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Nanoscaled-VPO (vanadium phosphate oxide) catalysts have been prepared on an alumina support with the objective of obtaining crystallites of the active phase with a high surface-to-volume ratio. Since the catalysed reaction occurs at the surface, these nanoscaled catalysts allows studying surface species transformations with a minimized signal from the bulk dominion. The chemical environment and oxidation state of these species has been determined by a combination of NMR, ESR and XANES spectroscopies. The results have allowed to optimize V + P coverage and V/P molar ratio to minimize the concentration of isolated vanadium oxide species and maximize V4+ concentration. The distribution of phosphorus islands ordered and significantly distorted of tetra- and penta-coordinated aluminium sites of the catalytic support, thus shaping the catalytic material to an optimal structure for the desired partial oxidation reaction. [ABSTRACT FROM AUTHOR]

This review is devoted to modern applications of solid state NMR (ssNMR) to the structural studies of heterogeneous catalysts. Recent developments of ssNMR combining high magnetic fields up to 23.5 T, high rotation frequencies up to 120 kHz, and the new software of spectra acquisition and processing coupled with quantum chemistry calculations allow unravel the 3D structure of active sites of the sophisticated structures, such as heterogeneous catalysts. Moreover, this opens a possibility of investigation of low sensibility nuclei that are generally assigned as inappropriate for NMR. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP-SENS) is another powerful technique extending NMR application for surface feature investigation. Combination of modern ssNMR (including DNP) and DFT calculations synthesize the most powerful approach (so called NMR crystallography) of structural research of catalysts which is demonstrated on several remarkable application examples on challenging nuclei. [ABSTRACT FROM AUTHOR]

Abstract: 93Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the 93Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the 93Nb NMR chemical shift scale for NbO x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360ppm; the shifts from −1200 to −1600ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO x –MO y polyhedra association has been suggested. The magnitude of the 93Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz. [Copyright &y& Elsevier]

We investigated the possibility of synthesizing Co nanoparticles in CoZrnH/AlOx(OH)y/Al ceramic–metal catalysts and controlling the catalytic properties of these nanoparticles in syngas conversion by changing the Co/Zr ratio. The CoZr nanocomposites were obtained from metal powders by mechanochemical activation in a high-energy mill under an argon atmosphere, followed by treatment with hydrogen at high pressure and room temperature. Ceramic–metal catalysts were prepared by mixing the corresponding CoZrnH powder nanocomposite (30 wt%) with powdered aluminum (70 wt%), hydrothermal treatment of the mixture and subsequent calcination. The materials were characterized with a set of physicochemical methods: powder X-ray diffraction, scanning electron microscopy, 59Co internal field nuclear magnetic resonance spectroscopy, and temperature programmed reduction. Catalytic studies were performed in a laboratory fixed-bed flow reactor at 2 MPA and 210–270 °C. It is shown that the activity in syngas conversion to C5+ hydrocarbons and selectivity to methane and C2–C4 hydrocarbons depend on the Co/Zr ratio. Thus, with an increase in the zirconium content in the samples, the interaction of metal cobalt with metal zirconium improves in the process of mechanical activation and subsequent treatment with hydrogen. The destruction of the agglomerates of crystallites of metallic cobalt in the form of β-Co (Cofcc) occurs as well as their transformation to α-Co (Cohcp) particles active in the syngas conversion to C5+ hydrocarbons. This can explain the highest specific yield of C5+ hydrocarbons on a cermet with the lowest Co/Zr ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • 59Co IF NMR reveals the crystal structure of Cobalt nanoparticles supported on alumina. • hcp structure is favored for cobalt on χ-Al 2 O 3 compared to cobalt on γ-Al 2 O 3. • DFT modeling of χ-Al 2 O 3 surface shows that its hydroxyl distribution favors the formation of large hcp cobalt particles. • Alumina structure affects hcp / fcc Co ratio with potential catalytic implications, for example in Fischer-Tropsch synthesis. Metallic Co nanoparticles, widely used and studied as supported heterogeneous catalysts for Fischer-Tropsch synthesis (FTS), display catalytic properties that can vary significantly depending on their size and crystal structure. In this work, we used 59Co Internal Field NMR (59Co IF NMR) complemented by high-resolution transmission electron microscopy (HRTEM) to demonstrate the influence of strong metal-support interaction on two noticeably different metastable alumina phases - γ -Al 2 O 3 and χ -Al 2 O 3. According to 59Co IF NMR and HRTEM, the metallic particles supported on χ -Al 2 O 3 were larger and displayed a significantly higher content of Co in its hcp phase, which is known to be more active and selective to C 5+ in FTS. The 1H NMR chemical shifts of hydroxyl groups anchored to the (1 1 0) and (1 1 1) spinel crystal planes were calculated by DFT. It revealed that the hydroxyl coverage of γ -Al 2 O 3 facilitates the dispersion of Co precursor over the surface of the support, ultimately leading to the formation of smaller metal Co nanoparticles on γ -Al 2 O 3 than on χ -Al 2 O 3. [ABSTRACT FROM AUTHOR]

