The IR spectra of Mg5C(2+)(CO) complexes on the (0 0 1) surfaces of polycrystalline and single crystal MgO

The FTIR spectroscopy of carbon monoxide adsorbed on polycrystalline MgO smoke has been investigated as a function of the CO equilibrium pressure at constant temperature (60 K) (optical isotherm) and of the temperature (in the 300-60 K range) at constant CO pressure (optical isobar). In both cases the spectra fully reproduce those of CO adsorbed on the (00 1) surface of UHV cleaved single crystals [Heidberg et al., Surf. Sci. 331-333 (1995) 1467]. This result, never attained in previous investigations on dispersed MgO, contribute to bridging the gap which is commonly supposed to exist between surface science and the study of "real" (defective) systems. Depending on the surface coverage theta the main spectral features due to the CO/MgO smoke interaction are a single band shifting from 2157.5 (at theta --> 0) to 2150.2 cm(-1) (at theta = 1/4) or a triplet, at 2151.5, 2137.2 and 2132.4 cm(-1) (at theta > 1/4). These manifestations are due to the v(CO) modes of Mg-5(C)2(+)...CO adducts formed on the (001) terminations of the cubic MgO smoke microcrystals The formation of the CO monolayer is occurring in two different phases: (i) a first phase with CO oscillators perpendicularly oriented to the surface (2157-2150 cm(-1)) and (ii) a second phase constituted by an array of coexisting perpendicular and tilted species (triplet at 2151.5, 2137.2 and 2132.4 cm(-1)). A much weaker feature at 2167.5-2164 cm(-1) is assigned to Mg-4(C)2(+)...CO adducts at the edges of the microcrystals. The heat of adsorption of the perpendicular Mg-5(C)2(+)...CO complex in the first phase has been estimated from the optical isobar and results to be 11 kJ mol(-1). (C) 2003 Elsevier B.V. All rights reserved.