Intermediate Reaction Steps Research Articles

(Invited) Resolving Catalytic Mechanism at an Electrode Surface: Thermodynamics & Kinetics of Reaction Steps

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution. Yet, when compared to experiment, kinetic models are applied. For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms. A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO3/aqueous interface. I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO3 surface. The intermediate population is of trapped holes measured by emission from the conduction band into unoccupied states in the middle of the semi-conductor bandgap. The isotherm is tuned by the pH of the electrolyte. The pH-dependent formation of these intermediates are connected to their pH-dependent decay at microsecond timescales, presumably related to later reaction steps of OER. I'll also present work on the side of vibrational spectroscopy that looks at how the potential energy surface of a particular trapped hole, the terminal oxyl radical, arises in time through the spectral kinetics of its normal mode. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely polymorphs of TiO2.

Biological Rhythms Generated by a Single Activator-Repressor Loop with Inhomogeneity and Diffusion.

Common models of circadian rhythms are typically constructed as compartmental reactions of well-mixed biochemicals, incorporating a negative-feedback loop consisting of several intermediate reaction steps essentially required to produce oscillations. Spatial transport of each reactant is often represented as an extra compartmental reaction step. Contrary to this traditional understanding, in this Letter we demonstrate that a single activation-repression biochemical reaction pair is sufficient to generate sustained oscillations if the sites of both reactions are spatially separated and molecular transport is mediated by diffusion. Our proposed scenario represents the simplest configuration in terms of the participating chemical reactions and offers a conceptual basis for understanding biological oscillations and inspiring invitro assays aimed at constructing minimal clocks.

Transient Absorption Spectroscopy of Films: Impact of Refractive Index.

Transient absorption spectroscopy is a powerful technique to study the photoinduced phenomena in a wide range of states from solutions to solid film samples. It was designed and developed based on photoinduced absorption changes or that photoexcitation triggers a chain of reactions with intermediate states or reaction steps with presumably different absorption spectra. However, according to general electromagnetic theory, any change in the absorption properties of a medium is accompanied by a change in the refractive properties. Although this photoinduced change in refractive index has a negligible effect on solution measurements, it may significantly affect the measured response of thin films. In this Perspective paper, we examine why and how the measured responses of films differ from their expected "pure" absorption responses. The effect of photoinduced refractive index change can be concluded and studied by comparing the transmitted and reflected probe light responses. Another discussed aspect is the effect of light interference on thin films. Finally, new opportunities of monitoring the photocarrier migration in films and studying nontransparent samples using the reflected probe light response are discussed. Most of the examples provided in this article focus on studies involving perovskite, TiO2, and graphene-based films, but the general discussion and conclusions can be applicable to a wide range of semiconductor and thin metallic films.

Controllable spatial distribution of La dopants in perovskite NaTaO3 enhancing its photocatalytic performance of formaldehyde solution degradation and simultaneous hydrogen evolution

Photocatalytic degradation of aqueous organic pollutant while hydrogen evolution has become an attractive and potential approach to address environmental pollution and energy shortages. The design of highly efficient photocatalyst and the mechanism study are the keys for this issue. For this purpose, a series of rare earth La doped perovskite NaTaO3 were prepared by an easy solid-phase synthesis method. La doping changed the morphology, structure and physicochemical properties of NaTaO3 crystal. More importantly, it also found that controllable La dopants formed a concentration gradient between the surface and bulk of NaTaO3 crystal, which would cause a gradient of energy band structure. Noteworthy, 0.5La/NaTaO3 was identified as the intermediate state with the La doping from A sites to B sites and from bulk to surface in NaTaO3 crystal. It possessed a large La concentration gradient, leading to the better photoelectrochemical properties and photocatalytic performance than any other xLa/NaTaO3 catalyst. For the photocatalytic reaction over 0.5La/NaTaO3, HCHO was partially mineralized and gradually converted to HCOOH due to the large energy barriers of the intermediate reaction steps, while H2 was produced stably in the five recycle runs. The active species of •O2−, •OH and h+ were detected by the control experiments with trapping agents, and •O2− was confirmed to be the dominant one in the photocatalytic reaction. On this basis, the possible mechanism was proposed to understand the photocatalytic system in depth.

Research progress on layered metal oxide electrocatalysts for an efficient oxygen evolution reaction.

