Molybdena deposited on titania by equilibrium deposition filtration: structural evolution of oxo-molybdenum(vi) sites with temperature.

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoOx)n/TiO2 catalysts. In situ Raman spectroscopy in the temperature range of 25-450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo-Mo(VI) species on TiO2 with increasing temperature as well as the influence of the supported (MoOx)n species on the photo-generation of electrons and holes of TiO2. This study concerns (MoOx)n/TiO2 samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm(2), thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.

Solvent-induced frequency shifts of 5-hydroxymethylfurfural deduced via infrared spectroscopy and ab initio calculations.

Solvent-induced frequency shifts (SIFS) of the carbonyl stretching vibration ν(C═O) of 5-hydroxymethylfurfural were measured in protic, polar aprotic, and nonpolar solvents. The Gutmann acceptor number (AN) was found to correlate with the measured frequency shifts. The SIFS in six solvents were investigated using ab initio electronic structure calculations, treating the solvent implicitly and with an explicit solvent ligand interacting with the carbonyl. The conductor-polarizable continuum model (CPCM) of solvation predicted that ν(C═O) shifted according with the dielectric constant as (ε - 1)/(2ε + 1), in agreement with the analytical predictions of the Kirkwood-Bauer-Magat (KBM) theory for a dipole in a dielectric continuum, but in disagreement with the experimental trend. The experimental SIFS were best predicted using gas-phase complexes of HMF and explicit solvent-ligand. Natural bond orbital (NBO) analysis and Bader's atoms in molecules theory were used to investigate the electronic structure of these complexes. Strong SIFS were found to arise from stronger H-bonding interactions, as observed in delocalization of carbonyl lone-pair electrons by H-bonding solvent σ*(X-H) orbitals, and an increase in charge density and a decrease in local potential energy at the H-bond (3, -1) critical point. Consequently, by predicting the experimental SIFS and examining the electronic structure, we find the first theoretical evidence for treating Gutmann's solvent AN as a measure of solvent Lewis acidity.

An operando Raman study of molecular structure and reactivity of molybdenum(VI) oxide supported on anatase for the oxidative dehydrogenation of ethane.

Supported molybdenum oxide catalysts on TiO(2) (anatase) with surface densities in the range of 1.8-17.0 Mo per nm(2) were studied at temperatures of 410-480 °C for unraveling the configuration and molecular structure of the deposited (MoO(x))(n) species and examining their behavior for the ethane oxidative dehydrogenation (ODH). In situ Raman and in situ FTIR spectra under oxidizing conditions combined with (18)O/(16)O isotope exchange studies provide the first sound evidence for mono-oxo configuration for the deposited (MoO(x))(n) species on anatase. Isolated O=Mo(-O-)(3) tetra-coordinated species in C(3v)-like symmetry prevail at all surface coverages with a low presence of associated (polymeric) species (probably penta-coordinated) evidenced at high coverages, below the approximate monolayer of 6 Mo per nm(2). A mechanistic scenario for (18)O/(16)O isotope exchange and next-nearest-neighbor vibrational isotope effect is proposed at the molecular level to account for the pertinent spectral observations. Catalytic measurements for ethane ODH with simultaneous monitoring of operando Raman spectra were performed. The selectivity to ethylene increases with increasing surface density up to the monolayer coverage, where primary steps of ethane activation follow selective reaction pathways leading to ∼100% C(2)H(4) selectivity. The operando Raman spectra and a quantitative exploitation of the relative normalized Mo=O band intensities for surface densities of 1.8-5.9 Mo per nm(2) and various residence times show that the terminal Mo=O sites are involved in non-selective reaction turnovers. Reaction routes follow primarily non-selective pathways at low coverage and selective pathways at high coverage. Trends in the initial rates of ethane consumption (apparent reactivity per Mo) as a function of Mo surface density are discussed on the basis of several factors.

Molybdenum(VI) oxosulfato complexes in MoO3-K2S2O7-K2SO4 molten mixtures: stoichiometry, vibrational properties, and molecular structures.

