Selective Desorption of Ethylene after Dimethyl Sulfide Reaction on

Jan 15, 2019 - Selective Desorption of Ethylene after Dimethyl Sulfide Reaction on Cold Gold Surface. H. Abdoul-Carime*† and F. Rabilloud‡. † In...
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Selective Desorption of Ethylene after Dimethyl Sulfide Reaction on Cold Gold Surface H. Abdoul-Carime*,† and F. Rabilloud‡ †

Institut de Physique Nucléaire de Lyon, Université Lyon 1, Villeurbanne, CNRS/IN2P3, UMR5822, Université de Lyon, F-69003, Lyon, France ‡ Institut Lumière Matière, Université Lyon 1, Villeurbanne, UMR5306, Université de Lyon, F-69003, Lyon, France

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S Supporting Information *

ABSTRACT: Ethylene is mostly generated from nonrenewable petroleum feedstocks. To reduce the environmental footprint, sustainable sources and less energy demanding methodologies are highly encouraged. Here we show that ethylene is produced from the reaction of dimethyl sulfide on the polycrystalline gold surface, maintained at 90 K. The ion detection ratio is measured to (8.5 ± 1.1)%, leading to a conversion rate of 52% in (450 ± 60)s. The reaction is investigated theoretically for three specific surfaces, Au(111), Au(110)(1 × 1) and Au(110)(1 × 3). The flat surface is found to be ineffective for conversion, while cooperative pairwise molecular mechanism is operative at the Au(110) surfaces. Pure ethylene is finally selectively desorbed from the surface by a thermal heating at 105 K. The laboratory findings can be applicable to nanoscale chemistry but may guide the optimization of larger scale methodologies.



INTRODUCTION The predicted industrial demand for ethylene is on a constant growth1 due to use in a large variety of applications.2−5 Ethylene is principally produced by high temperature pyrolysis/cracking from nonrenewable petroleum feedstock followed by multisteps purification comprising the refrigeration of the cracked gas mixture at 100 K. Although energy intensive, these processes dominate the industry. To support the global ethylene market, unconventional sources such as renewable feedstocks6,7 and less energy demanding methodologies8 are highly encouraged to improve energy balance, reduce gas emission from plants and dependency on the fossil fuels. A special interest has been turned to low temperature surface reaction of catalysis such as the HZSM-5 zeolite which has proven to effectively convert 99.9% ethanol to ethylene in 7200 s at the temperature of 530 K,8 before further gas purification steps. In the last past decade the concept of “clean and sustainable” chemistry compatible with the environment, has gained in importance as a means to reduce the environmental footprint. Ethanol produced from biomasses, i.e., corn or sugar cane, is nowadays commonly used as a substitute for fossil fuels (e.g., bioethanol) or to industrially generate ethylene.7 Dimethyl sulfide (DMS), a biogenic compound naturally synthesized by marine phytoplankton is one of the major source of sulfate aerosol that impact significantly the atmospheric chemistry.9 Moreover, the continuous production, i.e., 24 h/day, of DMS from algae (e.g., Symbiodinium) can now be controlled at a rate of about 30 mg per liter of algae and per hour,10 may open new © XXXX American Chemical Society

perspectives for sustainable chemistries. Indeed, DMS has already proven its potential as precursor in the preparation of nanotubes11 and in hydrocarbons production, where transformation techniques still rely on pyrolysis or thermal cracking at 700 K.12 However, such processes generate subproducts concomitantly to the ethylene, and further gas purification are necessary. We studied the surface reaction of dimethyl sulfide on a polycrystalline gold, Au(poly), substrate cooled down to 90 K. We found that DMS is decomposed at the surface of the substrate to produce ethylene for which we estimated the averaged transformation rate constant of 52% of the deposited molecules in 450 s. The decomposition of DMS at the substrate, rationalized by DFT calculations of reactions on three prototypical surfaces, Au(111), Au(110)(1 × 1) and (1 × 3), comprising the gold poly crystal, suggest that the flat surface is not effective for conversion while cooperative reaction is involved for the stepped surfaces. Finally the hydrocarbon can be selectively desorbed from the substrate by thermal heating at 105 K, suggesting a new methodology for producing pure ethylene gas with a priori a minimum of energy cost and without a further purification process. Received: November 22, 2018 Revised: January 3, 2019

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DOI: 10.1021/acs.jpcc.8b11311 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C



