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Atomically dispersed Pt on the surface of Ni particle – synthesis and catalytic function in hydrogen generation from aqueous ammonia borane Zhao Li, Teng He, Daiju Matsumura, Shu Miao, Anan Wu, Lin Liu, Guotao Wu, and Ping Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01790 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017
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Atomically dispersed Pt on the surface of Ni particle – synthesis and catalytic function in hydrogen generation from aqueous ammonia borane ┴
Zhao Li†‡, Teng He*†, Daiju Matsumura§, Shu Miao†, Anan Wu* , Lin Liu†, Guotao Wu†, Ping Chen†# †Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China ‡University of Chinese Academy of Sciences, Beijing 100049, China §Materials Sciences Research Center, Japan Atomic Energy Agency 1-1-1 Koto, Sayo, Hyogo 679-5148, Japan ┴
Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China #Collaborative Innovation Center of Chemistry for Energy Materials (iChEM·2011)
ABSTRACT: The development of cost-effective and highly efficient catalyst is of scientific importance and practical need in the conversion and utilization of clean energies. One of the strategies fulfilling that demand is to achieve high exposure of catalytically functional noble metal to reactants to maximize its utilization efficiency. We report herein that the single atom alloy (SAA) made of atomically dispersed Pt the surface of Ni particle (Pt is surrounded by Ni atoms,) exhibits improved catalytic activity on the hydrolytic dehydrogenation of ammonia borane, a promising hydrogen storage method for onboard application. Specifically, an addition of 160 ppm of Pt leads to ca. 3-fold activity to that of pristine Ni/CNT catalyst. The turnover frequency based on the isolated Pt is 12000 molH2·molPt-1·min-1, which is about 21 times of the best Pt-based catalyst ever reported. Our simulation results indicate that the high activity achieved stems from the synergistic effect between Pt and Ni, where the negatively charged Pt (Ptδ-) and positively charged Ni (Niδ+) in the Pt/Ni alloy are prone to interact with H and OH of H2O molecule, respectively, leading to an energetic favourable reaction pathway.
KEYWORD: Single atom alloy, Pt-Ni alloy, catalytic dehydrogenation, hydrogen storage, ammonia borane
INTRODUCTION The environmental deterioration and depletion of fossil fuel resources have brought about an imperious need to shift to a cleaner and more sustainable energy system. Hydrogen has been regarded as an ideal energy carrier because of its abundance, high energy density and cleanliness.1, 2 However, the large-scale utilization of hydrogen is hindered by the lack of safe and efficient hydrogen storage methods.3, 4 Intensive and extensive research activities over the past two decades have been thus devoted to the development of hydrides of light elements (HLEs) and physisorbents for hydrogen storage.4-7 Liquid-state HLEs with high hydrogen capacity, moderate dehydrogenation temperature and more importantly, the compatibility with existing gasoline infrastructure, are of particular interests as hydrogen carriers for both onboard application and large scale long-distance hydrogen transportation.8-10 Catalysing hydrogen generation from those liquid hydrides by traditional catalysts remains less effective and/or selective11-14 and thus, is an arena where recent advancements in catalysis can exert strong influence. Hydrolytic dehydrogenation of ammonia borane (AB), an
