Mechanism of activity enhancement of the Ni based hydrotalcite-derived materials in carbonyl sulfide removal
Graphical abstract
Introduction
Hydrotalcite-like compounds (HTLCs), also known as layered double hydroxide (LDH), are a family of anionic clays [1], [2], [3], [4], [5]. The chemical composition can be represented by the general formula: [M(II)1−xM(III)x(OH)2]x+(An−)x/n•mH2O, where M(II) and M(III) is divalent and trivalent cations in the octahedral positions within the hydroxide layers. An− is an exchangeable interlayer anion. The structure of HTLCs is similar to that of brucite where each Mg2+ ion is octahedrally surrounded by six OH ions and the different octahedra share edges to form infinite sheets. In the process of roasting, HTLCs can lose crystal water and the interlayer anions. The mixed oxides derived from the HTLCs possess large specific surface area and strong surface reactivity, therefore, the materials have received increased interest in anion exchange media, adsorbents, catalysis, and so forth [6], [7], [8], [9], [10].
Energy efficient and sustainable utilization has been an important subject for economic development of the world. Coal has been the main primary energy in the world, and the absolute amount of coal in average annual consumption will show an upward trend. Nevertheless, one of the well-known challenges associated with the use of coal gas is the presence of sulfur species which may damage or poison the downstream pipelines, instrument and catalysts. Therefore, the desulfurization plays an important role in purification of coal-derived gas [11], [12].
Several kinds of sulfur-containing compounds, for example hydrogen sulfide, carbonyl sulfide (COS), sulfur dioxide, etc., are inevitably generated in the transforming process of coal, while COS is very difficult to be directly removed from the coal gas because of its stability and neutral nature [13]. Trace amount of COS can result in the deactivation of catalysts and lead to corrosion of reaction equipments [14], [15]. Furthermore, not only does COS provides economic problems, but also affects the environment. Photolysis of COS is the principal source of stratospheric sulfate layer [16], [17]. Thus, deep removal of COS is in high demand.
To date, many articles have reported the removal of COS. Hutchings G. et al. investigated that the rate of COS hydrolysis can be significantly enhanced by the addition of K, Cs, Fe, Co, Ni, Cu and Zn to the alumina [18]. They noted that the activity is involved in basicity of the promoted catalyst. Liu Y. et al. [19] studied the heterogeneous reaction of COS on mineral oxides. The results showed that the activity series for heterogeneous oxidation of COS decrease in the following sequence: Al2O3 ≈ CaO > MgO > TiO2 ≈ ZnO > Fe2O3 > SiO2. The specific surface area and surface basicity of these oxides have great effect on the catalytic activity. Thus it can be seen that the materials which can provide abundant and active acid/base sites having a great applying potentiality.
In this paper, a series of Ni containing hydrotalcite-derived oxides (HTO) with various trivalent cations (Fe, Al, and Cr) were synthesized. The samples were characterized by X-Ray Diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), N2 adsorption/desorption techniques, CO2temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) in order to study the effect of trivalent cations on the electronic structure and activity of the mixed oxides. The density-functional calculations (DFT) method was adopted to simulate the desulfurization reaction process and electronic structures of the desulfurizer.
Section snippets
Preparation of the desulfurizer
The hydrotalcites with a Ni/MⅢ (MⅢ = Al, Fe, and Cr) molar ratio of 3.0 was prepared by urea pyrolysis method. Ni(NO3)2·6H2O, Al(NO3)3·9H2O (Fe(NO3)3·9H2O or Cr(NO3)3·9H2O) and urea were dissolved in distilled water first, wherein the total concentration of metal ions was fixed at 0.1 M, and the molar ratios of Ni/MⅢ and urea/([Ni] + [MⅢ]) were 3 and 40, respectively. The mixed solution was heated to and maintained at 110 °C for 24 h. The gel was washed with distilled water until the pH of the
Characterization of the HTLCs and the derived mixed oxides
Fig. 1 presents SEM images of the HTLCs precursors and samples calcined at 400 °C. The aggregates composed of small thin crystals which is morphology characteristic of hydrotalcite materials were observed for all samples [30]. This structure can bring developed pore structure and high surface areas as evidenced by results of N2 adsorption/desorption, which is conducive to the adsorption of COS. In addition, it can be seen that the morphology of the samples was nearly unchanged. Layer plate
Conclusions
In this paper, Ni3Al-HTO, Ni3Fe-HTO, and Ni3Cr-HTO were successfully prepared by thermal decomposition of the hydrotalcite-like compounds. Removal of COS at low temperature over the hydrotalcite-derived oxides was studied. The desulfurizer was characterized by SEM, XRD, FTIR, CO2-TPD, FTIR, XPS, XAFS, and N2 physical adsorption analysis. The results showed that the activity series for removal of COS decrease in the following sequence: Ni3Al-HTO > Ni3Fe-HTO > Ni3Cr-HTO. The characterization
Acknowledgments
This work is supported by China Postdoctoral Science Foundation (2016M600045), and Fundamental Research Funds for the Central Universities (FRF-TP-16-060A1).
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