Non-oxidizing Environments Research Articles

Design and processing of SiOC/SiC/Si3N4/C ceramics with hierarchical porosity and non-oxide nanostructures

An integral study on the development and characterization of porous SiOC/SiC/Si3N4/C-based ceramic bodies was conducted using a less-explored approach, which is based on the direct consolidation of previously unstudied composite systems (methacryloxypropyl silsesquioxane/sucrose/prepyrolyzed powder), burn-out, and N2 pyrolysis (1300–1400 °C). Their textural, physicochemical, and microstructural properties depended on the initial system composition and pyrolysis temperature; the latter was also decisive in the materials' oxidative stability. At the highest pyrolysis temperature, SiC, mainly in the form of whiskers, and Si3N4 formed, the amount of SiOC decreased, Si2N2O crystallized, and nanocrystalline graphite-type carbon underwent graphitization. The final bodies exhibited macro- and mesopores, with micropores present only in those pyrolyzed at 1300 °C. This latter material presented the highest specific surface area, making it a potential candidate for catalysis in non-oxidizing environments at high temperatures or in oxidizing conditions at lower temperatures. Meanwhile, the material developed at 1400 °C offered high-temperature oxidative stability.

OXIDATION CHARACTERISTICS OF THERMAL-SPRAYED COBALT-BASED SUPERALLOY COATINGS: A REVIEW

Superalloys go through cyclic oxidation which refers to the progression of reputed exposure of materials to alternating oxidizing and non-oxidizing environments at high temperatures. The oxidation behavior of cobalt-based superalloy coatings depends on several variables, including the environment, surface roughness, manufacturing conditions, & coating composition. Because of their higher temperatures and protective oxide layer, coatings with a greater cobalt concentration are more oxidation-resistant. The behavior of oxidation is influenced by surface roughness, where rough surfaces offer greater surface area to oxidation while smooth surfaces decrease interaction with the environment. The microstructure and porosity of the coating are further impacted by processing variables used in thermal spraying, including temperature, particles velocity, and spray distance. Adding reactive components to the coating composition, such as silicon and aluminum, and applying post-treatments like nitriding or sealing are two ways to increase oxidation resistance. A greater temperature and particle speed can lead to a denser with less porous covering, improving the oxidation process resistance. The two typically used heat spraying techniques are high-velocity oxy-fuel (HVOF) and plasma spraying. To increase the resistance to oxidation of thermal spray cobalt-based superalloy coating, some strategies have been devised, such as inclusion of reactive materials, such as aluminum and silicon, which is the coating composition. These elements may establish an oxide layer protecting the coating’s appearance, preventing further oxidation.

New Optimized Lubricating Blend of Peanut Oil and Naphthenic Oil Additivated with Graphene Nanoparticles and MoS2: Stability Time and Thermal Conductivity

Lubricants are essential to machinery life, as they play a crucial role in controlling and diminishing the friction and wear between moving parts when operated under extreme conditions. To this end, due to tight environmental conditions, manufacturers are looking for alternative solid lubricants to be dispersed in base liquid lubricants. MoS2 and graphene are solid lubricants that provide low frictional properties and high thermal stability in both oxidizing and non-oxidizing environments. This research offers a new lubricant with improved thermal conductivity that combines the synergistic effect of graphene and MoS2 in a blend of vegetable oil (peanut) and naphthenic oil. The ratio of peanut oil and naphthenic oil varies from 1:3–3:1. A fixed composition of 4.34 wt.% palm oil methyl ester (POME) is added to enhance the anti-wear property further. Graphene and MoS2 concentrations varied between 1:2–5:2, respectively. This nanoparticle additive oil blend is physically mixed using a water bath sonication for 4 h. The stability of the blend lubricant dispersed with MoS2 and graphene is studied using a UV-Vis spectrophotometer for 25 days. The effect of various concentrations of graphene, MoS2, peanut oil, and naphthenic oil on the thermal conductivity of the nanolubricant is also studied as a function of temperature (25 °C–55 °C). Artificial neural network models were used for the parametric investigation of the nanolubricant. It is found that the stability of the formulated nanolubricant increased with peanut oil composition above 25 wt.%. The results show that the 3:1 blend ratio showed higher stability for hybrid MoS2-based lubricants. Similarly, the highest thermal conductivity is observed for 100 wt.% naphthenic oil with a 1:2 ratio of graphene–MoS2 at 55 °C.

