Ruthenium is the least abundant and relatively the most affordable of the platinum-group metals. Though used in small tonnages, it is irreplaceable in several niches: data storage, fine chemicals, and specialty alloys. Its largest industrial use is in hard disk drives: ever since the mechanical hard-disk era, ruthenium has served as an intermediate alloy layer and seed layer in the magnetic recording medium, helping cobalt-chromium magnetic grains align more uniformly and remain finer-grained, thereby multiplying the areal recording density per disk platter. Today's data-center storage still depends on it. In chemicals, ruthenium is the active component of a number of high-performance catalysts, used both in supported catalytic hydrogenation and selective oxidation and in newer energy directions such as water electrolysis for hydrogen production and hydrogen-evolution electrodes for fuel cells. In alloys, ruthenium is frequently added as a hardener to platinum and palladium alloys to make spark-plug electrodes, electrical contacts, platinum-ruthenium thermocouples, and bushings for glass-fiber drawing, because of its high melting point, wear resistance, and resistance to high-temperature corrosion. In renewable energy, ruthenium-based dye-sensitized solar cells were once a research hotspot, holding leading photoconversion efficiencies among dye cells for a long time. Ruthenium is also used in wear-resistant electrical contacts, resistor films, and barrier layers for microelectronic packaging.
Ruthenium dioxide (RuO2) is a conductive metal oxide, deep blue to black in appearance, and is a key material for thick-film chip resistors and ruthenium-based thin-film resistor pastes. It also serves as the active component of titanium anodes (dimensionally stable anodes) used in the chlor-alkali industry and in electroplating anodes. Ruthenium trichloride (RuCl3), commonly available as both a trihydrate and an anhydrous form, is the entry-level starting material for preparing nearly all ruthenium catalysts and complexes; supported on activated carbon or alumina it becomes a widely used selective hydrogenation catalyst, and it is also used in ruthenium electroplating and electrode coatings. Tris(2,2'-bipyridine)ruthenium(II) dichloride (the Ru(bpy)3 complex) is a classic luminescent metal complex that glows with a bright orange-red fluorescence under visible light. It is widely used in chemiluminescence analysis, photosensitization, dye-sensitized solar cells, and photocatalytic water-splitting research, and is a common model compound in electrochemistry teaching.
