Dissociation Rates of Urea in the Presence of NiOOH Catalyst: A DFT Analysis
Dissociation Rates of Urea in the Presence of NiOOH Catalyst: A DFT Analysis
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DOI：
10.1021/jp105159t
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发表时间：
2010-11-04
影响因子：
2.9
通讯作者：
Botte, Gerardine G.
中科院分区：
文献类型：
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作者：
Daramola, Damilola A.;Singh, Deepika;Botte, Gerardine G.
已经研究了羟基氧化镍、尿素和氢氧根离子之间的单分子反应，以了解尿素解离成氨、异氰酸、氰酸根离子、二氧化碳和氮气的过程。在不存在氢氧根离子的情况下，羟基氧化镍将催化尿素形成氨和异氰酸，其中限速步骤是以1.5 x 10（-6）s（-1）的速率常数形成氨。在氢氧化物存在下，氨的析出也是速率限制步骤，速率常数为1.4 × 10（-26）s（-1）。此外，氰酸根离子的脱附呈现6190 kJ mol（-1）的能垒，这表明除非发生进一步的反应，否则氰酸根离子不能与NiOOH分离。最后，分析了氢氧根离子使尿素去质子化生成氮气和二氧化碳的基元解离反应。这些基元反应被研究沿着三个路径不同的顺序，质子被删除和氮原子被旋转。发现速率限制步骤是二氧化碳的去除，速率常数为4.3 × 10（-65）s（-1）。因此，催化剂可能因二氧化碳吸附引起的表面堵塞而失活。
Single molecule reactions have been studied between nickel oxyhydroxide, urea, and the hydroxide ion to understand the process of urea dissociation into ammonia, isocyanic acid, cyanate ion, carbon dioxide, and nitrogen. In the absence of hydroxide ions, nickel oxyhydroxide will catalyze urea to form ammonia and isocyanic acid with the rate-limiting step being the formation of ammonia with a rate constant of 1.5 x 10(-6) s(-1). In the presence of hydroxide, the evolution of ammonia was also the rate-limiting step with a rate constant of 1.4 x 10(-26) s(-1). In addition, desorption of the cyanate ion presented an energy barrier of 6190 kJ mol(-1) suggesting that the cyanate ion cannot be separated from NiOOH unless further reactions occurred. Finally, elementary dissociation reactions with hydroxide ions deprotonating urea to produce nitrogen and carbon dioxide were analyzed. These elementary reactions were investigated along three paths differing in the order that protons were removed and the nitrogen atoms were rotated. The rate-limiting step was found to be the removal of carbon dioxide with a rate constant of 4.3 x 10(-65) s(-1). Therefore, the catalyst could be deactivated by the surface blockage caused by carbon dioxide adsorption.