NO在赤铁矿(0001)表面吸附的第一性原理计算

Ab initio simulations of NO adsorption on hematite(0001)surface:PBE versus PBE+U

氮氧化物NO_(x)(NO,NO_(2))是对人类健康有严重危害的大气污染物。近年来研究表明赤铁矿(α-Fe_(2)O_(3))可作为高效光催化剂去除大气中NO_(x),但NO_(x)气体在赤铁矿表面的吸附特性还未明确,阻碍了对其催化机理的进一步认识。基于密度泛函理论,采用包括电子强关联效应的PBE+U以及色散力修正的方法,对NO气体分子在α-Fe_(2)O_(3)(0001)晶面的吸附行为进行深入研究,发现基于PBE+U方法获得的吸附能(Ead=-0.64 eV)比PBE获得的Ead(-1.31 eV)低近50%。这是由于电子强关联项U的引入降低了表面铁原子d轨道对价带顶的贡献,抑制了其化学活性,而NO具有一个未成对的π*轨道电子,使得其对吸附基体的电子结构格外敏感。与+U不同,色散力修正不会显著改变体系的电子结构,只是使Ead略有增加(-0.18 eV)。采用统计力学的Langmuir公式计算NO在α-Fe_(2)O_(3)(0001)表面的热力学平衡占据数,发现基于+U的吸附能得到的平衡占据数与实验观测更为一致。这些结果揭示了电子强关联效应在α-Fe_(2)O_(3)表面化学中的重要作用,并为进一步研究NO_(x)在α-Fe_(2)O_(3)表面的光催化反应机理奠定了基础。

NO_(x)(x=1,2)are major air-pollutants detrimental to human health and much effort has been devoted to find efficient photocatalysts capable of removing NO_(x) from air(de-NO_(x)).Recent experiments indicate that hematite(α-Fe_(2)O_(3))is a promising de-NO_(x) photocatalyst.However some key features of the NO adsorption on the hematite surface remain unclear,hindering further comprehension of the photocatalytic process.Here we study the adsorption of NO on the hematite(0001)surface using the PBE+U method with a dispersion correction(vdw)in the framework of density functional theory(DFT).We find the addition of a Hubbard U term in the DFT Hamiltonian strongly affects the adsorption properties,with the adsorption energy(-0.64 eV)decreased by 50%with respect to those of PBE(-1.31 eV).This decrease is attributed to two factors:(i)the U term shifts the energy of Fe 3d orbitals away from the valence band maximum,making them chemically less active;(ii)the NO molecule has an unpairedπ*electron and is more sensitive to the electronic structure of the substrate.In contrast to the inclusion of U,the dispersion correction causes little change to the adsorption properties except increases the adsorption energy by about-0.18 eV.We use the Langmuir formula to calculate the thermal equilibrium coverage of NO on the hematite(0001)surface and find predictions made with the PBE+U vdw are more consistent with experiments.These results highlight the importance of strong electronic correlations in describing the hematite surface reactions,and may serve as a starting point to unravel the complete photocatalytic mechanism.