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Ruthenium

Ruthenium — Material Page
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Ruthenium (Ru, atomic number 44) is a Group 8 HCP platinum-group metal (PGM) with a melting point of 2,334 °C, density of 12.45 g/cm³, hardness of ~220–240 HV (annealed), and outstanding chemical resistance — the hardest and most refractory of the lighter PGMs (Ru, Rh, Pd), and the only one with an HCP crystal structure. Ru is rare (~0.001 ppb crustal abundance) and is produced as a byproduct of Ni-Cu-Pt mining primarily from the Bushveld Complex (South Africa) and Norilsk (Russia), with annual production of ~35–40 tonnes. Ru forms compounds across oxidation states −2 to +8, with +2, +3, and +4 most common; RuO₄ (Ru⁸⁺, bp 40 °C) is a volatile, highly toxic oxidant analogous to OsO₄ that must be avoided during high-temperature processing. Ru has the electron configuration 4d⁷5s¹ and a rich organometallic chemistry — Ru-arene, Ru-carbene, and Ru-polypyridyl complexes are central to modern catalysis, energy conversion, and anticancer drug research.

The two largest applications of ruthenium are as a diffusion barrier and seed layer in advanced DRAM and logic semiconductor devices (Ru thin films replacing TiN and TaN in sub-5 nm node interconnects) and as the sensitizing dye ligand metal in dye-sensitized solar cells (DSSCs) — Ru(bpy)₂(NCS)₂ (N3 dye) and related Ru-polypyridyl complexes defined the DSSC field developed by Grätzel (Nobel Prize in Chemistry 2019). In DRAM capacitors, Ru bottom electrodes allow extremely thin (≤5 nm) high-κ dielectric (ZrO₂, HfO₂) deposition due to Ru's low interface defect density and resistance to oxidation, enabling continued capacitance scaling below 10 nm node. Ru is also a key component in CrRu and RuO₂ hard mask layers in EUV lithography patterning stacks. In heterogeneous catalysis, Ru/C and Ru/Al₂O₃ are the most active catalysts for ammonia synthesis at moderate temperatures and pressures — Ru replaced Fe as the catalyst of choice in the Kellogg Advanced Ammonia Process (KAAP) for new low-pressure ammonia plant designs.

Grubbs-type Ru-carbene catalysts (first-generation: Cl₂(PCy₃)₂Ru=CHPh; second-generation: Cl₂(IMes)(PCy₃)Ru=CHPh) transformed synthetic chemistry by enabling efficient, functional-group-tolerant olefin metathesis — reactions for which Robert Grubbs, Richard Schrock, and Yves Chauvin shared the Nobel Prize in Chemistry in 2005. Grubbs catalysts are now used in pharmaceutical manufacturing (e.g., ring-closing metathesis steps in synthesis of BILN 2061, a hepatitis C protease inhibitor), polymer synthesis (ROMP of cyclic olefins to produce specialty elastomers and photoresists), and fine chemical production. RuO₂ is the standard dimensionally stable anode (DSA) coating in the chlor-alkali process for industrial chlorine and NaOH production, carrying >50% of global chlorine production capacity; ¹⁰⁶Ru (t½ = 371.5 days) is a significant fission product in spent nuclear fuel and a key radionuclide for monitoring nuclear activities.

