(139f) High Throughput of Non-Steady-State Catalytic Activity Characteristics Using Temporal Analysis of Products

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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Gaffney, Mike Clark and Scott Han, Combining TAP-2 experiments with atomic beam
deposition of Pd on quartz particles, CatalysisToday, 121, 3-4 (2007)
170-186

J. Gleaves, G. Yablonskii, P. Phanawadee and Y. Schuurman,
TAP-2: Interrogative kinetics approach, Appl. Catal. A: Gen. 160
(1997), 55?88

G.S. Yablonsky, M. Olea and G. Marin, TAP-approach: theory and application, J.
Catal. 216 (2003) 120?134 

S.O. Shekhtman and G.S. Yablonsky, Thin-zone TAP Reactor (TZTR) versus
differential PFR, Ind. Eng. Chem. Res. 44 (2005),
6518?6522