Highly Active and Stable Metal Single-Atom Catalysts

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Highly Active and Stable Metal Single-Atom Catalysts Achieved by Strong Electronic Metal-Support Interactions Junjie Li, Qiaoqiao Guan, Hong Wu, Wei Liu, Yue Lin, Zhihu Sun, Xuxu Ye, Xusheng Zheng, Haibin Pan, Junfa Zhu, Si Chen, Wenhua Zhang, Shiqiang Wei, and Junling Lu J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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Journal of the American Chemical Society

Highly Active and Stable Metal Single-Atom Catalysts Achieved by Strong Electronic Metal-Support Interactions Junjie Li,†, ‡,⊥Qiaoqiao Guan,†, ‡,⊥ Hong Wu,¶,⊥Wei Liu,§ Yue Lin,† Zhihu Sun,§ Xuxu Ye,¶ Xusheng Zheng,§ Haibin Pan,§ Junfa Zhu,§ Si Chen,¶ Wenhua Zhang,†, ¶, ǁ Shiqiang Wei,§ Junling Lu,†, ‡,¶,* †Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 China ‡Department of Chemical Physics, iChem, University of Science and Technology of China, Hefei, Anhui 230026 China ¶CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026 China §National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029 China ǁSynergetic Innovation Centre of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026 China

Supporting Information ABSTRACT: Developing an active and stable metal single-

atom catalyst (SAC) is challenging due to the high surface free energy of metal atoms. In this work, we report that tailoring of the 5d state of Pt1 single atoms on Co3O4 through strong electronic metal-support interactions (EMSI) boosts an activity up to 68-fold higher than those on other supports in dehydrogenation of ammonia borane for roomtemperature hydrogen generation. More importantly, this catalyst also exhibit excellent stability against sintering and leaching, in sharp contrast to the rapid deactivation observed on other Pt single-atom and nanoparticle catalysts. Detailed spectroscopic characterization and theoretical calculations revealed that EMSI tailors the unoccupied 5d state of Pt1 single atoms, which modulates the adsorption of ammonia borane and facilities hydrogen desorption, thus greatly leading to the high activity. Such extraordinary electronic promotion was further demonstrated on Pd1/Co3O4 and in hydrogenation reactions, providing a new promising way to design of advanced SACs with high activity and stability.

Oxide/carbon supported metal catalysts are widely applied in industrial chemical reactions.1 On one hand, the support is used to disperse the metal with high surface areas for a high activity and low cost. Therein, metal-support interactions are used to stabilize the metal nanoparticles (NPs) for a long catalyst life.2 As reducing the particle size to the subnanometer region, even atomically-dispersed atoms,

these metal-support interactions can become extremely important due to the drastically increased surface free energy.3 On the other hand, the support might also induce substantial electronic perturbations to the admetal.4 Such “electronic metal-support interactions” (EMSI), rationalized in terms of charge transfer was suggested to modulate the metal d-band centers,5 thus often leading to the outstanding catalytic activity.4a-c, 5-6 Supported metal single-atom catalysts (SACs) with maximized atom efficiency and novel catalytic performance have recently attracted extensive attention in the catalysis field.7 In these materials, isolated metal atoms are directly bonded to the support with an ionic character,8 thus the influence on the catalytic activity of SACs by substantial charge transfer could be expected to be more prominent than that on the nanoparticulated counterpart.4b, 9 Consequently, atomic-level understanding and tuning of the 5d states of metal atoms through EMSI can be essential to develop an active and sinter-resistant SAC, which, however, has been rarely explored. Variation of supports is an effective way to tailor EMSI.2d With this in mind, here Pt1 single atoms with different 5d electronic states were fabricated on Co3O4, CeO2, ZrO2, and graphene, where the electronic properties of Pt1 atoms were well characterized by X-ray absorption near-edge structure (XANES), X-ray photoemission (XPS) and Infrared. We found that strong EMSI in Pt1/Co3O4 induced a larger depletion of 5d states of Pt1 atoms, which in turn makes this sample extremely active along with excellent stability in

