S. Lars T. Andersson

Effect of water on the catalytic oxidation of toluene over vanadium oxide catalysts

Abstract The effect of water on the catalytic oxidation of toluene over bulk V 2 O 5 and monolayer type 2 wt.-% V/TiO 2 catalysts was investigated by varying the feed composition and by IR studies. Water addition improves both the activity and the selectivity for benzoic acid. The response is fast and is suggested to be due to hydrolysis of adsorbed benzoate species at the higher concentration of surface hydroxy groups caused by the water addition or directly by water. The effects are larger for vanadium oxide in the absence of oxygen. Small irreversible improvements observed are probably caused by hydrolysis of V O V bonds increasing the vanadium oxide surface area. Low temperature deactivation by product accumulation is prevented by water addition.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over silica-supported vanadium oxides

Vanadium oxides on silica gel (Davison 952) with varying vanadium concentrations have been investigated for the activities and selectivities in the oxidation of toluene. These results were correlated with the features shown by X.r.d., ESCA, u.v.–visible and i.r. studies of adsorbed CO. The catalysts show a low activity in comparison with V/alumina. The support alone shows 60% selectivity for benzaldehyde formation. From i.r. data it is suggested that this benzaldehyde is formed on sites in association with sodium impurities. Introduction of vanadium increases the rate of reaction but the selectivity is shifted towards carbon oxides. The increased activity at low vanadium content is due to isolated four-coordinated vanadium species. U.v.–visible data show that the isolated species agglomerate at further loading to polyvanadium chains. These polymers possess an increased activity as a further route of reduction of VV to VIV is present. Five-coordinated vanadium of the same activity as the chains and producing selective oxidation products other than benzaldehyde is found with a loading of 2% V. Larger agglomerates and V2O5 crystallites are present on the 10% V catalyst. These species only give a somewhat increased activity, but with a product pattern different from crystalline V2O5 and 10% V/alumina.

Catalytic and Spectroscopic Studies of Vanadium Oxide Supported on Group IVb and Vb Metal Oxides for Oxidation of Toluene

Vanadia was deposited on TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5 supports using impregnation with either an oxalic acid solution of NH4VO3 or a solution of vanadyl acetylacetonate in ethanol. Prepared samples, with a nominal vanadia content in the range 0.5–2 monolayers, were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and were used in toluene oxidation. XRD did not reveal formation of any vanadia phase. XPS spectra showed deposited vanadium to be present mainly as V5+ on all the supports. A plot of the V:( support metal) ratio determined by XPS showed agglomeration of vanadia on TiO2, ZrO2, HfO2, and Nb2O5 to occur at loadings above 0.5 monolayer, while for Ta2O5 the dispersion seemed independent of the loading. Preparations from vanadyl acetylacetonate gave superior dispersion. Raman spectra showed bands from crystalline V2O5 on all the supports except Nb2O5. Raman and infrared bands from dispersed vanadia were present in the spectra of TiO2 (three species), ZrO2 and HfO2 (both one species) supported catalysts. No bands from vanadium-oxygen vibrational modes were seen in the spectra of Nb2O5 supported samples, but a V-OH band was observed, suggesting an amorphous structure. Some evidence was obtained for formation of amorphous VTa9O25 on Ta205. The activity for toluene oxidation increased with vanadia loading for each support, and the activity varied with respect to support at all loadings in the order TiO2 > ZrO2 > Nb2O5 > HfO2 > Ta2O5. The selectivity for formation of benzaldehyde was the highest using TiO2 and Nb2O5 supports, while for benzoic acid TiO2 was the best support.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over vanadium oxides supported on titania

