Synthesis of Well-Defined Pt-Based Catalysts for Methanol Oxidation

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Synthesis of Well-defined Pt-based Catalysts for Methanol Oxidation Reaction Based on Electro-hole Separation Effects Can Li, Yunteng Qu, Lei Du, Guangyu Chen, Shuaifeng Lou, Yunzhi Gao, and Geping Yin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00377 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Sustainable Chemistry & Engineering

Synthesis of Well-defined Pt--based Catalysts for Methanol Oxidation Reaction Based on Electro-hole Separation Effects Can Li a, †, Yunteng Qub, †, Lei Du a, Guangyu Chen a, Shuaifeng Lou a, Yunzhi Gao a*, Geping Yin a** C. Li, Dr. L. Du, Dr. G. Chen, Dr. S. Lou, Prof. Y. Gao, Prof. G. Yin aMIIT

Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, Harbin Institute of Technology, Harbin 150001, China; bCenter

of Advanced Nanocatalysis (CAN) and Department of Chemistry, University of

Science and Technology of China, Hefei 230026, China. †C.

L and Y. Q contributed equally.

E-mail: [email protected] (Y. Gao), [email protected] (G. Yin). Corresponding authors. Tel.: + 86 451 86403961; fax: +86 451 86403807.

Abstract In this work, we develop a green and facile photo-reduction strategy to fabricate Pt nanoparticles, which uniformly distributed on the graphitic carbon encapsulated titanium oxide nanorods (TNR@GC). The TNR@GC exhibits enhanced electron-hole separation effect due to the existence of trivalent titanium and excellent electric conductivity arising from ultra-thin carbon layer (1.5 nm). The excited electron from TNR can rapidly migrate to the surface of ultra-thin carbon layer and thus reduce the Pt ion, forming well-defined Pt NPs on carbon layer. The obtained p-Pt/TNR@GC catalyst shows impressive activity and stability for methanol electro-oxidation, exhibiting almost 3-times higher mass activity (1.12 A·mg1Pt)

compared with Pt/C. Furthermore, this strategy can be extended to access a series of

noble metal function materials, including Au, Ag and Pd. This work opens a new prospect to prepare functional nanomaterials with environmentally friendly and energy saving.

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Keywords: photo-reduction, enhanced electron-hole separation effect, ultra-thin carbon layer, Pt nanoparticles, methanol electro-oxidation

Introduction Noble metals nanoparticles are widely used due to the unique quantum-size effect and high catalytic activity in various fields, including environmental remediation, photochemistry, biomedicine, and electrochemistry, etc[1-11]. A series of physical and chemical methods have been applied to prepare noble metals nanoparticles, such as wet chemical reduction, microwave heating, electrochemical reduction, sputtering, sol-gel and hydrothermal, etc[1219]. However, the high-energy consumption and the pollution of reducing agent have severely impeded their widespread application in the actual production. To develop an efficient environment-friendly and energy-saving strategy used in the manufacture of well-dispersed noble metal nanoparticles has become an attractive area for researchers[20-24]. Recently, the exploration that photo-deposition noble metals nanoparticles under ultraviolet light irradiation has attracted significant interest due to its mild preparation process[25-28]. However, the noble metal nanoparticles directly supported on the surface of titanium dioxide, meaning poor conductivity and thus leading to an obstacle to its application in the field of electro-catalysis[29-33]. Furthermore, the photogenerated electron-holes easily recombined subsequently for the anatase phase of TiO2, thus reducing the quantum efficiency of photocatalysts [34]. Hence, surface modification of TiO2 has become an attractive direction to enhance electron-holes separation effect. Herein, we present a rational-designed photo-reduction strategy, deriving from enhanced electron-hole separation effect, to fabricate well-dispersed Pt nanoparticles. As shown in Figure 1, the titanium oxide nanorods were partially reduced to form Ti3+ by ultra-thin carbon layer, thus narrowing the band gap. In addition, the ultra-thin carbon layer changed the electronic structure and improved the conductivity of titanium oxide[35]. Under ultraviolet 2 ACS Paragon Plus Environment

