The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.

Catalysis with transition metal nanoparticles in colloidal solution: nanoparticle shape dependence and stability.

Nanoparticles are regarded as a major step forward to achieving the miniaturisation and nanoscaling effects and properties that have been utilised by nature for millions of years. The chemist is no longer observing and describing the behaviour of matter but is now able to manipulate and produce new types of materials with specific desired physicochemical characteristics. Such materials are receiving extensive attention across a broad range of research disciplines. The fusion between nanoparticle and nanoporous materials technology represents one of the most interesting of these rapidly expanding areas. The harnessing of nanoscale activity and selectivity, potentially provides extremely efficient catalytic materials for the production of commodity chemicals, and energy needed for a future sustainable society. In this tutorial review, we present an introduction to the field of supported metal nanoparticles (SMNPs) on porous materials, focusing on their preparation and applications in different areas.

Supported metal nanoparticles on porous materials. Methods and applications.

Metal nanoparticles have attracted much attention over the last decade owing to their unique properties as compared to their bulk metal equivalents, including a large surface-to-volume ratio and tunable shapes. To control the properties of nanoparticles with particular respect to shape, size and dispersity is imperative, as these will determine the activity in the desired application. Supported metal nanoparticles are widely employed in catalysis. Recent advances in controlling the shape and size of nanoparticles have opened the possibility to optimise the particle geometry for enhanced catalytic activity, providing the optimum size and surface properties for specific applications. This Review describes the state of the art with respect to the preparation and use of supported metal nanoparticles in catalysis. The main groups of such nanoparticles (noble and transition metal nanoparticles) are highlighted and future prospects are discussed.

Sustainable Preparation of Supported Metal Nanoparticles and Their Applications in Catalysis

Nouveau Traité de Chimie Minérale.

Study of nickel catalysts supported on silica of low surface area and prepared by reduction of nickel acetate in aqueous hydrazine

Nickel metal nanoparticles supported on silica of low surface area (15 m2 g-1) were prepared by reduction of nickel acetate by hydrazine in aqueous medium Their gas-phase stability and surface properties depended on thermal pre-treatment under H2 or air atmosphere Small nickel particles (<10 nm), in oxidized or reduced state, are strongly resistant to reductive or oxidative treatment respectively For H2-treated catalysts, H2 chemisorption and TPD results suggested the occurrence of spillover hydrogen between the metal nickel phase and silica For air then H2 treated catalysts, hydrogen spillover seemed to involve the NiO phase The activity of the catalysts in gas-phase benzene hydrogenation also depended on the thermal pre-treatment Pre-calcined then reduced catalysts exhibited higher TOFs than non pre-calcined catalysts, suggesting that the presence of NiO phase may have influenced the hydrogenation process

Nickel Nanoparticles Supported on Silica of Low Surface Area. Hydrogen Chemisorption and TPD and Catalytic Properties

We have studied the effect of silica of quartz-type on the reducibility of nickel acetate in aqueous hydrazine (80 °C, pH = 10−12) and metal particle formation. The obtained materials were characterized by X-ray diffraction, transmission electron microscopy, and thermodesorption experiments. With nickel acetate alone, the reduction was partial (45%) and a metal film at the liquid−gas interface or a powdered metal precipitate with an average particle size of 120 nm was obtained. In the presence of silica as the surfactant, the reduction of nickel acetate was total and the nickel phase deposited as a film on the support with an average particle size of 25 nm. Supported nickel acetate was also totally reduced. Crystallites of a mean particle size of about 3 nm were obtained. Decreasing the nickel content or increasing the hydrazine/nickel ratio decreased the metal particle size. Whiskers were formed for low nickel loadings. Hydrogen thermal treatment of the reduced phase showed that the organic acetate fragm...

Study of Support Effects on the Reduction of Ni2+ Ions in Aqueous Hydrazine

Non-conventional Ni catalysts supported on low surface area SiO 2 were prepared by reduction of nickel acetate by aqueous hydrazine, characterized by XRD and H 2 chemisorption properties or TPSR and tested in acetylene cyclotrimerization in the presence of hydrogen. Their performances were compared with those of classical catalysts supported on silica of low or high surface area. Hydrogen TPD profiles exhibited the presence of active sites involving nickel or nickel in interaction with the support. Hydrogen storage increased for non-classical catalysts or low surface area support. Under the reaction conditions used, no cyclotrimerization of acetylene was observed in the absence of hydrogen, probably as a result of strong adsorption of benzene precursor species. In the presence of hydrogen, benzene and ethylene were the main products in the low reaction temperature regime ( 2 and C 4 hydrogenation products at the expense of cyclotrimerization to benzene. Increasing the partial pressure of acetylene strongly deactivated the catalyst but favoured selectivity of both benzene and ethylene. The presence of both water vapour and hydrogen improved cylotrimerization to benzene at the expense of hydrogenation products, notably at low reaction temperatures. The effect of the different parameters on the sorptive and catalytic properties of the nickel catalysts prepared are discussed, notably the role of hydrogen and water in the determination of activity and reaction paths.

Acetylene cyclotrimerization over Ni/SiO2 catalysts in hydrogen atmosphere

We have studied a whisker-like nickel phase supported on silica and obtained by the reduction of nickel acetate in aqueous hydrazine media. Under H2/300 °C flow, the obtained phase remains stable when the reduction is carried out at 70 °C for 2 h whereas it totally tranforms to spherical-like particles when the reduction is performed at 80 °C for 30 min after a gradual heating from room temperature. It is shown that the whisker-like nickel phase adsorbs and stores more hydrogen than the spherical-like nickel particles. It also exhibits a higher activity in the gas phase hydrogenation of benzene (3-4 times greater). In acetylene hydrogenation, it strongly favors the cylotrimerization process to benzene at the expense of the processes to linear C2-C4hydrocarbons. This work is a direct demonstration of the influence of the metal phase morphology of nickel supported catalysts on their surface and catalytic properties.

Abdel-Ghani Boudjahem

Papers

Study of nickel catalysts supported on silica of low surface area and prepared by reduction of nickel acetate in aqueous hydrazine

Nickel Nanoparticles Supported on Silica of Low Surface Area. Hydrogen Chemisorption and TPD and Catalytic Properties

Study of Support Effects on the Reduction of Ni2+ Ions in Aqueous Hydrazine

Acetylene cyclotrimerization over Ni/SiO2 catalysts in hydrogen atmosphere

Nanonickel Particles Supported on Silica. Morphology Effects on Their Surface and Hydrogenating Properties