The equivalent of 5 monolayers of molybdenum trioxide were deposited on TiO₂ using ammonium heptamolybdate (NH₄)₆Mo₇O₂₄∙4H₂O (99.9%) supplied by STREM Chemicals. Titanium dioxide, TiO₂, is Degussa P-25 (25% rutile) with a pore volume of 0.5 cm³/g and a BET surface area of 50 ± 5 m²/g. Supported catalysts are prepared by impregnating the appropriate amount of molybdenum in ammonium heptamolybdate salt following the method described by Pines et al. [9] . After impregnation, the catalyst is calcined at 773 K which enabled to convert the molybdenum compound to MoO₃ as characterized by XPS. Small amount of 2.5% by mass of rubidium with respect to Mo were added to the impregnated catalyst. Catalyst activation is achieved following in- situ reduction of the MoO₃/TiO₂ system in a flow of 40 cm³ H₂/min. was carried out at 673 K. The hydrogen gas was a 99.9% pure product of KOAC (Kuwait).

Characterization of the samples by XPS was conducted using VG Scientific ESCALAB-250Xi spectrometer. The radiation source was monochromatic of Al Kα operating at a power of 300 W (15 kV, 20 mA). UPS He(I) resonance 584 Å radiation of 21.217 eV was employed for the VB energy region measurements. Vacuum in the analysis chamber was below 7 × 10⁻⁹ mbar during all measurements. In-situ reduction was carried out in a high-pressure gas cell housed in the preparation chamber, with hydrogen flow at 120 mL/min. Binding energies were based on the carbon contamination C1s at 284.8 eV within an experimental error of ±0.2 eV.

Ion scattering measurements were performed on the same sample using He⁺ with a kinetic energy of 1 KeV.

Time on stream catalytic reactions under atmospheric hydrogen pressure was studied. The reactant was drawn from the reservoir throw HPLC pump of flow rate of 0.1 ml/min then it passed though vaporizer and eventually it passed over a fixed bed quartz reactor containing the catalyst. A continuous H₂ flow of 40 cm³/min was allowed through 500 mg of the catalyst which contains 65 mg of Mo. The reaction mixture was separated and analyzed with an on-line gas chromatograph Chemito, India 1000 equipped with a Petrocol-DH column and a flame ionization detector.

X-ray photoelectron spectroscopy XPS, UPS and Ion Scattering ISS techniques are the most appropriate surface techniques which enable to define the nature and composition of the outermost surface layers, responsible for the catalytic activity of a system. Characterization experiments were carried out in-situ at the same conditions of catalytic measurements. The assignment of the different oxidation states of Mo oxides is based on the Mo (3d_3/2, _5/2) spin-orbit components. These energies are at 235.85, 232.65 eV for MoO₃, 234.9, 231.7 eV for Mo₂O₅ and 232.3, 229.1 eV for MoO₂. Continuous sample exposure to hydrogen as a function of reduction temperature results in the formation of large Mo (0) crystallites at 873 K. This metallic state of Mo is characterized by Mo(3d) spin-orbit components at 230.85 and 227.7 eV.

Two well-defined spectral lines at 235.9 and 232.7 eV were observed (Figure 1(a)) in the case of calcined Mo salt deposited on TiO₂. These lines are assigned to the Mo (3d) spin-orbit components of MoO₃. In situ exposure of the sample to hydrogen at 673 K for 12 h shows the presence of more complex structure of the Mo (3d) energy region (Figure 1(b)). Deconvolution of this band reveals the presence of three Mo oxides states. The most intense Mo (3d_3/2,_5/2) at 232.3 and 229.1 eV is assigned to MoO₂. Lower concentration states of Mo₂O₅ and MoO₃ are also present. Most probably, the MoO₂ structure constitutes the outermost surface structure of the sample. This statement is based on the fact that the XPS of the valence band following sample reduction at 673 K (Figure 2(b)) reveals the presence of DOS structure at the Fermi level, characteristic of the conductive π electrons of MoO₂. This DOS structure (0.4 eV) at the surface of the sample is well defined in the UP spectrum (Figure 3(b)). Consequently, it is concluded that Mo₂O₅ is present in the interphase between surface MoO₂ and unreduced MoO₃ in the bulk.

In the case of the reduced sample by hydrogen at 673 K (Figure 4(b)), the oxygen 1 s energy region shows the presence of a shoulder at 531.4 eV beside the main oxide oxygen. This is attributed to the formation of Brønsted acid Mo-OH function(s) on the sample surface. The Mo-OH formation takes place in a sequent of processes in which hydrogen molecules were dissociated by the delocalized π electrons to hydrogen atoms. Bonding of the produced active hydrogen atoms to surface oxygen results in the formation of Brønsted acid Mo-OH groups. As a result, a bifunctional MoO_2−x(OH)_y /TiO₂ structure is obtained.The XPS of the Ti 2p energy region before and after hydrogen reduction does not show any changes in neither the binding energy nor the spectral lines shape (Figure 5(a) and Figure 5(b)). In fact, TiO₂ is employed in this case as a support in order to increase the surface area and provide mechanical strength to the Mo catalyst. ISS spectra of the Mo sample shows the presence of Mo, Ti and oxygen on the sample surface before and after reduction (Figure 6(a) and Figure 6(b)). This indicates that Mo does not completely cover the TiO₂ surface, This is due, most probably to the formation of needle like crystallite structure of Mo on TiO₂ as observed by Bond et al. [10] .

Hydrogenation catalytic reactions of C5 - C7 alkenes and dehydration/ hydrogenation of 2-propanol will be presented.