Vanadium in water added to the top of mixed carbon/mortar is an interesting thing to see a month later. But I dont know if its helps.
I have no idea of what you're doing, but if you also add potassium carbonate you will have something close to the formulation used for industrial catalysts in petrochemistry (composed mainly of iron oxide/hematite and potassium oxide or carbonate as the active agents, plus other oxides in small amounts for structural stability and indirectly also carbon).
However, in that case carbon is inherently added in controlled amounts (completing the catalyst preparation/activation) during operating conditions when the catalyst reacts with the cracking/decomposing hydrocarbons molecules.
To make this information more in-topic, perhaps some of the graphite and iron carbide phases formed on the catalysts contribute to their excellent chemical activity in a manner similar to that of the doped nanodiamonds described in the last few pages.