Stability, catalyst

Many enzymes require metal ions for maximal activity. If the enzyme binds the metal very tightly or requires the metal ion to maintain its stable, native state, it is referred to as a metalloenzyme. Enzymes that bind metal ions more weakly, perhaps only during the catalytic cycle, are referred to as metal activated. One role for metals in metal-activated enzymes and metalloenzymes is to act as electrophilic catalysts, stabilizing the increased electron density or negative charge that can develop during reactions. Among the enzymes that function in this... 

The selection of optimum reactor inlet and outlet temperatures is affected by catalyst activity, and catalyst stability, and the need to minimize operating and investment costs. When the special BASF methanation catalyst is used, inlet temperatures of 260°-300°C or even lower are quite acceptable (see Table II). The final decision on design inlet temperature is affected by engineering requirements. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

A Small but important daSS of fOi mUlatiOnS comprises the Composite Solid Rocket Propellants. Composites typically contain a major amount of an oxidizer such as AP or HMX, a metal powder such as Al, a binder which is one or another type of rubber (or double-base), and up to a dozen trace ingredients such as catalysts, stabilizers, etc. There are literally hundreds of formulations, all to a degree similar and the choice comes down to specific missions, economics, and special requirements Loading of End Items. The blends and formulations described above may be loaded into their hardware in the plant where they are made, or they may be shipped to another plant for Load/... 

Olefin disproportionation 02/CH3C00H, caused a higher catalyst stability and activity. Catalysts based on Mo or W, especially 45... 

A pilot unit was built to confirm effective heat control and to establish long-term catalyst stability (Fig. 1) (ref. 17). The pilot unit was designed around a single tube... 

Two types of laboratory tests were conducted to evaluate contaminant tests, a catalyst stability test and a high-conversion bromine product test. For catalyst stability testing, only a small amount of catalyst was used (1.5 g) to ensure incomplete conversion of the HBr. If a feed contaminant causes catalyst deactivation, it is apparent as an immediate decrease in conversion. If an excess of catalyst was used instead, even if deactivation occurred at the inlet of the bed, it may not be detected until the region of deactivation moves considerably downstream. This could take many hours or days. 

The effect of HCl on catalyst stability was tested using an aqueous HBr stream containing 5 % HCl. No decrease in conversion occurred during 24 hours on stream with a 300°C inlet temperature (Fig. 5). 

Fig. 5. Catalyst stability testing at partial conversion using an HBr feed stream containing 5 % HCl.

Catalyst stability studies were conducted using a variety of model feeds. The results using 0.7 wt % propionic acid in aqueous HBr demonstrate the effectiveness of operating at partial conversion to monitor deactivation. Figure 6 shows that at 292 °C, the propionic acid-contaminated feed caused rapid deactivation. Subsequent analysis of the catalyst showed carbon deposits on the catalyst. 

Firstly, there are technical reasons concerning catalyst and reactor requirements. In the chemical industry, catalyst performance is critical. Compared to conventional catalysts, they are relatively expensive and catalyst production and standardization lag behind. In practice, a robust, proven catalyst is needed. For a specific application, an extended catalyst and washcoat development program is unavoidable, and in particular, for the fine chemistry in-house development is a burden. For coated systems, catalyst loading is low, making them unsuited for reactions occurring in the kinetic regime, which is particularly important for bulk chemistry and refineries. In that case, incorporated monolithic catalysts are the logical choice. Catalyst stability is crucial. It determines the amount of catalyst required for a batch process, the number of times the catalyst can be reused, and for a continuous process, the run time. 

As well as increasing the reaction rate and catalyst stability, at all-important low water concentrations and low CO partial pressures, the iridium system also produces lower levels of by-products. These improvements combine to give the CATIVA process the following advantages ... 

Fig. 3 showed the catalyst stability of Ni-Mg/HY, Ni-Mn/HY, and Ni/HY catalysts in the methme reforming with carbon dioxide at 700°C. Nickel and promoter contents were fixed at 13 wt.% and 5 wt.%, respectively. Initial activities over M/HY and metal-promoted Ni/HY catalysts were almost the same. It is noticeable that the addition of Mn and Mg to the Ni/HY catalyst remarkably stabilized the catalyst praformance and retarded the catalyst deactivation. Especially, the Ni-Mg/HY catalyst showed methane and carbon dioxide conversions more thrm ca. 85% and 80%, respectively, without significant deactivation even after the 72 h catalytic reaction. 

After these encouraging results, it is surprising that no further investigations have been performed on this reaction, which might be attributed to problems regarding catalyst stability, reproducibility, or the use of a problematic CO gas atmosphere. The comeback of the Hieber anion dates back to 2006 when our group... 

The main product of the reaction was CF2=CHC1. Moreover, two secondary alkenes CFC1=CHC1 (Z and E) were formed from CF2 = CHCl by successive chlorine-fluorine exchanges [10], After a fast initial deactivation, the catalyst stabilized. The catalytic activity for the dehydrofluorination reaction was estimated after a 5 hours reaction from the sum of the amount of alkenes. 

The N4 complexes are rather stable in acidic solutions. However, sometimes the stability is not high enough, particularly so at higher temperatures. It was quite unexpected, therefore, to hud that after pyrolysis at temperatures of 600 to 800°C the catalytic activity of these compounds not only failed to decrease but in some cases even increased. The major result of pyrolysis is a drastic increase in catalyst stability. Tests have been reported where after pyrolysis such catalysts have worked for 4000 to 8000 h without activity loss. The reasons for the conservation of high activity after pyrolysis are not entirely clear. The activity evidently is associated with the central ion that has attained a favorable enviromnent of pyrolysis products. 

Pd ternary alloys, including Pd-Co-Au [Fernandez et al., 2005a, b] and Pd-Co-Mo [Raghuveer et al., 2005] have been developed to further improve the stability of the catalyst. The addition of 10% Au to the Pd-Mo mixture improved catalyst stability. Another promising way to improve the activity and durability of Pd-M alloys is to deposit a Pt monolayer on them. Recently, a Pt monolayer deposited on PdsFe/C was found to possess higher activity than that of Pt/C [Shao et al., 2007b]. 

It appears that the heme/imidazole motif can be realized in metaUoporphyrins that are available in just two steps (Fig. 18.21b). However, it is not yet known how to accomplish objectives 2 and 3. It is also important to understand the mechanism of catalyst degradation during the ORR and to identify alternative functional groups that may increase catalyst stability to be useful in fuel cells, a metaUoporphyrin catalyst would probably have to retain its catalytic properties over at least 4 x 10 turnovers (about 1000 hours of operation at a turnover frequency of 1 s ), i.e., more than a hundred times longer than the most stable metaUoporphyrin catalysts reported to date. 

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