Cu/Zn Hydroxycarbonates obtained by co‐precipitation of Cu2+ and Zn2+ with Na2CO3 have been investigated regarding phase formation and thermal decomposition in two series with varying Cu/Zn ratios prepared according to the decreasing pH and constant pH method. Hydrozincite, aurichalcite and (zincian)‐malachite were found to form at differing Cu/Zn ratios for both series. For the constant pH preparation the Cu/Zn ratio in zincian‐malachite was close to the nominal values whereas excess values were found for the decreasing pH samples. The degree of crystallinity as well as the thermal decomposition temperatures were lower for the constant pH series. All samples containing aurichalcite revealed an unexpected decomposition step at high temperatures evolving exclusively CO2. The differences in composition and microstucture were traced back to the different pathways of solid formation for the two preparation methods. Substantial changes were observed during the post‐precipitation processes of ageing and washing. The effects were studied in detail on samples with a cation ratio of Cu/Zn 70:30 mol %. Ageing of the precipitates in their own solutions is accompanied by a spontaneous crystallization of the initially amorphous solids. The decreasing pH sample develops from a hydroxy‐rich material comprising basic copper nitrate (gerhardtite) as an intermediate. Only small changes in the chemistry of the samples were detected for the constant pH precipitation. The findings are summarised into a scheme of solid formation processes that explains the phenomenon of a “chemical memory” of the precipitates when they are converted into Cu/ZnO model catalysts.

Binary Cu/ZnO catalysts with varying molar ratios (90/10 through10/90) were studied under methanol synthesis conditions at 493 K and at atmo spheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N 2 O reactive frontal chromatography, N 2 O RFC) after reduction in 2 vol-% H 2 at 513 K. Activity data were supplemented with a detailed analysis of the microstructure i. e. crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N 2 O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn Kedge show that about 5 % ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e. g. size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.

The bulk structure of copper in various binary Cu/ZnO catalysts for steam reforming of methanol under activation and working conditions is studied by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The evolution of bulk phases from CuO/ZnO precursors during activation with hydrogen was studied using temperature programmed reduction (TPR) (448 -523 K, 2 vol-% H 2 with and without water vapor). With decreasing copper content the onset of reduction is shifted from 473 K (pure CuO) to 443 K (40 mol-% Cu) accompanied by a decrease in Cu crystallite sizes (from 210 Å to 40 Å). Using time -resolved in situ XANES measurements at the Cu K edge during TPR experiments the degree of reduction was monitored. It is shown that Cu(I) oxide forms prior to Cu. Adding oxygen to the feed gas leads to the formation of a mixture of Cu(II) and Cu(I) oxide accompanied by a complete loss of activity. After switching back to steam reforming conditions a higher activity is attained while the catalyst shows an increased Cu crystallite size (up to 40%). EXAFS measurements at the Cu K and the Zn K edge indicate a structural disorder of the Cu particles in the medium range order based on increasing Debye-Waller factors for higher Cu-Cu shells . Furthermore, the dissolution of Zn atoms (up to ~ 4 mol-%) in the copper lattice is detected. Upon oxidation/reduction cycles activity is increased, the disorder in the copper particles increases, and Zn segregates out of the copper bulk. A structural model is proposed which ascribes the enhanced activity to structurally disordered (strained) copper particles due to an improved interface interaction with ZnO.

Bulk catalysts are indispensable for the production of primary chemicals. A Cu/ZnO system, which has been tremendously improved through promotion with Al, is used for the lowpressure synthesis of methanol. [1][2][3] This material is synthesized through the coprecipitation of a hydroxycarbonate (HC) precursor followed by calcination to form CuO/ZnO. [4,5] The properties of the activated catalyst are dictated by the kinetic details of the synthesis of the solid, that is, by the properties of the precursor HC material; [3,[5][6][7] the catalyst retains a "chemical memory".The precipitation of large quantities of material is an illdefined process. Even under "constant" precipitating conditions, the chemical potential of the reagents are neither spatially nor temporally constant when a drop of the precipitating agent is added to the reactor in which precipitate and dissolved ions already coexist. During ageing, washing, and drying of the catalyst precursor, complex exchange reactions occur, particularly between the anions (carbonate/ hydroxide and hydroxide/nitrate). These exchange reactions are not controlled and thus result in variations in the properties of materials that are synthesized in a similar manner. [8] A remedy for such a situation could be a continuous precipitation process in which, in the shortest possible time, a precipitate is formed in a small volume of solution, and is then collected under inert conditions (room temperature, water) followed by conventional processing. The kinetically controlled steps of nucleation and growth are thus carried out under much more reproducible boundary conditions. Furthermore, one can hope to minimize ensuing processes such as redissolution, reprecipitation, and ion exchange.In our studies, a commercial microreactor designed for reactions in solution was used for the precipitation.[9] The reactor consists of a plate (with parallel channels approximately 100 mm long and about 200 m wide), which is sealed between two temperature-controlled plates. The reagent solutions (0.15 m metal nitrate and 0.18 m sodium carbonate; pH = 7.0) are combined in the channels at a constant throughput (5 mL min À1 ) under fully defined flow conditions (very short residence time and rapid, intense mixing in the reaction zone). Heat transfer is thus rapid and precise temperature control (328 K) is ensured. The product is collected in a cooled settling container and is processed with the conventional washing and drying steps, followed by calcination. In a future, improved version of this method, the postprecipitation steps of collection, washing, and drying could also be organized into a continuous processing system to produce an even more uniform product.The fundamental experiments described herein demonstrate that such a production scheme, which above all distinguishes itself from standard batch precipitation in that the precipitate is rapidly isolated from the reactive mother liquor, does not produce a worse catalyst. Thus, such a continuous (parallel) precipitation technique is possible.T...

The photoreduction of nitrate in aqueous medium was investigated at 292 K in a batch system open to the ambient. Titania was tested as a photocatalyst and humic acids were added as promoters. Conversions of 28% were reached after 80 hours when a 44 mg/l nitrate solution was irradiated with a high pressure Xe-lamp; the major product was nitrite. The addition of humic acids (20 mg/l) promoted reduction of nitrate to nitrite but the mechanism of promotion could not be unambiguously identified. Titania (0.1 g/l) itself did not catalyze the photoreduction of nitrate but rather seemed to act as a catalyst for the reoxidation of nitrite to nitrate. The most successful system was a combination of 44 mg/l nitrate, 20 mg/l humic acids and 0.1 g/l Kronos-1002 titania: the nitrate conversion reached 32% after 76 hours, with little nitrite formed. Photocatalytic nitrate degradation is accompanied by homogeneous reduction to the more toxic nitrite; requiring any effective catalyst system to also reduce nitrite concentration.