Nanoparticle-based catalysts

Both catalysts and all the reagents were added to a reaction vessel at time zero, and the mixture was stirred for 30 min complete conversion of 112 into 114 was observed. After reaction, the catalysts were separated by sonicating the reaction vessel and affixing a small permanent magnet externally to one wall of the vessel the nonmagnetic resin catalyst was removed from the reaction vessel by decantation while the magnetic nanoparticle base catalyst was held... 

Using underpotential deposition with the redox replacement technique a novel electrochemical approach for nanoparticle-based catalyst has been designed. Here, the as-prepared Pt-coated Au nanoparticle monolayer at atomic level on the electrode surface can reduce O2 predominately by 4e to H2O, which was confirmed by rotating ring disk electrode technique (Figure 8). 

Much work has been focused on the synthesis of metal, alloy, and metal oxide nanostructures in order to enhance their activities. Nanoparticle-based catalysts... 

The preparation of catalysts with small metal nanoparticles (NPs) is of primary importance for many chemical reactions [1]. The main hmitation of classical preparation methods is the difficulty to control the size of the NPs [2]. Indeed, the formation of the metal phase often rehes on a treatment at high temperature (e.g. reduction) during which sintering phenomena are hardly controlled. In turn, sintering is the main cause of deactivation of many metal nanoparticle-based catalysts [3]. 

Density Functional Theory (DFT) has shown that low-coordinated sites on the gold nanoparticles can adsorb small inorganic molecules such as O2 and CO, and the presence of these sites is the key factor for the catal5dic properties of supported gold nanoclusters. Other contributions, induced by the presence of the support, can provide parallel channels for the reaction and modulate the final efficiency of Au-based catalysts. Also these calculations extended for the adsorption of O and CO on flat and... 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Y. Yoo, M. Tuck, R. Kondakindi, C.-Y. Seo, Z. Dehouche, K. Belkacemi, Enhanced hydrogen reaction kinetics of nanostructured Mg-based composites with nanoparticle metal catalysts dispersed on supports , J. Alloys and Compounds, 446-447 (2007) 84-89. 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

Fischer-Tropsch synthesis making use of cobalt-based catalysts is a hotly persued scientific topic in the catalysis community since it offers an interesting and economically viable route for the conversion of e.g. natural gas to sulphur-free diesel fuels. As a result, major oil companies have recently announced to implement this technology and major investments are under way to build large Fischer-Tropsch plants based on cobalt-based catalysts in e.g. Qatar. Promoters have shown to be crucial to alter the catalytic properties of these catalyst systems in a positive way. For this reason, almost every chemical element of the periodic table has been evaluated in the open literature for its potential beneficial effects on the activity, selectivity and stability of supported cobalt nanoparticles. 

Another specific feature of the catalytic behavior of the structures under study consists in that the chemical nature of a metal becomes a factor less important for catalysis as the surface nanoparticles density increases. This is well seen in Figure 15.14, which shows the results obtained in measurements of the activity of copper- and nickel-based catalysts in the reaction of carbon tetrachloride addition to olefins. Presented in this figure are the activities of catalysts prepared by laser electrodispersion and the conventional deposition techniques. Two important features are worth noting. First, the activity... 

The third route to achieve higher catalytic activities is related to NdC -based catalysts and concerns the use of NdCU nanoparticles (Sect. 2.1.1.1). The nanoparticles are prepared by a sequence of preparative steps [132]. It is demonstrated that the size of the NdCb particles correlates with catalytic activity. 

Recently, experimental and theoretical evidence for a model of the active site of industrial methanol synthesis that combines the role of ZnO and defects in Cu has been presented [58]. Planar defects have been shown to lead to changes in surface faceting of the Cu nanoparticles (Figure 5.3.8C) associated with formation of steps and kinks that were assumed to represent high-energy surface sites of special catalytic activity. For a series of Cu/ZnO-based catalysts, a linear correlation of the defect concentration with the intrinsic activity of the exposed Cu surface was observed. In addition, (partial) surface decoration of Cu with ZnOx by SMSI has been... 

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