Reversible
formate (HCOO–) dehydrogenation and
bicarbonate (HCO3
–) hydrogenation would
be desirable for the utilization and storage of hydrogen (H2) as an effective energy carrier. Carbon-supported Pd-based nanoparticles
demonstrated enormous competitive advantages for these reactions.
However, the fundamental mechanisms underlying these reversible reactions
have not yet been elucidated. Herein, we report the reaction pathways
for reversible reactions on a Pd-based catalyst using density functional
theory (DFT) calculations and propose key factors for improving the
reaction efficiency. As the first essential step, the difficulty in
the conventional DFT modeling, that is simulation of an anion environment
caused by HCOO–, was overcome by designing two-sided
Pd12 nanoclusters supported on graphene (Pd12NC-G) with extra electrons. Using Pd12NC-G, we demonstrated
that the key factor determining the potential limiting steps for the
reversible reaction was desorption of hydrogen in HCOO– dehydrogenation (1.24 eV) and HCO3
– hydrogenation (1.49 eV). The key factor was the same in Pd12NC-N1G, Pd12NC-N2G, and Pd12NC-N3G (where N1, N2, and N3 represent the number of N atoms doped on carbon). Among these,
the Pd12NC-N2G model with the appropriate amount
of nitrogen doping showed optimal hydrogen adsorption strength corresponding
to the smallest d-band center and spin density values, resulting
in the lowest energy barriers for HCOO– dehydrogenation
(0.76 eV) and HCO3
– hydrogenation (0.96
eV). Based on harmonization between electronic and geometrical properties,
we demonstrated that the appropriate level of nitrogen doping can
provide the optimal balance between the magnitude of reactivity and
the number of sites for improving the efficiency of the reversible
reactions.
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