Pt2CeO2 Heterojunction Supported on Multiwalled Carbon Nanotubes for Robust Electrocatalytic Oxidation of Methanol

Herein, we prepared Pt2CeO2 heterojunction nanocluster (HJNS) on multiwalled carbon nanotubes (MWCNTs) in deep eutectic solvents (DESs) which is a special class of ionic liquids. The catalyst was then heat-treated at 400 °C in N2 (refer to Pt2CeO2/CNTs-400). The Pt2CeO2/CNTs-400 catalyst showed remarkably improved electrocatalytic performance towards methanol oxidation reaction (MOR) (839.1 mA mgPt−1) compared to Pt2CeO2/CNTs-500 (620.3 mA mgPt−1), Pt2CeO2/CNTs-300 (459.2 mA mgPt−1), Pt2CeO2/CNTs (641.6 mAmg−1) (the catalyst which has not been heat-treated) and commercial Pt/C (229.9 mAmg−1). Additionally, the Pt2CeO2/CNTs-400 catalyst also showed better CO poisoning resistance (onset potential: 0.47 V) compared to Pt2CeO2/CNTs (0.56 V) and commercial Pt/C (0.58 V). The improved performance of Pt2CeO2/CNTs-400 catalyst is attributed to the addition of appropriate CeO2, which changed the electronic state around the Pt atoms, lowered the d-band of Pt atoms, formed more Ce-O-Pt bonds acting as new active sites, affected the adsorption of toxic intermediates and weakened the dissolution of Pt; on the other hand, with the assistance of thermal treatment at 400 °C, the obtained Pt2CeO2 HJNS expose more new active sites at the interface between Pt and CeO2 to enhance the electrochemical active surface area (ECSA) and the dehydrogenation process of MOR. Thirdly, DES is beneficial to the increase of the effective component Pt(0) in the carbonization process. The study shows a new way to construct high-performance Pt-CeO2 catalyst for the direct methanol fuel cell (DMFC).


Introduction
The development of fuel cells is one of the important ways to achieve carbon neutrality [1][2][3][4]. Fuel cells have been the focus of attention in the field of energy, such as direct methanol fuel cells (DMFCs) [5,6], direct ethanol fuel cells (DEFCs) [7], direct formic acid fuel cells (DFAFCs) [8] and so on. Among them, with the advantages of easy operation, safe liquid methanol, high energy density and low operating temperature, DMFCs have received extensive attention as a hopeful power technology for vehicles and portable electronic devices [9,10].
As we know, the precious metal platinum (Pt) has excellent catalytic performance for the DMFC. However, the high price and scarcity of Pt hinder the commercialization of these technologies. Additionally, Pt-based catalysts are susceptible to poisoning by carbonaceous intermediates (mainly CO ads ) that are adsorbed on the Pt active sites and reduce the catalytic performance towards MOR. According to the bifunctional mechanism, in order to effectively alleviate the toxicity of CO ads to the Pt active site, introducing a second cheap metal is an effective method. Due to the effect of the added metal on the electronic structure, the Pt electronic state is changed to reduce the adsorption of poisonous intermediates (e.g., CO ads , COOH ads ). Among these alloy catalysts, the Pt-CeO 2 binary nanoparticles [38,39]. In our previous work, a series of Pt/Pd-based catalysts have been prepared using DES [21,24,40,41]. DES has been widely proved to be a green solvent for the preparation of high-performance catalysts. Therefore, DES is expected to be further widely used in more fields.
