单斜氧化锆表面水合和脱水的动态模拟

Dynamic Simulation on Surface Hydration and Dehydration of Monoclinic Zirconia

常用的氧化物负载金属催化剂通常在水相中制备,且在使用前常常需要经过煅烧因此,氧化物载体表面的水合和脱水过程对于负裁型金属催化剂的真实建模至关重要.通过第一性原理分子动力学模拟,本文考察了温和温度下无水单斜氧化锆(111)表面在显式溶剂水中的演化,在模拟过程中,所有的双重配位桥位氧位点很快被溶剂水质子化,形成酸性羟基(HOL),并在锆原子上留下碱性羟基(HO^(*)).这些碱性羟基(HO^(*))可以与表面未解离的吸附水分子(H_(2)O^(*))进行活跃的质子交换,进而在表面自由扩散,在278 K到373 K的温度范围下,第一性原理分子动力学水相模拟可以得到一种较为确定的、有代表性的平衡水合单斜氧化锆(11)表面,其表面锆原子上覆盖度(θ)为0.75.随后,为了模拟低于800 K的温和煅烧温度下的表面脱水过程,本文使用密度泛函理论计算了表面水分子的逐步脱附自由能,通过获得表面的脱水相图,总结了不同煅烧温度下有代表性的、部分水合的单斜氧化锆(111)表面(0.25<θ<0.75).这些水合单斜氧化锆(111)表面具有重要的理论意义,可以方便快捷地被应用于氧化锆催化剂及氧化锆负裁金属催化剂的真实建模与模拟.

The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase,which then often need to undergo calcination before usage.Therefore,the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts.In this work,by ab initio molecular dynamics(AIMD)simulations,the initial anhydrous monoclinic ZrO_(2)(111)surfaces are evaluated within explicit solvents in aqueous phase at mild temperatures.During the simulations,all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls(HO_(L)),remaining the basic hydroxyls(HO^(∗))on Zr.The basic hydroxyls(HO^(∗))can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption water(H_(2)O^(∗)).Within the temperatures ranging from 273 K to 373 K,in aqueous phase a certain representative equilibrium hydrated m-ZrO_(2)(¯111)surface is obtained with the coverage(θ)of 0.75 on surface Zr atoms.Later,free energies on the stepwise surface water desorption are calculated by density functional theory to mimic the surface dehydration under the mild calcination temperatures lower than 800 K.By obtaining the phase diagrams of surface dehydration,the representative partially hydrated m-ZrO_(2)(111)surfaces(0.25≤θ<0.75)at various calcination temperatures are illustrated.These hydrated m-ZrO_(2)(111)surfaces can be crucial and readily applied for more realistic modeling of ZrO_(2) catalysts and ZrO_(2)-supported metal catalysts.