Michael D. Amiridis

Selective catalytic reduction of nitric oxide by hydrocarbons

Abstract There are several problems related to the application of currently available NO x control technologies (i.e. three-way catalysts and the selective catalytic reduction of NO by NH 3 ) to lean burn diesel engines, and thus a very significant market potential exists for an improved technology in this area. As a result, the selective catalytic reduction of NO by hydrocarbons derived from on board fuel has received a lot of attention in the past five years. Different forms of base and noble metal exchanged zeolites, as well as supported noble metals appear to be the most active catalysts for this family of reactions. The available literature in this area is summarized in this review, with emphasis placed on the performance of different zeolite and supported noble metal catalysts, the effect of the various exhaust gas components and the mechanistic implications.

Hydrogen production via the direct cracking of methane over silica-supported nickel catalysts

We have investigated the catalytic cracking of methane as an alternative route for the production of hydrogen from natural gas. Nickel supported on silica was found to be active for this reaction producing stoichiometric amounts of hydrogen and carbon. SEM and TEM characterization of the spent catalyst indicated that carbon deposits on the catalyst in the form of hollow cylindrical filaments, with a nickel particle located on the tip of each filament. As a result, deactivation due to carbon deposition does not occur until the carbon filaments begin to interfere with each other and the silica surface, due to the spatial limitations of the pore structure of the catalyst. The deactivated catalyst can be fully regenerated by either oxidation in air or steam gasification of the deposited carbon. Additional hydrogen is produced during the steam regeneration process.

Comparison of Au Catalysts Supported on Mesoporous Titania and Silica: Investigation of Au Particle Size Effects and Metal-Support Interactions

Au catalysts supported on mesoporous silica and titania supports were synthesized and tested for the oxidation of CO. Two approaches were used to prepare the silica-supported catalysts utilizing complexing triamine ligands which resulted in mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the key for the formation of uniformly sized 2–3 nm Au nanoparticles in the silica pores. On mesoporous titania, high gold dispersions were obtained without the need of a functional ligand. Au supported on titania exhibited a much higher activity for CO oxidation, even though the Au particle sizes were essentially identical on the titania and the wormhole silica supports. The results suggest that the presence of 2–3 nm particle size alone is not sufficient to achieve high activity in CO oxidation. Instead, the support may influence the activity through other possible ways including stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O2−), or stabilization of optimal Au structures.

The effect of solvents on the heterogeneous synthesis of flavanone over MgO

Abstract The effect of several solvents on the heterogeneous synthesis of flavanone from benzaldehyde and 2-hydroxyacetophenone over a solid MgO catalyst was examined experimentally through kinetic and FTIR spectroscopic studies. High-boiling-point solvents considered were dimethyl sulfoxide, tetralin, mesitylene, benzonitrile, and nitrobenzene. Kinetic results indicate that the presence of different solvents in this reaction system affects the rate, kinetic dependencies, and selectivities toward flavanone and 2′-hydroxychalcone. Dimethyl sulfoxide (DMSO) in particular, significantly promotes the rates of both steps used in this synthesis (i.e., the Claisen–Schmidt condensation reaction of benzaldehyde with 2-hydroxyacetophenone and the subsequent isomerization of the 2′-hydroxychalcone intermediate to flavanone). The effect is more pronounced for the second reaction. Even the presence of small amounts of DMSO in other solvents, such as benzonitrile and nitrobenzene, results in strong promotion of the flavanone synthesis scheme. Results of FTIR spectroscopic studies indicate that the interaction of MgO with DMSO results in the formation of stable surface sulfate species. The presence of these species on the catalyst surface affects the adsorption behavior of benzaldehyde and 2-hydroxyacetophenone.

Infrared spectroscopic study of sol–gel derived mixed-metal oxides

Abstract Binary, mixed-metal oxides of silicon–aluminum, silicon–cobalt, silicon–copper, silicon–iron, and silicon–nickel were produced via a sol–gel method and characterized with in situ Fourier transform infrared (FTIR) spectroscopy over the temperature range from 30°C to 350°C. Characteristic Si–O–Si IR stretches, observed at 1086 and 798 cm −1 , shifted to lower frequencies as the second metal oxide was introduced to the system, suggesting a weakening of the Si–O–Si network. In addition, new peaks observed in the 600–700 cm −1 range were assigned to silicon–oxygen–metal vibrations providing further evidence that the second metal was chemically incorporated into the gel network. Heating these mixed-metal oxides to 350°C only dehydroxylated the surface, with no other structural changes observed. This result indicated that the mixed-metal oxide structure, including silicon–oxygen–metal bonds, was formed during the synthesis and drying stages at relatively low temperatures (i.e., 90–110°C).

