with the C 1s signal intensity, we find that ΔWF depends linearly on the adsorbate concentration up to a coverage of ∼0.1 ML, but the work function response to alkene coverage decreases when the coverage is increased further. Adsorption of propene causes a work function change that is very similar to that of adsorbed ethene. This indicates that the work function is sensitive only to the part of the molecule that interacts directly with the cobalt surface, i.e., the chemisorbed part, whereas the presence of an additional, noninteracting methyl group in the case of propene

Figure 2. Ethene and propene adsorption at 300 K and in the presence of 0.33 ML COad, respectively. (a) Top view of C 1s spectral region during exposure to ethene at 300 K and total C 1s intensity. (b) Work function change during adsorption of ethene and propene on clean Co(0001) held at 300 K compared to exposure to ethene of a 0.33 ML COad-covered surface held at 200 K.

eV during exposure to ethene signifies the formation of acetylene, the decomposition product of ethene. The slope of the C 1s signal intensity as a function of ethene dose is comparable to that found at low temperature, i.e., the initial sticking coefficient is approximately independent of surface temperature. However, we find that the signal saturates around 0.14 ML when dosing at 300 K. Work function measurements performed at 300 K during exposure to either ethene or propene are shown in Figure 2b. The work function change closely follows the C 1s signal intensity change as a function of dose, i.e., the work function change correlates linearly with the coverage of acetylene. For ethene, the work function reaches a saturation point at −700 meV, whereas for propene a ΔWF of −800 meV is found after saturation at 300 K. In our study of ethene decomposition discussed in the next section, we found that heating to 300 K leaves 0.12 ML acetylene on the surface, which produces a ΔWF of −600 meV. Using the linear ΔWF− coverage correlation, this means that the −700 meV found after exposure at 300 K corresponds to 0.14 ML acetylene, a good match with the concentration at the saturation point as derived from XPS. Similarly, for propene we found that 0.16 ML C


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The Journal of Physical Chemistry C propyne causes a ΔWF of −1050 meV (see section 3.3 for more details). With this value we can derive that the −800 meV found after adsorption of propene at 300 K corresponds to a propyne coverage of 0.12 ML. The influence of preadsorbed CO on the adsorption of ethene and propene was also briefly explored. After preparing a COad layer with θCO = 0.33 ML, we dosed ethylene or propene at 200 K (to promote dissociative, irreversible adsorption of the alkene) while measuring the work function. (It should be noted that 0.33 ML COad increases the work function by ∼500 meV compared to the clean surface reference. To facilitate direct comparison with the data in the absence of COad, these data are offset by 500 meV in Figure 2b.) We find that even a subsaturated COad layer leaves little room for either ethene or propene to reach the surface and adsorb: in the presence of COad we find a ΔWF of only 20% compared to that of the COfree case. 3.2. Ethene Surface Chemistry on Co(0001). The thermal behavior of molecular ethene adsorbed at 90 K was investigated in-depth using a combination of experimental techniques. Figure 3 summarizes the experimental findings during heating of ethene-covered Co(0001). The changes in the XP

