Oxygen surface lattice

The air pollutants of volatile organic compoimds emitted from many industrial processes and transportation activities could be abated by catalytic combustion processes. Scire et al. reported the catalytic combustion of 2-propanol, methanol, and toluene on ceria-gold catalysts. The catalysts were prepared with coprecipitation and deposition-precipitation methods. The gold significantly enhanced the catalytic activity of ceria for the oxidation of these volatile organic compounds. The supposed reason is that the gold NFs weakened the mobility/reactivity of surface lattice oxygen (Scire et al., 2003). 

Surfaces with four-fold coordinate Ti cations are capable of bimolecular coupling of surface formate to form formaldehyde these sites can be created by more severe thermal faceting [1,3,23]. Reaction (5) illustrates this, where R may be a hydrogen atom or an alkyl group, and where Os denotes surface lattice oxygen. 

A mechanism for catalytic dehydrogenation was proposed to explain these observations. The researchers used deuterium-labeled formic acid to discriminate against background water and hydrogen. In the scheme below. Os represents surface lattice oxygen, while oxygen originating from formic acid is... 

Methanol decomposes on titanium dioxide surfaces by mechanisms that are similar to those by which formic acid decomposes. Methanol can reversibly adsorb on single crystal surfaces of titania (reaction 16) in a molecular state, or it may dissociatively adsorb by interaction with surface lattice oxygen anions, forming a surface methoxide (reaction 17). Reaction (18) represents the disproportionation reaction of hydroxyl groups on the surface of the metal oxide. 

Use of Halide Ions to Improve Selectivity. Earlier work has claimed that enhanced selectivities for alkene oxidation can be achieved by the inclusion of electronegative elements such as S, Se, or halogens. This has been reviewed elsewhere. " More recent work has demonstrated substantial improvements in selectivity for propene (25—70%) and isobutene (35—80%) oxidation when either chloride or bromide is present. Both elements are added to the catalyst in the form of trace levels of organo-halide in the process gas stream. The selectivity increase is the result of a decrease in the rate of complete oxidation rather than an increase in the partial oxidation rate. Since the reaction is first order in oxygen pressure and zero order with respect to alkene in the presence and absence of halide, the reaction mechanism is probably similar in both cases. In the light of Anshits recent work, the effect of the halide is presumably to reduce the relative number and/or reactivity of surface lattice oxygen species and thus reduce the amount of irreversibly adsorbed alkene. 

Assuming that oxygen clusters of 2-5 adjacent surface lattice oxygens lead to selective oxidation while clusters of more than 5 lead to waste products, one would predict that maximum selective oxidation should occur on a surface that is about 65% reduced (Fig. 6) (6). The behavior of solid CuO in propylene oxidation at 300°C is consistent with this model (Fig. 7) (6) the highest selectivity occurring on semireduced surfaces. 

The oxidative coupling of isobutene can be performed in two separate steps, coimected with reduction of catalyst and reoxidation of the reduced catalyst afterwards. The two step process leads to an improvement of DMH selectivity as compared to the conventional process. The formation of carbon dioxide requires surface lattice oxygen from tbe catalyst, while formation of DMH occurs by abstraction of protons and electrons at the catalyst surface. They are absorbed on the catalyst bulk and, finally, react to water there. Thus, the rate of carbon dioxide formation is more affected by catalyst reduction than the rate of DMH formation. 

The theoretical work and consideration of the nature and the creation of active centres on the solid catalyst surface put more emphasis on the acidic ones rather than on basic ones. Several studies concerning the basic centres of the oxide surfaces dealt with the oxygen on the surface - lattice oxygens and adsorbed oxygens. 

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