Milan M. Jakšić

Electrocatalysis of hydrogen evolution in the light of the brewer—engel theory for bonding in metals and intermetallic phases☆

Abstract A comprehensive survey of achievements in electrocatalysis for the hydrogen evolution reaction (h.e.r.) has been reviewed with the main emphasis on the composite d -metal catalysts. The basic concept of the Brewer—Engel valence-bond theory that relates the electronic state of highest multiplicity which corresponds to the electronic configuration of lowest energy level or the structure of atoms, and corresponding phase structure both in individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer—Engel model it has been pointed out that whenever metals of the left-half of the transition series that have empty or half filled vacant d -orbitals are alloyed with metals of the right-half of the transition series that have internally paired d -electrons not available for bonding in the pure metal, there arises well pronounced synergism in electrocatalysis for the h.e.r., which often exceeds individual effects of precious metals and each other (the synergism condition) and approaches the reversible behaviour within the wide range of current density. It has been inferred that the upmost electrocatalytic activity reach the composite d -metal catalysts of improved d -orbital overlap in intermetallic phases of highest symmetry and minimal entropy such as Cr 3 Si(A 15 ) type like MoCo 3 , WNi 3 , MoNi 3 , LaNi 5 , that the Brewer—Engel theory predicts for the most stable systems.

Advances in electrocatalysis for hydrogen evolution in the light of the Brewer-Engel valence-bond theory☆

Abstract The Brewer-Engel valence-bond theory for bonding in metals and intermetallic phases has been employed to correlate the electrocatalytic features of both individual and composite transition metal catalysts for the hydrogen evolution reaction (h.e.r.). The basic concept of the Brewer-Engel valence-bond theory, which relates the electronic state of lowest energy level or the structure of atoms, with the corresponding phase structure in both individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer intermetallic bonding model as a generalized Lewis acid-base reaction, it is pointed out that whenever metals of the left half of the transition series, having empty or half-filled vacant d-orbitals, are alloyed with metals of the right half of the transition series having internally paired d-electrons not available for bonding in the pure metal, which proceeds with definite charge transfer, there arises a well-pronounced synergism in electrocatalysis for the h.e.r., which often exceeds the individual catalytic effects of precious metals by themselves or in combination (the synergism condition) and approaches reversible behaviour within a wide range of current density. It is inferred that the maximum electrocatalytic activity extends to the composite d-metal catalysts of improved d-orbital overlap in intermetallic phases of highest symmetry and minimal entropy, such as Laves phases and A15 or Cr3Si types such as MoCo3, WNi3, VNi3, HfPd3, ZrPt3, LaNi5, HfPt3, that the Brewer theory for intermetallic bonding predicts for the most stable systems. The theoretical explanation for an optimal (d8) electronic configuration in both individual and composite synergetic electrocatalytic systems is given and compared with similar catalytic processes. The induced hydridic feature of high activity synergetic transition metal composite electrocatalysts is also pointed out.

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.

The Electrochemical Activation of Catalytic Reactions

The use of electrochemistry to activate and precisely tune heterogeneous catalytic processes is a new development1-7 which originally emerged due to the existence of solid electrolytes. Depending on their composition, these specific anionic or cationic conductor materials exhibit substantial electrical conductivity at temperatures between 25 and 1000°C. Within this broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used as reversible in situ promoter donors or poison acceptors to affect the catalytic activity and product selectivity of metals deposited on solid electrolytes in a very pronounced, reversible, and, to some extent, predictable manner.

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

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 in electrocatalytic properties as the part of Periodic Table between initial constituents or their periods, and thereby features the Gschneidner type of volcano curve for the hydrogen electrode reactions (helr), in common with the well known electrocatalytic volcano plots for the hydrogen evolution reaction (her) upon individual transition metals. Several characteristic 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. Electrocatalysis appears as the hypo‐hyper-d-electronic interactive effect of transition element composites either within metallic lattice, or their ions upon the substrate surface.