in the same temperature window to produce acetylene, which is characterized by a photoemission peak at 283.3 eV as discussed in detail below. Molecular ethene desorption from the sample surface could not be measured directly by mass spectrometry because of excessive degassing of the sample holder. The work function data was used instead to determine the molecular desorption peak temperature. An ethene dose of 0.5 L produces a coverage of 0.12 ML, a coverage for which decomposition is the only reaction path. The gradual work function change observed between 120 and 200 K can in this case be solely attributed to the decomposition route. A 1 L ethene dose produces the ethene saturation coverage of 0.21 ML. In this case, the ethene in excess of 0.12 ML desorbs, and the difference between the two work function measurements corresponds to the ΔWF due to molecular ethene desorption. The derivative of this signal (included in Figure 3b) shows that the desorption peak maximum is located at 180 K (∼50 kJ mol−1). A second step occurs around 400 K, where a downward shift of the C 1s peak position is accompanied by a ∼600 meV increase of the work function back to the clean surface value. In the H2 TPD spectrum, shown in Figure 3c, this step is marked by a distinct high-temperature H2 desorption peak. The C 1s signal intensity at this point shows a decrease. Because there is no hint of desorption of any C-containing species in the mass spectrometer data in this temperature window, this apparent decrease is attributed to photoelectron diffraction effects. Quantitative evaluation of C 1s spectra recorded with 800 eV photon energy support this view, as the spectra recorded after heating to 300 and 500 K both correspond to a carbon atom concentration of 0.24 ML (equivalent to 0.12 ML of a C2 species). Ethene decomposition yields H2, which is detected in two separate desorption peaks. Quantitative evaluation of the desorption peak area confirms that 0.12 ML ethene decomposes, corroborating the XPS-derived quantification. The desorption-limited peak 300 K (the onset of HD desorption), we conclude that exchange between H bound to C in the acetylene adsorbate with Dad is rather fast. Similarly, the desorption of D in the 420 K peak can only be due to the decomposition of Dcontaining acetylene, which must have formed in the same exchange reaction. More extensive deuterium incorporation into the acetylene adsorbate was brought about by using a slightly different experimental approach. After ethene adsorption and heating to 360 K to produce 0.12 ML acetylene, the sample was kept at 360 K during subsequent exposure to 50 L (p = 1.2 × 10−7 mbar; ∼500 s) D2. After the sample is cooled in vacuum, the TPD spectrum, shown in Figure 6b, contains only the peak due to acetylene dehydrogenation. This procedure leads to 60% deuterium incorporation into the adsorbed acetylene, i.e., more than half of the original hydrogen atoms in the acetylene adsorbate have been replaced by deuterium. Extensive exchange between surface-bound deuterium and hydrogen present in CxHy adsorbates reveals that various hydrogenation−dehydrogenation reactions can apparently occur with great ease. Even the hydrogen atoms bound to carbon in the acetylene intermediate, the most stable C2Hx adsorbate which at first instance appears to be unreactive up to 400 K, still exchanges its hydrogen with surface-bound hydrogen at temperatures ≤300 K. 3.2.4. Ethylidyne Formation. The high-resolution spectrum after heating 0.12 ML ethene to 230 K shows additional features compared to the 0.07 ML ethene-covered surface heated to the same temperature (see Figure 4). Likewise, a careful inspection of the temperature-dependent series of XP spectra of the ethene-saturated surface shown in Figure 7a reveals a small difference between the spectra recorded at 200 and at 320 K. The shape of the 320 K spectrum corresponds to that of acetylene, and when the 320 K spectrum (multiplied by 0.96) is subtracted from the 200 K spectrum, the spectral shape of the minor decomposition product can be resolved: it contains a peak at 282.8 eV, without the vibrational fine structure characteristic of the presence of hydrogen bound to that carbon atom, and another component at 283.9 eV, with the vibrational fingerprint of a −CH3 group (see Figure 9b for methyl reference spectra). We assign these peaks to ethylidyne, CH3−C, the second-most stable C2Hx species on Co(0001) according to previously reported DFT calculations.32 The experiments show that it forms upon decomposition of ethene around 180 K, but only when the initial ethene coverage is high. It is stable up to ∼300 K, where its decomposition (dehydrogenation) to acetylene coincides with the onset of hydrogen desorption. Because of the extremely small concentration of ethylidyne (4%, 0.005 ML), we did not detect the characteristic symmetric CH3 bending mode of ethylidyne in our infrared absorption (RAIRS) experiments. Instead, we find additional support for our interpretation from DFT calculations. DFT-based predictions of the binding energies for the two carbon atoms of ethylidyne, reported by the group of Saeys and co-workers,33 show a decent match with the experimentally found binding energy values. Although the absolute BE calibration differs between the DFT prediction and the experiment, the ΔBE of 1.1 eV between the two carbon atoms matches the 1 eV shift predicted by DFT rather well. When ethene is adsorbed onto a hydrogen-saturated surface, molecular desorption is the dominant reaction path, and the quantity of C2Hx decomposition products found after heating to 200 K amounts to a C2Hx coverage of 0.01 ML. Figure 7b

decomposition pathway. The small increase of the work function around 400 K is attributed to the same process. 3.2.3. Isotope Exchange: Acetylene + Dad. The experiments where deuterium was used show that exchange of surfacebound hydrogen and hydrogen bound in CxHy adsorbates takes place already for T > 150 K. To gain further insight into the hydrogenation−dehydrogenation dynamics of CxHy adsorbates in the presence of surface hydrogen, the exchange reaction of the C-bound hydrogen of adsorbed acetylene with Dad adsorbed on the cobalt surface was studied in two experiments. In the first experiment, the surface was first saturated with ethene at 100 K. During subsequent heating in vacuum to 360 K, ethene decomposes to adsorbed acetylene, and because the surface hydrogen produced in this step leaves the surface as H2 below 360 K, this procedure yields a surface covered with 0.12 ML acetylene. After cooling in vacuum to 100 K, the acetylenecovered surface was exposed to 200 L D2. The desorption rates of H2, D2, and HD measured during subsequent heating in vacuum are shown in Figure 6a. A summation of all hydrogen

Figure 6. H−D exchange in adsorbed acetylene (0.12 ML), prepared by low-temperature ethene adsorption followed by heating to 360 K. (a) TPD after a subsequent dose of 200 L D2 at 100 K, and (b) TPD after dosing 50 L D2 with the sample held at 360 K. A heating rate of 2 K s−1 was used for the TPD experiments.

that desorbs yields a desorption profile that was similar to the profile found during heating of an ethylene-covered surface (see Figure 3c). (Note that for this particular experiment a 4× higher heating rate was used here compared to the typical 0.5 K s−1, to increase signal intensity. The higher heating rate causes an upward shift of the peak maxima in comparison to the spectra reported in Figure 3c.) The broad desorption peak between 300 and 400 K is attributed to the recombination reaction of surface-bound hydrogen, and a sharper, hightemperature desorption peak at 420 K is attributed to acetylene dehydrogenation. Because the preparation procedure of the acetylene layer does not leave any “free” Had adsorbed on the surface, the H that desorbs either as HD or as H2 in the lowtemperature peak must originate from the acetylene adsorbate and must have formed via an exchange with the Dad present on the surface. Because H starts to appear in the desorption G


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Figure 8. Coverage-dependent propene decomposition studied by (a) TPD and (b) TP-WF. A heating rate of 0.5 K s−1 was used. The initial propene coverage (ML) is indicated in the figure. Black lines mark the data for the saturated surface, and the blue line marks the subsaturated surface for which temperature-programmed XPS data was measured as well (Figure 9a).