attractive liquid hydride system (1) for hydrogen storage first investigated by Xu et al.,15-17 may also serve as a touchstone for new dehydrogenation catalysts. NH3BH3 + 2H2O = BO2- + NH4+ + 3H2 (1) Usually, the Pt18-20, Ru21 and Pd22 based catalysts outperform non-noble metals (such as Ni23, 24, Co25 and Fe26) in the hydrogen production from the above system. Through making bimetallic catalysts , such as Co-Pd27 or core-shell such as Au@Co catalysts28, enhanced catalytic activities in comparison with the monometallic counterparts can be achieved. Unfortunately, those noble metals are either buried substantially in the bulk phase of alloys or coated as the core in core-shell structures. It is, therefore, highly demanded to maximize the utilization efficiency of functional noble metals to be cost effective. The ideal scenario for this end is to atomically disperse noble metal catalyst on top of supports or non-noble metals, which can be nicely referenced to the exciting forum of single atom catalyst (SAC)29, 30. The presence of isolated atom in SAC has been regarded as the key favouring highly effective and/or selective site in CO oxidation31-33, hydrogenation34-36, water–gas shift
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reaction37-40, oxidations of methane41, formaldehyde42 or alcohols43, hydroformylation44, 45, reduction of nitric oxide46 and electrocatalytic reactions47, 48 etc. There are many interesting features involved in the SAC, among which the chemical and steric nature of the isolated functional atom has been actively investigated. Zhang and co-workers reported that single Pt forms coordinative bond with FeOx support, resulting in partially positive charged electronic state that favours CO oxidation.29 In a “Crown-Jewel” Au/Pd nanostructure, single Au atom controllably assemble at the top position of Pd clusters, on the other hand, receives electron from Pd and becomes negatively charged, which was found to exhibit excellent catalytic activity for aerobic glucose oxidation.49, 50 The lack of conjugated active site in SAC also creates a unique reacting environment in the activation of reactants. Therefore, the highly selective hydrogenation of nitroarenes to anilines on Pt1/FeOx catalyst36 and depression of coking on Pd1/C3N451 catalyst in hydrogenation of 1-hexyne to 1-hexene are obtained. Flytzani-Stephanopoulos and Sykes et al. reported bi-function Pd/Cu and Pt/Cu single atom alloy (SAA) for selective hydrogenations, where the isolated Pd and Pt atoms facilitate the activation of molecular H2, and Cu is responsible for hydrogenation, rendering highly selective hydrogenation of styrene to ethylbenzene and of 1,3-butadiene to 1-butene, respectively.52, 53 Therefore, both the electronic structure and the synergy between the isolated atom and its neighbour in catalysts are certainly forefront topics worthy of in-depth investigation. With all these intriguing scientific findings and understanding in SAC in the recent few years, it is about the right time to bridge such an advancement in heterogeneous catalysis with hydrogen storage research. Herein, we assembled atomically dispersed Pt atom on the surface of Ni particle (Pt is surrounded by Ni atoms), showing an extraordinary activity in catalysing hydrogen evolution from aqueous AB. Ca. 3-fold activity to that of neat Ni catalyst was achieved by adding only 160 ppm Pt to neat Ni catalyst. The single atomically dispersed Pt atom surrounded by Ni atoms was confirmed by X-ray absorption fine structure (XAFS) and electronic microscope. Of equivalent importance is the identification of synergistic effect between the isolated Pt and surrounding Ni in the activation of H2O by both experiments and theoretical simulations, which substantially reduces the kinetic barrier of the rate determining step and enables abnormally high turnover frequency (TOF) that is 21 times of the best Pt catalyst ever reported for this reaction.
