Methane dehydroaromatization (MDA) offers a direct route for the non-oxidative valorization of methane into aromatic hydrocarbons. Wurtzite gallium nitride has been reported experimentally to catalyze MDA near 450°C, yet the later stages of the reaction remain poorly understood. Here, we developed and applied a high-throughput combinatorial ab initio framework to propose two thermally accessible MDA pathways on GaN: the C3 + C3 and C4 + C2 mechanisms. Methylene and ethylene, which form readily with barriers of 1.79 eV and 1.44 eV, respectively, emerge as key C1 and C2 intermediates. The rate-determining step in the C3 + C3 pathway is ethyl formation (3.41 eV), whereas the C4 + C2 route proceeds through a lower-barrier vinyl dehydrogenation (2.89 eV) involving strongly bound acetylene intermediates. The unique geometry of the GaN surface, characterized by the spacing and orientation of surface Ga-N bonds, stabilizes both C3 and C4 intermediates and promotes the coupling steps that yield benzene. Several thermodynamically stable intermediates may act as kinetic traps, rationalizing observed by-products such as ethylene and cyclohexane. These findings provide a comprehensive mechanistic framework for MDA on GaN and underscore the catalytic potential of non-oxide nitrides for hydrocarbon activation and coupling.

Methane dehydroaromatization (MDA) offers a promising non-oxidative route for converting methane into value-added aromatic hydrocarbons. Wurtzite gallium nitride (GaN) has demonstrated catalytic activity for this transformation at temperatures as low as 450°C, although the underlying mechanism remains incompletely understood. In this study, the initial steps of MDA on the GaN m-plane surface were investigated using density functional theory. A low-barrier pathway for methylene (CH₂) formation was identified, enabled by the migration of surface methyl and hydrogen species with similarly low barriers. This mechanism significantly lowers the activation barrier relative to previous estimates. Beyond dehydrogenation and migration, we examined the coupling of C₁ intermediates to form C₂ species, including ethyl, ethane, and ethylene. C-C bond-formation steps leading to ethane and ethylene were found to proceed with barriers comparable to that of CH₂ formation. These results indicate that methane activation on GaN is not governed by a single high-barrier step; instead, dehydrogenation, migration, and coupling each contribute comparable kinetic bottlenecks. Overall, the findings revise the mechanistic understanding of methane activation on GaN and underscore the importance of site availability and adsorbate mobility in enabling selective low-temperature reactivity.

In this work, computational modeling is used to modify NacNac gallium imide, which has recently been shown to cleave unactivated sp3 C-H bonds in organic substrates, with the aim of making it suitable for methane activation. Density functional theory predicts that the 123 kJ mol-1 methane activation barrier for the experimentally employed gallium imide can be reduced to 93 kJ mol-1 by changing substituents around the active gallium center. Furthermore, pre-straining the gallium imide reduces the reaction barrier to just 62 kJ mol-1. Dimerization of the gallium imides can be prevented with bulky groups that do not affect the reaction barrier. Several modified NacNac gallium imides are thus shown to be viable for the homogeneous activation of methane and higher alkanes.

NHC states on two reactive metal surfaces.

Activation of the strong non-polar C-H bonds in methane is difficult, especially in solution. In this work, computational modeling is used to modify NacNac gallium imide, which has recently been shown to cleave unactivated sp3 C-H bonds in organic substrates with the aim of making it suitable for methane activation. Density functional theory predicts that the 123 kJ mol−1 methane activation barrier for the experimentally employed gallium imide can be reduced to 93 kJ mol−1 by changing substituents around the active gallium center. Furthermore, pre-straining the gallium imide reduces the reaction barrier to just 62 kJ mol−1. Dimerization of the gallium imides can be prevented with bulky groups that do not affect the reaction barrier. Several modified NacNac gallium imides are thus shown to be viable for the homogeneous activation of methane and higher alkanes.

