Acetylene and Ethylene Adsorption during Floating Fe Catalyst Formation at the Onset of Carbon Nanotube Growth and the Effect of Sulfur Poisoning: a DFT Study.

Here, we investigated the adsorption of acetylene and ethylene on iron clusters and nanoparticles, which is a crucial aspect in the nascent phase of carbon nanotube growth by floating catalyst chemical vapor deposition (FCCVD). The effect of sulfur on adsorption was also studied due to its indispensable role in the process and its commonly known impact on metal catalyst poisoning. We performed systematic density functional theory (DFT) computations, considering numerous adsorption configurations and iron particles of various sizes (Fen, n = 3-10, 13, 55). We found that acetylene binds significantly more strongly than ethylene and prefers different adsorption sites. The presence of sulfur decreased the adsorption strength only in the immediate proximity of the adsorbate, suggesting that the effect of sulfur is mainly of steric origin while electronic effects play only a minor role. Higher sulfur coverage of the catalyst surface significantly weakened the binding of acetylene or ethylene. To further investigate this interaction, Bader's atoms in molecules (AIM) analysis and charge density difference (CDD) were used, which showed electron transfer from iron clusters or nanoparticles to the adsorbate molecules. The charge transfer exhibited a decreasing trend as sulfur coverage increased. These results can also contribute to the understanding of other iron-based catalytic processes involving hydrocarbons and sulfur, such as the Fischer-Tropsch synthesis.

Stability and Electronic Structure of Nitrogen-Doped Graphene-Supported Cun (n = 1-5) Clusters in Vacuum and under Electrochemical Conditions: Toward Sensor and Catalyst Design.

Here, we present a detailed computational study of the stability and the electronic structure of nitrogen-doped graphene (N4V2) supported Cun (n = 1-5) clusters, which are promising carbon-dioxide electroreduction catalysts. The binding of the clusters to the nitrogen-doped graphene and the electronic structure of these systems were investigated under vacuum and electrochemical conditions. The stability analysis showed that among the systems, the nitrogen-doped graphene bound Cu4 is the most stable in vacuum, while in an electrolyte, and at a negative potential, the N4V2-Cu3 is energetically more favorable. The ground state electronic structure of the nitrogen-doped graphene substrate undergoes topological phase transition, from a semimetallic state, and we observed a metallic and topologically trivial state after the clusters are deposited. The electrode potential adjusts the type and density of the charge carriers in the semimetallic models, while the structures containing copper exhibit bands which are deformed and relaxed by the modified number of electrons.

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination.

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

CO2 and H2 Activation on Zinc-Doped Copper Clusters.

Here we systematically investigate the CO2 and H2 activation and dissociation on small Cun Zn0/+ (n=3-6) clusters using Density Functional Theory. We show that Cu6 Zn is a superatom, displaying an increased HOMO-LUMO gap and is inert towards CO2 or H2 activation or dissociation. While other neutral clusters weakly activate CO2 , the cationic clusters preferentially bind the CO2 in monodentate nonactivated way. Notably, Cu4 Zn allows for the dissociation of activated CO2 , whereas larger clusters destabilize all activated CO2 binding modes. Conversely, H2 dissociation is favored on all clusters examined, except for Cu6 Zn. Cu3 Zn+ and Cu4 Zn, favor the formation of formate through the H2 dissociation pathway rather than CO2 dissociation. These findings suggest the potential of these clusters as synthetic targets and underscore their significance in the realm of CO2 hydrogenation.

Multistep CO2 Activation and Dissociation Mechanisms on PdxPt4-x Clusters in the Gas Phase.

Palladium, platinum, and their alloys are promising catalysts for electrochemical CO2 reduction reactions (CO2RR), leading to the design of durable and efficient catalysts for the production of useful chemicals in a more sustainable way. However, a deep understanding of the CO2RR mechanisms is still challenging because of the complexity and the factors influencing the system. The purpose of this study is to investigate at the atomic scale the first steps of the CO2RR, CO2 activation and dissociation mechanisms on PdxPt4-x clusters in the gas phase. To do it, we use Density Functional Theory (DFT)-based reaction path and ab initio molecular dynamics (AIMD) computations. Our research focuses on the description of CO2 activation and dissociation processes via the computation of multistep reaction paths, providing insights into the site and binding mode dependent reactivity. Detailed understanding of the CO2-cluster interaction mechanisms and estimating of the reaction energy barriers facilitate comprehension of why and how catalysts are poisoned and identification of the most stable activated adducts configurations. We show that increasing the platinum content induces fluxional behavior of the cluster structure and biases CO2 dissociation; in fact, our computations unveiled several dissociated CO2 isomers that are very stable and that there are various isomerization processes leading to a dissociated structure (possibly a CO poisoned state) from an intactly bound CO2 one (activated state). On the basis of the comparison of the PdxPt4-x reaction paths, we can observe the promising catalytic activity of Pd3Pt in the studied context. Not only does this cluster composition favor CO2 activation against dissociation (thereby expected to facilitate the hydrogenation reactions of CO2), the potential energy surface (PES) is very flat among activated CO2 isomers.