RESULTS AND DISCUSSION Synthesis catalysts
and
characterization
of
Pt/Ni
SAA
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5% Ni-based catalysts on different supports (carbon nanotubes (CNT), covalent triazine framework (CTF) and activated carbon (AC)) were synthesized as parent catalysts. Different amounts of Pt in the initial Pt/Ni molar ratios from 1/2 to 1/1000 were introduced to the parent catalysts by using the galvanic reduction method, a facile and efficient approach to fabricate the low cost and high efficiency bimetallic catalysts.54 Specifically, a water solution of H2PtCl6 is added slowly to the pre-prepared 5% Ni-based catalysts under Ar flow to produce the bimetallic catalyst (See the Experiment Section). Since the reduction-replacement process starts from the outer surface, the deposited Pt will most likely stay at the Ni surface. The prepared catalysts are marked as nPt+Ni/support, where n stands for the initial molar ratio of Pt to Ni. Because Pt will partially substitute Ni in the parent catalyst, the Pt to Ni ratio in the final catalysts will be higher. The crystal structures of the Pt-modified Ni catalysts were characterized by X-ray diffraction (XRD) as shown in Figures S1-S3. Taking the nPt+Ni/CTF series as an example (Figure S1, Supporting information), the peaks at 7.8° and 27.2° are belonged to the support CTF. Furthermore, the Ni(111) and Ni(200) diffraction peaks at 46.5° and 51.8° with weak intensity can be found in 5% Ni/CTF and in the lower Pt loading samples (from 1/10 to 1/1000). Those diffraction peaks are getting weakened with the increase of Pt loading. In the higher Pt ratio catalyst such as 1/2Pt+Ni/CTF, only diffraction peaks assigned to Pt are detected (Figure S1, Supporting information), which hints the majority of Ni has been replaced by Pt. To maximize the utilization of Pt and to explore the Pt-Ni synergistic effect, catalysts with lower Pt loadings (1/100Pt, 1/500Pt and 1/1000Pt) were chosen for further investigation. Transmission electron microscopy (TEM) was employed to characterize the morphology of the parent Ni catalysts as shown in Figures S4-S5. It can be seen that the Ni particles are well dispersed on the supports with the average particle size of 6.5 nm and 4.8 nm for 5% Ni/CTF and 5% Ni/CNT, respectively. To further detect the morphology of Pt in the low Pt content catalysts, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique was used. As shown in Figure 1, individual heavier Pt atoms (marked by the red circles) can be discerned on the Ni crystal surface in the 1/500Pt+Ni/CNT and 1/1000Pt+Ni/CTF catalysts. Those isolated Pt atoms surrounded by Ni atoms were repeatedly observed in different regions of the samples, and Pt clusters or Pt nanoparticles were not evidenced (Figures S6-S7), showing the effectiveness of the galvanic reduction method49. The interplane spacing of the particle lattice is 0.204 nm (Figure S8) in good consistence with the Ni {111} lattice spacing.
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Figure 1. HAADF-STEM images of (a) 1/500Pt+Ni/CNT, and (b) 1/1000Pt+Ni/CTF. The atomically dispersed Pt atoms (red cycles) are clearly observed on the Ni surface.
XAFS spectroscopy was further employed to confirm the atomically dispersed Pt and to identify the coordination environment of Pt. Since the Pt content in the 1/1000Pt-based catalysts is too low to get a spectrum with acceptable signal to noise ratio, higher content of Pt-modified Ni catalysts were chosen for XAFS measurements. Figure 2a shows the normalized X-ray absorption near edge structure (XANES) spectra of Pt in 1/20Pt+Ni/CNT, 1/100Pt+Ni/CNT and 1/500Pt+Ni/CNT catalysts. All these three spectra exhibit similar profiles, indicating the similar chemical state of Pt in Ni catalysts. Their white-line intensities, an indication of the oxidation state of Pt species, resemble to that of Pt foil. However, a close inspection of these spectra can tell that the white-line intensities of Pt in the Pt/Ni catalysts are slight lower than that of Pt foil, indicating that Pt in the Pt/Ni catalysts is partially negatively charged (Figure S9, Supporting information). Moreover, their spectral oscillations are quite different from that of Pt foil, reflecting that the Pt in the catalyst has a different coordination environment. As expected, the Fourier transformed spectra from extended XAFS (EXAFS) of samples (Figure 2b and Figure S10) exhibit prominent peak at ~2.5-2.6 Å due to the Pt-Ni coordination and small shoulder peak at ~2.9 Å from the Pt-Pt coordination. Importantly, the shoulder peak at ~2.9 Å weakens with the decrease in the Pt loading, suggesting the tendency of formation of isolated Pt atoms. The fitted coordination numbers (N) as shown in Table 1 also confirm the decrease of Pt-Pt coordination number from 4.3 in 1/20Pt+Ni/CNT to 3.5 in 1/100Pt+Ni/CNT and further to 1.3 in 1/500Pt+Ni/CNT. The small coordination number of 1.3 in 1/500Pt+Ni/CNT evidences that predominant Pt atoms are dispersed as isolated atoms surrounded by Ni atoms, agreeing well with the HAADF-STEM observation of atomically dispersed Pt atoms in 1/500Pt+Ni/CNT. The overall reduction of Pt coordination number from 12 (Pt foil) to ca. 7 (1.3 for Pt-Pt and 5.9 for Pt-Ni in 1/500Pt+Ni/CNT) may serve as an indication that most of Pt atoms are at the surface. Furthermore, the weak second shell coordination of Pt reflects the absence of ordered Pt-Ni alloy configuration. Based on the HAADF-STEM and XAFS characterizations, we believe that Pt in the lower loading 1/1000Pt+Ni catalysts will retain essentially the atomically dispersed morphology. Similar phenomena (XANES and EXAFS) were also observed on the CTF supported catalysts (Figure S11-S13 and Table S1, Supporting information), i.e., similar
white-line of Pt in Pt/Ni alloy catalysts to Pt foil and similar tendency of formation of Pt/Ni SAA as decreasing the loading of Pt.
