Studies of CO2 hydrogenation over cobalt/ceria catalysts with in situ characterization: the effect of cobalt loading and metal-support interactions on the catalytic activity

The catalytic activities of cobalt/ceria catalysts (Co loadings of 1, 2 and 4 wt%) for CO2 hydrogenation at 250 degrees C were measured. When catalyzed by samples with the same mass of cobalt, the CO2 conversion showed a trend where 4 wt% approximate to 2 wt% > 1 wt%. XRD results combined with XANES data of pristine and reduced samples showed that the size of the cobalt particles was proportional to the admetal loading. Moreover, XANES data of reduced catalysts indicated that samples with higher loading have a higher percentage of metallic cobalt while a significant part of cobalt in the 1 wt%Co/CeO2 sample remains as CoOx after reduction. By analyzing EXAFS data, it was found that the Co-O bond in the 1 wt% and 2 wt%Co/CeO2 was about 0.16 angstrom shorter than that of standard CoO, which explains its stability during the reduction process. The positive charge on the cobalt atoms substantially affected the surface chemistry of the CO2 hydrogenation process. In situ DRIFTS experiments under reaction conditions showed that the Ce-3(+) site density on the surface of the 1 wt%Co/CeO2 catalyst was much lower than that of the other two samples. In addition, formates were found to be spectators in the 1 wt% sample while they could be further hydrogenated in the 2 wt% and 4 wt%Co/CeO2 surfaces. Based on these results, we propose that the strong perturbations on the structural and electronic properties, which cause positively charged cobalt atoms in the cobalt-ceria interface, make the highly dispersed cobalt on ceria less active for activating hydrogen leading to a lower CO2 hydrogenation efficiency in the case of 1 wt%Co/CeO2.