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ammonia borane (AB) dehydrogenation.10 Density functional theory (DFT) revealed that EMSI considerably tailors the unoccupied 5d state of Pt1 atoms which varies the adsorption of AB and hydrogen molecules and leads to the high activity. Such impressive improvement by EMSI was further illustrated on Pd1/Co3O4 and in hydrogenation reactions, shedding light on design of advanced SACs with high activity and stability. These Pt1 SACs were synthesized using atomic layer deposition (ALD), a technique relying on sequential molecular-level self-limiting surface reactions.11 The Pt loadings on Co3O4, CeO2, ZrO2, and graphene were 0.5, 1.1, 0.2, and 0.4 wt%, respectively (Table S1). Aberrationcorrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) showed that Pt was atomically dispersed in all these samples, without presence of any visible NPs/clusters at both low- and highmagnifications (Fig. 1 and Figs. S1-S4).

Figure 1. Representative aberration-corrected HAADF-STEM images of (a,b) Pt1/Co3O4, (c,d) Pt1/CeO2, (e,f) Pt1/ZrO2, and (g,h) Pt1/graphene at both low- and high-magnifications. The white circles highlight the Pt1 atoms. The insets in (b,d,f,h) show the line intensity profiles along the middle of the yellow dash squires, highlighting the positions of Pt1 atoms. X-ray absorption spectroscopy (XAS) is known for characterizing the electronic and structural properties of absorbing atom.12 For the XANES curves at the Pt L3-edge, the area under the white line peak is directly proportional to the population of the unoccupied Pt 5d states.13 As shown in Fig. 2a, the Pt1/Co3O4 XANES curve resembled that of PtO2,

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except that the white line peak exhibited a considerably stronger intensity, implying a high valence state of ~ 4+ and a larger population of unoccupied Pt 5d states for the Pt1 in Pt1/Co3O4. The XANES spectra of other SACs also showed similar shapes but with considerably lower white line peak intensities, suggesting the lower oxidation states of Pt1 and lesser populations of unoccupied Pt 5d states. Fourier transforms of the extended X-ray absorption fine structure (EXAFS) spectra of these samples showed a similar peak at 1.60 Å, assigned to the Pt-O coordination, with coordination numbers (CNs) of 5.7, 4.9, 4.8, and 4, for Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene, respectively (Fig. S5 and Table S2). The Pt-Pt coordination peak was absent in all these samples, suggesting the absence of Pt NPs/clusters, in line with the STEM observation. Additionally, the Pt1/Co3O4 curve showed a second shell peak at 2.56 Å, distinct with Pt foil (2.37 Å) and PtO2 (2.68 Å). EXAFS curve-fittings (Figs. S5a,b and Table S2) showed this shell is attributed to the PtCo coordination, reflecting the strong interaction between the Pt1 atom and the Co3O4 support.

Figure 2. (a) XANES spectra of Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene SACs as well as the Pt foil and PtO2 reference at the Pt L3-edge. (b) The corresponding K2weighted Fourier transform spectra. (c) DRIFTS of CO chemisorption on Pt1/Co3O4, Pt1/CeO2, and Pt1/ZrO2 at the saturation coverage. (f) XPS spectra of Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, Pt1/graphene and PtO2 in the Pt 4f region. Infrared spectroscopy of CO chemisorption is another convenient tool to differentiate noble metal single atoms from NPs and to disclose the population of metal 5d electronic states.14 Here diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO chemisorption showed that the linear CO peak located at 2105, 2093, and 2097 cm-1 on Pt1/Co3O4, Pt1/CeO2, and Pt1/ZrO2, respectively (Fig. 2c). The higher frequency of the CO peak on Pt1/Co3O4 suggests that the Pt1 atoms have less populated 5d state electrons which weaken the Pt(5d)-CO(2*) bonding


