Kinetic Consequences Research Articles

Thermochemical Correlations of Redox and Brønsted Sites on Bifunctional Polyoxometalate Clusters and Their Kinetic Consequences in Methanol-O2 Catalysis

Thermochemical Correlations of Redox and Brønsted Sites on Bifunctional Polyoxometalate Clusters and Their Kinetic Consequences in Methanol-O₂ Catalysis

Competing Kinetic Consequences of CO2 on the Oxidative Degradation of Branched Poly(ethylenimine).

Amine-functionalized porous solid materials are effective sorbents for direct air capture (DAC) of CO2. However, they are prone to oxidative degradation in service, increasing the materials cost for widespread implementation. While the identification of oxidation products has given insights into degradation pathways, the roles of some species, like CO2 itself, remain unresolved, with conflicting information in the literature. Here, we investigate the impact of CO2 on the oxidative degradation of poly(ethylenimine)-alumina (PEI/Al2O3) sorbents under conditions encompassing a wide range of CO2-air mixture compositions and temperatures relevant to DAC conditions, thereby reconciling the conflicting data in the literature. Degradation profiles characterized by thermogravimetric analysis, in situ ATR-FTIR, and CO2 capacity measurements reveal nonmonotonic effects of CO2 concentrations and temperatures on oxidation kinetics. Specifically, 0.04% CO2 accelerates PEI/Al2O3 oxidation more at low temperatures (<90 °C) compared to 1% and 5% CO2, but this trend reverses at high temperatures (>90 °C). First-principles metadynamics, machine learning accelerated molecular dynamics simulations, and 1H relaxometry experiments show that chemisorbed CO2 acid-catalyzes critical oxidation reactions, while extensive CO2 uptake reduces PEI branch mobility, slowing radical propagation. These contrasting kinetic effects of CO2 explain the complex degradation profiles observed in this work and in prior literature. Collectively, this work highlights the importance of considering atmospheric components in the design of DAC sorbents and processes. Additionally, it identifies the unconstrained branch mobility and local acid environment as two of the major culprits in the oxidation of amine-based sorbents, suggesting potential strategies to mitigate sorbent degradation.

Mechanistic Understanding of the Role of Bis(4-nitrophenyl) Carbonate (BNC) As Additive on the Solvation Structure and Polysulfides Shuttling Effect Towards Capacity Improvement in Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) batteries are considered as one of the most promising devices for next-generation energy storage applications, owing to its exceptional theoretical capacity and energy density. However, persistent challenges have hampered its performance, notably the sluggish polysulfides conversion kinetics and the polysulfides shuttling phenomenon. These issues lead to irreversible active material loss, rapid capacity decay, low coulombic efficiency, and a limited cycle life. This study demonstrates that bis(4-nitrophenyl) carbonate (BNC) can serve as an effective agent in anchoring dissolved polysulfides, thereby preventing their migration to the anode. Furthermore, this investigation indicates that BNC has the capability to modify the solvation structure of polysulfides in the electrolyte. The presence of 0.01M BNC induces a downfield shift in the 7Li NMR spectrum, signifying a strengthened solvation structure on Li-ion. This solvation modification results in altered kinetics, specifically a reduced Li2S2/Li2S conversion without sacrificing capacity, as confirmed by operando X-ray absorption spectroscopy (XAS). Operando Raman spectroscopy reveals the suppression of long-chain polysulfides (at 400 cm-1) and intermediate polysulfides (at 453 cm-1) with 0.01M BNC, mitigating sulfur shuttling to the anode. This observation is corroborated by X-ray fluorescence (XRF) mapping. Analysis of the corresponding X-ray absorption near-edge structure (XANES) demonstrates that both anodes and cathodes with 0.01M BNC exhibit lower levels of unconverted polysulfides over extended cycles. This improvement is attributed to the kinetic consequences resulting from solvation structure alteration. However, this investigation cautions against an indiscriminate increase in BNC concentration, as higher concentrations may lead to the over-conversion of soluble polysulfides. This nuanced understanding of the solvation structure's intrinsic mechanism, coupled with chemical reactions, provides valuable insights for practical design of Li-S batteries Figure 1

Dynamics of Elementary Steps on Metal Surfaces at High Coverages: The Prevalence and Kinetic Competence of Contiguous Bare-Atom Ensembles.