Figure 7. Ethylidyne formation during ethylene decomposition. (a) C 1s spectra at 200 and 320 K during heating of an ethylene-saturated surface, respectively. Ethylidyne is identified as a minor decomposition product after subtracting the acetylene spectrum (320 K, red) from the 200 K spectrum. (b) C 1s spectrum after heating ethylene adsorbed on 0.5 ML Had to 200 K. The inset in panel b shows the DFT-derived adsorbate geometry for acetylene (HC−CH) and ethylidyne (CH3− C).

mass. The desorption peak at 100 K is attributed to desorption of physisorbed propene, and the peak around 150 K (∼40 kJ mol−1) is attributed to intact desorption of chemisorbed propene. By using the quantitative information from the integrated H2 desorption signal (shown for the 0.19 ML propene-covered surface as a dashed line in Figure 8a) in combination with the spectroscopic information discussed below, we assign the four different H2 desorption peaks as follows: The H2 desorption peak between 280 and 350 K corresponds to desorption of two out of the six hydrogens present in the propene precursor. Similar to ethene, it is attributed to desorption of Had formed in the low-temperature dehydrogenation of propene to (predominantly) propyne (+ 2 Had). Because the sharp H2 desorption peak at 360 K is absent in the ethene results, it is attributed to (partial) dehydrogenation of the methyl group. This desorption peak accounts for two more hydrogen atoms present in the original propene adsorbate. Around 380 K only two out of the original six hydrogen atoms are still bound to the C3Hx adsorbate. One more hydrogen is lost around 400 K, at the temperature where we find complete acetylene dehydrogenation in the ethene experiments. However, for propene, the last bit of hydrogen is released only gradually during further heating to 630 K. The work function measurements provide complementary information, in particular on the low-temperature decomposition and desorption processes. Similar to the procedure used earlier for ethene, we disentangle molecular desorption from decomposition by subtraction of the 0.16 ML data from the 0.19 ML data. The derivative of the difference between the two, shown in Figure 8a, contains a peak at 150 K which coincides with the peak in the m/z = 41 desorption signal. Because physisorbed propene does not affect the work function

shows the C 1s spectrum at 200 K, after the majority of ethene has desorbed from the surface. The most prominent peak in the spectrum, at 283.3 eV, can readily be assigned to acetylene. The residual spectrum after subtraction of a peak with the shape of acetylene shows both the low-BE peak due to a hydrogen-free carbon atom and the high-BE peak with the spectral shape of a methyl group, i.e., ethylidyne is formed in this case, and under these hydrogen-rich conditions, it accounts for a significant fraction of the C2Hx product mixture, as opposed to the etheneonly case. 3.3. Propene Surface Chemistry on Co(0001). The decomposition of propene on Co(0001) was studied as a function of coverage in a combined TPD/temperatureprogrammed work function (TP-WF) study, shown in Figure 8. For a low propene coverage (0.02 ML; quantification is based on H2 peak area as well as the work function value at 90 K) the H2 desorption spectrum shows a broad peak centered around 390 K, similar to the low-coverage ethylene results shown in Figure 3. The differences between ethene and propene become increasingly pronounced with increasing coverage. For propene, an additional H2 desorption peak develops around 360 K when the propene coverage is increased, a peak that is absent in the H2 desorption spectra for ethene. In addition, a broad H2 desorption peak develops between 400 and 630 K with increasing quantities of propene. When the initial propene coverage is greater than 0.13 ML, a fourth H2 desorption peak appears around 300 K, at a temperature similar to that in the case of ethene. For the highest propene coverage, the desorption of m/z = 41, the most prominent mass signal for molecular propene, could be discerned from a large background desorption signal for this H


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Figure 9. Propene decomposition on Co(0001), studied by XPS and RAIRS. (a) Signal intensity plot of the C 1s spectral region during heating of a subsaturated (0.085 ML) and saturated (0.19 ML and physisorbed second layer) propene layer (0.5 K s−1). (b) Selected high-resolution spectra at the indicated temperatures, for the saturated and subsaturated surface. (c) IR absorption spectra of the saturated surface recorded after heating to the indicated temperatures. An ethoxy reference is included for comparison.

