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J. Phys. Chem. C 2009, 113, 7728–7734
Formation of Adsorbed Layers by Deposition of Dinitrogen Pentoxide, Nitric Acid, and Water on Graphite Liza S. E. Romero Lejonthun, Erik A. Svensson, Patrik U. Andersson, and Jan B. C. Pettersson* Department of Chemistry, Atmospheric Science, UniVersity of Gothenburg, SE-412 96 Gothenburg, Sweden ReceiVed: NoVember 9, 2008; ReVised Manuscript ReceiVed: March 12, 2009
The formation of adsorbed layers of dinitrogen pentoxide, nitric acid, and water on graphite has been studied by molecular beam and light-scattering techniques. The desorption kinetics of N2O5 on graphite were described by the Arrhenius equation with an activation energy of 0.24 ( 0.03 eV and a pre-exponential factor of 2.3 × 10(10 ( 0.73) s-1, and N2O5 is concluded to bind more strongly than H2O to the graphite surface. Elastic helium scattering and light scattering were used to probe the formation of adlayers on the surface. Adsorption of pure N2O5 resulted in formation of thin adlayers at temperatures below 160 K. In coadsorption experiments N2O5 was concluded to facilitate the formation of thick N2O5-H2O ice layers at 155 K. In a similar way coadsorption of HNO3 and H2O resulted in the formation of thick adlayers at 170 K. N2O5 and HNO3 both bind more strongly than water to the graphite surface and are concluded to facilitate nucleation and growth of ice. 1. Introduction The condensation of water on surfaces is of central importance in a number of disciplines including atmospheric and life sciences. In the atmosphere, condensation of water on solid particles is a prerequisite for the formation of liquid water and ice clouds. The condensation processes are influenced by several factors including surface properties of the solid particles and coadsorbed compounds, and the understanding is still far from complete. One important class of particles in the atmosphere is soot particles that are produced during incomplete combustion of hydrocarbons. These particles mainly consist of disordered graphitic layers and polycyclic aromatic hydrocarbon structures,1 and they may have an influence on the chemical composition of the atmosphere by facilitating reactions that are slow in the gas phase. Recently formed soot particles are generally found to be hydrophobic, but they may develop hydrophilic properties as other components present in the atmosphere adsorb and react on their surfaces. The hygroscopic properties of the modified soot particles will enhance water adsorption and the particles may potentially act as cloud-condensation nuclei. Any compound with a reasonably high concentration in the atmosphere that binds strongly to both water and soot particles may have a large impact on deposition freezing (nucleation of ice on the soot particles). The molecules may help to “anchor” water molecules or small clusters to the surface and thereby decrease the supersaturation required for water deposition to occur. We have recently initiated a program to improve the fundamental understanding of water adsorption on solid surfaces and coadsorption of multiple components on model surfaces of potential relevance to atmospheric processes. We recently investigated the formation of water ice layers on graphite using helium scattering to probe the conditions in the topmost layer on the surface.2 Water-graphite interactions are relatively weak and considerably weaker than water-water interactions, and water is normally observed not to wet graphite. The earlier study, * To whom correspondence should be addressed. Phone: +46 31 772 28 28. E-mail:
[email protected].
however, showed that an amorphous ice film that wets the graphite surface was formed at surface temperatures of 110-140 K, while three-dimensional ice structures with a height of hundreds of nanometers were formed at higher temperatures. Desorption of adsorbed water molecules competed with water incorporation into the ice film, and the ice formation rate was strongly temperature dependent. Molecular dynamics simulations of water cluster formation on graphite at 90-180 K showed that clusters formed at low temperature tended to have most molecules in direct contact with the graphite surface, while multilayer cluster structures were preferred at high temperatures. Suter et al.3 also studied the formation of mixed water-ammonia ice layers on graphite at 110-160 K. Ice formation was mainly governed by the partial water pressure, but the presence of ammonia enhanced the formation rate and influenced the layer properties. Ice formed at temperatures
