Crystal Structure, Diffusion Path, and Oxygen Permeability of a Pr₂NiO₄-Based Mixed Conductor (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ

We have investigated in situ the crystal structure, oxygen diffusion path, oxygen permeation rate, and electrical conductivity of a doped praseodymium nickel oxide, Pr2NiO4-based mixed conductor, (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ (PLNCG) in air between 27 °C and 1015.6 °C. The PLNCG has a tetragonal I4/mmm K2NiF4-type structure which consists of a (Pr0.9La0.1)(Ni0.74Cu0.21Ga0.05)O3 perovskite unit and a (Pr0.9La0.1)O rock salt unit in the whole temperature range. Both experimental and theoretical electron density maps indicated two-dimensional networks of (Ni0.74Cu0.21Ga0.05)-O covalent bonds in PLNCG. Highest occupied molecular orbitals (HOMO) in PLNCG demonstrate that the electron−hole conduction occurs via Ni and Cu atoms in the (Ni0.74Cu0.21Ga0.05)-O layer. The bulk oxygen permeation rate was high (137 μmol cm−2 min−1 at 1000 °C), and its activation energy was low (51 kJ mol−1 at 950 °C). The Rietveld method, maximum-entropy method (MEM), and MEM-based pattern fitting analyses of neutron and synchrotron diffraction data indicate a large anisotropic thermal motion of the apical O2 oxygen at the 4e site (0, 0, z; z ≈ 0.2) in the (Pr0.9La0.1)(Ni0.74Cu0.21Ga0.05)O3 perovskite unit. Neutron and synchrotron diffraction data and theoretical structural optimization show the interstitial oxygen (O3) atom at (x, 0, z) (x ≈ 0.6 and z ≈ 0.2). The nuclear density analysis indicates that the bulk oxide-ion diffusion, which is responsible for the high oxygen permeation rate, occurs through the interstitial O3 and anisotropic apical O2 sites. The nuclear density at the bottleneck on the oxygen diffusion path increases with temperature and with the oxygen permeation rate. The activation energy from the nuclear density at the bottleneck decreases with temperature, which is consistent with the decrease of the activation energy from oxygen permeation rate. The extremely low activation energy (12 kJ mol−1 at 900 °C) from the nuclear density at the bottleneck indicates possible higher bulk oxygen permeation rates in quality single crystals and epitaxial thin films

Crystal Structure, Diffusion Path, and Oxygen Permeability of a Pr₂NiO₄-Based Mixed Conductor (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ

We have investigated in situ the crystal structure, oxygen diffusion path, oxygen permeation rate, and electrical conductivity of a doped praseodymium nickel oxide, Pr2NiO4-based mixed conductor, (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ (PLNCG) in air between 27 °C and 1015.6 °C. The PLNCG has a tetragonal I4/mmm K2NiF4-type structure which consists of a (Pr0.9La0.1)(Ni0.74Cu0.21Ga0.05)O3 perovskite unit and a (Pr0.9La0.1)O rock salt unit in the whole temperature range. Both experimental and theoretical electron density maps indicated two-dimensional networks of (Ni0.74Cu0.21Ga0.05)-O covalent bonds in PLNCG. Highest occupied molecular orbitals (HOMO) in PLNCG demonstrate that the electron−hole conduction occurs via Ni and Cu atoms in the (Ni0.74Cu0.21Ga0.05)-O layer. The bulk oxygen permeation rate was high (137 μmol cm−2 min−1 at 1000 °C), and its activation energy was low (51 kJ mol−1 at 950 °C). The Rietveld method, maximum-entropy method (MEM), and MEM-based pattern fitting analyses of neutron and synchrotron diffraction data indicate a large anisotropic thermal motion of the apical O2 oxygen at the 4e site (0, 0, z; z ≈ 0.2) in the (Pr0.9La0.1)(Ni0.74Cu0.21Ga0.05)O3 perovskite unit. Neutron and synchrotron diffraction data and theoretical structural optimization show the interstitial oxygen (O3) atom at (x, 0, z) (x ≈ 0.6 and z ≈ 0.2). The nuclear density analysis indicates that the bulk oxide-ion diffusion, which is responsible for the high oxygen permeation rate, occurs through the interstitial O3 and anisotropic apical O2 sites. The nuclear density at the bottleneck on the oxygen diffusion path increases with temperature and with the oxygen permeation rate. The activation energy from the nuclear density at the bottleneck decreases with temperature, which is consistent with the decrease of the activation energy from oxygen permeation rate. The extremely low activation energy (12 kJ mol−1 at 900 °C) from the nuclear density at the bottleneck indicates possible higher bulk oxygen permeation rates in quality single crystals and epitaxial thin films

