Core-shell catalysts chemical

Cargnello, M., et al., Multiwalled carbon nanotubes drive the activity of metal oxide core-shell catalysts in modular nanocomposites. Journal of the American Chemical Society, 2012. 134(28) p. 11760-11766. 

To inspect and compare the activation overvoltage of the three catalysts in more detail, so-called Tafel plots are used, which plot the cell voltage as a function of the logarithm of the current density. Figure 3.3.16B shows the Tafel plots derived from Figure 3.3.15A. At a cell voltage of 0.9 V, where the overall reaction rate is limited by the chemical surface catalysis, the dealloyed core-shell catalysts perform three... 

Core-shell catalyst materials may also be prepared by non-electrochemical routes. Core-shell nanoparticles may be produced in solutimi using colloidal methods, by sequential deposition of the core and shell components [33], or Pt layers may be deposited chemically or via displacement reactions onto preprepared core nanoparticles, but in cmitrast to approaches described in Sect. 19.3.1, no applied potential is required typically core particles or colloidal core-shell particles are deposited onto carbon supports. 

Chapters 18-21 discuss core-shell and advanced Pt alloy catalysts (which also can be considered to have a core-shell structure). Chapter 18 studies the fundamentals of Pt core-shell catalysts synthesized by selective removal of transition metals from transition metal-rich Pt alloys. Chapter 19 outlines the advances of core-shell catalysts synthesized by both electrochemical and chemical methods. The performance, durability, and challenges of core-shell catalyst in fuel cell applications are also discussed. Chapter 20 reviews the recent analyses of the various aspects intrinsic to the core-shell structure including surface segregation, metal dissolution, and catalytic activity, using DFT, molecular dynamics, and kinetic Monte Carlo. Chapter 21 presents the recent understanding of activity dependences on specific sites and local strains in the surface of bulk and core-shell nanoparticle based on DFT calculation results. 

E nre 7.9 Changes in shell morphology for oxidized and reduced Pd Ce02/ AI2O3 core-shell catalysts. Reprinted from Wieder et al Copyright 2011 the American Chemical Society. 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a muldtude of phases containing the acdve metallic components. Of these many phases, a characteristic is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catchall term. An historical overview of the properties of alloys in connection with catalysis has been published by Ponec. At present, a broadly agreed view accepts that alloy components can be chemically recognised and, therefore, supports a somewhat localised interpretation of the alloy nature and properties. Obviously, a delocalised view loses most of its meaning in the case of clusters due to its finite dimensions. 

Polymer-capped bimetallic nanoclusters are very effective as catalysts. The combination of various metals can produce many kinds of bimetallic nanoclusters of various structures. We can now freely control the structure of bimetallic nanoclusters. Recently, we have succeeded in preparing triple core/shell structured trimetallic nanoclusters which have much higher catalytic activity than the corresponding bimetallic nanoclusters. Thus, the present author believes that researches on polymer-capped bi- or tri-metallic nanoclusters are progressing rapidly and that the results will be applied to various practical chemical processes in the near future. 

Galvanic displacement method is also often used for synthesizing catalysts. By this method, low Pt-content electrocatalysts can be obtained. For example, a carbon-supported core—shell structured electrocatalyst with bimetallic IrNi as the core and platinum monolayer as the shell has been successfully synthesized using this method. In this synthesis, IrNi core supported on carbon was first synthesized by a chemical reduction and thermal annealing method and a Ni core and Ir shell structure could be formed finally. The other advantage of this method is that the Ni can be completely encased by Ir shell, which will protect Ni dissolve in acid medium. Secondly, IrNi PtML/C core—shell electrocatalyst was prepared by depositing a Pt monolayer on the IrNi substrate by galvanic displacement of a Cu monolayer formed by under potential deposition (UPD). 

A series of PfML/Pd/C core-shell samples with varying nominal Pt shell thickness have also been prepared via a proprietary chemical method and explored using X AS and electrochemical techniques [26]. Analysis of EXAFS at the Pd K and ft L3 edges for catalyst pellet samples revealed the expected increase in ft-ft and decrease in Pd-ft and Pt-Pd neighbors with increasing nominal ft coverage from 0.5 to 2 monolayers of ft (calculated based on catalyst surface area of the Pd/C cores). Further EXAFS measurements under electrochemical control in liquid electrolytes revealed an increase in average Pd-Pd bond distance to 2.780 A at 0.0 V for the 0.5 of... 

Kim, H. W., Kang, K. M., Kwak, H. Y., Kim, J. H. (2011). Prqtaration of supported Ni catalysts on various metal oxides with core/shell structures and their tests for the steam reforming of methane. Chemical Engineering Journal, 168, 775—783. Scopus Exact. 

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