Surface acidity and basicity of functionalized silica particles

Tetraethoxysilane has been co-hydrolyzed with functionalized organosilanes in a modified Stöber process to produce silica particles with amino, carboxylate or dihydroimidazole groups on the surface. The effects of reaction conditions and the loading of the functionalized organosilane on particle size was examined by TEM. Fluorescence spectroscopy of the surface amino groups covalently modified with fluorescamine, and the surface carboxylate groups with 4-bromomethyl-6,7-dimethoxycoumarin, demonstrated that these functional groups were accessible for further reaction. Changes in surface acidity and basicity caused by the presence of functional groups (amine, dihydroimidazole, carboxylate) on the particle surface were determined using an indicator titration technique. Particles with surface imidazole and amine groups and particles with surface carboxylate groups have enhanced basicity and acidity, respectively. Dihydroimidazole-modified silica had greater surface basicity than the amine-modified silica. The effect on basicity and acidity increases as the amount of added functionalized silane increases. However, this increase is nonlinear with respect to the increase in added functionalized silane. Particles with both surface dihydroimidazole and carboxylate groups demonstrated reduced surface basicity and acidity.     

Surface acidity of supported vanadia catalysts

The surface acidity of SiO₂, γ-Al₂O₃ and TiO₂ supported vanadia catalysts has been studied by the microcalorimetry and infrared spectroscopy using ammonia as the probe molecule. The acidity in terms of nature, number and strength was correlated with surface structures of vanadia species in the catalysts, characterized by X-ray diffraction and UV-Vis spectroscopy. It was found that the dispersion and surface structure of vanadia species depend on the nature of supports and loading and affect strongly the surface acidity. On SiO₂, vanadium species is usually in the form of polycrystalline V₂O₅ even for the catalyst with low loading (3%) and these V₂O₅ crystallites exhibit similar amount of Brönsted and Lewis acid sites. The 25%V₂O₅/SiO₂ catalyst possesses substantial amount of V₂O₅ crystallites on the surface with the initial heat of 105 kJ mol^-1 and coverage of about 600 mmol g^-1 for ammonia adsorption. Vanadia can be well dispersed on g-Al₂O₃and TiO₂ to form isolated tetrahedral species and polymeric two-dimensional network. Addition of vanadia on γ-Al₂O₃ results in the change of acidity from that associated with g-Al₂O₃ (mainly Lewis sites) to that associated with vanadia (mainly Brönsted sites) and leads to the decreased acid strength. The 3%V₂O₅/TiO₂ catalyst may have the vanadia structure of incomplete polymeric two-dimensional network that possesses the Ti-O-V-OH groups at edges showing strong Brönsted acidity with the initial heat of about 140 kJ mol^-1 for ammonia adsorption. On the other hand, the 10%V₂O₅/TiO₂ catalyst may have well defined polymeric two-dimensional vanadia network, possessing V-O-V-OH groups that exhibit rather weak Brönsted acidity with the heat of 90 kJ mol^-1 for NH₃ adsorption. V₂O₅ crystallites are formed on the 25%V₂O₅/TiO₂ catalyst, which exhibit the acid properties similar to those for 25%V₂O₅ on SiO₂ and γ-Al₂O₃.     

Empirical parameters of lewis acidity and basicity for aqueous binary solvent mixtures

Empirical parameters of Lewis acidity, E^N_T, introduced by Reichardt et al., and Lewis basicity, B_KT , introduced by Kamlet and Taft, have been determined for mixtures of water with ten organic solvents. In the case of water/alcohol mixtures a distinct dependence between these acidity and basicity parameters have been found. For the other solvent mixtures the E^N_T on B_KT dependence is more complex even if these parameters are purified from non-specific solute/solvent interactions.     

Surface acidity in promoted molybdena and tungsta hydrodesulphurisation catalysts

Acidity of Ni- and Co-promoted molybdena and tungsta hydrodesulphurisation (HDS) catalysts have been determined by volumetric adsorption of ammonia. A direct correlation between the adsorption of ammonia, and hence the acidity, and the HDS activity could be observed for reduced catalysts A relation between ammonia adsorption and anion vacancy concentration is thus indicated, as the latter is related to HDS activity. Contrary to unpromoted molybdena there is a decrease in acidity on sulphidation of Ni-promoted molybdena catalyst. This decrease is attributed to the influence of, Ni in suppressing the formation of anion vacancies by catalysing the crystallisation of MoS₂. Promoted tungsta catalysts were found to have lower acidities as compared to promoted molybdena catalysts, which is consistent with their lower desulphurisation activity.     

