Rutile-Deposited Pt-Pd clusters: A Hypothesis Regarding the Stability at 50/50 Ratio
Rutile-Deposited Pt-Pd clusters: A Hypothesis Regarding the Stability at 50/50 Ratio
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DOI：
10.1021/cs5011426
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发表时间：
2014-10-01
期刊：
影响因子：
12.9
通讯作者：
Alexandroya, Anastassia
中科院分区：
文献类型：
--
作者：
Ha, Mai-Anh;Dadras, Jonny;Alexandroya, Anastassia
沉积在氧化物上的混合 Pt-Pd 簇对催化作用非常重要。发现含有大致相等比例的 Pt 和 Pd 的簇对烧结异常稳定，而烧结是催化剂失活的主要机制之一。此类催化剂老化后，50/50 Pt-Pd 和 Pd-O 簇似乎是最常见的两个相。这些等比例的簇稳定性增强的原因仍不清楚。接下来，模拟了不同初始 Pt 和 Pd 原子浓度以及一系列催化相关温度下混合 Pt-Pd 粉剂在 TiO2(110) 上的烧结。经证实，均匀混合的簇具有相对最高的存活率。令人惊讶的是，含有各种比例的 Pt 和 Pd 的亚纳米团簇具有非常相似的几何形状和化学键合，没有揭示出支持 1:1 Pt/Pd 比例的明显解释。然而，人们发现，在高温下，50/50 团簇比含有其他比例 Pt 和 Pd 的分散体具有更多的热可接近异构体。因此，稳定的原因之一是熵稳定。静电在微妙的电荷重新分布方面也发挥着关键作用，电子密度向电负性稍强的 Pt 移动，导致部分带电原子通过簇内库仑吸引力进一步稳定；对于 1:1 混合物，这种效果最大。
Mixed Pt-Pd clusters deposited on oxides have been of great interest to catalysis. Clusters containing Pt and Pd in roughly equal proportions were found to be unusually stable against sintering, one of the major mechanisms of catalyst deactivation. After aging of such catalysts, the 50/50 Pt-Pd and Pd-O clusters appeared to be the two most prevalent phases. The reason for the enhanced stability of these equally proportioned clusters has remained unclear. In the following, sintering of mixed Pt-Pd dusters on TiO2(110) for various initial atomic concentrations of Pt and Pd and at a range of catalytically relevant temperatures was simulated. It is confirmed that equally mixed clusters have the relatively highest survival rate. Surprisingly, subnanoclusters containing Pt and Pd in all proportions have very similar geometries and chemical bonding, revealing no apparent explanation for favoring the 1:1 Pt/Pd ratio. However, it was discovered that at high temperatures, the 50/50 clusters have considerably more thermally accessible isomers than dusters containing Pt and Pd in other proportions. Hence, one of the reasons for stability is entropic stabilization. Electrostatics also plays a key role as a subtle charge redistribution, and a shift of electron density to the slightly more electronegative Pt results in the partially charged atoms being further stabilized by intracluster Coulomb attraction; this effect is greatest for 1:1 mixtures.