In situ spectroscopic insights into the redox and acid-base properties of ceria catalysts

Cerium oxide (ceria) plays an important and fascinating role in heterogeneous catalysis as illustrated by its versatile use as a catalyst, a catalyst support, or a promotor in various oxidation and reduction reactions. Central to these reactions is the rich defect chemistry, facile redox capability, and unusual acid-base properties of ceria. Understanding the unique redox and acid-base properties of ceria is essential to build the structure-catalysis relationship so that improved catalytic functions can be achieved for ceria-based materials. Among the characterization toolbox, spectroscopic approach indisputably stands out for its unparalleled power in offering chemical insights into the surface properties of ceria at atomic and molecular level. In this review, we summarize advances in revealing the redox and acid-base properties of ceria via a variety of spectroscopic methods including optical, X-ray, neutron, electronic and nuclear spectroscopy. Both direct spectroscopy characterization and its coupling with probe molecules are analyzed to illustrate how the nature, strength and density of different surface sites are influenced by the pretreatment, the morphology and size of ceria nanoparticles. Further directions in taking advantage of in situ/operando spectroscopy for better understanding the catalysis of ceria-based materials are proposed in the summary and outlook section.