CeO2 Basic-Site-Enhanced Catalyst via in ... - ACS Publications

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2D NiFe/CeO2 Basic-Site-Enhanced Catalyst via in-Situ Topotactic Reduction for Selectively Catalyzing the H2 Generation from N2H4·H2O Dandan Wu, Ming Wen,* Chen Gu, and Qingsheng Wu School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China S Supporting Information *

ABSTRACT: An economical catalyst with excellent selectivity and high activity is eagerly desirable for H2 generation from the decomposition of N2H4·H2O. Here, a bifunctional two-dimensional NiFe/CeO2 nanocatalyst with NiFe nanoparticles (∼5 nm) uniformly anchored on CeO2 nanosheets supports has been successfully synthesized through a dynamic controlling coprecipitation process followed by in-situ topotactic reduction. Even without NaOH as catalyst promoter, as-designed Ni0.6Fe0.4/CeO2 nanocatalyst can show high activity for selectively catalyzing H2 generation (reaction rate (molN2H4 mol−1NiFe h−1): 5.73 h−1). As ceria is easily reducible from CeO2 to CeO2−x, the surface of CeO2 could supply an extremely large amount of Ce3+, and the high-density electrons of Ce3+ can work as Lewis base to facilitate the absorption of N2H4, which can weaken the N−H bond and promote NiFe active centers to break the N−H bond preferentially, resulting in the high catalytic selectivity (over 99%) and activity for the H2 generation from N2H4·H2O. KEYWORDS: two-dimensional NiFe/CeO2, H2 generation, N2H4·H2O decomposition, Lewis base, selectively catalyze

1. INTRODUCTION Nanocatalysts have been widely employed in many important chemical reactions to achieve the activity and selectivity approach to those of homogeneous catalysts.1,2 In the aim of high selectivity and activity along with long lifetime, nanoalloy composites supported on metal oxides have been widely researched.3 As one of the most widely used supports, ceria (CeO2) has been used in many catalytic applications, such as CO oxidation, water−gas shift, C−C coupling, syngas conversion, partial oxidation, selective hydrogenation, and so on.4−8 In catalysis, CeO2 serves as a support to prevent the agglomeration of active centers, which helps to improve the activity and selectivity as well as the stability of catalysts.9,10 Ceria is easily reduced to CeO2−x from CeO2, and oxygen vacancies are produced on the surface, which can often dramatically influence the substrate adsorption and subsequent reaction either on a pure surface or on the surface with metal particles supported.11−13 In addition, ceria with two-dimensional (2D) thin-layer structure could supply a great amount of coordinated-unsaturated atoms.14 The convenient transformation of chemical hydrogen storage material with considerable hydrogen content is one of the most challenging technologies in hydrogen energy society. Among various hydrogen storage materials, hydrazine hydrate (H2NNH2·H2O) with the hydrogen content of 8.0 wt %, which is a liquid within the temperature range of 221−392 K, © XXXX American Chemical Society

can be a promising hydrogen storage material applied for the infrastructure using liquid fuels to recharge.15−18 Hydrogen could be released by the complete decomposition of hydrazine through the reaction H2NNH2 → N2 + 2H2 (pathway 1). However, to efficiently exploit H2 generation from hydrazine, the undesired decomposition to NH3 by the reaction 3H2NNH2 → N2 + 4NH3 (pathway 2) must be avoided.19,20 To our knowledge, most of the current catalysts for the decomposition of N2H4·H2O either contain noble metals or add an alkali (e.g., NaOH) to increase catalytic performance, which results in a greater difficulty in both equipment and operation requirements.21−25 To solve this problem, some methods have already been tried by employing layered double hydroxides (LDHs) to introduce solid basic sites, such as Ni− Al2O3−HT26 and NiFe-alloy/MgO,27 but the activity and selectivity remain to be further improved. Therefore, it is necessary to construct a multifunctional catalyst, which is composed of low cost non-noble metal as the catalyst active site and solid base as the catalyst promoter, to attain excellent catalytic performance without the assistance of aqueous alkali. Herein, we designed a new 2D-structured NiFe/CeO2 nanocomposite only including non-noble metals via a Received: January 16, 2017 Accepted: April 28, 2017

