Abstract <p>By scanning electron microscopy and energy-dispersive X-ray (EDX) analysis, spent catalyst (spent carbon additive) from Veba Combi Cracking of heavy petroleum residues is investigated. Analysis of the distribution of chemical elements shows that, in suspended cracking, V and Ni are deposited in macro- and mesopores and at the surface of coal particles as a result of breakdown of porphyrin complexes. The correlation between the V and N contents at concentration points suggests that disintegration of porphyrin complexes occurs first, with the formation of Ni particles, which then act as a catalyst in the decomposition of vanadyl porphyrins, which are more thermostable. The ultrafine Ni and V particles formed at the surface react with sulfur-bearing components (for example, hydrogen sulfide or sulfonic acids released in cracking), with the formation of sulfides and sulfates. Thus, the removal of metals from the cracking products is accompanied by their desulfurization on account of the formation of ultrafine Ni and V particles binding sulfur at the surface of the carbon additive. The Ca and Fe present in the carbon additive also participate in desulfurization of the cracking products and are converted to sulfides and sulfates.</p>