Effects of Calcination Temperature and Acid-Base Properties on Mixed Potential Ammonia Sensors Modified by Metal Oxides

Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO3, Bi2O3 and V2O5, while the use of WO3, Nb2O5 and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO3 > Bi2O3 > V2O5, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified

Zinc oxide nanostructures with carbon nanotube and gold additives for co gas sensing application

Abstract: Zinc oxide (ZnO) nanostructures were synthesised for gas sensing application. In an attempt to improve the surface area and the electrical conductivity of the ZnO, nanomaterials such as the carbon nanotubes (CNTs) and gold nanoparticles (AuNPs) were used separately to produce CNTs/ZnO and Au/ZnO nanocomposites, respectively. The addition of these nanomaterials onto the ZnO nanostructures significantly improved the gas sensing properties such as the sensitivity and response time. Synthesis of gold nanoparticles was successfully achieved via gold salt (HAuCl4.3H2O) reduction using the Turkevich method. Citrate molecules were used as the stabiliser and to systematically control the sizes of the AuNPs. The sizes of AuNPs were found to increase from 14 nm to 40 nm when the concentration of citrate ions was reduced from 1 mM to 0.3 mM. The size distribution of AuNPs was relatively wider as the particle size increased. The synthesized AuNPs were stable for over a period of 4 weeks. Carbon nanotubes synthesis was achieved using chemical vapour deposition (CVD) method using acetylene gas as the carbon source and ferrocene as the catalyst. An increase in the flowrate of the precursor gas (acetylene) yielded an increase in amorphous carbon, which was attached to the walls of the carbon nanotubes. The optimum flowrate of acetylene was found to be 150 m3/min that yielded CNTs with an average diameter of 95 nm and a relatively narrow size distribution. The hydrothermal chemical precipitation method was used to synthesise ZnO nanostructures. Zinc sulphate (ZnSO4) and sodium hydroxide (NaOH) were used as a metal precursor and reducing agent, respectively. The NaOH concentration of 0.3 M yielded ZnO nanosheets with relatively the highest surface area of 102 m2/g. Gas sensing analysis was conducted using carbon monoxide (CO) gas at 250°C. The sensitivity and response time were calculated to be 9.8% and 114 seconds, respectively, at a CO concentration of 200 ppm. The composites CNTs/ZnO and Au/ZnO were prepared, separately. The average surface area of the Au/ZnO composite was 131 m2/g and that of CNTs/ZnO composite was 153 m2/g. The CNTs/ZnO composite showed an optimum sensitivity of 9.9% and the response time of 49 seconds when exposed to 200 ppm of CO gas at 250°C.M.Tech. (Chemical Engineering

Recent Advancements in TiO2 Nanostructures: Sustainable Synthesis and Gas Sensing

The search for sustainable technology-driven advancements in material synthesis is a new norm, which ensures a low impact on the environment, production cost, and workers' health. In this context, non-toxic, non-hazardous, and low-cost materials and their synthesis methods are integrated to compete with existing physical and chemical methods. From this perspective, titanium oxide (TiO2) is one of the fascinating materials because of its non-toxicity, biocompatibility, and potential of growing by sustainable methods. Accordingly, TiO2 is extensively used in gas-sensing devices. Yet, many TiO2 nanostructures are still synthesized with a lack of mindfulness of environmental impact and sustainable methods, which results in a serious burden on practical commercialization. This review provides a general outline of the advantages and disadvantages of conventional and sustainable methods of TiO2 preparation. Additionally, a detailed discussion on sustainable growth methods for green synthesis is included. Furthermore, gas-sensing applications and approaches to improve the key functionality of sensors, including response time, recovery time, repeatability, and stability, are discussed in detail in the latter parts of the review. At the end, a concluding discussion is included to provide guidelines for the selection of sustainable synthesis methods and techniques to improve the gas-sensing properties of TiO2

Binary metal oxides based on Fe (III) and Ti (IV) as efficient catalysts for total oxidation of volatile organic compound pollutants

