Platinum particle size

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

Giordano N, Passalacqua E, Pino L, Arico AS, Antonucci V, Vivaldi M, Kinoshita K. 1991. Analysis of platinum particle-size and oxygen reduction in phosphoric-acid. Electrochim Acta 36 1979-1984. 

Other work has been reported in the literature on the influence of platinum particle size (in supported catalysts) on isomerization and dehydro-cyclization reactions. However, the reaction conditions tend to vary widely... 

If mechanism (14) is correct, it is to be expected that the cyclization of this reactant should not be affected by a change in platinum particle size, while the converse should be true if reaction (15) occurs. This has not yet been checked experimentally. 

Average Platinum Particle Sizes of the Catalysts Estimated by XRD and CO Adsorption... 

Fourth, the PtC species is further chlorinated to form Pt4 + species which are strongly bound to the surface. This process leads to a completely new spatial distribution of platinum. After reduction there is an entirely new distribution of platinum particle sizes. One caveat is that metal is lost as volatile species and removed from the reactor. Operating conditions must be selected with care. 

Understanding the Influence of the Pretreatment Procedure on the Platinum Particle Size and Particle Size Distribution for Si02 Impregnated with... 

M.K. Oudenhuijzen, P.J. Kooyman, B. Tappel, J.A. van Bokhoven and D C. Koningsberger, Understanding the Influence of the Pretreatment Procedure on Platinum Particle Size and Particle-size Distribution for SiC>2 Impregnated with Pt2+(NH3)4(NC>3 )2 A Combination of HRTEM, Mass Spectrometry and Quick EXAFS , J. Catal., 205 (2002), 135-146. 

Savadogo O and Essalik A, 1996, Effect of Platinum Particle Size on the Oxygen Reduction Reaction on 2%Pt-l%H2W04 in Phosphoric Acid. Journal of the Electrochemical Society, 143(6), 1814-1821. 

Figure 4 shows that much of the platinum does not chemisorb CO at platinum and tin loadings of 3%, A simple explanation of this effect would be that the non-adsorbing platinum is not exposed to CO, This platinum could be in the large platinum particles or buried within a tin species. However, attributing the decrease In CO chemisorption to an Increase in platinum particle size does not give a satisfying answer to the enhanced coke deposition which parallels the decrease in CO chemisorption. 

For monometallic Pt catalysts, sulfur coverage increases with particle size while the sulfur adsorption capacity is higher for the monometallic Re catalyst. This last result is in accordance with previous work, the adsorption of an electron acceptor like sulfur being enhanced on metals of low electronic affinities [20]. Indeed, electronic affinities are 2.12 eV and 0.16 eV for platinum and rhenium, respectively. With regard to the effect of platinum particle size, the increasing 0s results fi-om the electron deficient character of small metal crystallites deposited on an acidic support [20, 21]. 

The platinum particle sizes found in our calcined oxidation catalysts are much larger than those observed [6] in the reduced microemulsion catalysts moving in the range 0.5 -3.5 nm. Calcination in air of platinum catalysts including microemulsion catalysts apparently evolves a growth of primary platinum particles by a sintering process. 

More precise effect of platinum particle size on the activity of catalysts having... 

Nevertheless, it is necessary to return to the main point of interest here, which is the effect of deactivat ion and the accumulation of carbonaceous species on the Pt surface on the structure sensitivity or insensitivity exhibited (as inferred from the dependence of K on the platinum surface area)- Turnover numbers so calculated after 10 min reaction time decrease rapidly with increasing platinum surface area in a hyperbolic manner (and increase almost linearly with the mean platinum particle size) for reactions followed at both 3L3K and 295K, in contrast to the results obtained by Boudart over a narrower range of Pt surface area, Such behaviour very unusual but has been reported for structure sensitivity in cyclopentadiene hydrogenation on supported copper. However, the turnover numhers were almost independent of platinum surface area and... 

The H2-chemisorption data indicated fliat the average platinum particle size was not affected by the metal uptakeAoading and all three catalysts had a platinum particle size of ca. 1.5 nm in diameter. The catalyst exhibited high activities in o-xylene hydrogenation. Cis and trans 1, 2-dimethylcyclohexane were the only reaction products. The hydrogenation rate was found to increase by increased hydrogen partial pressure. Coirplete hydrogenation of o-xylene could be achieved at 460 K at H2 to o-xylene molar ratio of 12. 

The temperature dependency of the TOF is given in Figure 3. Since all the catalysts had the same platinum particle size (ca. 1.5 nm) the TOF appears to depend on the support morphology. The catalyst in which the fiber support was calcined at 1173 K exhibited the highest activity. This due to the lack of internal dif-fiisional limitations. The higher calcinations temperature results in closure of smaller pores and decreased pore volume (Figure 2), hence, less internal mass-transfer limitation is taking place. 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

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