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A Characterization Study of Reactive Sites in ALD-Synthesized WOx/ZrO2 Catalysts

1
Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA
2
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA
3
Material Science & Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(7), 292; https://doi.org/10.3390/catal8070292
Received: 20 June 2018 / Revised: 11 July 2018 / Accepted: 17 July 2018 / Published: 19 July 2018
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Abstract

A series of ZrO2-supported WOx catalysts were prepared using atomic layer deposition (ALD) with W(CO)6, and were then compared to a WOx/ZrO2 catalyst prepared via conventional impregnation. The types of sites present in these samples were characterized using temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements with 2-propanol and 2-propanamine. Weight changes showed that the WOx catalysts grew at a rate of 8.8 × 1017 W atoms/m2 per cycle. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) indicated that WOx was deposited uniformly, as did the 2-propanol TPD-TGA results, which showed that ZrO2 was completely covered after five ALD cycles. Furthermore, 2-propanamine TPD-TGA demonstrated the presence of three types of catalytic sites, the concentrations of which changed with the number of ALD cycles: dehydrogenation sites associated with ZrO2, Brønsted-acid sites associated with monolayer WOx clusters, and oxidation sites associated with higher WOx coverages. The Brønsted sites were not formed via ALD of WOx on SiO2. The reaction rates for 2-propanol dehydration were correlated with the concentration of Brønsted sites. While TPD-TGA of 2-propanamine did not differentiate the strength of Brønsted-acid sites, H–D exchange between D2O and either toluene or chlorobenzene indicated that the Brønsted sites in tungstated zirconia were much weaker than those in H-ZSM-5 zeolites. View Full-Text
Keywords: solid acids; metal-oxide catalysts; tungstated zirconia; atomic layer deposition; Brønsted-acid strength; temperature-programmed desorption/thermogravimetric analysis; H–D exchange solid acids; metal-oxide catalysts; tungstated zirconia; atomic layer deposition; Brønsted-acid strength; temperature-programmed desorption/thermogravimetric analysis; H–D exchange
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Wang, C.; Mao, X.; Lee, J.D.; Onn, T.M.; Yeh, Y.-H.; Murray, C.B.; Gorte, R.J. A Characterization Study of Reactive Sites in ALD-Synthesized WOx/ZrO2 Catalysts. Catalysts 2018, 8, 292.

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