The rate of elementary steps on densely-covered surfaces depends sensitively on repulsive interactions within dense adlayers, situations ubiquitous in practice and with kinetic consequences seldom captured by Langmuirian treatments of surface catalysis. This study develops an ensemble-based method that assesses how such repulsion influences the prevalence and kinetic competence of bare-atom ensembles of different size. Chemisorbed CO (CO*) is used as an example because it forms dense adlayers on metal nanoparticles during CO2 hydrogenation (CO2-H2) and other reactions, leading to significant repulsion that weakens the binding of CO* and kinetically-relevant transition states (TS). This approach is enabled by density functional theory and probability formalisms and describes the prevalence of ensembles of contiguous bare atoms from their formation energy (via CO* desorption); it then determines their competence in stabilizing the TS and mediating the reaction rates. The specific conclusions reflect the extent to which a given TS and CO* desorbed to form bare ensembles "sense" repulsion and the contribution of each ensemble size to each reaction channel mediated by distinct TS structures. These formalisms are illustrated by assessing the relative contributions, kinetic relevance, and ensemble size requirements for two CO2-H2 routes (direct and H-assisted CO2 activation to CO and H2O) on Ru nanoparticles, but they are not restricted to specific bound species or reaction channels. This method is essential to assess the kinetic relevance of elementary steps in a given catalytic sequence and to determine the contributions from parallel reaction channels at the crowded surfaces that prevail in the practice of surface catalysis.

Comparative kinetics of humans and non-human primates during vertical climbing.

Climbing represents a critical behavior in the context of primate evolution. However, anatomically modern human populations are considered ill-suited for climbing. This adaptation can be attributed to the evolution of striding bipedalism, redirecting anatomical traits away from efficient climbing. Although prior studies have speculated on the kinetic consequences of this anatomical reorganization, there is a lack of data on the force profiles of human climbers. This study utilized high-speed videography and force plate analysis to assess single limb forces during climbing from 44 human participants of varying climbing experience and compared these data with climbing data from eight species of non-human primates (anthropoids and strepsirrhines). Contrary to expectations, experience level had no significant effect on the magnitude of single limb forces in humans. Experienced climbers did, however, demonstrate a predictable relationship between center of mass position and peak normal forces, suggesting a better ability to modulate forces during climbing. Humans exhibited significantly higher peak propulsive forces in the hindlimb compared with the forelimb and greater hindlimb dominance overall compared with non-human primates. All species sampled demonstrated exclusively tensile forelimbs and predominantly compressive hindlimbs. Strepsirrhines exhibited a pull-push transition in normal forces, while anthropoid primates, including humans, did not. Climbing force profiles are remarkably stereotyped across humans, reflecting the universal mechanical demands of this form of locomotion. Extreme functional differentiation between forelimbs and hindlimbs in humans may help to explain the evolution of bipedalism in ancestrally climbing hominoids.

Kinetic implications of (hierarchical) zeolite fouling during liquid-phase aromatics upgrading

The kinetics of fouling in large pore zeolites (BEA, MOR), including those with hierarchical pore systems, were probed during the Brønsted acid catalyzed reaction of benzyl alcohol (BA) with 1,3,5-trimethylbenzene (TMB) by varying reactant driving force (i.e., [TMB]0/[BA]0; 11–119). In BEA, initial deactivation rate constants (kD,0) decreased exponentially with [TMB]0/[BA]0, highlighting the significance of oxygenates as deactivation precursors. Further, seeding pores with oxygenates completely suppressed measured rates in parent MOR and BEA, while seeding with TMB had no effect. Comparisons of mass accumulations of different organics (low molecular weight reaction species and coke) as functions of ln([TMB]0/[BA]0) revealed that coke (derived from oxygenate-seeded polyalkylation of TMB) disproportionately controlled deactivation rates by damping apparent (diffusion-controlled) and intrinsic (diffusion-corrected) rate constants through proton losses and/or altered molecule confinement within shrinking pores. These kinetic consequences were delayed upon introduction of mesopores, demonstrating how zeolite porosity impacts coke proliferation and behavior in liquid-phase reactions.

