Controlled Catalytic Energy Release of the Norbornadiene

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Controlled Catalytic Energy Release of the Norbornadiene/ quadricyclane Molecular Solar Thermal Energy Storage System on Ni(111) - a Photoemission and DFT Study Udo Bauer, Lukas Fromm, Cornelius Weiß, Philipp Bachmann, Florian Späth, Fabian Düll, Johann Steinhauer, Wolfgang Hieringer, Andreas Gorling, Andreas Hirsch, Hans-Peter Steinrück, and Christian Papp J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03746 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

Controlled Catalytic Energy Release of the Norbornadiene/Quadricyclane Molecular Solar Thermal Energy Storage System on Ni(111) U. Bauer1+, L. Fromm2+, C. Weiß3, P. Bachmann1, F. Späth1, F. Düll1, J. Steinhauer1, W. Hieringer2, A. Görling2, A. Hirsch3, H.-P. Steinrück1,4, C. Papp1* 1

2

3

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität ErlangenNürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

Lehrstuhl für Organische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestr. 42, 91054 Erlangen, Germany

4

Erlangen Catalysis Resource Center (ECRC), Friedrich-Alexander-Universität ErlangenNürnberg, Egerlandstraße 3, 91058 Erlangen, Germany +

the authors contributed equally to this work *email: [email protected]

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Abstract We have investigated the surface chemistry of the molecular solar thermal energy storage system of the valence isomer pair norbornadiene (NBD)/quadricyclane (QC) on Ni(111). Our multimethod approach includes UV-photoelectron spectroscopy (UPS), high-resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. The NBD/QC system holds the potential to be utilized in future energy storage technologies due to its comparably high gravimetric energy storage density, and the release of energy in a catalytic and sustainable cycle. UPS shows molecular adsorption of both compounds at 120 K, as is also predicted by DFT. NEXAFS and DFT suggest an adsorption geometry of NBD with both double bonds binding to the surface (η2:η2). For QC, no preference is found, and both the η2: η2 and the η2: η1 adsorption geometry are stable. The conversion of QC to NBD is thermally activated. From UPS, a reaction temperature of ~175 K is determined. Possible detrimental decomposition reactions of NBD were investigated by XPS. At 190 K, benzene (C6H6) and methylidyne (CH) are formed, and further react to C-H fragments at 330 K and finally leave carbide on the surface above 475 K.


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Introduction In times of dwindling public acceptance for fossil fuel-based energy resources, their limited long-term availability and the need for renewable energy production due to environmental reasons, novel energy production, distribution and storage solutions are required. One possible route is the efficient use of solar energy. Unfortunately, solar power is intermittent and depends on geographical and climatic conditions. Therefore, challenges regarding energy storage and distribution are inevitable. A possible route for solar energy conversion and storage is the use of so-called molecular solar thermal (MOST) energy storage systems.1-4 Upon irradiation with sun light, these molecular systems store energy by forming new intramolecular bonds leading to an energy-rich isomer. These newly formed metastable isomers allow for energy storage and release at will, which is a prerequisite to overcome the challenges associated with the intermittency of solar energy. The applicability of such energy storage systems critically depends on understanding the relevant energy release processes. One route is a catalytic reaction transforming the (meta)stable energy-rich molecules to the energy-lean form under the release of thermal energy. Ideally, the reaction takes place at a heterogeneous catalyst without the loss of any storage material, with a minimum of undesired intermolecular side reactions, so that the molecules can be used for a large number of loading/unloading cycles. All solar fuels share a number of basic requirements to be fulfilled for a potential and large-scale application in future. First and most importantly is absorption in the UV and visible range, that is, from 300 to 700 nm, since this region (peaking at ~550 nm) contains ~50 % of the solar energy reaching earth. In addition, the formed photoisomer should not absorb in this region to avoid undesired back conversions stimulated by irradiation; ideally, both compounds are non-luminescent. In literature, a large variety of potential storage systems have been studied, e.g., based on anthracene3, 5-6, stilbene3, 7-8, azobenzene8-13 or (fulvalene) tetracarbonyl diruthenium.1, 14-18 Herein, we focus on norbornadiene (NBD) and its valence isomer quadricyclane (QC), schematically depicted in Figure 1, which is the prototype system. The underivatised NBD/QC system provides the largest endergonic reaction profile of all MOST candidates with ∆H = 89 kJ mol-1 released as potential heat during the back conversion19, which results in a specific energy storage density of ~0.96 MJ kg-1. This value is in the range of other storage technologies, e.g. lithium-ion batteries exhibiting values from 0.3820 to 3.24 MJ kg-1 21

