Particle size effects catalysis

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

It is almost impossible to distinguish clearly between those physical or catalytic effects that are intrinsically dependent on particle size from those which are conditioned by contact with the support, because at least in the context of catalysis small particles are necessarily employed, and their utility depends on their being supported. Furthermore, the smaller the particle, the greater will be the fraction of atoms directly in contact with the support and therefore influenced by it, while at the same time the fraction of coor-dinatively unsaturated surface atoms also increases, and this changes the physical properties of the whole particle. It is therefore virtually impossible to draw a clear distinction between intrinsic particle size effects and those that are due to metal-support interactions. In Section 3.4.2 we noted some effects of size on structure in systems where the influence of the support was likely to be minimal now we must examine effects of size in conjunction with the metal-support interaction. 

Thus, complex high-area catalysts are typically not the best for fundamental investigations at the atomic or molecular level. Although many broadly important characteristics of heterogeneous catalysis, such as metal particle size effects, support effects, metal—support interaction, and the influence of the promoters and poisons... 

The role of particle size in catalysis and electrocatalysis is a subject of longstanding interest. It is not our intention here to discuss in detail the available experimental and theoretical literature. Extensive reviews on particle-size effects in gas-phase catalysis and electrocatalysis can be found in the papers of Henry [25] and Kinoshita [26], respectively. Also, several monographs, reviews, and conference proceedings discuss particle-size effects from experimental, theoretical, and computational points of view [9,27,28]. 

The purpose of the last sections was to create an overview of the changes which occur in the physical properties of metal particles as their sizes alter, and the ways in which these may be explained, in order to provide a basis for understanding particle size effects in chemisorption and catalysis. The complications that may arise from the having a binodal distribution of particles size have however always to be kept in mind. The main conclusions have been summarised by Burch in the following way. 

Thomas JR (1997) Particle size effect in microwave-enhanced catalysis. Catal Lett 49 137... 

Particle size is an important parameter that strOTigly influences the activity of catalyst nanoparticles. There are extensive studies on the particle size effect of Pt particles on the ORR catalysis, showing that the area-specific activity increased as the particle size increases due to a lower portion of low-coordinated Pt atoms on the edges and comers [13, 74—77]. Taking the surface area into account, this leads to a maximum mass activity at particle size of around 3-5 imi. Similar trends were also reported in PtsM alloy catalysts [13, 78, 79]. Eor dealloyed nanoparticles, situation becomes more complex as the morphologies and core-shell stractures were found to be dependent on the particle size, being an additional contribution to the overall size effect. 

Hayden BE, Suchsland J-P (2009) Support and particle size effects in electrocatalysis. In Koper MTM (ed) Fuel cell catalysis a surface science approach. Wiley, Hoboken, NJ, pp 567-592... 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

Hayden, B. E. and J. Suchsland. 2009. Support and Particle Size Effects in Electrocatalysis, in Fuel Cell Catalysis—A Surface Science Approach, editor, M. T. M. Koper, 567-592. Hoboken, New Jersey John Wiley Sons. 

Hayden, B.E. and Suchsland, J.-P. (2009) Support and particle size effects in dec-trocatalysis, in Fuel Cell Catalysis (ed. M.T. Koper), John Wiley Sons, Inc., Hoboken, NJ, 567-592. 

Weaver and co-workers explained the particle size effect in terms of an ensemble effect related to surface morphology [230]. For Pt nanoparticles of diameter < 4 mn it was experimentally determined [231] that the fraction of flat terrace sites diminished considerably as compared to edge sites. Therefore, the probability of ensemble of active site formation situated on the terraces and needed for methanol dehydrogenation (especially for the removal of the first three hydrogen atoms) is lower for particles with diameters below 4 nm. It was estimated that the 2.5 nm diameter particle has an approximately five times lower availability of adjacent Pt atoms compared to the 8.8 mn diameter particle [230]. In the case of formic acid oxidation, on the other hand, it was proposed that Pt ensembles are not required for catalysis. Interestingly, the COad surface coverage decreased with particle size for both CFI3OFI and FICOOFI. 

Emerson, S. C. Synthesis of Nanometer-size Inorganic Materials for the Examination of Particle Size Effects on Heterogeneous Catalysis, Hi.D. thesis, WorcestCT Polytechnic Institute, 2000. May be accessed online at http //www.wpLedu/Pubs/ETD/Available/etd-0503100-105634/uniestricted/ emerson.pdf... 

Bond GC (1985) The origins of particle size effects in heterogeneous catalysis. Surf Sci 156(pt2) 966... 

The subject of particle size effects in heterogeneous catalysis, in particular the dependence or otherwise of catalytic activity and selectivity on metal particle size in the range 1-10 nm, is of interest to both industrial and academic circles. General trends that have emerged suggest that transformations such as those involving the reactions of unsaturated hydrocarbons, e.g., catalytic hydrogenation and isomerisation, are independent of metal particle size, i.e., are... 

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