Jan G. van Ommen

Structure and reactivity of titania-supported oxides. Part 1: vanadium oxide on titania in the sub- and super-monolayer regions

Vanadium oxide has been deposited on TiO2 (washed anatase, 10 m2g−1; Degussa P-25, 55 ±3 m2g−1; Eurotitania, 46 m2g−1) by aqueous impregnation of (NH4)2[VO(C2O4)2] and by reaction with VOCl3, VO(OR)3 (R=iBu) and VO(acac)2 in organic solvents. Single applications of the last tree reagents form not more than a monolayer of vanadium oxide VOx, a monolayer being defined as 0.10 wt.% V2O5 per m2 of surface. When less than about four monolayers of VOx are present, there is in most cases only a single TPR peak: Tmax values, which increase with V2O5 content, are almost independent of the method used but vary slightly with the support (P-25 < Eurotitania < washed anatase). The 995 cm−1 band, characteristic of VzO in V2O5, only appears when more than a monolayer of VOx is present. In the sub-monolayer region, VOx is best formulated as an oxohydroxy species bonded to two surface oxygens. As the V2O5 content is increased, layers of disordered V2O5 are formed on limited areas of the surface, but crystalline V2O5 only occurs, probably on top of the disordered V2O5, when the V2O5 content exceeds about four monolayers, and takes the form of acicular crystals exposing only planes perpendicular to the a and b axes.

The activity of supported vanadium oxide catalysts for the selective reduction of NO with ammonia

The activities of monolayer V2O5 catalysts for the selective reduction of NO with NH3 are compared with those of commercial available catalysts containing V and/or W. From steady state and pulse experiments it can be concluded that the reduction of surface sites proceeds either by NH3 + NO or by NH3 alone. The reoxidation of the reduced sites occurs by gaseous oxygen or NO. The experimental reaction stoichiometry can be explained in terms of suitable combinations of these four reactions.

Factors influencing the temperature-programmed reduction profiles of vanadium pentoxide

The temperature-programmed reduction (t.p.r.) of bulk V2O5 has been examined as part of a study of the reducibility of V2O5-containing catalysts. T.p.r. profiles have been studied as a function of flow rate, heating rate and sample weight. From experiments at different flow rates it is concluded that the order of the reduction rate in hydrogen is low or even zero. A rule of thumb has been derived to provide an easy check on possible exhaustion of hydrogen in the feed. The influence of sample weight and heating rate is explained in terms of the formation of water in the sample during reduction. The reduction of bulk V2O5 to V2O3 proceeds in several steps; intermediate species include V6O13 and VO2. The apparent activation energy of ca. 200 kJ mol–1 indicates that solid-state diffusion influences the reduction process of V2O5.

Immobilization of a layer of carbon nanofibres (CNFs) on Ni foam: A new structured catalyst support.

This work describes the preparation of new materials based on immobilizing carbon nanofibres (CNFs) on the surface of Ni foam. CNFs were catalytically synthesized by decomposition of ethene over the Ni foam. The influence of formation conditions on the morphology of the CNFs, on the mechanical stability of the CNFs?Ni-foam composite structures and on the attachment of the CNFs to the Ni foam is discussed. The surface area of the Ni?CNFs-foam composite increased with the loading of CNFs, from less than 1 m2 g?1 to 30 m2 g?1 for 50 wt.% CNFs on the foam. The layer of the CNFs was highly open with a pore volume of 1 cm3 g?1 CNFs. Stable Ni?CNFs-foam composite structures can be obtained under the conditions that the extent of corrosive metal dusting of Ni is limited, via decreasing the temperature and/or the formation time. Some metal dusting is, however, needed to form small Ni particles that allow formation of CNFs. The extent of corrosive metal dusting determines to what extent CNFs are weakly attached. The remaining CNFs, at least 80%, are remarkably strongly attached. Every single CNF is bonded to the Ni structure, probably via penetration of the CNFs into the polycrystalline Ni foam. Controlling the conditions of CNFs formation is vital in order to optimise the mechanical stability of the CNF?Ni-foam composite structures as well as the strong attachment of the CNFs to the surface of the Ni foam.

Growing a carbon nano-fiber layer on a monolith support; effect of nickel loading and growth conditions

