Recent Advances in Mechanistic Understanding of Metal-Free Carbon Thermocatalysis and Electrocatalysis with Model Molecules

Metal-free carbon, as the most representative heterogeneous metal-free catalysts, have received considerable interests in electro- and thermo-catalytic reactions due to their impressive performance and sustainability. Over the past decade, well-designed carbon catalysts with tunable structures and heteroatom groups coupled with various characterization techniques have proposed numerous reaction mechanisms. However, active sites, key intermediate species, precise structure-activity relationships and dynamic evolution processes of carbon catalysts are still rife with controversies due to the monotony and limitation of used experimental methods. In this Review, we summarize the extensive efforts on model catalysts since the 2000s, particularly in the past decade, to overcome the influences of material and structure limitations in metal-free carbon catalysis. Using both nanomolecule model and bulk model, the real contribution of each alien species, defect and edge configuration to a series of fundamentally important reactions, such as thermocatalytic reactions, electrocatalytic reactions, were systematically studied. Combined with in situ techniques, isotope labeling and size control, the detailed reaction mechanisms, the precise 2D structure-activity relationships and the rate-determining steps were revealed at a molecular level. Furthermore, the outlook of model carbon catalysis has also been proposed in this work.

High-Field (3.4 T) Electron Paramagnetic Resonance, 1H Electron-Nuclear Double Resonance, ESEEM, HYSCORE, and Relaxation Studies of Asphaltene Solubility Fractions of Bitumen for Structural Characterization of Intrinsic Carbon-Centered Radicals

Petroleum asphaltenes are considered the most irritating components of various oil systems, complicating the extraction, transportation, and processing of hydrocarbons. Despite the fact that the paramagnetic properties of asphaltenes and their aggregates have been studied since the 1950s, there is still no clear understanding of the structure of stable paramagnetic centers in petroleum systems. The paper considers the possibilities of various electron paramagnetic resonance (EPR) techniques to study petroleum asphaltenes and their solubility fractions using a carbon-centered stable free radical (FR) as an intrinsic probe. The dilution of asphaltenes with deuterated toluene made it possible to refine the change in the structure at the initial stage of asphaltene disaggregation. From the measurements of samples of bitumen, a planar circumcoronene-like model of FR structure and FR-centered asphaltenes is proposed. The results show that EPR-based approaches can serve as sensitive numerical tools to follow asphaltenes' structure and their disaggregation.

On the Aromatic Stabilization of Fused Polycyclic Aromatic Hydrocarbons

The thermodynamic properties and band gap energies were evaluated for six ortho- and peri-fused polycyclic aromatic hydrocarbons (PAHs): triphenylene; benzo[a]pyrene; benzo[e]pyrene; perylene; benzo[ghi]perylene; coronene. The standard molar enthalpies of formation in the crystalline state and the standard molar enthalpies of sublimation were measured by high precision combustion calorimetry and Knudsen effusion methodology, respectively. The combination of the molar enthalpies of formation in the crystalline state with the respective enthalpies of sublimation was used to evaluate the energetics of the progressive peri-fusion of the aromatic moieties from triphenylene to coronene aiming to investigate the hypothetical superaromaticity character of coronene. The linear trend of the enthalpy of formation in crystalline and gaseous phases in the series (from benzo[e]pyrene to coronene) is an irrefutable indication of a non-superaromaticity character of coronene. High accurate thermodynamic properties of sublimation (volatility, enthalpy, and entropy of sublimation) were derived by the measurement of vapor pressures as a function of temperature, using a Knudsen/quartz crystal effusion methodology. Furthermore, the π-electronic conjugation of these compounds was explored by evaluation of the optical band gaps along with this series of compounds. The morphology of perylene, benzo[ghi]perylene, and coronene thin films, deposited by physical vapor deposition onto transparent conductive oxide substrates (ITO and FTO), was used to analyze the nucleation and growth mechanisms. The morphologies observed were found to be related to the cohesive energy and entropy of the bulk.

Can Small Polyaromatics Describe Their Larger Counterparts for Local Reactions? A Computational Study on the H-Abstraction Reaction by an H-Atom from Polyaromatics

Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system.

Study on fluorescence spectroscopy of PAHs with different molecular structures using laser-induced fluorescence (LIF) measurement and TD-DFT calculation

Laser-induced fluorescence (LIF) is an effective technique for non-intrusive and on-line measurement of PAHs in sooting flames but it is still need further investigation due to the complexity of PAH fluorescence characteristics. Therefore, in-depth investigations on the fluorescence spectroscopy of PAHs with different molecular structures are relevant. In this study, we investigated the fluorescence spectrum characteristics of 13 gas-phase PAHs using LIF measurement and time-dependent density functional theory (TD-DFT) calculation. The experimental results showed that the fluorescence emission wavelengths increased with more aromatic (benzenoid) rings, but this relationship no longer existed when the PAH molecules contain the five-membered ring structures. The TD-DFT calculation showed that the fluorescence emission wavelength ranges of PAHs with different molecular structures were dominantly determined by the electronic structures of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and their energy gaps. It was found that the saturated aliphatic branched chains (methyl and ethyl) only slightly influenced the LIF spectra, while the unsaturated aliphatic branched chains (ethenyl and ethynyl) caused remarkable redshifts. The TD-DFT results indicated that the aliphatic branched chains changed the electric structures of HOMO and LUMO of the core aromatic rings, and then influence the fluorescence emission wavelength ranges.

Optical band gap of cross-linked, curved, and radical polyaromatic hydrocarbons

The impact of cross-linking, curvature, and radical character on the optical band gap of polyaromatic hydrocarbons has been investigated.

Predicting aromatic exciplex fluorescence emission energies

Analysis of PAH exciplex TDDFT fluorescence energies shows a linear relationship between the mean monomer HOMO–LUMO gap and complex fluorescence.

A quantitative topological descriptor for linear co-oligomer fusion

A purely topological descriptor for the quantitative classification of fully or partially fused linear quasi-one dimensional centre-symmetric molecules is proposed. The index also makes it possible to quantitatively correlate optoelectronic properties with the molecular topology without involving an explicit calculation of the molecular electronic structure.