TiC- and TiN-Supported Single-Atom Catalysts for Dramatic Improvements in CO₂ Electrochemical Reduction to CH₄

CO₂ electrochemical catalysis is limited by scaling relations due to a d-band theory of transition metals. As a means of breaking the scaling relation, it has recently been reported that hybridizing the d-orbitals of transition metal with p-orbitals of main group elements or using naturally hybridized materials such as metal carbides and nitrides is a promising strategy. In this Letter, by means of density functional theory calculations, we investigate the catalytic properties of TiC, TiN, and single-atom catalysts supported on them for CO₂ electrochemical reduction. In particular, we found that when single transition-metal atoms are inserted into the surface defect sites of TiC, denoted as M@d-TiC (M = Ag, Au, Co, Cu, Fe, Ir, Ni, Os, Pd, Pt, Rh, or Ru), the iridium-doped TiC (Ir@d-TiC) is found to have a remarkably low overpotential of −0.09 V, the lowest value among any catalysts reported in the literature to selectively produce CH₄ (−0.3 ∼ −1.0 V). It is also shown that possible surface protonation reactions on TiC as a side reaction can be ignored because the overpotential (−0.38 V) is significantly larger than that of the CO₂ electrochemical reduction reaction on single-atom catalysts (e.g., −0.09 V). The origin of an extraordinary catalytic activity of Ir@d-TiC is also explained. This work clearly demonstrates the great potential of carbides and single-atom catalysts supported on TiC as active and selective CO₂ reduction catalysts, and perhaps for other electrochemical applications as well

Can Metal–Organic Framework Separate 1‑Butene from Butene Isomers?

The separation of 1-butene from the other isomers is an industrially important but challenging task because these isomers mainly differ only by the position of CC double bond with many of their physical properties very similar. In this work, we propose using first-principles calculations that Fe-MOF-74 can be a promising candidate for the separation of 1-butene from all other isomers with high selectivity. We demonstrate that the underlying mechanism of this olefin separation is the steric interactions; that is, 1-butene with terminal double bond has the smallest steric interactions with the framework and therefore can approach the metal binding sites more closely for stronger π-complexation. This combined effect (π-complexation modulated by steric interactions) in MOFs with open metal sites can offer a promising design strategy for difficult separation of even longer olefin isomers by properly engineering the lengths and functional groups of the MOF linkers

Selective Heterogeneous CO₂ Electroreduction to Methanol

Catalytic electroreduction of carbon dioxide to useful chemical feedstocks is an environmentally and technologically important process, yet the low energy efficiency and difficulty in controlling product selectivity are great challenges. The reason for part of the latter is that there are presently no catalyst design principles to selectively control CO₂ electroreduction toward a desired product. In this work, as a first attempt, we suggest combining a few criteria (CO binding energy, OH binding energy, and H binding energy) that can be collectively used as activity- and selectivity-determining descriptors to preferentially produce methanol over methane from CO₂ electroreduction. We then apply these concepts to near-surface alloys (NSAs) to propose efficient and selective CO₂ electrochemical reduction catalysts to produce methanol. The W/Au alloy is identified as a promising candidate to have increased catalyst efficiency (decreased CO₂ reduction overpotential and increased overpotential for unwanted hydrogen evolution) as well as improved product selectivity toward methanol, in comparison to conventional Cu catalyst

Active Sites of Au and Ag Nanoparticle Catalysts for CO₂ Electroreduction to CO

Highly active and selective CO₂ conversion into useful chemicals is desirable to generate valuable products out of greenhouse gases. To date, various metal-based heterogeneous catalysts have shown promising electrochemical catalytic activities for CO₂ reduction, yet there have been no systematic studies of the active sites of these metal catalysts that can guide further experiments. In this study, we use first-principles calculations to identify active sites for the CO₂ reduction reaction for Ag and Au metals, the two metals that have been shown to be the most active in producing CO. We compare the catalytic activity and selectivity of three reaction sites of nanoparticles, namely, low-index surfaces, edge sites, and corner sites of these metals. For nanoparticle corner sites, in particular, we find that the size effect is critical, and 309-atom (or larger) nanoparticles should be used to appropriately describe realistic metal nanocatalysts. However, a 55-atom cluster model is often used in the literature to model nanoparticles. From a comparative study, we reveal that corner sites are the most active for the CO₂ reduction reaction in the case of Au, whereas edge sites are the most active in the case of Ag. Although Au is generally the more active CO₂ reduction catalyst than Ag due to the intrinsically stronger binding of *C-species, our results indicate that reducing the size of Au nanoparticles up to 2 nm also increases the unwanted H₂ evolution reaction, as observed in a recent experiment. However, reducing the size of Ag nanoparticles up to 2 nm enhances the CO₂ reduction reaction without suffering from the H₂ evolution reaction, and on this basis, Ag nanoparticles are a comparable or even better-performing, inexpensive catalyst than Au for electrochemical CO production. Our findings suggest that the catalyst design principle (elemental composition, morphology, and size) is metal-dependent and should be carefully tailored for each system

Two-Dimensional Transition Metal Dichalcogenide Monolayers as Promising Sodium Ion Battery Anodes

