(750g) Surface Characterization of 2D Metal-Supported Bilayer Silica and Aluminosilicates As Model Zeolites

Zeolites have been a cornerstone of industrial catalysis for many decades and are essential for a wide range of reactions. While three-dimensional silica and aluminosilicates have been studied widely, their typical micron particle sizes coupled with internal 5-10 Å pores containing active sites have limited the application of direct surface microscopic and spectroscopic techniques for measurement of reactions in these materials. Two-dimensional analogs have only recently been discovered and synthesized, with applications as model zeolite surfaces and potentially as atomically-thin size-selective membrane materials. In the silica bilayers, corner-shared SiO₄ tetrahedra arrange on metal substrate surfaces to form two mirror-image layers with all bonds saturated, ultimately consisting of all six-membered rings in the crystalline phase and a range of four- through nine-membered rings in the amorphous (vitreous) phase. Through the substitution of Al into the silica bilayers to form Al_xSi_1-xO₂, Brønsted acid sites analogous to those in zeolites can be generated, therefore the Al_xSi_1-xO₂ bilayer can be considered as an acid zeolite model system. In this work, we have focused on the synthesis and fundamental understanding of these two-dimensional silica and aluminosilicate bilayer materials. To this end, we have synthesized a range of bilayers over a continuously tunable Ni-Pd(111) alloy system in ultra-high vacuum to achieve the desired strain match to the bilayer composition. Whether the film becomes crystalline or amorphous depends on the atomic spacing of the underlying metal substrate and the growth conditions, and by careful substrate selection the resulting observed bilayer may be carefully controlled even though the bilayer sheets interact with the substrate through van der Waals forces alone. Surface diffraction, spectroscopy, and scanning tunneling microscopy reveal how the bilayers accommodate the underlying strain and form coherent, atomically flat surfaces for model study. Additionally, chemical and catalytic properties of the aluminosilicate bilayers have been explored through probe reactions (e.g. alcohols and amines) by conducting thermal desorption spectroscopy to demonstrate the applicability of these materials for direct study of zeolite catalysis.