Particle size dependence

The effect of decreased coordination of surface metal atoms compared to the bulk leads to small decreases in the bond distances with atoms in the subsurface layer. Because of bond order conservation, discussed in Section 10.3.5, this increased bonding of metal atoms in the subsurface layer to those in the surface layer will weaken bonds with metal atoms in the next subsurface layers. The result is a decaying alternating strengthening and weakening of the interaction energies between atoms in layers perpendicular to the surface layer. 

In Section 10.5, we analyze the consequences of lateral effects between adsorbed molecular species in the chemisorbed overlayer. The presence of steps and kinks in nanoparticles alters the steric constraints of packing of molecules in high-coverage overlayers, which will be seen to result in large increases in adsorption energies at high coverage (46, 47]. 

In Section 10.6.1, we study the consequences of the presence of steps and kinks in the overall reactivity of nanoparticles by altered geometric options for transition state formation. 

To illustrate the changes in adsorption energies on surface atoms that do not have altered surface topologies but become part of a different nanoparticle here, we use results of DFT calculations of low-coverage adsorption on Pd and Au particles. 

The increased delocalization of the Pd atom valence electrons with decreasing Pd-Pd atom bond distance causes the CO adsorption energy to decrease with decreasing particle size. The decreases in adsorption energies found are less than 10% compared to the adsorption energies on the same site for the fully extended surface. 

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During Stage II the growing particles maintain a nearly constant monomer concentration. The concentration of monomer is particle-size dependent, with smaller particles having lower concentrations (28). 

In the United States, a number of physical tests are performed on siUcon carbide using standard AGA-approved methods, including particle size (sieve) analysis, bulk density, capillarity (wettabiUty), friabiUty, and sedimentation. Specifications for particle size depend on the use for example, coated abrasive requirements (134) are different from the requirements for general industrial abrasives. In Europe and Japan, requirements are again set by ISO and JSA, respectively. Standards for industrial grain are approximately the same as in the United States, but sizing standards are different for both coated abrasives and powders. 

In addition to the surface area of the particles, several other parameters have some influence on the necessary resin consumption, e.g. the type of the boards, thickness of the sanding zone, type and capacity of the blenders, separation and spraying of the glue resin (only wiping effect or spraying of the resin by air or liquid pressure), shape of the panicles for the same particle sizes, dependence of... 

Considerable work has been done on the behavior of pollutant species at air-water and air-soil interfaces. For example, wet and diy deposition measurements of various gaseous and particulate species have been made over a wide range of atmospheric and land-cover conditions. Still, the problem is of such complexity that species-dependent and particle-size-dependent rates of transfer from the atmosphere to water and soil surfaces are not completely understood. There is much to be learned about pollutant transfer at water-soil interfaces. Concern about groundwater contamination by mineral... 

The different BEP behavior for the activation of a versus tt bonds, basic to the very different Class I and Class 11 particle size dependence compared to Class 111 particle size dependence, is summarized in Figure 1.16 [14]. 

The particle size depends on the chain length of the aluminium-alkyl group and the concentration of the tri-alkylaluminium compound applied. If mixtures of, e.g., Fe- and Co-carbonyl compounds are used, Fe/Co alloy particles are formed. When the magnetic particles... 

Prior to inclusion of PVP-protected Pt nanoparticles the SBA-15 silica is calcined at 823K for 12h to remove residual templating polymer. Removal of PVP is required for catalyst activation. Due to the decomposition profile of PVP (Figure 6), temperatures > 623 K were chosen for ex situ calcination of Pt/SBA-15 catalysts. Ex-situ refers to calcination of 300-500 mg of catalyst in a tube furnace in pure oxygen for 12-24 h at temperatures ranging from 623 to 723 K (particle size dependent) [13]. Catalysts were activated in He for 1 h and reduced at 673 K in H2 for 1 h. After removal, the particle size was determined by chemisorption. Table 2 is a summary of chemisorption data for Cl catalysts as well as nanoparticle encapsulation (NE) catalysts (see description of these samples in proceeding section). 