Hydrogen, highly valued for its pristine cleanliness and remarkable efficiency as an emerging energy source, is anticipated to ascend to a preeminent status within the forthcoming energy landscape. Electrocatalytic water splitting is considered a pivotal, eco-friendly, and sustainable strategy for hydrogen production. The substantial energy consumption stemming from oxygen evolution side reactions significantly impedes the commercial viability of water electrolysis. Consequently, the pursuit of a cost-effective and efficacious oxygen evolution reaction (OER) catalyst stands as an imperative strategy for realizing hydrogen production via water electrolysis. Layered metal oxides, owing to their robust anisotropic properties, versatile adjustability, and extensive surface area, have emerged as suitable candidates for OER catalysts. However, owing to the distinctive attributes of layered metal oxides, ongoing investigations into these materials are slightly fragmented, lacking universal consensus. This article comprehensively surveys the recent advancements in layered metal oxide-based OER catalysts, categorized into single metal oxides, alkali cobalt oxides, perovskites, and miscellaneous metal oxides. Initially, the main OER intermediate reaction steps of layered metal oxides are scrutinized. Subsequently, the design, mechanism, and application of several pivotal layered metal oxides in the OER are systematically delineated. Finally, a summary is provided, alongside the proposal of future research trajectories and challenges encountered by layered metal oxides, with the aspiration that this paper may serve as a valuable reference for scholars in the field.

Active Carbon from Date Stones for Phenol Oxidation in Trickle Bed Reactor, Experimental and Kinetic Study

The catalytic wet air oxidation (CWAO) of phenol has been studied in a trickle bed reactor&#x0D; using active carbon prepared from date stones as catalyst by ferric and zinc chloride activation (FAC and ZAC). The activated carbons were characterized by measuring their surface area and adsorption capacity besides conventional properties, and then checked for CWAO using a trickle bed reactor operating at different conditions (i.e. pH, gas flow rate, LHSV, temperature and oxygen partial pressure). The results showed that the active carbon (FAC and ZAC), without any active metal supported, gives the highest phenol conversion. The reaction network proposed accounts for all detected intermediate products of phenol oxidation that composed by several consecutive and parallel reactions. The parameters of the model estimated using experimental data obtained from a continuous trickle bed reactor at different temperatures (120-160 C) and oxygen partial pressures (8-12 bar). Simple power law as well as Langmuir-Hinshelwood (L-H) expressions accounting for the adsorption effects were checked in the modeling of the reaction network. A non-linear multi-parameter estimation approach was used to simultaneously evaluate the high number of model parameters. Approach by simple power law only succeeds in fitting phenol disappearance. Instead, when L-H expressions are incorporated for the intermediate reaction steps, the model accurately describes all the experimental concentration profiles, giving mean deviations below 10%.

Self-Organization of Enzyme-Catalyzed Reactions Studied by the Maximum Entropy Production Principle.

The self-organization of open reaction systems is closely related to specific mechanisms that allow the export of internally generated entropy from systems to their environment. According to the second law of thermodynamics, systems with effective entropy export to the environment are better internally organized. Therefore, they are in thermodynamic states with low entropy. In this context, we study how self-organization in enzymatic reactions depends on their kinetic reaction mechanisms. Enzymatic reactions in an open system are considered to operate in a non-equilibrium steady state, which is achieved by satisfying the principle of maximum entropy production (MEPP). The latter is a general theoretical framework for our theoretical analysis. Detailed theoretical studies and comparisons of the linear irreversible kinetic schemes of an enzyme reaction in two and three states are performed. In both cases, in the optimal and statistically most probable thermodynamic steady state, a diffusion-limited flux is predicted by MEPP. Several thermodynamic quantities and enzymatic kinetic parameters, such as the entropy production rate, the Shannon information entropy, reaction stability, sensitivity, and specificity constants, are predicted. Our results show that the optimal enzyme performance may strongly depend on the number of reaction steps when linear reaction mechanisms are considered. Simple reaction mechanisms with a smaller number of intermediate reaction steps could be better organized internally and could allow fast and stable catalysis. These could be features of the evolutionary mechanisms of highly specialized enzymes.

Efficient catalyst screening using graph neural networks to predict strain effects on adsorption energy.

