Platinum crystallite sizes

Bregoli LJ. 1978. The influence of platinum crystallite size on the electrochemical reduction of oxygen in phosphoric acid. Electrochim Acta 23 489-492. 

Fig. 2. Platinum crystallite size distribution for 0.12 ng cm-1 ultrathin platinum film (Fig. 1). Full line, number distribution broken line, surface area distribution. 

Fig. 4. Platinum crystallite size distribution for 2.5% (w/w) platinum/silica catalyst (Fig. 3). Full line, number distribution broken line, surface area distribution. After T. A. Dorling, and R. L. Moss, J. Catal. 7, 378 (1967) and R. L. Moss, Platinum Metals Rev. 11 (4), 1 (1967).

For fuel-cell technology development, it has been important to understand the characteristics and operation of highly dispersed platinum and platinum alloy electrocatalysts. A series of papers on platinum crystallite size determinations in acid environments for oxygen reduction and hydrogen oxidation was published together by Bert, Stonehart, Kinoshita and co-workers.5 The conclusion from these studies was that the specific activity for oxygen reduction on the platinum surface was independent of the size of the platinum crystallite and that there were no crystallite size effects. 

Initially, crystallite growth occurs rapidly either by ion dissolution/reprecipitation (Ostwald ripening) or by surface atom diffusion, due to the requirement for lowering the surface energy of any individual crystallite. This thermodynamic driving force will tend to eliminate the incomplete faces but with the drive to lower the surface energy, the crystallites also will strive towards sphericity. This means, that to all intents and purposes, the ratios ofthe (111) and (100) faces should be approximately the same. Bett et al.1 noted that as the platinum crystallite sizes grew, the size distribution increased. If this is so, then... 

Hydrocarbon activity is inversely related to the average platinum crystallite size with the same amount of Pt/Rh alloy formation. 

Platinum-alumina catalysts have been of profound importance in petroleum naphtha reforming and paraffin isomerization processes. Much information has been established concerning platinum crystallite size and interaction with the acidic support in terms of... 

Bett J, Lundquist J, Washington E, Stonehart P (1973) Platinum crystallite size considerations for electrocatalytic oxygen reduction. Electrochim Acta 18 343-348... 

In the case of platinum catalysts the addition of some metal oxides has also resulted in enhanced catalytic activity, which has been attributed to the modification of platinum crystallite size, and especially to the modification of the oxidation state of platinum. It has been reported that promoters with large electronegativities, such as molybdenum, vanadium, tungsten and niobium, enhance the catalytic activity compared with the unpromoted Pt/Al203, since more electronegative promoters present a higher resistance to oxidation, and platinum remains less oxidised than those with electropositive promoters, such as alkaline and alkaline-earth metals, which are even less reactive than the unpromoted Pt/Al203 catalyst. ... 

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