METHODOLOGY SECTION Experiment. The UHV apparatus (base pressure of 3 × 10−10 mb) is equipped with a cryogenically cooled polycrystalline Au substrate (10 × 8 × 1) mm3 mounted on a manipulator, and a commercial quadrupole mass spectrometer (QMS, Balzers) with a home-built extraction system.13 The mass spectrum of the background signal (Figure S1) is cleared from any species of the same m/z of ethylene (i.e., m/z 28). The gold substrate is cooled down to 90K by means of a closed cycle He refrigerator and resistively heated to several hundred kelvin (>450 K) required for the cleaning and for the desorption of unwanted species such as SCH3.14 The temperature is measured by a type E thermocouple fixed to the substrate. Dimethyl sulfide (>99% grade), provided from Acros Organics, is further purified by repeated freeze−pump− thaw cycles in vacuum. The purity has been verified by mass spectrometry (Figure S2). The molecules are condensed onto the substrate by exposing to a volumetrically calibrated effusing gas quantity. The thickness of the film can also be determined by correlating the film deposition times to the magnitude of the H− anion signal desorbed from the film under the electron bombardment15 Thus, the thickness of a film from 300 s deposition time is evaluated to 3 (±1) MLs. For thermal programed desorption (TPD) spectrometry, the species is monitored at a heating rate of 0.2 K/s. The desorbing neutral molecules are detected by recording the positive ions via the electron impact ionization at 70 eV in the QMS. Theory. Calculations were performed with the VASP package16 based on density functional theory (DFT), which implements plane waves and PAW pseudopotentials.17 The plane-wave kinetic-energy cutoff was set to 400 eV. The PBE18 exchange-correlation functional in the generalized-gradient approximation (GGA) framework was adopted, together with the D3 dispersion correction.19 The Brillouin zone was sampled with a Γ centered grid of 25 k-point (5 × 5 × 1). We have considered several surface slabs of Au atoms: a supercell with 150 atoms composed of 6 layers of a 5 × 5 surface unit cells for Au(111), a clean surface slab Au(110)-(1 × 1) and an Au(110)-(1 × 3) surface with missing row reconstructed as previously considered by Landmann et al.,20 the two latter were composed of 8 layers and contained 200 and 105 atoms, respectively. During the geometry relaxation, the atoms of the three bottom layers of the slab were kept fixed in their ideal bulk position. The naked slab was first relaxed, before adsorbing the molecule. The vacuum region was about 15 Å. Surface adsorption and reaction energies given in Table 1 are obtained from calculations with a single molecule absorbed

in the supercell. The structure of dimethyl sulfide on different oriented gold surfaces is shown in Figure 1. In the most stable configuration, the S atom of DSM is located at the bridge site on Au(110) with a sulfur−gold bond distance of 2.49 Å, while it is located at the bridge site slightly off centered toward the fcc-hollow site on Au(111) and the shortest S−Au bond distansces are found to be 2.59 and 2.85 Å, respectively. For the three investigated gold crystals, the sulfur is bound by two Au atoms. After the dissociation of the methyl group, the sulfur atom of the adsorbed SCH3 comes closer to the gold surface, i.e., for the Au(110) the S−Au bond distance is found to be 2.42 Å, and the two binding Au distance shifts from 3.96 to 3.66 Å. The dissociation of the methyl group is not operated through an intermediate state. Figure 1b presents the binding of the (top) S(CH3)2 and (bottom) SCH3 to the gold surface. Finally, from the structure calculation and the estimation of the experimental surface deposition, a 3 ML film contains an average of approximately 6.5 ng of DMS material.