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(UV) irradiation, the excited electron from TNR can rapidly migrate to the surface of ultrathin carbon layer and thus reduce the Pt ion, forming well-defined Pt NPs on carbon layer (named as p-Pt/TNR@GC). This observation is continent with previous work[36]. No surprisingly, the as-prepared p-Pt/TNR@GC shows efficient and stable electro-catalytic performances for the methanol electro-oxidation (MOR). Moreover, this photo-reduction strategy can also be applied to other noble metals, for example, Au, Pd and Ag, as demonstrated in this study[37, 38].

Figure 1. Schematic illustration of the fabrication of p-Pt/TNW@GC catalysts

Results and discussion Transmission electron microscopy (TEM) characterization (Figure 2a) indicates the uniformly Pt nanoparticles dispersed on the TNR@GC. The histogram of particle size distribution in Figure 2a reveals a mean diameter with 2.07 nm for Pt NPs. As shown in Figure 2b (and Figure S1), the anatase TiO2 nanorods is covered by graphitic carbon layer, and the thickness is approximately 1.5 nm. Furthermore, we can observe the d-spacing for anatase TiO2 is 0.35 nm, and 0.23 nm for the Pt nanoparticles, respectively. The high-angle annular dark-field scanning TEM (HAADF-STEM) images further confirmed the uniform distribution of Pt NPs (Figure 2c and 2d). Figure 2e shows the elemental mappings of the p-Pt/TNR@GC, indicating that the Ti, C, O and Pt elements are homogeneously dispersed in p-Pt/TNR@GC. 3 ACS Paragon Plus Environment


Figure 2f displays the representative XRD patterns, TNR@GC presents the remarkable diffraction peaks at 23.3 °, 37.8 °, 48.1 °, 54.0 ° and 62.8 °, revealing the crystal type is anatase TiO2. This observation indicates the carbon layer could successfully restrain the transformation of nano-scaled anatase phase to rutile phase under 800 °C. After photoreduction, p-Pt/TNR@GC shows the (110) crystal type Pt, with sharp peaks located at 39.7 °, 46.2 °, 67.4 ° and 81.3 °, respectively. Furthermore, the broad peaks suggested the existence of Pt nanoparticles, in line with the TEM results. Together, these results confirms strengthen evidence of the formation of well-dispersed Pt nanoparticles using the photo-reduction strategy.

Figure 2. (a) Typical TEM images of p-Pt/TNR@GC and associated particle size distribution, (b) HRTEM image of p-Pt/TNR@GC, (c) (d) HAADF-STEM images, (e) Element mappings of p-Pt/TNR@GC corresponding to the Figure 2(d), (f) XRD patterns of the as obtained TNR@GC and p-Pt/TNR@GC.


