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Structural Effects of Cu,Zn Substitution in the Malachite–Rosasite System Malte Behrens, Frank Girgsdies

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Structural Effects of Cu,Zn Substitution in the Malachite–Rosasite System Malte Behrens, Frank Girgsdies Structural Effects of Cu,Zn Substitution in the Malachite–Rosasite System Malte Behrens, Frank Girgsdies To cite this version: Malte Behrens, Frank Girgsdies. Structural Effects of Cu,Zn Substitution in the Malachite–Rosasite System. Journal of Inorganic and General Chemistry / Zeitschrift für anorganische und allgemeine Chemie, Wiley-VCH Verlag, 2010, 10.1002/zaac.201000028. hal-00552444 HAL Id: hal-00552444 https://hal.archives-ouvertes.fr/hal-00552444 Submitted on 6 Jan 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. ZAAC Structural Effects of Cu,Zn Substitution in the Malachite– Rosasite System Journal: Zeitschrift für Anorganische und Allgemeine Chemie Manuscript ID: zaac.201000028.R1 Wiley - Manuscript type: Article Date Submitted by the 02-Mar-2010 Author: Complete List of Authors: Behrens, Malte; Fritz-Haber-Institute, Inorganic Chemistry Girgsdies, Frank; Fritz-Haber-Institute, Inorganic Chemistry Keywords: Cu/ZnO catalyst, Jahn-Teller distortions, Malachite, Rosasite Wiley-VCH Page 1 of 32 ZAAC 1 2 3 4 5 6 Structural Effects of Cu,Zn Substitution in the 7 8 9 Malachite–Rosasite System 10 11 12 13 14 15 16 Malte Behrens a, * and Frank Girgsdies a 17 18 19 20 21 th 22 Dedicated to Professor Rüdiger Kniep on the Occasion of his 65 birthday 23 24 25 26 27 a) Fritz-Haber-Institut der Max-Planck-Gesellschaft 28 29 Abteilung Anorganische Chemie 30 31 Berlin, Germany 32 33 34 35 36 * Corresponding author: Dr. Malte Behrens 37 38 Fritz-Haber-Institut der Max-Planck-Gesellschaft 39 40 Abteilung Anorganische Chemie 41 Faradayweg 4-6 42 43 14195 Berlin 44 45 Germany 46 47 48 Email:[email protected] 49 50 Tel.: +49 30 8413-4408 51 52 Fax: +49 30 8413-4405 53 54 55 56 57 58 59 60 1 Wiley-VCH ZAAC Page 2 of 32 1 2 3 Abstract 4 5 Synthetic zincian malachite samples (Cu 1-xZn x)2(OH) 2CO 3 with x = 0, 0.1, 0.2 and 0.3 6 7 were characterized by powder X-ray diffraction and optical spectroscopy. The XRD 8 9 patterns of the samples up to x = 0.2 indicate single phase materials with an 10 11 approximately linear dependence of the refined lattice parameters on the Zn content. In 12 contrast, the sample with a nominal Zn content x = 0.3 shows the formation of a small 13 14 amount of aurichalcite (Zn,Cu) 5(OH) 6(CO 3)2 as an additional phase. Based on the lattice 15 16 parameter variations, the Zn content of the zincian malachite component in this sample is 17 18 estimated to be x ≈ 0.27, which seems to represent the maximum possible substitution in 19 20 zincian malachite under the synthesis conditions applied. The results are discussed in 21 relation to preparation of Cu/ZnO catalysts and the crystal structures of the minerals 22 23 malachite and rosasite. One striking difference between these two structurally closely 24 25 related phases is the orientation of the Jahn-Teller elongated axes of the CuO 6 octahedra 26 27 in the unit cell, which seems to be correlated with the placement of the monoclinic β 28 29 angle. The structural and chemical relationship between these crystallographically 30 distinct phases is discussed using a hypothetical intermediate Zn2(OH) 2CO 3 phase of 31 32 higher orthorhombic symmetry. In addition to the crystallographic analysis, optical 33 34 spectroscopy proves to be a useful tool for estimation of the Cu:Zn ratio in 35 36 (Cu 1-xZn x)2(OH) 2CO 3 samples. 37 38 39 Keywords 40 41 Malachite, Rosasite, Jahn-Teller distortions, Cu/ZnO catalyst 42 43 44 Introduction 45 46 Rosasite is a mineral of the chemical composition (Cu 1-xZn x)2(OH) 2CO 3 with 47 48 approximately 0.3 < x < 0.5. The related pure Cu mineral, Cu 2(OH) 2CO 3, is malachite 49 50 whose monoclinic crystal structure is reported in the literature [D1D]. While synthetic 51 52 malachite can be easily prepared by precipitation, synthetic rosasite samples, i.e. single 53 phase compounds with x > 0.3, are much harder to obtain, because usually aurichalcite, 54 55 (Cu 1-xZn x)5(OH) 6(CO 3)2 with x > 0.5, is formed if zinc-rich solutions are used for 56 57 coprecipitation [ D 2D]. The cell parameters and space group of rosasite have been reported in 58 59 60 2 Wiley-VCH Page 3 of 32 ZAAC 1 2 3 [D3D] and atomic coordinates were recently revealed on the basis of powder diffraction 4 5 studies [D4D]. Both malachite and rosasite are monoclinc, space group P21/a (No. 14), 6 7 crystallize in different crystal structures and are, thus, not isomorphous despite their 8 9 similar compositions. However, the close relation of the two phases is apparent from the 10 11 similar powder diffraction patterns (Fig. 1) and was recently discussed by Perchiazzi 12 [X4X,D5D]. It is interesting to note that rosasite analogous minerals were found also with Mg, 13 14 Ni or Co instead of Zn together forming the rosasite group of minerals. 