Dissociative Adsorption Of O2 Research Articles

Dissociative Adsorption of O2 on Ag3Au(111) Surface: A Density Functional Theory Study

The catalytic efficiency of oxygen reduction catalysts is notably influenced by the dissociative adsorption of O2. We conducted a systematic investigation into the dissociative adsorption of O2 on the Ag3Au(111) surface using ab initio density functional theory (DFT) calculations. Our computational findings indicate that adsorption the configuration designated t-b-t exhibits favorable energetics on the Ag3Au(111) surface. Regarding the dissociation of O2, we identified a reasonable dissociation pathway, which proceeds from the initial t-b-t state to the creation of two oxygen atoms that occupy a set of neighboring fcc sites. Furthermore, our analysis indicates that the adsorption of O2 on the Ag3Au(111) surface is less favored thermodynamically and more difficult to dissociate than that on the Ag(111) surface. This study furnishes a theoretical framework elucidating the prospective utilization of Ag-Au alloy in the capacity of oxygen reduction catalysts.

Dissociative Adsorption of Hydrogen Molecules at Al2O3 Inclusions in Steels and Its Implications for Gaseous Hydrogen Embrittlement of Pipelines

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H2) at specific “active sites”, such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as “active sites” to cause the dissociative adsorption of H2. Al2O3 is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the α-Al2O3(0001)/α-Fe(111) interface on the Fe011¯ plane was studied by density functional theory calculations. The impact of gas components of O2 and CH4 on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al2O3 inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al2O3 side. There is a greater tendency for the occurrence of dissociative adsorption of O2 and CH4 than of H2, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O2 is preferential over that of CH4. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface.

Graphene/Hexagonal Boron Nitride Heterostructures for O2 Activation and CO Oxidation: Metal-Free Catalysts by Design.

Pristine graphene and h-BN monolayers are chemically inert to oxygen and thus exhibit very limited catalytic activity toward O2 activation. Herein, we show that graphene/h-BN heterostructures exhibit a surprising O2 activation capability. We theoretically designed ten graphene/h-BN heterostructures with three types of interfaces and investigated their catalytic activities toward O2 activation and CO-oxidation. In general, O2 can be molecularly chemisorbed and activated on electron-rich graphene/h-BN heterostructures. Electron-deficient graphene/h-BN heterostructures can lead to dissociative O2 adsorption with relatively low dissociation energy barriers (<0.4 eV). For CO-oxidation, the computed energy barrier can be as low as 0.67 eV. The high catalytic activities toward O2 stem from either electron-deficient heterostructures' accumulated electrons or electron richness and low work function for the electron-rich heterostructures. Although the catalytic activities of graphene/h-BN heterostructures depend strongly on the interface type, they are insensitive to the patterns of BN-substitutes, hence benefiting applicability of a wide range of heterostructures.

Illuminating Oxygen Reduction Reaction Kinetics in High Temperature Polymer Electrolyte Membrane Fuel Cells Using EIS

Owing to their notable power efficiency and zero greenhouse emissions, High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) hold immense promise as a prospective candidate for generating clean energy. However, their commercialization is still impeded by the cathodic Oxygen Reduction Reaction (ORR) kinetics, which after almost half a century not only remain elusive, but constitute the most energy-intensive process, requiring large amounts of the expensive Pt catalyst to mitigate.In this work we provide solid experimental evidence, that the simple three elementary step Dissociative Adsorption Pathway (DAP) with two intermediate species (Oad and OHad), can accurately capture both the steady state (IV) and the Electrochemical Impedance Spectra (EIS) as well.Detailed EIS and IV experimental data were recorded at 180 °C under differential conditions, using a single serpentine HTPEMFC with a 4 cm2 active area. The fuel cell operated at the activation regime, where ORR potential losses dominate but mass transport limitations are negligible. The experimental data were successfully fitted with the derived analytic IV and EIS microkinetic model.The use of the DAP in accordance with the Transitions State Theory (TST), allowed the extraction and analysis of ORR kinetics and energetics. The deconvoluted EIS outlined the important role of the intrinsic kinetic inertia , i.e. the outcome of the competitive nature of the elementary reaction steps on the adsorbed intermediate species. Kinetic inertia not only dominated EIS and the polarization resistance, but demonstrated that the commonly used Butler Volmer and apparent Tafel slope kinetic approach is incapable of describing the multistep ORR kinetics. With the help of the Degree of Rate Control (DRC) analysis, we successfully identified the O2(g) dissociative adsorption as the rate limiting step.Finally, from an energetics perspective, by employing the TST kinetics formulation, we were able to calculate the free Gibbs’ transition states activation (kinetics) and reaction steps’ energies (thermodynamics). These results clarified that, the high ORR overpotential losses stem from the combined strength of both kinetically and thermodynamically imposed barriers, which originate from the high bonding strength of Oad on the Pt surface and the rather high activation energy of the O2(g) adsorption step.

