DOI: 10.1002/chem.201402691
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& Nanomaterials
General Facile Approach to Transition-Metal Oxides with Highly Uniform Mesoporosity and Their Application as Adsorbents for Heavy-Metal-Ion Sequestration Gulaim A. Seisenbaeva,*[b] Geoffrey Daniel,[c] Vadim G. Kessler,[b] and Jean-Marie Nedelec*[a] tremely high adsorption capacity in removal of CrVI anions from solutions (25.8 for TiO2, 73.0 for ZrO2, and 74.7 mg g1 for Nb2O5 in relation to the Cr2O72 anion), making them very attractive as adsorbents in water remediation applications. The difference in adsorption capacities for the studied oxides may be explained by variation in surface hydration and surface-charge distribution.
Abstract: Mesoporous powders of transition-metal oxides, TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5, pure from organic impurities were produced by a rapid single-step thermohydrolytic approach. The obtained materials display an impressively large active surface area and sharp pore-size distribution, being composed of partially coalesced uniform nanoparticles with crystalline cores and amorphous shells. They reveal ex-
Introduction
improve their crystallinity and then to calcination. Alternatively, such materials can be synthesized in the absence of template under rather extreme solvothermal conditions and still require calcination for removal of the organic residues.[10] Mesoporous oxides can be produced under mild conditions as films by using an evaporation-induced self-assembly (EISA) approach, but remain in that case quite thermally unstable and most often collapse on attempts to remove the applied surfactants by thermal treatment.[11] The use of sacrificial matrices comprised of mesoporous silica[12] or mesoporous carbon[13] with subsequent removal by leaching or calcination, render materials with the cost incompatible with applications in water remediation. Sol-gel synthesis by using metal alkoxides as precursors is potentially a very promising route for the production of mesoporous oxides. The oxides obtained by hydrolytic transformation of these precursors are nucleating in the form of polyoxometallate-type species (micelles templated by self-assembly of ligands, MTSALs), which behave, due to their size of 2–5 nm as phase-separated (nano)particles.[14] Their common structure comprises a crystalline core with the structure of the relevant metal oxide covered by an amorphous shell bearing residual organic ligands.[14b,c] Particles with thicker amorphous shells easily coalesce if not protected through electric charging, for example, by protonation, and this often results in closed mesoporosity formed through extended coalescence in the outer layer of particle aggregates.[15] Dependent on the nature of the alkoxide groups and introduced heteroligands it is possible to influence the size of the particles and their crystallinity.[16] The latter is improved on the use of increased temperature and with more efficient removal of the ligands. Thus highly crystalline single nanoparticles of titania have been produced by immersion of solid titanium alkoxide modified with highly hydrophilic amino acid ligands into boiling water.[17] In our recent study, we have shown that using solid titanium alkoxides de-
Mesoporous transition-metal oxides constitute a highly attractive class of materials due to a broad spectrum of applications, ranging from heterogeneous catalysts,[1] in particular, photocatalysts,[2] and creation of matrices for catalyst deposition,[3] electrode materials for metal ion batteries,[4] supercapacitors,[5] and dye-sensitized solar cells,[6] to functional adsorbents for removal from ground and wastewaters of radioactive[7] and, more generally, heavy-metal pollutants.[8] To be attractive for all these potential uses, metal oxides have to possess a high active surface area, in the first hand, and considerable chemical stability to different types of leaching and dissolution, and in many cases also be essentially pure from residual organics that may deteriorate the surface characteristics required for alkali metal-ion insertion, but also for heavy-metal adsorption. This makes obtaining such materials a considerable challenge. Most often they are produced using either large organic, and even macromolecular, templates[9] and need then to be subjected subsequently to relatively long-term solvothermal treatment to [a] Prof. J.-M. Nedelec ICCF, CNRS UMR 6296 Clermont Universit, ENSCCF BP 10448, 63177 Clermont-Ferrand (France) E-mail: [email protected] [b] Dr. G. A. Seisenbaeva, Prof. V. G. Kessler Department of Chemistry and Biotechnology, BioCenter Swedish University of Agricultural Sciences Box 7015, 75007 Uppsala (Sweden) E-mail: [email protected] [c] Prof. G. Daniel Department of Forest Products/Wood Science Swedish University of Agricultural Sciences Box 7008, 75007 Uppsala (Sweden) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402691. Chem. Eur. J. 2014, 20, 1 – 6
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Full Paper rived from highly volatile alcohols, MeOH and EtOH, in reaction in boiling water it is possible to produce materials with high crystallinity and open mesoporosity, exploiting the evaporation of ligands as a tool to prevent collapse of the pores.[18] Herein, we demonstrate that this approach has a general nature and can be used to produce, in a rapid one-step synthesis not requiring any long-term after-treatment, a broad variety of chemically different metal oxides with highly active surface area and rather uniform open mesoporosity that, due to their enhanced surface properties, are capable of acting as highly efficient adsorbents for the sequestration of heavymetal ions.
