Julen Munárriz

Analysis of the Magnetic Entropy in Oxygen Reduction Reactions Catalysed by Manganite Perovskites

Manganese oxides with a half‐metallic ground state are particularly active for oxygen reduction reactions (ORR). La0.67Sr0.33MnO3 (LSMO) perovskite is the archetypal example for compositions with a Curie temperature (TC) above room temperature and with a high intrinsic activity for the partial reduction of triplet‐state O2. The ferromagnetic (FM) character of the superexchange interactions in LSMO facilitates both charge and spin transport below 370 K. Other than the enhanced electronic conductivity, the reduced spin entropy seems to be relevant in oxygen catalysis because the magnetic ordering extends to the surface. The sign of the exchange interactions determines the adsorption of the triplet oxygen molecule with its spin antiparallel to the FM catalysts. Based on the transition‐state theory, we report that on LSMO, the hindrance resulting from the magnetic entropy for the initial reduction of O2 by two antiparallel electrons to diamagnetic intermediates (such H2O2) is minimum. On the other hand, the additional reduction of H2O2 to H2O, diamagnetic steps, prefers paramagnetic catalysts with higher magnetic entropy such as La0.4Sr0.6MnO3 to avoid spin accumulation.

Orthometallation of N-substituents at the NHC ligand of [Rh(Cl)(COD)(NHC)] complexes: its role in the catalytic hydrosilylation of ketones

The rhodium(I)-NHC (NHC = N-heterocyclic carbene) complex [Rh(Cl)(COD)(2-methoxyphenyl-NHC-(CH2)3Si(OiPr)3)] (2a) catalyzes the solvent-free homogeneous hydrosilylation of acetophenone with HSiMe(OSiMe3)2. Kinetic studies show that 2a behaves differently to the related homogeneous catalysts [Rh(Cl)(COD)(R-NHC-(CH2)3Si(OiPr)3)] (R = 2,6-diisopropylphenyl (2b); R = 2-methoxyethyl (2c)). This behavior could be attributable to the participation of different catalytic active species. Indeed, 1H NMR studies of the reaction of 2a with HSiMe(OSiMe3)2 evidenced the formation of a new hydrido-bridged binuclear complex, namely {[Rh(SiMe(OSiMe3)2)(κ-C,C′-R-NHC-(CH2)3Si(OiPr)3)]2(μ-H)2} (R = 2-methoxyphenyl, 3), featuring orthometallated NHC and terminal silyl ligands, which has been proposed as the resting species in the hydrosilylation of acetophenone with HSiMe(OSiMe3)2 catalyzed by 2a. Moreover, the heterogeneous catalyst 2a-MCM-41 evidenced a behavior similar to the homogeneous catalyst 2a in the solvent-free hydrosilylation of acetophenone with HSiMe(OSiMe3)2.

Orbital Physics of Perovskites for the Oxygen Evolution Reaction

The study of magnetic perovskite oxides has led to novel and very active compounds for O2 generation and other energy applications. Focusing on three different case studies, we summarise the bulk electronic and magnetic properties that initially serve to classify active perovskite catalysts for the oxygen evolution reaction (OER). Ab-initio calculations centred on the orbital physics of the electrons in the d-shell provide a unique insight into the complex interplay between spin dependent interactions versus selectivity and OER reactivity that occurs in these transition-metal oxides. We analyse how the spin, orbital and lattice degrees of freedom establish rational design principles for OER. We observe that itinerant magnetism serves as an indicator for highly active oxygen electro-catalysts. Optimum active sites individually have a net magnetic moment, giving rise to exchange interactions which are collectively ferromagnetic, indicative of spin dependent transport. In particular, optimum active sites for OER need to possess sufficient empty orthogonal orbitals, oriented towards the ligands, to preserve an incoming spin aligned electron flow. Calculations from first principles open up the possibility of anticipating materials with improved electro-catalytic properties, based on orbital engineering.

Highly Efficient Ir-Catalyst for the Solventless Dehydrogenation of Formic Acid: The Key Role of an N-heterocyclic Olefin

A sturdy iridium complex that features a PCP ligand based on an N-heterocyclic olefin shows remarkable activity for the solventless dehydrogenation of formic acid. Reactivity studies highlight the importance of NHO in the activity of the catalyst. A plausible intermediate has been isolated and the mechanism was substantiated by DFT calculations.

On the Role of Ferromagnetic Interactions in Highly Active Mo-Based Catalysts for Ammonia Synthesis

Reactions involving nitrogen fixation and transfer are of great industrial interest. In this regard, unveiling all the physical principles that determine their activity would be enormously beneficial for the rational design of novel catalysts with improved performance. Within this context, this work explores the activity of bulk molybdenum-based transition metal nitrides in ammonia synthesis. Our results highlight that the most active compositions show increasing ferromagnetism in the metal-nitrogen bonds, which constitute the active sites. We observe that the total spin accumulated in the bonds at the active sites is a physically meaningful descriptor to discriminate optimum catalysts. Higher activities are associated with ferromagnetic phases, and the underlying reason is an enhanced overlapping of the electronic wavefunctions; which also make the reaction steps spin-sensitive. These finding provides strong evidence of the general influence of electrons magnetic moment in catalysis, being part of the specific field of spintro-catalysis.