Tuning the degree of CO2 activation by carbon doping Cun- (n = 3-10) clusters: an IR spectroscopic study.

Copper clusters on carbide surfaces have shown a high catalytic activity towards methanol formation. To understand the interaction between CO2 and the catalytically active sites during this process and the role that carbon atoms could play in this, they are modeled by copper clusters, with carbon atoms incorporated. The formed clusters CunCm- (n = 3-10, m = 1-2) are reacted with CO2 and investigated by IR multiple-photon dissociation (IR-MPD) spectroscopy to probe the degree of CO2 activation. IR spectra for the reaction products [CunC·CO2]-, (n = 6-10), and [CunC2·CO2]-, (n = 3-8) are compared to reference spectra recorded for products formed when reacting the same cluster sizes with CO, and with density functional theory (DFT) calculated spectra. The results reveal a size- and carbon load-dependent activation and dissociation of CO2. The complexes [CunC·CO2]- with n = 6 and 10 show predominantly molecular activation of CO2, while those with n = 7-9 show only dissociative adsorption. The addition of the second carbon to the cluster leads to the exclusive molecular activation of the CO2 on all measured cluster sizes, except for Cu5C2- where CO2 dissociates. Combining these findings with DFT calculations leads us to speculate that at lower carbon-to-metal ratios (CMRs), the C can act as an oxygen anchor facilitating the OCO bond rupture, whereas at higher CMRs the carbon atoms increasingly attract negative charge, reducing the Cu cluster's ability to donate electron density to CO2, and consequently its ability to activate CO2.

C2 product formation in the CO2 electroreduction on boron-doped graphene anchored copper clusters.

A possible remedy for the increasing atmospheric CO2 concentration is capturing and reducing it into valuable chemicals like methane, methanol, ethylene, and ethanol. However, a suitable catalyst for this process is still under extensive research. Small sized copper clusters have gained attention in recent years due to their catalytic activity in the CO2 reduction reaction. Although C2+ products have a higher economic value, the formation of C1 products was investigated most thoroughly. Graphene is a promising support for small copper clusters in the electrochemical reduction of CO2. It exhibits good mechanical and electrical properties, but the weak interaction between copper and graphene is an issue. Our DFT computations reveal that small Cu clusters on the boron-doped graphene (BDG) support are promising catalysts for the electrochemical reduction of CO2. We found facile reaction pathways towards various C1 (carbon-monoxide, formic acid, formaldehyde, methanol or methane) and C2 (ethanol or ethylene) products on Cu4 and Cu7 clusters on BDG. The reactivity is cluster-size tunable with Cu4 being the more reactive agent, while Cu7 shows a higher selectivity towards C2 products.

Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics.

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au3 clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O2. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O2 on Au3 is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au3/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O2 has on the fluxional motion of the Au3 cluster.

The promoter role of sulfur in carbon nanotube growth.

We investigate the effect of sulfur on the interaction of iron catalyst nanoparticles and carbon nanotubes (CNTs), typically present in a floating catalyst chemical vapor deposition (FCCVD) process. As a reference, the interaction of graphene with the Fe fcc(111) surface is used. In both systems we performed a systematic density functional theory (DFT) study on the interaction with different sulfur contents. We found that the presence of sulfur changes the nature and strength of interaction between graphene and the iron surface from strong chemisorption to weak physisorption. Furthermore, sulfur significantly reduces the CNT-iron binding, indicating a beneficial effect on the CNT growth and its promoter role. We believe that these results induce further experimental studies and optimization of the CNT synthesis process.

Nanofibrous membranes comprising intrinsically microporous polyimides with embedded metal-organic frameworks for capturing volatile organic compounds.

