(462f) High Surface Area Nitrides: New Base Catalysts

Early transition metal nitrides have received considerable attention for use as replacements for Pt-group metal catalysts. The nitrides are interstitial compounds. Charge transfer between the transition metal and nitrogen atoms as a consequence of the significant difference in electronegativity has been reported [1]. This charge transfer is expected to facilitate the creation of acid and base sites. Acid sites for high surface area Mo₂N catalysts have been reported [2]. The purpose of work described in this paper was to continue our investigation of the base properties of high surface area Mo₂N using CO₂ and NH₃ temperature programmed desorption (TPD) and the thermal decomposition of vapor phase 2-methyl-3-butyn-2-ol (MBOH). MBOH decomposes over weak base sites to form acetone and acetylene. Dehydration and isomerization occur over acid sites to form 3-methyl-2-buten-1-al (prenal) and 3-methyl-3-butene-1-yne (MByne), respectively [2, 3]. Although the basic site pathway is of primary interest here, acid products will be considered in order to fully elucidate the effect of pretreatment on the acid-base characteristics of Mo₂N. Because of its pyrophoric nature, the Mo₂N catalyst was passivated following synthesis in a mixture containing 1% O₂ in He to prevent bulk oxidation upon exposure to air. This formed an oxide or oxynitride passivation layer. This layer must be removed prior to activity and desorption studies. This paper describes the influence of the pretreatment conditions on the surface character of Mo₂N. The effects of reduction in H₂, as well as renitridation of the passivated catalyst in 10% NH₃ in He on acetone and MByne formation as well as CO₂ and NH₃ desorption were investigated.

Results and Discussion

Renitriding the Mo₂N surface at the synthesis temperature (700 °C) maximized basic surface density while minimizing surface acidity. This was evidenced by a comparison of acetone and Mbyne formation rates following the various pretreatments. The unpassivated sample possessed rates comparable to the renitrided surface, while the H₂ reduced sample exhibited slightly lower acetone formation rates and higher MByne rates. The degassed sample did not form acetone and displayed Mbyne formation rates comparable to the reduced sample. The renitrided and unpassivated catalysts desorbed the most CO₂ and the least NH₃ during TPD. There was significant CO₂ uptake from the reduced sample while the degassed sample did not desorb any CO₂. The highest amount of NH₃ was desorbed from the reduced and degassed surfaces. The presence of surface oxygen appeared to contribute to the acid-base character of the Mo₂N catalyst and its removal via reduction of the passivation layer enhanced surface basicity. Renitridation at 700 °C resulted in a slightly higher CO₂ uptake and significantly lower NH₃ desorption than those for the unpassivated catalyst, suggesting that the basic character of Mo₂N was enhanced by the addition of nitrogen to the surface and lattice structure. The effect of pretreatment on nitrogen and oxygen content at the Mo₂N surface is currently being investigated using x-ray photoelectron spectroscopy (XPS) and will be discussed along with an investigation of the MBOH reaction mechanism and effect of pretreatment on surface basicity using Diffuse Infrared Reflectance Transmission spectroscopy (DRIFTs). Finally, corresponding data for W₂N and VN will be presented.

References

1. H.O. Pierson, Handbook of Refractory Carbides and Nitrides, Noyes Publications, New Jersey, (1996). 2. Keller V., Lauron-Pernot, H., Djega-Mariadassou, G., J. Mol. Catal. A, 188 (2002) 163. 3. Handa, H., Fu, Y., Baba, and Ono, Y. Catal. Lett., 59, (1999) 195.