Oxygen-vacancy induced structural changes of Co species in CoAl2O4 spinels for CO2 hydrogenation

Promotional effects of oxygen vacancies of spinel catalysts in CO 2 hydrogenation are reported in early works, but the mechanistic origins remain elusive. Here, CoAl 2 O 4 spinels with varying numbers of oxygen vacancies are deliberately designed by a sol-gel method and different post-treatments. By combining catalytic testing, advanced electron microscopic and spectroscopic characterizations, and computational studies, the unusual oxygen vacancy-dependent catalytic behaviors are rationalized. Our work reveals that i) perfect spinel crystals possessing least oxygen vacancies can effectively constrain the Co 2+ species at working conditions that are less active but selective to CO; and ii) vacancy-rich spinels promote both H 2 and CO 2 activations and COOH* formation, explaining the higher hydrogenation activity, but overwhelming vacancies cause Co 2+ reduction and promote direct CO 2 * dissociation to CO* and deep hydrogenation to CH 4 . These molecular-level understandings reinforce the idea of proper design of oxygen vacancies to achieve activity-selectivity balance.