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DOI: 10.1021/acsami.7b00652 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

of the NiFe/CeO2 nanocomposite, was initially fabricated by a dynamic controlling process by using the collodion membrane as director. The reaction was driven by the concentration difference between metal ions (Ni2+, Fe2+, and Ce3+) and NaOH which were separated on both sides of the semipermeable collodion membrane. Owing to the concentration gradient, OH− from NaOH aqueous can diffuse to the other side to react with metal ions (Ni2+, Fe2+, and Ce3+); thus, the above-mentioned mixture hydroxide precursor was formed. The NiFeCe−hydroxide complex was transferred to the 2D FeNi/CeO2 nanocomposite with NiFe NPs implanted on the CeO2 support by the following in situ topotactic reduction process in a solvothermal system. During the reduction process, ethylene glycol (EG) worked as reductant which was assisted by NaOH. Therefore, under the mild reduction conditions, the 2D nanosheet morphology and atomic thickness can be kept by the slow conversion kinetics.28,29 2.2. Morphologies and Structures. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the 2D NiFeCe−hydroxide (the precursor of NiFe/CeO2) and NiFe/CeO2 nanocomposites were shown in Figure 1. The distinct 2D thin-layer structure of NiFeCe−hydroxide can be observed in Figure 1A and 1E. As shown in Figure 1B and 1F, NiFe NPs with an average diameter of 5 nm are dispersed on the surface of CeO2 nanosheets to form 2D-structured NiFe/CeO2 nanocomposites. To further confirm whether the CeO2 nanosheets were composed by numbers of tiny CeO 2 NPs, the highmagnification TEM image of CeO2 nanosheets was measured and shown in Figure 1G, and it can be clearly seen that the crystal plane is continuous without gaps, indicating the 2Dnanostructured CeO2 is not composed by tiny CeO2 NPs. Besides, the high-magnification TEM image of CeO2 nanosheet edges in Figure 1C presents a multilayered structure which has a thickness of 3.2 nm, and the selected-area electron diffraction (SAED) pattern of NiFe/CeO2 nanocomposites in Figure 1D provides a mixed polycrystalline diffraction ring with the indexed planes (111) and (220) of NiFe and CeO2, indicating the fabrication of composite structure. In Figure 1H, the highresolution TEM (HRTEM) image of NiFe/CeO2 nanosheets presents the lattice fringes with spacing of 0.20 nm for NiFe (111) planes (white marked) and 0.31 nm for CeO2 (111) planes (red marked). Meanwhile, owing to the 2D thin-layer

coprecipitation process followed by in situ topotactic reduction, in which NiFe nanoparticles (NPs) are uniformly dispersed on the surface of CeO2 nanosheets. Applied as nanocatalyst, the 2D Ni0.6Fe0.4/CeO2 catalyst can supply an extremely large amount of Ce3+, and the high-density electrons of Ce3+ can work as Lewis base to facilitate the absorption of N2H4, which can weaken the N−H bond and promote NiFe active centers to break the N−H bond preferentially, resulting in high activity for selectively catalyzing the H2 generation from N2H4·H2O with the reaction rate (molN2H4 mol−1NiFe h−1) of 5.73 h−1 even without the assistance of NaOH. So this work puts forward a promising orientation for constructing high-performance nonnoble metal catalysts and demonstrates their applications to the H2 generation from N2H4·H2O.

