The energy storage cycle of the molecular solar thermal (MOST) system has been investigated. In particular, upon irradiation, the energy-lean 2-carbethoxy-3-phenyl-norbornadiene is converted into its energy-rich quadricyclane isomer. The energy-releasing back reaction can be triggered thermally by means of a suitable catalyst surface. The differences in the energy landscapes is visualized by sea level and mountains. More information can be found in the Research Article by C. Papp and co-workers (DOI: 10.1002/chem.202203759). Cover design by F. Hemauer.

Novel energy-storage solutions are necessary for the transition from fossil to renewable energy sources. Auspicious candidates are so-called molecular solar thermal (MOST) systems. In our study, we investigate the surface chemistry of a derivatized norbornadiene/quadricyclane molecule pair. By using suitable push-pull substituents, a bathochromic shift of the absorption onset is achieved, allowing a greater overlap with the solar spectrum. Specifically, the adsorption and thermally induced reactions of 2-carbethoxy-3-phenyl-norbornadiene/quadricyclane are assessed on Pt(111) and Ni(111) as model catalyst surfaces by synchrotron radiation-based X-ray photoelectron spectroscopy (XPS). Comparison of the respective XP spectra enables the distinction of the energy-rich molecule from its energy-lean counterpart and allows qualitative information on the adsorption motifs to be derived. Monitoring the quantitative cycloreversion between 140 and 230 K spectroscopically demonstrates the release of the stored energy to be successfully triggered on Pt(111). Heating to above 300 K leads to fragmentation of the molecular framework. On Ni(111), no conversion of the energy-rich compound takes place. The individual decomposition pathways of the two isomers begin at 160 and 180 K, respectively. Pronounced desorption of almost the entire surface coverage only occurs for the energy-lean molecule on Ni(111) above 280 K; this suggests weakly bound species. The correlation between adsorption motif and desorption behavior is important for applications of MOST systems in heterogeneously catalyzed processes.

Molecular solar thermal (MOST) systems are a promising approach for the introduction of sustainable energy storage solutions. We investigated the feasibility of the dicyano-substituted norbornadiene/quadricyclane molecule pair on Ni(111) for catalytic model studies. This derivatization is known to lead to a desired bathochromic shift of the absorption maximum of the parent compound. In our experiments further favorable properties were found: At low temperatures, both molecules adsorb intact without any dissociation. In situ temperature-programmed HR-XPS experiments reveal the conversion of (CN)(2)-quadricyclane to (CN)(2)-norbornadiene under energy release between 175 and 260 K. The absence of other surface species due to side reactions indicates full isomerization. Further heating leads to the decomposition of the molecular framework into smaller carbonaceous fragments above 290 K and finally to amorphous structures, carbide and nitride above 400 K. DFT calculations gave insights into the adsorption geometries. (CN)(2)-norbornadiene is expected to interact stronger with the surface, with flat configurations being favorable. (CN)(2)-quadricyclane exhibits smaller adsorption energies with negligible differences for flat and side-on geometries. Simulated XP spectra are in good agreement with experimental findings further supporting the specific spectroscopic fingerprints for both valence isomers.

The Front Cover shows the concept of molecular solar thermal (MOST) energy storage systems, whereby the light-harvesting process is combined with storage as molecular strain. The photoinduced isomerization of 2,3-dicyano-norbornadiene to its energy-rich counterpart is illustrated, after which the backward reaction is catalyzed under energy release. Cover design by Felix Hemauer. More information can be found in the Research Article by Christian Papp and co-workers.

We present detailed studies on the covalent adsorption of molecular oxygen and atomic hydrogen on the hexagonal boron nitride (h-BN) nanomesh on Rh(111). The functionalization of this two-dimensional (2D) material was investigated under ultra-high vacuum conditions using synchrotron radiation-based in situ high-resolution X-ray photoelectron spectroscopy, temperature-programmed X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. We are able to provide a deep insight into the adsorption behavior and thermal stability of oxygen and hydrogen on h-BN/Rh(111). Oxygen functionalization was achieved via a supersonic molecular beam while hydrogen functionalization was realized using an atomic hydrogen source. Adsorption of the respective species was observed to occur selectively in the pores of h-BN leading to spatially defined modification of the 2D layer. The adsorption of the observed molecular oxygen species was found to be an activated process that requires high-energy oxygen molecules. Upon heating to 700 K, oxygen functionalization was observed to be almost reversible except for small amounts of boron oxides evolving due to the reaction of oxygen with the 2D material. Hydrogen functionalization of h-BN/Rh(111) was fully reversed upon heating to about 640 K.

