Martin Schreyer

Dual-Phase Titanate/Anatase with Nitrogen Doping for Enhanced Degradation of Organic Dye under Visible Light

The need for environmental remediation processes on a large scale is becoming ever more urgent, especially in anticipation of the increasing demand (and potential shortage) of potable water supplies for a growing world population. Among the armory of advanced oxidation technologies (AOTs), photocatalytic (solar-light-driven) processes are particularly attractive, and photocatalysts have a well-demonstrated potential to mineralize harmful organic substances in air and water and even to act as regenerable adsorbents for toxic heavy metal ions, some of these being recovered as photodeposited metals. [1] Although anatase TiO2 remains the most popular photocatalyst due to high catalytic activity and chemical stability, there are some drawbacks associated with it. The activity is confined to UV-light stimulation, representing just a few percent of the solar-power spectrum. In this respect, much research has been done in modifying the bandgap of the material to extend the absorption into the visible-light region. [2] In addition, the adsorptive properties of TiO2 are not ideal either. [3] Since photoreactions take place at or near the catalyst surface, surface adsorption is critical for efficient interfacial charge transfer to and from the target molecules. In contrast, titanate materials have recently been identified as superior adsorbents for, for example, organic dyes and heavy metal ions. [4] The crystal structure consists of layers of TiO6 octahedra in edge connectivity with protons or alkali metal ions localized between the layers. [5] Various one-dimensional structures, including nano

Five-Dimensional Incommensurate Structure of the Melilite Electrolyte [CaNd]2[Ga]2[Ga2O7]2

Melilite-type gallium oxides are potential intermediate temperature electrolytes for solid oxide fuel cells. Single crystals of [CaNd](2)[Ga](2)[Ga(2)O(7)](2) grown using an optical floating zone furnace have been investigated using transmission electron microscopy and powder and single-crystal X-ray diffraction. The anion array topologically conforms to a [(3.5.4.5)(2), 3.5.3.5] network that contains distorted pentagonal tunnels. The distortion is necessary to achieve space filling and accommodate structural misfit between the layers. Satisfactory bond lengths and angles are obtained through two-dimensional modulation in the tetragonal based plane, leading to five-dimensional symmetry in the superspace group P(4⁻)2(1)m(α,α,0)00s((a⁻)a,0)000, α = 0.2319(2), with modulation vectors q(1) = α(a* + b*) and q(2) = α(-a* + b*). Both displacive and occupational modulations are found. Through this mechanism, melilites are primed to accommodate mobile oxygen interstitials, suggesting a rational approach to crystallochemical tailoring that will enhance ionic diffusion and optimize electrolyte performance.

Concurrent synergism and inhibition in bimetallic catalysis: catalytic binuclear elimination, solute-solute interactions and a hetero-bimetallic hydrogen-bonded complex in rh-mo hydroformylations.

Hydroformylations of cyclopentene and 3,3-dimethylbut-1-ene were performed using both Rh(4)(CO)(12) and (eta(5)-C(5)H(5))Mo(CO)(3)H as precursors in n-hexane at 298 K. Both stoichiometric and catalytic hydroformylations were conducted as well as isotopic labeling experiments. Six organometallic pure component spectra were recovered from the high-pressure FTIR experiments, namely the known species Rh(4)(CO)(12), (eta(5)-C(5)H(5))Mo(CO)(3)H, RCORh(CO)(4), and the new heterobimetallic complexes RhMo(CO)(7)(eta(5)-C(5)H(5)), a weak hydrogen bonded species (eta(5)-C(5)H(5))Mo(CO)(3)H-C(5)H(9)CORh(CO)(4), and a substituted RhMo(CO)(7-y)(eta(5)-C(5)H(5))L(y), where y = 1 or 2 and L = (pi-C(5)H(8)). The main findings were (1) catalytic binuclear elimination (CBER) occurs between (eta(5)-C(5)H(5))Mo(CO)(3)H and RCORh(CO)(4) resulting in aldehyde and RhMo(CO)(7)(eta(5)-C(5)H(5)), and this mechanism is responsible for ca. 10% of the product formation; (2) molecular hydrogen is readily activated by the new heterobimetallic complex(es); (3) FTIR and DFT spectroscopic evidence suggests that the weak hydrogen bonded species (eta(5)-C(5)H(5))Mo(CO)(3)H-C(5)H(9)CORh(CO)(4) has an interaction of the type eta(5)-C(5)H(4)-H...O=C; and (4) independent physicochemical experiments for volumes of interaction confirm that significant solute-solute interactions are present. With respect to the efficiency of the catalytic cycle, the formation of a weak (eta(5)-C(5)H(5))Mo(CO)(3)H-C(5)H(9)CORh(CO)(4) complex results in a significant decrease in the measured turnover frequency (TOF) and is the primary reason for the inhibition observed in the bimetallic catalytic hydroformylation. Such hydrogen bonding through the eta(5)-C(5)H(5) ring might have relevance to inhibition observed in other catalytic metallocene systems. The present catalytic system is an example of concurrent synergism and inhibition in bimetallic homogeneous catalysis.