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The photochemical properties of this molecule pair were already described decades ago22-23 and have drawn a lot of interest in literature up to now. While most investigations were conducted in solution19, 24-34 or by theoretical calculations,35-37 only rarely studies under model conditions in UHV (ultra-high vacuum) were reported.38-40 Latest research is reported by Moth-Poulsen et al. who investigate the influence of the substitution pattern of the molecule pair on all significant properties like absorption spectrum of NBD, half life of QC, and quantum yield and energy storage density of the overall cycle.41-45 Moreover, they also build first prototypes of reactors working under real conditions as a proof of concept.46 Our surface science approach, which uses highly sensitive spectroscopic tools such as XPS, UPS and NEXAFS under well-defined, clean conditions in UHV combined with state-of-the-art DFT calculations, allows for a detailed understanding of the surface chemistry of NBD/QC on Ni(111).

Figure 1: Schematic drawing of the norbornadiene (NBD)/quadricyclane (QC) MOST system. NBD is irradiated with sunlight to form the energy-rich QC. QC is then exposed to a catalyst surface, where it rearranges to NBD. Thereby the stored energy ∆H is released and the produced NBD is regained after desorption to be irradiated and photoconverted in further cycle.


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Since sunlight is readily available, the loading of the NBD / QC system with energy might not be the bottleneck. We therefore will focus mainly on the energy release, which yields 89 kJ/mol for the isomerization of QC to NBD. Three main routes have been proposed: (1) at homogeneous transition metal catalysts or Lewis acids in solution, (2) at heterogeneous catalyst surfaces, and (3) electrochemically. All might exhibit partial irreversible degradation of the molecules. The use of heterogeneous catalysts requires a fundamental understanding of the catalytic reaction at such surfaces. Thus, to implement and further improve the triggered energy release, a detailed knowledge of the reaction mechanism is desirable. The energy release, which is identified as the bottle neck in this reaction cycle, can be studied precisely using surface science techniques that will allow for a molecular level understanding of the triggered “at will” release of the thermal energy stored in QC. Herein, we present a model study on the adsorption, the energy release and degradation of the NBD/QC system on a Ni(111) surface. The model system is studied with different surface science techniques, including high-resolution X-ray photoelectron spectroscopy (HRXPS), ultraviolet photoelectron spectroscopy (UPS) and near edge X-ray absorption fine structure (NEXAFS) supported by density functional theory (DFT) calculations to gain a fundamental understanding of the important surface processes.



Experimental Section & Computational Details The experiments were carried out in a transportable UHV apparatus that has been described in detail elsewhere.47 Shortly, it consists of two chambers; the preparation chamber houses LEED optics, metal evaporators and a sputter gun for sample cleaning and the analyzer chamber contains an electron energy analyzer, a setup for NEXAFS, gas dosing facilities and a quadrupole mass spectrometer for gas analysis. For HR-XPS and NEXAFS measurements, the chamber was connected to beamlines U49/2 PGM 2 or UE56/2 PGM 2 at BESSY II of Helmholtz-Zentrum Berlin. Additional laboratory UPS and XPS measurements were conducted with a UV lamp with He I radiation (hν = 21.2 eV) and with Al-Kα radiation (hυ = 1486.6 eV), respectively. The sample can be cooled to 120 K using liquid nitrogen, and heated to 1400 K by direct resistive heating. For TPXPS48-49, the sample can be heated by an additional filament in the back of the crystal, without disturbing influences of induced magnetic fields. The Ni(111) crystal was cleaned by sputtering (Ar+, 1⋅10-5 Torr, 1.0 keV, 60 min, ~5.0 µA) and subsequent annealing to 1200 K. Surface order and cleanness were checked by LEED and XPS, respectively. The time per spectrum was typically in the order of 10 s for synchrotron XPS, and ~50 s for UPS. All spectra were taken at a light incidence angle of 50° and an electron emission angle of 0°, both with respect to the surface normal. For UPS, the sample was biased with -18 V. All binding energies are referenced to the Fermi edge. Carbon coverages were determined using XPS, by comparison to the C 1s signal from the saturated chemisorbed (√7x√7)R19.1° benzene layer on Ni(111) with a benzene coverage of 0.143 ML (=0.86 ML carbon atoms); 1 ML corresponds to one adsorbate species per substrate atom.50 The energy resolution was set to 150 meV for the C 1s region recorded with hν = 380 eV. All XP spectra were treated with a linear background correction, whereas UPS data are untreated. NEXAFS spectra were recorded using a separate partial electron yield detector (PEY) with a retarding voltage of -240 V. The photon energy was varied by simultaneous scanning undulator and monochromator with a speed of 0.1 eV/s and a step width of ~0.08 eV. After adsorption and measurement of the molecules at 130 K, the surface was flashed to the desired temperatures prior to the measurement. For background correction of the spectra, a clean reference spectrum was subtracted. Furthermore, all spectra are averaged from several individual spectra, which were recorded consecutively. The angle of the synchrotron light relative to the surface normal was 0° for normal incidence and 70° for grazing incidence.


