1,721,022 research outputs found
Opportunities for tailoring catalytic properties through metal-support interactions
No full textThe oxides used as supports for metal catalysts can be used to modify the catalyst properties. In this paper, we discuss three relatively new ways for optimizing the oxide–metal interactions and show examples where these methods have been used to improve catalytic performance. Opportunities still exist for using each of these approaches to produce materials with improved catalytic performance
Photocatalysis by Nanostructured TiO2-based Semiconductors
No full textThe shift towards being as environmentally-friendly as possible has resulted in the need for this important volume on the topic of green nanoscience. Edited by two rising stars in the community, Alvise Perosa and Maurizio Selva, this is an essential resource for anyone wishing to gain an understanding of the world of green chemistry, as well as for chemists, environmental agencies and chemical engineers
Playing with Structures at the Nanoscale: Designing Catalysts by Manipulation of Clusters and Nanocrystals as Building Blocks
No full textThe purpose of this Concept is to highlight some of the most recent and promising methods for the preparation of tailored catalysts by designing and preparing the component building blocks and by assembling them in a controlled fashion. We want to emphasize how rational design and synthesis of catalysts must be coupled to precise catalytic and structural characterization of the systems in an ideal feedback loop. New catalyst design and preparation techniques, dictated by information about the active sites that the specific application requires, are frequently available. The building blocks for developing these novel catalysts include colloidal methods for the preparation of uniform nanostructures, physical methods for rational assembly of the building blocks (Langmuir–Blodgett, liquid–air self-assembly), and development of rational interactions between the building blocks for enhanced activity of the assemblies. These methods, which apply techniques normally used in other fields of nanotechnology to catalysis, offer exciting opportunities to help improve currently available catalytic systems in terms of activity, stability and selectivity
High-temperature calcination improves the catalytic properties of alumina-supported Pd@ceria prepared by self assembly
No full textThe effect of varying the calcination temperature from 773 to 1073 K was examined on catalysts prepared from Pd@CeO2, core–shell nanoparticles adsorbed on silane-functionalized γ-Al2O3 (Pd@CeO2/Si–Al2O3). Calcining to higher temperatures increased rates per gram catalyst for the methane-oxidation reaction significantly. The Pd@CeO2/Si–Al2O3 catalyst calcined to 773 K was unstable for methane-steam reforming (MSR) due to deep reduction of the catalyst while the catalyst calcined at 1073 K showed reasonable stable rates. CO adsorption, monitored using volumetric uptakes and FTIR, indicated adsorption on the Pd was suppressed following reduction at 673 K in H2 when the catalyst had been calcined at only 773 K, but not after 1073 K. Pulse-reactor measurements demonstrated that catalysts calcined at either 773 K or 1073 K were heavily reduced under MSR reaction conditions at 673 K but that the catalyst heated to 1073 K could be re-oxidized by H2O at this temperature, while the 773-K sample could not be. It is suggested that increasing calcination temperature modifies the structure of the ceria shell, which in turn changes the ceria redox properties
Supported platinum–zinc oxide core–shell nanoparticle catalysts for methanol steam reforming
No full textPlatinum–zinc oxide (Pt@ZnO) and palladium–zinc oxide (Pd@ZnO) core–shell nanoparticles were synthesized in solution using a method based on self-assembly and deposited onto a functionalized alumina (Si–Al2O3) support. TEM investigations of the samples confirm the formation of core–shell structures of approximately 6 nm of diameter following calcination to remove the ligands. In situ TEM and coulometric titration experiments suggest that Pt–Zn alloys are formed upon reduction and that these are highly tunable in size. While methanol steam reforming (MSR) measurements on conventional Pt/Al2O3 and Pd/Al2O3 catalysts show poor CO2 selectivities, a Pt (1 wt%)@ZnO (9 wt%)/Si–Al2O3 system showed comparable activity and selectivity for CO2 as a conventional Pt/ZnO catalyst, providing further indication that Pt@ZnO forms a Pt–Zn alloy upon reduction due to the intimate contact between the two materials. The Pd@ZnO/Si–Al2O3 exhibited lower CO2 selectivities than Pt@ZnO/Si–Al2O3
Methane oxidation on Pd@ZrO2/Si-Al2O3 is enhanced by surface reduction of ZrO2
No full textThe catalytic properties of Pd@ZrO2 core–shell catalysts supported on Si-modified alumina were studied for application to methane oxidation and compared to the analogous Pd@CeO2 catalysts. In the absence of water (dry conditions), both Pd@ZrO2 and Pd@CeO2 were highly active and showed nearly identical reaction rates and thermal stabilities. However, unlike catalysts based on Pd@CeO2, the Pd@ZrO2 catalysts were also very stable in the presence of high concentrations of water vapor. By means of Coulometric titration and pulse-reactor studies, we demonstrate that ZrO2 in contact with Pd can be reduced. Additionally, Coulometric titration showed that the Pd-PdO equilibrium at 600 °C is shifted to much lower P(O2) in the Pd@ZrO2 catalyst compared to conventional Pd/ZrO2 or Pd/Al2O3 catalysts. Because PdO is more active for methane oxidation, this observation provides a possible explanation for the superior performance of the Pd@ZrO2 catalyst
