Vibrational Stark tuning rates from periodic DFT calculations: CO/Pt(1 1 1)

DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm⁻¹/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm⁻¹/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm⁻¹/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm⁻¹/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations.     

Allene adsorption isotherms and active site determination on various titanium trichloride catalysts

The techniques of gas-solid chromatography and temperature programmed desorption have been applied to the problem of evaluating the number of active polymerizing centres in variously prepared titanium trichloride catalysts. Using gas-solid chromatography the adsorption isotherms of allene on (i) hydrogen reduced TiCl₄ (Stauffer H), (ii) aluminium reduced TiCl₄ (Stauffer AA), (iii) Et₂AlCl activated Stauffer AA and (iv) Et₃Al activated Stauffer AA, have been obtained. The non-linear isotherms so derived have been analysed in terms of a two site Langmuir model from which the heats of adsorption and the number of adsorption sites have been evaluated. The number of surface sites is virtually identical on all four materials having values of 4.2 × 10¹³, 5.3 × 10¹³, 4.8 × 10¹³ and 4.9 × 10¹³ sites cm^?2 respectively ($\text{～200}\text{A}\text{?}{}^2$ per site), the majority of these sites (> 90%) having heats of adsorption of 8 kcal mol^?1. Since these catalysts have different surface areas which varied from 2.5 m²g^?1 to 8.7 m²g^?1 the near identity of the site densities suggests that a fundamental property of the solid is being measured. The only observed effect of the aluminium alkyl activator on Stauffer AA was to reduce the heat of adsorption of the high energy site from 16.5 kcal mol^?1 to 12.7 kcal mol^?1 (Et₂AlCl) and to 9.3 kcal mol^?1 (Et₃Al). Temperature programmed desorption experiments on Stauffer AA and Stauffer H confirmed the heat of adsorption and the site densities of the high energy sites showing coverages of 2.9 × 10¹² and 2.6 × 10¹² sites cm^?2 respectively for these sites (identical to those predicted by the two site Langmuir adsorption model) and desorption activation energies of 18.8 and 17.8 kcal mol^?1 respectively, inferring no activation energy to adsorption on to these sites.     

Surface analytical approach to TiCl3-based Ziegler-Natta catalysts combined with microstructure analysis of polymer

In the field of Ziegler-Natta catalysis (ZNC), most of the previous works have been focused on the characterization of either catalyst surface or polymer microstructure but the combined studies of these two aspects are still limited. A clearer understanding concerning the isospecific nature of active sites on the donor-free TiCl₃ - based catalyst has been presented in this work in terms of both surface characteristics of the catalyst and microstructure of polypropylene (PP) produced. Ti- and Al-species existing in the catalyst surface have been identified by high resolution X-ray photoelectron spectroscopic (XPS) and solid-state magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR) techniques, respectively. Moreover, grinding effect on catalyst performance has been carried out to investigate the nature of active sites by particle size distribution (PSD) method. Microstructures of PPs produced using the surface characterized catalysts have been investigated by temperature rising elution fractionation (TREF), ¹³C NMR and GPC methods. It can be demonstrated that the surface structure of catalyst is dominantly affected by the type of Al-alkyl cocatalyst used as well as the preparation method of catalyst whilst it precisely determine the nature of isospecificity of active sites and its distribution. Two different types of surface Ti⁺³-species have been identified on the catalyst surface depending on the different vacant coordination states of surface titanium species. On the other hand, ²⁷Al MAS NMR experiments have shown the presence of surface aluminum in four (Al^IV), five (Al^V) and six (Al^VI) coordination number of multiple environments. The states of surface Al-species are sensitive to coordination number and symmetry of the local environment around the aluminum nuclei. Higher isospecificity of active sites is seen to be related to the higher fulfilled coordinated states of surface Ti- and Al-species.     

Molecular weight distributions in novolac type phenol-formaldehyde polymerizations

In the acid-catalysed condensation polymerization of phenol and formaldehyde, the ortho and para positions of the phenol ring are known to exhibit different reactivities^1,2. Along with this on polymer molecules, the internal sites have lower reactivity due to molecular shielding. To model the novolac formation, five reactive sites have been proposed³ and their reactivities are assumed to be completely determined by the site involved. To find the molecular weight distribution (MWD) of the polymer from this information, a given polymer chain has been assumed to consist of these sites in the same relative ratio as in the reaction mass. A mass balance, on polymer molecules of given size, has been made and solved numerically as a function of time. The sensitivity analysis has been carried out to show that the MWD is little affected by the variation of reactivities of the internal sites. The reactivity of para external positions and the ratio of phenol and formaldehyde, however, play a major role in determining the MWD.     

