Effect of high-temperature treatment on the properties of an alumina-chromium catalyst for the dehydrogenation of lower paraffins

The crystal and pore structures of a microspherical alumina-chromium catalyst calcined at 800–1100°C were studied using a set of currently available physicochemical techniques (X-ray diffraction, lowtemperature nitrogen adsorption, diffuse reflectance UV-vis spectroscopy, Raman spectroscopy, and EPR spectroscopy); the state of its active component and the catalytic properties in isobutane dehydrogenation were examined. As the calcination temperature was increased from 800 to 900–1000°C, the properties of the catalyst were improved as a result of the formation of Cr₂O₃ clusters in an optimum amount and a decrease in the surface acidity of the catalyst due to the dehydroxylation and phase transformations of the aluminum oxide support. Calcination at 1100°C was accompanied by a decrease in the yield of isobutylene as a result of the formation of inactive macrocrystalline chromium (III) oxide and a chromium species inaccessible to reacting molecules; this chromium species was encapsulated in closed pores as the constituent of a solid solution of α-Al₂O₃-Cr₂O₃.     

Advancements in Perovskite-Based Cathode Materials for Solid Oxide Fuel Cells: A Comprehensive Review

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO₃ (LSM) and LaSrCOFeO₃ (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.     

Structural Investigations and Thermal Behavior of (NH4)[Cr(C2O4)2]·2H2O

Single crystals of ammonium chromium(III) dioxalate dihydrate (or ammonium diaquo bis(μ‐oxalato)chromate(III)) have been obtained from aqueous solution of oxalic acid and ammonium dichromate. A pale violet crystal of good optical quality was used for the structure determination at ?100(2) and 25(2) °C, respectively. The basic crystallographic data for the low temperature data set are as follows: monoclinic, space group C2/m, a = 6.597(2) Å, b = 7.301(2) Å, c = 9.983(3) Å, β = 92.32(2)°, V = 480.5(2) ų. The structure was solved by direct methods and refined (using anisotropic displacement parameters for all non‐hydrogen atoms) to a final residual of R1 = 0.032 for 503 independent observed reflections (I>2σ(I)). The compound is isotypic with the corresponding rubidium salt. The structure is built up from alternating layers parallel to (001) containing (NH₄)⁺ ions or Cr(C₂O₄)₂(H₂O)₂ octahedra, respectively. The corners of the octahedra consist of four O atoms from two oxalate groups and two additional water molecules. The ammonium cations (occupying Wyckoff‐site 2a) are disordered among two possible orientations. They provide linkage between different octahedral layers by hydrogen bridging. The water molecules in turn form hydrogen bridges with adjacent octahedra within the same layer. Further structural characterization included infrared spectroscopy. According to DTA/TG experiments the present compound shows several thermal processes in the range between room temperature and 900 °C.     

Chemical Stability and Electrical Property of BaCe0.7Zr0.2Nd0.1O3-α Ceramic

BaCe_0.7Zr_0.2Nd_0.1O_3?α ceramic was prepared by solid state reaction. Phase composition, surface and fracture morphologies of the material were characterized by using XRD and SEM, respectively. Chemical stability against carbon dioxide and water steam at the high temperature was tested. The conductivity and ionic transport number of the material were measured by ac impedance spectroscopy and gas concentration cell methods in the temperature range of 500–900°C in wet hydrogen and wet air, respectively. Using the ceramic as solid electrolyte and porous platinum as electrodes, the hydrogen‐air fuel cell was constructed, and the cell performance at the temperature from 500 to 900°C was examined. The results indicate that BaCe_0.7Zr_0.2Nd_0.1O_3?α was a single phase perovskite‐type orthorhombic system, with high density and good chemical stability in carbon dioxide and water steam atmospheres at the high temperature. The conductivity of the material in wet hydrogen and wet air was increased as the temperature rises. In wet hydrogen, the material was a pure protonic conductor with the protonic transport number of 1 from 500 to 600°C, a mixed conductor of proton and electron with the protonic transport number of 0.973–0.955 from 700 to 900°C. In wet air, the material was a mixed conductor of proton, oxide ion and electron hole. The protonic transport numbers were 0.002–0.003, and the oxide ionic transport numbers were 0.124–0.179. The fuel cell could work stably. At 900°C, the maximum short‐circuit current density and power output density were 156 mA·cm^?2 and 40 mW·cm^?2, respectively.     