Figure 2. (a) Normalized XANES spectra at Pt L3-edge of different amount of Pt modified 5% Ni/CNT samples. (b) The k3-weight Fourier transformed spectra from EXAFS. Phase shifts were not corrected. Δk = 3-10 (Å-1) was used for simulation.
Table 1. EXAFS data fitting results of Pt modified Ni catalysts with different Pt content. N, the coordination number; R, the average distance; σ2, the Debye-Waller factor; the data range used for data fitting in k-space (Δk) and R-space are 3.0-10 Å and 1.6-3.4 Å, respectively. *, fixed parameters. samples
Shell
N
R(Å)
σ2*102 (Å2)
r-fact or (%)
Pt foil
Pt-Pt
12.0(7)
2.767(4)
0.42(3)
0.07
Pt-Pt
4.3(4)
2.677(8)
0.6*
Pt-Ni
5.5(5)
2.573(4)
0.86(9)
Pt-Pt
3.5(6)
2.662(15 )
0.6*
Pt-Ni
5.9(9)
2.565(6)
0.89(14)
Pt-Pt
1.3(8)
2.650(50 )
0.6*
2.556(8)
0.88(17)
1/20Pt+Ni/ CNT
1/100Pt+Ni /CNT
1/500Pt+Ni /CNT
0.06
Pt-Ni
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0.16
0.36
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electron transfer between these two metals 49, 56, which agrees well with the XAFS results and Bader charge calculations. Metallic Ni in the 1/50Pt+Ni/CTF catalyst doesn’t show noticeable chemical shift because of its predominate content in the sample. The existence of Ni2+ species is probably caused by air oxidation during sample loading to the XPS chamber.
Catalytic hydrolysis of AB
Figure 3. (a) Calculated atomic Bader charges of Pt (yellow ball) and nearby Ni (green ball). (b) Pt 4f7/2, Pt 4f5/2 and (c) Ni 2p3/2, Ni 2p1/2 XPS spectra of 1/50Pt+Ni/CTF catalyst compared with pristine 5% Ni/CTF and 5% Pt/CTF. All catalysts were pre-treated with Ar+ sputtering.