It is a significant challenge to relate ligand-assisted bond activation on metal surfaces to specific adsorption and intermolecular binding structures. To address this objective, we studied carbonyl bond activation in single chirality transfer complexes formed by methyl 3,3,3-trifluoropyruvate (i.e., MTFP) and (R)-1-(1-naphthyl)ethylamine (i.e., (R)-NEA) on a Pt(111) surface. The experiments combined reflectance absorbance infrared spectroscopy (RAIRS), scanning tunneling microscopy (STM), and density functional theory (DFT) methods. While STM measurements, in combination with DFT calculations, permit the study of single surface complexes, RAIRS is an ensemble technique that yields a composite spectrum resulting from an often heterogeneous distribution of molecular structures on the sampled surface. We show that the intrinsic thermal behavior of the MTFP/(R)-NEA/Pt(111) system facilitates meaningful comparison between single complex measurements by STM and ensemble measurements by RAIRS in that the vibrational signal can be attributed to a small number of complexation configurations, one of which has a high relative abundance. We take advantage of mode mixing in a nu(CF3) + nu(CO)keto vibration to detect a spectroscopic signature for complexation-induced carbonyl bond activation. A red-shift of the band correlates with DFT-predicted lengthening of the bridge-bonded carbonyl group. While the intensity of the shifted band is in the majority due to the most abundant complexation configuration, minority states produce line broadening. In addition to providing insight on rate-enhancement in enantioselective reactions on catalysts bearing chiral auxiliaries, the study contributes to the development of ligand control of reactivity and selectivity in heterogeneous catalysis.

Dimerization, thermal and tip-induced partial dehydrogenation, and surface dynamics of 2,2,2-trifluoro-1-phenylethanol (TFPE) on Pt(111) were investigated in relation to the enantioselective hydrogenation of 2,2,2-trifluoroacetophenone (TFAP). Measurements were made on racemic and (R)-TFPE and compared to data for TFAP. The structures of homochiral and minority heterochiral TFPE dimers, formed using racemic TFPE, were determined from combined RAIRS, STM, and DFT data, revealing homochiral Z-structures rather than the linear-type dimers predicted by DFT. Interconversion between homochiral Z-structures and buckled heterochiral dimers was observed and is attributed to the transient formation of termolecular structures. While thermally driven partial dehydrogenation was observed at approximately 270 K, tip-induced dissociation of dimers through partial dehydrogenation was observed at 220-240 K. Time-lapsed STM measurements at different temperatures revealed a dimer dissociation process that we attribute to the tip-induced formation of alkoxy-TFAP followed by surface diffusion. The migration of the newly formed alkoxy species leads to the formation of relatively immobile monomers and hydroxy-TFAP/TFAP structures at undetermined sites distant from the parent dimer. Both tip-induced O-H bond scission and thermal C*-H bond scission pathways convert TFPE to hydroxy-TFAP/TFAP structures, the same structures that are formed through partial hydrogenation of TFAP on Pt(111) above 240 K. In the context of the hydrogenation of TFAP on chirally modified Pt, formation of stable hydroxy-TFAP/TFAP structures may reduce the enantiomeric excess obtained in that they compete with the formation of the modifier-substrate complexes required for chirality transfer and in that they may also induce carbonyl function activation through hydrogen bonding. However, we provide spectroscopic evidence that TFAP in model TFAP/1-(1naphthyl)ethylamine/Pt(111) diastereomeric complexes is subject to stronger activation of the carbonyl bond.