Thermal stability of anatase TiO2 aerogels

Titanium dioxide (TiO₂) aerogels were prepared with sol–gel ambient pressure drying method by using titanium tetrachloride (TiCl₄) as precursor and tetraethoxysilane as modifier, calcinated at different temperature and characterized by X‐ray diffraction, transmission electron microscopy and small angle X‐ray scattering. The results showed that the TiO₂ aerogels remained amorphous under 500 °C, changed to anatase from 600 °C and further changed to rutile from 900 °C. Between 60 °C and 500 °C, the primary particles within the samples concentrated mainly upon small sizes, enlarged and diverged remarkably above 600 °C. The crystalline grains grew and agglomerated with the rise of the calcination temperature. The TiO₂ aerogels at a temperature higher than 800 °C have better stability than anatase because of the formation of partial Ti―O―Si bonds. Copyright © 2016 John Wiley & Sons, Ltd.     

Deposition of chromium species on Sr-doped LaMnO3 cathodes in solid oxide fuel cells

The system of chromia-forming alloy/Sr-doped LaMnO₃ (LSM) electrode/3 mol%Y₂O₃–ZrO₂ (TZ3Y) electrolyte has been investigated at 900°C in air under cathodic polarization conditions. Deposition of chromium species was found to occur preferentially at the TZ3Y electrolyte surface, forming a deposit ring around the edge of the LSM electrode coating. The width of the ring was about 60 μm for an LSM electrode polarized for about 50 h. Overpotential (η) increases with polarization time. In contrast to η, electrode interface resistance (R_E) measured under open circuit conditions decreases initially after passing the current and remains almost constant with polarization. The results indicate that the deposition process of chromium species may not be dominated by electrochemical reduction processes in competition with O₂ reduction at an early stage of polarization.     

Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide over SBA‐15‐Supported Chromia‐Ceria Catalysts

A series of SBA‐15‐supported chromia‐ceria catalysts with 3% Cr and 1%–5% Ce (3Cr‐Ce/SBA) were prepared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N₂ adsorption, SEM, TEM‐EDX, Raman spectroscopy, UV–vis spectroscopy, XPS and H₂‐TPR, and their catalytic performance for isobutane dehydrogenation with CO₂ was tested. The addition of ceria to SBA‐15‐supported chromia improves the dispersion of chromium species. 3Cr‐Ce/SBA catalysts are more active than SBA‐15‐supported chromia (3Cr/SBA), which is due to a higher concentration of Cr⁶⁺ species present on the former catalysts. The 3Cr‐3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570 °C after 10 min of the reaction.     

Effects of calcination and activation temperature on dry reforming catalysts

The effect of calcination temperatures on dry reforming catalysts supported on high surface area alumina Ni/γ-Al₂O₃ (SA-6175) was studied experimentally. In this study, the prepared catalyst was tested in a micro tubular reactor using temperature ranges of 500, 600, 700 and 800 °C at atmospheric pressure, using a total flow rate of 33 ml/min consisting of 3 ml/min of N₂, 15 ml/min of CO₂ and 15 ml/min of CH₄. The calcination was carried out in the range of 500–900 °C. The catalyst is activated inside the reactor at 500–800 °C using hydrogen gas. It was observed that calcination enhances catalyst activity which increases as calcination and reaction temperatures were increased. The highest conversion was obtained at 800 °C reaction temperature by using catalyst calcined at 900 °C and activation at 700 °C. The catalyst characterization conducted supported the observed experimental results.     