2. RESULTS AND DISCUSSION 2.1. Mechanism of Fabrication. The designed 2Dstructured NiFe/CeO2 composites with NiFe NPs evenly dispersed on CeO2 nanosheets were successfully fabricated through the dynamic controlling coprecipitation process followed by in-situ topotactic reduction. As shown in Scheme 1, the 2D NiFeCe−hydroxide complex, which is the precursor Scheme 1. Schematic Illustration of the Controllable Synthesis of the 2D NiFe/CeO2 Nanocomposites Based on in-Situ Topotactic Reduction of the 2D NiFeCe−Hydroxide Complex

Figure 1. (A, B) SEM images of 2D NiFeCe−hydroxide and NiFe/CeO2 nanocomposites; (C, D) high-magnification TEM image and SAED pattern of 2D NiFe/CeO2 nanocomposites; (E, F) TEM images of 2D NiFeCe−hydroxide and NiFe/CeO2 nanocomposites; (G) highmagnification TEM image of CeO2 nanosheets marked area in F; (H) HRTEM image of 2D NiFe/CeO2 nanocomposites. B


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ACS Applied Materials & Interfaces structure, NiFe/CeO2 nanocomposites possess an extremely high special surface area of 214.8 m2·g−1 and massive coordinated-unsaturated surface atoms (Ce3+), which can greatly facilitate the catalytic reaction. In contrast, the SEM and TEM images of bulk NiFe/CeO2 nanocomposites were also investigated in detail (Supporting Information (SI), Figure S1). Figure 2 shows the powder X-ray diffraction (XRD) patterns and the energy-dispersive X-ray spectroscopy (EDS) analysis of

Figure 3. (A) Comparison of Ce 3d XPS analysis between 2D NiFe/ CeO2 nanocomposites (a) and bulk NiFe/CeO2 nanocomposites (b). Detailed XPS analysis of Ce 3d from 2D NiFe/CeO2 nanocomposites (B) and bulk NiFe/CeO2 nanocomposites (C); (D) CO2-temperature-programmed desorption (TPD) profiles of different catalysts.

Figure 2. XRD patterns (A) and EDS analysis (B) of bulk NiFe/CeO2 nanocomposites (a) and 2D NiFe/CeO2 nanocomposites (b).

2D-structured and bulk NiFe/CeO2 nanocomposites. In Figure 2A, both the XRD patterns of bulk NiFe/CeO2 (trace (a)) and 2D-structured NiFe/CeO2 (trace (b)) show three sharp peaks besides the peaks of CeO2 (PDF No. 34-0394). The peaks at 43.7°, 51.0°, and 75.4° can be indexed to the (111), (200), and (220) planes of the anchored NiFe NPs, which are located between the standard peaks of Ni (JCPDF No. 65-0380) and Fe (JCPDF No. 65-4150), indicating the existence of the NiFe alloy. In addition, it can be clearly observed that CeO2 peaks of bulk NiFe/CeO2 are sharper than those of 2D NiFe/CeO2, which is because the 2D thin-layer structure reduces the crystallization degree of CeO2. Figure S2 gives the element mappings of Fe, Ni, and Ce (Figure S2B∼S2D) which are overlapped with the corresponding SEM image (Figure S2A), further indicating the formation of NiFe/CeO2 nanocomposites. From the EDS analysis in Figure 2B, Ni, Fe, Ce, and O elements can be observed in bulk NiFe/CeO2 (trace (a)) as well as 2D NiFe/CeO2 (trace (b)), and the atomic ratio of Ni:Fe:Ce: is 6:4:1 which agrees well with the result of ICP. However, the trace (b) shows Ce/O atomic ratio of 2Dstructured NiFe/CeO2 to be 1:1.87; therefore, the 2D CeO2 is a nonstoichiometric compound because of the oxygen vacancy of its crystal defect.30 Then, trace (a) gives the Ce/O atomic ratio of bulk NiFe/CeO2 to be 1:2.14, which is consistent with the CeO2 molecule structure, and the excess oxygen can be attributed to the oxidation of trace amounts of NiFe alloy. Meanwhile, the loading content of NiFe is 76% which is calculated by the ICP data and confirmed by EDS analysis. In order to confirm and compare the chemical bonding information on 2D-structured and bulk NiFe/CeO2 nanocomposites, X-ray photoelectron spectroscopy (XPS) measurement was performed on the prepared samples. The wide spectra for the surface of 2D-structured and bulk NiFe/CeO2 nanocomposites were shown in Figure S3, in which both of them present five photoemission peaks (Ce 3d, Ni 2p, Fe 2p, O 1s, and C 1s). In addition, the magnified detailed XPS spectra of Ce 3d for 2D NiFe/CeO2 nanocomposites (trace (a)) and bulk NiFe/CeO2 nanocomposites (trace (b)) in Figure 3A contain eight and six peaks, respectively. The multiple states arise from different Ce 4f level occupancies in the final state,

