Platinum particle agglomeration

Platinum particle agglomeration due to migration on the support surface. 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

Honji A, Mori T, Tamura K, Hishinuma Y. 1988. Agglomeration of platinum particles supported on carbon in phosphoric acid. J Electrochem Soc 135 355-359. 

The results from both types of experiments lead to the same conclusion platinum agglomerates on the filter act as an NO oxidation catalyst. The resulting NO2 subsequently oxidises the soot. The observation that platinum treated filters are more effective after a high-temperature treatment is supported by studies rejtorted by Xue etal. [17]. There is a direct relation between the size of platinum particles and the effectiveness as an NO-oxidation catalyst. 

Especially at elevated temperature small particles tend to agglomerate, therefore a spatial separation is important. It was found [31] that the platinum surface area correlates with the BET surface of the carbon carrier material. This is easily understandable as a better dispersion of the noble metal particles leading to higher electrochemical activity. But as soon as the platinum particles reside in very small pores, smaller than 40 nm, they do not contribute to the electrochemical reaction anymore... 

Sintering is a physical process in which a loss of surface area of the catalyst is observed. On supported metal catalysts, the agglomeration of the metal particles leads to a growth in the particle size and consequently to a decrease in surface area. The support can also sinter in this case, the alumina can display a change in the pore structure with a decrease in the surface area. The sintering of the platinum particles is reversible, through the process called redispersion. The sintering of the alumina is irreversible. 

The dissolved platinum species, which are very mobile in the liquid electrolyte of a HT-PEM FC, tend to agglomerate on other metallic platinum particles via an electrochemical Ostwald ripening process [20]. When the fuel cell is cycled under different potentials, e.g., at start-stop conditions or under fast load changes, the... 

Similarly to the catalysts in H -air fuel cells, DMFC catalysts lose their ECS A over time, predominantly owing to catalyst particle agglomeration. Once again, the performance loss is more pronounced at the platinum cathode, which tends to undergo frequent oxidation-reduction cycles, than at the anode (possible protective role of oxophilic ruthenium). 

Two series of Pt/KL catalysts were synthesized by incipient wetness and vapor-phase impregnation (VPI) with a Pt(acac)2 complex [60]. The precalcined KL zeolite was physically mixed with the complex that had been sublimed up to 130 °C. To decompose the platinum precursor, the sample was heated in air at 350 °C. EXAFS and TEM showed that the VPI sample contains smaller particles than the catalyst prepared by incipient wetness, which are in close contact with the zeolite walls. This results in better diffusion of methylcyclopentane (no col-limation effect), better resistance to poisoning and coking, and a lower particle agglomeration rate. 

---