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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. 

Pt and Pd monolayer on transition metal nitrides as efficient and low-cost catalysts for hydrogen evolution reaction.

 

Electrochemical CO₂reduction reaction (CO₂RR) provides a potential pathway to mitigate challenges related to CO₂emissions. Pd nanoparticles have shown interesting properties as CO₂RR electrocatalysts, while how different facets of Pd affect its performance in CO₂reduction to synthesis gas with controlled H₂to CO ratios has not been understood. Herein, nanosized Pd cubes and octahedra particles dominated by Pd(100) and Pd(111) facets are, respectively, synthesized. The Pd octahedra particles show higher CO selectivity (up to 95%) and better activity than Pd cubes and commercial particles. For both Pd octahedra and cubes, the ratio of H₂/CO products is tunable between 1 and 2, a desirable ratio for methanol synthesis and the Fischer–Tropsch processes. Further studies of Pd octahedra in a 25 cm²flow cell show that a total CO current of 5.47 A is achieved at a potential of 3.4 V, corresponding to a CO partial current density of 220 mA cm⁻². In situ X‐ray absorption spectroscopy studies show that regardless of facet Pd is transformed into Pd hydride (PdH) under reaction conditions. Density functional theory calculations show that the reduced binding energies of CO and HOCO intermediates on PdH(111) are key parameters to the high current density and Faradaic efficiency in CO₂to CO conversion.

 

The electrochemical CO₂reduction reaction (CO₂RR) to syngas represents a promising solution to mitigate CO₂emissions and manufacture value‐added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO₂RR due to its transformation to Pd hydride under CO₂RR conditions, however, the pre‐hydridized effect on the catalytic properties of Pd‐based electrocatalysts has not been investigated. Herein, pre‐hydridized Pd nanocubes (PdH_0.40) supported on carbon black (PdH_0.40NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH_0.40NCs/C presented an enhanced CO₂RR performance due to its less cathodic phase transformation revealed by the in situ X‐ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH_0.40NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO₂RR. The pre‐hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO₂RR.

 

The electrochemical conversion of carbon dioxide (CO₂) into value‐added chemicals is regarded as one of the promising routes to mitigate CO₂emission. A nitrogen‐doped carbon‐supported palladium (Pd) single‐atom catalyst that can catalyze CO₂into CO with far higher mass activity than its Pd nanoparticle counterpart, for example, 373.0 and 28.5 mA mg⁻¹_Pd, respectively, at −0.8 V versus reversible hydrogen electrode, is reported. A combination of in situ X‐ray characterization and density functional theory (DFT) calculation reveals that the PdN₄site is the most likely active center for CO production without the formation of palladium hydride (PdH), which is essential for typical Pd nanoparticle catalysts. Furthermore, the well‐dispersed PdN₄single‐atom site facilitates the stabilization of the adsorbed CO₂intermediate, thereby enhancing electrocatalytic CO₂reduction capability at low overpotentials. This work provides important insights into the structure‐activity relationship for single‐atom based electrocatalysts.