Surface Reactions of Dicyclohexylmethane on Pt(111) - The Journal of

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Surface Reactions of Dicyclohexylmethane on Pt(111) Christoph Gleichweit,† Max Amende,† Oliver Höfert,† Tao Xu,† Florian Spaẗ h,† Nicole Brückner,‡ Peter Wasserscheid,‡,§,∥ Jörg Libuda,†,§ Hans-Peter Steinrück,†,§ and Christian Papp*,†

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†

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany § Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ∥ Helmholtz-Institute Erlangen-Nürnberg, IEK-11, Forschungszentrum Jülich, Nägelsbachstraße 49, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: We investigated the surface reaction of the liquid organic hydrogen carrier dicyclohexylmethane (DCHM) on Pt(111) in ultrahigh vacuum by high-resolution X-ray photoelectron spectroscopy, temperature-programmed desorption, near-edge X-ray absorption fine structure, and infrared reflection−absorption spectroscopy. Additionally, the hydrogen-lean molecule diphenylmethane and the relevant molecular fragments of DCHM, methylcyclohexane, and toluene were studied to elucidate the reaction steps of DCHM. We find dehydrogenation of DCHM in the range of 200−260 K, to form a doublesided π-allylic species coadsorbed with hydrogen. Subsequently, ∼30% of the molecules desorb, and for ∼70%, one of the π-allyls reacts to a phenyl group between 260 and 330 K, accompanied by associative hydrogen desorption. Above 360 K, the second π-allylic species is dehydrogenated to a phenyl ring. This is accompanied by C−H bond scission at the methylene group, which is an unwanted decomposition step in the hydrogen storage cycle, as it alters the original hydrogen carrier DCHM. Above 450 K, we find further decomposition steps which we assign to C−H abstraction at the phenyl rings.

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INTRODUCTION Chemical reactions of cyclic hydrocarbons on transition-metal model-catalyst surfaces have been investigated extensively in the past. The pathways, mechanisms, and kinetics of such reactions are prototypical for industrial processes such as hydrocarbon conversion.1−3 Recently, the hydrogenation of hydrocarbons has been proposed as an energy storage option for renewable energy sources;4−7 this approach is attractive, as it encompasses high pressures or cryogenic temperatures currently required for hydrogen containment.8 Possible candidates for storage molecules, so-called liquid organic hydrogen carriers (LOHC), are mostly composed of cyclic hydrocarbons. These LOHCs can be reversibly hydrogenated and dehydrogenated using noble metal catalysts,7,9 and their high similarity to hydrocarbon fuels allows for storage and transport of hydrogen in the existing infrastructure for liquid energy carriers.10 To optimize the efficiency of both the catalyst and the LOHC involved in this process, a detailed understanding of the catalytic hydrogenation and dehydrogenation reactions at the atomic scale is required. In the case of small adsorbates, this type of insight can be obtained from surface science experiments under well-defined conditions. However, due to the complexity of the rather large adsorbate systems, a spectroscopic identification of the reaction intermediates and pathway is challenging. Nevertheless, we have recently shown © 2015 American Chemical Society

that the dehydrogenation and decomposition of the H2-rich LOHC molecule dodecahydro-N-ethylcarbazole (H12−NEC) on Pd11,12 and Pt13−16 model catalysts can indeed be followed by UHV-based surface science techniques. In these investigations, high-resolution X-ray photoemission spectroscopy (HR-XPS) and infrared reflection−absorption spectroscopy (IRAS) were applied. In the XPS studies, the high number of nonequivalent carbon atoms of H12−NEC complicates the elucidation of the reaction mechanism. By combining this information with the one from the single nitrogen atom in the pyrrole subunit, we were able to identify key steps of the surface reaction. Focusing on heteroatom-free LOHC systems (hydrocarbons), the analysis becomes even more challenging, as only the C 1s core level can be used for analysis in XPS. The route to overcome this challenge is to combine various complementary techniques. Lately, the use of industrially well-established heat-transfer oils as LOHCs has been proposed.8 These liquids consist of isomeric mixtures of benzyltoluenes and dibenzyltoluenes and offer numerous advantages. Besides their comparably low price, the H2-lean liquids are fully characterized (including a full Received: June 28, 2015 Revised: August 4, 2015 Published: August 11, 2015 20299

DOI: 10.1021/acs.jpcc.5b06178 J. Phys. Chem. C 2015, 119, 20299−20311

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The Journal of Physical Chemistry C

carrier diphenylmethane (DPM) consists of two phenyl rings interconnected by a methylene (CH2) group. The energy-rich carrier dicyclohexylmethane (DCHM) is a fully saturated hydrocarbon with 12 additional hydrogen atoms (i.e., with two cyclohexyl entities connected by the methylene group). In practice, hydrogen loading is carried out over Ru catalysts, while unloading is typically performed over supported Pt catalysts.8 The system has a hydrogen storage capacity of 7.1 wt %. However, DPM contains carbon and hydrogen atoms only, which presents a challenge for spectroscopic analysis. However, the molecule exhibits a symmetric shape, which, together with the flat single crystal surface, leads to a sufficiently well-defined system. To elucidate the reaction mechanism of the dehydrogenation and possible decomposition steps of DCHM, we build on results from previous surface science studies of the building blocks and possible decomposition products on Pt(111), including benzene18−20 and cyclohexane.19−22 Moreover, there are surface science studies on methylcyclohexane22−24 (MCH) and toluene,20 which are also considered as LOHC candidates,25−27 giving us the opportunity to draw conclusions on the influence of methyl groups on the reactions. We show that with the aid of such model substances, we are able to obtain a detailed picture of the reaction of DCHM and DPM on Pt model catalysts.

