Transformation of titanium carbide into mesoporous titania for catalysed HBr oxidation

Abstract

Herein, we report TiC as precursor of a highly active and novel TiO₂–TiC composite for the catalytic oxidation of HBr into Br₂, which is a key technology for bromine looping in the halogen-mediated process for natural gas valorisation. In addition, we investigate the mechanism of TiO₂ formation that is pivotal in deriving synthesis–structure–performance relationships. In-depth material characterisation, by means of X-ray diffraction, thermogravimetric analysis, N₂ sorption, Raman, X-ray photoelectron, ^47,49Ti and ¹³C nuclear magnetic resonance spectroscopies, as well as ex situ and in situ transmission electron microscopy reveal that the transformation of TiC proceeds via a combination of shrinking core and spot-oxidation mechanisms. This results in the formation of a TiO₂–TiC composite, containing both anatase and rutile polymorphs, with enhanced mesoporosity, reaching up to a one order of magnitude higher surface area and pore volume compared to pristine TiC. Reactivity and kinetics studies are performed in HBr oxidation, showing that the catalyst activity follows a volcano behaviour, whose apex is found for the material calcined at 673 K. Its performance surpasses that of benchmark TiO₂-P25 and is also found stable for 25 h on stream. Accordingly, the reactivity of the TiO₂–TiC composite is rationalized based on the TiO₂ generation and porosity formation, which are both functions of the oxidation temperature.