In situ LA-ICPMS U[sbnd]Pb dating and geochemical characterization of fault-zone calcite in the central Tarim Basin, northwest China: Implications for fluid circulation and fault reactivation

Direct evidence for fault reactivation is crucial for understanding long-term fault behavior and reconstruction of fluid circulation and hydrocarbon migration and accumulation during active tectonics. However, absolute dating of fault reactivations and fluid circulation is still challenging due to dissolution and recrystallization of fault zone materials and obliteration of previous events by the latest deformation/hydrothermal event. The lower Ordovician limestone strata at 3.8 km depth in the Central Uplift of the Tarim Basin, northwest China experienced multi-stage regional tectonic activities that developed a series of NW–SE striking reverse faults and NE–SW oriented strike-slip faults. In this study, we performed in situ laser-ablation ICP-MS UPb dating and trace element characterization of calcites collected from borehole cores within one of the main fault zones in the Central Uplift of the Tarim Basin. A new in-house calcite standard AHX-1A (209.8 ± 1.3 Ma), collected from the Tarim Basin, calibrated against well-calibrated WC-1 and ASH-15 calcite standards, is used for mass-bias correction. We obtained in situ UPb ages of 9 calcite samples from two wells within a Paleozoic fault zone in the Tarim Basin. They are 456 ± 11 Ma, 454.7 ± 7.2 Ma, 450.4 ± 6.2 Ma, 435.2 ± 9.7 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma from well TZ2, and 371 ± 18 Ma, 390.6 ± 6.0 Ma and 395 ± 14 Ma from well TZ4 (all reported uncertainties are 2σ). Combined with petrographic evidence, these results define at least six generations of secondary calcite precipitations in the fault zone, that occurred during 456 ± 11 to 450.4 ± 6.2 Ma (weighted mean 452.8 ± 4.3 Ma), 435.2 ± 9.7 Ma, 395 ± 14 to 390.6 ± 6.0 Ma (weighted mean 391.3 ± 5.5 Ma), 371 ± 18 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma. Chondrite-normalized rare earth element (REE) + yttrium (Y) patterns of these calcite samples show prominent positive Gd and Y anomalies, lack of negative Ce anomaly, and enrichment in light REE but depletion in heavy REE, similar to the shallow marine platform host-rock limestone. The results suggest multi-stage fluid circulation under shallow burial, low temperature, and non-oxidizing environments along the fault zone, with the secondary calcite precipitating from a similar fluid source that originated from dissolution of host-rock carbonate or early-stage diagenesis. The detail petrographical and mineralogical analysis of this study, combined with the new interpretation of high-resolution seismic data, constrains two main episodes of faulting and related uplift during 456–435 Ma and 395–371 Ma, as well as a younger faulting episode with weaker intensity during 328–308 Ma. This study demonstrates that the central fault zone of the Tarim Basin experienced ~150 Ma of episodical reactivation during the Paleozoic, which has important implications for hydrocarbon exploration.

Corrosivity Study of Sensitized Welded and Unwelded Austenitic Stainless Steel AISI 304 in Oxidizing and Non-Oxidizing Environment

An experimental investigation was carried out to determine the corrosive impact of oxidizing and non-oxidizing environments on sensitized welded and unwelded samples of AISI 304. The selected samples were cut into several sizes. To induce sensitization, the samples were heated and soaked at 750oC at different soaking time intervals such as 30 minutes, 60 minutes, 180minutes, 300 minutes and 600 minutes followed by water quenching [1]. The resultant sensitized weldment and unwelded samples were subjected to immersion duration test each in oxidizing (H2SO4) and non-oxidizing (HCL) media for 5, 10, 15, 20, 25, 30, 35, 40, 45, & 50 minutes respectively. From the results obtained, it was concluded that corrosion rate decreases as soaking time/immersion duration increases, at constant soaking temperature in non-oxidizing medium of hydrochloric acid (HCL). However, the corrosion rate decrease of the unwelded samples of AISI 304 in non-oxidizing medium is greater than that of their welded counterpart as immersion duration/soaking time increases. Similarly, the unwelded and welded samples of AISI 304, in oxidizing medium of H2SO4 have one common characteristic with their corrosion rate decreasing as soaking time/immersion duration increases; but, surprisingly, the welded samples are having lower corrosion rate than that of the unwelded counterpart as the immersion duration/soaking time increases between 5-15minutes; but reverses to a higher corrosion rate than their unwelded counterpart as immersion duration increases between 15-50 minutes. The results showed that each sample reacted differently in oxidizing and non-oxidizing media; hence oxidizing and non-oxidizing media impacted the materials’ properties differently. The sensitization differences of the welded and the unwelded counterpart obtained during soaking time, significantly affected their reactions in oxidizing and non-oxidizing media which led to differences in the corrosive impacts, under the same environment.