15 16 These mixed basic transition metal carbonates have not only attracted the attention of 17 18 mineralogists, but also of solid state chemists active in the field of heterogeneous 19 catalysis, because (Cu Zn )(OH) CO is the relevant precursor compound for the 20 1-x x 2 3 21 preparation of nanostructured Cu/ZnO catalysts [ D6D-DDDDD 9D]. Such catalysts are employed in 22 23 industrial synthesis of methanol from syngas, one of the top ten chemical processes 24 25 worldwide. A highly substituted synthetic rosasite-like precursor would be desirable for 26 catalyst preparation, because the resulting Cu/ZnO material after calcination and 27 28 reduction is expected to exhibit a higher dispersion of the catalytically active Cu phase. 29 30 This is because the stabilizing function of the ZnO phase in the composite catalyst is 31 32 more efficient, if large amounts of Zn were homogenously distributed in the cation lattice 33 34 of the precursor (Fig. 2) [ X6X]. However, preparation of (Cu 1-xZn x)(OH) 2CO 3 with 35 substitution levels near x = 0.5, i.e. as high as in natural rosasite, remains a challenge and 36 37 values around x = 0.3 cannot be exceeded for state-of-the-art catalysts synthesized from 38 39 malachite/rosasite-like precursors. Zn-richer aurichalcite precursors were shown to lead 40 41 to inferior catalysts because aurichalcite is usually obtained in form of large platelets 42 exhibiting an unfavorable meso-structure compared to the needle-like zincian malachite. 43 44 We have started to investigate the structural properties of mixed basic Cu,Zn carbonates 45 46 from the perspective of Cu/ZnO catalyst preparation and first results have been reported 47 48 recently [ X2X,X6X]. Here, we report on the effects of Cu by Zn substitution on the crystal 49 50 structure and optical absorption properties of malachite and discuss the relationship 51 between synthetic (Cu 1-xZn x)(OH) 2CO 3 and to natural samples of malachite and rosasite. 52 53 54 55 Results and Discussion 56 57 58 59 60 3 Wiley-VCH ZAAC Page 4 of 32 1 2 3 A comparison of the crystal structures of natural malachite [X1X] and rosasite [X4X] based on 4 5 literature data, is shown in Figure 3. The main building blocks are (Cu,Zn)O 6 octahedra 6 7 and two distinct metal sites are present in both structures (Tab. 1). As pointed out by 8 9 Perchiazzi [X4X], the view along the c-axis (Fig. 3, top row) shows equal connectivity of the 10 11 octahedral units in both minerals in form of “double ribbons” of two edge-sharing 12 octahedra running along [001]. The major structural difference of the two phases 13 14 becomes apparent if the view is slightly tilted towards the (110) plane (Fig. 3, bottom 15 16 row), showing that the monoclinic angle is placed between the short and the medium axes 17 18 in malachite but between the short and the long axes in rosasite. Thus, one might get the 19 idea that both in malachite and rosasite the monoclinic angle β is shifted towards 90° as 20 21 the Cu:Zn ratio changes and that a hypothetical orthorhombic parent structure exists for 22 23 both minerals, possibly at an intermediate value of x. 24 25 The effects of substitution of Cu by Zn on the crystal structure of synthetic malachite, 26 27 leading to zincian malachite, (Cu 1-xZn x)(OH) 2CO 3, were first investigated in detail by 28 Porta et al. in 1988 [ D10 D,D11 D]. The authors report a contraction of the unit cell and a 29 30 systematic shift for the h0l and hkl reflections. We have recently reported a characteristic 31 32 and very pronounced shift of the (20 -1) and (21 -1) reflections to higher angles in the 33 34 XRD patterns as Cu is gradually substituted by Zn [ X2X]. The unusual effect on these 35 characteristic XRD peaks was explained as a ligand field effect.
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