Table 1. Textural characteristics of metal oxides produced by thermohydrolysis. Metal oxide[a]
Precursor
TiO2 ZrO2 ZrO2 Nb2O5 Ta2O5
Ti(OMe)4 Zr(OEt)4 Zr(OiPr)4(iPrOH) Nb(OMe)5 Ta(OMe)5
Active surface area [m2 g1][b] 278 257 321 410 98
Mean pore size [nm][c]
Pore volume [cm3 g1][d]
5.1 3.2 5.1 2.9 3.8
0.43 0.20 0.41 0.29 0.09
[a] Data after initial drying at 120 8C for 3 h. [b] Calculated using the BET method. [c,d] Calculated using the BJH method on the desorption branch and at p/p0 = 0.99.
Results and Discussion Metal alkoxides applied in this work were chosen either among the commercially available solid microcrystalline materials, such as Zr(OEt)4, Zr(OiPr)4(iPrOH), and Hf(OiPr)4(iPrOH) (Aldrich 339121, 339237, and 697508, respectively, and, for comparison, also nonsublimed Ti(OMe)4 Aldrich 404950), or were produced by a simple and highly productive method based on anodic oxidation of metals in the parent alcohol, MeOH, for Nb(OMe)5 and Ta(OMe)5.[19] Synthesis of the oxide material was carried out by using a unified procedure by quick immersion of the precursor powder into boiling water with subsequent refluxing in half-an-hour. All produced oxide materials are mesoporous and demonstrate nitrogen sorption isotherms of type IV according to the IUPAC classification (see Figure 1).[20] They possess impressively large active surface areas and pore volume fractions (see Table 1) with sharp pore-size distribution with a well-defined maximum in the mesoscale (Figure 1).
Figure 2. X-ray powder diffraction patterns for TiO2 produced from Ti(OMe)4 (anatase, bottom), ZrO2 obtained from Zr(OEt)4 (middle), and Nb2O5 produced from Nb(OMe)5 (top). For reference patterns, please see the Supporting Information.
the considerably smaller volumes of the crystalline cores in the obtained nanoparticles, not permitting true diffraction to be revealed. This supposition was confirmed very distinctly by the HRTEM investigation of the ZrO2 sample obtained from Zr(OEt)4. In Figure 3, the structures of a TiO2 nanorod obtained from Ti(OMe)4 and the ZrO2 nanorod produced by the same approach from Zr(OEt)4 nanocrystals are compared in the same scale. It can clearly be seen that while the spherical TiO2 nanoparticles (anatase, distance between fringes 0.35 nm, 011 planes) are considerably bigger, about 5 nm in size, and mostly crystalline, the ellipsoidal ZrO2 nanoparticles (tetragonal phase, distance between fringes 0.30 nm, 101 planes) are much smaller, about 2 4 nm, and clearly bear a relatively thick amorphous shell. Another easily observable important feature is that the TiO2 nanoparticles are apparently packed less densely in the emerging structure, which explains why the smaller size of ZrO2 particles is not resulting in higher surface area or pore volume. The nature of the precursor plays a considerable role in relation to the size of the produced nanoparticles and resulting porosity. Bigger and less volatile isopropoxide alcohol as the ligand results for the ZrO2 final material in a considerably bigger pore size, 5.1 nm, in comparison with 3,2 nm on average for ZrO2 derived from the ethoxide complex. The cumulative pore volume for the material produced from the isoprop-
Figure 1. Nitrogen sorption isotherms for mesoporous oxides obtained by thermohydrolytic treatment of Ti(OMe)4 (a), Zr(OiPr)4(iPrOH) (b), Nb(OMe)5 (c), Zr(OEt)4 (d), and Ta(OMe)5 (e).