Kinetics of the Temperature-Dependent eaq− and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (e_aq⁻) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr³⁺ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)²⁺, did with a rate coefficient of k= (7.2±0.3)×10⁸ M⁻¹ s⁻¹ at 25 °C. The oxidation of Cr(OH)²⁺ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.     

Effect of SO<Subscript>2</Subscript> on SCR activity of MnOx/PG catalysts at low temperature

Palygorskite (PG)-supported manganese oxide catalysts (MnOx/PG) were prepared for the selective catalytic reduction (SCR) of NO with ammonia in the presence of SO₂ at low temperature. The influence of gaseous SO₂ on the performance of the catalyst was studied by means of specific surface area (Brunauer-Emmett-Teller, BET) analysis, scanning electron microscopy (SEM), thermogravimetric (TG) analysis, temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The results showed that the SCR activity of Mn10/PG was significantly inhibited by gaseous SO₂ at temperatures below 300°C. However, the SCR activity of Mn10/PG was markedly promoted by SO₂ in a higher temperature range of 300°C to 500°C. The sulphating of surface active species (MnOx) was suggested to inhibit the oxidation of NH₃ to NO leading to enhancement of the SCR activity at a higher temperature range of 300°C to 500°C and decrease in the SCR activity at temperatures below 300°C.     

Anionic surfactant‐aided preparation of high surface area and high thermal stability ceria/zirconia‐mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

This work is focused on the ceria zirconia mixed oxide prepared through a surfactant‐introduced synthesis method. High surface area nanoparticle mesoporous ceria/zirconia‐mixed oxide was successfully synthesized and characterized using various techniques. High surface area mesoporous fluorite‐structured CeO₂? ZrO₂ was obtained from the elimination of surfactants upon calcination. A surface area in excess of 205.6 m²/g was obtained after calcination at 500 °C, and dropped to 75.96 m²/g by heating at 900 °C. Temperature‐programming reduction (TPR) results showed that the lowest reduction temperature was obtained from the sample containing 40% zirconia content. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermolysis of [Co(NH3)6][Cr(C2O4)3]

Thermolysis of the double-metal complex [Co(NH₃)₆][Cr(C₂O₄)₃] was studied in air at 200, 350, and 500°C and in a hydrogen atmosphere at 200, 350, 500, 700, and 900°C, as well as the composition and properties of thermolysis products. Oxidative thermolysis produces mixed oxides CoCr₂O₄ and Co₂CrO₄; reductive thermolysis produces Co + Cr₂O₃ mixture. Specific surface areas were measured for reductive thermolysis products; the maximal specific surface area and, therefore, maximal dispersion are reached at 500°C. The morphology of the reductive thermolysis products and the thermolysis chemism were studied in relation to the nature of the complex anion.     

ToF-secondary ion mass-spectrometric study of copper deposition and stripping on directly heated screen-printed gold electrodes

We report about a new kind of directly heated gold electrode. All electrodes including a directly heated gold loop electrode, a Ag pseudo reference, and a carbon counter electrode have been screen-printed on a ceramic alumina substrate. Thermal behaviour was studied by potentiometry using either an external or the integrated reference electrode. Stripping voltammetric copper signals were greatly improved at elevated deposition temperature. Secondary ion mass spectrometric studies (ToF-SIMS) revealed that different negative ionic species of copper complexes can be found on the gold electrode surface as a result of ion bombardment during SIMS analysis like Cu^?, CuCl^? and CuCl₂ ^?. SIMS surface imaging using a fine focussed ion beam over the surface allowed us to obtain ion images (chemical maps) of the analyzed sample. SIMS depth profile analysis of the gold loop electrode was performed after copper deposition at room temperature (23 °C) and at 60 °C. CuCl₂ ^? ion was used for the depth profile studies as it has shown the highest intensity among other observed species. Surface spectroscopic analysis, surface imaging and depth profile analysis have shown that the amount of deposited copper species on the gold loop electrode was increased upon increasing electrode temperature during the deposition step. Therefore, the presence of chloride in the solution will hinder underpotential deposition of Cu(0) and lead to badly defined and resolved stripping peaks.     

