Correction: 2D oxides on metal materials: concepts, status, and perspectives.

Correction for '2D oxides on metal materials: concepts, status, and perspectives' by Giovanni Barcaro et al., Phys. Chem. Chem. Phys., 2019, 21, 11510–11536, DOI: 10.1039/C9CP00972H

Nanocluster metallici: uno studio computazionale di struttura e crescita

In questa tesi è affrontato uno studio teorico-computazionale della struttura e della crescita di nanocluster formati da metalli di transizione. La ridotta dimensionalità di questi sistemi fa sì che essi manifestino proprietà peculiari, che ne favoriscono l’applicazione nell’ambito della catalisi e della fabbricazione di dispositivi opto- e magneto-elettronici. Lo stretto rapporto che lega la struttura di questi sistemi alle proprietà che essi manifestano ci ha motivato ad indagare in dettaglio la struttura dei cluster metallici in diversi tipi di intorno: cresciuti in fase gassosa oppure cresciuti o stabilizzati su superfici di supporto o in soluzione. Particolare rilievo è stato dato allo studio delle proprietà strutturali statiche e dinamiche di cluster cresciuti sulla superficie regolare o difettata (100) dell’ossido di magnesio (MgO)

Structure and diffusion of small Ag and Au clusters on the regular MgO (100) surface

The lowest energy structures and the diffusion energy barriers of small MN (N = 1–4) Ag and Au clusters absorbed on the regular MgO (100) surface are investigated via density-functional (DF) calculations, using two different xc-functionals (PBE and LDA). In agreement with previous work, it is found that the lowest-energy structures of Ag and Au clusters in this size-range exhibit a strong 'metal-on-top' effect, by which the clusters are absorbed atop oxygen ions in a linear (dimer) or planar (trimer and tetramer) configuration perpendicular to the surface. The corresponding diffusion mechanisms range from monomer hopping, to dimer leapfrog (Ag2) or hopping (Au2), trimer walking, tetramer walking (Ag4) or rocking and rolling (Au4), exhibiting interesting differences between Ag and Au. An analysis of the corresponding energy barriers shows that trimers can diffuse at least as fast as monomers, while tetramers and (especially in the case of gold) dimers present somewhat higher barriers, but are anyway expected to be mobile on the surface at the temperatures of molecular beam epitaxy (MBE) experiments. The calculated PBE diffusion energy barriers compare reasonably well with the values extracted from the analysis of recent MBE experimental data, with the LDA predicting slightly higher barriers in the case of gold

Diverse Phases of Carbonaceous Materials from Stochastic Simulations

Amorphous carbon systems are emerging to have unparalleled properties at multiple length scales, making them the preferred choice for creating advanced materials in many sectors, but the lack of long-range order makes it difficult to establish structure/property relationships. We propose an original computational approach to predict the morphology of carbonaceous materials for arbitrary densities that we apply here to graphitic phases at low densities from 1.15 to 0.16 g/cm³, including glassy carbon. This approach, dynamic reactive massaging of the potential energy surface (DynReaxMas), uses the ReaxFF reactive force field in a simulation protocol that combines potential energy surface (PES) transformations with global optimization within a multidescriptor representation. DynReaxMas enables the simulation of materials synthesis at temperatures close to experiment to correctly capture the interplay of activated vs entropic processes and the resulting phase morphology. We then show that DynReaxMas efficiently and semiautomatically produces atomistic configurations that span wide relevant regions of the PES at modest computational costs. Indeed, we find a variety of distinct phases at the same density, and we illustrate the evolution of competing phases as a function of density ranging from uniform vs bimodal distributions of pore sizes at higher and intermediate density (1.15 g/cm³ and 0.50 g/cm³) to agglomerated vs sparse morphologies, further partitioned into boxed vs hollow fibrillar morphologies, at lower density (0.16 g/cm³). Our observations of diverse phases at the same density agree with experiment. Some of our identified phases provide descriptors consistent with available experimental data on local density, pore sizes, and HRTEM images, showing that DynReaxMas provides a systematic classification of the complex field of amorphous carbonaceous materials that can provide 3D structures to interpret experimental observations

Atomistic Quantum Plasmonics of Gold Nanowire Arrays

The dielectric properties of a regular 2D array of Au nanowires are investigated using time-dependent density-functional theory employing a fully atomistic quantum description. Longitudinal modes produce a Drude-like peak in the infrared that is rather insensitive to geometrical parameters. Transverse modes, instead, give rise to a plasmonic peak in the optical region, which exhibits a nontrivial dependence on the spatial separation between the wires: a strong resonant enhancement and a shift from the optical to the far-infrared region is observed as the interwire distance is decreased, with the formation of "hot spots" in which induced field and charge distributions exhibit nondipolar shape and rapidly alternating quantum phase. The general character of this phenomenon is confirmed by its occurrence in Au nanoparticle arrays. Addition of ligand species in the hot spot region can lead to the appearance of new resonances due to strong coupling between plasmonic and molecular modes, as exemplified in a proof-of-concept case. This shows the possibilities of atomistic quantum plasmonics effects and subwavelength control of electromagnetic field intensity in properly engineered nanogaps

