Hypo–hyper-d-electronic interactive nature of interionic synergism in catalysis and electrocatalysis for hydrogen reactions

Abstract

The nature, causes and consequences of the existence of volcano plots along transition series with resulting theoretical conclusions have been extended on the Brewer hypo–hyper-d-electronic combinations of both intermetallic phases and interionic composites. It has been pointed out that every such phase diagram behaves as the part of the Periodic Table between initial constituents, and thereby features the Gschneidner type of volcano curve for the hydrogen electrode reactions (helr), in common with the well-known electrocatalytic volcano plots of H. Kita and M. H. Miles for the hydrogen evolution reaction (her) upon individual transition metals. Several typical Brewer-type intermetallic systems (Ti–Ni, Zr–Ni, Mo–Ni, Mo–Pt) were investigated and their comparable electrocatalytic and hydridic properties were revealed from volcanic plots along the corresponding phase diagrams. In the light of such conclusions, it has been pointed out that catalytic volcano plots represent in their essence the main criterion and guidance leading to both optimal and synergetic effects in broader (electro)catalytic sense for the helr. Electrocatalysis and its synergetic effect appear as the hypo–hyper-d -electronic interactive effect of corresponding transition element composites either within metallic lattice, or their ions upon the substrate surface. The d-band both as the bonding and adsorptive orbital for intermediates in the rate determining step (rds) has been inferred to be decisive in (electro)catalytic hydrogen reactions, while electronic density, in addition, defines the overall kinetics and reaction rate. Such statements have been supported by the novel self-consistent calculated functional density of one-electron states (DOS) of adsorbed H-adatom, wherefrom the Fermi level relative to the antibonding peak plays decisive role in electrocatalysis for the helr. The surface state of a given interionic bulk structure reflects the latter and appears decisive in its mutual electronic configuration for the overall electrocatalytic effect and resulting synergism in the helr.