Polyallylamine-Functionalized Platinum Tripods: Enhancement of

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Polyallylamine-Functionalized Platinum Tripods: Enhancement of Hydrogen Evolution Reaction by Proton Carriers Guang-Rui Xu,†,§ Juan Bai,†,§ Lin Yao,‡ Qi Xue,† Jia-Xing Jiang,† Jing-Hui Zeng,† Yu Chen,*,† and Jong-Min Lee*,‡ †

Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, China ‡ School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore S Supporting Information *

ABSTRACT: Tailoring the size, controlling the morphology, and designing the metal−organic interface are three promising strategies to improve the catalytic performance of monometallic noble-metal nanocrystals. In the “hydrogen economy” society, water electrolysis is viewed as one of the most promising technologies for hydrogen production. The design and synthesis of highly active and durable electrocatalysts for the hydrogen evolution reaction (HER) is vitally important for the development of the hydrogen economy. In this work, we successfully synthesized polyallylamine (PAA)-functionalized Pt tripods (Pttripods@PAA) with ultrathin and ultralong branches through a facile chemical reduction method in an aqueous solution of PAA. The morphology, structure, and composition of Pttripods@PAA were fully investigated by various physical techniques. The characterization results reveal that ultrathin and ultralong branches of Pttripods@PAA have a concave structure with high-index facets and that PAA strongly binds on the Pt surface as a proton carrier. Impressively, Pttripods@PAA display unexpected activity for the HER in acidic solution with an onset reduction potential of +19.6 mV vs RHE, which significantly outperforms currently reported monometallic Pt electrocatalysts. This activity is due to the increase in the local proton concentration on the Pt surface. KEYWORDS: platinum tripod, chemical functionalization, high-index facets, electrocatalysis, interface proton concentration

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anisotropic growth.4−8 For example, since Pt tripods with ultrathin and ultralong branches were synthesized by Maksimuk and Yang in organic media in 2006 and 2007,31,32 no other effective synthesis methods have been reported until now. In general, NMNCs with controllable morphology are synthesized in the presence of surfactants, which inevitably adsorb on the surface of the NMNCs. Although clean/pristine metal surfaces are preferable for the high activity of NMNCs, recent works have demonstrated that the catalytic activity and selectivity of NMNCs can be optimized by rationally designing the metal−organic interface because of the significant electronic and steric effects induced by the organic ligand on the metal surface.33−45 For example, ethylenediamine-functionalized Pt nanowires exhibited an unexpectedly high activity for the selective generation of N-hydroxylanilines due to the electronic effect,38 and chlorophenyl-39 and cyanide-modified34 Pt nanocrystals revealed enhanced reactivity for the oxygen reduction

oble-metal nanocrystals (NMNCs) have wide applications in heterogeneous catalysis. Tailoring the size and/ or morphology of NMNCs are two effective strategies for improving the catalytic activity and utilization of NMNCs.1−14 Since the morphology governs the surface structure of NMNCs, in a sense the morphology may provide better versatility than the size in tuning the catalytic activity because of the crystal facet effect.2 For example, tetrahexahedral Pt nanocrystals bounded by high-index facets displayed significantly enhanced (up to 400%) specific catalytic activity for formic acid oxidation compared with commercial Pt nanoparticles.9 Recently, branched NMNCs with structural anisotropy have attracted increasing attention because of their extremely enhanced catalytic activity, originating from the high density of atomic steps and kinks on their surfaces.15−30 Obviously, the metal utilization and catalytic activity of NMNCs can be further improved by decreasing the diameter of the branches and increasing their length, leading to higher surface area and higher porosity between NMNCs. Nevertheless, the synthesis of highly branched NMNCs is still an enormous challenge because the face-centered cubic (fcc) noble metals with high symmetry have no intrinsic driving force for © XXXX American Chemical Society

