Ba_xO_y小团簇修饰Ru(0001)表面的结构稳定性和氮分子吸附性质

Geometric stability and nitrogen adsorption properties of small Ba_xO_y cluster-modified Ru(0001) surface

为了说明钡助剂的存在形式,本文采用第一性原理方法研究了BaxOy小团簇修饰Ru(0001)表面的结构稳定性和氮分子吸附性质.基于总能的热力学分析发现,在实验条件下(500 K,PH2O/PH2<10-3),Ba2O团簇比Ba O2,Ba O,Ba和O等团簇(原子)更加稳定.这证实含有金属性钡原子的团簇也是氧化钡助剂可能的工作状态.表面电荷差分密度说明Ba2O团簇的氧和钡原子与衬底的作用不同.不过Ba2O团簇氧和钡原子附近的氮分子吸附行为相似,Ba2O团簇增强了氮分子和衬底的相互作用.Ba2O团簇氧和钡原子附近的氮分子吸附能分别为0.78和0.88 e V,均大于清洁表面的0.67 e V.氮分子间距和氮分子的拉伸振动频率都表明Ba2O团簇在一定程度上活化了吸附氮分子.Ba2O团簇氧和钡原子附近的N—N键长分别为0.117和0.116nm,大于清洁表面的0.114 nm.氧和钡原子附近氮分子的拉伸振动频率分别为1888和1985 cm-1,小于清洁表面的2193 cm-1.电荷差分密度的计算结果说明,削弱作用主要来自于Ba2O团簇中钡离子和氮分子间的静电作用.两者间的静电作用增加了氮分子π反键轨道的占据数,促进了氮分子极化,从而削弱氮分子键.

Barium promoter is widely used in the secondary ammonia synthesis catalysis, which could greatly improve the performance of a catalyst. Although barium oxide is confirmed as the main component of barium promoter, the existence of metallic barium has been argued. In order to theoretically clarify this issue, the first principles calculations have been performed to study the geometric stability and the nitrogen adsorption properties of small BaxOy cluster-modified Ru（0001） surface. It is found that Ba20 cluster is more stable than other small clusters or atoms （BaO2, BaO, Ba and O） on the Ru（0001） surface under the condition that the pressure rate of H2O/H2 is below 1%. This implies that BaO promoter could be partially reduced by hydrogen gas in the experiment. According to the results of the projected density of states and charge difference induced by modification of cluster, the O atom in BaRO cluster gains electrons from dz2 orbit of the underlying Ru atom, and forms O--Ru bonds; while Ba atom in Ba20 clusters transfers electrons to the nearest Ru atoms and forms Ba-Ru metallic bonds. As the adsorption of nitrogen is an initial reactant in ammonia synthesis, we also study the nitrogen adsorption properties near the Ba20 cluster. Compared with the different chemical properties of O and Ba atoms, the adsorption properties of nitrogen molecules on the sites close to O and Ba atoms are similar. The nitrogen adsorption energies at the corresponding sites are calculated to be 0.88 and 0.78 eV, respectively. The bond lengths of nitrogen molecules are about 0.187 nm near O atom, and 0.190 nm near Ba atom, both of which are shorter than those on a clean surface （ ～0.197 nm）. And the stretching vibrational frequency of a nitrogen molecule is calculated to be 1888 cm-1 near the O atom, 1985 cm-1 near the Ba atom, both of which are also less than those on a clean surface （～2193 cm-1）. This suggests that Ba20 cluster may weaken the bond strength of nitrogen molecules. According to the charge difference induced by nitrogen adsorption, the electrostatic interactions of Ba2 O clusters increase the occupation of π antibonding orbital and the electric polarization of the nitrogen molecule, and thus weaken the N--N bond.