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Letter pubs.acs.org/journal/ascecg
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. Kar̈ kas̈ ,*,† and Eric V. Johnston*,† †
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, Svante Arrhenius väg 16C, SE-106 91 Stockholm, Sweden ‡ Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16C, SE-106 91 Stockholm, Sweden S Supporting Information *
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. KEYWORDS: Artificial photosynthesis, Electron microscopy, Heterogeneous catalysis, Oxygen evolution, Ruthenium, Water oxidation
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INTRODUCTION 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.4−6 Here, one of the remaining hurdles is fabrication of robust and efficient water oxidation catalysts (WOCs).7−9 The harsh oxidative conditions required for oxidation of water using either heterogeneous or homogeneous molecular catalysts require crucial presynthetic considerations in both support and catalyst design in order to ensure high activity and structural integrity during catalysis.10−12 Oxidative degradation of molecular WOCs can to some extent be hindered by redoxactive 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.15−23 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 © 2017 American Chemical Society
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 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, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), 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 (C-RuO2).33,34
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RESULTS AND DISCUSSION 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.37−40 The synthesis of MP-RuO2 was performed through a self-assembly method using a soft template, using RuCl 3 as the ruthenium source and cetyltrimethylammonium bromide (CTAB) as a templating agent. The ruthenium salt was converted into its hydrated form Received: August 17, 2017 Revised: September 27, 2017 Published: October 19, 2017 9651
DOI: 10.1021/acssuschemeng.7b02845 ACS Sustainable Chem. Eng. 2017, 5, 9651−9656
Letter
ACS Sustainable Chemistry & Engineering
Figure 1. High-angle PXRD patterns recorded at room temperature in reflection mode for the synthesized MP-RuO2, recycled MP-RuO2 (four cycles), and C-RuO2.
under basic hydrothermal conditions (100 °C) within the CTAB template. Finally, the filtered 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 PXRD in low-angle transmission and high-angle reflection modes, nitrogen adsorption, 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 were 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. 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). 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/ p0) 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 non-local density functional theory (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, hightemperature treatment of the synthesized particles enlarges the pores as a result of the expansion of the hydrophobic core of the CTAB micellar structure. We 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 both the synthesized and recycled MPRuO2. Almost no porosity and N2 adsorption were observed for commercial ruthenium oxide nanoparticles (Figure 2). 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-
Figure 2. (a) N2 adsorption−desorption isotherm and (b) pore size distribution.
RuO2 measured through N2 adsorption isotherm. Particle size distributions for MP-RuO2 and C-RuO2 are shown in Figure S2. 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 C-RuO2 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 MP-RuO2 had essentially no contribution to the micropores as previously synthesized transition metal oxide mesoporous materials.42 Furthermore, the MP-RuO2 catalyst was studied by SEM and TEM for porosity, morphology, and pore structure. SEM images revealed high aggregation of the primary particles of 9652
DOI: 10.1021/acssuschemeng.7b02845 ACS Sustainable Chem. Eng. 2017, 5, 9651−9656
Letter
ACS Sustainable Chemistry & Engineering Table 1. Textural Properties of the Synthesized MP-RuO2, Recycled MP-RuO2 and C-RuO2 Sample MP-RuO2 recycled MPRuO2c C-RuO2
BET specific surface area (m2/g)
Average pore size (Å)a
Total pore volume (cm3/g)b
Average particle size (nm)
68.3 62.8
127 110
0.22 0.18
1931 2153
11.3
130
0.03
390
a
Calculated as 4× total pore volume/BET surface area. bTotal pore volume was measured at 0.9896 p/po. cRecycled catalyst after four catalytic runs.
MP-RuO2 (Figure 3a). The secondary large aggregates are above 500 nm in size, while the size of the smaller primary 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).
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. 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 CANdriven 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. 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). PXRD 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-RuO2type materials is highly sensitive to the calcination temperature.47 It was shown that calcination at different temperatures (280, 290, 300, 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
Figure 3. (a) SEM image for MP-RuO2. TEM images of (b) MPRuO2, (c) recycled MP-RuO2, and (d) C-RuO2.
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 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. Both particle size distribution and TEM images showed an absence of nanoparticles (