Kinetic simulation of heterogeneous catalytic processes: Ethane hydrogenolysis over supported group VIII metals

A kinetic model for ethane hydrogenolysis over Pt, Pd, Ir, and Co was formulated in terms of essentially two chemical parameters: the strength of bonding between atomic hydrogen and the metal surface and the strength of carbon-metal bonding between hydrocarbon fragments and the surface. These two surface bond strengths were estimated by calorimetric measurements of the heats of H{sub 2} and CO adsorption, combined with bond order conservation calculations. The results of the kinetic simulations suggest that ethane hydrogenolysis over Pt, Pd, Ir and Co takes place through irreversible C-C rupture of C{sub 2}H{sub 4} and C{sub 2}H{sub 3} surface species. Hydrogenation of monocarbon CH{sub y} fragments is kinetically insignificant. Dissociative adsorption of hydrogen is an equilibrated process, while dissociative adsorption of ethane is slow and reversible. Finally, the role of kinetic modeling in the formulation, interpretation, and generalization of experimental research in heterogeneous catalysis is discussed.

The selective catalytic reduction of NO by propylene over Pt supported on dealuminated Y zeolite

Abstract The reactivity of Pt supported on a stable dealuminated Y zeolite (Pt/DeY) for the Selective Catalytic Reduction of NO by hydrocarbons (HC-SCR) has been investigated with a monolith sample. The results show that the Pt/DeY catalyst has substantial activity for this reaction at temperatures between 200 and 300°C. Furthermore, the presence of water and sulfur dioxide, at levels similar to the ones expected in vehicle exhaust gas, does not significantly affect the performance of the catalyst, which makes it a promising candidate for further commercial development. In the same temperature range, oxygen promotes the rate of the NO reduction by assisting in the activation of the hydrocarbon. NO2 is also formed under the conditions studied as a result of the oxidation of NO. In the presence of the hydrocarbon however, it is preferentially reacting with the hydrocarbon, and reduces primarily back to NO. High selectivities were observed toward the formation of N2O, which is a primary product of the hydrocarbon-SCR reaction.

Dispersion polymerization of styrene in alcohol media: Effect of initiator concentration, solvent polarity, and temperature on the rate of polymerization

The effects of three different variables (initiator concentration, polarity of the solvent and reaction temperature) on the rate of dispersion polymerization of styrene in alcohols have been investigated. It was found that the rate of polymerization increases with the initiator (AIBN) concentration at the 0% conversion level and becomes independent of it at higher monomer conversions. More significant was the result that the rate was also found to increase with solvent polarity. This is consistent with thermodynamic equilibrium calculations which account for the partitioning behavior of monomer and solvent in both the solution and the particle phases. The results further suggest the existence of two different kinetic regions: one at low conversions, where the reaction takes place primarily in the solution phase, and one at high conversions, where the reaction takes place primarily in the particle phase.

On the mechanism of the NO reduction by propylene over supported Pt catalysts

Different mechanisms have been proposed in the literature for the selective catalytic reduction of NO by hydrocarbons (and in particular by propane and propylene) over supported Pt catalysts. This manuscript reviews the available experimental evidence in support of a scheme involving the initial partial oxidation of the hydrocarbon and the subsequent formation of a surface cyanide or isocyanate intermediate. Furthermore, we present results of reaction kinetics analyses which indicate that such a reaction scheme can describe the available kinetic data at a quantitative level.

Substitution effects in the heterogeneous catalytic synthesis of flavanones over MgO

Abstract Flavanones were synthesized via a series reaction scheme involving the Claisen–Schmidt condensation of 2′-hydroxyacetophenone and benzaldehydes to form 2′-hydroxychalcones, and the subsequent isomerization of the 2′-hydroxychalcones to flavanones. Reactions were carried out at 160 °C heterogeneously using MgO as the catalyst. Substitution effects were investigated by conducting reaction studies involving various para-substituted benzaldehydes (i.e., OH, OCH3, H, F, Cl, Br and NO2). Different solvents, including DMSO, nitrobenzene, benzonitrile and tetralin, were used. The results from this work suggest that a negatively charged ketone intermediate, similar to one previously proposed for the homogeneously base-catalyzed reaction, is also involved in the heterogeneous synthesis under these conditions.