Formation of Cr(II) species in the H-2/CrO3 system - Parameter control

The thermal behaviour of CrO3 on heating up to 600 degrees C in dynamic atm ospheres of air, N-2 and H-2 was examined by thermogravimetry (TG), differe ntial thermal analysis (DTA), IR spectroscopy and diffuse reflectance spect roscopy (DRS). The results revealed three major thermal events, depending t o 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) deco mposition of the chromate into Cr2O3 at 415-490 degrees C (significantly de pendent). The decomposition CrO3 --> Cr-2(CrO4)(3) is largely thermal and i nvolves exothermic deoxygenation and polymerization reactions, whereas the decomposition Cr-2(CrO4)(3) --> Cr2O3 involves endothermic reductive deoxyg enation reactions in air (or N-2) which are greatly accelerated and rendere d exothermic in the presence of H-2. TG measurements as a function of heati ng 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 me chanism whereby H-2 molecules are considered to chemisorb dissociatively, a nd 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 no n-isothermal kinetic parameters strongly supportive of the mechanism envisa ged.