General Properties

PropertyValueNotes
Atomic Number44Group 8, Period 5; 4d⁷5s¹; oxidation states −2 to +8; most common +2 (Ru(bpy)₃²⁺ DSSC dye), +3 (RuCl₃, Ru₂(OAc)₄), +4 (RuO₂, RuCl₄²⁻). RuO₄ (Ru⁸⁺) is a volatile, extremely toxic oxidant (similar to OsO₄) formed on heating Ru in air above ~800 °C; all high-temperature processing of Ru requires inert atmosphere or fume extraction.
Atomic Mass101.07 uSeven naturally occurring stable isotopes: ⁹⁶Ru (5.54%, Stable*), ⁹⁸Ru (1.87%), ⁹⁹Ru (12.76%), ¹⁰⁰Ru (12.60%), ¹⁰¹Ru (17.06%), ¹⁰²Ru (31.55%), ¹⁰⁴Ru (18.62%). ⁹⁹Ru and ¹⁰¹Ru are NMR-active (both I = 5/2), used to characterize Ru coordination complexes and organometallic intermediates.
Density (20 °C)12.45 g/cm³Moderate density for a PGM — similar to Rh (12.41 g/cm³) and much lower than the heavier PGMs Os, Ir, Pt. Ru's relatively lower density (compared to heavier PGMs) combined with its hardness and HCP structure makes it attractive as an alloying addition to Pt and Pd for hardening without excessive density increase.
Melting Point2,334 °C (2,607 K)Highest melting point of the lighter PGM triad (Ru, Rh, Pd). Processing requires arc melting or powder metallurgy under inert atmosphere; Ru is not readily hot-worked and is typically used in powder-sintered or sputtered forms. Ru alloying additions (0.1–1.0 wt%) to Pt and Pd improve high-temperature creep resistance in thermocouple wire and electrical contact applications.
Boiling Point4,150 °CHigh boiling point supports Ru sputtering target use in PVD deposition of Ru and RuO₂ thin films for DRAM capacitor electrodes, EUV hard masks, and DSA coatings. The more practically relevant vapor species at moderate temperatures is RuO₄ (bp 40 °C), which forms above ~800 °C in air and is the primary handling hazard for heated Ru.
Thermal Conductivity117 W/m·KHigh thermal conductivity for a PGM — higher than Pt (71.6 W/m·K) and Pd (71.8 W/m·K); relevant to Ru thin-film performance as a thermal interface and interconnect material in advanced semiconductor devices, and to RuO₂ DSA electrode heat dissipation in industrial chlor-alkali electrolyzers.
Electrical Resistivity71 nΩ·m (20 °C)Moderate-low resistivity — lower than Pt (105 nΩ·m) and competitive with Cu (17 nΩ·m) at sub-5 nm linewidths where Cu suffers severe surface/grain-boundary scattering. This is why Ru is being introduced as a replacement for Cu in the narrowest interconnect lines in advanced logic chips (TSMC, Intel, Samsung sub-5 nm nodes). Ru becomes superconducting below Tc = 0.49 K.
Crystal StructureHCP, a = 2.706 Å, c = 4.282 Å; c/a = 1.582HCP structure — the only lighter PGM with HCP rather than FCC symmetry. The HCP structure contributes to Ru's greater hardness and lower ductility compared to FCC Rh and Pd. Ru(0001) surface (basal plane of HCP) is a key model catalyst in surface science for CO oxidation (the "Ru controversy" — Ru/RuO₂ activity switching), NH₃ synthesis, and N₂ dissociation studies.

Mechanical Properties

PropertyValueNotes
Tensile Strength~240 MPaModerate tensile strength in pure annealed form; Ru is significantly harder and less ductile than Rh or Pt, limiting its use in drawn-wire form. Ru alloying additions to Pt and Pd dramatically increase tensile strength — 5% Ru in Pt raises tensile strength to ~450 MPa, used for thermocouple wire and electrical contact alloys.
Yield Strength~200 MPaHigher yield strength than Pt (~30–120 MPa) and Pd in comparable condition; consistent with Ru's greater hardness and d-electron bonding strength. The relatively high yield strength of Ru combined with HCP slip system limitations contributes to its brittleness at room temperature during deformation processing.
Young's Modulus447 GPaVery high modulus — among the highest of any metal after Os (550 GPa), Ir (528 GPa), and W (~411 GPa). Relevant to Ru thin-film stress management in semiconductor device stacks — Ru films under compressive or tensile stress can cause delamination or wafer bow, requiring careful deposition condition control.
Hardness~220–240 HV (annealed polycrystalline)Source value of "Vickers ~2160" is erroneous — this would exceed diamond (~8,000 HV by equivalent scale) and is not physically possible for a pure metal. Corrected to ~220–240 HV for annealed polycrystalline Ru, consistent with published literature. Work-hardened Ru can reach ~600 HV. Ru is the hardest of the lighter PGMs (harder than Rh ~110 HB, Pd ~37 HB).
Elongation at Break~10%Limited ductility — substantially less than Rh (20–30%) and Pt (30–50%) due to HCP crystal structure with fewer active slip systems at room temperature. Ru powder or sponge is typically processed by PM sintering rather than by melting and drawing, and is more commonly used as a thin film or coating than as a bulk formed material.
Poisson's Ratio0.30Consistent with HCP metals. Used in stress modeling of Ru thin films in semiconductor back-end-of-line (BEOL) interconnect stacks and in Ru-coated electrode simulations for electrochemical applications.