Pt in bulk phase usually has a coordination number of 12.29 In the 1/500Pt+Ni sample, however, its coordination number is largely reduced to ca. 7. With further reduction in Pt content, i.e., as in the 1/1000Pt+Ni samples where single atom alloy is highly likely to form, Pt should predominantly be coordinated with less Ni atoms. We thus calculated the Bader charges borne by Pt and adjacent Ni in a Ni (111) surface. As shown in Figure 3a, Pt is obviously negatively charged while Ni is slightly positively charged showing the electron transfer from Ni to Pt, which is derived from the difference in their electron negativities and in consistent with the change in the white-line intensity observed in XANES as shown in Figure S9(Supporting information). Similar phenomenon was also observed in the “Crown-Jewel” Au/Pd or Au/(Pd-Ir) SAA.49, 50 X-ray photoelectron spectroscopy (XPS) was also employed to characterize the chemical state of Pt in the alloy samples. Because the noise-to-signal is very poor for the lower Pt content samples, XPS measurement was done on the 1/50Pt+Ni/CTF sample. As shown in Figure 3b, Pt 4f in 5% Pt/CTF sample has binding energies of 71.5 eV (4f7/2) and 74.7 eV (4f5/2), evidencing that Pt is in metallic state.55 Similar XPS peaks can also be detected in the 1/50Pt+Ni/CTF catalyst. However, ca. 0.6 eV downshift in binding energy is apparently observed. Such a shift may arise from alloy formation, accompanying with the
The electron-rich Pt surrounded with electron-poor Ni in those alloy samples was employed in the hydrogen generation of aqueous AB, one of the promising chemical hydrogen storage methods. As shown in Figure 4, the parent catalysts, i.e., 5% Ni/CNT and 5% Ni/CTF (referred as 0Pt+Ni in the figures) show moderate activities for the hydrolysis with the TOFs of 5.8 and 7.0 molH2·molNi-1·min-1, respectively. The difference in the activity may be ascribed to the dispersion and electronic effect of the different support.25 With the modification of trace amount of Pt, however, their catalytic activities jump markedly. Take the Pt modified Ni/CNT catalysts as examples (Figure 4a), the addition of 1/1000 Pt to 1/1000Pt+Ni/CNT, where the Pt loading is as low as 160 ppm, could produce ca. 3-fold activity increase and a higher TOF of 17.8 molH2·molNi-1·min-1 (Figure 4c). By increasing the Pt loading to 1/100, a TOF of 33.3 molH2·molNi-1·min-1 that is more than 5 times of neat Ni catalyst can be achieved. Similar activity increases were observed on the Pt-modified Ni/CTF and Ni/AC catalysts (Figure 4b and Figure S14). It is worth mentioning that a TOF of 57.1 molH2·molNi-1·min-1 can be obtained for the 1/100Pt+Ni/AC, which is about 9-fold to that of pristine 5% Ni/AC catalyst and is the highest activity among the Ni-based catalysts ever reported23. We also prepared a 1/1000Pt+Ni catalyst by co-impregnation method (referred as co-IMP in Figure 4b), where Pt may deposit randomly with Ni on the support. The co-IMP exhibits a moderate dehydrogenation rate that is similar to that of pristine Ni, illustrating the importance of atomically dispersed Pt atoms on surface in catalysing the reaction. To highlight the Pt contribution in the catalysis, we further prepared 0.16% Pt/CNT and 0.16% Pt/CTF catalysts (referred as 100Pt+0Ni/Support in the figures), where the Pt loading is close to that in 1/100Pt+Ni catalysts. The 0.16% Pt/CTF exhibits negligible activity, which may be due to the diffusion of Pt into the micropores of the CTF.57 The 0.16% Pt/CNT catalyst has an TOF as high as 448 molH2·molPt-1·min-1 which is close to the highest activity of Pt/CNT-HT catalyst reported in the literature20. We then re-calculated the TOFs of the Pt/Ni SAA catalysts by assuming that Pt is the active centre through equation (2), where the nH2, npt and t stand for the amount of hydrogen generated, the amount of catalyst and reaction time, respectively, noting that Ni’s contribution to the reaction rate was removed in the equation.