The X-ray powder study results for produced oxides generally show considerably less defined patterns than that observed earlier for TiO2 (Figure 2) and reminiscent more of the X-ray scattering than diffraction. The reason lies most apparently in &
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Figure 4. Left) TGA curves for the as-prepared samples of ZrO2 from Zr(OEt)4 (a), HfO2 from Hf(OiPr)4(iPrOH) (b), and Nb2O5 from Nb(OMe)5 (c). Right) XRD patterns for resulting metal oxides, cubic ZrO2 (a), monoclinic HfO2 (b), and orthorhombic Nb2O5 and Ta2O5 (c, top and bottom, respectively). For the reference XRD pattern, please see the Supporting Information. Figure 3. HRTEM images of the TiO2 (top) and ZrO2 produced from the ethoxide precursor (bottom) nanostructures produced by the thermohydrolytic approach.
moval of the adsorbed matter from as-synthesized samples occurs as a single-step process at low temperatures even under atmospheric pressure. Analysis of the differential curves in TGA (DTA) indicates that the observed single-step transition usually is involving two overlapping effects, both with maxima below 100 8C and associated supposedly with evaporation of water from the surface of the particles and then from the mesoporous structure. Evacuation at 102 mm Hg and 120 8C in over 2 h produces materials that do not lose any more weight in TGA. This feature is very important for many practical applications of mesoporous metal oxide materials, in particular for their use as electrodes for metal (Li + and Na + )-ion batteries.[21] It is important to note that the oxide phases obtained by heating to 600 8C in TGA have relatively high symmetry. The phases revealed in TGA residues were cubic ZrO2 (for both precursors—only minor difference in the weight loss could be observed), orthorhombic Nb2O5 (and partly still amorphous Ta2O5 with already distinguishable orthorhombic phase present), and monoclinic HfO2 (no other phases are actually stable for this compound). Crystallization of relatively higher symmetry phases in this case is caused apparently by the fact that MTSAL cores acting as nuclei for crystallization involve generally higher symmetry packing of metal cations and oxide anions. In view of the growing interest in metal oxides as adsorbent materials, we wanted to evaluate the ability of the produced
oxide is twice as high as the one derived from the ethoxide. Even the active surface area is rendered noticeably larger when a derivative of bigger ligand is used as precursor. Earlier the same relationship was found even between TiO2 samples derived from the bigger ligand ethoxide relative to the smaller one methoxide.[18] The nature of the metal in the metal oxide also plays a considerable role. It is apparent that the highest crystallinity is observed for TiO2, which also displays bigger particles. Even if the particles in the case of ZrO2, produced from Zr(OiPr)4(iPrOH), were relatively bigger in size, their crystalline cores cannot exceed 2 nm as indicated by the scattering-type XRD pattern. It has to be mentioned that Nb2O5 displays clearly exceptional features, being characterized by the smallest pore (and apparently, particle) size and quite exceptional active surface area. For Ta2O5, the active surface area appears to be smaller, but this value is a bit misleading in view of the much higher density for this material in bulk form (8.18 compared to 3.78 g cm3 for anatase). The observed active surface area for Ta2O5 of 98 m2 g1 is corresponding thus to at least about 220 m2 g1 for the relevant anatase structure. An important feature of all the obtained materials is that they are not bearing any strongly bound or incorporated organic residues. According to TGA data (see Figure 4) the reChem. Eur. J. 2014, 20, 1 – 6
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Full Paper mesoporous matrices in the sequestration of heavy-metal ions. Understanding that cost-efficiency is an important factor in relation to practical applications of this type, we focused in our studies on ZrO2 and Nb2O5 in comparison with recently characterized TiO2.[18] A broadly used model for such evaluation is adsorption of dichromate ions, Cr2O72, a relatively widespread and dangerous pollutant displaying a recognized carcinogenic effect.[9] The adsorption takes place quite efficiently in both neutral and acidic medium. The reported data (Figure 5) correspond to a medium kept at pH 3 to facilitate the spectrophotometric quantification of the dichromate ions.[18] This value is potentially relevant for studies of CrVI removal from industrial wastewaters, especially those generated by corrosion-protection technologies.[22]
tion capacity is apparently results from higher surface area and possibly even higher concentration of the active adsorption sites for the samples produced by the thermohydrolytic approach in this work. The reason supposedly is that their preparation was template-free and did not involve any calcination step.
Conclusion The proposed thermohydrolytic approach using simply hydrolysis and transformation (nucleation) within solid metal alkoxide precursors in boiling water has proved to be a general reliable approach for the creation of mesoporous oxide matrices with truly large active surface area, uniform pore size, and considerable pore volume. The produced materials are free from organic impurities and do not require any long-term after treatment or calcination, which opens prospects for many attractive applications. The behavior in adsorption of dichromate ions revealed by the obtained mesoporous oxides sets them into the frontline of attractive materials for applications in heavy-metal sequestration from ground and wastewaters.
