Homolytic cleavage of molecular oxygen by manganese porphyrins supported on Ag(111).

Oxygen binding and cleavage are important for both molecular recognition and catalysis. Mn-based porphyrins in particular are used as catalysts for the epoxidation of alkenes, and in this study the homolytic cleavage of O2 by a surface-supported monolayer of Mn porphyrins on Ag(111) is demonstrated by scanning tunneling microscopy, X-ray absorption, and X-ray photoemission. As deposited, {5,10,15,20-tetraphenylporphyrinato}Mn(III)Cl (MnClTPP) adopts a saddle conformation with the average plane of its macrocycle parallel to the substrate and the axial Cl ligand pointing upward, away from the substrate. The adsorption of MnClTPP on Ag(111) is accompanied by a reduction of the Mn oxidation state from Mn(III) to Mn(II) due to charge transfer between the substrate and the molecule. Annealing the Mn(II)ClTPP monolayer up to 510 K causes the chlorine ligands to desorb from the porphyrins while leaving the monolayer intact. The Mn(II)TPP is stabilized by the surface acting as an axial ligand for the metal center. Exposure of the Mn(II)TPP/Ag(111) system to molecular oxygen results in the dissociation of O2 and forms pairs of Mn(III)OTPP molecules on the surface. Annealing at 445 K reduces the Mn(III)OTPP complex back to Mn(II)TPP/Ag(111). The activation energies for Cl and O removal were found to be 0.35 ± 0.02 eV and 0.26 ± 0.03 eV, respectively

Electronic structure of nickel porphyrin NiP: Study by X-ray photoelectron and absorption spectroscopy

Energy distributions and properties of the occupied and empty electronic states for a planar complex of nickel porphyrin NiP are studied by X-ray photoemission and absorption spectroscopy techniques. As a result of the analysis of the experimental spectra of valence photoemission, the nature and energy positions of the highest occupied electronic states were determined: the highest occupied state is formed mostly by atomic states of the porphine ligand; the following two states are associated with 3d states of the nickel atom. It was found that the lowest empty state is specific and is described by the σ-type b 1g MO formed by empty Ni3dx2−y2 -states and occupied 2p-states of lone electron pairs of nitrogen atoms. This specific nature of the lowest empty state is a consequence of the donor–acceptor chemical bond in NiP

Hexagonal boron nitride on transition metal surfaces

We validate a computational setup based on density functional theory to investigate hexagonal boron nitride (h-BN) monolayers grown on different transition metals exposing hexagonal surfaces. An extended assessment of our approach for the characterization of the geometrical and electronic structure of such systems is performed. Due to the lattice mismatch with the substrate, the monolayers can form Moire-type superstructures with very long periodicities on the surface. Thus, proper models of these interfaces require very large simulation cells (more than 1,000 atoms) and an accurate description of interactions that are modulated with the specific registry of h-BN on the metal. We demonstrate that efficient and accurate calculations can be performed in such large systems using Gaussian basis sets and dispersion corrections to the (semi-)local density functionals. Four different metallic substrates, Rh(111), Ru(0001), Cu(111), and Ni(111), are explicitly considered, and the results are compared with previous experimental and computational studies

Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface

The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror “anti-Moiré” reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions.We acknowledge funding from the Spanish MINECO (Grant MAT2014-54231-C4-1-P), the EU via the ERC-Synergy Program (Grant ERC-2013-SYG-610256 Nanocosmos), and computing resources from CTI-CSIC. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant agreement No. 604391 Graphene Flagship. J.I.M. acknowledges funding from both the CSIC-JaeDoc Fellowship Program (co-funded by the European Social Fund) and Nanocosmos. P.M. was supported by the “Rafael Calvo Rodés” Program.Peer reviewe