EXCEPTIONAL SUSCEPTIBILITY OF CERIA-SUPPORTED RHODIUM CATALYST TO INHIBITORY SMSI EFFECTS INCLUDING ACETONE HYDROGENATION

Complementary lines of evidence, as follows, are presented to show that hydrogen pretreatments of a ceria-supported rhodium catalyst (Rh-CeO2) at rather moderate temperatures (653-573 K) cause this system to progress deeply into a strongly inhibitory SMSI state: (i) by comparative measurements, with a continuous-flow microcatalytic reactor system at a fixed reaction temperature of only 343 K, of the activities of samples of the Rh-CeO2 material for hydrogenation of acetone vapour to isopropanol, after prereduction at temperatures in the range 413 - 653 K. Complete loss of hydrogenation activity is thus demonstrated after reduction at 653 K, but full restoration of activity is shown to be achieved upon reoxidation in O2 at 673 K followed by re-reduction at 413 K; (ii) by selective identifications, using proton NMR, of hydrogen chemisorbed on the rhodium of Rh-CeO2 and its differentiation from hydrogen absorbed by, or adsorbed onto, the ceria support. These results of selective characterizations of H-Rh upon Rh-CeO2 samples prereduced in H2 at temperatures in the range 473-573 K confirm a severe inhibition of the ability to produce H-Rh species after prereduction at 573 K, and thus provide complementary evidence for creation of an SMSI-state in Rh/CeO2 after prereduction at this relatively low temperature. Additional information on rhodium-induced modifications of surface and bulk properties of Rh/CeO2 samples exposed to vacuum, H2 or O2 at various temperatures emerges from comparative measurements of Rh/CeO2 and CeO2 using the techniques of electron spin resonance, volumetric evaluation of carbon monoxide chemisorption, and gravimetric monitoring of hydrogen absorption/adsorption. © 1990.