Experimental Section Synthesis Figure 5. The dichromate ion, Cr2O72, adsorption. Isotherms are fitted by Langmuir equations for ZrO2 derived from Zr(OEt)4 and Nb2O5—from Nb(OMe)5. The data for TiO2, reported earlier in ref. [18] are provided for comparison.
Solid metal alkoxide precursors used in this work, were either purchased from Aldrich (Ti(OMe)4, Zr(OEt)4, Zr(OiPr)4(iPrOH), and Hf(OiPr)4(iPrOH)) or produced by anodic oxidation of metals in the parent alcohol with subsequent purification by conventional techniques as described earlier (Nb(OMe)5 and Ta(OMe)5).[19] The synthesis of adsorbent materials was carried out by quick insertion of the microcrystalline alkoxide (about 0.50 g) into boiling water (typically 50 mL) with subsequent refluxing at ambient atmosphere for 30 min. Milli-Q water (pH 7.0) was used for solution preparation and synthesis.
It should be noted that the general appearance of the adsorption isotherms is in agreement with the data recently reported in the literature[9] for TiO2, ZrO2, and mixed metal mesoporous matrices, while the adsorption capacity observed for materials in this study is considerably higher than the best results reported so far (35.15 mg g1 CrVI compared to 25.40 reported in ref. [9] about a 40 % improvement!). The adsorption occurs in agreement with the Langmuir isotherm, defined by the following equation:
Characterization SEM-EDS studies were carried out with a Hitachi TM-1000-m-DeX tabletop scanning electron microscope. TEM investigations were made using a CM12 transmission electron microscope (Philips, Holland) with an accelerating voltage of 100 kV. X-ray powder patterns were obtained using a Bruker SMART Apex-II diffractometer operating with MoKa radiation (l = 0.71073 ). Bruker Apex-II and EVA software were used for integration and data treatment. Textural characteristics (specific surface area, mean pore size, porous volume) were measured by using nitrogen sorption at 77 K on a Quantachrome Autosorb 1 apparatus. TGA investigations were made with Perkin–Elmer Pyris 1 instrument (heating rate 10 8C min1, temperature range 25–600 8C) with simultaneous analysis of outgoing gases using Perkin–Elmer Spectrum 100 FTIR instrument. Spectrophotometric determination of dichromate ions was carried out with Hitachi UV-2001 UV/Vis instrument in the water solutions set to pH 3 by addition of nitric acid. The calibration curve was measured as absorbance at lmax at 420 nm (correlated at lower concentrations with a more intense peak at 350 nm) and remained strictly linear up to 300 mg L1, R2 = 0.9992.
qe ¼ Qmax a C e ð1 þ a C e Þ in which Qmax stays for the maximal adsorption capacity and a is a specific constant characterizing the affinity of adsorbed substrate to the adsorbent. The affinity to TiO2 determined by least-square refinement is characterized by a relatively higher value compared to those for ZrO2 and Nb2O5 (the latter—investigated for the first time as an adsorbent for CrVI removal in this work). It can be hypothesized that adsorption on titania is proceeding by formation of a “covalent” TiOCr bond, whereas for heavier transition metals both electrostatic interactions and hydrogen bonding can play a more considerable role. The general appearance of the Langmuir isotherms is very similar to those reported in ref. [9]. The observed difference in adsorp&
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Full Paper Acknowledgements [7]
The authors would like to express their gratitude to the Swedish Research Council for the support to the projects “Molecular precursors and molecular models of Nanoporous materials” and “Advanced magnetically removable adsorbents for complex water remediation”.
[8] [9] [10] [11]
Keywords: mesoporous oxides · nanostructures · sorption mechanism · thermohydrolysis · transition metals
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FULL PAPER & Nanomaterials
Mesoporous metal oxides with highly active surface area and pore volume, attractive for applications in heavy-metalion sequestration, can easily be produced from solid metal alkoxide precursors by a facile thermohydrolytic approach (see figure).