very much, the work function data indicates that this peak corresponds to desorption of chemisorbed propene. By comparing the work function data of ethene with that of propene, we find that propene adsorbs weaker than ethene and also decomposes at a lower temperature. The desorption peak maximum for ethene is located at 180 K versus 150 K for propene. For decomposition, we find the maximum slope of the work function change at 160 K (44 kJ mol−1) for ethene but at 144 K (39 kJ mol−1) for propene. Another difference between the two concerns the quantity of decomposition products found after heating to >200 K. Despite the larger size of the molecule, quantification by both XPS and the H2 peak area show that 0.16 ML propene decomposes versus only 0.12 ML ethene. The work function changes at a specific temperature during further heating, and for T > 350 K, all coincide with desorption of hydrogen. For the fully dehydrogenated carbonaceous layer formed at 630 K, we find that the work function is ∼200 meV lower than that of the clean surface reference, in contrast to the work function of carbon formed by ethene decomposition, which gives a negligible deviation from the clean surface reference. This difference can be traced back to the nature of the surface carbon formed, atomic carbon in the case of ethene and a mix of atomic and polymeric carbon for propene, as evident from the XPS spectra discussed in the next paragraphs. High-resolution X-ray photoemission spectroscopy as well as infrared (IR) absorption spectroscopy were used in addition to the TPD/TP-WF study to identify the surface intermediates formed during heating of an adsorbed propene layer in vacuum. Figure 9a shows a color-coded signal intensity plot of the C 1s spectral region during heating of a propene-covered surface, both for a low propene coverage and a saturated propene layer. The C 1s signal intensity as a function of temperature for the saturated layer is provided in Figure S2. It should be noted that for the subsaturated layer the adsorption was performed at 180 K, above the temperature where the first propene decomposition step takes place. For the saturated layer, the starting point consists of a mixture of (0.19 ML) chemisorbed propene

with in addition some physisorbed propene being present as well, which causes an additional photoemission peak around 285 eV. Before zooming in on the exact nature of the lowtemperature decomposition products, we first provide a brief, global description of the observed temperature-dependent spectral changes. For the saturated layer, the total C 1s signal intensity decreases around 100 K (see Figure S2), which can be attributed to desorption of the physisorbed layer. A second decrease of the C 1s signal intensity around 150 K is attributed to molecular desorption of chemisorbed propene, in line with the TPD and TP-WF results discussed previously. Above 200 K only the C3Hx decomposition products remain on the surface. At this point there is a distinct difference between the low- and high-coverage case. The most pronounced difference is the presence of a low binding energy component at 282.7 eV which is seen only for the high coverage. This peak disappears around 280 K, after which the spectral shape becomes similar to that of the low-coverage case. For both coverages a clear change is found around 360 K, which coincides with a sharp H2 desorption peak and the change of work function at this temperature (see Figure 8). This change is attributed to the onset of methyl dehydrogenation, involving intermediates such as H2C−C−CH and HC− C−CH. Because we are primarily interested in the lowtemperature propene decomposition products where the methyl group remains intact, we do not go into a detailed discussion of the exact nature of this intermediate here. During further heating the spectral shape gradually changes, forming a mixture of atomic carbon, identified by its characteristic binding energy of 282.8 eV, and “polymeric” carbon, at a binding energy of 284.5 eV.4 In this temperature regime, the main difference between the two coverages studied is the relative intensity of the two components, where the “polymeric” carbon content after complete dehydrogenation (630 K) is highest for the high propene coverage. 3.3.1. High-Resolution XPS and RAIRS. The high-resolution photoemission spectra shown in Figure 9b provide more I


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Co(0001). Figure 9c shows the IR absorption spectra recorded after heating a propene-saturated surface to 240 and 300 K. In addition to this, the difference spectrum between the two is included, as well as the absorption spectrum of adsorbed ethoxy species. The assignment of the absorption bands observed for the various adsorbates is reported in Table 1, alongside relevant

information on the chemical composition of the surface intermediates formed between 150 and 350 K during heating. For θC3Hx = 0.04 ML, we attribute the spectrum obtained after heating to 200 K to propyne, the methylated analog of acetylene. The methyl group at a binding energy of 284.1 eV can be readily identified by its vibrational fine structure, which is identical to the C 1s spectrum of a methoxy (CH3−O) reference spectrum, measured with the same resolution and included in Figure 9b. (Note that the C 1s binding energy for methoxy is 285.8 eV. It is shifted downward in Figure 9b to facilitate direct comparison.) The peak at 283.1 eV is attributed to the −CH end of propyne, because its binding energy is identical to that of acetylene. The peak shape of the third peak at 283.6 eV was deduced by subtracting the respective C 1s spectra of methoxy and acetylene from the propyne spectrum. The residual peak does not have vibrational fine structure, i.e., there are no hydrogen atoms attached to this carbon atom. It is therefore assigned to the central (−C−) atom in adsorbed propyne. Thus, we conclude that the principal decomposition product of propene is propyne (CH3−C−CH), the methylated analog of acetylene. This assignment is in agreement with the theory prediction that propyne is the most stable C3Hx species on Co(0001).4 For the propene-saturated surface, the C 1s spectrum after heating to 325 K contains the same three peak maxima, and in particular, the spectral fingerprint at 284.3 eV can be readily identified. On the basis of the similarity of the photoemission spectra for the two coverages, combined with the notion that propyne is the most stable C3Hx species on Co(0001),32 we conclude that heating of the propene-saturated surface to 325 K produces propyne as the dominant product. The minor differences between the low- and high-coverage propyne spectrum are attributed to broadening of the peaks due to the higher coverage. We now evaluate the spectrum recorded after heating the propene-saturated surface to 215 K. When the spectrum recorded after heating to 325 K (i.e., the pure high-coverage propyne spectrum), multiplied by a factor of 0.87, is subtracted from the 215 K spectrum, the photoemission peaks due to a second, minor decomposition product are revealed, which is attributed to propylidyne, CH3−CH2−C. From the magnitude of the multiplication factor used we estimate that the mixed layer present between 150 and 280 K consists of 87% propyne and 13% propylidyne. Similar to the previously discussed spectrum of ethylidyne on Co(0001), the 282.7 eV peak in the difference spectrum is attributed to the terminal C atom through which propylidyne is bound to the surface. The ethyl group produces a set of two peaks in the 283.5−284.6 eV region. From the DFT-derived adsorbate geometries of propylidyne and ethoxy,34 we infer that both adsorbates adopt a very similar adsorption geometry. As indicated in Figure 9b, the signal attributed to the ethyl group of propylidyne can indeed be accounted for by using the vibrational fine structure of the −CH2− and −CH3 groups of the previously published ethoxy C 1s photoemission spectrum.34 The methyl component in the spectrum attributed to propylidyne is found at the same binding energy as the methyl group in adsorbed propyne, whereas the peak attributed to −CH2− is located at a slightly lower binding energy. Reflection absorption infrared spectroscopy was used to further corroborate the formation of a mixed propyne/ propylidyne layer upon propene decomposition on

