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

Conventional (CTEM) and high-resolution transmission electron microscopy (H RTEM) of Ag/SiO2 and Ag/TiO2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silv er particle size on activity and selectivity toward the unsaturated alcohol . Nanostructural features of the catalysts, as their silver particle size d istribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity;t o the unsaturated alcohol was found to be (59 +/- 3)%, independent of the p article 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 simila r in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO2 catalysts appears t o be structure-insensitive. Titania-supported silver catalysts reduced in h ydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), h owever, showed a quite different behavior. These silver particles exhibit r ather 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 alc ohol (53%) than the ultradispersed HTR catalyst (28%). This pronounced chan ge in selectivity suggests the hydrogenation of crotonaldehyde over these A g catalysts to be qualified as structure-sensitive with the rate-determinin g step depending critically on the silver particle size and thus on the sil ver surface structure. If hydrogenation of the C=O group of the alpha,beta- unsaturated aldehyde is favored by face atoms, most likely the increased fr action of Ag(lll) planes of the larger silver particles will give higher fo rmation rates of the desired unsaturated alcohol.