NiO Clusters Coated by Hollow Sillica: Novel Design for Highly

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PtNi/NiO Clusters Coated by Hollow Sillica: Novel Design for Highly Efficient Hydrogen Production from Ammonia-Borane Yuzhen Ge, Wanyue Ye, Zameer Hussain Shah, Xi-Jie Lin, Rongwen Lu, and Shufen Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15020 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Applied Materials & Interfaces

PtNi/NiO Clusters Coated by Hollow Sillica: Novel Design for Highly Efficient Hydrogen Production from Ammonia−Borane Yuzhen Ge, Wanyue Ye, Zameer Hussain Shah, Xijie Lin, Rongwen Lu*, Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China. Corresponding Author: * Rongwen Lu E-mail: [email protected]

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ABSTRACT: Ammonia−borane (NH3·BH3; AB) has been considered as an excellent chemical material for the hydrogen storage. However, developing highly efficient catalysts for continuous hydrogen generation from AB is still a challenge for the future fuel cell applications. The combination of Pt with Ni is an effective strategy to achieve active bimetallic nanocatalyst, and the particle size has been proved to play a crucial role in determining its final activity. However, the synthesis of PtNi bimetallic catalyst in the size of highly dispersed clusters has always been a challenge. In this report, PtNi/NiO clusters coated by small sized hollow silica (RPtNi/NiO@SiO2) were designed for efficient hydrogen generation from the hydrolysis of ammonia-borane. The newly designed catalysis system showed extremely high activity with the initial turnover frequency (TOF) value reaching 1240.3 mol H2 ∙ mol-1 Pt ∙ min-1, which makes it one the most active Pt based catalyst for this reaction. Detailed characterization by means of STEM, XPS and EDS element mapping etc. revealed that the excellent performance of RPtNi/NiO@SiO2 is derived from the highly dispersed PtNi/NiO clusters and the reduction of extra Pt4+ on the surface of PtNi/NiO clusters to Pt0 at relatively low temperature. KEYWORDS: Ammonia−borane, Hydrogen storage, Heterogeneous catalysis, Platinum-nickel cluster, Hollow silica. INTRODUCTION Hydrogen is regarded as one of the cleanest fuels due to which it has received a lot of attention from the energy and environmental research community.1-4 Despite all the advantages of using hydrogen as the ultimate energy carrier, the controlled storage and release of H2 is still a challenging task for the future “hydrogen economy”.5-7 In recent years, although different strategies and materials have been developed for the storage of hydrogen, ammonia−borane (NH3·BH3; AB) is still considered as an intriguing candidate due to its high gravimetric


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hydrogen density (19.6 weight %), low molecular weight (30.87g mol-1), and high stability under ambient conditions.8-10 More importantly, the hydrogen stored in AB can be controllably released by its hydrolysis in the presence of a suitable catalyst, with 3 moles of H2 can be generated from 1 mole of AB (eq 1).8, 11 NH3BH3 + 2H2O → NH4+ + BO2− + 3H2

(1)

In the early reports, Pt has been proved among the most active metals for AB dehydrogenation.1215

Chen et al. reported carbon nanotube (CNT) supported Pt particles for catalyzing hydrolytic

dehydrogenation of AB with TOF value reaching 567 mol H2 ∙ mol-1 Pt ∙ min-1, also the particle size of Pt was proved to play a crucial role in the reaction.13, 16 Xu et al. also reported ultrafine Pt nanoparticles immobilized inside the pores of a metal-organic framework (MIL-101) for liquid phase ammonia borane hydrolysis, with TOF value of 414 mol H2 ∙ mol-1 Pt ∙ min-1.17 Although excellent results have been achieved, the relatively low abundance of Pt has been very daunting to extend its practical applications.3, 18 To reduce the usage of expensive earth-scarce Pt and enhance its catalytic activity, one effective strategy is the combination of Pt with earth abundant transition metals.19 The synergistic effect between different metal atoms has been widely reported in most bimetallic nanocatalysts and even a small amount of doping can markedly improve the activity and selectivity of catalytic reactions.20-22 Typically, the doping of Pt with Ni has been proved among one of the most effective strategy for many important reactions.21, 23-26 But the facile synthesis of highly dispersed PtNi nanocatalyst in the cluster size has always been a challenging task.21 This background suggests that developing new catalysis systems to catalyze hydrogen generation from AB with high-efficiency is a highly exciting and interesting area of research.


