C.R.F. Lund

A study of the nickel-titanium oxide interaction

Previous studies have demonstrated that, when nickel supported on titanium oxide is reduced in hydrogen at 450 °C and higher, the system exhibits SMSI properties. We have employed several complementary experimental approaches in an attempt to gain an insight into the intimate details surrounding the nickel-titanium oxide interaction. High resolution transmission electron microscopy was used to examine the changes in morphology of nickel particles following reduction at increasing temperatures. In situ ferromagnetic resonance studies have provided characterization of the state of the nickel as a function of reduction temperature. A geometrically designed catalyst in combination with scanning Auger surface analysis was used to probe transport phenomena involving nickel and titanium oxide during treatment in hydrogen. The combined results of these studies have enabled us to develop a model which involves the migration of titanium-oxygen moieties onto the surface of the nickel particles during reduction in hydrogen. This decoration model provides a mechanism whereby SMSI properties are observed.

Hydrogen storage in aligned carbon nanotubes

Aligned carbon nanotubes (CNTs) with diameters of 50–100 nm, synthesized by plasma-assisted hot filament chemical vapor deposition, were employed for hydrogen adsorption experiments in their as-prepared and pretreated states. Quadruple mass spectroscopy and thermogravimetric analysis show a hydrogen storage capacity of 5–7 wt% was achieved reproducibly at room temperature under modest pressure (10 atm) for the as-prepared samples. Pretreatments, which include heating the samples to 300 °C and removing of the catalyst tips, can increase the hydrogen storage capacity up to 13 wt% and decrease the pressure required for storage. The weight gains were measured after the samples moved out of the hydrogen environment. The release of the adsorbed hydrogen can be achieved by heating the samples up to 300 °C.

Strong oxide-oxide interactions in silica-supported Fe3O4. III. Water-induced migration of silica on geometrically designed catalysts

Abstract Geometrically designed catalysts were prepared by evaporating silica onto part of the surface of an oxidized iron foil. These model catalysts allowed scanning Auger electron microscopy to be used to study the mobility of silica in different gas environments at ca. 670 K and atmospheric pressure. For the iron oxide/silica model catalyst, silica migration was observed upon treatment in H 2 O CO or H 2 O H 2 gas mixtures, while significant migration was not observed upon treatment in CO 2 CO or O2. In particular, that portion of the iron oxide surface, initially free of Si, became covered by Si after treatment in H 2 O CO or H 2 O H 2 , while it remained essentially free of Si after treatment in CO 2 CO or O2. Preliminary investigation of a similarly prepared Pt SiO 2 catalyst indicated that the phenomenon of SiO2 mobility at ca. 660 K and atmospheric pressure may be of importance for other silica-supported catalyst systems.

Use of a membrane reactor to improve selectivity to intermediate products in consecutive catalytic reactions

Abstract Through modeling it has been shown that a concentric-tube catalytic membrane reactor can be used to increase the selectivity for the intermediate products of a consecutive reaction scheme. The reactants are fed to the tube-side of the reactor where the catalyst is also located. The wall of the tube is permeable, allowing the intermediate products to pass through to the annular space instead of undergoing further reaction. The annular space is swept by an inert gas flow and contains no catalyst. Both permselective and non-permselective membranes have been considered in both co-current and counter-current flow regimes. In contrast to most catalytic membrane reactor applications where reactions are reversible and thermodynamically limited, in the present study the reactions considered are irreversible and are under kinetic control.

Further studies of the formation of filamentous carbon from the interaction of supported iron particles with acetylene

Abstract A combination of controlled atmosphere electron microscopy and Mossbauer spectroscopy has been used to investigate the characteristics of supported α-iron and γ-iron particles during the formation of carbon filaments via decomposition of acetylene. γ-iron was found to exhibit a higher intrinsic activity than α-iron for this reaction when the metal was supported on graphite. In both systems, however, catalytic action decreased significantly at temperatures in excess of 700°C. Major changes were observed in the catalytic behavior of the metal particles when they were supported on silica. The rate of formation of carbon filaments from the α-iron/silica system showed a uniform increase up to 900°C. Mossbauer spectroscopy analysis of similarly treated samples revealed that under these conditions α-iron was the only metallic phase present, even though experiments were conducted through a temperature region where the transformation of α-iron to γ-iron can occur, suggesting that silica stabilizes the α-form of iron. In contrast, the catalytic activity displayed by γ-iron particles supported on silica was considerably reduced over that found for the corresponding graphite supported system. The results of this study are discussed in terms of some of the factors controlling the growth characteristics of filamentous carbon.

