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Ru and Ru-Ni Nanoparticles on TiO Support as Extremely Active Catalysts for Hydrogen Production from Ammonia Borane Kohsuke Mori, Kohei Miyawaki, and Hiromi Yamashita ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00715 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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Ru and Ru-Ni Nanoparticles on TiO2 Support as Extremely Active Catalysts for Hydrogen Production from Ammonia Borane Kohsuke Mori,a,b,c* Kohei Miyawaki,a Hiromi Yamashitaa,b*

a

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. b

Elements Strategy Initiative for Catalysts Batteries ESICB, Kyoto University, Katsura, Kyoto 615-8520, Japan.

c

JST, PREST, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan Tel&FAX: +81-6-6879-7460, +81-6-6879-7457

E-mail: [email protected], [email protected]

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Abstract Highly dispersed monometallic Ru nanoparticles can be successfully synthesized on TiO2 supports for effective hydrogen production from ammonia borane (NH3-BH3; AB). The choice of support material and reduction methods were confirmed to significantly influence the size of the Ru nanoparticles, and smaller sizes of Ru nanoparticles with a mean diameter of 1.7 nm could be formed on a TiO2 support material by H2 reduction at 200 °C. The catalytic activity of the Ru nanoparticles can be significantly enhanced by alloying with Ni atoms, whereby a significantly high total turnover number (TTO) of approximately 153,000 over 8 h was achieved with an excellent turnover frequency (TOF) of 914 min–1 and an activation energy of 28.1 kJ mol–1. Detailed characterization by means of TEM, EDX, H2-TPR, and in situ XAFS measurements revealed that a synergistic alloying effect originates from the random distribution of Ru-Ni nanoparticles with a mean diameter of 2.3 nm and plays a crucial role in the exceptional catalytic performance. This catalytic system has particular potential for industrial application in fuel cells due to advantages such as the facile preparation method, the use of relatively cheap metals, and the exceptionally high catalytic activity.

Keywords: Ru-Ni alloy, Titania, dehydrogenation, ammonia borane, hydrogen


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1. Introduction Hydrogen (H2) has been regarded as an ultimate energy carrier, and it has promise as a next-generation energy vector for solving many environmental and energy problems.1 However, the potential of hydrogen has not been entirely utilized due to difficulties in achieving controllable storage and safe delivery.2,3 Thus, the development of efficient and convenient strategies to store hydrogen is still a crucial task for the up-coming hydrogen economy. There has been emergent demand for the exploration of suitable hydrogen storage materials that can release hydrogen in situ under mild reaction conditions for direct use in fuel cells.4-8 Ammonia borane (NH3-BH3; AB) has emerged as a candidate hydrogen storage material9-17 on account of its nontoxicity, low molecular weight (30.87 g·mol−1), high theoretical hydrogen gravimetric capacity (19.6 wt%), and its high stability in solid form under ambient conditions.18 The hydrogen stored in AB can be released via either pyrolysis19 or hydrolysis routes.5 Since the pyrolysis route needs substantially high temperatures and results in the release of only 6.5 wt% (1 equivalent based on AB) hydrogen, hydrolysis is considered the more promising process because as much as 3 equivalents of hydrogen

based on AB can be obtained under mild reaction conditions with appropriate catalysts, according to Eq. 1.

NH3BH3 + 2H2O → NH4+ + BO2− + 3H2.

(1)

Ru nanoparticles (NPs) are among the most widely investigated catalyst materials for both homogeneous and heterogeneous AB dehydrogenation systems due to their superior catalytic properties.20-27 To improve the catalytic activity and simultaneously minimize the use of expensive Ru, the fabrication of bimetallic NPs with earth abundant and less expensive first-row transition metals, including Ni, Co, and Cu, has attracted much attention.28-32 The coordination of one metal to another can provide specific new active sites that originate from the unexpected interplay of electronic and lattice effects in adjacent metals.33-36 Bimetallic catalysts can exhibit significantly higher catalytic activity and selectivity than their monometallic counterparts due to such synergistic


