Jonathan C. Hanson

Rapid‐acquisition pair distribution function (RA‐PDF) analysis

An image-plate (IP) detector coupled with high-energy synchrotron radiation was used for atomic pair distribution function (PDF) analysis, with high probed momentum transfer Qmax ≤ 28.5 A−1, from crystalline materials. Materials with different structural complexities were measured to test the validity of the quantitative data analysis. Experimental results are presented for crystalline Ni, crystalline α-AlF3, and the layered Aurivillius type oxides α-Bi4V2O11 and γ-Bi4V1.7Ti0.3O10.85. Overall, the diffraction patterns show good counting statistics, with measuring time from one to tens of seconds. The PDFs obtained are of high quality. Structures may be refined from these PDFs, and the structural models are consistent with the published literature. Data sets from similar samples are highly reproducible.

Biological Control of Crystal Texture: A Widespread Strategy for Adapting Crystal Properties to Function

Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for sea urchin and mollusk calcite crystals, whereas none was found for the foraminifer, Patellina, and the control calcite crystals. These results show that the manipulation of crystal texture in different organisms is under biological control and that crystal textures in some tissues are adapted to function. A better understanding of this apparently widespread biological phenomenon may provide new insights for improving synthetic crystal-containing materials.

Inverse CeO2/CuO Catalyst As an Alternative to Classical Direct Configurations for Preferential Oxidation of CO in Hydrogen-Rich Stream

A novel inverse CeO(2)/CuO catalyst for preferential oxidation of CO in H(2)-rich stream (CO-PROX) has been developed on the basis of a hypothesis extracted from previous work of the group (JACS 2007, 129, 12064). Possible separation of the two competing oxidation reactions involved in the process (of CO and H(2), respectively) is the key to modulation of overall CO-PROX activity and is based on involvement of different sites as most active ones for each of the two reactions. Achievement of large size CuO particles and adequate CeO(2)-CuO interfacial configurations in the inverse catalyst apparently allows appreciable enhancement of the catalytic properties of this kind of system for CO-PROX, constituting an interesting alternative to classic direct configurations so far explored for this process. Reasons for such behavior are analyzed on the basis of operando-XRD, -XAFS, and -DRIFTS studies.

REDUCTION OF CUO IN H2: IN SITU TIME-RESOLVED XRD STUDIES

CuO is used as a catalyst or catalyst precursor in many chemical reactions that involve hydrogen as a reactant or product. A systematic study of the reaction of H2 with pure powders and films of CuO was carried out using in situ time-resolved X-ray diffraction (XRD) and surface science techniques. Oxide reduction was observed at atmospheric H2 pressures and elevated temperatures (150-300 °C), but only after an induction period. High temperature or H2 pressure and a large concentration of defects in the oxide substrate lead to a decrease in the magnitude of the induction time. Under normal process conditions, in situ time-resolved XRD shows that Cu1+ is not a stable intermediate in the reduction of CuO. Instead of a sequential reduction (CuO → Cu4O3 → Cu2O → Cu), a direct CuO → Cu transformation occurs. To facilitate the generation of Cu1+ in a catalytic process one can limit the supply of H2 or mix this molecule with molecules that can act as oxidant agents (O2, H2O). The behavior of CuO-based catalysts in the synthesis of methanol and methanol steam reforming is discussed in the light of these results.

Real space refinement of neutron diffraction data from sperm whale carbonmonoxymyoglobin

Abstract Neutron diffraction data from crystals of sperm whale carbonmonoxymyoglobin have been refined by the real space refinement technique. Estimates of the neutron occupancies at the end of the refinement show that the mean for each atom type (including hydrogen and deuterium) is close to the expected value and has a standard deviation from the mean of about 5%. Mean neutron occupancies of main-chain atoms involved in deuterium bonds versus those not involved in deuterium bonds demonstrate that the hydrogen/deuterium exchange of the latter group is higher. The oxygen and deuterium co-ordinates for 40 water molecules have been determined: 27 of these water molecules were involved in bridges between protein atoms, and nine were involved in deuterium bonds with main-chain atoms. The deuterium-bond angles in helical regions show significant deviations from linearity. The mean ND … O angle was 154(3) ° † and the mean CO … D angle was 145(3) °.

A versatile sample-environment cell for non-ambient X-ray scattering experiments

A compact reaction cell is described for in-situ experiments requiring control of both the temperature of the sample and the atmosphere over the sample. The cell incorporates an optional furnace capable of temperatures of up to {approx} 1273 K. The compact design and ability of the cell to mount directly on a standard goniometer head allows portability to a large number of diffraction instruments at synchrotron sources.