RESULTS AND DISCUSSION After the deposition of 3 MLs of fresh pure film of dimethyl sulfide, DMS, onto the polycrystalline gold substrate, Au(poly), maintained initially at 90 K, the latter is gradually heated and the desorbed neutral molecules enter the quadrupole mass spectrometer (QMS) for analysis. Figure 2 shows the yield of the detected species as a function of the substrate temperature. The black line corresponds to the temperature dependence of the m/z 62 molecules (DMS) which desorb from the surface at a characteristic temperature of 150 K. The red and the blue lines representing the signal measured for the m/z 47 and 28 species, respectively, and also observed at 150 K, are attributed to the fragmentation of DMS inside the QMS,21 in agreement with the electron ionization mass spectrum available from the NIST data.22 Pure films of ethylene have been observed to desorb at temperatures of about 100 K from Pt(111) substrate,15,23 but the desorption temperature depends on various parameters such as the surface orientation or the nature of the substrate. We are not able to observe other species, e.g. methane (CH4) and ethane (C2H6) at m/z 15 and 30, respectively, to desorb from the film. If formed via the surface reaction, these hydrocarbons are not likely to remain on the substrate since their low condensation and desorption temperatures (25 and 77 K, respectively24). Finally, we do not observe the formation of dimethyl disulfide (DMDS, m/z 94) in the probed temperature range. Indeed, DMDS could be formed from the associative desorption of methylthiolate radicals (vide inf ra) but at much higher temperatures (∼460 K).25 The yield of ethylene produced at different deposition times corresponding to a thickness of the film of 1.5 MLs and 10 MLs (Figure 3) are almost comparable demonstrating that (1) the m/z 28 species are not arising from contaminants, for which the contribution would increase accordingly, (2) and the reaction of DMS for the production of the hydrocarbon arises at the gold surface of the substrate and, thus, only low coverage of DMS is requested. The production of C2H4 from the reaction of CH3SCH3 on the Au(poly) surface observed presently can be initiated by the molecular dissociation for the methyl (CH3) and the corresponding methyl thiolate (SCH3) radicals. The latter species is likely to undergo chemisorption via a S−Au bonding, as it has been investigated for the reaction of analogue systems H−SCH3 or CH3S−SCH3 with the gold surface. The liberated methyl radical may migrate or directly react with the

Table 1. Calculated Surface Adsorption Energy of the Gas Phase DMS (S(CH3)2), the Dissociation Energy of the Adsorbed DMS for the Production of the Methyl Group, and the Total Reaction Energy for the Methyl Radical and Ethylene Production via the Surface Reaction

gold surface

surface adsorption energy of DMS

S(CH3)2Au → SCH3Au + CH3

total reaction energy for the methyl production

(111) (110) 1 × 1 (110) 1 × 3

−0.55 −1.23 −1.04

1.83 1.92 1.96

1.28 0.69 0.92

total reaction energy for the ethylene production 0.20 −0.98 −0.52 B

DOI: 10.1021/acs.jpcc.8b11311 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Views of (a) two dimethyl sulfide molecules, (b, top) one single molecule, and (b, bottom) one SCH3 radical adsorbed on Au(111), Au(110)-(1 × 1) surfaces, and the reconstructed missing row type surface Au(110)-(1 × 3).

neighboring DMS.12 Surprisingly, the energetics for the cleavage of the CH3−SCH3 bond on gold substrate is not known from the literature, in contrast to those for the H− SCH3 or CH3S−SCH3 bond breakage on gold substrate that have been thoroughly investigated25−29 since the key role of such a process in nanoscience and nanotechnology (e.g., selfassembled monolayers) 29. Thus, we here first explore the energetic of formation of the methyl radical accompanied by the chemisorption of the methyl-thiolate radical on the gold polycrystalline surface. Au(poly) is constituted by terraces of single crystal Au(hkl) domains. As the theoretical exploration of a realistic polycrystalline surface, i.e., including all the possible Miller hkl-indexes, defects, is cumbersome, we have restricted our study to two prototypical low-index surfaces, Au(111) and Au(110) (Figure 1), bearing in mind that reactions may depend on the surface orientation.30 The wellstudied Au(111) surface is smooth, flat and highly ordered, while Au(110) exhibits a grooved-structure.20 Tables 1 presents the energetics of the adsorbed DMS by the Au surface (Figure 1) and the dissociation of the adsorbed dimethyl sulfide into SCH3 radical and CH3, and the total energy for the reaction producing the methyl group or the ethylene. It has to be noted that the dissociation energies of the adsorbed DMS into a methyl group (1.83, 1.92, and 1.96 eV,

