S.R.G. Carrazán

An FT-IR spectroscopy study of the adsorption and oxidation of propene on multiphase Bi, Mo and Co catalysts

Abstract Multiphasic BiCoMo oxide catalysts have been prepared by co-precipitation or mechanically mixing the corresponding molybdates. Characterization using X-ray diffraction and FT-IR and V-UV/diffuse reflectance spectroscopies has shown that no new phase is formed after mixing the parent molybdates, while a rather low specific surface area indicates a strong interaction between the phases. FT-IR monitoring of propene adsorption indicates a large selectivity towards acrolein formation.

Selective oxidation of isobutene to methacrolein on multiphasic molybdate-based catalysts

Abstract Synergetic effects in the selective oxidation of isobutene to methacrolein have been studied over catalysts containing several molybdate phases. Two types of catalysts have been analysed: (1) pure bismuth molybdates with Bi/Mo ratios of 2/2 and 2/3[BiMo(2/2)and BiMo(2/3)] and cobalt molybdate [Co/Mo(1/1)]; and (2) multiphasic molybdates consisting of three or four molybdates prepared by: (a) coprecipitation of cobalt and bismuth salts and calcination at high temperatures, (b) mechanically mixing in dry conditions the separate molybdates and calcining at high temperatures and (c) dispersing the separate molybdates in n-pentane and calcining at high temperatures. Synergetic effects are observed in multiphasic Bi/Mo(2/2)CoMo and BiMo(2/3) catalysts. For BiMo (2/2)CoMO catalysts the highest synergetic effects in the conversion, yield and selectivity are observed. For the catalysts before test no new phase is formed after calcination while after test X-ray measurements indicate a practically complete transformation of α-Bi2Mo3O12 into γ-Bi2MoO6 and MoO2 and traces of MoO3 and Bi2xMo1−xO3. Neither contamination between phases is observed. The synergetic effects are explained by the cooperation between separate γ-Bi2MoO6, MoO3 and (α+β)-CoMoO4 phases in contact. The cooperation between phases is more effective when MoO3 is present in a large proportion.

A laser Raman spectroscopy study of molybdenum oxide supported on alumina and titania

Abstract Molybdenum oxide supported on alumina and titania prepared by three different methods (impregnation, calcination and hydrothermal treatment of mechanical mixtures), have been studied by laser Raman spectroscopy. Different oxomolybdenum species have been detected depending on the method of preparation and on the calcination conditions. Heptamolybdate and polymolybdate species were found in the impregnated and in the uncalcined, hydrothermally treated samples supported on alumina and titania, respectively, whereas polymolybdate species were formed on the surface of alumina when the mechanical mixtures and the hydrothermally treated samples were calcined in the presence of water vapour. However, two kinds of species, polymolybdate and mono-oxomolybdenum, were found in the case of the mechanical mixtures and the hydrothermally treated samples calcined in the presence of water vapour when supported on titania.

Surface dispersion of molybdena supported on silica, alumina and titania

Surface species formed upon impregnation of silica, alumina and titania with aqueous solutions of ammonium molybdate and calcination at 770 K have been studied by X-ray diffraction, specific surface area assessment, scanning electron microscopy, laser Raman spectroscopy and temperature-programmed reduction. Dispersion decreases in the order TiO2> Al2O3SiO2. Polymolybdate species are formed for low loadings, that lead to bulk MoO3 for loadings of 0.4 monolayer on SiO2, but persist for higher loadings on titania and alumina. On this last support, isolated molybdate species have been also found.

The effect of the preparation method on the nature and dispersion of surface species formed upon reaction of molybdenum trioxide with alumina and titania

Titania- and alumina-supported molybdenum-containing systems have been prepared from mechanical mixtures of the support and molybdenum trioxide. The systems have been characterized by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy, and Fourier transform-infrared monitoring of pyridine adsorption has been used to determine surface acidity. Dispersion of the supported phase, and its transformation to well-dispersed species has been attained by mechanical grinding and calcination in air, or by hydrothermal treatment of such mixtures and calcination. In the latter case, a higher dispersion is reached, despite the shorter time of calcination. Also hydrothermally treated samples show a larger concentration of surface Brönsted acid sites.

Dispersion and reactivity of molybdena on the surface of alumina

Several series of molybdenum oxide catalysts supported on γ-Al2O3 prepared by a mechanical mixture of both oxides have been studied by X-ray diffraction and Laser Raman spectroscopy. The formation of bulk molybdena is observed after monolayer coverage has been reached (0.71 theoretical monolayers). Calcination at 770K in wet oxygen favours the dispersion of molybdenum species leading to polymolybdate formation, while under dry oxygen a lower degree of dispersion is achieved, and the polymolybdate species are not formed. The formation of molybdenum species on the γ-Al2O3 surface is also studied as a function of calcination time.

A FTIR assessment of surface acidity and dispersion of surface species in titania and alumina-supported molybdena

Abstract Adsorption of pyridine on the MoO3/TiO2 and MoO3/Al2O3 systems has been studied by FTIR spectroscopy, in order to identify surface acid sites existing in samples with different molybdena loadings. The results show that both Lewis and Bronsted surface acid sites exist, whatever the molybdena loading. The percentage of Bronsted sites is larger for loadings below the theoretical monolayer, and should correspond to bidimensional molybdenum oxides species, while for loadings above the monolayer these sites are associated with bulk MoO3.

A laser Raman Spectroscopy study of surface species existing in MoO3/Al2O3 catalysts.

Abstract Laser Raman Spectra of molybdena supported at different concentration on the surface of y-A12O3 -prepared by both impregnation and physical mixtures- have been recorded. Formation of polymolybdate species, depending on Mo loading and time and temperature of calcination, has been detected in both series of samples.

A surface dispersion study of silica-supported molybdena

Surface species formed on MoO3/SiO2 catalysts prepared by manually grinding mixtures of both oxides in the presence of water vapour and with molybdenum loadings between 2% and 12% were studied by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy. Polymolybdate species have been identified for molybdenum loadings below 4%, these species condensing to form MoO3-like phases and orthorhombic MoO3, as the weight loading increases.

Surface vanadia species in V2O5-TiO2 systems prepared by mechanically mixing: A Fourier transform infrared spectroscopy study

Abstract Surface vanadium-containing species supported on titania (97% rutile) prepared by manually grinding both oxides and calcined between 773 and 973 K in the presence of water vapour have been studied by Fourier transform infrared spectroscopy. The application of band-shape analysis to the overlapping and weak bands using a computer programme has provided information about the surface vanadium species existing in these samples. Up to four different species have been identified in samples calcined above 923 K.