Steady-state activation

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

After reduction and surface characterization, the iron sample was moved to the reactor and brought to the reaction conditions (7 atm, 3 1 H2 C0, 540 K). Once the reactor temperature, gas flow and pressure were stabilized ( 10 min.) the catalytic activity and selectivity were monitored by on-line gas chromatography. As previously reported, the iron powder exhibited an induction period in which the catalytic activity increased with time. The catalyst reached steady state activity after approximately 4 hours on line. This induction period is believed to be the result of a competition for surface carbon between bulk carbide formation and hydrocarbon synthesis.(6,9) Steady state synthesis is reached only after the surface region of the catalyst is fully carbided. 

Hydrodearomatization (HDA) was investigated on the mentioned catalysts in temperature range 260-360°C, at total pressures 35-80 bar, hydrogen to hydrocarbon ratios (H2/HC thereafter) 200-1000 Nm3/m3 and liquid hourly space velocity (thereafter LHSV) 0.75-4.0 h"1. The experiments were carried out on catalysts of steady-state activity, in continuous operation. 

On the surface of the Ni catalyst, carbon is normally produced in a whisker (or filamentous) form. According to Rostrup-Nielsen, carbon formation is avoided when the concentration of carbon dissolved in Ni crystal is smaller than that at the equilibrium. The steady-state activity is proportional to [C ], which can be expressed by the following equation ... 

Luo, M., O Brien, R.J., Bao, S., and Davis, B.H. 2003. Fischer-Tropsch synthesis Induction and steady-state activity of high-alpha potassium promoted iron catalysts. Appl. Catal. A Gen. 239 111-20. 

The steady-state activity and selectivity characteristics of a monolithic Mo03 (2 wt % Mo) catalyst are shown in Figure 1A. Percent... 

One could find that physical mixing is a simple way to prepare molybdenum oxide loaded mesoporous or silica oxide catalysts. The rate of catalyst deactivation is expressed in terms of the percentage decrease in initial conversion after 2 h of reaction. Initial and steady state activities were taken after 0.5 and 24 h of reaction on stream, respectively. Generally, the selectivity of styrene, which is the major and desired product, is at least 96% at steady state. 

In both fresh and regenerated catalysts, the MCM-41 supported catalysts are better than amorphous silica supported ones. Physically mixed molybdenum oxide catalysts with Def-MCM41 support are particularly active. The steady state activity decreases in the order Mo/DM > Mo/M > Mo/Si02. Interestingly, the rate of deactivation also seems to depend on... 

It is noted that Mo/DM is the best performing catalyst with the highest steady state activity and lowest deactivation rate. The deactivation rate is the lowest even under the influence of intense acid-catalyzed side reactions known to produce coke, i.e. oligomerization of styrene and cracking of ethylbenzene. Obviously, the high surface area and high connectivity of the support have played a determining role in the catalytic reaction. The effects they exert can be looked at in two ways ... 

Figure 1 reveals that steady state activity was established after approximately 30 hours of reaction over 1.2PV(dir) at 300°C under standard conditions. Initially there was almost total conversion of MEK which fell to ca 60% when at the steady state. In these conditions AcH appeared in highest yield. Yield of DA reached a maximum of 10% after 10 hours on stream but gradually fell to 6.5% at the steady state. The other major products, CO2 and AcOH, gave steady state yields of ca. 11.5% and 19% respectively. 

The important roles of H2 and Pt are demonstrated in Fig. 68, in which the conversions for Pt-Cs2.5(A) and Cs2.5 are compared. It is clear that Pt and H2 enhance the activity and selectivity. In the absence of H2, the initial activity of Pt-Cs2.5 is high, but the stationary-state conversion (at 5 h) is very low. H2 suppresses the deactivation by the hydrogenation of coke or coke precursors, resulting in a high steady-state activity. 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

The air and followed HaS pretreatment of the HPMo/SiOa at 500° C has drastically declined the initial and steady state activity (Figure 3). Only M0O3 phase is revealed in the IR spectra of air calcined sample. Ho bands at about 960, 790 and... 

Based on the results of Dalla Betta and co-workers, it is clear that the steady-state activity of a completely sulfur-poisoned Ni or Ru methanation catalyst is 102-104 times lower than that of the fresh catalyst. However, a typical industrial methanation process would more probably involve a catalyst only partly poisoned by sulfur. Bartholomew and co-workers (23, 113, 157) attempted to assess how sulfur poisoning of only a portion of the catalyst would affect its activity/selectivity properties in fixed-bed and fluidized-bed reactors. Data in Table XII show the effects on specific activity and product distribution of partially presulfided Co/A1203 and Ni/Al203 catalysts in a fixed bed. Catalysts were presulfided with 10 ppm H2S at 725 K, and reaction was carried out with sulfur-free feedgas. Corresponding data are listed in Table XIII for catalysts partially presulfided and then studied in a fluidized-bed reactor under the same conditions. The decrease in H2 uptake... 

Bartholomew and co-workers also measured the loss of catalytic activity with time of Ni and Co bimetallics (157, 194), Ni-molybdenum oxide (23, 113), and borided Ni and Co catalysts (161) during methanation in the presence of 10 ppm H2S. Typical activity versus time plots are shown in Figs. 25 and 26. Activity is defined as the ratio of the mass-based rate of methane production at any time t divided by the initial rate. The activitytime curves are generally characteristic of exponential decay some catalysts decay more slowly than others, but all catalysts suffer at least two orders of magnitude loss in activity within a period of 100-150 hr. Accordingly, it does not appear that other metals or metal oxides in conjunction with Ni significantly change the sulfur tolerance defined in terms of steady-state activity of Ni. These materials can, however, influence the rate at which the... 

---