N-heterocyclic compounds such as octahydroindole and dodecahydro-N-ethylcarbazole have been proposed as suitable liquid organic hydrogen carriers for chemical hydrogen storage. Following these studies, we focused on hydrogen-rich 2,2'-bipiperidine with a hydrogen storage capacity of 7.1 wt % and hydrogen-lean 2,2'-bipyridine. Both were adsorbed on Ni(111), and the temperature-induced reaction mechanism and decomposition were studied. The reaction was investigated using synchrotron-based XPS, NEXAFS, and TPD experiments. Upon adsorption, the formation of a flat-lying chemisorbed layer is observed. Above 370 K, 2,2'-bipyridine is dehydrogenated in the alpha-position to the nitrogen atoms to form an alpha-2,2'-bipyridyl species with a tilted adsorption geometry. The hydrogen-rich 2,2'-bipiperidine is partially dehydrogenated above 180 K and deprotonated at the nitrogen atoms above 250 K. Above 320 K, alpha-2,2'-bipyridyl is formed, which is accompanied by a byproduct that is partially dehydrogenated at the carbon atoms. Above 400 K, we observe the decomposition of the alpha-2,2'-bipyridyl species.

By using an atomic hydrogen source and a supersonic molecular beam of oxygen, the hexagonal boron nitride nanomesh on Rh(111) was covalently functionalized with molecular oxygen and atomic hydrogen. Investigation of the system by high-resolution X-ray photoelectron spectroscopy reveals that only the pores of the nanomesh are functionalized whereas the wires remain unaffected. This selectivity leads to spatially defined chemical functionalization of the corrugated two-dimensional material. More information can be found in the Full Paper by C. Papp et al. (DOI: 10.1002/chem.202101946).

The reactivity of iron nanocluster arrays on h-BN/Rh(111) was studied using in situ high-resolution X-ray photoelectron spectroscopy. The morphology and reactivity of the iron nanoclusters (Fe-NCs) were investigated by CO adsorption. On-top and hollow/edge sites were determined to be the available adsorption sites on the as-prepared Fe-NCs and CO dissociation was observed at 300 K. C- and O-precovered Fe-NCs showed no catalytic activity towards CO dissociation because the hollow/edge sites were blocked by the C and O atoms. Therefore, these adsorption sites were identified to be the most active sites of the Fe-NCs.

N-heterocycles belong to the class of so-called liquid organic hydrogen carriers (LOHCs), which have been identified as suitable materials for chemical hydrogen storage due to favorable hydrogen storage capacity and reaction kinetics. In this contribution, we focus on the dehydrogenation reaction of hydrogen-rich octahydroindole, its dehydrogenation intermediate indoline, and hydrogen-lean indole. Octahydroindole has a hydrogen storage capacity of 6.4 wt %, and indoline has 1.7 wt %. We investigated the mechanism of the temperature-induced dehydrogenation of the three compounds after adsorption on Ni(111) at low temperatures. Nickel is attractive as an effective and low-priced dehydrogenation catalyst, which potentially could replace more expensive Pt and Pd in industrial applications. We compare the obtained results with our previous work on Pt(111) for the same LOHC system and for N-ethylcarbazole/H-12-N-ethylcarbazole. A comprehensive understanding of the reaction mechanism was obtained by combining high-resolution X-ray photoelectron spectroscopy with temperature-programmed desorption lab experiments. For all three compounds, we find dehydrogenation at the nitrogen atom above 270 K (indole, >= 130 K; indoline, >240 K; octahydroindole, >270 K). For indoline and octahydroindole, we observe simultaneous dehydrogenation at the carbon atoms, resulting in an indolide surface species. For octahydroindole, small amounts of side products and decomposition products are observed throughout the reaction pathway. Above 380 K, the indolide species decomposes into fragments for all three compounds.

We report the reversible functionalization of hexagonal boron nitride (h-BN) on Ni(111) with a molecular oxygen species and atomic hydrogen, as studied by high-resolution X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and near-edge X-ray absorption fine structure. As a first step, we prepare oxygen-functionalized h-BN by exposure to a supersonic beam of molecular oxygen. Subsequent exposure to atomic hydrogen leads to complete removal of the oxygen species and the formation of hydrogen-functionalized h-BN. In turn, this hydrogen-functionalized h-BN can be reconverted to oxygen-functionalized h-BN by exposure to the supersonic oxygen beam. This reaction occurs at a lower rate as the initial functionalization of pristine h-BN with oxygen. We further demonstrate that the core levels and valence bands of h-BN can be shifted at will between the boundaries obtained for the purely oxygen- and hydrogen-functionalized h-BN layers.

We present well-ordered Pt nanocluster arrays supported on the h-BN/Rh(111) Moiré as a model system for an ethylene dehydrogenation catalyst. Thereby, the h-BN nanomesh serves as a chemically inert eggbox-like template for clusters with a narrow size distribution. The thermal evolution of ethylene is investigated by synchrotron-based high-resolution in situ x-ray photoelectron spectroscopy on the Pt nanoclusters. We compare our results with data on Pt(111) and Pt(355). Interestingly, the Pt nanoclusters and Pt(355) behave very similarly. Both open a new reaction pathway via vinylidene in addition to the route via ethylidyne known for Pt(111). Due to the importance of coking in ethylene dehydrogenation on Pt catalysts, we also studied C2H4 adsorption and decomposition on carbon precovered Pt nanoclusters. While the amount of adsorbed ethylene decreases linearly with the carbon coverage, we found that edge sites are more affected than facet sites and that the vinylidene reaction pathway is effectively suppressed by carbon residues.