and the peaks labeled U and V refer to the 3d5/2 and 3d7/2 spin−orbit components, respectively. The additional peaks result from so-called “shakedown” states where electrons are transferred from the O 2p level to the Ce 4f level in the excited state.31,32 Meanwhile, the added peaks at 906.0 and 886.3 eV of Ce 3d in 2D NiFe/CeO2 nanocomposites are attributed to the partial reduction of ceria, demonstrating the existence of Ce3+ and oxygen vacancy, which is consistent with the result of EDS (Figure 2Bb). In addition, the narrow Ce 3d spectra were further analyzed by using XPSPEAK41 software, and it can be clearly observed that the peak areas of Ce3+ in 2D NiFe/CeO2 nanocomposites (Figure 3B) are much larger than that in bulk NiFe/CeO2 nanocomposites (Figure 3C), which proves the above conclusion. To further understand the catalyst basicity resulting from the 2D CeO2, CO2-TPD experiments were applied to 2D NiFe/ CeO2 nanocomposites and other contrast catalysts. As shown in Figure 3D, besides NiFe NPs all the samples have desorption peaks with different intensity. The peak centered at 100−200 °C may be attributed to the hydroxyl groups on the surface (weak Brønsted base site), and the peak centered at 300−500 °C is ascribed to the Ce3+ which has high-density electrons derived from the surface defects of 2D-structured CeO2 (strong Lewis base site).19,27 Also, the corresponding value by integral computation of basic strength was shown in Table 1. It can be Table 1. Basic Properties of As-Prepared Catalysts: 2D NiFe/CeO2, CeO2 Nanosheets, Bulk NiFe/CeO2, NiFe/ Al2O3, and NiFe NPs catalysts 2D NiFe/CeO2 CeO2 nanosheets bulk NiFe/CeO2 NiFe/Al2O3 NiFe NPs C

total amount of CO2 desorption (μmol−1)

strong basic site (μmol−1)

weak basic site (μmol−1)

