Treesukol Piti, Srisuk Kanokthip, Limtrakul Jumras, Truong Thanh N
Henry Eyring Center for Theoretical Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
J Phys Chem B. 2005 Jun 23;109(24):11940-5. doi: 10.1021/jp0511348.
The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Brønsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Brønsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Brønsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.
采用密度泛函理论/B3LYP泛函方法，利用嵌入簇模型和团簇模型，研究了分散在H-ZSM-5沸石上的Pt原子与金属载体的相互作用。我们发现，Pt原子与一个布朗斯台德质子和一个邻近的骨架氧相互作用。与骨架氧的相互作用导致电子从沸石转移到Pt原子。同时，一个布朗斯台德质子通过从Pt原子中提取过量的电子密度，使Pt原子在沸石表面稳定下来。这些电荷转移导致Pt原子净电荷为零，同时改变其轨道占据情况。Pt在布朗斯台德酸上的结合能为15 kcal/mol。采用静电嵌入势的表面电荷表示法（SCREEP）包含马德隆势，对Pt/H-ZSM-5的结构和电荷密度影响较小，但使CO的拉伸模式向高频移动了近40 cm(-1)。与实验中20 cm(-1)的红移相比，用嵌入簇模型计算的吸收CO的频移为8至11 cm(-1)的红移。这意味着，不仅Pt原子的电子态，而且载体的马德隆势，都导致了在Pt-H-ZSM-5上观察到的CO的小红移。