Here, we report the fabrication of nanofibrous air-filtration membranes of intrinsically microporous polyimide with metal-organic frameworks (MOFs). The membranes successfully captured VOCs from air. Two polyimides with surface areas up to 500 m2 g-1 were synthesized, and the impact of the porosity on the sorption kinetics and capacity of the nanofibers were investigated. Two Zr-based MOFs, namely pristine UiO-66 (1071 m2 g-1) and defective UiO-66 (1582 m2 g-1), were embedded into the nanofibers to produce nanocomposite materials. The nanofibers could remove polar formaldehyde and non-polar toluene, xylene, and mesitylene from air. The highest sorption capacity with 214 mg g-1 was observed for xylene, followed by mesitylene (201 mg g-1), toluene (142 mg g-1), and formaldehyde (124 mg g-1). The incorporation of MOFs drastically improved the sorption performance of the fibers produced from low-surface-area polyimide. Time-dependent sorption tests revealed the rapid sequestration of air pollutants owing to the intrinsic porosity of the polyimides and the MOF fillers. The porosity allowed the rapid diffusion of pollutants into the inner fiber matrix. The molecular level interactions between VOCs and polymer/MOFs were clarified by molecular modeling studies. The practicality of material fabrication and the applicability of the material were assessed through the modification of industrial N95 dust masks. To the best of our knowledge, this is the first successful demonstration of the synergistic combination of intrinsically microporous polyimides and MOFs in the form of electrospun nanofibrous membranes and their application for VOC removal.

IR spectroscopic characterization of the co-adsorption of CO2 and H2 onto cationic Cun+ clusters.

To understand elementary reaction steps in the hydrogenation of CO2 over copper-based catalysts, we experimentally study the adsorption of CO2 and H2 onto cationic Cun+ clusters. For this, we react Cun+ clusters formed by laser ablation with a mixture of H2 and CO2 in a flow tube-type reaction channel and characterize the products formed by IR multiple-photon dissociation spectroscopy employing the IR free-electron laser FELICE. We analyze the spectra by comparing them to literature spectra of Cun+ clusters reacted with H2 and with new spectra of Cun+ clusters reacted with CO2. The latter indicate that CO2 is physisorbed in an end-on configuration when reacted with the clusters alone. Although the spectra for the co-adsorption products evidence H2 dissociation, no signs for CO2 activation or reduction are observed. This lack of reactivity for CO2 is rationalized by density functional theory calculations, which indicate that CO2 dissociation is hindered by a large reaction barrier. CO2 reduction to formate should energetically be possible, but the lack of formate observation is attributed to kinetic hindering.

Screening of transition metal doped copper clusters for CO2 activation.

Activation of CO2 is the first step towards its reduction to more useful chemicals. Here we systematically investigate the CO2 activation mechanism on Cu3X (X is a first-row transition metal atom) using density functional theory computations. The CO2 adsorption energies and the activation mechanisms depend strongly on the selected dopant. The dopant electronegativity, the HOMO-LUMO gap and the overlap of the frontier molecular orbitals control the CO2 dissociation efficiency. Our calculations reveal that early transition metal-doped (Sc, Ti, V) clusters exhibit a high CO2 adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. Thus, early transition metal-doped copper clusters, particularly Cu3Sc, may be efficient catalysts for the carbon capture and utilization process.

Enhanced Methanol Electro-Oxidation Activity of Nanoclustered Gold.

Size-selected 3 nm gas-phase Au clusters dispersed by cluster beam deposition (CBD) on a conducting fluorine-doped tin oxide template show strong enhancement in mass activity for the methanol electro-oxidation (MEO) reaction compared to previously reported nanostructured gold electrodes. Density functional theory-based modeling on the corresponding Au clusters guided by experiments attributes this high MEO activity to the high density of exposed under-coordinated Au atoms at their faceted surface. In the description of the activity trends, vertices and edges are the most active sites due to their favorable CO and OH adsorption energies. The faceted structures occurring in this size range, partly preserved upon deposition, may also prevent destructive restructuring during the oxidation-reduction cycle. These results highlight the benefits of using CBD in fine-tuning material properties on the nanoscale and designing high-performance fuel cell electrodes with less material usage.

The effect of size, charge state and composition on the binding of propene to yttrium-doped gold clusters.

The catalytic activity of metal clusters can be easily tuned by their size, charge state, or the introduction of dopant atoms. Here, the dopant-, charge- and size-dependent propene adsorption on gold (Au n +) and yttrium doped gold (Au n-1Y+) clusters (n = 4-20) was investigated using combined gas-phase reaction studies and density functional theory computations. The increased charge transfer between the cluster and propene in the cationic clusters considerably enhances the propene binding on both pure and yttrium-doped species, compared to their neutral cluster counterparts, while yttrium-doping lowers the propene binding strength in a size-dependent way compared to the pure gold clusters. Chemical bonding and energy decomposition analysis indicate that there is no covalent bond between the cluster and propene. The preferred propene binding site on a cluster is indicated by the large lobes of its LUMO, together with the low coordination number of the adsorption site. In small yttrium-doped gold clusters propene can not only bind to the electron-deficient yttrium atom, but also to the partially positively-charged gold atoms. Therefore, by controlling the charge of the clusters, as well as by introducing yttrium dopants, the propene binding strength can be tuned, opening the route for new catalytic applications.

