02-09-2013, 03:44 PM
Generation of Pd Model Catalyst Nanoparticles by Spark Discharge
Introduction:-
To produce palladium model catalyst particles, a commercially available aerosol generator, Palas, model GFG 1000, primarily constructed for carbon soot particle production was used. The mechanism behind particle formation is based on spark discharge between two conducting electrodes positioned in the middle of a polymer chamber with their flat ends separated by a distance of 2 mm. A 20 nF capacitor is connected to one of the electrodes and charged by a high-voltage supply with an adjustable output current. The spark discharge frequency can be set between 0 and 300 Hz, and the carrier flow rate can be set between 2 and 8 L/min. In this investigation, particles were generated at spark discharge frequencies of 30, 60, 120, 180, 240, and 300 Hz and flow rates of 3.4, 3.9, 4.4, 4.9, 5.4, and 5.9 L/min. The cylindrical carbon electrodes used in the original setup were replaced by high-purity palladium rods (99.99%) with diameters of 3 mm, mounted to cylindrical stainless steel holders with diameters of 6 mm in order to fit the apparatus.
Experimental Methods:-
The spark generator was connected to an aerosol nanoparticle system setup (Figure 8) in order to enable size distribution measurements, reshaping of the agglomerate particles into compact particles, and controlling the deposition of particles. A β-emitting Ni source was used as a neutralizer in order to achieve a reproducible and known charge distribution on the agglomerate particles before size selection in a differential mobility analyzer (DMA), labeled DMA 1 in Figure 8. The DMA, a standard instrument in aerosol science, classifies charged particles according to their mobility inside an electric field. This mobility is roughly inversely proportional to the particle diameter. Following size selection, the agglomerate particles could be reshaped into more compact particles inside a compaction tube furnace (route 2 in Figure 8).
Alternatively, knowing that a majority of the particles that pass the DMA carry one single charge, each in the size range applied here, particle concentration measurements could be directly performed using an electrometer (route 1 in Figure 8). By stepwise scanning the voltage of DMA 1 and measuring the resulting particle concentration, size distribution measurements of the agglomerate particles were obtained. To size select and measure particle concentrations of the reshaped particles, a second DMA, labeled DMA 2 in Figure 8, was scanned in a similar fashion. The compaction behavior of the particles was examined by scanning the reshaping temperature and measuring the peak value of the size distributions for each temperature.