Co 3 O 4 /TiO 2 catalysts studied in situ during the preferential oxidation of carbon monoxide: the effect of different TiO 2 polymorphs

Co 3 O 4 nanoparticles were supported on different TiO 2 polymorphs, namely, rutile, anatase, and a 15 : 85 mixture of rutile and anatase (also known as P25), via incipient wetness impregnation. The Co 3 O 4 /TiO 2 catalysts were evaluated in the preferential oxidation of CO (CO-PrOx) in a H 2 -rich gas environment and characterised in situ using PXRD and magnetometry. Our results show that supporting Co 3 O 4 on P25 resulted in better catalytic performance, that is, a higher maximum CO conversion to CO 2 of 72.7% at 200 °C was achieved than on rutile (60.7%) and anatase (51.5%). However, the degree of reduction (DoR) of Co 3 O 4 to Co 0 was highest on P25 (91.9% at 450 °C), with no CoTiO 3 detected in the spent catalyst. The DoR of Co 3 O 4 was lowest on anatase (76.4%), with the presence of Ti x O y -encapsulated CoO x nanoparticles and bulk CoTiO 3 (13.8%) also confirmed in the spent catalyst. Relatively low amounts of CoTiO 3 (8.9%) were detected in the spent rutile-supported catalyst, while a higher DoR (85.9%) was reached under reaction conditions. The Co 0 nanoparticles formed on P25 and rutile existed in the fcc and hcp crystal phases, while only fcc Co 0 was detected on anatase. Furthermore, undesired CH 4 formation took place over the Co 0 present in the P25- and rutile-supported catalysts, while CH 4 was not formed over the Co 0 on anatase possibly due to encapsulation by Ti x O y species. For the first time, this study revealed the influence of different TiO 2 polymorphs (used as catalyst supports) on the chemical and crystal phase transformations of Co 3 O 4 , which in turn affect its activity and selectivity during CO-PrOx.