| Title | FTIR spectroscopy combined with quantum chemical calculations to investigate adsorbed nitrate on aluminum oxide surfaces in the presence and absence of co-adsorbed water. |
| Publication Type | Journal Article |
| Year of Publication | 2007 |
| Journal | Physical Chemistry Chemical Physics |
| Volume | 9 |
| Issue | 36 |
| Pagination | 4970 - 4980 |
| Authors | Baltrusaitis, Jonas, Jennifer Schuttlefield, Jan H. Jensen, and Vicki H. Grassian |
| Publisher | Royal Society of Chemistry |
| ISBN Number | 1463-9076 |
| Keywords | adsorption surface reaction nitrate alumina water coadsorption |
| Abstract | Surface reactions of nitrogen oxides with aluminum oxide particles result in the formation of adsorbed nitrate. Specifically, when α-Al2O3 and γ-Al2O3 particles are exposed to gas-phase NO2 and HNO3 adsorbed nitrate forms on the surface. In this study, Fourier transform IR (FTIR) spectroscopy is combined with quantum chem. calcns. to further our understanding of the adsorbed nitrate product on aluminum oxide particle surfaces in the presence and absence of co-adsorbed water at 296 K. FTIR spectra of adsorbed nitrate on α-Al2O3 and γ-Al2O3 particles are interpreted using calcd. vibrational frequencies of nitrate coordinated to binuclear Al oxide cluster models. Comparison of the calcd. and exptl. vibrational frequencies of adsorbed nitrate establishes different modes of coordination (monodentate, bidentate and bridging) of the nitrate ion to the surface in the absence of adsorbed water. In the presence of co-adsorbed water, the nitrate ion becomes fully solvated, as shown by a comparison of the exptl. nitrate IR spectra as a function of relative humidity with the calcd. nitrate vibrational frequencies for binuclear Al cluster compds. which contain both coordinated nitrate ions and water mols. These calcns. also suggest that adsorbed water can displace nitrate from direct coordination to the surface, leading to an outer-sphere nitrate adsorption complex as well as an inner-sphere complex. Furthermore, the relative humidity dependence of the spectra suggest that water does not evenly wet the surface even at high relative humidity, as there are open or bare surface sites where nitrate ions are not solvated. Besides adsorbed monodendate, bidendate, bridging and solvated nitrate, the presence of ion bound nitrate ion, partially solvated nitrate, mol. nitric acid, hydronium ion and H3O+:NO3- ion pairs on the oxide surface are also discussed. [on SciFinder(R)] |