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Cite This: Chem. Mater. 2018, 30, 1164−1177
Comprehensive Magnetic Study of Nanostructured Mesoporous Manganese Oxide Materials and Implications for Catalytic Behavior Ehsan Moharreri,† William A. Hines,‡ Sourav Biswas,§ David M. Perry,‡ Junkai He,† Dustin Murray-Simmons,§ and Steven L. Suib*,§,† †
Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States Department of Physics, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269, United States ‡
S Supporting Information *
ABSTRACT: Magnetic behavior of nanostructured mesoporous manganese oxide materials, designated UCT-1 and UCT18, were studied using a combination of superconducting quantum interference device (SQUID) magnetometry and 55 Mn zero-field spin−echo nuclear magnetic resonance (NMR). Curie−Weiss fits to the magnetic susceptibility for the UCT-1 and UCT-18 samples calcined at 550 °C yielded paramagnetic moment values consistent with spin-only Mn3+ ions in the α-Mn2O3 phase (S = 2, 4.90 μB). However, the magnetization and NMR results reported here clearly identify a small amount of the Mn3O4 second phase (ferrimagnetic with TC ≈ 43 K) that does not appear in X-ray diffraction (XRD). The study resulted in the observation of fascinating magnetic behavior: (1) exchange bias, which occurs in cases where a ferromagnetic (or ferrimagnetic) phase forms a boundary with an antiferromagnetic phase and (2) a magnetic contribution attributed to uncompensated spins on the surfaces of the α-Mn2O3 nanoparticles. The presence of Mn3O4 and the interplay of Mn3+ and Mn2+ impact the catalytic properties.
1. INTRODUCTION The introduction of mesoporous silica and aluminosilicates (MCM-41 and M-41S) by Mobile Oil Corporation researchers over two decades ago1 has resulted in considerable research activity,2−5 particularly in the application of mesoporous transition metal oxide materials as catalysts.6,7 Since that time, a variety of synthesis procedures, involving both soft and hard templates, chemical compositions, dopants, and promoter ions have been employed.6,8−10 The transition metal oxides can accommodate various cations either in the form of promoter ions on the surface or charge balancing ions in the structure.6,11,12 A unique type of mesoporous materials was developed by this group, recognized as UCT (University of Connecticut) materials.13 The UCT approach is a one-step synthetic process that relies on the use of sol−gel-based inverse micelles as soft templates along with unique NOx chemistry. The resulting nanoparticles are packed in a random close-packed structure to form micrometer-sized aggregates;14,15 the mesopores are created by the interconnected voids. The surface area and pore size of the mesoporous materials can be tuned efficiently by controlling the heat treatment cycles. This diverse synthesis method allows us to prepare mesoporous metal oxides from different parts of the periodic table. A major advantage the UCT process allows is the ability to synthesize mesoporous oxide materials involving the late transition metals (e.g., Mn, Co, Fe, and Ni). Mesoporous transition metal oxides prepared © 2018 American Chemical Society
by this process were found to have broad applicability in a variety of catalytic reactions including water oxidation, lowtemperature CO oxidation, photocatalytic dye degradation, aerobic oxidation, and cross-coupling of organic compounds.6,7,16−20 Among the transition metals, Mn is particularly interesting because various valence states and five stable oxides (MnO, Mn2O3, Mn3O4, Mn5O8, and MnO2) exist with a wide range of technological applications.21−24 Furthermore, there has been recent emphasis to understand the behavior of the oxides at the nanoscale.25−28 Because the manganese ion can possess the valence states Mn2+, Mn3+, and Mn4+, various magnetic moment values occur (e.g., low spin-states of S = 5/2, 2, and 3/2, respectively). The oxides exhibit a variety of complex magnetic structures and behavior (antiferromagnetism, ferrimagnetism, and