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  1. Development of ceria-supported metal-oxide (MOx/CeO2) catalysts via a one-pot chemical vapor deposition (OP-CVD) technique: Structure and reverse water gas shift reaction study

    Pophali, Amol ; Shimogawa, Ryuichi ; Zhang, Lihua ; ... - Chemical Engineering Journal
    Current synthesis techniques for metal oxide (MOx)-supported catalysts have certain limitations of undesired target loading, ineffective dispersion of active species over the surface, uncontrolled particle size of active species, and complicated synthesis steps. Here, we developed a one-pot chemical vapor deposition (OP-CVD) methodology; by using which a solid metal precursor forms a vapor in a controlled condition and gets supported over the surrounding matrix. The theoretical stability followed by experimental validation using TGA is crucial for selecting the metal precursors. Three simple steps viz. premixing, dispersion, and rapid fixation by calcination are involved in the catalyst development via the OP-CVDmore » approach. This study solely focused on the synthesis of 3d transition MOx over ceria support. The physicochemical characterizations of the prepared catalysts were performed by XRD, ICP-OES, SEM-EDX, CO pulse chemisorption, XANES, and EXAFS analyses to understand the crystal structure of involved species, target metal loading, dispersion, and particle size and prove the feasibility and viability of OP-CVD. The prepared catalysts were further tested for reverse water gas shift (RWGS) reaction to link their structural information with activity. The RWGS reaction data showed that the CO activity and CO selectivity were metal - and metal precursor-dependent. Higher CO activity of > 0.1 mol/h g-cat was observed for Cu and Co-based catalysts, with CO selectivity of ~100 %. This study provides an opportunity to produce efficient supported catalysts in a convenient way, providing effective catalytic activity.« less

  2. Does H2 Temperature–Programmed Reduction Always Probe Solid–State Redox Chemistry? The Case of Pt/CeO2

    Lee, Jaeha ; Christopher, Phillip - Angewandte Chemie (International Edition)
    Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature-programmed reduction (H2-TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H-spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2-TPR. Literature often attributes changesmore » in H2-TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H-spillover at Pt-CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Furthermore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single-atoms and nanoclusters demonstrated that H2-TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2-TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.« less

  3. Revisiting the influence of Ni particle size on the hydrogenation of CO2 to CH4 over Ni/CeO2

    Adhikari, Dipesh ; Whitcomb, Colby A. ; Zhang, Weijie ; ... - Journal of Catalysis
    Nickel nanoparticles were supported on CeO2 and evaluated in the hydrogenation of CO2 to CH4 at 538 K and 1 atm. Chemisorption of H2 was used to estimate the average Ni particle size, which ranged from 1.3 to 17 nm. The apparent activation energy for CH4 formation and turnover frequency for CO2 conversion was independent of Ni particle size. The smallest Ni particles (1.3 nm) were less selective to CH4. Comparisons to alumina-supported Ni reveal the same performance as Ni/CeO2 when normalized to active Ni atoms counted by H2 chemisorption. A beneficial effect of ceria relative to alumina is themore » enhanced reducibility of Ni as evaluated by H2 temperature-programmed reduction. The constancy of the turnover frequency on Ni particles suggests the acid-base and/or redox properties of the support do not play a significant role in the CO2 methanation reaction under the conditions of study, which contrasts many earlier works.« less

  4. Co-existence of atomically dispersed Ru and Ce3+ sites is responsible for excellent low temperature N2O reduction activity of Ru/CeO2

    Song, Inhak ; Wang, Yong ; Szanyi, János ; ... - Applied Catalysis. B, Environmental
    Nitrous oxide N2O reduction is a big challenge due to high global warming potential of N2O. (~300 times higher compared with CO2). The best known catalysts, such as Rh/ceria, require relatively high temperatures for N2O decomposition. Herein, we report that Ru/ceria catalysts with low Ru loading of ~0.25 wt% efficiently catalyze low temperature N2O reduction by CO starting at 100 °C (full N2O conversion below 200 °C) under industrially relevant flow rates and gas concentrations. Further, this remarkable performance stems from maintaining isolated Ru cations even on reduced ceria surface and, simultaneously, the propensity of Ru to affect ceria surfacemore » to form labile surface oxygen thereby creating large number of oxygen vacancies (Ce+3 cations) in the presence of CO. In contrast, for Rh/CeO2 catalysts with equivalent metal loading, the activity is much lower because atomically dispersed Rh sinters into metallic clusters at the onset reaction temperature (~200 °C): these clusters are much less effective than isolated single Ru ions, with lower Ce+3 concentration maintained on reduced Rh/CeO2 catalyst. Our study highlights the benefits of gaining molecular-level insight into the dynamic nature of catalytically active sites under reaction conditions for preparing catalysts containing low loading of precious metals with unsurpassed low temperature activity.« less

