Nanocatalysis I Synthesis of Metal and Bimetallic

Authors: Kwangjin An Gabor A Somorjai

Publish Date: 2014/12/24

Volume: 145, Issue: 1, Pages: 233-248

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Abstract

In recent heterogeneous catalysis much effort has been made in understanding how the size shape and composition of nanoparticles and oxidemetal interfaces affect catalytic performance at the molecular level Recent advances in colloidal synthetic techniques enable preparing diverse metallic or bimetallic nanoparticles with welldefined size shape and composition and porous oxides as a high surface support As nanoparticles become smaller new chemical physical and catalytic properties emerge Geometrically as the smaller the nanoparticle the greater the relative number of edge and corner sites per unit surface of the nanoparticle When the nanoparticles are smaller than a critical size 27 nm finitesize effects such as a change of adsorption strength or oxidation state are revealed by changes in their electronic structures By alloying two metals the formation of heteroatom bonds and geometric effects such as strain due to the change of metal–metal bond lengths cause new electronic structures to appear in bimetallic nanoparticles Ceaseless catalytic reaction studies have been discovered that the highest reaction yields product selectivity and process stability were achieved by determining the critical size shape and composition of nanoparticles and by choosing the appropriate oxide support Depending on the pore size various kinds of micro meso and macroporous materials are fabricated by the aid of structuredirecting agents or hardtemplates Recent achievements for the preparation of versatile core/shell nanostructures composing mesoporous oxides zeolites and metal organic frameworks provide new insights toward nanocatalysis with novel ideasNanotechnology is making huge strides and its discoveries are affecting catalysis in gigantic ways Colloidal synthesis has made great progress by controlling the size shape and composition of nanoparticles 1 2 3 4 An ultimate goal of industrial catalysis is to achieve the highest catalytic activity and selectivity toward only one desired product while maintaining high stability against deactivation For this purpose noble nanoparticle catalysts are developed by colloidal synthetic techniques and many reaction studies exhibit enhancement of reaction rates and change of selectivities with the optimum nanoparticle size and shape Bimetallic nanoparticle catalysts by alloying two metals create new chemical and catalytic properties which cannot be achieved by their parent single metal nanoparticles 5 6 High surface porous materials have also been developed as a support as well as a catalyst 7 8 As catalytic behaviors can also be altered at oxidemetal interfaces support materials with high surfaces and ordered pore structures become increasingly important for supported catalystsIn this review article we classify nanoparticle catalysts Through synthetic nanotechnology we explain how metal and bimetallic nanoparticles are generated with welldefined size shape and composition By means of versatile and elaborative synthetic approaches various kinds of core/shell nanostructures are introduced with specific functions for catalysis as well As a support porous oxide materials are classified into three categories depending on their pore size which are micro meso and macroporous materials The synthetic strategies and several examples are provided for the preparation of microporous zeolite mesoporous and macroporous materials with ordered pore structuresIn addition to nanoparticle catalysts we account for the latest model catalytic reactions to show how the designed nanocatalysts impact catalytic properties and what are the major molecular factors to change the surface chemistry With much progress on in situ surface characterization techniques we can monitor working catalysts under real catalytic conditions and identify molecular information during the reaction 9 10 In a separate review article an indepth study for in situ surface characterization techniques will be given Instead here we will focus more on the correlation between specific nanoparticle catalysts and catalytic properties We hope to provide basic knowledge of stateoftheart nanoparticle catalysts for readers Furthermore we expect to provide an understanding of how the designed nanoparticle catalysts influence catalytic properties through the latest catalytic reaction studiesSchematics showing synthetic strategies of Pt nanoparticles with controlled size and shape a TEM images of sizecontrolled Pt nanoparticles by the polyol reduction method b A dendrimertemplating strategy for the synthesis of Pt nanoclusters c Highresolution TEM images of Pt nanoparticles with shapes of cubes cuboctahedra and octahedra by adding Ag ions as a structure directing agent reproduced with permission from 13 17 copyright 2008 and 2005 American Chemical SocietyBy rational selection of surfactants reducing agents and additional foreign metal ions nucleation and growth kinetics are further regulated to produce nanoparticles with different sizes and shapes 3 For example Pt nanocubes were selectively produced when an alkylammonium salt was introduced in the presence of PVP 16 As shown in Fig 1c Pt nanoparticles with various shapes including cubes cuboctahedra and octahedra can be generated selectively by adding silver ions because the crystal growth rate along 〈100〉 was determined by the amount of silver ions present during the reaction 17 When small nanoclusters were introduced as a seed to the precursor solution various nanoparticles with controlled shapes can be produced by seed mediated growth 18Scanning transmission electron microscopy/energydispersive spectroscopy STEM–EDS images of bimetallic nanoparticles a Pt90Rh10 b PtCo and c d CoCu reproduced with permission from 20 22 copyright 2013 2011 Springer 23 copyright 2013 American Chemical Society The color points represent distribution of metals in the nanoparticles STEM/EDS images of assynthesized CoCu bimetallic nanoparticles show their Curich core/Corich shell nature c When a redox cycle was applied in H2/O2 at 350 °C the bimetallic CoCu nanoparticles suffered an intraparticle phase segregation of Co and Cu to form contact dimer particles dRecently oxidation states and compositions of nanoparticle surfaces have been investigated through synchrotronbased in situ techniques such as ambient pressure Xray photoelectron spectroscopy APXPS and near edge and extended Xray absorption fine structure NEXAFS/EXAFS spectroscopies 9 10 From many investigations it has been proven that bimetallic nanoparticles suffer profound structural and chemical changes in response to various gaseous environments For example RhPd bimetallic nanoparticles exhibited reversible changes in composition under alternating oxidizing or reducing conditions 21 Pd with a lower surface energy than Rh migrated to the surface when reduced while stable RhOx formed on the surface under oxidizing conditions Similarly CoPt bimetallic nanoparticles synthesized by the polyol reduction of Ptacac2 and Coacac2 precursor mixtures were identified as having Ptrich surfaces by atomic diffusion during exposure to 01 Torr of H2 Fig 2b 22 Alayoglu et al also reported that asprepared CoCu bimetallic nanoparticles were indentified as Curich core/Corich shell nanoparticles by STEM/EDS phase maps 23 In O2 conditions both metals were oxidized and Cu segregates to the surface having the Cu+ state while the segregation was reversed in H2 having reduced Co and fully reduced Cu on the surface Fig 2c d

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