Design of effective catalysts for the aftertreatment of CNG-fueled vehicle exhaust by doping 1% Pt/Ce0.4Zr0.5La0.05Y0.05-xO1.95+x with niobium

Abstract

With the improvement of current global vehicle emission standards increase and widespread application of natural gas vehicles (CNG), existing three-way catalytic converters (TWCs) need to be modified to meet the new emission regulations. In recent years, the price of palladium has been rising, while platinum, which is also used in catalytic systems, is relatively cheap, and the industry has come up with the idea of replacing palladium with platinum. Palladium loaded with a mass fraction of 1% on Ce0.4Zr0.5La0.05Y0.05O1.95 (CZLY) did show excellent catalytic activity, however, when palladium is replaced by platinum, the catalytic activity of the catalyst for methane combustion is significantly reduced. At 680 °C under oxygen-rich conditions (20%), methane conversion can typically reach 90%. The reduced 1% Pt/CZLY catalyst, on the other hand, was unstable in activity at high temperatures, although it was significantly higher at the starting temperature (T50 = 378 °C).In this work, it was found that the addition of niobium to CZLY improved the activity of Pt-based catalysts. The 1% Pt/Ce0.4Zr0.5La0.05Nb0.05O2 (1% Pt/CZLN) catalysts showed a decrease in T90 (CO) of 26 °C and T90 (CH4) of approximately 250 °C. The activity of this catalyst is consistent with that of the reduction pre-treatment 1% Pt/CZLY catalyst. Results from thermal deactivation and sulphur poisoning experiments show that the 1% Pt/CZLN catalyst exhibits superior thermal stability and anti-sulphur toxicity. For example, methane conversion of the 1% Pt/CZLN catalyst maintained at 85% at 600 °C for 32 h with a 5% decrease in activity.To clearly explain the effect of niobium doping on the catalyst, BET, XRD, H2-TPR, XPS and TEM tests were performed on the catalysts confirming that the strong Pt-Nb interaction stabilizes the valence state of the platinum and loads the platinum on the carrier surface in the active metallic platinum state. The amount of niobium doping was positively correlated with the metallic Pt content in the catalyst. In conclusion, the addition of niobium to platinum-based catalysts increases the methane combustion activity of the catalyst and delays thermal deactivation and sulphur poisoning of the platinum-based catalyst, extending the service life of the platinum-based catalyst.