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NBD and QC were dosed with a microcapillary array doser. The exposures are given in Langmuirs L (1 L = 1.0⋅10-6 Torr⋅s) using the uncorrected background pressure during dosing. NBD was purchased from Sigma-Aldrich with a purity of 98%. QC was synthesized by illuminating a mixture of NBD with 10 mol% acetophenone (Sigma-Aldrich, purity ≥ 99.0%) as photosensitizer in diethylether for 72 hours with a 125 W high pressure mercuryvapor lamp. The solvent was removed and fractional distillation at ambient pressure resulted in a pure QC fraction at 110°C. The purity of QC was checked by NMR spectroscopy. DFT calculations were performed using the VASP software51-54 with the exchangecorrelation functional of Perdew, Burke and Ernzerhof (PBE)55-56 and a projector-augmented plane wave basis set (PAW)57. The plane wave basis cutoff was set to 450 eV. The Methfessel-Paxton smearing58 with 0.2 eV was applied. The van der Waals correction method from Grimme DFT-D359-60 was used. The Ni(111) surface with and without adsorbates was described with a slab model with supercells consisting of a six layer slab with the three bottom layers frozen to bulk geometry with a lattice constant of 3.512 Å and a 20 Å vacuum layer between the slabs. Differently sized unit cells were used to model different coverages. The geometries were optimized until all forces were smaller than 0.01 eV/Å. Adsorption energies were calculated by total energy differences with Eads = Egas + Esurf - Esys. Therefore, higher adsorption energies correspond to more stable systems. Approximate K-edge NEXAFS spectra were simulated using ground-state density functional calculations. Excitation energies from C 1s core levels at a specified atom to a (partially) unoccupied band ߰௔,௞ (with band index a and k-point index k) were approximated as the difference ∆εi,a,k between the 1s core level energy ε1s,i at atom i (obtained using the ICORELEVEL=1 tag of the VASP program) and the single-particle energy of εa,k of the unoccupied band. The obtained ∆εi,a,k are typically too low compared to experimental core excitation energies (due to the use of ground state orbital energies) and were therefore uniformly shifted to obtain the best overall match of the simulated spectrum with the experimental spectrum. Approximate intensity information is obtained from the spd- and site projected wave function character of each band as provided in the VASP PROCAR file. These data are interpreted as approximate dipole intensities for electronic transitions from the C 1s core levels to the unoccupied bands. The line spectra were subsequently convoluted using a variable Gaussian broadening (constant broadening with 0.2 eV FWHM at low energies up to 5 eV over the onset, and constant increase to 2.0 eV FWHM over the next 10 eV). 7 ACS Paragon Plus Environment


Results and Discussion UV Photoelectron Spectroscopy We first present the UP spectra collected during adsorption of NBD and QC on Ni(111) at 120 K in Figure 2a and 2b, respectively. The low temperature was chosen to avoid an instantaneous reaction during adsorption. The clean Ni(111) surface shows the well-known spectrum with the 3d band between 0 and 2 eV binding energy, with a maximum at ~0.7 eV, and a satellite at 6.0 eV. Upon exposure to both NBD and QC, the Ni 3d band is damped by the molecular overlayer, and changes its shape. For NBD, four distinguishable adsorbate peaks at 3.2, 6.3, 7.5 and 9.1 eV are observed in Figure 2a, which increase in intensity with increasing exposure in the monolayer range. For QC, Figure 2b shows six characteristic features at 3.0, 4.3, 5.3, 6.4, 7.5 and 9.2 eV in the monolayer range. For both molecules, the observed peaks undergo shifts of up to 200 meV to higher values with increasing coverage in the monolayer range, which is attributed to lateral adsorbate – adsorbate interactions. Completion of the first layer is indicated by the bold blue spectrum for NBD and bold red spectrum for QC.

