Electron microscopy and catalytic study of silver catalysts: Structure sensitivity of the hydrogenation of crotonaldehyde

Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO2 and Ag/TiO2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silver particle size on activity and selectivity toward the unsaturated alcohol. Nanostructural features of the catalysts, as their silver particle size distribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity;to the unsaturated alcohol was found to be (59 +/- 3)%, independent of the particle size in the range of 3.7 nm less than or equal to (d) over bar(Ag) less than or equal to 6.3 nm. Moreover, the specific activities were similar in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO2 catalysts appears to be structure-insensitive. Titania-supported silver catalysts reduced in hydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), however, showed a quite different behavior. These silver particles exhibit rather narrow size distributions and very low mean particle sizes ((d) over bar(Ag) = 2.8 +/- 1.9 nm for Ag/TiO2-LTR, (d) over bar(Ag) = 1.4 +/- 0.5 nm for Ag/TiO2-HTR), i.e., an exceedingly high dispersion (D-Ag = 0.46 and 0.69, respectively). The LTR catalyst gave a higher selectivity to crotyl alcohol (53%) than the ultradispersed HTR catalyst (28%). This pronounced change in selectivity suggests the hydrogenation of crotonaldehyde over these Ag catalysts to be qualified as structure-sensitive with the rate-determining step depending critically on the silver particle size and thus on the silver surface structure. If hydrogenation of the C=O group of the alpha,beta-unsaturated aldehyde is favored by face atoms, most likely the increased fraction of Ag(lll) planes of the larger silver particles will give higher formation rates of the desired unsaturated alcohol.