A multi-domain gem-grade Brazilian apatite

Abstract A gem-grade apatite from Brazil of general composition (Ca,Na)10[(P,Si,S)O4]6(F,Cl,OH)2 has been studied using single-crystal X-ray and neutron diffraction together with synchrotron powder X-ray diffraction. Earlier electron microscopy studies had shown the nominally single-phase apatite contains an abundant fluorapatite (F-Ap) host, together with chloro-hydroxylapatites (Cl/OH-Ap) guest phases that encapsulate hydroxylellestadite (OH-El) nanocrystals. While the latter features appear as small (200-400 nm) chemically distinct regions by transmission electron microscopy, and can be identified as separate phases by synchrotron powder X-ray diffraction, these could not be detected by singlecrystal X-ray and neutron analysis. The observations using neutron, X-ray and electron probes are however consistent and complementary. After refinement in the space group P63/m the tunnel anions F− are fixed at z = ¼ along <001>, while the anions Cl− and OH− are disordered, with the suggestion that O-H···O-H··· hydrogen-bonded chains form in localized regions, such that no net poling results. The major cations are located in the 4f AFO6 metaprism (Ca+Na), 6h ATO6X tunnel site (Ca only), and 6h BO4 tetrahedron (P+Si+S). The structural intricacy of this gem stone provides further evidence that apatite microstructures display a nano-phase separation that is generally unrecognized, with the implication that such complexity may impact upon the functionality of technological analogues.

Nonstoichiometry, amorphicity and microstructural evolution during phase transformations of photocatalytic titania powders

The performance of photocatalytic titania powders is regulated, in part, by nonstoichiometry and the proportions of the crystalline and amorphous components. These variables can be quantitatively established by Rietveld analysis of diffraction data when internal standards are used to fix absolutely the crystallochemical parameters during quantitative phase analysis and to correct for mass absorption. Here, fixed-wavelength neutron and multiple-wavelength X-ray powder diffraction are used to assess phase development in alkoxide-derived titania gel as a function of temperature. In this manner, it is shown that the amorphous gel is progressively replaced by anatase for temperatures ≤ 773 K, and that during the reconstructive transition to rutile (773–873 K) aperiodicity increases as anatase is broken down to clusters of TiO6 octahedra, with a fraction (∼10 wt%) of this short-range order persisting to 1273 K. Microabsorption correlates with X-ray energy, leading to systematic aberrations in the Rietveld scale factors connected to microstructural evolution which accompanies phase development during heat treatment. These changes are consistent with encapsulation of anatase and rutile by ubiquitous non-diffracting materials. The appearance of significant quantities of an intervening disordered phase during the dimorph transformation is supportive of recent kinetic models; however, its impact on catalytic activity remains to be determined.

The influence of stereochemically active lone-pair electrons on crystal symmetry and twist angles in lead apatite-2H type structures

Abstract Lead-containing (Pb-B-X)-2H apatites encompass a number of [AF]4[AT]6(BO4)6]X2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb2+ on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb2+ at both the AF and AT results in substantial adjustments to the PbFO6 metaprism twist angle, φ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2H (ideally Pb10(VO4)6Cl2), pyromorphite-2H (Pb10PO4)6Cl2), mimetite-2H/M (Pb10(As5+O4)6Cl2) and finnema-nite-2H (Pb10(As3+O3)6Cl2). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s2 lone-pair electrons on the Pb2+ that occupy a volume similar to an O2- anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials arc found at similar positions to the lone-pair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the PbFO6 metaprism twist-angles (φ) as the temperature changes. Finally, crystal-chemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.