Hydrogen Peroxide-Induced Overoxidation of Fe-N-C Catalysts: Implications for ORR Activity.

Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as promising alternatives to precious metals for the oxygen reduction reaction (ORR), but they remain insufficiently stable for widespread adoption in fuel cell technologies. One plausible mechanism to explain this lack of stability, and the associated catalyst degradation, is oxidative attack on the catalyst surface by hydrogen peroxide, a non-selective byproduct of the ORR. In this work, we perform a detailed analysis of this degradation mechanism, using a combination of periodic Density Functional Theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations to probe the thermodynamics and kinetics of hydrogen peroxide activation on a series of candidate active sites for the Fe-N-C catalyst. The results demonstrate that carbon atoms neighbouring FeN4 active sites can be strongly over-oxidized via formation of hydroxyl or epoxy groups when hydrogen peroxide is present in the electrolyte. In most cases, the interaction between the over-oxidizing groups and the ORR reaction intermediates reduces the ORR activity, and we further propose that the over-oxidized sites are likely precursors to irreversible carbon corrosion and further catalyst deactivation.

Disproportionation chemistry in K2PtCl4 visualized at atomic resolution using scanning transmission electron microscopy.

The direct observation of a solid-state chemical reaction can reveal otherwise hidden mechanisms that control the reaction kinetics. However, probing the chemical bond breaking and formation at the molecular level remains challenging because of the insufficient spatial-temporal resolution and composition analysis of available characterization methods. Using atomic-resolution differential phase-contrast imaging in scanning transmission electron microscopy, we have visualized the decomposition chemistry of K2PtCl4 to identify its transient intermediate phases and their interfaces that characterize the chemical reduction process. The crystalline structure of K2PtCl4 is found to undergo a disproportionation reaction to form K2PtCl6, followed by gradual reduction to crystalline Pt metal and KCl. By directly imaging different Pt─Cl bond configurations and comparing them to models predicted via density functional theory calculations, a causal connection between the initial and final states of a chemical reaction is established, showcasing new opportunities to resolve reaction pathways through atomistic experimental visualization.

Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization that overlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Origin of Stability and Activity Enhancements in Pt-based Oxygen Reduction Reaction Catalysts via Defect-Mediated Dopant Adsorption.

Platinum alloys are highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in acidic conditions. However, these alloys are susceptible to metal loss through leaching and degradation, leading to reduced catalyst stability and activity. Recently, it has been shown that doping with oxophilic elements can significantly alleviate these problems, with a prominent example being Mo-doped Pt alloys. Here, to achieve atomic scale understanding of the exceptional activity and stability of these alloys, we present a detailed density functional theory description of the dopants' structures and impact on electrocatalyst properties. Beginning with the Mo/Pt system, we demonstrate that Mo can be stabilized in the form of low-dimensional oxyhydroxide moieties on Pt defects. The resulting structures enhance stability and activity via distinct physical processes, with the Mo moieties both directly inhibiting Pt dissolution at defects and indirectly enhancing ORR activity by generation of strain fields on surrounding Pt terraces. We then generalize these analyses to other metal dopant elements, and we demonstrate that similar low-dimensional oxyhydroxide structures control the electrocatalytic properties through an intricate interplay of the structures' acid stability, intrinsic activity for the ORR, and ability to induce ORR-promoting strain fields on Pt.

A first principles analysis of potential-dependent structural evolution of active sites in Fe-N-C catalysts.

Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as potential alternatives to precious metal-based materials for the oxygen reduction reaction (ORR). However, the structure of these materials under electrochemical conditions is not well understood, and their poor stability in acidic environments poses a formidable challenge for successful adoption in commercial fuel cells. To provide molecular-level insights into these complex phenomena, we combine periodic density functional theory (DFT) calculations, exhaustive treatment of coadsorption effects for ORR reaction intermediates, including O and OH, and comprehensive analysis of solvation stabilization effects to construct voltage-dependent ab initio thermodynamic phase diagrams that describe the in situ structure of the active sites. These structures are further linked to activity and stability descriptors that can be compared with experimental parameters such as the half-wave potential for ORR and the onset potential for carbon corrosion and CO2 evolution. The results indicate that pyridinic Fe sites at zigzag carbon edges, as well as other edge sites, exhibit high activity for ORR compared to sites in the bulk. However, edges neighboring the active sites are prone to instability via overoxidation and consequent site loss. The results suggest that it could be beneficial to synthesize Fe-N-C catalysts with small sizes and large perimeter edge lengths to enhance ORR activity, while voltage fluctuations should be limited during fuel cell operation to prevent carbon corrosion of overoxidized edges.

Intermetallic alloy structure-activity descriptors derived from inelastic X-ray scattering.