Corrosivity Study of Sensitized Welded and Unwelded Austenitic Stainless Steel AISI 304 in Oxidizing and Non-Oxidizing Environment

An experimental investigation was carried out to determine the corrosive impact of oxidizing and non-oxidizing environments on sensitized welded and unwelded samples of AISI 304. The selected samples were cut into several sizes. To induce sensitization, the samples were heated and soaked at 750oC at different soaking time intervals such as 30 minutes, 60 minutes, 180minutes, 300 minutes and 600 minutes followed by water quenching [1]. The resultant sensitized weldment and unwelded samples were subjected to immersion duration test each in oxidizing (H2SO4) and non-oxidizing (HCL) media for 5, 10, 15, 20, 25, 30, 35, 40, 45, & 50 minutes respectively. From the results obtained, it was concluded that corrosion rate decreases as soaking time/immersion duration increases, at constant soaking temperature in non-oxidizing medium of hydrochloric acid (HCL). However, the corrosion rate decrease of the unwelded samples of AISI 304 in non-oxidizing medium is greater than that of their welded counterpart as immersion duration/soaking time increases. Similarly, the unwelded and welded samples of AISI 304, in oxidizing medium of H2SO4 have one common characteristic with their corrosion rate decreasing as soaking time/immersion duration increases; but, surprisingly, the welded samples are having lower corrosion rate than that of the unwelded counterpart as the immersion duration/soaking time increases between 5-15minutes; but reverses to a higher corrosion rate than their unwelded counterpart as immersion duration increases between 15-50 minutes. The results showed that each sample reacted differently in oxidizing and non-oxidizing media; hence oxidizing and non-oxidizing media impacted the materials’ properties differently. The sensitization differences of the welded and the unwelded counterpart obtained during soaking time, significantly affected their reactions in oxidizing and non-oxidizing media which led to differences in the corrosive impacts, under the same environment.

Characterization of a Cast Duplex Stainless Steel with 3.0%Cu and Modeling of Precipitation Hardening

Duplex stainless steels (DSSs) are corrosion-resistant alloys extensively used in aggressive environments. Cast DSSs may be selected for pipes, valves and pumps in chemical, petrochemical and nuclear industries. The grade steel ASTM A890 1B is an example of cast DSS with 2.7-3.3%Cu addition. Copper increases the resistance to many types of corrosion, especially in non-oxidizing environments. When the copper content is higher than 2%, the steel can be precipitation-hardened. In this work, the precipitation hardening of a DSS ASTM A890 1B steel with 3.0%Cu was studied and modeled for aging temperatures in the 450-600 °C range. Copper precipitates in the ferrite phase, but remains in solid solution in the austenite. The age hardening curves were modeled by ΔH = K(t)n model, where ΔH is the increase in hardness, t is the aging time, and K and n are constants to be determined. The microstructure and substructure were investigated by scanning electron and transmission electron microscopes.

