Highly Oxidized Gold Nanoparticles: In Situ Synthesis, Electronic Properties, and Reaction Probability Toward CO Oxidation

Highly oxidized gold nanoparticles prepared by RF-discharge under an oxygen atmosphere were studied by X-ray photoelectron spectroscopy and transmission electron microscopy depending on the particle size. A surface-like gold oxide was found in the case of small nanoparticles (1–2 nm) obtained at the first steps of deposition. With an increase of the particle size up to 5 nm, a bulklike gold oxide was formed. The O 1s spectra exhibited an oxygen peak at binding energy <i>E</i>_b = 529.4 eV for the surface-like oxide and <i>E</i>_b = 530.7 eV for the bulklike gold-oxide. The reaction probability of oxidized gold nanoparticles was examined in the reaction of CO oxidation at room temperature. The surface-like gold oxide interacted with CO with a high reaction probability of approximately 0.005, while CO interaction with the bulklike oxide was characterized by an induction period with lower reaction probability (0.001). The mechanisms of the interaction of oxidized gold nanoparticles with CO depending on its size are discussed

XPS Study of Nanostructured Rhodium Oxide Film Comprising Rh⁴⁺ Species

Studies of highly oxidized rhodium species as potential active sites in catalytic oxidation reactions are of great interest. In this work, we investigated the properties of highly oxidized nanostructured rhodium film prepared by radio frequency discharge in an oxygen atmosphere. The charge states of Rh in RhO_<i>x</i> particles, their thermal stability, and reactivity toward CO were analyzed in comparison with the properties of thermally prepared Rh₂O₃ oxide. The formation of Rh⁴⁺ species in a composition of Rh⁴⁺/Rh³⁺ oxyhydroxide structures was shown to take place in plasma-synthesized films. The highly oxidized rhodium species was stable up to 150 °C and demonstrated reactivity in a CO oxidation reaction at 100 °C. The reoxidation of a partially reduced Rh/RhO_<i>x</i> film was observed at 100 °C under treatment with molecular O₂. However, Rh⁴⁺ species were not recovered under such conditions

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS₂-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methyl­imid­azolium (BuMeIm), 2-phenyl­imid­azolium, and 2-methyl­benz­imid­azolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS₂ sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5–20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS₂ sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS₂ sheets of the NCs are of the nonconventional 1T-MoS₂ (metallically conducting) structure. The sheets’ puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS₂ modification and raises the temperature for its transition to the conventional 2H-MoS₂ (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS₂ (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS₂

Stabilization of 1T-MoS₂ Sheets by Imidazolium Molecules in Self-Assembling Hetero-layered Nanocrystals

We report a facile, room-temperature assembly of MoS₂-based hetero-layered nanocrystals (NCs) containing embedded monolayers of imidazolium (Im), 1-butyl-3-methyl­imid­azolium (BuMeIm), 2-phenyl­imid­azolium, and 2-methyl­benz­imid­azolium molecules. The NCs are readily formed in water solutions by self-organization of the negatively charged, chemically exfoliated 0.6 nm thick MoS₂ sheets and corresponding cationic imidazole moieties. As evidenced by transmission electron microscopy, the obtained NCs are anisotropic in shape, with thickness varying in the range 5–20 nm and lateral dimensions of hundreds of nanometers. The NCs exhibit almost turbostratic stacking of the MoS₂ sheets, though the local order is preserved in the orientation of the imidazolium molecules with respect to the sulfide sheets. The atomic structure of NCs with BuMeIm molecules was solved from powder X-ray diffraction data assisted by density functional theory calculations. The performed studies evidenced that the MoS₂ sheets of the NCs are of the nonconventional 1T-MoS₂ (metallically conducting) structure. The sheets’ puckered outer surface is formed by the S atoms and the positioning of the BuMeIm molecules follows the sheet nanorelief. According to thermal analysis data, the presence of the BuMeIm cations significantly increases the stability of the 1T-MoS₂ modification and raises the temperature for its transition to the conventional 2H-MoS₂ (semiconductive) counterpart by ∼70 °C as compared to pure 1T-MoS₂ (∼100 °C). The stabilizing interaction energy between inorganic and organic layers was estimated as 21.7 kcal/mol from the calculated electron density distribution. The results suggest a potential for the design of few-layer electronic devices exploiting the charge transport properties of monolayer thin MoS₂

