Investigating the Surface Structure of γ-Al ₂ O ₃ Supported WO _X Catalysts by High Field ²⁷ Al MAS NMR and Electronic Structure Calculations

Journal Article · Thu Oct 13 00:00:00 EDT 2016 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.6b09060· OSTI ID:1340866

Wan, Chuan; Hu, Mary Y.; Jaegers, Nicholas R.; Shi, Dachuan; Wang, Huamin; Gao, Feng; Qin, Zhaohai; Wang, Yong; Hu, Jian Zhi

The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1340866
• Report Number(s): PNNL-SA-119580; 48332; KC0302010
• Journal Information: Journal of Physical Chemistry. C, Vol. 120, Issue 40; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

Density functional theory modeling
Surface structure
Penta-coordinated alumina
Tungsten species
27Al MAS NMR
Environmental Molecular Sciences Laboratory