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Journal of the American Chemical Society through  backdonation,14c, 15 coinciding perfectly with the XANES result (Fig. 2a). The bridge-bonded CO (~1840 cm-1) was not present in all these samples, again suggesting that there were no any detectable Pt NPs/clusters, consistent with the STEM and XAFS results (Figs. 1 and 2a,2b). It’s worth noting that DRIFTS of CO chemisorption on Pt1/graphene is challenging due to the strong light adsorption of the graphene support. XPS measurements showed that the Pt 4f7/2 binding energy peak of Pt1/Co3O4 located dominantly at 74.2 eV, along with a weaker shoulder at 72.4 eV on (Fig. 2d), indicating that the valence state of the Pt1 atoms in majority is close to 4+.16 Whereas, the Pt 4f7/2 binding energy of the Pt1 atoms in other samples mainly located at ~72.4-72.6 eV, implying a ~2+ state. This data consists excellently with the XANES and IR results (Fig. 2a,2c).

deactivated severely after only 3 cycles (Fig. 3d). Analysis of the used samples using STEM and ICP-AES revealed that Pt1/Co3O4 had no any visible Pt aggregation or leaching (Fig. S12, and Table S1). However, the Pt1 atoms in the used Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene samples all sintered aggressively to large Pt NPs, and the metal loss via leaching was very heavy up to 50% (Fig. S13-15, and Table S1). In particular, high degrees of B-induced catalyst poisoning might be the major reason for the severe deactivation of Pt1/graphene, since less pronounced Pt aggregation was observed (Fig. S15).10c, 18 We found that the stability of the Pt1 atoms in these four samples doesn’t seem to correlate with the Pt reducibility (Fig. S16). One possible reason is that the Pt1 atoms on Co3O4 were only reduced to a 2+ state after 15 cycles of recyclability test, while the Pt atoms on CeO2, and ZrO2 were reduced to zero state (Fig. S17). Surprisingly, Pt NPs on Co3O4 (Pt-NPs/Co3O4) also deactivated severely (Fig. S6c). These results unambiguously suggest that strong EMSI observed in Pt1/Co3O4 plays the decisive roles for the remarkably high activity and stability.

Figure 3. Catalytic performance of Pt1 SACs in hydrolytic dehydrogenation of AB at 25 °C. (a) Plots of the volume of H2 generated as a function of time, (b) mass specific rates, (c) Arrhenius plots and (d) recyclability test on Pt1/Co3O4, Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene SACs. The legends in (a) also apply to (c). AB with a high hydrogen content (19.6 wt%) and satisfactory air stability has been regarded as one promising hydrogen storage media for portable applications.17 Here hydrolytic dehydrogenation of AB for hydrogen production (Eq. (1)) was utilized as a probe reaction to investigate the “EMSI” effect on SACs.10b, 10d NH3BH3 + 2H2O  NH4+ +BO2- + 3H2

(1)

As shown in Fig. 3a, a total of 23.4 mL of H2 gas, the theoretical volume according to Eq. (1), was vigorously generated on Pt1/Co3O4 in only 3 min at 25 °C. However, less than 6.5 mL of H2 was generated on Pt1/CeO2, Pt1/ZrO2, and Pt1/graphene even after 9 min. The calculated mass specific rate of Pt1/Co3O4 was 1220 molH2 molPt-1 min-1, up to 68-fold higher than other Pt1 SACs (Fig. 3b). To note, all blank supports had no any activity (Fig. S6), and the rate achieved on Pt1/Co3O4 was also much higher than other Pt NPs catalysts (Figs. S7-S10 and Table S3). Kinetic studies further revealed a much lower apparent activation energy on Pt1/Co3O4 than other Pt SACs (Fig. 3c and S11). Recycling stability tests showed that only Pt1/Co3O4 exhibited excellent recyclability, while all other Pt1 SACs

Figure 4. The local partial density of state (LPDOS) projected on the Pt1 5d orbitals with Fermi level set at zero and the local configurations for AB adsorption. (a) Pt1/Co3O4, (b) Pt1/CeO2, (c) Pt1/ZrO2 and (d) Pt1/graphene. The ball in gray, white, pink, orange, red, green, dark blue, sky blue and black represent carbon, hydrogen, boron, nitrogen, oxygen, cerium, cobalt, zirconium and platinum, respectively. The adsorption energy of AB on these Pt1 SACs are also indicated. The legends in (d) also apply to (a-c). DFT calculations were further carried out to understand the underlying mechanism. The optimized structures of these Pt1 SACs are shown in Fig. S18, Tables S2 and S4. We found that the local partial density of states (LPDOSs) of the 5d orbitals of Pt1 atom in these four samples are very different (Fig. 4). In Pt1/Co3O4, the unoccupied 5d states locate at 0.39 and 0.26 eV above the Fermi level, for the spin up and spin down component, respectively. Such difference induced by spin manifests the remarkable electronic


