Article Cite This: Chem. Mater. 2017, 29, 8670-8678
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Enhanced Stability of Immobilized Platinum Nanoparticles through Nitrogen Heteroatoms on Doped Carbon Supports Wen Shi,†,‡ Kuang-Hsu Wu,† Junyuan Xu,§ Qiang Zhang,∥ Bingsen Zhang,*,† and Dang Sheng Su*,† †
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ‡ School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China § International Iberian Nanotechnology Laboratory, Avenida Mestre Jose Veiga, 4715-330 Braga, Portugal ∥ Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Catalysts in the form of dispersed platinum nanoparticles (Pt NPs) immobilized on carbon usually suffer from deactivation through sintering under reaction conditions. In this contribution, we report the enhanced stability of highly dispersed Pt NPs on surface-modified carbon nanotubes (CNTs) against thermal and electrochemical sintering by N heteroatoms in the N-doped carbon support. The improved antisintering property of Pt NPs under thermal condition is characterized by in situ transmission electron microscopy (TEM), while the stability in electrochemical methanol oxidation reaction (MOR) is further examined at identical location (IL) using an advanced IL-TEM technique. A correlation of the Pt NP growth with the electrochemical surface area (ECSA) and the mass activity in MOR has been inferred. Our results indicate that both the surface oxygen groups and nitrogen-doped species are responsible for the fine dispersion of Pt NPs on the surface-modified CNTs, while the Pt NPs can be effectively stabilized under thermal and electrochemical conditions through the strong metal−support interaction via N heteroatoms. We further reveal that the mass activity of Pt NP is closely associated with the ECSA rather than directly affected by N-doping to CNTs.
1. INTRODUCTION Platinum nanoparticles (Pt NPs) as the active components in heterogeneous reactions have been extensively studied for the excellent performance in industrial petrochemical transformation, fine chemical synthesis, and clean energy development.1−3 By the nature of metal-based nanomaterials, the catalytic NPs are prone to sintering and thus growing under energetic reaction conditions, reducing the exposure of the active sites and significantly breaking down the catalytic activity.4−6 For example, thermal and pressurized conditions and high applied redox potential nearly always lead to deleterious particle sintering and prohibit long-term utilization of the catalyst.7−10 Much effort has been devoted to catalyst design in order to achieve excellent activity while keeping it for long term by using a stable functional support with anchoring sites to allow strong metal−support interaction.11−15 Carbon nanotubes (CNTs) are promising supports for transition metal NPs in various catalytic reactions owing to their high specific surface area, physical and chemical stability, and excellent thermal and electrical conductivity.16−19 The convenient way for recycling of precious metal through combustion also adds to the advantages of carbon-based materials as support for scale-up use.20 Moreover, the inert © 2017 American Chemical Society
nature of pristine CNTs can be tailored through surface modification with functional groups and defects, and doping with a family of heteroatoms. The abundant surface functional groups or doping species can act as anchoring sites for metal NPs, which are beneficial in preventing the metal NPs from coalescence during fabrication and contribute to stabilize the highly dispersed NPs during the catalytic processes.21−26 Among all the surface-modified strategies, nitrogen doping has received particular interest and has been proven to be the most effective one to improve the stability of catalysts because of the strong binding with many transition metals.27−31 In addition, nitrogen species acting as electron donors in the graphitic lattice of CNTs also afford a pivot for tuning the metal−support interaction, considerably broadening its potential as a functional support.32,33 As such, improved intrinsic reactivity and enhanced stability of immobilized metal NPs are commonly observed when N-doped CNTs (NCNTs) are applied as catalyst supports.34−36 For example, Hu et al.37 have reported excellent catalytic activity for lower olefins and Received: June 26, 2017 Revised: September 19, 2017 Published: September 21, 2017 8670
DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
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Chemistry of Materials
