New process to control hydrolysis step during sol-gel preparation of sulfated zirconia catalysts

Sulfated zirconia catalysts were prepared by sol-gel process using a new method allowing the control of the hydrolysis step of zirconium alkoxides. Prepared samples were characterized by N₂ adsorption (desorption) at 77 K, XRD, chemical analysis of total sulfur and XPS. Catalytic properties have been evaluated in n-butane and n-hexane isomerization reactions. Obtained results show that time necessary for gelation is significant and depends considerably on the nature of the zirconium precursor. Concerning the properties of the solids obtained by this method, it has been noted that the use of the zirconium propoxide led to a better retention of the sulfate species, which improves activity during isomerization reactions. It has been found also that an initial S/Zr molar ratio equal to 0.5 allowed to obtain an optimal sulfur content and a high specific surface area.     

Analytic and numerical analysis of some statistical features of fragmentation processes

In this work, we investigate the following inhomogeneous model of fragmentation with an exponential splitting probability: suppose that an initial particle having a mass x₀>0 (large enough) is broken, with a size-dependent probability , into b>1 pieces. With probability 1−p(x₀), this fragment is left unchanged for ever. The process of fragmentation is repeated recursively for all pieces.Some statistical features are studied, chiefly the fragment size distribution, number of fragments and partition function of the random fragmentation tree. Also, we give details of the numerical analysis of these features.     

Comparative Study of the Acidity of Sulphated Zirconia Supported on Alumina Prepared by Sol-Gel and Impregnation Methods

Sulphated and unsulphated alumina-zirconia with atomic ratio Zr/Al = 0.5 were prepared by sol-gel and impregnation methods. The solid prepared by the sol-gel method exhibits the higher specific surface area. The Kelvin probe shows that the value of unsulphated sample is around 400 mV. This value grows up to 1100 mV for sample prepared by impregnation of aerogel alumina by sulphated propoxide zirconium and up to 1450 mV for sulphated alumina-zirconia aerogel catalyst. The modification of the work function is probably due to the charge transfer from the zirconium and aluminium to an oxygen species, responsible for the increase of Lewis acidity. XPS results show that the aluminium and zirconium exist in oxide form as Al₂O₃ and ZrO₂. The sulphur is present as sulphate species in the solids bonded to the Al—Zr—O framework. Furthermore, the oxygen species exist in different types created by the introduction of sulphur in the bulk of solids.Compared to the impregnated catalyst the sol-gel sulphated alumina zirconia exhibits higher activity in the isopropanol dehydration reaction in the temperature range 423 K–523 K.     

Textural,structural and catalytic properties of zirconia doped by heteropolytungstic acid: a comparative study between aerogel and xerogel catalysts

Zirconia doped by heteropolytungstic acid HPW have been synthesized by sol–gel method using two drying techniques of the solvent evacuation. Samples were analyzed with adsorption–desorption of N₂ at 77 K, and the aerogel catalyst was found to exhibit a higher surface area and a higher average pore diameter compared to xerogel. XRD results show that aerogel develops ZrO₂ tetragonal phase, whereas xerogel is amorphous. The thermal analysis studies show that the aerogel’s thermal stability is better than the xerogel one. The catalytic behavior of the aerogel and xerogel toward the nature of the isomerization products probably depends on the acidity and the presence of carbide species. This has been explained by XPS and isopropanol dehydration reaction. In fact, the deconvolution aerogel’s Cls bands reveals the presence of four carbon species assigned to C–C, C=O, C–O and carbide species.     

Kinetics of the catalytic transformation of isobutene into methacrylonitrile with NO on supported nickel oxide aerogel

Kinetic studies of the nitroxidation of isobutene by NO show that the reaction follows a redox mechanism in the temperature range of 300–400°C. This mechanism implies an interaction between dehydrogenated isobutene -allyl radical and atomic nitrogen due to the decomposition of NO by the reduced catalyst, which is then reoxidized.

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NO , 300–400°C. - , NO , .

