SGI Power Challenge Projekt Beschreibung (2001)

Transcription

1 SGI Power Challenge Projekt Beschreibung (2001) 1 Project Catalysis by solid acids (account tdemuth) The project aims at the ab-initio investigation of catalytic properties of solid acids i.e. zeolites. This field has gained great attention in the last years, because Kohn-Sham density functional theory seems to open the possibility to describe real catalytic processes on computers for the first time. First principles calculations and simulations based on Kohn-Sham density functional theory are currently a major tool of computational material science and computational chemistry. At the Institute, most such calculations are currently performed with VASP (Vienna ab-initio simulation package). Extensive descriptions of the techniques used in VASP can be found in: Efficiency of ab initio total energy calculations for metals and semiconductors G. Kresse and J. Furthmüller Comput. Mat. Sci. 6, (1996). From ultrasoft pseudopotentials to the projector augmented wave method G. Kresse, and J. Joubert Phys. Rev. B 59, 1758 (1999). Zeolites are widley used in reactions such as cracking or isomerization of hydrocarbons. We have studied the skeletal isomerization of a 2-pentene molecule in ZSM-22 (a one-dimensional medium pore size zeolite). The most likely mechanism of skeletal isomerization of the intermediate carbenium ion involves the rearrangement of the classical secondary carbenium ion into a non-classical carbonium ion, namely a protonated dialkyl-cyclopropane. This mechanism was originally proposed by Brouwer for the skeletal isomerization in liquid superacids. Starting from an alkoxy species a dimethyl-cyclopropane is formed by the backdonation of a proton from the molecule to the zeolite to form a Brønsted acid site. The intermediate configuration shows a physisorbed dimethyl-cyclopropane at the zeolite acid site. Afterwards the ring is reopened by protonation, a different bond is broken leading thus to a secondary pentyl-oxide. Hence if the reaction is catalyzed by a zeolite the creation of a carbonium ion, due to the proton backdonation is avoided. Moreover within this mechanism only the more stable secondary transition states and intermediates are involved. The complete reaction pathway is depicted in Fig. 1. The overall reaction starting from a linear alkoxy to a mono-branched one involves in total three transition states and two intermediate configurations. The highest barrier of 98 kj/mol was found for the proton backdonation (see TS 2). For the ring-opening via the reprotonation of the DMCP molecule an activation energy of 71 kj/mol was calculated. Compared to a pathway via the ethyl shift the barriers found for this mechanisms are much smaller (by 80 kj/mol). Ab-initio investigation of the adsorption of benzene in mordenite Th. Demuth, L. Benco, J. Hafner, H. Toulhoat, and F. Hutschka J. Chem. Phys. 114, (2001). Adsorption of water in mordenite - an ab-initio study Th. Demuth, L. Benco, J. Hafner, and H. Toulhoat Inter. J. Quantum. Chem. 84, 110 (2001). Ab-initio study of the adsorption and reactions of hydrocarbons in mordenite Th. Demuth, L. Benco, J. Hafner, H. Toulhoat, and F. Hutschka Proceeding of the 13 th Intern. Zeolite Conference, Montpellier (2001).

2 A Periodic Structure DFT Study of propylene chemisorption in Acidic Chabazite: Effect of the Zeolitic Crystal Structure Relaxation X. Rozanska, Th. Demuth, F. Hutschka, J. Hafner, and R. A. van Santen (submitted). Figure 1: Reaction energy diagram for the isomerization via a DMCP intermediate.

