Magnetism and NMR at IPN

Nuclear magnetic resonance (NMR) is extensively applied in several areas of research in heterogeneous catalysis. The study of zeolites and other solid networks is an important field; structure and dynamics of (relatively simple) adsorbates and reaction products on the surfaces of catalysts form another. A third category is concerned with the supported metal particles themselves; we have been involved in this particular subject for a number of years .

The NMR properties of metals are dominated by a magnetic coupling between the nuclear spin (in the case of catalytic work mostly ¹⁹⁵Pt) and the spin of the conduction electrons. The coupling is proportional to the density of one-electron energy states at the Fermi level, and results in shifts that are large compared with the usual chemical shift range, and also in a very characteristic temperature dependence of the nuclear spin-lattice relaxation rate.

In systems that lack the translational symmetry of the extended solid, such as the small metal particles in supported catalysts, these properties depend on the local electronic environment of the resonating nucleus (the local density of states). In favorable cases, this leads to distinct spectral ranges for atoms in the metal surface and those in the interior of the particles. Since the local density of Fermi-level energy states on the metal surface plays also a role in frontier-orbital theories of chemisorption, it becomes possible to test (qualitatively) the effects of chemisorption, promotion, poisoning, etc.

A rather spectacular example of at first sight unexpected correlations predicted by this type of theories concerns platinum particles in zeolites of different acidity. It is well known experimentally that the infrared stretch frequency of carbon monoxide adsorbed on such particles varies with zeolite acidity. Frontier orbital theory then predicts that the nuclear spin lattice relaxation rate of nuclei in the clean (i.e. before CO adsorption) platinum surface will be higher in those zeolites where (after the adsorption) the CO infrared stretch frequency will be lower. This correlation has experimentally been confirmed.

A detailed discussion of the relation between the density-functional theory of metals and NMR parameters has been given in:

J.J. van der Klink and H.B. Brom

NMR in metals, metal particles and metal cluster compounds

Progr. NMR Spectroscopy vol. 36 (2), pp. 89-201 (2000)

An extensive review of surface-related NMR in heterogeneous catalysis, including work by other groups, can be found in:

J.J. van der Klink

NMR spectroscopy as a probe of surfaces of supported metal catalysts

Advances in Catalysis vol. 44, pp. 1-117

A more concise review, focused on electrochemical applications, will appear in:

Y.Y. Tong and J.J. van der Klink

NMR investigation of supported metal catalysts

Catalysis and Electrocatalysis at Nanoparticle Surfaces, E.R. Savinova, C. G. Vayenas, A. Wieckowski, Eds.; Marcel Dekker, New York (2002)

See also: J.J. van der Klink, "The NMR reciprocity theorem for arbitrary probe geometry", J. Magn. Reson. vol.148 pp 147-154 (2001)

J.J. van der Klink

The NMR reciprocity theorem for arbitrary probe geometry

J. Magn. Reson. vol.148 pp 147-154 (2001)

Jean-Pierre Bucher	Propriétés électroniques de petites particules de platine étudiées par RMN	Thèse EPFL no. 747 (1988)
Jean-Jacques Bercier	Etude par RMN de petites particules d'argent supportées	Thèse EPFL no. 1202, (1993)
Yuye Tong	Metal-adsorbate and metal-matrix interactions in platinum catalysts studied by ¹⁹⁵Pt NMR	Thèse EPFL no. 1267 (1994)
Séverine Burnet	Etude de la structure électronique de petites particules de rhodium par RMN	Thèse EPFL (2001)