Reports: AC5
47388-AC5 Role of Specific Reactive Sites on Silicon Nitride Surface in Catalysis of Isomerization and Addition Reactions
The main focus of this research is to understand the role of specific surface reactive sites in addition and condensation reactions catalyzed by silicon nitride. We have prepared the reactive sites postulated to play a role in catalysis by exposing a clean Si(100)-2x1 surface to ammonia in ultra-high vacuum and briefly annealing this surface to a predetermined temperature. This approach yields almost exclusively a models surface with -NH2 functional groups if room temperature is used, =NH covered surface after a brief anneal to approximately 500 K, and nearly exclusively =N- surface sites after a brief heating to 800K. These sites should be effective to a very different extent in catalyzing alkene isomerization, addition reactions and fuel reforming. These highly basic structures have previously been successfully tested for Knoevenagel condensation of benzylazide and malonitrile and for Michael addition of malononitrile to acrylonitrile, test reactions to characterize the catalytic activity of materials. In the first year of this research we have successfully and reproducibly prepared the test reactive sites according to the strategy previously published by our group (Rodríguez-Reyes, J. C. F. and Teplyakov, A. V. Cooperative nitrogen insertion processes: Thermal transformation of NH3 on a Si(100) surface. Phys. Rev. B, 2007, 76, 075348-1-075348-16; Rodríguez-Reyes, J. C. F. and Teplyakov, A. V. Role of surface strain in the subsurface migration of adsorbates on silicon. Phys Rev. B. 2008, 78, 165314-1-165314-14). We have used a combination of multiple surface analytical techniques and Density Functional Theory calculations to determine the mechanism of interaction of a potential candidate for testing surface isomerization process, 2,3-dimethyl-2-butane with Si(100) (Madachik, M. and Teplyakov, A. V. Unique Lack of Chemical Reactivity for 2,3-Dimethyl-2-Butene on a Si(100)-2x1 Surface. J. Vac. Sci. Technol. A 2008, 26(5), 1241-1247). We discovered that 2,3-dimethyl-2-butene is uniquely unreactive with respect to this surface compared to any other alkenes studied, unexpectedly opening a wide range of applications for this compound. In the meantime, we have successfully prepared a surface with a unique pattern created by co-adsorption of two different molecules with very different reactivities on silicon (Madachik, M. R. and Teplyakov, A. V. Coadsorption of ethylene and nitrobenzene on Si(100)-2x1: Towards surface patterning at the molecular level. Accepted to J. Phys Chem. C) and we plan to use this approach to study reactivity of other surface reactions.
In addition, the first step of Knoevenagel condensation, the reaction of benzylazide with various aminoterminated model surfaces has been investigated, compared to the primary amine-terminated self-assembled monolayers and will be a subject of further research and upcoming publications. Further studies of catalytic properties of selectively prepared ammonia-induced sites on silicon crystalline substrates and on model aminoterminated self-assembled monolayers are under way.