Abstract: In this article we will discuss the potentiality of solid-state 51V NMR technique to characterize heterogeneous catalysts containing paramagnetic cations located on the surface or incorporated in the bulk of heterogeneous catalysts. 51V NMR data for a number of 3d and rare earth vanadates, where paramagnetic effects are caused by the presence of paramagnetic cations, are considered. We also summarize results available for vanadium bronzes, where some vanadium is in a paramagnetic V4+ state, and for some vanadium compounds, where closely spaced paramagnetic centers form diamagnetic pairs. The comprehensive 51V chemical shift scale (database) for paramagnetic solids was compiled. [Copyright &y& Elsevier]

• A new ammonium niobium oxalate phase with O-Nb-O bridges was found. • The combination of the XRD, NMR and DFT was efficient for structure refinement. • The well-developed protons and ligands mobility was detected in a new phase of niobium oxalate. Unusual structure transformation in NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ∙nH 2 O was discovered and investigated by combination of techniques including NMR crystallography (variable temperature 1H, 2H, 13C and 93Nb solid-state NMR methods), X-Ray powder diffraction, simultaneous thermal analysis and DFT calculations. It has been revealed that the mixture of known oxalate phases NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ∙3H 2 O (Phase I) and NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ·2H 2 O (Phase II) treated at mild condition (temperature up to 100 °C in vacuo) induces an unknown crystal phase, catena-NH 4 [NbO 2/2 (C 2 O 4) 2 (H 2 O)] (Phase III). The new orthorhombic phase has polymeric structure consisted of 7-coordinated niobium fragments [NbO 2/2 (C 2 O 4) 2 (H 2 O)] connected by oxygen bridges (Nb O Nb) forming the infinite zigzag chains along the c → axis. The bridging oxygen atoms are connected to Nb symmetrically with respect to the 2-fold axis. Each of them forms two identical bonds Nb-O of 1.892(6) Å length, indicating their origination from the condensation process. The niobium coordination polyhedron can be crossed by the plane passing through the Nb, (O) 2/2 , (H 2 O) atoms. The oxalate groups lie in the opposite sides from the plane. In the new phase, 93Nb has unusually small nuclear quadrupole coupling constant mainly due to the developed thermal mobility of the crystal lattice. Variable temperature 93Nb, 2H and 13C NMR experiments together with DFT calculations confirm that the new Phase III has proton and ligand mobility. This mobility of the lattice correlates with the fact that hydrogen bonds network stabilizing the structure of Phases I and II practically disappear in the new Phase III. [ABSTRACT FROM AUTHOR]

Decoration of one-dimensional multi-walled carbon nanotubes (MWCNTs) with zero-dimensional Co nanoparticles leads to hybrid structures with chemical and electromagnetic features that are not available to the individual components. This work addresses the influence of the nature and structure of MWCNTs on the localization of Co nanoparticles. Depending on synthesis conditions, Co can be deposited on the external or in inner surfaces of the nanotubes. Co/MWCNTs hybrids have been characterized by in situ X-ray powder diffraction, high-resolution transmission electron microscopy and 59 Co internal field nuclear magnetic resonance. It has been shown that the average diameter (7.2, 9.4 and 18.6 nm), number of walls (5–7, 12–15, 15–20), and functional composition of the MWCNTs have a remarkable effect on the size of Co nanoparticles and their distribution in the structure of MWCNTs. The observed phenomenon has been rationalized in terms of nanotubes surface properties. Parent MWCNTs being hydrophobic and having limited porosity do not stabilize Co nanoparticles and, therefore, they are localized on the outside surface with relatively large average size and broad size distribution. On the other hand, the oxidation of the MWCNTs resulted in the penetration of Co nanoparticles inside of the nanotubes, presumably because of pore opening as well as increased hydrophilicity of the nanotubes. [ABSTRACT FROM AUTHOR]