Figure 2: UP spectra taken during the adsorption of (a) NBD and (b) QC on a clean Ni(111) surface at 120 K; hυ = 21.2 eV. Multilayer spectra of NBD and QC show spectral differences for the case of physisorption, without interaction with the surface.


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At the highest exposures, the nickel peaks have nearly vanished due to dampening by physisorbed multilayers on top of the nickel surface and all molecular features grow in intensity; the corresponding spectra are indicated in light blue and light red for NBD and QC, respectively. Note that the occurrence of a new peak around 4.2 eV in the NBD multilayer spectrum, which could not be clearly resolved for the monolayer, is probably due to a specific orientation/interaction of the NBD molecules on the surface, which leads to this low intensity.61 The spectra of both molecules show characteristic differences, in the monolayer as well as in the multilayer. From this very different shape of the spectra after exposure to NBD and QC, we conclude that the molecules remain intact, that is, no reaction occurs under the applied experimental conditions, neither in the monolayer range, nor in the multilayer range. See Figure S1 for the calculated gasphase density of states (DOS) in the SI. The comparison of the monolayer and multilayer spectra of QC shows that in the multilayers all peaks are shifted to higher binding energies by 0.3 - 0.5 V. This is attributed to a lower degree of final state relaxation in the physisorbed multilayers and thus higher binding energies. Interestingly, for NBD multilayers, peak 2 - 4 are shifted to lower binding energy compared to the monolayer. These differential shifts are bonding shifts arising due to the interaction of molecular orbitals with the substrate states. This interaction occurs only for molecules directly bound to the surface. As a next step, we heated the adsorbed monolayers with a linear heating ramp of 0.2 K/s to investigate the conversion from QC to NBD on Ni(111). The corresponding spectra are shown in Figure 3a for NBD and Figure 3b for QC. The first spectrum in each case corresponds to the highlighted monolayer spectrum of Figure 2a and 2b, respectively. In Figure 3a, the two characteristic NBD peaks at binding energy of 7.5 eV (P3NBD) and 9.1 eV (P4NBD) are indicated with dashed orange lines. Similarly, in Figure 3b the characteristic QC peaks at 7.5 eV (P5QC) and 9.2 eV (P6QC) are indicated with dashed orange lines. When heating the sample from 115 to ~210 K, the spectrum of QC undergoes characteristic changes, which are most pronounced for peaks P5QC and P6QC; these changes are assigned to the reaction of QC to NBD on the surface. The intensity ratios P5QC:P6QC and P3NBD:P4NBD are plotted as a function of temperature in Figure 4 and serve as an indicator on the degree of conversion from QC to NBD. Up to ~130 K, the spectra in Figure 3 and the ratio P3NBD:P4NBD of ~1.06 for NBD, and P5QC:P6QC of ~0.82 for QC in Figure 4 remain unchanged. Above 130 K, the QC spectra and consequently the P5QC:P6QC ratio changes. At ~175 K, the spectra of QC exactly match those of NBD (cf. Figure 3a and b), which is 9 ACS Paragon Plus Environment


reflected by identical P5QC:P6QC and P3NBD:P4NBD ratios in Figure 4. The decrease of the P3NBD:P4NBD ratio between 130 and 160 K is attributed to desorption of small amounts of physisorbed NBD molecules.

Figure 3: Selected UP spectra of (a) NBD and (b) QC monolayers taken at different temperatures during the temperature-programmed experiments with a linear heating ramp of β = 0.2 K/s and hν = 21.2 eV. In both graphs, the characteristic peaks P3NBD and P5QC (7.5 eV), and P4NBD and P6QC (9.1 resp. 9.2 eV) are marked by the dashed orange line.

Upon further heating to ~195 K, the identical P5QC:P6QC and P3NBD:P4NBD ratios stay constant and the spectra do not show any significant changes indicating that NBD (directly adsorbed or formed from QC) is stable up to this temperature. At higher temperatures, identical changes occur, as is evident from the increase of the peaks ratio in Figure 4 and from the spectra at 210 K in Figure 3: For NBD (QC) we find a shift of peak P2NBD (P4QC) and peak P3NBD (P5QC) to lower binding energy by about 0.2 eV and a vanishing of the valley between P2NBD (P4QC) and P3NBD (P5QC). This behavior indicates that further reaction steps start to occur and continue until 300 K. We tentatively propose the formation of benzene (C6H6) and methylidyne (CH) in this range, because the spectra at 300 (310) K resemble those of deuterated benzene on Ni(111)62 with the exception of the benzene 1b2u band around 8.5 eV that is almost missing; this could be due to cross sectional effects61. The spectra at 400 K do not exhibit clear molecular features. As the intensity of the Ni 3d states is still damped,


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we attribute this behavior not to desorption but to an undefined carbon-containing overlayer. This reaction of NBD on the surface will be further discussed in the XPS section below.

