Lattice oxidation

Radical anions of tetracyanoethylene and trinitrobenzene have been observed on Ti02 and MgO. For nitro-benzene compounds adsorbed on well-degassed MgO, Tench and Nelson (90) reported spectra similar to those observed for the radical anions on alumina and silica-alumina. After discussing several possibilities for the source of electrons in MgO they suggested that some of the lattice oxide ions may act as electron donors. One of the difficulties with this proposal is the failure to detect the resulting... 

E. O Tracer Investigations Involvement of Lattice Oxide Ion in the Reaction... 

Incorporation of lattice oxide ion into the oxidized products has been systematically investigated in the oxidation of propylene to acrolein using 99.1% 1802 tracer on a series of tri- and tetracomponent bismuth molybdate catalysts in our laboratory (35, 36, 40-42). 

Slow heterogeneous exchange reactions between gaseous oxygen or oxidized products with catalyst oxides were observed but they were not quick to exert any serious effect on the evaluation of the participation of lattice oxide ion in the oxidation of propylene. 160 atom from the bulk of the catalyst was... 

Interestingly, the replacement of a part of Co2+ in the Mo BiiConO, by another divalent cation, Ni2+ or Mg2+, does not improve the degree of participation of lattice oxide ion at all and only the lattice oxide ion in the bismuth molybdate phase is active in the MonBiiCosNijO or MonBhCoitMgjO, catalyst. It is noteworthy that the situation mentioned above corresponds exactly to the catalytic activity of the tri- and tetracomponent bismuth molybdate catalysts. 

The mobility of lattice oxide ion under the working conditions was examined in the oxidation of propylene with 1802 for a series of... 

The mobility of lattice oxide ion was evaluated by the complete mixing volume of lattice oxide ion according to the method proposed by Keulks and co-workers (30). By assuming the 180 concentration of lattice oxide ions in a dilution volume of catalyst is equal to that in oxidized products in the reaction at a given time, the complete mixing volume can be calculated from the results of l8C>2 tracer experiments by the following ... 

Clear experimental evidence of the enhancement of catalyst life by the rapid migration of lattice oxide ion was demonstrated by our investigations (39). The stability of the tricomponent catalyst system Bii-x/3Vi-xMoJr04 was examined by the reduction and reoxidation of the samples using XRD... 

FIG. 24. Comparison of the specific activity of the Bii-j,V iMo 04 with the fraction of lattice oxide ions involved in the oxidation of propylene to the total lattice oxide ions in the catalyst (,38). 

III-7. Hydrogen Photogeneration from Low-Pressure Water Vapor. Water vapor can react with oxygen vacancies of illuminated pre-reduced SrTi03 surfaces to yield hydrogen and lattice oxide.(14) Vacuum-prepared (111) surfaces of pre-reduced and stoichiometric SrTi03 were heated to 400°C in 10 Torr D2O in a UHV system equipped with a quadrupole mass spectrometer. Illumination of the pre-reduced crystal caused an increase in the D2 pressure of the system equivalent to D2 production of 3 mono-layers/hr. No such effect was seen on the stoichiometric crystal. 

Eley-Rideal) mechanism, one of the reactants comes directly from the fluid phase to react with the other, which is already chemisorbed. This procedure was devised to explain the kinetics of the hydrogen-deuterium reaction on certain metals (see Section 9.2), but has also been suggested for other reactions. The Mars-van Krevelen mechanism applies to oxidations catalysed by oxides that are easily reducible, and are therefore able to release their lattice oxide ions for the purpose of oxidising the other reactant they are then replaced by the dissociation of molecular oxygen. With gold catalysts supported on such oxides, it is sometimes proposed that this mechanism plays a part in the total process. 

There are certain similarities between ceria and ferric oxide the cations Ce4+ and Fe3+ are both quite easily reduced, and in the case of ceria, the surface lattice oxide ions are readily mobilised, so that cation vacancies are... 

The photoluminescence of lattice oxide ions of transition-metal oxides mixed or supported on conventional carriers has also been reported (160b). The luminescence is shown to occur from oxo complexes (M04)" (M = V, Mo, W, Cr) in which the transition-metal ion exists in a high oxidation state with a d° electronic configuration. Since the d orbitals of the transition-metal ion are not occupied and therefore the d-d transitions impossible, S0 -)-S charge-transfer electronic transitions occur in the oxo complexes upon absorption of light. The result is that an electron is transferred from a filled molecular orbital localized mainly on the O2 anions to a d orbital of the transition-metal ion. This leads to the formation of an excited singlet electronic state S, with two unpaired electrons, in which the total electron spin,... 

Furthermore, it was shown that these ligands when competing with lattice oxide ions may well form defined coordination complexes with the transition metal cation (1-5). In some complexes. the ligands were loosely bound (almost reversibly) to the cation in others the complexes may have the same structure and similar stability as those formed in solution (1, 4). 

Whether the rhodium dicarbonyl was attached to the zeolite lattice or to an extra-framework anion such as OH, 0 or a labile ion, could be also decided upon using IR spectroscopy. Indeed lattice vibration between 1300 and 300 cm- characteristic of an NaY zeolite (16) are sensitive to the interaction of lattice oxide ions with cations. In particular, it was observed that an IR absorption band at 877 cm- grew simultaneously with the growth of CO absorptions at 2115-2048 characteristic of the dicarbonyl (13).This... 

Hence the rhodium III solvated by lattice oxide ions and presumably extra framework oxide ions or hydroxo ligands (depending on the dehydration state) could be carboxylated reductively to rhodium I dicarbonyl according to one of the following reaction scheme depending on the hydration state... 

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