Formation of Cr(II) species in the H2/CrO3 system -: Parameter control

The thermal behaviour of CrO3 on heating up to 600 degrees C in dynamic atmospheres of air, N-2 and H-2 was examined by thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy and diffuse reflectance spectroscopy (DRS). The results revealed three major thermal events, depending to different extents on the surrounding atmosphere: (i) melting of CrO3 near 215 degrees C (independent of the atmosphere), (ii) decomposition into Cr-2(CrO4)(3) at 340-360 degrees C (insignificantly dependent), and (iii) decomposition of the chromate into Cr2O3 at 415-490 degrees C (significantly dependent). The decomposition CrO3 --> Cr-2(CrO4)(3) is largely thermal and involves exothermic deoxygenation and polymerization reactions, whereas the decomposition Cr-2(CrO4)(3) --> Cr2O3 involves endothermic reductive deoxygenation reactions in air (or N-2) which are greatly accelerated and rendered exothermic in the presence of H-2. TG measurements as a function of heating rate (2-50 degrees C min(-1)) demonstrated the acceleratory role of H-2, which extended to the formation of Cr(II) species. This could sustain a mechanism whereby H-2 molecules are considered to chemisorb dissociatively, and then spill over to induce the reduction. DTA measurements as a function of the heating rate (2-50 degrees C min(-1)) helped in the derivation of non-isothermal kinetic parameters strongly supportive of the mechanism envisaged.