Intensive phase transformations of alumina are known to occur at temperatures above 1000 °C. In the present work, high temperature behaviour of pure Al 2 O 3 and the carbon coated Al 2 O 3 @C sample with core-shell structure was comparatively studied using low-temperature nitrogen adsorption, transmission electron microscopy, powder X-ray diffraction (XRD) analysis and solid-state nuclear magnetic resonance (NMR). The solid-state NMR 27 Al method has allowed us to identify and estimate the concentration of all phases appeared during the transformation of pseudoboehmite γ-Al 2 O 3 into corundum α-Al 2 O 3 . The data obtained correlate well with the results of XRD analysis and low-temperature nitrogen adsorption. It is shown that the deposition of carbon coating with formation of core-shell Al 2 O 3 @C system stabilizes the size of oxide core and prevents the formation of corundum phase until the temperatures of 1350–1400 °C, which are close to the temperature of carbothermal reduction of alumina. The stabilization of the size of the oxide core nanoparticles was considered as a main factor preventing the formation of corundum phase at high temperatures. At the same time, the carbon coating does not affect the phase transformation of γ-Al 2 O 3 to δ -Al 2 O 3 . [ABSTRACT FROM AUTHOR]

Two sets of silica-supported nickel phosphide catalysts with a nickel content of about 2.5 and 10 wt % and Ni/P molar ratio 2/1, 1/1 and 1/2 in each set, were prepared by way of a temperature-programmed reduction method using (Ni(CH3COO)2) and ((NH4)2HPO4) as a precursor. The NixPy/SiO2 catalysts were characterized using chemical analysis N2 physisorption, XRD, TEM, 31P MAS NMR. Methyl palmitate hydrodeoxygenation (HDO) was performed in a trickle-bed reactor at 3 MPa and 290 °C with LHSV ranging from 0.3 to 16 h-1. The Ni/P ratio was found to affect the nickel phosphide phase composition, POx groups content and catalytic properties in methyl palmitate HDO with the TOF increased along with a decline of Ni/P ratio and a growth of POx groups' content. Taking into account the possible routes of methyl palmitate conversion (metal-catalyzed hydrogenolysis or acid-catalyzed hydrolysis), we proposed that the enhancement of acid POx groups' content with the Ni/P ratio decrease provides an enhancement of the rate of methyl palmitate conversion through the acceleration of acid-catalyzed hydrolysis. [ABSTRACT FROM AUTHOR]

The Si/Al ratio is a key parameter of acid-base, structural, textural and, consequently, catalytic properties of amorphous crystalline (micro- and mesoporous) aluminosilicates. The changes of structural and textural characteristics of Al-Si aerogels with gradual increase of aluminum content are investigated. Aerogels were prepared via sol-gel method using prehydrolysed tetraethoxysilane and aluminium isopropoxide stabilized by acetylacetone. The gelation of the obtained sols took place in the presence of ammonia with the following drying in supercritical isopropanol. It was shown all aluminum reacts with prehydrolyzed tetraethoxysilane forming spherical particles in case the content of Al in the samples is less than 20 mol %. Aluminum drives the increase of interparticle coupling leading to the particle agglomeration, which is associated with the increase of the particle size and decrease of specific surface area and pore volume. For the samples with the aluminum content of >50 mol % the formation of pseudoboehmite plate-like particles is observed. The pseudoboehmite particles prevent the sintering of SiO 2 particles that leads to the increase of the aerogel specific surface area and pore volume. In case the high aluminum content (>80 mol %) the silica particles serve as a connector between boehmite plates. The ratio between Brønsted and Lewis acid sites decreases gradually with the increase of aluminum content of the aluminosilicate aerogels. [ABSTRACT FROM AUTHOR]

Obtaining stable metal nanoparticles is of high interest for various applications such as catalysis, batteries, supercapacitors and electro-magnetic devices. Cobalt/multi-walled carbon nanotubes (MWCNT) hybrids with an original set of magnetic and electric properties were formed by casting Co nanoparticles (3 – 5 nm) of high aspect ratios within the internal space of MWCNTs. The Co particles localization and size were analyzed by transmission electron microscopy and synchrotron x-ray diffraction. The magnetism of the cobalt nanoparticles was probed by 59 Co internal field nuclear magnetic resonance (IF NMR) and their electrical behavior by dielectric spectroscopy. The majority of Co particles were fully metallic. They resisted sintering up to 550 °C. Below 7.5 wt%, the Co was exclusively embedded inside the MWCNT. At higher loading, they coexisted with larger Co outside particles. While nanometer size particles are normally superparamagnetic at room temperature, the confinement of Co within MWCNTs resulted in a ferromagnetism revealed by 59 Co IF NMR. This spectroscopy provided original information about the structure, size, and shape anisotropy of the nanoparticles. Finally, the MWCNT modification by Co metal nanoparticles improved the electrical conductivity of polyethylene based composite thus extending the useful frequency band of Co/MWCNT/PE composites for applications requiring light-weight conduction or energy absorption. [ABSTRACT FROM AUTHOR]

