Gas–solid interaction of H2–Ce0.95Zr0.05O2: new insights into surface participation in heterogeneous catalysis

Abstract

A direct interaction between a reduction medium (H₂) and a Ce_0.95Zr_0.05O₂ (CZ) solid surface was explored through ambient pressure photoelectron spectroscopy (APPES), with conventional X-ray and He-I photon sources, in H₂ atmosphere up to 0.1 mbar pressure and 773 K. A porous CZ thin film was prepared by a combination of sol–gel and spin-coating methods, and was employed to understand the redox nature of ceria under reduction conditions. The maximum of 45% Ce³⁺ along with the corresponding oxygen vacancy was observed due to reduction. An unprecedented decrease in the valence band (VB) energy up to 1.4 eV was observed on reduction along with a narrowing of VB. The highest occupied energy band derived from Ce 4f also shifts closer to E_F. H₂ molecular vibration observed in VB-APPES was employed as a probe to explore the surface potential changes along with the dynamic change in the nature of the surface under reduction conditions. The surface potential decreases by 0.27 eV up to 673 K, and then it reverts by 0.24 eV on further reduction at 773 K for different reasons. Further, an enhancement in the Ce 5d–O 2p interaction occurs at the expense of the Ce 4f–O 2p interaction under the above reduction conditions. Vacuum annealing and H₂ reduction after that show significantly more VB shift and enhanced reduction than H₂ reduction alone. Although Ce reduction occurs on high temperature vacuum annealing of CZ, a significantly lower amount of oxygen vacancies appears. This study shows the dynamic changes in the nature of the surface due to a gas (H₂)–solid (CZ) interaction and ensuing electronic structure changes that influence heterogeneous catalysis. It also underscores the necessity to study the catalytic materials under in situ conditions or closer to that.