     

Mesoporous Silica Supported Sulfated Zirconia Prepared by a Sol–Gel Process

Mesoporous silica-supported sulfated zirconia aerogels with different Zr/Si and S/Zr molar ratios were prepared using a one-step preparation. The characterization of these solids was conducted using N₂ physisorption at 77 K, X-ray diffraction, mass spectrometry, and sulfur chemical analysis. The isomerization of n-Hexane was used as a catalytic test. Highly improved textural properties were obtained for these solids, and two types of mesopores were observed. Depending on the zirconium content, the size and amount of mesopores are highly affected. The importance of sulfates effectively bonded to Zr(IV) is elucidated, both for achieving high catalytic activity and for a good sulfur thermal stability. The catalytic activity (for a sample prepared with a particular S/Zr ratio) expressed per gram of Zr(IV) is comparable to that of pure sulfated zirconia.     

Analysis of the occurrence of stick-slip in AFM-based nano-pushing

The use of the Atomic Force Microscope (AFM) as a tool to manipulate matter at the nanoscale is promising. However, the complexity of the corresponding physics and mechanics makes such nanomanipulation difficult and not very accurate. In the present paper, we analyze the dynamics of AFM-based nano-pushing manipulation. Simulation results show that the choice of the manipulation speed and loading force highly affect the manipulation outcome. In addition, simulations predict the existence of several threshold manipulation speeds. These thresholds mark the transitions between no stick-slip motion and either unique or multiple coexisting stick-slip. The obtained results bear significant implications and help get more insight into AFM-based nano-pushing.     

One-pot deposition of palladium on hybrid TiO2 nanoparticles and catalytic applications in hydrogenation

One-pot deposition of Pd onto TiO(2) has been achieved through directly contacting palladium(II) salt with nanosized functionalized TiO(2) support initially obtained by sol-gel process using titanium isopropoxide and citric acid. Citrate groups act as functional moieties able to directly reduce the Pd salt avoiding any further reducing treatment. Various palladium salts (Na(2)PdCl(4) and Pd(NH(3))(4)Cl(2)·H(2)O) and titanium to citrate (Ti/CA) ratios (20, 50, and 100) were used in order to study the effect of the nature of the precursor and of the citrate content on the final Pd particle size and catalytic properties of the as-obtained Pd/TiO(2) systems. Characterization was performed using N(2) adsorption-desorption isotherms, ICP-AES, FTIR, XRD, XPS, and TEM. The as-obtained hybrid Pd/TiO(2) catalysts were tested in the selective hydrogenation (HYD) of an α,β-unsaturated aldehyde, i.e. cinnamaldehyde. Citrate-free Pd/TiO(2)-based catalysts present lower selectivity into saturated alcohol. However, citrate-functionalized Pd/TiO(2) catalyst seems to control the selectivity, the particle size and dispersion of Pd NPs leading to high intrinsic activity.     

EPR and UV–visible spectroscopic studies of alumina-supported chromium oxide catalysts

The oxidation state, the mobility and the molecular structure of chromium species present on CrO_x–Al₂O₃ catalysts have been studied by combined diffuse reflectance spectroscopy, EPR and reduction–extraction by ethane 1,2 diol. CrO₄²⁻ species exist on the alumina surface in the form of loosely-interacting species on hydrated surface (species A) and in the form of strongly bonded species on dehydrated Al₂O₃ surface (species B). The CrO₄²⁻ species show high mobility and are probably responsible for the formation of CrO_x clusters.     

catalytic activity of comgal, coal and mgal of mixed oxides derived from hydrotalcites in the selective catalytic reduction of no with ammonia

Summary Calcined CoMgAl LDHs and a Co-impregnated LDH were characterized by XRD, N₂ physisorption, TPR and ICP elemental analysis, then tested in the selective catalytic reduction (SCR) of NO with ammonia. Cobalt deposited by impregnation on LDH leads to the more active and selective catalyst. On this type of catalyst nearly 80% of conversion and selectivity to N₂ were reached at 578 K with a volume space velocity of 2200 h^-1.