Density functional theory study of bulk properties of transition metal nitrides.

Density functional theory (DFT) calculations are performed to compute the lattice constants, formation energies and vacancy formation energies of transition metal nitrides (TMNs) for transition metals (TM) ranging from 3d-5d series. The results obtained using six different DFT exchange and correlation potentials (LDA, AM05, BLYP, PBE, rPBE, and PBEsol) show that the experimental lattice constants are best predicted by rPBE, while the values obtained using AM05, PBE, rPBE and PBEsol lie between the LDA and BLYP calculated values. A linear relationship is observed between the lattice constants and formation energies with the mean radii of TM and the difference in the electronegativity of TM and N in TMNs, respectively. Our calculated vacancy formation energies, in general, show that N-vacancies are more favorable than TM-vacancies in most TMNs. We observe that N-vacancy formation energies are linearly correlated with the calculated bulk formation energies indicating that TMNs with large negative formation energies are less susceptible to the formation of N-vacancies. Thus, our results from this extensive DFT study not only provide a systematic comparison of various DFT functionals in calculating the properties of TMNs but also serve as reference data for the computation-driven experimental design of materials.

Metal-Coordinated Phthalocyanines as Platform Molecules for Understanding Isolated Metal Sites in the Electrochemical Reduction of CO2.

Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CO2RR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CO2RR activity, selectivity, and reaction mechanisms. In this study, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TM-Pcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CO2RR performance. The combined experimental measurements, in situ characterization, and density functional theory calculations of TM-Pc catalysts reveal a TM-dependent CO2RR activity and selectivity, with the free energy difference of ΔG(*HOCO) - ΔG(*CO) being identified as a descriptor for predicting the CO2RR performance.

Pt- and Pd-modified transition metal nitride catalysts for the hydrogen evolution reaction.

Hydrogen production via electrochemical splitting of water using renewable electricity represents a promising strategy. Currently, platinum group metals (PGMs) are the best performing hydrogen evolution reaction (HER) catalysts. Thus, the design of non-PGM catalysts or low-loading PGM catalysts is essential for the commercial development of hydrogen generation technologies via electrochemical splitting of water. Here, we employed density functional theory (DFT) calculations to explore Pt and Pd modified transition metal nitrides (TMNs) as low-cost HER catalysts. Our calculations show that Pt/Pd binds strongly with TMs on TMN(111) surfaces, leading to the formation of stable Pt and Pd-monolayer (ML)-TMN(111) structures. Furthermore, our calculated hydrogen binding energy (HBE) demonstrates that Pt/MnN, Pt/TiN, Pt/FeN, Pt/VN, Pt/HfN, Pd/FeN, Pd/TaN, Pd/NbN, Pd/TiN, Pd/HfN, Pd/MnN, Pd/ScN, Pd/VN, and Pd/ZrN are promising candidates for the HER with a low value of limiting potential (UL) similar to that calculated on Pt(111).

Achieving complete electrooxidation of ethanol by single atomic Rh decoration of Pt nanocubes.

SignificanceDirect ethanol fuel cells are attracting growing attention as portable power sources due to their advantages such as higher mass-energy density than hydrogen and less toxicity than methanol. However, it is challenging to achieve the complete electrooxidation to generate 12 electrons per ethanol, resulting in a low fuel utilization efficiency. This manuscript reports the complete ethanol electrooxidation by engineering efficient catalysts via single-atom modification. The combined electrochemical measurements, in situ characterization, and density functional theory calculations unravel synergistic effects of single Rh atoms and Pt nanocubes and identify reaction pathways leading to the selective C-C bond cleavage to oxidize ethanol to CO2. This study provides a unique single-atom approach to tune the activity and selectivity toward complicated electrocatalytic reactions.