The structural and vibrational properties of molybdenum(VI) oxosulfato complexes formed in MoO(3)­K(2)S(2)O(7) and MoO(3)­K(2)S(2)O(7)­K(2)SO(4) molten mixtures under an O(2) atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400­640 °C. The corresponding composition effects were explored in the X(MoO)(3)(0) = 0­0.5 range. MoO(3) undergoes a dissolution reaction in molten K(2)S(2)O(7), and the Raman spectra point to the formation of molybdenum(VI) oxosulfato complexes. The Mo═O stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo(═O)(2) configuration as a core. The stoichiometry of the dissolution reaction MoO(3) + nS(2)O(7)(2­) → C(2n­) was inferred by exploiting the Raman band intensities, and it was found that n = 1. Therefore, depending on the MoO(3 content, monomeric MoO(2)(SO(4))(2)(2­) and/or associated [MoO(2)(SO(4))(2)](m)(2m­) complexes are formed in the binary MoO(3)­K(2)S(2)O(7) molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO(2)(2+) cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high X(MoO)(3)(0)), the observed spectral changes can be explained by partial dissociation of [MoO(2)(SO(4))(2)](m)(2m­) by detachment of S(2)O(7)(2­) and formation of a Mo­O­Mo bridge. Addition of K(2)SO(4) in MoO(3)­K(2)S(2)O(7) results in a "follow-up" reaction and formation of MoO(2)(SO(4))(3)(4­) and/or associated [MoO(2)(SO(4))(3)](m)(4m­) complexes in the ternary MoO(3)­K(2)S(2)O(7)­K(2)SO(4) molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum(VI) oxosulfato complexes pertain to the Mo(═O)(2) stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm(­1) for the ν(s) and ν(as) Mo(═O)(2) modes of MoO(2)(SO(4))(2)(2­) and [MoO(2)(SO(4))(2)](m)(2m­) and (2) at 935 (polarized) and 895 (depolarized) cm(­1) for the respective modes of MoO(2)(SO(4))(3)(4­) and [MoO(2)(SO(4))(3)](m)(4m­). The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO(2)(SO(4))(3)](4­) and [{MoO(2)}(2)(SO(4))(4)(µ-SO(4))(2)](8­) ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program.

Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia.

Plastics waste has become a major environmental threat, with polyethylene being one of the most produced and hardest to recycle plastics. Hydrogenolysis is potentially the most viable catalytic technology for recycling. Ruthenium (Ru) is one of the most active hydrogenolysis catalysts but yields too much methane. Here we introduce ruthenium supported on tungstated zirconia (Ru-WZr) for hydrogenolysis of low-density polyethylene (LDPE). We show that the Ru-WZr catalysts suppress methane formation and produce a product distribution in the diesel and wax/lubricant base-oil range unattainable by Ru-Zr and other Ru-supported catalysts. Importantly, the enhanced performance is showcased for real-world, single-use LDPE consumables. Reactivity studies combined with characterization and density functional theory calculations reveal that highly dispersed (WO x )n clusters store H as surface hydroxyls by spillover. We correlate this hydrogen storage mechanism with hydrogenation and desorption of long alkyl intermediates that would otherwise undergo further C-C scission to produce methane.

Molten Salt Hydrates in the Synthesis of TiO2 Flakes.

Herein, we present a method for the preparation of titanium dioxide with a relatively large surface area, mesoporosity, and good thermal stability. We show that by utilizing molten salt hydrates (MSH) as non-trivial synthesis media, we prepare materials with thin, flake-like morphology with a large aspect ratio. The thickness of the synthesized flakes can be controlled by adjusting the salt/water (always in the MSH regime) and/or the salt/precursor molar ratio. The TiO2 flakes appear to be formed via the aggregation of small TiO2 nanoparticles (typically around 7-8 nm) in an apparent 2D morphology. We hypothesize that the ordered structure of water molecules within the ions of the salt in conjunction with the fast hydrolysis/condensation rates occurring in the presence of water of the precursor used are responsible for this agglomeration. We also report that the purity of materials (anatase vs brookite crystalline phase) appears to be a function of the LiBr/water ratio which is hypothesized to arise either from pH variation or due to lattice matching of the relevant orthorhombic structures (brookite and LiBr x ·3H2O). Discussion on the potential for scalability of the presented method is also highlighted in this article.

