Methane Chemisorption on Oxide-Supported Pt Single Atom

Methane chemisorption has been recently demonstrated on the rutile IrO₂(110) surface. However, it remains unclear how the general requirements are for methane chemisorption or complexation with a single atom on an oxide surface. By exploring methane adsorption on Pt₁ substitutionally doped on many rutile-type oxides using hybrid density functional theory, we show that the occupancy of the Pt d_z² orbital is the key to methane chemisorption. Pt single atom on the semiconducting or wide-gap oxides such as TiO₂ and GeO₂ strongly chemisorbs methane, because the empty Pt d_z² orbital is located in the gap and can effectively accept σ-electron donation from the methane C−H bond. In contrast, Pt single atom on metallic oxides such as IrO₂ and RuO₂ does not chemisorb methane, because the Pt d_z² orbital strongly mixes with the support-oxide electronic states and become more occupied, losing its ability to chemisorb methane. This study sheds further light on the impact of the interaction between a Pt single atom and the oxide support on methane adsorption.     

Theory of electronic structure of oxide surfaces

Recent theoretical studies on the electronic structures of oxide surfaces by the DV-Xα cluster method are reviewed. The aim is to elucidate the major material and structural factors governing the surface electronic structure. Special attention is paid to the properties of the electronic states which are related to the surface chemical activities. To give a comprehensive picture of the electronic structure of oxide surfaces, topics are selected from several different types of oxides including MgO, MO (where M denotes 3D transition metals), TiO₂, SrTiO₃, ReO₃, ZnO. Electronic structure of oxygen chemisorbed on metals, which is the initial step of the oxide formation, is also studied. The important role of the partially filled d band and the influence of the chemisorption geometry is clarified in detail. Oxygen vacancy and other surface imperfections introduce localized electronic states, often with the energy level deep in the gap. They form active centers in the surface chemical processes. The charge compensation mechanism and the related peculiar properties of the polar surfaces are discussed. The chemisorption of oxygen, proton and hydroxyl on the oxide surface is investigated and the mechanism of the acidic and basic sites are discussed.     

Enhancement of Lewis Acidity of Cr-Doped Nanocrystalline SnO2: Effect on Surface NH3 Oxidation and Sensory Detection Pattern

Understanding ammonia oxidation over metal oxide surfaces is crucial for improving its detection with resistive type gas sensors. Formation of NO_x during this process makes sensor response and calibration unstable. Cr-doping of nanocrystalline metal oxides has been reported to suppress NO₂ sensitivity and improve response towards NH₃, however the exact mechanism of such chromium action remained unknown. Herein, by using EPR spectroscopy we demonstrate formation of Cr(VI) lattice defects on the surface of nanocrystalline Cr-doped SnO₂. Enhancement of Cr-doped SnO₂ surface acidity and ammonia adsorption as a result has been revealed by using in situ IR spectroscopy. Moreover, a decrease in concentration of free electrons in the conduction band has been shown as a result of substitutional Cr(III) defects formation. Weaker NO_x chemisorption during ammonia oxidation over SnO₂ surface after Cr doping has been found with the use of mass-spectrometry assisted NH₃ thermo-programmed desorption. The given example of surface acidity adjustment and electronic configuration by means of doping may find use in the design of new gas-sensing metal oxide materials.     

CoN1O2 Single-Atom Catalyst for Efficient Peroxymonosulfate Activation and Selective Cobalt(IV)=O Generation

High-valent metal-oxo (HVMO) species are powerful non-radical reactive species that enhance advanced oxidation processes (AOPs) due to their long half-lives and high selectivity towards recalcitrant water pollutants with electron-donating groups. However, high-valent cobalt-oxo (Co^IV=O) generation is challenging in peroxymonosulfate (PMS)-based AOPs because the high 3d-orbital occupancy of cobalt would disfavor its binding with a terminal oxygen ligand. Herein, we propose a strategy to construct isolated Co sites with unique N₁O₂ coordination on the Mn₃O₄ surface. The asymmetric N₁O₂ configuration is able to accept electrons from the Co 3d-orbital, resulting in significant electronic delocalization at Co sites for promoted PMS adsorption, dissociation and subsequent generation of Co^IV=O species. CoN₁O₂/Mn₃O₄ exhibits high intrinsic activity in PMS activation and sulfamethoxazole (SMX) degradation, highly outperforming its counterpart with a CoO₃ configuration, carbon-based single-atom catalysts with CoN₄ configuration, and commercial cobalt oxides. Co^IV=O species effectively oxidize the target contaminants via oxygen atom transfer to produce low-toxicity intermediates. These findings could advance the mechanistic understanding of PMS activation at the molecular level and guide the rational design of efficient environmental catalysts.     

Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO₂ reduction with water. Among all the prepared Ag-loaded metal oxides, Ga₂O₃, ZrO₂, Y₂O₃, MgO, and La₂O₃ showed activities for CO and H₂ productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d⁰, d¹⁰, and s⁰ had the potential for CO₂ reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO₂ adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO₂ enhanced H₂ production rather than CO production due to very little CO₂ adsorption. Ag/Ga₂O₃ exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La₂O₃, Ag/Y₂O₃, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H₂ production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.     

