Michael Tovar

The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts

Mechanisms in Methanol Catalysis The industrial production of methanol from hydrogen and carbon monoxide depends on the use of copper and zinc oxide nanoparticles on alumina oxide supports. This catalyst is “structure sensitive”; its activity can vary by orders of magnitude, depending on how it is prepared. Behrens et al. (p. 893, published online 19 April; see the Perspective by Greeley) used a combination of bulk and surface-sensitive analysis and imaging methods—along with insights from density functional theory calculations—to study several catalysts, including the one similar to that used industrially. High activity depended on the presence of steps on the copper nanoparticles stabilized by defects such as stacking faults. Partial coverage of the copper nanoparticles with zinc oxide was critical for stabilizing surface intermediates such as HCO and lowering energetic barriers to the methanol product. Catalysis is favored by stepped copper nanoparticles decorated with zinc oxide, which promotes stronger intermediate binding. One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al2O3 methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

A neutron diffraction study of the stannite-kesterite solid solution series

The intermixing of stannite and kesterite, i.e. Cu 2 Fe 1−x Zn x SnS 4 , was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ x ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 750°C and quenched after the final annealing. The lattice parameter, cation site occupancies and the isotropic temperature factors were determined by simultaneously Rietveld analysis of X-ray and neutron powder diffraction data. The refined lattice constants are in agreement with the literature. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in stannite, kesterite and Cu 2 Fe 1−x Zn x SnS 4 solid solutions, giving new insight into the cation distribution. For Cu 2 FeSnS 4 the stannite type cation ordering was approved, whereas a new ordering was obtained in Cu 2 ZnSnS 4 . In the latter Cu occupies the 2a site, Zn and the remaining Cu are disordered on 2c (0, ½, ¼) and 2d (0, ¼, ¾). The disorder may be due to sample synthesis, i.e. to the quenching of the samples. The cross-over from stannite to kesterite by Fe 2+ ↔ Zn 2+ substitution in the Cu 2 Fe 1−x Zn x SnS 4 solid solution can be foreseen as a 3-stage process of cation exchange among the positions at (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu + , Zn 2+ and Fe 2+ . The Sn 4+ cation does not take part in this process and remains on the 2b site. Moreover the crossover is also visible in the ratio of the lattice parameter c/(2a), showing a characteristic dependence on chemical composition. The refined isotropic temperature factors were also taken into account. They give a consistent picture and to a degree support the developed cation distribution.

Comprehensive insights into point defect and defect cluster formation in CuInSe2

The concentration of native point defects in CuInSe2 powder material as a function of stoichiometry has been experimentally determined by neutron powder diffraction. A correlation between the Cu/In ratio and the density of VCu as well as InCu has been established and their concentrations are quantified. It is demonstrated, that assuming the spontaneous formation of defect pairs, the density of native point defects is reduced significantly by an order of magnitude. The functionality of a solar device, assuming same conditions like in the analyzed material, may be explained by a neutralization due to the formation of electrically inactive defect complexes.

Microstructural and Defect Analysis of Metal Nanoparticles in Functional Catalysts by Diffraction and Electron Microscopy: The Cu/ZnO Catalyst for Methanol Synthesis

The application of different methods for a microstructural analysis of functional catalysts is reported for the example of different Cu/ZnO-based methanol synthesis catalysts. Transmission electron microscopy and diffraction were used as complementary techniques to extract information on the size and the defect concentration of the Cu nano-crystallites. The results, strengths and limitations of the two techniques and of different evaluation methods for line profile analysis of diffraction data including Rietveld-refinement, Scherrer- and (modified) Williamson–Hall-analyses, single peak deconvolution and whole powder pattern modeling are compared and critically discussed. It was found that in comparison with a macrocrystalline pure Cu sample, the catalysts were not only characterized by a smaller crystallite size, but also by a high concentration of lattice defects, in particular stacking faults. Neutron diffraction was introduced as a valuable tool for such analysis, because of the larger number of higher-order diffraction peaks that can be detected with this method. An attempt is reported to quantify the different types of defects for a selected catalyst.

Magnetic and piezoelectric properties of the Bi1 − xLaxFeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi1 − xLaxFeO3 solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.

Shocked quartz in Sahara fulgurite

Comparative strain analyses were performed on quartz crystals within a Sahara fulgurite using neutron diffraction experiments. Besides, 3D-μXRCT (three-dimensional micro X-ray computed tomography) studies of the fulgurite were implemented in order to quantify the fusion behaviour, the expansion of the fulgurite as well as the distribution of the stressed quartz crystals. The study of the neutron diffraction pattern was carried out by applying an integral breadth method, more precisely a Rietveld and a LeBail strain–size evaluation with the software FullProf. The absorption contrast of the neutron diffraction pattern shows a concentration of the shocked quartz crystals in the outer shell of the Sahara fulgurite, while the inner part of the fulgurite is mostly glassy. On the basis of the diffraction results, the quartz crystals in the fulgurite can be assigned to a stage of deformation for which the causal shock pressure reaches about 25 GPa, as compared to a melting threshold pressure for quartz of about 50 GPa. Comparing the average density of the fulgurite with the average bulk density of quartz-based sand, it can be concluded that the fulgurite expanded by more than 35 % during its formation.

Long-range structure of Cu(InxGa1−x)3Se5: A complementary neutron and anomalous x-ray diffraction study

Distinguishing the scattering contributions of isoelectronic atomic species by means of conventional x-ray- and/or electron diffraction techniques is a difficult task. Such a problem occurs when determining the crystal structure of compounds containing different types of atoms with equal number of electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid solution. Our model is based on neutron and anomalous x-ray diffraction experiments. These complementary techniques allow the separation of scattering contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly identically in monoenergetic x-ray diffraction experiments. We have found that CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification exhibits a modified stannite structure (space group I4¯2m). Different occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies have been found at three different cationic pos...

Temperature evolution of the crystal structure of multiferroic solid solutions (1– x )Pb(Fe 2/3 W 1/3 O 3 )–( x )PbTiO 3

Neutron diffraction was used to study the temperature evolution of the structure of (1–x)Pb · (Fe2/3W1/3O3)–(x)PbTiO3 solid solutions of two compositions x = 0.2 and 0.3, in which the existence of the morphotropic phase boundary (MPB) is observed, in the temperature range 90–400 K. It is shown that the system is in the two-phase state, in which the cubic and the tetragonal phases coexist, even at temperatures higher than MPB. The temperature dependences of the phase percentages were obtained. The static displacements of lead ions from the crystallographic position (000) are estimated in terms of a multiwell potential.