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To investigate the effect of carbon layer, we fabricate TNR@GC supports with the identical condition except the Ti/C mass ratios, including 1:1, 1:3 and 1:5. The actual carbon ratio of the TNR@GC composite is detected by thermogravimetric analysis (TGA), as shown in Figure S2. The final graphitic carbon content of the sample was calculated to be 8.2 wt%, 19.5 wt% and 47.8 wt% for Ti/C mass ratio 1:1, 1:3 and 1:5 , respectively. The compositions of TNR@GC is conducted by Raman spectroscopy. As shown in Figure S3, the peaks at 1600 cm−1 can be assigned to G band, providing information on in plane vibrations of sp2 bonded carbon, while the D band (peaks at 1333 cm-1) is a common feature of sp2 units adjacent to structural defects[39]. The ID/IG value of Ti/C mass ratio (1:1, 1:3, 1:5) each sample is calculated to be 0.83, 0.85 and 0.87, respectively. The higher ID/IG ratio indicates possessing the higher density of defects are present in the carbon layer, this structure is conductive to anchored noble metal nanoparticles. Figure S4 shows the Brunauer-Emmett-Teller (BET) surface area value of TNR@GC-1:3 is 326.8 m2 g-1. The XPS spectrum of Ti 2p is shown in Figure 3a, which is composed of two peaks. The obvious negative shift can be observed with the increase of C contents, which can be contributed to the existence of possible Ti3+ species[40, 41]. Note that the thicker carbon layer prevented the detection of Ti when Ti/C ratio arrives to 1:5. Figure S 5a shows the deconvoluted XPS spectrum of Ti 2p, the peak of Ti4+ 2p3/2 shifts sharply to 457.9 eV for TNR@GC-1:3 (Figure. S5b), reconfirming the presence of Ti3+ in the sample. Notably, two peaks located at 457.9 and 463.4 eV were attributed to be 2p3/2 and 2p1/2 core levels of Ti3+. The ratio of Ti3+/Ti4+ is ~ 18 % in TNR@GC-1:3, which is obtained from the proportion of Ti3+ and Ti4+ in Figure. S5b. To further verify the presence of Ti3+, electron paramagnetic resonance (EPR) is employed to investigate the TNR@GC-1:3 and anatase TiO2 sample, the black line for the anatase TiO2 shows a negligible EPR signal of g = 2.005, while a hallmark signal peak for the TNR@GC-1:3 sample is observed. This observation confirms the existence of trivalent Ti in bulk TNR@GC-1:3 sample [42].



Figure 3. (a) Ti 2p XPS spectroscopic spectra of the different ratios TNR@GC samples, (b) Electron paramagnetic resonance (EPR) spectra of anatase TiO2 and TNR@GC-1:3 samples, (c) UV-vis absorption spectra and (d) Calculated band gap energy of the pure anatase TiO2 and different ratios TNR@GC samples.

UV Vis absorption spectrum has been measured to evaluate the photochemical activities of the as-prepared TNR@GCs nanocomposites (named as 1:1, 1:3, 1:5), together with pure anatase TiO2 (Figure 3c) [43, 44]. The TNR@GC samples show strong optical absorption in both UV and visible light region with an increasing loading amount of carbon coated, which is agreed with the change of color from white to black (Figure 3c inset). All of this indicates the enhanced range of spectrum response and light absorption ablility of TNR@GC, arising from the ultrathin carbon layer [45]. Meanwhile, the TNR@GC samples show red shift in the UV-vis absorption spectra with respect to the pure anatase TiO2, which suggested the narrowed band gap [46]. Moreover, the band gap of the TNR@GC samples (3.0 eV for 1:1, 2.5 eV for 1:3 and 1:5 is too weak to count) show lower than that of pure anatase TiO2 (3.2 eV) (Figure 3d). Based on UV-vis and XPS results, the Ti3+ concentration of TNR@GC is 6 ACS Paragon Plus Environment