Figure 4: Intensity ratio of the two characteristic peaks for NBD (peak 3 and 4) and QC (peak 5 and 6) observed during the TPUPS experiment as a function of temperature.

Density Functional Theory In order to gain information on adsorption geometries and energies, we performed DFT calculations of single molecules on Ni(111). For lower coverages with nearly non-interacting adsorbates, orthogonal 4x4 surface unit cells with 16 Ni atoms in the first layer were used. Geometry optimizations were started from different geometries. The results with the lowest calculated total energies are shown in Figure 5. For NBD, there are two binding motives to the surface. In the more stable structure (a) the NBD molecule interacts via both double bonds with the surface (η2:η1). There is a major change in geometry at the sp² carbons relative to the gas phase geometry. The hydrogen atoms are pointing away from the surface so that we conclude that there are bonds formed to the Ni surface atoms and the double bond with formerly 1.34 Å is elongated to 1.47 Å. The second structure shown in (b) binds instead with



one of the double bonds and via the CH2-bridgehead group to the surface (η2:η1). As in the η2:η2-structure, the geometry of the interacting double bond changes.

Figure 5: Adsorption geometries of NBD and QC. a) NBD adsorbed in η2:η2 (2.34 eV), b) in η2:η1 geometry (1.67 eV) and c) QC adsorbed in η2:η2 (0.71 eV) and d) in η2:η1 geometry (0.78 eV); first line: top view; second line: side view; the third line shows charge density differences between the surface and the adsorbate while charge accumulation is shown in green and depletion in magenta with an isovalue of 0.05 for NBD and 0.01 for QC.

For the second molecule, QC, the situation is different because it does not have double bonds but a rather rigid 4-membered ring. We still find the two different structures, one with the 4-membered ring towards the surface (c) and a second one interacting via the CH2bridgehead (d). For QC, the energy difference between the two structures is small with the η2:η1 structure preferred by 0.07 eV. A transformation of QC to NBD via the optimization was not observed unlike for the calculations on the Pt(111) surface38; this is in good agreement with the experimental data (see above). The stronger binding of NBD compared to QC leads to an increased conversion energy of about 215 kJ/mol on the surface as compared to 89 kJ/mol in the gas phase. Unfortunately, in the catalytic cycle this additional energy is required to remove NBD from the surface. 12 ACS Paragon Plus Environment

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For higher coverages, 4x3 surface unit cells were used with two adsorbate molecules per unit cell, corresponding to a carbon coverage of 1.18 ML. For NBD, the adsorption energy for the η2:η2 geometry is lowered relative to the isolated molecule by 0.6 eV. For the η2:η1 geometry, the adsorption energy is only slightly smaller by 0.08 eV per molecule. This is due to the smaller surface area that is occupied by the adsorbate molecule in the η2:η1 structure. In the η2:η2 geometry, the hydrogens interact with each other while in the η²:η1 geometry they can easily avoid each other. For QC, in case of the higher coverage, the adsorption energy of the η2:η2 geometry increases by 0.15 eV. This is ascribed to a higher significance of van der Waals interactions as compared to the molecule-surface bonds. The system is stabilized by these interactions between the QC molecules. For QC in η2:η1 adsorption mode, the higher coverage leads to a slightly smaller (0.03 eV) adsorption energy. Therefore, we can conclude that QC should become slightly more stable in the η2:η2 geometry at higher coverages, whereas the two different adsorption geometries for NBD approach similar adsorption energies at saturation coverage.

