H.-G. Lintz

Isothermal oscillations in the catalytic decomposition of nitrous oxide over Cu-ZSM-5

The catalytic decomposition of nitrous oxide was studied on a copper-exchanged ZSM-5 catalyst in the temperature range 648–723 K. Using a mixture of 1000 ppm N2O in nitrogen, isothermal oscillations both in nitrous oxide and oxygen concentrations occurred, accompanied by formation of small amounts of NO. While the addition of excess oxygen did not significantly change frequency and amplitude of the oscillations, even the presence of small amounts of NO immediately quenched the oscillations. The reacting system then remained in the ignited state at high nitrous oxide conversions.

Intrinsic kinetics of nitric oxide reduction by ammonia on a vanadia-titania catalyst

Abstract The kinetics of the selective catalytic reduction of nitric oxide by ammonia on a vanadia-titania (anatase) model catalyst have been investigated. Stationary values of the reaction rates are measured as a function of the concentrations of NO, NH3 and H2O in a recirculation system. Up to values usually present in flue gases the water content only weakly affects the rate of the main reaction, the reduction of nitric oxide to nitrogen. On the other hand the possible side reactions, the oxidation of ammonia and the formation of nitrous oxide are largely suppressed by the presence of water. The investigation of transport phenomena has shown that a strong influence of inner mass transfer resistance is expected in catalysts of technically relevant dimensions. The intrinsic kinetics can be quantified by use of a Lang-muir-Hinshelwood type rate equation.

Solid Electrolyte Potentiometry Aided Studies of Oxidic Catalysts

Abstract Solid electrolyte potentiometry (SEP) is a valuable tool in the investigation of catalytic gas–solid reactions as it is an in situ technique applicable under working conditions. It is commonly used in combined kinetic and electrochemical measurements and relates the steady state value of the oxygen activity at the catalyst to the observed reaction rates. After the description of principles and experimental procedure the main results obtained so far are discussed, namely: (a) the chemisorption of oxygen and the stability of single phases in the open system far from equilibrium; (b) model considerations influenced by or initiated through the knowledge of the oxygen activity in steady state; and (c) additional insight in the catalyst behavior in the case of partial oxidations of organic compounds at oxidic catalysts.

Oxygen transfer in mixed oxides in the case of CO oxidation over oxides containing Cu, Mo and V

The combined measurement of reaction rate and oxygen activity in the catalyst by solid electrolyte potentiometry (SEP) can be used in principle to examine the concept of oxygen transfer in mixed oxides, often used to explain synergetic effects between different solid phases. This is shown in the case of copper molybdate and a Mo-V-Ox mixed oxide, using the oxidation of CO as a test reaction. Both the pure oxidic phases and a physical mixture are used in the study. The rate measurements show that the addition of Mo-V-Ox, inactive in CO oxidation, enhances the catalytic activity of CuMoO4. On the other hand, SEP measurements indicate the existence of a gradient of oxygen activity between the two phases as the driving force of oxygen transfer from the Mo-V oxide to the copper molybdate.

Partial oxidation of isobutene to methacrolein on Bi-W/Fe-Co-Mo-K catalysts

The partial oxidation of isobutene to methacrolein and methacrylic acid was studied on oxidic catalysts containing mainly a mixture of bismuth tungstate and iron-cobalt-potassium molybdate. Catalyst shaping was done by extrusion. The kinetic measurements were performed by monitoring the gas-phase composition along the length of a fixed bed of catalyst. The reactor was treated as an isothermal plug-flow system. The network of parallel and consecutive reactions taking place in this system can be described by a simplified scheme. It is possible to predict the gas-phase composition prevailing in the reactor using five different kinetic parameters. In the temperature range 380<ν<420°C grain selectivities as high as 85% combined with yields of methacrolein and methacrylic acid about 72% at 92% conversion were obtained.

Simultaneous determination of reaction kinetics and oxygen activity during selective oxidation of an aldehyde over an oxidic multicomponent catalyst

Publisher Summary This chapter discusses an experimental setup that allows simultaneous determination of the gas phase composition and the oxygen activity of the solid catalyst under operating conditions along a tubular fixed bed reactor. Preliminary results of the partial oxidation of acrolein to acrylic acid at an oxidic catalyst illustrate the possibilities of the procedure. For this purpose, gas phase composition and the oxidation state of the catalyst, represented by its oxygen activity, during selective oxidation of acrolein over an oxidic multicomponent catalyst, were monitored both by simultaneous kinetic and potentiometric measurements. To the current knowledge, it is the first time that the oxygen activity profile along a bed of an oxidic catalyst during partial oxidation of an aldehyde could be determined by the use of solid electrolyte potentiometry (SEP). The results show that, in case of acrolein partial oxidation, there exist two domains of a nearly constant catalyst composition. This indicates that two values of the oxidation state of the catalyst are “buffered” in the system under consideration even if the composition of the oxygen acceptors (acrolein and acrylic acid) in the gas phase is dramatically changed.

Oxidation of carbon monoxide over platinum-tin(IV) oxide catalysts: an example of spillover catalysis?

A detailed study of the oxidation of carbon monoxide has been undertaken in order to validate the assumption that the synergism between platinum and tin dioxide may be due to oxygen spillover from the oxide to noble metal. Measurements of the reaction rate and the sorption capacity of the catalyst for the two reactants together with a Monte Carlo simulation show that the migration of adsorbed oxygen to the three phase boundary gas-noble metal-oxide is rate determining. However, although oxygen spillover from the oxide onto the metal can not be excluded, it is not necessary to explain the observed synergism.

Selective catalytic reduction of nitrogen oxide by use of the Ljungstroem air heater as reactor: a case study

Abstract A new NO abatement strategy proposes the use of the Ljungstroem air heater of the power plant as a chemical reactor, covering the air heater elements with a catalyst. A simple model has been developed to predict the performance of such a catalyst air heater. The model calculations are based on kinetic data obtained by laboratory scale experiments. The comparison of the calculated results with preliminary experimental data of a full-scale installed catalyst air heater proves the validity of the approach chosen. The catalyst air heater provides substantial NO reduction potential without requiring major modifications of existing power plant equipment.

Structure and Selectivity Changes in Vanadia-Titania-Denox Catalysts

Summary The rapid evaporation of thin liquid films of precursor solutions containing vanadium and titanium followed by calcination is an adequate method to prepare active and selective catalysts for the SCR process. Catalysts useful for operation at temperatures above 350 °C can be obtained by modifying this preparation method. Addition of sulfuric acid to the precursor solution leads to high surface area catalysts with a low level of vanadia species weakly interacting with the titania carrier, if the vanadia loading does not exceed 10 wt%. Catalytic tests demonstrate that sulfate stabilization increases NO conversion while N 2 O formation is suppressed. The addition of tungsten is equally beneficial for high temperature operation of the catalyst. Quantitative information about activity and selectivity of the developed catalysts is obtained by rate measurements in a gradientless system.