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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
A Novel Graphdiyne-Based Catalyst for Effective Hydrogenation Reaction Han Shen,†,‡ Yongjun Li,*,‡,§ and Zhiqiang Shi*,† †
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Shandong Normal University, 88 Wenhuadonglu Road, Jinan 250014, P. R. China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China § School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *
ABSTRACT: The platinum nanoparticles (Pt NPs) hybrided with nanostructured carbon materials with high stability are important for catalyzing hydrogenation reaction. Here we reported the fabrication of ultrastable Pt NPs anchored on graphdiyne, in which the strong interactions induced by the porous graphdiyne can prevent the thermal migration of Pt nanoparticles on the graphdiyne surface, exploiting the strong charge transfer interactions from Pt NPs to GDY substrate to tune the electron density of Pt NPs. Pt NPs catalyst with size of 2−3 nm showed high performance on hydrogenation of aldehydes and ketones to the corresponding alcohols compared with commercial Pt−C. Our results indicated that graphdiyne is a promising substrate for constructing metal nanoparticle-based heterogeneous catalysts, especially for those requiring strong interactions between metal nanoparticles and reactants. KEYWORDS: graphdiyne, Pt nanoparticle, hybrid material, microwave irradiation, hydrogenation catalysis
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nanomaterials, whose sp2- and sp-hybridized flat carbon networks provide this family with highly conjugated π-system, ordered porous structures, and adjustable electronic properties.20−26 The applications of GDYs in various research fields (such as catalysis,27−32 lithium-ion batteries,18,33−37 biomaterials,38−40 and solar cells41−43) have been realized experimentally. Simulations indicated that the strong charge transfer from Pt to the p-orbitals of the large C ring of graphdiyne can induce the higher stability of Pt NPs on graphdiyne than those on graphene.44 At the meantime, the uniformly distributed holes on the surface of graphdiyne can prevent the thermal migration of Pt NPs. The strong charge transfer interactions from Pt NPs to GDY substrate would also change the electron density of Pt NPs, which would increase the interactions between Pt NPs and reactants (Scheme 1). Herein, we develop a GDY-based catalyst (Pt−GDY) through microwave-assisted distribution of Pt nanoparticles on GDY. The hydrogenation of aldehydes and ketones to the corresponding alcohols was used as the model reaction to investigate the exceptional catalytic activities induced by the
INTRODUCTION Platinum nanoparticles (Pt NPs) are highly active catalysts for hydrogenation1 whose size-dependent catalytic activity has been revealed recently.2 However, the nanoparticle catalysts with high surface energy prefer to aggregate in the reaction system, which leads to catalytic activity decreasing. Because of their exciting electronic properties,3 nanostructured carbon materials like carbon nanofibers, carbon nanotubes, and graphene have exhibited excellent performance as important Pt-nanoparticle support materials. Pt NPs supported on these carbon materials were used as electrocatalysts for methanol electro-oxidation,4,5 hydrogen oxidation and oxygen reduction in fuel cell reactions,6−8 overall water splitting,9 and as hydrogenation catalysts.10,11 The challenge for Pt NP catalyzed hydrogenation is the requirement of harsh conditions (a high hydrogen pressure (1−4 MPa) or an elevated temperature (40−200 °C)), which is due to the weak interactions between the Pt NPs and the reactants. Although there have been some reports of catalysts worked under relatively mild conditions, optimal catalytic systems that can solve the aggregation of Pt NPs and increase the interaction between Pt NPs and reactants are still anticipated. To solve the aggregation problem of Pt nanoparticles and to improve the adsorption capability of Pt NPs toward the reactants, it is necessary to try another carbon substrate. Graphdiyne (GDY),12−19 a new allotropic form of carbon © XXXX American Chemical Society
Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: January 11, 2018 Accepted: February 20, 2018
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DOI: 10.1021/acsami.8b00566 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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water for three times, and the resulted Pt−GDY catalysts were dried in vacuum at 30 °C overnight. The actual loading of Pt in this nanocomposite was 13.4 wt %, as determined by inductively coupled plasma mass spectrometry (ICP-MS). The representative TEM images of Pt−GDY are given in Figure 1. From the pictures with different magnifications we can observe plenty of grain-like small particles distributing on GDY film (Figure 1a-e). Eighty of them were measured and counted the size distribution. Statistical results (Figure S1) showed a minimum diameter of 1.55 nm and a maximum diameter of 2.91 nm, with the mean value at 2.05 nm. Most of the particles grew in diameter between 1.8 and 3 nm. The visible lattice fringes of those nanoparticles in the highresolution TEM image (Figure 1f) show a spacing of 0.223 or 0.197 nm, which is corresponding to the well-known d-spacing of the Pt (111) or Pt (200) plane, respectively.46 The scanning TEM (STEM) image and the EDX mapping images matched well, and indicated the uniform distribution of C and Pt elements (Figure 1g−j). The X-ray powder diffraction (XRD) patterns of the GDY and Pt−GDY are illustrated in Figure 2a. The strong diffraction peaks corresponding to Pt (111), Pt (200), and Pt (220) indicated the fcc structure of Pt NPs obtained at present condition, and the predominant plane of Pt (111) was observed.47 X-ray photoelectron spectroscopy (XPS) was used to examine the qualities of the samples (Figure 2b, c). The area ratio value about 2 was maintained for the sp- and sp2-
Scheme 1. Interaction between the Pt Nanoparticles and Graphdiyne for Stabilization of the Pt NPs
increased absorption interactions between Pt NPs and reactants.
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RESULTS AND DISCUSSION The reduction potential of GDY was determined to be about −0.33 V vs SHE, which was much lower than that of PtCl42− ion (+0.76 V vs SHE), indicating its ability as the substrate for electroless deposition of Pt, as it was in the case of Pd.29 GDY powder was obtained through the modified Glaser reaction. For a typical preparation of Pt−GDY composite, a homogeneous colloid of GDY in ethylene glycol was mixed with 0.05 M chloroplatinic acid and 1 M sodium acetate glycol, and the reduction reaction was then performed in the microwave organic synthesis equipment (400 W) at 160 °C for 2 min under constant stirring.45 Finally, the obtained samples were ultracentrifuged, washed repeatedly with absolute ethanol and
Figure 1. TEM images of Pt−GDY composites: (a−c) low-magnification image; (d, e) high-magnification image; (f) HRTEM image; (g) scanning TEM (STEM) image; EDX elemental mapping of (h) C, (i) Pt, and (j) overlayer. B
DOI: 10.1021/acsami.8b00566 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. (a) XRD spectrum of Pt−GDY. The two diffraction peaks (39.76°, 67.45°) correspond to the platinum nanoparticles (111) and (220) crystal plane. (b) C 1s and (c) Pt 4f XPS spectra of Pt−GDY. (d) XANES and (e) FT-EXAFS of Pt−GDY and Pt-foil reference. (f) High-resolution valence-band Pt 5d XPS of Pt−GDY and Pt−C relative to the VBM, as an analogue of the density of states. Blue lines indicate the positions of the dband centers.
