Combined experimental and theoretical investigation of polar organic adsorption/desorption from model carbonaceous surfaces: Acetone on graphite

摘要

Adsorption on carbonaceous surfaces is important for applications ranging from atmospheric processes in the upper atmosphere to nanotechnology, yet little is known about adsorption of simple polar molecules on even the simplest carbonaceous surface, graphite. Optical differential reflectance (ODR) and temperature-programmed desorption (TPD) were combined to investigate the adsorption and desorption of a volatile polar organic compound (acetone) on the basal plane of a model graphite surface (highly oriented pyrolytic graphite) under ultrahigh vacuum conditions. The ODR change induced by adsorption/desorption was shown to correlate with relative coverage as determined by TPD experiments. TPD spectra revealed the existence of monolayer, bilayer, and multilayer adsorption states. Adsorption was found to follow a VolmerWeber rather than a layer-by-layer growth mode. Molecular simulation of acetone on planar graphite was used to compute the geometry and coverage of acetone in the first, second, and third layers. Acetone molecules at submonolayer coverages tend to lie approximately parallel to the graphite surface. The acetone dipoles are predicted to align to produce favorable dipole-dipole interactions in the adsorbed phase. The second and third layers show considerably more disorder than the monolayer. The bilayer begins to form when the total coverage is about 4.5 x 10(-6) mol/m(2). At this coverage, the monolayer is about 75% full. The third layer begins at a coverage of about 7.6 x 10(-6) mol/m(2) at which point the monolayer is over 90% complete. Thus, the simulations support the experimental observation of Volmer-Weber growth. [References: 32]