Study of the water-gas-shift reaction over Pd@CeO2/Al2O3 core-shell catalysts
No full textAbstract Image
An alumina-supported, Pd@CeO2, core−shell catalyst having 1 wt % Pd and 9 wt % ceria was characterized for the water-gas-shift (WGS) reaction. Although the catalyst initially exhibited similar WGS rates to that of a conventional Pd/ceria catalyst at 623 K in 25 Torr each of CO and H2O, the Pd@CeO2 catalyst deactivated severely over the period of 1 h. The WGS activity of the Pd@CeO2 could be completely restored by mild oxidation, and oxygen-titration measurements showed that the ceria shell in the Pd@CeO2 catalyst was significantly reduced after being used for the WGS reaction. These observations are in sharp contrast to those found with a conventional Pd/ceria catalyst, for which the ceria remains almost fully oxidized under WGS conditions. CO adsorption measurements, using FTIR at room temperature and CO uptakes at 195 K, indicated that Pd in the oxidized Pd@CeO2 catalyst was accessible to CO, but adsorption was completely suppressed on the reduced catalyst. A model is presented to explain the results, which assumes that cracks and fissures in the oxidized ceria shell allow access to the Pd core but that reduction blocks access, either due to changes in the density of the ceria, which closes the fissures, or to coverage of the metal surface with ceria
A Versatile Route to Core-Shell Catalysts: Synthesis of Dispersible M@Oxide (M = Pd, Pt; Oxide = TiO2, ZrO2) Nanostructures by Self-Assembly
No full textA method, based on self assembly, for preparing core–shell nanostructures that are dispersible in organic solvents is demonstrated for Pd and Pt cores with CeO2, TiO2, and ZrO2 shells. Transmission electron microscopy (TEM) of these nanostructures confirmed the formation of distinct metal cores, approximately 2 nm in diameter, surrounded by amorphous oxide shells. Functional catalysts were prepared by dispersing the nanostructures onto an Al2O3 support; and vibrational spectra of adsorbed CO, together with adsorption uptakes, were used to demonstrate the accessibility of the metal core to CO and the porous nature of the oxide shell. Measurements of water-gas-shift (WGS) rates demonstrated that these catalysts exhibit activities similar to that of conventional supported catalysts despite having lower metal dispersions. Pd-based CeO2 and TiO2 core–shell catalysts exhibit significant transient deactivation, which is probably caused by a decrease in the exposed metal surface area due to the ease of reduction of the shells. Alternatively, Pt-based analogous core–shell catalysts do not exhibit such a transient decrease. Both Pd- and Pt-based ZrO2 core–shell catalysts deactivate at a significantly lower rate due to the less reducible nature of the ZrO2 shell
Synthesis of dispersible Pd@CeO2 nanostructures by self-assembly
No full textA methodology is described for the preparation of Pd@CeO2 core−shell nanostructures that are easily dispersible in common organic solvents. The method involves the synthesis of Pd nanoparticles protected by a monolayer of 11-mercaptoundecanoic acid (MUA). The carboxylic groups on the nanoparticle surfaces are used to direct the self-assembly of a cerium(IV) alkoxide around the metal particles, followed by the controlled hydrolysis to form CeO2. The characterization of the nanostructures by means of different techniques, in particular by electron microscopy, allowed us to demonstrate the nature of core−shell systems, with CeO2 nanocrystals forming a shell around the MUA-protected Pd core. Finally, an example of the use of these nanostructures as flexible precursors for the preparation of heterogeneous catalysts is reported by investigating the reactivity of Pd@CeO2/Al2O3 nanocomposites toward CO oxidation, water−gas shift (WGS), and methanol steam reforming reactions
Au@TiO2 core-shell nanostructures with high thermal stability
No full textA catalyst system consisting of core–shell nanostructures with Au core and porous TiO2 shell was synthesized and characterized for room temperature CO oxidation. The core–shell structures were prepared by colloidal methods starting from pre-formed 3 nm Au particles in solution and then adsorbed on to high-surface area, functionalized hydrophobic Al2O3 support. The obtained Au@TiO2/Si–Al2O3 catalyst showed higher activity and thermal stability when compared to a conventional Au/TiO2 sample prepared by impregnation of the same Au particles on to commercial titania P25. The core–shell catalyst was able to maintain its activity and 3 nm Au particles size upon calcination up to 600 °C, whereas the Au/TiO2 sample was found to sinter. Furthermore, it was found that the crystallization of TiO2 was suppressed in the core–shell structure, resulting in a thin layer of small TiO2 particles, which is favorable for the dispersion and thermal stability of Au nanoparticles
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