Amorphous atomic size ratio

The metal size clearly increases when the decomposition takes place on the substrate. Nevertheless, the overall shift after complete decomposition is the same both on crystalline and amorphous substrates. This can be explained by the assumption that the increase of the number of the metal atoms in the cluster takes place also on an amorphous substrate, on a scale high enough to shift the core levels but low enough to maintain a constant emitted intensity ratio between the substrate and the metal core levels. The authors concluded therefore that the core-level position is highly size-sensitive in the range of very small particles, e.g. < 100 atoms where the associated electronic properties are primarily atomic. However, on approaching the metallic state for >100 atoms, the corelevel shift is a much poorer criterion of the cluster size. 

Irrespectively of the iron content, the applied synthesis procedure yielded highly crystalline microporous products i.e. the Fe-ZSM-22 zeolite. No contamination with other microporous phases or unreacted amorphous material was detected. The SEM analysis revealed that size and morphology of the crystals depended on the Si/Fe ratio. The ZSM-22 samples poor in Fe (Si/Fe=150) consisted of rice-like isolated crystals up to 5 p. On the other hand the preparation with a high iron content (Fe=27, 36) consisted of agglomerates of very small (<0.5 p) poorly defined crystals. The incorporation of Fe3+ into the framework positions was confirmed by XRD - an increase of the unit cell parameters with the increase in the number of the Fe atoms introduced into the framework was observed, and by IR - the Si-OH-Fe band at 3620 cm 1 appeared in the spectra of activated Fe-TON samples. 

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (EMT and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. However, the main reactions of meta-diisopropylbenzene are isomerization and alkylation. It is proposed that this alkylation can be used as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the weight ratio of 1,3,5- to 1,2,4-triisopropylbenzene formed. In most cases, this ratio increases with die increasing effective void size of the molecular sieves in the order SAPO-5 < mordenite < offretite < beta < EMT FAU < L < SAPO-37 < amorphous silica-alumina. 

Stoupin et al. assume a core stracture surrounded by a more amorphous shell rich in Ru, and therefore determine a parameter Sru = (n - (1-D)/xru)/D, where D is the dispersion (fraction of atoms at the surface) determined assuming spherical particles and based on the estimated particle size from the EXAFS (or XRD) results, and n is the nominal molar ratio (R Ru, perhaps determined from XRF). Here Sr represents the relative composition of the snrface (i.e., it is the fraction of Ru atoms relative to the total on the surface), and xr is an estimate of the relative composition of the core using in situ EXAFS results. Their final results suggest that the surface of a PtRu ETEK cluster is rich in Ru, and is heavily oxidized, similar to Hwang s and our conclusions. 

During the process, the aqueous silicate solution is introduced into the upper portion of the gas-fired spray dryer and passes through a spray nozzle or a disk atomizer (see Figure 22.5). The speed of the spray wheel may be about 11,000 rpm. The finely and evenly dispersed liquid comes into contact with upwardly directed hot air. Typical spray tower tanperatures are about 180°C [21] with inlet temperatures of about 260-300°C and outlet air temperatures of above lOO C. The resultant spray-dried droplets adopt the form of hollow microspheres. The silicate particles are collected at the spray dryer s bottom and are withdrawn by a screw conveyor. The amorphous sodium silicate may have a bulk density on the order of 250-500 g/L, an SiOjiNajO molar ratio of 2.04 1, and an ignition loss on the order of 19-20%. Its mean particle size can be on the order of 100-200 pm. The material may be subjected to further milling to modify the form and density of the powder [51,63]. 

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