Among other N-heterocycles, indole and its substituted derivatives, such as methylindoles, are considered promising Liquid Organic Hydrogen Carriers (LOHCs) for the storage of renewable energy. We used X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and density-functional theory (DFT) to investigate the low temperature adsorption and consecutive dehydrogenation reaction during heating of 2-methylindole, 2-methylindoline, and 2-methyloctahydroindole on Pt(111) and their viability as the LOHC system. In the photoemission experiments, for all Hx-2-methylindoles, we find deprotonation at the NH bond starting between 240 and 300 K, resulting in a 2-methylindolide species. Simultaneously or before this reaction step, the dehydrogenation of 2-methyloctahydroindole via 2-methylindoline and 2-methylindole intermediates is observed. For 2-methyloctahydroindole, we also find π-allyl intermediates above 230 K. Starting at ∼390 K, decomposition of the remaining 2-methylindolide species takes place under the conditions of our surface science experiments. DFT calculations give insight into the relative energies of the various species, reaction intermediates, and their isomers both in the gas phase and on the Pt(111) surface.

We investigated growth and temperature stability of Pt nanoclusters from 1 to 50 atoms on h-BN/Rh(111) using high-resolution XPS and STM.

We report on the formation of nanoscopic heterostructures composed of the semimetal graphene, the metal Pt, and the insulator hexagonal boron nitride (h-BN). Both graphene and h-BN are chemically inert two-dimensional materials with similar geometric but different electronic properties. Between these materials, a Pt nanoparticle array was encapsulated. Thereby, the h-BN/Rh(111) nanomesh served as a template for a well-ordered array of Pt nanoclusters, which were overgrown with graphene, forming single-crystal nanoheterostructures. We investigated this process in situ by high-resolution, synchrotron-radiation-based X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure. The nanographene layers proved tight against CO under the tested conditions. These nanoheterostructures could find possible application in optoelectronics or as a data storage material. At the same time, our approach represents a new route for the synthesis of nanographene.

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 (eta(2):eta(2)). For QC, no preference is found, and both the eta(2):eta(2) and the eta(2):eta(1) adsorption geometry are stable. The conversion of QC to NBD is thermally activated. From UPS, a reaction temperature of similar to 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.

Nanocluster arrays grown on a graphene Moire on Rh(1 1 1) are an excellent model system that can bridge the materials gap between single crystals and real catalysts. They feature different adsorption sites like edges, kinks, and facets, but are at the same time well-ordered with a small size distribution. Herein, we used platinum nanocluster arrays to investigate the oxidation of sulfur, which is a common catalyst poison, as a possible route to restore the catalyst. We studied the kinetics in a site-resolved manner with high-resolution XPS, despite the complex shape of the nanoparticles. Since SO3 and SO4 were the only observable SOx surface species, the reaction of S to SO was identified as the rate-determining step. As a secondary process, the displacement of sulfur from edge to facet sites caused by oxygen was observed.

Graphene-supported nanocluster arrays are interesting model systems to bridge the materials gap to real catalysts. This model system was used to investigate the poisoning of platinum particles by sulfur, which is crucial for many industrial processes. The adsorption and desorption of the probe molecule CO on sulfur-poisoned platinum nanoparticles on graphene were studied using high-resolution X-ray photoelectron spectroscopy. We observed a strong blocking of edge adsorption sites and a weaker blocking of on-top sites on facets; bridge sites are only blocked at higher sulfur coverages. During heating, CO diffuses to edge sites, thereby displacing the sulfur to the facets. Our results show interesting similarities to stepped platinum single crystals.

Indole derivatives are considered as promising liquid organic hydrogen carriers for renewable energy storage. Using X-ray photoelectron spectroscopy, temperature-programmed desorption, and infrared reflection–absorption spectroscopy, we investigated low-temperature adsorption and dehydrogenation during heating of indole, indoline, and octahydroindole on Pt(111). For all three molecules, we find deprotonation of the NH bond above 270 K, accompanied with dehydrogenation of indoline and octahydroindole via an indole intermediate, resulting in an indolide species above 300 K. For octahydroindole, we also find a side reaction yielding small amounts of a π-allyl species between 170 and 450 K. Above 450 K, decomposition of the remaining indolide species takes place.

Together with borazine, ammonia borane is a prominent precursor molecule for the formation of hexagonal boron nitride, which is of high interest as a 2D-material and graphene analog. Ammonia borane is also a possible solid hydrogen carrier for renewable energies with high storage density. Using X-ray photoelectron spectroscopy and temperature-programmed desorption, we investigated low-temperature adsorption and dehydrogenation during heating of borazine and ammonia borane on Ni(111) to form h-BN. For borazine, we observe the formation of disordered boron nitride above 300 K, which starts to form hexagonal boron nitride above 600 K. Ammonia borane shows multiple dehydrogenation steps at the boron and nitrogen atoms up to 300 K. This results in various BHxNHy species, including borazine-like intermediates, before the formation of disordered boron nitride and finally hexagonal boron nitride, analogous to the borazine decomposition.