Surface behavior of nickel powders in aqueous suspensions

The colloidal behavior of nickel aqueous suspensions is studied and compared to that of NiO suspensions. Under acidic conditions, nickel readily dissolves, but no dissolution takes place at basic pH. Zeta potential is studied as a function of pH, showing that the isoelectric point (IEP) occurs at pH 3.5-4. Above the IEP there is a zeta potential plateau, which is associated to the predominance of NiO(OH) species. At pH 9 a new decrease in zeta potential is associated to NiO predominance. XPS studies support that suspensions prepared at pH >9 lead to NiO-enriched species, while suspensions prepared at lower pH form NiO(OH) species.     

ESR studies of aqueous suspensions of zinc oxide

Results are presented from a technique which allows reproducible ESR spectra to be obtained from circulating aqueous suspensions of various zinc oxides. Singlet ESR signals with g ≈ 1.96 were observed in the dark from In-ZnO and from several zinc oxides under UV illumination. Fast and slow components of the photoinduced ESR signal are attributed respectively to surface and bulk electrons produced initially from photogenerated electron—hole pairs.     

Dipolarity, hydrogen-bond basicity and hydrogen-bond acidity of aqueous poly(ethylene glycol) solutions.

Poly(ethylene glycol) (PEG) in water is known to alter the structure and/or state of water to give a different polarity from that of pure water. We determined using the solvatochromic comparison method the dipolarity/polarizability (pi*), hydrogen bond (HB) accepting basicity (beta) and HB donating acidity (alpha) of aqueous solutions of PEGs of variegated molecular weights at different concentrations in order to understand the influence of the polymer on these properties of water. It was observed that PEG decreases alpha for water while it does not change pi* and beta appreciably in the range of the molecular weight and compositions studied.     

Solid aluminiumalkyl-based isomerization catalysts. Surface acidity and catalytic activity

Isomerization catalysts with high activity and selectivity were prepared by chemical anchoring of aluminiumethyldichloride on the surface of inorganic oxides. Surface acidity was studied by calorimetric and IR spectroscopy methods, and was found to correspond well with catalytic activity for m-xylene isomerization.

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Response to high acidity and basicity at a platinum electrode in chronopotentiometry

A simple and rapid method is developed to determine the high acidity and the basicity of solutions by chronopotentiometry with a platinum working electrode. The acidity range from 5.0 mol/l H⁺ to 1.0 mol/l OH⁻ can be measured by the adjustment of deposition potential and time. The response mechanism to acidity and basicity has been explored. The transition potential plateau in chronopotentiograms is caused from the oxidation of hydrogen adsorbed on electrode surface.     

Relation between the hydrogenating and dehydrogenating activity of chromium oxide catalysts and their basicity

Conclusions The results obtained in studying the hydrogenating and dehydrogenating activity of catalysts make it possible to assume that a relation exists between the activity of the catalysts and their basicity.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2832–2833, December, 1971.     

Surface complex characteristics of synthetic maghemite and hematite in aqueous suspensions

The acid-base properties of the maghemite (gamma-Fe2O3)/water and hematite (alpha-Fe2O3)/water interfaces have been studied by means of high precision potentiometric titrations and the experimental results are evaluated as surface complexation reactions. Synthetic maghemite and hematite were prepared and characterized using a combination of SEM, FT-IR and XRD. The specific surface area of the minerals was determined by the BET method. The titrations were performed at 25.0+/-0.2 degrees C within the range 2.8相似文献   

Surface characterization of silica-supported cobalt oxide catalysts

Silica supported cobalt oxides were prepared by the impregnation method, using an aqueous solution of cobalt nitrate hexahydrate (Co(NO₃)₂ · 6H₂O), then calcined at different temperatures (510, 620 and 870 K). Characterization of the samples was carried out by X-ray diffraction, N₂-adsorption at −196°C, UV–Vis diffuse reflectance spectroscopy and KBr-IR spectroscopy of the calcination products. The surface acidity was studied by IR spectroscopy of adsorbed pyridine at different temperatures (300, 370, 470 and 570 K). Results indicated that Co₃O₄ is the stable phase on silica, however, dispersion of minor amount of cobalt oxide could not be ruled out. Results also indicated that the crystallinity of the formed Co₃O₄ increased by increasing the loading level and/or the calcination temperature. Furthermore, the surface area of the support was decreased by increasing the loading level and the calcination temperatures. It has been also found that the surface of the supported catalysts exposed strong different Lewis acid sites.     

Relationships between basicity and reactivity of zeolite catalysts

A wide variety of characterization methods, including UV-vis spectroscopy of adsorbed I₂, microcalorimetry of CO₂ adsorption, and x-ray absorption spectroscopy at the Cs L_III edge of zeolite cations, was applied to a series of alkali containing zeolites in order to elucidate the nature of the basic sites on these materials. In addition, three catalytic reactions involving basic zeolites were studied. In the first case, alkali-exchanged zeolites (L, Beta, X and Y) were used as catalysts for the side-chain alkylation of toluene with methanol to form styrene and ethylbenzene. Zeolites with low base site densities and appropriate base strengths catalyzed toluene alkylation without decomposing methanol to carbon monoxide. In the second example, ruthenium metal clusters were supported on alkali and alkaline earth exchanged X zeolites and tested as catalysts for ammonia synthesis. Zeolites containing alkaline earth ions exhibited rates greater than those containing alkali ions. Finally, zeolite X loaded with alkali metal was an active catalyst for toluene alkylation with ethylene whereas zeolite X loaded with alkali oxide was inactive for the reaction. These results suggest that exciting opportunities exist for the use of basic zeolites as catalysts and catalyst supports.     