Table 1. Vibrational Frequencies of Various Propene Decomposition Products on Co(0001) species

mode assignment

propyne ethoxy propylidyne propyne propyne ethoxy propylidyne propyne ethoxy propylidyne propyne

υas-CH3 υas-CH3 υas-CH3 υ-CH (?) υs-CH3 υs-CH3 υs-CH2 2 × δass-CH3 δs-CH3 δs-CH3 δs-CH3

freq (cm−1) 2965 2962 2960 2928 2892 2869 2861 2833 1373 1373 1349

(w) (s) (s) (w) (s) (w) (w) (s) (w) (w) (s)

Ni(111)

Ru(0001)

− 29645 − − 288837 28755 − 282637 − − 135437

− 297235 296336 − 286835 286336 − − − −

literature values. The IR absorption spectrum measured after heating to 300 K is attributed to a propyne layer, and the absorption bands match those reported for propyne adsorbed onto Ni(111) reported by Roberts et al.37 In the C−H stretch region, the most prominent peak is visible at 2892 cm−1, attributed to the υs-CH3. Following the assignment made by Roberts et al., we attribute the second-most abundant band at 2833 cm−1 to the 2 × δass-CH3 mode. In addition to these two major bands, we find another minor peak at 2965 cm−1, attributed to the asymmetric methyl stretch, and a small band around 2928 cm−1. Because both bands are not present in the propyne reference,37 the assignment of these bands remains somewhat tentative, and in light of the relatively long acquisition time, adsorption from residual propene in the background gas cannot be excluded. In the CH-bending region, the symmetric δs-CH3 mode is prominently visible at 1349 cm−1, a wavenumber similar to that of the propyne reference. According to the XPS measurements, heating of the propenesaturated surface to 240 K produces a mixed 87% propyne/13% propylidyne layer. The IR absorption spectrum at this point indeed shows additional vibrational bands next to those assigned to propyne, where the band at 2960 cm−1, attributed to the asymmetric C−H stretching band of a methyl group, is most prominently visible. The difference spectrum between the 240 and 300 K spectra shows a remarkable resemblance with the spectrum of ethoxy, with the asymmetric methyl bending mode prominently visible at 2962 cm−1, the (weak) υs-CH2 at 2869 cm−1, and a very weak δs-CH3 at 1373 cm−1. We note that the easiest distinction between propyne and propylidyne can be made by looking at the prevalence of either the symmetric or asymmetric C−H stretching mode of the methyl group. Propyne interacts through two of its carbon atoms to the surface, and the CH 3 −C bond is oriented close to perpendicular to the surface plane. Because of the surface dipole selection rule, the symmetric methyl vibrations are therefore the most prominent bands in the absorption spectrum. Instead, for both propylidyne and the iso-structural ethoxy, the CH3−C2 bond is oriented parallel to the surface plane, which results in a prevalence of the asymmetric methyl vibrations. J