Modelling of phenol—formaldehyde polymerization reaction

In the condensation polymerization of phenol and formaldehyde, the ortho and para positions exhibit different reactivities. In addition, along a polymer chain, internal reactive sites have lower reactivity, possibly due to shielding phenomena^1,2. A detailed kinetic model for novalac formation has been proposed accounting for all these factors. Five different kinds of reactive sites have been shown to exist on a polymer chain and the reactivity of a given reaction has been assumed to be governed completely by the reactive site involved in it. A mass balance has been written for each site and the equations have been solved numerically to examine the effect of different parameters on the course of the condensation polymerization. The model predicts polymer chains with small levels of branching, which is consistent with experimental observations.     

Oxidative dehydrogenation of ethane on a magnesium-vanadium aluminophosphate (MgVAPO-5) catalyst

Vanadium and/or magnesium substituted aluminophosphate with ALPO₄-5 structure have been prepared by hydrothermal synthesis. These catalysts have been tested for the oxidative dehydrogenation (ODH) of ethane. ALPO₄-5 has a low activity and low selectivity for the ODH of ethane. The presence of Mg²⁺ ions in MgAPO-5 increases the selectivity to ethene, while the presence of V⁵⁺ species in VAPO-5 increases both the activity and the selectivity for this reaction. The presence of Mg²⁺ and V⁵⁺ species in the vanadium-magnesium alumino-phosphate (MgVAPO-5) results in a more selective catalysts for the ODH of ethane. The behavior of MgVAPO-5 could be attributed to the presence of acid sites (Mg²⁺) near to the redox sites (V⁵⁺) in the molecular sieve framework.     

Investigation of aromatic carbon sites in materials derived from petroleum and coal using 13C n.m.r. methods

Aromatic C and CH carbon sites in a variety of petroleum and coal derived materials have been investigated using a ¹³C n.m.r. technique termed spin echo broad band off-resonance decoupling (SEBBORD). Only resonances due to non-protonated aromatic carbon sites are observed in SEBBORD spectra such that comparison with conventional ¹³C n.m.r. spectra enables differentiation between aromatic C and CH group resonances. Relative abundances of non-protonated aromatic carbon sites calculated from SEBBORD data are in good agreement with values derived from a combination of conventional¹H n.m.r., ¹³C n.m.r. and elemental analysis data. The occurrence of significant proportions of aromatic C intensity to high field of 129–130 ppm and of aromatic CH intensity to low field of 129–130 ppm has been found to be quite common. Consequently attempts to determine aromatic C and CH group abundances by partitioning conventional ¹³C n.m.r. spectra in the vicinity of 129–130 ppm can lead to considerable quantitative errors. SEBBORD provides more detailed information about aromatic carbon sites than can be obtained from conventional ¹³C n.m.r. spectra.     

Electric field gradient tensor in positions of rare-earth metals in RBa2Cu3O7 lattices

The parameters of the electric field gradient tensor (EFG) created in rare-earth metal (REM) sites of RBa₂Cu₃O₇ lattices (R = Nd, Sm, Gd, Dy, Y, Ho, Er, Tm) by crystal lattice ions have been determined by the method of emission Mössbauer spectroscopy on ⁶⁷Ga(⁶⁷Zn) and ¹⁵⁵Eu(¹⁵⁵Gd) isotopes. The EFG tensor has been calculated for REM sites in the approximation of the point charge model. It was shown that the agreement between the experimental and calculated parameters of the EFG tensor can be achieved if the holes are preferably localized in the chain oxygen sublattice.     

The nucleation of lead from halide-containing solutions

The density of active sites and the nucleation rates per site for the nucleation of lead onto vitreous carbon from lead ion solutions in the presence of halides (Cl^–, Br^–, I^–) have been investigated. Mass transport to growing nuclei was not found to be affected by the formation of soluble complexes of lead with the halides. No systematic variation of the true nucleation rates per site was observed upon addition of halides to the electrolyte. The density of active sites for nucleation, on the other hand, increased in the presence of halides. It is also shown that, in the absence of halide ions, the density of active sites for the nucleation of both lead and silver are equal to and independent of the concentration of metal ions in solution.     

