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Synthesis and crystal structure of layered molybdate NH4Co2OH(MoO4)2⋅H2O

Published online by Cambridge University Press:  28 September 2023

Paweł Adamski Open the ORCID record for Paweł Adamski [Opens in a new window] ,
Aleksander Albrecht  and
Dariusz Moszyński
Paweł Adamski*
Affiliation:
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
Aleksander Albrecht
Affiliation:
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
Dariusz Moszyński
Affiliation:
Department of Inorganic Chemical Technology and Environment Engineering, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland
*
a)Author to whom correspondence should be addressed. Electronic mail: [email protected]
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Abstract

A new compound NH4Co2OH(MoO4)2⋅H2O was prepared by precipitation of aqueous solutions of cobalt nitrate and ammonium heptamolybdate at pH = 7.5. The crystal structure was identified by X-ray powder diffraction (XRPD) and Rietveld refinement as a known polymorph of layered molybdates (Φy) with general formula AT2OH(MoO4)2⋅H2O (A = NH4+, Na+, K+ and T = Zn2+, Co2+, Cu2+, Ni2+) and refined from a model based on that structure. The lattice parameters were refined with R-3 space group (148) a = 6.1014(2) Å, b = 6.1014(2) Å, c = 21.826(1) Å, α = 90°, β = 90°, and γ = 120°.

Keywords

Cobalt molybdateX-ray powder diffractionRietveld refinement
Type
New Diffraction Data
Information
Powder Diffraction , Volume 38 , Issue 4 , December 2023 , pp. 240 - 243
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Copyright
Copyright © The Author(s), 2023. Published by Cambridge University Press on behalf of International Centre for Diffraction Data

I. INTRODUCTION

Cobalt molybdates are interesting compounds, with many electrochemical (Mandal et al., Reference Mandal, Ghosh, Giri, Shakir and Das2014; Tian et al., Reference Tian, Zhou, Meng, Miao and Zhang2017; Kim et al., Reference Kim, Manikandan, Yu, Park, Kim, Park and Raj2019) and catalytical (Li et al., Reference Li, Yang, Liao, Luo, Wang, Cao, Zhou, Huang and Chen2018; Xu et al., Reference Xu, Xie, Li, Yang, Jiang and Chen2018) applications. Cobalt molybdates can be used as catalyst precursors to obtain cobalt molybdenum nitrides and sulfides (Kojima and Aika, Reference Kojima and Aika2001; Zhao et al., Reference Zhao, Yang, Gao, Pang, Kingman and Wu2016). There are three known polymorphic forms of cobalt molybdate CoMoO4: the pale green low-temperature α phase (monoclinic, space group C2/m) (Smith and Ibers, Reference Smith and Ibers1965), the black high-pressure hp phase (monoclinic, space group P2/c) (Livage et al., Reference Livage, Hynaux, Marrot, Nogues and Férey2002) and the pale violet high-temperature β phase (monoclinic, space group C2/m) (Courtine et al., Reference Courtine, Cord, Pannetier, Daumas and Montarnal1968). The hydrate phases CoMoO4⋅3/4H2O (triclinic, space group P-1) and CoMo4O13⋅2H2O (triclinic, space group P-1) are also known (Eda et al., Reference Eda, Uno, Nagai, Sotani and Whittingham2005, Reference Eda, Ohshiro, Nagai, Sotani and Whittingham2009). Additionally, two more hydrates were identified in the literature: CoMoO4⋅0.9H2O (PDF 00-014-0086) and CoMoO4⋅1.3H2O (PDF 00-014-0087) (Liu et al., Reference Liu, Kong, Ma, Lu, Li, Luo and Kang2012; Kim et al., Reference Kim, Manikandan, Yu, Park, Kim, Park and Raj2019). However, for these phases, no structural data were reported. The known structures of cobalt molybdate phases consist of cobalt atoms coordinated octahedrally with oxygen to form octahedra [CoO6]. In the β-phases and CoMoO4⋅3/4H2O hydrate, molybdenum is present in tetrahedra [MoO4], and in the α-, hp-phases, and CoMo4O13⋅2H2O hydrate in octahedra [MoO6] (Eda et al., Reference Eda, Uno, Nagai, Sotani and Whittingham2005, Reference Eda, Ohshiro, Nagai, Sotani and Whittingham2009).

