Molybdenum Nitride Catalysts
Molybdenum Nitride Catalysts
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DOI：
10.1006/jcat.1996.0121
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发表时间：
1996-04
影响因子：
7.3
通讯作者：
C. W. Colling;Jeong-Gil Choi;L. Thompson
中科院分区：
文献类型：
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作者：
C. W. Colling;Jeong-Gil Choi;L. Thompson
用热脱附光谱法对一系列比表面积达193 m2/g的氮化钼催化剂的表面化学进行了表征。这些材料是由MoO 3与NH3的程序升温反应制备的.钝化的催化剂在表面上或表面中含有多达一个单层的氧。在程序升温还原过程中，这些氧以H2 O的形式被除去：（a）在低于550 K的温度下与NH3分解产生的氢和合成后残留在表面上的氢反应;（B）在较高温度下与气相H2反应。氮和/或表面NHX物种也与气相H2反应产生NH3。的NH 3吸附容量是不同的每个减少钼氮化物，但是，在程序升温脱附光谱的其他变化定性相似。在低覆盖度下，NH3在500-700 K之间全部分解，产物以H2形式解吸，而在600 K以上以N2形式解吸。在300-500 K温度范围内，NH3在表面饱和后，大部分以分子形式脱附，其余部分分解.分子解吸分解的NH3的比例约为3：1的所有催化剂表明之间的连接NH3的解吸和分解。我们已经解释了结果，特别是对于高表面积的材料，饱和覆盖率低，在本地化的NH 3吸附方面可能形成岛屿在高覆盖率。吡啶HDN反应速率随NH3化学吸附量的增加而线性增加.在633 K时，相应的转换频率为4.1 × 10 −4 s −1。最后，NH3和H2脱附位点的类型数是表面积的函数。低比表面积、高活性的氮化钼具有低温和高温脱附位。对于高表面积、低活性材料，仅观察到低温位点。NH3从Mo氮化物上脱附的速率限制步骤为一级反应，低温位和高温位的脱附能分别为24 ± 4和32 ± 5 kcal/mol。
Abstract The surface chemistries of a series of Mo nitride catalysts with surface areas ranging up to 193 m 2 /g were characterized using thermal desorption spectroscopies. These materials were prepared by the temperature programmed reaction of MoO 3 with NH 3 . The passivated catalysts contained up to one monolayer of oxygen on or in the surface. This oxygen was removed as H 2 O via two pathways during temperature programmed reduction: (a) reaction with hydrogen from decomposed NH 3 and hydrogen residue left on the surface after synthesis at temperatures less than ∼550 K and (b) reaction with gas phase H 2 at higher temperatures. Nitrogen and/or surface NH x species also reacted with gas phase H 2 producing NH 3 . The NH 3 adsorption capacities were different for each of the reduced Mo nitrides; however, other variations in the temperature programmed desorption spectra were qualitatively similar. At low coverages, all of the NH 3 decomposed and the products desorbed as H 2 at 500–700 K and N 2 above 600 K. Following saturation of the surface, most of the NH 3 desorbed molecularly at 300–500 K and the balance decomposed. The ratio of molecularly desorbed to decomposed NH 3 was approximately 3 : 1 for all the catalysts suggesting a connection between the desorption and decomposition of NH 3 . We have interpreted the results, in particular for the high surface area materials where the saturation coverage was low, in terms of localized NH 3 adsorption perhaps forming islands at high coverage. The pyridine HDN reaction rate increased linearly with the amount of NH 3 chemisorbed. The corresponding turnover frequency was 4.1 × 10 −4 s −1 at 633 K. Finally, the number of types of NH 3 and H 2 desorption sites was a function of the surface area. The low surface area, high activity Mo nitrides possessed low and high temperature desorption sites. Only the low temperature sites were observed for the high surface area, low activity materials. The rate limiting step for NH 3 desorption from the Mo nitrides was first order with desorption energies for the low and high temperature sites of 24 ± 4 and 32 ± 5 kcal/mol, respectively.