Electrochemical properties and state of paramagnetic centers in copper-modified complex vanadium and titanium oxides

Complex vanadium and titanium oxides modified by copper ions are studied by the electrochemical and ESR methods. Oxides Cu _x V_2?y Ti _y O_5?δ·nH₂O (0<y<1.33) have a layered structure and oxides Cu _x Ti_1?y V _y O_5+δ·nH₂O (0<y<0.25), an anatase structure. The intercalation of cations Cu²⁺ into the hydrates leads to oxidation of V⁴⁺. According to ESR data, V⁴⁺ exists in the oxides in the form of VO²⁺ and an octahedral surround of oxygen (V⁴⁺?O₆), respectively. The electroreduction of ions of d-elements and chemisorbed oxygen in the oxides is analyzed. The intercalation of cations Cu²⁺ alters the content of V⁴⁺ and the chemisorption ability of the oxides. Possible reasons for this phenomenon are discussed.     

Exchange and correlation effects in transition-metal oxides 3dn (n = 4, 5 and 6)

In order to clarify the competition of exchange and correlation energies vs crystal-field stabilisation energy, a simple approach is proposed. The exchange and correlation contributions are described on the base of Brandow and Kanamori’s U, U′ and J_H parameters. The dependence of the crystal field effect on site distortion has simply been modelled using a linear interpolation between undistorted and fully distorted sites. This approach leads to establish phase diagrams for d⁴, d⁵ and d⁶ cations, allowing us to predict the spin-state stability range depending on exchange (J_H), crystal field (Dq) and distortion (k) parameters. It can be used to complement the Extended Hückel Tight Binding calculations of the electronic structure of materials having a noticeable ionic character such as 3d transition-metal oxides, in order to interpret the electronic behaviour, and, particularly, discuss the insulating-vs-metallic character of these oxides. To cite this article: M. Pouchard et al., C. R. Chimie 6 (2003) 000–000.     

Electronic state of Fe used as Mössbauer probe in the perovskites LaMO3 (M=Ni and Cu)

For the first time a comparative study of rhombohedral LaNiO₃ and LaCuO₃ oxides, using ⁵⁷Fe Mössbauer probe spectroscopy (1% atomic rate), has been carried out. In spite of the fact that both oxides are characterized by similar crystal structure and metallic properties, the behavior of ⁵⁷Fe probe atoms in such lattices appears essentially different. In the case of LaNi_0.99Fe_0.01O₃, the observed isomer shift (δ) value corresponds to Fe³⁺ (3d⁵) cations in high-spin state located in an oxygen octahedral surrounding. In contrast, for the LaCu_0.99Fe_0.01O₃, the obtained δ value is comparable to that characterizing the formally tetravalent high-spin Fe⁴⁺(3d⁴) cations in octahedral coordination within Fe(IV) perovskite-like ferrates. To explain such a difference, an approach based on the qualitative energy diagrams analysis and the calculations within the cluster configuration interaction method have been developed. It was shown that in the case of LaNi_0.99Fe_0.01O₃, electronic state of nickel is dominated by the d⁷ configuration corresponding to the formal ionic “Ni³⁺-O²⁻” state. On the other hand, in the case of LaCu_0.99Fe_0.01O₃ a large amount of charge is transferred via Cu-O bonds from the O:2p bands to the Cu:3d orbitals and the ground state is dominated by the d⁹L configuration (“Cu²⁺−O” state). The dominant d⁹L ground state for the (CuO₆) sublattice induces in the environment of the ⁵⁷Fe probe cations a charge transfer Fe³⁺+O⁻(L)→Fe⁴⁺+O²⁻, which transforms “Fe³⁺” into “Fe⁴⁺” state. The analysis of the isomer shift value for the formally “Fe⁴⁺” ions in perovskite-like oxides clearly proved a drastic influence of the 4s iron orbitals population on the Fe−O bonds character.     

Catalytic activity of solid solutions of nickelous and zinc oxides

Summary A comparison of the change in the rules of chemisorption of gas-acceptors and donors of electrons with catalytic activity for processes with gases of the same type showed that the change in the catalytic properties of NiO and ZnO and their solid solutions may be explained by the effect of dissolved oxides Li₂O, Ga₂O₃, Fe₂O₃, etc., on chemisorption. The change in chemisorption is considered to be connected with the effect of cations with an anomalous charge in the lattice points on the statistics of the active centers on the surface.     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps,Different Electronics

Metallocene alkyl complexes with d⁰ electron configuration and d⁸-configured square planar α-diimine late transition metal alkyl complexes show activity in both C−H activation through σ-bond metathesis and olefin insertion. Herein, we show by analysis of their M−CH₃ ¹³C chemical shift tensors that these reactions involve a π(M−C) interaction in the horizontal plane of the complex for both d⁰ and d⁸ systems. While in the case of d⁰ systems the interaction of an empty metal d-orbital and a filled carbon p-orbital causes partial alkylidene character of the M−C bond, the corresponding metal d-orbital is filled in d⁸ systems, thus generating a filled π*(M−C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d⁰ and d⁸ systems, which are reflected in the structures of the transition states: While d⁰ olefin insertion can be viewed as a [2+2] cycloaddition reaction, d⁸ olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to σ-bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.     