Chemical Properties

PropertyValue / BehaviorNotes
Oxidation States+2 (Ru(bpy)₃²⁺); +3 (RuCl₃); +4 (RuO₂, RuCl₄²⁻); +8 (RuO₄, volatile toxic)RuO₂ (Ru⁴⁺, rutile structure, metallic conductivity ~35 µΩ·cm) is simultaneously the most important Ru industrial compound (DSA chlor-alkali electrodes, DRAM capacitor electrodes) and the primary surface oxide formed in air above ~300 °C. Ru(bpy)₃²⁺ is the prototype photoredox sensitizer — its MLCT absorption at 452 nm, long excited-state lifetime (600 ns), and reversible Ru³⁺/Ru²⁺ reduction potential make it the model for artificial photosynthesis and photoredox catalysis research.
Corrosion ResistanceExcellent in most acids at RT; resistant to HCl, H₂SO₄, HNO₃; resists aqua regia at RT; attacked by oxidizing fused alkalis; forms volatile RuO₄ above ~800 °C in airRu's corrosion resistance in chloride media and acidic environments underpins its use as a DSA coating (RuO₂-TiO₂ on Ti substrate) in chlor-alkali electrolyzers — RuO₂ withstands continuous anodic polarization at +1.4 V vs. RHE in 4–5 M NaCl brine at 80–90 °C for years. The primary handling hazard for heated Ru metal or RuO₂ is formation of volatile RuO₄ (toxic, bp 40 °C), requiring inert atmosphere or fume extraction above ~600 °C.
Surface OxideRuO₂ (rutile, metallic conductor ~35 µΩ·cm) forms above ~300 °C in air; RuO₄ (volatile, toxic) above ~800 °CRuO₂ is a metallic conducting oxide — one of very few oxides with truly metallic conductivity — making it useful as an electrode material, pseudocapacitor active material (RuO₂·nH₂O, charge storage 720–1,000 F/g, the highest of any pseudocapacitive material), and as a conductive adhesion layer under high-κ dielectrics in DRAM capacitors. The RuO₂/Ru activity transition for CO oxidation (the "Ru controversy" in surface science) established the importance of surface oxide phases in heterogeneous catalysis.
IdentifierValue
SymbolRu
Atomic Number44
CAS Number7440-18-8
UN NumberUN3089 (powder)
EINECS Number231-127-1
IsotopeTypeNotes
⁹⁶Ru Stable* 5.54% natural abundance; I = 0; Stable* — double electron capture to ⁹⁶Mo is energetically allowed (Q = 2.718 MeV); t½ not yet measured experimentally (predicted >10²⁰ yr). Used as an enriched target for ⁹⁷Ru production (⁹⁶Ru(p,n)⁹⁷Rh → ⁹⁷Ru EC, t½ = 2.9 days, a PET imaging candidate) in proton irradiation at cyclotron facilities.
⁹⁸Ru Stable 1.87% natural abundance; I = 0. Least abundant stable Ru isotope. Used as an IDMS reference isotope for Ru concentration measurements in geochemical and environmental samples by MC-ICP-MS, and as a spike isotope for precise Re-Os and PGM geochronology work where Ru concentrations are needed alongside Os isotope ratios.
⁹⁹Ru Stable 12.76% natural abundance; I = 5/2, NMR-active. ⁹⁹Ru NMR (chemical shift range ~16,000 ppm — one of the widest of any NMR nucleus; quadrupolar relaxation limits use in low-symmetry environments) characterizes Ru oxidation state and coordination in Ru carbonyl clusters, Grubbs catalyst intermediates, and RuO₂ surface species. Also a nuclear fission product (fission yield ~6.1% from ²³⁵U thermal fission) that accumulates in spent nuclear fuel — monitored in nuclear effluent and environmental surveillance programs.
¹⁰⁰Ru Stable 12.60% natural abundance; I = 0. Used as a normalization isotope in Ru isotope ratio measurements (¹⁰⁰Ru/¹⁰²Ru) by MC-ICP-MS for PGM geochemistry and for stable isotope tracer studies of Ru mobility in environmental and hydrometallurgical systems.
¹⁰¹Ru Stable 17.06% natural abundance; I = 5/2, NMR-active. ¹⁰¹Ru NMR complements ⁹⁹Ru NMR for characterizing Ru complexes in higher-symmetry environments (narrower linewidths due to smaller quadrupole moment relative to ⁹⁹Ru). Also a significant fission product from ²³⁵U thermal fission (~5.1% yield), monitored in nuclear reactor cooling water and environmental surveillance alongside ¹⁰⁶Ru.
¹⁰²Ru Stable 31.55% natural abundance — the most abundant Ru isotope; I = 0. The primary reference isotope for Ru isotope ratio measurements. Its high abundance makes ¹⁰²Ru the preferred denominator in Ru isotope ratio conventions (e.g., ¹⁰⁶Ru/¹⁰²Ru in fission product monitoring). Used in IDMS-based Ru quantification in spent fuel dissolution solutions.
¹⁰⁴Ru Stable 18.62% natural abundance; I = 0. ¹⁰⁴Ru(n,γ)¹⁰⁵Ru (σ = 0.47 barn) produces ¹⁰⁵Ru (t½ = 4.44 hr, β⁻), used as a short-lived radiotracer in Ru electrochemistry studies and as a diagnostic activation product in research reactor irradiation experiments.
¹⁰⁶Ru Radioactive t½ = 371.5 days; β⁻ (pure, 39.4 keV max, no significant gamma) to ¹⁰⁶Rh (t½ = 29.8 s, β⁻ 3.54 MeV + gamma); produced by ²³⁵U thermal fission (~0.38% cumulative yield). The ¹⁰⁶Ru/¹⁰⁶Rh source is used in ophthalmic brachytherapy for choroidal melanoma. ¹⁰⁶Ru is a key environmental monitoring radionuclide — atmospheric detections in Europe in 2017 (IRSN, BfS) traced to a Ru processing incident in the southern Urals. Not listed in source; added here.