helical magnetic order). Moreover, the mixed valence nature of manganese gives it a wide range of catalytic applications in both the amorphous and crystalline states, which can be enhanced by the inclusion of other elements. In our previous study, we found that introduction of a minute amount ( TN(CoO). Because the oxides of many ferromagnetic transition metals are themselves antiferromagnetic, many of the early investigations involved nanoparticles consisting of a ferromagnetic core and an antiferromagnetic shell.42 However, Berkowitz et al.43 and Salazar-Alvarez et al.41 independently reported a “doubly inverted” core−shell system in which nanoparticles with an antiferromagnetic core (MnO) were surrounded with a ferrimagnetic shell (Mn3O4) shell and the magnetic ordering temperatures were such that TN(MnO) > TC(Mn3O4). The UCT-1 and UCT-18 materials studied here replicate a doubly inverted core−shell picture; i.e., nanoparticles with an α-Mn2O3 core having Mn3O4 inclusions or surface shells and TN(αMn2O3) ≈ 80 K > TC(Mn3O4) ≈ 43 K. Recently, electron energy loss spectroscopy (EELS) has provided a very advanced method in the characterization of nanostructured materials. An example is the study by Estradé et al.44 in which scanning transmission electron microscopy with EELS analysis (STEMEELS) was used to identify the core/shell structure of the MnOx/MnOy nanoparticles. The Mn3O4 second phase in our samples may, however, exist in the form of small inclusions and not simply as well-defined shells. It is of particular importance to compare the magnetic phase composition of the UCT-1 and UCT-18 samples calcined at 250 °C. In earlier work, it was demonstrated the Cs-promoted UCT-18 (250 °C) had greatly enhanced catalytic activity compared to UCT-1(250 °C).6 Although the Mn ion magnetic moment values are comparable as described above, the ZFC/ FC and TRM curves show significant differences. As shown in Figure 7a,c respectively, the UCT-1(250 °C) and UCT-18(250 °C) samples both show a peak in the ZFC at ≈22 K; however, the peak for the UCT-18(250 °C) is considerably broader. Although a weak first-order magnetic transition has been reported near 25 K for polycrystalline α-Mn2O3,45 the ZFC peaks shown in Figure 7a,c are more likely due to the existence of a superparamagnetic blocking temperature. This would be possible for small α-Mn2O3 particles with uncompensated surface spins; however, the α-Mn2O3 phase has not been directly identified in any of the samples calcined at 250 °C. The suggestion of a superparamagnetic blocking temperature at 22K is based on the ac susceptibility measurements reported by Mukherjee et al.;46 however, a spin-glass-like transition cannot be completely ruled out.47 The ac susceptibility measurements were carried out on α-Mn2O3 nanocrystals (9−18 nm, similar to ours) dispersed in a silica matrix. From their analysis of the shift of the peak in χ’ with frequency, they conclude that the parameters are consistent with those obtained for super-
how both magnetization and NMR measurements can detect and identify small amounts of a magnetic manganese oxide second phase, e.g., Mn3O4, which is not observed using XRD. Furthermore, there are situations where it is difficult to distinguish between two structurally similar crystalline phases using XRD, particularly when the peaks are broadened. (2) The coexistence of the antiferromagnetic α-Mn2O3 principal phase with the ferrimagnetic Mn3O4 second phase resulted in the observation of an “inverted” exchange bias (or exchange anisotropy) behavior. (3) Surface effects for nanoscale materials can yield magnetic properties different from the bulk materials. For the UCT-1 and UCT-18 materials, uncompensated spins at the surfaces of the α-Mn2O3 nanoparticles resulted in a magnetic contribution in addition to the Mn3O4 inclusions. (4) The enhanced catalytic activity of the Cs-promoted UCT18(250 °C) samples compared to the nonpromoted UCT1(250 °C) samples might be related to the presence of Mn3O4 in the