Eric N. Coker

Sr- and Mn-doped LaAlO3−δ for solar thermochemical H2 and CO production

The increasing global appetite for energy within the transportation sector will inevitably result in the combustion of more fossil fuel. A renewable-derived approach to carbon-neutral synthetic fuels is therefore needed to offset the negative impacts of this trend, which include climate change. In this communication we report the use of nonstoichiometric perovskite oxides in two-step, solar-thermochemical water or carbon dioxide splitting cycles. We find that LaAlO3 doped with Mn and Sr will efficiently split both gases. Moreover the H2 yields are 9× greater, and the CO yields 6× greater, than those produced by the current state-of-the-art material, ceria, when reduced at 1350 °C and re-oxidized at 1000 °C. The temperature at which O2 begins to evolve from the perovskite is fully 300 °C below that of ceria. The materials are also very robust, maintaining their redox activity over at least 80 CO2 splitting cycles. This discovery has profound implications for the development of concentrated solar fuel technologies.

Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3

A facile and general method has been developed to fabricate oxygen vacancies on perovskite SrTiO3 (STO) nanocrystals through a controllable solid-state reaction of NaBH4 and SrTiO3 nanocrystals. STO samples with tunable color, oxygen vacancy concentration on nanocrystal surface have been synthesized. TEM results reveal that these STO samples have a crystalline core/amorphous shell structure (SrTiO3@SrTiO3-x). XPS and EPR results disclose that the oxygen vacancy concentration increases with the increase of reaction time and temperature. The concentration of oxygen vacancies calculated from TGA data, could reach 5.07% (atom) in this study. UV-vis spectra and photocatalytic results indicate that oxygen vacancies on STO surface play an important role in influencing the light absorption and photocatalytic performance. However, an excess amount of oxygen vacancies leads to a decrease of photocatalytic performance. The optimal photocatalytic activity for H2 production under UV-vis irradiation is up to 2.2 mmol h(-1) g(-1), which is about 2.3 times than the original SrTiO3, corresponding to 3.28% (atom) of oxygen vacancy concentration.

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

Establishing processing–structure–property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔGH), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔGH from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

Porous one-dimensional nanostructures through confined cooperative self-assembly.

We report a simple confined self-assembly process to synthesize nanoporous one-dimensional photoactive nanostructures. Through surfactant-assisted cooperative interactions (e.g., π-π stacking, ligand coordination, and so forth) of the macrocyclic building block, zinc meso-tetra (4-pyridyl) porphyrin (ZnTPyP), self-assembled ZnTPyP nanowires and nanorods with controlled diameters and aspect ratios are prepared. Electron microscopy characterization in combination with X-ray diffraction and gas sorption experiments indicate that these materials exhibit stable single-crystalline and high surface area nanoporous frameworks with well-defined external morphology. Optical characterizations using UV-vis spectroscopy and fluorescence imaging and spectroscopy show enhanced collective optical properties over the individual chromophores (ZnTPyP), favorable for exciton formation and transport.

Ferrite-YSZ composites for solar thermochemical production of synthetic fuels: in operando characterization of CO2 reduction

Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy for the production of hydrogen from water, or carbon monoxide from carbon dioxide. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves the cyclability of the ferrites and enables a move away from powder to monolithic systems. This synergistic effect is only partially understood. In order to unravel the underlying mechanisms of the effect and to understand the evolution of thermochemically active phases, we have studied the behaviour of iron oxides co-sintered with 8YSZ (8 mol% Y2O3) using in operando X-ray diffraction and thermogravimetric analysis at temperatures up to 1500 °C and under environments representative of those present in a thermochemical cycle. The solubility of iron oxide in 8YSZ measured by XRD at room temperature, following calcination to 1500 °C in air, was 9.4 mol% Fe. The solubility increased to at least 10.4 mol% Fe when heated between 800 and 1000 °C under inert (He) atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing insoluble iron (i.e., containing >9.4 mol% Fe) stepwise heating to 1400 °C under helium caused reduction of Fe2O3 (hematite) to Fe3O4 (magnetite) to FeO (wustite). This gradual thermal reduction from hematite to wustite was accompanied by evolution of oxygen. The wustite remained stable upon cooling to room temperature in the helium environment, although after multiple consecutive cycles some of the wustite was observed to disproportionate to Fe metal and magnetite. Exposure of the wustite-containing material to CO2 at 1100 °C enabled re-oxidation of the wustite to magnetite with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with iron oxide contents between 1.8 and 27.6 mol% Fe showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a significant fraction of un-dissolved iron oxides.

