Synthesis, reactivity, and thermochemistry of vanadium carbide and nitride

Early transition metal carbides and nitrides have been found to possess catalytic activity resembling those of the noble group metals, such as platinum. This study presents and contrasts the synthesis, characterization, and reactivity of the carbides and nitrides of vanadium. Vanadium carbide and nitride powders and foams have been prepared in high surface area forms (60-120 m{dollar}rmsp2gsp{lcub}-1{rcub}{dollar}) by temperature programmed reaction (TPR) between vanadium oxides and a reactive gaseous stream. The gas was a mixture of methane and hydrogen for the carbide and pure ammonia for the nitride. The synthesis was followed by mass spectrometry and was found to proceed through various suboxide intermediates. The intermediate phases were identified by x-ray diffraction and their formation explained using both thermodynamics and kinetics. The synthesis parameters for producing maximum surface areas were determined. The hydrodenitrogenation activity of vanadium nitride for finished diesel oil was tested and compared with that of Mo{dollar}sb2{dollar}C, Mo{dollar}sb2{dollar}N, and a commercial catalyst, Ni-Mo-P/Al{dollar}rmsb2Osb3{dollar}.; The TPR process was mathematically modeled as a diffusion process, and a general solution embracing synthesis parameters were presented in analytical and graphical forms. The model was used to determine the diffusivity of oxygen in sintered vanadium oxide during reduction (D = 0.004 exp (101000/RT) cm{dollar}rmsp2ssp{lcub}-l{rcub}{dollar}, R = 8.3 J mol{dollar}rmsp{lcub}-l{rcub} Ksp{lcub}-1{rcub}{dollar}, T = 600-800 K). A new linear free energy relationship for diffusive mass transport in condensed phase was shown to exist for the series of intermediate reactions in vanadium nitride synthesis: ln D{dollar}rmsb{lcub}i{rcub} = -0.27 Delta Gsp{lcub}o{rcub}sb{lcub}i{rcub}/RTsb{lcub}p{rcub}{dollar} + const. Here T{dollar}rmsb{lcub}p{rcub}{dollar} is the peak temperature for a transformation as it is observed in a TPR trace. An Evans-Polanyi type relation between activation energy and the heat of reaction was also found for this series.