Xie Y, He S-G, Dong F, Bernstein E R
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Chem Phys. 2008 Jan 28;128(4):044306. doi: 10.1063/1.2813348.
Reactions of neutral V(n), Nb(n), and Ta(n) metal clusters (n< or =11) with CO+H(2) mixed gases and CH(3)OH in a flow tube reactor (1-50 Torr) are studied by time of flight mass spectroscopy and density functional theory calculations. Metal clusters are generated by laser ablation, and reactants and products are ionized by low fluence (approximately 200 microJ/cm(2)) 193 nm excimer laser light. Nb(n) clusters exhibit strong size dependent reactivity in reactions both with CO+H(2) and CH(3)OH compared with V(n) and Ta(n) clusters. A "magic number" (relatively intense) mass peak at Nb(8)COH(4) is observed in the reaction of Nb(n) clusters with CO+H(2), and CH(3)OH is suggested to be formed. This feature at Nb(8)COH(4) remains the most intense peak independent of the relative concentrations of CO and H(2) in the flow tube reactor. No other Nb(n), Ta(n), or V(n) feature behaves in this manner. In reactions of CH(3)OH with metal clusters M(n) (M=V, Nb, and Ta, n=3-11), nondehydrogenated products M(n)COH(4)/M(n)CH(3)OH are only observed on Nb(8) and Nb(10), whereas dehydrogenated products M(n)CO/CM(n)O are observed for all other clusters. These observations support the suggestion that CH(3)OH can be formed on Nb(8) in the reaction of Nb(n) with CO+H(2). A reaction mechanism is suggested based on the experimental results and theoretical calculations of this work and of those in the literature. Methanol formation from CO+H(2) on Nb(8) is overall barrierless and thermodynamically and kinetically favorable.
在流动管反应器（1 - 50托）中，通过飞行时间质谱和密度泛函理论计算研究了中性V(n)、Nb(n)和Ta(n)金属簇（n≤11）与CO + H₂混合气体以及CH₃OH的反应。金属簇通过激光烧蚀产生，反应物和产物通过低能量密度（约200微焦/平方厘米）的193纳米准分子激光进行电离。与V(n)和Ta(n)簇相比，Nb(n)簇在与CO + H₂以及CH₃OH的反应中表现出强烈的尺寸依赖性反应活性。在Nb(n)簇与CO + H₂的反应中，观察到在Nb(8)COH₄处有一个“幻数”（相对较强）质量峰，并推测形成了CH₃OH。在流动管反应器中，Nb(8)COH₄处的这一特征仍然是最强烈的峰，与CO和H₂的相对浓度无关。没有其他Nb(n)、Ta(n)或V(n)的特征表现如此。在CH₃OH与金属簇M(n)（M = V、Nb和Ta，n = 3 - 11）的反应中，仅在Nb(8)和Nb(10)上观察到未脱氢产物M(n)COH₄/M(n)CH₃OH，而在所有其他簇上观察到脱氢产物M(n)CO/CM(n)O。这些观察结果支持了在Nb(n)与CO + H₂的反应中，CH₃OH可在Nb(8)上形成的推测。基于这项工作以及文献中的实验结果和理论计算，提出了一种反应机理。在Nb(8)上由CO + H₂形成甲醇总体上无势垒，在热力学和动力学上都是有利的。
