Gálvez-González Luis E, Posada-Amarillas Alvaro, Paz-Borbón Lauro Oliver
Programa de Doctorado en Ciencias (Física), División de Ciencias Exactas y Naturales, Universidad de Sonora, Blvd. Luis Encinas y Rosales, Hermosillo, Sonora 83000, Mexico.
Departamento de Investigación en Física, Universidad de Sonora, Blvd. Luis Encinas y Rosales, Hermosillo, Sonora 83000, Mexico.
J Phys Chem A. 2021 May 27;125(20):4294-4305. doi: 10.1021/acs.jpca.0c11303. Epub 2021 May 19.
Bimetallic Re-Pt is a widely used catalyst in petroleum reforming to obtain high-octane gasoline, but experimental and theoretical information of such systems at the subnanometer scale-namely, as cluster aggregates-is currently lacking. Thus, in this work, we performed a density functional theory-based global optimization study to determine the physicochemical properties of the most stable Re-Pt gas-phase clusters up to six atoms for all compositions. Our results indicate that in these putative global minima (GM) geometries, Re atoms tend to aggregate, while most Pt atoms remain separated from each other. This is even observed in Pt-rich clusters-an indication of the strength of the Re-Re and Re-Pt bonds over pure Pt-Pt ones-due to a strong, directional hybridization of the Re half-filled 5d and the nearly full Pt 5d states. We observe that doping monometallic Pt clusters even with a single Re atom increases their binding energy values and widens the bimetallic cluster highest occupied molecular orbital-lowest unoccupied molecular orbital gap. As catalysis occurs at elevated temperatures, we explore the concept of cluster fluxionality for Re-Pt minima in terms of the calculated isomer occupation probability, (). This allows us to quantify the abundance of GM and low-energy isomer configurations as a function of temperature. This is done at size 5 atoms due to the wide isomer observed variety. Our calculations indicate that for pure Re, the () of the GM configuration substantially decreases after 750 K. Especially, for RePt, the GM is the dominant structure up to nearly 700 K when the second-energy isomer becomes the stable one. Although no ordering changes are seen for RePt, RePt, and RePt, we do observe a structural transition-between the GM and the second isomer-for pure Pt above 1000 K. We expect this type of combined analysis to add to the overall, continuous understanding of the stability and energetics of ultrafine and highly-dispersed Re-Pt petroleum-reforming catalysts and the scarce available information on this particular bimetallic system.
双金属Re-Pt是石油重整中广泛使用的催化剂，用于生产高辛烷值汽油，但目前缺乏关于此类系统在亚纳米尺度（即团簇聚集体）的实验和理论信息。因此，在本工作中，我们进行了基于密度泛函理论的全局优化研究，以确定所有组成的最稳定的含六个原子以内的Re-Pt气相团簇的物理化学性质。我们的结果表明，在这些假定的全局极小值（GM）几何结构中，Re原子倾向于聚集，而大多数Pt原子彼此分离。这在富Pt团簇中也能观察到——这表明Re-Re和Re-Pt键比纯Pt-Pt键更强——这是由于Re的半充满5d态与几乎全满的Pt 5d态之间存在强烈的定向杂化。我们观察到，即使只用一个Re原子掺杂单金属Pt团簇，也会增加它们的结合能值，并扩大双金属团簇的最高占据分子轨道-最低未占据分子轨道间隙。由于催化作用在高温下发生，我们根据计算得到的异构体占据概率（）来探讨Re-Pt极小值的团簇流动性概念。这使我们能够量化GM和低能异构体构型的丰度随温度的变化。由于观察到的异构体种类繁多，这是在5原子尺寸下完成的。我们的计算表明，对于纯Re，GM构型的（）在750 K后大幅下降。特别是，对于RePt，在接近700 K之前GM是主导结构，此时第二能量异构体成为稳定结构。虽然对于RePt、RePt和RePt没有观察到有序变化，但我们确实观察到纯Pt在1000 K以上会发生GM与第二异构体之间的结构转变。我们期望这种类型的综合分析能够增进对超细和高度分散的Re-Pt石油重整催化剂的稳定性和能量学的全面、持续理解，以及关于这个特定双金属系统的稀缺可用信息。