(eco)toxicological characterization), and the thermal stabilities are known to be excellent. Compared to the chemically similar, low-molecular-weight LOHC system toluene/methylcyclohexane,17 these benzyltoluenes and dibenzyltoluenes mixtures and their respective perhydro-analogues offer the advantage of lower vapor pressure, thus greatly facilitating the purification of the liberated hydrogen by simple condensation. However, the chemical complexity of these isomeric mixtures hampers their investigation by surface science methods, mainly due to the variety of coexisting species in the system. Herein, we present a multimethod surface science study of the catalytic surface reaction of the LOHC system dicyclohexylmethane/diphenylmethane on a Pt(111) single crystal surface (see Figure 1). These molecules are a model system for the

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RESULTS AND DISCUSSION Methylcyclohexane and Toluene on Pt(111). We start by investigating methylcyclohexane (MCH) and toluene on Pt(111). These molecules represent building blocks of the larger LOHC system dicyclohexylmethane (DCHM)/ diphenylmethane(DPM). Understanding of their surface chemistry is crucial for the following analysis of the reaction of the more complex systems. We use high-resolution X-ray photoelectron spectroscopy (HR-XPS) and temperatureprogrammed XPS (TP-XPS) experiments as our main tools. Briefly, the reaction of MCH on Pt(111) has been proposed28 to involve the following steps: A monolayer of MCH (cC6H11−CH3,a) dosed at low temperature partially desorbs (∼55−65%) and dehydrogenates (∼35−45%) to a π-allylic species (c-C6H8−CH3,a) upon heating to 240 K. At ∼310 K,

Figure 1. Schematic drawing of the investigated system: DCHM and DPM on Pt(111).

chemically more complex heat transfer oils. We used synchrotron-based HR-XPS, temperature-programmed desorption (TPD), infrared reflection−absorption spectroscopy (IRAS), and near-edge X-ray absorption fine structure (NEXAFS) to study its reaction in detail. The energy-lean

Figure 2. Selected C 1s XP spectra of the heating experiment of (a) toluene and (b) methylcyclohexane (MCH) on Pt(111) (hν = 380 eV). 20300


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binding energy (C1:284.4 eV, C2:284.7 eV). The relative intensity of the C2 contribution increases to ∼0.3, and an additional peak, C3, appears at 283.8 eV. Benzyl is stable from 325 to 370 K. Figure 4b shows the fit to the benzyl spectrum at 350 K.

this step is followed by dehydrogenation of the allyl to a phenyl ring and, in the same step, C−H bond scission at the methyl group occurs, leading to benzyl (Ph−CH2,a). Thus, toluene (Ph−CH3) is not a reaction intermediate in the surface reaction of MCH. Interestingly, in the reaction of toluene29 on Pt(111), also benzyl (Ph−CH2) is formed as a reaction intermediate at 300 K. We start by adsorbing 1.1 L toluene at 140 K. Subsequently, the sample is annealed to 200 K to desorb the multilayer. Thereafter, we perform a TPXPS experiment; that is, we heat the sample with a linear heating ramp of 0.5 K/s while continuously recording C 1s spectra approximately every 10 K from 200 to 500 K. Selected spectra are shown in Figure 2a. The toluene monolayer shows a sharp peak at 284.5 eV and a higher binding energy shoulder at ∼284.8 eV. In Figure 4a, the fit of the spectrum at 230 K is presented, yielding two components C1 and C2. C1 is attributed to the adiabatic peaks of the phenyl group and the methyl group; C2 is due to the vibrational excitation of C−H bonds of the core hole state; its relative intensity (denoted as S-factor) of 0.19 is in perfect agreement with the value of 0.19 expected for toluene considering a contribution of 0.17 ± 0.02 per C−H bond (with 7 carbon and 8 hydrogen atoms in toluene).30 The quantitative analysis of the surface reaction is performed by applying fitting envelopes to the occurring surface species, for example, the envelope of the C1 and C2 peaks for toluene shown in Figure 4, thereby enabling a quantification of the surface species. Figure 3a shows the analysis of the peak areas of

Figure 4. Comparative XP spectra of the surface species (a) toluene, (b) benzyl as from the reaction of toluene, (c) MCH, (d) the π-allylic MCH intermediate, (e) benzyl as from the reaction of MCH on the Pt(111) single crystal. See the text for information about the peak components of the spectra. (hν = 380 eV).

The new C3 component at 283.8 eV is assigned to the CH2 group of benzyl, which experiences a major change in the chemical environment. This leads to the observed large shift, and the C1 and C2 components originate from the phenyl ring. Interestingly, the intensity of the peak C3 related to the total peak area (C3:Ctot ∼ 1:8) is slightly lower than expected (1:7). We attribute this to differences in photoelectron diffraction (PED) for the different carbon atoms at the low kinetic energy of the photoelectrons (∼100 eV). Moreover, the C2 component displays a significantly higher intensity than expected for the C−H vibration. From NEXAFS, it is known that toluene lies flat on the surface, while benzyl is oriented almost perpendicular to the surface.32 Thus, the C1 and C2 components cannot be assigned to specific transitions. We rather use them to obtain a good fit of the overall peak shape, with the envelope of the fits C1 − C3 simply representing the spectrum of benzyl.

