Among the transition-metal oxides relevant for catalysis, MoO₃ is one of the few oxides that remains mesoscopically unstructured. This results in limited technological significance. Usually, sublimation of MoO₃ leads to a quick loss of the mesostructure. The Boudouard equilibrium, i.e., the equilibrium between CO₂/C and CO, can be used to convert MO₃ to non-volatile MoO_2–x to avoid this problem.

Josef Breu, University of Bayreuth, Germany, Thomas Lunkenbein, Fritz Haber Institute of the Max Planck Society, Berlin, Germany, and colleagues have taken advantage of this approach. They prepared high-surface-area, mesostructured MoO₃ nanowires via the oxidation of MoC/C nanocomposites in a CO₂ atmosphere.

The key to retaining the mesostructure was the avoidance of MoO₃ sublimation during thermal template removal. The team bypassed the sublimation of MoO₃ by reducing the oxygen partial pressure into the stability regime of MoO_2–x. The CO₂ atmosphere can act as an oxidation agent to remove the carbon template, but does not oxidize MoO_2–x to MoO₃, and the formed CO cannot reduce MoO_2–x.

Subsequent reoxidation at moderate temperatures can convert the resulting MoO_2–x nanowires to MoO₃ while retaining the mesostructure. The material can also be chemically converted to MoN nanowires. According to the researchers, the method could be extended to a general route towards mesostructured refractory compounds, i.e., compounds than can withstand high temperatures.

• Template Removal via Boudouard Equilibrium Allows for Synthesis of Mesostructured Molybdenum Compounds,
  Martin Schieder, Carina Bojer, Julia vom Stein, Sebastian Koch, Thomas Martin, Holger Schmalz, Josef Breu, Thomas Lunkenbein,
  Angew. Chem. Int. Ed. 2017.
  DOI: 10.1002/anie.201610786