This work describes how a new, extremely porous, hairy layer of carbon nano-fibers (CNFs) can be prepared on the surface of porous inorganic bodies, e.g. wash-coated monoliths. CNFs were prepared catalytically by methane and ethene decomposition over a Ni catalyst. The influence of the Ni particle size and growth conditions on the properties of the resulting material is reported. It turns out that the thickness of the CNF layer at the outermost surface (ca. 1 mm) as well as the diameter of the fibers increases with mean Ni particle size. The structure of this layer resembles the inverse structure of a traditional inorganic support material, combining high surface area, high porosity and low tortousity. Growing CNFs using methane leads to immediate fragmentation and doubling of the thickness of the washcoat which is independent of the amount of CNFs, forming a macro-porous composite layer of entangled alumina particles and CNFs with a typical diameter of 10?30 nm. Immediate fragmentation is due to the fact that some of the fibers are too thick for the pores in the washcoat. The total porosity decreases with the amount of CNFs whereas the surface area per gram of monolith increases. Large Ni particles are able to grow CNFs for longer times, resulting in detachment of the washcoat from the cordierite, which is caused by extensive growth of CNFs out of the washcoat. Furthermore, extended growth of CNFs inside the cordierite body causes disintegration of the monolith body when macro-pores are locally overfilled with CNFs. Methane is preferred over ethene for growing CNFs because ethene grows CNFs rapidly even on relatively large Ni particles, resulting in thick fibers up to 70 nm in the macro-porous cordierite, destroying the monolith. Controlling both the Ni particle size and Ni distribution as well as choosing the right activity of the hydrocarbon are essential to grow CNF washcoats without damaging the monolith structure.

Propane selective oxidation on alkaline earth exchanged zeolite Y: room temperature in situ IR study

The effect of zeolite Y ion-exchanged with a series of alkaline-earth cations on selective propane oxidation at room temperature was studied with in situ infrared spectroscopy. Isopropylhydroperoxide was observed as a reaction intermediate and can be decomposed into acetone and water. Contrary to previous studies, BaY was found to be active at room temperature. The reaction rate increased in the order BaY<MgY<SrY<CaY based on the rate of formation of adsorbed acetone. Surprisingly, the acetone/water ratio was found to increase with cation size, while no other products could be detected. Moreover, the acetone/isopropylhydroperoxide ratio decreased with decreasing number of Bronsted acid sites. Both observations mark the importance of Bronsted acid sites for this reaction, in addition to alkali (earth) cations. A two-step mechanism with two different active sites is proposed. Conversion of propane into isopropylhydroperoxide takes place on cations, while the decomposition into acetone occurs by Bronsted acid sites.

Influence of CO2 on the oxidative coupling of methane over a lithium promoted magnesium oxide catalyst

Li/MgO catalysts used for the oxidative coupling of methane are found to deactivate relatively rapidly in use; this deactivation can be reversed by treating the the catalyst with CO2 under the reaction conditions and the deactivation can be avoided completely if CO2 is added in low concentrations to the reaction mixture.

The influence of water on the oxygen–silver interaction and on the oxidative dehydrogenation of methanol

Experiments carried out using temperature-programmed desorption and reduction could detect no interaction between water and silver at 200 °C. However, separate experiments on the effect of water on the oxidative dehydrogenation of methanol over a silver catalyst showed that water affected the selectivity of the reaction, reducing the production of CO2. It is suggested that this change in selectivity arises from the adsorption of water on the weakly bound oxygen surface species which are responsible for the non-selective reaction to CO2. The interaction is apparently too weak to give rise to observable species in the t.p.d. and t.p.r. experiments.

The preparation and properties of lanthanum-promoted nickel-alumina catalysts: Structure of the precipitates

Precursors of La-promoted Ni-alumina catalysts have been prepared by precipitation from their nitrate solutions at pH 7 using solutions of NH4HCO3, Na2CO3 or K2CO3. The preparation was carried out either by coprecipitation from a mixed salt solution or by sequential precipitation of Al3+, La3+ and Ni2+ in succession. In the absence of promoter, the precipitate with Ni/Al ratio of 2.5 is of the pyroaurite structure and has the composition Ni5Al2(OH)14CO3.4H2O. Two types of lanthanum-containing precipitate were made in which either extra La was added (Ni/Al kept constant at 2.5) or the proportion Ni/(Al+La) was kept constant at 2.5. The majority of these precipitates were single compounds which also had the pyroaurite structure. At high La contents, the series in which La is added gives separation of the compounds La2O(CO3)2 and LaONO3 in addition to the layer structure; with the series in which the La is substituted for Al, all the samples appeared to have the pyroaurite structure, even one in which no Al was present. The sequential precipitation route yields smaller crystallites than does coprecipitation. Materials precipitated with NH4HCO3 in all cases contained NH4NO3 while those precipitated with Na2CO3 gave inclusion of NaNO3. In both cases, the presence of the nitrates causes a decrease of crystallinity of the layer compound. Potassium is not included in the precipitate in any of the samples examined. A model is presented for the structure of the lanthanum-containing precipitates.

An X-ray photoelectron spectroscopy study of the influence of hydrogen on the oxygen–silver interaction

Hydrogen treatment of a pure silver catalyst before adsorption of oxygen influences the position and form of the O(1s) peak of the X-ray photoelectron spectra. It is possible that both the formation of sub-surface OH groups and an increase in the concentration of sub-surface oxygen, found to be of importance in earlier work, are responsible for this effect. Furthermore, when oxygen adsorption is carried out with extremely high exposures and at elevated temperature, pronounced differences occur in spectra obtained at temperatures between 298 and 723 K as compared with the results of ultrahigh vacuum studies reported in the literature. These differences are again most probably caused by the presence of the sub-surface oxygen in the present study.