Selective oxidation crystal structure, role

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

Ample experimental evidence indicates that the habit of crystallites of the catalyst has a profound influence on the activity and selectivity of the reaction, which results in the appearance of structure sensitivity of the selective oxidation reactions at oxide catalysts (39). However, little is known about the origin of this phenomenon and about the differences of the structure of active sites present at various crystal planes. Even less is known about the role of defects present at the surface of an oxide, in determining the catalytic properties. Only recently studies of the properties of (100) surface of a monocrystal of NiO revealed that an ideal surface is chemically inert and the reactivity of the system increases only if defects are introduced in the surface (40). At such a surface, dissociation of molecular water to form hydroxyl groups is observed in contrast to an ideal surface which is inactive in water dissociation. 

There has been considerable speculation concerning the role of carbide in the iron base catalyst. The carbide was originally depicted as an intermediate in the reaction (7), but more recent work indicates the contrary (3). It now appears more probable that the lattice between carbide crystals or between groupings of carbide and relatively fewer oxide or even free iron crystals offers the form of pore structure required for both high activity and selectivity. 

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

The reason for the difference in the effectiveness between each of the crystal face to catalyze the ammoxidation of alkyl aromatics selectively is a result of the specific electronic character of the oxygen atoms associated with the vanadium atoms of the V2 O5 structure. As was learned about the role of lattice oxygen (0 ) in the selective ammoxidation of propylene to acrylonitrile (see above), hydrogen abstraction and oxygen insertion require oxygen atoms with nucleophilic character (79). On the other hand, nonselective oxidation is affected by electrophilic oxygen species, O2 and O . These are the intermediate species in the dissociative chemisorption and reduction of O2 to lattice (80). 

The size of the metal particle plays an important role in the structure-sensitive reaction when site coordination (edge, kink, or terrace sites) or crystal orientation affect the activity and selectivity. In recent work, transient curves of gas-phase COg produced with CO-pulse injection on Pd/CeOg at 500-700 °C, established the structure sensitivity of the CO oxidation reaction on supported... 

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