Scientific & Research Applications

Use CaseForm Typically UsedDescription
Dye-Sensitized Solar Cells (DSSCs)Ru(bpy)₂(NCS)₂ (N3 dye); Ru(dcbpy)₂(NCS)₂ (N719 dye); black dye Ru(tcterpy)(NCS)₃ (synthesized from RuCl₃)Ru-polypyridyl sensitizers adsorbed on nanocrystalline TiO₂ are the defining dye class in DSSCs — the N3/N719 dyes achieve >10% PCE by MLCT excitation at 452 nm, electron injection into TiO₂ conduction band, and regeneration by I⁻/I₃⁻ electrolyte. The Grätzel DSSC, based on Ru dyes, established the field of molecular photovoltaics (Grätzel, Nobel Prize in Chemistry 2019).
Olefin Metathesis Catalysis (Grubbs Catalysts)RuCl₂(PCy₃)₂(=CHPh) (G1); RuCl₂(IMes)(PCy₃)(=CHPh) (G2); synthesized from RuCl₃·nH₂OGrubbs-generation Ru-carbene catalysts enable ring-closing metathesis (RCM), cross metathesis (CM), and ring-opening metathesis polymerization (ROMP) with high functional-group tolerance in air and protic solvents — transformative for pharmaceutical total synthesis (BILN 2061 HCV protease inhibitor, Taxol analogs) and specialty polymer manufacturing (Nobel Prize in Chemistry 2005).
Photoredox Catalysis & PhotosensitizersRu(bpy)₃²⁺ (as RuCl₂(bpy)₃) and derivatives; Ru(phen)₃²⁺Ru(bpy)₃²⁺ is the archetype visible-light photoredox catalyst — absorption at 452 nm, long triplet MLCT lifetime (~600 ns), and well-calibrated excited-state redox potentials enable single-electron transfer to organic substrates. Used in photoredox C–H functionalization, dehalogenation, and radical cyclization reactions in synthesis, and as a photocatalyst for water oxidation and H₂ evolution in artificial photosynthesis model systems.
RuO₂ Pseudocapacitor Electrodes & SupercapacitorsHydrous RuO₂·nH₂O (electrodeposited or thermally prepared); RuO₂ sputtered filmsHydrous RuO₂·nH₂O has the highest gravimetric pseudocapacitance of any known material (~720–1,000 F/g), storing charge via fast, reversible Ru²⁺/Ru³⁺/Ru⁴⁺ surface redox transitions across a 1.2 V window in H₂SO₄. Used in high-power supercapacitor electrodes for pulse-power applications and as a model pseudocapacitive material in electrochemical energy storage research.
Ammonia Synthesis CatalysisRu/C, Ru/MgO, Ru/BaO-Al₂O₃ catalysts (prepared from RuCl₃ or Ru₃(CO)₁₂)Ru-based catalysts (Ba-promoted Ru/C or Ru/MgO) are 10–100× more active than Fe catalysts for NH₃ synthesis at low pressures (<100 bar) and temperatures (<400 °C) — the basis of the KAAP (Kellogg Advanced Ammonia Process) low-pressure ammonia process. Research into Ru catalysts for electrocatalytic N₂ reduction to NH₃ at ambient conditions is ongoing as an alternative to the Haber-Bosch process.