TOF = (nH (Pt + Ni)-nH (Ni))/(t × n Pt ) 2 2 (2) Unprecedented high specific activities as shown in Figure 4d, i.e., 12000, and 10900 molH2·molPt-1·min-1 for 1/1000Pt+Ni/CNT and 1/1000Pt+Ni/CTF, respectively, can be achieved. The TOF for 1/1000Pt+Ni/CNT is about
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27-fold to that of 0.16% Pt/CNT and about 428-fold to that of 5% Ni/CNT catalyst. Furthermore, the TOF of 1/1000Pt+Ni/CNT catalyst is ca. 21 times of the best Pt-based catalyst ever reported (Pt/CNT-HT).20 Even though a TOF of ca. 652 molH2·molPt-1·min-1 based on the surface Pt can be estimated from its particle size, 1/1000Pt+Ni/CNT catalyst still shows a ca. 18-fold TOF to that of Pt/CNT-HT. As shown in Figure S15, the activity of 1/1000Pt+Ni/CTF catalyst was essentially kept after 5 cycles albeit a dropped reaction rate was observed, which may be attributed to the diluted concentration of catalyst and reactant after injection of reaction solution during the stability tests (See Experiment Section). Furthermore, the used catalyst was recovered and characterized by HAADF-STEM technique. As shown in Figure S16, although some of the atomically dispersed Pt atoms can be observed (in red cycles), it is not as clear as that in fresh catalysts (Figure 1 and Figure S6), indicating that the surface of the catalyst may be covered by some species that may hinder the interaction between reactant and catalyst and will result in the deactivation of catalyst18. This may be the other reason for the dropped activity. It is worth mentioning that the alloy catalyst can be easily recycled due to the magnetism of Ni (Figure S17).
Kinetics The kinetics of AB hydrolysis with different contents of catalyst and concentrations of substrate provide further valuable kinetic information. Figures S18-S19 show the plots of volume of H2 generated versus time from AB hydrolysis catalysed by 1/1000Pt+Ni/CTF with different
contents of catalyst and concentrations of AB, where almost linear H2 gas evolution was observed. The hydrogen generation rates versus the concentration of AB and content of catalyst in natural logarithmic scale are also plotted in the Figures S18-S19, respectively. The slope of 0.11 of ln(rate) vs. ln[AB] indicates that the hydrolysis of AB catalysed by the 1/1000Pt+Ni/CTF catalyst can be interpreted as zero-order with respect to the AB concentration, which is consistent with the previous report.58 However, the slope of 0.98 of ln(rate) vs. ln[Ni] indicates that the catalytic hydrolysis is of the first-order with respect to the contents of catalyst. The zero-order kinetics in terms of AB concentration infers that activation of AB is not involved in the rate determining step (RDS). Kinetic isotope effect (KIE) method is effective in identifying the RDS of a reaction. Herein, deuterated AB at boron was prepared and subjected to kinetic measurements. As shown in Figure S20, 5% Ni/CTF catalyst exhibits moderate activity on the hydrolytic dehydrogenation of NH3BH3. Interestingly, a similar hydrogen evolution rate of NH3BD3 in H2O can be observed, indicating that no isotopic effect on the rate of hydrolysis occurred with deuteration at boron. However, the hydrolysis of NH3BH3 in D2O shows a slower dehydrogenation rate compared with those of NH3BH3 and NH3BD3 in H2O. Consequently, a KIE value of 4.0 was obtained, suggesting that the activation or conversion of H in H2O molecule should be involved in the RDS of hydrogen evolution from aqueous AB, which is similar to the previous observations in the Co catalysed hydrolysis.25
Figure 4. Hydrogen generation from aqueous NH3BH3 in the presence of Pt-modified 5% Ni catalysts at room temperature: (a) CNT and (b) CTF supported catalysts. The co-IMP means that the catalyst was prepared by co-impregnation of Ni and 1/1000Pt precursor, which shows a similar catalytic activity to that of pristine 5% Ni/CTF catalyst. (c) Comparison of TOFs among different catalysts based on Ni. (d) TOFs were calculated based on the Pt-centred active site (yellow bars) compared with that of pristine 0.16% Pt/CNT (referred as 1/100Pt+0Ni/CNT), where the Ni’s contribution was eliminated. The TOF of 5% Ni/CNT (green bar) was calculated based on the surface Ni estimated by the particle size.