3 fpr.zserv - SGI Power Challenge Reciprocal-space Tight-Binding LMTO First principles calculations and simulations based on Kohn-Sham density-functional-theory (KS-DFT) are currently a major tool of computational materials science. My calculations at the institute are performed with a reciprocal-space tight-binding linearized-mun-tin-orbital (TB-LMTO) method in the atomic-sphere-approximation (ASA), which is based on the Kohn-Sham density functional theory. The reciprocal-space TB-LMTO-ASA is a first-principles method, where energies and charges were treated fully self-consistently. The TB-LMTO is a fast method compared to full potential methods and projector augmented plane-wave codes and can therefore be used to treat complex systems with large number of inequivalent atoms per cell. Nevertheless these calculations are generally very time consuming, therefore a parallel version of the package has been developed. Detailed description of the techniques used in this package can be found in: O.K. Anderson, O. Jepsen and M. Sob The Electronic Structure and its Applications, ed. M. Yusouff (Springer, Berlin 1987) O.K. Anderson, O. Jepsen and D. Glötzel 1985 Highlights of Condensed Matter Theory, eds. F. Bassani, F. Fumi and M.P. Tosi Hans L. Skriver, The LMTO Method, Muffin-Tin Orbitals and Electronic Structure (Springer, Berlin 1984) Project: Thin Films The understanding of the magnetic behaviour of 3d transition-metal surfaces and thin films has increased due to experimental and theoretical effort in recent years. The magnetic ground state of thin Cr films epitaxially grown on bcc(100) Fe substrates has been investigated. The magnetic ground state of bcc Cr in DFT calculations is of commensurate antiferromagnetic order of structural type I, which however is in contrast to experimental results where a spin-density-wave (SDW) as ground state is predicted. The ab-initio study of bcc(100) Fe/Cr multilayer showed that SDWs can be stabilized as the magnetic ground state due to charge modulation at the Fe/Cr interface. The questions arises if SDWs could also be stabilized in thin Cr films on a Fe substrate. The results indicate that SDWs represent the magnetic ground state in thin Cr films depending on the film thicknesses. Publication: R. Hafner, D. Spišák, R. Lorenz and J. Hafner, Magnetic ground state of thin Cr films on bcc (100) Fe substrate A first principles study, to be published.

4 SGI Origin2000: Projekt Beschreibung Project Surface physics (account fmitten) The aim of this project is the structural and electronic characterization of solid surfaces and the modeling of reactions on the surface (physisorption, chemisorption and catalysis). This field has gained great attention in the last few years, because Kohn-Sham density functional theory seems to open the possibility to describe real catalytic processes on computers for the first time. First principles calculations and simulations based on Kohn-Sham density functional theory (KS-DFT) are currently a major tool of computational material science and computational chemistry. At the Institute, most such calculations are currently performed with VASP (Vienna ab-initio simulation package). Extensive descriptions of the techniques used in VASP can be found in: G. Kresse and J. Furthmüller, Efficiency of ab initio total energy calculations for metals and semiconductors using a plane wave basis set, Comput. Mat. Sci. 6, (1996). G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total energy calculations using a plane wave basis set, Phys. Rev. B 54, (1996). G. Kresse, and J. Joubert, From ultrasoft pseudopotentials to the projector augmented wave method, Phys. Rev. B 59, 1758 (1999). As catalytic reactions based on a Langmuir-Hinshelwood mechanism are based on the initial adsorption of the reactants, the first part of this project is based on the investigation of the interaction of aromatic molecules with metal surfaces. Whereas the aromatic molecules form strong chemisorptive bonds on group VIII metals via the interaction of the aromatic ring with the substrate d-states, benzene and thiophene are merely physisorbed on Al(111). In the following step the catalytic properties of Nickel surfaces are investigated by means of two important reactions: The hydrogenation of benzene/ni(111) and the hydrodesulfurization of thiophene/ni(100). For the hydrogenation of benzene the initial hydrogenation step has been identified to be rate determining. In the case of the desulfurization of thiophene several concurrent reaction mechanisms have been compared to clarify the role of a prior hydrogenation of the molecule. Although the hydrogenation lowers the reaction barrier for the desulfurization of the molecule it is energetically strongly disfavored. The calculations require large computer resources and fairly large amount of memory and were thus performed in most cases on the SGI cluster. The following systems have been studied in detail: Benzene on Ni(100), (110) and (111) surfaces F. Mittendorfer and J.Hafner Density-functional study of the adsorption of benzene on the (111), (100) and (110) surfaces of nickel Surf. Sci. 472 (2001) Thiophene on Al(111) R. I. R. Blyth, F. Mittendorfer, J. Hafner, S. A. Sardar, R. Duschek, F: P. Netzer and M. G. Ramsey An experimental and theoretical investigation of the thiophene/aluminium interface J. Chem. Phys. 114 (2001)

5 Adsorption of thiophene on Ni(100) F. Mittendorfer, J. Hafner, Adsorption of thiophene on Ni(100), Surf. Sci. 492 (2001) 27. The Hydrodesulfurisation of Thiophene on Ni surfaces F. Mittendorfer and J. Hafner The hydrodesulfurization of thiophene over Ni(100) : A DFT study in preperation. Hydrogenation of benzene/ni(111) F. Mittendorfer and J. Hafner Hydrogenation of benzene on Ni(111) in preperation. Figure 1: Schematic reaction pathway for the desulfurization of thiophene over a Nickel surface.