Slt Andersson

Characterisation of Silica-Titania Mixed Oxides

Abstract A series of coprecipitated silica-titanias containing between 0 and I00 mol% titania were characterized by various methods. These materials are often used as supports for catalysts in the reduction of NO x and are shown to be micro-, meso-, and macroporous. All textural quantities decrease with the addition of TiO 2 to SiO 2 . The macroporosity, of vital importance in the highly diffusion-controlled NO x reduction, reaches a maximum value at 50 mol% TiO 2 . X-ray diffraction, FT-IR, and XPS studies showed that the materials, calcined at 723 K, consisted of a SiO 2 -TiO 2 glass phase, an amorphous silica phase, and an anatase phase. At I0 mol% TiO 2 , rutile was observed, by XRD, in addition to anatase. XPS data indicate the presence of a TiO 2 phase in all Ti-containing samples, a SiO 2 phase at and below 50 mol% TiO 2 , and a SiO 2 -TiO 2 phase at and above 75 mol% TiO 2 . The quantitative XPS analysis indicates a heterogeneous distribution of phases with an increased surface concentration of Si phases. Similar results were obtained by the FT-IR studies, which additionally indicate the presence of surface free silanol groups in all samples and detect tetrahedrally substituted Ti 4+ in the SiO 2 phase.

Surface characterization and reactivity in ammoxidation reactions of vanadium antimonate catalysts

Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidation to benzonitrile and acrylonitrile, respectively, and were characterized by X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Activity data for toluene ammoxidation suggest that excess antimony with respect to the stoichiometric amount required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidation, on the other hand, due to a higher reaction temperature with respect to toluene (500°C vs. 370°C), free vanadia on the surface of the catalyst has a negative influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidation, in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidation mechanisms are interpreted to be primarily related to the different reaction temperatures.

Ammoxidation of toluene by YBa2Cu3O6+x and copper oxides: Activity and XPS studies

The catalytic activities of YBa2Cu3O6+x and copper oxides have been studied in the ammoxidation of toluene. At low oxygen pressures, over YBa2Cu3O6+x selective ammoxidation to benzonitrile occurs, whereas at high pressures, only total combustion of toluene to carbon oxides is observed. X-ray photoelectron spectroscopy (XPS) analysis of YBa2Cu3O6+x catalysts indicates predominance of Cu(I) states under partial ammoxidation conditions, while under total oxidation conditions, Cu(II) states are predominant. Comparison with the catalytic performance of copper oxide catalysts shows that differences do exist. The catalysis by YBa2Cu3O6+x is greatly influenced by the oxygen content of the bulk material despite its surface composition.

Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase

The kinetics of the direct synthesis of acrylonitrile from propane on V-Sb-Al-(W) mixed oxides indicate that acrylonitrile (ACN) forms by two parallel pathways, one directly from propane and the second, which is the prevalent path, through the intermediate formation of propylene (C3=). The limiting factor in the formation of ACN is the relative slowness of the step of allylic oxidation to ACN of the intermediate C3=, and the higher rate of C3= oxidation to carbon oxides as compared to that of ACN to COx. The step of C3= oxidation to ACN is controlled by the surface availability of NH3 which, in turn, depends considerably on the side reaction of NH3 oxidation to N2. The catalytic behavior of different modified V-Sb-(Al)-O systems and their characterization by X-ray diffraction analysis and Raman, Infrared and X-ray Photoelectron spectroscopies indicate that i) a reduction of both V and Sb occurs during the catalytic reaction, ii) the presence of vanadium not stabilized in the rutile-like phase is responsible for the side reaction of NH3 oxidation and lowering of the selectivity, iii) alumina reacts with antimony forming an AlSbO4 rutile phase which could be epitaxially intergrown or in solid solution with the VSbO4/Sb2O4 system, which, in turn, limits the presence of not stabilized (unselective) vanadium species, and iv) antimony oxide supported on alumina is also selective in propane ammoxidation, but forming acetonitrile as the main product. The doping with vanadium of this sample increases slightly the activity, but especially gives rise to the formation of acrylonitrile instead of acetonitrile.

Kinetics of carbon monoxide oxidation over YBaCuO perovskites

Abstract The kinetics of carbon monoxide oxidation over YBa 2 Cu 3 O 6+x and PrBa 2 Cu 3 O 6+x catalysts was investigated in the temperature range 160 to 200°C and a single rate expression for all catalysts was derived. A reaction mechanism consistent with the kinetics is proposed: it involves the formation of carbon dioxide via two routes. In these, carbon monoxide reacts with either mobile lattice oxygen or with adsorbed oxygen species to give carbon dioxide. Oxygen-deficient samples are more active than oxygen-rich samples due to higher concentrations of oxygen surface vacancies and defects. The presence of water significantly diminishes activity reversibly, probably as a result of competitive adsorption with carbon monoxide.

Oxidation of methane over Pt−Cr(Mo,W)/Al2O3 catalysts

Catalytic combustion of methane was carried out using alumina supported chromium or molybdena or tungsten catalysts as well as 0.3 wt.% Pt−Cr(Mo,W) catalysts. The catalysts contained either low (2 wt.%) or high (20 wt. %) concentration of metals from the 6th group. Catalysts with chromium alone showed higher methane conversion in lowere temperatures when compared to that of molybdena and tungsten catalysts. Addition of 0.3 wt.% Pt resulted in considerable increase in methane conversion on all examined catalysts. Platinum-chromium catalysts were found the most active and allowed full methane conversion at 600°C. Oxidation rates as well as selectivity in methane oxidation to carbon dioxide were also the highest for these catalysts.

Oxidation of carbon monoxide over cobalt and aluminium substituted YBaCuO perovskites

CO oxidation over YBa2Cu3−δMeδO6+x, where Me is Co or Al, has been studied. The activity for CO oxidation decreases with increasing degree of substitution in the B site, in the case of substitution both with a more active element (Co) and with an inactive element (Al). The active site is believed to consist of -Cu-()-Cu-, where () can be an oxygen species O or an oxygen vacancy []. Dilution of this site with less active-Cu-()-Me-results in a lower activity. Nitrogen-annealed samples show a considerably higher activity than oxygen-annealed samples. Oxidation-reduction experiments suggest that bulk influences such as oxygen diffusion and filling of vacancies also affect activity.