Fe (III)-modified Titania and Fe (III)-Ti (IV) binary oxides have received much less attention as possible catalysts compared to their parent single-metal oxides, Fe2O3 and TiO2. This could be due to the difficulty of obtaining pure desired monophases. In addition, the effect of different preparative conditions on the textural properties of sol-gel-prepared Ti-Fe mixed oxides was rarely studied. Furthermore, the effect of the composition on the reducibility and the catalytic activity of these composites was never studied. In the present work, Ti-Fe mixed oxides were prepared using a modified sol-gel method and their textural properties as well as their reducibility and their catalytic activity were investigated and were compared with those of parent single-metal oxides. The use of propylene oxides (PO) as a gelation promoter as well as the presence of hetero-ions was found to play a key role in promoting gel formation at certain concentrations. While in the presence of a single metal, colloidal solutions and very fine precipitates were obtained, gels formed readily from mixed solutions containing 5-15% Fe(III) in the presence of PO, and from solutions containing 40 and 66.7% Fe(III) even in the absence of PO. While Fe (III) concentration as high as 10% was well dispersed in the Titania lattice, which was also associated with enhanced stability of the anatase structure, higher concentrations resulted in the formation of anatase and pserudorutile (Pr) initially, which converted to rutile and pseudobrookite (Pb) upon heating at elevated temperatures. The preparation of a pure Pr phase was possible under the employed preparative conditions starting with a solution containing 40% Fe (III). However, the presence of higher concentrations resulted in the formation of some segregated α- Fe2O3. Xerogel and aerogel mixed oxides possessed significantly higher surface areas than their parent single metal oxides, and the surface area increased as the Fe (III) concentration increased. Furthermore, the mixed oxides showed an enhanced reducibility indicating a more labile lattice oxygen. The mixed oxides possessed significantly improved catalytic activity compared with their single-metal oxide counterparts, especially at lower temperatures. Using 4% air in the reaction mixture resulted in the formation small amounts of benzene besides CO2 as the major product. However, Using 16% air in the reaction mixture, resulted in deep oxidation to CO2 as the only product. Among the tested catalysts, TiFe67 showed the highest catalytic activity making it a promising catalyst for oxidative degradation of volatile organic compounds, VOCs

Multifunctional Oxide-Based Materials: From Synthesis to Application

The book deals with novel aspects and perspectives in metal oxide and hybrid material fabrication

Rational design of mesoporous gallium oxide and gallium-based mixed oxide catalysts.

In the present study, we report the synthesis of thermally stable mesoporous gallium oxide and novel gallium-niobium mixed oxides employing Evaporation Induced Self-Assembly (EISA), Self-Assembly Hydrothermal-Assisted (SAHA) and Self-Assembly Microwave-Assisted approaches. These methods offer the possibility to synthesize thermally stable mesoporous oxides with controlled morphological, textural and structural properties. EISA led to partially crystalline meso porous gallium oxide phases displaying unimodal pore size distribution in the ~2-5 nm range and surface areas as high as 300 m2/g. SAHA led to nanocrystalline mesoporous uniform micron-sized gallium oxide spheres (~0.3-6.5 11m) with narrow size distribution displaying cubic spinel type structure. These mesophases displayed surface areas as high as 220 m2/g and unimodal pore-size distribution in the 5-15 nm range. Microwave-assisted approach led to the formation of nanocrystalline mesoporous gallium oxide phases at low reaction temperature (l30°C) and short reaction times (~15-120 min). Novel semicrystalline mesoporous Gallium-Niobium mixed oxide phases were prepared via Self-Assembly Hydrothermal-Assisted (SAHA) method. This method led to the formation of uniform ~ 0.3-2 11m micron-sized mesoporous mixed gallium-niobium oxide spheres with narrow size distribution displaying surface areas as high as 360 m2/g and unimodal pore size distribution in the 3-6 nm range. Due to their high surface areas, tunability of pore sizes and their acidic nature these single phase and mixed mesoporous gallium-niobium oxides were employed as catalysts in the epoxidation of cyclooctene and isomerization of methyl oleate. For the epoxidation of cyclooctene to epoxycyclooctane carried out at 60°C the mesoporous gallium oxide displayed 100% selectivity towards epoxide with the conversion of cyclooctene in the 4 to 16% range. As the reaction temperature was increased to 80°C, an increase in the cyclooctene conversion was observed. The highest cyclooctene conversion observed was ~52% with a selectivity of 83% toward the epoxide. A clear correlation was observed between the cyclooctene conversion and gallium oxide particle size at both reaction conditions. Agglomerate size between 2-3 11m led to higher cyclooctene conversion, whereas the agglomerate sizes between 4.5-7.5 11m led to lower cyclooctene conversions. For the isomerisation of methyl oleate, highest conversion of 57% with the selectivity of 86% and yield of ~50% was observed over a sample with gallium-niobium composition of 0.3:0.7 wt%. The superior catalytic performance of the gallium-niobium mixed oxide was attributed to its high acidity, crystallinity and mesoporosity

Synthesis, Chracterization and Applications of Coated Composite Materials for Energy Applications

The formulation of coated composite materials is an important field of research around the world today. Coated composite materials include inhomogeneous and anisotropic materials. These materials are formulated by an amalgamate minimum of two or more materials that accommodate different properties. These materials have a vast field of appealing applications that encourage scientists to work on them. Due to their unique properties, such as their strength, liability, swiftness, and low cost, they are used as promising candidates for reliable applications in various fields, such as biomedical, engineering, energy devices, wastewater treatment, and agriculture. Different types of composite materials have had a noticeable impact in these fields already, such as glass, plastic, and, most promisingly, metal oxide nanoparticles

