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 Co2+ 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 Co2+ 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. [Display omitted] • CoAl 2 O 4 spinels with different numbers of oxygen vacancies were designed. • CoAl 2 O 4 spinels presented divergent CO 2 hydrogenation performances. • Complex interplay among oxygen vacancies, catalyst structure and CO 2 hydrogenation performances were revealed. • Mechanistic origins of the multifaceted roles of oxygen vacancies were unveiled by DRIFT and DFT. [ABSTRACT FROM AUTHOR]