Corrosion Resistance of Cu-Alloyed Precipitation Hardenable Duplex Stainless Steel ASTM A890 Grade 1B

Duplex stainless steels (DSS) are corrosion resistant alloys largely used in chemical and petrochemical industries. Some commercial DSS contain 0.5-1.0% copper addition to improve the corrosion resistance by reducing the corrosion rate in non-oxidizing environments. Higher copper addition (≥ 2%) can also hard by precipitation, especially when fine copper precipitates (e phase) are produced. In this work, a cast copper alloyed DSS type ASTM A890 grade 1B with 3.01%Cu was investigated. Different levels of hardness were produced by solution treatment and aging at 450, 500, 550 and 600 oC for periods of time up to 1 hour. The corrosion resistance of aged DSS was evaluated by electrochemical tests in three media: 0.6 mol/L NaCl, 0.3 mol/L H2SO4 and 0.6 mol/L NaCl + 0.3 mol/L H2SO4 solutions. The results indicate that the effect of Cu addition depends on the media studied. Polarization studies in 0.6 mol/L NaCl showed a small anodic current peak occurred at around 400 mV vs. Ag/AgCl, with a strong influence on the passive film stability. Additionally, chronoamperometric measurements at 400 mV vs. Ag/AgCl showed a high electrochemical activity for the samples in 0.6 mol/L NaCl.

Use of interphase in geopolymer matrix composites for improved toughness

Geopolymers are a lightweight material with excellent fire and thermal resistance. When combined with continuous fiber reinforcement, geopolymers demonstrate potential to replace heavier, structural components, especially those that are exposed to elevated temperature regimes (> 400 °C) in air. Oxidation resistance is essential to continuous operations in these environments. While oxide fibers can drastically improve the mechanical properties of geopolymers, their incorporation has been met with challenges. These geopolymer matrix composites (GMC) lack the necessary toughness to find much use as a structural component. In this study, concepts and materials used to reduce interfacial strength in ceramic matrix composites for the purpose of improving toughness are employed in GMCs. To investigate the potential to improve toughness, composites containing an alumina-based fiber and geopolymer were prepared with three different interfacial conditions: heat cleaned, carbon coated, and monazite coated. From the fabricated composites, specimens for four-point flexural testing and single fiber pushout were prepared. Additionally, composite samples were heat treated to 650 °C in oxidizing and non-oxidizing environments. To varying degrees, composites that utilized a fiber coating demonstrated improvements in toughness and relative damage tolerance as compared to those without any modification to the interface. In some cases, the improvement was over 160% as compared to those without a coating. Pushout testing showed specimens with coated fiber to have weaker interfacial conditions. The combined results support the use of tailored interface concepts in GMCs for improving toughness.

Grand Challenges in Carbon-Based Materials Research

CARBON IN ALL FLAVORS: CARBON MATERIALS CONSTITUTE THE MOST VERSATILE MATERIAL GROUP Carbon materials constitute a large family of diverse structures and textures that underlie an increasing range of applications suggested by the impressive number of papers published on this topic. Several hundreds of thousands of publications have appeared since 1900, out of which, thousands were published in 2013 only (Web of ScienceTM, 2014 Thomson Reuters, using as searching topics “carbon materials,” “carbon chemistry,” “carbon nanomaterials,” “carbon nanotubes,” and “graphene”). The popularity of carbon materials is due to a unique combination of physical and chemical properties such as high electrical conductivity, high surface area, good resistance to corrosion, high thermal stability, and high chemical stability in non-oxidizing environments and particular mechanical properties. Carbon materials are easy to process, provide a wide variety of structures and textures, have diverse surface chemical properties and are compatible with other materials, thus are ideal for composites as well (Inagaki et al., 2014). This unique combination of properties is a consequence of the different hybridization of orbitals in C atoms and the possibility of combining with other heteroatoms. This makes possible to derive the different allotropes known to the moment and the large number of textures discovered up to now (Inagaki et al., 2014). The possibility of having different hybridization of the carbon atoms in the same material and other heteroatoms, forming multicomponent systems, led to the definition of “carbon alloys” (Yasuda et al., 2003). Carbon is an “old but new material” (Walker, 1990) with continuous discoveries that allows us to recognize the amazing and challenging research frontiers in carbon science. An elegant way to introduce the huge synthetic possibilities of carbon materials is by classifying them into classic and new carbon forms (Inagaki and Radovic, 2002). Classic materials include activated carbons, graphite, and carbon black; new carbon forms contain carbons developed since 1960 such as carbon fibers, glassy carbons, diamond-like carbons, and recent lowdimensional carbons, e.g., fullerenes, carbon nanotubes, and graphene. The number of structures and textures that can be created by controlling the carbonization process are enormous, this leads to the discovery of numerous carbon nanoforms. Considering only sp2 carbon nanoforms, more than 25 different forms have been identified. These can be found under different names in the literature that highlights the urgent need for a standardized nomenclature in the field (Suarez-Martinez et al., 2012).