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RESULTS AND DISCUSSION In the current study, a novel catalysis system based on small sized hollow silica coated PtNi/NiO cluster (R-PtNi/NiO@SiO2) is well designed. Different from the early reported wet chemical method in which PtNi particles were synthesized by coreduction of Pt and Ni anions21,

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PtNi/NiO clusters in this report are prepared by atomic sacrifice method. Besides, catalysts with hollow structure have also been reported to exhibit higher catalytic activity and stability than the solid structure28-30. Typically, the synthesis of R-PtNi/NiO@SiO2 can be separated into two steps. Firstly, synthesis of silica coated Ni/NiO clusters (Ni/NiO@SiO2) by a microemulsion method; secondly, the displacement between Ni0 atoms on the surface of Ni/NiO clusters with Pt4+ anions in the H2PtCl6 solution for the synthesis of PtNi/NiO@SiO2 (Pt4+ + 2Ni → Pt + 2Ni2+, this can be explained by considering the reduction potentials of the respective ions Eo ([PtCl6]2-/[PtCl4]2-) = 0.68 V, Eo ([PtCl4]2-/Pt) = 0.847 V) vs Eo (Ni2+/Ni) = -0.257 V)2 and further reduction of the extra Pt4+ to Pt0. Scheme 1 illustrates the detailed construction process of R-PtNi/NiO@SiO2. In an early report, we cooperated with Cao and Veser, the first ever hollow Ni@SiO2 nanoparticles31 were synthesized, but the detailed procedure was not described. Typically, the reverse microemulsion formed by Brij®58, cyclohexane and water solution of Ni(NO3)2 was adopted as soft template. After the addition of N2H4·H2O, Ni2+ can be completely reduced to Ni clusters through 2Ni2+ + N2H4 + 4OH− → 2Ni + N2 + 4H2O. Dahlberg et al. also reported the similar process for the preparation of silica nanotube and the hollow structure of silica was ascribed to the generated N2.32 But when the similar procedure was adopted for the preparation of Fe@SiO2 and Co@SiO2, no ideal results were achieved. The real reason for the generation of perfect hollow silica is still unclear. After the process of coating with silica, drying process and calcination, silica protected nickel oxide (NiO@SiO2) were generated (the content of Ni is 3.2 wt%


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based on the ICP result). The chemical states of Ni species in NiO@SiO2 were studied by XPS, the binding energy at 856.8 eV proved the NiO state after calcination, and no signal for Ni0 was detected. The binding energy at 862.7 eV can be assigned to the satellites formed under high temperature as NiSiO333 (Supporting Information, Figure S1, line a).

Scheme 1 Illustration of the formation process of R-PtNi/NiO@SiO2. As the existence of Ni0 is crucial for the following replacement reaction between Pt4+ and Ni0, we tried to reduce the NiO@SiO2 under H2 and suitable temperature by calcination. Figure S2 (Supporting Information) line a shows the H2 temperature-programmed reduction (TPR) measurement of as prepared NiO@SiO2, three peaks around 350 oC, 500 oC, 650 oC can be detected, and NiO@SiO2 can be completely reduced at 800 oC. The XPS analysis of samples after the reduction at 350 oC, 500 oC, and 650 oC for 2 h revealed the coexistence of Ni0 and Ni2+ in all samples (Figure S3, Supporting Information). The reason for the detection of Ni2+ in the sample after reduction even at 650 oC might come from oxidized Ni particles during the XPS analysis. The analysis of XPS peaks revealed an interesting phenomenon that the binding energy of Ni0 2p3/2 in samples prepared at the reduction temperature of 350

o

C (named as

Ni/NiO@SiO2-350) and 500 oC (named as Ni/NiO@SiO2-500) were similar (854.5 eV and 854.7 eV, respectively) and much higher than that of the sample prepared at the reduction temperature of 650 oC (named as Ni/NiO@SiO2-650, 853.2 eV). The reason for the different binding energies


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of Ni0 2p3/2 can be ascribed to the different sizes of Ni/NiO particles. The TEM images of Ni/NiO@SiO2 prepared at different reduction temperature are shown in Figure S4 (Supporting Information), serious sintering of Ni/NiO particles can be observed in Ni/NiO@SiO2-650 and the particle size of Ni/NiO was much bigger than that of Ni/NiO@SiO2-350 and Ni/NiO@SiO2-500. Meanwhile, due to the small size of Ni/NiO clusters, the XRD patterns (Figure S5, Supporting Information) of Ni/NiO@SiO2-350 and Ni/NiO@SiO2-500 did not show clear diffraction peaks for Ni or NiO, but Ni/NiO@SiO2-650 showed obvious peak around 44o corresponding to the {111} facet of fcc Ni and the broad peak around 25o in all samples was attributed to the amorphous silica34. As it is well known that the surface Ni atoms can be easily oxidized by the oxygen in air35, the increasing of Ni/NiO particles size reduces the percentage of surface Ni atoms and prevents more Ni0 atoms from the direct contact with the oxygen and NiO which results in the lower binding energy. Figure 1A and B show the TEM images of Ni/NiO@SiO2-350, perfect hollow structure of silica with the average size of 26.8 ± 2.1 nm and hole size of 11.9 nm ± 1.5 nm can be observed (size distributions are shown in Figure S6, Supporting Information). As can be seen that Ni/NiO clusters with the average size of 1.0 ± 0.2 nm enriched at the interface of silica shell and the inner hole (see Figure S7 for the HRTEM image and size distribution of Ni/NiO clusters, Supporting Information). Figure 1C-F show the HAADF-STEM image of Ni/NiO@SiO2-350 and the corresponding element mapping for Si, O, and Ni. The signal for element Ni can be clearly observed and well distributed in the whole sample region.


