Oxidation Kinetics of Nanocrystalline Hexagonal RMn_1–xTi_xO₃ (R = Ho, Dy)

Abstract

Hexagonal manganites, RMnO 3 (R = Sc, Y, Ho-Lu), are potential oxygen storage materials for air separation due to their reversible oxygen storage and release properties. Their outstanding ability to absorb and release oxygen at relatively low temperatures of 250–400 °C holds promise of saving energy compared to current industrial methods. Unfortunately, the low temperature of operation also implies slow kinetics of oxygen exchange in these materials, which would make them inefficient in applications such as chemical looping air separation. Here, we show that the oxidation kinetics of RMnO 3 can be improved through Ti 4+ -doping as well as by increasing the rare earth cation size. The rate of oxygen absorption of nanocrystalline RMn 1– x Ti x O 3 (R = Ho, Dy; x = 0, 0.15) was investigated by thermogravimetric analysis, X-ray absorption near-edge structure, and high-temperature X-ray diffraction (HT-XRD) with in situ switching of atmosphere from N 2 to O 2 . The kinetics of oxidation increases for larger R and even more with Ti 4+ donor doping, as both induce expansion of the ab -plane, which reduces the electrostatic repulsion between oxygen in the lattice upon oxygen ion migration. Surface exchange rates and activation energies of oxidation were determined from changes in lattice parameters observed through HT-XRD upon in situ switching of atmosphere.

High-Entropy Hexagonal Manganites for Fast Oxygen Absorption and Release

“…This implies very fast oxidation kinetics compared to literature reports. 25,28,33 The samples containing both Ti and Tb (Figure 1b−d) show the highest oxygen storage capacities, as expected from previous studies of Ti and Tb in hexagonal manganites. 28,30,33,35 The oxygen content of these samples is also above the δ = 0.33 limit for oxidation of 2/3 of Mn only, giving 2 oxygen interstitials per 30-atom unit cell.…”

“…The oxidation time for HEO1-n is substantially shorter than previous findings for RMnO 3 without aliovalent dopants, with, e.g., Y 0.95 Pr 0.05 MnO 3 reported to need several hours to oxidize in air at multiple temperatures 25 and HoMnO 3 being far from fully oxidized after 20 min at 250 °C in an O 2 atmosphere. 33 This shows that high-entropy compositions greatly improve the oxidation rate of hexagonal manganites. This may be attributed to lattice site disorder causing overlap in site energies, which promotes ionic transport.…”

“…In contrast, literature reports on undoped Y-, Ho-, or Dy-based materials all show a significant reduction in OSC during cooling, even at a 5 °C min −1 cooling rate. 25,28,33 YMnO 3 substituted with larger R such as Tb, Gd, and Sm show no significant change in OSC when increasing the cooling rate from 1 to 5 °C min −1 , 28 indicating that the larger R improves the kinetics of oxidation. However, these R-substituted samples also show a significant increase in thermal hysteresis with increasing cooling rates, implying significantly slower oxidation kinetics than those for the high-entropy materials studied here.…”

“…38 Oxidation of Tb 3+ to Tb 4+ can charge compensate more oxygen interstitials than possible in samples with only "inert" R cations. Ti-doped samples (Figure 1b−e) show significant oxygen content at T > 350 °C, as Ti increases the thermal stability of interstitial oxygen, 30,33 and a stable plateau is not reached at T > 700 °C, where all Mn is presumed to be Mn 3+ , indicating a small underestimation in the assumption of δ for these samples. Compared to our recent work on single-R nanocrystalline materials, 30,33 the effect of high entropy is much stronger for materials without Ti donor doping (Figure 1a) than for Ti-doped (Figure 1d).…”

“…In contrast, the Ti-doped HEO2-n sample shows no thermal hysteresis in oxygen content, in accordance with previous findings for Ti 4+ -doped RMnO 3 . 30,33,35 The oxygen absorption of HEO2-n is less sensitive to faster heating rates than that for HEO1-n.…”