X-Ray Photoelectron Spectroscopy In Figure 6, XPS measurements in the C 1s region during adsorption and reaction of NBD on Ni(111) are shown; the corresponding quantitative analysis is shown in Figure 7. Figure 6a depicts the adsorption of NBD at 125 K up to a total carbon coverage of 0.70 ML (blue spectrum), for the corresponding TPXPS data see Figure 6b. The NBD species at 125 K can be described with three contributions at 284.51 (C1), 284.09 (C2) and 283.44 eV (C3), as evident from peak fitting in Figure 6c. With increasing coverage, their relative intensities do not change and the overall intensity increases linearly; the uniform shift to higher binding energies is attributed to lateral interactions of the intact molecules in the first layer on the surface. At higher exposures, the C 1s peak broadens at the high binding energy side, indicating the formation of a multilayer (data not shown). In Figure 6a, for comparison also the spectrum of QC with a carbon coverage of 0.63 ML is shown with an offset above the blue NBD spectrum at 0.70 ML. At those very coverages (and also at lower coverages), the spectra for both intact molecules are very similar, with only minor differences, which reflects their very similar structure. Notably the intensity ratio of 4:2:1 of peaks C1, C2 and C3 is in line with the number of equivalent carbon atoms in both molecules. 13 ACS Paragon Plus Environment


Figure 6: (a) C 1s XP spectra measured during the adsorption of NBD on Ni(111) at a sample temperature of 125 K with hν = 380 eV. For comparison, a QC spectrum for a similar coverage is inserted on top; coverages are given as carbon coverages, that is, carbon atoms per substrate atom. (b) Selected C 1s XP spectra measured during the heating ramp (β = 0.2 K/s) applied to the NBD layer in (a). (c)-(f) Selected fits of the TPXPS in (b).

To study the thermal evolution and the conversion of QC to NBD, we show C 1s spectra of both molecules in Figure 8a and b at 123 and ~180 K, for similar coverages, that is, 1.0 ML carbon for NBD and 0.8 ML carbon for QC; these coverages are close to the coverage of a saturated first layer.


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Figure 7: Quantitative analysis of the C 1s data acquired during the a) adsorption and b) the heating ramp of NBD on Ni(111). For clarity, the individual spectral contributions of the chemical species are summed up. In c) the proposed reaction pathway is schematically depicted.

For NBD, the spectrum in Figure 8a shows only a small increase of peak 2 when annealing to 180 K. For QC, the main peak in Figure 8b loses intensity at the high binding energy side whereas peak 1 and 2 gain intensity. These changes are assigned to the reaction of QC to NBD and possibly to the desorption of physisorbed QC molecules on top of chemisorbed QC islands. Despite the fact that the differences in the spectra of QC and NBD are small, we analyze the peak intensities of the fitted peaks to visualize the characteristic changes, see Figure 8c. The ratio of the area of peak 1 vs the total peak area is depicted upon heating for NBD (blue) and QC (red), and in Figure 8d the height ratio of peaks 1 and 2. The dashed vertical line at 175 K indicates the temperature, where according to UPS the conversion from QC to NBD is completed. From this temperature on, the UP spectra of adsorbed QC and NBD showed an identical appearance. Within the margin of error, the same



is true for the peak area ratios of Peak 1 and 2, which confirms the proposed transformation from QC to NBD in this temperature range.

Figure 8: (a) and (b) show HR-XP spectra of NBD and QC at 123 K and at 180 (179) K, that is, at a temperature, where the conversion from QC to NBD has been finished. In (c), the area ratio of peak 1 to the total peak area, and in (d) the height ratio of peaks 1 and 2 are plotted against the temperature for both compounds. Both ratios are characteristic for each compound and become equal between 165 and 175 K. The solid lines in (c) and (d) are guides to the eye.

To study the subsequent reaction and possible decomposition of NBD on Ni(111), we continued to heat with a linear heating ramp of 0.2 K/s, while simultaneously measuring XP spectra. Selected spectra acquired during this TPXPS experiment are shown in Figure 6b with typical fits depicted in Figure 6d-f. The corresponding quantitative analysis of the data is shown in Figure 7b with a scheme of the overall reaction pathway in Figure 7c In the following, we only discuss the experiment with NBD as QC converts to NBD at 168 K leading to identical spectra and reaction pathways. A first reaction of NBD starts at


