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Gold(III) Oxide

Au2O3 oxide
Molar Mass
441.931 g/mol
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Properties

StateSolid
ColorRed-brown to dark brown
SolubilityInsoluble in water; dissolves in concentrated acids and alkali
Melting PointDecomposes at 160 °C to Au + O2

About Gold(III) Oxide

Gold(III) oxide is a red-brown to nearly black solid and the only thermodynamically stable binary oxide of gold — Au2O exists but disproportionates slowly to Au + Au2O3, and even Au2O3 itself decomposes back to gold metal and oxygen at modest temperatures (about 160 °C) in a reaction that's essentially irreversible at atmospheric pressure. This thermal instability is a direct consequence of gold being the most electrochemically noble metal: E°(Au³⁺/Au) = +1.50 V means Au(III) has a strong thermodynamic incentive to grab electrons from anything available, and at elevated temperature the oxide ion (which is a poor electron acceptor) loses to the entropic drive of releasing O2 gas. The relativistic 6s contraction that makes Au(I) prefer linear coordination is the same effect that makes gold so noble: the contracted 6s lowers the energy of the metal's filled valence orbitals, raises its first ionization energy to 9.23 eV (higher than oxygen's 13.62 eV is the only reason most non-noble metals do oxidize), and makes Au(III) a marginal oxidation state at best. Au2O3 itself is rarely used as such, but it appears as the surface oxide on supported gold nanoparticle catalysts — and that surface chemistry is the entire foundation of modern gold catalysis. Haruta showed in 1987 that gold nanoparticles smaller than ~5 nm supported on TiO2 or Fe2O3 catalyze CO oxidation at temperatures as low as -70 °C, completely overturning the textbook claim that bulk gold was catalytically inert. The mechanism involves O2 activation at the Au-oxide interface, with the oxide partially reduced and the gold pulling electron density that activates adsorbed CO. Au2O3 is also the colorant in cranberry-red ruby glass, where it's incorporated into a borosilicate melt and then thermally treated to nucleate dispersed Au(0) nanoparticles whose plasmon absorbs green light.

Where you'll encounter it

If you've ever admired a stained-glass window's red panes, looked at the ruby-red Lycurgus Cup in the British Museum (4th-century Roman gold-nanoparticle glass that appears green in reflected and red in transmitted light), or read about platinum-free CO oxidation catalysts for indoor air purification, you've encountered the chemistry that runs through Au2O3. A glassblower making cranberry stemware drops a few hundred mg of Au2O3 into a borosilicate melt, then strikes the piece by reheating to ~600 °C to nucleate Au(0) nanoparticles whose surface plasmon absorbs green and gives the famous transmitted red. In a heterogeneous-catalysis lab, calcining HAuCl4 on a TiO2 support generates a transient Au2O3 surface layer that decomposes in situ to leave the active Au(0) nanoparticles for low-temperature CO oxidation.

Common Uses

  • Reference material for studying supported-gold CO oxidation catalysts
  • Colorant precursor for cranberry-red and ruby-red art glass
  • Source of gold for electroless plating bath formulations
  • Decomposition feedstock for thin-film gold deposition by thermal evaporation
  • Intermediate in some gold recovery processes from electronic waste
  • Pigment component in Purple of Cassius (gold-tin-oxide pigment)
  • Catalyst precursor for selective glucose oxidation to gluconic acid

Safety Information

GHS: Eye irritation Category 2 (H319), skin irritation Category 2 (H315). Acute toxicity is low — gold and its oxides are essentially non-bioavailable from the GI tract. Decomposes on heating above 160 °C, releasing O2 gas, so don't seal in a closed container during pyrolysis or you'll build pressure. Like other soluble gold compounds it can cause contact dermatitis ('gold rash') in pre-sensitized individuals — typically only after prior exposure to dental gold alloys or gold injection therapies. OSHA has no specific PEL. Standard lab PPE (nitrile gloves, dust mask if grinding) is sufficient. Don't dissolve in cyanide solution casually — Au(III) cyanide complexes are stable but the dissolution chemistry generates toxic free cyanide if pH drifts below 9.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

Au — Gold O — Oxygen

Frequently Asked Questions

What is the molar mass of gold(III) oxide?
Au2O3 weighs 441.931 g/mol: 2 golds (2 × 196.967 = 393.934) + 3 oxygens (3 × 15.999 = 47.997). Gold has the lowest IUPAC atomic weight uncertainty of any element (±0.001) because Au-197 is the only stable isotope, so this number is reliable to four decimal places — an unusual luxury when most metal oxides have molar mass uncertainties in the third decimal.
Why does Au2O3 decompose so easily?
Au has the most positive standard reduction potential of any metal: E°(Au³⁺/Au) = +1.50 V. That means metallic gold is thermodynamically preferred over Au(III) under almost any condition where electron donors are available, and even pulling oxygen off as O2 at modest temperature (releasing entropy) is favorable: 2 Au2O3 → 4 Au + 3 O2 starts to proceed measurably above 160 °C and is rapid by 250 °C. The chemistry traces back to relativistic effects on the 6s orbital that make gold's outer electrons unusually tightly held — high ionization energy, low electron affinity for further oxidation, exceptional nobility.
How do tiny gold nanoparticles catalyze reactions when bulk gold is inert?
Bulk gold has a fully terminated face-centered cubic surface where every atom has nine in-plane neighbors plus three above; coordination saturation kills reactivity. Drop the particle size below 5 nm and the fraction of corner and edge atoms (low coordination, dangling orbitals) explodes — at 2 nm, roughly 50% of all atoms are at edges or corners. Pair these reactive Au atoms with a reducible oxide support like TiO2 or CeO2, and you get a nanoparticle-support interface where O2 activates (often as superoxide) on the oxide just adjacent to gold, and adsorbed CO at the gold-oxide perimeter reacts to form CO2. Haruta's 1987 paper showing CO oxidation by Au/Fe2O3 at -70 °C upended decades of textbook chemistry and launched the field of gold catalysis; today commercial Au/TiO2 catalysts go into indoor air-quality CO removers.