with magnetic stirring to obtain a transparent yellow solution. Then the flask was placed in the center of an oil bath pot refluxing at 433 K for 4.0 h with stirring under the protection of N2 atmosphere. After cooling, a dark-brown homogeneous colloidal solution of Pt NPs with a Pt concentration of 1.0 mg mL−1 was obtained. 2.2. Materials Preparation. Pristine carbon nanotubes (CNTs) were grown in a fluidized bed reactor.54 The CNTs with an average diameter of 20 nm were further purified with water-diluted HCl several times to remove the residue (volume ratio is 1:1). The oxygen functionalized CNTs (oCNTs) were obtained by refluxing the pristine CNTs in concentrated HNO3 at 393 K for 4.0 h. The nitrogen-doped CNTs with distinct nitrogen-doping levels prepared by thermal treatment of oCNTs in ammonia at target temperature yielded products donated as NCNTs-T (T is the treatment temperature). The preparation of oCNTs supported Pt NPs catalyst (Pt/oCNTs) was prepared via the incipient wetness impregnation of Pt EG colloidal solution. The general procedure as follows: 100 mg of CNTs dispersed in 5.0 mL of H2O was ultrasonically treated to obtain a homogeneous suspension followed by the addition of as-synthesized Pt NPs containing colloidal solution with vigorous stirring for 2.0 h. Then the pH of the suspension was adjusted to pH = 2.0 by addition of 5.0 mol L−1 HCl aqueous solution with constant stirring at room temperature. The resulting suspension was collected by filter, and the as-obtained powder was washed with ethanol and deionized water until free of Cl− and subsequently dried at 353 K in a vacuum oven overnight. Pt NPs supported on different supports were similar to the description above, which were denoted as Pt/NCNTs-500, Pt/ NCNTs-700, and Pt/NCNTs-900, respectively. 2.3. Characterizations. A FEI Tecnai G2 F20 transmission electron microscope (TEM) equipped with high-angle annular darkfield scanning TEM (HAADF-STEM) detector operated at 200 kV was used for acquiring TEM, high-resolution TEM (HRTEM), and STEM images. The TEM images obtained through IL-TEM method were operated at 120 kV in order to avoid the electron beam irradiation damage. The X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer (D/MAX-2400) with Cu Kα source (λ = 1.5406 Å) at a scan rate of 2 deg min−1. X-ray photoelectron spectroscopy (XPS) spectra were carried out by ESCALAB 250 instrument with Al Kα X-rays as radiation source (1486.6 eV, 150 W, and 50.0 eV pass energy). The charging effects were eliminated by calibrating the observed spectra with the C 1s binding energy (BE) value of 284.8 eV. The valence-band spectra and the work function of the synthesized catalysts were acquired on an ESCALAB 250 instrument with He I as ultraviolet source (hν = 21.22 eV) and a bias of 5 eV. The Brunauer−Emmett−Teller (BET) surface areas of the samples were measured at 77 K using a Micrometrics ASAP 2020 instrument. UV−Raman spectroscopy was performed on powder samples by using a HORIBA LabRam HR Raman spectrometer with an excitation wavelength of 532 nm and a power of 0.2 mW. The actual loading amount of Pt was determined by a Leeman Laboratories Prodigy inductively coupled plasma mass spectrometry (ICP-MS). 2.4. In Situ Heating Experiment. A DENSsolutions heating specimen holder with wildfire nanochips was used in this contribution. The thermal stability of the samples was investigated in TEM from room temperature (RT) to 1073 K with a heating rate of 10 K min−1. 2.5. Catalytic Studies. The electrochemical measurement was performed in a conventional three electrode cell, using a glass carbon working electrode (GCE, ϕ = 3 mm), a graphite rod counter electrode, and a saturated calomel reference electrode (SCE). The catalyst ink was prepared by dispersing 5.0 mg of catalysts in the mixture of 700 μL of ethanol and 300 μL of H2O, followed by the addition of 50 μL of 5.0 wt % Nafion (Alfa Aesar). Then the suspension was ultrasonicated for 10 min to form a homogeneous ink with a Pt concentration of 0.50 mg mL−1. Finally, 5.0 μL of the dispersion was dropped onto the surface of GCE, which was employed as a working electrode after solvent evaporation. All electrochemical performance of the as-synthesized catalysts was characterized with an Epsilon electrochemical station (PAR2273) at room temperature. The electrochemical surface area (ECSA) of Pt for different catalysts was