Figure 3. Quantitative analysis of TPXPS experiments of the toluene (a) and MCH (b) shown in Figure 2.

the toluene heating experiment. Up to 270 K, toluene is the only surface species. A minor decrease in surface coverage (∼10%) between 200−270 K corresponds well to the TPD spectra of toluene,29 where a broad desorption peak (m/e = 91) is observed between 250 and 350 K with its maximum at 315 K. The higher temperatures in the TPD experiments as compared to our TPXPS experiments are attributed to the lower heating rate used in the latter. Above 270 K, we observe a further decrease in surface coverage and the reaction to benzyl occurs.29,31 The C 1s peaks in Figure 2a shift by 0.1 eV to lower 20301


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Figure 5. Selected C 1s XP spectra of adsorption and reaction of diphenylmethane (DPM) and dicyclohexylmethane (DCHM) on Pt(111); on the left-hand side in (a) and (b) the data for adsorption and reaction of DPM, and on the right-hand side in (c) and (d) the respective spectra for DCHM are shown. (hν = 380 eV).

al.28 Figure 4e shows the corresponding fitted spectrum at 350 K. The lower binding energy as compared to the benzyl spectrum in Figure 4b is due to a 55% lower surface coverage of benzyl formed from MCH. Such a coverage-dependent shift is in line with results for benzene on Pt(111),18 where significant lateral interactions at higher surface coverage led to a decrease of the adsorption energy and shifts of the C 1s peak. Above 380 K, similar decomposition reactions as for toluene occur, again leading to contributions D I and D II. Some minor shifts to lower binding energy and the somewhat slower reaction are also attributed to the lower surface coverage in this experiment. To conclude this section, we discuss the temperature difference observed for benzyl formation in toluene and MCH: When starting with toluene, we find only little molecular desorption during benzyl formation from 260−330 K. This leads to a carbon coverage of 0.6 ML. The situation is different for MCH: the first reaction to the π-allylic species leads to release of 3 H atoms and pronounced molecular desorption, resulting in a carbon coverage of only 0.25 ML. The subsequent reaction to benzyl between 290−360 K is shifted by 30 K to higher temperatures compared to toluene. This shift may be induced by the coadsorbed hydrogen blocking adsorption sites required for further dehydrogenation to benzyl. As associative desorption of hydrogen on Pt(111) takes place at around 300 K, the surface area available for MCH dehydrogenation increases above this temperature. Moreover, the hydrogen release during the formation of the π-allylic species may be the reason for molecular desorption of MCH in the first place. This is expected to be relevant for catalysis under realistic conditions because the reaction pathways of hydrocarbons are strongly influenced by the presence of coadsorbed hydrogen.33 Diphenylmethane on Pt(111). After studying the dehydrogenated and hydrogenated smaller building blocks toluene and MCH, we address the dehydrogenated LOHC molecule diphenylmethane (DPM). Later this will allow us to identify DPM as an intermediate in the dehydrogenation of

Above 380 K, further contributions to the C 1s spectra are observed. The peak at 283.8 eV strongly increases and the C2 contribution shifts to lower BE (284.3 eV) indicating further C−H bond scission. In the quantitative analysis in Figure 3a, this species is named D I. We find a strong acceleration of this reaction at ∼425 K. TPD shows a hydrogen desorption peak at 490 K, which is in line with our results considering the different heating rates mentioned above.29 At even higher temperatures, the feature D II is found. From XPS, it is not possible to assign the features D I and D II to specific chemical species, but TPD suggests further C−H bond scission and desorption of hydrogen. Next, we adsorbed 2.1 L MCH at 200 K, leading to a monolayer. Selected spectra of the TPXPS experiment are depicted in Figure 2b. Figure 4c shows the spectrum at 200 K, which can be fitted with two components, C1 and C2, with a ratio of C2/C1 = 0.32 and an energy separation of ∼0.5 eV. We cannot simply attribute the C2 component to C−H vibrations because the energetic separation is larger than expected for this case.30 Overall, the peaks of MCH are broader than those for toluene. This is expected for a saturated hydrocarbon with a bulkier shape and weaker interactions with the surface, leading to an inhomogeneous environment of the individual carbon atoms. Again, we use the envelope of the fit for MCH to describe this surface species quantitatively without an analysis of the single components and their physical origin. Upon heating, we observe desorption and reaction of MCH, best seen from the quantitative analysis in Figure 3b. 72% of MCH desorbs, in agreement with previous report,28 while 28% reacts to form a new species at 284.0 eV. This new species was assigned to a π-allylic species (c-C 6 H 8 −CH 3,a ); 28 the corresponding fit at 260 K is shown in Figure 4d. Above 280 K, a further reaction is observed. The C 1s peak shifts to higher binding energy (284.2 eV), as indicated in Figure 2b, and additional contributions appear at 284.6 and 283.6 eV. This species is attributed to benzyl, in agreement with our toluene experiment (see above) and as suggested by Xu et 20302


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The Journal of Physical Chemistry C DCHM, or to identify common reaction intermediates of the two molecules. Selected XP spectra for different exposures at 120 K are shown in Figure 5a. The peak at 284.3 eV with a high binding energy shoulder is attributed to the chemisorbed monolayer of intact DPM. With increasing coverage, the binding energy shifts to slightly higher values (284.4 eV). The fit of the spectrum at 1.0 L is shown in Figure 7a with two contributions C1 and C2

Figure 6. Quantitative analysis of the XPS experiments shown in Figure 5. In (a) and (b), the analysis of adsorption and reaction of DPM is displayed, and in (c) and (d) the respective results for DCHM are shown. The left (right) axis describes the coverage in carbon atoms (molecules) per nickel atom.

Figure 7. Comparative XP spectra of the surface species (a) DPM, (b) DPM II, (c) DCHM, (d) the double-π-allyl, and (e) the single π-allyl on Pt(111). See the text for information about the peak components of the spectra (hν = 380 eV).

