Hydrolysis of lanthanide dicarbides. Rates of reaction with water vapour

Article abstract of DOI:10.1016/0022-5088(91)90080-N

The kinetics of the reaction between water vapour and nine of the lanthanide dicarbides, YC₂ and UC₂ have been investigated by gravimetric and gas evolution techniques. Mild conditions were used with water vapour pressures in the range 5.8-16.6 × 10² N m^-2 and temperatures from 4.0-48 °C. Over 90% of the reactions studied showed linear kinetics with the remainder following parabolic rates. Linear reaction rate constants lie in the range 0.34-18.1 × 10^-2 mg cm^-2 min^-1 depending on the carbide, temperature and pressure. For any given temperature and water vapour pressure the rate constant increased in the series from LaC₂ to DyC₂ and then decreased in the series DyC₂ to LuC₂, suggesting that the light lanthanides are least reactive. YC₂ had rate constants close to those of GdC₂, whilst UC₂ reacted some 50 times more slowly. In general, at any given water vapour pressure, in the range studied, the rate constant decreased with increasing temperature - a behaviour leading to negative values for the activation energy. From the activation energies a mechanism controlled by water adsorption on the product layers is postulated which suggests that the data do not give a true picture of LaC₂ + H₂O reactions. Increasing the water vapour pressure always increases the reaction rate constants. The results are discussed in the light of data from UC₂ and CaC₂.