Surface tungsten reduction during thermal decomposition of ammonium paratungstate tetrahydrate in oxidising atmosphere: A paradox?

• Detection of reduced tungsten ions at the solid’s surface in oxidising atmosphere.
• Detection of gaseous ammonia liberated as oxidising agent.
• Detection of ammonia’s oxidation products.
• Quantification of the ammonia/tungsten redox process.

The interaction of ammonia, liberated during thermal decomposition of ammonium paratungstate tetrahydrate in oxidising atmosphere, with tungsten has been studied employing a conventional microbalance combined with MS (Setaram’s instrument Sensys). Applying XPS a partial reduction of tungsten at the surface with the minimal tungsten oxidation number of +5.3 for a sample generated at 293 °C was detected. The balancing oxidation of ammonia to nitrogen/nitrogen oxides has been proven by MS. An amount of 0.049 mol e− per mol W was transferred which resulted in an ammonia conversion degree from 2.1 mol% (NO2 formation) to 3.0 mol% (N2 formation).

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