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High Throughput of Non-Steady-State Catalytic Activity Characteristics Using Temporal Analysis of Products

Gregory S. Yablonsky1, John T. Gleaves2, Xiaolin Zheng2, Renato Feres3, and Denis Constales4. (1) Parks College, Department of Chemistry, Saint Louis University, 3450 Lindell Blvd, St. Louis, MO 63103, (2) Department of Energy, Environmental, and Chemical Engineering, Washington University in St. Louis, Campus Box 1180, Cupples II 207, One Brookings Drive, Saint Louis, MO 63130, (3) Department of Mathematics, Washington University in St. Louis, Cupples Hall I 0017, One Brookings Drive, Saint Louis, MO 63130, (4) Department of Mathematical Analysis, Ghent University, Building S-22, Galglaan 2, B-9000, Ghent, Belgium

The new approach of high throughput monitoring of non-steady-state catalytic characteristics is based on the combination of the original technique of catalyst preparation, which combines in a single apparatus an atomic beam deposition (ABD) system with a temporal analysis of products (TAP-2) reactor system, and new methodology of non-steady-state kinetic characterization. This approach is focused on establishing direct, reproducible correlations between changes in surface composition and changes in catalyst activity. Catalyst samples were prepared by directly adding metal atoms in sub-monolayer amounts to the surface of micron-sized particles. The method is illustrated by the examples of CO oxidation over a series of Pd/PdO catalysts and hydrocarbon selective oxidation over modified VPO catalysts. The Pd deposits were characterized using XPS, SEM and TEM as well. CO2 production during TPR experiments exhibited oscillatory behavior related to the self-assembly of the catalyst micro-structure. For VPO-catalysts, the different concentrations of chosen metallic modifiers (Te, Co) are compared and the best one is determined.

‘State-by-state' catalyst kinetic screening of catalytic properties is illustrated by the example of hydrocarbon selective oxidation.  For many substances of the reactive mixture (butane, furane, butadiene, maleic aldehyde, aldehyde, CO2, CO), intrinsic catalytic properties, i.e. apparent kinetic constant of substance transformation, apparent time delay, apparent storage, were determined as functions of controlled oxidation/reduction degree.  Non-steady-state characterization was performed using pulse response methods and temperature-programmed reaction (TPR). Based on the systematically obtained characteristics, the hypotheses on the detailed mechanism and its dependence on the catalyst state are proposed.

Methodologically, the non-steady-state kinetic catalyst characterization procedure is based on the application of the idea to perform the TAP-experiment in thin-zone reactors. (1999, Shekhtman& Yablonsky) is a very useful special case of the three-zone TAP-reactor configuration, in which the thickness of catalyst zone is very small compared to the reactor length.  Having the catalyst zone very thin making the change in the gas concentration across the catalyst zone small compared to its average value. A key advantage of the TZTR is that the catalyst bed can be changed uniformly by exposing the catalyst to a long series of small pulses at values of conversion up to 80% (It is much higher than in the differential PFR).  Recently the new configuration of the thin-zone reactor has been proposed in which the reaction zone is collapsed to the surface of a single micron-sized catalyst particle in a bed of inert particles. The particle occupies less than 0.3% of the cross-sectional area of the microreactor, so that the reaction zone can be considered as a point source. In a typical experiment, the microreactor was packed with approximately 100, 000 quartz particles (210-250 microns in diameter) and a single catalyst particle (300-400 microns in diameter) usually positioned in the bed. An important advantage of this configuration is that for the most reactions, concentration and temperature gradients can be assumed to be negligible.

New results in the theory of TZTR are presented, particularly the method for extracting chemical transformation rate from reaction-diffusion data with no assumption on the kinetic model (“kinetic model-free procedure”), so called Y-procedure. The mathematical foundation of the Y-procedure is a Laplace-domain analysis of two inert zones in TZTR followed by transposition to the Fourier domain. When combined with time discretization and filtering, the Y-procedure leads to an efficient method for determining the reaction and reaction rate in the active zone. Using the Y-procedure the concentration and reaction rate of a non-steady catalytic process can be determined without any pre-assumption regarding the type of kinetic dependence. The Y-procedure is the basis for advanced software for non-steady-state kinetic data interpretation. The Y-procedure can be used to relate changes in the catalytic reaction rate and kinetic parameters to changes in the catalyst surface composition. Particularly, it may provide information about the set of the ‘fast' transformations observed within the single pulse. Regarding a set of slow transformations, such information will be provided by the approach based on the moment analysis which presents an evolution of averaged catalyst kinetic characteristics.

Literature

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