Magnetite surface area titration using nitric oxide

Abstract The adsorption of nitric oxide on unsupported Fe 3 O 4 has been investigated. Consistent with previous studies, the rate of adsorption follows Elovich kinetics, and the adsorption isotherm obeys the Freundlich equation over the range from 0.133 to 27 kPa at 273 K. However, the time of evacuation of the sample at 650 K prior to NO adsorption is shown to be critical for obtaining reproducible results. Specifically, if the evacuation is carried out for 1 hr, then consistent values of the NO uptake are subsequently obtained. Furthermore, this NO uptake normalized by the nitrogen BET monolayer capacity gives an estimate of the surface cation density that agrees with model calculations based on various low index faces of magnetite. On the other hand, long (ca. 25 hr) evacuation of the sample leads to a reduction of the surface oxidation state resulting in additional NO uptake and reduction of NO to primarily N 2 O.

Possible role of nitrite/nitrate redox cycles in N2O decomposition and light-off over Fe-ZSM-5

Surface nitrite/nitrate redox cycles were proposed to explain light-off behavior that was observed during the decomposition of N2O over Fe-ZSM-5. Further study has demonstrated that while the nitrite/nitrate model can explain the original observations as an isothermal, mechanistic phenomenon, the light-off behavior is thermal, and not a mechanistic effect. Nonetheless, a pathway involving nitrite/nitrate redox cycles appears to be more consistent with experimental observation than the simple two-step pathway involving cation redox cycles. In particular, the nitrite/nitrate pathway can explain the effect of added NO upon the reaction kinetics and the reported isotopic product composition when unlabeled N2O reacts over an oxygen-labeled catalyst. Further, a nitrite/nitrate pathway is consistent with the steady-state kinetics as well as published thermal desorption and infrared spectroscopic results.

Hydrodenitrogenation-selective catalysts I. Fe promoted Mo/W sulfides

This paper describes the development of a new catalyst system which gives an unusual combination of high hydrodenitrogenation and low hydrodesulfurization. This has been observed in tests with a highly aromatic distillate feed. The catalyst is an unsupported iron-promoted molybdenum or tungsten sulfide formed from thermal decomposition of bis(diethylenetriamine) iron thiomolybdate or thiotungstate. The results from a brief accelerated aging experiment have shown that this bulk sulfide system is thermally stable. Characterization of the Fe-Mo catalyst has indicated that it consists of a single sulfide phase which during activity testing partially transforms into an iron sulfide mixed with an MoS[sub 2]-like phase. 30 refs., 8 figs., 6 tabs.

Membrane reactors for catalytic series and series-parallel reactions

Abstract The use of membrane reactors to improve the yield of intermediate products of series and series-parallel heterogeneous catalytic reactions has been investigated. For series reactions the most critical dimensionless parameters related to improving the yield are a Damkohler-Peclet number product (rate of reaction relative to rate of membrane permeation) and the permeability of the intermediate product relative to the reactant. The relative inert gas sweep rate, the pressure differential between the reactant and sweep zones, and the rate of the desired reaction relative to the undesired reaction also affect performance. When series-parallel reactions are considered the relative permeabilities of the reactants are also important in addition to the critical parameters identified for series reactions. For both classes of reactions, mathematical models have been used to identify ranges of these parameters where a membrane reactor can substantially increase the yield of intermediate products compared to a conventional packed bed reactor.

A model for the catalytic growth of carbon filaments

Abstract A two-dimensional model of the growth of filamentous carbon using an iron catalyst is presented. A single catalyst particle in the shape of a pear, producing filament diameters in the range from 30 to 100 nm is considered. The steady-state mass balance on the carbon within the particle is solved numerically. The model predicts the hollow structure of carbon filaments accurately and also indicates a strong dependence of growth rate on diameter. These findings are consistent with previous one-dimensional models and experimental observations. This two-dimensional model refines previous one-dimensional models in that it accounts for the variation in diffusion path length with filament radius, and provides predictive capability both with respect to filament inner radius and growth rate. However, the results are not significantly better than those obtained via one-dimensional approximations, and they require significantly more computational effort.