Reactivity oxide-supported metal particles

Although several studies have been dedicated to analyzing the relationship between nanostructure of supported metal particles and catalytic behaviour, fewer studies have been dedicated to growing controlled oxide nanostructures and their relationship to catalytic reactivity. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

A further appealing consequence of good control of particle size and shape is that the selectivity of the catalyst can improve. The selectivity is the capability to produce a large fraction of just one product over the many products that can be obtained in a chemical reaction. Supported metal particles exhibit atoms with different coordination, hence different reactivity, and in this way give rise to a mixture of products. Thus, the preparation of small well-ordered metal species supported on an oxide substrate is at the heart of the research on supported metal catalysts. 

The latter point brings us to an important question in the field of catalysis by supported metal particles to which extent is the chemical reactivity of a (sub-) nanocluster affected by the interaction with the substrate Very few theoretical studies were dedicated to this problem, and most of them are related to the surface of MgO, an oxide which interacts weakly widi the supported particle, as shown above. Still, the knowledge accumulated in the course of the years on the structure of surface defects and morphology of the MgO surface allows one to analyze some of the mechanisms which can modify the chemical properties of a supported cluster as a function of the site where nucleation has occurred. 

In subsequent catalyst activation steps, a complex set of reactions can take place, and depend on catalyst loading as well as the chemistry. For surface complexes that are not easily reduced, such as for Co + or Co + reacted with alumina or silica, the support-metal particle interface may exist as a Co-aluminate or -silicate layer. Under reducing conditions, this interface is covered with reduced Co. In addition, subsequent catalyst preparation steps carried out on a reactive support such as TiOg can increase the interface with a reduced metal particle by partially covering the reduced metal particle with the oxide or by an increased wetting of the particle surface which will increase the interfacial area[ l. 

In conclusion, the CO oxidation mechanism on Ft, and other d-metals is well understood and serves as a benchmark reaction to characterize reactivity. However, with respect to behavior for supported metal particles and small clusters under ambient conditions, there is still the need for studies in order to fully understand the role of the size, particularly with respect to the electronic structure. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

In recent years, metal nanoparticles and thin films supported on oxides have become fundamental components of many devices as their small dimensions present structures with new chemical and physical properties, often enhancing the reactivity of these surfaces relative to their bulk counterparts. Numerous theoretical and experimental studies show that the metal particle size and shape as well as direct adsorbate interactions with the oxide support can each play a key role in enhancing the reactivity of these surfaces. Further investigations imply that the support material may... 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

At the end of the reactivity measurements the sample is transferred back to the preparation chamber where a carbon film is deposited on the sample. This carbon layer has a double use. First, it protects the clusters against a further evolution (coalescence, restructuring, oxidation during air exposure) and second, in the case of MgO crystals it will serve as a thin support for the metal particles (after floating in acidic solution) for subsequent TEM characterization [12]. 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

From catalysis it is well-known that the metal-substrate interaction influences the reactivity of supported nanoparticles. For instance, for noble metal particles on oxidic supports, the hydrogenation and hydrogenolysis activity is much greater if the support has a higher acidity (high concentration of acidic —OH groups at the surface) than for neutral or alkaline oxidic supports. The influence of the presence of a support on the catalytic activity of metal nanoparticles has been ascribed to [70, 75-79] ... 

---