Catalytic measurements

A second source of difficulty is caused by the unavoidable empty space in recycle reactors. This limits their usefulness in some studies. Homogeneous reactions in the empty gas volume may interfere with heterogeneous catalytic measurements. Transient experiments could reveal much more information on various steps in the reaction mechanism but material in the empty space can obscure sharp changes. 

Catalytic measurements were carried out in a stirred-bed reactor of stainless steel. The details of the reactor and experimental procedures have been described elsewhere [4]. 

The main purpose of this paper is to draw conclusions from a rather long list of papers and dissertations published from Leiden. Therefore, for details on the techniques used (catalytic measurements. Auger spectroscopy, TPD, IR) as well as for details of catalyst preparation, the reader is kindly referred to the literature quoted. 

CO conversion data relative to (N1 SI ) and (ThNl Fe, series were taken from ref. ( ) and (,9), respectively. Catalytic measurements were obtained for oxygen treated N1 Th Intermetallics. Prior to each run, a sample mixture (50 mg cata ys + 50 mg ground quartz) was reduced In H. at 275 C for 16 hours. CO hydrogenation was carried out at 275 C using H /C0 ratio 9. More experimental details are given elsewhere (10). 

Commercial porous ceramic tubes (SCT /US Filter Membralox Tl-70 [7]) were used in this study as support for the zeolite material. They are made (Figure 1) of three consecutive layers of tnacroporous a-Al203 with average pore sizes decreasing from the external to the internal layer. A thin toplayer made of mesoporous y-Al203 was also present in some samples. For gas permeability, gas separation and catalytic measurements the tubes were first sealed at both ends with an enamel layer before zeolite synthesis. Tubes with porous lengths up to 20 cm were used in this study. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

Classical low values for the mammalian enzyme that have appeared in the literature are the result of enzyme inactivation by hydrogen peroxide when measurements were carried out with peroxide levels in excess of 10 mM over time scales of 10 minutes or longer. The rapid sampling/titration method of Bonnichsen overcame the inactivation problem and permitted a satisfactory correlation of the overall catalytic measurements and Chance s observations on the intermediate complex (compound 1). Eventually, the introduction of the UV detector/spectrophotometer and the consequent assay based upon the UV absorbance of peroxide (35) further simplified the process by eliminating the discontinuous titrimetric assay. 

Bruker E-2000), MASNMR (Bruker MSL-300), FTIR (Micolet 60SXB) and MSssbauer spectroscopies, magnetic susceptibility (Cahn Ventron), adsorption (McBain balance) and catalytic measurements. The procedures have been fully... 

The catalytic measurements were performed in a continuous flow system (23.5 cm3/min total flow) in a fixed bed reactor. In this reactor a standard amount of (0.30 g) catalyst was used. The carrier gas used was helium. The partial pressure of nitrobenzene was 1.2xl03Pa (total pressure 1.01xl0sPa). The composition of the reaction mixture was determined by GC analysis. 

The two-electron reduction of Compound I to Fe(III) and the one electron reduction of Compound I to Compound II and Compound II to Fe(III) have been estimated by different methods. These methods will be described in the next section, but they differ in the strategy to estimate the redox potential. Two of them rely on spectral determination of equilibrium between redox species [63-65], whereas a third one proposes the use of catalytic measurements [53]. The values obtained with the different methods are shown in Table 4.4. It should be noted these are not standard values. As expected, one of the more oxidant enzymes is the versatile peroxidase, which is able to catalyze the oxidation of Mn(II) to Mn (III) ( 0,=1.5 V). MPO has the highest two-electron redox potential, supporting the fact that only this enzyme is able to catalyze the oxidation of chloride to hypochlorite at neutral pH [72, 73], whereas eosinophil peroxidase performs better at acidic pH [74]. 

The catalyst must be as homogeneous as possible to get good spectroscopic data. On the other hand, basic engineering rules such as flow patterns through the reactor, heat- and mass-transport properties, dead volume, and catalytic measurements need to be fulfilled. Therefore, preferentially, a thin layer of a catalyst or a sieved catalyst fraction should be applied, especially if the reactions are rapid [31], Moreover, such studies should be performed under realistic conditions (i.e. in gas phase, liquid phase [including catalyst preparation], or even at high pressure). 

Before making catalytic measurements, all oxygen species and oxygen-containing phases that can exist on the Ru(0001) surface were investigated by using postreaction XPS analysis that allowed not only the determination of the conditions of their formation, but also assignments of spectroscopic characteristics to various surface species. 

