Catalysis experiments

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

A wide selection of metal reference foils and powder films of ideal thickness for tranmission EXAFS is available from The EXAFS Materials Company, Danville, CA, USA. The transmission method is well-suited for in situ measurements of materials under industrially relevant conditions of extreme temperature and controlled atmosphere. Specially designed reactors for catalysis experiments and easy-... 

C.G. Vayenas, and S. Brosda, Spillover-modified catalysis Experiment and mathematical modeling, Stud. Surf. Sci. Catal. in press, (2001). 

Application of the largest dendritic catalyst 8 (Figure 4.15) in a continuous process showed activity over 15 exchanged reactor volumes (Figure 4.16). The decrease in activity caused by wash out was calculated to be only 25% (retention of ligand 98.1%). The drop in activity was therefore ascribed to the decomposition of the palladium catalyst. Addition of membrane material to batch catalysis experiments did not change the conversion showing that this was not the cause of decomposition. 

Solvent evaporation in a rotary evaporator afforded the amino alcohol product (4.14g, 90% yield). The purity of the crude product is high enough to be used in the catalysis experiments, but it can be purified further by vacuum distillation in a Kugelrohr [90-100 °C, 0.030-0.035 mbar] cooling the recipient flask with dry ice (84% yield from the amino ester, as white needles). 

Catalysis experiments were performed to investigate the telomerization of butadiene with ethylene glycol in selected TMS systems (e.g. si toluene DMF 1 5 4 or sl 2-octanol DMSO 1.35 3 5.2). With Pd/TPPTS as the catalyst a maximum yield of only 10% of the desired products could be achieved. With Pd/TPPMS the yield increased up to 43% in the TMS system si toluene isopropyl alcohol, but additional water had to be added to obtain a phase split after the reaction. The catalyst leaching is very high and 29% of the palladium used is lost to the product phase. 

With ethanol and DMSO as mediators catalysis experiments were performed. By use of DMSO, about 70% of the product can be obtained, if the reaction takes place in one single phase in a two-phase system the yield decreases to about 30%. With ethanol almost no product can be detected, because a biphasic system was formed with this solvent under all conditions. In all cases the inorganic components K2CO3 and KCl were insoluble in the reaction mixture. 

To the precipitate was added dichloromethane, and then the combined organic extracts were filtered and concentrated in vacuo to afford the corresponding azacrown ether-type quaternary ammonium salt (0.27 g, 85%) as a light brown crystalline solid. The purity of the cmde product is high enough to be used in the catalysis experiments, m.p. 118-119 °C [a]p 71 (c= 1.0, CHCI3). 

The formation of boundary layers at the surface interface between semiconductor and gas influences also the luminescence and the electro-optical qualities of semiconductors. These effects offer interesting possibilities for studying experimentally the mechanism of chemisorption, the stationary state of chemisorption, and electron defects in the catalyst during catalysis. Experiments along this line have been carried out by some investigators (40,41) who have studied in a qualitative way the factors influencing the oxidation of phenols catalyzed by zinc oxide under the influence of light. Further work on this subject is desirable. 

How are NMR measurements of adsorbates and reactive species on catalysts made This has been the subject of several recent reviews 6-8). In brief, there are a number of approaches that differ in the extent to which they are intended to reproduce the conditions of an actual catalysis experiment. Heterogeneous catalysis in a microreactor is typically carried out in a carrier gas flow stream at temperatures of 673 K or even higher. Products emerging from the microreactor are most commonly analyzed with an on-line gas chromatograph (GC). The contact time of the reactant on the catalyst typically ranges between several tenths of a second to several tens of seconds. In contrast, the acquisition of a 13C spectrum of an adsorbate on a typical catalyst at 298 K takes several minutes or tens of minutes, even with enriched compounds. As the sample temperature increases to the range commonly used in catalysis, NMR sensitivity rapidly decreases, and the discrepancy in time scale becomes greater. 

This chapter is largely concerned with the characterization by PL of various types of solids under inert and reactive atmospheres, and at low temperatures and pressures (i.e., under "spectroscopic" conditions). In contrast, to the best of our knowledge, there has been no report so far of PL spectra of a catalyst under working conditions (i.e., in the presence of flowing reactants at pressures and temperatures typically involved in catalysis experiments in the laboratory). 

Figures 3 and 4 summarise the catalysis experiments results for the mixed copper-cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen fi om Figure 4 the...

Table 1 Examples of Mediators Currently Used in Redox Catalysis Experiments With Their Redox Potentials Established in DMF Containing 0.1 M TBABF4, with Redox Potentials Referred to the Aqueous SCE...

Modification method b the salen ligand was stirred for 4hrs at 80°C with the [Pt(NH3)4](N03)2 in an ethanol/water solution. The obtained platinum salen complex was filtered and dried Then the platinum salen complex was solved in methylene chloride and stirred with the zeolite for 24 h. Finally the methylene chloride was separated with destination from the zeolite. After drying in vacuum the catalyst was ready for the catalysis experiments.(CAT 3)... 

The cobalt catalyst prior to catalysis experiments have been shown to be less than 10 A in size by several methods including chemisorption methods, ferromagnetic resonance and spin echo nuclear magnetic resonance techniques (11, 20, 27). The transmission... 

Because of the possible interest of these substances for catalysis, experiments for the further characterization of the state of the Ni have been carried out. 

In a recent communication, Ohde et al. showed the synthesis of palladium nanoparticles by hydrogen reduction of Pd ions dissolved in the water core of a CO2 microemulsion 18, 21). The Pd nanoparticles so produced are uniformly dispersed in the supercritical fluid phase and are stable over an extended period of time long enough for catalysis experiments. Reduction of a... 

From the preceding discussion it is evident that in order to obtain information about the intermediates in enzyme catalysis, experiments must be done at sufficiently high enzyme and substrate concentrations so that the intermediates can be detected with available experimental methods. However,... 

Fig. 6.27 - Dimensionless plots for determining rate constants from redox catalysis experiments (a) Case 1 systems where ki may be determined, (b) Case 2 systems where k may be estimated. The numbers on the curves indicated the excess factor y = c lcp. The figure is drawn from tables supplied by J. M. Saveant.

Case 2 is the one of interest for the determination of the half life of an unstable intermediate. The redox catalysis experiments illustrated by the data of Fig. 6.28, lead only to a value of kkilk2- Fortunately k can be extracted by several procedures, the simplest by combining the redox catalysis data with data from cyclic voltammograms for the direct reduction of A. 

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