Advances in electrocatalysis for hydrogen evolution in the light of the Brewer-Engel valence-bond theory

The Brewer-Engel valence-bond theory for bonding in metals and intermetallic phases has been employed to correlate the electrocatalytic features of both individual and composite transition metal catalysts for the hydrogen evolution reaction (h.e.r.). The basic concept of the Brewer-Engel valence-bond theory, which relates the electronic state of highest multiplicity that corresponds to the electronic configuration of lowest energy level or the structure of atoms, with the corresponding phase structure in both individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer intermetallic bonding model as a generalized Lewis acid-base reaction, it is pointed out that whenever metals of the left half of the transition series, having empty or half-filled vacant d-orbitals, are alloyed with metals of the right half of the transition series having internally paired d-electrons not available for bonding in the pure metal, which proceeds with definite charge transfer, there arises a well-pronounced synergism in electrocatalysis for the h.e.r., which often exceeds the individual catalytic effects of precious metals by themselves or in combination (the synergism condition) and approaches reversible behaviour within a wide range of current density. It is inferred that the maximum electrocatalytic activity extends to the composite d-metal catalysts of improved d-orbital overlap in intermetallic phases of highest symmetry and minimal entropy, such as Laves phases and A15 or Cr3Si types such as MoCo3, WNi3, VNi3, HfPd3, ZrPt3, LaNi5, HfPt3, that the Brewer theory for intermetallic bonding predicts for the most stable systems. The theoretical explanation for an optimal (d8) electronic configuration in both individual and composite synergetic electrocatalytic systems is given and compared with similar catalytic processes. The induced hydridic feature of high activity synergetic transition metal composite electrocatalysts is also pointed out.

Selective Interactive Grafting of Composite Bifunctional Electrocatalysts for Simultaneous Anodic Hydrogen and CO Oxidation I. Concepts and Embodiment of Novel-Type Composite Catalysts

The equivalence of interionic hypo-hyper-d-interelectronic interaction (HHDII) in both metallic and any other ionic state and its effect upon electrocatalytic properties for hydrogen electrode reactions has been proved and inferred. Thermal gravimetry (TG) analysis of temperature programmed reduction (TPR) of mixed hypo-hyper-d-electronic oxides of transition elements was broadly employed to prove the interionic bonding effect (the extended Brewer theory) as reflected in dramatically decreased individual temperatures of their mutual reduction into intermetallic phases or alloys. The same interionic (and/or intermetallic) bonding effect has been confirmed both by under potential deposition of hyper-d- upon hypo-d-electronic substrates and vice versa, and by the shift of bonding peaks in X-ray photoelectron spectroscopy analysis. The former affords the basis for new trends in submonolayer hypo-hyper-d-interelectronic electrocatalysis of transition metals. Strong metal support interaction (SMSI) of both individual and composite, prevailingly hyper-d-electronic metallic electrocatalysts upon individual and/or composite, usually hypo-d-electronic oxide substrates have been employed to create and graft (anchor) bifunctional electrocatalysts for simultaneous anodic hydrogen and CO oxidation in low temperature polymer exchange membrane fuel cells. The selective interionic bonding method upon predestined active centers of hypo-d-electronic oxide supports has been adapted to avoid nanostructured colloidal precursors and directly graft (anchor) a priori defined nanosized intermetallic phases and synergetic bifunctional electrocatalysts from decomposition of corresponding stoichiometric mixtures of various individual or intermetallic acetylacetonates. An adapted TG method based on TPR has been properly used to define, control and/or stimulate the homogeneity of the intermetallic crystal bonding and growth of nanostructured composite catalysts, mostly of rather extra strong bonding Brewer intermetallic phases upon proper SMSI oxide supports. Thus, it has been pointed out that the term SMSI has a broader HHDII sense in both the bonding effectiveness and bifunctional catalytic meaning, and in fact stays in the core of such extended Brewer interionic bonding theory.