Small-molecule adsorption energies correlate with energy barriers of catalyzed intermediate reaction steps, determining the dominant microkinetic mechanism. Straining the catalyst can alter adsorption energies and break scaling relationships that inhibit reaction engineering, but identifying desirable strain patterns using density functional theory is intractable because of the high-dimensional search space. We train a graph neural network to predict the adsorption energy response of a catalyst/adsorbate system under a proposed surface strain pattern. The training data are generated by randomly straining and relaxing Cu-based binary alloy catalyst complexes taken from the Open Catalyst Project. The trained model successfully predicts the adsorption energy response for 85% of strains in unseen test data, outperforming ensemble linear baselines. Using ammonia synthesis as an example, we identify Cu-S alloy catalysts as promising candidates for strain engineering. Our approach can locate strain patterns that break adsorption energy scaling relations to improve catalyst performance.

(Invited) Resolving a Catalytic Mechanism on an Electrode Surface: Using Time Resolution to Identify Theoretical Descriptors

Catalytic mechanisms at electrode surfaces guide the development of electrochemically controlled energy storing reactions and chemical synthesis. The intermediate steps of these mechanisms are challenging to identify experimentally but are critical to understanding the speed, stability, and selectivity of product evolution for a given energy input. One of the most studied reactions is the water-splitting reaction which breaks apart H2O into H2 and O2. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct the energy intensive part, the oxygen evolution reaction (OER), into its component parts. The first step of this oxidative reaction, which is to release a proton and take an electron from water, has often been thought of as a theoretical “descriptor” that differentiates the OER activity of materials: if this is hard to do, then the material is less energy efficient and if it is easy to do, then a later, bond-forming step limits the energy efficiency. Yet, the formation free energy of this step had yet to be measured experimentally. Here, I will begin by summarizing the work done previously by the group: the structure and kinetics of forming these first reaction intermediates (Ti-OH*) by their optical and vibrational signatures upon driving OER with an ultrafast photo-trigger of the electrode surface (SrTiO3). I will then discuss the most recent work, which shows that by separating the reaction steps in time and optically detecting these intermediates created upon hole-transfer from the valence band, we can determine their formation free energy1. In addition to the optical spectroscopy, we employ reaction isotherm methodology borrowed from surface studies of reactions. By tuning the pH, we obtain a Langmuir isotherm on the driven surface (Figure 1) regulated by the proton transfer that increases surface hydroxylation for higher pH prior to photo-triggering OER. From the midpoint of this isotherm, one can obtain the formation free energy of Ti-OH* from valence band holes. Such an application of isotherms is new in that the product is not the final one, for which one generally knows the reaction free energy, but rather an intermediate of a reaction. Overall, the innovation of the work is to utilize kinetic measurements to obtain the calculated free energy of an intermediate reaction step at a material surface. 1I. Vinogradov, S. Singh, H. Lyle, M. Paolino, A. Mandal*, J. Rossmeisl, T. Cuk *, Nature Materials, 2022, 21, 88.Figure 1: The time-resolved optical data as a function of pH show how the population of intermediates generates a reaction isotherm (black trace). The emission (in blue) counts the intermediate Ti-OH* population. Figure 1

Electrooxidation of per- and polyfluoroalkyl substances in chloride-containing water on surface-fluorinated Ti4O7 anodes: Mitigation and elimination of chlorate and perchlorate formation

Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFASs, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti4O7 anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti4O7 anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveal the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti4O7 anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti4O7 anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.

DelaySSAToolkit.jl: stochastic simulation of reaction systems with time delays in Julia.

DelaySSAToolkit.jl is a Julia package for modelling reaction systems with non-Markovian dynamics, specifically those with time delays. These delays implicitly capture multiple intermediate reaction steps and hence serve as an effective model reduction technique for complex systems in biology, chemistry, ecology and genetics. The package implements a variety of exact formulations of the delay stochastic simulation algorithm. The source code and documentation of DelaySSAToolkit.jl are available at https://github.com/palmtree2013/DelaySSAToolkit.jl.

In-depth Sequence-Function Characterization Reveals Multiple Pathways to Enhance Enzymatic Activity.