G. A. Seisenbaeva,* G. Daniel, V. G. Kessler, J.-M. Nedelec* && – && General Facile Approach to TransitionMetal Oxides with Highly Uniform Mesoporosity and Their Application as Adsorbents for Heavy-Metal-Ion Sequestration
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& Nanomaterials
General Facile Approach to Transition-Metal Oxides with Highly Uniform Mesoporosity and Their Application as Adsorbents for Heavy-Metal-Ion Sequestration Gulaim A. Seisenbaeva,*[b] Geoffrey Daniel,[c] Vadim G. Kessler,[b] and Jean-Marie Nedelec*[a] tremely high adsorption capacity in removal of CrVI anions from solutions (25.8 for TiO2, 73.0 for ZrO2, and 74.7 mg g1 for Nb2O5 in relation to the Cr2O72 anion), making them very attractive as adsorbents in water remediation applications. The difference in adsorption capacities for the studied oxides may be explained by variation in surface hydration and surface-charge distribution.
Abstract: Mesoporous powders of transition-metal oxides, TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5, pure from organic impurities were produced by a rapid single-step thermohydrolytic approach. The obtained materials display an impressively large active surface area and sharp pore-size distribution, being composed of partially coalesced uniform nanoparticles with crystalline cores and amorphous shells. They reveal ex-
Introduction
improve their crystallinity and then to calcination. Alternatively, such materials can be synthesized in the absence of template under rather extreme solvothermal conditions and still require calcination for removal of the organic residues.[10] Mesoporous oxides can be produced under mild conditions as films by using an evaporation-induced self-assembly (EISA) approach, but remain in that case quite thermally unstable and most often collapse on attempts to remove the applied surfactants by thermal treatment.[11] The use of sacrificial matrices comprised of mesoporous silica[12] or mesoporous carbon[13] with subsequent removal by leaching or calcination, render materials with the cost incompatible with applications in water remediation. Sol-gel synthesis by using metal alkoxides as precursors is potentially a very promising route for the production of mesoporous oxides. The oxides obtained by hydrolytic transformation of these precursors are nucleating in the form of polyoxometallate-type species (micelles templated by self-assembly of ligands, MTSALs), which behave, due to their size of 2–5 nm as phase-separated (nano)particles.[14] Their common structure comprises a crystalline core with the structure of the relevant metal oxide covered by an amorphous shell bearing residual organic ligands.[14b,c] Particles with thicker amorphous shells easily coalesce if not protected through electric charging, for example, by protonation, and this often results in closed mesoporosity formed through extended coalescence in the outer layer of particle aggregates.[15] Dependent on the nature of the alkoxide groups and introduced heteroligands it is possible to influence the size of the particles and their crystallinity.[16] The latter is improved on the use of increased temperature and with more efficient removal of the ligands. Thus highly crystalline single nanoparticles of titania have been produced by immersion of solid titanium alkoxide modified with highly hydrophilic amino acid ligands into boiling water.[17] In our recent study, we have shown that using solid titanium alkoxides de-
Mesoporous transition-metal oxides constitute a highly attractive class of materials due to a broad spectrum of applications, ranging from heterogeneous catalysts,[1] in particular, photocatalysts,[2] and creation of matrices for catalyst deposition,[3] electrode materials for metal ion batteries,[4] supercapacitors,[5] and dye-sensitized solar cells,[6] to functional adsorbents for removal from ground and wastewaters of radioactive[7] and, more generally, heavy-metal pollutants.[8] To be attractive for all these potential uses, metal oxides have to possess a high active surface area, in the first hand, and considerable chemical stability to different types of leaching and dissolution, and in many cases also be essentially pure from residual organics that may deteriorate the surface characteristics required for alkali metal-ion insertion, but also for heavy-metal adsorption. This makes obtaining such materials a considerable challenge. Most often they are produced using either large organic, and even macromolecular, templates[9] and need then to be subjected subsequently to relatively long-term solvothermal treatment to [a] Prof. J.-M. Nedelec ICCF, CNRS UMR 6296 Clermont Universit, ENSCCF BP 10448, 63177 Clermont-Ferrand (France) E-mail: [email protected] [b] Dr. G. A. Seisenbaeva, Prof. V. G. Kessler Department of Chemistry and Biotechnology, BioCenter Swedish University of Agricultural Sciences Box 7015, 75007 Uppsala (Sweden) E-mail: [email protected] [c] Prof. G. Daniel Department of Forest Products/Wood Science Swedish University of Agricultural Sciences Box 7008, 75007 Uppsala (Sweden) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402691. Chem. Eur. J. 2014, 20, 1 – 6
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Full Paper rived from highly volatile alcohols, MeOH and EtOH, in reaction in boiling water it is possible to produce materials with high crystallinity and open mesoporosity, exploiting the evaporation of ligands as a tool to prevent collapse of the pores.[18] Herein, we demonstrate that this approach has a general nature and can be used to produce, in a rapid one-step synthesis not requiring any long-term after-treatment, a broad variety of chemically different metal oxides with highly active surface area and rather uniform open mesoporosity that, due to their enhanced surface properties, are capable of acting as highly efficient adsorbents for the sequestration of heavymetal ions.