Electrocatalysis for hydrogen electrode reactions in the light of fermi dynamics and structural bonding FACTORS—I. individual electrocatalytic properties of transition metals

Abstract The Brewer valence-bond theory for bonding in metals and intermetallic phases has been employed, together with Fermi dynamics, to correlate with the electrocatalytic properties of both individual and composite transition metal catalysts for the hydrogen electrode reactions (HELR). It has been inferred that the electrocatalytic activity of both individual transition metals and their intermetallic phases and alloys for both hydrogen evolution (HER) and its oxidation (HOR), primarily correlates with the electronic density of states and obeys typical laws of catalysis reflected in the first place in the existence of volcano plots along the Periodic .Table. Since the bonding effectiveness of both individual and intermetallic hypo–hyper- d -electronic transition metal composite electrocatalysts correlates in a straightforward manner with their electrocatalytic activity, such evidence strongly suggests Fermi energy, as a typical elementary binding energy, which otherwise stays in the linear relation with cohesive energy, this forms the basis in investigation and correlation of electrocatalytic activity. Due to the fact that the Fermi wave-vector represents the individual and collective (alloys and intermetallic phases) bulk property of the available electronic number density (or its concentration, n , i.e., k F = (3 π fn2 n) 1 fn3 ), and in a straightforward manner correlates with the electronic density of states at the Fermi level, and thereby defines all metallic properties of a metal (and intermetallics) as a solid with a Fermi surface, including electrocatalytic features, it has been taken as the main parameter to correlate with the exchange current density in the hydrogen electrode reactions.

Towards the reversible electrode for hydrogen evolution in industrially important electrochemical processes

Abstract It has been pointed out that appropriate intermetallic combinations of transition metals, when employed as composite catalysts in the hydrogen evolution reaction (HER), almost approach reversibility and provide favorable long lasting cathode characteristics. A comprehensive survey of recent electrocatalytic achievements by the composite d -metal catalysts is given. The basic concept of the Brewer-Engel valence bond theory relating the electronic state of highest multiplicity (which corresponds to the electronic configuration or structure of atoms of lowest energy) and corresponding phase structure, both in individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer-Engel model it is pointed out that whenever metals of the left half of the transition series that have empty or half-filled vacant d -orbitals are alloyed with metals of the right half of the transition series that have internally paired d -electrons not available for bonding in the pure metal, there arises well pronounced synergism in electrocatalysis of the HER, which often exceeds the individual effects of precious metals and approaches reversible behavior in a wide range of current density. It is inferred that the highest electrocatalytic activity is attained by composite d -metal catalysts with improved d -orbital overlap in intermetallic phases with high symmetry and minimal entropy such as Cr 3 Si (β-W or A 15 ) types like MoCo 3 , WNi 3 , HfPd 3 , VNi 3 , ZrPt 3 , LaNi 5 , as predicted by Brewer-Engel theory for the most stable systems. There are two approaches to improving electrode characteristics of substrate cathodic materials with composite d -metal electrocatalysts: (a) thermal formation and processing of appropriate intermetallic phases on the carrying surface, and (b) in situ addition of activating ionic species during the electrochemical process, as well as their mutual combination. The main aim of the paper is to present an electrocatalytic method of producing a reversible hydrogen electrode for industrially important electrochemical processes at high current densities.

Interionic nature of synergism in catalysis and electrocatalysis

Fermi dynamics and the Brewer structural bonding factors have been employed to define synergism and electrocatalytic properties for hydrogen electrode reactions upon individual transition metals and their hypo-hyper-d-electronic combinations of intermetallic phases. It has clearly been shown and pointed out that electrocatalysis and its synergetic effect appear as the interionic interaction of hypo-hyper-d-electronic components either within metallic lattice, or upon the substrate surface. The induced external polarization or non-Faradaic (NEMCA) effect upon the electronic density of states and thereby advanced overall (electro)catalytic activity has been correlated with the basic catalytic properties and emphasized, too.