Thermal and chemical aging of model three-way catalyst Pd/Al2O3 and its impact on the conversion of CNG vehicle exhaust

Abstract

Thermal and chemical aging induced physical and chemical modifications in the model three-way catalyst Pd/Al2O3 for compressed natural gas vehicles exhaust were studied. The fresh catalyst Pd/Al/F (calcined at 500°C for 4h) was subjected to either thermal (Pd/Al/700) or chemical (poisoned with phosphorous (P), XP/Pd/Al; X=1.8 or 7.5wt%) aging under air flow at 700°C for 5h. The catalysts were analyzed by N2-physisorption, CO-chemisorption, high angular annular dark-field scanning transmission electron microscopic (HAADF-STEM), X-ray diffraction (XRD), solid state 31P MAS NMR, H2-temperature programmed reduction (H2-TPR) and in situ X-ray absorption near edge structure spectroscopy (XANES) during H2-TPR. Characterization results reveal that thermal aging leads to merely physical modifications such as decreased surface area of the support and palladium (Pd) dispersion in Pd/Al/700. Whereas, chemical aging causes both severe physical and chemical modifications in XP/Pd/Al: (i) physical modifications are clogging of support pores and fouling of Pd nanoparticles with P as evident from N2-physisorption, CO-chemisorption and HAADF-STEM and (ii) chemical alterations are the conversion of support aluminum oxide into aluminum phosphate as evident from solid state 31P MAS NMR, and decreased reducibility of PdOx species as evident from H2-TPR and in situ XANES. Consequently, chemical aging is more detrimental than thermal aging for three-way catalytic (TWC) performance as evident from the activity data. Undoubtedly, P (chemical aging) profoundly decreases the TWC efficiency. On the other hand, mere thermal aging considerably decreases CO and CH4 oxidation efficiency, but interestingly improves the NO reduction efficiency.