The support effect and the chemical composition and structure space of solid oxide catalysts

Joachim Sauer

Solid oxide catalysts as industrially used for selective oxidation reactions, e.g. of small alkanes to alkenes or of methanol to formaldehyde, consist of an active oxide, we will look at V2O5, and a supporting oxide such as Al2O3, SiO2, TiO2, ZrO2, CeO2. The support is known to strongly affect the catalytic properties, The supporting oxides may have different bulk struc¬tures (?-Al2O3 vs. ?-Al2O3), they may expose different crystal planes, they may stabilize active oxide species and particles of different size and shape. Under reaction conditions (O2, H2O, and hydrocarbon partial pressure) the relative stabilities of different surfaces and of different surface species may change. Finally, the different electronic structure (electronega¬tivity, redox-active vs. inert support) may contribute to the support effect.
To understand how the activity and the selectivity of the catalysts varies in this complex chemical compound space, we calculate the energies of O defect formation that relate to the reaction energies [1], and the ener¬gies of hydrogenation [2] that relate to the energy barriers for the rate-determining C-H activation step [3, 4].
We do this for structures that are likely to be stable under reaction conditions according to calculated phase diagrams [5, 6]. We calculate spectroscopic signatures (mainly infrared spec¬tra) to assist in the identification of the structure and size distribution of active oxide species on the surface
[7]. We use density functional theory for evaluating the potential energy surfaces of the systems considered.

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