Nanoparticles surface reactivity studies

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

In our relativistic density-functional study of mixed Pt-M nanoparticle surfaces is represented by a two-layer cluster with seven surface and three second-layer atoms, Ptio-nMn(7,3) [6]. The subnano cluster model does not simulate bulk surface properties because of its limited size and undercoordinated metal atoms. However, the model is suitable for simulating the properties of nanoscale particle catalysts, e.g., Pt-Ru alloy nanoparticles wife an fee surface. Catalytically much more active than bulk metal surfaces, these nanocrystals exhibit a transition from metallic to insulator properties [48]. The cluster model is also suitable for rough Pt-M electrode surfaces that exhibit a high surface density of reactive Pt-M sites [49]. 

The stable nanoparticles were used to study the surface reactivity of nanoobjects. To probe the surface state of these RuNPs, the adsorption of carbon monoxide (CO) was studied by IR. This shows that CO can coordinate at the surface of the particles in two different coordination modes linear at 2,030 cm and bridging at 1,968 cm. ... 

Characterization by WAXS showed crystalline nanoparticles displaying the expected hexagonal close packed (hep) structure of bulk ruthenium. Reactivity studies were carried out in particular with CO to show the availability of the nanoparticles surface for reactivity. Recently, the synthesis of these PVP-stabilized RuNPs has been reproduced to perform an exhaustive study of the coordination of CO to their surface by a combination of IR and NMR techniques [62]. It has been observed that (1) the coordination mode of the CO to the NP surface depends on the reaction time and (2) CO is mobile. Low reaction-times give rise to CO adsorption in the bridging mode while longer reaction times result in the adsorption of more CO molecules only adsorbed in the linear or multicarbonyle modes. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

Before studying the reactivity of the nanoparticles, it is necessary to evaluate whether the synthetic method employed would lead to particles of clean unoxidized surface, able to react with incoming molecules. For this purpose we used, besides physical techniques (which are sometimes difficult to handle due to the high oxidability of particles prepared in this way), molecular methods, namely IR and NMR spectroscopy, as well as magnetic measurements which can give a precise description of the surface properties of the particles. 

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

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. 

Previous studies in conventional reactor setups at Philip Morris USA have demonstrated the significant effectiveness of nanoparticle iron oxide on the oxidation of carbon monoxide when compared to the conventional, micron-sized iron oxide, " as well as its effect on the combustion and pyrolysis of biomass and biomass model compounds.These effects are derived from a higher reactivity of nanoparticles that are attributed to a higher BET surface area as well as the coordination of unsaturated sites on the surfaces. The chemical and electronic properties of nanoparticle iron oxide could also contribute to its higher reactivity. In this work, we present the possibility of using nanoparticle iron oxide as a catalyst for the decomposition of phenolic compounds. 

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