Fundamentals of metal oxide catalysis

Hari Nair, Purdue University

Abstract

The properties of metal oxide catalysts and hence, catalytic activity are highly dependent on the composition and structure of these oxides. This dissertation has 3 parts – all directed towards understanding relationships between structure, composition and activity in metal oxide catalysts. The first part of this dissertation focuses on supported metal oxide catalysts of tungsten, vanadium and molybdenum. Mechanisms are proposed for ethanol oxidative dehydrogenation which is used to probe the acidity and reducibility of these oxide catalysts. These studies are then used to develop a novel method to quantify active redox sites and determine the nature of the active site on these catalysts – our results show that the intrinsic redox turn-over frequency is independent of the nature of the metal oxide and its loading and that the actual rate obtained over an oxide is only a function of the number of removable oxygen atoms linking the metal to the support. The extension of Ultraviolet-visible Diffuse Reflectance Spectroscopy (UV-vis DRS) to the study of active oxide domains in binary oxide catalysts is demonstrated for distinguishing between interacting and non-interacting domains in binary MoO x-WOx catalysts on alumina. We show also how the rigorous analysis of pre-edge features, absorption white-line intensity and the full width at half maximum of the white-line in X-ray Absorption Spectra provide determinants for metal atom coordination and domain size in supported metal oxide catalysts. The second part of this work looks at effects of structure variations on the activity of polyoxometalate catalysts that are promising for the production of Methacrylic Acid from Isobutane. The use of these catalysts is limited by structural changes that impact their performance – an “activation” period is required before the catalysts become active for methacrylic acid production and structural changes also lead to degradation of the catalyst, which are also seen during thermal degradation. The affect of reaction conditions on the lifetime of these catalysts is investigated. The structural changes occurring in these catalysts are studied during thermal degradation at the bulk-scale (using UV-vis DRS, XAS and X-ray Diffraction) and at the atomic-scale (using High-Resolution Transmission Electron Microscopy). These studies show that the actual mechanism of structural reorganization occurs through a twinning process which eventually leads to MoO3, which is inactive for methacrylic acid production. This gives new insight into how to make these catalysts commercially attractive. Finally, the design and fabrication of a micro-reactor to perform complete spectroscopic and reactive characterization of supported metal oxide catalysts is presented; we aim to integrate semiconductor microfabrication technology with conventional spectroscopic tools to create cost-effective, simple and flexible tools for catalytic studies.

Degree

Ph.D.

Advisors

Baertsch, Purdue University.

Subject Area

Chemical engineering|Materials science

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