Catalytic Hydrogenation Reactions on Molybdenum Oxide

Partially reduced molybdenum trioxide deposited on titania under hydrogen at 673 K for 12 h ena- bled to convert several surface atomic layers to the catalytically active bifunctional (metal-acid) MoO2-x(OH)y/TiO2 (MoTi) structure. The formed metallic function is the result of π bonding between adjacent Mo-Mo atoms placed along the C-axis of the rutile structure of MoO2. Delocalization of these π electrons produces a wire like atomic metal. This resembles in a way, the small Pt particles deposited on a support. Moreover, dissociated hydrogen atoms are bonded to sample surface oxygen to produce Brønsted acid Mo-OH function(s). These metal-acidic properties have been tested for several catalytic reactions requiring one or bothcatalytic functions. In this order, 2-propanol species could be considered as a model test of the acidic function via dehydration of the molecule to propene, while hydrogenation of the produced propene to propane is performed by the metallic function. Moreover, hydrogenation of 2-propanol to acetone, requires relatively strong metallic function. In this order, addition of small amount of alkali metal like rubidium will suppress the acidic function in MoO2-x(OH)y/TiO2 and enhance the metallic function strength. The performance of the metallic function alone in this case will be evaluated. Titanium dioxide is employed in this catalytic system as a support. It does not have any catalytic effect. Association of XPS-UPS, ISS sur-face techniques with catalytic performances of this catalytic MoTi system will be presented.

Catalytic Active Sites in Molybdenum Based Catalysts

In situ metal, acid and metal-acid (bifunctional) catalytic active functions were prepared following partial reduction by hydrogen of MoO3 deposited on TiO2 at temperatures between 623 K and 673 K. The bifunctional structure is obtained following the reduction of MoO3 to MoO2. The metallic properties of MoO2 are attributed to the delocalized p electrons above the Mo atoms place along the C-axis of the deformed rutile structure of this phase and observed as a density of states at the Fermi level. Hydrogen dissociation by this metallic function and bonding of the produced H atoms to surface oxygen atoms results in the formation of Bronsted acid Mo-OH function(s). Accordingly, a bifunctional (metal-acid) MoO2-x(OH)y structure is formed on the TiO2 support. The bifunctional properties enabled to perform isomerization reactions of light naphtha hydrocarbons into branched species of higher octane number. This catalyst is proposed as a possible replacement of the commercially used Pt deposited on chlorinated alumina catalysts in which toxic chlorine is employed and benzene is produced as a byproduct of n-hexane isomerization. The acid function in this bifunctional Mo system is quenched following the addition of controlled amount of sodium. The presence of only the metallic function in this modified NaMoTi system is monitored via the hydrogenation of olefins and enabled to define the bifunctional mechanism of the hydrocarbon isomerization process performed by MoO2-x(OH)y structure.