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effects. The replacement of a portion of precious noble metal NPs with low-cost metals also contributes to atomic economy.37,38 Thus, precise control of the geometric and electronic effects of bimetallic Ru NPs by changing the size, shape and composition plays an important role in the design of highly functionalized catalysts. In addition, the catalyst support is a key component to improve the catalytic activity and stability of metal NPs.39 The use of suitable supports, such as polymers, metal oxides, mesostructured materials, and carbon materials can also afford unique catalytic functions that include site isolation of active metal NPs from agglomeration and the synergistic effects due to strong metal-support interactions (SMSI). Herein we report the synthesis, characterization, and catalytic application of Ru and Ru-Ni NPs supported on TiO2 for AB dehydrogenation. TiO2 and H2 have proven to be a promising combination as a support material and reductant for the optimization of catalytic activity. Investigation using a series of RuxNi1-x bimetallic NPs deposited on TiO2 indicated a volcano-shaped relationship with change in the Ru/Ni ratio. The catalyst with Ru:Ni =1:0.3 afforded a significantly high total turnover number (TTO) of approximately 153,000 over 8 h with an excellent turnover frequency (TOF) of 914 min–1. To the best of our knowledge, these are some of the highest values reported for Ru-based catalytic systems. Characterization by means of TEM, EDX, H2-TPR, and in situ XAFS measurements revealed the formation of randomly distributed Ru-Ni bimetallic NPs with a narrow size distribution, which plays an important role in achieving extraordinary catalytic performance in addition to the synergistic alloying effect.

2. Results and Discussion 2.1.

Ru catalyst

Several Ru catalysts were prepared with commercially available inorganic supports (TiO2, SiO2, Al2O3, ZrO2, and MgO) by a simple impregnation method from an aqueous solution of RuCl3·3H2O, followed by reduction under a H2 atmosphere (20 mL/min) at 300 °C without a specific calcination step before reduction. The XRD pattern is shown in Figure S1, in which the peaks attributable to the metals could not be confirmed.


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The dehydrogenation of AB was conducted using prepared Ru catalysts (Figure S2). The results are summarized in Figure 1, which also contains the Brunauer-Emmett-Teller (BET) surface area of the supports determined from N2 adsorption-desorption measurements and the average diameter of Ru NPs determined from CO adsorption measurements. The reaction proceeded with high reproducibility for all cases. Any induction period was not observed and H2 was stoichiometrically produced in a 3:1 (H2/NH3BH3) molar ratio during the course of the reaction for all samples, which represents complete dehydrogenation. No reaction occurred in the control experiment (without catalyst) under the same condition. All Ru/TiO2 catalysts exhibited the highest activity among those investigated, regardless of their crystalline phase, while the Ru/Al2O3, Ru/ZrO2, and Ru/MgO catalysts showed substantially low catalytic activity. More detail inspection under reduced amount of catalyst conditions revealed that the TOF obtained by Ru/TiO2 P25 (anatase + rutile), Ru/TiO2 (rutile), and Ru/TiO2 (anatase) were 558, 510, 455 min-1, respectively, suggesting that the TiO2 P25 acts as a best support for the Ru-catalyzed AB dehydrogenation. There was no distinct relationship between the catalytic activity and BET surface area. The BET surface areas of TiO2 P25 (anatase + rutile) and TiO2 (rutile) are 39.7 and 40.0 m2·g−1, respectively, which are lower than 338 m2·g−1 for TiO2 (anatase). Amorphous SiO2 and Al2O3 also showed low activity, despite their relatively large surface areas. In contrast, the average diameter of Ru NPs determined by CO adsorption measurement corresponded well with the catalytic activity. The catalytic activities increased with decreasing average particles size; the average particle sizes of all Ru/TiO2 samples were less than 5 nm, while relatively larger Ru NPs were formed in the cases of Ru/ZrO2 and Ru/MgO. Transmission electron microscopy (TEM) measurements of the active Ru/TiO2 P25 and less active Ru/MgO catalysts was performed to compare the average diameters of the Ru NPs. Figure 2A shows that Ru/TiO2 P25 has highly dispersed Ru NPs with a mean diameter of 1.7 nm and a narrow size distribution throughout the observation area. Similarly, small Ru NPs can be observed in the Ru/TiO2 (anatase) and Ru/TiO2 (rutile) catalysts, of which the average diameters were 1.8 and 1.9 nm, respectively. We suppose that the formation of slightly smaller Ru NPs in Ru/TiO2 P25 is 5 ACS Paragon Plus Environment

