M. de Boer

Effect of the reduction treatment on the structure and reactivity of silica-supported copper particles

Abstract Silica-supported copper particles of high thermostability have been subjected to oxidation-reduction treatments after which the metal particle size, the surface structure, and the catalytic hydrogenolysis of methyl acetate were investigated. The metal particle size was assessed from the dissociative adsorption of nitrous oxide, X-ray line broadening, and transmission electron microscopy. The surface structure of the copper particles was derived from infrared spectra of adsorbed carbon monoxide. The hydrogenolysis of methyl acetate was used as a structure-sensitive test reaction to illustrate the effect of the surface structure on the activity of the catalyst. The copper particle size is not affected by reduction treatments up to 873 K, whereas the surface structure of the copper particles and thereby the oxygen uptake during dissociative adsorption of nitrous oxide and the activity of the catalyst in the hydrogenolysis of methyl acetate strongly depend upon the temperature and duration of the reduction treatment. Without a change of the copper particle size, prolonged reduction of the catalyst results in more densely packed copper surfaces that are more susceptible to penetration of oxygen during passivation with nitrous oxide and less active in the hydrogenolysis of methyl acetate. The rearrangement of the surface structure of the copper particles is reversible upon repeated oxidation-reduction cycles.

Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Selective oxidation of ammonia to nitrogen over SiO2-supported MoO3 catalysts

Abstract The (dis)similarities between the Selective Catalytic Reduction (SCR) of NO and the Selective Catalytic Oxidation (SCO) of NH3 are briefly reviewed. Since the SCO reaction acts counterproductively in the SCR reaction the two processes are related. A series of V2O5, MoO3, and WO3 catalysts on various supports have been tested in the SCO reaction. It appears that an acid metal oxide in combination with an acid support gives the most active catalysts. The main byproduct is NO; minor amounts of N2O are formed. NO formation occurs in the temperature regime where the reduction of oxide lattice is faster than the recombination of two nitrogen atoms. High partial pressures of NH3 give rise to elevated selectivity to N2. This can be understood by assuming that the recombination probability of two nitrogen atoms becomes higher. A reaction mechanism, entitled the ‘internal SCR mechanism’ is proposed to explain the formation of N2O.

Molybdena on silica catalysts: selective catalytic oxidation of ammonia to nitrogen over MoO3 on siO2 catalysts

Abstract MoO3 on SiO2 catalysts are suitable for the selective oxidation of ammonia into nitrogen (N2) and water. The catalysts were prepared by homogeneous deposition precipitation using electrochemically reduced precursors. The experiments were carried out at atmospheric pressure and at temperatures ranging from 200 to 400 °C. Nitrogen and water yields of up to 100% at temperatures over 375 °C were obtained when ammonia and excess oxygen in helium were passed over a MoO3 on SiO2 catalyst. Catalysts containing less than 15 wt.% MoO3 gave rise to no conversion of ammonia. The presence of small amounts of lead in the catalyst appeared to influence the conversion of NH3 in a positive way. However, the presence of nitric oxide in the feed did not influence either the conversion of ammonia or the selectivity to nitrogen. The nitric oxide concentration remained constant during the conversion of ammonia. The results indicate that species reducible by hydrogen at 540 °C are related to the activity of the catalysts.

The cobalt-molybdenum interaction in CoMo/SiO2 catalysts: A CO-oxidation study

Abstract The interaction among cobalt, molybdenum and silica in CoMo/SiO 2 catalysts has been investigated in a series of catalysts with the same overall composition and varying interaction among the components. XRD and DRIFTS give a rough indication of the phases present, and the formation of CoMoO 4 can be easily monitored. The formation of CoMoO 4 is observed in most of the catalysts due to the poor interaction between molybdenum oxide and silica. The presence of other minor, non-crystalline phases must be attested by other methods, such as, TPR and catalytic oxidation of carbon monoxide. The presence of “free” cobalt can be demonstrated by a low temperature peak in TPR. The catalytic oxidation of CO may function as a useful indicator for the interaction between cobalt and molybdenum, because the oxidation activity of cobalt species incorporated in a molybdate lattice (i.e. CoMoO 4 ) is considerably less than cobalt oxide.

