Details

Autor(en) / Beteiligte

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

Titel

Direct Spectroscopic Evidence for Isolated Silanols in SiO x /Al2O3 and Their Formation Mechanism

Ist Teil von

• Journal of physical chemistry. C, 2017-03, Vol.121 (11), p.6060-6064

Ort / Verlag

American Chemical Society

Erscheinungsjahr

2017

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• 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 SiO x sites on Al2O3 catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiO x sites are probed with dynamic nuclear polarization (DNP)-enhanced 29Si–29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiO x /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 SiO x /Al2O3 reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiO x /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 SiO x /Al2O3 catalysts.