Synchrotron spectroscopy and Density Functional Theory (DFT) are combined to develop a new descriptor for the stability of adsorbed chemical intermediates on metal alloy surfaces. This descriptor probes the separation of occupied and unoccupied d electron density in platinum and is related to shifts in Resonant Inelastic X-ray Scattering (RIXS) signals. Simulated and experimental spectroscopy are directly compared to show that the promoter metal identity controls the orbital shifts in platinum electronic structure. The associated RIXS features are correlated with the differences in the band centers of the occupied and unoccupied d bands, providing chemical intuition for the alloy ligand effect and providing a connection to traditional descriptions of chemisorption. The ready accessibility of this descriptor to both DFT calculations and experimental spectroscopy, and its connection to chemisorption, allow for deeper connections between theory and characterization in the discovery of new catalysts.

Universal properties of metal-supported oxide films from linear scaling relationships: elucidation of mechanistic origins of strong metal-support interactions.

The properties of ultrathin (1-2 monolayer) (hydroxy)oxide films on transition metal substrates have been extensively studied as models of the celebrated Strong Metal-Support Interaction (SMSI) and related phenomena. However, results from these analyses have been largely system specific, and limited insights into the general principles that govern film/substrate interactions exist. Here, using Density Functional Theory (DFT) calculations, we analyze the stability of ZnO x H y films on transition metal surfaces and show that the formation energies of these films are related to the binding energies of isolated Zn and O atoms via linear scaling relationships (SRs). Such relationships have previously been identified for adsorbates on metal surfaces and have been rationalized in terms of bond order conservation (BOC) principles. However, for thin (hydroxy)oxide films, SRs are not governed by standard BOC relationships, and a generalized bonding model is required to explain the slopes of these SRs. We introduce such a model for the ZnO x H y films and confirm that it also describes the behavior of reducible transition metal oxide films, such as TiO x H y , on metal substrates. We demonstrate how the SRs may be combined with grand canonical phase diagrams to predict film stability under conditions relevant to heterogeneous catalytic reactions, and we apply these insights to estimate which transition metals are likely to exhibit SMSI behavior under realistic environmental conditions. Finally, we discuss how SMSI overlayer formation for irreducible oxides, such as ZnO, is linked to hydroxylation and is mechanistically distinct from the overlayer formation for reducible oxides such as TiO2.

Adsorbate chemical environment-based machine learning framework for heterogeneous catalysis.

Heterogeneous catalytic reactions are influenced by a subtle interplay of atomic-scale factors, ranging from the catalysts' local morphology to the presence of high adsorbate coverages. Describing such phenomena via computational models requires generation and analysis of a large space of atomic configurations. To address this challenge, we present Adsorbate Chemical Environment-based Graph Convolution Neural Network (ACE-GCN), a screening workflow that accounts for atomistic configurations comprising diverse adsorbates, binding locations, coordination environments, and substrate morphologies. Using this workflow, we develop catalyst surface models for two illustrative systems: (i) NO adsorbed on a Pt3Sn(111) alloy surface, of interest for nitrate electroreduction processes, where high adsorbate coverages combined with low symmetry of the alloy substrate produce a large configurational space, and (ii) OH* adsorbed on a stepped Pt(221) facet, of relevance to the Oxygen Reduction Reaction, where configurational complexity results from the presence of irregular crystal surfaces, high adsorbate coverages, and directionally-dependent adsorbate-adsorbate interactions. In both cases, the ACE-GCN model, trained on a fraction (~10%) of the total DFT-relaxed configurations, successfully describes trends in the relative stabilities of unrelaxed atomic configurations sampled from a large configurational space. This approach is expected to accelerate development of rigorous descriptions of catalyst surfaces under in-situ conditions.

Deep reaction network exploration at a heterogeneous catalytic interface.

Characterizing the reaction energies and barriers of reaction networks is central to catalyst development. However, heterogeneous catalytic surfaces pose several unique challenges to automatic reaction network characterization, including large sizes and open-ended reactant sets, that make ad hoc network construction the current state-of-the-art. Here, we show how automated network exploration algorithms can be adapted to the constraints of heterogeneous systems using ethylene oligomerization on silica-supported single-site Ga3+ as a model system. Using only graph-based rules for exploring the network and elementary constraints based on activation energy and size for identifying network terminations, a comprehensive reaction network is generated and validated against standard methods. The algorithm (re)discovers the Ga-alkyl-centered Cossee-Arlman mechanism that is hypothesized to drive major product formation while also predicting several new pathways for producing alkanes and coke precursors. These results demonstrate that automated reaction exploration algorithms are rapidly maturing towards general purpose capability for exploratory catalytic applications.

High-entropy nanoparticles: Synthesis-structure-property relationships and data-driven discovery.

