Temporal analysis of products 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. 

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. 

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

A further advance is the TAP (Temporal Analysis of Products) reactor, where very fast reaction steps can be followed, but the equipment needed is sophisticated, and has not been much used for reactions of interest to us. Catalytic reactions can also be run in an infrared spectrometer cell, so that adsorbed species can be inspected after if not during reaction. Finally reactions can now readily be performed inside UHV apparatus, so that detailed knowledge of the state of the surface single crystals during and after reaction is accessible. ... 

Temporal Analysis of Products Reactor A Basic Reactor-Diffusion... 

Perez-Ramirez, J., Kondratenko, E., Kondratenko, V., et al. (2004). Selectivity-Directing Factors of Ammonia Oxidation over PCM Cauzes in the Temporal Analysis of Products Reactor Primary Interactions of NH3 and O2, J. Catal., Ill, pp. 90-100. 

Rothaemel, M. and Baerns, M. (1996). Modehng and Simulation of Transient Adsorption and Reaction in Vacuum Using the Temporal Analysis of Products Reactor, Ind. Eng. Chem. Res., 35, pp. 1556-1565. 

Karthereuser, B., Hodnett, B.K., Zanthoff, H., and Baerns, M. Transient experiments on the selective oxidation of methane to formaldehyde over V205/S102 studied in the temporal analysis of products reactor. Catal. Lett. 1993, 21, 209-214. 

The preferential CO oxidation in H2-rich streams is a reaction of great relevance due to its application in the purification of feeds for hydrogen fuel cells, and because of the scientific interest. This reaction is known to be very sensitive to catalytic surface structures and to the pretreatments. Au catalysts supported on metal oxides with high metal dispersions have been demonstrated as very effective in this PROX reaction [1]. However, the studies carried out over these systems have allowed us to conclude that several factors affect the performance of the catalyst, such as particle sizes, preparation method, supports, etc. Nevertheless, there is still some controversy with respect to the nature of the active site or about the mechanism of reactioa In the present conununication and with the aim to obtain further information to elucidate these unresolved points a study on the role of the support in the CO PROX mechanism, particularly emphasizing in the aspects related to the preparation of Au-supported catalysts is presented. The use of a TAP (Temporal Analysis of Products) reactor is also applied to reveal elementary processes that are taking place on the surface under reaction conditions, since this reactor system allows the detection of reactants and products with a submilhsecond time resolution [2]. 

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. 

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. 

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... 

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. 

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 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)... 

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... 

In this contribution the application of an improved Temporal Analysis of Products (TAP) reactor system [7] in the analysis of NO-decomposition is described. The sensitivity of the Multitrack set-up (MULTIple Time Resolved Analysis of Catals ic Kinetics) is an important advantage compared to conventional TAP-reactors. The detection of small product amounts has become possible and thus, the analysis of low conversion reactions. Furthermore, without the necessity of averaging, transient processes can be monitored in real time. Here, we focused on the transformation of NO on reduced Pt-sites and the effect of the oxidation state of the Pt catalyst on N2O selectivity and (adsorbed) NO2 formation. 

Insignificant change of some substance amount/eoneentration in comparison with its initial amount/concentration during a non-steady-state process. For example, in pulse-response experiments under vaeuum eonditions in a temporal analysis of products (TAP) reactor, the total number of active sites on the catalyst surface is much larger than the amount of gas molecules injected in one pulse. Therefore, the concentration of active catalyst sites may be assumed to remain equal during a pulse experiment. 

The temporal analysis of products (TAP) reactor is an experimental device for the systematic study of gas-solid diffusion-reaction systems. The TAP reactor can serve various purposes, such as... 

Schematic of aTAP reactor coupled to a time-of-flight mass spectrometer. Reprinted from Cleaves,]. T., Yabhnsky, G., Zheng, X., Fushimi, R., Mills, P.L, 2010. Temporal analysis of products (TAP)—recent advances in technology for kinetic analysis of multi-component catalysts. J. Mol. Catal. A Chem. 315, 108—134, Copyrigfit (2010), with permission from Elsevier.

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