Deep mutational scanning (DMS) has recently emerged as a powerful method to study protein sequence-function relationships but it has not been well-explored as a guide to enzyme engineering and identifying pathways by which their catalytic cycle may be improved. We report such a demonstration in this work using a Phenylalanine ammonia-lyase (PAL), which deaminates L-phenylalanine to trans-cinnamic acid and has widespread application in chemo-enzymatic synthesis, agriculture, and medicine. In particular, the PAL from Anabaena variabilis (AvPAL*) has garnered significant attention as the active ingredient in Pegvaliase®, the only FDA-approved drug treating classical Phenylketonuria (PKU). Although an extensive body of literature exists on the structure, substrate-specificity, and catalytic cycle, protein-wide sequence determinants of function remain unknown, as do intermediate reaction steps that limit turnover frequency, all of which has hindered rational engineering of these enzymes. Here, we created a detailed sequence-function landscape of AvPAL* by performing DMS and revealed 112 mutations at 79 functionally relevant sites that affect a positive change in enzyme fitness. Using fitness values and structure-function analysis, we picked a subset of positions for comprehensive single- and multi-site saturation mutagenesis and identified combinations of mutations that led to improved reaction kinetics in cell-free and cellular contexts. We then performed QM/MM and MD to understand the mechanistic role of the most beneficial mutations and observed that different mutants confer improvements via different mechanisms, including stabilizing transition and intermediate states, improving substrate diffusion into the active site, and decreasing product inhibition. This work demonstrates how DMS can be combined with computational analysis to effectively identify significant mutations that enhance enzyme activity along with the underlying mechanisms by which these mutations confer their benefit.

Study on CO/CO2 formation mechanism of Zigzag model coke with high oxygen coverage based on DFT theory

The clean and efficient utilization of carbon resources is becoming more and more important, in energy, material, and chemical engineering field, but the mechanism of coke oxidation, especially that of the CO2/CO desorption is not fully studied yet. In this paper, density functional theory was used to study the oxidation mechanism of Zigzag char structure with high coverage of O2, which is related to an oxidation under lower temperature or high pressure. Based on the corresponding quantum chemistry calculation, it is shown that there are several possible pathways for the CO2 desorption process, which may need rearrange to form the structure containing O–C–O clusters. And successively, multiple intermediate reaction steps are required to complete the desorption of CO2. Other than in the literature that the COO–O–C functional group formed first, with then the C–O bond broken and CO2 desorbed respectively, a novel pathway with two C–O bonds broken simultaneously to generate CO2 was found. It results from a functional group of COO–char formed, and certain alternative pathways via C–C bonds breaking were also dealt with, as well as related CO desorptions. The reaction model built was validated by theoretical and experimental results from literature satisfactorily.

Queuing Models of Gene Expression: Analytical Distributions and Beyond

Activation of a gene is a multistep biochemical process, involving recruitments of transcription factors and histone kinases as well as modification of histones. Many of these intermediate reaction steps would have been unspecified by experiments. Therefore, classical two-state models of gene expression established based on the memoryless (or Markovian) assumption would not well describe the reality in gene expression. Recent experimental data have indicated that the inactive phases of gene promoters are differently distributed, showing strong memory. Here, we use a nonexponential waiting-time distribution to model the complex activation process of a gene, and then analyze a queuing model of stochastic transcription. We successfully derive the analytical expression of the stationary mRNA distribution, which provides insight into the effect of molecular memory created by complex activating events on the mRNA expression. We find that the reduction in the waiting-time noise may result in the increase in the mRNA noise, contrary to the previous conclusion. Based on the derived distribution, we also develop a method to infer the waiting-time distribution from a known mRNA distribution. Data analysis on a realistic example verifies the validity of this method.

Cyclic Carbonate Formation from Epoxides and CO2 Catalyzed by Sustainable Alkali Halide-Glycol Complexes: A DFT Study to Elucidate Reaction Mechanism and Catalytic Activity.

We provide a comprehensive DFT investigation of the mechanistic details of CO2 fixation into styrene oxide to form styrene carbonate, catalyzed by potassium iodide–tetraethylene glycol complex. A detailed view on the intermediate steps of the overall reaction clarifies the role of hydroxyl substances as co-catalysts for the alkali halide-catalyzed cycloaddition. The increase of iodide nucleophilicity in presence of tetraethylene glycol is examined and rationalized by NBO and Hirshfeld charge analysis, and bond distances. We explore how different alkali metal salts and glycols affect the catalytic performance. Our results provide important hints on the synthesis of cyclic carbonates from CO2 and epoxides promoted by alkali halides and glycol complexes, allowing the development of more efficient catalysts.