The functionalization of graphene (Gr) on Pt(111) and Ru(0001) substrates by an N-heterocyclic carbene (NHC) is described. The formation, thermal stability, and bonding geometry of the grafted NHC were probed using reflection absorption infrared spectroscopy (RAIRS). The carbene contains a CF3 substituent, which provides a set of three very strong absorption bands with which to monitor the chemical modification. By employing Pt(111) and Ru(0001) substrates, it is possible to compare markedly different graphene systems: while graphene forms a quasi-freestanding p-doped layer on Pt(111), its interaction with Ru(0001) involves periodic chemical bonding to form an n-doped layer. The RAIRS data show that the benzimidazolium hydrogen carbonate precursor transforms to a relatively strongly adsorbed NHC on both systems. Clean formation of the surface carbene at 300-350 K is attributed to a process activated by electron transfer from the supported graphene layer. The RAIRS signal attributed to the NHC on full-coverage Gr/Pt(111) is removed on heating to similar to 450 K, while it disappears from Gr/Ru(0001) at similar to 400 K. The difference in thermal stability is interpreted in terms of weaker bonding of the electron-donor NHC to the n-doped graphene layer on Ru(0001). This hypothesis is explored using oxygen intercalation to decouple graphene from Ru(0001) to form a p-doped layer. The study outlines a method to modify graphene and also reveals that vibrational spectra of an adsorbed NHC provide a highly sensitive method to probe and distinguish between different graphene-on-metal systems.

Scanning tunneling microscopy (STM) data for α-ketoester/1-(1-naphthyl)ethylamine complexes on Pt(111) reveal a tumbling motion that couples two neighboring binding states. The interconversion, resulting in prochiral inversion of the α-ketoester, occurs in single complexes without breaking them apart. This is a surprising observation because the overall motion requires rotation of the α-ketoester away from the surface without branching exclusively into diffusion away from the complex or desorption. The multi-step interconversion is rationalized in terms of sequences of bound states that combine transient H-bond interactions with the chiral molecule and weakened adsorption interactions with the metal. The observation of tumbling in single long-lived complexes is of relevance to self-assembly and directed molecular motion on surfaces, to ligand-controlled surface reactions, and most directly to stereocontrol in asymmetric heterogeneous catalysis.

The surface Mo-doped Cu(111) catalyst feature improved performance towards deoxygenation reactions, acting as a single-atom alloy capable of breaking Brønsted–Evans–Polanyi relations for carbonyl bond scissions.

Alumina-doped resorcinol–formaldehyde carbon supported Pt nanoparticles were synthesized and employed in asymmetric hydrogenation with high enantioselectivity, good reusability, and unprecedentedly high TOF.

Enantioselectivity in heterogeneous catalysis can be induced through the adsorption of optically active compounds known as chiral modifiers. The modifiers typically stereodirect prochiral reactants on the metal surface through the formation of diastereomeric complexes. Surface science measurements on model systems show several examples where such complexes display multiple abundant binding configurations. Insights into the factors determining the relative populations of complexation states can be potentially gained from comparing slightly variant chiral modifiers. Here, we compare scanning tunneling microscopy (STM) data for chirality transfer complexes formed by (R)-1-(1-naphthyl)ethylamine and (R)-1-(8-methyl-1-naphthyl)ethylamine coadsorbed with the prochiral substrate 2,2,2-trifluoroacetophenone in order to probe for changes induced by the methyl substituent. The comparison shows changes in relative abundances beyond those predicted by only direct steric interaction. Density functional theory (DFT) methods are used to calculate complexation energies. A number of rationalizations are proposed for the experimentally determined differences in relative abundances and also for differences between DFT-predicted and STM-measured populations.

There is evidence that there is a potential overlap between olefin metathesis and deoxygenation chemistries on molybdenum carbide catalysts. Ketones and aldehydes are known to undergo selective deoxygenation on Mo2C to form surface alkylidenes, and metal alkylidenes are initiator and propagator sites in olefin metathesis reactions. Moreover, a recent report correlated increased olefin metathesis activity with molybdenum carbide or oxycarbide formation on pre-treatment of a supported molybdenum oxide catalyst in methane. Taken together, these three strands prompt a DFT study of the elementary carbonyl bond scission step of selected aldehydes and ketones to form surface alkylidenes on Mo2C. This theoretical study is extended to an analysis of previously reported experimental reflectance infrared data for the interaction of cyclopentanone and acetaldehyde with a polycrystalline bulk hexagonal Mo2C sample. (C) 2020 Elsevier Inc. All rights reserved.

