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  Synthesis and Characterization of Cu-based Catalysts resulting from Cu,Zn,X Hydrotalcite-like Compounds

Kühl, S. (2011). Synthesis and Characterization of Cu-based Catalysts resulting from Cu,Zn,X Hydrotalcite-like Compounds. PhD Thesis, Technische Universität, Berlin.

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 Urheber:
Kühl, Stefanie1, Autor           
Schlögl, Robert1, Gutachter           
Ressler, Thorsten2, Gutachter
Muhler, Martin3, Gutachter
Affiliations:
1Inorganic Chemistry, Fritz Haber Institute, Max Planck Society, ou_24023              
2Institut für Chemie, Technische Universität Berlin, ou_persistent22              
3Laboratory of Industrial Chemistry, Ruhr Universität Bochum, ou_persistent22              

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 Zusammenfassung: Cu,Zn,Al layered double hydroxide (LDH) are formed as a by-phase during the coprecipitation step of the well known Cu/ZnO/Al2O3 catalysts for methanol synthesis. In the present work the LDH system was investigated systematically.

First of all, a phase pure Cu,Zn,Al LDH was prepared by co-precipitation, resulting in the composition (Cu0.5Zn0.17)Al0.33(OH)2(CO3) 0.17 •mH2O and showing the typical platelet-like morphology and homogeneous elemental distribution. Upon decomposition in air a carbonate-modified, amorphous mixed oxide was formed. This mixed oxide can be described as well dispersed CuO within ZnAl2O4 still containing stabilizing carbonate and showing a strong interaction of Cu2+ ions with the Zn-Al matrix. After reduction, the main nanostructuring step of ex-LDH catalyst, a Cu based catalyst was obtained with Cu dispersed in a ZnAl2O4 spinel-like matrix. Due to the high embedment of the
small Cu particles (7nm) it exhibits just a small absolute activity in methanol synthesis compared to the conventionally prepared reference catalyst but a high intrinsic one (relative to the Cu surface area) showing the high importance of the interface contact of Cu and its surrounding.

By pH-controlled co-precipitation within microemulsion droplets a Cu-based catalysts was prepared from a phase pure LDH with smaller particles. Thus, smaller Cu/ZnAl2O4 aggregates with less embedment of the resulting Cu particles were obtained and a higher activity in methanol steam reforming was observed. However, a smaller intrinsic activity was observed in methanol steam reforming.

Furthermore, two sample series were prepared using Cr and Ga as substituting elements for Al to investigate their effects onto the ex-LDH catalyst system and to gain understanding of the functionality of Al. After decomposition in air carbonate-modified mixed oxides were obtained for Cr containing samples as well as for Ga contents <15 at%, similar to the Cu,Zn,Al system. Higher Ga contents led to the formation of crystalline Zn(Al,Ga)2O4 or ZnGa2O4 spinel. Both sample series did not show a correlation of Cu surface area and catalytic activity. After all, these sample series show that it is possible to prepare a series of Cu based catalysts with the same preparation history, which are characterized by a homogeneous elemental distribution and a similar microstructure but significant differences in catalytic performance attributed to differences in intrinsic activity in methanol synthesis.
In all prepared sample series, the interaction between Cu and the oxide matrix was identified as an important parameter to control this intrinsic activity.

In summary, interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface. The modification of the interface area (Cr) as well as the type of the interface interaction (Ga) were reported to be a possibility to use these positive interactions and modify the resulting Cu based catalyst.

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Sprache(n): eng - English
 Datum: 2011-09-30
 Publikationsstatus: Angenommen
 Seiten: 151
 Ort, Verlag, Ausgabe: Berlin : Technische Universität
 Inhaltsverzeichnis: List of Figures . . . . . . . . . . . . . . . . . . . . ix
List of Tables . . . . . . . . . . . . . . . . . . . . xii
List of Abbreviations . . . . . . . . . . . . . . . . xiii
1 Introduction 1
1.1 Methanol Synthesis . . . . . . . . . . . . . . . . . 2
1.1.1 Catalysts for Methanol Synthesis . . . . . . . . . 2
1.2 Layered Double Hydroxides . . . . . . . . . . . . . .4
1.2.1 LDH as precursor for Cu based catalysts . . . . . .6
1.3 Outline of the work . . . . . . . . . . . . . . . 8
2 Methodology – fundamentals and theoretical details 11
2.1 X-ray Diffraction . . . . . . . . . . . . . . . . . 11
2.2 Temperature Programmed Reduction . . . . . . . . . .12
2.3 Determination of Cu surface area . . . . . . . . . .17
2.4 X-ray Absorption Spectroscopy . . . . . . . . . . . 17
2.5 Electron Microscopy . . . . . . . . . . . . . . . . 18
2.5.1 Scanning Electron Microscopy . . . . . . . . . . .18
2.5.2 Transmission Electron Microscopy . . . . . . . . .19
2.6 Other Characterization Methods . . . . . . . . . . .20
2.6.1 Nitrogen adsorption isotherm . . . . . . . . . . .20
2.6.2 Thermogravimetric Analysis . . . . . . . . . . . .22
2.6.3 Photoelectron Spectroscopy . . . . . . . . . . . .23
3 Experimental 25
3.1 Materials . . . . . . . . . . . . . . . . . . . . . 25
3.2 Co-Precipitation of LDH compound . . . . . . . . . .25
3.2.1 LDH synthesis from microemulsion . . . . . . . . 26
3.3 Conventionally prepared reference catalyst . . . . 27
3.4 Synthesis of Zn,Al spinel as reference material . . 27
3.5 Catalyst characterization . . . . . . . . . . . . 27
3.6 Testing for catalytic activity . . . . . . . . 30
4 Microstructure of Cu,Zn,Al LDH material and resulting catalyst 33
4.1 Choice and influence of precipitation pH . . . . 33
7 Final Discussion 121
A Appendix 127
A.1 Additional diagrams to the analysis of the CZA catalyst system . . . 127
A.2 Diagrams and tables for the conventionally prepared reference catalyst 130
A.3 Results of DLS measurements of microemulsions . . .131
A.4 Additional informations to the analysis of the Cr substituted catalyst system . 133
A.5 Additional informations to the analysis of the Ga substituted catalyst system . . 138
A.6 Sample identification - FHI intern . . . . . . . . 142
 Art der Begutachtung: -
 Identifikatoren: URN: nbn:de:kobv:83-opus-34464
URI: http://dx.doi.org/10.14279/depositonce-3140
 Art des Abschluß: Doktorarbeit

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