Characterizing the Kinetics of Nanoparticle‐Catalyzed Reactions by Surface‐Enhanced Raman Scattering

Abstract

Determining the catalytic activity and the reaction kinetics are key issues when new catalysts are developed, characterized, and introduced. Catalysis at the nanoscale employing nanoparticles has great potential because of their new catalytic properties, high surface-to-volume ratios, and high surface reactivity. [1] In principle, reactions at the surface of metal structures can be studied using molecular surfacespecific spectroscopic techniques. [2] The most versatile of these is surface-enhanced Raman scattering (SERS), which has been frequently applied in investigations of different types of reactions at electrochemical interfaces in situ [3] to address, for example, the formation of reaction intermediates, [4] the dependence of electroorganic reactions on electrode potential, [5] and electron transfer in protein systems. [6] Herein we demonstrate that SERS can be used to study directly the kinetics of a catalytic reaction in situ. Our approach is novel by allowing the structural characterization of the reactant and product surface species in the reaction as well as investigating rate constants in the same experiment. This was possible by using separate gold and platinum nanoparticles that were simultaneously attached to the same glass surface. Our method is independent of the optical absorption properties of the reaction products and/or the catalysts.In order to investigate a metal-catalyzed reaction with SERS or other plasmon-supported approaches, [7] bifunctional metal structures are needed that have plasmonic properties and also act as a catalyst. [8] A number of catalytically active composite nanostructures have been reported to enhance the Raman signals of dye molecules, [9] and SERS has been used to monitor the structural evolution of bimetallic catalytically active Au-Pt nanoparticles. [10] However, direct observations of a catalytic process by SERS have been rare as they require bifunctional nanomaterials. [11] Our approach is different from those previously reported based on composite nanostructures with plasmonic (Au) and catalytic (Pt, [11a] or Pd) [11b] properties, as we have used separate gold and platinum nanoparticles that are simultaneously immobilized on a glass surface. Scanning tunneling microscopy (STM) data indicate that owing to the proximity of the platinum and gold nanoparticles, the molecules can interact with the platinum nanoparticles whilst they reside in the local optical fields provided by the localized surface plasmons of the gold nanoparticles. The versatility, stability, and general applicability of the immobilized gold nanoparticles for studying catalytic reactions are demonstrated by the quantification of the reaction products and the determination of the kinetics with different catalysts. The results reported therefore have implications both for basic catalysis research and analytical applications.Gold nanoparticles 40 nm in diameter and platinum nanoparticles less than 2 nm in diameter were prepared by reported procedures. [12] Mixtures of these gold and platinum nano...

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