Studies Of Molybdenum-Based Model Catalysts Supported On Alumina Thin Films

There are prominent differences between traditional surface science underUHV conditions and catalysis in the real world, focusing on the so-called"materials gap"and "pressure gap". In order to bridge the materials gap, theordered oxide films are fabricated under UHV conditions. The molybdenumsystem on the alumina film was chosen for the model studies. On the variousmodel surfaces under UHV conditions, the important results are shown as follows:1. The compact and well-defined Al₂O₃ film was fabricated under UHVconditions. The HREELS exhibits good signals for the lattice vibrations.Aluminum has been completely oxidized from AES and XPS, and the oxidesurface is stoichiometric. This model surface can be used to simulate theextensively dehydroxylated alumina carrier in the real catalysis.2. Via thermal decomposition of Mo(CO)₆, the Mo(0)/Al₂O₃ model surfacewas fabricated under moderate conditions, whereas it is difficult to obtain themetallic molybdenum species on the alumina carrier over the real catalysts. Onthe basis of the Mo(0)/Al₂O₃ model surface, the model surfaces of molybdenumcarbides and oxides were obtained, and transformation between molybdenumcarbides and oxides was carried out under special conditions as well.3. There are two kinds of coordinated unsaturated Al³⁺ sites on the Al2O3surface. CO titration indicates that the metallic molybdenum preferentiallyoccupies the octahedrally coordinated Al3+ sites. Oxidation of the surfacemolybdenum makes more distinct the preferential occupation tendency. 4. This back donation of the d electrons is reduced from the metallicmolybdenum nanoclusters to the coordinated CO molecules. It is likely for CO tomolecularly adsorb, different from the dissociative chemisorption on the bulkmolybdenum. TDS of CO on the Mo(0)/Al2O3 model surface exhibits a desorptionsignal at ²40 K, whereas no signals at such low temperatures were detected onthe Al2O3 thin film and MoO3/Al2O3 model surface. The XPS results show that asurface species was formed when CO was exposed to the Mo(0)/Al2O3 modelsurface at liquid nitrogen temperatures, which was similar to the molybdenumsubcarbonyls. This indicates a multi-coordinated CO chemisorption was formedon the surface of the metallic molybdenum nanoclusters. The appearance of thishighly-coordinated CO chemisorption displays the difference betweennanoclusters and bulk metals, indicating an obvious nanometer size effect.