The Impacts of Nanotechnology on Catalysis by Precious Metal Nanoparticles

Nanotechnol Rev 1 (2012): 31–56 © 2012 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/ntrev-2011-0003 Review The impacts of nanotechnology on catalysis by precious metal nanoparticles Rongchao Jin synthesis of ammonium in 1913, industrial catalysis has Department of Chemistry , Carnegie Mellon University, 4400 been practiced for nearly a century; its signifi cance for the Fifth Avenue, Pittsburgh, PA 15213 , USA , petrochemical industry was particularly realized when the e-mail: [email protected] oil crisis occurred in the 1970s. The importance of catalysis is also refl ected in environmental protection and public health; a well-known example is the catalytic converters for Abstract removing toxic emissions of automobiles, which were fi rst developed by General Motors Corporation and Ford Motor This review article focuses on the impacts of recent Company in 1974. advances in solution phase precious metal nanoparticles Catalysis is a complex and highly interdisciplinary sci- on heterogeneous catalysis. Conventional nanometal cata- ence; it integrates chemistry, materials science, and chemi- lysts suffer from size polydispersity. The advent of nano- cal reaction engineering. Apparently, catalysis constitutes a technology has signifi cantly advanced the techniques for central theme of chemical research as the primary activity of preparing uniform nanoparticles, especially in solution chemists is to perform reactions, which almost exclusively phase synthesis of precious metal nanoparticles with excel- involve catalysts, such as metals. Generally speaking, a cata- lent control over size, shape, composition and morphology, lyst is a special substance that can speed up chemical reac- which have opened up new opportunities for catalysis. This tions without itself being consumed in the reaction process; review summarizes some recent catalytic research by using note that in some (but rare) cases, a catalyst is used to slow well-defi ned nanoparticles, including shape-controlled down chemical reactions. The power of a catalyst lies in its nanoparticles, high index-faceted polyhedral nanocrys- capability in accelerating chemical reactions by reducing the tals, nanostructures of different morphology (e.g., core- energy barrier (i.e., activation energy) for the transition state shell, hollow, etc.), bi- and multi-metallic nanoparticles, as and in controlling reaction pathways for selective synthesis well as atomically precise nanoclusters. Such well-defi ned of desired product. With respect to materials with catalytic nanocatalysts provide many exciting opportunities, such as power, it is interesting that almost all types of substances identifying the types of active surface atoms (e.g., corner (e.g., acids, bases, metals, semiconductors, clays, carbon, and edge atoms) in catalysis, the effect of surface facets on organometallic complexes, nucleic acids, proteins, etc.) can catalytic performance, and obtaining insight into the effects serve as catalysts for certain chemical processes. The earliest of size-induced electron energy quantization in ultra-small observation of catalytic action dates back to the 19th century metal nanoparticles on catalysis. With well-defi ned metal when chemistry fi rst came into being. J ö ns Jacob Berzelius nanocatalysts, many fundamentally important issues are (1779 – 1848) is generally credited for being the fi rst person expected to be understood much deeper in future research, who carried out systematic, scientifi c study on catalysis. In such as the nature of the catalytic active sites, the metal- 1836, he integrated early observations of catalytic power support interactions, the effect of surface atom arrange- of special substances into a systematic body of knowledge ment, and the atomic origins of the structure-activity and and coined for the fi rst time the term “ catalysis ” . Currently, the structure-selectivity relationships. catalysis is recognized widely, not only in organic or inor- ganic chemistry (e.g., homogeneous and heterogeneous Keywords: heterogeneous catalysis; nanocatalysis; catalysis) but also in life and all living things (e.g., enzymatic nanoclusters; nanocrystals; nanoparticles; precious metals. catalysis). This review article focuses on heterogeneous catalysis by precious metal nanoparticles. Unlike homogeneous and 1. Introduction enzymatic catalysis, heterogeneous catalysis refers to a catalytic process that typically involves solid catalysts and reac- Catalysis is of tremendous importance for the chemical tants of different phases (e.g., gas or liquid). The solid-state industry. Approximately two-thirds of the chemical prod- catalysts are rather versatile, such as metals (e.g., Pt, Pd, Ag, ∼ ucts and 90 % of the chemical processes involve cataly- Au), semiconductors (e.g., TiO 2 , CdS), zeolites, molecular sis (e.g., homogeneous, heterogeneous, or enzymatic sieves, and so on. Precious metal catalysis constitutes an type). Since the fi rst industrialized catalytic process – the important branch of heterogeneous catalysis in the chemical 32 R. Jin: Nanocatalysis industry; for example, the well-known catalytic converter 2. Conventional heterogeneous metal catalysts utilizes three metals (Rh, Pt, and Pd) of the platinum group. In retrospect, Paul Sabatier (1854 – 1941, awarded a Nobel Heterogeneous catalysts of metals are composed of two Prize in 1912) was the fi rst to carry out hydrogenation of major components: the active metal particles and the sup- organic compounds using metal (Ni) powder catalysts, and port. Typical supports are Al2 O3 , SiO2 , MgO, Fe2 O3 , TiO2 , Irving Langmuir (at General Electric) performed CO oxida- CeO2 , and many others. The most widely used conventional tion with Pd catalyst. In the Periodic Table, precious metals method for preparing metal catalysts is the wet impregnation (also called noble metals) refer to Ag, Au, Pd, Pt, Rh, Ir, Ru, method [1] . In this method, the support (usually in powders) and Os, with the latter six elements known as the platinum is soaked in a solution of metal salt (with metal loading, say, group. Rhenium (Re) may also be included in the group of of ∼ 0.1 – 10 wt % ), followed by drying, then by thermal treat- precious metals. ment (i.e., calcination) [2] . The metal salt decomposes under Heterogeneous catalysts of precious metals almost exclu- high temperatures and converts to nanoparticles, which are sively involve the small particle form, often on the nanoscale fi nally dispersed on the support surfaces. Hydrogen pretreat- (e.g., 1 – 100 nm in diameter), although in some cases microscale ment is often applied to turn such pro-catalysts into active particles are also used (e.g., Ag particles of a few µ m in size ones (i.e., to reduce the surface oxidized metal nanoparticles are used in industrial epoxidation of ethylene). Physical “ see- into metallic state). ing ” of nanoparticles was apparently not feasible prior to the Apparently, the impregnation method has a poor invention of electron microscopy and, thus, early studies of control over the metal particle size and the size distribu- heterogeneous catalysis (e.g., during the fi rst half of the 20th tion. However, in practical work, researchers may not care century) were rather “ dark ” . Even today, catalysis is still some- about those aspects, as long as high activity and selectivity times called “ black art ” owing to the nature of catalysts being of the catalyst can be attained, which is why catalysis was largely unknown. However, in the past two decades, signifi cant once called “ black art ” . Nevertheless, in modern research, developments in nanotechnology have enabled researchers not the availability of many nanoscale characterization tools, only to see nanoparticles clearly at atomic resolution and create especially electron microscopy, has enabled researchers to well-defi ned nanoparticles but also to look into the very fun- obtain a great deal of invaluable information of catalysts damentals of catalytic processes. Such remarkable, signifi cant and to understand some mechanistic aspects of catalysis, progress in catalytic research repudiates the previous “ black hence no longer “ black art ” . The impregnation method, art ” notoriety of catalysis and brings catalysis into an exciting although very old, is still a very popular method for prepar- era of scientifi c research at the ultimate atomic level, which has ing heterogeneous catalysts, even today. Many variations long been dreamed by catalysis chemists. or new methods have been developed in the past decades, In this review article, I will fi rst briefl y discuss conven- such as coprecipitation, which involves simultaneous pre- tional nanocatalysts (Section 2) to usher new researchers into cipitation of the active metal and the support, deposition- the catalysis fi eld, then present the major advances in the cre- precipitation [3] , sol-gel method [4] , deposition of colloids ation of well-defi ned nanoparticles in solution phase (Chart [5] , and so on. 1 ) and their catalytic applications (Section 3), and fi nally pro- The metal nanocatalysts prepared by conventional, salt- vide my personal perspective on some future developments of based methods often consist of polydisperse nanoparticles, catalytic science, including challenging issues and prospects which preclude studies of particle size dependence or obscure of the catalysis fi eld (Section 4). As this review focuses on the size-dependent catalytic results. This situation is even the catalytic materials, theoretical aspects of catalysis and worse when it

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