Kinetics catalyst characterization

Catalysis as a science developed along three major directions kinetics, catalyst characterization, and the synthesis of catalysts. Kinetics is the aspect of catalysis that we emphasize in this book. The basic kinetic characteristic of a catalytic reaction, as we recognize it now, is that it is a reaction cycle consisting of several coupled reactions. The kinetic formalism needed to treat coupled reaction systems dates from the first quarter of this century, as will be reviewed in Section 1.3. 

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

Most of the techniques discussed above are typically used ex situ for catalyst characterization before and after reaction. This is normally the easiest way to carry out the experiments, and is often sufficient to acquire the required information. However, it is known that the reaction environment plays an important role in determining the structure and properties of working catalysts. Consequently, it is desirable to also try to perform catalytic studies under realistic conditions, either in situ [113,114,157, 191-193] or in the so-called operando mode, with simultaneous kinetics measurements [194-196], In addition, advances in high-throughput (also known as combinatorial) catalysis call for the fast and simultaneous analysis of a large number of catalytic samples [197,198], This represents a new direction for further research. 

Of the three regimes described by Forster, it is the second, the ionic regime, which has proved the most attractive for commercial operation and subsequent quantitative model studies have concentrated on interconversion of complexes in that cyde. As with model studies of Rh catalysts, IR and NMR spectroscopy have been the main techniques used to follow kinetics and characterize complexes. 

By following this multistep procedure, the kinetics were evaluated on a wide range of data (both conditions and feedstock) not used in the parameter estimation. The start-of-cycle kinetics were extended to other catalysts and catalyst states by defining an appropriate catalyst state vector (a = aD, al5 a , ac), which is different from 1 at the start of cycle. A catalyst characterization test was developed to estimate parameters for new catalysts. 

In the mentioned studies the selective hydrogenations were made with the aim to obtain kinetical data, and the catalysts characterization was scarce. On the other hand, It is known that metal supported catalysts exhibit important particle size and support effects in the selectivity patterns (ref. 7). 

Concepts of Modem Catalysis and Kinetics, I. Chokendorff and J. W. Niemantsverdriet, Wiley-VCH 2003, 452 pp., ISBN 3-527-30574-2. This specialized book deals only with classic gas/solid heterogeneous catalysis. It contains excellent technical explanations and has a strong mathematical and physical approach, which makes for rather heavy reading. It covers many surface reaction mechanisms and catalyst characterization techniques. 

Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

Block and co-workers [35] modified the atom probe to develop a method called pulsed-field desorption mass spectrometry (PFDMS), whereby a short high-voltage pulse desorbs all species present on the tip during a catalytic reaction. The repetition frequency of the field pulse controls the time for which the reaction is allowed to proceed. Hence, by varying the repetition frequency between desorption pulses in a systematic way, one can study the kinetics of a surface reaction [35], In fact, this type of experiment - where one focuses on a facet of desired structure, which may include steps and defects - comes close to one of the fundamental goals of catalyst characterization, namely studying a catalytic reaction on substrates of atomically resolved structure with high time resolution. 

Adsorption (Chemical Engineering) Catalyst Characterization Catalysis, Homogeneous Inclusion (Clathrate) Compounds Kinetics (Chemistry) Petroleum Refining Pharmaceuticals... 

Development of fundamental kinetics for improved understanding of complex reaction systems is another frontier. More advanced catalyst characterization tools, including on-line and in-line measurements, need to be developed to provide better understanding of critical catalyst parameters. This should involve application of predictive chemistry capability to design better catalysts which carry out desired conversions in complex reaction systems. 

One of the most important issues of macrokinetic studies is distinguishing the kinetic region in which the observed rate is governed by kinetic dependences. Obviously, intrinsic kinetic data are necessary for catalyst characterization and reactor design. Traditionally, in the kinetic studies of heterogeneous catalysis, to distinguish the kinetic regime from fhe... 

During the last four decades, various chemical reactions concerning processes of technological and environmental interest have been related to the development of catalysts. Catalyst characterization is a necessary step, and it usually involves activity tests and investigation of the kinetics of the related reactions, as well as of the nature of the active sites. 

The RF-GC methodology is technically very simple and it is combined with a mathematical analysis that gives the possibility for the estimation of various physicochemical parameters related to solid catalysts characterization in a simple experiment under conditions compatible with the operation of real catalysts. The experimentally determined kinetic quantities are not only consistent with the results of other techniques, but, moreover, they can give important information about the mechanism of the relevant processes, the nature of the active sites, and the topography of the heterogeneous surfaces. 

The vapor phase catalytic oxidation of toluene to benzaldehyde has been studied over V20s-K2S04-Si02 catalysts in an isothermal differential reactor. The experiments were carried out at atmospheric pressure, temperatures from 410 C to 470 C and the modified spatial time (W/Fto) ranging from 0 to 180 g cat/mol toluene/h. The experimental tests showed the best performance for the catalyst obtained by co-precipitation. These results may be due to a crystalline phase identified in the process of catalyst characterization. Reaction kinetics was determined using the Mars and van Krevelen model. 

Catalyst characterization tests include measurement of surface areas, chemisorption, pore-size distributions, crystal structure as determined by X-ray crystallography, reaction mechanisms as revealed by kinetics, and isotopic tracers and diagnostic catalytic reactions to test functional capabilities. These have been interpreted in terms of variation of catalyst preparation-structure-performance relationships. 

The few examples discussed here, added to the large number anyone can find in the literature, are definite proof of the usefulness X-ray spectroscopy has today in the field of catalyst characterization. We have restricted our illustrations to the EXAFS domain but it becomes clearer every day that the edge region is also of great interest XANES (X-ray Absorption Near Edge Structure) provides information on the electronic states taken by the active species during the reaction. As we very briefly reported, the number of empty d states can be followed accurately and relations with the different chemical pathways of the reaction may be established (see for example The whole will undoubtedly contribute to the development of time-resolved studies done under real reaction conditions, so that kinetic measurements will be one of the major uses of the technique in the near future. 

The release kinetics are characterized by an initial lag-phase, a zero release phase and a depletion phase. During the lag-phase water intrudes into the polymer matrix and activates the latent catalyst. During the zero-order release phase, an equilibrium between water intrusion and polymer erosion is established and an eroding front, (V2), that penetrates the device is established. Because thin disks were used, device geometry remains essentially constant and zero order release uncomplicated by a decrease in total surface area is observed. The depletion phase characterizes a decrease in device and depletion of the incorporated acidic excipient. 

There is reason to believe that catalyst samples prepared in different laboratories may not be identical, despite the best efforts to make them so. The most likely differences between samples are probably due to differences in adsorption properties, which are rarely well identified during catalyst characterization. Yet differences in adsorption equilibrium constants can lead to great variability in kinetic and selectivity behaviours, even in simple catalytic rate expressions, as we have just seen. 

Temperature-programmed desorption (TPD) is extensively applied for catalyst characterization. Commonly used molecules are NH3, H2, CO and CO2. From the desorption pattern much useful information can be obtained. TPD allows kinetic experiments in which the desorption rate from the surface is followed while the temperature of the substrate is increased continuously in a controlled way, usually in a linear... 

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