Solid catalysts with experimental

Gasteiger and Mathias assume a thin-film structure of the ionomer of 0.5-2 nm covering the entire solid catalyst surface. Experimental support for this electrode structure comes from double-layer capacitance measurements using cyclic voltammetry and AC impedance techniques. Gasteiger and Mathias observed values that are typical of Pt and carbon interfaces with electrolyte and imply that the entire solid surface was in contact with electrolyte for these electrodes. Under several assumptions regarding structure, diffusion, and reactivity, a minimum permeability was derived for a maximum of 20 mV loss. 

In this study, a simulated countercurrent moving bed reactor (SCMBR) with four parts by switching the inlets and outlets of the parts cyclically is employed in order to avoid abrasion occurring from the movement of a solid catalyst. Based on the above concepts, we focused on the performance of a SCMBR for the oxidation of CO at low concentration in absence of H2 over Pt/AlaOs catalyst adsorbent. For the first stqj of the overall nractor draign, the performance of a SCMBR is experimentally investigated and compared with that of a PBR for the reaction. 

The following experimental results are presented on the use of solid catalysts in esterification of dodecanoic acid with 2-ethylhexanol and methanol. In the next figures, conversion is defined as X[%] = 100 (1 - [Acid]finai / [AcidJinitiai), and the amount of catalyst used is normalized [%] = M=at / (Mod + MicohoD-... 

Fig. 46. TPR validation run over small monolith catalyst with 200 cpsi. Feed l,020ppm NH3, 960ppmNO, 10% H2O, 10% 02 in N2, SV= 36,000h 1 symbols experimental, solid line model predictions.

This review starts with an introduction to the principles and techniques of solid-state NMR spectroscopy and the description of the most important experimental approaches for NMR investigations of solid catalysts in the working state (Sections II and III). Section IV is a summary of experimental approaches to the characterization of transition states of acid-catalyzed reactions under batch reaction conditions. 

In the preceding decade, solid-state NMR spectroscopy has provided important and novel information about the nature and properties of surface sites on working solid catalysts and the mechanisms of these surface reactions. This spectroscopic method offers the advantages of operation close to the conditions of industrial catalysis. A number of new techniques have been introduced and applied that allow investigations of surface reactions by solid-state NMR spectroscopy under both batch and flow conditions. Depending on the problems to be solved, both of these experimental approaches are useful for the investigation of calcined solid catalysts and surface compounds formed on these materials under reaction conditions. Problems with the time scale of NMR spectroscopy in comparison with the time scale of the catalytic reactions can be overcome by sophisticated experimental... 

In some cases the estimated temperature of preparation of solid catalysts seems to be close to 0 and therefore in accordance with the foregoing working hypothesis moreover, adsorption measurements on some catalysts show a dependence on the temperature of catalyst pretreatraent in accordance with a Boltzmann distribution of their surface centers. However for catalysts such as chlorides (15) and oxides of the type Me2O3 (7), which were first considered to be suitable objects for rationalizing the compensation effect, Equations (10) and (17), which interrelate the overall rate constant k with the temperature 9 of the pretreatment of the catalyst, did not fit the experimental data. 

In the first chapter, Bates and van Santen summarize the theoretical foundations of catalysis in acidic zeolites. Being the most important crystalline materials used as catalysts, zeolites have been the obvious starting point for applications of theory to catalysis by solids and surfaces. Impressive progress has been made in the application of theory to account for transport, sorption, and reaction in zeolites, and the comparisons with experimental results indicate some marked successes as well as opportunities for improving both the theoretical and experimental foundations. 

The results of the steady-state model for the reactor under the same operating conditions are displayed as the solid lines in Figure 2. The predicted catalyst and gas temperatures are shown at each of the axial collocation points. As discussed earlier, a priori values of kinetic parameters were used ( 1, 2) similarly, heat and mass transfer parameters (which are listed in Table II) were taken from standard correlations (15, 16, 17) or from experimental temperature measurements in the reactor under non-reactive conditions. The agreement with experimental data is encouraging, considering the uncertainty which exists in the catalyst activity and in the heat transfer parameters for beds with such large particles. 

Experiments were executed in an autoclave at temperature between 130 and 180 °C, with alcohol/acid ratios between 1/9 to 27/1, as well as sulfated zirconia catalyst concentration up to 5 wt%. The experimental conditions preserved the chemical equilibrium constraint. Details are given elsewhere [2]. Two contributions in forming the reaction rate can be distinguished enhancement due to the solid catalyst and an autocatalysis effect by the fatty acid. Consequently, the following expression can be formulated for the overall reaction rate ... 

Several reactor types have been described [5, 7, 11, 12, 24-26]. They depend mainly on the type of reaction system that is investigated gas-solid (GS), liquid-solid (LS), gas-liquid-solid (GLS), liquid (L) and gas-liquid (GL) systems. The first three arc intended for solid or immobilized catalysts, whereas the last two refer to homogeneously catalyzed reactions. Unless unavoidable, the presence of two reaction phases (gas and liquid) should be avoided as far as possible for the case of data interpretation and experimentation. Premixing and saturation of the liquid phase with gas can be an alternative in this case. In homogenously catalyzed reactions continuous flow systems arc rarely encountered, since the catalyst also leaves the reactor with the product flow. So, fresh catalyst has to be fed in continuously, unless it has been immobilized somehow. One must be sure that in the analysis samples taken from the reactor contents or product stream that the catalyst docs not further affect the composition. Solid catalysts arc also to be fed continuously in rapidly deactivating systems, as in fluid catalytic cracking (FCC). 

Transport Limitation For the estimation of the mass transport limitation, Equation (20) has an important drawback. In many cases neither the rate constant k nor the reaction order n is known. However, the Weisz-Prater criterion, cf. Equation (21), which is derived from the Thiele modulus [4, 8], can be calculated with experimentally easily accessible values, taking < < 1 for any reaction without mass transfer limitations. However, it is not necessary to know all variable exactly, even for the Weisz-Prater criterion n can be unknown. Reasonable assumptions can be made, for example, n - 1, 2, 3, or 4 and / is the particle diameter instead of the characteristic length. For the gas phase, De can be calculated with statistical thermodynamics or estimated common values are within the range of 10-5 to 10 7 m2/s. In the liquid phase, the estimation becomes more complicated. A common value of qc for solid catalysts is 1,300 kg/m3, but if the catalyst is diluted with an inert material, this... 

Furthermore, quantitative structural phase analysis, for instance, is important for investigations of solid catalysts, because one frequently has to deal with more than one phase in the active or precursor state of the catalyst. Principal component analysis (PCA) permits a quantitative determination of the number of primary components in a set of experimental XANES or EXAFS spectra. Primary components are those that are sufficient to reconstruct each experimental spectrum by suitable linear combination. Secondary components are those that contain only the noise. The objective of a PCA of a set of experimental spectra is to determine how many "components" (i.e., reference spectra) are required to reconstruct the spectra within the experimental error. Provided that, first, the number of "references" and, second, potential references have been identified, a linear combination fit can be attempted to quantify the amount of each reference in each experimental spectrum. If a PCA is performed prior to XANES data fitting, no assumptions have to be made as to the number of references and the type of reference compounds used, and the fits can be performed with considerably less ambiguity than otherwise. Details of PCA are available in the literature (Malinowski and Flowery, 1980 Ressler et al., 2000). Recently, this approach has been successfully extended to the analysis of EXAFS data measured for mixtures containing various phases (Frenkel et al., 2002). 

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