Oxygen vacancy-regulated atomic dispersed Au on CeFeOx for preferential oxidation of CO in H2-rich stream

CO preferential oxidation in H2-rich stream (CO-PROX) is a critical reaction for H2 purification in emerging hydrogen economy including advanced fuel cell technology. Au-based catalysts are viewed as some of the most promising catalysts but achieving high catalytic efficiency and selectivity is still challenging. Defect engineering is deemed as a powerful strategy to improve the intrinsic catalytic activity. Herein, CeFeOx-supported Au catalysts with controllable oxygen vacancy (Ov) concentrations were obtained and applied to CO-PROX. The results of characterizations showed that the Ovs were preferentially generated in conjunction with Au single atoms (SAs) rather than nanoparticles (NPs), which further modulated the surface electronic properties via surplus electrons in Ovs and enhanced the interfical interaction between Ovs and Au SAs. The CO-PROX results indicated that the reduced Au SA catalyst (Au1/CeFeOx-R) with the highest Ov concentration completely converted CO at temperature of 80 °C with relatively low reactivity towards H2 oxidation. The remarkable catalytic activity and selectivity were attributed to the modified electronic properties by Ovs that played a bridging role in electron transfer and accelerated reaction process, as well as the coupling effect of Ovs and Au SAs that enhanced the dissociative adsorption of O2 and the adsorption-oxidation of CO, resulting in a reduced reaction energy barrier and making CO oxidation more favorable than H2 oxidation. This research opens up an alternative strategy to simultaneously stabilize and activate single-atom catalysts.

Illuminating Oxygen Reduction Reaction Kinetics in High Temperature Polymer Electrolyte Membrane Fuel Cells Using EIS

High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) commercialization, is impeded by the sluggish, power intensive and elusive cathodic Oxygen Reduction Reaction (ORR) kinetics. In this work we provide solid experimental evidence that the simple three elementary step Dissociative Adsorption (DA) pathway, with two intermediate species (Oad and OHad), can accurately describe the steady state (IV) and the Electrochemical Impedance Spectra (EIS) response of a HTPEMFC. Deconvoluted EIS outlined the important role of Intrinsic Kinetic Inertia, which dominated both EIS and polarization resistance. The Degree of Rate Control (DRC) analysis, identified the O2(g) dissociative adsorption as the rate limiting step. Finally, Transition State Theory (TST) allowed the extraction and analysis of ORR energetics, demonstrating that the high ORR overpotential losses originate from the combined strength of both kinetically and thermodynamically imposed barriers, due to the high bonding strength of Oad on Pt and the high activation energy of O2(g) adsorption.

Mechanistic study on the effect of ammonia co-firing with pulverized coal on NO formation and reduction

In this study, the heterogeneous formation and reduction mechanisms of NO by ammonia (NH3) were determined to evaluate the performance of NH3 co-firing with coal in NO emissions by density functional theory and canonical variational transition-state theory. NO formation is initialized by the dissociative adsorption of O2 on the char, and the subsequent dehydrogenation reactions after NH3 adsorption are prioritized. NO reduction via char synergism with NH3 are also elucidated by the NNH dissociation or ONNH conversion. Thermodynamic and kinetic results indicate that introducing NH3 has the advantage of promoting heterogeneous NO reduction, which is characteristic of the synergistic effect afforded by the properties of char and reducing agent NH3. The reduction effect of NH3 on NO is more significant than its oxidation effect for NO formation at elevated temperatures, which provides theoretical evidence for the potential advantages of NH3 co-firing on NO emissions in coal-fired boilers.

