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Fourier-transform Infrared Spectroscopic Studies of the Reactivity of Vanadia Titania Catalysts Toward Olefins .1. Propylene
No full textThe adsorption and transformation of propylene and of several O-containing C3 organic molecules (namely isopropyl alcohol, acetone, allyl alcohol, acrolein, and acrylic acid) on vanadia-titania in the temperature range 150-673 K have been investigated by IR spectroscopy. At low temperatures (150-200 K), propylene adsorbs as such through pi-bonding and reacts later to give an adsorbed species identified tentatively as an allyl radical. Alternatively, it undergoes addition by a surface OH group to give an isopropoxy species. At least two main surface pathways for propylene transformation are active, leading to surface species functionalized at C2 (isopropoxy species and acetone) and at C1 (acrolein and acrylates) as well as products arising from coupling of the allyl radical. The results are discussed in relation to literature data concerning the heterogeneously catalyzed selective oxidation of propylene carried out on vanadia-titania as well as on bismuth-molybdate-based catalysts
Ft-ir Studies of the Reactivity of Vanadia-titania Catalysts Toward Olefins .3. Normal-butenes and Isobutene
No full textThe interaction of vanadia-titania catalyst with 1-butene, cis-2-butene, trans-2-butene, and isobutene has been investigated by FT-IR spectroscopy in the temperature range 150-673 K. The three normal butenes give at room temperature 2-butoxide species that can undergo at higher temperatures oxidative dehydrogenation to methyl ethyl ketone and later oxidative C-C bond cleavage to acetate species. Methyl-allyl species are also formed and can be isolated by low-temperature experiments in the case of 1-butene. These speces follow a different oxidative path with formation of butadiene, furan, and finally maleic anhydride. Starting from near 200 K, isobutene oligomerizes giving its dimer 2,4,4-trimethylpent-1-ene and higher oligomers. At room temperature, hydration to t-butoxide species is also observed. Further heating causes oxidative decomposition to acetone, carboxylate species, and, from the oligomers, cyclic anhydrides. Only traces of allylic oxidation products of isobutene can be found. All these reactions occur in the absence of gas-phase oxygen
MnOx-TiO2 catalyzers for valatil organic compounds elimination.
No full textMn-Ti mixed oxides with stoichiometry TixMn1-xOy (with x = 0, 0.1, 0.2, 0.5, 0.8, 0.9, 1) have been prepared by a coprecipitation method. Mn3O4 (hausmannite) and TiO2 (anatase) were found to be the main crystalline phases, while solid solutions appeared to be formed at the extreme values of x. Thermal stability of these materials reaches 873 K, transforming into thermodynamically stable phases, TiO2 (rutile) y alpha-Mn2O3, at higher temperatures. From catalytic point of view, they are active in propane and propene oxidation, though the increase of Ti content seems to deactivate Mn oxide for propene oxidation. Conversely, the sample with the Mn/Ti = 1 ratio presents a maximum for propane oxidation
A FT-IR study of the FeGa mixed oxide surface by probe molecule adsorption
No full textSeveral Fe Ga mixed oxide samples denoted as Fe1-xGaxOy (x = 0, 0.20, 0.25, 0.50, 0.80, 0.90) have been synthesised by a conventional coprecipitation method at controlled pH, dried at 90 degrees C for 24 hours and calcined at 400 y 800 degrees C for 3 hours. All samples were characterised by X-ray diffraction, chemical analyses and thermogravimetry and differential thermal analysis (TG-DTA) and FT-IR and UV-VIS spectroscopies. It was found all materials formed a solid solution, alpha-(Fe,Ga)(2)O-3, in the entire Fe/Ga compositional range until 400 degrees C. Samples calcined at 800 degrees C gave rise to biphasic samples for x > 0.5 due to different structure of the beta-Ga2O3 with respect to the alpha-Fe2O3 one. Surface acidity of samples was studied by pivalonitrile molecule adsorption on the pressed pellet of each material, founding weak Lewis acid in all cases and not being possible distinguish both cation sites on the surface and Bronsted one that increased by increasing Ga content. Methanol molecule was also adsorbed, showing an increase of reactivity for those samples with a little bit of Ga content
Synthesis and characterization of Fe-Ga mixed hydroxide powders
No full textSeveral samples of iron gallium mixed oxyhydroxides with stoichiometry Fe1-xGax (with x = 0, 0,10, 0.25, 0.50, 0.75, 0.90) have been synthesized by a coprecipitation method. They have been characterised from the point of view of their solid state structure by X-ray diffraction (XRD), thermogravimetry and differential thermal analyses (TG-DTA), FTIR and diffuse reflectance (DR) UV-VIS spectroscopy. After drying at 393 K, only one crystalline phase denoted as alpha-(Fe,Ga)OOH, is found in the samples, ie. a solid solution in the whole composition range in agreement with the continuous lowering of the unit cell parameter. When Ga is added in small amounts to FeOOH, it tends to notably hinder alpha-(Fe,Ga)OOH crystallisation. The electronic spectra suggest that clustering of Fe3+ in dilute samples may occur. Low-temperature thermal decomposition of such mixed oxyhydroxides gives rise to corundum-hematite type solid solutions in the whole compositional range