Received: October 25, 2016 Revised: December 1, 2016

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of the polarization curves were corrected by iR compensation. The current density in this work is normalized to the geometric surface area of the GC electrode. All of the electrochemical data were averages of eight measurements. Instruments. The morphology and crystal structure of the sample were examined by transmission electron microscopy (TEM) on a JEOL JEM-2800 transmission electron microscope with a scanning transmission electron microscopy (STEM) detector and an energy-dispersive X-ray analysis (EDX) accessory, by scanning electron microscopy (SEM) on a JSM2010 scanning electron microscope, and by X-ray diffraction (XRD) on a D/max-rC diffractometer. TEM samples were prepared by placing a drop of a dilute ethanol dispersion of nanocrystals on the surface of a copper grid. The surface chemical composition and surface charge of the sample were investigated by X-ray photoelectron spectroscopy (XPS) on an AXIS ULTRA spectrometer, by thermogravimetric analysis (TGA) on a TA Q600SDT thermal analyzer, and by zeta potential measurements on a Nano ZS90 Malvern Zetasizer. The reduction of the PtII−PAA complex was monitored by UV−vis spectroscopy on a Shimadzu UV2600U UV−vis spectrometer.

reaction compared with naked Pt nanocrystals due to the steric effect. Hydrogen, as a green fuel, has been widely considered as an alternative clean energy carrier. Among various hydrogen generation methods, the electrocatalytic hydrogen evolution reaction (HER) is viewed as one of the most effective and environmentally friendly approaches because of its sustainability, absence of emissions, and energy security.46,47 To date, the noble metal Pt is still considered as the most efficient electrocatalyst for the HER because of its advantageous features, such as excellent electrochemical stability, highly competitive electrocatalytic activity, and low overpotential.48−54 In view of the prohibitive price of Pt metal, many efforts have been devoted to reducing the use of Pt by downsizing the Pt particle size or combining Pt with a less-expensive component.55,56 However, the effect of the interface structure of Pt nanocrystals on the HER has rarely been investigated. In the present work, we developed a facile route to synthesize polyallylamine (PAA)-functionalized Pt tripods (Pttripods@PAA) with ultrathin and ultralong branches in an aqueous solution. Electrochemical results showed that the PAA layers on the Pt tripod surface can effectively serve as proton carriers by means of the protonation of −NH2 groups under strongly acidic solutions. The as-prepared Pttripods@PAA exhibit improved HER activity and durability compared with commercial Pt black because of their distinctive surface structure and interface structure.

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RESULTS AND DISCUSSION Characterization of Pttripods@PAA. Pttripods@PAA were readily obtained by reducing K2PtCl4 with N2H4·H2O in the presence of PAA (see the Experimental Section for details). The morphology and size of the products were investigated by TEM (Figures 1A and S1). As can be observed, the tripods are the main products in a high yield, accompanied by a small quantity of concave nanocubes (Figure S2). The PXRD pattern of the products matches well with the standard diffraction pattern of the fcc Pt crystal (JCPDS no. 04-0802), indicating that the products are Pt tripods (Figure 1B). XPS analysis showed that the percentage of elemental Pt is 87.8%, confirming the reduction of the PtII precursor (Figure 1C). The higher-magnification TEM image shows that the diameter and length of the branch in the Pt tripods are ca. 4 and 50 nm, respectively (Figure 1D). Three branches grow in three different directions with an angle of 120°, suggesting that the generation of the Pt tripods results from threefold-symmetric growth. The corresponding selected-area electron diffraction (SAED) pattern reveals discrete diffraction spots, suggesting a high degree of crystallinity (Figure 1E). To confirm the planar tripod structure of the products, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements were performed. The brightness in the tripod center is same as that in the branches, indicating that these Pt nanostructures are not 3D tetrapods but rather 2D tripods (Figure 1F). Usually, the formation of 1D noble-metal nanowires originates from the oriented attachment or overgrowth process. The high-resolution TEM (HRTEM) image of a branch of a Pt tripod shows perfect and continuous lattice fringes with a ca. 0.226 nm interval at the same direction (Figure 1G), suggesting that the formation of the ultralong branches originates from overgrowth rather than oriented attachment. Generally, NMNCs with a concave structure show enhanced catalytic activity compared with their convex counterparts because of the higher density of atomic steps and kinks. The concave features can be clearly observed in the HRTEM images of some branches (Figure 1G,H), indicating that the branches are covered by high-index facets or lowcoordinate atomic steps. The distinct atomic steps can be

