62nd Annual Report on Research 2017

This project has made significant progress in the past year on multiple fronts. First, our collaboration with University of Washington (Prof. Charles Campbell) and Washington State University (Prof. Jean-Sabin McEwen) has concluded with a solid publication in the Journal of Physical Chemistry C, after receiving several constructive reviews ("DFT-based method for more accurate adsorption energies: An adaptive sum of energies from RPBE and vdW density functionals," Hensley et al., J. Phys. Chem. C 2017, 121, 4937-4945; DOI: 10.1021/acs.jpcc.6b10187). This work not only provides useful benchmarking information on two recently developed, self-consistent van der Waals (vdW) functionals (optB86b and optB88), but proposes an adaptive method that can 1) be used to gauge the importance of vdW contribution to adsorption energies; 2) provide accurate predictions of adsorption energies for adsorbates regardless of whether the adsorption mechanism is dominated by charge transfer or dispersion forces, achieving superior accuracy to any current standard DFT functional alone. We have furthermore proposed how it can be applied to treat the adsorption energies of transition states and co-adsorbates.

The adaptive sum method is already being actively utilized in theoretical catalysis research by the collaborators' groups, and has helped initiate additional, related research directions, including a collaboration between the PI (Xu) and Prof. Michael Trenary at University of Illinois Chicago. Prof. Trenary's group is investigating the decomposition of hydrogen cyanide on several metals, including palladium and platinum, primarily by using reflection absorption infrared spectroscopy. The interpretation of the reaction pathway is hampered by possible vdW contribution that is only partially significant in the reaction pathway. We are collaborating with Prof. Trenary to provide theoretical support for his effort based on the adaptive sum method, which we expect to generate new insights not possible with mainstream DFT functionals. If our method proves successful in treating a reaction system that is relatively simple but not included in the original benchmarking dataset, we will be able to apply it to many more catalytic reactions on metals with greater confidence.

Our work on the ring opening of model cyclic paraffins has also generated new scientific insights. We have revisited cyclohexane decomposition on Pt(111), a topic previously studied by experimentalists and theorists alike, in the context of benzene hydrogenation. We chose to investigate the reverse pathway of cyclohexane decomposition, and in doing so revealed intrinsic limitation to the selectivity for benzene due to hitherto not-considered dehydrogenation pathways. We have further discovered that Ir(111) is intrinsically more selective away from benzene and toward ring opening than Pt(111), a reflection of the greater stability of the Ir-C bond vs. Pt-C bond, and perhaps more importantly, the greater stability of C₂-C₃ moieties on Ir vs. Pt. These results are being prepared in an upcoming publication ("A comparative theoretical study of cyclohexane decomposition on Ir(111) vs. Pt(111)," Ghale et al., in preparation). In addition, our work on cyclohexane and dimethylcyclohexane decomposition on Ir(111) and Ir(211) has also made substantial progress. It is worth mentioning that both the J. Phys. Chem. C paper and the manuscript under preparation on cyclohexane decomposition contain contribution from undergraduate students who sought to gain research experience in the PI's group.