Investigating the Structural, Functional, and Biochemical Properties of PPi -dependent PEPCK Paralogs from Entamoeba histolytica

Phosphoenolpyruvate carboxykinase (PEPCK) is an important metabolic enzyme which functions to interconvert oxaloacetic acid (OAA) and phosphoenolpyruvate (PEP) in the Krebs cycle, a key process of generating cellular energy. There exist three known classes of PEPCK - two of which are nucleotide-dependent, using ATP and GTP. Very little is known about the third, PPi-dependent PEPCK. Comparing classes, nucleotide-dependent PEPCKs are both functionally and structurally similar (~60-70 kDa) whereas PPi-dependent PEPCK bears significant functional and structural differences (~130 kDa). This presented work investigates PPi-dependent PEPCK from a human parasite Entamoeba histolytica (EhPEPCK). It is unique from previous work done on another homolog from Propionibacterium freudenreichii (PfPEPCK) in that there are three paralogs instead of one. This suggests increased complexity in function and regulation. This work has determined that the interaction between EhPEPCK paralogs gives rise to dimers and heterotrimers, and certain interactions show substrate induced inhibition. Kinetic measurements were completed to determine the metal cofactor of EhPEPCKs, and to determine the kinetic consequences of the aforementioned oligomeric states. The experiments support the conclusion that aggregation causes substrate inhibition, and that dimers are more active than trimers.

Virtual pivot point: Always experimentally observed in human walking?

A main challenge in human walking is maintaining stability. One strategy to balance the whole body dynamically is to direct the ground reaction forces toward a point above the center of mass, called virtual pivot point (VPP). This strategy could be observed in various experimental studies for human and animal gait. A VPP was also observed when VPP input variables like center of mass or ground reaction forces were perturbed. In this study, the kinetic and kinematic consequences of a center of pressure manipulation and the influence on the VPP are investigated. Thus, eleven participants walked with manipulated center of pressure (i.e. barefoot, backwards, with a rigid sole, with stilts, and in handstand compared to shoe walking). In all conditions a VPP could be observed, only one participant showed no VPP in handstand walking. The vertical VPP position only differs between shoe walking and rigid sole walking, there are no significant differences between the conditions in the horizontal VPP position and the spread around the VPP. However, it is conceivable that for more severe gait changes, walking without VPP could be observed. To further analyze this issue, the authors provide a VPP calculation tool for testing data regarding the existence of the VPP.

Individual Contributions of Amido Acid Residues Tyr122, Ile168, and Asp173 to the Activity and Substrate Specificity of Human DNA Dioxygenase ABH2.

Human Fe(II)/α-ketoglutarate-dependent dioxygenase ABH2 plays a crucial role in the direct reversal repair of nonbulky alkyl lesions in DNA nucleobases, e.g., N1-methyladenine (m1A), N3-methylcytosine (m3C), and some etheno derivatives. Moreover, ABH2 is capable of a less efficient oxidation of an epigenetic DNA mark called 5-methylcytosine (m5C), which typically is a specific target of DNA dioxygenases from the TET family. In this study, to elucidate the mechanism of the substrate specificity of ABH2, we investigated the role of several active-site amino acid residues. Functional mapping of the lesion-binding pocket was performed through the analysis of the functions of Tyr122, Ile168, and Asp173 in the damaged base recognition mechanism. Interactions of wild-type ABH2, or its mutants Y122A, I168A, or D173A, with damaged DNA containing the methylated base m1A or m3C or the epigenetic marker m5C were analyzed by molecular dynamics simulations and kinetic assays. Comparative analysis of the enzymes revealed an effect of the substitutions on DNA binding and on catalytic activity. Obtained data clearly demonstrate the effect of the tested amino acid residues on the catalytic activity of the enzymes rather than the DNA-binding ability. Taken together, these data shed light on the molecular and kinetic consequences of the substitution of active-site residues for the mechanism of the substrate recognition.