at 284.4 and 284.9 eV, respectively. The large relative intensity of the C2 component and the separation of 0.5 eV indicates that the latter is not only due to vibrational excitations. Interestingly, the C 1s spectra of benzyl (Figure 4a) and DPM (Figure 7a) resemble each other, apart from the lower binding energy shoulder (C3) observed for benzyl. Similar to benzyl, the spectrum reflects a situation with nonequivalent carbon atoms. In DPM, the two phenyl rings cannot both be planar on the surface due to the tetrahedral bonds in the methylene group. The nonplanar adsorption geometry is also reflected by our NEXAFS measurements discussed below (see Figure 8). At exposures over 0.9 L, an additional signal at 284.8 eV appears, which is due to the adsorption of the (physisorbed) multilayer. The quantitative analysis in Figure 6a shows that, after the onset of the multilayer peak at 0.9 L, the monolayer signal continues to grow up to exposures of approximately 1.5 L. This indicates that multilayer growth starts before the monolayer is completed. This behavior may be due to a low mobility, which could be induced by strong intermolecular π−π−interactions of the phenyl rings. At even higher exposure, formation of further layers leads to damping of the signal from the first layer. Additional information is obtained from NEXAFS. C 1s NEXAFS spectra acquired after an exposure of 4 L DPM at 140 K are shown in Figure 8a, for both normal incidence (NI, 90°) and grazing incidence (GI, 20°). The spectra contain

contributions both from the chemisorbed monolayer and the multilayer. Note the lower surface sensitivity compared to our XPS experiments, resulting from the higher the kinetic energy of the electrons collected in partial electron yield (PEY) mode (Ekin ∼100 eV in XPS, Ekin > 240 eV in PEY).34 Both in GI and NI in Figure 8a, we find two π* peaks at 285.0 eV (A) and 288.8 eV (C), and two σ* peaks at 293.5 eV (D) and ∼300.5 eV (E); see also Table 1. Between the two π* resonances, we also find a Rydberg excitation at 286.6 eV, similar as for benzene,32,35,36 styrene, and comparable materials.37 As for cyclohexane35 and saturated hydrocarbons,38 the contributions from the CH2 group in between the phenyl rings are also expected between 286.8 and 293.5 eV. The reaction of DPM upon heating was monitored by TPXPS, NEXAFS spectra at selected temperatures, and TPD (note that due to the different experimental setups the exposure values for the TPD data are higher by a factor of ∼4 compared to the NEXAFS and TPXPS experiments). Figure 9 displays TPD spectra of DPM adsorbed on Pt(111) for increasing exposures; only DPM (m/e = 91) and hydrogen (m/e = 2) are found to desorb from the surface. Up to 4 L, only desorption of hydrogen is observed. At 1 L, the first hydrogen desorption peak (m/e = 2) is observed at 380 K, most likely due to 20303


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Figure 8. Near edge X-ray absorption fine structure (NEXAFS) spectra for (a) DPM and (b) DCHM recorded in (i) gracing (20°) and (ii) normal angle (90°) of the incident X-rays. The spectra were acquired in partial electron yield (PEY) mode.

Table 1. Energy Positions (± 0.5 eV) and Assignments of Resonances in the Carbon K Shell Spectra of DPM on Pt(111); Denoted Orbital Assignment Corresponds to Benzene Spectra36,37 feature no.

energy [eV]

assignment DPM

A B C D E

π*, phenyl rings (C 1s → e2u) σ-like valence, Rydberg (rings and methylene group)36,38,39 π*, phenyl rings (C 1s → b2g) σ*, rings (C 1s → e1u) σ*, rings (C 1s → e1u+a2g)

285.0 286.8 288.8 293.5 300.5

dehydrogenation at the methylene group. This interpretation is in line with results for a similar molecule, bibenzyl (1,2diphenylethane)40 on Pt(111) for which first the ethylene group undergoes dehydrogenation. At higher exposures, we find the desorption onset to shift to lower temperatures. At the highest exposure, the first hydrogen desorption is already observed at 300 K. This behavior is related to the second order desorption of hydrogen and the increasing coverage of DPM. From an integration of the peak area it also becomes evident that, besides the more reactive methylene group, dehydrogenation occurs also at the phenyl rings at ∼390 K: At exposures above 4 L, we find desorption of three hydrogen atoms up to 470 K, while another three hydrogens are released up to 600 K, followed by the remaining 6 atoms up to 820 K (see Supporting Information (SI) for selected integrated spectra). Note that above 4 L, we also detect a broad desorption peak of DPM (m/e = 91) at ∼325 K, indicative of multilayer desorption. Note that the hydrogen desorption states above 470 K are all already related to the dehydrogenation of the phenyl rings, similar as in benzene, toluene, and other aromatic molecules.17,35 Such processes are not desired for the application as LOHC. In Figure 5b, selected TPXP spectra are shown, with the quantitative analysis from 120 to 500 K depicted in Figure 6b. The starting point of the TPXPS experiment is the end point of the adsorption experiment. Above 240 K, a decrease of the signal at 284.8 eV is observed, indicating desorption of the

physisorbed multilayer; at the same time, damping of the monolayer signal vanishes, as seen by the increase of the peak at 284.4 eV. At 300 K, the DPM monolayer signal has reached its maximum intensity, corresponding to ∼0.85 C atoms per surface Pt atom, or ∼0.07 ML DPM molecules. Note that the apparent maximum DPM monolayer coverage (∼0.75 C atoms/Pt) during the adsorption experiment is lower, which is due to the higher mobility of the molecules at higher temperatures. This leads to a higher packing density in the monolayer with the multilayer as a reservoir. Between 260 and 450 K, a peak grows at 283.8 eV, at the cost of the DPM signals at 284.4 and 284.8 eV, which shift slightly to lower binding energy by 0.1 eV. In Figure 7a and 7b, we compare the XP spectra of the DPM monolayer and the species formed at 450 K. From TPD, we deduced desorption of three hydrogen atoms between 300 and 470 K at exposures above 4 L. Therefore, we would expect the peak at 283.8 eV to exhibit an area of 3/13 (23%) of the total C 1s peak area. The much larger area (∼40%) indicates a change in adsorption geometry which, together with the dehydrogenation, leads to a larger number of C atoms with a changed local environment. This slight change in adsorption geometry is also found in NEXAFS as discussed in the next paragraph. To further investigate this first intermediate, we also measured NEXAFS spectra (see Figure 8a). The red curves are collected after annealing a DPM monolayer at 323 K for 1 min. At this temperature, C−H bond scission is minor and thus 20304