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Figure 1 (A), (B) TEM image and (C) HAADF-STEM image of Ni/NiO@SiO2 prepared under the reduction temperature of 350 oC, and the corresponding element mapping of Si (D), O (E), Ni (F). The introduction of Pt element to Ni/NiO@SiO2 was conducted by a facilely spontaneous displacement reaction (Scheme 1, step 2, check the Supporting Information for the detailed procedure) and the product was collected for the catalytic activity evaluation for the hydrolysis of AB (the product after this step was named as PtNi/NiO@SiO2). Prior to the catalytic studies,


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N2 sorption measurements were carried out to study the surface area and pore size distribution of Ni/NiO@SiO2 and PtNi/NiO@SiO2. The results are shown in Figure S8 and Table S1 (Supporting Information). The surface area of Ni/NiO@SiO2 and PtNi/NiO@SiO2 were determined to be 177 m2/g and 220 m2/g, respectively, which are higher than the solid nanoparticles as reported earlier34. According to the IUPAC classification the sorption isotherms are typical type IV and the clear increase at low pressure indicates the micro- or mesoporous feature. As no obvious porous structure can be clearly observed from the silica shell by the TEM, the mesoporous nature can be assigned to the inner hole of silica particles or interparticle voids36. Meanwhile, the micropore size was calculated mainly to be 0.6 nm and 1.0 nm which are much larger than the calculated molecular size of AB2. Figure 2 shows the time course for hydrogen production from hydrolysis of AB using PtNi/NiO@SiO2-350, PtNi/NiO@SiO2-500, and PtNi/NiO@SiO2-650 as catalyst. For comparison, the activity of Ni/NiO@SiO2 was firstly tested which was found to be inactive for the hydrogen generation. The TOF value of PtNi/NiO@SiO2350 (ICP content of Pt is 0.48 wt%) was 558.1 mol H2 ∙ mol-1 Pt ∙ min-1 with 2 min for the complete hydrolysis of AB which is dramatically higher than that of PtNi/NiO@SiO2-500 and PtNi/NiO@SiO2-650 (the TOF values were 239.3 mol H2 ∙ mol-1 Pt ∙ min-1 and 39.3 mol H2 ∙ mol-1 Pt ∙ min-1, respectively). By the XANES study, Wang et al. confirmed Ni oxidation state in the PtNi nanoparticles seems to be influential for the catalytic activity in AB decomposition. As the chemical states of Ni species were similar for all PtNi/NiO@SiO2 catalysts tested (Figure S3, Supporting Information), the main reason for the activity difference can be attributed to the size effect of PtNi/NiO particles as prepared. Figure S9 (Supporting Information) shows the TEM images of PtNi/NiO@SiO2-350, the metal clusters can be identified and the holes were maintained after the displacement reaction, but due to the etching of hot water, silica nanosheet


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was generated around the inner side of the hole37. The XRD pattern of PtNi/NiO@SiO2-350 is shown in Figure S10 (Supporting Information), no obvious peaks for Ni and Pt species can be detected which is consistent with its ultra small size. To evaluate the chemical states of Pt and the changing of chemical states of Ni species in PtNi/NiO@SiO2, the XPS spectrum of PtNi/NiO@SiO2-350 was conducted and the high-resolution spectrum of Ni species was acquired (Supporting Information, Figure S1, line c). The result indicates the coexistence of Ni0 and Ni2+ which is similar with the valence state distribution of Ni species in Ni/NiO@SiO2.

