Electronic characteristics as descriptors for catalytic efficacy of perovskite-type cobalt oxides

• Elektronische Eigenschaften als Deskriptoren der katalytischen Effektivität von Cobaltoxiden des Perowskit-Typs

Simböck, Johannes Peter; Palkovits, Regina (Thesis advisor); Simon, Ulrich (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Transition metal (TM) oxides show significant potential as catalysts in various redox reactions that are related to automotive exhaust gas catalysis. Cobalt-based perovskite-type oxides, in particular, have been addressed as potential replacement of platinum group metals in oxidation and decomposition reactions, where exhaust gas abatement is one field of relevant application. The development of highly active catalysts includes the determination of design principles that guide the elemental composition of an ideal catalyst. The picture of the role of electronic characteristics in catalytic efficacy and the influence of compositional variation, however, is incomplete. This work uses in situ X ray absorption spectroscopy (XAS) to characterize B-site substituted LaCoO3 derivatives and to determine electronic characteristics, which are analyzed for their suitability as descriptors for intrinsic reaction rates and selectivity, where applicable. The catalysts were tested in NO oxidation, CO oxidation, N2O decomposition and the system of NO + CO and N2O + CO reactions. A detailed mechanistic analysis was conducted to understand the interaction of the catalyst and respective reactants. Transition metal t2g and eg orbital occupancy yield volcano type or non linear correlations with all reaction rates and the selectivity in the NO + CO reaction. Covalency between O 2p and TM 3d states was determined as single-valued parameter and is a linear descriptor for all reaction rates and the NO + CO selectivity. This covalent interaction of O 2p-TM 3d electronic states crucially determines the ability of the catalytically active sites to interact with surface species during the kinetically relevant step of the reaction. All correlations of electronic descriptors with reaction rates are remarkably sensitive to the XAS measurement conditions, which lead to partially reduced catalysts in ultra-high vacuum and fully oxidized catalysts in O2 presence. This sensitivity originates from the different characteristics of the respective kinetically relevant step in terms of involved reactants and the oxidation state of the active site during the reaction step. The mechanistic understanding provided by kinetic analysis is essential to identify the reactants that interact with the active site during the kinetically relevant step. This work therefore establishes firm links between mechanistic understanding and the role of catalyst characteristics in the determination of descriptors for catalytic efficacy. It further emphasizes that the varying specifics of reactions are mirrored in the necessity of different XAS conditions that are required to elucidate valid correlations with descriptors. The correlations of catalytic efficacy with covalency, in all examined reactions in this work underline the major impact of this particular electronic characteristic. These results mark a starting point towards the establishment of covalency as relevant descriptor for catalytic efficacy, which can be further compounded by DFT analysis and extended analysis of further reaction systems and materials. Overall, as a brief analysis illustrates, covalency may offer the possibility to compare catalytic efficacy in different reactions among TM oxides with varying crystal structures.