Effect of Interactions between Alkyl Chains and Solvent Structures on Lewis Acid Catalyzed Epoxidations

Solvent molecules within zeolite pores provide interactions that influence the stability of reactive intermediates and impact rates and selectivities for catalytic reactions. We show the kinetic and thermodynamic consequences of these interactions and reveal their origins using alkene epoxidations in titanium-substituted *BEA (Ti-BEA) zeolites. Epoxidation turnover rates vary widely among primary n-alkenes (C6–C18) in hydrophilic (Ti-BEA-OH) and hydrophobic (Ti-BEA-F) catalysts in aqueous acetonitrile (CH3CN). Apparent activation enthalpies (ΔHapp‡) and entropies (ΔSapp‡) increase with alkene carbon number in both catalysts; however, the span of ΔHapp‡ values in Ti-BEA-OH (68 kJ mol–1) greatly exceeds that in Ti-BEA-F (18 kJ mol–1). These trends, and commensurate gains in ΔSapp‡, reflect the displacement and reorganization of solvent molecules that scale with the size of transition states and the numbers of solvent molecules stabilized by silanol defects near active sites. Experimental and computational assessments of intrapore solvent composition from 1H NMR, infrared spectroscopy, and grand canonical molecular dynamics (GCMD) simulations show that Ti-BEA-OH uptakes larger quantities of both CH3CN and H2O than Ti-BEA-F. The Born–Haber decomposition of simulated enthalpies of adsorption (ΔHads,epox) for C6–C18 epoxides attributes ΔHads,epox that become more endothermic for larger adsorbates to the displacement of greater numbers of solvent molecules bound to silanol defects into the bulk solvent. A strong correlation between ΔHapp‡ and ΔHads,epox (from GCMD and isothermal titration calorimetry) gives evidence that the disruption of solvent structures provides excess thermodynamic contributions (e.g., Gε) that depend on the solvent composition in the pores, the excluded volume of reactive species, and the density of silanol groups near active sites. Altering Gε values offers opportunities to control selectivities and rates of reactions through the design of extended active site environments.

Cybersecurity Education

Recent cyber events within the U. S. cyber ecosystem present the alarming fact that attacks with both denial of service and kinetic consequences are now prevalent in non-governmental systems. This paper examines the need to expand studies of cyber and other warfare modalities into the cybersecurity curricula now being taught in American universities.

Mass Transport Limitations and Kinetic Consequences of Corn Stover Deacetylation

Alkaline pretreatment of herbaceous feedstocks such as corn stover prior to mechanical refining and enzymatic saccharification improves downstream sugar yields by removing acetyl moieties from hemicellulose. However, the relationship between transport phenomena and deacetylation kinetics is virtually unknown for such feedstocks and this pretreatment process. Here, we report the development of an experimentally validated reaction–diffusion model for the deacetylation of corn stover. A tissue-specific transport model is used to estimate transport-independent kinetic rate constants for the reactive extraction of acetate, hemicellulose and lignin from corn stover under representative alkaline conditions (5–7 g L−1NaOH, 10 wt% solids loadings) and at low to mild temperatures (4–70°C) selected to attenuate individual component extraction rates under differential kinetic regimes. The underlying transport model is based on microstructural characteristics of corn stover derived from statistically meaningful geometric particle and pore measurements. These physical descriptors are incorporated into distinct particle models of the three major anatomical fractions (cobs, husks and stalks) alongside an unsorted, aggregate corn stover particle, capturing average Feret lengths of 917–1239 μm and length-to-width aspect ratios of 1.8–2.9 for this highly heterogeneous feedstock. Individual reaction–diffusion models and their resulting particle model ensembles are used to validate and predict anatomically-specific and bulk feedstock performance under kinetic-controlled vs. diffusion-controlled regimes. In general, deacetylation kinetics and mass transfer processes are predicted to compete on similar time and length scales, emphasizing the significance of intraparticle transport phenomena. Critically, we predict that typical corn stover particles as small as ∼2.3 mm in length are entirely diffusion-limited for acetate extraction, with experimental effectiveness factors calculated to be 0.50 for such processes. Debilitatingly low effectiveness factors of 0.021–0.054 are uncovered for cobs—implying that intraparticle mass transfer resistances may impair observable kinetic measurements of this anatomical fraction by up to 98%. These first-reported quantitative maps of reaction vs. diffusion control link fundamental insights into corn stover anatomy, biopolymer composition, practical size reduction thresholds and their kinetic consequences. These results offer a guidepost for industrial deacetylation reactor design, scale-up and feedstock selection, further establishing deacetylation as a viable biorefinery pretreatment for the conversion of lignocellulosics into value-added fuels and chemicals.