Preparation of highly loaded vanadium oxide―silica catalysts

Abstract A technique for the application of vanadium oxide onto silica supports starting from trivalent vanadium precursors is described. At loadings up to 30 wt.%, transmission electron microscopy and X-ray diffraction show the presence of highly dispersed vanadium oxide particles in calcined samples, which is very unusual for silica-supported vanadium oxide. Combined with recorded precipitation curves and temperature-programmed reduction profiles, these results indicate a good interaction between the silica support and the vanadium oxide precursor. The kinetic parameters of the catalytic oxidation of carbon monoxide are significantly different from those of both bulk V 2 O 5 and impregnated silica-supported vanadium oxide on the one hand and from TiO 2 - and Al 2 O 3 -supported vanadium oxide on the other. X-ray photoelectron spectroscopic and IR data indicate that a considerable concentration of vanadium (IV) remains after calcination. Selectivity profiles of this catalyst in the selective catalytic reduction of nitric oxide with ammonia proved to be significantly different from those of both TiO 2 - and Al 2 O 3 -supported vanadium oxide and bulk V 2 O 5 . The typical selectivity profiles, the deviating activity in the carbon monoxide oxidation and the spectroscopic results for these catalysts all indicate a strong interaction between the active vanadium oxide and the silica support.

Selective Fischer-Tropsch wax hydrocracking : opportunity for improvement of overall gas-to-liquids processing

Abstract Fischer-Tropsch (F-T) based Gas-to-Liquids (GTL) processing is recognized as an industrially proven and economically competitive route to high quality diesel. Furthermore, it is generally accepted that for this purpose, GTL processing is most effective when comprising an F-T synthesis driven to wax production, followed by hydrocracking to produce middle-distillate products. Applying a CoMo/SiO 2 -Al 2 O 3 catalyst, optimised for hydrocracking crude oil refinery feedstocks in a sulphur-containing environment, to the processing of a linear paraffin F-T wax model compound, n-tetradecane, shows that a significant opportunity exists for utilisation of base metal catalyst, having the advantage of producing less branched hydrocracking products, i.e. high cetane number diesel via a hydrogenolytic cracking mechanism. A drawback of such a catalyst, if applied in non-sulphided form and in a non-sulphur containing environment, is the comparably high yield of light gases, in particular methane. It is shown, and proved by a simple kinetic model, that methane is formed via ‘methanolysis’, i.e. successive hydrogenolytic demethanisation reaction of the feed compounds, presumably of islands of metallic cobalt on the catalyst.

Chapter 6 Morphology and nanometric characterization of V2O5/TiO2 (Eurocat) catalysts

Abstract Three V2O5/TiO2 samples were investigated by members of the ‘oxide group’ of the Eurocat project by means of electron microscopic techniques. The bare support consisted of well crystallized TiO2 particles. No crystalline V2O5 could be observed in the 1% V2O5/TiO2 (EL10V1) catalyst, indicating a good dispersion of the active phase. The 8% V2O5/TiO2 (EL10V8) catalyst exhibited substantial inhomogeneities, viz. enrichment of the vanadium in parts of the external edge of the TiO2 crystallites, and unsupported V2O5 platelets. Prolonged exposure of the samples to the electron beam caused damage to the oxide lattice, which transformed into a suboxide.

Application of acoustophoresis in a study on solute-support interactions for the preparation of supported cobalt-molybdenum catalysts

Abstract Acoustophoresis has been used to investigate the interaction between suspended mineral oxides (SiO 2 and Al 2 O 3 ) and aqueous solutions of cobalt (II) chloride, molybdenum (III) chloride and ammonium heptamolybdate (AHM) as a function of the pH. The measured electrokinetic sonic amplitude (ESA) is directly related to the ζ-potential and gives information on the iso-electric point of the mineral oxides, that are used as supports for catalysts. The interaction between the solutes and the suspended mineral oxides is an important parameter for the preparation of supported catalysts, which can be studied perfectly by means of acoustophoresis. The specific adsorption of cobalt and molybdenum on silica and alumina has been considered.

Selective oxidation of ammonia to nitrogen over silica supported molybdena catalysts: A structure-selectivity relationship

Abstract The selective catalytic oxidation of NH3 to N2 over unsupported and silica-supported MoO3 catalysts has been investigated. The defect structure of the catalysts greatly affects the selectivity towards N2O. The performance of the silica supported catalysts is controlled by the thermal pretreatment and the structure, which is installed by the preparation procedure. Water dramatically decreases the selectivity to N2 of MoO3–on-SiO2 catalysts with low loadings. The selectivity to N2 highly depends on the ability of the catalyst to decompose released N2O, which proceeds on oxygen vacancies.