Mesoporous Ruthenium Oxide: A Heterogeneous Catalyst for Water

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Mesoporous Ruthenium Oxide: A Heterogeneous Catalyst for Water Oxidation M. Naeem Iqbal, Ahmed F. Abdel-Magied, Hani Nasser Abdelhamid, Peter Olsén, Andrey Shatskiy, Xiaodong Zou, Björn Johan Åkermark, Markus D. Kärkäs, and Eric Johnston ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02845 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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ACS Sustainable Chemistry & Engineering

Mesoporous Ruthenium Oxide: A Heterogeneous Catalyst for Water Oxidation M. Naeem Iqbal,†,‡,§ Ahmed F. Abdel-Magied,†,§,¶ Hani Nasser Abdelhamid,‡ Peter Olsén,† Andrey Shatskiy,† Xiaodong Zou,‡ Björn Åkermark,† Markus D. Kärkäs,*,† and Eric V. Johnston*,† † Department of Organic Chemistry, Arrhenius Laboratory, Svante Arrhenius väg 16C, Stockholm University, SE‐106 91, Stockholm, Sweden ‡ Department of Material and Environmental Chemistry, Svante Arrhenius väg 16C, Stockholm University, SE‐106 91, Stockholm, Sweden * E-mail: [email protected]; [email protected] KEYWORDS: artificial photosynthesis, electron microscopy, heterogeneous catalysis, oxygen evolution, ruthenium, water oxidation

ABSTRACT: Herein we report the synthesis of mesoporous ruthenium oxide (MP-RuO2) using a template-based approach. The catalytic efficiency of the prepared MP-RuO2 was compared to commercially available ruthenium oxide nanoparticles (C-RuO2) as heterogeneous catalysts for water oxidation. The results demonstrated superior performance of MP-RuO2 for oxygen evolution compared to the C-RuO2 with respect to recyclability, amount of generated oxygen, and stability over several catalytic runs.

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Water splitting can be used for production of high-energy renewable fuels, which constitutes an essential part of the global sustainable development. 1,2 In Nature, water oxidation occurs via intricate processes within the photosynthetic machinery, 3 providing us with a model for development of artificial photosynthetic systems. The overall goal in artificial photosynthesis is to design and implement systems for production of solar fuels on an industrial scale. 4,5,6 Here, one of the remaining hurdles is fabrication of a robust and efficient water oxidation catalyst (WOC). 7,8,9 The harsh oxidative conditions required for oxidation of water using either heterogeneous or homogeneous molecular catalysts require crucial pre-synthetic considerations in both support and catalyst design in order to ensure high activity and structural integrity during catalysis. 10,11,12 Oxidative degradation of molecular WOCs can to some extent be hindered by redox-active ligands or oxidation-resistant organic scaffolds. 13,14 However, increasing the long-term stability of such catalysts remains a distant goal. In contrast, some heterogeneous WOCs based on Mn, Co, Ir, Ru, and Fe have been found to be highly active. 15,16,17,18,19,20,21,22,23 We have developed nanoparticle-based WOCs, where the nanoparticle catalysts were anchored to mesoporous silica support. 24,25 The hypothesis is, not solely based on our own work, that the mesoporous structure is highly beneficial for achieving high catalytic activity. 26 It has been found that for these catalysts the structural and textural features of the support play crucial part in the catalytic behavior. 27,28 Synthesis of mesoporous RuO2 generally requires the use of hard templates, such as metal oxides. 29,30 These templates are difficult to remove completely even at harsh reaction conditions, which leads to the contamination of the desirable catalyst. Contrary to this, soft templates, such as surfactants and surfactant-like polymers, are easy to remove. 31,32


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Herein, we describe the preparation of a WOC based on RuO2 with a mesoporous architecture that provides a highly accessible surface area for catalysis. The developed mesoporous ruthenium oxide (MP-RuO2) catalyst was characterized using a variety of techniques, such as Xray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption-desorption isotherm. The catalytic performance and recyclability of the catalyst for water oxidation were subsequently evaluated using ceric ammonium nitrate (CAN) as a chemical oxidant and compared to the commercial RuO2 (CRuO2). 33,34 Preparations of mesoporous ruthenium oxide materials using hard templates have been reported;29,30,35,36 however, limited studies have been performed to evaluate their stability under strongly oxidizing conditions. 37,38,39,40 The synthesis of MP-RuO2 was performed through a selfassembly method using a soft template, using RuCl3 as the ruthenium source and cetyltrimethylammonium bromide (CTAB) as a surfactant templating agent. The ruthenium salt was converted into its hydrated form under basic hydrothermal conditions (100 °C) within the CTAB template. Finally, the material was calcined at 400 °C under a flow of air to provide the desired MP-RuO2 catalyst. The synthesized MP-RuO2 catalyst was characterized using powder XRD (PXRD) in low-angle transmission and high-angle reflection modes, nitrogen adsorption, dynamic light scattering (DLS), SEM and TEM. Figure 1 depicts a PXRD pattern with clearly-resolved diffraction peaks. The diffraction peaks for MP-RuO2 were compared with those of C-RuO2 that was indexed using the rutile-type tetragonal structure (P42/mnm, ICDD PDF Database no. 00-040-1290). This shows that the synthesized MP-RuO2 catalyst has the same rutile-type phase as C-RuO2.


