Reactivation of an Aged Commercial Three-Way Catalyst by Oxalic and Citric Acid Washing

Abstract

The efficiency of dilute oxalic and citric acid solutions on improving the oxygen storage capacity (OSC) and catalytic activity of a severely aged (83,000 km) commercial three-way catalyst (TWC) has been investigated. Washing procedures applied after optimization of experimental parameters, namely, temperature, flow-rate, and concentration of acid solution, led to significant improvements of OSC and catalytic activity (based on dynamometer test measurements) of the aged TWC. The latterwas made possible due to the removal of significant amounts of various contaminants accumulated on the catalyst surface (e.g., P, S, Pb, Ca, Zn, Si, Fe, Cu, and Ni) during driving conditions, as revealed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and X-ray Photoelectron Spectroscopy (XPS) analyses. For the first time, it is demonstrated that dilute oxalic acid solution significantly improves the catalytic activity of an aged commercial TWC toward CO, Cx,Hy, and NOx conversions under real exhaust gas conditions (dynamometer tests) by two to eight times in the 250-450 degrees C range and the OSC quantity by up to 50%. Oxalic acid appears to be more efficient than citric acid in removing specifically P- and S-containing compounds from the catalyst surface, whereas citric acid in removing Pb- and Zn-containing compounds, thus uncovering surface active catalytic sites.

Efficient In-Situ Regeneration Method of the Catalytic Activity of Aged TWC

“…5b). Similar results were also observed for X CO and X NO under real exhaust-gas conditions (dynamometer tests) [11]. Less improvements in the catalytic behaviour were observed after washing with acetic acid, citric acid, EDTA and NTA solutions compared with the oxalic acid [11,12].…”

“…The oxalic acid solution regeneration method was applied on the cubic monolith fragments extracted from the aged TWC using a lab-constructed apparatus [11]. The TWC samples were treated with the oxalic acid solution (0.1 M) for 1 h, where the latter was then replaced by a fresh one used for an additional 1 h. The flow rate of the acid solution recirculated was 600 mL/ min, whereas its temperature was kept constant at 50°C [11].…”

“…The TWC samples were treated with the oxalic acid solution (0.1 M) for 1 h, where the latter was then replaced by a fresh one used for an additional 1 h. The flow rate of the acid solution recirculated was 600 mL/ min, whereas its temperature was kept constant at 50°C [11]. After acid washing, each catalyst specimen was washed thoroughly with de-ionized water for 30 min followed by drying at 120°C overnight and at 200°C for further 2 h for complete removal of acid residues.…”

“…Thus, it could be said that cerium removal would probably correspond to a large extent to that present in CePO 4 (XRD results). Based on XPS results [11], Ce and Zr surface concentrations were found to significantly increase only after oxalic acid washing. These results imply that the latter procedure largely uncovers the TWC washcoat surface through the removal of most of P-containing overlayer which was reported to be constituted of Zn, Ca, Mg, Ce and Al phosphates [4][5][6]9].…”

“…The interest in finding suitable in situ regeneration methods of commercial TWCs has recently grown. The use of liquid solvents, such as weak organic acids (e.g., acetic, oxalic and citric acids) for extraction of contaminants has been reported [11,12], as well as the application of various thermo-chemical treatments using O 2 , H 2 and Cl-containing gas mixtures [13,14] or other Cl-containing reagents (e.g., 1,2-dichloropropane) [15] for the redispersion of large noble metal particles.…”