N2在Pd金属表面的吸附行为

对Pd原子采用相对论有效原子实势(RECP/SDD), N原子采用AUG-cc-pVTZ基函数, 利用B3LYP方法计算了PdN和PdN2分子的微观结构以及不同温度下的热力学函数. 以气态分子总能量中的振动能EV代替该分子处于固态时的振动能量, 以电子运动和振动运动熵SEV代替分子处于固态的熵的近似方法, 计算了不同温度下金属Pd与N2反应的ΔHӨ、ΔSӨ、ΔGӨ及氮化反应平衡压力, 导出了氮化反应平衡压力与温度的关系. 由此可看出, 在常压及298.15～998.15 K温度条件下, N2在金属Pd表面的吸附过程以Pd(s) + N2 = PdN2(s)反应进行. 计算得出在标准条件下, PdN(s)的生成焓为254.37 kJ·mol-1, PdN2(s)的生成焓为－80.59 kJ·mol-1. 并与Pd氢化反应平衡压力比较, 得到平衡常数Kp(N2)比Kp(H2)约小两个数量级, 说明N2较难被金属Pd表面吸附, 在热力学上有利于氢置换氮.

Adsorption Behavior of N2 on Pd Surface

Based on the relativistic effective core potential (RECP/SDD) for Pd atom and AUG-cc-pVTZ basis function for N atom, the structure of PdN and PdN2 have been optimized using B3LYP method, and the thermodynamic functions for PdN and PdN2 have been calculated. The vibrational energy, electronic and vibrational entropy of the molecules in their solid states are assumed, ΔHӨ, ΔSӨ and ΔGӨ and nitrogen equilibrium pressures of the nitrogenating reaction have been calculated based on this approximation. It is deduced that at 1.01325×105 Pa and 298.15～998.15 K, adsorption of N2 on Pd surface proceeds as Pd(s) + N2 = PdN2(s). The formation enthalpy for PdN(s) at 298.15 K is 254.37 kJ·mol-1, and the formation enthalpy for PdN2(s) at 298.15 K is －80.59 kJ·mol-1. The nitrogen equilibrium pressure is about 100 times of the hydrogen equilibrium pressure, so it is difficult for Pd to adsorb N2.