former and not the latter. In some catalytic reactions, transformation of Mn3+ to Mn2+ is driving the catalytic reaction.20 The coexistence of Mn3+ and Mn2+ provides the source and sink of labile lattice oxygen as the means for driving dynamic redox during reactions. For the UCT-1 and UCT-18 samples in this study, as the final heat treatment (calcination) temperature increased, the corresponding XRD scans progressed from XRD amorphous (250 and 350 °C), to showing α-Mn2O3 with some MnO2 second phase (450 °C), and finally to showing only the αMn2O3 phase (550 °C). Consistent with the XRD results for the UCT-1(550 °C) and UCT-18(550 °C) samples, the magnetic susceptibility temperature dependence fit the Curie− Weiss law quite well over the range 100 K ≤ T ≤ 300 K yielding paramagnetic moment values very close to that for spin-only Mn3+ (S = 2 or 4.90 μB). On the other hand, the Curie−Weiss fits for all of the UCT-1(250 °C) and UCT18(250 °C) samples, which were XRD amorphous, yielded consistently smaller paramagnetic moment values, i.e., closer to the spin-only value for Mn4+ (S = 3/2 or 3.87 μB). These results might suggest the presence of the MnO2 phase in the 250 °C samples; yet due to smaller nanoparticles, this connection is very tenuous. However, the appearance of the Mn3O4 phase in the UCT-18 (250 °C) samples and not the UCT-1(250 °C) samples is quite clear. All of the Curie−Weiss fits yield Θ-values that are large and negative indicating relatively strong antiferromagnetic interactions between the Mn ions (see Table 1), which is typical of manganese oxides.39 For all of the UCT-1 and UCT-18 samples with final heat treatment temperatures of 250 and 550 °C, magnetization versus magnetic field (or hysteresis) loops were obtained at 10 and 60 K. Over a large magnetic field range, the curves appear essentially linear and through the origin. However, when the curves are viewed around the origin, there is the appearance of hysteresis (coercive field, Hc, and remanent magnetization). Furthermore, when the samples are cooled in a large magnetic field (H = ±10 kOe) from T = 100 to 10 K, there is a clear shift along the field axis, Heb, consistent with the occurrence of exchange bias, or exchange anisotropy. From the data listed in Table 2, Hc values are significantly larger: (1) at 10 K compared to 60 K and (2) for the samples calcined at 250 °C compared to the samples calcined at 550 °C. For the former, the explanation lies in the fact that there is a small amount of Mn3O4 second phase with TC ≈ 43 K. For the latter, a possible explanation lies in the fact that the samples calcined at higher temperature have larger nanoparticle size and smaller 1174
DOI: 10.1021/acs.chemmater.7b05280 Chem. Mater. 2018, 30, 1164−1177
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Chemistry of Materials
mention of magnetic behavior related to uncompensated surface spins, probably due to the relatively large particle size. On the other hand, Mukherjee et al.46 have prepared much smaller α-Mn2O3 nanocrystals in a silica matrix using a sol−gel procedure followed by calcination. Their α-Mn2O3 nanocrystals show weak ferromagnetic-like order for T ≤ 77 K along with the features of a superparamagnetic/ferromagnetic blocking temperature near 40 K. They rule out the presence of Mn3O4 nanocrystals and attribute all of the magnetic behavior to the uncompensated surface spins. On the basis of the results presented here, we suggest that this conclusion be revisited. In a recent work, Thota et al.49 prepared samples of MnO, Mn2O3, Mn5O8, and Mn3O4 nanoparticles by decomposing sol−gel produced manganese oxalate in oxygen, air, nitrogen, or oxygen at different temperatures. In particular, the α-Mn2O3 nanoparticles showed the antiferromagnetic transition at TN = 75 K along with a bifurcation in the ZFC/FC magnetization curves below 50 K. It is concluded that small amounts of the Mn3O4 phase are always present and cannot be completely eliminated in the preparation of various manganese oxide materials.
