V. Sánchez Escribano

Influence of tile synthesis parameters on the structural and textural properties of precipitated manganese oxides

Abstract A systematic study has been conducted in order to compare the structural and morphological properties as well as the genesis course of manganese oxides prepared by a precipitation-calcination method under different conditions. In particular, the roles of the precursor salt, the pH, the precipitating agent and thermal treatment were investigated. It was found that calcination at 873 K of the starting materials gives rise in all cases to crystalline bixbyite. However, the phases formed at lower temperatures are quite different and depend largely on the synthesis conditions. The systems prepared from a Mn(III) ionic salt present maximum specific surface areas according to a greater development medium-sized pores.

Chemistry of olefins at metal oxide surfaces: a tool for surface science investigation of oxide catalysts

Abstract The results of IR investigations of the adsorption of simple olefins (ethylene, propylene and isobutene) on a number of metal oxide catalysts are summarized. The spectroscopic features of molecularly adsorbed species interacting with the surface via H-bonding, via σ-type electron donation over do cations and via σ-type donation with π-type back donation on d-electron-containing cations are briefly discussed. Evidence for surface reactivities including: 1. i) oxidative breaking of the C=C bond; 2. ii) hydroxylation of the C = C bond giving alkoxides; 3. iii) proton induced cationic polymerization; 4. iv) proton abstraction at the allylic position giving anionic allyl species; 5. v) hydrogen abstraction at the allylic position giving radical-like allyl species; 6. vi) adsorption with hydrogen transposition giving metal alkylidene species; is also discussed on particular surfaces.

Characterization of MoO3-P2O5-ZrO2 catalysts: an oxide-supported mixed oxide

Abstract P2O5/ZrO2, MoO3/ZrO2 and MoO3-P2O5/ZrO2 samples have been prepared by impregnation of a commercial zirconia support with ammonium phosphate and/or heptamolybdate aqueous solutions. The structural and morphological properties have been studied by XRD diffraction, nitrogen adsorption—desorption experiments and skeletal FT-IR and FT-Raman spectra. Original zirconia is in monoclinic phase with traces of tetragonal phase. Only Mo and P amount higher than the theoretical monolayer molybdite is observed. The incorporation of P and/or Mo causes a slight decrease in the value of the specific surface area with respect to pure support. All samples are mesoporous, whose texture can be described by tubular pores with widened parts. Surface MexOy (where Me = P or Mo) groups have been detected. Fourier transform infrared spectra of adsorbed ammonia and carbon dioxide show that the addition of Mo and/or P on zirconia causes a relevant increase of the surface acidity and destroys the surface nucleophilic oxygen present on the pure support.

Methanol conversion over a Pd5Cu/Al2O3-CeO2 catalyst: an FT-IR study and reaction mechanism.

A catalyst composed of a Pd(5)Cu mixed oxide supported over Al(2)O(3)-CeO(2) with general formula Pd(5)CuO(x)/Al(2)O(3)-CeO(2) (Al/Ce atomic ratio=1/1) has been prepared by a wet impregnation method and tested in the methanol conversion. The structural and morphological characterization of the catalyst evidences that it is a mesoporous material thermally stable up to 873 K. At that temperature the specific surface area value is 170 m(2)/g, and a CeO(2) cubic phase is identified together with ill-defined diffraction peaks tentatively assigned to Cu-Pd clusters, suggesting that the active phase is well dispersed over the support. Infrared studies prove that methanol conversion takes place over the catalyst to a high extent yielding syngas as main product in the range 473-723 K and methane at higher temperatures. Oxygenated intermediates containing methoxy, carbonile or formiate species are not detected, which evidences that methanol conversion to methane very probably takes place according to a via-carbide mechanism.

Propane catalytic oxidation and oxy-dehydrogenation over manganese-based metal oxides.

Pure Mn oxides (Mn 3 O 4 and α-Mn 2 O 3 ) and Mn-Fe and Mn-Al mixed oxides have been prepared by (co)precipitation methods, thoroughly characterized and tested as catalysts for propane oxy-dehydrogenation to propene using He-O 2 -propane mixtures, using different flows, different contact times and different propane to oxygen molar ratios. These materials, due to their very high reactivities, allow propene production with yields not higher than 6-8 % but with productivities in excess of 3 kg C3H6 / kg CAT h and in a quite low temperature range.

INTERACTION AND FORMATION OF ETHOPROPHOS-GRAPHITIC OXIDE INTERLAYER COMPOUNDS

Abstract A study by X-Ray, FT-IR, and Gravimetric Adsorption, was made of the interlayer compounds that the pesticide ethoprophos (O-ethyl s,s-dipropylphosphorodithioate) forms with graphitic oxide, the latter taken as a model of the humic acids existing in soil.

Effect of K-doping on 2-propanol adsorption, desorption and catalytic oxidation over vanadia-titania

The adsorption, desorption and catalytic oxidation of 2-propanol has been investigated over a vanadia-titania catalyst without and with potassium doping. 70 % yields in acetone can be obtained at 443 K over vanadia-titania, with propylene and carbon oxides as the main by-products. Potassium doping causes a slight reduction in activity but a significant increase of acetone selectivity. Acetone yields exceeding 85 % are obtained at 573 K. FT-IR and TPD data show that the key adsorbed intermediates are 2-propoxide groups that give propylene by elimination of an hydroxy- group, and acetone by oxidative dehydroge-nation. K-doping is found to decrease strongly the surface acidity. Consequently, the elimination reactions giving propylene is inhibited, while acetone adsorption is also weakened, making easier its desorption as such. Both these effects favour higher acetone selectivities.

An infrared spectroscopic study of the interaction of olefins on vanadia-titania and PdCl2-vanadia-titania selective oxidation catalysts

The adsorption and oxidation of the simple olefins ethylene, propylene and n-butenes on the surface of vanadia-titania and PdCl 2 -containing vanadia titania have been investigated by FT-IR spectroscopy. Two oxidation paths have been observed on vanadia-titania, one producing species functionalized at C 2 (acetaldehyde from ethylene, acetone from propylene, methyl-ethyl-ketone and acetic acid from n-butenes), and the other giving compounds functionalized at C 1 and C 1 /C 4 (acrolein and acrylic acid from propylene, butadiene, furan and maleic anhydride from n-butenes). On PdCl 2 -containing vanadia-titania olefins are oxidized much faster to carbonylic compounds, that are also more stable with respect to overoxidation, without the intermediacy of alkoxide species. A mechanism involving the formation of Pdalkylidene species and their oxidation by V 5+ is proposed.