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and a broadening at the higher binding energy shoulder at 284.8 eV is seen (“Decomp” in Figure 6b). These decomposition steps include most probably also the phenyl rings, as already discussed in the TPD analysis above. Dicyclohexylmethane (DCHM) on Pt(111). Next, we address the adsorption and thermal evolution of the hydrogenrich DCHM on Pt(111). Figure 5c shows selected XP spectra for adsorption at 140 K. The monolayer signal is observed at 283.6 eV, which is about 0.7 eV lower compared to DPM. The peaks have a larger full width at half-maximum (fwhm) of ∼0.8 eV compared to DPM (∼0.6 eV). From the width of the peaks, a less-defined adsorption state with more inequivalent carbon is deduced, an effect that is expected due to the lower molecule− surface interaction of the saturated hydrocarbon. In Figure 7c, the fit and fitting envelope after a DCHM exposure at 140 K are depicted, with two contributions C1 and C2 representing adsorbed DCHM. This envelope is used to describe this particular surface species in a quantitative manner. We cannot, at this point, analyze the origin of the single components and their physical origin. Above 3 L, we find a broad multilayer contribution at ∼284.4 eV which damps the monolayer signal (see Figure 6c). Overall, DCHM shows a ∼50% higher surface coverage for the chemisorbed monolayer as compared to DPM, which is attributed to a different adsorption geometry and a higher mobility at lower temperatures. NEXAFS spectra at a DCHM exposure of 4 L at 140 K are shown in Figure 8b in black, and Table 2 lists our peak Table 2. Energy Positions (± 0.5 eV) and Assignments of Resonances in the Carbon K Shell Spectra of DCHM and Reaction Products on Pt(111) feature no.

Figure 9. Thermal desorption spectra representing the evolution of (a) Mass 91 (DPM) and (b) Mass 2 (H2) following increasing exposures of DPM on Pt(111).

energy [eV] 90°

assignment 20°

DCHM (140 and 173 K) 1 2 3 4

the spectra represent intact DPM. At NI, the π* peak at 285.0 eV (A) is clearly visible, indicating that the phenyl rings are not lying flat on the surface. Thus, the nonplanar geometry of DPM is preserved when adsorbing on the Pt(111) surface. In GI, this peak is still more pronounced than the σ* contributions, indicating that the rings are not perpendicular to the surface. After heating to 473 K (blue spectra), in NI the π* state is more pronounced and shifts to slightly higher binding energy. In GI, it also shifts but decreases in intensity. This suggests a change in the adsorption geometry, probably a slight increase of the average angle of the phenyl rings relative to the surface. Moreover, a lower photon energy shoulder appears in NI at approximately 284 eV. A similar shoulder was found for condensed styrene previously, where it was assigned to the vinyl group.37 This hints to formation of CC double bonds, probably at the methylene position. These changes are moderate, however, because only 1 out of 13 carbon atoms is affected and the phenyl rings are intact. Therefore, we believe that the species observed at 473 K is still very similar to DPM. Consequently, we call this species “DPM II” in our discussion and in the quantitative analysis. The fit and the fitting envelope of the XP spectra at 450 K are depicted in Figure 7b. From the quantitative analysis in Figure 6b, we see that the intact DPM (red) reacts to DPM II (blue) and entirely replaces it at around 450 K. Note that there is no carbon loss during this process. Above 450 K, we observe additional C 1s contributions due to the next decomposition reaction. A small peak at ∼283.9 eV

284.8 286.6/287.4 288.9 292.0

285.0 286.6 288.9 292.2

see Hitchcock et al.35 mixed Rydberg/valence * (CH2)35 unassigned * (C−C)35

assignments. At this exposure, the multilayer contribution is expected to be still small (cf. Figure 6c). The main σ* resonance in the GI spectrum is found at 292.2 eV (“4” in Figure 8b). The features around 285 eV (1) both in GI and NI are assigned to transitions to the substrate valence level in analogy to observations by Hitchcock et al.35 for cyclobutane and cyclohexane. The features at 286.6 eV (2) in GI and at 287 eV in NI are attributed to a mixed Rydberg/valence (CH2) state.35 An additional peak at 288.9 eV cannot be assigned unambiguously. In comparison to cyclohexane, the additional methylene bridge might cause this state, but no clear assignment is possible without detailed calculations. Since the DCHM spectra do not show significant differences as compared to those for cyclohexane (for details see Hitchcock et al.35), we conclude that the molecule is adsorbed intact on the surface at low temperatures. To further elucidate the adsorption behavior of DCHM on Pt(111), we conducted isothermal IRAS experiments. The surface was exposed to a constant flux of approximately 1 ML/ min of DCHM, and IR spectra were obtained in a time-resolved manner. Figure 10a shows the IRAS spectra acquired after 30 min at the denoted temperatures. The following band 20305


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Figure 10. (a) IR spectra taken during dosing of DCHM on Pt(111) under isothermal conditions at various temperatures (exposure time: 30 min). (b) Integrated areas of the bands observed in the IR experiment.

desorption occurs which leads to the disappearance of the broad signal at 284.4 eV until 240 K. In the NEXAFS experiment, only 4 L were adsorbed to minimize the contribution from the multilayer in the first place. At 223 K (Figure 8b), the small resonance at 285 eV disappears compared to the spectrum at 140 K. The integrated IRAS intensities in Figure 10b clearly indicate multilayer growth at 173 K. Please note that this temperature is close to the multilayer desorption temperature, thus leading to an adsorption/desorption equilibrium that results in strongly temperature-dependent coverage. This also explains the somewhat smaller coverage at 173 K as compared to 123 K (Figure 10a). In contrast to IRAS, no additional gas was supplied during the NEXAFS, TPD, and TPXPS experiments at higher temperatures, and therefore, the corresponding coverages at a given temperature are lower. In TPXPS, a new species evolves at 284.0 eV between 200 and 260 K. For MCH, we found the dehydrogenation of the cyclohexyl ring at temperatures of ∼200 K to form a π-allylic species with a peak at 284.0 eV and maximum intensity at 260 K. In Figure 7d, the spectrum of DCHM at 260 K is presented, showing 85% of the reaction intermediate and a small contribution of 15% of remaining DCHM. The peak shape and position are similar to the peak found for the π-allylic reaction intermediate of MCH. This hints to a similar reaction also for DCHM. However, this argument is done simply by comparison of the binding energy in XPS. To further support this assignment IRAS and NEXAFS data is discussed in the following. The IRAS data helps us to further characterize this intermediate. The spectrum at 223 K (Figure 10a) shows a

