Benjamin Kniep

Fritz Haber Institute of the Max Planck Society, Berlín, Berlin, Germany

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Publications (13)49.36 Total impact

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    ABSTRACT: A novel continuous method for the preparation of a ternary Cu/ZnO/Al2O3 catalyst based on a cascade of micromixers and a tubular aging reactor is presented as a promising alternative route to the conventional batch process. Its application, in combination with immediate spray drying, enables monitoring of the formation of the final precursor by exchange reactions between initially separated phases during the aging step. These exchange reactions were successfully simulated by consecutive precipitation by using micromixers in series as analytical tool. After 60 min of continuous aging, calcination, and reduction, a catalyst is produced that exhibits an almost equal mass-related activity in methanol synthesis compared to a commercial catalyst and an area-related activity that is about 50 % higher.
    ChemCatChem 01/2011; 3(1). · 5.18 Impact Factor
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    ABSTRACT: Co-precipitation of Cu,Zn,(Al) precursor materials is the traditional way of synthesizing Cu/ZnO/(Al2O3) catalysts for industrial methanol synthesis. This process has been investigated by titration experiments of nitrate and formate solutions. It was found that the solidification of the single components proceeds sequentially in case of nitrates: Cu2+ is precipitated at pH 3 and Zn2+ (as well as Al3+) near pH 5. This behavior prevents a homogeneous distribution of all metal species in the initial precipitate upon gradual increase of pH and requires application of the constant pH micro-droplet method.This effect is less pronounced if formate instead of nitrate is used as counter ion. This can be explained by the strong modification of the hydrolysis chemistry of the metal ions due to the presence of formate anions, which act as ligands and buffer.A formate-derived Cu/ZnO/Al2O3 catalyst was more active in methanol synthesis compared to a nitrate-derived sample although the same crystallographic phases were present in the precursor after co-precipitation and ageing.The effect of precipitation temperature was studied for the binary CuZn nitrate model system. Increasing the temperature of co-precipitation above 50 °C leads to down-shift of the precipitation pH of Zn2+ by a full unit. Thus, in warm solutions more acidic conditions can be used for complete co-precipitation, while in cold solutions, some Zn2+ may remain dissolved in the mother liquor at the same precipitation pH. The higher limit of temperature is given by the tendency of the initial Cu precipitate towards formation of CuO by oxolation. On the basis of these considerations, the empirically determined optimal pH and temperature conditions of the industrially applied synthesis can be rationalized.Graphical abstract.View high quality image (92K)Research highlights▶ Precipitation of Cu, Zn and Al species proceed sequentially in nitrate solution. ▶ Microdroplet-technique or complexing anions can lead to a homogeneous distribution. ▶ Conditions have been optimized for the zincian malachite precursor. ▶ New precursors require renewed optimization of the synthesis conditions.
    Applied Catalysis A: General. 01/2011; 392:93-102.
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    ABSTRACT: The activity of Cu/ZnO/Al2O3 catalysts for methanol synthesis is determined by the microstructural arrangement of the oxides among other factors. A large metal/oxide interface area seems to enhance the intrinsic activity of Cu. A novel type of Cu/ZnO/Al2O3 catalyst demonstrates the potential of this positive effect, which can outweigh the simultaneous loss of gas accessible Cu surface area in comparison to a conventionally prepared sample.
    ChemCatChem 01/2010; · 5.18 Impact Factor
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    ABSTRACT: Structural modifications of Cu/ZnO catalysts for methanol steam reforming (MSR) as a function of precipitate aging in catalysts preparation process has been investigated comparatively. Freshly precipitated Cu,Zn-hydroxycarbonate (HC) and Cu,Zn-hydroxynitrate (HN) were aged in their mother liquor for a period of 120 min followed by washing, drying, calcination and reduction. Pronounced effect of aging was found for aged HC precipitates while no significant effect of aging was observed for aged HN solids. The bulk structure of the Cu/ZnO catalysts was investigated by means of TG/MS, in situ XRD and 63Cu NMR. The increase in the activity of the catalysts prepared by HC aging did not correlate linearly with the specific Cu surface area but coincides with an increase in the microstrain in the copper clusters presumably because of the improved interface between Cu and ZnO. Meanwhile, aging of HN precipitates results in large, separated and less strained Cu and ZnO particles with an inferior catalytic activity. Finally, both aged Cu/ZnO catalysts revealed smaller copper crystallite size compared to unaged samples.
    Catalysis Today. 01/2008;
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    Igor Kasatkin, Benjamin Kniep, Thorsten Ressler
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    ABSTRACT: A technique of contact angle measurement was applied to the nano-scale oxide-supported metal particles. For Cu supported on ZnO and ZrO2 the angles were found to increase and the work of adhesion to decrease with increasing particle size. Such a trend is interpreted as an effect of negative contact line tension of 2.1 x 10(-9) J m(-1) and 1.0 x 10(-9) J m(-1) in the Cu/ZnO and Cu/ZrO2 system, correspondingly. For the small-sized Cu particles the apparent work of adhesion on ZnO support is higher than that on ZrO2.
    Physical Chemistry Chemical Physics 03/2007; 9(7):878-83. · 3.83 Impact Factor