Synthesis and anion-sensing property of a family of Ru(II)-based receptors containing functionalized polypyridine as binding site

A family of fluoroionophores have been synthesized incorporating Ru(II)-bipyridine moiety as fluorogenic unit and amino/benzenesulphonamido functionalized 1,10-phenanthroline moiety, attached to Ru(II), as binding sites. Two of the ligands and one of the complexes have been characterized crystallographically. Anion recognition property, studied by luminescence, UV–vis and ¹H NMR spectroscopy, with a large number of anions exhibit strong complexation with F^?, H₂PO₄^? and AcO^?. Binding constants have been determined from luminescence titration and ¹H NMR study gave insight about binding site of anions. Bidentate chelating nature of the H₂PO₄^? and AcO^? anions and steric crowding created by benzenesulphonamide moiety has significantly influenced binding constants and selectivity.     

Infrared Study of Benzene Hydrogenation on Pt/SiO2 Catalyst by Co-adsorption of CO and Benzene

FT-IR spectra of the co-adsorption of benzene and CO have been performed to identify the preferred adsorption sites of hydrogen and benzene on a Pt/SiO₂ catalyst for hydrogenation of benzene. Results of CO adsorbed on atop sites on Pt/SiO₂ includes: an α peak at 2091 cm⁻¹, a β peak at 2080 cm⁻¹ and a γ peak at 2067 cm⁻¹ indicating three kinds of adsorption sites for dissociative hydrogen on Pt/SiO₂. The site of lowest CO stretching frequency offers stronger adsorbates–metal interaction for benzene and hydrogen. Hydrogen binding on the site of lowest CO stretching frequency before benzene adsorption significantly enhances the reaction rate of benzene hydrogenation.     

11B and 15N solid state NMR investigation of a boron nitride preceramic polymer prepared by ammonolysis of borazine

A poly(aminoborazine), precursor for hexagonal boron nitride (h-BN) obtained by reaction of borazine B₃N₃H₆ with ammonia, and its pyrolysis derivatives have been extensively characterised by ¹⁵N and ¹¹B MAS NMR. The various B and N sites have been identified according to their first neighbouring atoms, as well as to the second ones in the case of ¹⁵N, and have also been quantified. This study demonstrates that a suitable choice of NMR techniques together with the use of isotopic enrichment can lead to a large improvement in spectral resolution, which allows a better understanding of such complex BN preceramic polymer structures and permits to follow the polymer-to-ceramic transformation.     

The Hydration of Vanadyl Doped into Magnesium Acetate: Single Crystal Proton ENDOR at 20 K

The single crystal ESR and proton ENDOR of VO²⁺ doped into magnesium acetate tetrahydrate have been studied at 20 K. The VO²⁺ ion occupies Mg²⁺ sites, replacing Mg²⁺ together with one water molecule. Hyperfine tensors for the six protons in the three coordinating water molecules have been determined and their dipolar components used to construct a model of the proton geometry. The relation of the structure to that of {VO(H₂O)₅}²⁺ in a Tutton salt and in frozen aqueous solutions of vanadyl sulphate is discussed.     

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp² clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp² sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp²-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.     

NMR Study on the Acidity of TS-1 Zeolite

The acidic properties of TS-1 and Silicalite-1 zeolites have been investigated by the solid-state MAS NMR technique capable of in situ sample pretreatment. As shown by a combination of the ³¹P MAS NMR and ¹H MAS NMR techniques with trimethylphosphine, not only Brønsted acid sites but also Lewis acid sites exist in the TS-1 zeolites. Moreover, TS-1 zeolite is more acidic compared with Silicalite-1. The ¹H, ²⁹Si MAS NMR spectra and the resonance related to Brønsted acid species in the ³¹P MAS NMR spectra demonstrate clearly that the presence of titanium in the framework results in the formation of a new hydroxy group, titanols, which is more acidic than silanols of Silicalite-1. The ³¹P MAS NMR measurements also illustrate convincingly the existence of at least two different Lewis acid species on the TS-1 zeolites. The conversion of propylene oxide into methoxypropanol catalyzed by TS-1 or Silicalite-1 zeolite in methanol solution as a test reaction has also been described. With the increase of titanium in zeolite, TS-1 appears to have a higher activity during the reaction of propylene oxide to methoxypropanol.     

