Platinum core-shell catalysts

Hsu IJ, Kimmel YC, Jiang XJ, Willis BG, Chen JG (2012) Atomic layer deposition synthesis of platinum-tungsten carbide core-shell catalysts for the hydrogen evolution reaction. Chem Commun 48 1063-1065... 

Examples of both approaches will be discussed below, starting with the so-called core shell catalysts, which mostly consist of non-noble metal cores covered by a noble metal such as platinum. Platinum-free materials, especially when based on non-noble components, have to fulfill the criterion of stability in acidic media. Recently, electrocatalysts including cobalt and iron proved then-suitability in fuel cell applications where the metal ion is incorporated in a nitrogen macrocycle comparable to the natural porphyrin ring system. 

Our cubo-octahedral structures were of the core-shell t5rpe with inner core, formed by second component atoms - transition metals, while the shell in just one atomic layer was constructed by platinum - active catalyst of surface processes. Such structures in our own calculations [25] and others [26-27] are optimal in catalytic sense, because they cause effective way of surface reactions for oxygen reduction. On the other side such nanoclusters possess stability in aggressive acid environments, which lead to electrochemical corrosion of materials of catalysts. 

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). 

Kaplan, D., Burstein, L., Rosenberg, Y., and Peled, E. (2011) Comparison of methanol and ethylene glycol oxidation by alloy and core-shell platinum based catalysts. J. Power Sources, 196, 8286-8292. 

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

This section starts with a short overview of state-of-the-art catalysts, all of which are based on platinum, followed by an explanation of the carbon monoxide issue when designing a catalyst for fuel cell applications. As the oxygen reduction reaction is the main source of kinetic performance losses of the cell, the reaction is discussed in more detail to set guidehnes for new catalyst concepts. New approaches cover platinum-containing core shell catalysis as well as de-alloyed approaches, where both classes aim for a severely reduced overall loading, and platinum-free alternatives are discussed in more detail. Finally, the influence of carbon supports on performance is discussed and alternative catalyst supports are presented. 

To enhance the mass activity of Pt, the modification of the catalytic platinum surface by the addition of a second or multiple metals has been developed to remarkably reduce Pt loading while enhancing the catalytic performance. For CO and methanol oxidation reactions, while PtRu is the practical electro-catalyst, PtMo (core-shell) and other ternary catalysts show very high activity in the oxidation of CO and/or methanol. Well-defined Pt3Ni surfaces and Pt3(CoNi) are two good examples to show how judicious design can be beneficial to dramatically enhanced activity in catalyzing ORRs. 

Zhang, J., Lima, F. H. B., Shao, M. H., SasaM, K., Wang, J. X., Hanson, J. and Adzic, R. R. (2005b) Platinum monolayer on nonnoble metal-noble metal core-shell nanoparticle electro-catalysts for Oj reduction, J. Phys. Chem. B 109, 22701-22704. 

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