improved stability on a Fe/NCNT catalyst in a Fischer− Tropsch process, which is ascribed to the strong surface basicity and metal−support interaction. Moreover, Schlögl et al.38 have systematically studied the nature of Pd−N interaction in NCNT catalysts by a combined theoretical and spectroscopy analysis for a comprehensive understanding of the specific metal−support interaction. The N heteroatoms can be also served as anchoring sites for polar intermediates in a working lithium−sulfur battery.39,40 Although there is a growing number of works in the literature on the applications of NCNTs as support in catalysis, they do not yet focus on the direct observation of nitrogen species on the thermal or sintering behavior of supported metal NPs under operation conditions.41,42 Considering the complex materials systems, various operation conditions, and the lack of systematical characterizations, it is difficult to obtain direct evidence for a fair and convincing conclusion. Transmission electron microscopy (TEM) is a useful technique for visualizing morphological and structural changes of nanomaterials due to its high spatial resolution.43−45 Conventional approaches to reveal the degradation process of catalysts are typically characterized by long-term performance evaluation and assisted with post reaction TEM analysis. The conclusions drawn from the extensive statistical analysis based on the comparison between fresh and used catalysts at different locations can be useful, but not always straightforward. Despite the development of environmental TEM (ETEM) and in situ−TEM holder would be possible to provide time-resolved in situ observation, the high-cost and the limited environmental types and also the irradiation damage impede their widespread application.46−48 However, these disadvantages can be well-improved by the introduction of an identical location TEM (IL-TEM) method, which enables the observation of one catalyst at identical locations before and after reaction without any changes to the TEM equipment and also allows one to track the catalysts of different reaction stages at the same location on an atomic scale to further explore the detailed structure evolution process of catalyst during operation conditions.49−51 For the purpose of clarifying the critical role of CNTs with different surface properties as supports for Pt NPs, we characterize the detailed structural evolution of Pt NPs supported on oxidized as well as nitrogen-doped CNTs under thermal and electrochemical conditions by in situ/ILTEM methods. It was observed that both the oxygen functional groups and nitrogen-doped species are responsible for the fine dispersion of Pt NPs, which affords the catalysts with excellent pristine mass activity in methanol electron-oxidation reaction (MOR), while only the nitrogen species can effectively stabilize the Pt NPs under thermal and applied redox potential conditions through the strong metal−support interaction, thus contributing to improved durability for MOR. These findings afford fundamental understanding of catalyst sintering behavior under operation conditions and provide a guidance to rationally design catalysts toward high stability.
2. EXPERIMENT 2.1. Synthesis of Pt Nanoparticles. The Pt nanoparticles (NPs) were synthesized by a facile modified ethylene glycol (EG) method according to the previous literature.52,53 In a typical process, a EG solution of NaOH (15 mL, 0.50 M) was added slowly into the glycol solution containing chloroplatinic acid hexahydrate (H2PtCl6·6H2O > 98.0%; purchased from Sinopharm Chemical Reagent Co., Ltd.) as precursor salts (0.135 g in 50 mL) in a 100 mL round-bottom flask 8671
DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
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Chemistry of Materials measured by conducting cyclic voltammograms (CVs) with a potential range from −0.25 to 1.0 V in a nitrogen-statured 0.50 M H2SO4 aqueous solution at scanning rate of 50 mV s−1, and electrochemical catalytic activity for methanol oxidation was measured by collecting CVs with a potential range from −0.2 to 1.0 V in 0.50 M H2SO4 + 0.50 M methanol aqueous solution at a scanning rate of 20 mV s−1. The ECSA of the as-fabricated catalysts was also characterized by CO stripping experiment. The CVs with a potential range from −0.25 to 1.0 V was carried out in 0.50 M H2SO4 at 50 mV s−1 after CO bubbled into the electrolyte for 30 min and then followed by purging purified Ar into the electrolyte for 30 min to remove redundant CO. The detailed description for the durability tests were conducted as follows: the electrode with catalysts were first put into the aqueous solution of 0.5 M H2SO4 by nitrogen staturation, and the pristine ECSA of the catalysts was obtained by conducting CVs with a potential range from −0.25 to 1.0 V at 50 mV s−1. Then, the catalytic activity in MOR is gained by transferring the electrode with catalysts to the solution of 0.5 M H2SO4 + MeOH and carrying out the CVs with continuous potential sweeps between −0.2 and 1.0 V vs SCE for 25 cycles at a scanning rate of 20 mV s−1. After 25 cycles, the ECSA of the Pt is detected again by putting the electrode with catalysts into the 0.5 M H2SO4 solution. The long time stability was examined by conducting the CVs in the solution of 0.5 M H2SO4 and 0.5 M H2SO4 + MeOH alternatively, and after every 25 cycles in methanol, the ECSA of the Pt was examined and the value was calculated based on the integrated Coulombic charge of the hydrogen desorption peak according to the formula ECSA = QH/WPt × 0.21 (QH is the measured total charge (μC) for hydrogen desorption after excluding the double-layer region, WPt is the real loading amount of platinum, and 0.21 is the charge (mC cm−2) required for standard Pt electrode to oxidize a monolayer of hydrogen on a bright Pt surface). The electrochemical performance for MOR was evaluated in terms of oxidation current for the forward peak and was normalized to Pt loading in order to compare the mass activity of the catalysts at different reaction stages. 