Lattice oxygen ions

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Hence, the formation of Pd(I) during the calcination in vacuo results from the reducing effect of ammonia which desorbs during the heat treatment. Attack of lattice oxygen ions by ammonia would form trigonal aluminum ions. 

These results parallel those found on the V20,/Si02 system where O was formed by adsorption (72). The photoreactivity of transition-metal oxides deposited onto PVG or silica supports has been investigated by several authors (71b-7Ie) and is discussed in more detail in Section VI,C under reactivity of the lattice oxygen ions. 

The Rietveld refinements show that the crystalline structure contains microstrain that is mainly caused by the crystal distortion related to zirconium replacement. It is also found that cationic occupancy number in the crystalline structure is smaller than their normal value 0.02083 present in an ideal crystal, indicating that the crystalline structure is of cationic deficient. As far as the local environment of the cationic defect is concerned, the lattice oxygen ions around it are not fully bonded, which are mobile and more active under the reaction condition compared to the normal ones. Therefore, the creation of cationic defects in the structure is a possible origin of the unusual reducibility and high mobility of oxygen species from bulk to surface exhibited on the ceria-zircnia solids, that are reported by other groups [5, 6]. 

It is well known that in oxidation reactions oxygen acts by adsorption on the oxide catalyst as O, O2, etc. or incorporation as lattice O species. The solid is oxidized in this step, and the electrons received by adsorbed oxygen could come fi om reduced surface cations or anionic vacancies with trapped electrons. If oxygen is incorporated as lattice oxygen ion, the sites for adsorption of oxygen and for oxygen attack in the catalytic reaction may be different, and migration of oxide ions in the solid between the two sites would occur [9]. 

Very recently, ultrafme metal oxides have attracted much research interests in terms of materials science and heterogeneous catalysis[10-12]. These new catalytic materials are expected to have unique catalytic properties because of their nano-scale particle sizes. In this work, a novel catalyst for selective oxidation of toluene to benzaldehyde, i.e. ultrafme complex molybdenum based oxide particles, has been developed. It has been found that the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale and that the ultrafme oxide particles exhibit unique catalytic properties for selective oxidation. Our results have revealed that the ultrafme complex oxide particles are potentially new catalytic materials for selective oxidation reactions. 

Data on ionic radii can be utilized for the evaluation of a. For instance, for lattice oxygen ions cr 6 x 10 20m2 can be accepted. Concentrations of surface... 

ELS = electron energy loss spectroscopy 0 - = lattice oxygen ion... 

On and D represent lattice oxygen ions and surface anion vacancies, respectively, and ( ) designates a free adsorption site. The CO2 produced can remain adsorbed at the surface or be readsorbed later as a stable carbonate at the active surface, thus effecting the self-poisoning that is invariably observed. The direct participation of lattice oxygen in the oxidation reaction has been substantiated in this study by in-situ electrical conductivity measurements. 

A drawback of using ammonia as a probe molecule is the large variety of possible adsorption forms. For example, an ammonium ion formed in the adsorption process could be bound to lattice oxygen ions through two, three, or four hydrogen atoms. A similar variety of interactions can also stabilize coordinated ammonia or ammonia H-bonded to weakly acidic hydroxyls. In the latter case, measurement of the hydroxyl acidity through the H-bond method should be made with another probe for which such... 

The supported palladium catalyst known to be the most active for total methane oxidation was the subject of considerable amount of research [1-9]. However, no agreement about the mechanism reaction was observed in the literature [1-8]. The Langmulr-Hinshelwood [1-4], the Eley-Rideal [5-7], and the Mars-Van Krevelen [8], mechanisms were proposed for the total oxidation of methane on the supported palladium catalysts. This diversity is explained by the variation of the active surface in each case. Indeed, according to Burch et al.[9], the active sites can be modified by the pre-treatment conditions, by the particle size, by the support nature and by the presence of some poisons such as chlorides. Others difficulties result from the fact that it is not confirmed if the active site is a partial or a total oxidized palladium particle. In addition, little is known about the reactive oxygen form. Indeed, it is not yet established if the reactive oxygen is a chemisorbed molecular or ionic form or a lattice oxygen ion. The aim of this paper is to identify the palladium oxidation state under catalytic stream, to study the reactive form of oxygen and to propose a mechanism of the reaction. 

Knowing that there are three oxygen sites per unit cell in a perovskite, the lattice oxygen ion concentration can be directly linked to the vacancy concentration ... 

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