Herein, we fabricated Pt 2 CeO 2 HJNS supported on MWCNTs catalyst successfully with the help of DES and the pyrolysis process. The related results demonstrated that we used the combination of (Pt + CeO 2 + MWCNTs + DES + calcination) to get a material with rich structure, so as to obtain good MOR catalytic performance. The prepared Pt 2 CeO 2 /CNTs-400 exhibits enhanced catalytic activity and stability for the MOR compared with Pt 2 CeO 2 /CNTs-500, Pt 2 CeO 2 /CNTs-300, Pt 2 CeO 2 /CNTs and Pt/C catalysts. Figure 1 shows the XRD patterns of Pt 2 CeO 2 /CNTs and Pt 2 CeO 2 /CNTs-400 catalysts. The peak at approximately 26.2 • for both catalysts was due to the (002) crystal phase of the MWCNTs [42]. The two catalysts show peaks characteristic of Pt, that is, 40.8 • , 47.8 • , 68.5 • , 82.9 • and 87.3 • of (111), (200), (220), (311) and (222), respectively [38]. Four diffraction peaks (111), (200), (311) and (420) of CeO 2 were observed in Pt 2 CeO 2 /CNTs-400 corresponding to 28.5 • , 33 • , 56.7 • and 79 • , respectively [24]. Interestingly, the diffraction peaks of CeO 2 (220) are combined with Pt(200) approximately at 2θ = 47.8 • . It is worth noting that the peak of CeO 2 (311) appeared strongly in the non-pyrolytic Pt 2 CeO 2 /CNTs material, while after 400 • C heat treatment, the peaks of CeO 2 (311) disappeared and CeO 2 (111) and CeO 2 (200) appeared in the Pt 2 CeO 2 /CNT-400. This result indicates that proper pyrolysis is conducive to the formation of different crystalline of CeO 2 , and these CeO 2 distributed in different places can promote the catalytic effect of Pt better. nanoparticles in DES. It is shown that the GCE (glassy carbon elec trode)/Pd@Pd(OH)2-modified electrode displays a high catalytic activity towards the MOR in alkaline solution [37]. Fan et al. have prepared high-performance Pt-based alloy catalysts by chemical reduction or electrochemistry method in DES that plays an im portant role in controlling the shape of the nanoparticles [38,39]. In our previous work, a series of Pt/Pd-based catalysts have been prepared using DES [21,24,40,41]. DES has been widely proved to be a green solvent for the preparation of high-performance cata lysts. Therefore, DES is expected to be further widely used in more fields.

Results and Discussion
Herein, we fabricated Pt2CeO2 HJNS supported on MWCNTs catalyst successfully with the help of DES and the pyrolysis process. The related results demonstrated tha we used the combination of (Pt + CeO2 + MWCNTs + DES + calcination) to get a materia with rich structure, so as to obtain good MOR catalytic performance. The prepared Pt2CeO2/CNTs-400 exhibits enhanced catalytic activity and stability for the MOR com pared with Pt2CeO2/CNTs-500, Pt2CeO2/CNTs-300, Pt2CeO2/CNTs and Pt/C catalysts. Figure 1 shows the XRD patterns of Pt2CeO2/CNTs and Pt2CeO2/CNTs-400 catalysts The peak at approximately 26.2° for both catalysts was due to the (002) crystal phase o the MWCNTs [42]. The two catalysts show peaks characteristic of Pt, that is, 40.8°, 47.8° 68.5°, 82.9° and 87.3° of (111), (200), (220), (311) and (222), respectively [38]. Four diffrac tion peaks (111), (200), (311) and (420) of CeO2 were observed in Pt2CeO2/CNTs-400 cor responding to 28.5°, 33°, 56.7° and 79°, respectively [24]. Interestingly, the diffraction peaks of CeO2 (220) are combined with Pt(200) approximately at 2θ = 47.8°. It is worth noting that the peak of CeO2(311) appeared strongly in the non-pyrolytic Pt2CeO2/CNTs material, while after 400 °C heat treatment, the peaks of CeO2(311) disappeared and CeO2(111) and CeO2(200) appeared in the Pt2CeO2/CNT-400. This result indicates tha proper pyrolysis is conducive to the formation of different crystalline of CeO2, and these CeO2 distributed in different places can promote