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

Abstract

The catalytic activities of cobalt/ceria catalysts (Co loadings of 1, 2 and 4 wt%) for CO₂ hydrogenation at 250 °C were measured. When catalyzed by samples with the same mass of cobalt, the CO₂ conversion showed a trend where 4 wt% ≈ 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 CoO_x 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 Å 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 Ce³⁺ 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.