Salil Varma

Synthesis, characterization, and redox behavior of mixed orthovanadates La1-xCexVO4

A series of mixed orthovanadates with general formula La1−xCexVO4 (0<x<1) were synthesized and characterized using powder XRD and FTIR techniques. The monoclinic phase of LaVO4 was retained for the samples with x≤0.2, while the tetragonal phase of CeVO4 was stabilized in x≥0.5 compositions. On the other hand, the mixed phases of LaVO4 and La0.5Ce0.5VO4 existed for the values of 0.2<x<0.5. The lattice parameters, deduced by indexing of XRD patterns, were found to decrease with the increasing cerium content in the case of both the monoclinic and the tetragonal phases of substituted orthovanadates having single-phase compositions. This trend can be attributed to the presence of Ce in +3 oxidation state and to its smaller ionic size compared to that of La3+. The temperature-programmed reduction/oxidation studies showed that, compared to CeVO4 or LaVO4, the single-phase mixed orthovanadates exhibited a better reproducibility during the repeated cycles of reduction/oxidation.

Synthesis, characterisation, TPR/TPO and activity studies on LaMnxV1-xO4-δ catalysts

Abstract LaMnxV1−xO4−δ samples (0≤x≤1) were synthesised and characterised using XRD and FTIR techniques, while their oxygen content, reducibility, stability and ionic mobility were investigated by TPR, TPO and TPD methods. All mixed oxides were found to be of single phase; they were stable in air and exhibited reproducible TPR/TPO patterns over consecutive six to seven cycles. The doping of Mn resulted in lowering of reduction and oxidation temperatures. The substitution of B cations with Mn4+ also facilitated CO oxidation reaction in a lower temperature range of 200–500°C, while LaVO4 or LaMnO4−δ showed very poor catalytic activity. The XRD powder patterns of all the Mn-doped samples revealed a redox behaviour, i.e. conversion of LaVO4 to LaVO3 and back to LaVO4, on subjecting a sample to H2-reduction and re-oxidation treatments. Our results substantiate the important role of oxygen vacancies generated in the crystal lattice on introduction of Mn4+, such vacancies in turn lead to improved catalytic activity because of a greater oxygen mobility in the lattice. An ionic redox mechanism, where the re-oxidation step is a limiting process, is applicable to CO oxidation at temperatures above 200°C.

Redox behavior and catalytic activity of La–Fe–V–O mixed oxides

Abstract The mixed oxides of composition LaFe x V 1− x O 4− δ (0.0≤ x ≤1.0) were characterized using XRD and FTIR techniques, and their redox behavior was examined by recording temperature-programmed reduction/oxidation profiles. The samples with x ≤0.2 and x ≥0.8 compositions were of monoclinic LaVO 4 and orthorhombic LaFeO 3 phase, respectively. The intermediate compositions consisted of a mix of these two phases in addition to small concentration of a non-stoichiometric LaV(Fe)O 4− δ phase, where iron occupied some of the lattice positions of vanadium. Substitution of Fe at a small level resulted in considerable lowering of the reduction temperature and also in the lowering of the temperature required for subsequent re-oxidation. At the same time, these samples showed higher catalytic activity for CO oxidation reaction and demonstrated a greater stability during the repeated cycles of reduction interspersed by oxidation steps. Our results reveal that the lattice defects in the host matrix and the synergistic effect between the multiple oxide phases may together control the overall performance of a multi-component oxide sample. Also, the data of this study support the predominance of an ionic redox mechanism where the re-oxidation step is a rate-limiting process.

Gas‐Phase Photooxidation of Alkenes by V‐Doped TiO2‐MCM‐41: Mechanistic Insights of Ethylene Photooxidation and Understanding the Structure–Activity Correlation

Nanoparticles of Ti(0.95)V(0.05)O(2) were found to be impregnated in the hexagonal channels of the MCM-41 host, with a distribution of some particles on the surface, thus leading to an effective variation in the particle size as a function of loading host MCM-41 matrix. These catalysts were subjected to the photocatalytic degradation of alkenes under the ambient conditions in which the photocatalytic activity varied as a function of the loading percentage of Ti(0.95)V(0.05)O(2) in the host MCM-41.This is explained in light of the structure-activity correlation, and the better catalytic activity can be attributed to an electronic interaction between the host and guest molecules, as established from X-ray photoelectron spectroscopy. To understand the mechanistic aspect of the photooxidation of ethylene on the vanadium-doped titania dispersed in the MCM-41 matrix, extensive in situ FTIR experiments were undertaken. The intermediate species produced on bare Ti(0.95)V(0.05)O(2) are different from that produced on the Ti(0.95)V(0.05)O(2)/MCM-41 surface. Moreover, different intermediates were produced during ethylene oxidation under UV and visible irradiation, thus leading to different rates. The ethylene decomposition over bare Ti(0.95)V(0.05)O(2) occurs by means of formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates, and then to CO(2) under both UV and visible irradiation. However, in the case of Ti(0.95)V(0.05)O(2)/MCM-41 catalyst with UV irradiation, the adsorbed acetaldehyde thus formed undergoes aldol condensation over the Lewis acid sites to lead to the formation of crotonaldehyde, which is subsequently oxidized to acetate and consequently to CO(2). It was observed that during visible irradiation labile ethyl acetate is produced either by the Tischenko reaction or by the reaction between the labile acetic acid and the unreacted ethoxy groups. The ethyl acetate produces acetic acid monomer, which is oxidized to CO(2). Furthermore, in this work the effects of particle size on the intermediate species were also studied.

