Sulfur dioxide-tolerant strontium chromate for the catalytic oxidation of diesel particulate matter

Abstract

A sulfur dioxide (SO₂)-tolerant, stable mixed oxide as an alternative to platinum (Pt) group metal (PGM) catalysts has been explored for the catalytic oxidation of particulate matter (PM) due to its low-cost, desirable physiochemical properties, thermal stability, tailoring options, etc. Herein, a mixed oxide of Sr and Cr was prepared via a solution combustion method by mixing stoichiometric amounts of Sr- and Cr-precursors with citric acid and urea as fuels followed by calcination at 800 °C in air. A pure mixed oxide phase of Sr and Cr (SrCrO₄) (P2₁/n [14]) (a = 0.7090 nm, b = 0.7394 nm, and c = 0.6755 nm) has been successfully prepared, and the SrCrO₄ particles are larger in size with semi-oval shapes due to their agglomeration at elevated temperatures. The SrCrO₄ catalyst shows significantly improved intrinsic catalytic performance for PM oxidation in the tested temperature range as compared to the reference catalysts, Pt/Al₂O₃ and Pt-dispersed SrCrO₄ (Pt/SrCrO₄) (loading weight: 5 wt%) are used as internal reference catalysts. The onset temperature (i.e., intrinsic catalytic activity at 10% conversion, T₁₀) is observed at 426 °C, which is significantly lower than that of the reference Pt/Al₂O₃ (T₁₀ = 537 °C) and comparable with that of the Pt/SrCrO₄ catalyst (T₁₀ = 414 °C). The SrCrO₄ catalyst shows a stable, multi-cycle PM oxidation performance for the tested five cycles, and both its crystalline structure and morphology remain unchanged even after its multiple cycles of use. The structure of the SrCrO₄ catalyst is stable even after moisture and SO₂ treatments, and the catalytic PM oxidation activity of SrCrO₄ is not compromised even after these harsh treatments. Importantly, the SrCrO₄ catalyst is also stable after PM oxidation conducted using real world PM from a heavy-duty vehicle. These results demonstrate that the mixed oxide phase of SrCrO₄ shows promise for PM oxidation.