Influence of Gas Packing and Orientation on FTIR Activity for CO Chemisorption to the Cu Paddlewheel


In situ Fourier-transform infrared (FTIR) spectroscopy is able to probe structural defects via site-specific adsorption of CO to the Cu-BTC (BTC = 1,3,5-benzenetricarboxylate) metal-organic framework (MOF). The temperature-programmed desorption (TPD) of CO chemisorbed to Cu-TDPAT (TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) is virtually identical to Cu-BTC, suggesting CO chemisorbs to the open metal site at the axial position of the copper paddlewheel that is the building unit of both MOFs. Yet, despite an increased gravimetric CO : Cu ratio, CO chemisorbed to Cu-TDPAT is FTIR inactive. We rule out the presence of residual solvent, thermal degradation, adsorption temperature, and ligand-induced electronic effects at the adsorption site. TPD at increased pressure suggests the multiple CO per Cu site rearrange in Cu-TDPAT as a dynamic function of temperature and pressure. Thus, the FTIR inactivity of CO chemisorbed to Cu-TDPAT is attributed to orientation and/or packing of the CO relative to the Cu binding site. The results suggest dynamic chemisorption complicate extension of a site-specific in situ FTIR probe of gas adsorption. For both Cu-BTC and Cu-TDPAT, the in situ FTIR probe is a less sensitive probe of defects than X-ray photoelectron spectroscopy and nitrogen adsorption.


Chemical and Biochemical Engineering


United States. Department of Energy.Office of Energy Efficiency and Renewable EnergyUnited States. Department of Energy. Office of Basic Energy Sciences


The initial spectroscopic measurements were supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy program, Award DE-FG36-08GO18139. Additional spectroscopic measurements, analysis, and adsorption studies were supported by the U.S. Department of Energy, Basic Energy Sciences Awards DE-FG02-09ER466556 and DE-SC0002157.

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