2.6. Durability of Catalysts Investigated by IL-TEM Method. A diluted catalysts ink was sonicated and pipetted onto a gold TEM grid (ϕ = 3.0 mm) with alphabetical code. After the TEM images of the initial state of catalysts were taken, the TEM grid was then transferred and attached to the top of the glass carbon working electrode. Accelerated durability testing was carried out by applying cyclic potential range from −0.2 to 1.0 V at 100 mV s−1 in 0.50 M MeOH + 0.5 M H2SO4 for 150 cycles. Then the grid was transferred to the electron microscope again to acquire the TEM images of the final state of catalysts at identical locations.
Figure 1. STEM images, EDX element maps (the areas within white markings), and coresponding PSD histograms of Pt/oCNTs (a) and Pt/NCNTs-500 (b) catalysts.
3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. The morphology of as-prepared oCNTs and NCNTs-T supports with different nitrogen-doping levels was characterized by transmission electron microscope (TEM), as shown in Supporting Information Figure S1. The samples are in a well-defined tubular structure without obvious damage on the tube wall. Such observation is consistent with the slight changes in the ID to IG ratio of the samples in the Raman spectra (Supporting Information Figure S2a and Table S1). N2 absorption− desorption isotherm was employed to measure the BET surface area and pore size distribution of the surface-modified CNTs. All samples exhibit a similar BET surface value and similar pore size distribution (Supporting Information Figure S2b and Table S1). The surface property of CNTs is revealed by X-ray photoelectron spectroscopy (XPS). In Figure S3 and Table S2, the spectra of O 1s exhibit that the treatment by concentrated nitric acid introduces some oxygen functional groups to decorate the graphitic surface of CNTs. With the increasing treatment temperature from 773 to 1173 K under
Figure 2. XRD patterns of the pristine CNTs, Pt/oCNTs, and Pt/ NCNTs-500 catalysts.
Figure 3. XPS spectra of Pt 4f (a) for Pt/oCNTs and Pt/NCNTs-500 catalysts and N 1s (b) for NCNTs-500 and Pt/NCNTs-500 samples. 8672
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Figure 4. TEM images of Pt/oCNTs (a−d) and Pt/NCNTs-500 (e− h) catalysts at temperatures of 293 (a, e), 573 (b, f), 873 (c, g), and 1073 K (d, h), respectively. Panels i, j, k, and l are the corresponding PSD histograms.
ammonia atmosphere, the oxygen functional groups on the surface of CNTs decrease seriously, which is ascribed to their less thermal stability at high-temperature treatment. Similarly, the drop in the nitrogen content on the surface of CNTs can also be observed in the N-doped CNTs prepared at high temperature. The decreased nitrogen-doping level is due to the decomposition of oxygen groups at high temperature, which are responsible for the formation of a C−N bond.55 The reduced oxygen and nitrogen content on the surface of CNTs leads to the low functionalization of CNT supports. In addition, three kinds of oxygen functional groups (carbonyl group at 531.0 eV, carboxyl group at 532.1 eV, and hydroxyl group at 533.3 eV) and four kinds of nitrogen groups (pyridine at 398.4 eV, amine or pyrrole at 399.6 eV, graphitic N at 400.7 eV, and N oxide at 402.3 eV, respectively) coexist on the surface of NCNT-T supports.56,57 The highest nitrogen content was obtained in NCNTs-500. The actual Pt content in the as-prepared catalysts determined by inductively coupled plasma−optical emission spectrometry are all close to 10 wt %. The morphology and structure of the catalysts were characterized by TEM. The representative highangle annular dark-field scanning TEM (HAADF-STEM) images are shown in Figure 1. The Pt NPs were uniformly dispersed on the oxygen functionalized as well as nitrogendoped CNT supports. The mean particle size distribution (PSD) of Pt/oCNTs and Pt/NCNTs-500 catalysts were estimated to be 1.5 and 1.6 nm, respectively. A small mean particle size of Pt NPs with narrow PSD was observed for Pt/ oCNTs sample with further analysis based on the statistic which may be attributed to the high surface functionalization of the support, providing abundant anchor sites for Pt NPs.