In situ measurements of nanotube dimensions in suspensions by depolarized dynamic light scattering

We show that the dimensions of carbon nanotubes (CNTs) in suspension can be characterized by depolarized dynamic light scattering. Taking advantages of this in situ technique, we investigate in detail the influence of sonication procedures on the length and diameter of CNTs in surfactant solutions. Sonication power is shown to be particularly efficient at unbundling nanotubes, whereas a long sonication time at low power can be sufficient to cut the bundles with limited unbundling. We finally demonstrate the influence of CNT dimensions on the electrical properties of CNT fibers. Slightly varying the sonication conditions, and thereby the suspended nanotube dimensions, can affect the fibers conductivity by almost 2 orders of magnitude.     

Complex investigation of adsorbed solvation layers in polymer-containing aqueous suspensions of zinc oxide

Theoretical and Experimental Chemistry -     

A computational study of gas-phase acidity and basicity of azulene-based uracil analogue

Structural Chemistry - In this paper, we suggest a computational scheme for the theoretical estimation of gas-phase acidity and basicity of azulene-based uracil analogue. The proton affinities...     

In situ preparation of weakly flocculated aqueous anatase suspensions by a hydrothermal technique

Weakly flocculated aqueous anatase suspensions were prepared in situ by hydrothermally treating amorphous titania particles peptized with different amounts of tetraethylammonium hydroxide (TENOH). The simultaneous formation of hydrous TiO2 polyanions in the presence of OH- and tetraethylammonium cations are two essential conditions for the peptization process to occur. The absence of either of these conditions will cause reprecipitation. Transmission electron microscopy (TEM) revealed that the morphology of the particles formed at low TENOH concentrations consisted of well-dispersed anatase crystals, changing to asterisk-like structured particles with increasing concentrations of TENOH. Because of the extremely high absolute zeta potential (over -70 mV in all the samples) and ionic strength values, nontouching particle networks may be formed in situ in the mother solution in all samples, as predicted by DLVO theory. A trend to coagulation was observed in the suspensions with increasing concentrations of TENOH due to a more pronounced secondary minimum in the particle pair potential curves. Assuming the particles remained in the secondary minimum throughout the hydrothermal treatment may lead to the formation of the asterisk-like hard agglomerates. This may arise from the condensation of the -OH-rich TiO2 particles or from the deposition of material in the interparticle gap during the particle growth process. The green packing density of slip-cast bodies from a suspension containing 20 wt% solids was around 46%.     

In situ characterization of aluminum-containing mineral-microorganism aqueous suspensions using scanning transmission X-ray microscopy

In situ characterization of colloidal particles under hydrous conditions is one of the key requirements for understanding their state of aggregation and impact on the transport of pollutants in aqueous environments. Scanning transmission X-ray microscopy (STXM) is one of the few techniques that can satisfy this need by providing element- and chemical-state-specific 2-D maps at a spatial resolution better than 50 nm using soft X-rays from synchrotron radiation wiggler or undulator sources tuned to the absorption edges of different elements. X-ray absorption near-edge structure (XANES) spectra can also be collected simultaneously at a similar spatial resolution and can provide phase identification in many cases. In this study, we report STXM images and XANES spectroscopy measurements at or above the Al K-edge (E = 1559.6 eV) of various Al-containing minerals and synthetic oxides [alpha-Al2O3 (corundum), gamma-Al2O3, gamma-AlOOH (boehmite), alpha-Al(OH)3 (bayerite), KAl2(AlSi3O10)(OH)2 (muscovite), (Al,Mg)8(Si4O10)4(OH)8.nH2O (montmorillonite), and Mg6Al2(OH)16CO3.4H2O (hydrotalcite)] and demonstrate the capability of this spectromicroscopic tool to identify different Al-containing mineral colloids in multiphase mixtures in aqueous solution. We also demonstrate that STXM imaging at or above the C K-edge (E = 284.2 eV) and Al K-edge can provide unique information on the interactions between bacteria and Al-containing nanoparticles in aqueous suspensions. STXM images of a mixture of Caulobacter crescentus and montmorillonite and corundum particles just above the C and Al K-edges show that the mineral particles and bacteria are closely associated in aggregates, which is likely due to the binding of bacteria to clay and corundum particles by extracellular polysaccharides.