One-dimensional γ-Al2O3 growth from the oxidation of NiAl

We report an oxide nanowire growth phenomenon that morphologically resembles the vapor-liquid-solid process but with a fundamentally different mechanism. This is demonstrated with the high-temperature NiAl oxidation that results in the unidirectional γ-Al2O3 growth with a NiO nanoparticle at the nanowire tip. The NiO nanoparticle surface serves as the active site for dissociative O2 adsorption that produces an inward flux of atomic oxygen toward the γ-Al2O3/NiAl interface for γ-Al2O3 growth by reacting with bottom-up diffusing Al3+ cations. The nanowire lengthening follows the parabolic kinetics with the upward lattice diffusion of Al3+ cations in the γ-Al2O3 nanowires as the rate-limiting factor.

Enhanced degradation of few-layer black phosphorus by fulvic acid: Processes and mechanisms

The oxidation of the emerging nanomaterial black phosphorus (BP) affected by pH and oxygen has been carefully documented. However, in natural waters, there is a large amount of chemically reactive organic matters like fulvic acid (FA), whose impacts on degradation and stability of few-layer BP or BP nanosheets (BPNS) are scarcely disclosed. Hence, we investigated the kinetics of BPNS degradation products (H2PO2−, HPO32−, and PO43−) in the presence of FA. The results showed that the apparent reaction rate constants of BPNS were 0.026, 0.050, and 0.060 d−1 under oxygen-and-light condition and 0.005, 0.016, and 0.023 d−1 under hypoxia-and-darkness condition at FA gradients of 0, 2.5, and 5 mgC/L, respectively. Microscopic observations, simple molecular simulation experiment, and density functional theory computation explained that FA significantly enhanced the degradation of P atoms on the BPNS surface through the indirect pathway of reducing the energy barrier of O2 dissociative adsorption and the direct pathway of chemical adsorption, which caused the P-P bond on the BPNS surface to break down and formed P-O bonds or C-P bonds. This study revealed for the first time the degradation mechanism of BPNS in the presence of FA, which is a chemical mechanism of the BPNS transformation behavior. It helps to make a more scientific risk assessment of BP in natural waters.

Crystal face-dependent pyrite oxidation: An electrochemical study

The oxidation of pyrite has attracted significant attention due to its predominant contribution to the serious environmental issue of acid mine drainage (AMD). However, relatively few studies have been conducted investigating the oxidation characteristics of individual pyrite crystal faces. The oxidation rates, under acidic, neutral and alkaline conditions, of three typical pyrite surfaces have been examined with their contrasting behaviors explored in terms of surface structure. Electrochemical and surface analytical measurements were conducted across the pH range 1 to 11, showing surface formation of Fe(OH)3, S0 and SO42−. Tafel plots, electrochemical impedance spectroscopy and surface analyses demonstrate that the oxidation rate follows the order of {210} > {111} > {100}. This rate trend is directly related to the breakage of bonds on surface formation with formation of the {210} surfaces requiring the greatest bond breakage; hence this surface is the most unstable, and the {100} surface owns the least bond breakage and the greatest stability. The oxidation rate of all pyrite surfaces under strongly acidic conditions (pH = 1) is greater than for strongly alkaline conditions (pH = 11), with the slowest rates being observed under neutral condition, since S0, iron oxyhydroxide (Fe-OOH) and Fe(OH)3 formed under neutral or alkaline conditions cover the pyrite surface, thus inhibiting further oxidation. However, a more aggressive oxidant of hydroxo–Fe(III) complex is present under alkaline conditions, resulting in greater oxidation rate compared to that under neutral conditions that determined by the oxidation of Fe2+ by O2. DFT calculations suggest that H2O has the greatest affinity for the {210} surface as compared to the {100} and {111} surfaces but does not dissociate. However, after the dissociative adsorption of O2, H2O can dissociate on all the pyrite surfaces, enhancing further oxidation. Under acidic conditions this process is likely to be rate determining. Under alkaline and neutral conditions the oxidation process is dependent on the transfer of hydroxyls to the surface S group and surface cycling of Fe(II) − Fe(III).

