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Transient kinetic experiments were used to investigate the microkinetic model of ammonia decomposition over iron, cobalt, and a bimetallic preparation of the two. Pulse response experiments of ammonia were performed using the temporal analysis of products (TAP) technique, and the Y-Procedure analytical method was used to calculate the time dependence of the transformation rates of NH3, H2, and N2. From this, a dynamic accumulation of surface species could be determined, and a region of coherent time behavior was identified for H- and N-storage on the surface. We found that the materials regulate surface species differently, with iron supporting hydrogenated species and materials with cobalt supporting dehydrogenation. These observations, together with the coherent time behavior, motivated a reduction of the kinetic model. The first reduction was based on the observation of hydrogen release behavior (both fast and slow processes in the case of cobalt-containing materials) from the surface, the second reduction was based on analysis of the H/N surface accumulation indicating negligible (quasi-steady-state species) versus dominant surface species. Simplified models are proposed such that each step can be verified by gas phase data. Together with the dynamic accumulation data, the rate coefficients of each reaction step were presented as a function of surface N species. This provides a unique window for comparing the intrinsic behaviors of materials as they regulate surface species and control chemical reactions. Through the example of ammonia decomposition, we present the observation of accumulation dynamics as a new method for comparing fundamental chemicokinetic behavior of complex catalytic surfaces.

This paper discusses on different kinetic devices and strategies to ensure concentration uniformity within the catalyst zone. For the differential PFR and thin-zone TAP reactor (TZTR), in which uniformity is achieved by making the active zone thin, the generated nonuniformity was quantified and related to the type of transport. It was shown for the TZTR that diffusion mixes up the nonuniformity created by reaction and allows maintaining small nonuniformity (15−20%) in a wide domain of conversions (up to 80%). Contrary to the TZTR, in the differential PFR, the convectional transport does not mix nonuniformity so that its estimate is equal to the conversion, which limits this reactor to the operation at low conversions (e.g., below 20%). This comparison demonstrates a remarkable role of diffusional transport in achieving concentration uniformity in a kinetic experiment.

The economic advantages of switching to a propane feedstock in the production of acrylic acid have been a motivating force for extensive research studies focused on developing alkane partial oxidation catalysts. While catalyst systems containing MoVTeNb oxides have received considerable attention, the activity of these catalysts has not been systematically characterized over a wide domain of working conditions (different gas compositions, temperatures, etc.) and a detailed kinetic model of the propane-to-acrylic-acid reaction has not been developed yet. Extensive steady-state studies (conversion/product yields vs temperature at different feed compositions and contact times) and non-steady-state studies (pulse-response temporal analysis of products (TAP) experiments) of the propane-to-acrylic-acid reaction were performed using a TAP-2 reactor system under normal and vacuum conditions. TAP pump−probe experiments indicate that there is an optimum time for the introduction of water and that water reacts with an adsorbed intermediate to produce acrylic acid. Under steady-state conditions, the temperature was slowly cycled and the reaction progress was monitored with a temperature hysteresis clearly distinguished. The upper branch of the hysteresis corresponds to a cooling regime and is characterized by complete oxygen conversion. No distinctive hysteresis behavior was observed for propane, while the product yields exhibit clear hysteresis loops, clockwise for acrylic acid and counterclockwise for CO2/CO and acrolein. In this paper, the hysteresis-type dependence is interpreted in terms of a possible detailed mechanism of selective oxidation of propane into acrylic acid.

A theoretical model for the growth of a spherical gas bubble in a quiescent liquid with chemical reaction at the surface of the bubble is presented. The growth of the bubble is assumed to be controlled by momentum and mass transfer, while the usual chemical equilibrium boundary condition at the surface of the bubble is replaced by a first-order heterogeneous chemical reaction. Using the integral method, the differential transport equations were transformed into ordinary differential equations, which were numerically solved. Some analytical solutions for simple cases are also presented. The effects of heat transfer on the growth process are also discussed.

A systematic analysis has been carried out to distinguish “kinetic” and “transport” limited regimes for photocatalytic degradation in a TiO2 slurry system using benzoic acid as a model component. Experiments were performed to investigate the effect of both monoparametric (e.g., catalyst loading, circulation rate, and initial concentration) and biparametric (e.g., catalyst loading and circulation rate, initial concentration of solute, and light intensity) on the degradation rate to differentiate the kinetic regime from the transport regime. It is shown that there is no external mass-transfer limitation at any catalyst loading, but at high catalyst loading, there exist both internal mass-transfer limitation and light limitation. The domain of kinetic limitation, the domain of internal mass-transfer limitation (particle agglomeration effect), and the domain of light transport limitation (shielding effect) were distinguished. Experimental results demonstrate a significant difference in the rate values between the “optimal catalyst loading” regime and the kinetic regime. The true kinetic dependences of the photocatalytic degradation rate on light intensity and the substance initial concentration were determined.

The analytical solution for the mean residence time of a nonreacting gas in a three-zone TAP reactor packed with nonporous pellets is presented. It is shown that the mean residence time is a sum of products of the weighting coefficient and reciprocal of the gas diffusivity of each zone. The weighting coefficients depend on the geometrical characteristics, i.e., lengths and fractional voidages of the three zones. This simple equation is useful for determination of the unknown gas diffusivity in the second zone (catalyst zone) from an experimental diffusion response when diffusivities in the inert-material zones (zone 1 and 3) are known.