Nanocomposite catalysts: Dendrimer encapsulated nanoparticles immobilized in sol–gel silica

Beakley, Lindsey W.; Yost, Sarah E.; Cheng, Raymond; Chandler, Bert D.

doi:10.1016/j.apcata.2005.05.049

Cited by 43 publications

(53 citation statements)

References 28 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Our second approach was to synthesize the solid support in situ in the presence of the colloidal particles, to encapsulate the latter within the pores of the former during the growth process (42). The results obtained from preliminary experiments using this methodology are encouraging.…”

mentioning

confidence: 51%

Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Lee

Morales

Albiter

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

224

173

View full text Add to dashboard Cite

Colloidal and sol-gel procedures have been used to prepare heterogeneous catalysts consisting of platinum metal particles with narrow size distributions and well defined shapes dispersed on high-surface-area silica supports. The overall procedure was developed in three stages. First, tetrahedral and cubic colloidal metal particles were prepared in solution by using a procedure derived from that reported by El-Sayed and coworkers [Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996) Science 272: 1924 -1926]. This method allowed size and shape to be controlled independently. Next, the colloidal particles were dispersed onto high-surface-area solids. Three approaches were attempted: (i) in situ reduction of the colloidal mixture in the presence of the support, (ii) in situ sol-gel synthesis of the support in the presence of the colloidal particles, and (iii) direct impregnation of the particles onto the support. Finally, the resulting catalysts were activated and tested for the promotion of carbon-carbon doublebond cis-trans isomerization reactions in olefins. Our results indicate that the selectivity of the reaction may be controlled by using supported catalysts with appropriate metal particle shapes. On the basis of their kinetic behavior, catalytic reactions are often classified as either mild or demanding (1-3). Demanding reactions-such as the oxidation of CO, NO, or hydrocarbons; the synthesis of ammonia; and most oil processing conversionsusually require high temperatures and pressures, and involve small concentrations of intermediates similar to those identified under vacuum. The performance of these reactions often depends strongly on the structure of the catalyst used (4, 5). In contrast, mild reactions-in particular, hydrogenations and isomerizations of unsaturated hydrocarbons-take place under less-demanding temperature and pressure conditions. Mild reactions have historically been considered structure-insensitive (6-8), but that conclusion has been drawn from studies on reactivity vs. metal dispersion that used ill-defined supported catalysts (9, 10) and has been questioned by more recent studies using better catalytic models (11). For instance, both experimental (12-14) and theoretical (15) studies on the selective catalytic hydrogenation of CAO bonds in unsaturated aldehydes have suggested that such reactions may be promoted by close-packed (111) surfaces. In another example, the dehydrogenation of cyclohexene was found to be faster on Pt (111) than on Pt (100) single-crystal surfaces (16). Our recent surface-science investigations on the isomerization of unsaturated olefins (17-19) strongly suggest that selectivity toward the formation of the cis isomer may be favored by Pt (111) facets. Additional surfacescience reports on the conversion of alkyl and alkene adsorbates under vacuum conditions (20-25), as well as studies with more realistic model systems (26, 27), point to a potential structure sensitivity in the conversion of other olefins and unsaturated hydrocarbons.These results not only s...

show abstract

mentioning

confidence: 51%

Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Lee

Morales

Albiter

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

224

173

View full text Add to dashboard Cite

show abstract

“…Synthesis of DENs (dendrimer encapsulated nanoparticles) is widely reported in the literature [13][14][15][16][17][18][19], and this process usually initiates with the formation of intradendrimer metal clusters, then chemically reduced with excess of NaBH 4 . Changes in solution colour and UV-visible absorbance bands are the evidences for this cluster formation [20] even giving some clue about their size [21,22].…”

Section: Introductionmentioning

confidence: 99%

Use of Dendrimers during the Synthesis of Pt-Ru Electrocatalysts for PEM Fuel Cells: Effects on the Physical and Electrochemical Properties

Calderón

Calvillo

Lázaro

et al. 2011

International Journal of Electrochemistry

View full text Add to dashboard Cite

In this work, Pt-Ru catalysts were synthesized by a novel methodology which includes the use as encapsulating molecules of dendrimers of different generation: zero (DN-0), one (DN-1), two (DN-2), and three (DN-3). Synthesized catalysts were heattreated at 350• C, and the effects of this treatment was established from the physical (X-ray dispersive energy (XDE) and X-ray diffraction (XRD)) and electrochemical characterization (cyclic voltammetry and chronoamperometry). Results showed that the heat-treatment benefits the catalytic properties of synthesized materials in terms of CO and methanol electrochemical oxidation. The curves for CO stripping were more defined for heat-treated catalysts, and methanol oxidation current densities were higher for these materials. These changes are principally explained from the removal of organic residues remaining on the surface of the Pt-Ru nanoparticles after the synthesis procedure. After the activation of the catalysts by heating at 350• C, the real importance of the use of these encapsulating molecules and the effect of the generation of the dendrimer become visible. From these results, it can be concluded that synthesized catalysts are good catalytic anodes for direct methanol fuel cells (DMFCs).

show abstract

“…Peak areas were standardized against external calibration solutions of toluene in hexanes and methylcyclohexane in toluene. 39 Toluene hydrogenation rates were determined at 60°C…”

Section: Introductionmentioning

confidence: 99%

“…The first stage was held at approximately 40°C, and the second stage was maintained at 23°C. 39 The reaction was monitored using an SRI 8610C gas chromatograph with a gas automatic sampling valve, 1 mL sample loop, and flame ionization detector. Products and reactants were quantified using an SiO 2 column operated at 260°C…”

mentioning

confidence: 99%

See 1 more Smart Citation

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt−Cu Nanoparticles

2006

Self Cite

View full text Add to dashboard Cite

The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions. IntroductionHighly dispersed supported metal nanoparticle catalysts are an important class of industrial materials, as one-third of material U. S. gross national product involves a catalytic process somewhere in the production chain. 1 State of the art heterogeneous catalysts often contain dopant metals, which are incorporated to promote a desired reaction, prevent undesirable side reactions, or enhance catalyst longevity. 2,3 Traditional preparative routes involve impregnating metal salts onto high-surfacearea supports, followed by various thermal activation steps. 3 Most important industrial supports can also be employed as separation media; consequently, chromatographic separation of salt precursors as they pass through the support pore structure is unavoidable. Further, nanoparticle preparation via traditional routes depends on the surface (and even gas phase) mobility of the species present during the thermal treatments. 4,5 These processes vary widely for different metals and are poorly understood, at best. 4,5 Consequently, bimetallic materials prepared by such methods are often complicated, have significant compositional inhomogenaity from one particle to the next, and are difficult to characterize.Recent advances in computational methods are beginning to shed light onto the fundamental properties of bimetallic catalyst systems, and predictive models are beginning to emerge. [6][7][8][9] The potential to controllably tune catalyst properties through the nanoscale design of bimetallic nanoparticles is becoming apparent, and designed bimetallic catalysts with novel behavior for important reactions already appear in the recent literature. [10][11][12] However, the utilization of traditional preparative techniques and the complicated materials that arise from these methods remain a significant hurdle to studying new catalysts, eva...

show abstract

Nanocomposite catalysts: Dendrimer encapsulated nanoparticles immobilized in sol–gel silica

Cited by 43 publications

References 28 publications

Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Use of Dendrimers during the Synthesis of Pt-Ru Electrocatalysts for PEM Fuel Cells: Effects on the Physical and Electrochemical Properties

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt−Cu Nanoparticles

Contact Info

Product

Resources

About