Figure 2 Time course for hydrogen production from AB using Ni/NiO@SiO2-350, PtNi/NiO@SiO2-350, PtNi/NiO@SiO2-500, and PtNi/NiO@SiO2-650. The chemical states of Pt in PtNi/NiO@SiO2-350 were also determined by XPS and the result is shown in Figure 3A, line a. Interestingly, the atomic ratio of Pt4+ (4f7/2, 73.0 eV) and Pt0 (4f7/2, 67.5 eV) was found to be nearly equal. In an early report, metallic Pt0 was proved to be more active for the dehydrogenation of AB than Pt2+ or Pt4+.13 In order to convert Pt4+ to Pt0, the PtNi/NiO@SiO2-350 as prepared was further reduced under H2 and 250 oC for 2 h (the product after this step was named as R-PtNi/NiO@SiO2-350). The XPS spectrum of Pt in RPtNi/NiO@SiO2-350 is shown in Figure 3A, line b. The result indicates that most Pt4+ has been


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converted to Pt0, and small amount of Pt2+ was also generated. The catalytic activity of RPtNi/NiO@SiO2-350 for hydrogen production from AB was tested, and the result is shown in Figure 3B. Compared with PtNi/NiO@SiO2-350 the activity of R-PtNi/NiO@SiO2-350 was much improved, with TOF value reaching 1217.6 mol H2 ∙ mol-1 Pt ∙ min-1, which makes it one of the most active Pt based catalyst ever reported for this reaction (the comparison of TOF values with the early reported Pt based catalysts is shown in Table S2, Supporting Information).

Figure 3 (A) The high-resolution XPS spectra of Pt in PtNi/NiO@SiO2 and R-PtNi/NiO@SiO2, (B) time course for hydrogen production from AB using PtNi/NiO@SiO2 and R-PtNi/NiO@SiO2. In order to study the influence of reduction temperature of PtNi/NiO@SiO2-350 on the catalytic performance of R-PtNi/NiO@SiO2-350, the H2-TPR measurement for PtNi/NiO@SiO2-350 was conducted and the result is shown in Figure S2 line b (Supporting Information). No obvious difference can be detected from the H2-TPR spectrum of PtNi/NiO@SiO2-350 with NiO@SiO2 due to the low loading of Pt. The detailed study was conducted by changing the reduction temperature of PtNi/NiO@SiO2 from 150 oC, 250 oC, 350 oC, to 450 oC, meanwhile, Ni/NiO@SiO2 which was used as the material for the synthesis of PtNi/NiO@SiO2, was also prepared under the reduction temperature of 250 oC, 350 oC, and 450 oC. The time course for hydrogen production from AB using this series of catalysts are shown in Figure 4A and the corresponding TOF values are shown in Figure 4B. One interesting result is that the reduction temperature of NiO@SiO2 for the preparation of Ni/NiO@SiO2 did not show much effect on the


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final activity of R-PtNi/NiO@SiO2, but the reduction temperature of PtNi/NiO@SiO2 significantly influenced the final activity of R-PtNi/NiO@SiO2. The best activity was achieved when the R-PtNi/NiO@SiO2 was prepared at the reduction temperature of 250 oC. When the reduction temperature of PtNi/NiO@SiO2 was raised to 450

o

C, the activity of R-

PtNi/NiO@SiO2 significantly decreased. The TEM images of R-PtNi/NiO@SiO2-350 prepared under the reduction temperature of 450 oC are shown in Figure S15 (Supporting Information), serious aggregation of PtNi/NiO clusters was observed which can be the reason for the dramatic decrease in the activity.

Figure 4 (A) Time courses for hydrogen production from AB using R-PtNi/NiO@SiO2 prepared under different conditions, (B) the corresponding TOF value calculated from (A).


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Figure 5A shows the TEM image of R-PtNi/NiO@SiO2-350 prepared at the reduction temperature of 250 oC. The TEM images do not show clear difference with PtNi/NiO@SiO2-350, which means the further reduction at 250 oC do not change the original morphology and dispersity of PtNi/NiO particles. Figure 5B-F show the HAADF-STEM image and the corresponding element mapping of R-PtNi/NiO@SiO2-350. The tiny bright dots in Figure 4B represent the generated PtNi/NiO clusters. The signal for element Ni and Pt can be clearly observed through the sample region in Figure 5E and F indicating the highly dispersed PtNi/NiO clusters. The EDS analysis indicated the loading of Pt is 0.41 wt% (Figure S14, Supporting Information), which is slightly lower than the ICP result (0.48 wt%), the reason might lie in the selected sample region for this characterization.

Figure 5 (A), (B) TEM image and HAADF-STEM image of R-PtNi/NiO@SiO2-350 prepared under the reduction temperature of 250 oC, (C) - (F) element mapping for O, Si, Ni, Pt corresponding to (B).