Table 1) are found (1) to depend on the crystal orientation and (2) to be lower than in the gas phase (3.30 eV in the present DFT calculations, in agreement with previous theoretical work 3.235 eV31 or experimental measurements 3.34831). In Table 2, the gas phase energetics for the methyl− methyl recombination and the dissociation of ethane into ethylene are reported. For the Au(111), Au(110) (1 × 1) and (1 × 3) surfaces, the production of the free methyl radical accompanied by the chemisorption of the methylthiolate radical is found to be endothermic by 1.28, 0.69, and 0.92 eV, respectively (Table 1). These calculations indicate clearly that the production of such free methyl radical that could thereafter migrate for potential sequential reaction scheme, is not energetically accessible and thus, unlikely. Alternatively, it has been demonstrated that surface cooperative reaction induced by a neighboring pair of radicals can also arise.32 In the present case, the adsorption of two neighboring DMS produces two methyl radical as the first step that would undergo recombination. It is the energetics of this process that we shall explore in the following. The association of a pair of CH3 radicals could form the C2H6 intermediate via a highly exothermic associative reaction CH3 + CH3 → C2H6 (−4.14 eV, Table 2). Therefore, this intermediate is most likely formed as an excited C2H6*. The redistribution of the excess C

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and Au(110)(1 × 3), respectively, rending the production of ethylene energetically accessible (Table 1). For the Au(111) flat surface, the reaction is found to be endothermic by 0.2 eV, indicative that this surface orientation might not favor for triggering the surface reaction (Table 1). Therefore, we suggest that the production of ethylene via the decomposition of DMS by gold surface to DMS are very likely to arise via cooperative pairwise. Figure 4a shows the ethylene desorption yield from 3MLs of freshly deposited films after different reaction times of the

Figure 2. Temperature programed desorption of fresh dimethyl sulfide (DMS) films after 120 s deposition. The signature of DMS (m/z 62, black) is observed at 150 K. Since the desorbed neutral molecules are ionized in the mass spectrometer by 70 eV electrons, the m/z 47 (red) and m/z 28 (blue) also reported at 150 K are identified as the fragmentation of the molecule.21,22 An additional contribution is observed at 100 K for m/z 28 and attributed to ethylene.

Figure 4. (a) Desorption yield of ethylene after 60 (black), 300 (red), 560 (blue), and 1200 s (green) reaction time of a 3MLs fresh deposited film with the gold surface and (b) integrated ethylene yield as a function of the reaction time. Each data point presented is an averaged value from 2 to 4 fresh films. The Integrated yield reaches a saturation corresponding to (8.5 ± 2.2)% of the 3MLs deposited molecules. The characteristic Reaction Time, τR, is estimated to (450 ± 60)s, i.e., 90% of the maximum yield. The dashed lines are a guideto-the-eye.

Figure 3. Temperature programed desorption of C2H4 after 150 s (black) and 900 s (red) deposition of DMS onto the gold substrate. The yields are comparable, indicative that the reaction arises at the substrate surface.

Table 2. Gas Phase DFT Values of the Energetics (in eV) for Different Reactions Involved in the Conversion Processes for the Production of Ethylene reaction

energy

CH3 + CH3 → C2H6 C2H6 → C2H4 + H2

−4.14 1.78

adsorbed DMS molecules on the gold substrate. The TPD yield of the ethylene increases with the reaction time, i.e., the time delay between the precursor molecules deposition and the desorption of the hydrocarbon, up to 560s (blue), while higher reaction times do not improve the ethylene conversion (1200s, green). The integrated TPD yield (Figure 4b) exhibits an increase as the function of the reaction time, which is followed by an asymptotical value corresponding to the conversion limit. By comparing directly the integrated TPD yield of the produced ethylene in the 100 K range to that of the deposited DMS in the 150 K range in Figure 2 (including all possible fragments from the DMS ionization in the QMS), we can estimate the ratio for the ions detection by the QMS to (8.5 ± 1.1)%; i.e., the ions are characteristic of the desorbed ethylene and the deposited dimethyl sulfide molecules. However, this number of 8.5% must be weighted by the ionization cross sections of ethylene (1.77 × 10−16cm2)34 and dimethyl sulfide (10.86 × 10−16cm2),35,36 i.e., by a factor of 6.136, since the neutral molecules are ionized inside the mass spectrometer by 70 eV electrons. Indeed, the number of detected ethylene or

energy may further stabilize the excited ethane intermediate. Since we do not observed ethane in while desorbing the films, as discussed above, we can conclude that the excited hydrocarbon, if formed at all, leaves the film prior to stabilization. Alternatively, the C2H6* may decay into C2H4 and H2. Although the C2H6 → C2H4 + H2 reaction is found to be endothermic by 1.78 eV (Table 2), this step can nevertheless be driven by the excess energy gained from the methyl−methyl associative reaction. The formed H2 is also not likely to remain in the 90 K film since the desorption of dihydrogen has been reported at 17 K.33 Thus, the global reaction process of DMS triggered by the gold surface is found to be exothermic by −0.98 and −0.52 eV for Au(110)(1 × 1) D

