Comparative DFT study of N2 and no adsorption on vanadium clusters Vn (n = 2-13)

New Multicomponent Optimization Scheme for Equiatomic Vanadium-Titanium Nanoparticle Study

We present a new multicomponent structure prediction method named MRS-Swap searching, which is inspired by space symmetry and swap of different atomic species. For the pure titanium cluster, a new ground-state structure of the Ti20 cluster with higher symmetry relative to a previous study was found by our method. Based on the structural analysis of Tin (n = 2k, k = 2-11), Vn (n = 2k, k = 2-11), and TinVm (n = m = 2-11) systems, we found that the lowest energy structures of these three systems are very similar, which indicates that equiatomic vanadium-titanium-mixed clusters do not change their ground-state structure relative to the same size pure vanadium and titanium cluster. According to the structure-activity relationship, we conclude that the yield strength (σ) of macro vanadium-titanium alloy is between pure titanium and pure vanadium metal, and this can be expressed through σ(Ti) > σ(TiV) > σ(V). The X-ray diffraction results show that the V2Nb, TiVNb, and Ti2Nb alloys also have the same BCC structure, which may be related to their microstructure. Our method and results can be helpful for future multicomponent alloy design.

First-Principles Study of Stability and N2 Activation on the Octahedron RuRh Clusters

The geometric and electronic structures of different octahedron RuRh clusters are studied using density functional theory calculations. The binding energy, electronic structure, and energy gap of the clusters have been obtained to determine the possible stable structures. The results show that the Ru4Rh2 cluster is the most stable structure which has D4h symmetry with the largest ionization potential, smallest affinity energy and larger energy gap. Furthermore, the information on adsorption and dissociation of multiple nitrogen molecules and the density of state for the octahedral Ru4Rh2 cluster is analyzed. The dissociation barrier of three nitrogen molecules further decreases to 1.18 eV with an increase in the number of N2 molecules. The co-adsorption of multiple N2 molecules facilitates the dissociation of N2 on the Ru4Rh2 cluster. The strong interaction between the antibonding orbital of N2 and the d orbital of the Ru4Rh2 cluster is illustrated by calculating and analyzing the results of PDOS, which stretches the N−N bond length and reduces the activation energy to dissociation. The antibonding orbital of the nitrogen molecule shows distinct and unique catalytic activity for the dissociation of the adsorbed nitrogen molecule on the octahedral Ru4Rh2 cluster.

Theoretical Study of NO Dissociative Adsorption onto 3d Metal Particles M55 (M = Fe, Co, Ni, and Cu): Relation between the Reactivity and Position of the Metal Element in the Periodic Table

NO dissociative adsorption onto 3d metal particles M55 (M = Fe, Co, Ni, and Cu) was investigated theoretically using density functional theory computations. A transition state exists at higher energy in the Cu case but at lower energy in the Fe, Co, and Ni cases than the reactant (sum of M55 and NO), indicating that Cu55 is not reactive for NO dissociative adsorption because NO desorption occurs more easily than the N-O bond cleavage in this case, but Fe55, Co55, and Ni55 are reactive because NO desorption needs a larger destabilization energy than the N-O bond cleavage. This result agrees with the experimental findings. The energy of transition state E(TS) becomes higher in the order of Fe < Co < Ni ≪ Cu. Exothermicity E exo (relative energy to the reactant) decreases in the order of Fe > Co > Ni ≫ Cu. These results indicate that the reactivity for NO dissociative adsorption decreases kinetically and thermodynamically in this order. In addition, the E(TS) and E exo values show that 3d metal particles are more reactive than 4d metal particles when a comparison is made in the same group of the periodic table. Charge transfer (CT) from the metal particle to NO increases as the reaction proceeds. The CT quantity to NO at the TS increases in the order of Cu < Ni < Co < Fe, identical to the increasing order of reactivity. The negative charges of the N and O atoms of the product (N and O adsorbed M55) increase in the order of Ni < Co < Cu < Fe, identical to the increasing order of E exo except for the Cu case; in the Cu case, the discrepancy between the order of E exo and those of the N and O negative charges arises from the presence of valence 4s electron of Cu because it suppresses the CT from N and O to Cu55. From these results, one can infer that the d-valence band-top energy of M55 plays an important role in determining the reactivity for NO dissociative adsorption. Truly, the d valence orbital energy decreases in the order of Fe > Co > Ni ≫ Cu and the 3d metal > 4d metal in the same group of the periodic table, which reflects the dependence of reactivity on the metal element position in the periodic table.