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The porous structure of the MP-RuO2 catalyst was characterized by low-angle XRD analysis (Figure S1). A single diffraction peak (denoted 100) was observed for MP-RuO2 at 0.68° (2ϴ) displaying the presence of mesopores with an interplanar distance of 12.7 nm. This is in agreement with the expected pore structures obtained from the CTAB templated synthesis of metal oxide materials under hydrothermal conditions.35,41 The synthesized MP-RuO2 displayed well resolved peaks as shown in the PXRD pattern, which can be attributed to the highly crystalline pore wall structure (Figure 1).

Figure 1. High-angle reflection PXRD pattern recorded at room temperature for the synthesized MP-RuO2, recycled MP-RuO2 (four cycles), and C-RuO2. N2 sorption isotherm shows the type-III isotherm and adsorption-desorption curves can clearly be distinguished for the synthesized MP-RuO2 (Figure 2a). A relative pressure (p/p) of 0.7–0.8 corresponds to the main peaks around 72 Å. Figure 2b shows the sharp pore size distribution of MP-RuO2 measured using a NLDFT model. Typically, CTAB-templated synthesis of mesoporous oxides without hydrothermal treatment gives pore sizes in the range of 20–40 Å. When CTAB is still present, high-temperature treatment of the synthesized particles enlarges the pores as a result of the expansion of the hydrophobic core of the CTAB micellar structure. We


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have observed here that all the pores from the original size of 31 Å are not transformed into larger sizes and a small portion of the pores can still be detected at 31 Å in MP-RuO2 and in the recycled MP-RuO2. Almost no porosity and N2 adsorption were observed for commercial ruthenium oxide nanoparticles (Figure 2).

Figure 2. (a) N2 adsorption-desorption isotherm and (b) pore size distribution. The particle size of MP-RuO2 was measured by DLS after sonication. The measurements revealed an absence of nanoparticles in the synthesized MP-RuO2 catalyst. The data are presented in Table 1 along with the physical properties of C-RuO2 measured through N2


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adsorption isotherm. Particle size distributions for MP-RuO2 and C-RuO2 are shown in Figure S2. Table 1. Textural properties of the synthesized MP-RuO2, recycled MP-RuO2 and C-RuO2.

a

Sample

BET specific surface area (m2/g)

MP-RuO2

68.3

127

0.22

1931

recycled MP-RuO2c

62.8

110

0.18

2153

C-RuO2

11.3

130

0.03

390

Average pore size Total pore volume Average particle size (Å)a (cm3/g)b (nm)

Calculated as 4 × total pore volume/BET surface area. b Total pore volume was measured at

0.9896 p/p˳. c Recycled catalyst after four catalytic runs. Nitrogen adsorption measurements on C-RuO2 at 77 K revealed practically no porosity and almost no specific surface area and pore volume (Figure 2). The BET plots for MP-RuO2 and CRuO2 are presented in Figure S3. For MP-RuO2, a lower surface area (68.3 m2/g) was observed in contrast to the previously reported mesoporous ruthenium oxide (190 m2/g), where the surface area of synthesized particles had a large contribution from micropores.21 The synthesized MPRuO2 had essentially no contribution to the micropores as previously synthesized transition metal oxide mesoporous materials. 42 Furthermore, the MP-RuO2 catalyst was analyzed by SEM and TEM for porosity, morphology, and pore structure. SEM images revealed high aggregation of the primary particles of MP-RuO2 (Figure 3a). The secondary large aggregates are above 500 nm in size, while the size of the smaller primary particles is in the range of 2–5 nm. Tailing in the pore size of these MP-RuO2 particles as shown in Figure 2b originates from the secondary aggregates of the primary


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particles. TEM demonstrated that MP-RuO2 particles had a wormlike pore network structure (Figure 3b), originating from the assembly of the primary nanoparticles of 2–5 nm in size. It is worth mentioning that for MP-RuO2 we did not observe any discrete nanoparticles outside the network of the primary particles.