High-entropy nanoparticles have become a rapidly growing area of research in recent years. Because of their multielemental compositions and unique high-entropy mixing states (i.e., solid-solution) that can lead to tunable activity and enhanced stability, these nanoparticles have received notable attention for catalyst design and exploration. However, this strong potential is also accompanied by grand challenges originating from their vast compositional space and complex atomic structure, which hinder comprehensive exploration and fundamental understanding. Through a multidisciplinary view of synthesis, characterization, catalytic applications, high-throughput screening, and data-driven materials discovery, this review is dedicated to discussing the important progress of high-entropy nanoparticles and unveiling the critical needs for their future development for catalysis, energy, and sustainability applications.

Understanding the Solid-State Electrode-Electrolyte Interface of a Model System Using First-Principles Statistical Mechanics and Thin-Film X-ray Characterization.

Intermixing of atomic species at the electrode-electrolyte boundaries can impact the properties of the interfaces in solid-state batteries. Herein, this work uses first-principles statistical mechanics along with experimental characterization to understand intermixing at the electrode-electrolyte interface. For the model presented in this work, lithium manganese oxide (LiMn2O4, LMO) and lithium lanthanum titanate (Li3xLa2/3-xTiO3, LLTO) are employed as the cathode and electrolyte, respectively. The results of the computational work show that Ti-Mn intermixing at the interface is significant at synthesis temperatures. The experimental results in this work find that, at some critical temperatures between 600 and 700 °C for material preparation, the interface of LLTO-LMO becomes blurred. Calculations predict that the interface is unstable with regard to Ti-Mn intermixing starting at 0 K, suggesting that the critical temperature found in the experiment is related to kinetics. The work overall suggests that, in designing a solid-state battery, the fundamental reactions such as intermixing need to be considered.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces.

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10-7 mbar of O2, and a D2/O2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D2/O2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides.

Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non-intrinsic activity metrics, thus hampering the construction of consistent structure-activity-relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline α-MA (II)MB (III) LDH and ß-MA (OH)2 electrocatalysts (MA =Ni, Co, and MB =Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe-free Co-containing catalysts > Fe-Co-free Ni-based catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual-metal-site nature of the reaction centers, which lead to composition-dependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance.

Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2.

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni2+ and Cr3+. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn2+ and Ga3+ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni2+. The molecular weight distribution of products formed on Zn2+ is similar to Ni2+, while Ga3+ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and ß-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

Structure and solvation of confined water and water-ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis.

Aqueous-phase reactions within microporous Brønsted acids occur at active centers comprised of water-reactant-clustered hydronium ions, solvated within extended hydrogen-bonded water networks that tend to stabilize reactive intermediates and transition states differently. The effects of these diverse clustered and networked structures were disentangled here by measuring turnover rates of gas-phase ethanol dehydration to diethyl ether (DEE) on H-form zeolites as water pressure was increased to the point of intrapore condensation, causing protons to become solvated in larger clusters that subsequently become solvated by extended hydrogen-bonded water networks, according to in situ IR spectra. Measured first-order rate constants in ethanol quantify the stability of SN2 transition states that eliminate DEE relative to (C2H5OH)(H+)(H2O) n clusters of increasing molecularity, whose structures were respectively determined using metadynamics and ab initio molecular dynamics simulations. At low water pressures (2-10 kPa H2O), rate inhibition by water (-1 reaction order) reflects the need to displace one water by ethanol in the cluster en route to the DEE-formation transition state, which resides at the periphery of water-ethanol clusters. At higher water pressures (10-75 kPa H2O), water-ethanol clusters reach their maximum stable size ((C2H5OH)(H+)(H2O)4-5), and water begins to form extended hydrogen-bonded networks; concomitantly, rate inhibition by water (up to -3 reaction order) becomes stronger than expected from the molecularity of the reaction, reflecting the more extensive disruption of hydrogen bonds at DEE-formation transition states that contain an additional solvated non-polar ethyl group compared to the relevant reactant cluster, as described by non-ideal thermodynamic formalisms of reaction rates. Microporous voids of different hydrophilic binding site density (Beta; varying H+ and Si-OH density) and different size and shape (Beta, MFI, TON, CHA, AEI, FAU), influence the relative extents to which intermediates and transition states disrupt their confined water networks, which manifest as different kinetic orders of inhibition at high water pressures. The confinement of water within sub-nanometer spaces influences the structures and dynamics of the complexes and extended networks formed, and in turn their ability to accommodate the evolution in polarity and hydrogen-bonding capacity as reactive intermediates become transition states in Brønsted acid-catalyzed reactions.

Oriented LiMn2O4 Particle Fracture from Delithiation-Driven Surface Stress.