The time dependences of H2 generation at different temperatures are recorded (Figure S21), from which the activation energy (Ea) can be determined through the Arrhenius equation (3).
lnk=lnA-Ea/RT (3) As shown in Figure S22, the Ea of 5% Ni/CTF catalysed AB hydrolysis is ca. 27.4 kJ/mol, which is consistent with the literatures59, 60. However, the activation energy is reduced
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to 17.0 kJ/mol for the 1/1000Pt+Ni/CTF, showing an energy favourable hydrolytic pathway with the presence of trace amount of Pt in the catalyst.
Mechanistic understanding To clarify the catalytic mechanism, quantum calculations were carried out using density functional method. Single Pt atom may locate at the corner, edge and surface of a Ni particle, which may have a coordination number range from 5 to 9. Considering that the coordination number of Pt in 1/500+Ni/CNT is around 7, we may deduce that the Pt should be randomly dispersed on the corner, edge and surface. Therefore, the Ni (111) plane was selected and substituted by single Pt atoms periodically as model surface for AB hydrolysis in the optimized structure models with the coordination number less than 9(Figure 5 top). As an AB molecule and a water molecule approaching to the Pt/Ni SAA surface, the entire system releases 1.24 eV to form the stable intermediate (II in Figure 5 top), with AB adsorbing on two Ni atoms through B-H bonds and H2O adsorbing on one Ni atom next to Pt. Both of the B-H bond in AB and O-H bond in H2O elongate from 1.281 to 1.296 Å and from 0.973 to 0.977 Å, respectively, indicating highly activated B-H bond after adsorption and less activated O-H bond. The largest kinetic barrier in calculation involves the dissociation of O-H bond to form III via transition state TS-I, where the Pt atom will capture an H atom from H2O. It should be noted that the energy of TS-I is about 0.75 eV which could be reduced if the solvent effect is considered. However, the kinetic barrier of dissociation of B-H bond in the following steps is relatively low (0.17 eV), which will then proceed energetically downhill to the intermediates IV and V to form B-O bond. From the intermediate V to product VI, hydrogen molecule can be formed and desorbed from the surface, leaving clean surface for further dehydrogenation. Consequently, the following H2O molecules will further be activated and interact with the activated H of B-H bond in AB, producing three equivalent H2 in total. For comparison, the hydrolytic pathways were simulated at Ni site on pristine Ni surface (Figure 5 bottom). The calculated hydrolytic dehydrogenation pathway shows 0.88 eV kinetic barriers in the dissociation of O-H bond in H2O, which are 0.13 eV higher than that on Pt/Ni SAA surface, suggesting unfavourable reaction pathway. At the same time, another pathway was simulated at Ni site on Pt-substituted Ni crystal (Figure S23, Supporting information), which exhibits a similar hydrogen evolution kinetic barrier (0.88 eV) to that on pristine Ni surface. Clearly, the simulation results indicate that the key step for the hydrogen evolution reaction is the activation of H2O molecule which is in line with our KIE experimental result as mentioned above. The addition of Pt atoms obviously decreases the reaction barrier (0.13 eV), agreeing well with the reduction of Ea in the kinetic analysis as mentioned above.
Discussion The excellent performance of the Pt/Ni SAA in the hydrogen evolution from aqueous AB may serve as a successful example demonstrating that the advancement in
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heterogeneous catalysis, i.e., SAC, would benefit significantly the hydrogen storage research where the activation of hydrides, H2, H2O and/or transportation of reacting species are usually the kinetically slow steps. Therefore, the in-depth understanding of the dehydrogenation mechanism is essentially needed to promote further application of SAC to this area.
