Igor Kasatkin

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.

Counting of Oxygen Defects versus Metal Surface Sites in Methanol Synthesis Catalysts by Different Probe Molecules

Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2.

The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles

Optimal dispersion of the active metal usually is the main goal in preparation of supported metal catalysts. Whereas the oxide support often can be considered as inert, in many systems it is known to chemically affect the catalytic material under reaction conditions exceeding the simple role as a physical carrier of metal particles. Strong metal support interaction (SMSI) is an example, which was first detected for Pt/TiO2 [1] and was also found in Cu/ZnO. Cu/ZnO/(Al2O3) catalysts are active in industrial methanol synthesis and a Cu/ZnO synergy is discussed in literature, the nature of which is still under debate. However, it seems clear that synergetic effects should have their origin in the Cu–ZnO interface region. The catalytic properties are, thus, not only determined by the amount of Cu surface area (SCu) accessible to gas but also by the nature of the Cu–ZnO interface contacts. As a result of the latter effect, deviations from linearity of SCu–activity correlations can be observed. Usually, the value of SCu is regarded as the dominating effect and a catalyst of considerably lower SCu would be estimated to exhibit inferior activity compared to a high SCu sample of the same Cu loading. The potential of intrinsic or synergistic effects, which are responsible for different specific activities of Cu, is not easily determined because surface and interface area are interrelated by morphology and microstructure, which are difficult to control during preparation of applied Cu/ZnO/(Al2O3) systems. Today, the Cu/ZnO/Al2O3 system has been industrially employed for more than 40 years. During this period of time the preparation of successful catalysts has been extensively studied in academic and industrial laboratories and a considerable optimization regarding activity and stability could be achieved. The resulting technically relevant synthesis route is based on preparation of Cu, Zn basic carbonate precursors by coprecipitation, for example, from metal nitrate solutions and sodium carbonate as precipitating agent. The precipitate is aged, recovered, and calcined, which results in an intimate mixture of the oxides. Finally, the CuO component is reduced to obtain active catalysts with the typical atomic Cu/Zn ratio near 70:30 and a molar Al content of 10–20 %. Catalysts prepared according to this procedure usually possess high values of SCu and are highly active due to their efficient mesoand nanostructuring during synthesis. Whereas the optimization of Cu dispersion can be regarded as widely advanced in these systems, optimization of the Cu–oxide contact interface, that is, the microstructural arrangement of the components, presents further unexplored room for improvement. Herein, we present a novel type of Cu/Zn/Al2O3 catalyst that was prepared according to the technically applied synthesis route described above with two important modifications: 1) Instead of pure sodium carbonate solution and Cu, Zn, Al–nitrate solution, as used in conventional catalyst preparation, a combination of sodium aluminate and carbonate solution was used as precipitating agent for a Cu, Zn–nitrate solution during coprecipitation; 2) the ageing period was suppressed by realizing a continuous process with direct spray-drying of the fresh precipitate suspension. Structural and catalytic characteristics of this sample, denoted catalyst A, are compared to a conventionally prepared catalyst, referred to as catalyst B. The latter sample showed an activity similar to a commercial methanol synthesis catalyst, provided by S d-Chemie AG. As revealed by X-ray diffraction (XRD), the washed precipitate consisted of crystalline hydroxy carbonate phases for catalyst B, whereas the precursor of catalyst A was completely X-ray amorphous. The precursors were subjected to the same treatments during washing, calcination, and reduction. It is important to note that both samples exhibit exactly the same industrially relevant metal composition of Cu/Zn/Al =60:25:15 and, thus, experimental differences can be related to a different microstructure. Investigation of the microstructure of the reduced catalysts was carried out by means of high resolution transmission electron microscopy (HRTEM). Recently, a comprehensive characterization of the microstructure of a series of industrially relevant Cu/ZnO/Al2O3 samples was presented by some of the authors. These bulk catalysts were quite different from classical supported systems and Cu particles of an average diameter(<10 nm) were detected to be separated by small ZnO particles, which prevented them from sintering, and forming a porous framework of individual particles. This type of morphology was also found in catalyst B and is represented in Figure 1, right. Unlike catalyst B, individual separated oxide particles are hardly detected and, consequently, the porous Cu/ZnO particle arrangement is absent in the novel material. The Cu particles are partly embedded in the oxide matrix re[a] Dr. M. Behrens, Dr. A. Furche, Dr. I. Kasatkin, Dr. A. Trunschke, Prof. Dr. R. Schlçgl Fritz Haber Institute of the Max Planck Society Department of Inorganic Chemistry, Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49) 30-8413-4405 E-mail : behrens@fhi-berlin.mpg.de [b] Dr. W. Busser, Prof. Dr. M. Muhler Ruhr University Bochum, Industrial Chemistry Universit tsstraße 150, 44801 Bochum (Germany) [c] Dr. B. Kniep, Dr. R. Fischer S d-Chemie AG, Research & Development Catalysts Waldheimer Straße 13, 83052 Bruckm hl (Germany)