paramagnetic particles and well outside those for spin-glass systems. In any case, there is a clear presence of Mn3O4 in the UCT-18(250 °C) sample as indicated by the bifurcation of the ZFC and FC curves at ≈46 K (Figure 7c), which is not apparent with the UCT-1(250 °C) sample (Figure 7a). This is also indicated in the corresponding TRM curves (Figure 7b,d). These results were reproduced in the duplicate samples. The presence of Mn3O4 in the UCT-18(250 °C) samples and not the UCT-1(250 °C) samples provides a link to an understanding of the enhanced catalytic activity.6 From prior studies, we know that the mixed valence of manganese and easy mobility of labile oxygen are driving factors of catalytic activity. Mesoporous MnOx materials were correctly deemed to have “mixed Mn species”, but rarely understood as mixed phase. Lattice size differences between α-Mn2O3 and Mn3O4 are rather challenging to pinpoint with HR-TEM. Here from a magnetic route, the coexistence of α-Mn2O3 and Mn3O4 provides another sign of interplay of Mn3+ and Mn2+ species. Though Mn3+ is mostly the more active species even in nonoxidative reaction,20 with oxygen being required for its replenishment, the presence of Mn2+ is the consequence and the indicator of its activity. We showed that the secondary phase of Mn3O4 is oxidized during heat treatment (Figure S6a,b) for a commercial α-Mn2O3 sample standard. The phase identification obtained from the magnetization measurements was supported by the 55Mn zero-field spin−echo NMR spectra. The uniqueness of NMR is that this method can identify the actual phase of small magnetic inclusions, which are detected in magnetization measurements and, perhaps, not detected by XRD. NMR is also a useful technique in cases where there are two phases with similar structures that are difficult to distinguish by XRD or TEM. For UCT-1, characteristic spectra for both the principal α-Mn2O3 phase and the Mn3O4 second phase were observed for the samples calcined at 450 and 550 °C. These two phases were also observed in UCT-18(550 °C). A relatively strong 55Mn spectrum characteristic of Mn3O4 was observed in a 99.9% pure commercial sample of α-Mn2O3. Although the commercial is “pure” with regard to elemental impurities, this material is not completely phase pure. There are some previous reports in the literature concerning the magnetic properties of nanostructured manganese oxides, which are relevant to the work reported here. F. Jiao et al.8 first reported on a synthesis procedure for highly ordered mesoporous Mn3O4 with crystalline walls. The process involved an initial synthesis of highly ordered mesoporous α-Mn2O3 using the hard template KIT-6 and then reducing it to form highly crystalline Mn3O4 retaining the ordered pore structure. They observed a distinct bifurcation of their ZFC/FC magnetization curves for the initial mesoporous α-Mn2O3 material just above 40 K and attributed this behavior to a trace amount (≈0.05%) of Mn3O4 in the sample, which was not detected in the XRD scans. T. Ahmad et al.48 have prepared a sample ≈50 nm α-Mn2O3 nanoparticles (along with samples of MnO and Mn3O4 nanoparticles) using a reverse micellar route followed by a heat treatment in air. They also observed a broad transition near 80 K due to the antiferromagnetic α-Mn2O3 as well as a “magnetic anomaly” near 45 K, which was attributed to ≈1% Mn3O4 impurity. They were able to fit the magnetic susceptibility for their α-Mn2O3 nanoparticles to the Curie− Weiss law 100 K ≤ T ≤ 300 K obtaining an effective moment value of 5.0 μB, which is consistent with the spin-only value for Mn3+, and Θ = −160 K. However, in their work, there is no
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05280. XRD patterns for UCT-18 samples, nitrogen sorption isotherms, and pore size distribution for UCT-18 samples, scanning electron microscope images for UCT-1 and UCT-18 samples, transmission electron microscopy images for UCT-18 samples, thermoremanent magnetization curves for commercial and treated αMn2O3 samples, refinement analysis of XRD measurements of UCT-1 and UCT-18, textural properties of mesoporous manganese oxide samples, and parameters of refinement on XRD patterns (PDF)
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AUTHOR INFORMATION
ORCID
Ehsan Moharreri: 0000-0002-3585-2962 Steven L. Suib: 0000-0003-3073-311X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.H. and D.P. thank Richard Bibeault for assistance with the magnetization and NMR experiments. E.M., S.B., J.H., and S.S. received funding from the Chemical, Geochemical and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy for supporting this work under grant DE-FG02-86ER13622-A000. We thank the mentioned division for the support. The TEM studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA).
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