Molybdenum-tungsten blue nanoparticles as a precursor for ultrafine binary carbides

Binary or mixed molybdenum-tungsten carbides with high surface area and small particle size are of great interest for catalytic applications, for example dry reforming of methane (DRM) or hydrogen evolution reduction (HER). The promising method for synthesis of ultafine carbide particles is sol-gel method using dispersions of molybdenum-tungsten nanoparticles. For further use it is necessary to know main properties of molybdenum-blue nanoparticles, including size, structure and stability under different conditions.

The synthesis of dispersions of molybdenum-tungsten blue was carried out as a result of the reduction of molybdate and tungstate ions in an acidic medium. Ascorbic acid was used as a reducing agent and acted as a carbon source.Dispersions and nanoparticles were investigated by UV/vis, infrared (FTIR) and X-ray photoelectron (XPS ) spectroscopy, transmission electronic microscopy (TEM) and dynamic light scattering (DLS).

It was shown, that molybdenum-tungsten blues are highly dispersed systems based on polyoxometalate complexes of molybdenum and tungsten. A unique property of POM is monodispersity and small particle size about 3-5 nm. The effect of molar ratio reducing agent/ metal (molybdenum and tungsten) and molar ratio molybdenum/tungsten on the properties of dispersions was investigated. It was shown, that stable nanoparticles were formed in the range of molar ratio reducing agent/metal from 0.8 to 1.0 and at the molar ratio molybdenum/tungsten [Mo]:[W]= 95:5; 90:10; 80:20; 50:50.