Phase Stability in the Mo-Ti-Zr-C System via Thermodynamic Modeling and Diffusion Multiple Validation

Alloys in the Mo-rich corner of the Mo-Ti-Zr-C system have found broad applications in non-oxidizing environments requiring structural integrity well beyond 1273 K (1000 °C). Alloys such as TZM (Mo-0.5Ti-0.08Zr-0.03C by weight %) and TZC (Mo-1.2Ti-0.3Zr-0.1C by weight) owe much of their high temperature strength and microstructural stability to MC and M2C carbide phases. In turn, the stability of the respective carbides and the subsequent mechanical behavior of the alloys are strongly dependent on the alloying additions and thermal history. A CALPHAD-based thermodynamic modeling approach is employed to develop a quaternary thermodynamic database for the Mo-Ti-Zr-C system. The thermodynamic database thus developed is validated with diffusion multiple experiments and the validated database is exercised to elucidate the effects of alloying and thermal history on the phase equilibrium in Mo-rich alloys.

Thermal and Irradiation Creep Behavior of a Titanium Aluminide in Advanced Nuclear Plant Environments

Titanium aluminides are well-accepted elevated temperature materials. In conventional applications, their poor oxidation resistance limits the maximum operating temperature. Advanced reactors operate in nonoxidizing environments. This could enlarge the applicability of these materials to higher temperatures. The behavior of a cast gamma-alpha-2 TiAl was investigated under thermal and irradiation conditions. Irradiation creep was studied in beam using helium implantation. Dog-bone samples of dimensions 10 × 2 × 0.2 mm3 were investigated in a temperature range of 300 °C to 500 °C under irradiation, and significant creep strains were detected. At temperatures above 500 °C, thermal creep becomes the predominant mechanism. Thermal creep was investigated at temperatures up to 900 °C without irradiation with samples of the same geometry. The results are compared with other materials considered for advanced fission applications. These are a ferritic oxide-dispersion-strengthened material (PM2000) and the nickel-base superalloy IN617. A better thermal creep behavior than IN617 was found in the entire temperature range. Up to 900 °C, the expected 104 hour stress rupture properties exceeded even those of the ODS alloy. The irradiation creep performance of the titanium aluminide was comparable with the ODS steels. For IN617, no irradiation creep experiments were performed due to the expected low irradiation resistance (swelling, helium embrittlement) of nickel-base alloys.

Partially reduced heteropolyanions for the oxidative dehydrogenation of ethane to ethylene and acetic acid at atmospheric pressure

Niobium- and pyridine-exchanged salts of phosphomolybdic (NbPMo 12Pyr) and phosphovanadomolybdic acids, NbPMo 12Pyr and NbPMo 11VPyr, respectively, were investigated as precursors to materials that catalyze the oxidative dehydrogenation of ethane to ethylene and acetic acid at atmospheric pressure. The effects of feed composition, steam flow, temperature, and precursor composition on catalytic activity and selectivity are presented for both ethane and ethylene oxidation. The productivity of ethylene and acetic acid from ethane using the catalytic materials formed from these precursors exceeds those reported in the literature for Mo–V–Nb–O x systems under atmospheric or elevated pressure. Also, the production of acetic acid from ethylene is greater than that observed from Mo–V–Nb–O x systems. Water is found to aid in desorption of acetic acid, thereby limiting deep oxidation to carbon oxides. Addition of vanadium reduces catalytic activity and selectivity to both ethylene and acetic acid, whereas niobium is essential for the formation of acetic acid from ethane. Other metals, such as titanium, tantalum, zirconium, antimony, iron, and gallium, do not provide the same beneficial effect as niobium. A balance of niobium and protonated pyridine in the catalyst precursor is required to produce an active catalyst by thermal treatment in nonoxidizing environments. Molybdenum and niobium are also necessary for the creation of the active site for ethane activation, and niobium is directly involved in the transformation of ethylene to acetic acid. A reaction scheme is proposed for the production of acetic acid from ethane over the catalytic materials.