On the akaganeite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts

Abstract The crystal structure of akaganeite and the akaganeite to hematite transition has been studied by means of conventional and synchrotron X-ray and neutron powder diffraction. The chemical formula of akaganeite can be written as FeO 0.833 (OH) 1.167 Cl 0.167 . The crystal structure does not contain free water. Heating below 200 °C will not alter the akaganeite structure. Initial water loss can be attributed to a large amount of adsorbed water due to a very small particle size; 0.15 μm by 0.03 μm. Chloride is released from the structure only in connection with the transformation to hematite. Due to its stability, the presence of akaganeite does not in itself posses a threat to iron artifacts, but it is rather a symptom of the presence of high concentrations of chloride in an acidic environment.

Non-framework cation migration and irreversible pressure-induced hydration in a zeolite

Zeolites crystallize in a variety of three-dimensional structures in which oxygen atoms are shared between tetrahedra containing silicon and/or aluminium, thus yielding negatively charged tetrahedral frameworks that enclose cavities and pores of molecular dimensions occupied by charge-balancing metal cations and water molecules. Cation migration in the pores and changes in water content associated with concomitant relaxation of the framework have been observed in numerous variable-temperature studies, whereas the effects of hydrostatic pressure on the structure and properties of zeolites are less well explored. The zeolite sodium aluminosilicate natrolite was recently shown to undergo a volume expansion at pressures above 1.2 GPa as a result of reversible pressure-induced hydration; in contrast, a synthetic analogue, potassium gallosilicate natrolite, exhibited irreversible pressure-induced hydration with retention of the high-pressure phase at ambient conditions. Here we report the structure of the high-pressure recovered phase and contrast it with the high-pressure phase of the sodium aluminosilicate natrolite. Our findings show that the irreversible hydration behaviour is associated with a pronounced rearrangement of the non-framework metal ions, thus emphasizing that they can clearly have an important role in mediating the overall properties of zeolites.

The behavior of mixed-metal oxides: Physical and chemical properties of bulk Ce1−xTbxO2 and nanoparticles of Ce1−xTbxOy

The physical and chemical properties of bulk Ce(1-x)Tb(x)O(2) and Ce(1-x)Tb(x)O(y) nanoparticles (x<or =0.5) were investigated using synchrotron-based x-ray diffraction (XRD), x-ray adsorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density-functional (DF) calculations. DF results and Raman spectra point to a small tetragonal distortion after introducing terbium in ceria. The results of XRD show a small contraction (< or = 0.08 A) in the cell dimensions. The presence of Tb generates strain in the lattice through the variation of the ionic radii and creation of crystal imperfections and O vacancies. The strain increases with the content of Tb and affects the chemical reactivity of the Ce(1-x)Tb(x)O(y) nanoparticles towards hydrogen, SO(2), and NO(2). DF calculations for bulk Ce(1-x)Tb(x)O(2) and Ce(8-n)Tb(n)O(16) (n=0, 1, 2, or 4) clusters show oxide systems that are not fully ionic. The theoretical results and XANES spectra indicate that neither a Ce<-->Tb exchange nor the introduction of oxygen vacancies in Ce(1-x)Tb(x)O(y) significantly affect the charge on the Ce cations. In contrast, the O K-edge and Tb L(III)-edge XANES spectra for Ce(1-x)Tb(x)O(y) nanoparticles show substantial changes with respect to the corresponding spectra of Ce and Tb single oxide references. The Ce(0.5)Tb(0.5)O(y) compounds exhibit a much larger Tb(3+)/Tb(4+) ratio than TbO(1.7). A comparison with the properties of Ce(1-x)Zr(x)O(y) and Ce(1-x)Ca(x)O(y) shows important differences in the charge distribution, the magnitude of the dopant induced strain in the oxide lattice, and a superior behavior in the case of the Ce(1-x)Tb(x)O(y) systems. The Tb-containing oxides combine stability at high temperature against phase segregation and a reasonable concentration of O vacancies, making them attractive for chemical and catalytic applications.

Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies

CeO2 and Cu/CeO2 are effective catalysts/sorbents for the removal or destruction of SO2. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO2 with pure and reduced CeO2 powders, ceria films (CeO2, CeO2−x, Ce2O3+x) and model Cu/CeO2 catalysts. The results of XANES and photoemission provide evidence that SO4 was formed upon the adsorption of SO2 on pure powders or films of CeO2 at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO2 and some SO3 evolving into gas phase. At 670 K, there was still a significant amount of SO4 present on the CeO2 substrates. The introduction of O vacancies in the CeO2 powders or films favored the formation of SO3 instead of SO4. Ceria was able to fully dissociate SO2 to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO2 new active sites for the destruction of SO2 were created improving the catalytic activity of the system. The surface chemistry of SO2 on the Cu‐promoted CeO2 was much richer than on pure CeO2. The behavior of ceria in several catalytic processes (oxidation of SO2 by O2, reduction of SO2 by CO, automobile exhaust converters) is discussed in light of these results.