Optimization of the Washcoat Slurry for Hydrotalcite-Based LNT Catalyst

Abstract

This work aimed to optimize the washcoat slurry for hydrotalcite-based lean NOx trap (LNT) catalyst. The effects of the slurry properties including pH, solid content, binder and additive on the hydrotalcite-based slurry viscosity were investigated. The particle size distribution of the optimal hydrotalcite-based slurry was measured. A cordierite material was used to coat the optimal slurry, and the washcoat was characterized by X-ray diffraction, scanning electron microscopy and N2 adsorption. The optimal slurry containing Pt and Ba was coated on the cordierite for the preparation of hydrotalcite-based LNT catalyst, and the performances of this catalyst were evaluated by NOx storage test, temperature programmed desorption and NOx reduction. For comparison, the performance of the commercial LNT catalyst with Pt/BaO/Al2O3 was analyzed. After coating, the hydrotalcite-based washcoat was closely contacted with the support, being the main phase MgO and presenting a specific surface area of 86.3 m2/g. The hydrotalcite-based LNT catalyst had better NOx storage and desorption ability, selectivity to N2 and LNT efficiency than the Pt/BaO/Al2O3 catalyst.

Elements of the Manufacture and Properties of Technical Catalysts

“…After washcoating a monolith, the surface area available for dispersing the catalytic phases can increase up to 1000 times. For example, literature reports 145,160,239 have shown that Y zeolite-based washcoats can raise the surface area of cordierite honeycombs from 0.7 to 212 m 2 /g. Washcoats are usually large nanoporous inorganic oxides such as γ-Al 2 O 3 or zeolites.…”

“…Indeed, highly viscous suspensions tend to deposit irregularly on the walls of the honeycomb and the removal of the excess suspension with the hot air current is not always homogeneous and reproducible. 227,228,239,245 On the other hand, a suspension with a very low viscosity may produce low washcoat loads, see Figure 59a. 239,246 As discussed in section 1.3.3.2, the viscosity of concentrated suspensions is determined by factors such as particle size distribution, the content of solids, the pH, and by the concentration of additives, e.g., binders.…”

“…227,228,239,245 On the other hand, a suspension with a very low viscosity may produce low washcoat loads, see Figure 59a. 239,246 As discussed in section 1.3.3.2, the viscosity of concentrated suspensions is determined by factors such as particle size distribution, the content of solids, the pH, and by the concentration of additives, e.g., binders. 247 Therefore, these factors must be considered for preparing washcoat suspensions.…”

“…Therefore, these factors must be considered for preparing washcoat suspensions. At this point, it is worth mentioning that additives such as polyethylene glycol also play the role of improving the adhesion of the washcoat over the monolith . However, the amount of these types of additives must be carefully considered since they may produce undesirable effects over the catalytic properties of the material.…”

“…In this case, it is important to control the rheological properties of the suspension. Indeed, highly viscous suspensions tend to deposit irregularly on the walls of the honeycomb and the removal of the excess suspension with the hot air current is not always homogeneous and reproducible. ,,, On the other hand, a suspension with a very low viscosity may produce low washcoat loads, see Figure a. , …”