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one possible reason for high catalytic activity compared to Ru/TiO2 (anatase) and Ru/TiO2 (rutile). However, Ru atoms on the MgO support were partially segregated on the surface and randomly distributed (Figure 2B). The mean diameter of Ru NPs in the Ru/MgO catalysts was determined to be 8.4 nm. The difference in the dispersity of Ru NPs on each support may be ascribed to the reducibility of Ru3+ ions. In the H2-temperature-programmed reduction (TPR) measurement of as-deposited Ru/TiO2 P25 before H2 reduction, one sharp reduction peak centered at 130 °C was observed (Figure 3A). In contrast, Ru/MgO displayed bimodal reduction peaks at around 160 and 350 °C, which are attributed to the reduction of Ru3+ → Ru2+ and Ru2+ → Ru0, respectively (Figure 3B).40 This result suggests that the TiO2 support allows a more rapid and homogeneous reduction of Ru ions than MgO at lower temperatures. It is well accepted that the kinetics for the reduction of metal ions play an important role in determining the final particle size, where rapid reduction generates more metal nuclei in a shorter period and efficiently suppresses the growth of metal NPs.41 Therefore, TiO2 supports may produce smaller Ru NPs than the MgO support. The catalytic activity of Ru/TiO2 P25 was also significantly influenced by the reducing reagent; H2 reduction is the best method to achieve high catalytic activity compared to chemical methods using NaBH4, NH3BH3, or HCOONa, as shown in Figure 4 and Figure S3. The H2 reduction temperature also influences the catalytic activity; maximum activity was achieved by H2 reduction in the range of 200–300 °C, while further increase of the temperature gradually decreased the catalytic activity. No significant reduction peak was observed in the H2-TPR spectra of bare TiO2, suggesting that change of oxidation state of Ti species is negligible. The average diameter of the Ru NPs reduced at 600 °C was determined to be 3.1 nm from TEM analysis (Figure S4), which is larger than those reduced at 300 °C. Therefore, it was concluded that the size of the Ru NPs rather than the surface area of the support is an important factor to achieve high catalytic activity. Logarithmic plots of the H2 generation rate as a function of the AB concentration indicated a 0.7 order dependence, while a first-order dependence was observed on the catalyst concentration (Figure S5). Therefore, the H2 generation rate is considered to be controlled by the surface reaction, so that an increase of active sites with smaller NPs is expected to yield higher catalytic activity for


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the dehydrogenation of AB. The activation energy (Ea) for the dehydrogenation of AB using Ru/TiO2 P25, determined from Arrhenius plots, was 37.7 kJ·mol−1, as shown in Figure S6. This value is similar to those for Ru/C (34.8 kJ·mol−1),20 Ru@/MWCNT (33.0 kJ·mol−1),21 and Ru NP@ZK-4 (28.0 kJ·mol−1),26 as summarized in Table 1. Furthermore, the TOF based on Ru atoms was 604 min−1. It is noteworthy that the TTO achieved with Ru/TiO2 P25 approached more than 100,000 within 11 h, which was significantly higher than those reported for other monometallic Ru catalyst systems, such as Ru@MWCNT,21 Ru@PSSA-co-MA,23 Ru/HAP,24 and [email protected]

2.2.