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The Journal of Physical Chemistry C 3.4. Comparing C2Hx and C3Hx Adsorbates: Electronic and Steric Effects. Our experiments show several distinct differences between ethene and propene. Molecular propene desorbs at a 30 K lower temperature than ethene and also dehydrogenates at a lower temperature. In both cases, alkyne is the most prominent product, but for propene the alkylidyne species forms in significant quantities as a secondary product. Several factors come into play when comparing ethene and propene. In the gas phase, hyperconjugation38 of the π bond with the σ bonds in the adjacent alkyl group provides additional stability to propene but not to ethene. Hydrogenation of the double bond eliminates this additional stabilizing effect, which is reflected by a higher reaction enthalpy39 for ethene hydrogenation, −137.1 kJ mol−1, compared to that of propene and longer 1-alkenes (ΔH = −124.2 kJ mol−1).40 The ΔE of 13 kJ mol−1 can thus be seen as a quantitative measure of the contribution of hyperconjugation to 1-alkene stabilization. This effect, referred to as “electronic” effect in the following discussion, sets ethylene apart from the 1-alkenes typically produced in LT-FTS. In addition to such electronic effects, adsorption onto a planar surface introduces potential steric repulsion. When both carbon atoms of the C2Hx adsorbate interact with the surface, the substitution of a hydrogen atom by a more bulky methyl group can cause decreased stability due to increased steric repulsion. Thus, electronic but also steric effects can give rise to distinct differences between equivalent C2Hx and C3Hx adsorbates. “Equivalent” here means that the C3Hx species is obtained by substituting one of the hydrogen atoms in the C2Hx adsorbate by a methyl group. Cheng et al.41 used DFT calculations to compare the chemisorption energies of the (1-)alkene series from ethene up to 1-hexene on a Co(0001) surface. They find that ethene adsorption is 17.3 kJ mol−1 more exothermic than adsorption of propene, whereas it is practically identical for all other 1-alkenes. (It should be noted that these authors mention that “the van der Waals interaction is not included in the exchange-correlation (XC) functional at the PBE level.”41) Filot et al.42 also used a DFT approach to compare C2Hx and C3Hx adsorbates on a stepped ruthenium surface. They find that the 20 kJ mol−1 higher adsorption energy of ethene compared to propene coincides with a comparatively larger distortion of the electron density upon adsorption. We employed DFT calculations to further explore how electronic and steric effects influence the stability of C2Hx and C3Hx species adsorbed on the flat, close-packed cobalt surface. Figure 10 shows the computed most stable adsorbate geometries for a number of equivalent C2Hx and C3Hx species. Several approaches can be used to quantify stability differences between equivalent CxHy species with a different carbon number. Cheng et al.41 report the chemisorption energies of alkyl and alkene adsorbates relative to the gaseous state. Stability differences of the gas-phase species then contribute to the total energy difference. Like for the 1-alkenes, hyperconjugation by an adjacent alkyl group stabilizes gas-phase alkyl radicals, as illustrated by the 18 kJ mol−1 higher reaction enthalpy for the formation of the methyl radical from methane compared to that for the formation of the ethyl radical from ethane.39 However, the reaction enthalpy of n-propyl formation is similar to that of the ethyl, i.e., for the alkyl species the gasphase stability affects the comparison between the CHx and C2Hx species but not the comparison between the C2Hx and C3Hx discussed here. Thus, by comparison of the stability of a C2Hx fragment relative to adsorbed ethyl with the stability of

Figure 10. Most stable adsorbate geometries for a number of equivalent C2Hx and C3Hx species. The reported relative energy difference (in kilojoules per mole) provides a measure of the effect of H substitution by −CH3 on the adsorbate stability.

the equivalent C3Hx fragment relative to adsorbed n-propyl, the influence of differences in gas-phase stabilities can be eliminated from the comparison of equivalent C2Hx and C3Hx adsorbates. On the basis of the relative stability differences between equivalent C2Hx and C3Hx adsorbates, we distinguish four different categories: (a) Mild destabilization due to steric repulsion occurs when a hydrogen atom in close proximity of the surface in the C2Hx fragment is replaced by a more bulky methyl group. For the case of molecular alkene adsorption (i), we attribute the 5.8 kJ mol−1 lower stability of adsorbed propene to steric repulsion. The top left set of images in Figure 10 compares the most stable adsorption geometries of ethene and propene. Both alkenes adopt a similar, asymmetric adsorption geometry where the −CH2 end adsorbs in a hollow site and the R−CH− end occupies a top site. The close proximity of the methyl group to the surface leads to a slightly larger tilt angle of the C−C bond in the R−CH−CH2 adsorbate relative to the surface plane. This in turn brings the hydrogen atoms at the −CH2 end closer to the surface and makes C−H bond cleavage easier for propene compared to ethene, which would then rationalize the experimentally observed more facile propene decomposition. The steric repulsion introduced by replacing hydrogen by a methyl group causes a modest destabilization of 5.8 kJ mol−1, much less than the computed 17.2 kJ mol−1 lower adsorption energy of propene. The gap of 11.4 kJ mol−1 is attributed to the influence of hyperconjugation on the relative stabilities of the gas-phase alkenes. This indicates that the double-bond character disappears to a large extent upon chemisorption, thereby eliminating the contribution of hyperconjugation to the C3Hx stability. In analogy to the decreased enthalpy of doublebond hydrogenation, this makes propene adsorption less exothermic than ethene adsorption. Thus, our evaluation reveals that the lower adsorption energy of propene is primarily caused by the elimination of hyperconjugation due to K


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ethene hydrogenation proceeds via an ethyl adsorbate, the low temperature at which ethane formation is found indirectly shows the high reactivity of alkyl species on a cobalt surface. Their high reactivity makes alkenes and alkyl adsorbates unlikely as intermediates for chain growth. A typical reaction barrier for reactions occurring around 200 K is 50−60 kJ mol−1, which translates to a reaction rate constant of 105−106 s−1 at LT-FTS temperatures (470−500 K). The corresponding lifetimes of alkene and alkyl species under these conditions are thus extremely short, which means that they simply do not survive long enough on the surface to grow into long chains. Instead, our experiments show that hydrogen-lean alkyne and alkylidyne species are the most stable CxHy intermediates on the close-packed surface of cobalt. We previously proposed that chain growth on cobalt proceeds primarily via the alkylidyne mechanism,32,44 shown graphically in Figure 11. Methylidyne

