Acid Sites on SiO₂−Al₂O₃ Monolayer Catalysts:  ³¹P NMR Probes of Strength and Accessibility

Abstract

We have examined the acidic properties of a SiO 2 -on-Al 2 O 3 monolayer catalyst by 31 P NMR of adsorbed tricyclohexylphosphine and triphenylphosphine. These large probe molecules permit the assessment of steric effects, by comparison with trimethylphosphine, and reveal differences in acid strengths of surface sites. We find that there are significant constraints to accessing Lewis sites, but not Brønsted sites. The concentration of strong Brønsted sites is slightly less than half of the total concentration of Brønsted sites.

Quantitative Distinction between Noble Metals Located in Mesopores from Those on the External Surface

“…It has a BET surface area of 350 m 2 /g but no mesopores and a negligible micropore volume. These properties fit the literature on comparable systems [16] . The 27 Al MAS NMR spectrum of ASA show peaks at chemical shifts of δ 27Al =56, 27, and 4 ppm that can be assigned to aluminum in tetrahedral, pentahedral, and octahedral coordination, respectively.…”

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

“…Asterisks denote spinning sidebands. [120] a Rapid exchange requires NMR measurements at low temperatures (T¼ 120 K). by (CH 3 ) 3 PH þ complexes (TMPH þ ) arising from chemisorption of TMP at Brønsted acid sites.…”

“…Only 6% of the Brønsted acid sites of this zeolite could interact with the probe molecules, which hints to a corresponding low content of these hydroxyl groups at the outer particle surface. In a similar manner, the accessibility and acidic properties of Brønsted sites of SiO 2 -Al 2 O 3 monolayer catalysts were investigated using tricyclohexylphosphine as bulky probe molecule [120].…”

Improvement of the proton exchange membrane fuel cell (PEMFC) performance at low-humidity conditions by adding hygroscopic γ-Al2O3 particles into the catalyst layer

“…The aim of this study is to explore the feasibility of adding hygroscopic ␥-alumina particles into the anode catalyst layer to improve the wettability of the anode under low-humidity conditions by utilizing the Lewis acid sites on the surface of ␥-alumina that can attract the hydroxyl group of the water molecular [33][34][35]. High BET surface area ␥-alumina particles were added to the anode catalyst layer by using an ultrasonic technique.…”