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To prove the formation of PtNi/NiO clusters, we tried to dissolve the as prepared RPtNi/NiO@SiO2-350 by hydrochloric acid of two different concentrations for different time periods. The proposed mechanism is shown in Figure S17 (Supporting Information). There are only two kind of catalysis systems that can be formed after the displacement reaction: one is the isolated Pt particles and Ni/NiO particles which are supported by silica (Figure S17A, Supporting Information); the other one is silica supported PtNi/NiO cluster with tight interaction between Pt and Ni atoms (Figure S17B, Supporting Information). The ICP results showed that the initial content of Ni and Pt in R-PtNi/NiO@SiO2-350 was 3.2 wt% and 0.48 wt%, respectively. After the etching of hydrochloric acid with the concentration of 0.1 M, for 5 h at 30 o

C, the content of Ni and Pt in R-PtNi/NiO@SiO2-350 was decreased to 2.7 wt% and 0.17 wt%

corresponding to 15.6% Ni and 64.6% Pt were dissolved. Meanwhile, by increasing the concentration of hydrochloric acid to 0.5 M, etching time for 12 h at 50 oC, the content of Ni and Pt in R-PtNi/NiO@SiO2-350 was significantly decreased to 1.4 wt% and 0.032 wt% corresponding to 56.3% Ni and 93.3% Pt were dissolved. As isolated Pt particles do not react with diluted hydrochloric acid, these etching results match the procedure which is given by Figure S17B (Supporting Information), hence, the tight interaction between Pt atoms and Ni atoms was proved. Besides the highly dispersed PtNi/NiO clusters, the synergistic effect between Pt and Ni atoms should be another important reason for the excellent performance of RPtNi/NiO@SiO2-350 for the hydrogen generation from the hydrolysis of ammonia borane. R-PtNi/NiO@SiO2-350 with the Pt content of 0.095 wt%, 0.18 wt%, 0.34 wt%, 0.48 wt% and 0.71 wt% can also be prepared by only changing the concentration of H2PtCl6 solution. The activities for the hydrogen production from AB were tested and the corresponding time dependent H2 evolution curves are shown in Figure S18A (Supporting Information). Different


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from the early report that the activity of catalyst show significant dependence on the loading of active component38, the TOF values of R-PtNi/NiO@SiO2-350 showed slight difference with the changing loading of Pt (Figure S18B, Supporting Information), the highest TOF value can reach 1240.3 mol H2 ∙ mol-1 Pt ∙ min-1 with the Pt loading of 0.34 wt%. The reason for this phenomenon can be attributed to the highly dispersed Pt0 atoms on the PtNi/NiO clusters which are generated by the random displacement between Pt4+ and Ni0. To study the reaction kinetics of AB hydrolysis catalyzed by R-PtNi/NiO@SiO2-350, the reactions were conducted in the presence of different concentrations of AB and Pt. By changing the initial amount of substrate, the influence of AB concentration on the reaction rate was evaluated. Figure S19A (Supporting Information) shows the relation between generated hydrogen volume versus time. The volume of generated hydrogen increased accordingly with the increasing of AB concentrations, as the reaction rate can be determined from the linear portion of each plot and a zero order reaction can be observed for the hydrolysis of AB (Figure S19B, Supporting Information) indicating the reaction rate is independent with the concentration of AB. Figure S19C (Supporting Information) shows the time courses for hydrogen production from AB using different Pt concentrations. It can be seen that the reaction rate increased with the increasing concentration of Pt and the hydrolysis of AB is first order with respect to the Pt concentration (Figure S19D, Supporting Information). The temperature dependent activity of R-PtNi/NiO@SiO2-350 (Pt loading of 0.48 wt%) was also studied. The results are shown in Figure S20 and S21 (Supporting Information), the TOF value was nearly doubled as the temperature increased by every 10 K. An Arrhenius plot shows that the activation energy for the catalytic hydrolysis of AB is 43 kJ mol -1 which is similar with the recently reported highly active Pt based catalysts.14, 16 Generally, the durability of catalyst plays


