Synthesis and Catalytic Application of Ag44 Clusters Supported on

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Synthesis and Catalytic Application of Ag Clusters Supported on Mesoporous Carbon Masaru Urushizaki, Hirokazu Kitazawa, Shinjiro Takano, Ryo Takahata, Seiji Yamazoe, and Tatsuya Tsukuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08903 • Publication Date (Web): 11 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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The Journal of Physical Chemistry

Synthesis and Catalytic Application Supported on Mesoporous Carbon

of

Ag44 Clusters

Masaru Urushizaki,1 Hirokazu Kitazawa,1,2 Shinjiro Takano,1 Ryo Takahata,1 Seiji Yamazoe,1,2 and Tatsuya Tsukuda1,2* 1 2

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan

ABSTRACT: 4-(Fluorophenyl)thiolate-protected Ag44 clusters (Ph4P)4[Ag44(SC6H4F)30] were calcined on mesoporous carbon (MPC) under a vacuum at 300–500 °C for 2 h. X-ray absorption spectroscopy, transmission electron microscopy, and thermaldesorption mass spectrometry revealed that sulfur-free Ag44 clusters were successfully produced by calcination of [Ag44(SC6H4F)30]4– at 300 °C, in sharp contrast to the formation of silver sulfide nanoparticles by calcination of dodecanethiolateprotected Ag nanoparticles (3.0 ± 0.6 nm). The Ag44/MPC was applied for catalytic dehydrogenation of ammonia–borane (NH3BH3) as a test reaction. It turned out that the Ag44/MPC catalysts produced one equivalent of H2 from NH3BH3, but only in the presence of O2 (turnover frequency; 1.9 × 103 h–1 Ag atom–1). Given that nanoparticles of other metals (Pt, Pd, Rh, Ni, or Ru) produced three equivalents of H2 under an inert atmosphere, this result indicates that the Ag44/MPC-catalyzed dehydrogenation of NH3BH3 proceeds by a different mechanism from that on other nanoparticles. 1. INTRODUCTION The development of non-platinum group metal catalysis is desirable not only from the viewpoint of cost reduction, but also because of concern regarding the conservation of limited resources. In this regard, silver (Ag) has recently gained much attention because it is much cheaper and more abundant than the platinum group metals. It has been reported in the last decade that heterogeneous Ag clusters and nanoparticles catalyze a variety of chemical transformations, including selective reduction of NOx,1–3 oxidant-free dehydrogenation of alcohols,4,5 N-alkylation of anilines with benzyl alcohol,6 oxidation of silanes to silanols with water as an oxidant,7 hydration of nitriles to amides in water,8 deoxygenation of epoxides to give alkenes,9 conversion of terminal alkynes to propiolic acids with carbon dioxide,10 hydrogenation of crotonaldehyde,11 acrolein,12 chloronitrobenzenes,13 or nitroaromatics,14 Diels–Alder cycloaddition,15 and crosscoupling of secondary and primary alcohols.16 These examples show that Ag clusters and nanoparticles are potentially useful catalysts, although harsh reaction conditions, such as high temperatures, are required in many cases. There is plenty of latitude in the ultrasmall (8 at Tc ≥ 400 °C is ascribed to aggregation of the Ag clusters due to thermally induced diffusion. The size of the aggregated clusters is ~150 at most, considering that the CN value for cuboctahedral 147-atom cluster is 9.0. Dramatic structural reconstruction at Tc = 300 °C is also recognizable by the remarkable increase of the Ag–Ag bond length from 2.794 to 2.868 Å (Table 1). The Ag–Ag bond length (r) after calcination (2.868 Å) is slightly shorter that of bulk Ag (2.899 Å) as frequently observed in metal nanoparticles. It is interesting to note, however, that the r value is reduced with increase in Tc, as opposed to the expectation that the bond lengths increase with the cluster size. This counterintuitive behavior is explained in such a way that the Ag–Ag bonds of the smallest clusters prepared at Tc = 300 °C are elongated owing to significant interaction with the MPC, whereas the contribution of the Ag–Ag bonds within the cluster become more important for aggregated Ag clusters. The EXAFS data could be fitted well (R factor < 10%) without considering the contribution of Ag–O bonds although the calcined samples were exposed to air before the measurement. This indicates that the oxidation of Ag clusters on MPC in air is negligible.