Effect of calcination temperature on the physicochemical and catalytic properties of FeSO<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> in hydrogen sulfide oxidation

The effect of calcination temperature on the state of the active component of iron-containing catalysts prepared by the impregnation of silica gel with a solution of FeSO₄ and on their catalytic properties in selective H₂S oxidation to sulfur was studied. With the use of thermal analysis, XPS, and Mössbauer spectroscopy, it was found that an X-ray amorphous iron-containing compound of complex composition was formed on the catalyst surface after thermal treatment in the temperature range of 400–500°C. This compound contained Fe³⁺ cations in three nonequivalent positions characteristic of various oxy and hydroxy sulfates and oxide and sulfate groups as anions. Calcination at 600°C led to the almost complete removal of sulfate groups; as a result, the formation of an oxide structure came into play, and it was completed by the production of finely dispersed iron oxide in the ?-Fe₂O₃ modification (the average particle size of 3.2 nm) after treatment at 900°C. As the calcination temperature was increased from 500 to 700°C, an increase in the catalyst activity in hydrogen sulfide selective oxidation was observed because of a change in the state of the active component. A comparative study of the samples by temperature-programmed sulfidation made it possible to establish that an increase in the calcination temperature leads to an increase in the stability of the iron-containing catalysts to the action of a reaction atmosphere.     

Preparation of thin films of platinum group metals by pulsed MOCVD. II. Deposition of Ru layers

In situ high-temperature mass spectrometry is used to analyze the thermal decomposition of Ru(acac)₃ and Ru(nbd)(allyl)₂ vapor and possible schemes of thermal transformations on the heated surface. By pulsed MOCVD with in situ mass spectrometric control of deposition processes ultrathin Ru layers with a thickness of several nanometers are obtained. The role of the reaction medium, precursor nature, and deposition temperature in the formation of a nanocrystalline structure of the films is revealed. Ruthenium films with a compact continuous structure are formed from Ru(acac)₃ and hydrogen at a deposition temperature of 340°C and below; an increase in the temperature results in the growth of nanogranular Ru layers. Regardless of deposition conditions, from Ru(nbd)(allyl)₂ granular nanocrystalline Ru layers are formed     

Graphene Grown from Flat and Bowl Shaped Polycyclic Aromatic Hydrocarbons on Cu(111)

The growth of carbon layers, defective graphene, and graphene by deposition of polycyclic aromatic hydrocarbons (PAHs) on Cu(111) is studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Two different PAHs are used as starting materials: the buckybowl pentaindenocorannulene (PIC) which contains pentagonal rings and planar coronene (CR). For both precursors, with increasing sample temperature during deposition, porous carbon aggregates (350 °C), dense carbon layers (400–450 °C), disordered defective graphene (500 °C–550 °C), and extended graphene (≥600 °C) are obtained. No significant differences for defective graphene grown from PIC and CR are observed. C 1s X-ray photoelectron spectra of PIC and CR derived samples grown at 350–550 °C exhibit a characteristic C−Cu low binding energy component. Preparation at ≥600 °C eliminates this C−Cu species and only C−C bonded carbon remains.     