Roles of excess minority carrier recombination and chemisorbed O2 species at SiO2/Si interfaces in Si dry oxidation: Comparison between p-Si(001) and n-Si(001) surfaces.

This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO2/Si interfaces is associated with O2 dissociative adsorption; (2) the x-ray induced enhancement of SiO2 growth is not caused by the band flattening resulting from the surface photovoltaic effect but by the electron-hole pair creation resulting from core level photoexcitation for the spillover of bulk Si electronic states toward the SiO2 layer; and (3) a metastable chemisorbed O2 species plays a decisive role in combining two types of the single- and double-step oxidation reaction loops. Based on experimental results, the unified Si oxidation reaction model mediated by point defect generation [S. Ogawa et al., Jpn. J. Appl. Phys., Part 1 59, SM0801 (2020)] is extended from the viewpoints of (a) the excess minority carrier recombination at the oxidation-induced vacancy site and (b) the trapping-mediated adsorption through the chemisorbed O2 species at the SiO2/Si interface.

Unraveling the elusive oxygen reduction reaction electrokinetics and energetics in PEM fuel cells

Unraveling the elusive electrokinetics and energetics of the sluggish Oxygen Reduction Reaction (ORR) is critical for the development of efficient Proton Exchange Membrane Fuel Cells (PEMFCs). Physical microkinetic modeling, Transition State Theory (TST), Transmission Line Modeling and Degree of Rate Control (DRC) analysis was employed to simulate analytically both the experimental Electrochemical Impedance Spectra (EIS) and the polarization curve data of a High Temperature PEMFC. The excellent fitting results revealed that: i) O2(g) dissociative adsorption is the main limiting step responsible for the high ORR overpotentials, ii) both EIS and polarization resistance are dominated by the intrinsic ORR kinetic inertia due to the competitive nature of the elementary reaction steps on the coverages of the adsorbed species (Oad and OHad). Finally, from TST the activation energies (kinetics) and the reaction steps’ free energies (thermodynamics) were extracted, which can serve as a guide for catalyst design optimizing the oxygen binding strength.

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Catalytic Oxidation of Methane on IrO2(110) Films Investigated Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The catalytic oxidation of CH4 over IrO2(110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2(110) films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500–650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CHyO2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CHyO2 surface species became optimal at an intermediate composition of the reactant mixture (∼90% CH4), consistent with a site competition between CH4 and O2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2(110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes.

Atomistic simulations of surface reactions in ultra-high-temperature ceramics: O[formula omitted], H[formula omitted]O and CO adsorption and dissociation on ZrB[formula omitted](0001) surfaces

Understanding surface reactivity is crucial in many fields, going from heterogeneous catalysis to materials oxidation and corrosion. In order to decipher the surface reactions of ZrB2 exposed to the harsh environment of aerospace components, the chemical activity of both Zr- and B-surfaces is predicted and compared by using density functional theory and nudged elastic band methods. In particular the adsorption, dissociation and diffusion of O2, CO and H2O are extensively examined through the calculation of surface adsorption energies and reaction pathways. We find the dissociative adsorption of O2 dominating the reactivity of ZrB2 surfaces, while the dissociation of H2O and CO is weakly active on Zr-surfaces, and even less activated on B-terminated ones. Importantly, we discover that the reaction of O2 and CO can trigger strong surface reconstruction at B-surfaces. Our work thus provides significant insights into the diverse adsorption and reaction mechanisms on ZrB2 surfaces.

A novel approach for metal extraction from metal sulfide ores with NH4Cl: A combined DFT and experimental studies

