Direct spectroscopic evidence for isolated silanols in SiOx/Al2O3 and their formation mechanism

Aidan R. Mouat, Takeshi Kobayashi, Marek Pruski, Tobin J Marks, Peter C Stair

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica−alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiOx sites on Al2O3 catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiOx sites are probed with dynamic nuclear polarization (DNP)enhanced29Si−29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiOx/Al2O3 material prepared by ALD and CLD, followed by calcination under an O2 stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al2O3 reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiOx/Al2O3 reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiOx/Al2O3 catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønsted-acidic SiOx/Al2O3 catalysts.

Original languageEnglish
Pages (from-to)6060-6064
Number of pages5
JournalJournal of Physical Chemistry C
Volume121
Issue number11
DOIs
Publication statusPublished - Jan 1 2017

Fingerprint

Atomic layer deposition
catalysts
atomic layer epitaxy
Catalysts
Calcination
roasting
Liquids
liquids
Fourier transform infrared spectroscopy
spectroscopy
reflectance
preparation
Nuclear magnetic resonance spectroscopy
liquid phases
signatures
silanol
Polarization
nuclear magnetic resonance
acids
Acids

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

Cite this

Direct spectroscopic evidence for isolated silanols in SiOx/Al2O3 and their formation mechanism. / Mouat, Aidan R.; Kobayashi, Takeshi; Pruski, Marek; Marks, Tobin J; Stair, Peter C.

In: Journal of Physical Chemistry C, Vol. 121, No. 11, 01.01.2017, p. 6060-6064.

Research output: Contribution to journalArticle

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AB - The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica−alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiOx sites on Al2O3 catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiOx sites are probed with dynamic nuclear polarization (DNP)enhanced29Si−29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiOx/Al2O3 material prepared by ALD and CLD, followed by calcination under an O2 stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al2O3 reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiOx/Al2O3 reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiOx/Al2O3 catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønsted-acidic SiOx/Al2O3 catalysts.

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