Selective Nanocatalysis of Organic Transformation

Authors: Gabor A Somorjai Yimin Li

Publish Date: 2010/05/11

Volume: 53, Issue: 13-14, Pages: 832-847

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Abstract

Monodispersed transition metal Pt Rh Pd nanoparticles NP in the 08–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions For NP systems the turnover rates and product distributions depend on their size shape oxidation states and their composition in case of bimetallic NP systems Dendrimersupported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis Transition metal nanoparticles in metal core Pt Co––inorganic shell SiO2 structure exhibit exceptional thermal stability and are wellsuited to perform catalytic reactions at high temperatures 400 °C Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions SFG ambient pressure XPS and high pressure STM These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems heterogeneous homogeneous and enzyme excellent catalysts and provides opportunities to develop hybrid heterogeneoushomogeneous heterogeneousenzyme and homogeneousenzyme catalyst systemsStockholm is the cradle of catalysis where in 1835 J Berzelius defined the phenomenon when he concluded in his famous paper 1 “Thus it is certain that substances both simple and compound in solid form as well as in solution… promote the conversion of chemical compounds…into other states without necessarily participating in the process…even if this should occasionally occur” He defined this phenomenon “I shall…call it the catalytic power of the substances and decomposition by means of this power catalysis…” He also envisioned “Turning with this idea to the chemical processes in living nature we regard them in a new light… It gives reason to believe that within living plants and animals thousands of catalytic processes are going on between the tissues and fluids producing a multitude of different chemical compounds…”In the last century catalysis developed into one of the most powerful technologies in the petroleum bulk chemical fine chemical and pharmaceutical industries 2 In parallel to developing technologies our fundamental understanding of catalytic processes has been advancing rapidly by developing model catalytic systems and then studying these model systems using experimental and theoretical techniques at the molecular level 3 4 The molecular level knowledge assists the rational design of new catalysts with optimal properties 5Activity selectivity the resistance to deactivation and the ability for regeneration are the key macroscopic properties that characterize the usefulness of catalysts As the concern for environmental protection is continuously growing in the 21th century the major challenge we face is to achieve high selectivity without significant degradation of other catalytic properties in order to reduce the cost for product separation and waste disposal 6 7 One of the promising strategies in this regard heterogenization of homogeneous or enzymatic catalysts aims at combining the superior selectivity of homogeneous and enzymatic catalysts with the recyclability of heterogeneous catalysts 8 9Many industrial heterogeneous catalysts 10 11 currently in use consist of metal nanoparticles with large variations in size and shape This variation makes it very difficult to control the distribution of the active sites for different reaction products over the catalyst surfaces In this paper we are concerned with how to tune the selectivity of metalbased heterogeneous catalysts using size and shape controlled metal nanoparticles 12 13 We start with the concept of the active sites on metal surfaces and its relation to catalytic selectivity Sect 2 Then we give a brief introduction to the colloid chemistry controlled nanoparticle synthesis Sect 3 Several hydrocarbon conversion reactions using model nanoparticle catalysts are reviewed and the nanoparticle size and shape dependence of the selectivity is discussed Sect 4 Section 5 discusses surface science techniques that provide catalyst surface information the structure composition oxidation state mobility of surface adsorbates and charge transfer under reaction conditions and on the molecular scale Finally in Sect 6 we point to future challenges that we believe will propel further development of catalysis scienceThe upper panel the model kinked Pt surface the upper panel The C–C bond and C–H bond are dissociated at the kink and step site respectively The lower left panel the schematic free energy potential surface for twopathway reaction Product 1 is formed by breaking the C–C bond and Product 2 is formed by breaking the C–H bond The activation barrier for product 1 is lowered at the kink site which leads to the difference in selectivity between the step and kink site as shown in the lower right panelThree types of the model catalysts developed to study the active sites on the catalyst surfaces Surface structure of single crystal catalysts is relatively easy to control and to characterize by available surface science techniques The selectivity studies over the single crystal surfaces provide information about the active sites which can serve as references for nanoparticle catalyst studies By dispersing the size and shape controlled nanoparticles onto 2D supports many of surface science techniques are still applicable for characterizing the surface of these nanoparticles Nanoparticle size and shape effects on catalytic selectivity can be studied systematically The effect of strong metal support interaction can also be investigated Loading nanoparticles onto highsurfacearea 3D supports produces systems similar to those used in industry and enables the selectivity studies under reaction conditions practiced in chemical technologies

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