Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation

The Co/ZrO₂ catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO₂ nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO₂. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO₂ which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO₂ crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO₂ prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO₂ catalysts were increased with increasing Co loading and precursor feed rate. According to H₂ chemisorption and H₂ temperature program reduction results, the improvement of catalytic activity and C₂–C₆ selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO₂ support obtained via the FSP synthesis.