776

745

31

407

395

12

251

216

35

196 0

14 0

182 0


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this is that when the content of NiFe is too high the dispersity of NiFe NPs and the amount of basic sites would decrease. On the contrary, the catalyst would have a small NiFe active center, thereby leading to the decrease of catalytic activity and H2 selectivity. Meanwhile, by the ICP data the loading content of NiFe was calculated to be 76%, which was confirmed by the EDS analysis. The catalytic performance over 2D NiFe/CeO2 nanocatalysts was further examined from 323 to 353 K (Figure S6A). As expected, the reaction rate increases with reaction temperature; meanwhile, H2 selectivity decreases a little. On the basis of the reaction rate data at different temperatures, the Arrhenius plot of ln k versus 1/T was plotted in Figure S6B, and the activation energy (Ea) for the decomposition of N2H4·H2O was calculated to be 44.06 kJ·mol−1 from the slope, which is comparable to the reported values of 47.6 kJ·mol−1 for Ni−Al2O3−HT,26 47.0 kJ· mol−1 for Ni−0.080CeO2,33 and 55.1 kJ·mol−1 for NiMoB− La(OH)3 catalysts.34 The catalytic performances of different reported nanocatalysts for H2 generation from N2H4·H2O were summarized in Table S1. To acquire the influence of support material on the catalytic performances of the H2 generation from N2H4·H2O, bulk NiFe/CeO2, NiFe/Al2O3, NiFe NPs, and CeO2 nanosheets were also prepared, and their catalytic performance at 50 °C was studied. As shown in Figure 4C, the 2D-structured NiFe/ CeO2 nanocatalyst exhibits excellent catalytic performance, which is superior to other contrast samples. Although CeO2 cannot catalyze the decomposition of N2H4·H2O, it can significantly facilitate the catalytic reaction especially when it has 2D structure. The reason can be suggested as that the Ce3+ with high-density electrons from the surface of 2D-structured CeO2 can serve as a strong Lewis base to facilitate the breakage of N−H bonds, thus promoting the catalytic activity and H2 selectivity. All the catalytic performance data of prepared catalysts were summarized in Table 2. It can be clearly observed that both catalytic activity and H2 selectivity of the 2D NiFe/ CeO2 nanocatalysts are superior to those of other contrast catalysts, as the 2D NiFe/CeO2 nanocatalysts contain much more basic sites than others due to the 2D structure of CeO2, which can also be proved by CO2-TPD experiment (Figure 3D and Table 1). In addition, the lifetime was also evaluated by successive catalytic tests over 10 times via magnet separation. As shown in Figure 4D, there is a slight decrease of catalytic activity and H2 selectivity of 2D NiFe/CeO2 nanocomposites because of the minor agglomeration of NiFe NPs after 10 cycles (Figure S7), which is much better than those of bulk NiFe/CeO 2 nanocomposites (Figure S8), because 2D-structured CeO2 can more effectively prevent NiFe NPs from aggregating together than bulk CeO2. From the above, for 2D-structured NiFe/CeO2 nanocomposite catalysts, the synergy of Ni−Fe along with the strong basic sites from the reduced Ce3+ gives rise to the excellent catalytic performance for H2 generation from N2H4·H2O decomposition. As a magnetic retrievable catalyst, the hysteresis loop of the 2D NiFe/CeO2 nanocomposites was plotted in Figure S9. As shown, the magnetization saturation value is 80.30 emu/g; therefore, the as-prepared 2D NiFe/CeO2 nanocomposites suspended in the aqueous solution can be separated by an external magnet easily (Figure 4D, inset). Moreover, it can be readily redispersed in water by stirring due to its low coercivity (53.45 Oe) and retentivity (18.05 emu/g), indicating the excellent magnetic separation ability of 2D NiFe/CeO2 nanocomposites.

clearly seen that the total amount of CO2 desorption and strong basic site of 2D NiFe/CeO2 are obviously larger than those of other as-contrast samples including bulk NiFe/CeO2, CeO2 nanosheets (Figure S4A), NiFe/Al2O3 (Figure S4B), and NiFe NPs (Figure S4C); meanwhile the corresponding reaction activity and H2 selectivity of 2D NiFe/CeO2 are better than those of others (Table 2). Table 2. Catalytic Performances of 2D NiFe/CeO2, Bulk NiFe/CeO2, NiFe/Al2O3, and NiFe NPs catalysts 2D NiFe/CeO2 bulk NiFe/CeO2 NiFe/Al2O3 NiFe NPs

reaction temperature (°C)

reaction time (min)

selectivity (%)

50 50

28 80

99 84

50 50

85 50

63 53

2.3. Catalytic Performance. The catalytic performances of the as-prepared 2D NiFe/CeO2 nanocomposites and other contrast catalysts for H2 generation from hydrazine aqueous solution at 323 K were illustrated in Figure 4. The catalytic

Figure 4. (A) n(N2 + H2)/n(N2H4) vs time for 2D NiFe/CeO2 nanocomposites at various Ni/Fe molar ratios; (B) n(N2 + H2)/ n(N2H4) vs time for 2D NiFe/CeO2 nanocomposites with different loading content of NiFe; (C) n(N2 + H2)/n(N2H4) vs time for different as-synthesized catalysts; and (D) recyclability test of 2D NiFe/CeO2 nanocomposites (inset: 2D NiFe/CeO2 nanocatalysts quickly respond to a magnet).

