An investigation of the surface chemistry of crown ethers: the adsorption and reaction of 12-crown-4 on palladium (1 1 1)

The adsorption of 12-crown-4 on Pd(1 1 1) is investigated using reflection–absorption infrared spectroscopy and temperature-programmed desorption. Analysis of the infrared spectrum of 12-crown-4 suggests that it adsorbs in a flat-lying geometry with C4v symmetry with a saturation coverage of 0.073±0.008 monolayers. This implies that the crown ether bonds to the surface via the lone pair orbitals on the ether oxygen atoms. Estimating the saturation coverage for this geometry using Van der Waalsʹ radii yields a value of 0.088 monolayers in good agreement with the measured value. 12-crown-4 thermally decomposes to leave predominantly CO and ethylidyne species on the surface where the desorbing CO removes three of the four crown ether oxygen atoms, the fourth remaining on the surface. Small amounts of ethylene are also found to desorb along with a high molecular weight product which is tentatively assigned to the formation of C2H5OCH2CHO.