Universal Kinetic Model to Simulate Two-Step Biodiesel Production from Vegetable Oil

To date, to simulate biodiesel production, kinetic models from different authors have been provided, each one usually applied to the use of a specific vegetable oil and experimental conditions. Models, which may include esterification, besides transesterification simulation, were validated with their own experimental conditions and raw material. Moreover, information about the intermediate reaction steps, besides catalyst concentration variation, is either rare or nonexistent. Here, in this work, a universal mathematical model comprising the chemical kinetics of a two-step (esterification and transesterification) vegetable oil-based biodiesel reaction is proposed. The proposed model is universal, as it may simulate any vegetable oil biodiesel reaction from the literature. For this purpose, a mathematical model using the software MATLAB has been designed. Using the mathematical model, the estimation of mass variation with time, of both reactants and products, as well as glyceride conversion and homogeneous catalyst concentration variation (instead of only alcohol/catalyst solution) are allowed. Moreover, analysis of the influence of some important variables affecting the reaction kinetics of biodiesel production (e.g., catalyst concentration), along with comparison and model validation with data from different authors may be carried out. In addition, Supplementary material with a collection of 290 rate constants, derived from 55 different experiments using different vegetable oils and conditions is provided.

Kinetic Effect of Surface Chemisorbed Oxygen on Platinum-Catalyzed Hydrogen Peroxide Decomposition

The chemical decomposition of H2O2 to H2O and O2 catalyzed by platinum nanocatalysts is important in many technologies such as steam propulsion, biosensors, fuel cells, and synthetic chemistry. Surface chemisorbed oxygen strongly impacts the kinetics of this reaction. However, the effect of this surface species has not been quantified in the context of the reaction mechanism. This study determined the rate constants of the elementary reaction steps of H2O2 decomposition on platinum nanocatalysts. For that, hydrogen peroxide decomposition rate measurements were analyzed on samples with variable surface chemisorbed oxygen (Pt(O)) abundance. The order of reaction in terms of surface Pt(O) abundance is 0.83, which indicates a nearly first order effect on the rate of H2O2 decomposition. This result is consistent with a reaction mechanism for H2O2 decomposition on platinum that involves two cyclic steps, where step 1 is the rate limiting step. $$\begin{array}{*{20}l} {{\text{Step}}\;1} \hfill & {{\text{Pt}} + {\text{H}}_{2} {\text{O}}_{2 } \xrightarrow{k = 0.0028} {\text{H}}_{2} {\text{O}} + {\text{ Pt}}({\text{O}})} \hfill \\ {{\text{Step}}\;2} \hfill & {\underline{{{\text{Pt}}({\text{O}}) + {\text{H}}_{2} {\text{O}}_{2} \xrightarrow{k = 0.038}{\text{Pt}} + {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}}} } \hfill \\ {{\text{Overall}}} \hfill & {2{\text{H}}_{2} {\text{O}}_{2 } \xrightarrow{k = 0.037}{\text{O}}_{2 } + 2{\text{H}}_{2} {\text{O}}} \hfill \\ \end{array}$$ The rate constant $$(k)$$ of step 2 is 14 times higher than that of step 1 at a reaction temperature T = 295 K. This is consistent with a 4.4 time larger activation energy for step 1. Reaction rates are initially faster on samples with more surface Pt(O) sites because these sites decompose H2O2 through step 2 in the first cycle of the reaction. The relative rates of step 1 and step 2 become smaller at higher temperature. The method presented in this study can be used for determining the rate constants of the intermediate reaction steps of many other chemical reactions that are important for numerous scientific and technological applications.

Origin and turnaround of enantioselectivity in a chiral organocatalysed Diels‐Alder reaction: A mechanistic study

AbstractThe present work comprises exploring the efficacies of the chiral organocatalysts employed in the classic Diels‐Alder reaction between anthrone and maleimide in generating an enantioselective product. The complete mechanism of the reaction has been figured out by performing the electronic structure calculations at the density functional theory (DFT) level. Our calculations suggest that the reaction commences with the abstraction of acidic proton of anthrone instigated by organocatalysts, (S)‐(‐)‐N‐(methyl‐2′‐pyridyl)‐2‐(α‐hydroxyethyl)benzimidazole[Pyr‐HEB]and (S)‐(‐)‐N‐(methyl‐1′‐benzotriazolyl)‐2‐(α‐hydroxyethyl)benzimidazole[Btz‐HEB]. Subsequently, N‐methyl maleimide interacts with the nucleophilic carbon of anthrone in a stepwise pathway, either from left‐hand side or right‐hand side, resulting in the generation of an enantioselective chiral product. A unique feature in the above reaction was thatPyr‐HEBandBtz‐HEBfavoured the formation of SS‐product and RR‐product, respectively, with high enantioselectivity. The turnaround of enantioselectivity upon changing the catalyst has been attributed to the noncovalent interaction and steric orientation in the molecule during the intermediate reaction steps. An estimated value of degree of enantioselectivity, for both the catalysts, was obtained in accordance with the experimental findings.