Pt−GDY, indicating the strong association of Pt NPs with the substrate. To reveal their interaction with reactant molecules, we characterized the d-band electron structures of the Pt NPs by high-resolution valence-band (VB) XPS spectra (Figure 2f), with the valence-band maximum (VBM) as a reference for the d-band center. The d-band electron structures, are proportional to the density of states (DOS), and directly related with the strength of interaction between the Pt NPs and reactant molecules.49−51 According to the d-band center theory, the reactant molecule adsorbed on the metal surface would lead to hybridization between the metal d-band and an induced state by the guest molecule, which will lead to fully filled bonding DOS and partially filled antibonding DOS states. The bond strength is determined by the filling degree of the antibonding states that is related to the d-band center position.52−54 Compared with Pt−C, Pt−GDY showed a relative shift of the d-band center toward the VBM, leading to an upward shift of the antibonding DOS states and thus stronger interaction with reactant molecules. This is due to the fact that there are fewer electrons available for filling the antibonding DOS states, that is, increased d-band vacancy of the Pt NPs, which results from the strong electron transfer from Pt NPs to highly conjugated GDY. These results suggest that GDY could act as stabilizer for Pt NPs, which could be attributed to the strong anchoring effect of
hybridized carbon atoms, suggesting that the anchoring of Pt nanoparticles occurred without breaking any covalent bonds. The appearance of the signals of the − C−OH and−CO indicated the occurrence of oxidation of the terminal alkynes (Figure 2b). The Pt 4f core level XPS spectrum of Pt−GDY gives values of 71.5 and 74.8 eV. These binding energies are related to Pt0 (4f7/2 BE 71.0 eV) and Pt2+ (4f7/2 BE 72.6 eV), respectively. As indicated in Figure 2c, Pt2+ suggested the appearance of oxide layers on the Pt NPs surface. Current results indicate that the small Pt nanoparticles are generated by reduction the Pt ion anchored on the GDY surface to the welldispersed nanoparticles.4 Pt L-edge X-ray absorption near edge structures (XANES) were analyzed to investigate the oxidation state of the Pt nanoparicles (Figure 2d). Pt−GDY show fine XANES features resembling those of Pt foil, with the intensity of the “white lines” at 11.56 keV increases a little bit compared to that of the Pt-foil, indicating an increase in the d-band vacancy.48 Fourier transformation of extended X-ray absorption fine structure (FT-EXAFS) further confirms the similar Pt−Pt coordination shells like that of Pt-foil, except of the stronger signal at 0.21 nm, which is ascribed to the oxide species of Pt (Figure 2e). The comparison of GDY and Pt−GDY was made through Raman spectroscopy (Figure S4), where the signal of dialkyl interactions shifted from 2144 cm−1 of GDY to 2112 cm−1 of C
DOI: 10.1021/acsami.8b00566 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a) Illustration of hydrogenation of ethyl-2-oxo-4-phenylbutyrate (EOPB). (b) Comparison of conversion of ethyl-2-oxo-4-phenylbutyrate hydrogenation over Pt−C and Pt−GDY catalysts. Conditions: 1 mM reactant, 5 mg of catalysts, H2 at ordinary pressure, room temperature. (c) Influence of temperature on its conversion. Conditions: 1 mM reactant, 5 mg of catalysts, H2 at ordinary pressure, within 1 h. (d) Influence of initial EOPB concentration on its conversion. Conditions: 5 mg of catalysts, H2 at ordinary pressure, room temperature, within 1 h. (e) Conversion ratio as a function of cycle, with catalyst recovery via precipitation. Conditions: 1 mM reactant, 5 mg of catalysts, H2 at ordinary pressure, 50 °C, within 1 h. TEM images of the as-synthesized (f) Pt−GDY and (g) Pt−GDY after cycling.
Pt nuclei by GDY, presumably due to the high π-conjugated structure of GDY that can interact with the as-formed Pt NPs, which was similar to the case of graphene.55,56 This Pt−GDY showed good stability, which was reflected by almost no change in morphology or size after the as-prepared Pt−GDY was stored at room temperature for 3 years (Figure S2), except that the CO species was increased due to oxidation of the terminal alkynes during the storage (Figure S3). On the basis of the analysis of the electron structure, it is expected that Pt−GDY may show significantly enhanced catalytic activity in hydrogenation of unsaturated hydrocarbons. The catalytic performance of the Pt−GDY was studied with the hydrogenation of ethyl-2-oxo-4-phenylbutyrate (EOPB) as the model reaction (Figure 3a). In CH3CH2OH, high conversion yield about 100% was obtained in the presence of Pt−GDY catalyst in 12 h at room temperature. The hydrogenation activities of Pt−GDY and commercial Pt−C (Pt loading of 20%) are compared concerning the reaction time, reaction temperature, and initial concentration
through controlling variables. In the initial stage, the hydrogenation rate in the presence of Pt− GDY is faster than that of the commercial catalyst (Figure 3b). This result indicates that the activity of Pt−GDY is superior to Pt−C considering the conversion yield in short reaction time (
