| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Nanomaterials | Monday Sessions |
| Session NM-MoE |
| Session: | NanoCatalysis |
| Presenter: | H. Henry Lamb, North Carolina State University |
| Authors: | H.H. Lamb, North Carolina State University P. Novotný, North Carolina State University S. Yusuf, North Carolina State University F. Li, North Carolina State University |
| Correspondent: | Click to Email |
Ethylene, a major petrochemical building block, is produced industrially by endothermic steam cracking of ethane and petroleum naphtha at 800-850°C. Catalytic oxidative dehydrogenation (ODH) of ethane offers huge potential for reductions in energy consumption and greenhouse gas emissions; however, ethane ODH using co-fed O2 also requires costly cryogenic air separation [1]. As an alternative, we are investigating redox catalysts that operate in a cyclic mode (chemical looping, CL) and utilize lattice oxygen (O2-) as the sole oxidant [2-3]. One monolayer (1 ML) equivalent MoO3/Al2O3 catalysts prepared by conventional impregnation and calcination methods contain highly dispersed supported molybdate species and exhibit high ethylene selectivity (>95%) in CL-ODH at 500-550°C. In contrast, at higher MoO3 loadings the Al2(MoO4)3 bulk phase predominates, as evidenced by Raman spectroscopy and x-ray diffraction (XRD); these catalysts have somewhat higher activity albeit 10–12% lower ethylene selectivity under equivalent conditions. X-ray photoelectron spectroscopy (XPS) studies indicate that high ethylene selectivity correlates with reduction of Mo+VI to Mo+V. The presence of Mo in lower oxidation states (+V, +IV) triggers H2 and CH4 generation resulting in selectivity loss. Interaction of molybdate species with the Al2O3 support appears to decrease reducibility and enhance ethylene selectivity in CL-ODH. Even after extended reduction in ethane at 550°C, the monolayer catalyst retained about 25% Mo+VI. In contrast, for 3 ML equivalent catalysts, surface Mo+VI is rapidly consumed, and its concentration eventually approaches zero. To better elucidate the nature of the catalytically active sites, model catalysts consisting of nanostructured MoO3 films on c-plane sapphire were deposited at 580°C by molecular beam epitaxy using a conventional Knudsen cell. The sapphire substrate after annealing at 700°C in UHV exhibited a streaky reflection high-energy electron diffraction (RHEED) pattern. Films deposited at short times (<1 min) were polycrystalline with relatively smooth surfaces (1.1 nm RMS roughness by atomic force microscopy). XPS revealed that MoO3 films deposited in vacuo were oxygen-deficient. Surface asperities grew with deposition time and with annealing at 700°C in UHV. The catalytic properties of the films are currently under investigation using molecular beam methods.
References:
1. C.A. Gärtner, A.C. van Veen, J.A. Lercher, ChemCatChem 5 (2013) 3196–3217.
2. S. Yusuf, L. Neal, V. Haribal, M. Baldwin, H.H. Lamb, F. Li, Appl. Catal. B 232 (2018) 77-85.
3. P. Novotný, S. Yusuf, F. Li, and H.H. Lamb, Catal. Today (2018) in press.