Vanadium oxide monolayer catalysts. I. Preparation, characterization, and thermal stability

Abstract

A b s t r a c t . Vanadium oxide catalysts of the monolayer type have been prepared by means of chemisorption of vanadate(V)-anions from aqueous solutions and by chemisorption of gaseous V,O,(OH),. Using A1,0,, Cr,03, TiO,, CeO, and ZrO,, catalysts with an approximately complete monomolecular layer of vanadium(V) oxide on the carrier oxides can be prepared, if temperature is not too high. Divalent metal oxides like CdO and Zno may already form threedimensional surface vanadates at moderate temperature.The thermal stability of a monolayer catalyst is related t o the parameter z/a, i. e. the ratio of the carrier cation charge to the sum of ionic radii of carrier cation and oxide anion. Thus, monolayer catalysts will be thermally stable only under the condition that z/a is not too high (aggregated catalyst) nor too small (ternary compound formation). Vanadiumoxid Monoschichtkatalysatoren. I. Darstellung, Charakterisierung und thermische StabilitiitI n h a l t s i i b e r s i c h t . Durch Chemisorption von Vana,dat(V)-Anionen aus waBrigen Losungen, bzw. Cheniisorption von gasformigem V,O,(OH), warden Va.nadinoxidkatalysatoren des Monoschichttyps dargestellt. Mit A1,0,, Cr,O,, TiO,, CeO, nnd IntroductionVanadium oxides have been used extensively as catalysts in gas phase oxidation of hydrocarbons. I n spite of extensive studies [l-121 the interpretation of the role of vanadium in the mechanisms of catalytic oxidations is still contradictory.One of the problems is the difficulty in discriminating between surface atoms and the atoms in the interior of the oxide phase.HANKE et al.[13] suggested that more favourable conditions for studying the role of transition metal ions as active centres may be present, if the metal 26 F. ROOZEBOON, T. FRAXSEN, P. MARS, and P. J. GELLKNCS oxide can be obtained as a monomolecular layer, in which all atoms are dispersed on a carrier material. I n this case all atoms may take part in catalytic reactions. However, the active component is chemically influenced by the carrier, i. e., it has an intimate contact with the latter species without loosing its surface character. This is reflected by the ~ta~bility towards certain reactions of the monolayer vanadium(V) oxide compared to pure V,O,, such as reduction by hydrocarbons [ I l , 141 and dissolution in aqueous ammonia [14].The chemical influence of the carrier is not necessarily a disadvantage: by carrier selection the selectivity and activity may be influenced [lSJ. Thus, the importance of monolayer catalysts is based on the large surface area exposed per amount of active component and on the influence of the carrier on catalytic activity and selectivity.However, monolayer formation and monolayer stability are limited and depend strongly on the nature of the carrier and on temperature. In preparing monolayer catalysts it is essential that no ternary compounds of the type M,V,O, (which we shall call a three dimensional "salt") nor solid solutions are formed between the two oxides by interdiffusion of the cations. Otherwise the active c...

The selective reduction of NOx with NH3 over zirconia-supported vanadia catalysts

“…Table 1 The ZrOZ-supported vanadium oxide catalysts used in this work These contents were in reasonable agreement measured with these calculated from the concentrations of the original solutions used for impregnation, assuming total adsorption. The surface coverages achieved are given in the last column of Table 1; these were calculated assuming that each VO2.5 or similar species was attached directly to the support and that a surface area of 10.3A 2 was required for each [15]. All the surface coverages corresponded to less than a complete monolayer.…”

Nature and Estimation of Functional Groups on Solid Surfaces

“…Similar coordinatively unsaturated metal compounds on silica gel have also been described with manganese [511] and with vanadium [512,513]. It is assumed that vanadium(III) is linked to the surface as a tripod.…”

Deep Oxidation of Benzene on Pt/V2O5–TiO2 Catalysts

“…This result is consistent with the formation of a V-oxide monolayer on titania support when reaching about 5%wt V 2 O 5 . In fact, taken into account the unit cell dimensions of V 2 O 5 [11,12], it can be calculated that the V 2 O 5 monolayer would be reached for 1.2 mg of V 2 O 5 per m 2 of support. For our TiO 2 support (Sg = 50 m 2 g -1 , Table 1) this monolayer value corresponds to 6%wt V 2 O 5 .…”