RuNi catalyst

To improve the catalytic activity of the Ru/TiO2 P25 catalyst and simultaneously minimize the quantity consumed, the deposition of both Ru and Ni on the surface of TiO2 P25 was performed by the same impregnation method from an aqueous solution of RuCl3·3H2O and NiCl2·6H2O. The RuNi NPs were synthesized by H2 reduction without calcination before the reduction process and then employed for the catalytic dehydrogenation of AB. The composition of the RuNi/TiO2 P25 samples (Ru:Ni = 1:0, 1:0.1, 1:0.3, 1:1, and 0:1) was easily adjusted by variation of the initial molar ratio of the Ru and Ni precursors used in the impregnation process. Time courses for hydrogen production from AB using various RuNi compositions are shown in Figure 5. The reactions using pure Ni/TiO2 were extremely sluggish, whereas an enhancement was observed with the RuNi alloy catalysts. This result indicates a synergistic effect between Ru and Ni atoms. Maximum catalytic activity was obtained at a Ru:Ni ratio of 1:0.3 (Figure S7). Such a volcano-shaped order of activity demonstrates that the formation of a uniform RuNi alloy structure on the surface of TiO2 and the synergistic effect originate from the integration of both Ru and Ni. In comparison, RuNi/MgO (Ru: Ni = 1: 0.3) did not show any enhancement by the addition of Ni, which indicates that no RuNi alloy was formed on the surface of MgO, as will be discussed later. In the preliminary experiments, the positive enhancement effect was observed in the case of Ni, while the Co and Cu did not show significant enhancement in the


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present catalytic system. An important advantage of the RuNi/TiO2 P25 catalyst is its extraordinarily high catalytic activity and long durability. As shown in Table 1 and Figure S8, Ea for RuNi/TiO2 P25 was 28.1 kJ·mol−1, which is comparable with that for RuNi/Ti3C2X2 (25.7 kJ·mol−1)28, but substantially higher than those observed for other RuNi-based catalytic systems.29-31 The present catalytic system gave a significantly higher TTO of approximately 153,000 over 8 h with an excellent TOF of 914 min–1. This implies that the present catalytic system suppresses the negative effect of accumulated BO2- ions to a minimum. When the catalyst was removed by filtration after first reaction, further addition of AB to the filtrate did not show any H2 production, suggesting the no leaching of active components. This comment was added in the text. To the best of our knowledge, these are one of the highest values ever reported for the dehydrogenation of AB. The high catalytic activity and durability achieved with RuNi/TiO2 are particularly desirable for an ideal hydrogen vector in terms of potential industrial applications for polymer electrolyte membrane fuel cells. The local structure of the RuNi NPs generated on the surface of TiO2 may play an important role in attaining high catalytic activity. In an effort to elucidate the structure-activity relationship of the Ru1Ni0.3/TiO2 P25 catalyst, detailed characterization was performed. The XRD pattern of Ru1Ni0.3/TiO2 P25 shows diffraction peaks with negligible intensities, presumably due to the smaller size of the particles. Figure 6A shows a TEM image of RuNi/TiO2 P25, which indicates that highly dispersed RuNi NPs were successfully formed with a narrow size distribution. The mean diameter of the RuNi NPs was determined to be 2.3 nm (Figure 6B), which was slightly larger than that of monometallic Ru/TiO2 with an average particle size of 1.7 nm. This result also suggests that the synergistic effect originates from the integration of Ru and Ni. A representative high-resolution TEM image shows a d-spacing of 0.210 nm, which differs from both the Ru crystal plane (0.214 nm) and the Ni plane (0.206 nm), and indicates the crystalline nature of the RuNi alloy NPs. Energy dispersive X-ray spectroscopy (EDS) mapping revealed that both Ru and Ni atoms were at almost the same position without the segregation of each atom throughout the image area; Ni, Ru, and Ti atoms are represented as blue, green, and yellow, respectively in Figures S9. Moreover, the


