Solid solutions of CdS and ZnS: Comparing photocatalytic activity and photocurrent generation

The series of Cd1-xZnxS (x = 0–1.0) photocatalysts and Cd1-xZnxS/FTO thin film photoelectrodes were prepared. The obtained samples were studied by X-ray diffraction method (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and N2 low temperature adsorption. The photocatalysts were tested in the reaction of hydrogen production from Na2S/Na2SO3 solution under visible light irradiation. The photocurrents were measured in aqueous solution of Na2Sn and NaCl. It was shown that the target properties in the hydrogen evolution and photocurrent production are changed differently dependent on the composition of the mixed solid solutions. The highest photocatalytic hydrogen production rate was observed over Cd0.3Zn0.7S while the most effective photoelectrode was Cd0.8Zn0.2S/FTO. The Cd1-xZnxS/FTO samples were studied by the electrochemical methods in details. The factors affecting the photocatalytic activity and the photocurrent generation were found and listed for the first time. Conduction band potential (or flat band potential) and electron lifetime play a crucial role for effective photocatalytic hydrogen production over Cd1-xZnxS. Optimal photoelectrochemical characteristics were obtained in case of high values of electron lifetime. If the electron lifetimes of the tested samples have similar values, the high concentration of charge carriers is required for high photocurrent generation over Cd1-xZnxS/FTO photoelectrodes

Nonstoichiometry Defects in Double Oxides of the A₂BO₄-Type

Double oxides with the structure of the Ruddlesden–Popper (R-P) layered perovskite An+1BnO3n+1 attract attention as materials for various electrochemical devices, selective oxygen-permeable ceramic membranes, and catalytic oxidative reactions. In particular, Sr2TiO4 layered perovskite is considered a promising catalyst in the oxidative coupling of methane. Our high-resolution transmission electron microscopy (HRTEM) studies of Sr2TiO4 samples synthesized using various methods have shown that their structure often contains planar defects disturbing the periodicity of layer alternation. This is due to the crystal-chemical features of the R-P layered perovskite-like oxides whose structure is formed by n consecutive layers of perovskite (ABO3)n in alternating with layers of rock-salt type (AO) in various ways along the c crystallographic direction. Planar defects can arise due to a periodicity violation of the layers alternation that also leads to a violation of the synthesized phase stoichiometry. In the present work, a crystallochemical analysis of the possible structure of planar defects is carried out, structures containing defects are modeled, and the effect of such defects on the X-ray diffraction patterns of oxides of the A2BO4 type using Sr2TiO4 is established as an example. For the calculations, we used the method of constructing probabilistic models of one-dimensionally disordered structures. For the first time, the features of diffraction were established, and an approach was demonstrated for determining the concentration of layer alternation defects applicable to layered perovskite-like oxides of the A2BO4 type of any chemical composition. A relation has been established between the concentration of planar defects and the real chemical composition (nonstoichiometry) of the Sr2TiO4 phase. The presence of defects leads to the Ti enrichment of particle volume and, consequently, to the enrichment of the surface with Sr. The latter, in turn, according to the data of a number of authors, can serve as an explanation for the catalytic activity of Sr2TiO4 in the oxidative coupling of methane

Sol-Gel Synthesis and Characterization of the Cu-Mg-O System for Chemical Looping Application

A sol-gel technique was applied to prepare the two-component oxide system Cu-Mg-O, where MgO plays the role of oxide matrix, and CuO is an active chemical looping component. The prepared samples were characterized by scanning electron microscopy, low-temperature nitrogen adsorption, and X-ray diffraction analysis. The reduction behavior of the Cu-Mg-O system was examined in nine consecutive reduction/oxidation cycles. The presence of the MgO matrix was shown to affect the ability of CuO towards reduction and re-oxidation significantly. During the first reduction/oxidation cycle, the main characteristics of the oxide system (particle size, crystallization degree, etc.) undergo noticeable changes. Starting from the third cycle, the system exhibits a stable operation, providing the uptake of similar hydrogen amounts within the same temperature range. Based on the obtained results, the two-component Cu-Mg-O system can be considered as a prospective chemical looping agent

The effect of ruthenium promotion of the Co/d-Al2O3 catalyst on the hydrogen reduction kinetics of cobalt

The effect of ruthenium content on the reductive activation of the Co/δ-Al2O3 catalyst was investigated using thermal analysis and in situ synchrotron radiation X-ray diffraction. Data of thermal analysis and phase transformations can be described by a kinetic scheme consisting of three sequential steps: Co³⁺ → Co²⁺ → (Co⁰Co²⁺) → Co⁰. The first step is the generation of several CoO clusters within one Co3O4 crystallite followed by their further growth obeying the Avrami–Erofeev kinetic equation (An1) with dimensional parameter n1 < 1, which may indicate the diffusion control of the growth. The second step is the kinetically controlled sequential process of the metallic cobalt phase nucleation (An2), which is followed by the third step of slow particle growth limited by mass transport according to the Jander model (D). Ruthenium promotion of Co/δ-Al2O3 catalysts significantly accelerates the reduction of cobalt. As the ruthenium content is raised to 1 wt%, the characteristic temperature of metal phase formation decreases by more than 200 °C and Ea for An2 step decreases by 25%. For step D, a joint decrease in activation energy and pre-exponential factor in case of ruthenium promotion corresponds to a weaker diffusion impediment at the final step of cobalt reduction. In the case of unmodified Co/δ-Al2O3, the characteristic temperature of the metal phase formation reaches very high values, the metallic nuclei rapidly coalesce into larger ones, and the further process is inhibited by diffusion of the reactants through the product layer. For ruthenium promoted catalysts, each CoO crystallite generates one metal crystallite; thus, ruthenium enhances the dispersion of the active component

