HCl Oxidation on IrO₂-Based Catalysts: From Fundamentals to Scale-Up

IrO 2 has been investigated as alternative rutile-type catalyst to RuO 2 for gas-phase HCl oxidation. The HCl conversion level over IrO 2 at 723 K was comparable to that of RuO 2 at ca. 170 K lower temperature, in line with the higher computed energy barrier for chlorine evolution over the former oxide. Similarly to RuO 2, chlorination took place only at the IrO 2 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, IrO 2 is more resistant than RuO 2 against oxidation, since the latter forms volatile RuO 4 species at high temperatures. IrO 2 (2 wt %) supported on TiO 2 -rutile displayed a 6 times higher activity than on TiO 2 -anatase. Although this corroborates the crucial role of the structural similarity between the carrier and active phase highlighted in the development of RuO 2 -based catalysts, some differences were uncovered. ( i ) Small and highly dispersed IrO 2 clusters rather than thin films are present on TiO 2 -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 TiO 2 -rutile are predicted not to lead to improved HCl oxidation activity for 1 and 2 epilayers of IrO 2 over the carrier. ( iii ) The superior performance of IrO 2 /TiO 2 -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 TiO 2 -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, IrO 2 /TiO 2 -rutile represents a suitable high-temperature HCl oxidation catalyst that could be applied in staged fixed-bed reactors along with the low-temperature RuO 2 -based materials.