Sylwester Gawinkowski opened a general discussion of the paper by Jorge Salmon-Gamboa: Why you are using SiO2 nanoparticles? Do they have any function or are they only the substrate to attach other active nanoparticles to? You have shown that only gold nanoparticles attached to silica nanoparti

A preliminary study of the surface reactivity of 2D--Mo2C crystallites grown on a copper foil was performed using x-ray photoelectron spectroscopy. Different sample preparation protocols for the as-received materials were explored in order to remove hydrocarbon surface contamination. Annealing in vacuum and in argon led to the formation of graphitic layers, while annealing in O-2 lead to almost the complete disappearance of the Mo signal. Gentle argon ion sputtering proved effective at removing the hydrocarbon contamination to reveal pristine molybdenum carbide. XPS spectra were recorded following the exposure of the prepared sample at 297K to furfural. The results are commented on in relation to deoxygenation and olefin metathesis surface chemistry.

The formation of N-heterocyclic carbenes on a platinum surface is demonstrated. Surface vibrational spectroscopy measurements isolate steps in the transformation from 1,3-diisopropylbenzimidazolium hydrogen carbonate to surface carbene. As the temperature is further increased, the reactive metal transforms the isopropyl groups leading to the effective formation of a new surface carbene.

The behavior of naphthalene on Pt(111) surfaces is studied by combining insight from scanning tunneling microscopy (STM) and van der Waals enabled density functional theory. Adsorption, diffusion, and rotation are investigated by a series of variable temperature STM experiments revealing naphthalene ability to rotate on-site with ease with a rotational barrier of 0.69 eV. Diffusion to neighbouring sites is found to be more difficult. The experimental results are in good agreement with the theoretical investigations which confirm that the barrier for diffusion is slightly higher than the one for rotation. The theoretical barriers for rotation and translation are found to be 0.75 and 0.78 eV, respectively. An automatic mapping of the possible diffusion pathways reveals very detailed diffusion paths with many small local minima that would have been practically impossible to find manually. This automated procedure provides detailed insight into the preferred diffusion pathways that are important for our understanding of molecule-substrate interactions.

The molecular description of chirality transfer on chirally modified metal surfaces is unclear due to the complexity of the systems and the weak energetic biases favoring specific enantioselective pathways. In order to move to an enhanced level of molecular understanding it is critical to define the chemisorption geometries of the modifiers responsible for enantiodifferentiation. Model surface science studies of chiral molecules on single crystals provide an indirect method to probe catalytic chirality transfer molecular mechanisms. Apart from the great difference between ultrahigh vacuum (UHV) and reaction conditions, the extent to which such studies can inform the interpretation of data obtained under reaction conditions depends on the degree to which the model systems are correctly defined. The same holds true for any comparison of UHV surface spectroscopy data to spectroscopic measurements made at the catalyst-solution interface under either in-situ or operando conditions. We present detailed reflection absorption infrared spectroscopy (RAIRS) data on (R)-1-(1-naphthyl)ethylamine, (R)-NEA, and its simple derivative, (R)-1-(8-methyl-1-naphthyl)ethylamine, on Pt(111). Various conflicting proposals for the structure of NEA and analogous modifiers on Pt catalyst particles and extended Pt surfaces are found in the literature. Here, we find that below high sub-monolayer coverages on Pt(111), (R)-NEA adopts a chemisorption geometry where the entire naphthyl group is sr-bonded to the surface and the amine group forms a dative bond to the surface. The same general adsorption geometry is found for the methyl-substituted derivative. The study provides reference spectra for these chiral modifiers and confirms the molecular structures described in our previous studies of diastereomeric complexes formed by the (R)-NEA/Pt(111) system. (C) 2018 Elsevier B.V. All rights reserved.