The insertion and removal of Li+ ions into Li-ion battery electrodes can lead to severe mechanical fatigue because of the repeated expansion and compression of the host lattice during electrochemical cycling. In particular, the lithium manganese oxide spinel (LiMn2O4, LMO) experiences a significant surface stress contribution to electrode chemomechanics upon delithiation that is asynchronous with the potentials where bulk phase transitions occur. In this work, we probe the stress evolution and resulting mechanical fracture from LMO delithation using an integrated approach consisting of cyclic voltammetry, electron microscopy, and density functional theory (DFT) calculations. High-rate electrochemical cycling is used to exacerbate the mechanical deficiencies of the LMO electrode and demonstrates that mechanical degradation leads to slowing of delithiation and lithiation kinetics. These observations are further supported through the identification of significant fracturing in LMO using scanning electron microscopy. DFT calculations are used to model the mechanical response of LMO surfaces to electrochemical delithiation and suggest that particle fracture is unlikely in the [001] direction because of tensile stresses from delithiation near the (001) surface. Transmission electron microscopy and electron backscatter diffraction of the as-cycled LMO particles further support the computational analyses, indicating that particle fracture instead tends to preferentially occur along the {111} planes. This joint computational and experimental analysis provides molecular-level details of the chemomechanical response of the LMO electrode to electrochemical delithiation and how surface stresses may lead to particle fracture in Li-ion battery electrodes.

In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution.

NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

Structural trends in the dehydrogenation selectivity of palladium alloys.

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior. To provide molecular-level insights into these effects, a series of Pd intermetallic alloy catalysts with Zn, Ga, In, Fe and Mn promoter elements was synthesized, and the structures were determined using in situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). The alloys all showed propane dehydrogenation turnover rates 5-8 times higher than monometallic Pd and selectivity to propylene of over 90%. Moreover, among the synthesized alloys, Pd3M alloy structures were less olefin selective than PdM alloys which were, in turn, almost 100% selective to propylene. This selectivity improvement was interpreted by changes in the DFT-calculated binding energies and activation energies for C-C and C-H bond activation, which are ultimately influenced by perturbation of the most stable adsorption site and changes to the d-band density of states. Furthermore, transition state analysis showed that the C-C bond breaking reactions require 4-fold ensemble sites, which are suggested to be required for non-selective, alkane hydrogenolysis reactions. These sites, which are not present on alloys with PdM structures, could be formed in the Pd3M alloy through substitution of one M atom with Pd, and this effect is suggested to be partially responsible for their slightly lower selectivity.

Defect-Mediated Ordering of Condensed Water Structures in Microporous Zeolites.

Ab-initio molecular dynamics simulations and transmission infrared spectroscopy are employed to characterize the structure of water networks in defect-functionalized microporous zeolites. Thermodynamically stable phases of clustered water molecules are localized at some of the defects in zeolite Beta, which include catalytic sites such as framework Lewis acidic Sn atoms in closed and hydrolyzed-open forms, as well as silanol nests. These water clusters compete with ideal gas-like structures at low water densities and pore-filling phases at higher water densities, with the equilibrium phase determined by the water chemical potential. The physical characteristics of these phases are determined by the defect identity, with the local binding and orientation of hydroxyl moieties around the defects playing a central role. The results suggest general principles for how the structure of polar solvents in microporous solid acids is influenced by local defect functionalization, and the thermodynamic stability of the condensed phases surrounding such sites, in turn, implies that the catalysis of Lewis acids will be influenced by local water ordering.

A Long-Cycle-Life Lithium-CO2 Battery with Carbon Neutrality.

Lithium-CO2 batteries are attractive energy-storage systems for fulfilling the demand of future large-scale applications such as electric vehicles due to their high specific energy density. However, a major challenge with Li-CO2 batteries is to attain reversible formation and decomposition of the Li2 CO3 and carbon discharge products. A fully reversible Li-CO2 battery is developed with overall carbon neutrality using MoS2 nanoflakes as a cathode catalyst combined with an ionic liquid/dimethyl sulfoxide electrolyte. This combination of materials produces a multicomponent composite (Li2 CO3 /C) product. The battery shows a superior long cycle life of 500 for a fixed 500 mAh g-1 capacity per cycle, far exceeding the best cycling stability reported in Li-CO2 batteries. The long cycle life demonstrates that chemical transformations, making and breaking covalent CO bonds can be used in energy-storage systems. Theoretical calculations are used to deduce a mechanism for the reversible discharge/charge processes and explain how the carbon interface with Li2 CO3 provides the electronic conduction needed for the oxidation of Li2 CO3 and carbon to generate the CO2 on charge. This achievement paves the way for the use of CO2 in advanced energy-storage systems.