Catalysts containing 0.1–10 wt% V have been prepared by impregnating TiO2(Degussa P25) by aqueous NH4VO3 and calcining at 773 K. The catalytic performance in the oxidation of toluene was investigated and correlated with the features shown by X.r.d., ESCA, u.v.–visible spectroscopy and i.r. spectroscopy of adsorbed CO. Both the activity in the oxidation of toluene and the initial selectivity for benzaldehyde increase rapidly with increasing vanadium loading up to 2 wt% V. The changes between 2 and 10 wt% V are small. Surface areas, pore structure, primary particle size and anatase/rutile ratio change with the vanadium content. Up to 2 wt% V, a small increase in the anatase/rutile ratio is obtained. In these catalysts, surface-bonded vanadium species are present, and it is suggested that they induce a reconstruction in the titania surface. Larger and opposite effects are observed for 10 wt% V, where crystalline V2O5 facilitates the anatase to rutile phase transition. Sintering of the primary particles and a large reduction in the specific surface area are also obtained. U.v.–visible spectroscopy of reduced catalysts indicated the predominance of VIV species in tetrahedral coordination. VIII species could not be observed by i.r. spectroscopy of adsorbed CO. I.r. spectra indicate the presence of V—OH groups. Breakpoints in the spectral data were obtained at ca. 1.1 wt% V corresponding to the consumption of two support OH groups per vanadium species. It is suggested that the isolated vanadium species are oxohydroxyvanadium bonded to titania through two oxygen bridges. These constitute the active site at low loadings. Two adjacent vanadium species are thought to form the reduced tetrahedral structure. New active sites present at higher loadings may be formed by the reaction between V—OH groups and vanadium precursors. The new sites possibly represent a VV/VIV redox couple: It is suggested that its presence is an important factor governing the catalytic activity.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over V2O5/Al2O3-C catalysts

Vanadium oxide catalysts based on Al2O3-C (γ-Al2O3) as support contain at least four different forms of vanadium. The distribution between these depends on the vanadium loading. The activity (per gram of vanadium) of these catalysts for toluene oxidation increases with increasing loading. The selectivity for benzene formation decreases from 2% for the support to 0% for 0.5 wt% V, while the selectivity for benzaldehyde formation first appears at this concentration and rises to 29% for 10 wt% V. It is suggested that benzene is formed at Lewis acid sites on the support, whereas benzaldehyde is formed on vanadium sites. At low loadings (0.1 and 0.2 wt% V) single vandium species with tetrahedral coordination are formed. The oxidised forms have u.v. bands at 35 500 and 42000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2200 cm–1(room temperature) and at 2190 and 2158 cm–1(133 K). At medium loadings, vanadium surface clusters with varying degrees of agglomeration are formed in addition to the other species. These are suggested to be single chain species with vanadium in tetrahedral coordination, double chain species with vanadium in square pyramidal coordination and aggregates of octahedral vanadium formed by the coupling of double chains. The oxidised forms have u.v. bands at 34 500 and 43000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2178 cm–1(room temperature) and at 2178 and 2158 cm–1(133 K). The agglomerates are more active than the isolated species and show some selectivity for benzaldehyde. Both the activity and the selectivity appear to increase with the degree of agglomeration. At high vanadium loadings (10 wt% V), surface crystallites of vanadium oxide are formed in addition to the other species. The oxidised forms have u.v. bands at 30 000 and 43 000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2181 cm–1(room temperature) and at 2181 and 2145 cm–1(133 K). These crystallites are more active and selective than other species with less agglomeration. It is suggested that the increased activity for the larger species is due to the possibility of a transition from corner to edge sharing octahedra at the release of oxygen, which increases the activity of the double-bonded oxygen.

An ESCA study of metal deposition on cracking catalysts

Abstract Equilibrium and fresh samples of FCC catalysts impregnated with V and Ni naphthenates have been analysed both by ESCA and atomic absorption spectrophototmetry. The samples were heated in air and in steam atmospheres. It was found that Ni may exist as Ni3+ or Ni2+ and the V as V5+ after the regenerator of the FCCU. It was also found that Ni was homogeneously distributed throughout the catalyst up to a concentration of at least 2% Ni. Higher loadings of Ni (3.7%) gave a surface enrichment. Further it was concluded that V is segregated in three different ways in an FCC catalyst: (i) Vanadium is deposited in the outer part of the catalyst particle during the cracking stage because of the polar nature of the V porphyrin complex. (ii) Vanadium migrates to the surface of the catalyst particles during the regeneration stage because of the low melting point of V2O5 (690°C). (iii) Vanadiun migrates from the matrix surface where it was originally deposited, to the zeolite, and reacts destructively with the zeolite [7].

