Felix Hemmann, I. Agirrezabal-Telleria, E. Kemnitz
Jul 9, 2013
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0
Influential Citations
7
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Journal
Journal of Physical Chemistry C
Abstract
15N NMR of pyridine has been used to study Lewis and Bronsted sites at the acid surface of magnesium hydroxide fluoride at two different pyridine loadings with ratios of excess pyridine compared to pyridine molecules bound to Lewis and Bronsted sites of about 1:1 (highly loaded) and 0.15:1 (low loaded), respectively. The Lewis and Bronsted sites can be distinguished by the 15N chemical shift of pyridine at about −100 ppm (LPy) and −175 ppm (BPy). In both samples a very broad 15N resonance is found around −88 ppm assigned to weakly adsorbed (excess) pyridine molecules via hydrogen bridges (HPy). Additionally, another signal at about −49 ppm is observed in the highly loaded sample only representing physisorbed pyridine (PPy). A slow chemical exchange process of strongly bound LPy and BPy molecules with excess HPy and PPy molecules can be monitored using 15N exchange NMR. It takes place on a time scale of about 50–100 ms at room temperature. All pyridine molecules have the same 15N T1 and its value increases...