A family of transition metal dichalcogenide (TMD) nanosheets has recently shown its potential as negative electrodes in lithium ion batteries (LIBs). Herein, Na ion adsorption and migration properties as well as the possibility of phase transition induced by the Na adsorption on TiS₂, VS₂, CrS₂, CoTe₂, NiTe₂, ZrS₂, NbS₂, and MoS₂ are predicted using first-principles calculations. In terms of average voltage and capacity, M = Ti, Zr, Nb, and Mo are found to be suitable as anodes for sodium ion batteries (SIBs) with voltages of 0.49–0.95 V and theoretical capacities of 260–339 mA h g^–1. Among the latter four screened TMDs, in particular, TiS₂ and NbS₂ are expected to maintain the same configurational phase upon sodiation (favorable kinetics) with Na ion migration barriers of 0.22 and 0.07 eV, respectively, suggesting that these TMD compounds could be promising for high-power energy storage applications. It is shown that a proper treatment of phase transitions during sodiation, though often neglected in the literature, is critical in an accurate theoretical description and interpretation of these two-dimensional materials

Understanding the Effects of Au Morphology on CO₂ Electrocatalysis

Toward efficient CO₂ electrocatalysis for CO production, nanostructured Au catalysts have been extensively investigated by the morphology control of oxygen plasma-induced Au islands, oxide-derived Au, Au nanowires (NWs), Au nanoparticles (NPs), nanoporous Au thin films, and Au needles, yet the better performance of one morphology from another is presently not well-understood, making a rational design difficult. Here, the effects of metal morphologies are investigated by focusing on Au NWs and NPs using density functional theory calculations. It is revealed that activity of two key undercoordinated active sites, namely, edge and corner sites, varies delicately with different local coordination environments of various NWs and NPs, and the observed activity trend is remarkably well-rationalized with a generalized coordination number. Furthermore, it is identified that the type of planes and the dihedral angle of the constituent planes are two key factors determining the catalytic activity. A general activity trend for CO₂ reduction and H₂ evolution with the consideration of the density of each type of sites explains why Au NWs exhibit better catalytic performance than Au NPs, as in experiments. On the basis of the theoretical understandings, atomic-level insights and design principles are provided toward efficiently catalyzing CO₂ reduction using nanostructured metal catalysts

Correction and Addition to “Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution”

Correction and Addition to “Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution

Rollover Cyclometalation Pathway in Rhodium Catalysis: Dramatic NHC Effects in the C–H Bond Functionalization

Organometallic chelates are readily obtained upon coordination of metal species to multidentate ligands. Because of the robust structural nature, chelation frequently serves as a driving force in the molecular assembly and chemical architecture, and they are used also as an efficient catalyst in numerous reactions. Described herein is the development of a Rh­(NHC) catalytic system for the hydroarylation of alkenes and alkynes with 2,2′-bipyridines (bipy) and 2,2′-biquinolines; the most representative chelating molecules. Initially generated (bipy)­Rh­(NHC) chelates become labile because of the strong <i>trans</i>-effect of <i>N</i>-heterocyclic carbenes, thus weakening a rhodium–pyridyl bond, which is <i>trans</i> to the bound NHC. Subsequent rollover cyclometalation leads to the C–H bond activation, eventually giving rise to double functionalization of chelate molecules. Density functional calculations are in good agreement with our mechanistic proposal based on the experimental data. The present study elucidated for the first time the dramatic NHC effects on the rollover cyclometalation pathway enabling highly efficient and selective bisfunctionalization of 2,2′-bipyridines and 2,2′-biquinolines

Mechanistic Study on C–C Bond Formation of a Nickel(I) Monocarbonyl Species with Alkyl Iodides: Experimental and Computational Investigations

An open-shell reaction of the nickel­(I) carbonyl species (PNP)­Ni-CO (<b>1</b>) with iodoalkanes has been explored experimentally and theoretically. The initial iodine radical abstraction by a nickel­(I) carbonyl species was suggested to produce (PNP)­Ni-I (<b>4</b>) and the concomitant alkyl radical, according to a series of experimental indications involving stoichiometric controls employing iodoalkanes. Corresponding alkyl radical generation was also confirmed by radical trapping experiments using Gomberg’s dimer. Molecular modeling supports that the nickel acyl species (PNP)­Ni-COCH₃ (<b>2</b>) can be formed by a direct C–C bond formation between a carbonyl ligand of <b>1</b> and a methyl radical. As an alternative pathway, the five-coordinate intermediate species (PNP)­Ni­(CO)­(CH₃) (<b>5</b>) that involves both CO and CH₃ binding at a nickel­(II) center is also suggested with a comparable activation barrier, although this pathway energetically favors the formation of (PNP)­Ni-CH₃ (<b>3</b>) via a barrierless elimination of CO over a CO migratory insertion. Thus, our present work supports that the direct C–C bond coupling occurs between an alkyl radical and the carbonyl ligand at a monovalent nickel center in the generation of an acyl product

Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution

Reducing anthropogenic carbon emission is a problem that requires immediate attention. Metal–organic frameworks (MOFs) have emerged as a promising new materials platform for carbon capture, of which Mg-MOF-74 offers chemospecific affinity toward CO₂ because of the open Mg sites. Here we tune the binding affinity of CO₂ for M-MOF-74 by metal substitution (M = Mg, Ca, and the first transition metal elements) and show that Ti- and V-MOF-74 can have an enhanced affinity compared to Mg-MOF-74 by 6–9 kJ/mol. Electronic structure calculations suggest that the origin of the major affinity trend is the local electric field effect of the open metal site that stabilizes CO₂, but forward donation from the lone-pair electrons of CO₂ to the empty d-levels of transition metals as in a weak coordination bond makes Ti and V have an even higher binding strength than Mg, Ca, and Sc