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    ABSTRACT: Copper-based catalysts are industrially applied in various reactions including water-gas shift, synthesis of fatty alcohols from fatty acid methyl esters, and methanol synthesis. Today, methanol is produced at low pressures (35-55 bar) and 200-300°C over Cu/ZnO/Al 2 O 3 catalysts. [1] Due to the great commercial relevance, Cu/ZnO-based catalysts have been extensively studied and many different models have been proposed regarding the nature of active sites and the valence of copper under conditions of methanol formation, such as Cu 1+ dispersed in ZnO, [2,3] metallic copper supported on ZnO, [4] dynamic surface and bulk alloy formation depending on the reduction potential of the synthesis gas, [5,6] Cu – at the so-called Schottky junction between metallic Cu and the semiconductor ZnO, [7] and ZnO segregated on Cu 1+ . [8] The catalytic activity of the binary catalyst has been reported to be several orders of magnitude greater than that of metallic Cu or pure ZnO, respectively, indicating a synergetic interaction of the two components. [9] ZnO is regarded either as provider of atomic spillover hydrogen for further hydrogenation of adsorbed reaction intermediates on Cu sites, [10,11] or as a structure directing support controlling dispersion, morphology, and specific activity of the metal particles. [12-18] Strong interaction between the metal and the support, especially in the case of large lattice mismatch, is known to cause strain in the metal particles, to which an increase in catalytic performance has been attributed. [19-21] On the other hand, 1-ML-high and thicker Cu islands epitaxially grown on the ZnO (000⎯1) surface were experimentally found to be strain-free. [22] In most of the earlier studies model catalysts with low Cu loadings (Cu/Zn << 1) containing large ZnO single crystals have been investigated, although, usually, in com-mercial catalysts copper represents the main component (Cu/Zn > 1) and the ZnO particles, acting rather as a spacer than as a support, are comparable in size, or even smaller than the Cu particles. In this paper we report the results of TEM and in situ XRD characterization of a series of Cu/ZnO/Al 2 O 3 catalysts exhibiting different catalytic activ-ities. The molar ratio Cu:Zn:Al = 60:30:10 is characteristic of commercial catalysts. [1] The microstructural features of the materials prepared by coprecipitation with sodium car-bonate from metal nitrate solution are analyzed after calci-nation in air at 330°C and subsequent reduction in hydrogen at 250°C. A quantitative estimation of imperfec-tions in metal particles determined by combination of inde-pendent TEM and in situ XRD investigations is established. The implications of strain in Cu crystallites and the defect frequency associated therewith on the catalytic activity of Cu/ZnO/Al 2 O 3 catalysts in methanol synthesis are dis-cussed. The TEM and HRTEM images shown in Figure 1 il-lustrate the microstructure typical of the catalysts studied. Generally, 10 to 15 clusters similar to the one shown in Figure 1a, which had the size that varied from 100 nm to several micrometers, were analyzed in each of the catalysts with Energy-Dispersive X-Ray spectroscopy (EDX) to determine the concentrations of Al, Cu, and Zn. The aver-age value (Al: 10.6 ±4.5 at.%; Cu: 62.7 ±7.2 at.%; Zn: 26.7 ±4.2 at.%) is close to the nominal composition. Figure 2 that contains data of all catalysts illustrates the scattering due to local inhomogeneities. The particles of Cu and ZnO form an irregular framework (Figure 1b). The particles of ZnO, which are comparable in size or smaller than the Cu particles, serve as spacers between the latter, preventing them from sintering.
    Angewandte Chemie International Edition 02/2007; 46(38):7324-7. · 11.34 Impact Factor
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    ABSTRACT: Cu/Zn/Al2O3 catalysts with industrially relevant high copper content have been investigated. With the aid of combined TEM (see picture) and XRD analysis, imperfections in the Cu lattice arising from twinning, fault defects, and strain are found to be related to increased activity of the catalysts in methanol synthesis.
    Angewandte Chemie 01/2007;
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    B.L. Kniep, F. Girgsdies, T. Ressler
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    ABSTRACT: Microstructural characteristics of the copper phase in Cu/ZnO catalysts for methanol steam reforming (MSR) were investigated as a function of aging of the precipitated hydroxycarbonates during catalyst preparation. The bulk structure of active catalysts under MSR reaction conditions was determined by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) combined with on-line mass spectrometry. Reduction kinetics and phase compositions obtained from XAS data analysis were compared with conventional TPR and TG/MS results. With increasing aging time of the precipitate, the onset of reduction of the CuO/ZnO precursor shifted from 462 to 444 K, whereas a decrease in crystallite size from 110 Å (0 min) to 70 Å (120 min) was detected. A strong increase in catalytic activity was observed for Cu/ZnO catalysts obtained from precipitates aged for more than 30 min. The microstrain in the copper particles as detected by XRD and XAS was determined as an additional bulk structural parameter correlating with the increase in catalytic activity. Moreover, continuous precipitate aging led to a decreasing amount of Zn in the copper clusters of the Cu/ZnO catalysts. A schematic model of the structural characteristics of Cu/ZnO catalysts as a function of precipitate aging is proposed. This model emphasizes the defect-rich state of the homogeneous microstructure of Cu/ZnO catalysts and its implication for the catalytic activity in the steam reforming of methanol.
    Journal of Catalysis. 11/2005;
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    Angewandte Chemie International Edition 08/2005; 44(30):4704-7. · 11.34 Impact Factor
  • Angewandte Chemie 06/2005; 117(30):4782 - 4785.
  • Angewandte Chemie International Edition 02/2004; 43(1):112-5. · 11.34 Impact Factor
  • T. Ressler, B. L. Kniep, F. Girgsdies
    Zeitschrift für anorganische Chemie 01/2004; 630(11):1695-1695. · 1.16 Impact Factor