Figure 2. (A) Ag K edge FT-EXAFS of (a) (Ph4P)4[Ag44(SC6H4F)30] dispersed in DMF and calcined on MPC at (b) 300, (c) 400, and (d) 500 °C. (B) Ag K edge XANES spectra of (a) 0.2Ag44(SC6H4F)30/MPC calcined at 300 °C, (b) Ag foil, and (c) Ag2S.

Table 1. Results of Curve-Fitting Analysis of Ag K Edge EXAFS Tc a Atomc CNd r (Å)e σ2 f R (%)g S 1.5(4) 2.505(7) 0.011(3) –b 1.8 Ag 3.4(6) 2.794(4) 0.011(7) 300 Ag 6.9(8) 2.868(3) 0.0092(14) 9.4 400 Ag 8.3(1.0) 2.861(2) 0.0090(14) 7.2 500 Ag 8.6(1.0) 2.860(3) 0.010(1) 7.0 a

Calcination temperature. bDispersed in DMF. cBonding atom. Coordination number. eBond length. fDebye–Waller factor. gR factor: R = (Σ(χdata – χfit)2/Σ(χdata)2)1/2. d

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A typical TEM image of 0.2(Ph4P)4[Ag44(SC6H4F)30]/MPC calcined at 300 °C is shown in Figure 3. The Ag clusters with a diameter of 1–2 nm are finely dispersed on MPC although it is not easy to identify the small Ag clusters on the spotted background of the MPC support. We conclude that the SC6H4F thiolates are completely removed without aggregation of the clusters on calcination of 0.2Ag44(SC6H4F)30/MPC at 300 °C for 2 h. Hereafter, we will refer to the product of this treatment as 0.2Ag44/MPC.

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the above observations, we propose that the 0.2Ag44/MPC catalyzed the following oxidative dehydrogenation of NH3BH3: NH3BH3 + O2 → NH4+ + BO2– + H2. (2) The TOF value of 0.2Ag44/MPC was estimated to be 1.9×103 h–1 Ag atom–1 in the presence of O2. This value is in the range of those reported for other metal nanoparticles (105–102 h–1 metal atom–1),50–54 To elucidate the effect of the removal of thiolate on the catalytic activity, the H2 production rate was compared before and after calcination of 0.2(Ph4P)4[Ag44(SC6H4F)30]/MPC (Figure S6). The catalytic activity of the calcined 0.2Ag44/MPC was significantly higher than that of the noncalcined catalyst. This shows that removal of the thiolates accelerates the reaction shown in eq. (2). Finally, we studied the reusability of 0.2Ag44/MPC. The yield of H2 gradually decreased as the catalyst was reused (Table S1). The deactivation mechanism is under study to obtain clues regarding ways of enhancing the durability of the Ag catalysts.

Figure 3. TEM image of 0.2Ag44(SC6H4F)30/MPC calcined at 300 °C.

