61st Annual Report on Research 2016

The scientific research in this project has made significant progress in two areas. One is in understanding the reactivity of model cycloalkanes on iridium surfaces. The other is in achieving an accurate prediction of cycloalkanes on metals via density functional theory (DFT) calculations.

Our work on cyclohexane (C₆H₁₂) on iridium aims to elucidate how this prototypical cycloalkane interacts with iridium surfaces at the molecular level, which has not been addressed in the literature. We have exhaustively investigated all possible decomposition steps that cyclohexane and its successively dehydrogenated derivatives can undergo. This allowed us to identify the minimum energy pathway for cyclohexane decomposition on the close-packed Ir(111) surface under vacuum conditions. A significant difference is seen between Ir and Pt. Experimentally and theoretically it has been shown that cyclohexane decomposes on Pt(111) through a metastable cyclic C₆H₉intermediate to form benzene, but bypassing cyclohexene. On Ir(111), however, the decomposition of cyclohexane primarily goes through a carbene intermediate to ring opening in C6H8, with cyclohexene being a secondary product. The results will be described in an upcoming publication (“Decomposition of cyclohexane on Ir(111)”, Ghale et al., in preparation). This difference is consistent with Ir being a more effective ring-opening catalyst than Pt, and reflects the greater stability of the Ir-C bond. Understanding how the metals that show promise for the selective ring-opening reaction, including Ir and Pt, interact with cycloalkanes differently is a key aspect of this project. As all C-C bonds in cyclohexane are identical, cyclohexane allows us to compare ring opening on different metals but not ring-opening selectivity at different types of C-C bonds. We have therefore begun to also study the decomposition of methylcyclohexane on Ir surfaces. Preliminary results show that it is highly sensitive to surface structure, with a step edge site lowering the initial C-H activation barrier in methylcyclohexane by ca. 0.6 eV. The investigation for the detailed decomposition pathway of methylcyclohexane is currently under way.

A major potential issue in the theoretical description of the ring-opening reaction on metals is the inability of most DFT functionals to describe van der Waals (vdW) forces. This can result in significant under-prediction for the adsorption energies of large hydrocarbon species, including cycloalkanes, and likely also for the energy of the transition states of their surface reaction steps. It would have serious consequences for the accuracy of kinetic analyses that are based on DFT calculation. To assess the vdW contribution in the reactivity of cycloalkanes on metal surfaces, in collaboration with Prof. Jean-Sabin McEwen (Washington State U.) and Prof. Charles Campbell (U. Washington) we tested the recently developed optB86b and opB88 vdW functionals against a data set of 39 experimentally measured accurate adsorption energies (CE39). The CE39 data set includes small adsorbates such as CO, NO and O but also species relevant to this project such as benzene, cyclohexene, and cyclohexane. As these vdW funcitonals turned out to offer much improved accuracy compared to DFT-GGA for vdW adsorbed species but overbind those with significant chemisorption, we have developed a new adaptive method for that accurately predicts adsorption energies for adsorptions regardless of whether they are dominated by charge transfer or dispersion forces, and produces superior accuracy to any current standard DFT functional alone, achieving a mean absolute error and root mean squared error of 13.5 and 19.6 kJ/mol relative to the measured adsorption energies in the CE39 data set. This method is described in an upcoming publication (“A DFT-based method for more accurate adsorption energies: An adaptive sum of energies from RPBE and vdW density functionals,” Hensley et al., submitted). We expect it to have an impact beyond this particular research and on computational catalysis research in general.

In the first reporting year this project has provided the opportunity for a graduate student to further his training in molecular modeling techniques, DFT calculations, and microkinetic modeling. It has also provided some research experience to two undergraduate students and opportunities to present their work to their fellow students in oral and poster formats. They will be coauthors on the two upcoming publications mentioned above.