Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 17

A Review of Recent Literature

By J.J. Spivey, G.W. Roberts

The Royal Society of Chemistry

Copyright © 2004 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-229-6

Contents

Chapter 1 Role of Metal Ion-Metal Nanocluster Ensemble Sites in Activity and Selectivity Control by J. Margitfalvi and S. Gobölös, 1,
Chapter 2 The Destruction of Volatile Compounds by Heterogeneous Catalytic Oxidation By C.S. Heneghan, G.J. Hutchings and S.H. Taylor, 105,
Chapter 3 CO Oxidation Over Supported Au Catalysts By M.C. Kung, C.K. Costello and H.H. Kung, 152,
Chapter 4 Coke Characterization By C.A. Querini, 166,
Chapter 5 Deactivation of Oxidation Catalysts for VOC Abatement by Si and P Compounds By M. Rahmani, K. Badii, M. Faghihi, M. Sanati, N. Cruise, O. Augustsson and J.J. Spivey, 210,
Chapter 6 Microemulsion: An Alternative Route to Preparing Supported Catalysts By S. Rojas, S. Eriksson and M. Boutonnet, 258,
Chapter 7 Catalysis of Acid/Aldehyde/Alcohol Condensations to Ketones By K.M. Dooley, 293,
Chapter 8 Turnover Frequencies in Metal Catalysis: Meanings, Functionalities and Relationships By J.G. Goodwin Jr, S. Kim and W.D. Rhodes, 320,

CHAPTER 1

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 of their oxide (ca. 60-90 kcal/mol) are frequently used as promoters in supported metal catalysts. The metals with an intermediate heat of formation of oxide are usually present in the heterogeneous metal catalysts in the form of isolated ions or nano-sized oxide crystallites even under reducing or reaction conditions. These metal ions behave as Lewis acid sites. These sites can be involved in the polarization of multiple bonds or electron donor groups of substrate molecules, and they can activate carbonyl compounds, nitriles, nitro-compounds, and CO molecule chemisorbed on the surface of heterogeneous catalysts.

The Lewis acid strength of metal ions is said to be proportional to the generalized electronegativity of metals in their oxide form (Xi = (1 + 2Z)X0, where Xi and X0 are electronegativity of metal in the oxide and elemental form, respectively, and Z = valence state of metal in the oxide). The Lewis acid strength, expressed as the electronegativity of metals in their highest oxides, is significantly higher for Cr(20.8), Mo(16.9), W(18.2), Mn(11.2), Re(22.5), Ge(18.0) and Sn(15.3), than for other metals. Therefore, these metals can be used as promoters in different reduction or oxidation catalysts requiring activation of the substrate molecule by Lewis acid sites.

It is also known that large number of catalytic reactions, such as catalytic naphtha reforming, hydrogenation of unsaturated carbonyl compounds, oxidation of CO or methanol, require both metallic sites and Lewis acid sites for activating hydrogen or oxygen and substrate molecules, respectively. Metal nanoclusters of Group VIII or Group IB catalysts supported and stabilized on irreducible oxides and promoted by a metal ion can fulfill this requirement. In these catalysts metal ions or metal oxide species of the promoter interact with metal nanoclusters at the cluster-support interface, or can be stabilized on the top of the metal nanocluster. Due to the intimate contact between the metal ion of the promoter and the metal nanocluster bimetallic ensemble sites are formed. These types of sites can also be formed by high temperature reduction of metal catalysts supported on a slightly reducible oxide support, such as TiO2.

Based on a careful literature survey and recent results published by the authors of this review, it can be assumed that in a number of heterogeneous catalytic reactions ‘metal ion — metal nanocluster’ ensemble sites are operative. In these catalysts the metal ions have to be located in atomic closeness to the metal nanocluster.

As far as the action of supported bimetallic catalysts is concerned, the main theories suggest either geometric and/or electronic effects to account for the improved catalytic properties. For instance, in platinum based naphtha reforming catalysts, the electronic modification of platinum particles may be induced by an interaction with an oxide layer of the promoter or by alloy formation. The electronic modification results in a change in the Pt-C bond strength of adsorption of hydrocarbons and hence alters the activity and selectivity of the reforming type catalysts.

Geometric or ensemble effects arise due to the dilution of the surface of the given active metal by an inactive one. For instance, this is the case for AxBy type binary alloy catalysts containing Pt, Pd or Ni as active metals and Au, Cu, Sn, etc. as diluting elements. The ensemble effect can induce different structure sensitivities of the reactions. For instance, the dilution of active metal (Pt) surface into smaller ensembles by addition of inactive species, such as Sn or Ge, selectively poisons demanding reactions (e.g. hydrogenolysis and coke formation) that requires relatively large clusters or ensembles of adjacent metal atoms. While structure insensitive reactions (double bond hydrogenation, aromatization or isomerization) can occur on single isolated atoms.

In bimetallic reforming-type catalysts the presence of separate oxidized promoter species, e.g. Sn(II) and Ge(IV), results in a change of the acidity, affecting both the activity and selectivity of the catalyst.

