Óxidos e carbetos de Mo com adição de Co: estudo do efeito da metodologia de adição de dopante e da cinética da obtenção por reação gás-sólido

Abstract

The development of new materials with improved properties is interesting for several industrial segments from the catalysis of chemical reactions for environmental compliance, to the development of new electronic devices. Mo carbides and oxides with cobalt addition may exhibit a diversity of intrinsic properties of great interest. In this work two methodologies for the addition of Co in these compounds with Mo (CoMoO4 and Mo2CCo) were studied: solid and wet, having commercial ammonium molybdate [(NH4)6Mo7O24]4H2O] and cobalt nitrate [Co(NO3)2.6H2O] as starting materials. The doped precursors were characterized and the products of the thermal treatment (oxides) and the carbo-reduction reaction (carbides) were evaluated according to morphological (SEM), compositional (FR-X) and crystallographic (XRD) aspects. It was verified that the dopant’s addition via solid state methodology presented itself, in both cases, as a more robust route; producing materials with smaller crystal sizes (~ 30nm for the carbide) and with higher dopant retention (δ ~ 10%) than the wet method (dp ~ 50nm and δ ~ 40% also for the carbide). A study of the conditions for obtaining Mo2C with and without dopant addition was performed using interval reactions. In the kinetic evaluation, Avrami’s nucleation and growth model was tested, as well as those proposed by Levenspiel of Shrinking Core. It was verified that the reaction proceeds according to MoO2Mo Mo2C and that the model that best represents the actual behavior of the gas-solid reaction is that of Avrami with grain growth rate of M =12,86 and dimensional parameter N = 0,34. Cobalt addition (5%) caused an increase in reaction rate for the same soaking time an reaction temperature. It was observed a 42% conversion for the synthesis of Mo2C-5%Co with 30 minutes, while for the synthesis of Mo2C this level was attained with 60 minutes at 660°C. Nucleation and growth model also explained this reactive process, with M=69,65 and N=2,86 as kinetics parameters.