  5. Conversion of CO2 to Methanol and Ethanol on Pt/CeOx/TiO2(110): Enabling Role of Water in C–C Bond Formation

    Graciani, Jesús ; Grinter, David C. ; Ramírez, Pedro J. ; ... - ACS Catalysis
    The surface chemistry of alcohol synthesis from CO2 hydrogenation has been investigated using kinetic testing, ambient pressure x-ray photoelectron spectroscopy (AP-XPS) and DFT calculations over a multicomponent system where Pt and ceria nanoparticles coexisted on a titania template, Pt/CeOx/TiO2(110). Due to its high ability to bind and activate CO2, not seen for typical Cu-ZnO catalysts, the Pt-CeOx-TiO2 interface is excellent for the hydrogenation of CO2 to methanol with some ethanol also being produced (21% selectivity). The results of AP-XPS and DFT calculations indicate that the active state involves a mixture of Ti4+/Ti3+, Ce3+ and Pt0/Pt+. A fast pathway for themore » formation of CH3O species is only plausible when Ce3+ and Pt are present. The addition of water to the reaction feed facilitates the first hydrogenation of CO2 and substantially enhances the surface coverage of C-containing species (CH3O, HCOO, CO3, CHx), facilitating the formation of C-C bonds and the production of ethanol (38% selectivity).« less

  6. Yield strength of CeO2 measured from static compression in a radial diamond anvil cell

    Huston, Larissa Q. ; Couper, Samantha C. ; Jacobsen, Matthew ; ... - Journal of Applied Physics
    Cerium oxide (ceria, CeO2) is frequently used as a standard in applications such as synchrotron and x-ray free electron lasers for calibrating x-ray wavelengths and offers the potential for understanding the high pressure properties and deformation mechanisms in a wide range of similar face centered cubic (fcc) materials. In this study, the pressure dependence of the strength of ceria was investigated up to 38 GPa using angle dispersive x-ray diffraction in a radial geometry in a diamond anvil cell. In this experiment, the difference in the stress along the axis of compression and perpendicular to the direction of compression canmore » be determined, giving a quantity known as the differential stress. It was found that the differential stress (t), a measure of the lower bound for yield strength, initially increases rapidly from 0.35 ± 0.06 GPa to 2.2 ± 0.4 GPa at pressures of 1.8 and 3.8 GPa, respectively. Above 4 GPa, t increases more slowly to 13.8 ± 2.6 GPa at a pressure of 38 GPa. The changes in the preferred orientation (texture) of CeO2 with pressure were also measured, allowing for the determination of active deformation mechanisms using an elasto-viscoplastic self-consistent model (EVPSC). It was found that as pressure increased, the [001] direction had a slight preferred orientation along the axis of compression. Our EVPSC model of experimental fiber (cylindrically symmetric) textures and lattice strains were most consistent with dominant slip activity along $${111}$$ $$\langle$$$$1\bar10$$$$\rangle$$.« less

  7. Dependency of CO2 methanation on the strong metal-support interaction for supported Ni/CeO2 catalysts

    Pu, Tiancheng ; Chen, Jiacheng ; Tu, Weifeng ; ... - Journal of Catalysis
    The strong metal-support interaction (SMSI) for supported Ni/CeO2 catalysts with different CeO2 nanomorphologies was systematically explored. The degree of encapsulation of Ni particles originating from the SMSI effect was found to follow the trend of Ni/CeO2-(1 1 1) > Ni/CeO2-(1 0 0) > Ni/CeO2-(110 + 100), which parallels the CO2 hydrogenation activity. Quasi in situ XPS reveals the presence of Ce3+ sites in accordance with the formation of an amorphous surface CeOx layer encapsulating the Ni nanoparticles. In situ DRIFTS indicates the reaction pathway and rate-determining step are dependent on the degree of the SMSI effect, leading to distinct selectivitiesmore » towards CH4, especially at a high weight hourly space velocity (WHSV). Finally, these findings present a fundamental strategy about tailoring catalytic performance through support facet dependent susceptibility of SMSI phenomena.« less

