Catalyst reusability is an important study to test effectiveness of catalytic materials in a specific reaction. The effectiveness may remain constant or decreases with successive run time, which is usually measured in terms of conversion and selectivity. The loss of activity may arises due to the strong chemisorption on the active sites of  impurities (may be some by-products) present in the feed stream. 

In your case, the activity of catalyst decreases in each run except run 2. The retardness in activity with successive cycle shows that your catalyst getting deactivated that reduces activity in 1,3 and 4th cycle. In my mind, once the catalyst deactivated, it will not remain active to show exactly same activity as in the previous cycle. The increase in activity in 2nd run is probably due to experimental handling errors that can be minimized by triplicating the experiment.

dear Imteyaz Alam 

I have run the experiment 6 time all give the same results 

Hi

Usually, the effectiveness may decreases with successive run time , The explination of Dr. Imteyaz Alam is reasonable, The increase in activity in 2nd run is probably due to experimental handling errors that can be minimized by triplicating the experiment.

Hi,

maybe the surface is blocked by impurities from the catalyst synthesis process, these are washed out in the first run, after the second run classical deactivation. Also a possible explanation is that your active catalyst species is formed in the first run, so you start the second run directly with the active species, not with the precursor, therefor a bit faster.       

Similar results were obtained when using a Ru-SiO2-Fe3O4 catalyst in the process of cellulose hydrogenolysis. In the second experiment we observed increase product selectivity. But in subsequent experiments the catalyst activity decreased. Apparently, this is indeed associated with the activation of the catalyst surface after the first experiment.

I am not fully convinced with the explanation by Edward Nürenberg and Oleg Manaenkov. There should be a strong and logical explanation. If the active site is get introduced during the first cycle of reaction then the activity in second run should comparatively more higher (not only 3%) than the obtained in first cycle, and in the third and fourth cycle it should follow the same trend without the loss of significant activity. But actually this is not the case. The activity droop by >10% after the two consecutive cycle and is following the pattern like most of the catalytic materials exhibit due to poisoning or deactivation of catalyst. Poisons are usually substance that interact very strongly and irreversibly with the catalyst’s active sites, while the adsorption of inhibitors on the catalytic surface is weak and most-often reversible.

Thus it would be better to test some textural and spectroscopic properties of fresh and recycled catalyst. Of course it will give some idea about the deactivation/poisoning effect, and thereby your results can be explained more clearly.