H_2在Al_nCr(n=1-7)团簇上吸附和解离的密度泛函研究

Density functional theoretical investigation for the adsorption and dissociation of molecular hydrogen on Al_nCr(n=1-7) clusters

采用密度泛函理论中的B3LYP方法研究了H2在AlnCr(n=1-7)团簇上的吸附和解离.结果表明:AlnCr团簇结构与Aln+1团簇结构相似;物理吸附是H2以侧向的形式吸附在Cr原子上,H-H键长略微增长,H2的振动频率发生了红移;除了n=5外,其它AlnCr H2团簇的最稳定结构均是AlnCr团簇的最稳定结构与两个氢原子成键而成;AlnCr团簇向H原子转移了电荷;AlnCr H2团簇的平均结合能,垂直电离势和能隙均大于AlnCr团簇的,即AlnCr H2团簇比AlnCr团簇更稳定;Al7Cr对H2的化学吸附表现出较强的惰性,而AlnCr H2(n=1,2,6)则表现出较强的化学活性;由化学反应路径跟踪可知,通过改变AlnCr团簇中Al原子的个数可以调节H2的物理化学吸附行为.

The adsorption and dissociation of molecular hydrogen on AlnCr （n = 1 -7 ） clusters are investigated by using the method of B3LYP in density functional theory. The results show that the structures of AlnCr clusters are similar to those of Aln ＋ 1 clusters. H2 is easily adsorbed physically on Cr atom with a side - on orientation. The bond length of H - H increases slightly. The vibration frequency of H2 adsorbed is smaller than that of free molecular H2, namely, a red - shift occurs. Except for n = 5, the most stable structures of the other AlnCrH2 clusters are composed by the most stable structures of AlnCr clusters and two H atoms. Charge transfers from AlnCr clusters to H atoms. Average binding energy, vertical ionization potential and energy gap of AlnCrH2 clus- ters are all greater than those of AlnCr clusters, namely AlnCrH2 clusters are more stable than AlnCr clusters. A17Cr cluster shows stronger inertness for chemical adsorption of H2, while AlnCrH2 （n = 1,2,6） show stronger chemical activity. The physical and chemical adsorption behavior of H2 on AlnCr clusters can be adjusted by changing the number of Al atom in AlnCr clusters from tracking the chemical reaction path.