:Temporal analysis of products
{{Short description|Experimental technique to study kinetics}}
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Temporal Analysis of Products (TAP), (TAP-2), (TAP-3) is an experimental technique for studying
the kinetics of physico-chemical interactions
between gases and complex solid materials, primarily heterogeneous catalysts.
The TAP methodology is based on short pulse-response experiments at low background pressure (10−6-102 Pa),
which are used to probe different steps in a catalytic process on the surface of a
porous material including diffusion, adsorption,
surface reactions, and desorption.
History
Since its invention by Dr. John T. Gleaves (then at Monsanto Company) in late 1980s,J. T. Gleaves, J. R. Ebner, T. C. Kuechler, Catal. Rev. Sci. Eng. 30 (1988) 49
TAP has been used to study a variety of industrially and academically relevant catalytic reactions, bridging the gap between surface science
experiments and applied catalysis.J. T. Gleaves, G. S. Yablonsky, X. Zheng, R. Fushimi, P. L. Mills, J. Mol. Calat. A Chem. 315 (2010) 108-134
The state-of-the-art TAP installations (TAP-3) do not only provide better signal-to-noise ratio than the first generation TAP machines (TAP-1),
but also allow for advanced automation and direct coupling with other techniques.
Hardware
TAP instrument consists of a heated packed-bed microreactor connected to a high-throughput vacuum system,
a pulsing manifold with fast electromagnetically-driven gas injectors, and a Quadrupole Mass Spectrometer (QMS)
located in the vacuum system below the micro-reactor outlet.
Experiments
In a typical TAP pulse-response experiment, very small (~10−9 mol) and narrow (~100 μs) gas pulses are introduced into the evacuated (~10−6 torr) microreactor
containing a catalytic sample. While the injected gas molecules traverse the microreactor packing through the interstitial voids,
they encounter the catalyst on which they may undergo chemical transformations. Unconverted and newly formed gas molecules eventually
reach the reactor's outlet and escape into an adjacent vacuum chamber, where they are detected with millisecond time resolution
by the QMS. The exit-flow rates of reactants, products and inert molecules recorded by the QMS are then
used to quantify catalytic properties and deduce reaction mechanisms. The same TAP instrument can
typically accommodate other types of kinetic measurements, including atmospheric pressure flow experiments (105 Pa),
Temperature-Programmed Desorption (TPD), and Steady-State Isotopic Transient Kinetic Analysis (SSITKA).
Data analysis
The general methodology of TAP data analysis, developed in a series of papers by Grigoriy (Gregory) Yablonsky
J. T. Gleaves, G. S. Yablonsky, P. Phanawadee, Y. Schuurman, Appl. Catal. A Gen. 160 (1997) 55–88
S. O. Shekhtman, G. S. Yablonsky, S. Chen, J. T. Gleaves, Chem. Eng. Sci. 54 (1999)
,D. Constales, G. S. Yablonsky, G. B. Marin, J. T. Gleaves, Chem. Eng. Sci 56 (2001) 133–149
is based on comparing an inert gas response which is controlled only by Knudsen diffusion
with a reactive gas response which is controlled by diffusion as well as adsorption and chemical reactions on the catalyst sample.
TAP pulse-response experiments can be effectively modeled by a one-dimensional (1D) diffusion equation with uniquely simple combination of boundary conditions.