Preparation, characterization and surface structure of coprecipitated high-area SrxTiO2+x(0<=x<=1) powders
No full textPowders with composition SrxTiO2+x (x = 0, 0.02, 0.11, 0.42 and 1) have been prepared by coprecipitation from strontium hydroxide and titanium isopropoxide and have been characterized after drying at 393 K, and after calcination at 773 and 973 K, using XRD, TG-DTA, BET surface area and porosity measurements, FTIR-FTFIR and FT-Raman skeletal vibrational characterization, diffuse reflectance UV-VIS spectroscopy and FTIR spectra of the hydroxy groups and of adsorbed pyridine and CO2. Strontium was found to inhibit anatase crystallization, sintering and transformation to rutile. Strontium tended to be deposited at the surface of anatase and gave rise to a decrease of the surface acidity and an increase of surface basicity. Moreover, it shifted the absorption edge of anatase to higher energies. For x = 1, the truly cubic SrTiO3 perovskite phase was produced at room temperature with high surface area. The surface of SrTiO, was definitely basic, with only strontium and oxygen ions exposed at the surface, and hydrogen impurities in the bulk
Solid-state and Surface-chemistry of CuO-TiO2 (anatase) Powders
No full textCuO-TiO2 samples have been prepared by impregnation of two different preformed TiO2 supports with copper nitrate, and have been characterized by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), Fourier-transform infrared (FTIR), Fourier-transform far-infrared (FT-FIR) and ultraviolet-visible (UV-VIS) spectroscopies. The surface properties have been investigated through surface area and porosity measurements, and FTIR spectra of surface OH groups and of adsorbed carbon monoxide at low temperature. The surface is thought to consist of complexes of Cu2+ and Cu+, involving surface Ti-O groups of the support as the ligands. These complexes can be reduced by CO under very mild conditions with the appearance of small Cu metal clusters. Ti4+ centres are also in part exposed on the surface, but their electron-withdrawing effect is weakened by the surface copper complexes. Surface copper ions act as electron-donor centres, giving rise to absorption over in the entire visible range, with a parallel weakening of the TiO2 absorption edge. Surface copper complexes have a pronounced accelerating effect on the anatase-to-rutile phase transition. A parallelism between the CuO-TiO2 and the V2O5-TiO2 systems is proposed
Characterisation of Fe-Cr-Al mixed oxides
No full textSeveral samples of iron chromium aluminium mixed oxides with different composition have been prepared by coprecipitation at controlled pH starting from the corresponding nitrate salts and following dried at 393 It and calcination at 673 K for 3 h and 1173 K for 3 h. The powders have been characterised by XRD, FT-IR and DR W-Vis spectrocopies, DTA-TG thermal analyses and measurements of BET surface area. It has been found alumina is soluble into alpha-FeCrO3 phase up to near 20%. These samples are stable at 1243 K with a relative high specific surface area. The gamma,theta --> alpha phase transition is shifted towards higher temperatures by increasing Al content, being not detectable when alpha-FeCrO3 phase is the main phase. Surface chromates species are identified by the different techniques used and their amount seem to depend directly on the specific surface area of each sample. (C) 1999 Elsevier Science S.A. All rights reserved
Ft-ir Study of the Surface-properties of the Spinels Nial2o4 and Coal2o4 In Relation To Those of Transitional Aluminas
No full textAnatase Crystal-growth and Phase-transformation To Rutile In High-area TiO2, MoO3-TiO2 and Other TiO2-supported Oxide Catalytic-systems
No full textAnatase-to-rutile phase transition has been studied on high-area TiO2-anatase and bicomponent systems containing molybdena and, for comparison, cobalt oxide, copper oxide, vanadia and corresponding TiO2-rutile based systems, using TG-DTA, XRD and surface-area measurements. These materials were prepared by impregnation and (co)precipitation methods. Conversion to rutile and crystal size growth strongly depend on the minority phase oxide. It is found that, while Cu and V oxides speed up both phase transition and particle sintering, Mo and Co oxides inhibit them. Moreover, the rutile particles obtained by phase transition are always much larger than both the starting and residual anatase particles. A mechanism of rutile formation by coalescence and phase transformation of anatase particles is proposed
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