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200 K, leading to the growth of a new and sharp feature at 283.08 (C5) and a smaller contribution at 283.42 eV (C4); a corresponding fit for 250 K is shown in Figure 6d. Moreover, the main peak and its low binding energy shoulder slightly shift to higher binding energy by 0.06 eV to 284.57 (C6) and 284.16 eV (C7), respectively, and C7 rises in intensity. We attribute these changes to the reaction of NBD to benzene and CH on the surface. The assignment to benzene is based on the binding energies of C6 and C7 and their relative intensity of ~4:2, which agrees well with benzene on Ni(111), where the binding energy difference of C6 and C7 was assigned to different coordination of the respective carbon atoms to Ni substrate atoms.63 The assignment of C5 to CH also results from its binding energy.49, 64-65. Please note that the absolute binding energies are slightly different to what is found in literature, but these changes are assigned to the different surface coverages leading to changes in lateral interactions. C4 corresponds to a vibrational excitation of CH in the photoemission process with a defined binding energy difference of 340 meV and area ratio of 0.17 as compared to the adiabatic transition C5.49, 64-65. Corresponding vibrational satellites of benzene are hidden in the asymmetric line shape of C6 and C7. Further confirmation for our assignment stems from the carbon ratio of ~5:1 between the benzene peaks (C6 + C7) and the methylidyne (C5), which is close to the expected amount of 6:1. From earlier studies on the thermal stability of 0.1 ML CH the reaction of two CH to form acetylene could be expected at ~270 K.65-67 This reaction seems to be hindered in our case due to the higher surface coverage and the coadsorbed benzene. Further confirmation of this reaction path, e.g. by vibrational spectroscopy or DFT calculations, would be helpful, but is out of the scope of the present work. Starting at around 315 K, benzene and CH react further to undefined CxHy fragments. Considering the different heating ramps (0.2 vs 5 K/s) in the temperature programmed experiments, the decomposition temperature is in line with literature.50 We observe a broadening of the spectrum, yielding a main peak at 284.39 eV (C8) and a shoulder at 283.64 eV (C9), see brown spectrum in Figure 6b and the corresponding fit in Figure 6e. At these temperatures, the decomposition and/or dehydrogenation of hydrocarbons is wellknown on nickel surfaces.68-71 Above 500 K (see green spectrum in Figure 6b and the corresponding fit in Figure 6f), further decomposition occurs, yielding a peak at 284.54 eV, in the region of amorphous or graphitic carbon structures,72-73 and carbide with a peak at 283.32 eV.74



Near Edge X-Ray Absorption Fine Structure As additional method, we performed NEXFAS to investigate the processes occurring on the surface. The corresponding spectra of 1.22 ML NBD and 1.08 ML QC are shown in Figure 9 in grazing (70°) and normal (0°) incidence, at different temperatures. In addition, we compare the low temperature spectra with calculated spectra for η2:η2 and η2:η1 adsorption geometries in Figure 9c and 9f. The quality of the spectra is hampered by the strong carbon dip of the beamline monochromator, which leads to unavoidable small structures around 284-286 eV. The spectra after exposure to NBD at 130 K show peaks at 284.5 (A), 287.1 (C), 290.0 (D) and 295.5 eV (E). We assign feature A and D to π* resonances, E is attributed to a σ* transition and feature C is due to mixed σ* (C-H)/Rydberg resonances or to pure excitations into diffuse non-bonding Rydberg orbitals.75-77 When comparing the grazing and normal incidence spectra, we find a pronounced π* resonance (A) at grazing incidence, while only a weak feature at normal incidence is seen, which is attributed to the above mentioned normalization problems. The observed behavior indicates that both C-C double bonds of NBD are aligned parallel to the surface. Overall, the spectra of QC at 130 K in Figure 9d and 9e show a similar appearance as the corresponding spectra for NBD, with all peaks found at similar positions. One significant difference is that for QC the π* resonance (A) has a lower intensity at grazing emission as compared to NBD. The presence of a π* resonance for QC is unexpected, as no π* unoccupied π* states should exist. This leads to the conclusion that the used QC contains non-negligible amounts of reacted NBD as impurity or that the much higher X-ray dose necessary to measure NEXAFS with our setup (factor of 100 per spectrum) leads to a reaction of the photoactive molecule. Normal incidence spectra of QC show no π* resonance, giving rise to very similar spectra for both compounds. Upon heating to 175 K, that is, to slightly above the conversion temperature from QC to NBD (see XPS and UPS data above), we find more or less identical spectra than at 130 K for both molecules at normal incidence. At grazing incidence, however, the intensity of the QC π* resonance increases, whereas it remains unchanged for NBD. This points towards a conversion of the remaining unreacted QC molecules to NBD. Heating to 250 K leads to characteristic changes in NBD and QC spectra that reflect the reaction of NBD to C6H6 (benzene) and CH (methylidyne). These changes are identical for both molecules, which is to be expected, since already at 175 QC has fully reacted to 18 ACS Paragon Plus Environment

