(673e) A Computational Study of Role of Surface Hydroxylation in Enhancing Propene Selectivity during Propane Oxidative Dehydrogenation over Ni Doped CeO2 Nanorods

Propane oxidative dehydrogenation (ODH) is a potential alternative for the conventional cracking processes, as it is an exothermic and a direct process for obtaining propene. Ni doped ceria nanorods, predominantly exposing (110) surface exhibit good surface reducibility properties and are promising propane ODH catalysts, provided the overoxidation of products is controlled. One of the ways of achieving this is by Lewis Base addition to the reaction system, which enhance desorption of propane ODH products i.e., propene and acetone. Using plane-wave based periodic Density Functional Theory (DFT) calculations, this work aims at understanding the role of hydroxyls (OH^-) in improving propene selectivity of a typical propane ODH over Ce_0.83Ni_0.17O_1.83 (110) surfaces. The ab-initio thermodynamic studies confirmed the stability of hydroxyls over Ce_0.83Ni_0.17O_1.83 (110) surfaces up to a temperature of 870 K and existence of 1 ML hydroxylated surface at typical Propane ODH conditions. With increase in hydroxyl coverage, the desorption of propene and acetone was enhanced and also their overoxidation was less favoured. Analyses showed that this was due to induced lateral repulsive interaction and hyperconjugation effects by hydroxyls. The adsorbed hydroxyl atoms proved to be better hydrogen abstractors with propane C-H activation energy barrier of 1.1 eV, which was 0.3 eV lower than that by surface lattice oxygen and adsorbed oxygen atoms (E_a= 1.4 eV). At high hydroxyl coverages (> 0.83 ML), propene formation was found to be more favourable than acetone, as at these conditions hydroxyl atoms are the active centres for propene formation while surface lattice oxygen atoms are the active centres for acetone formation. This predicts improved propene selectivity at high hydroxyl coverages. Hence this work highlights the importance and applicability of surface hydroxylation as an efficient pre-treatment strategy for Ni doped ceria nanorods to make them more selective towards propene formation during propane ODH.