Nanoparticles surfaces catalytic studies

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

CD-modified nanoparticles sites. These studies afford an interesting example of tunable catalyst design at the molecular level. Manipulation of the surface of cat-alytically active metal nanoparticles seems possible, and can be used to modulate the catalytic activity on demand. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

Catalysis and Electrocatalysis at Nanoparticle Surfaces reflects many of the new developments of catalysis, surface science, and electrochemistry. The first three chapters indicate the sophistication of the theory in simulating catalytic processes that occur at the solid-liquid and solid-gas interface in the presence of external potential. The first chapter, by Koper and colleagues, discusses the theory of modeling of catalytic and electrocatalytic reactions. This is followed by studies of simulations of reaction kinetics on nanometer-sized supported catalytic particles by Zhdanov and Kasemo. The final theoretical chapter, by Pacchioni and Illas, deals with the electronic structure and chemisorption properties of supported metal clusters. 

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]. 

Other aspects of the high surface area in nanoparticles appear in catalytic studies. In fact, it is well known that, in heterogeneous catalysis, the rate of reaction is assumed to be proportional to the surface coverage [33]. Therefore, the greater... 

The (i-band center model has been used extensively to describe experimentally measured catalytic activities, as a descriptor of catalyst behavior. Most computations have been performed on flat surfaces or surfaces with steps and kinks [7, 24,46 9]. The electronic stmcture of nanoparticles is expected to be deeply affected by the characteristic particle size and morphology. Particle size is therefore a critical parameter. The surface science studies that involve the reactions on a uniform single crystal surfaces and introduce the complexity characteristic to real nanoparticles by involving the defects, kinks, and steps in the models may not be sufficient to model the catalytic behavior at nanoscale. Such model does not take into account an inherent particle property sensitively dependent on structural parameters such as the particle size, strain, and local surface morphology. 

In nanoparticle electrocatalysis, the area that Michael entered just some time ago in Munich, he and his coworkers rationalized the sensitivity of electrocatalytic processes to the stmcture of nanoparticles and interfaces. Studies of catalytic effects of metal oxide support materials revealed intriguing electronic structure effects on thin films of Pt, metal oxides, and graphene. In the realm of nanoparticle dissolution and degradation modeling, Michael s group has developed a comprehensive theory of Pt mass balance in catalyst layers. This theory relates surface tension, surface oxidation state, and dissolution kinetics of Pt. 

Somorjai GA, Aliaga C (2010) Molecular studies of model surfaces of metals from single crystals to nanoparticles under catalytic reaction conditions. Evolution from prenatal and postmortem studies of catalysts. Langmuir 26 16190... 

In conclusion, these observations show that the surface oxide formed on Pt crystal surfaces and on Pt nanoparticles is catalytically active for the CO oxidation reaction. Whether the oxidized Pt surface is catalytically active, however, is still up for some debate. Therefore, studies to bridge the connection between surface structures and catalytic performance are now needed to clearly understand how surface oxide layers affect the catalytic activity of CO oxidation. 

Among metallic particles used in plasmonics, silver nanoparticles are widely studied due to the particular optical, spectroscopic and catalytic properties of silver [4-8]. They have been largely used in catalysis [9,10], biological labeling [11,12], photonics [13-15] and surface-enhanced spectroscopies [16,17]. Moreover a rich literature is now available for the synthesis of Ag nanoparticles [18, 19] (see also Chapter 10). 

Study the influence of MNP morphology on their surface properties and catalytic performances. In parallel to these methods for the preparation of MNPs, heterogeneous catalysis has developed powerful tools to model and characterize the surface of the nanoparticles. Interestingly, these studies can be achieved during the catalytic process (Transmission Electron Microscopy (TEM), powder X-Ray Diffraction (DRX), X-ray Photoelectron Spectrometry (XPS), Extended X-Ray Absorption Fine Structure (EXAFS), IR in operando) [26-34]. However, simple spectroscopic methods, such as UVA is, IR, or NMR both in solution and in solid state, which are adapted from molecular chemistry and homogeneous catalysis, offer interesting alternatives to precisely characterize the metallic surface of MNPs. 

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