23 The NMR Study of Proton Exchange between Adsorbed Species and Oxides and Silicates Surfaces J. J. FRIPIAT
Downloaded by EAST CAROLINA UNIV on October 26, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch023
Centre de Recherche sur les Solides à Organisation Cristalline Imparfaite, 45045 Orleans Cedex, France
Heterogeneous acid catalysts have an important i n d u s t r i a l role and i n recent years a great deal of research has been devoted to their study and to t h e i r improvement. The recognition of the nature of the acid centers specially by the study of the inter action of these acid centers with various molecular species has made important progrès. Infrared spectroscopy (IR) has been from t h i s respect a very useful tool i n making possible the direct observation of molecules interacting with surface s i t e s . The l i f e time of a specified molecular configuration revealed by infrared spectroscopy i s larger or at least of the order of the inverse of the characteristic frequencies for that configurâtion,e.g.>10" ^ sec. Any process including intermediates with s i g n i f i c a n t l y longer life-times w i l l therefore appear as s t a t i c for IR. For example consider an "acid surface s i t e " SH and an activated intermediate AH i n the A -*· Β transformation: 1
SH + A î S + AH AH + Β + Η Η + S + SH —ιρ
I f the l i f e - t i m e of AH i s , say, 10 sec, i t s characteristic v i brational spectrum i s observed but infrared w i l l give no informa t i o n on the rate l i m i t i n g step though i t may well be the proton "turn-over" on A. The proton jump frequencies measured i n homogeneous l i q u i d phasesare i n general smaller than 1 0 sec" (j_)and i t may be an ticipated that they w i l l be smaller when a s o l i d donor and an ad sorbed acceptor are involved. In t h i s domain of frequency nuclear magnetic resonance (N.M.R.) and especially proton pulse N.M.R. are of particular interest. Indeed measurements of the s p i n - l a t t i c e and spin-spin proton relaxation times, T-j and T2 respectively, ha ve been used for gaining information on proton transfer reactions. It i s the aim of t h i s paper to review some of recent works done i n this area and relevant to acid heterogeneous catalysis. Proton motions generate fluctuations i n the l o c a l magnetic f i e l d and therefore i n the spectral density functions J-j and J 2 + 1 2
1
261
In Magnetic Resonance in Colloid and Interface Science; Resing, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
M A G N E T I C
262
RESONANCE
at w and 2w ( w = 2 π ν = proton resonance frequency, radian sec ~ ) . I t may be shown that œ (mean square l o c a l f i e l d ) [ J ^ ( w ) + J ( 2 W ) ] (1 ) Q
Q
0
Q
1
Q
2
Q
J-j (or Jp) are the Fourier transform?of the autocorrelation funct i o n which for a random walk i s G(x)
f ( t + τ) > = < f ( 0 ) , f (0) > exp - T/T : :
=