Observation of the Reaction Intermediates of Methanol Dehydrogenation by Cationic Vanadium Clusters.

A mass spectrometric study of the reactions of vanadium cationic clusters with methanol in a low-pressure collision cell is reported. For comparison, the reaction of methanol with cobalt cationic clusters was studied. For vanadium, the main reaction products are fully dehydrogenated species, and partial dehydrogenation and non-dehydrogenation species are observed as minors, for which the relative intensities increase with cluster size and also at low cluster source temperature cooled by liquid nitrogen; no dehydrogenation products were observed for cobalt clusters. Quantum chemical calculations explored the reaction pathways and revealed that the fully dehydrogenation products of the reaction between Vn + and methanol are Vn (C)(O)+ , in which C and O are separated owing to the high oxophilicity of vanadium. The partial dehydrogenation and non-dehydrogenation species were verified to be reaction intermediates along the reaction pathway, and their most probable structures were proposed.

Non-covalent Interactions and Charge Transfer between Propene and Neutral Yttrium-Doped and Pure Gold Clusters.

The dopant and size-dependent propene adsorption on neutral gold (Aun ) and yttrium-doped gold (Aun-1 Y) clusters in the n=5-15 size range are investigated, combining mass spectrometry and gas phase reactions in a low-pressure collision cell and density functional theory calculations. The adsorption energies, extracted from the experimental data using an RRKM analysis, show a similar size dependence as the quantum chemical results and are in the range of ≈0.6-1.2 eV. Yttrium doping significantly alters the propene adsorption energies for n=5, 12 and 13. Chemical bonding and energy decomposition analysis showed that there is no covalent bond between the cluster and propene, and that charge transfer and other non-covalent interactions are dominant. The natural charges, Wiberg bond indices, and the importance of charge transfer all support an electron donation/back-donation mechanism for the adsorption. Yttrium plays a significant role not only in the propene binding energy, but also in the chemical bonding in the cluster-propene adduct. Propene preferentially binds to yttrium in small clusters (n<10), and to a gold atom at larger sizes. Besides charge transfer, relaxation also plays an important role, illustrating the non-local effect of the yttrium dopant. It is shown that the frontier molecular orbitals of the clusters determine the chemical bonding, in line with the molecular-like electronic structure of metal clusters.

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring.

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-ß-nitrostyrene. The 1.6-1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization.

Reaction of N2O and CO Catalyzed with Small Copper Clusters: Mechanism and Design.

Highly active catalytic clusters are observed during the reaction mechanism study of the copper cluster Cux (x = 4-15) catalyzed N2O + CO → N2 + CO2 reaction. It was shown that N2O adsorbs on the copper cluster without an activation barrier and the nitrogen-oxygen bond is broken during the next step. The analysis of the chemical bonding showed that the oxide ion formation is an important driving force of the reaction. Among the different clusters Cu12 was the most active as the nitrogen-oxygen bond is cleaved without an energy barrier, while the nitrogen molecule is eliminated. It was shown that the resulting copper oxide cluster is reduced easily with carbon-monoxide. The elimination of the thus formed carbon-dioxide is thermodynamically a highly favored process, even at low temperature. Thus, Cu12 cluster is a potentially highly active catalyst at ambient condition for the N2O + CO → N2 + CO2 reaction.

The structure of Au(6)Y(+) in the gas phase.

The geometric and electronic structure of the Au(6)Y(+) cation is studied by gas phase vibrational spectroscopy combined with density functional theory calculations. The infrared photodissociation spectrum of Au(6)Y(+)·Ne is measured in the 95-225 cm(-1) energy range and exhibits two characteristic absorption bands at 181 cm(-1) and 121 cm(-1). Based on DFT/BP86 quantum chemical calculations, the infrared spectrum is assigned to the lowest energy species found, an eclipsed C(3v) geometry. The 3D structure of Au(6)Y(+) is considerably different from those previously found for both the neutral Au(6)Y (quasi-planar circular geometry) and the anionic Au(6)Y(-) (planar D(6h) symmetry). The different geometries are related to different electronic structures in agreement with 2D and 3D phenomenological shell models for metal clusters.

Mercury dications: linear form is more stable than aromatic ring.

Analysis of the molecular orbitals and the nucleus independent chemical shift showed that the D(4h) and D(6h) symmetry Hg(4)(2+) and Hg(6)(2+) rings are aromatic. However, accurate quantum chemical methods indicated that the linear forms are more than 100 kJ mol(-1) lower in energy. This surprising case, where the non-aromatic species are considerably more stable than the aromatic rings, was explained by the charge distribution and the ring strain. Electronic structures of these species were consistently described using the phenomenological shell model of metal clusters.