Characterization of supported-palladium catalysts by deuterium NMR spectroscopy

Abstract

Alumina-supported Pd, Rh and Pd–Rh bimetallic alloys have been prepared by the method of incipient wetness. Sorption of deuterium by these samples was investigated by means of ²H NMR spectroscopy and uptake measurements at room temperature. Deuterium chemisorbed on palladium and rhodium atoms was characterized by ²H NMR peaks with chemical shifts of –12 and –170 ppm, respectively, from monometallic samples under a deuterium pressure of 1 Torr or less. These two characteristic peaks remained for the bimetallic samples and their relative intensities unambiguously revealed the surface composition of the bimetallic crystallites. Deuterium atoms were absorbed or weakly chemisorbed at greater pressures of deuterium. The weakly sorbed deuterium atoms exchanged with the strongly chemisorbed deuterium atoms, indicated by the coalescence of the ²H NMR characteristic peaks of the bimetallic samples. The position of the coalesced peak depended on the composition of the alloy crystallites.