Table 1. Textural characteristics of metal oxides produced by thermohydrolysis. Metal oxide[a]
Precursor
TiO2 ZrO2 ZrO2 Nb2O5 Ta2O5
Ti(OMe)4 Zr(OEt)4 Zr(OiPr)4(iPrOH) Nb(OMe)5 Ta(OMe)5
Active surface area [m2 g1][b] 278 257 321 410 98
Mean pore size [nm][c]
Pore volume [cm3 g1][d]
5.1 3.2 5.1 2.9 3.8
0.43 0.20 0.41 0.29 0.09
[a] Data after initial drying at 120 8C for 3 h. [b] Calculated using the BET method. [c,d] Calculated using the BJH method on the desorption branch and at p/p0 = 0.99.
Results and Discussion Metal alkoxides applied in this work were chosen either among the commercially available solid microcrystalline materials, such as Zr(OEt)4, Zr(OiPr)4(iPrOH), and Hf(OiPr)4(iPrOH) (Aldrich 339121, 339237, and 697508, respectively, and, for comparison, also nonsublimed Ti(OMe)4 Aldrich 404950), or were produced by a simple and highly productive method based on anodic oxidation of metals in the parent alcohol, MeOH, for Nb(OMe)5 and Ta(OMe)5.[19] Synthesis of the oxide material was carried out by using a unified procedure by quick immersion of the precursor powder into boiling water with subsequent refluxing in half-an-hour. All produced oxide materials are mesoporous and demonstrate nitrogen sorption isotherms of type IV according to the IUPAC classification (see Figure 1).[20] They possess impressively large active surface areas and pore volume fractions (see Table 1) with sharp pore-size distribution with a well-defined maximum in the mesoscale (Figure 1).
Figure 2. X-ray powder diffraction patterns for TiO2 produced from Ti(OMe)4 (anatase, bottom), ZrO2 obtained from Zr(OEt)4 (middle), and Nb2O5 produced from Nb(OMe)5 (top). For reference patterns, please see the Supporting Information.
the considerably smaller volumes of the crystalline cores in the obtained nanoparticles, not permitting true diffraction to be revealed. This supposition was confirmed very distinctly by the HRTEM investigation of the ZrO2 sample obtained from Zr(OEt)4. In Figure 3, the structures of a TiO2 nanorod obtained from Ti(OMe)4 and the ZrO2 nanorod produced by the same approach from Zr(OEt)4 nanocrystals are compared in the same scale. It can clearly be seen that while the spherical TiO2 nanoparticles (anatase, distance between fringes 0.35 nm, 011 planes) are considerably bigger, about 5 nm in size, and mostly crystalline, the ellipsoidal ZrO2 nanoparticles (tetragonal phase, distance between fringes 0.30 nm, 101 planes) are much smaller, about 2 4 nm, and clearly bear a relatively thick amorphous shell. Another easily observable important feature is that the TiO2 nanoparticles are apparently packed less densely in the emerging structure, which explains why the smaller size of ZrO2 particles is not resulting in higher surface area or pore volume. The nature of the precursor plays a considerable role in relation to the size of the produced nanoparticles and resulting porosity. Bigger and less volatile isopropoxide alcohol as the ligand results for the ZrO2 final material in a considerably bigger pore size, 5.1 nm, in comparison with 3,2 nm on average for ZrO2 derived from the ethoxide complex. The cumulative pore volume for the material produced from the isoprop-
Figure 1. Nitrogen sorption isotherms for mesoporous oxides obtained by thermohydrolytic treatment of Ti(OMe)4 (a), Zr(OiPr)4(iPrOH) (b), Nb(OMe)5 (c), Zr(OEt)4 (d), and Ta(OMe)5 (e).