Figure 5. CV curves of the as-synthesized catalysts in 0.5 M H2SO4 + 0.5 M methanol aqueous solution (a), mass activities of the Pt/oCNTs and Pt/NCNTs-500 catalysts in 0.5 M H2SO4 + 0.5 M methanol aqueous solution during a stability test (b), and the mass activities of Pt vs ECSA after different cycle numbers (c).
Meanwhile, STEM−energy dispersive X-ray spectroscopy (EDX) elemental maps of the as-prepared catalysts in Figure 1 confirm the existence of element O or N and Pt in the final products. Typical selected area electron diffraction (SAED) patterns were also acquired to reveal the crystal structure of the catalysts. Obvious diffraction rings for the CNTs were observed in Figure S4. The ambiguous isolated diffraction spots inhibited by diffraction rings of CNTs demonstrate a polycrystalline structure of Pt NPs and the measured lattices of 0.196 and 0.223 nm for Pt nanocrystals from HRTEM images in Figure S4d are corresponding to (111) and (200) crystal planes of face-centered cubic (FCC) phase Pt, respectively. The TEM results for the other catalysts with different nitrogen-doped levels are shown in Figures S4 and S5. The crystalline structures of the catalysts were also investigated by XRD (Figure 2). All catalysts exhibit typical diffraction peaks of CNTs at the 2θ values of ca. 25.8°, 42.2°, 8673
DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
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Figure 6. STEM (a, e), TEM (b, f), HRTEM images and corresponding PSD histograms (c, g) of Pt/oCNTs-used (a−c) and Pt/NCNTs-500-used (e−g) catalysts.
Figure 7. TEM images of Pt/oCNTs (a, b) and Pt/NCNTs-500 (c, d) catalysts at identical locations before (a, c) and after (b, d) MOR reaction. The corresponding PSD histograms (e, f, g, and h) were obtained by quantifying a series of TEM images at identical locations.
The chemical state of the supported Pt and the metal− support interaction were investigated by XPS. The Pt 4f XPS spectra of the series Pt-based catalysts in Figure S7a exhibit two main peaks at the binding energies (BEs) of about 71.8 and 75.1 eV corresponding to the spin−orbit split doublet of Pt 4f 7/2 and Pt 4f 5/2 (with a splitting energy of 3.3 eV), respectively. The spectra were deconvoluted into three components, namely, Pt0 (peaks at 71.6 and 74.9 eV), Pt2+ (peaks at 72.5 and 75.8 eV), and Pt4+ (peaks at 73.9 and 77.2 eV). From the deconvoluted results, it can be seen that slight differences for the Pt0 component are observed for the Pt/oCNTs and Pt/ NCNTs-500 samples and a higher Pt0 content is present on the
44.5°, and 54.5°, which are indexed to the graphitic-type planes of (002), (004), (101), and (110), respectively. The diffraction peaks at ca. 39.7°, 46.2°, and 67.4° are ascribed to the (111), (200), and (220) crystal planes of Pt NPs with FCC structure and a rather broad diffraction peak observed at 39.7° indicates that Pt NPs are all with small size. This is in agreement with the structure feature reflected by TEM results. Besides, an obvious sharp diffraction peak belonging to the (111) plane of Pt appears obviously with the diminishing functionalization of CNT supports (Figure S6), implying some large Pt NPs formed on the surface of CNTs as a consequence of reduced anchoring sites. 8674
DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
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nitrogen species contribute to stabilize the Pt NPs from preventing their migration and aggregation under thermal condition. Such stabilization effect is ascribed to the higher thermal stability of nitrogen species themselves as well as the presence of Pt−N bonding at the metal−support interfaces, as revealed by XPS analysis. The observation is also in agreement with the reported work summarized by Xia.65 The in situ TEM images of the Pt/oCNTs, Pt/NCNTs-500, Pt/NCNTs-700, and Pt/NCNTs-900 catalysts of other regions under thermal conditions are shown in Figure S11. 