Calcium vapor adsorption on the metal-organic framework NU-1000: Structure and energetics

The nature and energy of the reactions between calcium vapor and the internal surfaces of the metal-organic framework (MOF) NU-1000 have been studied by adsorption microcalorimetry, low energy He⁺ ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and Kohn-Sham density functional theory (DFT). NU-1000 is one of the most stable MOFs with transition-metal-oxide nodes, and thus it is of interest as a potential catalyst or catalytic support when modified with other metals. The reaction heats of Ca with NU-1000 are high below 2 monolayers (ML) Ca coverage (570-366 kJ/mol), attributed (based on DFT) to Ca reacting first with free benzoic acid functionalities or water impurities, then with H₂O and OH groups on the Zr₆ nodes to produce Ca(OH)₂ clusters. With higher Ca doses, the heat of Ca reaction decreases asymptotically to the sublimation enthalpy of bulk Ca (178 kJ/mol), attributed to the formation of Ca(solid) nanoparticles on the external surface, which only occurs after all of the H₂O and OH groups are titrated deeply enough (∼20 nm) such that slow Ca diffusion prevents further reaction.