Bond and co-workers have classified bimetallic or modified supported catalysts as follows:

i. Formation of bimetallic and/or alloy type particles from a pair of elements showing substantial or complete miscibility (for example, alloy type sup- ported Sn-Pt, Sn-Pd catalysts, see section 2.1.2);

ii. Formation of bimetallic clusters from pairs of elements showing limited solubility; (for example, supported Sn-Ru catalysts, see section 2.3);

iii. Incorporation of a third component that cannot be reduced to the zero-valence state but coming into contact with the metal particle Pt-MoO3/SiO2, Ru-TiO2/SiO2). It was suggested that in these catalysts the character of interactions is similar to catalysts with Strong Metal Support Interaction (SMSI);

iv. Addition of a third component that mainly interacts with the support. In this way either the character of metal-support interaction is altered or the electron density of supported metal nanocluster is changed (for example, alumina supported Sn-Pt catalysts prepared by conventional methods, see section 2.1.1);

v. Addition of other species (for example species electronegative in character, which act as selective or non-selective temporary poisons).

The above classification suggests that under properly chosen condition the subject of this chapter, i.e. ‘metal ion-metal nanocluster’ ensemble sites (MIMNES) can be formed in most of the above types of catalysts. For instance, from bimetallic clusters of type (i) and (ii) MIMNES can be formed under conditions of mild oxidation. In catalysts type (iii) MIMNES should exist both under oxidative and reductive environment. In catalysts type (iv) any metal- support interaction with the involvement of non-reducible oxide can also be considered as MIMNES. The only requirement for the formation of MIMNES is the atomic closeness of the two types of sites.

1.4 Promotion of Supported Metal Nanoclusters. – In the last decade a growing body of data provided evidence for the presence of specific active sites on the periphery of metal particles composed of metal site and specific sites on the support surface (adlineation sites). For example the enhanced activity of transition metal catalysts supported on/or promoted with reducible oxides TiO2, WO3, Nb2O5, etc. in carbonyl bond hydrogenation was attributed to the formation of specific sites. Boffa, Bell and Somorjai have proposed that the hydrogenation of carbonyl bonds on the surface of rhodium promoted by oxide species such as TiOx, ZrOx, TaOx, WOx, etc. proceeds via the activation of C=O bond through simultaneous adsorption of the carbon end to the metal site and oxygen end to the Lewis acid site on the oxide.

A similar model was proposed by Vannice et al. to explain the extremely high activity of 0.95%Pt/TiO2 reduced at 500°C in acetophenone hydrogenation, and the enhanced selectivity toward crotyl alcohol in crotonaldehyde hydrogenation. The model also implies the creation of special sites at the metal-support interface that can coordinate the oxygen end of the C=O bond and thereby specifically activate the carbonyl bond.

The enhanced selectivity of Ru/ZrO2 toward cinnamyl alcohol in cinnamal-dehyde hydrogenation was also ascribed to the formation of Ru-Zrn+ sites at the periphery of the nanoparticles. The presence of mixed Ru-Zrn+ sites appeared to decrease the strength of the C=O bond, thus facilitating the hydrogenation.

Similar interfacial active sites created in Pt/MoO3 and Pt/WO3 upon high temperature reduction were suggested to favor the isomerization of allyl alcohol to propanal at the expense of hydrogenation to propanol.

Bell and Somorjai proposed the concept of the interfacial active site involving the coupling of a metal center and a Lewis acid/base site to form adjacent centers. The latter sites are formed either in the oxide support or the added promoter. It was suggested that these active sites might be crucial in the conversion of the molecules with polar functional groups (such as CN, CS and NH). Close analysis of data presented in the above references shows that in all cases the character of interactions strongly resembles the presence of ‘metal ion-metal nanocluster’ ensemble sites.

1.5 Characterization of Supported Metal Catalysts. – Chemisorption of different probe molecules and Temperature Programmed Reduction (TPR) studies are frequently used to study the metal dispersion, surface composition and oxidation state of metals in mono- and bimetallic supported catalysts. Combined use of CO, hydrogen and oxygen chemisorption as well as oxygen-hydrogen titration can provide information about the dispersion and surface composition of metal nanoclusters. TPR studies of bimetallic catalysts can give information about the type, the reducibility, and the oxidation state of metal components. In addition, the position of TPR peaks can be used to characterize the type of interactions of the metal species in the catalysts.

Traditionally, IR spectroscopy of adsorbed CO serves as a tool to gain knowledge about the electronic state and dispersion of supported metals. The spectra of adsorbed CO are known to be the result of the interplay of the interaction between metal d-orbitals and σ-bonding and π-antibonding orbitals of adsorbed CO. X-ray photoelectron spectroscopy (XPS), X-ray absorption near fine structure (EXAFS) and X-ray absorption near edge structure (XANES) are also used to determine both the electronic state and the environment of metal species in supported catalysts.

There are indications in the literature suggesting the formation of electron deficient metal particles in e.g. Al2O3-based and halogenated solid catalysts. However, the mechanism of this process and the nature of anchoring sites are not quite clear. Broensted acid sites, as well as strong Lewis acid sites may be considered as surface centers stabilizing small metal particles (Pt, Pd, Ir, Ni) and causing their positive charging.

(Continues…)Excerpted from Catalysis Volume 17 by J.J. Spivey, G.W. Roberts. Copyright © 2004 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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