Water chemistry on two-dimensional silicates studied by density functional theory and temperature-programmed desorption

Abstract

The interactions of water with two-dimensional (2D) silica and aluminosilicate bilayers were studied using density functional theory (DFT) and temperature-programmed desorption (TPD). The DFT calculations encompassed a systematic investigation of the impacts of silanol groups, Si:Al ratio, and the proximity of a Pd(111) growth substrate on water adsorption, permeation, and dissociation, as well as the thermal stability of charge balancing protons at the 2D aluminosilicate surface. Water adsorption on pure, defect-free 2D SiO2 was weak; however, the barrier for water penetration to the Pd(111) substrate was small. Water adsorption at the 2D SiO2/Pd(111) surface was slightly weaker than bare Pd(111) but once at the metal surface water dissociation energetics were scarcely affected by the SiO2 overlayer. Silanol groups at 2D SiO2 edges substantially strengthened water adsorption but by only roughly half as much as hydroxyls bridging Al and Si atoms in 2D aluminosilicates. The metal substrate did not fundamentally alter the surface chemistry of the outer 2D aluminosilicate surface; water adsorption energies on the bridging hydroxyls differed by only a few meV for freestanding versus Pd-supported bilayers. Removal of the charge balancing proton via a dehydration process that produces an oxygen vacancy was favored over a dehydrogenation pathway irrespective of the presence of the metal. The dehydration pathway is uphill by at least 3.37 eV suggesting that temperatures in excess of 1100 K would be required to remove charge balancing protons as neutral species. In contrast, chemical bonds between the 2D aluminosilicate and the metal substrate make it difficult to protonate the metal side of the 2D aluminosilicate bilayer via water dissociation. The TPD experiments for pure 2D SiO2/Pd(111) revealed desorption curves consistent with adsorption at the metal surface, supporting weak adsorption on a defect-free bilayer and permeation through and around the silica overlayer to the metal. Incorporating Al into the bilayer shifted the water TPD peak to higher temperatures but not as much as would be expected based on the computed adsorption strength at bridging hydroxyls on 2D aluminosilicate surfaces. No water or hydrogen desorption consistent with removal of charging balancing protons from 2D aluminosilicate surfaces via dehydration or dehydrogenation was observed. The results suggest thermally robust Brønsted acid sites on the surface of 2D aluminosilicates supported on Pd(111).