Figure 3. (a) SEM image for MP-RuO2. TEM images of (b) MP-RuO2, (c) recycled MP-RuO2 and (d) C-RuO2. Both particle size distribution and TEM images showed an absence of nanoparticles (< 100 nm) outside the three-dimensional porous network of MP-RuO2 after the removal of the templating agent. The average particle sizes of MP-RuO2 and C-RuO2 obtained from three measurements were determined to be 1931 nm (polydispersity index 0.35) and 390 nm (polydispersity index 0.44), respectively. Chemical water oxidation catalyzed by the MP-RuO2 catalyst was carried out using the strong one-electron oxidant CAN, 43 and the evolved oxygen was detected by real-time mass spectrometry. Upon addition of a deoxygenated aqueous solution to a solid mixture of MP-RuO2


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and CAN, immediate O2 evolution was triggered. For a comparison, catalytic activity of C-RuO2 was also evaluated under similar conditions (Figure 4). The turnover number (TON; defined as moles of O2 evolved per mole of catalyst), initial turnover frequency (TOF; defined as moles of O2 evolved per mole of catalyst per unit time) and O2 yield (defined as 4 times the amount of O2 evolved per amount of oxidant) of each run were determined. The calculated TONs, TOFs (at 3 min) and the O2 yields for MP-RuO2 were determined to be 14, 0.07 s–1 and 99%, respectively, vastly exceeding the performance of C-RuO2 (TON = 1, TOF = 0.002 s–1, O2 yield = 9%). This difference in the catalytic activity can be attributed to the enhanced physical properties of the synthesized MP-RuO2.

Figure 4. Oxygen evolution catalyzed by MP-RuO2 and C-RuO2. The numbers 1‒4 refer to the number of catalytic water oxidation cycle using the recycled MP-RuO2. Reaction conditions: Deoxygenated water (1.0 mL) was added to a solid mixture of MP-RuO2 (0.5 mg) and CAN (120 mg, 0.22 mmol).


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The recyclability of MP-RuO2 was subsequently investigated. After completion of the catalytic reaction, the MP-RuO2 residue was centrifuged, washed with water, dried under vacuum, and reused under identical catalytic conditions. Although the kinetics of the O2 evolution curves varied slightly over the recycling experiments, the catalyst retained its catalytic activity over four cycles, generating 79–99% of the theoretical amount of O2 within 7 min (Figure 4). A minimal loss in the O2 yield and decrease in TOF can be observed over the four catalytic cycles, which can be attributed to a change in RuO2 aggregation in the higher pore size region, as previously observed in CAN-driven water oxidation using unsupported RuO2 nanoparticles. 44 Since the catalyst remained active over four catalytic cycles, the overall catalytic activity for the oxidation of water should substantially exceed the TON of 54 calculated from the four catalytic runs. A comparison between MP-RuO2 and other previously reported RuO2-based heterogeneous catalysts for water oxidation is shown in Table 2. Table 2. Catalytic activities of RuO2-based heterogeneous catalysts for water oxidation using CAN as a chemical oxidant.

Catalyst

TON

TOF (s–1)

Reference

MP-RuO2a

54

0.07

This work

C-RuO2a

1

0.002

This work

RuO2-PPU-MCFb

10

0.006

[25]

RuO2 NP-SBA-15c

200d

0.27

[44]

RuO2 on TiO2e

66f

≈0.02

[45]

RuO2 NP-loaded mesoporous silicag,h

400

0.038

[46]


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a

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Conditions: RuO2 (0.5 mg catalyst, 3.76 μmol Ru) and CAN (120 mg, 220 μmol) in water (1.0

