Steven H. Overbury

Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption.

Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman spectroscopy and O(2) adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on one-electron defect sites and peroxide on two-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphology. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which is of significance for ceria-catalyzed oxidation reactions.

Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO2 Supported Au25(SCH2CH2Ph)18 Nanoclusters

The effect of thiolate ligands was explored on the catalysis of CeO2 rod supported Au25(SR)18 (SR = -SCH2CH2Ph) by using CO oxidation as a probe reaction. Reaction kinetic tests, in situ IR and X-ray absorption spectroscopy, and density functional theory (DFT) were employed to understand how the thiolate ligands affect the nature of active sites, activation of CO and O2, and reaction mechanism and kinetics. The intact Au25(SR)18 on the CeO2 rod is found not able to adsorb CO. Only when the thiolate ligands are partially removed, starting from the interface between Au25(SR)18 and CeO2 at temperatures of 423 K and above, can the adsorption of CO be observed by IR. DFT calculations suggest that CO adsorbs favorably on the exposed gold atoms. Accordingly, the CO oxidation light-off temperature shifts to lower temperature. Several types of Au sites are probed by IR of CO adsorption during the ligand removal process. The cationic Au sites (charged between 0 and +1) are found to play the major role for low-temperature CO oxidation. Similar activation energies and reaction rates are found for CO oxidation on differently treated Au25(SR)18/CeO2 rod catalysts, suggesting a simple site-blocking effect of the thiolate ligands in Au nanocluster catalysis. Isotopic labeling experiments clearly indicate that CO oxidation on the Au25(SR)18/CeO2 rod catalyst proceeds predominantly via the redox mechanism where CeO2 activates O2 while CO is activated on the dethiolated gold sites. These results point to a double-edged sword role played by the thiolate ligands on Au25 nanoclusters for CO oxidation.

Comparison of Au Catalysts Supported on Mesoporous Titania and Silica: Investigation of Au Particle Size Effects and Metal-Support Interactions

Au catalysts supported on mesoporous silica and titania supports were synthesized and tested for the oxidation of CO. Two approaches were used to prepare the silica-supported catalysts utilizing complexing triamine ligands which resulted in mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the key for the formation of uniformly sized 2–3 nm Au nanoparticles in the silica pores. On mesoporous titania, high gold dispersions were obtained without the need of a functional ligand. Au supported on titania exhibited a much higher activity for CO oxidation, even though the Au particle sizes were essentially identical on the titania and the wormhole silica supports. The results suggest that the presence of 2–3 nm particle size alone is not sufficient to achieve high activity in CO oxidation. Instead, the support may influence the activity through other possible ways including stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O2−), or stabilization of optimal Au structures.

Colloidal deposition synthesis of supported gold nanocatalysts based on Au–Fe3O4 dumbbell nanoparticles

Highly active Au catalysts with a dumbbell-like heterostructure for CO oxidation were prepared through colloidal deposition method; both activities and stabilities were investigated under different experimental conditions.

The structure and composition of the NiAl(110) and NiAl(100) surfaces

Abstract The clean NiAl(110) and NiAl(100) surfaces have been studied by LEED, AES and low energy alkali ion scattering. The composition of the NiAl(100) surface was determined from Li + scattering intensities using the previously studied NiAl(110) surface for calibration. Depending upon annealing conditions c(3√2 × √2)R45° and p(1 × 1) LEED patterns are formed on the (100) surface, which are associated with first layer Al atom fractions of 0.65±0.02 and 0.78±0.07, respectively. The surface structure corresponding to these two LEED patterns was determined from the dependence on azimuthal and polar angle of incidence in Li + scattering and K + multiple scattering analysis. The c(3√2 × √2)R45° is attributed to an ordered layer of mixed Al and Ni adsorbed in four-fold sites above a second layer composed almost entirely of Ni. The p(1 × 1) is associated with a random mixture of Ni, Al, and a small fraction of vacancies above a second layer composed of Ni. The surface composition of this surface was found to be difficult to equilibrate, as indicated by a long-term variation in composition with repeated cleaning and annealing cycles. The surface rippling previously reported by LEED on NiAl(110) was confirmed by Li + ion scattering which determined that the first layer Al atoms protrude 0.21±0.05 A above the first layer Ni atoms.

