58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Propylene (C₃⁼) is the second largest feedstock for the petrochemical industry and a shortage is developing that will increase over the next decade. To meet this demand, industry is turning to heterogeneous olefin metathesis catalysis via C₂⁼ + C₄⁼ → 2C₃⁼. A key challenge is to improve performance of catalysts for this reaction. Unlike the well-defined, single-site homogeneous olefin metathesis catalysts, lack of fundamental information about the active sites on commercial heterogeneous olefin metathesis catalysts based on supported metal oxides (Re₂O₇, MoO₃ and WO₃) has hindered further development. Our goal is to improve the understanding at the molecular level of the catalytic active sites, their activation and deactivation, reaction intermediates, and the overall mechanism over supported metal oxide catalysts by elucidating molecular/electronic structure-activity/selectivity relationships for heterogeneous olefin metathesis to provide a fundamental basis of improved catalysts.  The research is divided into several parts: (1) synthesis and characterization of well-defined precursor states; (2) activation of precursor states to form active reduced sites; and (3) probing the reactions of active sites after exposure to olefins and during olefin metathesis reaction conditions.

We have initiated a series of experiments to synthesize and characterize well-defined ReO_x active sites on model planar catalysts.  In order to achieve this, we are preparing to carry out controlled depositions of rhenium onto Al₂O₃.  Initial steps taken thus far involve synthesizing ordered Al₂O₃ films on NiAl(110) single crystal surfaces, and characterizing the film with respect to its electronic and geometric structure, using a number of surface analytical methods including X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED).  Ordered Al₂O₃ films can be formed on the NiAl(110)  substrate by sputtering the hot surface, which results in a two domain structure with a clear Ni enrichment, and then oxidizing the sample at 625 K using an exposure of 300 L O₂ (at a pressure of 3x10^-7 mbar O₂).  The final result of the Al₂O₃ film is a superposition of two nearly rectangular (88.7°) reciprocal lattices due to two possible domains, with a unit cell size of 1.06 x 1.79 nm² and rotated 24° from the [110]-like directions.  The resulting thickness of the Al₂O₃ thin film is 0.5 nm, corresponding to about three layers.  Previous work using high-resolution electron energy loss spectroscopy (HREELS) suggests a ratio of Al in octahedral and tetrahedral sites at about 3:1.  The advantages of producing an Al₂O₃thin film from NiAl(110) is that the thin film is exceptionally well-ordered and can be reproducibly prepared with a thickness of 0.5 nm.  Additionally, it grows with an atomically flat uniform structure with long-range order as well as a uniform defect structure without holes. 

During the past grant period, much of our effort was spent in installing and commissioning a new UHV instrument that combines high-resolution XPS (HR-XPS), low energy ion scattering (LEIS), and HREELS.  This instrument includes state-of-the-art capability for surface characterization using a 270-mm-diameter VG Scienta R3000 XPS/UPS/ARPES hemispherical analyzer with a CCD-MCP detector and a 7-crystal, 650-W MX-650 monochromatic X-ray source for HR-XPS, and an LK ELS5000-MCA multichannel HREELS spectrometer for measuring vibrational and optical excitations with high detection efficiency. A differentially pumped ion gun used with the R3000 analyzer provides LEIS capability to determine elemental concentrations in the topmost atomic layer of samples, complementary to the near-surface concentration and chemical state information provided by HR-XPS.  

Rhenium has an extremely high melting point of 3180 °C, and this requires an electron-beam evaporator within the UHV chamber in order to evaporate and deposit Re films.  Of key importance to this project, is the installation of a new tectra 2-pocket e--flux2 Mini E-Beam Evaporator. This evaporator is specially designed for use in UHV for highest purity thin films, enabling deposition and co-evaporation of small and medium quantities of refractory metals such as Re, Mo, and W at temperatures of up to >3100 K. A built-in flux monitor working on the basis of ion current measurement allows for highly controllable deposition rates to form thin films from sub-monolayer up to multi nanometer.

Following the preparation of the Al₂O₃ thin film on NiAl(110), controlled depositions of Re will be carried out.  Detailed information about the molecular structure of rhenium-oxide dispersed monomer sites and active reduced sites on the Al₂O₃ film will be determined by using HR-XPS and HREELS. In order to determine the optimum oxidation state of the rhenium-oxide, the ReO_x species will be reduced with CO, H₂, or H atoms to prepare surface ReO_x with specific oxidation states (e.g., Re⁷⁺, Re⁶⁺, Re⁵⁺,…).  During the upcoming grant period, the structures associated with each oxidation state will be determined and related to the surface reactivity of specific active sites.