Theoretical Revisit of the Direct Synthesis of H₂O₂ on Pd and Au@Pd Surfaces: A Comprehensive Mechanistic Study

The direct synthesis of H₂O₂ from H₂ and O₂ on Pd(111) and Au@Pd(111) surfaces is studied with periodic density functional theory calculations. Ten possible reactions and processes involved in the H₂O₂ synthesis steps are considered: For O₂, (1) O₂* + H* → OOH*, (2) O₂* → 2O*, and (3) O₂* → O₂; for OOH, (4) OOH* + H* → H₂O₂*, (5) OOH* + H* → H₂O* + O*, (6) OOH* + H* → 2OH*, (7) OOH* → O* + OH*, and (8) OOH* → OOH; for H₂O_2, (9) H₂O₂* → 2OH* and (10) H₂O₂* → H₂O₂, where the asterisks indicate these species to be surface species. All side reactions involve O–O bond dissociation. On the Pd(111) surface with H atoms coadsorbed, O₂ dissociation is suppressed; OOH dissociation is more favorable than all OOH hydrogenation reactions; three OOH hydrogenation reactions have comparable activation barriers; the barrier for H₂O₂ dissociation is also comparable to that for H₂O₂ desorption. However, on the H atoms coadsorbed Au@Pd(111) surface, the main reactions for H₂O₂ production exceed all side reactions. The competition between the main reactions and the side reactions is actually the competition between the O–O bond and the O–M bond, where M is Pd in the case of the Pd(111) surface and Au in the case of the Au@Pd(111) surface. The O–Pd bond is usually stronger than the O–O bonds in the OOH intermediate and H₂O₂; however, the O–Au bond is weaker than the O–O bonds. Consequently, the final product H₂O₂ is easily produced and released from the Au@Pd(111) surface, and the side reactions involving O–O bond dissociation are suppressed. The role of the metal surface in the direct synthesis of H₂O₂ from H₂ and O₂ is to provide H atoms as the feedstock for the hydrogenation of O₂

Doped CeO₂–LaFeO₃ Composite Oxide as an Active Anode for Direct Hydrocarbon-Type Solid Oxide Fuel Cells

Direct utilization of hydrocarbon and other renewable fuels is one of the most important issues concerning solid oxide fuel cells (SOFCs). Mixed ionic and electronic conductors (MIECs) have been explored as anode materials for direct hydrocarbon-type SOFCs. However, electrical conductivity of the most often reported MIEC oxide electrodes is still not satisfactory. As a result, mixed-conducting oxides with high electrical conductivity and catalytic activity are attracting considerable interest as an alternative anode material for noncoke depositing anodes. In this study, we examine the oxide composite Ce(Mn,Fe)O2–La(Sr)Fe(Mn)O3 for use as an oxide anode in direct hydrocarbon-type SOFCs. High performance was demonstrated for this composite oxide anode in direct hydrocarbon-type SOFCs, showing high maximum power density of approximately 1 W cm–2 at 1073 K when propane and butane were used as fuel. The high power density of the cell results from the high electrical conductivity of the composite oxide in hydrocarbon and the high surface activity in relation to direct hydrocarbon oxidation

Self-Ordering of Disorderly Arranged 2D Crystal Layers to 3D Regular Arrangement Using a Heat-Induced Chemical Reaction between 2D Crystal Layers

Two-dimensional (2D) materials with a thickness of ∼1 nm are candidate nanobuilding blocks to fabricate electronic devices with a three-dimensional (3D) structure using a bottom-up technology. They can be stacked in a precisely controlled hierarchical structure with a controlled number of building layers. However, the atomic arrangements between individual stacked 2D crystal layers are generally not ordered as in a single crystal. The interface and the disordered atomic arrangements result in decrease in the performance of electronic devices prepared from 2D crystals, because the electron flow between 2D crystals is blocked by the interface and the disordered atomic arrangements. Therefore, ordered atomic arrangement of the stacked layers is one of the most critical challenges in the preparation of 3D electronic devices from 2D materials. Here, a successful example of self-ordering of disorderly arranged 2D crystal layers to 3D regular arrangement is described. The multilayer films of nickel hydroxide 2D crystal with a thickness of one NiO₆ octahedral unit was focused as the disorderly arranged 2D crystal layers. The 2D layered films deposited on a substrate were heated to 400 °C. This heat treatment converted the disordered 2D system to ordered 3D NiO with (111)-orientation. The heat-induced chemical reaction between 2D materials allowed the disordered layers to self-order to 3D regular arrangement. The NiO film exhibited a photocathodic current assigned to reduction of water, and then the photocurrent increased with increasing the number of layers. The improvement of the photocurrent property is due to the ordered atomic arrangements without interface