assignments are based on previous studies of cyclohexane on Pt(111).33,41−44 Besides a weak complex fingerprint below 1400 cm−1, the spectrum at 123 K shows bands that are assigned to CH stretching vibrations of the CH2 groups pointing away from the Pt surface (symmetric at 2850 cm−1, asymmetric at 2927 cm−1), CH stretching vibrations (2792 cm−1), and CH2 deformations at 1449 cm−1. The shoulder at 2887 cm−1 is assigned to symmetric stretching vibrations of the equatorially oriented CH bonds of the remaining methylene groups, which are located closer to the surface. In agreement with literature, the corresponding (softened) vibrations of the axial CH−Pt bonds give rise to a weak band at 2660 cm−1. The integrated areas of selected bands as a function of time are shown in Figure 10b. They clearly show that at 123 K the bands continue to increase even after 30 min, indicating adsorption of the intact molecule and successive multilayer growth, supporting the findings from NEXAFS and XPS. The deviations from a linearity at high exposure may be due changes in the average orientation for thicker multilayers. Next, we discuss the thermal evolution of the DCHM on Pt(111). TPXPS was performed after the adsorption experiment. In a separate experiment, NEXAFS spectra were acquired at selected temperatures after heating the sample to the denoted temperatures. Moreover, TPD experiments with exposures up to 5.4 L were performed to monitor the desorbing species in the gas phase, including the released hydrogen in the dehydrogenation reactions. To complete the picture of the reaction, IRAS spectra were obtained in isothermal adsorption experiments for selected temperatures. Selected TPXP spectra are presented in Figure 5d, along with their quantitative analysis in Figure 6d. First, multilayer 20306


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The Journal of Physical Chemistry C drastic decrease of the bands compared to lower temperatures, which is due to the absence of a physisorbed multilayer. The integrated areas (Figure 10b) saturate after ∼10 min, which also suggests that no multilayer forms at 223 K (note that the IRAS experiments were conducted in a separate setup, leading to exposures that are somewhat different from the other experiments; we believe that due to continuous supply from the gas phase combined with thermal diffusion of the molecules, the packing density on the surface increases over time). Neither the relative ratio of the bands changes, nor do new spectral features appear. Exclusively CH and CH2 vibrations are visible at 223 K. Dehydrogenation of DCHM to the π-allyl is connected to formation of CC double bonds, whose vibrational modes are, however, not observed at this point. This is attributed to the metal surface selection rule (MSSR)45 in IRAS, according to which only components of the dynamic dipole moments, which are not parallel to the surface, contribute to absorption. The absence of the mode indicates that the CC entity of the intermediate is oriented parallel to the Pt surface, and thus, the respective modes are forbidden. The experiment at 273 K shows a similar behavior, with even lower intensities of the CH2 bands. We speculate that this decrease is connected to reorientation and/or further dehydrogenation of the intermediates. The formation of CC double bonds goes along with the existence of a π*-resonance in the NEXAFS spectra. Indeed, in GI at 223 K, we find an increasing contribution to the spectrum at 285.0 eV (A) in Figure 8b. This π*-resonance is even more pronounced at 273 K, whereas in NI, no signal is visible. This indicates a perpendicular orientation of this orbital with respect to the surface, that is, the double bond is parallel to the surface. This agrees with the absence of the CC vibrations and the decrease of the CH2 vibrations in IRAS. The reaction step from DCHM to this intermediate should release six hydrogen atoms. In TPD (see Figure 11b) this reaction that proceeds around 240 K according to TPXPS (Figure 6) cannot be followed directly, because associative desorption of hydrogen is expected at higher temperatures.46 This leads to the conclusion that both cyclohexyl rings changed into π-allylic C6H8 group that is similar to the π-allylic species found for MCH. The reaction intermediate is then a double-sided π-allylic intermediate, which we write as c-C6H8− CH2−c-C6H8,a. Starting at 260 K, the C 1s signal of the double π-allyl species at 284.0 eV decreases while a new peak at 284.5 eV appears with a higher binding energy shoulder at 284.8 eV. This reaction is completed around 340 K and goes along with a decrease of the surface coverage by ∼30%. A spectrum of the new surface species along with the fits is presented in Figure 7e. Although some of the intensity of the peak at 284.0 eV (component C3) is remaining, the two new components C1 and C2 nicely resemble the line shape of DPM in Figure 7a. The remaining lower binding energy shoulder indicates that both rings are not yet fully converted to phenyl rings. One of the π-allyls dehydrogenated to form a phenyl ring, whereas the other π-allyl is still remaining. This would explain why we still find a contribution at the binding energy of the double π-allyl species. Hereinafter, we will address this surface species as the single π-allyl (Ph−CH2−c-C6H8,a). The above conclusion is in line with the NEXAFS spectra in Figure 8b (in purple) at 323 K. We observe a further increase of the π*-state at 285.0 eV in GI. In NI, there is still no evidence for an increase of a π*-state. Only the state at 300.5 eV (E)

Figure 11. Thermal desorption spectra representing the evolution of (a) Mass 91 (DCHM) and (b) Mass 2 (H2) following increasing exposures of DPM on Pt(111).