DOI: 10.1021/acs.jpcc.8b11311 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C dimethylsulfide molecules ionized by the mass spectrometer, Ndet, (Figure 4) is proportional to the number of preionized neutral molecules, Nmol, the number of the 70 eV ionizing electrons, Ne, the ionization cross section σ, and the detection efficiency P of the QMS: Ndetc = NeNmolσmolP. Assuming that the detection efficiency for ethylene and dimethyl sulfide by the QMS are reasonably similar over the mass range, the dimethyl sulfide-to-ethylene conversion factor is found to be Nethyl/NDMS = 8.5σDMS/σethylen, that is, 52.2%. Moreover, ethylene produced at the gold surface may be underestimated since the molecules can be embedded in the 3MLs DMS matrix,37 and thus, a fraction of the hydrocarbon evaporates only at the desorption temperature of dimethyl sulfide. The characteristic reaction time, tR, can simply be estimated at 90% of the total production of ethylene to (450 ± 60) s (Figure 4b). In other words, the reaction on the gold substrate can produce approximately 55 pmol of ethylene for 6.5 ng of deposited dimethyl sulfide in 450 s. This average rate measured here can be put in perspective with the figures obtained from the catalysis of ethanol for ethylene by zeolite (99% in 7200 s)8 or to the catalysis of CH4 into CH3OH by rhodium surface (230 μmol per gram in 3 h at 420 K, i.e., ∼ 10 fmmol per ng· h),38 bearing in mind that the comparison of this surface reaction process and the flow reactor catalysis might difficult since the differences in methodology for the hydrocarbon production. Nevertheless, the process is found to be sufficiently fast in time (i.e., in terms of conversion rate of the immobilized molecules) and the density of surface reacting molecules high in comparison to reported flow techniques that may allow the use of cold gold substrate as a surface reactor for the biogenic compound to be competitive.



AUTHOR INFORMATION

Corresponding Author

*(H.A.-C.) E-mail: [email protected]. ORCID

H. Abdoul-Carime: 0000-0002-9382-4310 F. Rabilloud: 0000-0002-5011-3949 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS H.A.-C. is indebted to Prof. Eugen Illenberger for the transfer of the experimental set up from the Freie Universität Berlin (Germany) to the Institut de Physique Nucléaire-Université de Lyon 1 (France). H.A.-C. also thanks Prof. Jean-Pierre Dutasta and Dr. Jean-Christophe Mulatier from the Ecole Normale Supérieure de Lyon, Prof. Henry Chermette from the Institut des Sciences AnalytiquesUniversité de Lyon 1 for providing dimethyl sulfide, Dr. Yann Chevolot from the Institut des Nanotechnologies de LyonEcole Centrale de Lyon, and Dr. Michael Steinke from the School of Biological Science, Essex University, for stimulating discussions. F.R. thanks the GENCI-IDRIS (Grant 20173087662) center for a generous allocation of computational time.





CONCLUSION Sustainable production of “green” hydrocarbon gas is a tremendous challenge. We have found that ethylene can also be produced from the reaction of dimethyl sulfide on gold surface at low temperature, 90 K, with a substantially appreciable conversion rate. Any side-products are not observed to desorb within the investigated temperature range, therefore obtaining high purity ethylene requires only a slight heating of the substrate up to 105 K, rendering this process temperature selective with a minimum of the energy cost, in that contrast to any methods proposed yet. It is noteworthy this high conversion rate is measured for gold substrate without any specific surface state requirements (i.e., polycrystal), thus the method can be easily accessible industrially. But the use of peculiar surface such as Au(110) (1 × 1) might increase the conversion rate, which can then be further optimized by investigating the reaction on different specific (h,k,l) single-crystal surfaces. Finally we have demonstrated the feasibility to generate “green” ethylene, and the precursor, dimethyl sulfide, can be continuously obtained from natural biomass, i.e., algae/planktons, via wellcontrolled methods.12 Thus, the present laboratory process can be suitable for nanoscale applications,4 but it may guide the optimization for a larger scale industrial manufacture of the hydrocarbon.



Figure S1, background signal; Figure S2, gas phase mass spectrum of the dimethyl sulfide sample; Figure S3, temperature desorption spectra of a fresh dimethyl sulfide film (PDF)

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b11311. E

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