Theoretical Calculation of the Dehydrogenation of Ethanol on a Rh/CeO2(111) Surface

We applied periodic density-functional theory (DFT) to investigate the dehydrogenation of ethanol on a Rh/CeO2 (111) surface. Ethanol is calculated to have the greatest energy of adsorption when the oxygen atom of the molecule is adsorbed onto a Ce atom in the surface, relative to other surface atoms (Rh or O). Before forming a six-membered ring of an oxametallacyclic compound (Rh-CH2CH2O-Ce(a)), two hydrogen atoms from ethanol are first eliminated; the barriers for dissociation of the O-H and the beta-carbon (CH2-H) hydrogens are calculated to be 12.00 and 28.57 kcal/mol, respectively. The dehydrogenated H atom has the greatest adsorption energy (E(ads) = 101.59 kcal/mol) when it is adsorbed onto an oxygen atom of the surface. The dehydrogenation continues with the loss of two hydrogens from the alpha-carbon, forming an intermediate species Rh-CH2CO-Ce(a), for which the successive barriers are 34.26 and 40.84 kcal/mol. Scission of the C-C bond occurs at this stage with a dissociation barrier Ea = 49.54 kcal/mol, to form Rh-CH(2(a)) + 4H(a) + CO(g). At high temperatures, these adsorbates desorb to yield the final products CH(4(g)), H(2(g)), and CO(g).

In Situ Studies of the Active Sites for the Water Gas Shift Reaction over Cu−CeO2 Catalysts: Complex Interaction between Metallic Copper and Oxygen Vacancies of Ceria

New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce1-xCuxO2 sample) and an impregnation method (impregnated CuOx/CeO2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-Ovacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the Ovacancy sites or the Cu-Ovacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.

Unusual Physical and Chemical Properties of Cu in Ce1-xCuxO2 Oxides

The structural and electronic properties of Ce(1-x)Cu(x)O(2) nano systems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Cu atoms embedded in ceria had an oxidation state higher than those of the cations in Cu(2)O or CuO. The lattice of the Ce(1)(-x)Cu(x)O(2) systems still adopted a fluorite-type structure, but it was highly distorted with multiple cation-oxygen distances with respect to the single cation-oxygen bond distance seen in pure ceria. The doping of CeO(2) with copper introduced a large strain into the oxide lattice and favored the formation of O vacancies, leading to a Ce(1-x)Cu(x)O(2-y) stoichiometry for our materials. Cu approached the planar geometry characteristic of Cu(II) oxides, but with a strongly perturbed local order. The chemical activities of the Ce(1-x)Cu(x)O(2) nanoparticles were tested using the reactions with H(2) and O(2) as probes. During the reduction in hydrogen, an induction time was observed and became shorter after raising the reaction temperature. The fraction of copper that could be reduced in the Ce(1-x)Cu(x)O(2) oxides also depended strongly on the reaction temperature. A comparison with data for the reduction of pure copper oxides indicated that the copper embedded in ceria was much more difficult to reduce. The reduction of the Ce(1-x)Cu(x)O(2) nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu(2)O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials.

TPR, TPO, and TPD examinations of Cu0.15Ce0.85O2−y mixed oxides prepared by co-precipitation, by the sol–gel peroxide route, and by citric acid-assisted synthesis

Temperature-programmed reduction (TPR), oxidation (TPO), and desorption (TPD) studies were performed on three copper-ceria mixed oxide samples having the same nominal composition, Cu0.15Ce0.85O(2-y), but prepared in three different ways: by co-precipitation, the sol-gel peroxide route, and the sol-gel citric acid route. The obtained results reveal that despite a drastic initial drop in specific surface area after consecutive redox cycles, the hydrogen consumption remains constant. This is because CuO is highly dispersed over the surface of CeO2 nanocrystallites and remains highly dispersed even after the agglomeration of CeO2 nanocrystallites in a denser secondary structure. The dispersed CuO is reduced to Cu(0) during the TPR, forming agglomerated metal particles on the surface of partially reduced CeO2. However, after subsequent temperature-programmed oxidation all the Cu(0) is oxidized back into CuO and redispersed over the CeO2 crystallites.

Structural Characterization of Nanosized CeO2−SiO2, CeO2−TiO2, and CeO2−ZrO2 Catalysts by XRD, Raman, and HREM Techniques

Structural characteristics of nanosized ceria-silica, ceria-titania, and ceria-zirconia mixed oxide catalysts have been investigated using X-ray diffraction (XRD), Raman spectroscopy, BET surface area, thermogravimetry, and high-resolution transmission electron microscopy (HREM). The effect of support oxides on the crystal modification of ceria cubic lattice was mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahighly dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO(2)-SiO(2) sample primarily consists of nanocrystalline CeO(2) on the amorphous SiO(2) surface. Both crystalline CeO(2) and TiO(2) anatase phases were noted in the case of CeO(2)-TiO(2) sample. Formation of cubic Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) (at 1073 K) were observed in the case of the CeO(2)-ZrO(2) sample. Raman measurements disclose the fluorite structure of ceria and the presence of oxygen vacancies/Ce(3+). The HREM results reveal well-dispersed CeO(2) nanocrystals over the amorphous SiO(2) matrix in the cases of CeO(2)-SiO(2), isolated CeO(2), and TiO(2) (anatase) nanocrystals, some overlapping regions in the case of CeO(2)-TiO(2), and nanosized CeO(2) and Ce-Zr oxides in the case of CeO(2)-ZrO(2) sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO(2) is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of the mixed oxide systems is more than that of pure CeO(2) and is system dependent.

Active nonmetallic Au and Pt species on ceria-based water-gas shift catalysts

Traditional analysis of reactions catalyzed by supported metals involves the structure of the metallic particles. However, we report here that for the class of nanostructured gold- or platinum-cerium oxide catalysts, which are active for the water-gas shift reaction, metal nanoparticles do not participate in the reaction. Nonmetallic gold or platinum species strongly associated with surface cerium-oxygen groups are responsible for the activity.