Hydrogenation pulsed

Figure 7 depicts the chain length distribution in liquid product, collected with repeated CO/Hj pulses (interrupted by hydrogenation pulses) with durations of 8 and 20 min. Steady-state distributions obtained with ruthenium exhibit highly linear Schulz-Flory plots, and the distribution in Fig. 7 is therefore substantially off steady state. 

With regard to Assumption 2, it is clear that a hydrogenation pulse may give rise to metal-catalyzed hydrogenolysis of the primary product, and there is recent evidence from infared spectroscopy (IR) that with Ru catalysts this actually occurs, at temperatures as low as 150°C (31). 

There is also considerable uncertainty as to the fraction of the surface atoms that act as active sites. As mentioned, results that support a Taylor fraction near unity are opposed by other results from which this fraction is concluded to be small. As pulse experiments such as those done by Dautzenberg et al. are open to, among other factors, the criticism that the hydrogenation pulse could have introduced artifacts, it would be desirable to check the product distribution in experiments in which chemically, the steady state is well defined, but pulses are used where normal synthesis gas is replaced... 

Fig. 1.12 Reduction by a train of hydrogen pulses of previously C>2-oxidised Pt catalysts. For pre-treatment conditions of catalysts Ml and M2, see text above.

Much better characterisation of the redox behaviour of the catalyst is achieved by PulseTA , carried out in incremental steps at temperatures at which the catalytic reactions occur. The results presented in Figure 18 indicate that, after each hydrogen pulse, there is a distinct mass loss due to ceria reduction marked as AFI2. 

Figure 4 shows an example of a gridsearch in which the split ratio and the pulse ratio are varied. The optimal solution presented earlier lies outside this picture on the dark ridge of high selectivities. The whole central dark area which continues up to pulse ratios of about 47 shows higher selectivity than the steady state solution. The decrease in the goalfimction at lower split ratios can be explained by short duration of the hydrogen pulse. 

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