Hydrogen diffusion in ceria: solid state NMR, combined scattering and spectroscopic studies, and ab initio calculations

TL;DR

This study combines experimental techniques and ab initio calculations to elucidate hydrogen diffusion mechanisms in ceria, revealing jump diffusion behavior with low activation energy barriers and providing direct evidence of microscopic hydrogen dynamics.

Contribution

It offers the first direct experimental evidence of hydrogen jump diffusion in ceria and combines multiple methods to comprehensively analyze hydrogen dynamics at the microscopic level.

Findings

Hydrogen jump diffusion in ceria confirmed by QENS.

Activation energy barriers for hydrogen diffusion are less than 0.2 eV.

Hydrogen diffusion coefficient estimated around 10^-11 m^2/s.

Abstract

Ceria has been extensively studied since it has many applications in diverse research fields. However, the mechanism of the hydrogen dynamics, especially the diffusion kinetics on a microscopic level is still unclear as the experimental data has been very limited. In this work, the CeO$_2$-H interaction has been comprehensively studied by a combination of $^1$H NMR transverse relaxation time ($T_2$) measurement, neutron powder diffraction, quasi-elastic neutron scattering (QENS), X-ray total scattering, and ab initio calculations. Based on QENS measurements, the first direct evidence for hydrogen jump diffusions of the Chudley-Elliot type in the bulk ceria has been given, with a jump distance of ~3.98 angstrom. The theoretically calculated activation energy barriers $E_a$ for hydrogen diffusion are relatively low (less than 0.1 eV), further supporting that such hopping can readily occur. A larger barrier value of $E_a$ ~0.2 eV is directly estimated by $T_2$ NMR data, suggesting possible slower hydrogen dynamics with the pre-exponential factor $D_0$ of diffusion coefficient ~10$^{-11}$ m$^2/$s.