We report the phase and microstructure evolution of the Cu–13 wt.% Al mixture during treatment in a high-energy planetary ball mill with a particular focus on the early stages of mechanical alloying. Several characterization techniques, including X-ray diffraction phase analysis, nuclear magnetic resonance spectroscopy, differential dissolution, thermal analysis, and electron microscopy/elemental analysis, have been combined to study the evolution of the phase composition of the mechanically alloyed powders and describe the microstructure of the multi-phase products of mechanical alloying at different length scales. The following reaction sequence has been confirmed: Cu + Al → CuAl 2 (+Сu) → Cu 9 Al 4 + (Cu) → Cu(Al). The phase evolution was accompanied by the microstructure changes, the layered structure of the powder agglomerates disappearing with milling time. This scheme is further complicated by the processes of copper oxidation, reduction of copper oxides by metallic aluminum, and by variation of the stoichiometry of Cu(Al) solid solutions with milling time. Substantial amounts of X-ray amorphous phases were detected as well. Differential dissolution technique has revealed that a high content of aluminum in the Cu(Al) solid solution-based powders is due to the presence of Al-rich phases distributed between the Cu(Al) crystallites. [ABSTRACT FROM AUTHOR]

For the first time, silicate-substituted hydroxyapatites have been prepared from mixtures containing different amounts of silicon (0.2-2 mol per mol of apatite unit cell) by dry mechanochemical synthesis at room temperature in a planetary ball mill. The XRD, FTIR, TEM, and NMR spectroscopic data show that the product of the mechanochemical synthesis is a single-phase nanocrystalline apatite containing different amounts of carbonate and silicate ions and adsorbed water. In the annealed samples, three silicon concentration subranges can be distinguished, each of which is characterized by specific evolution of the lattice parameters. The formation mechanism of the silicate-substituted hydroxyapatite obtainable by this method is discussed. The studies indicate that the silicon substitution limit in the silicate-substituted lattice achievable by the dry mechanochemical synthesis followed by heat treatment is 1.2 mol per mol of apatite unit cell. [ABSTRACT FROM AUTHOR]

Vanadium and niobium species (together and separately) were loaded on gamma alumina, and the resulting catalysts were run in the methanol conversion. This reaction was studied by both GC analysis and FTIR study in the flow system. The catalytic properties are discussed based on the combined FTIR and 27Al, 51V and 1H MAS NMR studies. The NMR studies revealed a different mechanism of interaction between Nb and Al2O3 than that between V and Al2O3. This predetermines the structure of vanadium sites in bimetallic VNb/Al samples. The effect of coverage was considered for various metal loadings ranging from below to above monolayer. One of our most interesting findings is that the surface Nb oxide species exhibited a redox character below monolayer but were acidic above monolayer. 27Al MAS NMR revealed a strong alumina–Nb interaction that may account for its redox performance. Moreover, the role of sulfate from vanadium precursor is evidenced. [Copyright &y& Elsevier]

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis. [ABSTRACT FROM AUTHOR]

Abstract: A series of catalytic filamentous carbons (CFC) obtained from gas phase reaction on iron subgroup metal catalysts was studied by 129Xe NMR. A connection between 129Xe NMR parameters and structural, textural and paramagnetic properties of CFC was discussed. The chemical shift (δ) was shown to depend on the structure of the CFC surface formed by edge, basal or both (edge and basal) graphite faces. This dependence follows a general trend of the chemical shift to increase with adsorption potential of a surface. The term describing Xe–Xe interaction (δ Xe–Xe) in confined space decreased with the average pore size of CFC granule. For hollow multi-wall carbon nanotubes two 129Xe NMR signals were attributed to voids inside nanotubes and to interstices between the interlaced nanotubes. However, analysis of experimental data is complicated due to the presence of paramagnetic metal particles necessary to the filament growth. Therefore, ESR data are given and correlated with 129Xe NMR results. [Copyright &y& Elsevier]

