Metal-N_(4) model single‐atom catalyst with electroneutral quadri‐pyridine macrocyclic ligand for CO_(2) electroreduction

Metal–N–C single‐atom catalysts,mostly prepared from pyrolysis of metalorganic precursors,are widely used in heterogeneous electrocatalysis.Since metal sites with diverse local structures coexist in this type of material and it is challenging to characterize the local structure,a reliable structure–property relationship is difficult to establish.Conjugated macrocyclic complexes adsorbed on carbon support are well‐defined models to mimic the singleatom catalysts.Metal–N_(4) site with four electroneutral pyridine‐type ligands embedded in a graphene layer is the most commonly proposed structure of the active site of single‐atom catalysts,but its molecular counterpart has not been reported.In this work,we synthesized the conjugated macrocyclic complexes with a metal center(Co,Fe,or Ni)coordinated with four electroneutral pyridinic ligands as model catalysts for CO_(2) electroreduction.For comparison,the complexes with anionic quadri‐pyridine macrocyclic ligand were also prepared.The Co complex with the electroneutral ligand expressed a turnover frequency of CO formation more than an order of magnitude higher than that of the Co complex with the anionic ligand.Constrained ab initio molecular dynamics simulations based on the well‐defined structures of the model catalysts indicate that the Co complex with the electroneutral ligand possesses a stronger ability to mediate electron transfer from carbon to CO_(2).

Non‐noble metal single‐atom catalyst with MXene support:Fe1/Ti_(2)CO_(2) for CO oxidation

MXenes have attracted considerable attention owing to their versatile and excellent physicochemi‐cal properties.Especially,they have potential applications as robust support for single atom cata‐lysts.Here,quantum chemical studies with density functional theory are carried out to systemati‐cally investigate the geometries,stability,electronic properties of oxygen functionalized Ti_(2)C(Ti_(2)CO_(2))supported single‐atom catalysts M_(1)/Ti_(2)CO_(2)(M=Fe,Co,Ni,Cu Ru,Rh,Pd,Ag Os,Ir,Pt,Au).A new non‐noble metal SAC Fe_(1)/Ti_(2)CO_(2) has been found to show excellent catalytic performance for low‐temperature CO oxidation after screening the group 8‐11 transition metals.We find that O_(2) and CO adsorption on Fe_(1) atom of Fe_(1)/Ti_(2)CO_(2) is favorable.Accordingly,five possible mechanisms for CO oxidation on this catalyst are evaluated,including Eley‐Rideal,Langmuir‐Hinshelwood,Mars-van Krevelen,Termolecular Eley‐Rideal,and Termolecular Langmuir‐Hinshelwood(TLH)mechanisms.Based on the calculated reaction energies for different pathways,Fe_(1)/Ti_(2)CO_(2) shows excellent kinet‐ics for CO oxidation via TLH mechanism,with distinct low‐energy barrier(0.20 eV)for the rate‐determining step.These results demonstrate that Fe_(1)/Ti_(2)CO_(2) MXene is highly promising 2D materials for building robust non‐noble metal catalysts.

Mechanistic insight into methanol electro‐oxidation catalyzed by PtCu alloy

In this work,we have performed density functional theory(DFT)calculations to investigate the methanol electro‐oxidation reaction(MOR)catalyzed by the Pt,PtCu alloy and Cu.The complex reaction networks,including the intermediate dehydrogenation,water dissociation and anti‐poison reaction steps,are systematically investigated to explore the mechanisms.At the standard condition of pH=0 and zero potential,for Cu,most dehydrogenation steps along the favorable pathway are endergonic,making it less active in MOR.For the Pt and PtCu alloy,their dehydrogenation steps are mainly exergonic,but the formed CO intermediate binds too tightly on Pt,that can accumulate on active sites to poison the electro‐catalyst.The CO can be consumed by the thermodynamic reaction with OH*,which comes from water dissociation.DFT calculation shows alloying the Pt with Cu could not only reduce the free energy barrier for binding between CO*and OH*,but also assist the water dissociation to produce more OH*for that anti‐poison reaction.That makes the PtCu alloy more active than the pure Pt electrode in experiment.The results reveal the importance of anti‐poison reaction and water dissociation in MOR,which could be applied to the rational design of more active alloy electro‐catalysts in future.

Investigation of water adsorption and dissociation on Au_1/CeO_2 single-atom catalysts using density functional theory

We examined the water adsorption and dissociation on ceria surfaces as well as ceria‐supported Au single‐atom catalysts using density functional theory calculations.Molecular and dissociative water were observed to coexist on clean CeO2and reduced Au1/CeO2?x surfaces because of the small difference in adsorption energies,whereas the presence of dissociative water was highly favorable on reduced CeO2?x and clean Au1/CeO2surfaces.Positively charged Au single atoms on the ceria surface not only provided activation sites for water adsorption but also facilitated water dissociation by weakening the intramolecular O-H bonds.In contrast,negatively charged Au single atoms were not reactive for water adsorption because of the saturation of Au5d and6s electron shells.This work provides a fundamental understanding of the interaction between water and single‐atom Au catalysts.