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high enough to create a lower band gap. Subsequently, the electrochemical impedance spectra (EIS) was used to investigate the interfacial charge dynamics and recombination behavior (Figure S7). The TNR@GC-1:3 shows a smaller arc radius than pure anatase TiO2, suggesting the higher efficiency for photogenerating electron–hole pairs and interfacial charge transfer rate. As expected, the narrowed band gap of TNR@GC is beneficial to photogenerate electron-hole pairs[47-49]. The ultra-thin carbon layer ensure the excellent conductivity and strong interaction between Pt and support for p-Pt/TNR@GC. These features frame the p-Pt/TNR@GC as an ideal catalyst for the small organic molecule (such as methanol) electro-oxidation, which is paramount for direct liquid fuel cells. To investigate the electrocatalytic activity of the pPt/TNR@GC, cyclic voltammetry experiments were performed to investigate methanol electro-oxidation. Figure.4a depicts cyclic voltammogram curves of commercial Pt/C (20 wt %) and p-Pt/TNR@GC catalysts in an Ar-saturated 0.5 M HClO4. The electrochemical active specific surface areas (ECSA) of p-Pt/TNR@GCs are calculated according to the charge of the hydrogen adsorption/desorption (HAD) integrals, as shown in Equation (1): (1) QH is the hydrogen desorption charge, 0.21 mC cm-2 is the electrical charge related to the monolayer hydrogen adsorbed on Pt, and [Pt] is the platinum loading on the working electrode surface. Based on the ICP-OES measurement (Figure S6), the Pt loading is 22.5, 10.2 and 1.3 wt % for Ti/C content ratio 1:1, 1:3 and 1:5, respectively. The p-Pt/TNR@GC1:3 shows a higher ECSA (62.6 m2 g-1) with respect to p-Pt/TNR@GC-1:1 (46.3 m2·g-1) and p-Pt/TNR@GC-1:5 (negligible). The methanol electro-oxidation is measured in an Arsaturated 0.5 M CH3OH+ 0.5 M HClO4. As can be seen in Figure 4b, p-Pt/TNR@GC-1:3 catalyst exhibits the highest MOR activity. The forward peak mass current densities for pPt/TNR@GC-1:3 is 1.12 A·mg-1Pt, which is almost three-fold as high as commercial Pt/C 7 ACS Paragon Plus Environment


catalysts (0.38 A·mg-1 Pt). To investigate the durability of the p-Pt/TNR@GC-1:3, the electrodes are conducted by repeating CVs for 1000 cycles of accelerated potential cycling tests (APCT) in the range of -0.7–0.45 V (vs Hg/Hg2SO4) in 0.5 M HClO4+ 0.5 M CH3OH. Figure 4c shows the mass activity retention ratio for catalysts. After the degradation test, the mass activity is 0.91 A·mg-1 Pt (82% retention of the initial sample) for p-Pt/TNR@GC-1:3, compared with only 0.23 A·mg-1 Pt (61%) for Pt/C. This result shows that p-Pt/TNR@GC-1:3 has a remarkably higher stability than Pt/C. Subsequently, the stability of the catalyst is also explored by chronoamperometry (CA). The CA curves for MOR at a constant potential (at 0.15 V vs Hg/Hg2SO4) for 2400 s [50]. The p-Pt/TNR@GC-1:3 exhibits a slower current attenuation in comparison with Pt/C, indicating a stronger resistance to the CO-poisoning (Figure 4d). Additionally, the residual current of p-Pt/TNR@GC-1:3 is 3.5 times as high as that of Pt/C, confirming its better electroactivity for MOR.


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Figure 4. (a) Cyclic voltammogram of Pt/C and different p-Pt/TNR@GCs in 0.5 mol·L-1 HClO4 and (b) in 0.5 mol·L-1 HClO4+ 0.5 mol·L-1 CH3OH, scan rate: 50 mV·s-1. (c) Simultaneously recorded CV of catalysts. Relationship of normalized peak current density and cycle number of catalysts (d) Chronoamperometry curves of Pt/C and p-Pt/TNR@GC-1:3 (-0.15 V vs Hg/Hg2SO4).

On the basis of these observations, we postulate that a strong metal / support interaction exists between Pt NPs and TNR@GC (Figure 5a). Here we also evaluate the ability of CO antpoisoning of catalysts. The CO stripping tests show the p-Pt/TNR@GC-1:3 exhibits an obviously more negative onset (about 40 mv) and peak potential compared with Pt/C (Figure 5b), indicating faster reaction kinetics for CO oxidation[51, 52]. The enhanced activity and durability can be assigned to the strong interaction between Pt NPs and support, as determined by XPS results (Figure 5b). A clear positive shift of in the binding energy of Pt 4f7/2 for pPt/TNR@GC-1:3 with respect to the Pt/C indicating the stronger interaction between anchored Pt atoms and the TNR@GC supports[53, 54]. Moreover, to quantify the difference, 9 ACS Paragon Plus Environment


the predominant peaks of Pt 4f are deconvoluted into two pairs of doublets (Pt(0) and Pt(Ⅱ)) (shown in Table 1). It can be observed that the binding energy of Pt/C is lower than that of pPt/TNR@GC-1:3, which confirms the strong interaction exists between anchored Pt atoms and the TNR@GC [55]. Thus, a faster MOR reaction was obtained for p-Pt/TNR@GC-1:3 catalyst than that of Pt/C [56].