the catalytic effect of Pt better. Figure S1 from Supplementary Materials shows the XRD patterns o Pt2CeO2/CNTs-300 and Pt2CeO2/CNTs-500 catalysts. The average crystallite size of the P nanoparticles was determined to be 4.5 ± 1.14, 4.5 ± 1.06, 4.3 ± 1.07 and 4.4 ± 1.11 nm for the Pt2CeO2/CNTs-400, Pt2CeO2/CNTs-300, Pt2CeO2/CNTs-500 and Pt2CeO2/CNTs, re  Figure S1 from Supplementary Materials shows the XRD patterns of Pt 2 CeO 2 /CNTs-300 and Pt 2 CeO 2 /CNTs-500 catalysts. The average crystallite size of the Pt nanoparticles was determined to be 4.5 ± 1.14, 4.5 ± 1.06, 4.3 ± 1.07 and 4.4 ± 1.11 nm for the Pt 2 CeO 2 /CNTs-400, Pt 2 CeO 2 /CNTs-300, Pt 2 CeO 2 /CNTs-500 and Pt 2 CeO 2 /CNTs, respectively, calculated from the Pt(220) diffraction peak using Scherrer's equation [43][44][45]. The result shows that pyrolysis has no significant effect on particle size for all catalysts. Figure 2 shows the TEM and HRTEM images, HAADF-STEM elements mapping and the corresponding elements Pt, Ce and O of Pt 2 CeO 2 /CNTs-400. As shown in Figure 2a,b, the Pt 2 CeO 2 HJNS are evenly dispersed on MWCNTs with no aggregation. The average size of Pt nanoparticles in the Pt 2 CeO 2 /CNTs-400 is approximately 4.5 ± 1.14 nm, which is very close to the XRD data above. The HRTEM image of Pt 2 CeO 2 /CNTs-400 ( Figure 2c) shows the crystal plane distances of 0.312 nm obtained for the CeO 2 (111) plane and 0.225 nm for the Pt(111) plane; both agree very well with the known crystal plane distances [24]. In addition, as shown in Figure S2 from Supplementary Materials, the Pt nanoparticles are also dispersed well with no aggregation and the average particle size is approximately 4.4 ± 1.11 nm of Pt 2 CeO 2 /CNTs. Figure S3 from Supplementary Materials shows the TEM and HRTEM images of Pt 2 CeO 2 /CNTs-300 and Pt 2 CeO 2 /CNTs-500 catalysts. We can see that these catalysts Pt 2 CeO 2 /CNTs-400, Pt 2 CeO 2 /CNTs-300, Pt 2 CeO 2 /CNTs-500 and Pt 2 CeO 2 /CNTs which were fabricated in DES probably act as a kind of surfactants, additives, or stabilizers to induce a uniform distribution for Pt and CeO 2 nanoparticles. However, systematic studies aimed at understanding the role of DES in the prepared process are still underway. EDX spectrum ( Figure S4 from Supplementary Materials) of the Pt 2 CeO 2 /CNTs-400 catalyst displays the signals of C, O, Pt and Ce elements, confirming the pyrolysis does not affect the metal composition in the catalyst. spectively, calculated from the Pt(220) diffraction peak using Scherrer's equation [43][44][45]. The result shows that pyrolysis has no significant effect on particle size for all catalysts. Figure 2 shows the TEM and HRTEM images, HAADF-STEM elements mapping and the corresponding elements Pt, Ce and O of Pt2CeO2/CNTs-400. As shown in Figure  2a,b, the Pt2CeO2 HJNS are evenly dispersed on MWCNTs with no aggregation. The average size of Pt nanoparticles in the Pt2CeO2/CNTs-400 is approximately 4.5 ± 1.14 nm, which is very close to the XRD data above. The H R T EM i ma ge o f Pt2CeO2/CNTs-400 ( Figure 2c) shows the crystal plane distances of 0.312 nm obtained for the CeO2 (111) plane and 0.225 nm for the Pt (111) plane; both agree very well with the known crystal plane distances [24]. In addition, as shown in Figure S2 from Supplementary Materials, the Pt nanoparticles are also dispersed well with no aggregation and the average particle size is approximately 4.4 ± 1.11 nm of Pt2CeO2/CNTs. Figure S3 from Supplementary Materials shows the TEM and HRTEM images of Pt2CeO2/CNTs-300 and Pt2CeO2/CNTs-500 catalysts. We can see that these catalysts Pt2CeO2/CNTs-400, Pt2CeO2/CNTs-300, Pt2CeO2/CNTs-500 and Pt2CeO2/CNTs which were fabricated in DES probably act as a kind of surfactants, additives, or stabilizers to induce a uniform