The electrocatalytic N2 reduction activity of core-shell iron nanoalloy catalysts: a density functional theory (DFT) study.

A molecular level understanding of the property evolution in binary nanoalloy catalysts is crucial for designing novel electrocatalysts for ammonia synthesis. In this regard, designing core-shell catalyst structures has been a versatile approach to achieve the product selectivity. Herein, we investigated the activity evolution of Fe-based core-shell (M15@Fe50) (M = Co, Ni, or Cu) clusters for the nitrogen reduction reaction (NRR). Nitrogen reduction following the associative mechanistic pathway is significantly activated over the Cu15@Fe50 cluster. The d-band center from the electronic structure analysis is found to be upshifted, justifying the activity towards the NRR. The reduction reaction occurs via the surface restructuring of the catalyst, in which the *NH2 formation is found to be the lowest endergonic potential determining step compared to pristine Fe(110). Based on this, the high NRR activity of the Cu15@Fe50 cluster has been proposed, which, we envision, will provide useful insights into the position and compositional effects of core-shell structures for the discovery of efficient NRR electrocatalysts.

Control over product formation and thermodynamic stability of thiolate-protected gold nanoclusters through tuning of surface protecting ligands.

Surface-protecting ligands can regulate the structure of a cluster's core either through electronic or steric effects. However, the influence of the steric effect along with the electronic effect over controlling the structure during ligand exchange reactions remains elusive. To understand this, we have carried out ligand exchange on [Au23(CHT)16]- (CHT: cyclohexane thiol) using aromatic thiolates where we have tuned the bulkiness at the para position of the thiolate group on the incoming ligands. The outcome of the experiments reveals that each of the ligands in the chosen series is precisely selective towards the parent cluster transformation through specific intermediates. The ligand with more steric crowding directed the reaction pathway to have Au28 nanocluster as the major product while Au36 was the final product obtained with the gradual decrease of bulkiness over the ligand. The combined experimental and theoretical results elucidated the mechanism of the reaction pathways, product formation, and their stability. Indeed, this study with the series of ligands will add up to the ligand library, where we can decide on the ligand to obtain our desired cluster for specific applications through the ligand exchange reaction.

Importance of Adatom on Pure Iron Catalyst Towards Electrocatalytic N2 Reduction Reaction.

The Haber-Bosch process using Fe-based catalysts is still the predominant technique for ammonia production despite tough reaction conditions and high energy consumption. In the present work, we have investigated iron adatom on the (110) surface of pure iron catalyst towards the electrocatalytic N2 reduction reaction (NRR) activity using density function theory (DFT) calculations. We demonstrate that the presence of adatom over the iron catalyst favours the NRR via alternating associative mechanistic pathway through a barrierless rate determining step (*NNH formation). Besides, the adatom-based catalyst requires lower working potential than the previously reported Fe(110) surface and Fe-nanocluster based catalysts. These findings may open a scope in terms of scrutinizing the atomicity effects over catalyst surface for various catalytic reactions.

Computational Screening of First-Row Transition-Metal Based Alloy Catalysts-Ligand Induced N2 Reduction Reaction Selectivity.

Large-scale ammonia production through sustainable strategies from naturally abundant N2 under ambient conditions represents a major challenge from a future perspective. Ammonia is one of the promising carbon-free alternative energy carriers. The high energy required for N≡N bond dissociation during the Haber-Bosch process demands extreme reaction conditions. This problem could be circumvented by tuning Fe catalyst composition with the help of an induced ligand effect on the surface. In this work, we utilized density functional theory calculations on the Fe(110) surface alloyed with first-row transition-metal (TM) series (Fe-TM) to understand the catalytic activity that facilitates the electrochemical nitrogen reduction reaction (NRR). We also calculated the selectivity against the competitive hydrogen evolution reaction (HER) under electrochemical conditions. The calculated results are compared with those from earlier reports on the periodic Fe(110) and Fe(111) surfaces, and also on the (110) surface of the Fe85 nanocluster. Surface alloying with late TMs (Co, Ni, Cu) shows an improved NRR activity, whereas the low exchange current density observed for Fe-Co indicates less HER activity among them. Considering various governing factors, Fe-based alloys with Co (Fe-Co) showed enhanced overall performance compared to the periodic surface as well as other pure iron-based structures previously reported. Therefore, the iron-alloy based structured catalysts may also provide more opportunities in the future for enhancing NRR performance via electrochemical reduction pathways.