Figure 5. Schematic drawing representing the cross-section of p-Pt/TNR@GC-1:3, (b) The CO-stripping voltammogram of p-Pt/TNR@GC-1:3 and Pt/C, (c) XPS spectrum of Pt 4f performed on p-Pt/TNR@GC1:3 and Pt/C.


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Table 1. Platinum component (%) of the following catalysts:

Catalyst

Pt/C

p-Pt/TNR@GC-1:3

Pt(0) 4f7/2

71.9 eV

71.1 eV

Pt(0) 4f5/2

75.4 eV

74.6 eV

Content

63.3 %

57.6 %

Pt(Ⅱ) 4f7/2

73.8 eV

72.2 eV

Pt(Ⅱ) 4f5/2

77.3 eV

75.7 eV

Content

36.7 %

42.4 %

We expand the photo-redution strategy to prepare other noble metal nanoparticles. As shown in Figure. 6a-c, the uniformly dispered noble metal nanoparticles (Pd, Au, Ag) on the TNR@GC support. The exact quantities of metal loading on each sample quantified was found to be 11.4, 13.5 and 22.3 wt %, respectively. The corresponding histograms of particle size distribution ( inset Figure. 6a-c) reveal a narrow particle size distribution with 2.39, 2.80, and 3.02 nm for Pd, Au, and Ag, respectively. The X-ray diffraction (XRD) (Figure 6d-f) confirms that the (fcc) crystal structure is equally applicable to Pd, Au and Ag. These results suggest that this photo-reduction strategy can be efficiently to synthesize various noble nanoparticles.



Figure 6. Typical TEM images of (a) p-Pd/TNR@GC, (b) p-Au/TNR@GC, (c) p-Ag/TNR@GC and (d) (e) (f) associated XRD patterns of the as obtained TNR@GC and TNR@GC support Pd, Au and Ag, respectively.

Conclusions In conclusion, we have demonstrated an effective photo-reduction strategy to prepare welldispersed Pt metal nanoparticles, which uniformly distributed on the graphitic carbon encapsulated titanium oxide nanorods (TNR@GC). The TNR@GC exhibits enhanced electron-hole separation effect due to the existence of trivalent titanium and ultra-thin carbon layer (1.5 nm). The excited electron from TNR can rapidly migrate to the surface of ultra-thin carbon layer and thus reduce the Pt ion, forming Pt NPs on carbon layer. The as-prepared pPt/TNR@GC exhibits a high MOR activity, with the 3-times higher reaction rate compared with Pt/C. This strategy has been successfully expended for constructing other noble meal nanoparticles, including Pt, Au and Ag. This work provides a valuable guidance for direct preparation of noble metal NPs with low pollution and cost. 12 ACS Paragon Plus Environment

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Supporting Information Experimental section and Materials characterization; TEM images; XRD data; XPS data; TGA curves; Raman spectroscopy; Nitrogen adsorption–desorption isotherms. Acknowledgements The authors was gratefully acknowledge financial supported by the National Natural Science Foundation of China through the Key Project of National Natural Science Foundation of China (Grant No. 21433003). Can Li and Yunteng Qu make the same contribution to this work.



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The well-dispersed Pt NPs (p-Pt/TNR@GC) is fabricated via a photo-reduction strategy and exhibits a high MOR activity. The ultra-thin carbon layer ensures the existence of Ti3+ and excellent electric conductivity to achieve an enhanced electron-hole separation effect.


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Synthesis of Well-Defined Pt-Based Catalysts for Methanol Oxidation

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