High temperature thermo-photocatalysis driven carbon removal in direct biogas fueled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) can directly convert renewable biogas into electricity with high efficiency at high temperature, however the long-term stability of SOFCs is significantly affected by the carbon deposition on the anode during cell operation. Herein, we report a novel carbon removal approach by high temperature infrared light driven photocatalytic oxidation. Upon the comparison of electrochemical performance of Ni-YSZ anode and TiO₂ modified Ni-YSZ anode in the state-of-the-art single cell (Ni-YSZ/YSZ/LSCM), the modified anodes exhibit markedly improved peak powder density with simulated biogas fuel (70% CH₄+ 30% CO₂) at 850 °C with less coking after 40 h operation. The high activity and carbon deposition resistance of the modified anode is possibly attributed to the in situ generated hydroxyl radical from the reduced TiO_x powder under high temperature infrared light excitation, which is supported by detailed analysis of microstructural information of anodes and the powder-based thermo-photocatalytic experiments.     

The effect of active oxygen species in nano-ZnCr2O4 spinel oxides for methane catalytic combustion

The nano-ZnCr₂O₄ spinel oxides was synthesized by a ethylene glycol mediated solvothermal method. Catalytic combustion of methane test showed that an excellent activity over nano-ZnCr₂O₄ with T_10% = 300 °C and T_90% = 400 °C. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption-desorption measurements (BET) indicated that a uniform nano-ZnCr₂O₄ spinel oxides particles with the high surface area (96.2 m²g⁻¹) was successfully synthesized. Oxygen temperature programmed desorption (O₂-TPD) profile revealed there were two obvious desorption of oxygen species from nano-ZnCr₂O₄ in the range of 300–400 °C and 500–700 °C. It was clear that the desorption temperature range of the first oxygen species coincided with the methane catalytic combustion temperature. X-ray photoelectron spectroscopy (XPS) analysis exhibited that Cr⁶⁺ was present in the lattice of ZnCr₂O₄ apart from Cr³⁺. High valence cations of chromium in crystal lattice probable caused the presence of interstitial oxygen species in the structure to maintain the electroneutrality. Additionally, Raman spectra proved that there is the interstitial oxygen species in the crystal lattice of ZnCr₂O₄. Therefore, the excellent catalytic activity for methane combustion was contributed to the flexible interstitial oxygen in the ZnCr₂O₄.     

Ultrathin cobalt silicide film formation on Si(100)

The interfacial reaction between ultrathin Co film and substrate Si(100) was studied by in situ XPS using monochromatized Al Kα. When the Co is deposited on Si(100) at room temperature, CoSi₂ is formed during the initial stage of Co deposition and then metallic Co starts to grow sequentially. For 8 ML Co deposition on Si(100) the interfacial reaction layer is relatively thin compared with the pure Co overlayer, which is not involved in the interfacial reaction in depth. The Co layers change rapidly to CoSi layers after annealing at 270°C, and then CoSi₂ layers form after annealing at 540°C for 2 min. The CoSi₂ layers are changed to CoSi₂ islands by post‐annealing at >650°C. Copyright © 2003 John Wiley & Sons, Ltd.     

On the oxidation behaviour of monolithic TiB2 and Al2O3-TiB2 and Si3N4-TiB2 composites

In view of the susceptibility of TiB₂ to oxidation, the thermal stability of monolithic TiB₂ and of Al₂O₃-30 vol% TiB₂ and Si₃N₄-20 vol% TiB₂ composites was investigated. The temperature at which TiB₂ ceramic starts to oxidize is about 400°C, oxidation kinetics being controlled by diffusion up toT≈900°C and in the first stage of the oxidation at 1000°C and 1100°C (up to 800 min and 500 min respectively), and by a linear law at higher temperatures and for longer periods. Weight gains in the Al₂O₃-TiB₂ composite can be detected only at temperatures above ≈700°C and the rate governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700°C andT=800°C and by two-dimensional diffusion at higher temperatures. Concerning the Si₃N₄-TiB₂ composite, three different oxidation behaviours related to the temperature were observed, i.e. up to ≈1000°C the reaction detected regards only the second phase; at ≈1000<T<≈1200°C, the diffusion of O₂ or N₂ through an oxide layer is proposed as the rate-governing step; atT〉=1200°C, a linear kinetic indicates the formation of a non protective scale.