The hydrate was found to lose its water of crystallization at 330 °C, transforming into amorphous CoMoO4, which then crystallizes as α-CoMoO4 (Haber, Reference Haber1974). Cobalt molybdate α-CoMoO4 can transform into β-CoMoO4 when cooled to 100 °C or during grinding at room temperature (Haber, Reference Haber1974). This transformation is reversible at 420 °C (β-CoMoO4 into α-CoMoO4) (Haber, Reference Haber1974). It was found that when the solid-phase reaction takes place between molybdenum(VI) oxide MoO3 and cobalt oxides, the main product is cobalt molybdate α-CoMoO4. Both cobalt molybdates, α-CoMoO4 as well as β-CoMoO4, are reduced in hydrogen into the Co2Mo3O8 and Co2MoO4 phases (Haber et al., Reference Haber, Sosnowska and Ziółkowski1976).

The most typical synthesis method of cobalt molybdates consists of precipitation from aqueous solutions of cobalt nitrate and ammonium heptamolybdate (Rodriguez et al., Reference Rodriguez, Chaturvedi, Hanson, Albornoz and Brito1998). Also, hydrothermal synthesis (Ding et al., Reference Ding, Yong, Min, Zhang and Yu2008), solid state reaction of MoO3 with CoO (Leyzerovich et al., Reference Leyzerovich, Bramnik, Buhrmester, Ehrenberg and Fuess2004), and other methods (Peng et al., Reference Peng, Gao, Yang and Sun2008) were applied successfully. In this work, a synthesis method and crystal structure of previously unreported compound NH4Co2OH(MoO4)2⋅H2O is presented.

II. EXPERIMENTAL

A. Synthesis

The material was obtained by precipitation from saturated aqueous solutions of cobalt(II) nitrate hexahydrate Co(NO3)2⋅6H2O (Chempur Poland, analytical grade) and ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24⋅4H2O (Chempur Poland, analytical grade). The solutions were heated to 80 °C and stirred with the use of a magnetic stirrer. In the next step, the solutions were mixed, and pH of the obtained mixture was adjusted by dropwise addition of 25 vol% aqueous ammonia to maintain at pH 7–7.5. The synthesis of purple precipitate was observed. The precipitation was continued at the same temperature, pH, and stirring for 25 min. Then, the precipitate was separated by vacuum filtration, rinsed with distilled water, and dried at 150 °C for 12 h. The obtained powder was ground in an agate mortar.

B. XRD data collection and structure refinement

X-ray powder diffraction measurements (XRPD) were conducted with a Philips X'pert Pro MPD diffractometer. The instrument was working in Bragg-Brentano geometry, with a goniometer radius of 240 mm, in continuous scan mode. An X-ray tube with the copper anode was used. The high voltage generator working parameters were 40 kV and 40 mA. A nickel filter was placed in the incident beam. In the diffracted beam, a graphite monochromator was mounted before the Pixcel1D detector. The diffraction data was collected in the angular range of 10–110° 2θ with a step size of 0.026 and ca. 230 s per step. Incident beam optics consisted of: the incident slit of 1/16°, the mask of 10 mm, and the Soller slit of 0.04 rad. Divergent beam optics used consisted of an anti-scatter slit of 7.7 mm and a 0.04 rad Soller slit.

The analysis of diffraction data was conducted with the use of PANalytical HighScore Plus v.3.0e software (Degen et al., Reference Degen, Sadki, Egbert, König and Nénert2014) coupled with International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF) 4+ (Gates-Rector and Blanton, Reference Gates-Rector and Blanton2019). Determination of the lattice parameters was conducted with the use of McMaille version 4.0 (Le Bail, Reference Le Bail2004). Rietveld refinement method (Rietveld, Reference Rietveld1967) was used to refine the structural parameters of the unknown phase. The following parameters were refined: scale factor, zero point error, sample displacement, unit-cell parameters, Caglioti parameters (U, V, W), profile coefficients, atomic coordinates, occupancies, individual isotropic thermal factors, and preferred orientation. A pseudo-Voigt type function was used. The structure imaging was performed with Crystal Impact Diamond software.

