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

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. IrO2 (2 wt %) supported on TiO2-rutile displayed a 6 times higher activity than on TiO2-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.