Figure 5. Simulated pathways of hydrolytic dehydrogenation of AB at Pt centre on atomic Pt substituted Ni surface (top) and at Ni centre on pristine Ni surface (bottom). (Ni: green ball, Pt: yellow ball, O: red ball, B: pink ball, N: blue ball, and H: grey ball)
Usually, isolated atom in an alloy possesses unique electronic properties due to the electron transfer from their neighbouring metals and will exhibit high catalytic activity. Toshima and co-workers reported that the atomically dispersed Au could receive electrons from Pd and/or Ir in “Crown-Jewel” Au/Pd49 or Au/(Pd-Ir)50 alloy catalysts, resulting in a partially negative charged Au (Auδ-) that showed extremely high activity for aerobic glucose oxidation reaction. Similarly, the atomically dispersed Pd in Pd/Ag SAA received electrons from Ag and exhibited excellent performance toward the selective hydrogenation of acetylene under ethylene-rich conditions, which may be due to that the higher electron density of single Pd atoms.56 However, the function of neighbouring atom in the activation of reacting species needs to be further investigated in the SAA catalysts. Recently, Flytzani-Stephanopoulos and Sykes et al. reported that the bi-functional Pd/Cu52 and Pt/Cu53 SAAs gave superior properties in the selective hydrogenation of unsaturated hydrocarbons, where the Pd (or Pt) and Cu are responsible for the activation of H2 and hydrogenation of substrates, respectively. Alternatively, Li and Tao et al. found that the isolated Ru atom in the Rh1Co3 configuration exhibited superior performance to that of Rh-Co bimetallic nanoparticles in reduction of nitric oxide with carbon monoxide, showing the important role of neighbouring Co atom and the unique configuration.46
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Although the Pt-Ni bimetal catalysts were employed and showed promoted activity in this hydrogen evolution from aqueous AB previously61-63, the concerted effect between Pt and Ni was not well illustrated. In the present study, XAFS, Bader charge simulation and XPS results suggested that Pt possesses higher electron density than metallic state as shown in Figure 3. Consequently, Ni atoms next to Pt are essentially in reduced state but share a certain positive charge due to the electron donation to Pt. The slightly positive charged Ni atoms prefer to absorb O in H2O because of the electrostatic attraction as determined by calculation (Figure 5 II). Meanwhile, the negative charged Pt (Ptδ-) is prone to interact with H in H2O, forming an energetic favourable transition state (Figure 5 TS-I). Consequently, Pt extracts one H atom from activated H2O, leaving -OH on Ni. Ni atom could activate AB molecule and extract one H from B-H bond in AB, forming Ni-H group and H3NH2B-OH species. Moreover, the synergistic effect is also involved in the H2 desorption from catalyst surface, in which the two H atoms of H2 are derived from Pt-H and Ni-H, respectively. Therefore, the synergistic effect between Pt and Ni is responsible for the reduction of the kinetic barrier of this reaction. And, the active center in the present study is the single Pt surrounded by Ni atoms. It is worth mentioning that the TOF calculated based on the Pt content is actually the activity of Ni surrounded Pt center.
CONCLUSIONS In conclusion, supported Ni catalysts modified by trace amount of Pt (160 ppm) showed superior activity in the hydrogen evolution from aqueous AB, where atomically dispersed Pt surrounded by Ni atoms was detected and identified as active centre for H2O activation both experimentally and theoretically. The calculated TOF at the isolated Pt centre is 12000 mol H2·molPt-1·min-1 for 1/1000Pt+Ni/CTF, which is about 21-fold to that of the best Pt-based catalyst ever reported. The high activity of the Ni surrounded Pt centre could be associated with its synergistic effect between these two metals and thus may shine light on the design of highly active catalysts not only for hydrogen evolution from aqueous AB but also for other hydrogen storage materials.
ASSOCIATED CONTENT Supporting Information Experimental details, method for DFT calculations, XRD characterization, TEM characterization, HAADF-STEM characterization, XANES characterization, reusability test, kinetic isotope effect, activation energy, simulated pathway. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author E-mail:
[email protected] E-mail:
[email protected] Author Contributions All authors contributed equally. All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT T.H. would like to acknowledge financial support from the project of National Natural Science Foundation of China (51671178), the project from DICP (DICP ZZBS201616), and project from the Youth Innovation Promotion Association (CAS). P.C. would like to thank the support from iChEM·2011. This research is partially supported by the CAS/SAFEA International Partnership Program for Creative Research Teams. A.W. would like to thank the fundamental research Funds for the Central Universities (20720160031). The authors thank BL14W beam line in SSRF for providing beam time.
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