Zukhra T. Fattakhova

On the origin of near-IR luminescence in Bi-doped materials (II). Subvalent monocation Bi+ and cluster Bi5(3+) luminescence in AlCl3/ZnCl2/BiCl3 chloride glass

Broadband NIR photoluminescence (from 1000 to 2500 nm) was observed from partially reduced AlCl₃/ZnCl₂/BiCl₃ glass, containing subvalent bismuth species. The luminescence consists of three bands, assigned to Bi⁺ , Bi₂⁴⁺, and Bi₅³⁺ ions. The physical and optical characteristics of these centers and possible contribution to NIR luminescence from bismuth-doped oxide glasses are discussed.

Kinetics of oxidative coupling of methane: bridging the gap between comprehension and description

The development of notions about the mechanism of the oxidative coupling of methane (OCM) over oxide catalysts and corresponding progress in its kinetic description are reviewed and discussed. The latter becomes essential at the stage of scaling up and optimization of the process in pilot and industrial reactors. It is demonstrated that the main achievements in the development of kinetic models can be reached by combining the approaches conventionally used in homogeneous gas-phase kinetics and in heterogeneous catalysis. In particular, some important features of the OCM process can be described if several elementary reactions of free radical species (formation and transformation) with surface active sites are included into the detailed scheme of methane oxidation in gas. However, some important features, such as a non-additive character of the reciprocal influence of methane and ethane in the case of their simultaneous presence in the reaction mixture, cannot yet be described and comprehended in the framework of schemes developed so far. Possible ways towards an advanced kinetic model, accounting the main principles of catalyst functioning (redox nature of active sites) and pathways of product formation (via free radicals) are traced.

Hydrogen formation during dehydrogenation of C2–C4 alkanes in the presence of oxygen: oxidative or non-oxidative?

Abstract The main advantage of oxidative dehydrogenation (ODH) of light alkanes over non-oxidative dehydrogenation (NODH) is the absence of thermodynamic limitations for olefin per-pass yield. It is shown, however, that considerable amounts of molecular hydrogen are formed (in some cases—equivalent to the amounts of produced olefin or even higher) over typical ODH oxide catalysts when C 2 –C 4 alkanes and oxygen are present in the feed gas. The analysis of product yields as functions of reaction parameters (temperature, flow rate, hydrocarbon-to-oxygen ratio) over a series of V- and Sb-containing ODH catalysts has shown that the main kinetic features cannot be described in the framework of a single mechanism. NODH, coking and free-radical oxidation are considered as the most important H 2 formation pathways. The ways to optimize the olefin production depending on the predominant mechanism of hydrogen formation over a particular catalyst are discussed.

Near infrared photoluminescence of the univalent bismuth impurity center in leucite and pollucite crystal hosts

The bismuth doped aluminosilicate phases leucite (KAlSi2O6), gallium leucite (KGaSi2O6) and pollucite (CsAlSi2O6) display broadband NIR photoluminescence. The active center, responsible for this luminescence, is the Bi+ monocation, which substitutes for the large alkali metal cations. The Al,Si-disorder in the aluminosilicate framework of studied crystal phases results in the heterogeneity of Bi+ luminescent center population, which manifests itself in the characteristic dependency of the luminescence spectrum shape on the excitation wavelength. The relation of NIR emission in Bi+-doped leucite and pollucite phases to the luminescent properties of bismuth-doped glasses is also discussed.

Redox properties and catalytic performance of complex oxides in oxidative coupling of methane

Abstract The redox properties of model Mn-containing mixed oxide are studied. It is shown that water evolution can determine the rate of oxide reduction. The kinetic model of the redox process taking into account surface interactions and diffusion of oxygen, hydrogen and hydroxy ions in the oxide lattice can describe successfully the experimental data. The influence of slow water evolution on catalytic performance of the oxides in oxidative coupling of methane is discussed.

Dynamics and Thermochemistry of Oxygen Uptake by a Mixed Ce–Pr Oxide

The dynamics of oxygen uptake by mixed Ce0.55Pr0.45O2–x oxide is studied in a pulsed oxygen supply mode using in situ high-temperature heat flow differential scanning calorimetry. It is stated that the oxidation proceeds in two regimes: a fast one at the beginning of the oxidation process, and a slow one, which is controlled by the diffusion of oxygen through the bulk of the solid at the later stages of the process. Analysis of the shape of calorimetric profiles reveals some processes, accompanied by heat release, that occur in the sample in the absence of oxygen in the gas phase. These could be due to both the redistribution of consumed oxygen in the oxide lattice and the lattice relaxation associated with the transformation of phases with different arrangements of oxygen vacancies in them. The heat effect (which diminishes from ~60 to ~40 kJ/mol in the course of oxygen uptake) associated with the oxidation of the reduced form of mixed Ce–Pr oxide, corresponds to the oxidation of praseodymium ions from (3+) to (4+).

New route to Bi + -doped crystals: Preparation and NIR luminescence of K, Rb and Cs ternary chlorides, containing univalent bismuth

In Lewis acidic melts it is possible to obtain the metal cations in unusually low oxidation (subvalent) states with unusual electronic configurations. Specifically, univalent bismuth cation possesses the open shell 6p² configuration with several low-lying electronic exited states, causing the absorption bands in visible/NIR and intense long-lived (hundreds of μs) and broad luminescence in NIR [1-4]. Quantum chemical estimation of Bi⁺ ionic radius in different coordination states gives the values, which place the radius of Bi⁺ very close to Rb⁺ cation. This fact open the possibility to prepare the crystal hosts with alkali cations of similar size (K⁺, Rb⁺, Cs⁺), which can be doped with Bi⁺ by isomorphous substitution. The experimental investigation of NIR luminescence in bismuth-doped RbPb₂Cl₅ [1], KAlCl₄ [2,3], KMgCl₃ [3] and CsI [4] crystals support this conclusion.

Optical properties of the Bi + centre in KAlCl4 crystal

For the first time near infrared (NIR) luminescence of Bi-doped media was observed in 2001 by Fujimoto et al. in silicate glasses. A lot of various Bi-centres that could be responsible for observed NIR luminescence were suggested but there are more and more evidences that it originates from some of low-valence bismuth species like Bi⁺ ions and subvalent Bi clusters (Bi₂⁴⁺, Bi₅³⁺, Bi₈²⁺ etc). Often several luminescent species are observed simultaneously in a Bi-doped media. Nevertheless to date several Bi-luminescent centres (e.g. Bi⁺, Bi₅³⁺ and Bi₈²⁺) was obtained in a “pure” form, i.e. without any additional ones.

IR luminescence from subvalent bismuth in phosphate glass and glass-ceramic matrices: A new insight into the nature of luminescent Bi species

It was demonstrated, that broadband NIR luminescent bismuth species can be prepared by synproportionation reaction of Bi metal and bismuth ions in usual oxidation state +3. This reaction was observed in highly Lux-Flood oxoacidic borate or phosphate glass melts [1], and Lewis acidic chloroaluminate ionic liquid [2]. The various subvalent Bi species, like univalent cation Bi⁺ and polycation clusters (Bi<inf>5</inf>³⁺, Bi<inf>8</inf>²⁺ etc.) were considered as possible products of synproportionation 2Bi + Bi³⁺ ↔ 3Bi⁺ or 4Bi + Bi³⁺ ↔ Bi<inf>5</inf>³⁺ and candidate NIR luminescent species. The equilibrium shifted toward subvalent species by increasing acidity or/and temperature.