In-Situ Pretreatment Approach for Surface Deterioration Alleviation Amidst Thermal Desorption of GaAs(100)

AbstractWithin this study, a novel in-situ pretreatment is proposed theoretically and demonstrated experimentally, in which the formation of surface pits is subsequently stifled during thermal desorption. The proposed method involves fueling the well reviewed chemical oxide reduction reaction with a segregated source of material other than that ordinarily utilized in pit formation. The proposed method is implementable in virtually all deposition systems subject to the constraints of providing material deposition, substrate heating, and the creation of non-oxidizing environments either via vacuum or inert atmo sphere.

In-Situ Pretreatment Approach for Surface Deterioration Alleviation Amidst Thermal Desorption of Si(100)

AbstractWithin this article, a novel in-situ method is proposed as a modification of thermal desorption utilizing pretreatment which can be applied to systems subject to material deposition, substrate heating, and creation of non-oxidizing environments (vacuum or inert atmosphere). Following the theoretical development of this proposed method, involving the fue ling of the oxide-reduction reaction with segregated sacrificial material, the method is demonstrated experimentally on Si (100) wafers utilizing ex-situ atomic force microscopy for resulting surface topography analysis and in-situreflective high-energy electron diffraction for crystalline information during the modified thermal desorption progression.

Effect of carbon fibers on the thermophysical properties of TiC composites

Ceramics have a high degree of brittleness, relatively low thermal conductivity, and high elastic modulus, which renders them highly susceptible to catastrophic failure under severe thermal transient conditions [1, 2]. In the past 40 years, carbon fibers have been extensively used to reinforce glass, glass-ceramic, and ceramic matrix composites because they producing an excellent combination of strength and toughness [3–6]. In our previous work, TiC composites containing 20 vol% short carbon fibers (Cf/TiC) were prepared by an optimum vacuum hot pressing method, and they exhibited a high fracture toughness and good high temperature strength compared with the monolithic TiC materials [7], which demonstrated that carbon fiber reinforced TiC composites would be a potential high temperature material in non-oxidizing environments. It is hoped that the addition of carbon fibers to the TiC matrix can improve the thermal shock resistance of TiC ceramics by modifying the thermophysical properties. However, not enough attention was paid to the thermophysical properties, such as the thermal expansion coefficient, α, specific heat, Cp, thermal diffusivity, λ, and thermal conductivity, κ , which are at least as important as strength and fracture toughness, especially for applications in thermal shock environments. The aim of this work was to investigate the temperature dependence of the thermal expansion, specific heat, thermal diffusion, and thermal conduction of Cf/TiC composites, and discuss the effects of carbon fibers on these thermophysical properties. TiC composites containing 20 vol% short carbon fibers were prepared by hot-pressing at 2100 ◦C, 30 MPa for 1 h in vacuum; the short fibers, with a length from 20–180 μm and a diameter of 6–8 μm, were randomly and uniformly distributed in the TiC matrix. The detailed information of the materials preparation procedure can be found elsewhere [7]. Monolithic TiC was also fabricated by the same procedure for comparison reinforce. Thermophysical properties measurements were carried out in the temperature range from room tempera-

Rare-earth elements as tracers of the genetic relationship between smectite and palygorskite in marine phosphorites

AbstractDetrital smectite in a sandy claystone and a phosphorite, and authigenic palygorskite in a dolomitic marl and a porcellanite from Cretaceous-Tertiary phosphorite deposits of the Ganntour Basin (Morocco) were purified using cation exchange resin, leached with dilute acid, and analysed for the contents and distribution patterns of their REE before and after acid treatment. The normalized patterns confirm a detrital origin for the smectite in the sandy claystone, whereas the origin of the smectite from the phosphorite is obscured by the addition of REE from the phosphogenic environment. The normalized REE patterns of the palygorskite suggest formation in non-oxidizing restricted environments. The Al2O3/ΣREE ratio of the two clay types suggests formation of diagenetic palygorskite (and mixed-layer illite-smectite) from Al-bearing detrital smectite by a dissolution-crystallization process.

