Temporal analysis of product

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

Micro reactors operated in the pulsed mode were introduced by Kokes et al. in 1955 [91], but have been intensively used only in the last 10 years. Such transient studies to obtain insight into reaction mechanisms were undertaken by Cleaves et al. with the temporal analysis of products (TAP) reactor 1997 [100], They observed rate coefficients of elementary reaction steps such as adsorption and desorption by applying pulses of reactants to a catalytic micro reactor combined with a quadrupole mass spectrometer. 

Temporal analysis of products Critical temperature Transmission electron microscopy Tetraethyl-ortho-silane Thermo gravimetry Tetrahydrofurane Turnover frequency Time on stream... 

The plug flow reactor is increasingly being used under transient conditions to obtain kinetic data by analysing the combined reactor and catalyst response upon a stimulus. Mostly used are a small reactant pulse (e.g. in temporal analysis of products (TAP) [16] and positron emission profiling (PEP) [17, 18]) or a concentration step change (in step-response measurements (SRE) [19]). Isotopically labeled compounds are used which allow operation under overall steady state conditions, but under transient conditions with respect to the labeled compound [18, 20-23]. In this type of experiments both time- and position-dependent concentration profiles will develop which are described by sets of coupled partial differential equations (PDEs). These include the concentrations of proposed intermediates at the catalyst. The mathematical treatment is more complex and more parameters are to be estimated [17]. Basically, kinetic studies consist of ... 

Other types of discontinuous flow reactors may be used for transient kinetic studies in which, for example, isotopically labelled reactants are widely used in kinetic studies. This kind of investigation has been particularly developed in the case of discontinuous flow reactors. A different instrument called TAP (for Temporal Analysis of Products) permits extremely sensitive detection of intermediates or products. The results obtained with these various instruments can provide information about reaction mechanisms. 

There have been two recent kinetic studies of the dismutation behaviour of CHC12F, CHC1F2 and other members of the CHC13 F series, in one case over activated y-alumina under conventional flow conditions [105] in the other, over activated chromia using plug-flow and temporal analysis of products (TAP) reactors... 

A consecutive reaction mechanism was also proposed by Cleaves and Centi (11). This mechanism was based on experimental work to back up the theoretical calculations of Schitt and Jorgensen. Although the proposed intermediates were not detected under reaction conditions, they have been observed with fuel-rich gas feeds and under conditions of transient reactor operation. Using a TAP (temporal analysis of products) reactor, the researchers detected products in the following order of formation butane —> butene butadiene furan. However, the... 

TAP-2 (Temporal Analysis of Products) reactor was used to obtain adsorption parameters and diffusion coeflScients of the system NO-carbon catalyst. The dififiisivity of NO as well as the enthalpy of adsorption does not change after increasing the number of fimctional groups by an oxidative pretreatment of the carbon. 

This work shows the ability of the temporal analysis of products technique to obtain the transport and sorption parameters for the NO-carbon system. These parameters are directly related to the nature and number of functional groups on the carbon. 

Butene, cis/trans-2-butenes, butadiene and furan have been detected in the oxidation of n-butane on the VPO catalysts under very unusual conditions, such as under oxygen deficiency at high n-butane concentration and very short contact times [9], or in high vacuum in a temporal analysis of products (TAP) reactor [70]. 

Keipert, O.P. Baers, M., Determination of the intraciystalline diffusion coefficients of alkanes in H-ZSM-5 zeolite by a transient technique using the temporal-analysis-of-products (TAP) reactor. Chem. Eng. Sci. S3 (1998) pp. 3623-3634. 

IR (5. ) and temporal analysis of products (TAP) (54) have been used to investigate the origin of the improved performance of fhe Pt/MO catalysts in CO oxidation. The TAP experiments shown in Fig. 12 demonstrate that the high activity of Pt/Co0 /Si02 in CO oxidation is related to the absence of CO inhibition effects at low temperatures. On the basis of these results it was proposed that CoO, is the suppher of O, which reacts with CO adsorbed on Pt. It is hkely that the reaction takes place at the Pt-CoO, interface. 

Neutron Scattering (QENS) [13], and Temporal-Analysis of Products (TAP) [14] (Figure 6, solid symbols). The isobutane diffusivities determined by the macroscopic methods, such as MEMBRANE, TAP, and FR show reasonable agreement. The self-difiusivity coefficient derived from QENS is about one order of magnitude lower. The E, values of 34 kJ mof and 25 kJ mol obtained by the MEMBRANE and the TAP methods, respectively, are higher than those determined by methods where the conditions of the mesurements correspond to sorption equilibrium or quasi-equilibrium, such as QENS (17 kJ mol )orFR(21 kJ mol ). 

In this paper, we analyze these aspects in more detail combining data from flow reactor studies, infrared studies in a flow reactor cell and results obtained in a high-speed transient micrcu eactor coupled with a mass quadrupole detector (TAP - Temporal Analysis of Products - Reactor). Furthermore, we show and discuss how it is possible to limit and control these effects of deactivation by the addition of SO during Q-Cs alkane oxidation. 

Temporal analysis of products (TAP) and flow reactor catalytic measurements were done as reported elsewhere (2,4,8,121. Fourier-transform infrared (ET-IR) studies were carried out using a Perkin Elmer 1750 instrument and a flow reactor infnued cell connected to conventional vacuum... 

The platinum-catalyzed CO oxidation was studied as a model reaction to evaluate the performance of the Multitrack system. Multitrack, developed in our laboratory, is an advanced version of a TAP (Temporal Analysis of Products) apparatus [1]. The advantages of this model reaction are that the reaction is fast, the only possible reported by-product is carbon, and that the reaction has already been studied in conventional TAP systems [2,3]. 

The evidence to date suggests that methane does not chemisorb on the catalyst surface [4,15]. The partial oxidation of methane has been studied in a temporal analysis of products (TAP) reactor, over 1 V-cabosil. A feature of the TAP system is that comparisons of residence times of various components in the reactor, and hence on the catalyst surface, can be made. Kartheuser has shown that methane and an inert gas with molecular mass = 16 g mol had the same residence times in the TAP reactor, over 1 V-cabosil, implying that methane did not adsorb on the catalyst surface [15]. This is consistent with the Eley-Rideal and Mars-van Krevelen mechanisms. Hence, methane from the gas phase (Rxn. 4) reacted with surface oxygen to form CH, radicals. It must be noted that both forms of Rxn. 4 describe the conversion of methane, but under different conditions. Reaction 4a predominated in methane lean cases, while Rxn. 4b was more relevant to methane rich conditions. Radicals generated... 

The principle of the temporal-analysis-of-products (TAP) reactor unit has been described detailed elsewhere [14,15]. The equipment was used for the investigation of the effect of a prereductionZ-oxidation of a VPO catalyst nearsurface area on its catalytic properties [16] and for isotope experiments ( NHa)... 

Miscellaneous techniques temporal analysis of products (TAP) and temperature programmed desorption (TPD)... 

Kinetics and diffusion Steady-state isotopic transient kinetic analysis (SSITKA) Temporal analysis of products (TAP) Tapered element oscillating microbalance (TEOM) Temperature scanning reactor (TSR) Zero length chromatography (ZLC) Pulsed field gradient NMR... 

The temporal analysis of products reactor system (TAP) developed recently by J. Cleaves is another technique to study fast responses. 

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