Electrostatic Origin of Stabilization in MoS₂–Organic Nanocrystals

Negatively charged molybdenum disulfide layers form stable organic–inorganic layered nanocrystals when reacted with organic cations in solution. The reasons why this self-assembly process leads to a single-phase compound with a well-defined interlayer distance in given conditions are, however, poorly understood to date. Here, for the first time, we quantify the interactions determining the cation packing and stability of the MoS₂–organic nanocrystals and find that the main contribution arises from Coulomb forces. The study was performed on the series of new layered compounds of MoS₂ with naphthalene derivatives, forming several distinct phases depending on reaction conditions. Starting with structural models derived from powder X-ray diffraction data and TEM, we evaluate their cohesion energy by modeling layer separation with periodic PW-DFT-D calculations. The results provide a reliable approach for estimation of the stability of MoS₂-based heterolayered compounds

Electrostatic Origin of Stabilization in MoS₂–Organic Nanocrystals

Negatively charged molybdenum disulfide layers form stable organic–inorganic layered nanocrystals when reacted with organic cations in solution. The reasons why this self-assembly process leads to a single-phase compound with a well-defined interlayer distance in given conditions are, however, poorly understood to date. Here, for the first time, we quantify the interactions determining the cation packing and stability of the MoS₂–organic nanocrystals and find that the main contribution arises from Coulomb forces. The study was performed on the series of new layered compounds of MoS₂ with naphthalene derivatives, forming several distinct phases depending on reaction conditions. Starting with structural models derived from powder X-ray diffraction data and TEM, we evaluate their cohesion energy by modeling layer separation with periodic PW-DFT-D calculations. The results provide a reliable approach for estimation of the stability of MoS₂-based heterolayered compounds

Different Efficiency of Zn²⁺ and ZnO Species for Methane Activation on Zn-Modified Zeolite

Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn²⁺ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods (¹H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn²⁺ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C–H bond with the zeolite Brønsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn²⁺ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by 2 and 3 orders of magnitude for the zeolite loaded with ZnO or Zn²⁺ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn²⁺ cations is more profound than that of ZnO species for H/D exchange reaction. This implies that the synergistic effect of Zn-sites and BAS for C–H bond activation in methane is significantly higher for Zn²⁺ cations compared to small ZnO clusters. It has been revealed, however, that only Zn²⁺ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn²⁺ cations provide the formation of zinc-methyl species, which are further transformed to zinc-methoxy species. The latter is the key intermediate for the performance of the alkylation reaction. Hence, while both zinc oxide clusters and Zn²⁺ cationic species are able to provide a synergistic effect for the activation of C–H bonds of methane displayed by the dramatic acceleration of H/D exchange reaction, only the Zn²⁺ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn²⁺ cations in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes

Bimetallic NiM/C (M = Cu and Mo) Catalysts for the Hydrogen Oxidation Reaction: Deciphering the Role of Unintentional Surface Oxides in the Activity Enhancement

Emerging interest in the platinum group metal (PGM)-free electrocatalysts calls for a fundamental understanding of the key factors determining their activity, the latter being critical for the development of efficient catalysts. Ni-based materials show high promises as PGM-free anodes of anion exchange membrane fuel cells (AEMFCs). However, their hydrogen oxidation reaction (HOR) activity can differ by several orders of magnitude, and the factors responsible for this are still being debated. In this work, the effect of unintentional surface oxides in Ni/C and NiM/C (M = Cu and Mo) is revealed by benchmarking either the catalysts conventionally stored under ambient conditions or purposely reduced “oxide-free” materials. The analysis of electrocatalytic data complemented by detailed material characterization, Monte Carlo simulations, and density functional calculations, underlines the key importance of surface oxides in the HOR catalysis on Ni, NiCu, and NiMo electrodes. These findings underscore the need to measure the HOR activity of Ni-based catalysts in the absence of surface oxides in order to unambiguously interpret the influence of other factors (such as the electronic effect of the second element) on activity enhancement

Platinum Acetate Blue: Synthesis and Characterization

Platinum acetate blue (PAB) of the empirical formula Pt­(OOCMe)_2.5±0.25, a byproduct in the synthesis of crystalline platinum­(II) acetate Pt₄(OOCMe)₈, is an X-ray amorphous substance containing platinum in the oxidation state between (II) and (III). Typical PAB samples were studied with X-ray diffraction, differential thermal analysis–thermogravimetric, extended X-ray absorption fine structure, scanning electron microscopy, transmission electron microscopy, magnetochemistry, and combined quantum chemical density functional theory–molecular mechanics modeling to reveal the main structural features of the PAB molecular building blocks. The applicability of PAB to the synthesis of platinum complexes was demonstrated by the preparation of the new homo- and heteronuclear complexes Pt^II(dipy)­(OOCMe)₂ (<b>1</b>), Pt^II(μ-OOCMe)₄Co^II(OH₂) (<b>2</b>), and Pt^III₂(OOCMe)₄(O₃SPhMe)₂ (<b>3</b>) with the use of PAB as starting material