Unravelling stereoisomerism in acid catalysed lignin conversion:An integration of experimental trends and theoretical evaluations

For the effective valorization of lignin, which is a significant component in agricultural residues, its reactivity needs to be understood in detail. Selective acid-catalysed depolymerisation of the lignin β-O-4 linking motif with stabilization of the formed aldehydes by diols is a promising approach to obtain phenolic monomers in high yields. However, the lignin β-O-4 linking motif exists in both the erythro and threo isomeric forms, and very little information is available on the influence of stereochemistry on the efficiency of the lignin diol-stabilised acidolysis. This is especially true for the set of intermediates in which the presence of stereochemistry persists. In this study, the stereoisomer ratios of two key intermediates, namely the diol (here ethylene glycol) adducts and C2-vinyl ethers, are monitored carefully in ytterbium(iii) trifluoromethanesulfonate [Yb(OTf)3]-catalysed conversion of an erythro β-O-4 model compound. The reactions showed the preferential formation and consumption of the ethylene glycol adduct in the erythro configuration, and the favored formation of trans C2-vinyl ether. Multiscale computational methods (including classical reactive molecular dynamics simulations and quantum chemistry calculations) were applied to elucidate the catalytic origins of the observed stereo-preferences and suggested that a proto-trans intermediate complex is stabilised by a hydrogen bond network connecting the carbocation, ethylene glycol, and the anionic [OTf]− species. The synergistic combination of experiments and computational studies disclosed the stereo-preference and the underlying mechanism in triflate-catalysed acidolysis, especially the catalytic role of [OTf]−, which can be helpful for a further improvement of the chemical process

New Mechanistic Insights into the Lignin beta-O-4 Linkage Acidolysis with Ethylene Glycol Stabilization Aided by Multilevel Computational Chemistry

Acidolysis in conjunction with stabilization of reactive intermediates has emerged as one of the most powerful methods of lignin depolymerization that leads to high aromatic monomer yields. In particular, stabilization of reactive aldehydes using ethylene glycol results in the selective formation of the corresponding cyclic acetals (1,3-dioxolane derivatives) from model compounds, lignin, and even from softwood lignocellulose. Given the high practical utility of this method for future biorefineries, a deeper understanding of the method is desired. Here, we aim to elucidate key mechanistic questions utilizing a combination of experimental and multilevel computational approaches. The multiscale computational protocol used, based on ReaxFF molecular dynamics, represents a realistic scenario, where a typical experimental setup can be reproduced confidently given the explicit molecules of the solute, catalyst, and reagent. The nudged elastic band (NEB) approach allowed us to characterize the key intermolecular interactions involved in the reaction paths leading to crucial intermediates and products. The high level of detail obtained clearly revealed for the first time the unique role of sulfuric acid as a proton donor and acceptor in lignin beta-O-4 acidolysis as well as the reaction pathways for ethylene glycol stabilization, and the difference in reactivity between compounds with different methoxy substituents

Erratum:New mechanistic insights into the lignin β-O-4 linkage acidolysis with ethylene glycol stabilization aided by multilevel computational chemistry (ACS Sustainable Chemistry and Engineering (2021) 9:5 (2388-2399)

In the Supporting Information of the original article (https://pubs.acs.org/doi/10.1021/acssuschemeng.0c08901), the x-axes in Figures S6c and d are wrongly labeled. It is stated that the reaction times are in “minutes”; however, the reaction times are actually in hours. Additionally, for Figure S6d, the indicated times are incorrect. A new Supporting Information file is provided here in which these mistakes have been corrected

The atomistic origin of the extraordinary oxygen reduction activity of Pt_3Ni_7 fuel cell catalysts

Recently Debe et al. reported that Pt_3Ni_7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface (“Pt-skin effect”). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt_3Ni_7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt–Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ~300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt_3Ni_7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O_(ad)-hydration reaction (O_(ad) + H_2O_(ad) → OH_(ad) + OH_(ad)) but including the barriers of O_2 dissociation (O_(2ad) → O_(ad) + O_(ad)) and water formation (OH_(ad) + H_(ad) → H_2O_(ad)). We find that the reaction barrier for the O_(ad)-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt–Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt_3Ni_7, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials