Comprehensive study of the ceria-h₂system: Effect of the reaction conditions on the reduction extent and intermediates

The interaction of hydrogen with ceria is a fundamental process of high importance for various catalytic reactions. Theoretical calculations have been performed to identify the elementary steps in this reaction and the intermediate species, as they strongly influence the selectivity and efficiency of the hydrogenation reactions catalyzed by ceria. However, these studies do not provide direct information on the effect of temperature or H₂ partial pressure on the reaction mechanism and intermediates. In the present work, thermogravimetric and differential thermal measurements were used to study the influence of temperature (in the 300-600°C range), hydrogen partial pressure (0.2- 3%), and the presence of oxygen vacancies on ceria-H₂ interaction. In addition, temperature-programmed reduction (TPR) and desorption measurements were performed to monitor the adsorbing/desorbing gases as a function of temperature. It was found that at T ≤ 400°C, H₂ adsorbs chemically on stoichiometric ceria but nearly does not dissociate, whereas at higher temperatures, H₂ adsorbs dissociatively, leading to hydroxyl formation followed by H₂O desorption. Some of the hydroxyl formed remains on the surface at the end of the reduction stage, allowing very fast re-oxidation of ceria by O₂ through H₂O formation. Under the studied conditions, increasing H₂ partial pressure led to a higher rate of H₂ adsorption and H₂O desorption but did not lead to an increased extent of reduction, suggesting that H₂O formation/desorption is the rate-determining step in ceria reduction (with an activation energy of 105 ± 4 kJ/mol according to the TPR measurements). The presence of oxygen vacancies, before exposing ceria to H₂, was found to strongly affect the characteristics of H₂ adsorption and to lead to the penetration of hydrogen into the sub-surface and bulk, probably as hydridic hydrogen. Penetration of hydrogen into the lattice, competing with hydroxyl formation, was found to occur preferentially at low temperatures.