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J. Phys. Chem. 1989, 93, 504-506
Observation of Surface Intermediates by NMR below 1 K: CO Chemisorbed on a SnO, Oxidizing Catalyst 0. Gonen, P. L. Kuhns, J. S. Waugh,* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39
and J. P. Fraissard Laboratoire de Chimie des Surfaces, UniversitP Pierre et Marie Curie, 75230 Paris CPdex 05, France (Received: December 12, 1988)
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We performed "C NMR on 13C0chemisorbed on SnOz, a metal oxide oxidizing catalyst, immersed in liquid 3He between 0.01 and 1 K. The observations confirm IR evidence that the catalyzed reaction CO + '/202 C02proceeds via a stable C03intermediate at the SnOZsurface. The I3C spin-lattice relaxation time T I is 540 30 s, temperature independent over 2 decades, indicating the Friedman-Millet-Richardson mechanism is responsible and that all the 13C are surface bound. The sensitivity gain below 0.1 K is sufficient for >10 signal-to-noise ratio from the 10l8low y spins with a single pulse.
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Introduction hinges on the existence of a relaxation mechanism that will make the lattice temperature accessible to the nuclear spins. The catalyst-mediated oxidation of C O to COz, in addition to Recently, we have shown that the Friedman-Millet-Richardson its commercial importance, also serves as a model reaction for (FMR)15,16relaxation mechanism is in fact general and applicable fundamental research in heterogeneous catalysis.' Stable interto any spin-bearing nucleus immersed in pure 3He.14J7 This mediates on metal oxide and supported metal catalysts have been mechanism however pertains to the surface nuclei only: for the investigated by I R techniques and yield evidence that for the magnetization to be transported into the bulk spins, it has to rely former this intermediate has a trigonal C 0 3 structure,'-3 while on spin d i f f ~ s i o n , ' ~ , shown ' ~ , ' ~ by Bl~embergen'~ to be extremely for the latter it is a M - C = 0 . 4 Solid-state N M R spectroscopy slow over macroscopic distances. Fortunately, for surface NMR, is a technique particularly well suited for investigation of such this is not a problem at all: the FMR Tl mechanism will produce molecular-structure level problem^.^-^ In fact, N M R studies of surface polarization in a time over scale of a few seconds to a few C O chemisorbed on supported metal catalysts have been conducted hours.14 via l i n e - ~ h a p e $relaxation ~~~~ and various elegant In this paper we report on a sub-kelvin N M R study of 13C0 double-resonance techniques."' The major impediment for widespread N M R application, however, is low s e n s i t i ~ i t y . ~ J ~ J ~chemisorbed on the surface of a metal oxide oxidation catalyst @noz).Using line-shape comparison, combined with surface Thus, although the catalytic samples are generally powders of large relaxation ( T I S )measurements based on the FMR effect, we surface area, the single-shot signal-to-noise (S/N) ratio obtained obtain the coverage, structure, and configuration for the stable by "usual" methods, a t room temperature, is