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elemental distribution along a single NP gave strong evidence for the formation of random homogeneous RuNi alloy structures in the RuNi/TiO2 catalyst, without the formation of a core-shell structure, where both Ru and Ni atoms were homogeneously observed in the cross-section (Figure 6C). However, partially segregated NPs with a wide particle size distribution were observed on the MgO support (Figure S10), of which the mean diameter was determined to be greater than 6.1 nm. Figure 3 shows H2-TPR profiles for the as-deposited mono- and bimetallic RuNi samples before H2 reduction. Ru/TiO2 P25 showed a single sharp peak with a maximum at around 130 °C, while monometallic Ni/TiO2 P25 exhibited a broad reduction peak centered at 350 °C, which demonstrates that the Ni precursor is more difficult to reduce than the Ru precursor. This is reasonably explained by considering the reduction potentials of the respective ions (E0(Ni2+/Ni0) = −0.257 V, E0(Ru3+/Ru0) = −0.47 V vs. NHE). Bimetallic RuNi/TiO2 P25 had only a single reduction peak with a maximum temperature at around 160 °C, which is intermediate between those corresponding to the monometallic samples. This simultaneous reduction of the two precursors indicates that the Ru and Ni atoms are in close interaction with each other, which leads to the formation of bimetallic RuNi NPs.42 In contrast, Ru/MgO displayed bimodal reduction peaks at around 160 and 350 °C, which are attributed to the reduction of Ru3+ → Ru2+ and Ru2+ → Ru0, respectively. Monometallic Ni/MgO showed a small reduction peak centered at around 360 °C. No significant shift of the reduction peak was observed in the case of the RuNi/MgO, which also exhibited bimodal reduction peaks at around 160 °C and 330 °C. Such reduction profiles indicate that the Ru and Ni atoms do not couple with each other on the surface of MgO, which results in segregated monometallic NPs rather than the bimetallic NPs. In order to further elucidate the change of local structure during the reduction sequence, in situ X-ray absorption fine structure (XAFS) measurements were conducted under a H2 atmosphere at elevated temperature. The XAFS results were consistent with the H2-TPR profiles. Figure 7 shows Fourier transforms (FT) of extended X-ray absorption fine structure (EXAFS) spectra at the Ru K-edge. The monometallic Ru/TiO2 P25 sample showed a single peak at around 100 °C due to the Ru–O bond at approximately 2.0 Å. The additional peak centered at 2.5 Å, which is assigned to the


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contiguous Ru–Ru bonds in metallic form, appeared at around 150 °C. With increasing temperature, the Ru species are completely reduced above 150 °C and are present as metallic Ru NPs. In contrast, reduction of the Ru species was retarded in the case of RuNi/TiO2 P25, which started at around 160 °C and was completed at higher than 220 °C. The inverse FT of the peak due to the metallic bonds was well fitted with Ru−Ru and Ru−Ni shells (Figure S11A). The interatomic distances (R) and coordination numbers (CN) were determined to be R = 2.64 Å and CN = 5.4, and R = 2.54 Å and CN = 4.2 for the Ru−Ru and Ru−Ni shells, respectively (Table S1). The peak for the metallic bonds was slightly shifted when compared with monometallic Ru bonds by the introduction of Ni atoms, which suggests the formation of heteroatomic Ru–Ni bonding in RuNi/TiO2 P25. The X-ray absorption near-edge structure (XANES) spectra also followed the different shape changes between Ru/TiO2 and RuNi/TiO2 P25 with increasing reduction temperature (Figure S12). The small differences in the spectral shape after treatment at 250 °C may be attributed to alloying with Ni. The edge position (measured at the half-height of the edge jump) is dependent on the electronic charge of the Ru atoms, and the energy of RuNi/TiO2 P25 is slightly lower than that of Ru/TiO2 P25 (ca. 1 eV), which could be attributed to the change of electronic state of Ru due to the electron-donating ability of Ni. The electronic charge could transfer from Ni atoms to Ru atoms because of the differences in their electron negativities (Ni: 1.91 and Ru: 2.20). Similar phenomena can also be observed in the Ni K-edge FT-EXAFS spectra (Figure 8). At the initial stage, monometallic Ni/TiO2 exhibits a single peak at approximately 2.0 Å due to the Ni–O bonds. The peak intensity gradually decreased and an additional shoulder peak ascribed to the contiguous Ni–Ni bonds in the metallic form appeared at around 280 °C. Further increase in the reduction temperature intensified the peak due to the metallic bonds, and complete reduction was achieved over 350 °C. In contrast, reduction of the Ni species was enhanced for RuNi/TiO2 P25, by which the additional shoulder peak for metallic bonds was observed below 200 °C with complete reduction achieved at around 250 °C. Moreover, the metallic bond in RuNi/TiO2 P25 was determined to be slightly longer than that of the monometallic Ni metal observed in Ni/TiO2 P25. The best fit was achieved with Ni–Ni bonds having R and CN of 2.45 Å and 5.7, and Ni–Ru bonds


