Many heterogeneous reactions catalyzed by nanoparticles occur at relatively high temperatures, which may modulate the surface morphology of nanoparticles during reaction. Inspired by the discovery of dynamic formation of active sites on gold nanoparticles, we explore theoretically the nature of the highly mobile atoms on the surface of nanoparticles of various sizes for 11 transition metals. Using molecular dynamics simulations, on a 3 nm Fe nanoparticle as an example, the effect of surface premelting and overall melting on the structure and physical properties of the nanoparticles is analyzed. When the nanoparticle is heated up, the atoms in the outer shell appear amorphous already at 900 K. Surface premelting is reached at 1050 K, with more than three liquid atoms, based on the Lindemann criterion. The activated atoms may transfer their extra kinetic energy to the rest of the nanoparticle and activate other atoms. The dynamic studies indicate that the number of highly mobile atoms on the surface increases with temperature. Those atoms with a high Lindemann index, usually located on the edges or vertices, attain much higher kinetic energy than other atoms and potentially form different active sites in situ. When the temperature passes the surface premelting temperature, a drastic change in the coordination number (SCN) of the surface atoms occurs, with attendant dramatic broadening of the distribution of the SCN, suppling active sites with more diverse atomic coordination numbers. The electronic density of states of a nanoparticle tends to “equalize”, due to the breaking of the translational symmetry of the atoms in the nanoparticle, and the d-band center of the nanoparticle moves further away from the Fermi level as the temperature increases. Besides Au, other nanoparticles of the transition metals, such as Pt, Pd, and Ag, may also have active sites easily formed in situ.

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用于非均相催化的过渡金属纳米粒子的表面活化：我们可以从分子动力学中学到什么

纳米粒子催化的许多异质反应都在相对较高的温度下发生，这可能会在反应过程中调节纳米粒子的表面形态。受发现在金纳米粒子上动态形成活性位的启发，我们从理论上探索了11种过渡金属的各种尺寸纳米粒子表面上高度可移动原子的性质。使用分子动力学模拟，以3 nm Fe纳米颗粒为例，分析了表面预熔融和整体熔融对纳米颗粒的结构和物理性能的影响。当纳米粒子被加热时，外壳中的原子在900 K时就已经显示为非晶态。根据Lindemann准则，表面预熔融在1050 K时达到了三个以上的液态原子。活化的原子可以将其额外的动能转移至其余的纳米粒子，并活化其他原子。动力学研究表明，表面上高度可移动的原子数随温度增加而增加。具有高Lindemann指数的原子（通常位于边缘或顶点）比其他原子具有更高的动能，并可能在原位形成不同的活性位点。当温度超过表面预熔温度时，表面原子的配位数（SCN）发生剧烈变化，随之而来的是SCN的分布急剧变宽，从而提供了具有更多原子配位数的活性位。由于破坏了纳米粒子中原子的平移对称性，纳米粒子状态的电子密度趋于“均衡”，随着温度的升高，纳米粒子的d带中心逐渐远离费米能级。除金以外，过渡金属的其他纳米粒子（例如Pt，Pd和Ag）也可能具有易于在原位形成的活性位。