Ethylene Dehydrogenation on Pt_4,7,8 Clusters on Al₂O₃: Strong Cluster Size Dependence Linked to Preferred Catalyst Morphologies

Abstract

Catalytic dehydrogenation of ethylene on size-selected Pt n (n = 4, 7, 8) clusters deposited on the surface of Al 2 O 3 was studied experimentally and theoretically. Clusters were mass-selected, deposited on the alumina support, and probed by a combination of low energy ion scattering, temperature-programmed desorption and reaction of C 2 D 4 and D 2 , X-ray photoelectron spectroscopy, density functional theory, and statistical mechanical theory. Pt 7 is identified as the most catalytically active cluster, while Pt 4 and Pt 8 exhibit comparable activities. The higher activity can be related to the cluster structure and particularly to the distribution of cluster morphologies accessible at the temperatures and coverage with ethylene in catalytic conditions. Specifically, while Pt 7 and Pt 8 on alumina have very similar prismatic global minimum geometries, Pt 7 at higher temperatures also has access to single-layer isomers, which become more and more predominant in the cluster catalyst ensemble upon increasing ethylene coverage. Single-layer isomers feature greater charge transfer from the support and more binding sites that activate ethylene for dehydrogenation rather than hydrogenation or desorption. Size-dependent susceptibility to coking and deactivation was also investigated. Our results show that size-dependent catalytic activity of clusters is not a simple property of single cluster geometry but the average over a statistical ensemble at relevant conditions.

Boron Switch for Selectivity of Catalytic Dehydrogenation on Size-Selected Pt Clusters on Al2O3

“…Thus, the nucleophilici--ty of the Pt 7 B/alumina ensemble is substantially reduced compared to pure Pt 7 /alumina. There is charge separation 1 , thus a substantial reduction in ethylene adsorption energy would be predicted just based on the effects of boron on clusters' charge, consistent with the TPD/R results.…”

“…Only a small recovery of Pt ISS signal is seen after 700 K heating to desorb adventi--tious CO, indicating that heating also causes structural changes that result in a smaller fraction of Pt in the sur--face layer. 1 Initial exposure of a Pt 7 /alumina sample to 1.5 L of B 2 H 6 at 130 K results in ~80 % attenuation of the Pt ISS signal (Figure 1a), demonstrating that diborane binds efficiently on top of Pt 7 . 1,24 Note that 1.5 L exposure may lead to ad--sorption of more than one diborane per cluster.…”

“…Size--selected Pt 4,7,8 on alumina have been prepared as discussed in detail previously, 1,24 and then borated by ex--posure to diborane (B 2 H 6 ). Boration and its effects on binding and dehydrogenation of a model alkene, eth--ylene, were probed by temperature programmed desorp--tion/reaction (TPD/R), low energy ion scattering (ISS), plane wave density--functional theory (PW--DFT) calcula--tions, and molecular dynamics (MD) simulations.…”

“…Moreover, with increasing coverage, ethylene reflects less cooperative adsorption on Pt 7 B than observed on Pt 7 : it destabilizes the system by ~0.2 eV/ethylene in Pt 7 B, but stabilizes by 0.3 eV/ethylene in Pt 7 . 1 The Pt 7 B cluster ensemble also activates a decreasing fraction of ethylene as compared to Pt 7 . 1 Pt 7 binds and activates for dehydrogenation more and more ethylene as the temperature and coverage increase.…”

“…We previously showed that the onset tem--perature for D 2 desorption is determined by the activation energy for C 2 D 4 dehydrogenation rather than for D 2 de--sorption. 1 Crampton et al recently reported a study of ethylene hydrogenation to ethane over size--selected Pt n deposited on MgO that provides an interesting point of compari--son. 36 In their experiment, they coadsorbed hydrogen and ethylene before carrying out TPR, and measured desorp--tion of ethane.…”