Organic acid-assisted preparation of highly dispersed Co/ZrO2 catalysts with superior activity for CO2 methanation

Various organic acids were investigated for preparing the highly dispersed cobalt catalysts on zirconia support, Co/ZrO2, by an acid-assisted incipient wetness impregnation. The organic acid-assisted preparation results in highly dispersed Co/ZrO2 catalysts which demonstrate superior catalytic activity with only 2 wt.% Co loading for CO2 methanation. Among the acids, the carboxylic acids with bigger molecular weight, more carboxyl groups and hydroxyl groups are more effective. Furthermore, the amino acids can also change the charge property of ZrO2 surface through controlling the solution pH and increase the metal dispersion through strong electrostatic adsorption. It is worth mentioning that, among the numerous organic acids, citric acid-assisted preparation gives Co/ZrO2 catalyst with superior catalytic activity and increased the CO2 conversion from 38% to 85% with highest turnover frequency. The linear relationship between metal dispersion and TOF with the molar ratio of acid to cobalt nCA/Co ranging from 0 to 2 demonstrates the importance of well dispersed metal particles in the enhanced activity. However, when the nCA/Co value exceeds 2 and the metal particle size further decreases, but no further enhancement is observed and the CO2 conversion even slightly decreased at the nCA/Co value of 5. The organic acids-assisted strategy can lead to increased metal dispersion and higher TOF for Co/ZrO2 catalysts; the appropriate metal dispersion corresponds to the suitable Co-ZrO2 interaction with oxygen vacancy which leads to more catalytically active sites for CO2 adsorption and catalytic hydrogenation. The organic acid-assisted strategy provides a new design approach for highly dispersed and active catalysts.