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EXPERIMENTAL SECTION Reagents and Chemicals. Potassium tetrachloroplatinate(II) (K2PtCl4, 99.9%), hydrazine hydrate (N2H4·H2O, 85%), and sulfuric acid (H2SO4, 98%) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polyallylamine hydrochloride (Scheme S1, MW = 15 000) was purchased from Sigma-Aldrich (Milwaukee, WI, USA). Commercial Pt black was supplied by Johnson Matthey Corporation (London, UK). Ultrapure water (resistance >18 MΩ cm−1) was used in all of the experiments. Preparation of PAA-Functionalized Pt Tripods (Pttripods@PAA). In a typical procedure, Pttripods@PAA were prepared as follows. K2PtCl4 (20 mg), PAA (200 mg), and ultrapure water (8 mL) were mixed under violent stirring, which resulted in the formation of the water-soluble PtII−PAA complex.57,58 Then the pH of the PtII−PAA complex solution was adjusted to 8.0, and the solution was heated to 80 °C. Subsequently, 1 mL of N2H4·H2O was added to reduce the PtII precursor under the static conditions. After 2 h, the black products were separated by centrifugation and further purified with water six times. Electrochemical Measurements. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometric tests were carried out on a CHI 660 D electrochemical analyzer with a Gamry RDE710 rotating disk electrode at 30 ± 1 °C using a three-electrode assembly including a 200 mL glass cell, a saturated calomel electrode as the reference electrode, a Pt wire as the counter electrode, and a catalyst-modified glassy carbon (GC) electrode as the working electrode. The working electrode was prepared according to the previous work.53 The catalyst ink was prepared by dispersing 10 mg of the catalyst in 5.0 mL of water containing 25 μL of 5 wt % Nafion under strong sonication conditions. Then 10 μL of the catalyst ink was carefully loaded on the surface of the 5 mm diameter GC electrode, and the electrode was dried at room temperature. All 453

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capping agent, in agreement with our previous reports.57,58 In order to understand the formation mechanism of Pttripods@ PAA, the reaction intermediates at different times were collected and characterized by TEM (Figure 2). At the initial

Figure 2. TEM images of the reaction intermediates collected at (A) 0.25, (B) 0.5, (C) 1, and (D) 2 h, respectively.

stage, the ultrafine and monodispersed triangular platelike Pt seeds were generated (Figure 2A). Then Pt tripods with short branches begin to appear at 0.5 h (Figure 2B). With increasing reaction time, the length of the branches gradually increases (Figure 2C), and the Pt tripods with ultrathin and ultralong branches ultimately evolve at 2 h (Figure 2D). On the basis of the continuous lattice fringes on a branch (Figure 1G), the strong dependence of the branch length on the reaction time indicates that autocatalytic tip overgrowth is responsible for the growth of the branches.19 Under kinetic control conditions, the final morphology of NMNCs is strongly related to the atomic deposition rates on certain facets (Rdeposition) and the atomic diffusion rates (Rdiffusion) on the surfaces.6 The previous excellent works have demonstated that amino groups can selectively bind to Pt facets.46,59 Thus, the newly generated Pt atoms preferentially deposit on Pt{111} facets. When Rdeposition/Rdiffusion ≫ 1, atomic surface diffusion is ignored, and the branches continuously grow along the Pt ⟨111⟩ direction. Indeed, the yield of Pt tripods decreases sharply when the Pt atom deposition rate (i.e., the reduction rate of the PtII precursor) is decreased by changing the solution pH (Figure S6), concentration of N2H4· H2O (Figure S7), or PtII/PAA molar ratio (Figure S8). These control experiments indicate that the high Pt atom deposition rate (i.e., the high rate of reduction of the PtII precursor) plays an important role in the formation of Pt tripods with ultrathin and ultralong branches, which was further confirmed by reaction-temperature-dependent branch length experiments (Figure 3). As can be observed, the diameter and length of the branches of the Pt tripods can readily be controlled by decreasing the reaction temperature. Electrocatalytic Activity for the HER. The electrochemical performance of Pttripods@PAA and commercial Pt black (Johnson Matthey Corporation) were first evaluated by cyclic voltammetry (CV) in a N2-saturated 0.5 M H2SO4 solution at 50 mV s−1 (Figure 4A). All of the potentials reported in this work were referenced to a reversible hydrogen electrode (RHE) according to the equation ERHE = ESCE + 0.059·pH + 0.242. The electrochemical active surface areas