Catalytic measurements. The catalytic tests were performed in fixed bed reactors operating at 463-498 K and total pressure of 1-20 bar. The H2/CO ratio was 2 in all experiments. Prior to the reaction, the catalysts were reduced in the flow of hydrogen at 753-773 K for 5 h. FT catalytic rates and selectivities were measured at the stationary regime after 24 h time-on-stream. FT reaction rates were normalized by the number of cobalt auims in the reactor. The reaction products were analyzed by gas chromatography. 

Ostrowski T, Giroir-Fendler A, Mirodatos C, and Mleczko L. Comparative study of the partial oxidation of methane to synthesis gas in fixed-bed and fiuidized-bed membrane reactors Part 11—development of membranes and catalytic measurements. Catal Today 1998 40(2-3) 191-200. 

The basic apparatus of Schwab presents a fundamentally sound approach to catalytic measurement capable of extension toward many reactions and types of investigation. It can be extended to reactions... 

Calculations of the effective diffusivity of porous catalysts in terms of other measurable quantities can be made (Wheeler, 1). They rely on certain simplifying assumptions of the geometry of the porous structure. More generally, confidence can be derived from direct measurements of diffusivity, or its indirect determination by way of appropriate catalytic measurements, as will be described. 

After a measured value of Dm has been obtained at room temperature and pressure, it is usually necessary to extrapolate this value to the conditions of the catalytic experiment, as regards gas pressure, temperature, and molecular species. In order to make the appropriate extrapolations, it is necessary to have some knowledge regarding the type of diffusion which is taking place in the particular solid in question. This is not only of importance regarding the desired numerical extrapolation but wdl also determine distinctly different behavior patterns in some catalytic measurements as has been described earlier. 

MFI ferrisilicates with different iron contents (Si/Fe ratios of 27.5, 35, 39 and 67) were investigated under reducing and catalytic conditions by in situ techniques (ESR and Mossbauer spectroscopies). Framework substituted and extra-framework iron species with different coordination and oxidation states were identified, transformations of various forms of iron in hydrogen and hydrocarbon treatments are compared. Results obtained from catalytic measurements in hydrocarbon conversion and data obtained from in situ techniques are correlated, and the roles of various iron species in catalytic processes are suggested. 

Here we refer briefly also to the results of corresponding catalytic measurements performed on the same samples [5,6]. In the toluene disproportionation (770 K, 18.5 vol % toluene in nitrogen) it was found that the overall catalytic activity increases with the iron content (1.8, 3.6 and 5.3 % conversions were determined), as well as the deactivation rate (16.7, 25.0 and 35.8 per cent decreases of the original rates were determined). In selectivity studies the portion of p-xylene among the products decreased with the increase of iron content (32.2, 29.8 and 27.1 per cent selectivities, respectively). (For explanation, it is mentioned that in pure MFI ferrisilicates increased p-selectivity for xylene is expected, due to the comparable size of components and diameter of channels in the zeolite.)... 

Catalytic measurements were performed in acetic acid media at n)om temperature, the amounts of consumed oxygen were determined periodically by volumetry during one-day long experiments [14]. 

To start the discussion it is worth to note that the relative intensity data of Fe(Pc) and Fe(lll) components obtained before and after the treatments of the Fe(Pc)(Py)2-Y sample are similar (see the Rl values in the four last rows in the 300 K part of Table 1.) Thus, in full correspondence with the results of catalytic measurements, it can be concluded from Mossbauer data as well that the catalyst was stable, no loss of the Fe(ll) ions has taken place from the Fe(Pc) complex during the treatments. 

The data are in good correspondence with data of catalytic measurements. At these studies oxygen uptake was detected for several hours, and, in correspondence, only a partial exchange of ligans around the iron in the encaged complex was observed within 2 h, in the interval used for the treatments before Mbssbauer studies. Further, the high stability of the encaged complex is also demonstrated. 

Fig. 1. Scheme of a typical combined UHV system with an atmospheric pressure cell for catalytic measurements. 

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... 

MORPHOLOGY CHANGES OF SUPPORTED Rh PARTICl.ES DURING THERMAL TREATMENTS AS PROBED BY CATALYTIC MEASUREMENTS... 

Catalytic Experiments Catalytic measurements were carried out by means of a vibrating glass microreactor [4]. The rotating vibration of the reactor simulates fluidized bed conditions. The composition of the reaction products and the methyl chloride conversion was measured by on-line gas chromatography. Under the assumption of a differential behavior of the reactor, initial reaction rates were calculated from the methyl chloride conversion as a measure of the catalytic activity of the contact masses. The selectivity of the reaction is not a matter of discussion in this paper and will be mentioned only exceptionally. 

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