The oxidation of toluene on various molybdenum-containing catalysts

The activities for the vapour-phase oxidation of toluene of various molybdenum-containing catalysts have been measured in a flow microreactor operating at 1 atm pressure. The catalysts comprised (a) unsupported crystalline MoO3, (b) Mo oxide monolayers on Al2O3, CeO2, TiO2, and ZrO2 supports, and (c) Al-, Ce-, and Zr-molybdate salts. The rates of oxidation of toluene per unit surface area on the monolayer catalysts are higher than those on the corresponding salts, but the selectivities for the partial oxidation products show the opposite trend. The catalytic behaviour of unsupported MoO3 resembles more closely that of the salts than that of the monolayer catalysts. The kinetic data for the oxidation of toluene on all catalysts can be interpreted on the basis of a reduction-oxidation mechanism. The energies of activation for the reduction and oxidation steps are correlated with the ionic potentials of the cations in the supports. Poisoning by pyridine of MoOx/TiO2 has a profound effect on the activity and selectivity of the monolayer catalyst and gives some information about the nature and concentration of the active sites

The sulphidation properties of pre-oxidized yttrium-ion-implanted Incoloy 800H

On sulphidation of pre-oxidized yttrium-implanted Incoloy 800H an improvement in sulphidation resistance is obtained. Pre-oxidized krypton-implanted alloy and aluminium-implanted alloy have sulphidation rates which are about the same as that of non-implanted pre-oxidized Incoloy 800H. Scanning electron microscopy observations show that one of the effects of yttrium implantation is to decrease the corrosion rate at the alloy grain boundaries

The effect of yttrium ion implantation on the sulphidation of incoloy 800H

Previous work has shown the beneficial effect of yttrium implantation on the oxidation behaviour of chromium oxide forming alloys. Because of the limited applicability of Fe-Cr-Ni alloys in sulphur containing gases the effect of yttrium implantation on the sulphidation of Incoloy 800H was studied. In short time tests a significant improvement was observed upon implantation of about 1016 yttrium ions/cm2 if an oxidizing treatment was applied before exposition to the sulphiding environment. The explanation of this effect is probably similar to that proposed in the literature for the improved oxidation resistance

Oxidation kinetics of chromium and morphological phenomena

The oxidation kinetics of chromium at 900°C are independent of the oxygen partial pressure. Although this observation gives evidence for a defect mechanism where chromium interstitials account for the chromium transport in the oxide scale, the experimental phenomena do not support one single model. The occurrence of oxide whiskers and oxide ridges are explained by the energy of activation for the breakup of the oxidant molecule. Large oxide pegs are formed at metal multiple-grain junctions after scale breakdown.\u

On the influence of cold work on the oxidation behavior of some austenitic stainless steels: High temperature oxidation

AISI 304, 314, 321, and Incoloy 800H have been subjected to several pretreatments: polishing, milling, grinding, and cold drawing. In the temperature range 800–1400 K, cold work improves the oxidation resistance of AISI 304 and 321 slightly, but has a relatively small negative effect on the oxidation resistance of AISI 314 and Incoloy 800H. Milling results in an enlargement of the surface area with a factor 2.5

The protective properties of thin alumina films deposited by metal organic chemical vapour deposition against high-temperature corrosion of stainless steels

Coatings of Al2O3 were deposited on Incoloy 800H and AISI 304 by means of metal organic chemical vapour deposition. Diffusion limitation was the rate-determining step above 420 °C. Below this temperature, the activation energy of the reaction appeared to be 30 kJ mol−1. Coating with Al2O3 increases the sulphidation resistance by at least 4–10 times. The sulphidation resistance is influenced by the growth rate of the coating and by the thermomechanical properties of the coating - substrate combination. The sulphidation resistance also depends on the growth rate of the coating. The weight gain of coated specimens is about 10–14 times less than that of uncoated specimens. Treatment of the coated samples in air at 850 °C results in a still higher sulphidation resistance. This is due to the chromium oxide grown into the cracks in the coating during the treatment. These cracks are the result of mechanical stresses at irregularities

Properties of protective oxide scales containing cerium on Incoloy 800H in oxidizing and sulfidizing environments. I. Constant-extension-rate study of mechanical properties

The mechanical properties of ceramic coatings containing cerium oxide, prepared by the sol-gel method and used to protect Incoloy 800H against aggressive environments, are reported. Deformation and cracking behavior in oxidizing and sulfidizing environments has been investigated by constant-extension-rate tests. Extension rates were between 9.3×10−6 and 3.7×10−7 sec−1 at 823 <T<973 K. Under these conditions, cerium oxide sol-gel-coated specimens do not show any failure at extensions of 1.0% or more, but in hydrogen, sulfide failure is found at lower extensions than in air

Investigation of the surface structure and activity of molybdenum oxide-containing catalysts : I. An infrared study of the surface structure of molybdena-alumina catalysts

A comparison has been made of the infrared spectra of alumina with molybdenum oxide-alumina in both the oxidized and reduced forms. In the case of molybdena-alumina prepared via adsorption of gaseous MoO2(OH)2, the spectra show that a practically complete monolayer of Mo6+ oxide covers the alumina. After reduction with hydrogen the hydroxyls of the carrier appear. From the reversibility of reduction and oxidation under mild conditions it has been established that the reduced oxide is present as an interrupted monolayer

The effect of thermal annealing on the properties of alumina films prepared by metal organic chemical vapour deposition at atmospheric pressure

Thin films deposited at 330°C by metal organic chemical vapour deposition on stainless steel, type AISI 304, were annealed in a nitrogen atmosphere for 1, 2 and 4 h at 600, 700 and 800°C. The film properties, including the protection of the underlying substrate against high temperature corrosion, the chemical composition of the film and the microstructure, were investigated.\ud \ud Corrosion experiments performed at 450°C in a hydrogen sulphide containing gas, showed that the cracks in the alumina films almost completely disappeared after a post-deposition heat treatment, probably as a result of stress relaxation. The porosity of the alumina films was not affected by this heat treatment. X-ray diffraction measurements of these films, deposited at 330°C, revealed an amorphous structure. Owing to the thermal annealing process, the amorphous alumina films were converted to λ-alumina, and OH-groups disappeared.\u

Corrosion resistant coatings (Al2O3) produced by metal organic chemical vapour deposition using aluminium-tri-sec-butoxide

The metal organic chemical vapour deposition (MOCVD) of amorphous alumina films on steel was performed in nitrogen at atmospheric pressure. This MOCVD process is based on the thermal decomposition of aluminium-tri-sec-butoxide (ATSB). The effect of the deposition temperature (within the range 290–420 °C), the precursor vapour pressure (5.33×10-3−2.67×10-2 kPa), and the gas flow (6.5−12.5 1 min-1) of the MOCVD process have been studied in relation to corrosion properties at high temperatures. The corrosion experiments were performed at 450 °C in a gas atmosphere containing 1% H2S, 1% H2O, 19% H2, and balanced Ar.\ud \ud It was found that the amount of corrosion products on an alumina film (0.20±0.05 mg cm-2)-AISI 304 combination decreased with increasing deposition temperature of the coating. This was more pronounced for the products formed through the coating owing to a certain porosity. The crack density, where products were also formed, was almost unaffected.\u