Silicon-wafer based planar models for hydrotreating catalysts

In the present study the use of planar silicon-based models for hydrotreating catalysts is discussed. The specific flat nature makes these models amenable to high resoln. surface-sensitive techniques such as XPS. The addnl. possibility to measure the activity of such model catalysts in thiophene hydrodesulfurization provides a powerful tool to det. structure-activity relations. Here, we study the effect of chelating agents (NTA and EDTA) on the sulfidation order of NiMo catalysts. The XPS results clearly show that the use of these chelating agents retards the sulfidation of Ni with respect to Mo. Esp., EDTA is effective by postponing Ni sulfidation after Mo is completely sulfided. The latter catalyst also displays the highest activity which is attributed to the highest amt. of 'Ni-Mo-S'-type phases. In essence, the results underpin the notion that such 'Ni-Mo-S' phase can be formed effectively when Ni sulfidation proceeds after MoS2 has been formed. When Ni sulfides at too low temp., inactive bulky Ni3S2 clusters are formed.