Preparation, characterization, and activity of Cu/TiO2 catalysts .2. Effect of the catalyst morphology on the hydrogenation of 1,3-cyclooctadiene and the CO-NO reaction on Cu/TiO2 catalysts

The hydrogenation of 1,3-cyclooctadiene and the GO-NO reaction have been studied on Cu/TiO2 catalysts, with reference to the effects of the preparation method and of the reductive pretreatments of the samples. In the hydrogenation of 1,3-cyclooctadiene at 433 K, a marked difference in activity is observed between catalysts prepared by a chemisorption-hydrolysis method (C) and those prepared by wet impregnation (I), the former showing turnover numbers about 100 times greater than the latter. This large difference in activity is ascribed to differences in the morphology of the copper species present at the surfaces of the two different kinds of catalysts: there are three-dimensional stepped small particles of Cu in the first case (C), very efficient in hydrogen dissociation, whereas there are isolated copper sites embedded in the reduced support in the second case (I), very much less active toward hydrogen dissociation. In the GO-NO reaction, C samples show a decrease in the activity on increasing the catalyst prereduction temperature, while an opposite behavior is observed on I samples. For this reaction the rate determining step is NO dissociation; the interpretation of the apparently conflicting data is that NO dissociation occurs efficiently on copper sites on mildly reduced C catalysts exposing three-dimensional copper particles and also on reduced titania sites formed in the high-temperature reductive treatments on I catalysts. (C) 1997 Academic Press