Chalcocite (Cu2S) is the main source for metallurgical production of copper worldwide but it usually co-exist with nickel sulfide in nature. To develop an efficient and green process for the separation and extraction of valuable metals from high-grade matte, we performed density functional theory (DFT) calculations and experimental studies to elucidate chlorination mechanism of Cu2S with ammonium chloride (NH4Cl). First, NH4Cl can react with Cu2S directly in the absence of O2, leading to the formation of H2S and NH3 with negative adsorption energies. Second, O2 can promote the chlorination process by providing adsorbed O and promoting the dissociation of HCl decomposed from NH4Cl, leading to the formation of H2O and CuCl species eventually. Besides, SO2 generated at the Cu2S surface after O2 dissociative adsorption, may leading to the generation of Cl2. Finally, the chlorination can also be achieved by the interaction between Cu2S and Cl2, which is favored thermodynamically and kinetically. Based on the computational results, low-temperature roasting experiments were performed. Cu2S was successfully converted into the corresponding metal chloride (CuCl) by NH4Cl in the absence of O2, but the chlorination process is incomplete with unreacted Cu2S. However, the conversion from Cu2S to CuCl is complete under air atmosphere, consistent with our computational results. Furthermore, metallic copper was successfully obtained at the cathode through electrodeposition with CuCl. The present study reveals detailed reaction mechanism between Cu2S and NH4Cl, and shows the chlorination of Cu2S by NH4Cl followed by electrodeposition of the resulting CuCl as a highly efficient and green process.

Angstrom-Thick Oxide Overcoat on Decorated Nanoclusters for Enhanced Performance and Durability of SOFC Cathodes

A solid oxide fuel cell (SOFC) is an energy conversion system with high conversion efficiency, high power density and fuel flexibility.1 Since its durability and cost issues are largely originated from the high temperature operation, intense efforts have been made to reduce the operational temperature to an intermediate range of 600 – 800 °C.2 The decrease in temperature, however, results in a significant decrease in the electrochemical kinetics in particular of the oxygen reduction reaction (ORR) occurring at the cathode.3,4 Atomic layer deposition (ALD) is an emerging low-temperature chemical vapor deposition variant that can deposit well-dispersed islands or uniform films at the atomic scale even on a highly corrugated geometry.8,9 By leveraging the capability, researchers have successfully improved SOFC durability mainly by suppressing the agglomeration of high-surface-area electrodes10–13 and/or dopant segregation to the surface of perovskite-based cathodes.14,15 Considering the capability of ALD to perform a conformal atomic-scale treatment on complex geometry, an ALD treatment can be best leveraged when applied to a structure of extreme surface area as opposed to conventional porous backbone structures. However, its application to a high-surface-area electrodes (e.g. infiltrated electrodes) and associated analysis is rarely reported.In this presentation, we demonstrate that a uniform atomic-scale oxide overcoat (either ceria or yttria with a nominal thickness of 0.7 – 1.5 Å) by ALD is highly effective not only in the enhancement of thermal stability of underlying ceria nanoparticles (5 – 20 nm) infiltrated on LaNi0.6Fe0.4O3- d (LNF) backbone (~100 nm) but also in the facilitation of oxygen reduction kinetics. Since LNF does not suffer from Sr segregation, a major degradation mechanism,17 the analysis of cell performance degradation is simplified. Based upon electrochemical and physical characterization of a series of infiltrated and/or ALD-treated samples, we discuss the impact of surface treatment on the nanoscale morphology, surface chemistry and electrode performance.We provide a quantitative analysis of the thermal agglomeration of NPs with and without ALD treatment and prove the close correlation between the NP agglomeration and electrode performance degradation. To the best of our knowledge, this is the first work showing the correlation in a quantitative manner. In addition, we demonstrate that an angstrom-scale ALD overcoat dramatically enhances the electrode performance in terms of polarization resistance and its activation energy for both bare LNF and ceria-infiltrated LNF electrodes. The improved electrode activity from ALD treatment is mainly ascribed to a significant facilitation of O2 dissociative adsorption with a partial reduction, with the aid of surface-specific oxygen-deficient and catalytically active ceria.The authors acknowledge the support from the Korea Institute of Industrial Technology (KITECH) and U.S. National Science Foundation CAREER Award (DMR 1753383). The use of TEM at the Molecular Foundry was supported by the U.S. Department of Energy (Contract No. DE-AC02-05CH11231). The XPS work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (Contract DE-AC52-07NA27344). Sossina M. A high-performance cathode for the next generation of solid-oxide fuel cells, 431, 170, 2004Kang H., Grewal S., Li H., Lee M. H. Effect of Surface-Specific Treatment by Infiltration into LaNi6Fe4O3-δ Cathodic Backbone for Solid Oxide Fuel Cells, 166, 255, 2019

Probing the structural, electronic, and adsorptive properties of Au16O2- clusters.