High surface area silicon carbide as catalyst support characterization and stability

High surface area silicon carbide (SiC) of 30 m 2/g has been synthesized by the catalytic conversion of activated carbon. The stability of this SiC in aqueous hydrogen fluoride and a boiling nitric acid solution is shown to be excellent. No corrosion is encountered by treatment with boiling HNO 3, HF treatment causes the dissolution of the silica surface layer present on the SiC while the SiC remains intact. Oxidation in air at elevated temperatures has been analyzed by thermal gravimetric analysis, diffuse reflectance infrared spectroscopy, nitrogen adsorption, and X-ray diffraction. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has been observed after ageing in nitrogen for 4 h at 1273 K. The presence of 2 v% steam at 1273 K results in partial SiC oxidation into SiO 2 and considerable sintering. Air oxidation at 1273 K of pure SiC, SiC loaded with 5 wt.% nickel, and HNO 3 treated SiC is shown to cause substantial SiC conversion, viz. 60% to 70% after 10 h. Air oxidation at 1080 K will result in complete conversion in about 100 days. This rate of oxidation agrees with reports on the oxidation of non-porous Acheson SiC and SiC coatings formed by Chemical Vapour Deposition. It is concluded that at high surface area SiC cannot be used as a catalyst support in processes operating in oxidizing environments and temperatures above 1073 K. SiC based catalysts are very well suited for (1) high-temperature gas-phase reactions operating in the absence of oxidizing constituents (O 2 or H 2O) and (2) strong acidic liquid-phase processes.

A Proposed Mechanism for Silica Supported Chromium HDPE Catalyst Activation

The decomposition of Cr3(C2H3O2)7(OH)2/SiO2in oxidizing, inert, and reducing environments was studied using isothermal kinetics, temperature-programmed reaction (TPRxn), and variable-temperature diffuse-reflectance infrared spectroscopy (VT-DRIFTS). Based upon these results a mechanism is proposed for the activation of silica-supported basic chromium acetate. The decompositions in N2and CO/N2mixtures appear very similar. The activation energies are within experimental error, and both exhibit second-order rate dependencies with respect to the surface acetate species. TPRxn results show that the Cr compound influences support dehydroxylation, but VT-DRIFT spectra show no evidence of Cr bonding to the SiO2surface. When CO is present, silica dehydroxylation appears to proceed via a water–gas shift type of reaction producing CO2and H2rather than H2O. In oxygen, Cr compound decomposition occurs at temperatures 90°C lower than in inert or reducing environments. The reaction orders are 1/2 for oxygen and 1 for the surface acetate species. The activation energy is comparable to that calculated for the other two media. VT-DRIFT spectra show that oxygen induces a greater degree of hydroxyl removal and formation of Cr–O/Cr=O bonds concurrent with and subsequent to recorded decomposition temperatures. In this way they support TPR findings and suggest Si–O–Cr bond formation. It appears that the rate-limiting step in the activation mechanism is removal of the acetate methyl group. In oxygen this involves dissociative activation of oxygen on the Cr center and subsequent combustion. In the other environments migration is necessary to allow hydrogen transfer between two acetate groups to form methane and leave behind a hydrocarbon fragment. This explains the different temperatures of decomposition in the various media and the second-order rate dependence upon acetate in N2and CO/N2. The lack of apparent Cr–SiO2bond formation in nonoxidizing environments allows surface migration of Cr compounds which makes this mechanism feasible.

Nickel Catalyzed Conversion of Activated Carbon into Porous Silicon Carbide

ABSTRACTHigh surface area silicon carbide (SiC) of 31 m2/g has been synthesized by the catalytic conversion of activated carbon. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has been observed after ageing in nitrogen for 4 hours at 1273 K. The presence of 2v% steam or the use of air results in SiC oxidation into SiO2 and considerable sintering at 1273 K. Air oxidation of SiC is shown to cause substantial SiC conversion, viz. 60 % after 10 hours at 1273 K. Complete conversion is achieved at 1080 K in about 100 days. This rate of oxidation agrees with reports on the oxidation of non-porous Acheson SiC and Chemical Vapour Deposited SiC coatings. The use of SiC based catalysts is, therefore, limited to (1) high temperature gas phase reactions operating in the absence of oxidizing constituents (O2 or H2O) and (2) liquid phase processes at demanding pH. Syntheses of highly dispersed and highly loaded Ni/SiC catalysts are feasible by applying an ion-exchange technique, resulting in supported nickel particles of 4 nm.