After supporting these sols on activated carbon, however, the obtained particle size depends on the capability of the protective agent to maintain the particle dimension. The obtained three catalysts, having different characteristics, are summarized in Table 3. As it is shown, mean size of gold nanoparticle obtained by TEM measurement did not always match with X-ray powder diffraction (XRPD) data. This result is not surprising as TEM measurements represent particle sizes, whereas from X-ray diffraction (XRD) it is possible to obtain crystallite dimensions that do not necessarily coincide with the size of... 

In the early work on the thermolysis of metal complexes for the synthesis of metal nanoparticles, the precursor carbonyl complex of transition metals, e.g., Co2(CO)8, in organic solvent functions as a metal source of nanoparticles and thermally decomposes in the presence of various polymers to afford polymer-protected metal nanoparticles under relatively mild conditions [1-3]. Particle sizes depend on the kind of polymers, ranging from 5 to >100 nm. The particle size distribution sometimes became wide. Other cobalt, iron [4], nickel [5], rhodium, iridium, rutheniuim, osmium, palladium, and platinum nanoparticles stabilized by polymers have been prepared by similar thermolysis procedures. Besides carbonyl complexes, palladium acetate, palladium acetylacetonate, and platinum acetylac-etonate were also used as a precursor complex in organic solvents like methyl-wo-butylketone [6-9]. These results proposed facile preparative method of metal nanoparticles. However, it may be considered that the size-regulated preparation of metal nanoparticles by thermolysis procedure should be conducted under the limited condition. 

Park S, Xie Y, Weaver MJ. 2002. Electrocatal3dic pathways on carbon-supported platinum nanoparticles Comparison of particle-size-dependent rates of metbanol, formic acid, and formaldehyde electrooxidation. Langmuir 18 5792-5798. 

COads + OHads recombination occurs in an electrochemical step and follows a Tafel law with a 0.5, and is particle-size-dependent when the size decreases to 1.8 nm, feox falls by about an order of magnitude. 

An important result of this study is the conclusion of a particle-size-dependent COads surface mobility. The value obtained for large Ft particles is significantly smaller than Deo at a solid/gas interface. However, Kobayashi and co-workers, using solid state NMR, performed measurements of the tracer diffusion coefficient Deo at the solid/electrolyte interface and for Ft-black particles (about 5nm grain... 

Schalow T, Brandt B, Starr DE, Laurin M, Shaikhutdinov SK, Schauermann S, Libuda J, Freund HJ. 2007. Particle size dependent adsorption and reaction kinetics on reduced and partially oxidized Pd nanoparticles. Phys Chem Chem Phys 9 1347-1361. 

Recent studies of Pt supported on titania synthesized using the high throughput method on arrays have also revealed a particle size dependence of the ORR... 

Figure 16.11 Titania (TiO c)-suppoited (a) and niobium-doped titania (Nbo.o8Tio.920j /Au)-suppoited (b) Au particle-size-dependent specific activities for—CO oxidation at a potential step of 0.5 V vs. RHE. The current densities have been corrected for the TEM-deteimined...

CO electro-oxidation exhibits a strong particle size dependence on both carbon-and titania-supported Au catalysts a strong deactivation of the reaction is observed for particle sizes below about 3 nm. In the case of the titania supports, however, a distinct activation of the reaction is also evident. This manifests itself in a strong decrease in the overpotential for the reaction, and an increase in activity as the particle size decreases in the range 8-3 nm. The result is a maximum in the catalytic activity with particle size. 

Walter HJ, Rutgers van der Loeff MM, Hoeltzen H (1997) Etrhanced scavenging of Pa relative to h in the South Atlantic south of the Polar front Implicatiotts for the use of the Pa/ h ratio as a paleoproductivity proxy. Earth Planet Sci Lett 149 85-100 Wheatcroft RA (1992) Experimental tests for particle size-dependant biotirrbidation in the deep oceans. Linmol Oceanogr 37(1) 90-104... 

L.A. Lyon, D.J. Pena, and M.J. Natan, Surface plasmon resonance of Au colloid-modified Au films particle size dependence. J. Phys. Chem. B 103, 5826-5831 (1999). 

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