Metal oxides of one type (M_IO) supported by a metal oxide of another type (M_IIO) in which the supported oxide is believed to be present in submonolayer to monolayer quantities comprise an important class of catalytic materials. Examples of such catalysts are (1) vanadia supported on TiO₂ or on Al₂O₃, (2) rhenia on Al₂O₃, MoO₃, WO₃, (3) chromia on Al₂O₃ and (4) WO₃ on ZrO₂. Among the many reactions catalyzed by these materials are ammoxidation of aromatics and alkyl pyridines (vanadia/Al₂O₃), oxidation of o-xylene to phthalic anhydride (vanadia/TiO₂), selective reduction of NO with NH₃ (vanadia/TiO₂), olefin metathesis (rhenia/Al₂O₃) and hydrodesulfurization (MoO₃,WO₃/Al₂O₃). There is very limited fundamental understanding of how these oxide systems realize their unique catalytic properties. The purpose of this research program is to provide a more fundamental understanding of the surface reactivity of these interesting composite materials.

Since the "active" oxide (guest) can be supported on a host oxide with high surface area, these catalysts can possess a high specific activity. However, because these guest oxides are highly dispersed, the potential exists for a strong influence of the support. The oxide system of host and guest thus can possess unique surface electronic and geometric structures and surface reactivity. Consequently, we have been working toward a better understanding of the structures, bonding, electron transfer processes and reactivity of the composite surface. Because they offer a support with well-defined surface properties, we are utilizing single crystal surfaces as models for such studies.

As a first step toward developing model surfaces for the study of supported metal oxide catalysts, we have made an extensive study of the electronic and physical structure of vanadium and vanadium oxides deposited on single crystal surfaces of titania and alumina. Specifically, we have utilized vapor deposition of vanadium, with and without ambient oxygen, to grow vanadium and vanadium oxide from submonolayer to multilayer coverages on TiO₂(110), Al₂O₃(0001) and alumina films grown on NiAl(110). The electronic properties of the deposits were studied by a variety of electronic spectroscopies, and the physical structures of the overlayers were determined directly by scanning tunneling microscopy. In addition, during this period we designed and constructed a new ultrahigh vacuum apparatus which combines XPS, UPS, AES, and mass spectrometry with facilities for TPRS and steady state kinetic measurements. The apparatus also is equipped for high pressure or aqueous dosing of the oxide surface.

Since these experiments are far from trivial, a brief description of the experimental methods employed is included. The photoemission experiments were performed in a UHV chamber at the Stanford Synchrotron Research Laboratory. Generally, the Al₂O₃ (0001)surface was cleaned by prolonged heating between 1000 and 1200 K in vacuum, followed by annealing in oxygen at 1100 K until a sharp (1x1) LEED pattern was obtained. Surface charging on this bulk alumina single crystal (an insulator) was minimized using a low energy electron gun, and all photoemission spectra were referenced to the known energies of the Al 2p and O 1s peaks of the clean Al₂O₃ surface. The TiO₂ single crystal was rendered bulk-conducting by annealing it at 1100 K for one hour. The surface stoichiometry was restored by annealing at 900 K in an oxygen pressure of 5x10^-5 torr, and restoration of the stoichiometric surface was judged by the absence of reduced titanium in the photoemission spectrum and the presence of a sharp (1x1) LEED pattern.

FIGURE 1: STM images obtained for (a) clean TiO2(110), and deposition of vanadium metal thereon; (b) 0.03 ML, (c) 0.25 ML , (d) 2.0 ML.

The growth of the vanadium and vanadium oxide on these surfaces is illustrated by the STM images in the figure above. Panel a shows the terrace-step structure of the clean TiO₂(110) (1x1) surface. The light rows correspond to the five-fold coordinated titanium cations aligned along the (001) direction; the rows of capping oxygen anions produce the darker lines. Close inspection of the image reveals oxygen defects, which appear as bright bridges between the rows of titanium cations. Some larger, subsurface defects appear as dark "holes." Basically, the surface is well ordered, and the order persists over the entire terrace up to the step edge. Deposition of a small fraction of a monolayer of vanadium at room temperature severely disrupts that structure, as shown in panel b. The very bright circular features, 10-20 A in diameter appear and are attributed to vanadium, which has reacted with the surface. Small isolated particles are formed. Further deposition of vanadium yields a higher density of vanadium particles of similar diameter. Agglomeration to form larger particles does not occur. As the vanadium loading is increased, a granular, monolayer film is formed.