Electroceramics for Fuel Cells, Batteries and Sensors

Electroceramics are ceramic materials with specific electrical, electromagnetic or optical properties. They are becoming increasingly important in many key technologies including communication, energy conversion and storage, electronics, sensors, and so on. Their properties can be tailored to operate as insulators, ferroelectric materials, highly conductive ceramics, electrodes as well as sensors and actuators. The performance of electroceramic materials and devices depends on the complex inter play between processing, chemistry and structure. Understanding the defect chemistry and electronic and ionic conduction in many high-temperature ceramics over the past four decades has attracted increasing attention for a variety of applications such as fuel cells, oxygen generators for life-support systems, high-temperature electrochemical sensors, and so on. The development of electroceramics depends upon the presence of ionic–covalent bonding, microstructure that comprises inorganic crystalline compounds and/or amorphous glass in varying proportions and thermal processing conducted at elevated temperatures. In this chapter, we give a broad overview of electroceramics, their synthesis, processing, electrical properties and their applications in advanced electrochemical devices.

In situ FT-IR studies on mechanistics of heterogeneous photocatalytic oxidation of ethene over uranyl species anchored on MCM-41

The uranyl species encapsulated within the mesopores of siliceous MCM-41 serves as efficient heterogeneous photo-catalyst for the sunlight-assisted direct oxidation of ethene. The mode of oxidation is through abstraction of H-atom from ethene by the photolytically excited uranyl species and the consequent formation of peroxy species. The in situ IR spectroscopy results indicate that these peroxy species give rise to final products such as carbon dioxide and water on further oxidation via formation of formate-type transient species. Furthermore, the silanol groups of the host matrix help in immobilization of these peroxy species through hydrogen bonding and, at the same time, they participate in the subsequent oxidation reactions also.

Study of the oxidation state and the structural aspects of the V-doped TiO2

A modified sol-gel method for synthesizing vanadium doped titania is being reported. These materials were thoroughly characterized for their oxidation states by electron paramagnetic resonance and x-ray absorption near edge structure and the local environment of the V-atom were investigated by the x-ray absorption fine structure. V-doped titania was found to be more active than nanotitania for photo-oxidation of methane in air under ambient conditions using UV-visible irradiation. The vanadium doping in the crystal lattice of titania leads to a mixture of oxidation states of 4+ and 5+ in the crystal lattice sites of the TiO2, which is crucial for its catalytic activity.

The Temperature-dependent Adsorption Behaviour of Benzene Molecules in ZSM-5 Zeolite Pores: TPD and FT-IR Spectroscopy Studies

Temperature-programmed desorption (TPD) and in situ Fourier-transform infrared (FT-IR) spectroscopic methods were employed to investigate the effect of loading and sample temperature on the state of benzene molecules inside the channels of NaZSM-5 zeolite. TPD profiles revealed the existence of at least three distinct states of benzene adsorption, characterized by desorption peak maxima at ca. 120°C, 170°C and 220°C, respectively. Based on the growth behaviour of these bands, it is suggested that the benzene molecules occupy sinusoidal channels, straight channels and external surfaces, in that order. A reverse trend was observed during the subsequent flushing of the sample at varying temperatures. A virtually fixed amount of benzene was occluded at these three locations, depending upon the loading. The FT-IR studies revealed that the benzene molecule exists in a compressed state in the zeolitic channels, with the molecular clusters formed in the process dispersing only at temperatures above 150°C. For initial benzene loadings of up to ca. 1.5 molecules/unit cell, the spectrum obtained showed that in the O—H stretch region the bridge-bonded OH groups and hydroxyl groups associated with the internal zeolitic channels were perturbed simultaneously. The results show that even for a loading lower than necessary for saturation, a considerable amount of benzene remains condensed at the external surface of ZSM-5 zeolite.

XPS and Raman studies of Pt catalysts supported on activated carbon

Activated carbon is a widely used support for dispersing noble metals in addition to its many applications. We have prepared platinum catalyst supported on activated carbon for HI decomposition reaction of I-S thermochemical process of hydrogen generation. These catalysts were characterized by XPS and Raman before and after using for the reaction. It was observed that platinum is present in zero oxidation state, while carbon is present is both sp2 and sp3 hybridized forms along with some amount of it bonded to oxygen.Activated carbon is a widely used support for dispersing noble metals in addition to its many applications. We have prepared platinum catalyst supported on activated carbon for HI decomposition reaction of I-S thermochemical process of hydrogen generation. These catalysts were characterized by XPS and Raman before and after using for the reaction. It was observed that platinum is present in zero oxidation state, while carbon is present is both sp2 and sp3 hybridized forms along with some amount of it bonded to oxygen.

XPS studies of Pt catalysts supported on porous carbon

Pt catalysts supported on porous carbon were prepared by hard templating route and used for HI decomposition reaction of Sulfur Iodine thermochemical cycle. These catalysts were characterized by X-ray photoelectron spectroscopy for oxidation state of platinum as well as nature of carbon present in the catalysts. It was found that platinum is present in metallic state and carbon is present in both sp2 and sp3 hybridization states. The catalysts were evaluated for their activity and stability for liquid phase HI decomposition reaction and it was observed that mesoporous carbon based catalysts were more active and stable under the reaction conditions.