The X-ray powder study results for produced oxides generally show considerably less defined patterns than that observed earlier for TiO2 (Figure 2) and reminiscent more of the X-ray scattering than diffraction. The reason lies most apparently in &
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Figure 4. Left) TGA curves for the as-prepared samples of ZrO2 from Zr(OEt)4 (a), HfO2 from Hf(OiPr)4(iPrOH) (b), and Nb2O5 from Nb(OMe)5 (c). Right) XRD patterns for resulting metal oxides, cubic ZrO2 (a), monoclinic HfO2 (b), and orthorhombic Nb2O5 and Ta2O5 (c, top and bottom, respectively). For the reference XRD pattern, please see the Supporting Information. Figure 3. HRTEM images of the TiO2 (top) and ZrO2 produced from the ethoxide precursor (bottom) nanostructures produced by the thermohydrolytic approach.
moval of the adsorbed matter from as-synthesized samples occurs as a single-step process at low temperatures even under atmospheric pressure. Analysis of the differential curves in TGA (DTA) indicates that the observed single-step transition usually is involving two overlapping effects, both with maxima below 100 8C and associated supposedly with evaporation of water from the surface of the particles and then from the mesoporous structure. Evacuation at 102 mm Hg and 120 8C in over 2 h produces materials that do not lose any more weight in TGA. This feature is very important for many practical applications of mesoporous metal oxide materials, in particular for their use as electrodes for metal (Li + and Na + )-ion batteries.[21] It is important to note that the oxide phases obtained by heating to 600 8C in TGA have relatively high symmetry. The phases revealed in TGA residues were cubic ZrO2 (for both precursors—only minor difference in the weight loss could be observed), orthorhombic Nb2O5 (and partly still amorphous Ta2O5 with already distinguishable orthorhombic phase present), and monoclinic HfO2 (no other phases are actually stable for this compound). Crystallization of relatively higher symmetry phases in this case is caused apparently by the fact that MTSAL cores acting as nuclei for crystallization involve generally higher symmetry packing of metal cations and oxide anions. In view of the growing interest in metal oxides as adsorbent materials, we wanted to evaluate the ability of the produced
oxide is twice as high as the one derived from the ethoxide. Even the active surface area is rendered noticeably larger when a derivative of bigger ligand is used as precursor. Earlier the same relationship was found even between TiO2 samples derived from the bigger ligand ethoxide relative to the smaller one methoxide.[18] The nature of the metal in the metal oxide also plays a considerable role. It is apparent that the highest crystallinity is observed for TiO2, which also displays bigger particles. Even if the particles in the case of ZrO2, produced from Zr(OiPr)4(iPrOH), were relatively bigger in size, their crystalline cores cannot exceed 2 nm as indicated by the scattering-type XRD pattern. It has to be mentioned that Nb2O5 displays clearly exceptional features, being characterized by the smallest pore (and apparently, particle) size and quite exceptional active surface area. For Ta2O5, the active surface area appears to be smaller, but this value is a bit misleading in view of the much higher density for this material in bulk form (8.18 compared to 3.78 g cm3 for anatase). The observed active surface area for Ta2O5 of 98 m2 g1 is corresponding thus to at least about 220 m2 g1 for the relevant anatase structure. An important feature of all the obtained materials is that they are not bearing any strongly bound or incorporated organic residues. According to TGA data (see Figure 4) the reChem. Eur. J. 2014, 20, 1 – 6
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Full Paper mesoporous matrices in the sequestration of heavy-metal ions. Understanding that cost-efficiency is an important factor in relation to practical applications of this type, we focused in our studies on ZrO2 and Nb2O5 in comparison with recently characterized TiO2.[18] A broadly used model for such evaluation is adsorption of dichromate ions, Cr2O72, a relatively widespread and dangerous pollutant displaying a recognized carcinogenic effect.[9] The adsorption takes place quite efficiently in both neutral and acidic medium. The reported data (Figure 5) correspond to a medium kept at pH 3 to facilitate the spectrophotometric quantification of the dichromate ions.[18] This value is potentially relevant for studies of CrVI removal from industrial wastewaters, especially those generated by corrosion-protection technologies.[22]
tion capacity is apparently results from higher surface area and possibly even higher concentration of the active adsorption sites for the samples produced by the thermohydrolytic approach in this work. The reason supposedly is that their preparation was template-free and did not involve any calcination step.
Conclusion The proposed thermohydrolytic approach using simply hydrolysis and transformation (nucleation) within solid metal alkoxide precursors in boiling water has proved to be a general reliable approach for the creation of mesoporous oxide matrices with truly large active surface area, uniform pore size, and considerable pore volume. The produced materials are free from organic impurities and do not require any long-term after treatment or calcination, which opens prospects for many attractive applications. The behavior in adsorption of dichromate ions revealed by the obtained mesoporous oxides sets them into the frontline of attractive materials for applications in heavy-metal sequestration from ground and wastewaters.