Colloidal plasmonic nanostar antennas with wide range resonance tunability.

Gold nanostars display exceptional field enhancement properties and tunable resonant modes that can be leveraged to create effective imaging tags, phototherapeutic agents, and hot electron-based photocatalytic platforms. Despite having emerged as the cornerstone among plasmonic nanoparticles with respect to resonant strength and tunability, some well-known limitations have hampered their technological implementation. Herein we tackle these recognized intrinsic weaknesses, which stem from the complex, and thus computationally untreatable morphology and the limited sample monodispersity, by proposing a novel 6-spike nanostar, which we have computationally studied and synthetically realized, as the epitome of 3D plasmonic nanoantenna with wide range plasmonic tunability. Our concerted computational and experimental effort shows that these nanostars combine the unique advantages of nanostructures fabricated from the top-down and those synthesized from the bottom-up, showcasing a unique plasmonic response that remains largely unaltered on going from the single particle to the ensemble. Furthermore, they display multiple, well-separated, narrow resonances, the most intense of which extends in space much farther than that observed before for any plasmonic mode localized around a colloidal nanostructure. Importantly, the unique close correlation between morphology and plasmonic response leads the resonant modes of these particles to be tunable between 600 and 2000 nm, a unique feature that could find relevance in cutting edge technological applications.

Mechanism of Brønsted acid-catalyzed glucose dehydration.

We present the first DFT-based microkinetic model for the Brønsted acid-catalyzed conversion of glucose to 5-hydroxylmethylfurfural (HMF), levulinic acid (LA), and formic acid (FA) and perform kinetic and isotopic tracing NMR spectroscopy mainly at low conversions. We reveal that glucose dehydrates through a cyclic path. Our modeling results are in excellent agreement with kinetic data and indicate that the rate-limiting step is the first dehydration of protonated glucose and that the majority of glucose is consumed through the HMF intermediate. We introduce a combination of 1) automatic mechanism generation with isotopic tracing experiments and 2) elementary reaction flux analysis of important paths with NMR spectroscopy and kinetic experiments to assess mechanisms. We find that the excess formic acid, which appears at high temperatures and glucose conversions, originates from retro-aldol chemistry that involves the C6 carbon atom of glucose.

Origin of 5-hydroxymethylfurfural stability in water/dimethyl sulfoxide mixtures.

In the present work, we combined vibrational spectroscopy with electronic structure calculations to understand the solvation of HMF in DMSO, water, and DMSO/water mixtures and to provide insights into the observed hindrance of HMF rehydration and aldol condensation reactions if it is dissolved in DMSO/water mixtures. To achieve this goal, the attenuated total reflection FTIR spectra of a wide composition range of binary and ternary mixtures were measured, analyzed, and compared to the findings of ab initio DFT calculations. The effect of solvent on the HMF C-O and O-H vibrational modes reveals significant differences that are ascribed to different intermolecular interactions between HMF and DMSO or water. We also found that DMSO binds to HMF more strongly than water, and interactions with the HMF hydroxyl group are stronger than those with the HMF carbonyl group. We also showed the preferential solvation of HMF C-O groups by DMSO if HMF is dissolved in DMSO/water mixed solvent. Frontier molecular orbital theory was used to examine the influence of the solvent on side reactions. The results show that HMF solvation by DMSO increases its LUMO energy, which reduces its susceptibility to nucleophilic attack and minimizes undesirable hydration and humin-formation reactions. This result, together with the preferential solvation of HMF by DMSO, provide an explanation for the enhanced HMF stability in DMSO/water mixtures observed experimentally.