PTQ Q3 2025 Issue

approximately 0.50 to 3 microns. There are two forms of iron seen at the surface of Ecat particles: nano iron oxide crystals (acting as nuclei for the formation of eutectics nodules with silica) and amorphous phase iron (with silica/alumina). Iron in the amorphous phase, which is the predominant form of iron, binds with silica on the outer surface of Ecat particles. The resulting low-melting-point eutectic seals off the interior of the catalyst particles. 6 This study also suggests that the intra-particle mobility of the added iron is minimal. Regenerator hydrothermal conditions However, iron can transfer from particle to particle, most likely through collisions, especially in the FCC regenera - tor dense bed, where sticky surfaces can facilitate matter transfer upon impact. Nevertheless, the exact mechanism of iron transfer between particles remains unclear. It has been reported that silica (SiO₂) promotes the formation of iron nodules and may also enhance the inter-particle mobil- ity of iron-containing species.⁹ - 11 There are various sources of silica in the FCC unit: silica from the feed and silica from the base catalyst, mainly in the Y-zeolite. It is suggested that silica in the Y-zeolite is highly mobile under FCC regenerator hydrothermal conditions. It decomposes and migrates from particle to particle. 12 Distinguishing externally introduced silica from the silica originally present in the Ecat remains challenging. The high mobility of silica has been clearly observed on Johnson Matthey’s proprietary Cat-Aid metals trap additive. When the metals trap additive, initially free of silica, is introduced into FCC units, it gradually accumulates sil - ica. EDS mapping (see Figure 3 ) reveals the formation of distinct silica-rich rings on the particle surfaces. Chemical analysis of Cat-Aid particles isolated from Ecats via a sink- float procedure shows that more than 20% silica is present on their surfaces. Since feed-derived silica is known to be minimal, mobile silica from the base catalyst appears to be the primary source of silica seen on the surface of the Cat-Aid particles. This study provides clear evidence of the high mobility of silica under FCC regenerator hydrothermal conditions. Iron rings are also visible on the surface of the additive particles, as shown in Figure 3. The inter-particle mobility

Iron

As shown in Figure 1 , a TEM-EDS study found that only the outer surface of Ecat particles is iron enriched. 5 A closer examination (see Figure 2 left and centre) by HR-TEM reveals that the iron-rich surface layer consists of a high density of randomly oriented iron oxide nanoparticles, ranging in size from 5 to 20 nm. These nanoparticles are embedded within an amorphous matrix. Figure 2 (right) is an HR-TEM bright-field image (highlighting heavier metal components) of cross-sections of Ecat particles capturing the interface between the Fe-contaminated surface and the inner catalyst matrix. This image provides a direct vis- ualisation of iron oxide nanoparticles obstructing a pore within the catalyst structure of an Ecat particle retrieved from a commercial unit, distinguishing it from iron contam- ination introduced via cyclic deactivation in the laboratory, as reported in the literature. 3 The vitrified (glasslike) outer surface of the catalyst, as shown in Figures 1 and 2, ranges in thickness typically from Figure 1 TEM- EDS pictures of Ecat iron nodules, indicating iron enrichment on the surface

Iron enriched layer

A n a no pore

Figure 2 HR-TEM images of one iron nodule on an iron-poisoned Ecat particle. Left and centre: Nano Fe₂O3 crystallites are embedded in a glassy substrate. Right: HR-TEM image of interface of iron-enriched nodules on the top layer of an Ecat particle indicating the blockage of nano pore by the iron oxide

PTQ Q3 2025