The electronic structure of aryl thiol esters

A combination of experimental data and the CNDO/2-SCF-MO method are used to evaluate the importance of d-orbitals to the electronic structure of aryl thiol esters. The thioester group is calculated to withdraw electron density through σ and 2p_π-3d_π bonding and donate by p_π-p_π bonding. Electron donating substituents para to the thiol ester group cause the latter group to accept electron density regardless of d-orbital participation.     

Reversible Entropy-Driven Defect Migration and Insulator-Metal Transition Suppression in VO2 Nanostructures for Phase-Change Electronic Switching

Oxygen defects are among essential issues and required to be manipulated in correlated electronic oxides with insulator-metal transition (IMT). Besides, surface and interface control are necessary but challenging in field-induced electronic switching towards advanced IMT-triggered transistors and optical modulators. Herein, we demonstrated reversible entropy-driven oxygen defect migrations and reversible IMT suppression in vanadium dioxide (VO₂) phase-change electronic switching. The initial IMT was suppressed with oxygen defects, which is caused by the entropy change during reversed surface oxygen ionosorption on the VO₂ nanostructures. This IMT suppression is reversible and reverts when the adsorbed oxygen extracts electrons from the surface and heals defects again. The reversible IMT suppression observed in the VO₂ nanobeam with M2 phase is accompanied by large variations in the IMT temperature. We also achieved irreversible and stable IMT by exploiting an Al₂O₃ partition layer prepared by atomic layer deposition (ALD) to disrupt the entropy-driven defect migration. We expected that such reversible modulations would be helpful for understanding the origin of surface-driven IMT in correlated vanadium oxides, and constructing functional phase-change electronic and optical devices.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+?As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co? As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Effect of the oxidation state of manganese in the manganese oxides used in the synthesis of Mn-substituted cordierite on the properties of the product

Catalysts based on Mn-substituted cordierite 2MnO · 2Al₂O₃ · 5SiO₂ have been synthesized using different manganese oxides (MnO, Mn₂O₃, and MnO₂) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn₂O₃ or MnO₂ are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn³⁺ cations. The highest NO_x yield (about 76% at 800–850°C) is observed for the MnO₂-based catalyst, whose surface contains the largest amount of manganese oxides.     

A simple model for predicting the electronic configuration of a dn ion in a tetragonally distorted octahedral environment

A simple, but general model which enables prediction of the electronic structure adopted by ad ⁿ ion in an axially distorted octahedron of ligands (D _4h) is described. The variousd ⁿ configurations (n = 1 to 9) are briefly reviewed with some recent results concerning the stabilization of unusual electronic configurations or oxidation states in oxides.     

Ground state electronic structures of some metal atom cluster compounds

The ground state electronic structures of several metal atom cluster compounds, Re₃Cl₉, Re₃Cl^3?₁₂ and Mo₆Cl^2?₁₄, have been calculated by the SCF Xα SW method. The results are consistent with the earlier d-orbital overlap patterns and with the reported spectroscopic properties.     

Electronic Structure and Optical Properties of the Surface F-Centers in MgO: A Theoretical Analysis by DFT Approach

Electronic structure of Mg9O9 and Mg9O8 clusters modeling nano-crystalline powders of magnesium oxide has been analyzed within the frames of the density functional theory (DFT). In the framework of time-dependent DFT method (TD-DFT), the relationship between the surface and bulk properties of nano-crystals is analyzed based on variations in the density of electronic states (DOS) and changes of electronic spectra. The spectroscopy of spatial defects like low-coordinated oxygen ions and of surface point defects like F⁺- and F-centers is investigated. Optical properties of the nano-sized crystalline magnesium oxide are characterized by a spectrum of absorption bands in the range of 1-5 eV. Point defects such as F-centers absorb light in the range of 1.2-1.5 eV. Spatial defects O_LC in nano-crystals generate absorption bands in the range of 2.5-5.0 eV. According to calculations, there is no direct relation between coordination numbers of surface ions and excitation energies. Theoretical excitation energies are compared with experimental optical properties of the F⁺- and F-centers.     

Variation De L'énergie Superficielle D'une Bentonite Par Traitement Chimique Et Thermique

Variation of surface energy of a bentonite by chemical and thermal treatments. The effect of an attack by hydrochloric acid and of thermal treatments on the dispersive surface energy, γ^d_s, and on the morphology index of a bentonite has been studied by inverse gas chromatography at infinite dilution. The results show that γ^d_s is very sensitive to two parameters: one is the atomic scale morphology of the clay material, the other one is the mineral impurity contents, mainly composed of alkaline and alkaline-earth oxides. The large increase of γ^d_s after a moderate treatment [t < 10 h, HC1 < 2N and thermal treatment temperature (TTT)≤300°C] is attributed to the bentonite purification and to the lateral surface appearence. However, a more severe treatment (t>10h, HC1 >3N et TTT > 500°C) destroys the lamellar structure of the montmorillonite and in this case the measured γ^d_s is attributed to the basal surface.