Calcium Vapor Adsorption on the Metal-Organic Framework NU-1000: Structure and Energetics

Lownsbury, J. M.; Santos-López, I. A.; Zhang, W.; Campbell, C. T.; Yu, H. S.; Liu, W.-G.; Cramer, C. J.; Truhlar, D. G.; Wang, T.; Hupp, J. T.; Farha, O. K.
J. Phys. Chem. C 2016, 120, 16850 (doi:10.1021/acs.jpcc.6b05707).

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 nodes, and thus 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-OH complexes 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 the H₂O-OH moeities are titrated deeply enough (~20 nm) such that slow Ca diffusion prevents further reaction.