Datensatz

Zusammenfassung

IrO₂ has been investigated as alternative rutile-type catalyst to RuO₂ for gas-phase HCl oxidation. The HCl conversion level over IrO₂ at 723 K was comparable to that of RuO₂ at ca. 170 K lower temperature, in line with the higher computed energy barrier for chlorine evolution over the former oxide. Similarly to RuO₂, chlorination took place only at the IrO₂ surface, which is predicted to exhibit full occupation of the coordinatively unsaturated iridium sites and replacement of 50% of the oxygen bridge positions by chlorine. Advantageously, IrO₂ is more resistant than RuO₂ against oxidation, since the latter forms volatile RuO₄ species at high temperatures. ₂2 wt %) supported on TiO₂-rutile displayed a 6 times higher activity than on Ti₂-anatase. Although this corroborates the crucial role of the structural similarity between the carrier and active phase highlighted in the development of RuO₂-based catalysts, some differences were uncovered. (i) Small and highly dispersed IrO₂ clusters rather than thin films are present on TiO₂-rutile, in line with the expected preference for Stranski–Krastanov-type growth rather than epitaxial growth due to strain. (ii) Geometric and electronic effects of TiO₂-rutile are predicted not to lead to improved HCl oxidation activity for 1 and ₂ epilayers of IrO₂ over the carrier. (iii) The superior performance of IrO₂/TiO₂-rutile thus mainly originates from the higher metal dispersion. A rational approach was applied to manufacture this catalyst in technical form. The successful protocol comprised TiO₂-anatase-aided extrusion of the rutile support followed by metal impregnation. The catalytic activity and kinetic fingerprints were unaltered upon shaping, and its robustness was highlighted in a 50 h test. On the basis of these findings, IrO₂/TiO₂-rutile represents a suitable high-temperature HCl oxidation catalyst that could be applied in staged fixed-bed reactors along with the low-temperature RuO₂-based materials.