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an essential role for its practical applications. Thus, the stability of R-PtNi/NiO@SiO2-350 was tested by recycling the catalyst after each hydrolysis reaction (check Supporting Information for the detailed procedure). The results indicated that the catalyst had good stability, even after the fifth cycle, 100% of AB was decomposed in 5 min (Figure S22, Supporting Information). However, the hydrogen-generating rate decreased gradually, the deactivation of catalyst can be attributed to the aggregation of PtNi/NiO clusters during the recycling which can be clearly observed from the TEM images of R-PtNi/NiO@SiO2-350 obtained after the recycling (Figure S23, Supporting Information). Another important reason for the deactivation of catalyst might be the poisoning of PtNi/NiO clusters by the increasing amount of generated BO2− during the recycling19. The XPS analysis of catalyst after the recycling was tested to prove the interaction between PtNi/NiO clusters and BO2− which is shown in Figure S24 (Supporting Information). The XPS content of element B was tested as 3.5 wt% with (B 1s) binding energy of 193.6 eV corresponding to the B3+ in BO2−. Due to the binding of BO2− with the surface atoms of PtNi/NiO cluster, the binding energy of Pt0 4f7/2 increased from the initial 67.5 eV to 68.8 eV after the recycling, the similar changing of binding energy for element Ni was also observed (from the initial 854.5 eV to 856.9 eV after the recycling). CONCLUSIONS In summary, we report a newly designed strategy to construct highly active small sized hollow silica coated ultra small PtNi/NiO clusters for hydrogen generation from the hydrolysis of ammonia−borane (AB). By adjusting the valence states of Pt element on the surface of PtNi/NiO clusters and maintaining its high dispersity, the highest TOF value can reach 1240.3 mol H2 ∙ mol-1 Pt ∙ min-1 which represents one of the highest TOF value for dehydrogenation of AB ever reported for Pt based catalysts. We hope our findings may provide a new insight into the


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synthetic chemistry of supported bimetallic clusters and offer a reference for the designing of highly active catalyst for the hydrogen production from chemical hydrogen storage materials. ASSOCIATED CONTENT Supporting Information Experimental details, TEM, XRD, XPS, H2-TPR, nitrogen adsorption−desorption isotherms, pore size distributions of Ni/NiO@SiO2 and PtNi/NiO@SiO2; complementary EDS, HAADFSTEM image and element mapping of R-PtNi/NiO@SiO2; the kinetics study, activation energy calculation and durability test of R-PtNi/NiO@SiO2 for hydrogen generation from the hydrolysis of ammonia borane. AUTHOR INFORMATION Corresponding Author * Rongwen Lu, E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgement This work is supported by the Joint Funds of the National Natural Science Foundation of China (U1608223), National Natural Science Foundation of China (21576044, 21536002), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21421005), Dalian University of Technology Innovation Team (DUT2016TB12).

Reference 1.

Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for Mobile Applications. Nature

2001, 414 (6861), 353-358.


16

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2.

Mori, K.; Miyawaki, K.; Yamashita, H. Ru and Ru–Ni Nanoparticles on TiO2 Support as

Extremely Active Catalysts for Hydrogen Production from Ammonia–Borane. ACS Catal. 2016, 6 (5), 3128-3135. 3.

Zhan, W.-W.; Zhu, Q.-L.; Xu, Q. Dehydrogenation of Ammonia Borane by Metal

Nanoparticle Catalysts. ACS Catal. 2016, 6 (10), 6892-6905. 4.

Wen, M.; Cui, Y.; Kuwahara, Y.; Mori, K.; Yamashita, H. Non-Noble-Metal Nanoparticle

Supported on Metal-Organic Framework as an Efficient and Durable Catalyst for Promoting H2 Production from Ammonia Borane under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2016, 8 (33), 21278-21284. 5.

Jiao, W.; Hu, X.; Ren, H.; Xu, P.; Yu, R.; Chen, J.; Xing, X. Magnetic Ni and Ni/Pt

Hollow Nanospheres and Their Catalytic Activities for Hydrolysis of Ammonia Borane. J. Mater. Chem. A 2014, 2 (43), 18171-18176. 6.

Guo, L.; Gu, X.; Kang, K.; Wu, Y.; Cheng, J.; Liu, P.; Wang, T.; Su, H. Porous Nitrogen-

Doped Carbon-Immobilized Bimetallic Nanoparticles as Highly Efficient Catalysts for Hydrogen Generation from Hydrolysis of Ammonia Borane. J. Mater. Chem. A 2015, 3 (45), 22807-22815. 7.

Hu, J.; Chen, Z.; Li, M.; Zhou, X.; Lu, H. Amine-Capped Co Nanoparticles for Highly

Efficient Dehydrogenation of Ammonia Borane. ACS Appl. Mater. Interfaces 2014, 6 (15), 13191-13200. 8.

Yao, Q.; Lu, Z.-H.; Huang, W.; Chen, X.; Zhu, J. High Pt-Like Activity of the Ni–

Mo/Graphene Catalyst for Hydrogen Evolution from Hydrolysis of Ammonia Borane. J. Mater. Chem. A 2016, 4 (22), 8579-8583. 9.

Yang, L.; Luo, W.; Cheng, G. Graphene-Supported Ag-Based Core-Shell Nanoparticles

for Hydrogen Generation in Hydrolysis of Ammonia Borane and Methylamine Borane. ACS


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

Appl. Mater. Interfaces 2013, 5 (16), 8231-8240. 10.

