An NMR study of the geometry of a CO layer chemisorbed on Pt particles

Abstract

The authors have analyzed the shape of the 1 3C spin echo envelope of a 1 3CO layer chemisorbed on supported Pt particles. They show that it contains information about the geometry of the array of molecules on the surface. They find that two mechanisms are responsible for the shape. The coupling of the C spins with the surface Pt spins gives rise to a T 2 of 2.9 ms at 77 K. By applying a Carr–Purcell sequence to the 1 3C spins, the effects of the coupling to the Pt can be eliminated. The dipolar coupling between 1 3C molecules then remains, producing a ‘‘slow beat’’ in the spin echo envelope. It can be analyzed exactly since mutual spins flips among C spins are not allowed, owing to large field inhomogeneities at the surface of the Pt particles. Analysis of the slow beat yields information about the structure of the array. The authors report evidence for the coalescence of islands of CO at low coverage and demonstrate that the coverage is uneven among the Pt particles if the sample powder is exposed to less than a monolayer of CO.