Activity of catalysts

In summary, for the most active of catalysts, the copper(II) ion, the diamine ligands that were investigated seriously hamper catalysis mainly by decreasing the efficiency of coordination of the dienophile. With exception of the somewhat deviant behaviour of N,N -dimethylethylenediamine, this conclusion also applies to catalysis by Ni" ions. Hence, significant ligand-accelerated catalysis using the diamine ligands appears not to be feasible. 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

The activity of catalyst degrades with time. The loss of activity is primarily due to impurities in the FCC feed, such as nickel, vanadium, and sodium, and to thermal and hydrothermal deactivation mechanisms. To maintain the desired activity, fresh catalyst is continually added to the unit. Fresh catalyst is stored in a fresh catalyst hopper and, in most units, is added automatically to the regenerator via a catalyst loader. 

The reducibility of the catalyst is demonstrated in Figure 6 which shows the activity of catalysts, measured as described above, after reduction to constant activity at temperatures of 280°-350°C (536°-662°F). It will be seen that ICI catalyst 11-3 compares favorably with other catalysts which contain larger amounts of alumina and consequently are more difficult to reduce at acceptable temperatures. 

J. Nicole, D. Tsiplakides, S. Wodiunig, and C. Comninellis, Activation of catalyst for gas-phase combustion by electrochemical pretreatment, J. Electrochem. Soc. 144(12), L312-L314 (1997). 

Surface Characterization and Methanation Activity of Catalysts Derived from Binary and Ternary Intermetallics... 

Two space velocities, i.e. 0.03 and 0.3 h l, have been used in the evaluation of catalytic activities of catalysts B and C at 823 K. Figure 6 shows a decrease in activity of the catalyst B when space velocity increases. The accessible sites are saturated at the lowest space velocity. This explains thus the lower conversion levels at a higher space velocity. However, for catalyst C, the evolution of the conversion, which is also depicted in Figure 6, is almost identical for both space velocities. This result could be explained by a better dispersion of the platinum due to the presence of tin. 

Figure 7 compares in function of time the activities of catalyst B and platinum supported on magnesium oxide MgO, a well-known basic industrial catalyst. 

The activity of catalysts for electrochemical reactions (like that of catalysts for chemical reactions in general) as a rnle faffs olf with time. The degree and rate of this decline depend on a large nnmber of factors the catalyst type, its method of preparation, its working conditions (composition of the electrolyte solution, temperature. 

Together with its central aspect, of studying the activity of catalysts in electrochemical reactions as a function of the nature and state of the catalyst, the term electro-catalysis is sometimes used as well to describe other areas of interest ... 

Table 10.4. Activity of catalysts prepared by modified recipe compared to original catalysts.

The reduction of nitrobenzene to aniline is a major industrial process at the heart of the production of polyurethanes, and it is also often used as a marker reaction to compare activities of catalysts [1,2], It can be performed over a variety of catalysts and in a variety of solvents. As well as its main use in polymethanes, aniline is used in a wide range of industries such as dyes, agrochemicals, by further reaction and functionalisation. Reductive alkylation is one such way of functionalising aromatic amines [3, 4], The reaction usually takes place between an amine and a ketone, aldehyde or alcohol. However it is possible to reductively alkylate direct from the nitro precursor to the amine and in this way remove a processing step. In this study we examined the reductive alkylation of nitrobenzene and aniline by 1-hexanol. 

As it has been previously shown (Figure 4), the activity of catalysts from active carbon promoted with Ag increases with the increase of the content of the promoting silver in the catalyst. But the transport hindrances are strongly influenced by the content of silver in the catalyst. 

Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 g. i.s.ml 1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (A1203, HMS, MFI). 

It was found that the hydrogen-producing activity of catalysts declined during consecutive reduction and oxidation cycles. The authors concluded that the catalyst deactivation could be prevented by careful balancing of the stoichiometry of the reduction oxidation reactions. The amount of H2 produced was estimated at 210 Nm1 2 3/kL of vacuum HRO [57],... 

FTS Activity of Catalysts Prepared by the Stepwise Impregnation Method... 

Pd(dba)2 [palladium(O)] generally affords the best results and thus an oxidation to the metal center must occur. The most likely mechanism for this to occur is by net oxidative addition of the acidic phosphonium P-H moiety (Scheme 3). This hypothesis is supported by the observation that the pKa of the phosphonium-hydro-gen bond directly affects the activity of catalysts generated in situ with more basic ligands being inactive. 

Catalyst (a) was inactive catalyst (b) had low activity catalyst (c) is very active. The much greater activity of i-BuAlCl2 as a cocatalyst over i-Bu2-A1C1 is shown in Fig. 7. Figure 7 also shows that the activity of catalyst system (b) can be increased to nearly that observed with catalyst system (c) by addition of some i-BuAlCl2, i.e., formation of catalyst system (d). 

For meaningful comparisons of the activity of catalysts in various solvents under seemingly equal conditions these factors must, of course, be considered. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

While the H20/CO ratio is crucial for the performance of LT WGS, it was particularly interesting to study the activity of catalysts at stoichiometric ratio and at H20/CO ratio of 3 1. Both are lower than those used in the commercial LT WGS processing of the gas exiting HT WGS. This was done deliberately for two reasons. The first is that there was no C02 present in the feed. Hence, the H20/CO ratio could be lower because there was no need to compensate the C02 influence on equilibrium with higher H20 concentration (due to reverse WGS reaction). The second reason was the intention to study the behavior of LT WGS catalysts at relatively low inlet CO concentration (0.5 vol%) with respect to the usual inlet CO concentrations used in the industrial process (1.5 to 3 vol%). The feed composition used here was similar to that reported in Refs. [45,46], except that the CO concentration and the H20/C0 ratio were lower. 

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