THE CHARACTERIZATION OF DISTINCT ADSORPTION SITES FOR HYDROGEN ON COPPER IN COPPER ALUMINA CATALYSTS BY IN-SITU H-1-NMR SPECTROSCOPY/

Tea distinct hydrogen adsorption sites on the copper component of a co pper/alumina catalyst have been identified by monitoring the adsorptio n of hydrogen using in situ H-1 NMR spectroscopy. The NMR spectra reve al that one type of adsorption site is associated with relatively larg e, smooth copper particles whilst the other comprises a family of adso rption sites probably associated with faceting and defect sites on sma ller copper particles. Exchange between the two types of adsorption si te is surprisingly slow, with an upper Limit for exchange of 2500 s(-1 ), whilst exchange amongst the family of sites associated with the sma ller particles, although still slow, is faster, of the order of millis econds. Spillover exchange of hydrogen between the copper surface and the support is shown to be negligible on the timescale of the spin-lat tice relaxation (tens of milliseconds). TPR, XRD, and optical and elec tron microscopy investigations of the catalyst precursors did not pred ict the differences between the catalyst samples. (C) 1996 Academic Pr ess, Inc.