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with 2.54 Å and 4.2, respectively (Figure S11B and Table S1), which provides evidence for the presence of heteroatomic Ru−Ni bonding in RuNi/TiO2. XANES spectra also tracked the different shape changes between the Ni and RuNi/TiO2 P25 samples with increasing reduction temperature (Figure S13), which supports the FT-EXAFS spectra results. The H2-TPR and in situ XAFS analyses revealed a strong interaction between the Ru and Ni species on the surface of the TiO2 support, which resulted in the improved reducibility of the Ni species and the retarded reducibility of the Ru species compared with that for the monometallic samples. The lowering of the reduction temperature for the Ni species in RuNi/TiO2 can be explained by a mechanism called intra-particle hydrogen spillover from the Ru surface to the Ni surface.43,44 The Ru3+ precursors are first reduced by H2 to form its nuclei, and H2 molecules are dissociated on its surface to form Ru-hydride species. Such hydride species can be transferred into the neighboring Ni2+ precursor in close proximity by spillover and reduce to the Ni0 species. The synthesis of Ru-Ni alloys is not easily achieved using conventional techniques, because Ru and Ni atoms are essentially immiscible at equilibrium due to their positive enthalpy of formation.45 Thus, the interaction between the Ru and Ni precursors and their high mobility are important factors for the simultaneous reduction of the two precursors and the formation of homogeneous RuNi alloy NPs with a narrow size distribution using TiO2 as a support. It has been reported that a possible mechanism for the catalytic dehydrogenation of AB involves the following three steps: i) the formation of an activated complex species between AB and the metal particle surface, ii) dissociation of the B–N bond upon attack by a H2O molecule, and iii) hydrolysis of the resulting BH3 intermediate to produce H2 along with the formation of a BO2– ion.46,47 With respect to this mechanism, we point out the participation of multiple interactions between Ru-Ni paired sites and AB molecules in the formation of an activated complex, which is most likely the rate-determining step. It is well accepted that the difference in the electron negativities of B and N atoms in AB molecules results in electropositive H atoms bound to B atom and electronegative H atoms bound to N atom. The electron transfer from Ni atoms to Ru leads to the change of the local charge density due to the different electronegativities. Therefore, we predict


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that the neighboring Ru-Ni site within a regular arrangement of RuNi NPs has a positive effect as an anchor on interaction with AB molecules by electrostatic interaction, which enhances the catalytic activity. Indeed, the distance of the Ru-Ni bond (2.54 Å) determined by EXAFS analysis is in accordance with the distance between the H atom bound to B atoms and an H atom bound N atom of ammonia borane (ca. 2.68 Å), as depicted in Figure 9. Such cooperative actions could not be attained with monometallic Ru-Ru bonds, which can explain the prominent activity of the RuNi catalyst for the dehydrogenation of AB.

Conclusion The synthesis and characterization of monometallic Ru and bimetallic RuNi/TiO2 P25 and evaluation of their high catalytic activity for the dehydrogenation of AB are demonstrated. TiO2 were proven to be an appropriate catalyst support to optimize the catalytic activity, despite its crystal phase. Characterization by several physicochemical methods revealed that highly dispersed Ru and RuNi NPs were generated on the surface of TiO2 P25, whereas Ru/MgO and RuNi/MgO, which exhibited significantly lower activities, had large NPs with wide size distributions. In the case of monometallic Ru/TiO2, the TiO2 support can allow a more rapid and homogeneous reduction of Ru ions at lower temperatures. On the other hand, a strong interaction between the Ru and Ni species can be attained on the surface of the TiO2 support, which resulted in simultaneous reduction of the two precursors to form homogeneous RuNi alloy NPs. Investigation using a series of RuNi/TiO2 P25 combinations with different Ru/Ni ratios resulted in a volcano-shaped relationship, which indicates a synergistic effect between Ru and Ni atoms. The catalytic system described here is a powerful protocol for application to chemical hydrogen storage-release devices due to the significantly high TTO of 153,000 over 8 h with an excellent TOF of 914 min–1.

