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Palladium(II) Oxide

PdO oxide
Molar Mass
122.419 g/mol
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Properties

StateSolid
ColorBlack to dark brown
SolubilityInsoluble in water; slowly dissolves in HCl, faster in aqua regia
Melting Point750 °C (decomposes to Pd + O2 above 850 °C in air)

About Palladium(II) Oxide

Palladium(II) oxide is a black-brown solid with the formula PdO and a molar mass of 122.419 g/mol. It crystallizes in a tetragonal lattice (the eponymous PdO structure) where each Pd(II) sits in a square-planar four-coordinate site of oxide ions — a clean break from the rocksalt octahedral coordination of NiO, MgO, and the other 3d monoxides. That structure is a direct readout of the d8 Pd(II) preference for square planar geometry: with 8 d-electrons, occupying dxy/dxz/dyz/dz2 and leaving dx2-y2 empty is energetically much better than the octahedral t2g6 eg2 of NiO. PdO is the thermodynamically stable palladium oxide at ambient pressure; PdO2 (Pd in the +4 state) is metastable and decomposes above 200 °C. PdO forms spontaneously as a thin native passivation layer on Pd metal in air, and that layer modulates the catalytic activity of supported Pd surfaces in ways that matter for emission-control catalysis. Industrially, PdO supported on alumina or ceria is the most active known catalyst for methane combustion below 500 °C — not a trivial reaction because the C-H bond in CH4 is 439 kJ/mol, the strongest of the light hydrocarbons. Pd/Al2O3 and Pd/CeO2 are the two industrial workhorses for natural-gas vehicle aftertreatment and lean-burn power generation.

Where you'll encounter it

If you've ever driven a CNG-powered city bus or worked on a methane-fueled stationary turbine, the catalytic converter scrubbing methane out of the exhaust runs on PdO. Methane slip — unburned CH4 escaping the engine — is a regulatory headache for natural-gas engines because methane is a 28x stronger greenhouse gas than CO2, and Euro VI rules cap it tightly. PdO/Al2O3 and Pd/CeO2 catalysts hold the line. In a research lab, PdO is also the air-oxidation product you see on long-stored Pd black or Pd/C — a faint surface darkening that explains why some hydrogenation catalysts seem to lose activity sitting on the shelf, then recover after a brief H2 reduction at 200 °C that converts surface PdO back to Pd(0). Specialty dental brazing alloys and some white-gold formulations also use PdO as an intermediate in alloy preparation.

Common Uses

  • Active phase of Pd/Al2O3 and Pd/CeO2 catalysts for methane combustion in CNG vehicle aftertreatment
  • Component in three-way catalytic converters and oxygen sensors for stoichiometric engine control
  • Calcination intermediate in synthesizing supported Pd hydrogenation catalysts (Pd/C, Pd/Al2O3)
  • Component of dental brazing alloys and specialty white-gold and platinum-group jewelry alloys
  • Research material in oxygen-reduction electrocatalysis for fuel-cell cathode studies

Safety Information

GHS: H317 (skin sensitizer Category 1). Low acute toxicity by ingestion or inhalation, but chronic skin contact causes palladium sensitization that can progress to contact dermatitis and, rarely, occupational asthma. OSHA PEL is 0.002 mg/m3 (2 micrograms per cubic meter) as soluble Pd, though PdO itself is poorly soluble. Wear an N95 respirator when handling dry powder, since fine PdO dust suspends easily and you don't want chronic palladium exposure. Calcination of Pd(OAc)2 or Pd(NO3)2 to PdO releases acetic acid or NOx fumes — do that step in a well-vented oven.

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

Pd — Palladium O — Oxygen

Frequently Asked Questions

What is the molar mass of palladium(II) oxide?
The molar mass of PdO is 122.419 g/mol from Pd (106.42) + O (15.999). Palladium content is 86.9 wt percent, so a gram of PdO contains 0.869 g of Pd — useful when calculating the metal loading on a supported catalyst. A typical 5 wt percent Pd/Al2O3 catalyst calcined from Pd(NO3)2 contains roughly 5.75 wt percent PdO before reduction to Pd(0).
Why does PdO have a square-planar structure?
Pd(II) is a 4d8 ion. In an octahedral crystal field, d8 fills t2g6 eg2 — both eg orbitals partially occupied, which is destabilizing because they point right at the ligands. Going to square planar geometry lifts the dx2-y2 orbital high and empty, drops dxy/dxz/dyz/dz2 down, and the d8 configuration cleanly fills the lower four orbitals. The crystal-field stabilization energy for Pd(II) square planar is large enough to overcome the loss of two ligand bonds, so PdO crystallizes with each Pd in a 4-coordinate square plane rather than the 6-coordinate octahedron that NiO, CoO, and FeO adopt.
Why is PdO important for methane combustion?
Methane is the hardest light hydrocarbon to oxidize because the first C-H bond is 439 kJ/mol — that activation barrier is what platinum and rhodium can't get around at the temperatures lean-burn natural-gas engines run (below 500 °C). Supported PdO, particularly Pd/Al2O3 and Pd/CeO2, is the most active known catalyst for full CH4-to-CO2 combustion in this temperature window. The active site appears to be a Pd(II)-O-Pd(0) interface where the lattice oxygen abstracts the methane H atom. Without PdO catalysts, modern Euro VI and EPA methane-slip limits on CNG engines would not be achievable.