Nanostructured catalysts for the development of the hydrogen economy

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2015-2016La catalyse joue un rôle essentiel dans de nombreuses applications industrielles telles que les industries pétrochimique et biochimique, ainsi que dans la production de polymères et pour la protection de l'environnement. La conception et la fabrication de catalyseurs efficaces et rentables est une étape importante pour résoudre un certain nombre de problèmes des nouvelles technologies de conversion chimique et de stockage de l'énergie. L'objectif de cette thèse est le développement de voies de synthèse efficaces et simples pour fabriquer des catalyseurs performants à base de métaux non nobles et d'examiner les aspects fondamentaux concernant la relation entre structure/composition et performance catalytique, notamment dans des processus liés à la production et au stockage de l'hydrogène. Dans un premier temps, une série d'oxydes métalliques mixtes (Cu/CeO2, CuFe/CeO2, CuCo/CeO2, CuFe2O4, NiFe2O4) nanostructurés et poreux ont été synthétisés grâce à une méthode améliorée de nanocasting. Les matériaux Cu/CeO2 obtenus, dont la composition et la structure poreuse peuvent être contrôlées, ont ensuite été testés pour l’oxydation préférentielle du CO dans un flux d'hydrogène dans le but d’obtenir un combustible hydrogène de haute pureté. Les catalyseurs synthétisés présentent une activité et une sélectivité élevées lors de l'oxydation sélective du CO en CO2. Concernant la question du stockage d'hydrogène, une voie de synthèse a été trouvée pour le composét mixte CuO-NiO, démontrant une excellente performance catalytique comparable aux catalyseurs à base de métaux nobles pour la production d'hydrogène à partir de l'ammoniaborane (aussi appelé borazane). L'activité catalytique du catalyseur étudié dans cette réaction est fortement influencée par la nature des précurseurs métalliques, la composition et la température de traitement thermique utilisées pour la préparation du catalyseur. Enfin, des catalyseurs de Cu-Ni supportés sur silice colloïdale ou sur des particules de carbone, ayant une composition et une taille variable, ont été synthétisés par un simple procédé d'imprégnation. Les catalyseurs supportés sur carbone sont stables et très actifs à la fois dans l'hydrolyse du borazane et la décomposition de l'hydrazine aqueuse pour la production d'hydrogène. Il a été démontré qu'un catalyseur optimal peut être obtenu par le contrôle de l'effet bi-métallique, l'interaction métal-support, et la taille des particules de métal.Catalysis plays an essential role in many industrial applications such as petrochemical and biochemical industries, as well as in the production of polymers and in environmental protection. Design and fabrication of efficient catalysts in a cost-effective way is an important milestone to address a number of unresolved issues in the new generation of chemical and energy conversion technologies. The objective of the studies in this thesis is the development of facile synthetic routes to prepare efficient catalysts based on non-noble metals, and elucidate fundamental aspects regarding the relationship between structure/composition and catalytic performance, in particular in the case of processes related to production and storage of hydrogen fuel. At first, a series of nanostructured porous mixed metal oxides (Cu/CeO2, CuFe/CeO2, CuCo/CeO2, CuFe2O4, NiFe2O4) have been synthesized via an improved nanocasting method. The porous structure of the nanocast products was tailored by tuning the mesostructure of the mesoporous silica phases used as templates. The obtained Cu/CeO2 materials with controlled composition and porous structure were then tested in preferential oxidation of CO in a hydrogen stream to achieve high purity hydrogen fuel. The synthesized catalysts exhibit high activity and selectivity in selective oxidation of CO to CO2. Regarding hydrogen storage, we reported a cost-effective synthetic way towards bi-component CuO-NiO catalyst showing excellent catalytic performance, which is comparable to noble metal catalysts, in the hydrogen generation from ammonia-borane. Moreover, we demonstrate that the interaction between Cu and Ni species is essential in accelerating hydrogen evolution of ammonia borane. The catalytic activity of the obtained catalyst investigated in this reaction is strongly influenced by the nature of the metal precursors, the composition and the thermal treatment temperature employed for the catalyst preparation. Finally, silica- and carbon-supported Cu-Ni nanocatalysts, with tunable composition and metal particle size, were synthesized by simple incipient wetness method. The carbon supported catalysts are stable, highly active and selective in both ammonia-borane hydrolysis and the decomposition of hydrous hydrazine for hydrogen evolution. We showed that optimal catalysts can be achieved through manipulation of bimetallic effect, metal-support interaction, and adequate metal particle size

Multilayer Thin Films

This book, "Multilayer Thin Films-Versatile Applications for Materials Engineering", includes thirteen chapters related to the preparations, characterizations, and applications in the modern research of materials engineering. The evaluation of nanomaterials in the form of different shapes, sizes, and volumes needed for utilization in different kinds of gadgets and devices. Since the recently developed two-dimensional carbon materials are proving to be immensely important for new configurations in the miniature scale in the modern technology, it is imperative to innovate various atomic and molecular arrangements for the modifications of structural properties. Of late, graphene and graphene-related derivatives have been proven as the most versatile two-dimensional nanomaterials with superb mechanical, electrical, electronic, optical, and magnetic properties. To understand the in-depth technology, an effort has been made to explain the basics of nano dimensional materials. The importance of nano particles in various aspects of nano technology is clearly indicated. There is more than one chapter describing the use of nanomaterials as sensors. In this volume, an effort has been made to clarify the use of such materials from non-conductor to highly conducting species. It is expected that this book will be useful to the postgraduate and research students as this is a multidisciplinary subject