Experimental Section Materials: P-25 TiO2 (JRC-TIO4, anatase:rutile = 7:3), anatase TiO2 (JRC-TIO3), rutile TiO2 12 ACS Paragon Plus Environment

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(JRC-TIO-8), CeO2 (JRC-CEO-1) were kindly supplied by the Catalysis Society of Japan. RuCl3·3H2O and MgO were purchased from Wako Pure Chemical Industries, Ltd. NiCl2·6H2O, and ZrO2 were purchased from Nacalai Tesque. Al2O3, SiO2, and ammonia borane (NH3-BH3, AB) were obtained from Aldrich Chemical Co. All commercially-available compounds were used as-received. Distilled water was used as the reaction solvent.

Preparation of catalyst: TiO2 (0.3 g) was mixed with 100 mL of an aqueous solution containing RuCl3·3H2O (0.0162 g, 0.062 mmol) and stirred at room temperature for 1 h. The suspension was evaporated under vacuum and the obtained powder was dried overnight. Subsequently, the sample was reduced with H2 (20 mL/min, 300 °C) for 2 h to yield Ru/TiO2 (Ru: 2.0 wt%). Ru/SiO2, Ru/Al2O3, Ru/ZrO2, Ru/MgO, Ru/ZrO2 with same amount of Ru loading were also synthesized according to the same procedure. In the case of RuNi/TiO2 (Ru: 1.0 wt%), NiCl2·6H2O was employed as a Ni source for impregnation to prepare RuNi/TiO2 samples with different Ru/Ni molar ratios (Ru:Ni = 1:0, 1:0.1, 1:0.3, 1:1, and 0:1). ICP analysis clearly indicated that the desired amount of metal species are successfully loaded onto the each supports.

Characterization:

BET

surface

area

measurements

were

performed

using

an

N2

adsorption-desorption instrument (BEL-SORP max, BEL Japan, Inc.) at 77 K. The sample was degassed under vacuum at 353 K for 24 h prior to data collection. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were performed using a Nippon Jarrell-Ash ICAP-575 Mark II spectrometer. CO pulse adsorption (BEL-METAL-1, BEL Japan, Inc.) was performed to measure the average metal particle size. TEM micrographs were obtained with a field emission (FE)-TEM (Hf-2000, Hitachi) equipped with a energy-dispersive X-ray detector (Kevex) operated at 200 kV. Temperature programmed reduction with H2 (H2-TPR) was conducted using a BEL-CAT (BEL Japan, Inc.) instrument by heating a 0.1 g sample at 10 °C/min from 80 to 700 °C under a 4.75% H2/Ar flow. In this experiment, as-deposited samples before H2 reduction were employed. Ru K-edge and Ni K-edge in situ XAFS spectra were recorded in


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transmission mode at the beam line 01B1 station with an attached Si(111) monochromator. In a typical experiment, a pellet sample was placed into a batch-type in situ XAFS cell. XAFS data were examined using the REX2000 program (Rigaku). Fourier transformation (FT) of k3-weighted normalized EXAFS data was performed over the range of 3.0 Å < k/Å-1 < 12 Å to obtain the radial structure function. Backscattering amplitude and phase shift parameters for a curve fitting analysis were theoretically calculated with FEFF8.40 code.