Total oxidation of lean methane over cobalt spinel nanocubes—Mechanistic vistas gained from DFT modeling and catalytic isotopic investigations

Abstract Mechanistic investigations based on combination of catalytic isotopic experiments with DFT modeling were used to probe the pathways by which CH4 in transformed into H2O and CO2 over cobalt spinel nanocubes exposing the (100) surface exclusively. The catalyst synthetized by the hydrothermal method was thoroughly characterized by means of XRD, XRF, Raman and SEM techniques. Combustion of methane was performed in a TPSR mode using sole 16O2 or a 1:1 mixture of 16O2 and 18O2. Two distinct mechanistic pathways of the C H bond activation, and subsequent oxidation of the consecutive intermediates involve a suprafacial mechanism operating in the oxygen covered surface and an interfacial mechanism operating on the oxygen depleted/bare surface. It was found that depending on the conditions they may act separately in the early and late stages of the reaction, and simultaneously in the middle phase, in response to differences in the nature and accessibility of the surface and lattice oxygen species, however the activation of the first C– bond remains the kinetically relevant step. The cobalt-oxo CoT2c–O species exhibit the highest activity in the suprafacial C H bond breaking, and the peroxo O22– species are engaged in the intermediate reaction steps leading to formation of final H2O and CO2 products. On the oxygen exhausted surface, where the interfacial mechanism operates, dual Co–O sites constituted by CoO5c–O3O and CoT2c–O2O,1T pairs may activate the C H bond in a concerted way, with the former being more active. For all the mechanistic steps the transition and stable states of the involved intermediates and products were identified and quantified in terms of their structure and the associated free enthalpy profiles. The three dimensional (log(kintra/ksupra) versus (ΔGasupra-ΔGasupra) and ΘO) diagrams were constructed to discriminate between the suprafacial (Eley-Rideal/Langmuir-Hinshelwood) and interfacial (Mars-van Krevelen) mechanism of CH4 combustion. It is believed that the main features of the established mechanism are also applicable in the case of the methane oxidation over other electron rich transition metal oxides.

H2Reduction Annealing Induced Phase Transition and Improvements on Redox Durability of Pt Cluster-Decorated Cu@Pd Electrocatalysts in Oxygen Reduction Reaction

Hierarchical structures in shell with transition metal underneath is a promising design for high-performance and low-cost heterogeneous nanocatalysts (NCs). Such a design enables the optimum extent of synergetic effects in NC surface. It facilitates intermediate reaction steps and, therefore, boosts activity of NC in oxygen reduction reaction (ORR). In this study, carbon nanotube (CNT)-supported ternary metallic NC comprising Cucluster-in-Pdcluster nanocrystal and surface decoration of atomic Pt clusters (14 wt %) is synthesized by using the wet chemical reduction method with sequence and reaction time controls. By annealing in H2 environment (H2/N2 = 9:1, 10 sccm) at 600 K for 2 h, specific activity of Cu@Pd/Pt is substantially improved by ∼2.0-fold as compared to that of the pristine sample and commercial Pt catalysts. By cross-referencing results of electron microscopic, X-ray spectroscopic, and electrochemical analyses, we demonstrated that reduction annealing turns ternary NC into complex of Cu3Pt alloy and CuxPd1–x alloy. Such a transition preserves Pt and Pd in metallic phases, therefore improving the activity by ∼29% and the stability of NC in an accelerated degradation test (ADT) as compared to those of pristine Cu@Pd/Pt in 36 000 cycles at 0.85 V (vs RHE). This study presents robust H2 annealing for structure stabilization of NC and systematic characterizations for rationalization of the corresponding mechanisms. These results provide promising scenarios for facilitation of heterogeneous NC in ORR applications.