Role of 'Metal Ion-Metal Nanocluster' Ensemble Sites in Activity and Selectivity Control

BY JOZSEF L. MARGITFALVI AND SANDOR GOBÖLÖS

1 Introduction

1.1 Historical Background. – In heterogeneous catalysis, the entity involved in the catalytic cycle is an active site or active center located at the surface of a solid material. This idea goes back to the second half of the nineteenth century. For example, Loew suggested that when a molecule interacts with the catalyst the 'sharp corners' of the catalysts are involved in the break up of the molecule into atoms, i. e., these sites are more reactive than others are. More precise definition of the active sites was first given with respect to metal catalysts. Langmuir has described active sites as an array of sites that can chemisorb an atom or molecule in a localized mode. In his model Langmuir suggested that all available active sites are identical. Taylor was the first who proposed that a solid surface with catalytic properties may contain not one, but many types of active sites. He focused on the heterogeneity of the surface of catalysts, ascribing special activity to surface atoms whose coordination to other surface atoms is low. The other very important prediction made by Taylor is related to the 'reaction induced' formation of active sites. He stated 'the amount of surface which is catalytically active is determined by the reaction catalysed'. This principle has been evidenced in several catalytic reactions. It will also be shown in this review that surface species formed in situ play an important role in the generation of a new type of active site containing 'metal ion-metal nanocluster' ensembles.

1.2 Type of Active Sites. — In heterogeneous catalysis the following type of actives sites can be distinguished: (i) metallic, (ii) acid-base, (iii) red-ox type, and (iv) anchored metal-complex. The catalytic sites may contain one of the above types of active sites or can include several types of sites. In case of different type of sites the catalysts are bifunctional or multifunctional. For instance, Pt/Al2O3 and Pt/mordenite are typical bifunctional catalysts containing both metallic and acidic types of active sites. On the other hand, Pt or Pd supported on silicon carbide, nitride, or Pt/L-zeolite are mono-functional catalysts. There are important industrial reactions, such as isomerization and aromatization of linear hydrocarbons, which requires bifunctional catalysts, such as chlorinated Pt/Al2O3. In these catalysts the two types of sites have to be located sufficiently close to each other so that transport between the sites would not be rate limiting in the overall process.

Metal catalysed reactions are differentiated introducing the concept of facile and demanding reactions. In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form multiplets or ensembles. In other words, some reactions depend on the surface geometry (e.g. hydrogenolysis of hydrocarbons), while other may not (e. g. hydrogenation of olefinic double bond).

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions.

1.3 Mono- and Bimetallic Supported Catalysts. – The key factor in designing supported metal catalysts is the knowledge about the reaction mechanisms and information about the role of different types of active sites in a given step of the catalytic reaction. The performance of supported mono-functional monometallic catalysts is governed by the metal particle size, metal dispersion, overall morphology of the metal nanocluster, the character of metal-support interaction, and the electronic properties of the metal. In bifunctional supported metal catalysts in addition to the above listed factors the metal/acid balance, and the type and strength of the acid function play a key role in the overall performance.

In case of bimetallic catalysts, other properties, such as surface composition and the potential stabilization of one of the metal components in ionic form, are the most crucial determining the performance of the catalyst. It is noteworthy that combination of modern methods enables the chemist to characterize both active sites of supported metals and the reaction intermediates formed. Additionally, quantum chemical calculations become more and more powerful tools in understanding chemical interaction controlling and governing both the catalyst structure and the catalytic performance.

In the last decade much attention has been paid to metal nano-clusters including supported nanoparticles as one of the promising advanced nanoscopic materials. Elements easily forming supported metal nanoclusters are Group VIII and IB transition metals as follows: Pt, Ir, Pd, Rh, Ru, Ni, Co, and Au, Ag, Cu. It is interesting to note that the heat of formation of the oxides of these metals is low (usually below -ΔHf = 40 kcal/mol at 25 °C referred to one oxygen atom). Therefore, the oxides of these metals can easily be reduced to zero valence. The reduction of metal oxides with high heat of formation (above 100 kcal/mol) (e. g. SiO2, TiO2, ZrO2, Al2O3, CeO2, Nb2O5, MgO, La2O3) is rather difficult, therefore they are usually applied as catalyst supports. Other transition metals, such as V, Cr, Mo, W, Mn, Re, Fe, Zn, and sp-metals such as Ga, In, Ge and Sn with an intermediate value for the heat of formation...