Complex sentence: It must be emphasized that the spectroscopic probing be preformed on the same reaction because having two experiments on two almost identical reactions introduces makes it harder to determine a direct link between the structure of the catalyst and intermediates, and the catalytic activity/selectivity.
Split into simpler sentences: It must be emphasized that the spectroscopic probing be performed on the same reaction. Having two experiments on two almost identical reactions makes it more difficult to determine a direct link between the structures of the catalyst and intermediates, and of the catalytic activity/selectivity.