The effect of ruthenium promotion of the Co/d-Al2O3 catalyst on the hydrogen reduction kinetics of cobalt

The effect of ruthenium content on the reductive activation of the Co/δ-Al2O3 catalyst was investigated using thermal analysis and in situ synchrotron radiation X-ray diffraction. Data of thermal analysis and phase transformations can be described by a kinetic scheme consisting of three sequential steps: Co³⁺ → Co²⁺ → (Co⁰Co²⁺) → Co⁰. The first step is the generation of several CoO clusters within one Co3O4 crystallite followed by their further growth obeying the Avrami–Erofeev kinetic equation (An1) with dimensional parameter n1 < 1, which may indicate the diffusion control of the growth. The second step is the kinetically controlled sequential process of the metallic cobalt phase nucleation (An2), which is followed by the third step of slow particle growth limited by mass transport according to the Jander model (D). Ruthenium promotion of Co/δ-Al2O3 catalysts significantly accelerates the reduction of cobalt. As the ruthenium content is raised to 1 wt%, the characteristic temperature of metal phase formation decreases by more than 200 °C and Ea for An2 step decreases by 25%. For step D, a joint decrease in activation energy and pre-exponential factor in case of ruthenium promotion corresponds to a weaker diffusion impediment at the final step of cobalt reduction. In the case of unmodified Co/δ-Al2O3, the characteristic temperature of the metal phase formation reaches very high values, the metallic nuclei rapidly coalesce into larger ones, and the further process is inhibited by diffusion of the reactants through the product layer. For ruthenium promoted catalysts, each CoO crystallite generates one metal crystallite; thus, ruthenium enhances the dispersion of the active component

Efficient Photocatalytic Hydrogen Production over NiS-Modified Cadmium and Manganese Sulfide Solid Solutions

In this work, new photocatalysts based on Cd1−xMnxS sulfide solid solutions were synthesized by varying the fraction of MnS (x = 0.4, 0.6, and 0.8) and the hydrothermal treatment temperature (T = 100, 120, 140, and 160 °C). The active samples were modified with Pt and NiS co-catalysts. Characterization was performed using various methods, including XRD, XPS, HR TEM, and UV-vis spectroscopy. The photocatalytic activity was tested in hydrogen evolution from aqueous solutions of Na2S/Na2SO3 and glucose under visible light (425 nm). When studying the process of hydrogen evolution using an equimolar mixture of Na2S/Na2SO3 as a sacrificial agent, the photocatalysts Cd0.5Mn0.5S/Mn(OH)2 (T = 120 °C) and Cd0.4Mn0.6S (T = 160 °C) demonstrated the highest activity among the non-modified solid solutions. The deposition of NiS co-catalyst led to a significant increase in activity. The best activity in the case of the modified samples was shown by 0.5 wt.% NiS/Cd0.5Mn0.5S (T = 120 °C) at the extraordinary level of 34.2 mmol g−1 h−1 (AQE 14.4%) for the Na2S/Na2SO3 solution and 4.6 mmol g−1 h−1 (AQE 2.9%) for the glucose solution. The nickel-containing samples possessed a high stability in solutions of both sodium sulfide/sulfite and glucose. Thus, nickel sulfide is considered an alternative to depositing precious metals, which is attractive from an economic point of view. It worth noting that the process of photocatalytic hydrogen evolution from sugar solutions by adding samples based on Cd1−xMnxS has not been studied before

Synthesis of Vanadia-Mayenite Nanocomposites and Characterization of Their Structure, Morphology and Surface Sites

Calcium aluminates (CA) with a mayenite structure have attracted a growing interest during the last decades. The present paper reports the preparation of vanadia-mayenite composites performed via an impregnation of pure CA with ammonium vanadate solution. The properties of the prepared materials were explored by a low-temperature nitrogen adsorption/desorption technique, X-ray diffraction analysis, transmission electron microscopy, and spin probe method. As revealed, the addition of vanadium significantly affects the textural properties and the porous structure of mayenite. The blockage of micropores by vanadium species is supposed. The spin probe electron paramagnetic resonance technique based on the adsorption of 1,3,5-trinitrobenzene, phenothiazine, and diphenylamine has been applied to study the active sites on the surface of the composite samples. The results demonstrated an increase in the concentration of weak electron-acceptor sites when the vanadium loading was 10 wt%. X-ray diffraction analysis and transmission electron microscopy studies showed that the composites consist of few phases including mayenite, CaO, and calcium vanadates