Figure 1. (A) TEM image, (B) XRD pattern, and (C) Pt 4f XPS spectrum of Pttripods@PAA. (D) TEM image and (E) SAED pattern of an individual tripod of Pttripods@PAA. (F) HAADF-STEM image of Pttripods@PAA. (G) HRTEM image of a branch at Pttripods@PAA. Insets: Magnified HRTEM images taken from regions G-1, G-2, G-3, and G-4 marked by the red squares. (H) HRTEM image of another branch at Pttripods@PAA. (I) Magnified HRTEM images taken from regions H-1 and H-2 marked by the rectangles in (H), respectively.

observed in the magnified HRTEM images (Figure 1I), proving that the Pt branches contain high-index facets, in agreement with the previously reported noble-metal tripods.18,19,23,25,50 Our previous works have demonstrated that polymeric amine can firmly bind on the Pt surface through the strong interaction between −NH2 groups and Pt atoms.57,58 In the present work, the N 1s signal was detected (Figure S3), demonstrating the binding of PAA on the Pt tripods. Meanwhile, the zeta potential for the Pt tripods was measured to be ca. +41 mV at pH 7, further confirming the binding of PAA on the Pt tripods. Formation Mechanism of Pttripods@PAA. Our previous works have indicated that K2PtCl4 can interact with PAA to generate a water-soluble PtII−PAA complex,57,58 which slows the reduction of the PtII precursor and consequently induces the kinetically controlled synthesis of Pt nanocrystals. UV−vis measurements showed that the PtII−PAA complex is reduced completely within 2 h after the addition of N2H4·H2O (Figure S4). In the absence of PAA, only Pt aggregates were obtained (Figure S5), demonstrating that PAA efficiently acts as a 454

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Figure 3. TEM images of Pttripods@PAA synthesized at different reaction temperatures: (A) 30, (B) 50, and (C) 80 °C.

Figure 5. (A) LSV curves of Pttripods@PAA and Pt black for the HER in N2-purged 0.5 M H2SO4 at 2 mV s−1 and a rotation rate of 1600 rpm. (B) Large-area and (C) locally amplified EDX elemental maps of Pttripods@PAA. (D) HER current densities for Pttripods@PAA and Pt black at −0.025 and −0.075 V. (E) Tafel plots of Pttripods@PAA and Pt black for the HER in N2-saturated 0.5 M H2SO4.

Figure 4. (A) CV curves of Pttripods@PAA and Pt black in a N2saturated 0.5 M H2SO4 solution at 50 mV s−1. (B) SEM image of Pttripods@PAA on the GC electrode.

is slight bigger than the width of the Pt element pattern (Figure 5C), revealing that the thickness of PAA is ca. 0.5 nm. The TGA curve shows that the mass fraction of PAA in the Pttripods@PAA is ca. 20.52% (Figure S9). After removal of the PAA, the ECSA of the naked Pt tripods was measured to be ca. 46.6 m2 g−1 (Figure S10). If it is assumed that all of the −NH2 groups on the Pttripods@PAA are completely protonated (i.e., −NH3+), the proton concentration on the Pttripods@PAA is calculated to be 1.6 × 103 M by combining the ECSA of the Pt tripods, the PAA mass fraction on the Pttripods@PAA, and the thickness of PAA layer. This is much higher than the proton concentration in the 0.5 M H2SO4 electrolyte. Thus, the PAA layers on the Pt tripods effectively increase the local proton concentration on the Pt surface compared with the bulk proton concentration of the 0.5 M H2SO4 electrolyte, which is responsible for the HER activity enhancement. Although the ECSA of the naked Pt tripods is bigger than that of Pttripods@ PAA, the onset potential of the HER on the naked Pt tripods negatively shifts ca. 20.7 mV relative to that on Pttripods@PAA (Figure S11), which surely confirms that PAA serves as proton carriers to improve the HER activity of the Pt tripods. Meanwhile, it was observed that the onset potential of the HER on the naked Pt tripods is higher than that on Pt black (Figure S12), indicating that the surface structures of the Pt tripods also play an important role in the HER performance enhancement. The TEM image (Figure 1I) and previous works have indicated that the edges of branches contain abundant step atoms, which contribute to the HER activity enhancement because of the high catalytic activity of low-coordinated atoms.50 Besides the overpotential, the cathodic current density of the HER is another key parameter for evaluating the electocatalytic performance of the catalyst. The Pttripods@PAA show about 3.92 and 2.84 times the activity of commercial Pt black at potentials of −0.025 and −0.075 V, respectively (Figure 5D), confirming the high reactivity of Pttripods@PAA for the HER. To investigate the reaction mechanism of the HER, Tafel plots were recorded