chemisorption and only to a minor extent due to a different, less stable adsorption geometry. The second example of modest destabilization due to steric repulsion is found for the alkylidyne species (ii). Although alkylidyne bind to the surface with only one carbon atom, propylidyne is still slightly destabilized in comparison to ethylidyne by approximately 3.4 kJ mol−1. This is attributed to the fact that the R−C−CH2 bond is oriented close to perpendicular to the surface. With the typical bond angle of 109.5° for tetrahedrally coordinated carbon, the methyl group of the propylidyne species then ends up in rather close proximity to the surface, and in this way gives rise to slight steric repulsion. (b) For the second category, shown in Figure 10b, substitution of the hydrogen atom pointing away from the surface in the C2Hx adsorbate by a methyl group has very little impact on the relative stability of the C3Hx adsorbate. For both the propyl (iii) and propyne (iv) species we find that the R− CH3 bond is oriented close to perpendicular to the surface plane, and the hydrogen atoms that form part of the methyl group point away from the surface. (c) In a third class of adsorbates, shown in Figure 10c, we find that hydrogen substitution has a stabilizing effect. This is particularly evident in the R−C−CH2 species, where the C3Hx species is more stable by ∼25 kJ mol−1. Substitution of a hydrogen at the −CH2 end of the HC−CH2 adsorbate also gives rise to stabilization, but not as much as for the CH2−C species. In both cases we attribute this stabilization to an electronic effect. We suggest that it relates to the relatively large angle between the C−C bond and the surface plane. Because of this, the bond maintains a strong π-bond character, and as in the gas-phase species, such a bond is stabilized by a neighboring alkyl group. (d) In some cases the C3Hx fragment adopts an adsorption geometry that is very different from that of the equivalent C2Hx adsorbate, and it is also significantly less stable. This is the case for the CH3−C−CH2 species (vii). In the equivalent vinyl (HC−CH2) fragment, the hydrogen atom that is substituted by methyl to produce this C3Hx species is in close proximity to the surface. Steric repulsion introduced by the methyl group makes it impossible to maintain this particular adsorption geometry; therefore, a different, less favorable adsorption geometry has to be adopted. 3.5. Relevance for FTS. Although the conditions used here are far removed from those in applied catalysis, the type of intermediates observed in the experiments such as alkyls, alkenes, alkylidynes, and alkynes are among the species that appear in the various chain growth mechanisms that have been proposed in the literature.43−46 Our high-coverage experiments to some extent mimic the “crowded” surface expected under FTS conditions,47 but in particualr the influence of COad, expected to be the most abundant adsorbate under reaction conditions, is studied to only a limited extent. This should therefore be taken into account when considering the results from this model study in the context of applied Fischer− Tropsch synthesis. Our study of C2Hx and C3Hx species on Co(0001) shows a high reactivity of hydrogen-rich adsorbates, such as alkenes and alkyl species. For low Had concentrations, chemisorbed alkenes either decompose or desorb below 200 K. Under the conditions employed here, a high H ad concentration suppressed decomposition, and instead molecular desorption and hydrogenation is found, at temperatures between 150 and 250 K. As

Figure 11. Graphical representation of the alkylidyne chain growth mechanism. Alkylidyne species feature as chain growth intermediates and are found to be stabilized by a high surface coverage. (Adapted with permission from ref 44. Copyright 2016 Elsevier.).

(CH), the most stable CHx species, is the inserting monomer, which couples with a R−C adsorbate, that is, an alkylidyne fragment, to produce the n+1 alkyne (R−C−CH). Further growth requires the net addition of a hydrogen atom to form the n+1 alkylidyne species, an endothermic reaction step. Despite their comparatively lower stability, we do find the formation of alkylidyne species alongside the (most stable) alkyne adsorbate in our experiments. In view of their prominent role in the alkylidyne chain growth mechanism, it is worthwhile to consider how and when alkylidyne species form, in particular because they are less stable than alkyne adsorbates. Surface coverage, and in particular θH, plays a decisive role in the formation of propylidyne as a secondary decomposition product. When the initial alkene coverage is high, the formation of the maximum quantity of the most stable product that can form, alkyne, is limited by space restrictions: alkyne formation requires two vacant (3-fold hollow) sites to accommodate the hydrogen produced, and the product itself, the alkyne species, occupies two hollow sites as well. Instead, alkylidyne formation requires only one site to accommodate surface hydrogen, and because alkylidyne species occupy only a single hollow site they take up less space on the surface. Thus, in the course of alkene decomposition, the growing Had coverage disfavors alkyne formation, and instead the (energetically speaking) second-best option, alkylidyne formation, becomes favorable because it takes up less space on an already crowded surface. Alkylidyne species remain intact during heating to around 280 K. At this point, H2 desorption produces free sites so that alkylidyne can decompose further to the alkyne, the dominant adsorbate found after heating to 320 K. Thus, our study highlights that the chemical identity of the CxHy adsorbate is determined by the interplay between relative stabilities and surface coverages. Here we show that coverage effects affect the composition of the product mixture formed by decomposition of chemisorbed alkene species. Likewise, Zhu and White L