3.2. Catalysis. The catalytic performance of 0.2Ag44/MPC was investigated for the dehydrogenation of NH3BH3. NH3BH3 has gained considerable attention in relation to the hydrogen economy because it is an efficient (19.6 wt% H2), lightweight (30.8 g mol–1), and stable medium for the storage of H2.44–46 The most promising method for releasing H2 from NH3BH3 is metal nanoparticle-catalyzed hydrolysis in water:47,48 NH3BH3 + 2H2O → NH4+ + BO2– + 3H2. (1) Although nanoparticles of a variety of metals, such as Pt, Pd, Rh, Ni, and Ru, have been widely tested for this reaction,47–56 Ag nanoparticles have been scarcely used.55,56 Figure 4 shows the temporal evolution of the yield of H2 from NH3BH3 at room temperature in the presence of 0.2Ag44/MPC. Catalysis by 0.2Ag44/MPC showed two notable differences from those of the previously reported metal nanoparticles. First, almost no H2 was produced under a N2 atmosphere (Figure 4; gray). A control experiment suggested that the slow production of H2 in N2 is caused by the MPC support. The poor catalytic activity of Ag44/MPC under an inert atmosphere is consistent with previous reports on Ag nanoparticles (~4 nm)/SBA-15 and on Ag nanoparticles (1.9 nm)/CeO2/SBA-15; the turnover frequencies (TOFs) of those Ag nanoparticles were reported to be 0.2 and 2.4 h–1Ag atom–1, respectively. 55,56 However, we found that the production of H2 proceeded efficiently in air (Figure 4; red) or under pure O2 (Figure 4; blue). Consumption of O2 during the reaction was confirmed by the reduction in the O2 concentration in the closed reaction system. This finding suggests that H2 production proceeds by a different mechanism from that of eq. (1), which does not involve O2. A need for O2 has been also reported for Rh catalysts.57 Secondly, the reaction was terminated after production of one equivalent of H2, whereas the dehydrogenation shown in eq. (1) results in the formation of three equivalents of H2. 1wt%Pt/Al2O3 actually produced about three equivalents of H2 (Figure S5). Although the decreased production of H2 is disadvantageous from the viewpoint of utilization of NH3BH3 as an H2-storage material, this result suggests that the dehydrogenation mechanism on 0.2Ag44/MPC is different from that of eq. (1). On the basis of

Figure 4. Time course of H2 generation from NH3BH3 in the presence of 0.2Ag44/MPC in various environments. The curves are eye guides.

4. SUMMARY Sulfur-free Ag44 clusters were successfully immobilized on mesoporous carbon (MPC) by calcination of (Ph4P)4[Ag44(SPhF)30] at 300 °C for 2 h. Ag44/MPC generated one equivalent of H2 in the catalytic dehydrogenation of NH3BH3, but only in the presence of O2. This indicates that the dehydrogenation by Ag44 proceeds by a different mechanism from that of other metals (Pt, Pd, Rh, Ni, or Ru), which generate three equivalents of H2 under an inert atmosphere. The high activity and new reaction mechanism suggest an opportunity for the development of novel Ag cluster catalysts.

ASSOCIATED CONTENT Supporting Information Details of experimental procedures and characterization results. This material is available free of charge via the Internet at http://pubs.acs.org. Characterization of X4[Ag44(SC6H4F)30] (X = Na or Ph4P) (Figure S1), characterization of Ag:SC12 (Figure S2), characterization of MPC (Figure S3), TPD of Ag:SC12 (Figure S4), catalysis of 1.0wt%Pt/Al2O3 (Figure S5), catalysis of 0.2(Ph4P)4[Ag44(SC6H4F)30]/MPC before and after calcination (Figure S6), reusability of 0.2Ag44/MPC (Table S1), and complete refs. 20, 32, 33, 37 and 38.

AUTHOR INFORMATION Corresponding Author *E–mail: [email protected]–tokyo.ac.jp

ACKNOWLEDGMENT


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We thank Prof. Hiroshi Nishihara (The University of Tokyo) for providing us with access to the TEM apparatus. This research was financially supported by the Elements Strategy Initiative for Catalysis and Batteries (ESICB) and by a Grant-in-Aid for Scientific Research (No. 26248003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. The Ag K edge XAFS measurements were carried out at beamline BL01B1 of the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014B1430, 2015A1590).

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Synthesis and Catalytic Application of Ag44 Clusters Supported on

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