Within the types segment, Residue Hydrodesulfurization Catalysts are classified by their active metal composition and support structure, each influencing performance and cost. Cobalt‑molybdenum (Co‑Mo) on alumina remains the dominant formulation, valued for its robust activity under high‑temperature, high‑pressure conditions typical of residue processing. Nickel‑molybdenum (Ni‑Mo) on alumina captures a secondary share, offering superior hydrogenation activity for feeds with elevated nitrogen content. Emerging bifunctional catalysts combine acidic supports (e.g., zeolites) with metal sites to enable simultaneous cracking and desulfurization, targeting integrated refinery configurations. Nanostructured metal sulfides represent a niche but growing category, delivering higher surface area and enhanced diffusion characteristics that reduce catalyst loading. Finally, promoted catalysts incorporating additives such as phosphorus or rare‑earth elements aim to improve resistance to metal poisoning and sintering. Collectively, these types delineate the market taxonomy, with each sub‑type contributing proportionally to the total catalyst spend based on feedstock characteristics and refinery integration strategies.