Great progress has been made in O2 adsorption on gold clusters. However, systematic investigations of O2 adsorption on [Formula: see text] clusters have not been reported. Here, we present a systematic study of the structural, electronic, and adsorptive properties of [Formula: see text] clusters by density functional theory (DFT) calculations coupled with stochastic kicking method. Global minimum searches for [Formula: see text] reveal that exohedral derivatives are more favored. Furthermore, the obtained ground-state structure exhibits significant stability, as judged by its larger adsorption energy (1.16eV) and a larger HOMO-LUMO gap (0.57eV). The simulated photoelectron spectra (PES) of [Formula: see text] isomers will be instructive to identify the structures in future experiments. There are three interesting discoveries in the present paper: (1) O2 undergoes chemical adsorption onto the parent [Formula: see text] clusters, but the amount of the adsorption energy is related to the parent [Formula: see text] clusters; (2) the process that O2 undergoes dissociative adsorption onto the parent [Formula: see text] clusters is exothermic; (3) [Formula: see text] isomers show smaller X-A energy gaps than those of parent [Formula: see text] clusters, reflecting that their geometric and electronic structures are distorted remarkably due to dissociative adsorption of O2.

Synthetically Tuned Pd-Based Intermetallic Compounds and their Structural Influence on the O2 Dissociation in Benzylamine Oxidation.

Intermetallic compounds (IMCs) have diverse electronic and geometrical properties to offer. However, the synthesis of intermetallic nanoparticles is not always easy; developing new methodologies that are conventional for many systems can be challenging, especially when incorporating highly electropositive metals to reduce to IMCs using solution synthesis methodologies. In this study, we report a comprehensive approach to access nanocrystalline PdxMy (M = Cu, Zn, Ga, Ge, Sn, Pb, Cd, In) intermetallic (IM) via the coreduction method employing sodium borohydride as the reductant. A combination of diffraction, spectroscopic, and microscopic techniques were performed to characterize the formed nanoparticles in terms of their phase composition, purity, particle size distribution, and surface oxidation properties of metals, respectively. IMCs of Pd with the elements such as Cu, Zn, Ga, and Ge exhibited higher catalytic activity that with elements such as In, Sn, Pb, and Cd. The DFT studies on these compounds revealed that the adsorption of benzylamine at the Pd site and the dissociative adsorption of O2 on the IM surface play a significant effect on catalytic activity. Among them, PdCu IM exhibited an excellent conversion of benzylamine (94.0%), with 92.2% of dibenzylimine selectivity compared to other IMCs. Moreover, PdCu exhibited decent recyclability and activity for the oxidation of different substituted primary amines.

Detailed description of pulse isotopic exchange method for analyzing oxygen surface exchange behavior on oxide ion conductors

A novel pulse 18O-16O isotopic exchange (PIE) technique for measurement of the rate of oxygen surface exchange of oxide ion conductors was presented. The technique employs a continuous flow packed-bed micro-reactor loaded with the oxide powder. The isothermal response to an 18O-enriched pulse passing through the reactor, thereby maintaining chemical equilibrium, is measured by on-line mass spectrometry. Evaluation of the apparent exchange rate follows from the uptake of 18O by the oxide at given reactor residence time and surface area available for exchange. The developed PIE technique is rapid, simple and highly suitable for screening and systematic studies. No rapid heating/quenching steps are required to facilitate 18O tracer anneal or analysis, as in other commonly used techniques based upon oxygen isotopic exchange. Moreover, the relative distribution of the oxygen isotopologues 18O2, 16O18O, and 16O2 in the effluent pulse provides insight into the mechanism of the oxygen exchange reaction. The PIE technique has been demonstrated by measuring the exchange rate of selected oxides with enhanced oxide ionic conductivity in the range of 350-900 ° C. Analysis of the experimental data in terms of a model with two consecutive, lumped steps for the isotopic exchange reaction shows that for mixed conductors Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La2NiO4+δ the reaction is limited by the apparent rate of dissociative adsorption of O2 molecules at the oxide surface. For yttria-stabilized zirconia (YSZ), a change-over takes place, from rate-limitations by oxygen incorporation below ~800 ° C to rate-limitations by O2 dissociative adsorption above this temperature. Good agreement is obtained with exchange rates reported for these materials in literature.