Co3O4/TiO2 catalysts studied in situ during the preferential oxidation of carbon monoxide: the effect of different TiO2 polymorphs

Abstract

Co₃O₄ nanoparticles were supported on different TiO₂ polymorphs, namely, rutile, anatase, and a 15 : 85 mixture of rutile and anatase (also known as P25), via incipient wetness impregnation. The Co₃O₄/TiO₂ catalysts were evaluated in the preferential oxidation of CO (CO-PrOx) in a H₂-rich gas environment and characterised in situ using PXRD and magnetometry. Our results show that supporting Co₃O₄ on P25 resulted in better catalytic performance, that is, a higher maximum CO conversion to CO₂ of 72.7% at 200 °C was achieved than on rutile (60.7%) and anatase (51.5%). However, the degree of reduction (DoR) of Co₃O₄ to Co⁰ was highest on P25 (91.9% at 450 °C), with no CoTiO₃ detected in the spent catalyst. The DoR of Co₃O₄ was lowest on anatase (76.4%), with the presence of Ti_xO_y-encapsulated CoO_x nanoparticles and bulk CoTiO₃ (13.8%) also confirmed in the spent catalyst. Relatively low amounts of CoTiO₃ (8.9%) were detected in the spent rutile-supported catalyst, while a higher DoR (85.9%) was reached under reaction conditions. The Co⁰ nanoparticles formed on P25 and rutile existed in the fcc and hcp crystal phases, while only fcc Co⁰ was detected on anatase. Furthermore, undesired CH₄ formation took place over the Co⁰ present in the P25- and rutile-supported catalysts, while CH₄ was not formed over the Co⁰ on anatase possibly due to encapsulation by Ti_xO_y species. For the first time, this study revealed the influence of different TiO₂ polymorphs (used as catalyst supports) on the chemical and crystal phase transformations of Co₃O₄, which in turn affect its activity and selectivity during CO-PrOx.