Synthesis and catalytic properties of porous Ta carbide crystallites for hydrogen production from the decomposition of ammonia

Synthesis and catalytic properties of porous tantalum carbide crystallites for hydrogen production from the decomposition of ammonia were investigated using eight different samples which have been synthesized via temperature-programmed reduction method of Ta2O5 with pure CH4. The Ta carbide crystallites synthesized using two different heating rates and space velocity showed the different surface areas. These results indicated that the structural properties of these materials have been related to the preparative conditions:heating rate and space velocity employed. The O-2 uptake has a linear relation with the BET surface area of tantalum carbides. The slope of the straight line corresponds to a site density showing an oxygen capacity of 1.36 x 10(13) O/cm(2). The tantalum carbide crystallites were very active for ammonia decomposition with catalytic properties that were superior to commercial Ta carbide catalysts (Aldrich). It was also observed that the tantalum carbide crystallites were as much as two orders of magnitude more active than Pt/C catalyst (Engelhard). The activity increased with the decrease of particle size, suggesting the presence of the structure-sensitivity. The highest activity in tantalum carbides was observed at an atomic ratio of C-1/Ta delta+ = 0.85, suggesting the presence of electron transfer between Ta metals and carbon (C-1) in Ta carbide crystallites.