Oxygen transport and isotopic exchange in iron oxide/YSZ thermochemically-active materials via splitting of C(18O)2 at high temperature studied by thermogravimetric analysis and secondary ion mass spectrometry

Ferrites are promising materials for enabling solar-thermochemical cycles for the production of synthetic fuels. Such cycles utilize solar-thermal energy for the production of hydrogen from water, or carbon monoxide from carbon dioxide. Recent work studying the thermochemical behaviour of iron oxides co-sintered with yttria-stabilised zirconia (YSZ) using thermogravimetric analysis revealed a striking difference in behaviour of iron that is in solid solution with the YSZ and that which exists as a second iron oxide phase. Materials in which the majority of iron was dissolved in the YSZ exhibited enhanced utilization of iron over those which possessed larger fractions of un-dissolved, bulk iron oxides. To illuminate this phenomena further, several samples of thermally-reduced iron oxide/8YSZ were re-oxidised using isotopically labelled C(18O)2. Post mortem characterization by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), with the application of multivariate analysis tools, enables the differentiation between 18O and 16O signals emanating from iron oxide particles. The distribution of 18O is uniform throughout the iron-doped 8YSZ, but concentrated at the surface of iron oxide particles embedded in this matrix. After identical thermal reduction and re-oxidation treatments, the gradient of 18O/16O across the iron oxide particles is found to depend on the size of the iron oxide particles, as well as the method of synthesis of the iron oxide/YSZ material. Comparative thermogravimetric analyses of the 18O-labelled materials and analogous un-labelled materials revealed that exposure to CO2 at 1100 °C results in rapid oxygen isotopic exchange.

Chapter 10 Ion Exchange in Zeolites

Summary Aspects of ion exchange in zeolites are reviewed, with special reference to those properties of zeolites which give rise to characteristic, and sometimes unique, ion exchange behaviour. As well as discussing basic principles, some thermodynamic and kinetic aspects of the theory of ion exchange are covered with particular reference to their utility for predicting exchange behaviour in zeolites and for the use of ion exchange technqiues in detergency building, radionuclide separation, waste water treatment and the preparation of zeolitic catalysts and sorbents. Recent developments on the use of zeolites in detergency are next reviewed, in more detail, together with progress in the field of ion exchange using high silica zeolites or solid state techniques.

Predicting the solar thermochemical water splitting ability and reaction mechanism of metal oxides: a case study of the hercynite family of water splitting cycles

A screening method is developed to determine the viability of candidate redox materials to drive solar thermal water splitting (STWS) and the mechanism by which they operate using only the reduction enthalpy of the material. This method is applied to the doped-hercynite water splitting cycle, as well as FeAl2O4 and CoAl2O4, materials which have not been previously experimentally demonstrated for STWS. Density functional theory (DFT) calculations of reduction energies coupled with our screening method predict H2 production capacities for iron and cobalt aluminate spinels to be in the order FeAl2O4 > Co0.5Fe0.5Al2O4 > CoAl2O4 with relative H2 production capacity ratios of approximately 1.0 to 0.7 to 2 × 10−4, respectively. Experimental measurements for 1500/1350 °C redox temperatures validate the H2 production capacity predicted by the screening method by demonstrating H2 production ratios of 1.0 to 0.6 to 0. Un-doped hercynite (FeAl2O4) is shown to be a viable STWS material for the first time with a higher H2 production capacity than traditional doped-hercynite materials. Theory and experiments show that redox of the aluminate family of spinel materials operates via an O-vacancy mechanism rather than a stoichiometric one, which is more typical for ferrites. The screening approach is generally useful for predicting the ability of new complex materials to drive STWS and the mechanism by which they operate, thus, providing a method to identify promising new candidate STWS materials.

The preparation and characterization of novel Pt/C electrocatalysts with controlled porosity and cluster size

Small platinum clusters have been prepared in zeolite hosts through ion exchange and controlled calcination/reduction processes. To enable electrochemical application, the pores of the Pt-zeolite were filled with electrically conductive carbon via infiltration with carbon precursors, polymerization, and pyrolysis. The zeolite host was then removed by acid washing, to leave a Pt/C electrocatalyst possessing quasi-zeolitic porosity and Pt clusters of well-controlled size. The electrocatalysts were characterized by TEM, XRD, EXAFS, nitrogen adsorption and electrochemical techniques. Depending on the synthesis conditions, average Pt cluster sizes in the Pt/C catalysts ranged from 1.3 to 2.0 nm. The presence of ordered porosity/structure in the catalysts was evident in TEM images as lattice fringes, and in XRD as a low-angle diffraction peak with d-spacing similar to the parent zeolite. The catalysts possess micro- and meso-porosity, with pore size distributions that depend upon synthesis variables. Electroactive surface areas as high as 112 m2 gPt−1 have been achieved in Pt/C electrocatalysts which show oxygen reduction performance comparable to standard industrial catalysts.

Ion exchange in beryllophosphate-G. Part 1.—Ion-exchange equilibria

Ion-exchange isotherms and selectivity plots are presented for beryllophosphate-G, the synthetic beryllophosphate analogue of the zeolite gismondine. Exchange reactions involving Na+ and K+ or NH+4 were accompanied by changes in crystal symmetry and hysteresis loops were seen in the isotherms. Exchange of K+ by NH+4 was fully reversible without hysteresis and the two end members were found to have very similar symmetry. Only Limited exchange of Na+ by Ca2+ or Mg2+ could be achieved under the conditions used and no symmetry change was observed.The phases present across the Na+–K+ exchange field were studied in depth using X-ray powder diffraction, solid-state NMR and scanning electron microscopy. Unit-cell parameters were calculated for fully K-exchanged beryllophosphate-G.