Immobilizing Metal Nanoparticles on Single Wall Nanotubes. Effect of Surface Curvature

One to several nanometer-size nanoparticles possess supreme catalytic activity for a variety of important synthetic reactions compared to larger particles and bulk surfaces. However, a significant drawback is the catalyst durability as small, active nanoparticles tend to merge to form larger, less active nanocolloids. Tailoring the nanoparticle–surface support interaction could provide a means to limit nanoparticle mobility and thus prevent aggregation. In this study, we demonstrate the stabilization of fine-metal nanoparticles on nanotube surfaces by manipulation of surface curvature. Systematic density functional theory calculations of a large variety of nanoparticle–nanotube complexes revealed that the nanoparticle–nanotube binding interaction depends on, and can be controlled by, the surface curvature. Thus, an effective mechanism is demonstrated for the immobilization of small metal clusters with high catalytic activity on support surfaces. Furthermore, we provide experimental verification of our theory by comparing the aggregation of palladium nanoparticles decorating carbon nanotube and graphene surfaces as a function of time. Our theoretical predictions and experimental observations provide fundamental understanding to the physics of nanoparticle–support interaction and demonstrate how tailoring the support geometry can improve the durability of high-performance nanocatalysts

Role of Ga³⁺ and Cu²⁺ in the High Interstitial Oxide-Ion Diffusivity of Pr₂NiO₄‑Based Oxides: Design Concept of Interstitial Ion Conductors through the Higher-Valence d¹⁰ Dopant and Jahn–Teller Effect

We have investigated the crystal structure, nuclear- and electron-density distributions, electronic structure, and oxygen permeation rate of three K2NiF4-type oxides of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ, Pr2Ni0.75Cu0.25O4+δ, and Sr2Ti0.9Co0.1O4–ε, in order to study the role of d10 Ga3+, Jahn–Teller Cu2+, and interstitial oxygen O3 in the high oxygen diffusivity of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ. The composition Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ has a larger amount of interstitial oxygen O3 atoms (δ = 0.31 at room temperature (RT)) compared with Pr2Ni0.75Cu0.25O4+δ (δ = 0.19 at RT) and the oxygen deficient Sr2Ti0.9Co0.1O4–ε (ε = 0.02 at RT). The interstitial O3 atom is stabilized by (1) the substitution of (Ni,Cu)2+ by higher valence Ga3+, (2) static atomic displacements of the apical O2 oxygen, and (3) local relaxation near d10 Ga3+. Nuclear-density distributions of Pr2(Ni0.75Cu0.25)0.95Ga0.05O4+δ and Pr2Ni0.75Cu0.25O4+δ at high temperatures have visualized the −O2–O3–O2– diffusional pathway of oxide ions, which indicates an interstitialcy diffusion mechanism. Doping of the Jahn–Teller Cu2+ in Pr2NiO4+δ stabilizes the high-temperature disordered tetragonal I4/mmm phase and makes the apical O2 atoms more mobile. The apical O2 is more mobile compared to the equatorial O1, because the longer covalent (Ni,Cu,Ga)–O2 bond is weaker than the shorter (Ni,Cu,Ga)–(equatorial O1) one, as evidenced by the experimental and theoretical electron-density analysis. The interstitial O3 is more mobile due to the lower coordination number (CN = 4) compared with the lattice O1 and O2 atoms (CN = 6). It was found that the minimum nuclear density on the O2–O3 pathway ρN(T) is a useful microscopic parameter for the oxygen diffusivity. The ρN(T) is regarded as the oxygen probability density at the bottleneck for diffusion. The oxygen permeation rate ρP(T) increases with an increase of ρN(T). The activation energy for oxygen diffusion estimated by the plots of log (the normalized oxygen permeation rate ρP(T)/δ) against T–1 (reciprocal of absolute temperature) is relatively independent of temperature as well as the formation energy of oxygen atoms at the bottleneck from the plots of log­(ρN(T)/δ) against T–1. These results indicate that the amount of interstitial oxygen δ is proportional to the carrier concentration for the oxide-ion diffusion. Doping of higher-valence Ga3+ at (Ni,Cu)2+ site in Pr2Ni0.75Cu0.25O4+δ does not change largely the activation energy for the oxygen permeation and formation energy of oxygen atoms at the bottleneck but increases the amount of excess interstitial oxygen (carrier concentration), which yields the high oxygen permeation rate of 262 μ mol min–1 cm–2 in Pr2(Ni0.75Cu0.25)0.95Ga0.05O4.13 at 900 °C. The present work demonstrates the design concept of interstitial ion conductors through the higher-valence d10 dopant and Jahn–Teller effect