  8. Chemical Grafting of Highly Dispersed VOx/CeO2 for Increased Catalytic Activity in Methanol Oxidative Dehydrogenation

    Sudduth, Berlin ; Sun, Junming ; Wang, Yong - Catalysis Letters
    Vanadia supported on well-defned ceria nanocubes are synthesized at various loadings using a liquid-phase chemical grafting technique and are compared to those prepared using traditional incipient wetness impregnation. Raman and IR characterization reveal that, as vanadia loading is increased to near monolayer coverage, vanadia deposited using grafting shows greatly enhanced dispersion (i.e., improved VOx monomer/dimer distribution). Methanol is used as a probe molecule to explore the redox behavior of the catalysts. IR and temperature programmed desorption of methanol show increased CO2 formation occurs on the bare ceria support and increasing the dispersion of vanadia promotes dehydrogenation to formaldehyde due tomore » the inhibited oxygen vacancy formation on VOx/CeO2. Hydrogen temperature programmed reduction demonstrates that the catalyst reducibility and formation of surface oxygen vacancy are directly related to the degree of vanadia oligomerization. In conclusion, the samples prepared using grafting exhibit superior catalytic performance for methanol oxidative dehydrogenation to formaldehyde compared to the impregnated samples, due to the presence of highly dispersed VOx species (i.e., monomers and dimers).« less

  9. Evaluation of ceria as a surrogate material for UO2 in experiments on fuel cracking driven by resistive heating

    Patnaik, S. ; Lopes, D. A. ; Spencer, B. W. ; ... - Nuclear Engineering and Design
    A variety of normal operation and accident scenarios can generate thermal stresses large enough to cause cracking in light-water reactor (LWR) fuel pellets. Cracking of fuel pellets can lead to reduced heat removal, larger centerline temperatures, and localized stress in cladding all of which impact fuel performance. Furthermore, pellet cracking also contributes to a temperature reduction in the pellet since the pellet fragments tend to move towards the heat sink (cladding), and the heat flow remains predominantly radial despite the presence of cracks. It is important to understand the temperature profile on the pellet before and after cracking to improvemore » cracking models in fuel performance codes However, in-reactor observation and measurement of cracking is very challenging owing to the harsh environment and design of fuel rods. Recently, an experimental pellet cracking test stand was developed for separate effects testing of normal operations and accident temperature conditions, using thermal imaging to capture the pellet surface temperature for evaluation of thermal stresses and optical imaging to capture the evolution of cracking in real time. Cracking experiments were initially performed using ceria (CeO2) as a surrogate fuel material, which is useful for developing and demonstrating the experimental approaches but is also valuable in its own right for cracking model development and validation. A combination of induction and resistance heating was used for volumetric heat generation in the pellet creating a thermal gradient. The material properties of CeO2 and UO2 are reviewed and compared for use in model development. Simulations of the experiment were performed to evaluate the behavior of the surrogate (CeO2) fuel in BISON. The measured temperature profiles from BISON models match reasonably well with the observed experiments for the ceria pellets before cracking. The findings from this work will help improve confidence in fracture models used for fuel pellets under similar in-reactor conditions.« less

  10. In situ spectroscopic insights into the redox and acid-base properties of ceria catalysts

    Wang, Xiang ; Li, Meijun ; Wu, Zili - Chinese Journal of Catalysis
    Cerium oxide (ceria) plays an important and fascinating role in heterogeneous catalysis as illustrated by its versatile use as a catalyst, a catalyst support, or a promotor in various oxidation and reduction reactions. Central to these reactions is the rich defect chemistry, facile redox capability, and unusual acid-base properties of ceria. Understanding the unique redox and acid-base properties of ceria is essential to build the structure-catalysis relationship so that improved catalytic functions can be achieved for ceria-based materials. Among the characterization toolbox, spectroscopic approach indisputably stands out for its unparalleled power in offering chemical insights into the surface properties ofmore » ceria at atomic and molecular level. In this review, we summarize advances in revealing the redox and acid-base properties of ceria via a variety of spectroscopic methods including optical, X-ray, neutron, electronic and nuclear spectroscopy. Both direct spectroscopy characterization and its coupling with probe molecules are analyzed to illustrate how the nature, strength and density of different surface sites are influenced by the pretreatment, the morphology and size of ceria nanoparticles. Further directions in taking advantage of in situ/operando spectroscopy for better understanding the catalysis of ceria-based materials are detailed in the summary and outlook section.« less
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