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10^–3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10^–2 molecules/sites, under the same reaction conditions. The initial C₂ formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.     

Estimation of Sc3+ solubility in dodecahedral and octahedral sites in YSAG:Yb

A method for synthesizing YSAG:Yb optical ceramics has been developed. Additionally, the phase composition and unit cell parameters for YSAG, YbSAG, and their solid solutions have been determined. The limits of scandium solubility in the dodecahedral and octahedral sites of garnet crystal lattice have been estimated after annealing at 1600°C. The scandium solubility limit in the dodecahedral sites of YSAG:Yb was found to be 66 ± 2 at.%. The limit of scandium solubility in octahedral sites depended upon the type of garnet-forming cations. The solubility limit for Sc³⁺ decreased from 97.5 ± 0.5 to 68.5 ± 1 at.% with simultaneous increase of ytterbium content in the YSAG-YbSAG system. Therefore, the region of solid solutions existing with a garnet structure in the Al₂O₃–Sc₂O₃–Yb₂O₃–Y₂O₃ system was determined. Samples having scandium in both dodecahedral and octahedral sites, which had a Sc³⁺ content of ~30 at.%, were synthesized as a precursor for optical ceramic production.     

Impact of magnesium on the structure of aluminoborosilicate glasses: A solid-state NMR and Raman spectroscopy study

Seven magnesium-containing aluminoborosilicate glasses, with three to five oxides, have been studied through comprehensive multinuclear solid-state NMR (¹¹B, ²⁷Al, ²⁹Si, ²³Na, ¹⁷O, and ²⁵Mg) and Raman spectroscopy. The progressive addition of cations and the substitution of sodium and calcium by magnesium illuminate the impact of magnesium on the glass structure. The proportion of tri-coordinated boron drastically increased with magnesium addition, demonstrating the poor charge-compensating capabilities of magnesium in tetrahedral boron units. Oxygen-17 NMR showed the formation of mixing sites containing both Na and Mg near nonbridging oxygen sites. Furthermore, a high magnesium content appears to result in the formation of two subnetworks (boron and silicon rich) with different polymerization degrees as well as to promote the formation of high-coordination aluminum sites (Al[V] and Al[VI]). Finally, magnesium coordination ranging from 4 to 6, with a mean value shifting from 5 to 6 along the series, suggests that magnesium might endorse an intermediate role in these glasses.     

Synthesis, structure and catalytic properties of Fe-substituted barium hexaaluminates

A sol–gel method using Ba and Al isopropylates and iron nitrate has been used to synthesise barium hexaaluminate partially substituted with iron. After calcination under oxygen at 1200°C the -alumina structure was obtained. Formation of the mixed BaFe_xAl_12-xO₁₉ phase occurred for x=1–4. XRD measurements showed a good crystallinity of the structure and expansion of unit cell parameters due to the presence of larger Fe³⁺ ions substituting Al³⁺ ones in octahedral sites only. Mössbauer spectroscopy revealed that Fe³⁺ ions are present in four different octahedral sites slightly distorted. Catalytic activity in methane combustion showed that an optimum was obtained for solid containing 2 Fe ions per unit cell: the increase of the amount of introduced iron was counterbalanced by the decrease of specific surface area. Intrinsic activities have been calculated for the four solids in both the fresh and aged states. It is observed that increasing iron content increases relative activities in the same ratio as the populations of iron located in two sites as deduced from Mössbauer spectroscopy. It is then tentatively assumed that activity is attributed to octahedrally coordinated Fe³⁺ ions in some specific sites.     

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. II. Action of Promoters Zn and Li

By correlating the results of the NH₃-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydro-aromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg²⁺, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn²⁺ or Li⁺ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg²⁺ and Zn²⁺, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn²⁺ or Li⁺ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W⁶⁺ species and an increase in quantity of the reducible W⁶⁺ species at the reaction temperature for DHAM, as has been evidenced by the results of a H₂-TPR study on the reducibility of the Zn²⁺ (or La³⁺, Li⁺, Mn²⁺)-promoted W/HZSM-5 system. The above two roles that Zn²⁺ and Li⁺ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.