Spinel-type oxides

In a relatively recent study, Rasiyah et al. [300, 303] suggested that the evolution of oxygen proceeds via the formation of higher valence oxides of Ni or Co on the surface of the NiCo204 

In other work on spinels, Iwakura et al. [312, 313] observed that the overpotential at a fixed current density on spinel-type ferrite film electrodes 

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The low-temperature Mossbauer spectra of the spinel type oxides, NiCr204 [14,18] (Fig. 7.6b) and NiFe204 [3, 18], have been found to exhibit combined magnetic dipole and electric quadrupole interaction (Fig. 7.7). For the evaluation of these spectra, the authors have assumed a small quadrupolar perturbation and a large magnetic interaction, as depicted in Fig. 7.3 and represented by the Hamiltonian [3]... 

Stainless steels, all of which contain at least 11% Cr, owe their usual passivity to an oxide layer that can be approximately formulated as FeCr204, or (if nickel is also present) as (Ni,Fe)Cr2C>4. These spinel-type oxides are similar to the mineral chromite (Section 4.6.2), which is extremely insoluble in aqueous media. Again, however, the oxide film may be weakened or lost under strongly reducing conditions, and the stainless steel may then become active. This problem is analyzed in electrochemical terms in Section 16.6. 

Assignment of A or B sites to cations in spinel type oxides based upon their chemical formulae can offen be misleading. Thus, for the mixed-valence oxide NiFe204, we might conclude that the Ni+ ions are on the tetrahedral A... 

Spinel-type oxides of Cr and Co were prepared by coprecipitation of a solution of Cr(N03)3 9H20 and Co(N03)2 6H20 in a mass ratio of 2.74 at pH=8. The precipitation was accomplished by adding dropwise a diluted solution of ammonium hydroxide to a cosolution of precursor salts under continuous stirring until a pH=8 was reached. The precipitate was then filtered, washed with distilled water and dried overnight at 100 C. Afterwards, three samples were taken and calcined each one at different temperature, 400, 600 and 800 C for 5 hours. All chemicals used were of analytical grade, supplied by Panreac. 

The reaction between the active compound and the washcoat can be avoided following two approaches. The first approach is to stabilize the active phase by applying it as a complex oxide. The spinel-type oxides, mentioned above, are relatively inactive. On the other hand, perovksites, often AMO3 (A = rare earth metal, e.g.. La, Sr M — transition metal, e.g., Co, Cr, Fe), exhibit promising behavior, which has attracted much attention [55,90,91]. Perovskites may also be used as unsupported oxides, but their surface area is small and not particularly thermostable. The activity of perovskites is dependent mostly on the M cation, but the A cation also has a significant effect [91]. Partial substitution... 

Tanaka, Y., Utaka, T., Kikuchi, R., Takeguchi, T., Sasaki, K., and Eguchi, K. Water gas shift reaction for the reformed fuels over Cu/MnO catalysts prepared via spinel-type oxide. Journal of Catalysis, 2003, 215, 271. 

Lu, G., Miura, N. and Yamazoe, N. (1997) High-temperature sensors for NO and NO2 based on stabilized zirconia and spinel-type oxide electrodes. J. Mater. Chem., 7 (8), 1445-9. 

Recently, the development of the impedance-based zirconia sensors using the specific spinel-type oxide SE for NO measurement at high temperatures allowed detecting the total NOx concentrations regardless of the NO-NO2 ratio in a gas mixture [55-57]. Development of the impedancemetric NO sensors was described in detail in Chapter... 

Once the spinel-type oxide layer is formed, the stabilized current vs. potential profile at 1.3 V obeys... 

For all these reasons an increase in chromium concentration may be the best solution to increase the sulphidation resistance of iron-aluminium alloys. The present work has shown that chromium has a very beneficial effect on the sulphidation resistance in oxygen deficient gases. Such improvement of sulphidation resistance in H2S environments has also been found in nickel-aluminium alloys, where the corrosion rate decreased significantly, when 8 wt% chromium were added [22], Whether chromium increases the rate of alumina formation or contributes to the formation of protective spinell type oxides Fe(Cr,Al)203 cannot be decided from the available results. It is also possible that chromium catalyses the transition from a less to a more protective type of A1203. It was for example found that chromium enhances the transition from 0-Al2O3 to a-Al203 in Ni-Al at temperatures above 1000°C [30],... 

When the above-mentioned data on the very small cobalt concentrations usually found in PWR primary coolant are taken into consideration, such solubilities would mean that cobalt traces in the primary coolant should be present in the ionic dissolved state. However, from the routine analyses of PWR primary coolant it is well known that the overwhelming fraction of cobalt present in the solution can be retained by filtering, together with the particulate corrosion products. This discrepancy raises the question of what mechanism is responsible for the uptake of the cobalt traces into the spinel-type oxides. According to Matijevic (1986), there are three possibilities ... 

With increase in the Co/Al atomic ratio (compare CatA and Cat C), an increase in the activity is observed which could be explained by the fact that the number of adsorption centers increases on the surface. Our XPS results also confirmed tiiat the Co/Al surface composition increases with increase in Uie bulk composition althouf the former is lower in value. The activity of the present samples are compared witii some of the most active catal3rsts reported in the literature [28]. Under our experimental conditions, Co-Al-HTs showed appreciable conversion even at 150 C and Cat C showed 100% conversion at 250°C which is 100°C less than the most active catalyst reported so far. Analysis of the spent catalysts by XRD showed non-stoichiometric spinel type oxides. The high activity for Co-Al-HTs can be explained based on the specific interaction between Co and Co ion with the support responsible for the generation of the active sites. However, a detailed study is required in understanding the nature of active centers. 

Metal oxide catalysts have been extensively studied over the last several decades for their ORR catalytic activities and electrochemical behaviors. Two types of metal oxide, the perovskite-type and spinel-type oxides, are the most attractive... 

Li N, YanX, Jin Y, Li S, Lin B. Spinel-type oxides LiMn2.xMx04 [M=Co, Fe, (CoFe)] as elecfrocatalyst for oxygen evolution/reduction in alkaline solution. J Appl Elecfrochem 1999 29 1351-4. 

RCh204, a spinel-type oxide which is formed across the R series. The crystal stmcture (Chen et al. 1995a) is monocUnic owing to a distortion and contains, somewhat surprisingly for a spinel, square-planar copper coordination polyhedra [NdCu204 J2/a, 582.2(2), 969.5(3), 574.8(2), 92.34]. 

Pino, D., Russo, N., Saracco, G., et al. (2006). Catalytic Removal of Nox and Diesel Soot over Nanostructured Spinel-type Oxides, 7. Catal., 242, pp. 38 7. 

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