Industrial & Commercial Applications

SectorForm / Grade UsedDescription
Semiconductor Interconnects & DRAM ElectrodesRu sputtering targets (99.9–99.95%); Ru ALD precursors (Ru(EtCp)₂, RuO₄)Ru thin films (2–10 nm) are replacing TiN and TaN as diffusion barrier/liner material in sub-5 nm logic node Cu interconnects (TSMC N3/N2, Intel 20A/18A) due to Ru's lower resistivity at narrow linewidths and compatibility with bottom-up Cu fill. Ru is also the standard bottom electrode for high-κ DRAM capacitors (ZrO₂ or HfO₂ dielectric on Ru) in 1α–1γ DRAM nodes, deposited by ALD for conformal coverage in high-aspect-ratio capacitor trenches.
Chlor-Alkali DSA ElectrodesRuO₂-TiO₂ mixed oxide coating on Ti substrate (DSA, dimensionally stable anode); RuO₂ ~30–40 mol%RuO₂-TiO₂ DSA coatings on titanium are the universal anode for industrial chlorine production — >90% of global Cl₂ capacity (~70 million tonnes/year) uses DSA technology. RuO₂ provides electrocatalytic activity for Cl₂ evolution (overpotential ~50 mV) while TiO₂ provides structural stability; anode lifetime is ~5–8 years before recoating. DSA technology replaced graphite anodes in the 1970s, dramatically reducing energy consumption and eliminating graphite erosion contamination of NaOH product.
Hard Disk Drive (HDD) Magnetic LayerRu interlayer (2–8 nm, 99.95%+ sputtered) in CoPtCr-based perpendicular magnetic recording mediaRu is used as a non-magnetic exchange coupling interlayer in anti-ferromagnetically coupled (AFC) recording media and as an underlayer to control the crystallographic texture and grain size of the magnetic recording layer (CoPtCrB or FePt) in hard disk drives. Nearly every modern HDD (>800 million units cumulative) contains a Ru interlayer, making HDD media one of the largest volume applications of Ru thin films. Ru interlayer thickness (typically 6–8 Å) is critical for controlling AFC coupling strength.
Electrical Contacts & Alloying AgentPt-Ru, Pd-Ru, and Pt-Pd-Ru alloy contacts (99.9%+ Ru); Ru addition 0.5–10 wt%Small additions of Ru (0.1–1.0 wt%) to Pt dramatically increase hardness, tensile strength, and creep resistance for thermocouple wire and electrical contact applications. Pd-Ru and Pt-Ru alloy contacts are used in telecommunications relay switches and precision instrumentation requiring millions of operating cycles with stable contact resistance and minimal material transfer under low-energy arcing conditions.
PurityDescription
99.9% (3N)High-purity ruthenium suitable for catalytic systems, sputtering targets, and electronic components requiring reliable corrosion and oxidation resistance.
Synonym / Alternative NameContext
RuChemical symbol; from Ruthenia, the Latin name for the Rus' region (modern Russia/Ukraine/Belarus) — named by Karl Ernst Claus in 1844, who isolated the element at Kazan University. Used as the primary identifier in ICP-MS databases, semiconductor process specifications (SEMI standards), and PGM commodity market reports.
Ruthenium metalCommercial designation for elemental Ru in powder, sponge, pellet, foil, rod, or sputtering target form; used in LPPM PGM market documentation, Goodfellow and Umicore materials datasheets, and procurement specifications for semiconductor and HDD thin-film deposition applications.
Ruthenium precious metalTrade designation classifying Ru among the platinum-group precious metals in commodity market documentation; used in LPPM trading rules, Johnson Matthey annual PGM reviews, and insurance/logistics documentation for Ru transport — though Ru is the least expensive PGM by unit price.
Elemental RutheniumScientific designation distinguishing the pure element from RuO₂, RuCl₃, Ru(bpy)₃²⁺, Grubbs catalysts, and other Ru compounds; used in surface science, semiconductor process, and electrochemistry literature specifying Ru metal foil, single crystals (Ru(0001)), or thin films as distinct from ruthenium oxide or complex species.
Element 44Periodic table designation; used in XRF/ICP-MS analytical software, nuclear data libraries (ENDF/B-VIII), and fission product monitoring databases where atomic number is the primary identifier for tracking ⁹⁹Ru, ¹⁰¹Ru, and ¹⁰⁶Ru in spent nuclear fuel and environmental surveillance programs.
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