High-voltage structural evolution and its kinetic consequences for the Na4MnV(PO4)3 sodium-ion battery cathode material

The recently demonstrated possibility of high-voltage extraction of more than 2 Na ions per Na4MnV(PO4)3 formula unit is considered as a possible way to increase the specific energy of this material, which would open up prospects for its application in sodium-ion batteries. However, for the Na4MnV(PO4)3 material and for a wide group of related NASICON-structured materials, high-voltage Na extraction was demonstrated to lead to a rapid decrease in capacity and a sharp increase in hysteresis between the charge and discharge curves. In this work, we use electrochemical and structural analysis to elucidate the reasons for the degradation observed after deep desodiation of Na4MnV(PO4)3. X-ray diffraction analysis of the electrodes after cycling demonstrates that a phase transformation is initiated when the anodic potential limit exceeds 3.8 V, which results in the formation of distorted NASICON-type phases and gradual amorphization. X-ray absorption spectroscopy studies suggest that the observed detrimental phase transitions might be triggered by the changes in the local environment of oxidized vanadium, which destabilize the structure. We thus demonstrate that the degradation of the Na4MnV(PO4)3 material is of thermodynamic origin, with its consequences being reflected in a dramatically decreased de/intercalation rates and a rapid deterioration in the material performance.

Brønsted acidity of H-adatoms at protic solvent-transition metal interfaces and its kinetic consequences in electrophilic addition reactions

H-adatom (H*) at protic solvent-metal interfaces can undergo ionization and form interfacial protons (H-Metalsolvated → H+solvated + Metal-solvated, Ka,HM). A three-dimensional Born-Haber thermochemical framework quantitatively decomposes the H* acidity (as Ka,HM) into the metal’s work function, H-binding energy, H+/H• reduction potential, and thermodynamic properties of gaseous H2 or H•, capturing the separate contributions from metal, adsorbate, and solvent. On Group VIII metals, the Ka,HM increases from left to right and top to bottom of periodic position and spans 20 orders in magnitude. The interfacial protons, together with transition metal surfaces, are much more effective than free protons for the electrophilic addition of deuteron that leads to the selective Car-H/Car-D exchange on the ortho and para carbon atoms of phenol because of the additional stabilization afforded by the transition metals acting as a Brønsted base. Hydrogen is required, but the reactive protons inherit the chemical fingerprint from the protic solvent.

Mutation of a conserved glutamine residue does not abolish desensitization of acid-sensing ion channel 1.