performances over 2D-structured NixFe1−x/CeO2 nanocomposites with various Ni/Fe molar ratios were shown in Figure 4A. It can be seen that by alloying Fe to Ni the catalytic activity and H2 selectivity of the catalysts increase obviously, and the catalytic performances of monometallic Ni/CeO2 and Fe/CeO2 are the poorest among these 2D NixFe1−x/CeO2 nanocomposites, revealing the synergistic effect of the NiFe alloy. Meanwhile, 2D Ni0.6Fe0.4/CeO2 nanocomposites exhibit optimal catalytic performance by showing over 99% selectivity for H2 generation and high reaction rate of 5.73 h−1 without the assistance of NaOH. Moreover, the effect of NiFe content on the catalytic performance of 2D NiFe/CeO2 nanocomposites was also discussed in detail. In Figure 4B, it shows that the catalyst Ni0.6Fe0.4−0.1CeO2 harvests the best catalytic activity and H2 selectivity. It can be suggested that the reason behind D



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To further investigate the catalytic reaction of 2D NiFe/ CeO2 nanocomposites for the H2 generation from N2H4·H2O, a mechanism was put forward and illustrated in Scheme 2.

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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/acsami.7b00652. Experiment section, SEM and TEM images, XPS patterns, Arrhenius plots, magnetic hysteresis loops, and so on (PDF)

Scheme 2. Illustration of the Catalysis Mechanism for the H2 Generation from N2H4·H2O by 2D NiFe/CeO2 Nanocomposites

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86)-21-65981097. ORCID

Ming Wen: 0000-0002-2327-5459 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos: 21471114, 51271132, and 91122025) and 973 Project of China (No: 2011CB932404).

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ABBREVIATIONS 2D, two-dimensional; LDHs, layered double hydroxides; NPs, nanoparticles; EG, ethylene glycol; Ea, activation energy; SEM, scanning electron microscopy; EDS, energy-dispersive X-ray spectroscopy; TEM, transmission electron microscopy; SAED, selected-area electron diffraction; HRTEM, high-resolution TEM; XRD, powder X-ray diffraction; XPS, X-ray photoelectron spectroscopy; TPD, temperature-programmed desorption

Generally, the heterogeneous catalytic reactions take place on the surface of catalysts; therefore, the surface structure of catalysts has a great effect on their catalytic performance.35−37 The 2D NiFe/CeO2 catalysts can offer a large number of Ce3+, and the high-density electrons of Ce3+ can work as a Lewis base to facilitate the absorption of N2H4, which can weaken the N− H bond and then promote NiFe active centers to break the N− H bond preferentially, resulting in high activity and H2 selectivity for catalyzing the decomposition of N2H4·H2O. Since the reaction of N2H4·H2O decomposition may include the cleavage of N−N and N−H bonds, the cleavage order of the bonds has great influence on the catalytic performance. It would be much easier to generate NH3 rather than H2 if the N−N bonds are broken first to produce •NH2 because the further cleavage of a N−H bond needs more energy than the combination of •NH2 with H•.19,38−40 So the 2D-structured CeO2 support not only serves as a matrix but also works as a solid alkali which can promote the catalytic performance significantly.

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REFERENCES

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3. CONCLUSION In summary, a new 2D-structured NiFe/CeO2 nanocomposite was synthesized via a coprecipitation process followed by in situ topotactic reduction. Promoted by the strong Lewis base site which is derived from the reduced Ce3+ with high-density electrons, a 2D Ni0.6Fe0.4/CeO2 nanocatalyst can harvest the excellent catalytic activity (reaction rate 5.73 h−1) and selectivity (over 99%) for the H2 generation from N2H4·H2O decomposition without NaOH as catalyst promoter. What’s more, as-designed nanocatalyst exhibited an outstanding recyclability, and no obvious performance decay can be observed after being used over 10 times. It puts forward an effective strategy for constructing superior bifunctional catalysts and demonstrates their applications to recyclable catalyst for selectively catalyzing the H2 generation from N2H4·H2O decomposition. E


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CeO2 Basic-Site-Enhanced Catalyst via in ... - ACS Publications

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