increased to some extent, which we believe is due to the phenyl ring. This is also in line with the spectrum of DPM (Figure 8a, in red), where we find a pronounced resonance at that position. The absence of a π*-state for DCHM in NI leads to the conclusion that all carbon atoms with CH groups (phenyl ring and parts of the π-allyl) are in-plane with the surface. The IRAS experiments are supporting this assumption because we observe a continuous decrease of all CH2 bands between 223 and 323 K and no evidence for CC vibrations. Due to the MSSR, the experiment is blind to dynamic dipole changes parallel to the surface. Thus, all unsaturated entities of the one-sided π-allyl must be in an in-plane orientation with the surface at this stage, but some CH2 vibrations are still visible, in line with the interpretation that the dehydrogenation is not yet complete. In TPD, the desorption peak between 280 and 400 K includes the six hydrogen atoms of the first reaction step, because associative desorption of hydrogen is expected around 300 K, and the three hydrogen atoms of the second step. Note that during the second step the decrease of the total C 1s signal in the TPXP spectra (Figure 6d), and the desorption signal for m/e = 96 at 320 K (Figure 11a) indicate desorption of larger molecular fragments. In summary, the TPD results show that the desorption behavior around 300 K is strongly coverage dependent. From the decrease of the desorption temperature with increasing exposure, we conclude that the rate-limiting step is the associative desorption of H atoms, rather than C−H bond scission. This leads to a second order behavior in the spectra and is reflected in a change in the dehydrogenation temperature of DCHM (see Figure 11b). 20307


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second step involves the catalytic conversion of one of the π− allylic groups to a phenyl ring, and the release of three hydrogen atoms. It yields a one-sided π−allylic species or, alternatively, leads to desorption of the double-π−allylic species (30%). The third step is the dehydrogenation of the remaining π−allyl entity, yielding a second phenyl ring. In the same process, it is suggested that the methylene group undergoes C− H bond scission. This last reaction and the following hydrogen abstraction at the phenyl rings give rise to carbon formation on the surface under surface science conditions. Such undesired side reactions need to be avoided in the hydrogen storage cycle of DCHM/DPM under real catalytic conditions (e.g., by modified catalysts). Corresponding studies with systematically modified surfaces and catalysts will be performed in the future.

The next surface reaction step is observed between 370 and 470 K. In XPS, it is manifested by the increase of the shoulder at 283.8 eV (see Figure 5d) and a shift of the main peak from 284.5 to 284.3 eV. The new surface intermediate shows exactly the envelope of the species “DPM II” found in the thermal evolution of DPM. The NEXAFS spectra at 423 K include a π*-state now also in NI. This leads to the conclusion that at this temperature the second π−allyl containing ring is also dehydrogenated to a phenyl ring, and at least one ring is not parallel to the surface. This reaction step is also evident in TPD, as a peak at 460 K (Figure 11b), with an area corresponding to about 4−5 hydrogen atoms. From the above discussion on DPM, we know that C−H bond scission at the methylene group occurs already below 300 K, which is more than 100 K lower than the reaction discussed here. Consequently, we claim that, as the second phenyl ring is formed in the reaction of DCHM, also the methylene bridge experiences C−H bond scission. Interestingly, the peak in TPD shifts to higher temperatures with increasing exposures. Obviously, lateral interactions between the molecules/intermediates play an important role in the reaction to form this second phenyl ring, leading to a lower reaction temperature at lower coverage. The proposed dehydrogenation and rearrangement of the species formed can also be followed by IRAS. At 373 K, we observe a band at 1540 cm−1. From our previous studies on the dehydrogenation of the LOHC dodecahydro-N-ethylcarbazole on noble metal catalysts we know that this frequency region is characteristic for CC stretching modes.11,12,14 Accordingly, the presence of this band confirms an assumed change in orientation and hints at the decoupling of at least one of the aromatic rings from the Pt surface. At the same time, the bands in the CH stretching region almost completely vanish, which is in good agreement with the suggested proceeding dehydrogenation. Interestingly, adsorption of DCHM at 423 K leads to the disappearance of the CC stretching vibration, while the CH modes do not undergo any noticeable changes. According to our XPS and NEXAFS results, we can rule out that desorption of the intermediate is the reason for the absence of the CC band. A reason for the different behavior in the IRAS experiment may be that at 423 K, the reaction initially follows a different pathway, which does not lead to the formation of nonparallel CC double bonds. As this preparation route has not been followed by the other techniques, where the thermal evolution of layers adsorbed at low temperatures have been studied, we refrain from a further discussion here. Above 470 K, both the XPS and the NEXAFS spectra of DPM and DCHM are very similar, independent of coverage. Similar to other molecules,29,40 above 500 K, the phenyl rings undergo dehydrogenation. In the range of 500−800 K, in TPD, we find that the remaining hydrogen atoms desorb, indicating complete decomposition of the remaining hydrocarbon fragments on the surface.