Vanadium Cluster Neutrals Reacting with Water: Superatomic Features and Hydrogen Evolution in a Fishing Mode

Hydrogen evolution reaction (HER) is known as the heart of various energy storage and conversation systems of renewable energy sources. Here we observe the cluster reactions of a light transition metal, vanadium, with water in a gas-phase flow tube reactor. While HER products of V₁ and V₂ were not observed, the effective HER of water on neutral V_n (n ≥ 3) clusters reveals reasonable and size-dependent reactivity of the vanadium clusters. Superatomic features and reaction dynamics of V₁₀, V₁₃, and V₁₆ are highlighted. Among the three typical superatoms, V₁₀ and V₁₆ exhibit an abnormal superatomic orbital energy level order, 1S|2S|1P|1D..., where the energy-reduced 2S orbital helps to accommodate the geometric structure and hence reinforce the cluster stability. In comparison, V₁₃ bears a less symmetrical structure and reacts readily with water, allowing for recombination of a hydroxyl atom with an adsorbed hydrogen atom, akin to a fishing-mode HER process. The joint experimental and theoretical study on neutral V_n clusters clarifies the availability of superatom chemistry for transition metals and appeals further development of cluster theory based on electronic cloud/orbital analysis instead of simply counting the valence electrons. Also, we provide insights into the HER mechanism of metal clusters and propose a strategy to design new materials for portable fuel cells of hydrogen energy.

Spectroscopic properties and activated mechanism of NO on isolated cationic tantalum clusters: A first-principles study

The adsorption and dissociation of NO on the cationic Ta₁₅⁺ cluster were investigated using the density-functional theory (DFT) calculations, and the Ta-centered bicapped hexagonal antiprism (BHA) structure of cationic Ta₁₅⁺ cluster can be identified as the global minimum, which reproduces well the infrared multiple photo dissociation (IR-MPD) spectrum. Our results show that the cationic BHATa₁₅⁺ cluster provides the hollow region for NO to interact effectively, and possess larger adsorption strength on the region than other sites. The density of states, charge density differences and frontier molecular orbitals were analyzed to understand the electronic properties of the stable NO-adsorbed isomers. The characteristic IR peaks of the firstly two low-lying isomers are properly assigned, in which the strongest IR peak originates from the N - O stretching vibration. For the dissociation of NO on the BHATa₁₅⁺ cluster, it is found that the reaction path II easily occurs rather than path I due to small reaction barrier, and the cluster may possess the great catalytic behavior to dissociate NO molecule. The present results will inevitably stimulate future theoretical and experimental studies for the design of novel Ta-based catalytic materials for the NO dissociation.

Electronic Structure of Cubane-Like Vanadium–Nitrogen Cationic Clusters [V4N4]+ and [V6N6]+

Density Functional Theory and Complete Active Space Self-Consistent Field (CASSCF) methodologies are used to explore the electronic structure of the cationic V–N clusters, [V4N4]+ and [V6N6]+, that have been identified in recent mass spectrometric experiments. Our calculations indicate that both clusters are based on cubane-like fragments of the rock-salt lattice. In the smaller [V4N4]+ cluster, the V–V bonding is delocalized over the tetrahedron, with net bond orders of 1/3 per V–V bond. In [V6N6]+, in contrast, the V–V bonding is strongly localized in the central V2N2 unit, which has a short V=V double bond. CASSCF calculations reveal that both localized and delocalized V–V bonds are highly multi-configurational.

Free-radical gases on two-dimensional transition-metal disulfides (XS2, X = Mo/W): robust half-metallicity for efficient nitrogen oxide sensors

The detection of single gas molecules is a highly challenging work because it requires sensors with an ultra-high level of sensitivity. By using density functional theory, here we demonstrate that the adsorption of a paramagnetic unpaired free radical gas (NO) on a monolayer of XS₂ (X = Mo, W) can trigger the transition from semiconductor to half metal. More precisely, the single-layer XS₂ (X = Mo, W) with NO adsorbed on it would behave like a metal in one spin channel while acting as a semiconductor in the other spin orientation. The half-metallicity is robust and independent of the NO concentration. In contrast, no half-metallic feature can be observed after the adsorption of other free radical gases such as NO₂. The unique change in electronic properties after the adsorption of NO on transition-metal sulfides highlights an effective strategy to distinguish NO from other gas species by experimentally measuring spin-resolved transmission. Our results also suggest XS₂ (X = Mo, W) nanosheets can act as promising nanoscale NO sensors.

Highly effective catalysis of the double-icosahedral Ru(19) cluster for dinitrogen dissociation - a first-principles investigation

The N2 bond cleavage is the rate-limiting step in the synthesis of ammonia, and ruthenium is a catalyst well known for this reaction. The double-icosahedral (D5h) Ru19 cluster is famous as an active catalyst, and has a remarkable stability towards the adsorption of H2, N2 and CO. Using first-principles calculations, we have investigated the adsorption and dissociation of dinitrogen on a double-icosahedral Ru19 cluster. Our results show that the hollow site in the rhombus region (BHB site) of the Ru19 cluster possesses the greatest catalytic activity to dissociate N2, with the reaction barrier of 0.89 eV and an exothermicity of -1.45 eV. Multiple coadsorption of N2 on the cluster (i.e. coadsorption of 2N2 and 3N2 on a single Ru19 cluster) causes the barrier to dissociate N2 to be less on a BHB site than for adsorption of a single N2. To understand the catalytic properties of a Ru19 cluster towards N2 bond cleavage, we calculated the electron population, vibrational wavenumbers and local densities of states; the results are explicable.