Structural and Chemical Properties of Subnanometer-Sized Bimetallic Au₁₉Pt Cluster

Structure and chemical reactivity of the bimetallic Au₁₉Pt cluster has been investigated within the framework of the relativistic density functional theory. It is observed that all isomers of the tetrahedral Au₁₉Pt cluster are energetically more stable as compared to pure Au₂₀ as well as cage-like isomers of the Au₁₉Pt cluster. The high stability of the bimetallic Au₁₉Pt cluster can be attributed to the strong interaction of the Au and Pt atoms, which is caused by the hybridization of s- and d-orbitals of guest Pt and the host Au atoms in the energy span of 5 eV below the HOMO level. To explore the chemical reactivity of the isomers of the bimetallic Au₁₉Pt cluster, we investigate the adsorption behavior of a CO molecule on various nonequivalent sites of these isomers. We calculate CO adsorption energy, C–O bond length, and bond stretching frequency for all the possible cluster–CO complexes. We find that a CO molecule is preferably adsorbed on Pt sites when both the Au and Pt sites are exposed for adsorption. Interestingly, we observe that the CO adsorption energy increases by more than 1.3 eV when a CO molecule gets adsorbed on the Pt site in the tetrahedral Au₁₉Pt cluster as compared to the adsorption on corresponding Au atoms in the pure Au₂₀ cluster. Moreover, we have shown that due to the charge transfer from the cluster to the CO molecule C–O bond length increases by around 0.02 Å, which causes a substantial amount of red shift (104–121 cm^–1) in C–O stretching frequency. These results indicate that the electronic structure of the CO molecule is highly disturbed when it is adsorbed on the bimetallic clusters, which in turn suggests that the oxidation of the adsorbed CO molecule becomes easy

Effect of Hydrogen Atom Doping on the Structure and Electronic Properties of 20-Atom Gold Cluster

We test the validity of gold–hydrogen analogy in a hydrogen-atom-doped larger gold cluster, namely, Au₂₀, which has attracted considerable interest in recent years because of its unique nature. For this purpose, we carry out density functional theory based calculations to determine the structures of various possible isomers of Au₁₉H cluster by employing GGA and meta-GGA functionals. To obtain the optimized structures of Au₁₉H cluster, several possible initial geometries have been explored. We find that the structure of Au₁₉H cluster is very close to that of tetrahedral Au₂₀ cluster, and the dopant H atom prefers to sit on one of the vertices of the tetrahedron. On the other hand, for the cases of Li, coinage metal (Cu and Ag), and Pt atom doping, the dopant atom has been shown to preferably sit on the surface site of the tetrahedral Au₂₀ cluster. The structure and HOMO–LUMO gap of the Au₁₉H cluster are found to be very close to that of the pure Au₂₀ cluster. Moreover, we observe that the adsorption energies and the extent of activations of CO and O₂ molecules on Au₁₉H cluster are similar to those on the Au₂₀ cluster. On the other hand, it has been reported in the literature that in the smaller sized gold clusters the catalytic activity of the clusters is found to be enhanced significantly due to the doping with a hydrogen atom. Hence, it is clear from the present study that the structure and the electronic properties of hydrogen-atom-doped 20-atom gold cluster almost remain the same as that of Au₂₀ cluster, thereby demonstrating the existence of gold–hydrogen analogy in a larger sized gold cluster

Mechanism of iron integration into LiMn1.5Ni0.5O₄ for the electrocatalytic oxygen evolution reaction

Abstract Spinel-type LiMn1.5Ni0.5O₄ has been paid temendrous consideration as an electrode material because of its low cost, high voltage, and stabilized electrochemical performance. Here, we demonstrate the mechanism of iron (Fe) integration into LiMn1.5Ni0.5O₄ via solution methods followed by calcination at a high temparature, as an efficient electrocatalyst for water splitting. Various microscopic and structural characterizations of the crystal structure affirmed the integration of Fe into the LiMn1.5Ni0.5O₄ lattice and the constitution of the cubic LiMn1.38Fe0.12Ni0.5O₄ crystal. Local structure analysis around Fe by extended X-ray absorption fine structure (EXAFS) showed Fe3+ ions in a six-coordinated octahedral environment, demonstrating incorporation of Fe as a substitute at the Mn site in the LiMn1.5Ni0.5O₄ host. EXAFS also confirmed that the perfectly ordered LiMn1.5Ni0.5O₄ spinel structure becomes disturbed by the fractional cationic substitution and also stabilizes the LiMn1.5Ni0.5O₄ structure with structural disorder of the Ni²⁺ and Mn⁴⁺ ions in the 16d octahedral sites by Fe²⁺ and Fe³⁺ ions. However, we have found that Mn³⁺ ion production from the redox reaction between Mn⁴⁺ and Fe²⁺ influences the electronic conductivity significantly, resulting in improved electrochemical oxygen evolution reaction (OER) activity for the LiMn1.38Fe0.12Ni0.5O4 structure. Surface-enhanced Fe in LiMn1.38Fe0.12Ni0.5O₄ serves as the electrocatalytic active site for OER, which was verified by the density functional theory study