Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA.
Faraday Discuss. 2013;162:9-30. doi: 10.1039/c3fd00094j.
Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These are key elements in technologies for the clean production and use of fuels and chemicals. We show here that the chemical reactivity of the surface metal atoms on these nanoparticles is closely related to their chemical potential: the higher their chemical potential, the more strongly they bond to small adsorbates. Controlling their chemical potential by tuning the structural details of the material can thus be used to tune their reactivity. As their chemical potential increases, this also makes the metal surface less noble, effectively pushing its behavior upwards and to the left in the periodic table. Also, when the metal atoms are in a nanoparticle with higher chemical potential, they experience a larger thermodynamic driving force to sinter. Calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces in ultrahigh vacuum show that the chemical potential increases with decreasing particle size below 6 nm, and, for a given size, decreases with the adhesion energy between the metal and its support, Eadh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, Eadh increases with (deltaHsub,M--deltaHf,MOx)/OmegaM2/3 for the metal, where deltaHsub,M is its heat of sublimation, deltaHf,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and OmegaM is the atomic volume of the bulk solid metal. The value deltaHsub,M--deltaHf,MOx equals the heat of formation of that metal's oxide from a gaseous metal atom plus O2(g), so it reflects the strength of the chemical bonds which that metal atom can make to oxygen, and OmegaM2/3 simply normalizes this energy to the area per metal atom, since Eadh is the adhesion energy per unit area. For a given metal, Eadh to different clean oxide surfaces increases as: MgO(100) approximately TiO2(110) < or = alpha-Al2O3(0001) < CeO2-x(111) < or = Fe3O4(111). Oxygen vacancies also increase Eadh, but surface hydroxyl groups appear to decrease Eadh, even though they increase the initial heat of metal adsorption.
许多催化剂由金属纳米粒子锚定在氧化物载体的表面组成。这些是燃料和化学品清洁生产和使用技术的关键要素。我们在这里表明，这些纳米粒子表面金属原子的化学活性与其化学势密切相关：化学势越高，它们与小吸附物的结合就越强。通过调整材料的结构细节来控制其化学势，可以用来调节它们的反应性。随着化学势的增加，金属表面也变得不那么惰性，有效地将其行为向上推并向左推到元素周期表中。此外，当金属原子处于具有较高化学势的纳米粒子中时，它们经历更大的烧结热力学驱动力。在超高真空下测量金属蒸气吸附能到清洁氧化物表面的量热法表明，化学势在粒径小于 6nm 时随粒径减小而增加，并且对于给定尺寸，随金属与其载体之间的粘附能 Eadh 减小而减小。控制金属/氧化物粘附能的结构因素因此也是调节催化性能的关键。对于给定的氧化物，Eadh 随金属的 (deltaHsub,M--deltaHf,MOx)/OmegaM2/3 增加，其中 deltaHsub,M 是其升华热，deltaHf,MOx 是该金属最稳定氧化物的标准生成热（每摩尔金属），OmegaM 是块状固体金属的原子体积。值 deltaHsub,M--deltaHf,MOx 等于从气态金属原子和 O2(g)形成该金属氧化物的生成热，因此它反映了该金属原子与氧形成化学键的强度，并且 OmegaM2/3 将该能量归一化为每个金属原子的面积，因为 Eadh 是单位面积的粘附能。对于给定的金属，Eadh 到不同的清洁氧化物表面的增加顺序为：MgO(100)~TiO2(110)<=alpha-Al2O3(0001)<CeO2-x(111)<=Fe3O4(111)。氧空位也会增加 Eadh，但表面羟基似乎会降低 Eadh，尽管它们会增加金属吸附的初始热。