Desensitization is a common feature of ligand-gated ion channels, although the molecular cause varies widely between channel types. Mutations that greatly reduce or nearly abolish desensitization have been described for many ligand-gated ion channels, including glutamate, GABA, glycine, and nicotinic receptors, but not for acid-sensing ion channels (ASICs) until recently. Mutating Gln276 to a glycine (Q276G) in human ASIC1a was reported to mostly abolish desensitization at both the macroscopic and the single channel levels, potentially providing a valuable tool for subsequent studies. However, we find that in both human and chicken ASIC1, the effect of Q276G is modest. In chicken ASIC1, the equivalent Q277G slightly reduces desensitization when using pH 6.5 as a stimulus but desensitizes, essentially like wild-type, when using more acidic pH values. In addition, steady-state desensitization is intact, albeit right-shifted, and recovery from desensitization is accelerated. Molecular dynamics simulations indicate that the Gln277 side chain participates in a hydrogen bond network that might stabilize the desensitized conformation. Consistent with this, destabilizing this network with the Q277N or Q277L mutations largely mimics the Q277G phenotype. In human ASIC1a, the Q276G mutation also reduces desensitization, but not to the extent reported previously. Interestingly, the kinetic consequences of Q276G depend on the human variant used. In the common G212 variant, Q276G slows desensitization, while in the rare D212 variant desensitization accelerates. Our data reveal that while the Q/G mutation does not abolish or substantially impair desensitization as previously reported, it does point to unexpected differences between chicken and human ASICs and the need for careful scrutiny before using this mutation in future studies.

Cyber Conflict and the Thresholds of War

Cyberspace has revolutionized the way in which states engage in conflict with one another. While it is widely accepted that international law applies to cyberspace, there remains the question of whether cyberspace is truly novel and distinctive in its effects to the extent that our existing legal thresholds for war have become outdated. Namely, this chapter defines the “thresholds of war” as use of force, armed attack, and armed conflict. Through an analysis of the novelty of cyber conflict, this chapter seeks to make two contributions. First, to demonstrate that cyber conflict can be understood not only through its kinetic consequences, but also through a variety of mass-scale and devastating effects that lie below the thresholds of war as articulated through the decades. Second, that these thresholds of war are outdated and in need of recalibration to properly capture actions that legitimate and illegitimate in cyberspace. This chapter argues that the current international legal order is at danger of creating a lawless cyberspace, where states engage in offensive cyberspace behavior based on an assumption that any effects below the thresholds of war are permissible. Redrawing the line between the permissible and impermissible in cyberspace is of utmost importance to tackle this concern.

The role of the three body photodissociation channel of water in the evolution of dioxygen in astrophysical applications.

A recent experiment at the Dalian Coherent Light Source (DCLS) has provided measurements of the partial cross sections for the photodissociation of water vapor over an unprecedented range of wavelengths in the vacuum ultraviolet (VUV) region. It was found that the three body dissociation channel, H + H + O(3P/1D), becomes prominent at wavelengths shorter than the Lyman α-line at 121.6 nm. The present work explores the kinetic consequences of this discovery for several astrophysically motivated examples. The irradiation of a dilute low-temperature gas by unscreened solar radiation, similar to early stage photochemical processing in a comet coma, shows significant increase in the production of O2-molecules at shorter times, <1 day, that might physically correspond to the photochemical reaction zone of the coma. Several examples of planetary atmospheres show increased O-atom production at high altitudes but relatively little modification of the equilibrium O2 concentrations predicted by conventional models.

Kinetic consequences of the endogenous ligand to molybdenum in the DMSO reductase family: a case study with periplasmic nitrate reductase.

The molybdopterin enzyme family catalyzes a variety of substrates and plays a critical role in the cycling of carbon, nitrogen, arsenic, and selenium. The dimethyl sulfoxide reductase (DMSOR) subfamily is the most diverse family of molybdopterin enzymes and the members of this family catalyze a myriad of reactions that are important in microbial life processes. Enzymes in the DMSOR family can transform multiple substrates; however, quantitative information about the substrate preference is sparse, and, more importantly, the reasons for the substrate selectivity are not clear. Molybdenum coordination has long been proposed to impact the catalytic activity of the enzyme. Specifically, the molybdenum-coordinating residue may tune substrate preference. As such, molybdopterin enzyme periplasmic nitrate reductase (Nap) is utilized as a vehicle to understand the substrate preference and delineate the kinetic underpinning of the differences imposed by exchanging the molybdenum ligands. To this end, NapA from Campylobacter jejuni has been heterologously overexpressed, and a series of variants, where the molybdenum coordinating cysteine has been replaced with another amino acid, has been produced. The kinetic properties of these variants are discussed and compared with those of the native enzyme, providing quantitative information to understand the function of the molybdenum-coordinating residue.

