Quantum Calculations of Hydrogen Absorption and Diffusivity in Bulk $\mathrm{CeO_2}$

TL;DR

This study uses advanced quantum DFT calculations to analyze hydrogen absorption and diffusion in bulk CeO2, providing bounds on key properties and insights into diffusion barriers relevant for catalysis.

Contribution

It offers a comprehensive DFT-based analysis of hydrogen in CeO2, establishing bounds on physical properties and diffusion barriers, aiding future modeling and experimental efforts.

Findings

Hydrogen absorption energies are bounded within specific ranges.

Activation energy barriers for hydrogen diffusion are uniformly low (<0.15 eV).

Hydrogen tunneling effects are minimal at ambient temperatures.

Abstract

CeO$_2$ (ceria) is an attractive material for heterogeneous catalysis applications involving hydrogen due to its favorable redox activity combined with its relative impermeability to hydrogen ions and molecules. However, to date, many bulk ceria/hydrogen properties remain unresolved in part due to a scarcity of experimental data combined with quantum calculation results that vary according to the approach used. In this regard, we have conducted a series of Density Functional Theory (DFT) calculations utilizing generalized gradient (GGA), meta-GGA, and hybrid functionals as well as several corrections for electronic correlations, applied to a number of properties regarding hydrogen in bulk stoichiometic $\mathrm{CeO_2}$. Our calculations place reasonable bounds on the lattice constants, band gaps, hydrogen absorption energies, and O-H bond vibrational frequencies that can be determined by DFT. In addition, our results indicate that the activation energy barriers for hydrogen bulk diffusion are uniformly low ($ < 0.15 \ \mathrm{eV} $) for the calculation parameters probed here and that, in general, the effect of hydrogen tunneling is small at ambient temperatures. Our study provides a recipe to determine fundamental physical chemical properties of Ce-O-H interactions while also determining realistic ranges for diffusion kinetics. This can facilitate the determination of future coarse-grained models that will be able to guide and elucidate experimental efforts in this area.