We present novel polyethylene (PE) composites for electromagnetic interference (EMI) shielding application. They are based on cobalt modified multi-walled carbon nanotubes (MWCNTs) produced via in situ polymerization of ethylene, with the Ti-Ziegler–Natta catalyst preliminarily immobilized on Co/MWCNT hybrids. The electromagnetic properties of the composites were tuned by varying the filler loading and Co:MWCNT ratio. The microstructure of the composites and electromagnetic absorption process were carefully characterized by transmission and scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, ferromagnetic resonance and vector network analysis. The electromagnetic wave absorbing properties of the nanocomposite were investigated in the 10 MHz−18 GHz frequency range revealing that the EMI absorption properties can be tuned by varying the Co:MWCNT weight ratio in the filler. Interestingly, the Co/MWCNT-PE composite with a total filler and Co loading of only 12 and 1.7 wt%, respectively, showed extremely high reflection loss (RL) of −55 dB. More importantly, an effective bandwidth of 12.8–17.8 GHz (RL below −10 dB) was achieved for a matching thickness of only 1.5 mm. The specific RL value (RL/filler loading) of the composite was superior in comparison with the previously reported nanostructured carbon materials. The highly effective absorbing properties of Co/MWCNT-PE composites are explained primarily by the unprecedented uniform filler distribution in the polyethylene as well as by the synergistic effect of MWCNTs and Co nanoparticles. This approach thus offered an effective strategy to design cost-effective, lightweight and flexible EMI shielding materials with tunable dielectric and magnetic performance. [Display omitted] • New Co/MWCNT/polyethylene composites for EMI shielding applications were designed. • In situ polymerization ensured uniform distribution of Co/MWCNT filler in PE. • EMI shielding properties can be tuned by varying filler loading and Co:MWCNT ratio. • The effective bandwidth was 12.8–17.8 GHz at reflection loss below −10 dB. • Co/MWCNT/PE composite shows a minimum reflection loss value of −55 dB at 5.2 GHz. [ABSTRACT FROM AUTHOR]

χ-Al 2 O 3 is a rarely studied low-temperature alumina phase that can be prepared in a finely dispersed form and attracts interest as a potential catalyst or catalyst support. Phase transformations of nanocrystalline χ-Al 2 O 3 and χ-Al 2 O 3 @C core-shell materials in the temperature range of 650–1400 °C were studied by XRD and 27Al solid-state NMR. Deposition of the carbon shell was demonstrated to stabilize the oxide core size and extend the stability range of the low-temperature χ-Al 2 O 3 phase to 1100 °C. Relatively high surface area of the oxide core consisting of χ-and κ-Al 2 O 3 , which was close to 80 m2/g after burning of the carbon shell, was preserved after the heat treatment at 1100 °C. Meanwhile, calcination of pure χ-Al 2 O 3 at the same temperature yielded a sample consisting of α-and κ-Al 2 O 3 with the surface are as low as 15 m2/g. Solid-state 27Al NMR with magic-angle spinning proved to be an efficient tool for investigation of χ- → κ- → α-Al 2 O 3 phase transformations that can be used to obtain quantitative information on the alumina phase composition. • Carbon shell hinders sintering of χ-Al 2 O 3 and its transformation to α-Al 2 O 3. • Stabilization of the oxide core nanoparticle size prevents the phase transformation. • High Al 2 O 3 surface areas are preserved in the carbon-coated samples till 1100 °C. [ABSTRACT FROM AUTHOR]

An abstract of the article "Synthesis of Alumina Through Hydrothermal Oxidation of Aluminum Powder Conjugated With Surfactant-Directed Oriented Growth" by Sergey F. Tikhov et al. is presented.

The effect of support nature, SiO2 and γ-Al2O3, on physicochemical and catalytic properties of nickel phosphide catalysts in methyl palmitate hydrodeoxygenation (HDO) has been considered. Firstly, alumina-supported nickel phosphide catalysts prepared by temperature-programmed reduction method starting from different precursors (phosphate–Ni(NO3)2 and (NH4)2HPO4 or phosphite–Ni(OH)2 and H3PO3) were compared using elemental analysis, N2 physisorption, H2-TPR, XRD, TEM, NH3-TPD, 27Al and 31P MAS NMR techniques and catalytic experiments. The mixture of nickel phosphide phases was produced from phosphate precursor on alumina while using of phosphite precursor provides Ni2P formation with the higher activity in methyl palmitate HDO. Besides, the comparative study of the performances of Ni2P/SiO2 and Ni2P/Al2O3 catalysts demonstrates the apparent superiority of alumina-supported Ni2P in the methyl palmitate hydrodeoxygenation. Considering the tentative scheme of methyl palmitate transformation, we proposed that cooperation of Ni2P and acid sites on the surface of alumina provides the enhanced activity of alumina-supported Ni2P through the acceleration of acid-catalysed hydrolysis. [ABSTRACT FROM AUTHOR]