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NBD, according to our UPS and XPS data. At grazing emission, the most pronounced changes are the observation of a new π* resonance at 285.4 eV (B), the disappearance of feature D, and the broadening of the σ* resonance (E) towards lower photon energy. Simultaneously, peak A decreases significantly. From the very low intensity of the new π* resonance at normal emission, we conclude that the decomposed fragment (C6H6) adsorbs in a flat geometry on the surface. In the NEXAFS data, the decomposition to benzene and CH is not completed at 250 K whereas from the XPS data we concluded that at 250 K the reaction has already occurred. This is explained by the different heating rates for TPXPS (β = 0.2 K/s) and NEXAFS (flash to desired temperature, β ~ 5-10 K/s) which leads to the higher reaction temperatures found for NEXAFS. Finally, we compare the experimental data collected at 130 K to the simulations for the different adsorption geometries of NBD and QC; see Figure 9c and 9f, respectively. The intensity of the calculated spectra is referenced to the highest peak of the corresponding measured data in each case, and the simulated spectra were shifted on the energy axis to fit best with the experimental data. We start with the analysis of the NBD data. For grazing incidence (bottom), a good agreement is found with the calculated spectrum for flat (η2:η2) adsorption geometry, which reproduced the dominating peaks at 284.5 and 290 eV. In contrast, the spectrum for the side-on (η2:η1) geometry has its main π* intensity at ~286.5 eV, where the measured spectrum exhibits only little intensity. This good agreement confirms the DFT results on the total adsorption energies (see above), which showed that the flat adsorption geometry is the more stable structure at low and high coverage. For normal incidence, again only the calculation for the flat (η2:η2) adsorption geometry fits to the measured data. For QC, we find no agreement of the spectra for grazing incidence regardless of which adsorption geometry is considered (both calculated spectra are very similar). In particular, the strong π* resonance at ~284.5 eV cannot be reproduced. This is due to the reasons discussed above.



Figure 9: C K-edge NEXAFS spectra for NBD (a-c) and QC (d-e) acquired in (a and d) grazing (20°) and (b and e) normal (90°) incidence at different temperatures (annealing time was 1 min). In (c) and (f) a comparison between measured low temperature spectra and simulated spectra for η2:η2 and η2:η1 adsorption geometries is shown. Note that the simulations were only performed within the energy range from 282 to 293 eV. The intensities of the simulated spectra were referenced to the most intense peaks in the measured spectra and multiplied by arbitrary factors. Simulated spectra were shifted by 17.55 eV to higher energy. 20 ACS Paragon Plus Environment

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Conclusions We investigated the surface chemistry of the molecular solar thermal energy storage system of the valence isomer pair norbornadiene (NBD) / quadricyclane (QC) on Ni(111) using photoemission techniques and density functional theory calculations. The combination of the different methods enables us to provide a detailed analysis of the adsorption and reaction of the molecules. Both compounds produce characteristic photoemission spectra in chemisorbed and physisorbed layers. DFT calculations suggest the flat (η2:η2) geometry to be the most stable adsorption at low temperature for NBD. For QC, a flat (η2:η2) and a side-on (η2:η1) geometry are energetically fairly close. For NBD, NEXAFS results confirm the adsorption geometry with both double bonds of NBD being parallel to the surface, in line with theory. Upon heating, QC is converted to NBD; this conversion is completed at 175 K, as deduced from temperature-programmed photoemission UPS and XPS. At higher temperatures, the spectra of both are identical. If we compare our observations on Ni(1111) to those on Pt(111), we find the Ni(111) surface to be less reactive, because on Pt(111) rapid cycloreversion of QC to NBD is observed already at 125 K.40 Upon further heating, on Ni(111) the thermal decomposition of NBD (and of QC reacted to NBD) occurs. Starting at 190 K, the formation of benzene (C6H6) and methylidyne (CH) is observed. This contrasts the situation on Pt(111), where a norbornadienyl species has been identified at similar temperatures. Finally, starting at 330 K C-H fragments are found that leave carbide on the surface above ~475 K.

Acknowledgements The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through the Excellence Cluster “Engineering of Advanced Materials”. Further, we acknowledge support by the Fonds der Chemischen Industrie. Special thanks go to the Helmholtz-Zentrum Berlin for allocation of synchrotron beam time and to the BESSY II staff for support during beam time.

Supporting Information Available

The supporting information contain a comparison of the gas phase of NBD and QC calculated with Turbomole and VASP, the adsorbed molecules calculated with VASP, and calculated density of states. See the Computational Details for more information. This material is available free of charge via the Internet at http://pubs.acs.org. 21 ACS Paragon Plus Environment


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Controlled Catalytic Energy Release of the Norbornadiene

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