mL). b Conditions: RuO2 nanocatalyst (0.25 mg nanocatalyst, 0.18 μmol Ru) and CAN (60 mg, 109 μmol) in water (1.0 mL). c Conditions: RuO2 on SBA-15 (0.5 mg catalyst, 0.15 μmol Ru) and Ce(SO4)2 (1.66 mg, 5 μmol) in 0.49 M H2SO4 (2.55 mL). d TON calculated after 15 cycles. e Conditions: RuO2 on TiO2 (30 mg, 2.25 μmol Ru) and Ce(SO4)2 (10 mg, 30 μmol) in 1 M HClO4 (30 mL). f TON calculated after five cycles. g Conditions: RuO2 nanocatalyst (10 mg, 6.83 μmol Ru) and Ce(SO4)2 (6.64 mg, 20 μmol) in 1 M H2SO4 (20 mL). h TON and TOF calculated based on large-scale studies on liter quantities of solution. After four consecutive runs, MP-RuO2 was recovered and analyzed for any change in its structural and textural properties. The textual properties of the MP-RuO2 catalyst after these four cycles are presented in Table 1, highlighting its stability, with almost no change in the surface area. Pore size distribution after recycling essentially remained unaltered after several catalytic runs (Figure 2b). Powder XRD revealed that the intensity of the Ru peak increased slightly after four catalytic runs (Figure 1), but apparently this did not have a large influence on the catalytic activity. It is shown in a previous atomic force microscopy study of mesoporous thin films of RuO2 that the synthesis of MP-RuO2-type materials is highly sensitive to the calcination temperature. 47 It was shown that calcination at different temperatures (280 °C, 290 °C, 300 °C, and 310 °C) over a period of 1 min resulted in coalescence of nanoparticles of RuO2 into larger ones, and the appearance of a crystalline structure. The generation of a crystalline structure of MP-RuO2 is in agreement with these results as our calcination temperature is 400 °C. Kinetic evolution of a mesoporous structure and its crystalline structure stability is dependent on the controlled coalescence of nanoparticles within the walls of the porous network and the slow heating rate.


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Here, our heating rate of 2 °C per minute seems to be optimal. Reactivity of this type of nanoporous structure is significantly higher compared to crystalline nanoparticles alone. TEM images of the material after the second and third run clearly show that MP-RuO2 retains its pore structure without deterioration of the pore network after catalysis (Figures 3c and S4). This observation indicates the strong connection between the crystallites inside the network of MPRuO2. Furthermore, it also suggests that atomic and porous structure stability improves catalytic activity and the recyclability. We have also observed small nanoparticles after the third and fourth runs when recycled samples were examined by TEM. It is likely that this small fraction of formed RuO2 nanoparticles is due to mechanical friction generated between the stirring bar and the reaction vial glass walls during the catalytic runs. Figure S5 shows the recycled MP-RuO2 (labeled 1) and C-RuO2 (labeled 2) after the catalytic runs. A comparison of the synthesized MP-RuO2 and C-RuO2 revealed higher stability and better recyclability of the former. The superior performance of MP-RuO2 compared to C-RuO2 is attributed to its mesoporous structure provided by the hydrothermal treatment during the synthesis that is further enhanced by the annealing. C-RuO2 on the other hand displayed low O2 yield and did not survive even the first catalytic run and to our surprise was dissolved completely. Therefore, it was not possible to recover and characterize the used C-RuO2. Currently, iridium oxide is considered the most viable heterogeneous WOC and is employed in the commercially available polymer electrolyte membrane electrolyzers. However, iridium is approximately 15 times 48 less abundant and about 15 times 49 more expensive compared to ruthenium. Therefore, addressing the known instability of RuO2 under water oxidation conditions is an important advancement in terms of the total cost of water splitting devices.


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Clearly, the synthetic strategy described here provides a viable framework for design of more stable and cheaper heterogeneous WOCs based on RuO2. In conclusion, mesoporous ruthenium oxide (MP-RuO2) was synthesized using a surfactantbased template approach. The synthesized MP-RuO2 displayed higher catalytic activity and excellent O2 yield compared to commercial RuO2 in chemical water oxidation using CAN as the chemical oxidant. The excellent O2 yield for the synthesized MP-RuO2 is attributed to the stability of the heterogeneous catalyst both from a structural and textural perspective. Furthermore, the catalyst showed excellent recyclability and stability over four catalytic runs. We are currently exploring the relationship between the surface area of synthesized MP-RuO2 materials and the catalytic water oxidation activity, recyclability, as well as ordered and nonporous structures of MP-RuO2. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:XX. Experimental procedures and characterization (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Present Addresses


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¶ Nuclear Materials Authority, P. O. Box 530, El Maadi, Cairo, Egypt. Author Contributions §These authors contributed equally to this work (M.N.I., and A.F.A.-M.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from Swedish Research Council (621-2013-4872 and 621-2013-4868), Stiftelsen Olle Engkvist Byggmästare, the Knut and Alice Wallenberg Foundation, and the Carl Trygger Foundation is gratefully acknowledged. We also thank the Knut and Alice Wallengberg Foundation for a grant for purchasing the electron microscopes. REFERENCES

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For Table of Content Use Only

Mesoporous ruthenium oxide (MP-RuO2) has been synthesized and compared to commercial ruthenium oxide (C-RuO2) in chemical water oxidation.


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Mesoporous Ruthenium Oxide: A Heterogeneous Catalyst for Water

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