Model particle size effects

Cherstiouk OV, Simonov PA, Savinova ER. 2003a. Model approach to evaluate particle size effects in electrocatalysis Preparation and properties of Pt nanoparticles supported on GC and HOPG. Electrochim Acta 48 3851-3860. 

Although there appears to be good evidence that redispersion can occur, Smith et 0/.251,252 observe no redispersion either for model or industrial Pt catalysts heated in 3% 02/N2 at 673-873 K. Instead they suggest that re-dispersion involves the recovery of Pt sites lost to the chemisorption of H2 rather than a particle size effect. Why re-dispersion does not occur in this case is not clear, but may be related to the use of a low partial pressure of 02 (see earlier), so that a Pt-Al2 03 complex does not form. 

This paper,11 which is a model of its kind, reported a study of the reaction on two Au/SiC>2 catalysts, having respectively 0.15 and 5% gold unfortunately both had somewhat broad particle size distributions, namely 3 9 nm (0.15% Au) or 3-7 nm (5% Au), with a significant number of very large (>10nm) particles. This complicated the interpretation of the results, as no clear particle size effect could be seen. However, silicalite-1 (Si-MFI) and TS-1 (a titanium-containing silicalite, Ti-MFI) were also used, and the size of the channels constrained the particle size to be less than 3nm in both cases. These size differences accounted for the marked variations in activity observed at 433 K ... 

R. Ceschino, I. Fenoglio, M. Tomatis, D. Ghigo, B. Fubini, and G. Martra, Ultrafine versus fine iron oxide particles size effects on the surface reactivity towards model biological systems, Chem. Res. Toxicol, submitted for publication. 

Frake and co-workers " extensively evaluated numerous chemometric techniques for the NIRS prediction of mass median particle size determination of lactose monohydrate. Models evaluated in zero order (untreated) and second derivative were MLR, PLS (partial least squares), and ANN (artificial neural network). The researchers concluded that there is more than one way to treat data and achieve a good calibration model. The group also confirms previous observations that derivitization of data does not remove particle size effects (previously thought to contribute to baseline shift). 

Much of the work to date on particle size effects on phase transformation kinetics has involved materials of technological interest (e.g., CdS and related materials, see Jacobs and Alivisatos, this volume) or other model compounds with characteristics that make them amenable to experimental studies. Jacobs and Alivisatos (this volume) tackle the question of pressure driven phase transformations where crystal size is largely invariant. In some ways, analysis of the kinetics of temperature-motivated phase transformations in nanoscale materials is more complex because crystal growth occurs simultaneously with polymorphic reactions. However, temperature is an important geological reality and is also a relevant parameter in design of materials for higher temperature applications. Thus, we consider the complicated problem of temperature-driven reaction kinetics in nanomaterials. 

In his interesting article on particle size effects, Bond (27) gives the results of model calculations based on spheres and on cubes with five faces exposed, for Ni, Pt, and Pd. For these simple systems, the fraction of total metal atoms exposed, FE, is related to the diameter or length of a side d (in the figures, d is written with a bar above it to highlight the fact that it is an average value) by the approximate formula... 

VI. Possible Explanations of Particle Size Effects Experiments versus Models... 

Somorjai and his colleagues have developed a model for the states of carbon on a platinum surface containing steps and kinks, in which much of the surface was obscured by a carbonaceous overlayer with islands of 3D carbon , leaving only a few single atoms or pairs at steps uncovered. It was felt that the higher activity of sites at steps would cause hydrogen if present to break C—M bonds. If this is so, then very small metal particles that expose only atoms of low coordination number should be more resistant to carbon deposition than larger particles, powders or macroscopic forms. Quantitative evidence on a particle-size effect is... 

Particle-size effects may also be addressed in terms of the model outlined in Schemes 13.2-5. In the case of the reference platinum catalysts, a decrease in size was accompanied by a marked decrease in S2 and in 5i, and in F this implies stronger hydrogen chemisorption on the Pt/Al203 reforming catalyst. On rhodium and iridium, however, the effect of size on S2 and F (Table 13.9) is more probably due to a change in the surface s ability to accommodate multiple C=M bonds. 

Markovic et al. [17] review the data on platinum particles and suggest that the data in dilute sulfuric acid is consistent with Kinoshita s model and further suggest that essentially all of the reactivity can be attributed to the (100) surface. They go on to suggest that this difference in reactivity between the crystal faces is due to structure sensitivity of anion adsorption that impedes the reaction. They point out that in PEM systems, where anion adsorption by the sulfonic acid groups is unlikely, there might be considerably less of a particle-size effect. Still, most PEM catalyst layers employ platinum particles on the order of 3nm, roughly the same size as the maximum in mass activity identified by Kinoshita. 

The intrinsic exchange current density,/ , is not a mere materials constant, but it depends on size distributions of catalyst nanoparticles, their surface structure, as well as surface composition in the case of alloy catalysts like PtRu. In this section, we discuss modeling approaches that highlight particle size effects and the role of surface heterogeneity in fuel cell electrocatalysis. 

Until recently, it was most common to use SMLR to develop a calibration model. A condition for the use of the technique is that the relationship between absorbance expressed as logio 1/R or logio 1/T and the laboratory measurement should be very close to linear. The specular reflectance and particle size effects seen in spectra discussed below also affect the optimal reduction of the SEC and SEP. 

---