3.3. Electrochemical Stability in Methanol Electrooxidation Characterized by IL-TEM. The MOR was used as a probe reaction to study the structure−performance relationship of the oxidized as well as nitrogen-doped CNTs supported Pt NPs catalysts, respectively. The ECSAs of the catalysts were first measured by conducting CVs in 0.50 M H2SO4 solution as shown in Figure S12a. The ECSA values of Pt/oCNTs and Pt/ NCNTs-500 catalysts were calculated to be 241.9 and 234.2 m2 gPt−1 based on the integrated Coulombic charge of the hydrogen desorption peak,66,67 which are much higher than that of commercial Pt/C catalyst (49.8 m2 gPt−1). The higher ECSA can be a result of the small particle size and fine dispersion of the Pt NPs. The as-synthesized catalysts together with commercial Pt/C electro-catalyst were both evaluated for MOR. All CV curves in MOR consisted of two main peaks: one peak observed at 0.6 V in the forward scan (If) is ascribed to the oxidation of methanol molecular, and the other at 0.4 V in the reverse scan (Ib) is related to the oxidation of accumulated intermediates (mainly absorbed carbonaceous and OH species).68,69 The ratio of If to Ib has been proposed as an index to evaluate the tolerance of catalyst to CO poisoning in MOR. Higher If/Ib value indicates a higher CO tolerance, which means a better ability of removing carbonaceous species on the Pt NP surface. Pt/oCNTs catalyst exhibits a similar onset peak potential and CO tolerance as compared with Pt/ NCNTs-500 (Figure 5a). The nitrogen incorporation does not improve the CO tolerance as previously reported,70,71 and the result is further supported by the constant peak potential of CO oxidation in CO stripping (Figure S12b), which may be attributed to the low nitrogen content and the high Pt loading. Therefore, the electrocatalytic activity of Pt/oCNT and Pt/ NCNT-500 catalysts in MOR can be compared in terms of the methanol oxidation peak current for the forward peak. A better pristine mass activity was found for the Pt/oCNT catalyst due to a better dispersion of Pt NPs. Thus, the catalytic performance of the oxidized as well as nitrogen-doped CNTs supported Pt catalysts is closely associated with the dispersion of supported Pt NPs rather than that induced by N-doping. Detailed investigation for the other catalysts was gained by analyzing the onset oxidation potential, forward peak potentials, and backward potentials as summarized in Table S4. The durability tests for the Pt/oCNTs and Pt/NCNTs-500 catalysts are probed to investigate the different surface properties of CNTs on the stability of the Pt-based catalysts under electrochemical process. The rapid deactivation along with the cycle number for the Pt/oCNT catalyst is observed (Figure 5b), which is consistent with the dramatic decrease in ECSA (Figure S13a), while the catalytic activity of Pt/NCNT500 catalyst gradually declines in the durability test followed by the slowly dropped ECSA (Figure S13b). The correlation between the ECSA and the mass activity for the degradation process can be established (Figure 5c). The collected data for the Pt/oCNT and Pt/NCNT catalysts all stand around the
NCNTs-500 support (Figure 3a and Table S3) because of the nitrogen participation. Accordingly, there is a noticeable change of the nitrogen distribution in NCNTs-500 from the N 1s XPS spectra before and after Pt loading (Figure 3b and Table S3). A corresponding decrease in the pyridinic/pyrrolic nitrogen species content was observed together with an increased amount of graphitic N. The regular presence of increased Pt0 along with the decreased pyridinic/pyrrolic N on N-doped CNTs is in accordance with the DFT calculation results.58−61 Through systematic calculation, Holmes et al.62 demonstrated that Pt nucleation is more favorable on pyridinic- and pyrrolictype dopants and the electronic structure of supported Pt can be modified through charge donation from the dopant to the Pt, thus attributing to a down-shift in the Pt D-band center. Therefore, the valence-band spectra for the prepared samples was acquired to get a fundamental understanding of the electronic interaction (Figure S8a). However, almost no differences for the valence-band spectra of the compared samples were observed, which may be attributed to the contaminated surface, on the one hand, and, on the other hand, the equipment resolution. The work function (Φ = hν − (EFermi − ECut)) can be detected by applying a bias (Figure S8b), and the results show that a lower work function was detected on Pt/NCNTs-500 sample than that on Pt/oCNTs, indicating that Pt/NCNTs-500 is more easy to give electrons and has a more reduced surface state, consistent with the XPS results.63,64 Thus, a hypothesis that the pyridinic/pyrrolic nitrogen species on the surface of N-doped CNTs being the electron donor for the coordination of Pt at the metal−support interface can be proposed. When the lone-pair electrons are shared with the metal, there is a reduction in the electron density and hence results in a shift to a higher binding energy, i.e., 401 eV (graphitic N). This phenomenon is also noticed for other nitrogen-doped catalysts as shown in Figures S7c and S8 and Table S3, and a more detailed analysis of the error bars of the X-ray fitted results is shown in Figure S9. 3.2. In Situ Heating in TEM. The thermal stability of modified CNT supported Pt NPs was visually investigated by TEM in situ under heating to explore the effect of oxygen functional groups or nitrogen dopants to the antisintering ability. The TEM images of Pt/oCNT and Pt/NCNTs-500 were taken at different stages of heating under high vacuum, as shown in Figure 4, and the corresponding PSD histograms of the Pt NPs on the catalysts are also provided. The Pt NPs supported on oCNTs exhibit an obvious growth with the mean particle size increased from 1.6 to 4.2 nm, when the temperature is increasing from 293 to 1073 K (Figure 4a−d). In contrast, the Pt NPs supported on NCNTs-500 only exhibit a slight increase in the mean particle size from 1.7 to 2.1 nm (Figure 4e−h) during the in situ thermal treatment. The results strongly demonstrate the nitrogen species on the surface of CNTs play a critical role in stabilizing Pt NPs under thermal condition. Moreover, along with a reduced nitrogen content of the supports, the mean size of the Pt NPs also shows a tendency to increase at the elevated heating temperatures. The results are shown in Figure S10. The obviously grown Pt particles on the acquired TEM images are highlighted with green circles, and particle dissolution is marked with blue circles. The number of green circles increases with the diminishment of nitrogen content on the CNTs surface, which shows a similarity with regular statistical results. Clearly, there is a close relationship between nitrogen content and particle growth upon heating. This means that the surface 8675
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N- and O-modified CNTs under thermal and electrochemical conditions by in situ TEM and IL-TEM techniques. Both the oxygen groups and nitrogen species are responsible for the fine dispersion of supported Pt NPs, which afford the electrocatalysts with high ECSA and excellent pristine mass activity in methanol electro-oxidation reaction. However, the thermal stability and electrochemical stability of the Pt NPs could only be achieved through nitrogen doping to the CNTs as a consequence of the strong metal−support interaction. Because of the thermal stability of nitrogen species themselves as well as strong chemical binding with Pt, the migration and agglomeration of Pt NPs on the surface of nitrogen-doped CNTs were strongly impeded as intuitively revealed by in situ heating process in TEM. The durability test of the catalysts in MOR reaction studied by IL-TEM method uncovered the different ways degradation procedures occurred on the oxidized as well as nitrogen-doped CNTs supported Pt NPs catalysts. The obvious antisintering ability of the supported Pt NPs under an oxidizing potential is achieved by nitrogen participation through inhibiting the formation of irregular large NPs, which leads to the dramatically reduced ECSA and deactivation of the catalyst. Therefore, the current study provides several visualized characterizations to understanding the fundamentals of catalysts sintering behavior involving carbon materials as supports and sheds light on the strategies to design carbon-based catalysts with high stability through tuning the interaction between the surface-modified carbon and supported metal NPs.