Theoretical Study of the Direct Synthesis of H₂O₂ on Pd and Pd/Au Surfaces

The direct synthesis of hydrogen peroxide on Pd and Pd/Au catalysts was investigated with first-principle DFT methods for periodic two-dimensional surfaces. A two-step reaction mechanism was proposed starting from a superoxo precursor state of the dioxygen molecule on Pd surface and its subsequent reaction with two hydrogen atoms situated over neighboring 3-fold positions. A competitive reaction of dioxygen dissociation leading to the nonselective formation of water was found. We have shown that the presence of surface gold atoms blocks this dissociation and increases the selectivity toward the main product, H2O2, which explains the experimentally reported data

Theoretical Study of the Decomposition and Hydrogenation of H₂O₂ on Pd and Au@Pd Surfaces: Understanding toward High Selectivity of H₂O₂ Synthesis

Three possible pathways for the conversion of hydrogen peroxide to water on Pd and Au@Pd catalysts are investigated with periodic density functional theory calculations: (1) the decomposition of H2O2 (H2O2 → H2O + O), including the dissociation of H2O2 to two OH groups (H2O2 → 2OH) and the disproportionation of two OH groups to water and oxygen (OH + OH → H2O + O); (2) the hydrogenation of the OH group to water (OH + H → H2O); and (3) the direct hydrogenation of H2O2 to water (H2O2 + 2H → 2H2O). The results show that the decomposition of H2O2 and the hydrogenation of OH groups are two available channels for the formation of water, and the former plays a main role. A key step in the overall process is the dissociation of H2O2, which is facile and irreversible. The direct hydrogenation of H2O2 to water has a very high activation barrier and is unlikely to occur. The competitions between the dissociation of H2O2 and the release of H2O2 on Pd and Au@Pd surfaces are analyzed. The high selectivity of H2O2 synthesis cannot be explained simply by the relatively increased barrier for H2O2 dissociation on the Au@Pd surface. Actually, the less active Au atoms on the Au@Pd surface weaken the interaction of the metal surface with H2O2, and thus suppress the dissociation of H2O2, and, on the other hand, facilitate the release of H2O2. The opposite effects of Au atoms on the dissociation and release of H2O2 move the balance to the release side, which is responsible for the high H2O2 selectivity of the Au@Pd catalysts. The effects of the unreacted H atoms are also considered. It is found that the H atoms coadsorbed on Pd and Au@Pd surfaces can decrease the interaction between the metal surfaces and H2O2 as well and, consequently, facilitate the release of H2O2 and suppress the dissociation of H2O2

Oxygen Activation on Nanometer-Size Gold Nanoparticles

The structure, electronic properties, and catalytic activity toward oxygen activation of gold nanoclusters with size between 10 and 42 atoms were investigated with first principle methods. Nanoparticle symmetry, bond lengths, and surface charge distribution were analyzed and compared to those of macroscopic gold surfaces. Irregular charge distribution was found on the surfaces of nanoparticles consisting of fewer than 30 gold atoms. Nanoparticles with more than 30 atoms were characterized with core–shell charge separation, e.g, positively charged core and negatively charged surface. The charge distribution on those nanoparticles significantly differs from the charge distribution on macroscopic gold surface. The structure and electronic properties of the gold nanoparticles were related to their catalytic activity toward the aerobic oxidation of organic molecules, e.g., cyclohexane. It was found that oxygen is activated by partially negatively charged surface gold atoms. Nanoparticles with sizes between 10 and 30 gold atoms could only activate oxygen over the negatively charged surface active sites, whereas larger nanoparticles could activate oxygen over the whole surface. The results are in good agreement and provide detailed understanding of recently published experimental data of aerobic oxidation on subnanometer gold nanoparticles (<i>ACS Catal.</i> <b>2011</b>, <i>1</i>, 2–6)