Synthesis, Characterization of Ruthenium-Titanium Mixed Oxide and the Effect of Hydrogen Incorporation on Catalytic Propane and HCl Oxidation Reactions

Abstract

Mixed oxide catalysts play a pivotal role in both fundamental research and industrial applications due to their ability to integrate advantageous properties from the different elements contained. Among all, RuxTi1-xO2 is a kind of highly regarded mixed oxide material since it exhibits favorable electro and thermocatalytic properties. There is a variety of studies on the synthesis and characterization of the Ru-Ti mixed oxides, while the solid solution limit or solubility of the ruthenium and titanium into each other’s oxide phase is still ambiguous and the phase purity has not been clarified yet. Therefore, it is necessary to prepare RuxTi1-xO2 varying the Ru and Ti compositions at full range and subsequently gain comprehensive insights into their physical and chemical properties via dedicated characterization methods. Hydrogen induced catalyst engineering has been recently reported and demonstrated as a promising strategy to obtain superior catalytic performance in both hydrogenation and dehydrogenation reactions. The incorporation of hydrogen into the lattice of the oxide material will normally result in the formation of a hydride species or hydroxyl groups, accompanied by changes of the lattice strain, which as a consequence may influence the catalytic behavior of the material. For Ru-Ti mixed oxides, it is not clear yet whether hydrogen can be inserted into the bulk region. Moreover, the effect of hydrogen insertion into the bulk oxides in thermal catalysis has not yet been reported from the literature. Hence, the main objective of the present thesis is to explore the potential application of hydrogen-incorporated Ru-Ti mixed oxides in oxidation catalysis, such as propane combustion and HCl oxidation reactions. In the present research, rutile-phase RuxTi1-xO2 are synthesized via the conventional sol-gel method and reveal a clear miscibility gap in a wide composition range (0.2 ≤ x ≤ 1). Hydrogen exposure at 250 °C (250R) results in hydrogen incorporation accompanied with lattice strain, which in turn significantly promotes the oxidation catalysis of propane combustion. Among all, Ru0.6Ti0.4O2_250R represents the catalytically most active catalyst. Furthermore, Ru0.3Ti0.7O2 sample is chosen to investigate the structural and electronic evolution upon hydrogen treatment at elevated temperatures. 17.6 mol% of hydrogen can be incorporated into the mixed oxide Ru0.3Ti0.7O2, while this is not possible for pure rutile RuO2 and TiO2 that is either reduced to metallic Ru or does not allow for hydrogen absorption, respectively. It is demonstrated that hydrogen-incorporated Ru0.3Ti0.7O2 improves substantially the catalytic performance in oxidation reactions such as the total oxidation of propane and HCl oxidation reaction. Hydrogen-induced lattice strain in Ru0.3Ti0.7O2 accompanied by altered electronic properties is likely to be the reason for the observed enhanced catalytic activity. The formation of a solid solution of a reducible oxide with a (non or) less reducible oxide may open the way to incorporate substantial amounts of hydrogen via simple exposure to H2 at elevated temperatures, providing an additional parameter to fine-tune in situ the catalytic performance in various catalytic reactions.