Correlation of oxygen storage capacity and structural distortion in transition-metal-, noble-metal-, and rare-earth-ion-substituted CeO₂ from first principles calculation

Abstract

Oxygen storage/release (OSC) capacity is an important feature common to all three-way catalysts to combat harmful exhaust emissions. To understand the mechanism of improved OSC for doped CeO₂, we undertook the structural investigation by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂-TPR (temperature-programmed hydrogen reduction) and density functional theoretical (DFT) calculations of transition-metal-, noble-metal-, and rare-earth (RE)-ion-substituted ceria. In this report, we present the relationship between the OSC and structural changes induced by the dopant ion in CeO₂. Transition metal and noble metal ion substitution in ceria greatly enhances the reducibility of Ce_1-xM_xO_2-δ (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare-earth-ion-substituted Ce_1-xA_xO_2-δ (A = La, Y) have very little effect in improving the OSC. Our simulated optimized structure shows deviation in cation-oxygen bond length from ideal bond length of 2.34 Å (for CeO₂). For example, our theoretical calculation for Ce₂₈Mn₄O₆₂ structure shows that Mn-O bonds are in 4 + 2 coordination with average bond lengths of 2.0 and 3.06 Å respectively. Although the four short Mn-O bond lengths spans the bond distance region of Mn₂O₃, the other two Mn-O bonds are moved to longer distances. The dopant transition and noble metal ions also affects Ce coordination shell and results in the formation of longer Ce-O bonds as well. Thus longer cation-oxygen bonds for both dopant and host ions results in enhanced synergistic reduction of the solid solution. With Pd ion substitution in Ce_1-xM_xO_2-δ (M = Mn Fe, Co, Ni, Cu) further enhancement in OSC is observed in H₂-TPR. This effect is reflected in our model calculations by the presence of still longer bonds compared to the model without Pd ion doping. The synergistic effect is therefore due to enhanced reducibility of both dopant and host ion induced due to structural distortion of fluorite lattice in presence of dopant ion. For RE ions (RE = Y, La), our calculations show very little deviation of bonds lengths from ideal fluorite structure. The absence of longer Y-O/La-O and Ce-O bonds make the structure much less susceptible to reduction.