III. RESULTS AND DISCUSSION

For the obtained material, the ICDD PDF4+ database did not contain any matching patterns for a compound containing cobalt and molybdenum. It was established that the closest match to the acquired diffractogram was the PDF 04-018-0438 pattern (Wu et al., Reference Wu, Lu, Lin, Lu, Zhuang and Huang2004). This diffraction dataset was ascribed to a compound with NH4H3Cu2Mo2O10 formula. It is an example of layered molybdates of transition metals with the general formula AT 2OH(MoO4)2⋅H2O, where A = NH4+, Na+, or K+, T = Co, Ni, Cu, or Zn (Mitchell et al., Reference Mitchell, Gómez-Avilés, Gardner and Jones2010). The mentioned above transition metal molybdates, firstly described by Pezerat (Reference Pezerat1965), occur in two distinct polymorphs designated as Φx and Φy. In the Φx structure, the layers consist of the edge-connected [TO6] octahedrons and [MoO4] tetrahedrons (Clearfield et al., Reference Clearfield, Michael and Ramanathan2002). The Φy structure consists of alike octahedrons and tetrahedrons, however, in the [TO6] net ordered cation vacancies are present (Levin et al., Reference Levin, Soled and Ying1996). In both these polymorphs layers are stacked along the z-axis, they have a negative charge (balanced by cations in the interlayer positions), and hydrogen bonds are present between them.

It was assumed the as-obtained material was isostructural with that phase, and thus, its formula may be written as NH4H3Co2Mo2O10. Applying these assumption, the Rietveld refinement was performed, where all the Cu atoms were substituted by Co. The experimental diffraction pattern with the Rietveld refinement is depicted in Figure 1. The model structure obtained from Rietveld refinement is shown in Table I. On the basis of the above, it was assumed that the obtained material was with NH4Co2OH(MoO4)2⋅H2O formula and a Φy structure. Its lattice parameters are equal to a = 6.1014(2) Å, b = 6.1014(2) Å, c = 21.826(1) Å, α = 90°, β = 90°, and γ = 120°. The compound is in a trigonal system with an R-3 space group (148). The atomic positions are given in Table I. In Rietveld refinement, a March-Dollase (Dollase, Reference Dollase1986) preferred orientation model was included; the 1st preferred orientation parameter alongside [001] direction was refined as 0.974(3). The R factors were: R exp = 3.54%, R wp = 7.82%, Rp = 5.80%, and GOF = 4.88. The structural model of the material is depicted in Figure 2. The refined structure was deposited with the ICDD PDF4+ database with the 00-071-0747 number.

Figure 1. Diffraction pattern of the material with the Rietveld refinement indicated.

TABLE I. Atomic positions and occupancy obtained with Rietveld refinement.

Figure 2. Crystal structure of NH4Co2OH(MoO4)2⋅H2O. The vacancies in Co positions are omitted for clarity.

IV. CONCLUSION

It was established that the precipitation of cobalt molybdate from aqueous solutions of cobalt nitrate and ammonium heptamolybdate at pH 7.5, modified with aqueous ammonia resulted in the synthesis of previously unpublished compound NH4Co2OH(MoO4)2⋅H2O. Its structure is a polymorph of layered molybdates (Φy) AT 2OH(MoO4)2⋅H2O (A = NH4+, Na+, K+ and T = Zn2+, Co2+, Cu2+, Ni2+), and consist of the layers of the edge-connected [CoO6] octahedrons and [MoO4] tetrahedrons.

V. DEPOSITED DATA

Files containing the raw diffraction data of material along with a file containing the refined structural parameters were deposited with the ICDD. The data can be requested at [email protected]. As observed powder diffraction pattern is included in the .cif file.

Supplementary material

The supplementary material for this article can be found at https://doi.org/10.1017/S0885715623000350.

ACKNOWLEDGEMENTS

The scientific work was financed by the Polish National Centre for Research and Development, grant “Lider”, project No. LIDER/10/0039/L-10/18/NCBR/2019. The support of ICDD through a Special Grant-in-Aid Program is kindly acknowledged.

CONFLICT OF INTEREST

The authors have no conflicts of interest to declare.

References

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Figure 0

Figure 1. Diffraction pattern of the material with the Rietveld refinement indicated.

Figure 1

TABLE I. Atomic positions and occupancy obtained with Rietveld refinement.

Figure 2

Figure 2. Crystal structure of NH4Co2OH(MoO4)2⋅H2O. The vacancies in Co positions are omitted for clarity.

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