Vanadium Oxide Monolayer Catalysts I. Preparation, Characterization, and Thermal Stability

Vanadiumoxid Monoschichtkatalysatoren. I. Darstellung, Charakterisierung und thermische Stabilitiit Inha l t s i ibers ich 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 ZrOz als Trageroxiden, konnen Katalysatoren mit, einer ungeflhr vollstandigen monomolekularen Schicht von'Vana,din(V)-Oxid auf den Trkgeroxiden dargestellt werden, falls die Temperatur nicht zii hoch ist,. Zweiwertige Metalloxide wie Z. B. CdO und ZnO konnen schon bei niedriger Temperat,nr dreidimensionale Oberflachenvanadate bilden. Die thermische Stabilitiit. eines Monoschicht,katalysators ist verbunden mit dem Parameter zja,: dem Quotient der Ladung des Trggerkations und der Summe der Ionenradien von Tragerkation und Oxidanion. Monoschichten werden thermisch nur stabil sein, falls z/a nicht zu grol3 (aggregierter Katalysator) nnd nicht zu klein (Bildang einer ternaren Verbindung) ist,. Introduction Vanadium 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. In 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. There is a variety of possible interactions between carrier and van. <L d‘ ium oxide : a) Formation of crystallites of the active component. b) Formation of a molecular dispersion of the active component (monolayer). c) Formation of a new bulk compound. d) Diffusion of the vanadium into the carrier (solid solution). 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 component might already disappear from the surface during annealing of the catalysts. In Fig. 1 a scheme is given of the various combinations of two oxides in order of increasing interaction (or affinity) (see also [IS]). As this interaction, if any, often concerns the formation of very thin surface layers on the catalyst i t is usually difficult to study or even to detect [l’i]. Only in a few cases this problem can be solved to a greater or lesser extent by using a combination of advanced experimental techniques. This paper deals with the preparation and thermal stability of V(V) oxide “monolayers” supported on some carrier oxides. ?Tie tried to obtain thcse with maximum monolayer coverage. I n [16] i t is argued that the trend in stabilities as found by heating mechanical mixtures may also apply to the stabilities of monolayer catalysts. Thus, equimolar mixtures of V,O, (Merck, reagent grade) and some relevant oxides were prepared. Their thermal behaviour was studied by GuinierLenn6 X-ray analysis. A theoretical background of the thermal stability of monolayer catalysts is also given. Vanadium Oxide Monolayer Cat,alysts