Theoretical design of nanocatalysts based on (Fe2O3)<i>n</i> clusters for hydrogen production from ammonia

The catalytic activities of high-spin small Fe(III) oxides have been investigated for efficient hydrogen production through ammonia decomposition, using the artificial force induced reaction method within the framework of density functional theory with the B3LYP hybrid exchange-correlation functional. Our results reveal that the adsorption free energy of NH3 on (Fe2O3)n (n = 1-4) decreases with increasing cluster size up to n = 3, followed by a slight increase at n = 4. The strongest NH3 adsorption energy, 28.55 kcal/mol, was found for Fe2O3, where NH3 interacts with a two-coordinated Fe site, forming an Fe-N bond with a length of 2.11 Å. A comparative analysis of NH3 dehydrogenation and H2 formation on various Fe(III) oxide sizes identifies the rate-determining steps for each reaction. We found that the rate-determining step for the full NH3 dehydrogenation on (Fe2O3)n (n = 1-4) is size-dependent, with the NH* → N* + H* reaction acting as the limiting step for n = 1-3. In addition, our findings indicate that H2 formation is favored following the partial decomposition of NH3 on Fe(III) oxides.