Kaya, M.; Zahmakiran, M.; Özkar, S.; Volkan, M. Copper(0) Nanoparticles Supported on

Silica-Coated Cobalt Ferrite Magnetic Particles: Cost Effective Catalyst in the Hydrolysis of Ammonia-Borane with an Exceptional Reusability Performance. ACS Appl. Mater. Interfaces 2012, 4 (8), 3866-3873. 11.

Li, F.; Jiang, X.; Zhao, J.; Zhang, S. Graphene Oxide: A Promising Nanomaterial for

Energy and Environmental Applications. Nano Energy 2015, 16, 488-515. 12.

Chandra, M.; Xu, Q. Room Temperature Hydrogen Generation from Aqueous Ammonia-

Borane Using Noble Metal Nano-Clusters as Highly Active Catalysts. J. Power Sources 2007, 168 (1), 135-142. 13.

Chen, W.; Ji, J.; Duan, X.; Qian, G.; Li, P.; Zhou, X.; Chen, D.; Yuan, W. Unique

Reactivity in Pt/CNT Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane. Chem. Commun. 2014, 50 (17), 2142-2144. 14.

Hu, Y.; Wang, Y.; Lu, Z.-H.; Chen, X.; Xiong, L. Core–Shell Nanospheres Pt@SiO2 for

Catalytic Hydrogen Production. Appl. Surf. Sci. 2015, 341, 185-189. 15.

Ge, Y.; Shah, Z. H.; Lin, X.-J.; Lu, R.; Liao, Z.; Zhang, S. Highly Efficient Pt Decorated

CoCu Bimetallic Nanoparticles Protected in Silica for Hydrogen Production from Ammonia– Borane. ACS Sustainable Chem. Eng. 2016. (10.1021/acssuschemeng.6b02430) 16.

Chen, W.; Ji, J.; Feng, X.; Duan, X.; Qian, G.; Li, P.; Zhou, X.; Chen, D.; Yuan, W.

Mechanistic Insight into Size-Dependent Activity and Durability in Pt/CNT Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane. J. Am. Chem. Soc. 2014, 136 (48), 1673616739. 17.

Aijaz, A.; Karkamkar, A.; Choi, Y. J.; Tsumori, N.; Ronnebro, E.; Autrey, T.; Shioyama,


18

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H.; Xu, Q. Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of MetalOrganic Framework: A Double Solvents Approach. J. Am. Chem. Soc. 2012, 134 (34), 1392613929. 18.

Khalily, M. A.; Eren, H.; Akbayrak, S.; Susapto, H. H.; Biyikli, N.; Ozkar, S.; Guler, M.

O. Facile Synthesis of Three-Dimensional Pt-TiO2 Nano-Networks: A Highly Active Catalyst for the Hydrolytic Dehydrogenation of Ammonia-Borane. Angew. Chem. Int. Ed. 2016, 55 (40), 12257-12261. 19.

Kang, J.-X.; Chen, T.-W.; Zhang, D.-F.; Guo, L. PtNiAu Trimetallic Nanoalloys Enabled

by A Digestive-Assisted Process as Highly Efficient Catalyst for Hydrogen Generation. Nano Energy 2016, 23, 145-152. 20.

Rej, S.; Hsia, C. F.; Chen, T. Y.; Lin, F. C.; Huang, J. S.; Huang, M. H. Facet-Dependent

and Light-Assisted Efficient Hydrogen Evolution from Ammonia Borane Using Gold-Palladium Core-Shell Nanocatalysts. Angew. Chem. Int. Ed. 2016, 55 (25), 7222-7226. 21.

Zhu, Q.-L.; Zhong, D.-C.; Demirci, U. B.; Xu, Q. Controlled Synthesis of Ultrafine

Surfactant-Free NiPt Nanocatalysts toward Efficient and Complete Hydrogen Generation from Hydrazine Borane at Room Temperature. ACS Catal. 2014, 4 (12), 4261-4268. 22.

Yen, H.; Seo, Y.; Kaliaguine, S.; Kleitz, F. Role of Metal–Support Interactions, Particle

size, and Metal–Metal Synergy in CuNi Nanocatalysts for H2 Generation. ACS Catal. 2015, 5 (9), 5505-5511. 23.

Wang, S.; Zhang, D.; Ma, Y.; Zhang, H.; Gao, J.; Nie, Y.; Sun, X. Aqueous Solution

Synthesis of Pt-M (M = Fe, Co, Ni) Bimetallic Nanoparticles and Their Catalysis for the Hydrolytic Dehydrogenation of Ammonia Borane. ACS Appl. Mater. Interfaces 2014, 6 (15), 12429-12435.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

24.