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The Journal of Physical Chemistry C showed that ethylidyne can form from acetylene on Ni(111),26 but only for a high surface coverage and in the presence of surface hydrogen. In the context of Fischer−Tropsch synthesis, reactions that are promoted by a high surface coverage are of particular interest. At the typical LT-FTS (relatively low) reaction temperature of 470−500 K, in combination with high reactant pressures in the order of 10−30 bar, the surface of an active FTS catalyst is expected to be highly covered with reactants and intermediates.47 Our finding that surface coverage affects the nature and concentration of surface intermediates illustrates that lateral interactions can have a pronounced influence and should be taken into account when considering FTS chain growth mechanisms. The observation that a high surface coverage favors in particular the alkylidyne species indicates that such species might in fact be of importance as chain growth intermediates under realistic conditions as well. We find distinctly different reactivity of ethene and propene in our experiments, which, after detailed consideration, can be traced back by the influence of a methyl group in the C3Hx species. We find that propene desorbs at a lower temperature, and it also decomposes at a lower temperature. Extrapolated to FTS, we propose that the anomalously low C2 production rate in Co-catalyzed FTS as well as the predominant production of ethane instead of ethene can in part be attributed to the different surface chemistry of C2Hx adsorbates. The comparatively higher desorption temperature for ethene translates to a slower rate of termination as olefin for C2Hx species. Because of this, the residence time of C2Hx species is longer, and hydrogenation can compete more efficiently with alkene desorption, giving rise to a comparatively lower olefin/paraffin ratio for the C2Hx products. Thus, we find that next to readsorption of ethene followed by hydrogenation or reinsertion, the different surface chemistry of C2Hx adsorbates can provide an additional explanation for the C2 selectivity difference. In the context of alkene readsorption, we further note that the abundant presence of COad during FTS is expected to severely hinder (re)adsorption of relatively weakly interacting alkenes, as indicated by the strong suppression of ethene and propene adsorption when 0.33 ML CO is preadsorbed on the surface.

hydrogen suppresses ethene decomposition. In addition, we find both hydrogenation to ethane as well as decomposition as minor pathways. In the decomposed fraction ethylidyne is prominently visible. Propene decomposes around 145 K to propyne for low initial propene coverage, but heating of a propene-saturated surface yields a mixture of 87% propyne and 13% propylidyne with a total coverage of 0.16 ML C3Hx. Surface hydrogen produced in the low-temperature decomposition step starts to desorb around 280 K, and at this point propylidyne decomposes to form a pure propyne layer. Around 360 K, dehydrogenation of the methyl group occurs, and further dehydrogenation proceeds in a gradual manner, producing a mixture of atomic and polymeric forms of surface carbon. In the context of FTS, we find that alkyne and alkylidyne species are particularly stable. Consequently, they have relatively long residence times at FTS-relevant temperatures and can stay long enough to grow a long hydrocarbon chain. We further find that a high surface coverage promotes the formation of, in particular, alkylidyne species, the second-most stable CxHy species. A DFT-based comparison between equivalent C2Hx and C3Hx species shows that the comparatively lower molecular desorption temperature and higher reactivity for dehydrogenation of propene can be attributed to both electronic and steric factors which vary significantly depending on the exact structure of the adsorbate. These and other distinct differences between C2Hx and C3Hx adsorbates are proposed to contribute to the anomalous C2 selectivity pattern in LT-FTS on Co.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09760. C 1s photoemission spectra recorded during the adsorption of ethene and propene; C 1s signal intensity as a function of temperature during heating of a saturated propene layer (PDF)

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AUTHOR INFORMATION

Corresponding Author

4. SUMMARY AND CONCLUSIONS The adsorption and decomposition of ethene and propene were studied on a Co(0001) surface to gain insight into the chemistry of hydrocarbon adsorbates which play an important role in the Fischer−Tropsch synthesis. Ethene and propene both adsorb with a high sticking probability on the Co(0001) surface. Spectroscopic evidence combined with DFT calculations show that an asymmetric adsorption geometry is preferred, with the R−CH−CH2 end occupying a 3-fold hollow site and the R−CH−CH2 adsorbed atop. Relatively small quantities of COad strongly suppress adsorption of both alkenes. Preadsorbed hydrogen suppresses the adsorption capacity for both alkenes and alters the ethene adsorption geometry to a more symmetric, atop adsorption geometry. Ethene dehydrogenates predominantly to acetylene and minor quantities of ethylidyne around 160 K, whereas adsorbed ethene in excess of 0.12 ML desorbs around 180 K. Surface hydrogen produced in the low-temperature decomposition step desorbs around 320 K, whereas acetylene dehydrogenation around 400 K produces a high-temperature H2 desorption peak and forms atomic carbon on the surface. Preadsorbed surface

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. J. N. Andersen for beamtime at beamline I311 (MAX-lab, Lund). ELETTRA is acknowledged for beamtime at the SuperESCA beamline. Henno Gericke and Tiny Verhoeven are acknowledged for their participation in the XPS measurement sessions. The help of the technical staff at ELETTRA (Trieste, Italy) and at TU/e Eindhoven is also greatly appreciated. Pieter van Helden, Jan-Albert van den Berg, Werner Janse-van Rensburg and Melissa Petersen are acknowledged for fruitful discussions. Syngaschem BV acknowledges funding from Synfuels China Technology Co. Ltd.

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REFERENCES

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Adsorption and Decomposition of Ethene and Propene on Co (0001

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