Oxygen removal from oxides

G-41 A chromia-alumina catalyst, used for hydrodealkylation and dehydrogenation reactions G-S8 Palladium-on-alumina catalyst, for selective hydrogenation of acetylene in ethylene G-52 Approximately 33 wt % nickel cm a refractory oxide support, prereduced. Used for oxygen removal from hydrogen and inert gas streams... 

Degree of reduction, in such a case, is defined by the ratio of oxygen removed from the oxide to the total removable oxygen. and is represented by the symbol a. Thus,... 

The reduction occurs by direct oxygen removal from the solid oxides (solid-state diffusion). The basic underlying mechanism is not known (diffusion of O, OH, H2O) and is likely to vary for different for different phase transitions. On the final reduction the metal remains pseudomorphous to the starting oxide, forming a polycrystalline metal sponge. Solid-state reactions are characteristic for low reduction temperatures (<750 °C) and the early WO3 - WO2 9 transition ( crystallographic shear transition). 

From the definition of the periodic steady state it follows, that on one side the oxygen removed from the solid by the reduction must be replaced during the oxidation step. On the other side the heat released by the exothermic reaction must be used by the endothermic reaction or leave the reactor by convection. Since equation 8 is a combination of heat and mass balance and temperature is a measure for heat it can be used for the... 

Phospholene oxides from the McCormack reaction also stand as useful precursors of phos-pholanes as noted, the double bond can be easily hydrogenated, and then the oxygen removed from phosphorus with silanes, etc. (Schemes 56 and 57 Equations (51) to (53)). Phospholanes can be prepared by other cyclization methods, however, some of which are outlined below. 

Despite a lack of data on active oxidation behavior some thermal stability tests give good hints. It relates to the oxygen impurities of the precursor material. The mass loss observed and clearly attributed to oxygen removal from siUcocarbo-boronitrides [166] starts from T w 1300°C. Thus in an environment with low ( 2), at which no protective layer is formed, we will have a substantial loss by active oxidation. 

The effect of temperature in the 700° to 900° C range needs more investigation relative to hydrogen reductions. Preliminary results indicate that a more reduced surface was obtained at 700°C as compared to 800°C or especially 900°C. The higher amounts of oxygen removed from the surface at 800°C and 900°C (see Table II) viere probably caused by the increased levels of surface oxides present at the start of the reduction steps for these two uns. 

The CO oxidation reaction has been extensively studied in M Sri. NiOs (M = Pr, Sm, and Eu) perovsldtes [47]. Sr-substituted samples were found to be more active than the unsubstituted M Ni03 counterparts. A close relationship between the CO oxidation activity and the reducibility was observed, indicating that the facilitation of oxygen removal from crystal lattice is the rate-determining step [47,49]. 

The comparison of P- and Pd-promoted catalysts shows that Pd-promoted catalysts are more selective towards the formation of acetic acid. Other advantages are that they do not produce CO as a reaction product and display better redox behavior. A study of catalyst redox property revealed that Pd facilitated both the reduction of the MoVNb oxide catalyst with ethane and its re-oxidation with oxygen. This is very important because the reaction occurs via the redox mechanism of a Mars-van Krevelen type. Such a conclusion comes from the data presented in Fig. 11.1. With increasing the amount of oxygen removed from the surface of the MoVNbPd... 

For ease of representation consider the overall rate of oxygen removal from the surfaces of oxide, 31, through the reaction... 

HCl gas reacts with metal oxides to form chlorides, oxychlorides, and water. Therefore, all the steel equipment should be pickled to remove the oxide scales before it is put in service. Because oxidi2ing agents in the HCl gas such as oxygen or chlorine significantly affect the corrosion rate, it is essential that the operating temperature of the steel equipment be kept below the temperature (316°C) at which ferric chloride is vapori2ed from the metal surface. 

The oxidant preheater, positioned in the convective section and designed to preheat the oxygen-enriched air for the MHD combustor to 922 K, is located after the finishing superheat and reheat sections. Seed is removed from the stack gas by electrostatic precipitation before the gas is emitted to the atmosphere. The recovered seed is recycled by use of the formate process. Alkali carbonates ate separated from potassium sulfate before conversion of potassium sulfate to potassium formate. Sodium carbonate and potassium carbonate are further separated to avoid buildup of sodium in the system by recycling of seed. The slag and fly-ash removed from the HRSR system is assumed to contain 15—17% of potassium as K2O, dissolved in ash and not recoverable. 

Impurities can be removed by formation of a gaseous compound, as in the fire-refining of copper (qv). Sulfur is removed from the molten metal by oxidation with air and evolution of sulfur dioxide. Oxygen is then removed by reduction with C, CO, in the form of natural gas, reformed... 

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

Hydrogen sulfide and methane can be removed by aeration, although the largest reduction in hydrogen sulfide may result from oxidation by the dissolved oxygen introduced during the aeration. At low pH values, the product is sulfate, whereas at high pH values, the product is free sulfur. 

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