Oxygen catalysts

Figure 40.3. Conversion of reactants in n-hexane ammoxidation as a function of the reaction temperature. Symbols conversion of n-hexane ( ), annnonia (A) and oxygen ( ). Catalyst SnA /Nb/Sb 1/0.2/1/3.

Formation of Oxo-bridged Dimers and Oligomers. A facile and pervasive type of process, and one that often leads to inactivation or significant attenuation in activity of the oxygenation catalysts, particularly under mild operating conditions, involves the trapping of oxometal intermediates by some form of the catalyst to... 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

The first chelate found to be electrocatalytic was cobalt phthalocyanine x>, which functions as an oxygen catalyst in alkaline electrolytes. Soon afterwards we were able to show 3,4,10,11) -that several phthalocyanines are also active in commercially important, sulfuric acid containing media. A comparison of various central atoms showed that activity increased in the order Cu Ni iron phthalocyanine, the nature of the carbon substrate plays a very important part FePc is more active on a carbon substrate with basic surface groups than on one with acid surface groups3). This property is however specific to phthalocyanines (Pc). 

The conditions encountered in studying the stability of catalysts under electrochemical load are very complicated. Stability depends strongly on the potential and on the nature of the working substance. For example, pure CoTAA, when used as an oxygen catalyst at potentials of about 800 mV, is active only for a period of some hours. If, however, it is used in the anode for the oxidation of formic acid at 350 mV, it will give more than 6 months (4000 hours ) continuous service under the same conditions. 

The reaction with 8 only occurred in stoichiometric ratios, hence we searched for a way to use copper-based molecular clips as true oxygenation catalysts. To this end the host molecule was altered by changing the pyrazole ligands for pyridine to obtain a model system that could mimic dicopper proteins, which can bind molecular oxygen between the copper centers in a bridging fashion [16]. After the binding of two Cu ... 

To learn more about the PremAir ozone-to-oxygen catalyst, type PremAir into the search engine of Engelhard. 

Interestingly, the analogous tetra-p-tolyl porphyrinato complex, which is known to form a p-oxo dimer upon oxygenation, was inactive as an oxygenation catalyst. This led the authors to conclude that inhibition of p-oxo dimer formation, via steric hindrance from bulky substituents in the porphyrin ring, is essential for catalytic activity. 

As mentioned earlier, for the ammonia oxidation reaction carbides can also be characterized in terms of selectivity. It has been found that the selectivity towards N20 decreases in the sequence Cr7C3 > ZrC > Mo2C, WC > VC > TiC, TaC, NbC, HfC. This is similar to the pattern observed in activity (the obvious exception is ZrC). Here, with increasing q the selectivity to N20 decreases. This finding may be due to the fact that for N20 formation more oxygen-catalyst bonds need to be broken than for N2 formation.16... 

The difference in the number of oxygen-catalyst bonds cleaved upon formation of N2 and N20 should result in a larger activation energy for N20 formation in comparison with N2 formation and should lead to an increase in selectivity to N20 with temperature.16 Accordingly, for Cr7C3 (N2) = 5 kJ/mol (N20) = 160 kJ/mol. For all carbides studied the selectivity to NaO increases with temperature. 

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