Experimental Evidences on Magnetism-Covalent Bonding Interplay in Structural Properties of Solids and during Chemisorption.

Valence electrons are one of the main players in solid catalysts and in catalytic reactions, since they are involved in several correlated phenomena like chemical bonding, magnetism, chemisorption, and bond activation. This is particularly true in the case of solid catalysts containing d-transition metals, which exhibit a wide range of magnetic phenomena, from paramagnetism to collective behaviour. Indeed, the electrons of the outer d-shells are, on one hand, involved in the formation of bonds within the structure of a catalyst and on its surface, and, on the other, they are accountable for the magnetic properties of the material. For this reason, the relationship between magnetism and heterogeneous catalysis has been a source of great interest since the mid-20th century. The subject has gained a lot of attention in the last decade, thanks to the orbital engineering of quantum spin-exchange interactions and to the widespread application of external magnetic fields as boosting tools in several catalytic reactions. The topic is discussed here through experimental examples and evidences of the interplay between magnetism and covalent bonding in the structure of solids and during the chemisorption process. Covalent bonding is discussed since it represents one of the strongest contributions to bonds encountered in materials.

Review on Magnetism in Catalysis: From Theory to PEMFC Applications of 3d Metal Pt-Based Alloys.

The relationship between magnetism and catalysis has been an important topic since the mid-20th century. At present time, the scientific community is well aware that a full comprehension of this relationship is required to face modern challenges, such as the need for clean energy technology. The successful use of (para-)magnetic materials has already been corroborated in catalytic processes, such as hydrogenation, Fenton reaction and ammonia synthesis. These catalysts typically contain transition metals from the first to the third row and are affected by the presence of an external magnetic field. Nowadays, it appears that the most promising approach to reach the goal of a more sustainable future is via ferromagnetic conducting catalysts containing open-shell metals (i.e., Fe, Co and Ni) with extra stabilization coming from the presence of an external magnetic field. However, understanding how intrinsic and extrinsic magnetic features are related to catalysis is still a complex task, especially when catalytic performances are improved by these magnetic phenomena. In the present review, we introduce the relationship between magnetism and catalysis and outline its importance in the production of clean energy, by describing the representative case of 3d metal Pt-based alloys, which are extensively investigated and exploited in PEM fuel cells.

Magnetism and Heterogeneous Catalysis: In Depth on the Quantum Spin-Exchange Interactions in Pt3M (M = V, Cr, Mn, Fe, Co, Ni, and Y)(111) Alloys.

Bimetallic Pt-based alloys have drawn considerable attention in the last decades as catalysts in proton-exchange membrane fuel cells (PEMFCs) because they closely fulfill the two major requirements of high performance and good stability under operating conditions. Pt3Fe, Pt3Co, and Pt3Ni stand out as major candidates, given their good activity toward the challenging oxygen reduction reaction (ORR). The common feature across catalysts based on 3d-transition metals and their alloys is magnetism. Ferromagnetic spin-electron interactions, quantum spin-exchange interactions (QSEIs), are one of the most important energetic contributions in allowing milder chemisorption of reactants onto magnetic catalysts, in addition to spin-selective electron transport. The understanding of the role played by QSEIs in the properties of magnetic 3d-metal-based alloys is important to design and develop novel and effective electrocatalysts based on abundant and cheap metals. We present a detailed theoretical study (via density functional theory) on the most experimentally explored bimetallic alloys Pt3M (M = V, Cr, Mn, Fe, Co, Ni, and Y)(111). The investigation starts with a thorough structural study on the composition of the layers, followed by a comprehensive physicochemical description of their resistance toward segregation and their chemisorption capabilities toward hydrogen and oxygen atoms. Our study demonstrates that Pt3Fe(111), Pt3Co(111), and Pt3Ni(111) possess the same preferential multilayered structural organization, known for exhibiting specific magnetic properties. The specific role of QSEIs in their catalytic behavior is justified via comparison between spin-polarized and non-spin-polarized calculations.

Ferromagnetic ligand holes in cobalt perovskite electrocatalysts as an essential factor for high activity towards oxygen evolution.

The definition of the interplay between chemical composition, electro-magnetic configuration and catalytic activity requires a rational study of the orbital physics behind active materials. Apart from Coulomb forces, quantum spin exchange interactions (QSEI) are part of the potentials that differentiate the activity of magnetic oxides, strongly correlated electrocatalysts, in electron transfer reactions. Ferromagnetic (FM) cobalt oxides can show low overpotentials for the oxygen evolution reaction (OER) and the La1-XSrXCoO3-δ (0 ≤ X ≤ 1) family of perovskites is good ground to gain understanding of the electronic interactions in strongly correlated catalysts. In this case, Sr-doping raises the OER activity and the conductivity and increases FM spin moments. The efficiency of electrocatalysts based on Earth-abundant 3d-transition metals correlates with the interrelated factors: mild-bonding energies, the reduction of the electronic repulsions because of the QSEI in the open-shells, and enhanced spin delocalization in FM ordering. The reason for the outstanding OER activity of SrCoO3-δ is the accumulation of FM holes in the 3d-2p bonds, including the ligand orbitals, thus facilitating spin-selected charge transport and production of triplet O2 moieties from the oxidation of diamagnetic precursors. Spin-polarized oxygen atoms in the lattice can participate in O-O coupling and release of O2 in a Mars-Van Krevelen mechanistic fashion. We show that the stabilizing FM QSEI decrease the adsorption and activation energies during oxygen evolution and spin-dependent potentials are one of the factors that govern the catalytic activity of magnetic compositions: spintro-catalysis.

Gold(I) Metallo-Tweezers for the Recognition of Functionalized Polycyclic Aromatic Hydrocarbons by Combined π-π Stacking and H-Bonding.

Two gold(I)-based metallo-tweezers with bis(Au-NHC) pincers and a carbazole connector have been obtained and used for the recognition of polycyclic aromatic hydrocarbons (PAHs). In the case of the tweezer with pyrene-NHC ligands, the presence of the pyrene fragment and the N-H bond in the carbazole linker enable the receptor to show significant enhanced binding abilities toward PAHs functionalized with H-bonding groups, through combined π-π stacking and H-bonding.