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September 3, 2004

Chemistry of SO₂ on Ce_1-xZr_xO₂ Nanoparticles and Ce_1-xZr_xO₂(111) Surfaces

J.A. Rodriguez¹, X. Wang¹, G. Liu¹, J. Hanson¹, J. Hrbek¹, C.H.F. Peden², A. Iglesias-Juez³, and M. Fernandez-Garcia^3
1Brookhaven National Laboratory, Upton, NY; ²Pacific Northwest National Laboratory, Richland; ³Instituto de Catálisis y Petroleoquímica, Madrid, Spain

A major effort in environmental cleanup is controlling the emission of toxic pollutants produced during the combustion of fuels in factories, power plants, and automotive engines. Cerium oxide (CeO₂) -based materials are utilized as catalysts for the destruction of one of these pollutants, sulfur dioxide (SO₂), and are also used to prevent acid rain. High-resolution photoemission, time-resolved x-ray diffraction (TR-XRD), and x-ray absorption near-edge spectroscopy (XANES) were used to investigate the chemistry of SO₂ on cerium oxide-zirconium (Ce_1-xZr_xO₂) nanoparticles and Ce_1-xZr_xO₂(111) surfaces (x ≤ 0.5). S K-edge XANES spectra pointed to sulfate (SO4) as the main product of the adsorption of SO₂ on these mixed-metal oxides. Full SO₂ dissociation was seen on the nanoparticles, but not on the Ce_1-xZr_xO₂(111) surfaces. The metal cations at corner or edge sites of the Ce_1-xZr_xO₂ nanoparticles probably play a very important role in interactions with the SO₂ molecules.

The Ce_1-xZr_xO₂ system is one of the most studied mixed-metal oxides in the literature. Typically, the Ce and Zr cations are randomly distributed in a fluorite-type structure. The focus has been on examining possible correlations between the CeO_2·ZrO₂ interactions and differences in the behavior of Ce_1-xZr_xO₂ and CeO₂. Oxide nanostructures can have special chemical properties due to size-induced structural distortions and the presence of corner sites and oxygen vacancies. Recent studies have shown that Ce_1-xZr_xO₂ nanoparticles can be prepared by a novel microemulsion method that leads to materials with highly homogenous chemical compositions (i.e. Ce,Zr distribution) and a narrow distribution of particle sizes. Following this approach, we synthesized nanoparticles with sizes between 4 and 7 nm. We investigated their reactivity, and the reactivity of Ce_1-xZr_xO₂(111) surfaces, towards SO₂.

Figure 1 shows the structure of an ideal Ce_1-xZr_xO₂(111) surface (x < 0.4). The top layer consists of O atoms, but within this layer there are holes that expose the Ce or Zr cations in the second layer. This well-defined surface allows the detailed study of O↔SO₂ and Ce,Zr↔SO₂ interactions. For the Ce_1-xZr_xO₂ nanoparticles, the results of transmission electron microscopy show rough surfaces that can be O or cation-terminated and have a high density of edge or corner sites. These different structural properties affect the chemical reactivity of these mixed-metal oxides.

The top of Figure 2 shows S K-edge XANES spectra for the adsorption of SO₂ on CeO₂(111) and Ce_0.7Zr_0.3O₂(111) surfaces at room temperature. A comparison to the corresponding peak positions for sulfates and sulfites indicates that SO₄ is the main species formed on the oxide surfaces, with a minor concentration of SO₃ (SO_2,gas + nO_lattice → SO_3,ads or SO_4,ads). The cations in the second layer have all of their O neighbors (eight in total), and interact very weakly with an adsorbed SO₂ molecule. One must introduce O vacancies in CeO₂(111) and Ce_0.7Zr_0.3O₂(111) to see the interaction of SO₂ with the metal cations and the subsequent dissociation of the molecule.

Figure 2 also shows S K-edge XANES spectra taken after exposing nanoparticles of CeO₂, Ce_0.66Zr_0.33O₂ and Ce_0.66Ca_0.33O_2-y to SO₂ at 25 °C. Again, we found that SO₄ is the main sulfur-containing species present on the oxides, but, in addition, we saw features at photon energies between 2470 and 2472 eV that denote the existence of metal-S bonds as a consequence of full SO₂ dissociation. Thus, the nanoparticles have metal cations at corner or edge sites that can interact well with the SO₂ molecule. In addition, there may be O vacancies in the surface of the Ce_0.66Zr_0.33O₂ and Ce_0.66Ca_0.33O_2-y nanoparticles that facilitate S-O bond cleavage.

Figure 3 shows the effect of temperature on the SO₄ signal for the CeO₂ and Ce_1-xZr_xO₂ systems in Figure 2. As the temperature is raised, SO₄ decomposes. The SO₄ adsorbed on the nanoparticles is somewhat more stable than that present on the (111) surfaces. For both types of systems, the presence of Zr seems to induce an increase in the thermal stability of the adsorbed sulfate.

BEAMLINE
U7A, X7B, X19A

FUNDING
U.S. Department of Energy
Interministerial Committee on Science and Technology (CICYT, Spain)

PUBLICATION
G. Liu et al, “Adsorption and Reaction of SO₂ on Model Ce_1-xZr_xO₂(111) Catalysts”, J. Phys. Chem. B, 108, 2931 (2004).

FOR MORE INFORMATION
José A. Rodriguez
Chemistry Department
Brookhaven National Laboratory
Upton, NY
Email: rodrigez@bnl.gov