Mechanical unfolding of a knotted protein unveils the kinetic and thermodynamic consequences of threading a polypeptide chain

Knots are remarkable topological features in nature. The presence of knots in crystallographic structures of proteins have stimulated considerable research to determine the kinetic and thermodynamic consequences of threading a polypeptide chain. By mechanically manipulating MJ0366, a small single domain protein harboring a shallow trefoil knot, we allow the protein to refold from either the knotted or the unknotted denatured state to characterize the free energy profile associated to both folding pathways. By comparing the stability of the native state with reference to the knotted and unknotted denatured state we find that knotting the polypeptide chain of MJ0366 increase the folding energy barrier in a magnitude close to the energy cost of forming a knot randomly in the denatured state. These results support that a protein knot can be formed during a single cooperative step of folding but occurs at the expenses of a large increment on the free energy barrier.

Correlation of Exchange Current Density j₀ and the Standard Potential of the Metal Electrodes EM⁰ for the Hydrogen Evolution Reaction (HER)

The hydrogen evolution reaction (HER) is critical technologically in electrochemically energy systems. HER is also important fundamentally as 2H⁺ + 2e ⇌ H₂ provides the standard reference potential E0 H = 0.000 V in thermodynamics and HER serves as a primary and critical test case for kinetics theory.For HER electrocatalytic rates, the mechanism is historically broken down into three steps. The reaction sequence begins with adsorption and reduction of proton to form electrochemically adsorbed hydrogen. This is the Volmer step. H⁺ + e ⇄ Hads (Volmer)After the first Hads is formed, H2 can be formed by two pathways. In the Heyrovsky pathway, a second proton undergoes electrochemical hydrogen adsorption at the same metal atom to form hydrogen gas. Hads + H⁺ + e ⇄ H2 (Heyrovsky)In the Tafel pathway, electrochemically adsorbed hydrogens on adjacent metal atoms react to form H2. Hads + Hads ⇄ H2 (Tafel)HER electrode kinetics measured as exchange current densities j₀ are evaluated in accord with the Volmer-Heyrovsky-Tafel scheme.For HER at metal electrodes, log j₀ is known highly dependent on the metal of the electrodes with rates that vary > 10 orders of magnitude. HER data are evaluated according to these three steps, despite that properties of the metal are not explicit in the Volmer-Heyrovsky-Tafel scheme.Here, properties of the metal electrode are introduced through the standard potential E0 M. For metal M0, Mz+ + ze ⇌ M⁰ E0 M Data for log j₀ are taken from a paper by Trasatti (Electroanalytical Chemistry and Interfacial Electrochemistry (1972) 39, 163-184). Trasatti grouped the 31 metal electrodes into d and sp metals. For the d metals (transition metals), log j₀ is well and linearly correlated with E0 M, whereas for the sp metals, little to no correlation is found.Several points are considered. Higher HER rates are observed at transition metal electrodes with more positive values of E0 M.It is sketched that the initial step in the metal dependent HER process is release of electron(s) to form Mz+ immediately at the electrode surface.Where formation of Mz+immediately at the electrode solution interface is the initial step, log j₀ is anticipated linearly dependent on E0 M.The behavior is sketched within transition state or activated complex theory and is based on a transition state formed of electron(s) shared between the metal cation Mz+ and the proton H+.Formation of the transition state [(1/z)Mz+⋯e⋯H+]‡ precedes the Volmer step, and where applicable, is a rate determining step.Implications for overpotential η pinned by E0 M are considered. This may account for the counter intuitive observation that higher log j₀ are found on metals such as Pt where E0 M is more positive. A kinetic model is sketched where a component of the activation energy is set by F(E0 H-E0 M). Finally, the process of proposing an electrode dependent transition state and evaluating the thermodynamic and kinetic consequences may yield a general method for modeling interfacial electron transfer and electrocatalysis. Such a process may introduces electrode specific properties into the rate expressions a priori.