XANES studies of the reduction behavior of (Ce1-yZry)O2 and Rh/(Ce1-yZry)O2

Using X-ray absorption near-edge spectroscopy (XANES) at the Ce LIII edge, we have measured the extent of reduction of Rh-loaded and Rh-free, mixed Ce-Zr oxides under hydrogen as a function of temperature. The high surface area, mixed oxides were synthesized by sol-gel techniques and hypercritical drying. Using a simple spectrum subtraction method, the degree of reduction has been measured and compared with previous results for CeO2 and (Ce0.5Zr0.5)O2. Addition of Zr lowers the temperature of reduction and increases the extent of Ce reduction. Rh catalyzes the reduction process at low temperatures but does not substantially affect the extent of reduction achieved at high temperature. A synergism between Rh and Zr is found which leads to very high reducibility in the range of 400–600 K.

Structure of Au15(SR)13 and Its Implication for the Origin of the Nucleus in Thiolated Gold Nanoclusters

Au15(SR)13 is the smallest stable thiolated gold nanocluster experimentally identified so far, and its elusive structure may hold the key to the origin of the nucleus in the formation of thiolated gold nanoclusters. By an extensive exploration of possible isomers by density functional theory, we arrive at a novel structure for Au15(SR)13 with high stability and whose optical absorption characteristics match those of the experiment. Different from the previous structures and the prevailing working hypothesis about the construction of thiolated gold nanoclusters, the Au15(SR)13 model features a cyclic [Au(I)-SR] pentamer interlocked with one staple trimer motif protecting the tetrahedral Au4 nucleus, together with another trimer motif. This structure suggests that Au15(SR)13 is a transitional composition from an [Au(I)-SR]x polymer such as Au10(SR)10 to larger Aun(SR)m (n > m) clusters that have only the staple motifs and that the nucleation process starts from the Au4 core.

Investigation of the structure of Au(110) using angle resolved low energy K+ ion backscattering☆

Abstract The reconstructed Au(110) surface has been studied by angle resolved low energy ion scattering using 600 eV K + as a probe ion. The backscattered intensities and strong multiple scattering features are consistent with very low neutralization probability and indicate the relatively higher sensitivity of alkali ion compared to inert gas ion scattering. The strong dependence upon incidence azimuthal angle attests to sensitivity of the technique as a probe of surface structure. Energy distributions are obtained for the clean and annealed Au(110) surface as a function of polar and azimuthal angles of incidence and of in-plane total scattering angle. The energies and the intensities of the single and multiple scattering features are characterized. Initial analysis of the data indicates that the distorted hexagonal overlayer and the disordered missing row models are unlikely descriptions of the surface. Comparison with computer simulation results indicates that an unrelaxed missing row model is also improbable.

Open-cage fullerene-like graphitic carbons as catalysts for oxidative dehydrogenation of isobutane.

We report herein a facile synthesis of fullerene-like cages, which can be opened and closed through simple thermal treatments. A glassy carbon with enclosed fullerene-like cages of 2-3 nm was synthesized through a soft-template approach that created open mesopores of 7 nm. The open mesopores provided access to the fullerene-like cages, which were opened and closed through heat treatments in air and inert gas at various temperatures. Catalytic measurements showed that the open cages displayed strikingly higher activity for the oxidative dehydrogenation of isobutane in comparison to the closed ones. We anticipate that this synthesis approach could unravel an avenue for pursuing fundamental understanding of the unique catalytic properties of graphitic carbon nanostructures.

In situ phase separation of NiAu alloy nanoparticles for preparing highly active Au/NiO CO oxidation catalysts.

In this communication, we report the synthesis of NiAu alloy nanoparticles (NPs) and their use in preparing Au/NiO CO oxidation catalysts. Because of the large differences in Ni and Au reduction potentials and the immiscibility of the two metals at low temperatures,1, 2 NiAu alloy NP colloids are inherently difficult to prepare by reducing metal salts with common reducing agents. This study describes the first solution-based synthesis of NiAu alloy NPs by way of a fast butyllithium reduction method. By supporting the particles on SiO2 and subsequent conditioning, one obtains a NiO-stabilized Au NP catalyst that exhibits remarkable resistance to sintering and is highly active for CO oxidation. The active NiO-stabilized Au NP catalyst is prepared by in situ phase transformation of NiAu alloy NPs through an Au@Ni core-shell like NP intermediate. In contrast, the corresponding NiO-free Au NPs prepared by an analogous method show negligible low-temperature catalytic activity and a high propensity for coalescence.