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perturbation through EMSI from the magnetic Co3O4 support. In contrast, the non-magnetic samples show symmetrical curves for the spin up and spin down components with unoccupied 5d states at 1.20, 2.66 and 0.23 eV on Pt1/CeO2, Pt1/ZrO2 and Pt1/graphene, respectively. The binding energy of Pt1 on Co3O4 is -8.43 eV, higher than on CeO2 (-8.34 eV), ZrO2 (-7.23 eV) and graphene (-2.31 eV) (Table S5), consistent excellently with the high stability of the Pt1/Co3O4 sample (Fig. 3d). When AB is absorbed on Pt1/Co3O4, the B-H distance is elongated from 1.21 to 1.37 Å with an adsorption energy of 2.65 eV. On Pt1/graphene, the AB adsorption becomes considerably stronger (-2.75 eV). Impressively, H2 adsorbs molecularly on Pt1/Co3O4 (-0.75 eV) but dissociatively on Pt1/graphene (-2.39 eV) (Fig. S19). According to our previous work,10b relatively higher energy positions of the unoccupied 5d states of the Pt1 atoms in Pt1/Co3O4, allows moderate adsorption of AB to reduce B poisoning and much weaker H2 adsorption to facilitate H2 desorption, which together boost the activity tremendously. Nonetheless, the unoccupied 5d states of the Pt1 atoms in Pt1/CeO2 and Pt1/ZrO2 both locate too high above the Fermi level to effectively adsorb AB. In fact, AB forms hydrogen bonds with the Pt neighboring O atoms, instead of the Pt1 atom, thus yielding the poor activity. These results implies that besides the dominant EMSI effect, the support itself might also contribute to the activity to a certain extent. Such strong EMSI promotion of activity and stability was further demonstrated on a Pd1/Co3O4 SAC, with a mass specific rate of 1470 molH2 molPd-1 min-1, even higher than Pt1/Co3O4 (Fig. S20-S24 and Table S1). In conclusion, atomically-dispersed Pt1 atoms have been successfully synthesized on the supports of Co3O4, CeO2, ZrO2, and graphene to investigate the role of EMSI in SACs. As revealed by XANES and DRIFTS CO chemisorption, a significantly larger depletion of 5d states of Pt1 atoms in Pt1/Co3O4 caused by EMSI, prompted a tremendous activity enhancement in hydrolytic dehydrogenation of AB, along with excellent stability against sintering and leaching. DFT calculations further revealed that unoccupied state of the Pt 5d orbital of Pt1/Co3O4 tailored through EMSI renders a moderate adsorption of AB and a much weaker adsorption of the H2 product, thus greatly improving the H2 generation rate. Such remarkable EMSI effect on SACs could be general in many catalytic reactions (eg. hydrogenation reaction as shown in Fig. S25), thus sheds light on rational design of advanced SACs with high activity and stability.

ASSOCIATED CONTENT Supporting Information Experimental section, additional characterizations and catalytic performances tests. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions ⊥L.J.,

G.Q. and W.H., contributed equally to this work.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 21673215, 21533007, U1632263). the Fundamental Research Funds for the Central Universities (WK2060030029, WK3430000005), Users with Excellence Program of Hefei Science Center CAS (2019HSC-UE016), and the Max-Planck Partner Group. The calculations were performed on the supercomputing system in USTC-SCC and Guangzhou-SCC. The authors also gratefully thank the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF), and the BL10B beamlines at National Synchrotron Radiation Laboratory (NSRL), China.

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Highly Active and Stable Metal Single-Atom Catalysts

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