Relating Catalytic Activity of d⁰ Semiconducting Metal Oxides to the Fermi Level Position

Abstract

The catalytic activity of semiconducting transition metal oxides varies with the addition of dopants. To deconvolute the chemical and electronic effects of dopants and to control the electronic effects with precision, models are needed that relate the Fermi level position to reaction rate. The present work develops such a model for oxides of transition metals in groups IV, V, and VI of the periodic table whose cations have a d 0 configuration. The model employs a series of semiempirical relations that connect the surface Fermi level of the solid to its activity through the point of zero charge and overall Hammett acidity. The effects of successful p-doping on reaction rate are predicted to be quite large, especially for reactions exhibiting strong ionicity.

Manipulating Reaction Rates of Metal-Oxide Heterogeneous Catalysts via Semiconductor Heterojunctions

“…Yet the present work extends the physical observations of ref to catalytic rate measurements as embodied in eq , whose applicability is supported by a rather sparse literature of directly relevant measurements . In principle, the functional form implied by eq could fail for a number of reasons relating to issues with spatial nonuniformity of the vanadia, possible contamination, or the effects of surface or bulk defects.…”

“…The parameter λ is analogous to the Bronsted coefficient defined for liquid-phase reactions. Its value should be positive for reactions catalyzed by acids, and negative for those catalyzed by bases.…”

“…For gas-phase reactions, even larger values are possible. For example, λ = +6.1 has been computed for gaseous CO oxidation on doped NiO based on the variation of activation energy with work function. Very few measurements of λ exist for surface reactions, however, and most fall between 0.1 and 0.6. , The theoretical interpretation of such cases remains unclear, and the present work treats λ simply as a phenomenological parameter.…”

“…That potential varies quite substantially with V 2 O 5 thickness, as the potential reflects the strong and nonlinear spatial variation of band bending in an accumulation region. The parameter λ is analogous 13 to the Bronsted coefficient defined for liquid-phase reactions. Its value should be positive for reactions catalyzed by acids, and negative for those catalyzed by bases.…”

“…Polycrystalline V 2 O 5 films of various thicknesses were grown atop the anatase by chemical vapor deposition at 200 °C according to methods detailed elsewhere . Additionally, some V 2 O 5 films were grown directly on n-type silicon (100) substrates (with native oxide).…”