Electron Microscopy and Catalytic Study of Silver Catalysts:  Structure Sensitivity of the Hydrogenation of Crotonaldehyde

Abstract

Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO 2 and Ag/ TiO 2 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 e d h Ag e 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/SiO 2 catalysts appears to be structureinsensitive. 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 h Ag ) 2.8 ( 1.9 nm for Ag/TiO 2 -LTR, d h Ag ) 1.4 ( 0.5 nm for Ag/TiO 2 -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 CdO group of the R,β-unsaturated aldehyde is favored by face atoms, most likely the increased fraction of Ag(111) planes of the larger silver particles will give higher formation rates of the desired unsaturated alcohol.

Silver Cluster Catalysts for Green Organic Synthesis

“…Claus et al 29), 30) reported selective hydrogenation by Ag nanoparticle catalysts. The group of Kaneda 32)～36) showed that hydrotalcite-or hydroxyapatite-supported Ag nanoparticles acted as effective catalysts for green organic reactions such as oxidation of silane and deoxygenation of epoxides into alkenes.…”

“…This is attributed to the completely filled d-band of Ag as well as the position of the d-band center relative to the Fermi level 7) . However, Claus et al first reported excellent selectivity of Ag nanoparticle catalysts for the hydrogenation of a C=O group in the presence of a C=C bond 29) . They confirmed experimentally that dissociation of H2 occurs on their catalyst by H2/D2 exchange experiments 30) .…”

“…For the selective hydrogenation of crotonaldehyde by TiO2-or SiO2-supported Ag particles with size of 1 to 7 nm, the larger Ag particles gave higher selectivity to the desired product, unsaturated alcohol. Bao et al reported that Ag/SiO2 with Ag nanoparticle size of 7-9 nm catalyzed the selective hydrogenation of chloronitrobenzenes to their corresponding chloroanilines 29) . Selective hydrogenation of nitro group in the presence of other reducible functional groups (such as olefinic groups) is an important reaction to produce functionalized anilines as industrial intermediates for a variety of specific and fine chemicals 101) .…”

Silver Nanoparticles From Trianthema Portulacastrum: Green Synthesis, Characterization, Antibacterial and Anticancer Properties

“…Especially in the biomedical fields such as anti-malarial, bio-labeling, antimicrobial, and cytotoxic properties of SNPs ensures a non-toxic biotherapeutic agent. In addition, green synthesized SNPs were applied in photographic reactions, catalysis, and chemical analysis [2][3][4][5][6].…”

Activation of dihydrogen on supported and unsupported silver catalysts

“…Reported data include H 2 chemisorption on Ag/TiO 2 [46], kinetics of H 2 /D 2 exchange on Ag/Al 2 O 3 [47], H 2 -promoted ethene isotopic homoexchange on Ag/SiO 2 [31], hydrogenation of ethene on Ag/SiO 2 , and selective hydrogenation of ,-unsaturated aldehydes [15][16][17]48,49].…”