Wet spinning of fibers made of chitosan and chitin nanofibrils

Biocompatible and bioresorbable composite fibers consisting of chitosan filled with anisotropic chitin nanofibrils with the length of 600-800 nm and cross section of about 11-12 nm as revealed by SEM and XRD were prepared by coagulation. Both chitin and chitosan components of the composite fibers displayed preferred orientations. Orientation of chitosan molecules induced by chitin nanocrystallites was confirmed by molecular modeling. The incorporation of 0.1-0.3 wt.% of chitin nanofibrils into chitosan matrix led to an increase in strength and Young modulus of the composite fibers.

HRTEM observation of the monoclinic-to-tetragonal (m-t) phase transition in nanocrystalline ZrO2

The orientation relations m(100) || t(001), m[001] || t[110]; m(011) || t(100), m[100] || t[001]; m(100) || t(110), m[001] || t[001]; m(013) || t(116), m[001] || t[001] (indices for the primitive tetragonal cell) have been found between the tetragonal (t) and monoclinic (m) domains during the electron irradiation-induced m-t phase transition observed in-situ with HREM within isolated zirconia nanoparticles. Geometric models of the m-t interfaces are proposed.

Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts

Nanostructured Cu(x)Zn(1-x)Al(2)O(4) with a Cu:Zn ratio of ¼:¾ has been prepared by a microwave-assisted hydrothermal synthesis at 150 °C and used as a precursor for Cu/ZnO/Al(2)O(3)-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m(2) g(-1)). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m(2) g(-1). The catalytic performance of the material in CO(2) hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al(2)O(3) reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn(2+) in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500 °C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m(2) g(-1) due to less copper incorporation is not a significant drawback for the methanol steam reforming.

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.

X-ray diffraction study of vertically aligned layers of h-BN, obtained by PECVD from borazine and ammonia or helium mixtures

The method of grazing incidence X-ray diffraction (GIXRD) is used to study the hexagonal boron nitride (h-BN) films produced by plasma-enhanced chemical vapor deposition (PECVD) from borazine and ammonia or helium mixtures. The diffraction patterns of boron nitride layers aligned vertically on the substrate are obtained for the first time. The films deposited consist of the amorphous phase and the nanocrystalline h-BN phase. The nanocrystallite sizes in the films obtained from the mixtures of borazine (B3N3H6) with both ammonia and helium increase with an increase in the synthesis temperature. Nanocrystallites are heteraxial and have a layered structure with the interplanar spacing of ∼0.35 nm.

Facile synthesis of scandium fluoride oriented single-crystalline rods and urchin-like structures by a gas–solution interface technique

A facile and effective method to obtain urchin-like structures and ordered arrays of rod-like crystals of scandium fluoride by an interfacial interaction between solution and gaseous reagents (Gas–Solution Interface Technique, GSIT) is reported. The rod crystals with tetragonal crystal lattice can be as thin as ca. 200 nm and as long as ca. 7 μm. The effect of solution concentration and reaction time on the morphology of the product is reported. The synthesized crystals were characterized by SEM, TEM, HRTEM, XRD analyses, EDS, XPS, and FT-IR spectroscopy. A hypothesis is proposed on the formation of rod crystals during the GSIT process.

Supramolecular structure of chitin nanofibrils

The structure of chitin nanofibrils as a promising filler for bioresorbable suture materials and matrixes for cellular technologies and tissue engineering is investigated via the methods of X-ray diffraction and scanning electron microscopy. It is shown that the powder microparticles obtained via lyophilization of an aqueous dispersion of chitin nanofibrils have a band structure with a cross-sectional size of 30 μm and a thickness of 0.1 μm. The bands consist of nanoparticles 25 nm in thickness and 400–500 nm in length. The chitin nanofibrils are composed of two crystallites with cross-sectional sizes of 11–12 nm and b axes perpendicular to the nanofibril axis. The chitin nanofibrils tend to form planar elements with a layered structure on both the microlevel and the nanolevel. The addition of chitin nanofibrils to a chitosan solution leads to a rise in its viscosity. However, the action of shear stresses leads to a substantial decrease in the chitosan-chitin solution viscosity, a phenomenon that is due to the presence of planar anisodiametric nanoparticles of chitin.