Surface reaction kinetics vacuum system

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

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. 

The results discussed in this article were mostly obtained with ultrahigh vacuum systems at total pressures not exceeding 10"4 Torr, whereas real catalysis is performed in the atmospheric pressure regime. This general pressure gap raises the serious question to which extent experiments of the type described using the spectroscopic techniques of "surface science are relevant at all for real-life catalysis. A general answer to this problem can certainly not yet be offered. However, a rather favorable situation is found in the present case, as long as the discussion is confined to temperatures below 7 ax at which the reaction rate reaches is maximum rmax (cf., for example, Fig. 35). This situation has been discussed in detail in Section IV for palladium and holds as well for the other platinum metals since the shape of the r(T) curve is always quite similar. It has been shown that the kinetics may then approximately be described by... 

Before applying the vacuum microbalance or any similar method to the study of the rate of a particular surface reaction it is essential to understand as much as possible concerning the chemistry of the main reaction and the possible side reactions which may occur in a given system. This requires detailed thermochemical calculations to be made for all conceivable reactions to determine the specifications for the vacuum system and furnace tubes, the preparation of the specimens, the experimental procedures, and the interpretation of the data obtained. Kinetic theory calculations should be applied to aid in interpretation of the rates of certain vacuum and low-pressure reactions. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Abstract A review is provided on the contribution of modern surface-science studies to the understanding of the kinetics of DeNOx catalytic processes. A brief overview of the knowledge available on the adsorption of the nitrogen oxide reactants, with specific emphasis on NO, is provided first. A presentation of the measurements of NO, reduction kinetics carried out on well-characterized model system and on their implications on practical catalytic processes follows. Focus is placed on isothermal measurements using either molecular beams or atmospheric pressure environments. That discussion is then complemented with a review of the published research on the identification of the key reaction intermediates and on the determination of the nature of the active sites under realistic conditions. The link between surface-science studies and molecular computational modeling such as DFT calculations, and, more generally, the relevance of the studies performed under ultra-high vacuum to more realistic conditions, is also discussed. 

For a further discussion of the structure and properties of bimetallic systems, see Sections 2.6 and 3.2.3 for the preparation of bimetallic catalysts, see Section 4.6 and for the mechanisms by which they work in oxidations, see Section 8.2.2. Most textbooks of physical chemistry have sections on adsorption and catalysis, but they frequently focus on studies made under ultra-high vacuum conditions with single crystal surfaces. While this work produces beautiful pictures, it has limited relevance to the more mundane world of practical catalysis. Other introductory treatments of about the level of this chapter, or slightly more advanced, are available,5,7,11 as are deeper discussions of the kinetics of catalysed reactions.12 14 Industrial processes using catalysts have also been described in detail.15,16... 

A system that links microcalorimetry to the volumetric measurement of quantities of adsorbed reactants makes it possible to study gas-solid interactions and catalyhc reactions. This system works under stahc vacuum. The admission of gases into the calorimeter can be performed either in a discontinuous way (by successive doses) by means of a valve, or in a continuous manner by means of a capillary. The classical technique of adsorption calorimetry by doses is the most appropriate way to measure the energy of interaction between the adsorbed species and the catalyst. If the surface can be a priori considered as heterogeneous, the heat of adsorption, the amount adsorbed and the kinetics of adsorption must be measured for very small successive doses of the adsorbate so as to obtain accu-... 

Over the past 20 years, fundamental studies of catalysis by metals have been carried out more and more over evaporated metal films which, at least in ultra-high vacuum, provided the guarantee of an atomically clean surface at the beginning of the reaction. Unfortunately, films do not lend themselves readily to investigations in flow systems and to kinetic studies. Nevertheless, films appear more reliable than supported metals unless any interaction between metal and support can be safely ruled out for the particular reaction under study. 

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