College Park, Maryland June 6 - 10 , 2004
M3-D1 (1:30 PM): Applications of vibrational spectroscopy by inelastic neutron scattering to problems in heterogeneous catalysis (Invited)
J. Eckert (LANSCE-12, Los Alamos National Laboratory, Los Alamos, NM 87545 and Materials Research Laboratory, University of California, Santa Barbara, CA 93106)
Neutron scattering techniques offer some particularly powerful ways of accessing issues related to molecular binding and reactions at active sites in real catalysts. This is because the low absorption cross-sections of neutrons make it relatively easy to carry out in-situ measurements in actual reactors or other realistic sample environments. Vibrational spectroscopy by means of inelastic neutron scattering (INS) is one such application of neutrons that may be used for the study of adsorbate dynamics and thereby to identify reaction intermediates and products. Experimentally, the contrast between adsorbate and framework vibrational spectra is achieved by using molecules with strongly scattering atoms (i.e., H) relative to those in the framework, whereby access to the entire vibrational spectrum of the sorbate is provided. INS spectra can also readily be computed from any theoretical approach that gives frequencies and atomic vibrational amplitudes because of the simplicity of the interaction of the neutron with the atomic nucleus. In our application of INS spectroscopy to problems in heterogeneous catalysis we use (1) a combined experimental and theoretical approach to the study of equilibrium sorption complexes, or (2) quench a reaction carried out in a reactor ex-situ, and examine the results by placing the cold reactor directly in the INS spectrometer. I will describe a number of such studies including the sorption of methylamines in zeolite rho, the decomposition of methane on supported metal catalysts, H spillover effects for metal clusters in zeolites, and the epoxidation of propene in H2 and O2 over Au nanoparticles supported on titania. Several important findings have emerged from these studies including direct evidence for the active species formed in in the propene epoxidation with H2 and O2. This work was carried out in collaboration with the groups of D. W. Goodman (Texas A&M University), B. C. Gates (UC Davis) and LANL colleagues L. L. Daemen, M. Hartl, and N. J. Henson. It has benefited from the use of facilities at the Manuel Lujan Jr. Neutron Scattering Center, a National User Facility funded as such by the Office of Science, U. S. Department of Energy.
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