Metal-Support Interactions in CeO2- and SiO2-Supported Cobalt Catalysts: Effect of Support Morphology, Reducibility, and Interfacial Configuration.

With the increasing demand for highly efficient and durable catalysts, researchers have been doing extensive research to engineer the shape, size, and even phase (e.g., hcp or fcc Co) of individual catalyst nanoparticles, as well as the interface structure between the catalyst and support. In this work, cobalt oxides were deposited on ceria with rod-like morphology (CeO2NR) and cube-like morphology (CeO2NC) and silica with sphere-like morphology (SiO2NS) via a precipitation-deposition method to investigate the effects of support morphology, surface defects, support reducibility, and the metal-support interactions on redox and catalytic properties. XRD, Raman, XPS, BET, H2-TPR, O2-TPD, CO-TPD, TEM, and TPR/TPO cycling measurements have been mainly employed for catalysts characterization. Compared with CeO2NC and SiO2NS supports, as well as CeO2NC- and SiO2NS-supported cobalt catalysts, CeO2NR counterparts exhibited enhanced reducibility and CO oxidation performance at a lower temperature. Both the apparent activation energy and CO conversion demonstrated the following catalytic activity order: 10 wt % CoO x/CeO2NR > 10 wt % CoO x/CeO2NC > 10 wt % CoO x/SiO2NS. These results showed a strong support-dependent reducibility, CO oxidation, and redox cycling activity/stability of the as-prepared catalysts. Moreover, the significantly enhanced catalytic CO oxidation of the 10 wt % CoO x/CeO2NR catalyst indicated the vital role of CeO2NR support with rich surface oxygen vacancies, superior oxygen storage capacity and mobility, and excellent adsorption/desorption behavior of CO and O2 species.

Enhanced intermediate-temperature CO2 splitting using nonstoichiometric ceria and ceria-zirconia.

CO2 splitting via thermo-chemical or reactive redox has emerged as a novel and promising carbon-neutral energy solution. Its performance depends critically on the properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface ion-incorporation pathways, along with the role of surface defects and the adsorbates remain largely unknown. This study presents a detailed kinetics study of CO2 splitting using CeO2 and Ce0.5Zr0.5O2 (CZO) in the temperature range 600-900 °C. Given our interest in fuel-assisted reduction, we limit our study to relatively lower temperatures to avoid excessive sintering and the need for high temperature heat. Compared to what has been reported previously, we observe higher splitting kinetics, resulting from the utilization of fine particles and well-controlled experiments which ensure a surface-limited-process. The peak rates with CZO are 85.9 µmole g-1 s-1 at 900 °C and 61.2 µmole g-1 s-1 at 700 °C, and those of CeO2 are 70.6 µmole g-1 s-1 and 28.9 µmole g-1 s-1. Kinetic models are developed to describe the ion incorporation dynamics, with consideration of CO2 activation and the charge transfer reactions. CO2 activation energy is found to be -120 kJ mole-1 for CZO, half of that for CeO2, while CO desorption energetics is analogous between the two samples with a value of ∼160 kJ mole-1. The charge-transfer process is found to be the rate-limiting step for CO2 splitting. The evolution of CO32- with surface Ce3+ is examined based on the modeled kinetics. We show that the concentration of CO32- varies with Ce3+ in a linear-flattened-decay pattern, resulting from a mismatch between the kinetics of the two reactions. Our study provides new insights into the significant role of surface defects and adsorbates in determining the splitting kinetics.

Toward enhanced hydrogen generation from water using oxygen permeating LCF membranes.

Hydrogen production from water thermolysis can be enhanced by the use of perovskite-type mixed ionic and electronic conducting (MIEC) membranes, through which oxygen permeation is driven by a chemical potential gradient. In this work, water thermolysis experiments were performed using 0.9 mm thick La0.9Ca0.1FeO3-δ (LCF-91) perovskite membranes at 990 °C in a lab-scale button-cell reactor. We examined the effects of the operating conditions such as the gas species concentrations and flow rates on the feed and sweep sides on the water thermolysis rate and oxygen flux. A single step reaction mechanism is proposed for surface reactions, and three-resistance permeation models are derived. Results show that water thermolysis is facilitated by the LCF-91 membrane especially when a fuel is added to the sweep gas. Increasing the gas flow rate and water concentration on the feed side or the hydrogen concentration on the sweep side enhances the hydrogen production rate. In this work, hydrogen is used as the fuel by construction, so that a single-step surface reaction mechanism can be developed and water thermolysis rate parameters can be derived. Both surface reaction rate parameters for oxygen incorporation/dissociation and hydrogen-oxygen reactions are fitted at 990 °C. We compare the oxygen fluxes in water thermolysis and air separation experiments, and identify different limiting steps in the processes involving various oxygen sources and sweep gases for this 0.9 mm thick LCF-91 membrane. In the air feed-inert sweep case, the bulk diffusion and sweep side surface reaction are the two limiting steps. In the water feed-inert sweep case, surface reaction on the feed side dominates the oxygen permeation process. Yet in the water feed-fuel sweep case, surface reactions on both the feed and sweep sides are rate determining when hydrogen concentration in the sweep side is in the range of 1-5 vol%. Furthermore, long term studies show that the surface morphology changes and silica impurities have little impact on the oxygen flux for either water thermolysis or air separation.

Temperature measurements in a rapid compression machine using mid-infrared H2O absorption spectroscopy near 7.6 µm.

A method for measuring the temporal temperature and number density in a rapid compression machine (RCM) using quantum cascade laser absorption spectroscopy near 7.6 µm is developed and presented in this paper. The ratios of H(2)O absorption peaks at 1316.55 cm(-1) and 1316.97 cm(-1) are used for these measurements. In order to isolate the effects of chemical reactions, an inert mixture of argon with 2.87% water vapor is used for the present investigation. The end of compression pressures and temperatures in the RCM measurements are P(C)=10, 15, and 20 bar in the range of T(C)=1000 to 1200 K. The measured temperature history is compared with that calculated based on the adiabatic core assumption and is found to be within ±5 K. The measured temporal number density of H(2)O to an accuracy of 1%, using the absolute absorption of the two rovibrational lines, show that the mixture is highly uniform in temperature. A six-pass, 5.08 cm Herriott cell is used to calibrate the line strengths in air and broadening in an Ar bath gas.