Page 20 of 22

Cao, N.; Yang, L.; Dai, H.; Liu, T.; Su, J.; Wu, X.; Luo, W.; Cheng, G. Immobilization of

Ultrafine Bimetallic Ni-Pt Nanoparticles Inside the Pores of Metal-Organic Frameworks as Efficient Catalysts for Dehydrogenation of Alkaline Solution of Hydrazine. Inorg. Chem. 2014, 53 (19), 10122-10128. 25.

Dubau, L.; Asset, T.; Chattot, R.; Bonnaud, C.; Vanpeene, V.; Nelayah, J.; Maillard, F.

Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction. ACS Catal. 2015, 5 (9), 5333-5341. 26.

Zhang, Z.; Lu, Z.-H.; Chen, X. Ultrafine Ni–Pt Alloy Nanoparticles Grown on Graphene

as Highly Efficient Catalyst for Complete Hydrogen Generation from Hydrazine Borane. ACS Sustainable Chem. Eng. 2015, 3 (6), 1255-1261. 27.

Song, F.-Z.; Zhu, Q.-L.; Xu, Q. Monodispersed PtNi Nanoparticles Deposited on

Diamine-Alkalized Graphene for Highly Efficient Dehydrogenation of Hydrous Hydrazine at Room Temperature. J. Mater. Chem. A 2015, 3 (46), 23090-23094. 28.

Zhang, W.; Lin, X. J.; Sun, Y. G.; Bin, D. S.; Cao, A. M.; Wan, L. J. Controlled Formation

of Metal@Al2O3 Yolk-Shell Nanostructures with Improved Thermal Stability. ACS Appl. Mater. Interfaces 2015, 7 (49), 27031-27034. 29.

Zhang, W.; Yang, L. P.; Wu, Z. X.; Piao, J. Y.; Cao, A. M.; Wan, L. J. Controlled

Formation of Uniform CeO2 Nanoshells in A Buffer Solution. Chem. Commun. 2016, 52 (7), 1420-1423. 30.

Wang, J.; Liu, H. The Effect of the Support Structure on Catalytic Activity: A Case Study

on Hollow and Solid MoO3/SiO2. RSC Adv. 2016, 6 (3), 2374-2378. 31.

Cao, A.; Lu, R.; Veser, G. Stabilizing Metal Nanoparticles for Heterogeneous Catalysis.

Phys. Chem. Chem. Phys. 2010, 12 (41), 13499-13510.


20

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32.

Dahlberg, K. A.; Schwank, J. W. Synthesis of Ni@SiO2 Nanotube Particles in A Water-in-

Oil Microemulsion Template. Chem. Mater. 2012, 24 (14), 2635-2644. 33.

Ahmadi, M.; Cui, C.; Mistry, H.; Strasser, P.; Cuenya, B. R. Carbon Monoxide-Induced

Stability and Atomic Segregation Phenomena in Shape-Selected Octahedral PtNi Nanoparticles. ACS Nano 2015, 9 (11), 10686-10694. 34.

Ge, Y.; Shah, Z. H.; Wang, C.; Wang, J.; Mao, W.; Zhang, S.; Lu, R. In Situ

Encapsulation of Ultrasmall CuO Quantum Dots with Controlled Band-Gap and Reversible Thermochromism. ACS Appl. Mater. Interfaces 2015, 7 (48), 26437-26444. 35.

Ge, Y.; Gao, T.; Wang, C.; Shah, Z. H.; Lu, R.; Zhang, S. Highly Efficient Silica Coated

CuNi Bimetallic Nanocatalyst from Reverse Microemulsion. J. Colloid Interface Sci. 2017, 491, 123-132. 36.

Almana, N.; Phivilay, S. P.; Laveille, P.; Hedhili, M. N.; Fornasiero, P.; Takanabe, K.;

Basset, J.-M. Design of A Core–Shell Pt–SiO2 Catalyst in A Reverse Microemulsion System: Distinctive Kinetics on CO Oxidation at Low Temperature. J. Catal. 2016, 340, 368-375. 37.

Lin, C. H.; Chang, J. H.; Yeh, Y. Q.; Wu, S. H.; Liu, Y. H.; Mou, C. Y. Formation of

Hollow Silica Nanospheres by Reverse Microemulsion. Nanoscale 2015, 7 (21), 9614-9626. 38.

Akbayrak, S.; Tonbul, Y.; Özkar, S. Ceria Supported Rhodium Nanoparticles: Superb

Catalytic Activity in Hydrogen Generation from the Hydrolysis of Ammonia Borane. Appl. Catal., B 2016, 198, 162-170.


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NiO Clusters Coated by Hollow Sillica: Novel Design for Highly

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