Experimental Section Synthesis Figure 5. The dichromate ion, Cr2O72, adsorption. Isotherms are fitted by Langmuir equations for ZrO2 derived from Zr(OEt)4 and Nb2O5—from Nb(OMe)5. The data for TiO2, reported earlier in ref. [18] are provided for comparison.
Solid metal alkoxide precursors used in this work, were either purchased from Aldrich (Ti(OMe)4, Zr(OEt)4, Zr(OiPr)4(iPrOH), and Hf(OiPr)4(iPrOH)) or produced by anodic oxidation of metals in the parent alcohol with subsequent purification by conventional techniques as described earlier (Nb(OMe)5 and Ta(OMe)5).[19] The synthesis of adsorbent materials was carried out by quick insertion of the microcrystalline alkoxide (about 0.50 g) into boiling water (typically 50 mL) with subsequent refluxing at ambient atmosphere for 30 min. Milli-Q water (pH 7.0) was used for solution preparation and synthesis.
It should be noted that the general appearance of the adsorption isotherms is in agreement with the data recently reported in the literature[9] for TiO2, ZrO2, and mixed metal mesoporous matrices, while the adsorption capacity observed for materials in this study is considerably higher than the best results reported so far (35.15 mg g1 CrVI compared to 25.40 reported in ref. [9] about a 40 % improvement!). The adsorption occurs in agreement with the Langmuir isotherm, defined by the following equation:
Characterization SEM-EDS studies were carried out with a Hitachi TM-1000-m-DeX tabletop scanning electron microscope. TEM investigations were made using a CM12 transmission electron microscope (Philips, Holland) with an accelerating voltage of 100 kV. X-ray powder patterns were obtained using a Bruker SMART Apex-II diffractometer operating with MoKa radiation (l = 0.71073 ). Bruker Apex-II and EVA software were used for integration and data treatment. Textural characteristics (specific surface area, mean pore size, porous volume) were measured by using nitrogen sorption at 77 K on a Quantachrome Autosorb 1 apparatus. TGA investigations were made with Perkin–Elmer Pyris 1 instrument (heating rate 10 8C min1, temperature range 25–600 8C) with simultaneous analysis of outgoing gases using Perkin–Elmer Spectrum 100 FTIR instrument. Spectrophotometric determination of dichromate ions was carried out with Hitachi UV-2001 UV/Vis instrument in the water solutions set to pH 3 by addition of nitric acid. The calibration curve was measured as absorbance at lmax at 420 nm (correlated at lower concentrations with a more intense peak at 350 nm) and remained strictly linear up to 300 mg L1, R2 = 0.9992.
qe ¼ Qmax a C e ð1 þ a C e Þ in which Qmax stays for the maximal adsorption capacity and a is a specific constant characterizing the affinity of adsorbed substrate to the adsorbent. The affinity to TiO2 determined by least-square refinement is characterized by a relatively higher value compared to those for ZrO2 and Nb2O5 (the latter—investigated for the first time as an adsorbent for CrVI removal in this work). It can be hypothesized that adsorption on titania is proceeding by formation of a “covalent” TiOCr bond, whereas for heavier transition metals both electrostatic interactions and hydrogen bonding can play a more considerable role. The general appearance of the Langmuir isotherms is very similar to those reported in ref. [9]. The observed difference in adsorp&
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Full Paper Acknowledgements [7]
The authors would like to express their gratitude to the Swedish Research Council for the support to the projects “Molecular precursors and molecular models of Nanoporous materials” and “Advanced magnetically removable adsorbents for complex water remediation”.
[8] [9] [10] [11]
Keywords: mesoporous oxides · nanostructures · sorption mechanism · thermohydrolysis · transition metals
[12]
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FULL PAPER & Nanomaterials
Mesoporous metal oxides with highly active surface area and pore volume, attractive for applications in heavy-metalion sequestration, can easily be produced from solid metal alkoxide precursors by a facile thermohydrolytic approach (see figure).
G. A. Seisenbaeva,* G. Daniel, V. G. Kessler, J.-M. Nedelec* && – && General Facile Approach to TransitionMetal Oxides with Highly Uniform Mesoporosity and Their Application as Adsorbents for Heavy-Metal-Ion Sequestration
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