Chemoselective hydrogenation of unsaturated aldehydes and ketones over heterogeneous catalysts is a demanding task. The achieved selectivity levels depend both on the electronic and geometric structures of reactants and metal surfaces. Recent development in catalyst preparation, selection of new solvents, catalyst and reactor structures have been reviewed in this work. Additionally, catalyst characterization technique, especially alloy formation and in situ characterizations have been discussed in detail. The deep understanding of the catalyst structure is necessary for development of tailor-made catalysts for chemoselective hydrogenations. The state of art in both kinetic and molecular modelling used in chemoselective hydrogenations was presented. Some special features, like conversion–selectivity relationship and the performance in gas and liquid phase hydrogenations were discussed.

Chemoselective hydrogenation of carbonyl compounds over heterogeneous catalysts

DOI: 10.1002/adma.201002763 Metal nanocrystals supported on metal oxides often exhibit improved catalytic activity and selectivity as compared to unsupported ones, [ 1–4 ] which is known to arise from several factors, including the shape and size of metal nanocrystals, the metal oxidation state, and the support effect. [ 5–11 ] Early methods for preparing metal nanocrystal on metal oxide hybrid nanostructures mainly involved calcination of metal oxide powders impregnated with metal precursors at high temperatures ( > 400 ° C). [ 12–17 ]

Aqueous‐Phase Synthesis of Pt/CeO2 Hybrid Nanostructures and Their Catalytic Properties

The reduction and oxidation features of nanostructured CuO/CeO2 catalysts prepared by the deposition-precipitation method were extensively investigated by TPR, FIF-IR and in situ XPS techniques. Both the chemical states of copper and the reduction degree of ceria could be well controlled during the reduction with hydrogen by adjusting the temperature. Noticeably, the fully reduced Cu-0 could be further oxidized into Cu+ in hydrogen by increasing the reduction temperature through the interaction between Cu-0 and lattice oxygen of ceria immigrating to the surface. Structure-reactivity relationship was established between the structural features of CuO/CeO2 formed during the pre-reduction with hydrogen and its catalytic activities for CO oxidation. It was observed that reduction with hydrogen at 473-573 K, which leads to the full presence of metallic copper in the catalyst, gives rise to higher CO conversion. These phenomena were interpreted in terms of the reduction degrees of ceria, the changes of surface morphology and the chemical states of copper species. The interface oxygen activation as well as its transfer from the interface to the adsorbed reactant was found to play decisive roles in determining the reaction rate of CO oxidation. (c) 2005 Elsevier B.V. All rights reserved.

CuO/CeO2 catalysts: Redox features and catalytic behaviors

Selective hydrogenation of unsaturated aldehydes to unsaturated alcohols is a valuable but challenging task for synthesizing fine chemicals. We report that single Rh atoms anchored to the edges of 2D MoS2 sheets can efficiently convert crotonaldehyde to crotyl alcohol with 100% selectivity via a steric confinement effect of pocketlike active sites. Characterization results suggest that the synthesized Rh1/MoS2 single-atom catalysts (SACs) possess a unique geometric and electronic configuration, which confines the adsorption mode of the reactant molecule by a steric effect. The DFT calculations suggest that the MoS2 sheets terminate with oxidized Mo edges and the Rh1 stably anchors at the Mo cation vacancy site, which can facilely dissociate H2 to H atoms. The dissociated H atoms spill over to react with the edge O atoms to form OH species and create an HO-Mo-Rh1-Mo-OH configuration, resembling a pocketlike active site, which confines the adsorption mode of the crotonaldehyde due to steric effects. Such specific adsorption configuration yields 100% selectivity. The strategy of constructing pocketlike active centers with single metal atoms and 2D nanosheets opens new approaches to designing highly selective SACs for specific classes of catalytic transformations.

Pocketlike Active Site of Rh1/MoS2 Single-Atom Catalyst for Selective Crotonaldehyde Hydrogenation.

https://rua.ua.es/dspace/bitstream/10045/2658/4/PtSnCeAl.pdf

Effect of Sn addition to Pt/CeO2–Al2O3 and Pt/Al2O3 catalysts: An XPS, 119Sn Mössbauer and microcalorimetry study

The nanostructural properties of Pt/CeO 2 catalysts, already studied for their specific catalytic behaviours in the selective hydrogenation of crotonaldehyde [Catal. Lett. 69 (2000) 139] have been examined by high resolution transmission electronic microscopy (HRTEM). Three different catalysts were prepared by impregnation of a commercial ceria support (54 m 2 /g) with three different precursors, Pt(NH 3 ) 4 (NO 3 ) 2 , H 2 PtCl 6 and Pt(NH 3 ) 4 Cl 2 , calcined at 673 K and reduced at low and high temperatures (473 and 973 K). The chlorine-free catalyst showed a nanostructural evolution when increasing the reduction temperature from 473 to 973 K with the formation of epitaxial CePt 5 particles on the support. This phenomenon did not occur when chlorine was present in the catalyst, independently of the particle size. The specific catalytic behaviours of these catalysts obtained in the selective hydrogenation of crotonaldehyde have been related to these observations. It clearly appeared that the presence of chlorine partly preserves the catalytic properties of platinum metal (hydrogenation of CC bond) while the catalyst made of CePt 5 patch units epitaxial on the CeO 2 support could provide more than 80% crotyl alcohol selectivity, stable as a function of the time-on-stream, up to 45% conversion.

Pt/CeO2 catalysts: correlation between nanostructural properties and catalytic behaviour in selective hydrogenation of crotonaldehyde

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Pt/CeO2 catalysts: correlation between nanostructural properties and catalytic behaviour in selective hydrogenation of crotonaldehyde