Effect of metal-organic compounds on thiophene hydrodesulphurization over sulphided forms of fluoride-containing CoMo/Al2O3 catalysts

Abstract The effect of adding fluoride at various stages of preparation of CoMo/γ-Al 2 O 3 catalysts was investigated with X-ray photoelectron spectroscopy (XPS) and with a pulse reactor method for activity in hydrodesulphurization (HDS) of thiophene and deactivation by nickel and vanadium naphthenate (NiN and VN). Activity is lowered by increasing fluoride content, except for initial activity at a fluoride-to-aluminium (F/Al) ratio of 0.1. It may be correlated with a decreased surface area and dispersion of the molybdenum sulphide phase. Deactivation in HDS of thiophene strongly increases with the fluoride concentration, probably due to an increased acidity induced by the fluoride anions. XPS data suggest that fluoride is highly dispersed except at the highest concentration, i.e. F/Al=0.2. Fluorination of the catalysts has an inhibiting effect on deactivation by NiN and VN and the highest initial and final activity is observed at F/Al=0.1. XPS intensity data suggest that the dispersion of deposited metals decreases with increasing fluoride concentration and that the deposition occurs to a higher extent at the exterior of the particles than the interior under the conditions applied. Both sulphidic and oxidic forms are detected. The order of adding fluoride and the salts of cobalt and molybdenum results in minor differences in activity, except when metal naphthenates are deposited.

Catalytic oxidation of toluene over V2O5-WO3 catalysts

Abstract The catalytic performance of V2O5-WO3 catalysts in the oxidation of toluene was investigated and correlated with features shown by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and chemical analysis of vanadium valence states. At low vanadium concentrations high selectivity and activity was obtained and these catalysts consisted of highly dispersed V2O5 supported on WO3. V4+ formation upon use was highest in this region. Increased vanadium concentrations resulted in decreased dispersion with formation of less active and less reducible crystallites, and both activity and selectivity decreased towards values for V2O5. Dissolution of W6+, increasing with WO3 concentration, affects crystallization of V2O5. The (010) to (101) ratio, from XRD data, increases at the lowest addition levels, but decreases strongly at higher levels. Activity and selectivity are not simply correlated with this ratio, but overall are inversely correlated with it. Activity and selectivity correlate with the increase in the V4+-to-ΣV ratio with use in the reaction, indicating the importance of reduced phases and surface defects.

Oxidation of toluene over V2O5/Nb2O5catalysts

Abstract Nb2O5 supported vanadium oxide catalysts of theoretical monolayer coverages from 0.05 up to 5 were prepared by impregnation with vanadyl acetylacetonate in ethanol. Nb2O5 of the high temperature form [niobia(2)] and a less well-defined Nb2O5 [niobia(1)] obtained by calcining the hydrated form, were used. Phase and surface composition were characterized by X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS) studies. XPS spectra showed that vanadium was present at V5+ on all fresh samples. At very low loading, V4+ was also detected. The V/Nb atom ratio showed considerable agglomeration of vanadia to occur at loadings above approximately one monolayer. The dispersion of the vanadia phase appears to be higher over niobia (2) than over niobia (1). Crystalline V2O5 could be detected by XRD at the highest loading over niobia (1). The catalytic activity for oxidation of toluene could, with niobia (2) as support, be correlated to the V/Nb atom ratio measured by XPS. Thus, the activity was roughly proportional to the surface area of exposed vanadium oxide species. Benzaldehyde selectivity was almost constant for these catalysts. Niobia (1) as support behaved differently with a large increase in activity and a decrease in benzaldehyde selectivity at loadings above approximately one monolayer, without a corresponding increase in the V/Nb atom ratio. A lower activity of niobia bonded vanadium species than vanadia bonded ones was suggested.

Surface and bulk composition of YBa2Cu3O6+x compounds studied by XPS

Abstract YBa2Cu3O6+x compounds prepared by a formic acid route where characterized by XPS. The samples were investigated both after scraping in air and in vacuum to reveal the surface composition after partial degradation during short time air exposure and to gather data on the pure bulk state. The surface layer is characterized by the XPS data of O 1s = 530.8–531.5 eV, Y3d5/2 = 157.5 eV, Ba3d 5 2 = 779.9 eV , Ba4d 5 2 = 89.4 eV and Cu2p 3 2 = 934.4 eV with a strong satellite structure at 942.5 eV. The data indicate the presence of barium hydroxide and carbonate, copper(II) oxide, the green phase and some yttrium compound. The bulk state is characterized by XPS data of O 1s = 528.8 eV, Y3d 5 2 = 156.2 eV , Ba3d 5 2 = 778.6 eV , Ba4d 5 2 = 88.3 eV and Cu2p 3 2 = 932.6 eV without a satellite and a similar copper state as for the surface layer. There exist single states of yttrium and barium in the bulk, both copper(I) and copper(II) valence states and for oxygen possibly several components closely spaced accounting for the structurally different oxide ions.