Heat of reoxidation

Selectivity and Metal-Oxygen Bond Strength. The metal-oxygen bond strength could be measured as the differential heats of reoxidation of the reduced... 

We have measured the differential heats of reoxidation and the reaction characteristics of a number of V2O5/Y-AI2O3 catalysts. The chemical transformation corresponding to the heat measurement in this system is ... 

Figure 3. Differential heat of reoxidation and selectivity for oxidative dehydrogenation of butane on V2O5/Y-AI2O3 samples, a 8.2 V/nm sample, reaction at 400°C and b 2.9 V/nm sample, reaction at 480°C.

The portion of this work on the vanadates has been supported by the Department of Energy, Basic Energy Sciences, Division of Chemical Sciences, and that on the heats of reoxidation by the National Science Foundation. PM acknowledges support by the Battelle s NASA Advanced Materials Center for the Commercial Development of Space, and LO acknowledges fellowship support from the 3M Company and a Faculty Minority Internship from the Monsanto Company. 

Fig. 7. Differential heat of reoxidation and selectivity for oxidative dehydrogenation of butane on V2Ov y -AFO, samples. For the 2.9 V/nm2 sample, the selectivity was calculated for the detected gaseous products, (a) 8.2 V/nm2 sample, reaction at 400°C (b) 2.9 V/ntn2 sample, reaction at 480°C (c) 8.2 V/nm2 sample, reduction by CO at 530°C, butane reaction at 400°C and (d) 2.9 V/nm2 sample, reduction by CO at 400°C, butane reaction at 480°C. (a) and (b) are from Ref. 50 (c) and (d) and from P. J., Andersen, Ph D. thesis, Northwestern University, 1992.

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Remote control model, 40 183-184 Reoxidation, 41 198-199 heat of, 40 19-22 Resolution, 33 251-254 Resonance-enhanced multiphoton ionization, in detection of surface-generated gas-phase radicals, 35 181-182... 

In particular, Iwaki and Miura have ascribed the observed decrease of the heat of immersion in water of an anatase sample, reduced by hydrogen at 873 K, to the partial removal of the surface ions. They have also noted that the normal interaction with water (typical of unreduced Ti02) was not restored after the reoxidation of the sample at 873 K. The question arises, however, whether such behaviour is associated with that particular mode of Ti02 reduction or is also characteristic of the samples reduced by other means. 

The catalyst solution is pumped from the flash drum to the bottom of another vertical cylindrical reactor. Air is compressed and enters the bottom of the reactor where it flows concurrently upward with the catalyst solution. Oxygen from the compressed air reacts with the catalyst solution at about 130°C (265 F) and 10 atm. (130 psig) to reoxidize cuprous chloride to cupric chloride. The exothermic heat of this reaction raises the temperature slightly and the reoxidized catalyst flows back to the ethylene oxidation... 

It is interesting to examine the quantities used for the abscissa of Figure 3. Bond suggests using the initial heat of adsorption. Satterfield, discussing metal oxide catalysts more specifically, suggests using the heat of reaction, Q, for reoxidation of the catalyst." Balandin specified that the maximum VOC oxidation rate would occur when QJ2 where Q, is the overall heat of reaction for conversion of VOC to products (e.g., VOC + - CO + H20). ° ... 

For the reduction of alkenes or alkynes to alkanes in laboratory we use metal catalysts such as Pt or Pd and often high pressures. The heating of alkane precursors with these metal catalysts reoxidizes alkanes to alkenes. In biosynthesis these reactions proceed with special reagents like flavine adenine dinucleotide FAD or its reduced form FADH2. 

The reaction is exothermic, with the heat of reaction amounting to 3.S80 Btu per pound of sulfur. The overall reaction can be subdivided into three separate consecutive steps. These steps are a) hydrogen sulfide absorption, b) conversion of hydrogen sulfide to elemental sulfur, and c) vanadium reoxidation. 

From calorimetric TPR-TPO experiments heats of reduction and reoxidation of eationic or metal species can be determined. Data obtained from these experiments may provide kinetie data of theoretical significance as well as an insight into the mechanism of the reduetion processes. 

The heats of oxidation of the reduced oxides can be further measured using O2 adsorption. Large variations of the reoxidation heat can be sometimes observed when any further oxidation is limited by the diffusion of oxygen into the reduced portion of the particle [61]. 

After the samples had been heated in an H2/N2 atmosphere, the XRD powder patterns (Fig. 2b) again showed the presence only of spinel-type phases for Cu/Cu-t-Zn < 0.5, while for the copper-rich samples the main phases present were Cu (Cat D and E) or CuO (Cat F). The lack of reoxidation for the metallic copper in Cat D and E, cannot be justified on the basis of differences of crystal size, but most probably can be attributed to the formation of copper-rich alloys at the surface of the particles. The presence of small amounts of zinc or cobalt does not modify the XRD powder pattern of the copper particles, but may strongly influence their physicochemical or catalytic properties (25-27). For all catalysts, the IR spectra show the presence, together with small amounts of residual carbonates, of the typical bands of spinels (even if not well resolved), except the Cu/Cr= 1.0 sample (Cat F) (Fig. 3) for which only... 

Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen in the air to form either Ee O or Ee202. The reaction continues as long as the DRI remains hot and sufficient oxygen is avadable. Because reoxidation reactions are exothermic and DRI is a good insulator, it is possible that once reoxidation begins inside a pde, the DRI temperature increases and accelerates the reoxidation rate. Although the inner core of the pde may reach temperatures up to the fusion point of iron, the maximum temperature of the outer parts of the pde will be much lower because of heat dissipation. 

Allowing DRI to become wet does not necessatily cause it to overheat. When large pdes of DRI are wetted with rain, the corrosion reactions are limited to the outer surface area of the pde and the resultant heat from the corrosion reactions is dissipated into the atmosphere. However, if water penetrates into the pde from the bottom, or if wet DRI is covered with dry DRI, the heat from corrosion reactions can budd up inside the pde to the point where rapid reoxidation begins. Corrosion occurs significantly faster with salt water than with fresh water. DRI saturated with water can cause steam explosions if it is batch charged into an electric arc furnace. 

These formerly involved the use of banks of externally heated, horizontal retorts, operated on a batch basis. They were replaced by continuously operated vertical retorts, in some cases electrically heated. Unfortunately none of these processes has the thermal efficiency of a blast furnace process (p. 1072) in which the combustion of the fuel for heating takes place in the same chamber as the reduction of the oxide. The inescapable problem posed by zinc is that the reduction of ZnO by carbon is not spontaneous below the boiling point of Zn (a problem not encountered in the smelting of Fe, Cu or Pb, for instance), and the subsequent cooling to condense the vapour is liable, in the presence of the combustion products, to result in the reoxidation of the metal ... 

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