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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 Monstanto Company) in late 1980's , 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. 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.

Experimental principles
In a typical TAP pulse-response experiment, very small ($~10^{-9}$ mol) and narrow ($~100$ $\mu s$) gas pulses are introduced into the evacuated ($~10^{-6}$ tott) 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 adjascent 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).

Knudsen diffusion as transport standard
Under the conditions of TAP pulse-response experiments, the mean free path of gas molecules traveling through the microreactor packing is greater than an average interstitial distance. This well-defined transport regime, known as Knudsen diffusion, provides a remarkably reliable standard process for measuring the rates of chemical transformations. A combination of very fast injection of minute amounts of gas with Knudsen transport regime leads to a number of advantages for the precise kinetic characterization of catalytic reactions. The Knudsen diffusion coefficient of each gas is \textbf{composition-independent} and can be estimated from the diffusion coefficient of an inert gas added to the feed-mixture as an internal standard. Since gas molecules undergoing Knudsen diffusion are more likely to collide with the packing rather than with each other, gas-phase reactions are practically eliminated. The absence of gas-phase reactions significantly simplifies data interpretation, especially under elevated temperatures. Millisecond time resolution of TAP pulse-response experiments allows for the isolation of separate elementary steps of complex catalytic mechanisms and for measuring the life-times of short-lived surface intermediates.

State-defining vs. State-altering experiments
The number of reagent molecules in a typical TAP pulse is much smaller than the number of active sites available in the catalytic sample. These small pulses do not change the catalyst state significantly, while in the same time they provide a "snapshot" characteristic of its catalytic properties. A long sequence of such state-defining pulses can be used to gradually change the catalyst state in a controlled state-altering experiment and elucidate how this change affects the catalytic properties.

Thin Zone configuration and catalyst uniformity
Conventional reactors for kinetic testing of heterogeneous catalysts, such as CSTR or PFR, often suffer from essential spatial non-uniformities of a catalytic zone. TAP pulse-response experiments are usually conducted using the Thin-Zone configuration of the microreactor packing. In this configuration a narrow catalytic layer with the thickness more than ten times smaller than the total reactor length is packed between two zones of inert powder, such as quartz or silicon carbide. As a direct consequence of the Knudsen transport regime, this narrow \textbf{catalytic zone remains spatially highly uniform} during pulse-response experiments for a broad range of chemical transformation rates.

Data analysis
The general methodology of TAP data analysis, developed in a series of papers by Grigoriy (Gregory) Yablonsky , 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 robust mathematical model based on a one-dimensional (1D) diffusion equation and a uniquely simple combination of boundary conditions.