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A thorough study is presented concerning the interfacial chemistry of the impregnation step involved in the preparation of molybdenum(VI) supported titania catalysts. This is based on a recently developed picture for the "titania/electrolyte solution" interface. In the frame of this work, we investigated the mode of interfacial deposition of the Mo(VI) oxo-species at the titania/electrolytic solution interface, the Mo(VI) interfacial speciation, and the structure of the deposited Mo(VI) oxo-species. Several methodologies based on potentiometric titrations, microelectrophoretic mobility, and macroscopic adsorption measurements were applied. The deposition model developed describes very well the experimental "proton-ion" linear curves and the "adsorption edges". Moreover, it was verified by laser Raman spectroscopy. At Mo(VI) solution concentration up to 3 × 10 -2 M and in the pH range 9-5, the mounted Mo(VI) is practically deposited as monomer MoO 42species in two configurations: an inner sphere mononuclear monosubstituted complex with the terminal surface oxygen atoms of titania [TiOMoO 3 ] 0.35and a surface species where the MoO 4 2ions are retained above a bridging surface hydroxyl through a hydrogen bond [Ti 2 OH • • • O-MoO 3 ] 1.57-. In both configurations, the Mo atom is situated between the surface plane and plane 1, whereas the solution oriented oxygen atoms are situated at plane 1 of the compact layer of the interface. The concentration of the [Ti 2 OH • • • O-MoO 3 ] 1.57increases with pH, while the concentration of the [TiOMoO 3 ] 0.35decreases. Thus, at pH > 8, the [Ti 2 OH • • • O-MoO 3 ] 1.57predominates, whereas at pH < 5.5 the [TiOMoO 3 ] 0.35is the most important species. In the pH range 5-4 and for the maximum initial Mo(VI) solution concentration, the contribution of the polymer species to the whole deposition process is not negligible. The deposited polymer species, Mo 7 O 24 6and HMo 7 O 24 5-, are adsorbed through electrostatic forces and located in a range extended from plane 1 up to the first layers of the stagnant-diffuse layer being close to plane 2 of the interface. The adsorption sites involve five bridging and five terminal neighboring (hydr)oxo-groups. A preferential deposition of the monomers, MoO 4 2-, with respect to the polymer ones was generally found. The above findings could prove useful for controlling the impregnation-equilibration step involved in the preparation of the molybdenum supported titania catalysts by equilibrium deposition filtration.

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The pH and temperature evolution of the molecular configuration of oxo-tungsten(VI) species deposited on TiO2 by equilibrium deposition filtration and the temperature evolution of the samples texture are studied using in situ Raman spectroscopy, DR spectroscopy, N2 adsorption–desorption measurements, and TGA. Prior to drying, WO4 2– monomers retained above a bridging surface hydroxyl of TiO2 through H bonding (with a Ti2OH···O–WO3 umbrella configuration) prevail at pH = 9. A pH decrease results in a gradual transformation to an inner sphere monosubstituted species located above terminal surface oxygen (Ti–O–WO3). At pH = 7 both species coexist, whereas at pH = 5 the latter prevails with electrostatically retained polyoxotungstes also present. Heating to 100 °C causes the same effect resulted by the pH decrease at room temperature, that is, transformation of Ti2OH···O–WO3 into Ti–O–WO3, attributed to the removal of H2O molecules upon heating, thereby liberating surface Ti atoms. Progressive heating to higher temperatures causes further removal of surface hydroxyls (confirmed also by TGA) liberating more neighbor surface Ti atoms and favoring further anchoring. This leads to formation of bisubstituted dioxo (Ti–O)2–W(O)2 sites in the range 120–250 °C and to the prevalence of trisubstituted mono-oxo (Ti–O)3-WO sites at 430 °C. Polyoxotungstates are also detected in the sample with the highest W loading. The DRS spectra indicate a charge transfer effect between the surface O and Ti atoms proceeding via Ti–O–W surface bridges. Calcination of the support causes a decrease of the pore volume and pore area in the range of pores with small width and an increase in the range of pores with large width. This effect is not affected by the presence of the well dispersed “bi-dimensional” W species inferred by the Raman study.

Nickel catalysts are the most popular for steam reforming, however, they have a number of drawbacks, such as high propensity toward coke formation and intolerance to sulfur. In an effort to improve their behavior, a series of Ni-catalysts supported on pure and La-, Ba-, (La+Ba)-and Ce-doped γ-alumina has been prepared. The doped supports and the catalysts have been extensively characterized. The catalysts performance was evaluated for steam reforming of n-hexadecane pure or doped with dibenzothiophene as surrogate for sulphur-free or commercial diesel, respectively. The undoped catalyst lost its activity after 1.5 h on stream. Doping of the support with La improved the initial catalyst activity. However, this catalyst was completely deactivated after 2 h on stream. Doping with Ba or La+Ba improved the stability of the catalysts. This improvement is attributed to the increase of the dispersion of the nickel phase, the decrease of the support acidity and the increase of Ni-phase reducibility. The best catalyst of the series doped with La+Ba proved to be sulphur tolerant and stable for more than 160 h on stream. Doping of the support with Ce also improved the catalytic performance of the corresponding catalyst, but more work is needed to explain this behavior.

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoOx)n/TiO2 catalysts. In situ Raman spectroscopy in the temperature range of 25-450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo-Mo(VI) species on TiO2 with increasing temperature as well as the influence of the supported (MoOx)n species on the photo-generation of electrons and holes of TiO2. This study concerns (MoOx)n/TiO2 samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm(2), thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.