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 Ce1−x M x O2−δ (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare-earth-ion-substituted Ce1−x A x O2−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 Ce1−x M x O2−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.

An experimental and DFT study of the adsorption and oxidation of NH3 on a CeO2 catalyst modified by Fe, Mn, La and Y

“…The results indicate that the decrease in the lattice distance results from doping with cations that have small radii (0.64Å for Fe 3+ , 0.66Å for Mn 3+ , 0.90Å for Y 3+ ) [26]. Their replacement of Ce 4+ (0.92Å) in the fluorite structure appears to be possible due to their similar structures and ionic radii [5][6][7]. We assume that dopant atoms could be well dispersed on the CeO 2 surface (solid solution) [5,27] and substitute surface cerium atoms to form point detects [8,13].…”

“…S3, the peaks of which can be attributed to the +3 oxidation state [5][6][7]. The spectra of Ce 3d and O 1s for the four catalysts are presented in Fig.…”

“…3. The bands labeled u, u and u and v, v and v represent the 3d 10 4f 0 state of Ce 4+ , whereas u and v represented the 3d 10 4f 1 state of Ce 3+ [7,28,29]. The O 1s peaks can be fitted into two peaks referenced to the lattice oxygen at 529.3-530 eV (denoted as O ␤ ) and the chemisorbed surface oxygen at 531.3-531.9 eV (denoted as O ␣ ), respectively.…”

“…Gupta et al [5][6][7] systematically studied transition-metal, noble-metal and rare-earth cations doped onto CeO 2 and revealed that transition-metal and noble-metal, rather than rare-earth, greatly enhanced the OSC of CeO 2 and the weak, longer bonds that formed on the active surfaces were responsible for the higher reducibility of these solid solutions. Nolan [8,9] modeled the adsorptions of NO and CO molecules on the Ti, Zr, Hf doped CeO 2 surfaces by density functional theory (DFT) calculations and proposed that the dopants atoms significantly distort the surface structure compared to the undoped surface.…”

Synthesis, characterization and catalytic performance of ZnO–CeO2 nanoparticles in wet oxidation of wastewater containing chlorinated compounds

“…The high activity, mobility in solution, specific surface areas, stability and easy separation from reaction mixture, make them the ultimate candidate for catalysis (Khin et al 2012;Xia et al 2016). CeO 2 gained considerable attention as an oxidative catalyst due to its oxygen storage and release capacity (Gupta et al 2010). The interaction of CeO 2 with & Anushree anushreejatrana@gmail.com low valent element strongly enhances its redox property, which leads to the promoted oxidation efficiency (Anushree and Kumar 2015a; Arena 2014).…”