Dehydrogenation of AB: In a typical experiment, the Ru/TiO2 catalyst (0.01 g) was placed into a Schlenk-type reaction vessel (30 mL) connected to a gas burette. After purging three times with Ar, 10 mL of an aqueous solution of AB (0.2 M) was added to the reaction vessel and reacted at 298 K. The reaction was started when the AB solution was added to the reaction vessel and the progress was monitored periodically. TOF values were defied as mol H2/mol Ru·min. The total turnover number (TTO) was determined by the sum of turnover numbers in the successive reaction. The first reaction was started with Ru catalyst (0.01 g) in 10 mL of an aqueous solution of AB (12.0 mmol). When all the AB in the solution was completely hydrolyzed, another portion of AB was added the reaction was continued in this way for several times. The rate constant k (mL/min·gRu) were determined by the H2 generation at first 1 min for Ru/TiO2 and 3 min for RuNi/TiO2, where the volume of evolved H2 linearly increased with increasing reaction time. These rate constants were employed for the evaluation of activation energy (Ea) based on the Arrhenius equation (1). lnk = lnA-Ea/RT (1)

Acknowledgements: The present work was supported by Grants-in-Aid for Scientific Research (Nos. 26220911, 25289289, 26630409 and 26620194) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). XAFS spectra were recorded at the beam line 01B1 station in SPring-8, Japan Synchrotron Radiation Research Institute (JASRI), Harima, Japan (Proposal Nos. 2014B1041,


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2015A1149). This work was also supported by the MEXT program “Elements Strategy Initiative to Form Core Research Center”.

Supporting Information Available: XRD pattern, XANES spectra, TEM images, curve fitting results, Arrhenius plots, and kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Effect of support materials on hydrogen production from AB, and the BET surface area and particle diameter for Ru NPs of various catalysts.


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Figure 2. TEM images and particle size distribution of (A) Ru/TiO2 P25 and (B) Ru/MgO catalysts.


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Figure 3. H2-TPR profiles for (A) TiO2 P25- and (B) MgO-supported Ru, Ni, and RuNi NPs.


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Figure 4. Catalytic activity for hydrogen production from AB for Ru/TiO2 P25 prepared under various reducing conditions.


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Figure 5. Time courses for hydrogen production from AB using different Ru/Ni molar ratios.


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Figure 6. (A) HR-TEM image, (B) particle size distribution, (C) elemental distribution along a single RuNi NP of Ru1Ni0.3/TiO2 P25.


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Figure 7. In situ FT-EXAFS spectra at the Ru K-edge of (A) Ru/TiO2 P25 and (B) RuNi/TiO2 P25.


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Figure 8. In situ FT-EXAFS spectra at the Ni K-edge of (A) Ni/TiO2 P25 and (B) RuNi/TiO2 P25.


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Figure 9 Schematic illustration of the interaction between Ru-Ni paired site and AB.


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Table 1. Comparison of the catalytic activity and lifetime of Ru and RuNi catalysts for H2 production from AB at 298 K.

Catalyst

catalyst/AB (mol·mol-1)

Ea (kJ/mol)

TOF (mol H2/(mol Ru·min)

TTO

Ref.

Ru/TiO2 (P25)

0.001

37.7

604

104,500(11 h)

This study

Ru/C

0.00425

34.8

429

–

20

Ru@MWCNT

0.00216

33

329

26,400 (29 h)

21

Ru/TiO2

0.00118

70

241

–

22

Ru@PSSA-co-MA

0.007

54

180

51,720 (48 h)

23

Ru@HAP

0.0392

58

137

87,000 (202 h)

24

Ru@Al2O3

0.005

48

83.3

–

25

Ru NPs@ZK-4

0.005

28

90

36,700

26

Ru@X-NW

0.00271

134,100 (116 h)

27

RuNi/TiO2

0.001

28.1

914

153,000 (8 h)

This study

RuNi/Ti3C2X2

0.0153

25.7

825

–

28

Ru@Ni/graphene

0.004

36.6

340

–

29

Ni0.74Ru0.26 alloy NPs

0.0125

37.2

195

–

30

Ru@Ni/C

0.004

37.9

250

–

31


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Graphic Abstract


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Figure 1 173x122mm (150 x 150 DPI)

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Figure 2 127x135mm (150 x 150 DPI)


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Figure 3 178x108mm (150 x 150 DPI)


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Figure 4 127x101mm (150 x 150 DPI)


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Figure 5 179x113mm (150 x 150 DPI)


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Figure 6 118x174mm (150 x 150 DPI)


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Figure 7 106x125mm (150 x 150 DPI)


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