(ECSAs) of Pttripods@PAA and Pt black were estimated by integrating their hydrogen adsorption charges. The ECSA value of Pttripods@PAA was 25.9 m2 g−1, which is 1.41 times bigger than that of the commercial Pt black (18.4 m2 g−1) because of the small diameter of the Pt branches. Benefiting from the highly branched structure, the Pttripods@PAA easily form a freestanding membrane with high porosity (Figure 4B), which is expected to be an ideal electrode material for the HER because of the favorable mass transfer and low hydrogen bubble adhesion force on the electrodes.60 Meanwhile, the unique 1D structure of the branches in Pttripods@PAA effectively provides a continuous conducting pathway for the electron transport,61 which can further accelerate the reaction kinetics of the HER. The HER activities of Pttripods@PAA and Pt black were evaluated by LSV in a N2-saturated 0.5 M H2SO4 solution at 2 mV s−1 and a rotation rate of 1600 rpm (Figure 5A). The current densities were normalized to the geometric area of the GC electrode. The onset potentials of the HER on Pttripods@ PAA and Pt black are about +19.6 and −8.6 mVvs RHE, respectively. Surprisingly, the onset potential of the HER on Pttripods@PAA (+19.6 mV vs RHE) is much higher than the theoretical value for the HER in an acidic solution (0 mV vs RHE). According to the HER mechanism in an acidic medium (2H+ + 2e− → H2), the HER activity of Pt nanocrystals is related to the surrounding proton concentration. A previous work demonstrated that the addition of amines to a H2SO4 solution could effectively improve the electrocatalytic activity of Pt nanocrystals for the HER as a result of adsorption of the amines on the Pt surface, which was ascribed to the increased proton concentration at the Pt−electrolyte interface due to protonation of the nitrogen atoms.62 XPS (Figure S3) and zeta potential measurements confirmed the binding of PAA on the Pt tripods, which was also visualized by EDX maps (Figure 5B). The similarity between the N and Pt element patterns indicates the uniform distribution of PAA on the Pt tripods. Further detailed analysis shows that the width of the N element pattern 455

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the continuous LSV scans, the durability of the catalyst was also evaluated by the chronoamperometry technique in a N2-purged 0.5 M H2SO4 solution (Figure 6D). After the reaction was run for 6000 s, Pttripods@PAA completely kept their initial activity, whereas Pt black generated a 0.2 mA cm−2 current density decay, manifesting the superior durability of Pttripods@PAA during the long-term test. According to the reaction mechanism of the HER (2H+ + 2e− → H2), no poisoning effect occurs at the Pt surface during the electrolysis. Thus, the excellent selfstability of Pttripods@PAA is responsible for their good durability for the HER.

