The trend of chemisorption of hydrogen and oxygen atoms on pure transition metals: Magnetism justifies unexpected behaviour of Mn and Cr

Abstract

The spin of the electron is decisive to understand electronic interactions in heterogeneous catalysis, mainly because the stabilizing Quantum Spin Exchange Interactions (QSEI) are always a significant contribution to the energy of magnetic catalysts and reactants during catalytic events. Cooperative QSEI in compositions with multi-atomic open-shell configurations (QSEI-OS) maximize the influence of the spin dependent potentials, determining the electronic properties of magnetic materials and shaping their reactivity. Because of the intraand inter-atomic QSEI-OS, high-spin (3d 5 ) antiferromagnetic (AFM) metals like Cr and Mn can be more inert (or "noble") than Au itself towards the formation of covalent bonds with hydrogen atoms. AFM QSEI-OS lead to a relative higher destabilization of the unoccupied (spin-)orbitals (Mott upper band) in Cr and Mn compared with other metals. In oxygen adsorption, QSEI-OS lead to the reduction of the electronic repulsions in occupied 3d 5 orbitals with a concomitant decrease of the chemisorption enthalpy of the oxygen atoms. Since hydrogen and oxygen atoms are the most important intermediates in relevant catalytic processes like oxygen reduction reaction and water splitting, the effect of QSEI-OS needs to be properly incorporated as a critical factor to understand the activity of catalysts based on magnetic metals.

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

“…Dominant intra-atomic QSEI in the 3d-orbitals and optimal interatomic nuclei attractions due to the AFM bonds provide an extra stabilization to high-spin 3d 5 manganese and chromium. The outcome is a larger energy gap between the 3d α 5 occupied bonding orbitals (the Lower Hubbard band) and the 3d β 5 empty antibonding orbitals (the Upper Hubbard band) of Mn and Cr atoms . This converts Pt 3 Mn­(111) and Pt 3 Cr­(111) in more inert catalysts (Figures and ) compared with FM compositions, as experiments also confirm .…”

“…The outcome is a larger energy gap between the 3d α 5 occupied bonding orbitals (the Lower Hubbard band) and the 3d β 5 empty antibonding orbitals (the Upper Hubbard band) of Mn and Cr atoms. 34 This converts Pt 3 Mn(111) and Pt 3 Cr(111) in more inert catalysts (Figures 3 and 4) compared with FM compositions, as experiments also confirm. 2 G-type AFM Pt 3 V(111) presents more favorable O* chemisorption (Figure 3), but in this case, the electron pair localization typical of AFM compositions and stability issues will yield high activation barriers for the ORR.…”

“…For 3d-metal Pt-based catalysts, QSEIs become a very relevant electronic factor in determining the energetics and kinetics of heterogeneous catalytic processes. ,,− Computational studies demonstrate that dominant FM interactions play a crucial role in the ORR/OER , and experiments confirmed the enhanced catalytic activity of magnetic particles in oxygen electrocatalysis because of spin-dependent delocalization of the charge carriers. , The QSEI stabilizing effect can also be macroscopically boosted by an external magnetic field when magnetic oxides are involved. , Experiments demonstrate that the application of an external magnetic field can induce an overall improvement in the PEMFC catalytic performance, by accelerating the oxygen transport toward the catalytic layer of the electrodes or activating the hydrogen molecules or aligning proton-conductive channels in the fuel cell membrane . In summary, exogenous and endogenous magnetic potentials must be unavoidably considered to achieve a comprehensive description of chemical, physical, and thus catalytic properties.…”

Strongly Correlated Electrons in Catalysis: Focus on Quantum Exchange

“…QSEIs shells open make the strongly correlated itinerant unpaired electrons collectively more stable at the Fermi level, causing, for instance, the almost similar HER activity seen for Fe, Co, and Ni metals. 89 FM QSEIs shells open yield to moderate binding energies, spin-selectivity, and enhanced redox transfers during catalytic reactions at the covalent active sites. For instance, FM conducting structures based on electron-rich magnetic metals (like Mn ∼3.25+ , Fe ∼3.25+ , and Co ∼3.25+ oxides) may become excellent oxygen reduction catalysts, 63,34 while configurations with fewer electrons (Mn ∼3.75+ or Co ∼3.75+ oxides) are particularly suitable to accelerate oxygen evolution.…”

Cooperation and competition between magnetism and chemisorption

“…These materials offer additional degrees of freedom such as internal strain and magnetic moment when alloyed with traditional heavy metals. Therefore effect of magnetism on surface reactivity also becoming focus of many recent studies [17][18][19].…”