well-fitted curve. Only a difference in the deactivation rate for the two counterparts indicates the reactivity degradation is mainly caused by the diminished ECSA of supported Pt. The results are also supported by the post TEM analysis as well as EDX maps results from the used catalysts in Figure 6 amd S17a,b. A significant difference in the PSD of Pt NPs was observed. The highly dispersed Pt nanocrystals on oCNTs have grown into larger NPs with a mean particle size of 6.5 nm, with a small mean particle size of 2.8 nm for Pt/NCNTs-500 after the durability test. The results strongly declare that the incorporation of nitrogen can decrease the sintering speed of supported Pt NPs occurring under electrochemical conditions and contribute to the enhanced stability of the catalyst in MOR. For comparison, the durability test involving catalysts with different nitrogen containing levels and commercial Pt/C and the corresponding post reaction TEM characterizations are shown in Figures S14−16 and S17c,d. All results further exhibit that the mass activity of Pt NP is closely associated with the ECSA rather than directly affected by N-doping to CNT, and an enhanced stability of Pt under electrochemical conditions can be obtained through nitrogen doping. IL-TEM was employed to directly observe the sintering behavior of Pt NPs on the catalysts at identical location before and after the electrochemical MOR. The TEM images at selected locations are shown in Figures 7, S18, and S19 and the red arrows are used to recognize the same locations. The carbon supports almost retained their structure and shape after electrochemical treatment; therefore, the deactivation of catalysts is not probably caused by the carbon corrosion under the applied condition. Furthermore, a notably different behavior for the supported Pt NPs is observed for the two catalysts after electrochemical process. The disappearance of Pt particles with various sizes and the change of their positions are directly observed on the Pt/oCNT sample, which are related to the migration of Pt NPs on the surface of CNTs due to a weak metal−support interaction. Besides, along with the irregular larger particles with a size more than 10 nm (marked with yellow circles) formed on the CNTs as a consequence of migration and agglomeration, a few small-sized Pt particles (highlighted with blue circles) appeared simultaneously, which is similar to the Ostwald-ripening-type degradation.4,72 The Pt dissolution occurred at the operation conditions, and the dissolved Pt re-deposited on the existing Pt NPs or the support, contributing to formation of the large- or small-sized NPs. Therefore, the rapid decline of the ECSA as well as mass activity for the Pt/oCNTs as displayed by the ex situ durability test are attributed to the large formed Pt NPs on the surface of oCNTs. Whereas the Pt NPs on NCNTs-500 process a homogeneous growth from a mean particle size of 1.6-2.8 nm, the barely changed position of the Pt NPs suggesting the particle coalescence as a main rule occurred at the Pt NPs adjacent to each other due to the high loading amount, and further growth is suppressed by the strong metal−support interaction induced by the Pt−nitrogen bonding at the interface. As mentioned above, the ways and the degree of degradation may be a consequence of their different metal− support interfacial interaction and an overall enhanced stability under electrochemical conditions in MOR was achieved on Pt supported on CNTs through nitrogen doping.
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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/acs.chemmater.7b02658. Characterization of the oxygen functional as well as nitrogen-doped CNTs supports and the corresponding supported Pt catalysts including TEM images and corresponding SAED, N2 adsorption−desorption isotherms, pore size distributions, and Raman and XPS analyses, results from in situ heating experiment, catalytic performance of the surface-modified CNTs supported Pt NPs catalysts in electron methanol oxidation reaction, and TEM results obtained from the IL-TEM method (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(B.Z.) E-mail:
[email protected]. Tel.: 86-24-83970027. Fax: 86-24-83970019. *(D.S.S.) E-mail:
[email protected]. Tel.: 86-24-23971577. Fax: 86-24-83970019. ORCID
Kuang-Hsu Wu: 0000-0002-7670-7948 Qiang Zhang: 0000-0002-3929-1541 Bingsen Zhang: 0000-0002-2607-2999 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant Nos. 91545119 and 21773269), the Youth Innovation Promotion
4. CONCLUSION We have probed the effect of nitrogen doping on the antisintering ability of highly dispersed Pt NPs supported on 8676
DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
Article
Chemistry of Materials
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Association CAS (Grant No. 2015152), the Joint Foundation of Liaoning Province National Science Foundation and Shenyang National Laboratory for Materials Science (Grant No. 2015021011), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09030103).
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DOI: 10.1021/acs.chemmater.7b02658 Chem. Mater. 2017, 29, 8670−8678
