(758b) Determining the Thermodynamic Stability of Single-Site Iron Complexes in Nitrogen-Doped Carbon: A Computational Study

Determining
the Thermodynamic Stability of Single-Site Iron Complexes in Nitrogen-Doped
Carbon: A Computational Study

M. Ayiania, A. Garcia, A. Hensley, K. Groden, Y. Wang, M. Garcia-Perez, J.-S.
McEwen

Carbonaceous
materials, such as graphene, activated carbon, and amorphous carbons, have
garnered great interest in recent years as their physical, chemical, and
electronic properties can be finely tuned through functionalization with heteroatomic
dopants.
In particular, nitrogen-doped variants have received interest for their
potential as electrodes, adsorbents, and supports for metallic single-site
catalysts [1,2]. While a wealth of experimental data exists on the
classification and applications of these compounds, a complete thermodynamic picture
of the functionalities that are likely to form under different experimental
conditions is lacking. To address this, we provide atomistic insight into the
most thermodynamically stable nitrogen and nitrogen-iron complexes on graphene-based
materials through a unique combination of density functional theory and
thermodynamic calculations. We report Gibbs free energies of reaction for the
formation of different graphitic, pyridinic, and pyrrolic nitrogen functional
groups as a function of temperature and NH₃ pressure on perfect
graphene, graphene nanoribbons terminated with hydrogen, and nanoribbons
terminated with hydrogen and oxygen. The resulting phase diagram shows a general
preference for the formation of conjugated nitrogen edge groups such as
pyridines and pyrroles, but such complexes are thermodynamically competitive to
a graphitic functionality under certain circumstances [3]. However, the presence
of iron can significantly favor the introduction of more complicated structures
within the sheet that would be otherwise untenable, as illustrated in Figure 1.
In addition, these supports have a significant effect on the oxophilicity of
iron, potentially providing a rationale for the durability of certain nitrogen-supported
iron catalysts.

[1] H. Xu, D. Cheng, D. Cao and X. C. Zeng, Nature Catalysis 1 (2018) 339-348.

[2] W. Liu, L.
Zhang, X. Liu,
X. Liu, X.
Yang, S. Miao, W. Wang, A. Wang and T.
Zhang, J. Am Chem. Soc 139 (2017) 10790-10798.

[3]
M. Ayiania, A. J. R. Hensley, K. Groden, M. Garcia-Perez; J.-S.
McEwen (submitted).

Figure 1:
Phase diagram illustrating the thermodynamic stability dependence on temperature
and pressure of N-doped carbon in the presence and in the absence of Fe.