by plotting the overpotential versus the logarithm of the current density (Figure 5E). Generally, the HER obeys Volmer− Heyrovsky mechanism (electrochemical hydrogen adsorption: H3O+ + e− → Hads + H2O and electrochemical desorption: Hads + H3O+ + e− → H2 + H2O) and/or the Tafel mechanism (Hads + Hads → H2), which give theoretical Tafel slope values of about 40−120 and 30 mV per decade, respectively.63−65 Thus, the Tafel slope can be used to decide the rate-determining step during the HER process. As observed, Pttripods@PAA and commercial Pt black have close Tafel slopes (26 vs 31 mV/ decade), suggesting that the recombination of two Hads atoms is the rate-limiting step for both Pttripods@PAA and commercial Pt black. The similar Tafel slope values also demonstrate that the control over the interface structure of Pt nanoparticles does not have a distinct influence on the HER reaction process. For Pttripods@PAA, the HER currents above 0 V vs RHE make the HER overpotential values negative in the Tafel plots. Similar situations have been reported in previous literature.50,66 Durability for the HER. For any electrocatalytic reaction, the self-stability of the electrocatalyst plays a critically important role in their durability. The self-stabilities of Pttripods@PAA and Pt black were investigated under harsh electrochemical conditions, which were completed by repeating CV between 0 and 1.2 V at 50 mV s−1 in a N2-saturated 0.5 M H2SO4 solution. After 10 000 cycles, Pttripods@PAA retain 96.7% of the initial ECSA value (Figure 6A). In contrast, the commercial Pt

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CONCLUSIONS In summary, we have demonstrated a facial and efficient wetchemical approach for the synthesis of Pttripods@PAA with ultrathin and ultralong branches. In the developed approach, PAA acts as a complexant, structure-directing agent, and functional molecule. Meanwhile, the length of the branches in the Pt tripods could be easily tuned by controlling the reaction temperature. When used as the electrocatalyst for the HER, Pttripods@PAA exhibited better activity compared with Pt black, originating from the high interface proton concentration and high-energy surface. Additionally, the 3D interconnected structure of the Pt tripods and 1D ultrathin structure of the branches resulted in the high self-stability of Pttripods@PAA, which contributed to the excellent durability of Pttripods@PAA for the HER. The present work highlights that the rational design of surface/interface structures can effectively improve the electrocatalytic activity and durability of NMNCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03049. Experimental section and characterization details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Chen). *E-mail: [email protected] (J.-M. Lee). ORCID

Yu Chen: 0000-0001-9545-6761 Jong-Min Lee: 0000-0001-6300-0866

Figure 6. Continuous CV curves of (A) Pttripods@PAA and (B) Pt black in a N2-purged 0.5 M H2SO4 solution at 50 mV s−1. (C) LSV polarization curves for Pttripods@PAA and Pt black initially and after 100 LSV runs at 2 mV s−1 and a rotation rate of 1600 rpm. (D) Timedependent current density curves of Pttripods@PAA and Pt black at potential of −50 mV for 6000 s.

Author Contributions §

G.-R.X. and J.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was sponsored by the National Natural Science Foundation of China (21473111), the Natural Science Foundation of Shaanxi Province (2015JM2043), the Fundamental Research Funds for the Central Universities (GK201602002 and 2016TS063), Innovation Funds of Graduate Programs at Shaanxi Normal University (2015CXS048), and the Academic Research Fund of the Ministry of Education in Singapore (RGT27/13).

black loses 49.6% of the initial ECSA (Figure 6B), originating from the ripening/aggregation of Pt black. The continuous CV tests demonstrate that Pttripods@PAA are more stable than Pt black, originating from the structural advantages of the former. Mainly, the 3D interconnected structure of the Pt tripods 23,27−29,60,67 and the 1D ultrathin structure of branches61,68 can effectively restrain the physical ripening and electrochemical dissolution of the Pt nanoparticles. The durability of the electrocatalyst for the HER was evaluated by repeating LSV runs in 0.5 M H2SO4 between 0.1 and −0.1 V vs RHE (Figure 6C).59 After 100 rounds, Pttripods@PAA exhibit no measurable potential shift in the LSV curves, whereas Pt black experiences a 7.5 mV potential shift at −0.1 V. In addition to

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DOI: 10.1021/acscatal.6b03049 ACS Catal. 2017, 7, 452−458

Research Article

ACS Catalysis (68) Oh, A.; Baik, H.; Choi, D. S.; Cheon, J. Y.; Kim, B.; Kim, H.; Kwon, S. J.; Joo, S. H.; Jung, Y.; Lee, K. ACS Nano 2015, 9, 2856− 2867.

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DOI: 10.1021/acscatal.6b03049 ACS Catal. 2017, 7, 452−458