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EXPERIMENTAL SECTION XPS and NEXAFS experiments: The XPS and NEXAFS measurements were carried out at the third generation synchrotron BESSY II of Helmholtz-Zentrum Berlin (beamline U 49/2 PGM 1) using a transportable UHV setup.47,48 The setup consists of a preparation chamber, equipped with sputter gun, LEED and dosing facilities and an analysis chamber. The latter is connected to the beamline, a hemispherical electron analyzer (EA 125 HR U7) and an additional dosing facility for LOHCs. The Pt(111) crystal was cooled with liquid nitrogen to approximately 110 K and heated resistively to 1300 K. Cleaning was performed with Ar+ sputtering (1 kV, 5 × 10−6 mbar, 140 K) and subsequent annealing to 1300 K for 1 min, followed by checks using XPS for impurities. Carbon contaminations were removed by cycles of O2 exposure at 700 K (oxidation) followed by annealing to 800 K to desorb the remaining oxygen. A filament at the back of the sample allows for heating the sample to about 600 K while measuring XP spectra. The XPS C 1s data were recorded in normal emission to the electron analyzer (NEA, 90°) with a photon energy of 380 eV and a resolution of 180 meV in a time-dependent fashion (acquisition time: typically ∼15 s/spectrum). The binding energy scale was calibrated by reference to the Fermi edge. Due to varying operational modes (multi bunch, low-α) at the synchrotron facility, the photon flux of each experiment was different. The carbon coverage was calibrated by the C 1s peak area of the known c(4 × 2) superstructure on Pt(111).49 For each peak component, as presented in Figure 4 and Figure 7, an asymmetric Doniach−Sunjic function convoluted with a Gaussian function was used (see refs 50 and 51 for details). Except for toluene, we are unable to assign the peak components to specific transitions; we only use the envelope to describe the surface species quantitatively. NEXAFS spectra were recorded in partial electron yield (PEY) mode, with a retarding voltage of −240 V for the carbon K-shell spectra using a grid in front of a separate electron detector (Photonis CEM 4716 TRI) in the analyzer chamber. This analyzer is arranged in-plane with the plane of polarization of the incident X-rays. Monochromator and undulator were moved simultaneously in continuous mode with a speed of 0.2 eV/s, while data points were recorded every second. The molecules were adsorbed at 140 K, and subsequent annealing for 1 min to the specified temperatures was followed by measurement of the spectra. The spectra were backgroundcorrected using the method of division by clean sample spectrum (see, e.g., ref 52). The absolute energy scale was calibrated by comparison to the π* resonance of solid

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CONCLUSION To conclude, we performed a multimethod study of the surface reaction of the LOHC system DCHM/DPM and the relevant molecular fragments, MCH and toluene, on Pt(111) in the temperature range from 140 to 500 K, under UHV conditions. Using the information gained for DPM, MCH, and toluene, we identify three major dehydrogenation steps of DCHM: the first takes place at 200−260 K, leading to formation of a double π− allylic species and six hydrogen atoms on the surface. The 20308



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benzene39 at 285.0 eV. For grazing (20°) and normal incidence (90°) spectra, separate adsorption experiments were performed. The TPD measurements were performed in a different UHV setup,53,54 equipped with a LEED optics, a simple XPS setup including Al Kα X-ray source and a hemispherical analyzer to check the sample cleanness. Moreover, a sputter gun and a capillary array gas doser, and a quadrupole mass spectrometer (QMS, Pfeiffer QME 200) are attached. TPD spectra were acquired with the QMS in a “Feulner cap”55 arrangement. The measurements were performed by adsorbing the molecules at low temperature (typically 140 K), and subsequent heating ramps at a rate of 2 K/s, where signals of the cracking patterns of the adsorbed molecules (including mass m/e = 2 for H2) and CO were recorded. Note that the difference in heating rate compared to the XPS experiments leads to a relative shift of the peaks in TPD of about 20 K compared to the XPS heating ramp. All IRAS experiments were performed in a UHV system with a base pressure below 2 × 10−10 mbar. A detailed description of the setup can be found elsewhere.56 Briefly, the system allows to conduct IRAS, molecular beam (MB), and mass spectroscopic studies. The IRAS experiments were performed with a vacuum FTIR spectrometer (Bruker IFS 66/v) coupled to the UHV system. During deposition, IR spectra were continuously acquired in a time-resolved manner (spectral resolution: 2 cm−1, acquisition time: 60 s/spectrum). DCHM was deposited via a supersonic molecular beam source, which is connected to the main UHV chamber and separated by a gate valve. It consists of three differentially pumped stages and several beam-defining elements, which enable us to modulate a spatially well-defined supersonic molecular beam of DCHM and Ar (Linde, 99.9999%) as a carrier gas. The amount of DCHM vapor in the carrier gas was controlled by the temperature of the DCHM reservoir (∼120 °C). Details on the setup and calibration of the molecular beam can be found in the literature.11,14 The MB technique enables us to dose DCHM in a well-controlled fashion on the sample surface while avoiding contact of the molecules with the chamber walls. (see, e.g., 57−59 for a more detailed discussion of the method). The Pt(111) single crystal (MaTecK GmbH) was cleaned by several cycles of Ar+ sputtering (E = 1.5 keV, 8 × 10−5 mbar Ar, 300 K) and annealing to 1000 K in vacuum. The cleanness of the Pt crystal was then controlled by low energy electron diffraction (LEED). In addition, prior to DCHM deposition, CO adsorption experiments at 100 and 300 K were conducted. Synthesis of chemicals: Toluene, MCH and DPM were purchased from Sigma-Aldrich (≥99%). The hydrogenation of diphenylmethane was carried out using a 300 mL stainless steel Parr batch-autoclave equipped with a four blade gas-entrainment stirrer (1200 rpm). The volume of 150 mL unloaded LOHC was placed in the pressure vessel, and a constant molar ratio of 400/1 (LOHC/catalyst metal) of the catalyst (Ru/C, 5 wt %) was added. To ensure an inert atmosphere, the gas volume of the reactor was replaced three times with Argon (4.6, Rießner Gase) by flushing the reactor. After heating up the LOHC to the desired reaction temperature (120 °C) using an external electrical heating jacket, the autoclave was pressurized with 10 bar of hydrogen 5.0 (Linde), and this pressure was kept constant during the experiment by continuous dosing of hydrogen.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06178. Integrated spectra of the hydrogen QMS signal of the DPM TPD experiment, XPS fit components and assignments of toluene, MCH, DCHM, and DPM (PDF) Experimental schema (PPTX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+49)9131-8527326. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. The present work was supported by BMW Forschung und Technik GmbH. P.W. acknowledges support by the ERC through his Advanced Investigator Grant (No. 267376). The European Union (COST Action CM 1104), the DFG, the Fonds der Chemischen Industrie, and the DAAD are gratefully acknowledged for further support. The authors thank the BESSY staff for support during the beamtime and the Helmholtzzentrum for travel support and the allocation of synchrotron beamtime.

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REFERENCES

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Surface Reactions of Dicyclohexylmethane on Pt(111) - The Journal of

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