distribution for Pt and CeO2 nanoparticles. However, systematic studies aimed at understanding the role of DES in the prepared process are still underway. EDX spectrum ( Figure S4   The surface composition and chemical oxidation states of these catalysts were characterized by XPS. Figure 3a shows the XPS survey spectra of Pt2CeO2/CNTs-400 and Pt2CeO2/CNTs. The signals corresponding to C 1s (283.8 eV), O 1s(531.9 eV), Ce 3d (~900.8 eV), Pt 4f (73.4 eV) and Pt 4d (315.1 eV) were observed for these two catalysts. Figure 3b shows the Ce3d spectrum of Pt2-CeO2/CNTs-400; the deconvolution of the asymmetric Ce3d photoemission of the Pt2CeO2/CNTs-400 produced four peaks at 881.8, 885.1, 900.5 and 903.8 eV. To further determine the presence of Ce3d peaks, we locally enlarged the peak shape of Ce3d in Figure S5 from Supplementary Materials. From the The surface composition and chemical oxidation states of these catalysts were characterized by XPS. Figure 3a shows the XPS survey spectra of Pt 2 CeO 2 /CNTs-400 and Pt 2 CeO 2 /CNTs. The signals corresponding to C 1s (283.8 eV), O 1s (531.9 eV), Ce 3d (~900.8 eV), Pt 4f (73.4 eV) and Pt 4d (315.1 eV) were observed for these two catalysts. Figure 3b shows the Ce3d spectrum of Pt 2 -CeO 2 /CNTs-400; the deconvolution of the asymmetric Ce3d photoemission of the Pt 2 CeO 2 /CNTs-400 produced four peaks at 881.8, 885.1, 900.5 and 903.8 eV. To further determine the presence of Ce3d peaks, we locally enlarged the peak shape of Ce3d in Figure S5 from Supplementary Materials. From the enlarged figure, it can be seen that the Ce3d peaks of the two catalysts do exist, but the peaks are small, which may be attributed to the small content of Ce. Figure 3c,d show the Pt 4f spectra for Pt 2 CeO 2 /CNTs and Pt 2 CeO 2 /CNTs-400 catalysts. The Pt 4f spectra of the Pt 2 CeO 2 /CNTs, two pairs of peaks, indicate the existence of two different Pt oxidation states on the surface, and two intense peaks located at binding energies of 70.7 eV (Pt 4f 7/2) and 74.1 eV (Pt 4f 5/2) originated from metallic Pt(0), and the weak peaks located at 71.5 eV (Pt 4f 7/2) and 74.4 eV (Pt 4f 5/2 ) were assigned to the Pt(II) state in the form of PtO or Pt(OH) 2 [46]. For the Pt 2 CeO 2 /CNTs-400 catalyst, the Pt 4f 7/2 peak located at 71.0 eV and Pt4f 5/2 peak located at 74.4 eV correspond to metallic Pt(0), and the Pt 4f 7/2 peak located at 71.8 eV and Pt 4f 5/2 peak located at 75.3 eV correspond to Pt(II) in PtO or Pt(OH) 2 . The fractions of the Pt(0) and Pt(II) species in Pt 2 CeO 2 /CNTs-400 and Pt 2 CeO 2 /CNTs were calculated as (40.3%, 59.7%) and (46.2%, 53.8%), respectively. The content of Pt(0) in Pt 2 CeO 2 /CNTs-400 is higher than that in Pt 2 CeO 2 /CNTs, which is also due to the carbonization of DES in the pyrolysis process [47]. In addition, the positive shift (about 0.3 eV) in the Pt peaks was observed in the Pt 2 CeO 2 /CNTs-400, which indicates the interaction of CeO 2 and Pt, exposing the strong electronic interactions between CeO 2 and Pt nanoparticles by the formation of Ce-O-Pt [24]. The electronic interaction between CeO 2 and Pt nanoparticles can alter the electronic environment of Pt atoms, thus affecting the bonding between Pt and intermediates (such as CO ads ). Thus, the electrocatalytic performance of MOR was improved. Figure S6 from Supplementary Materials shows the Pt 4f and Ce3d spectra for Pt 2 CeO 2 /CNTs-300 and Pt 2 CeO 2 /CNTs-500 catalysts. For the Pt 2 CeO 2 /CNTs-300 catalyst, the Pt 4f 7/2 peak located at 71.5 eV and Pt4f 5/2 peak located at 74.8 eV correspond to metallic Pt(0), and the Pt 4f 7/2 peak located at 71.4 eV and Pt 4f 5/2 peak located at 74.9 eV correspond to Pt(II) in PtO or Pt(OH) 2 . In addition, for the Pt 2 CeO 2 /CNTs-500 catalyst, the Pt 4f 7/2 peak located at 71.4 eV and Pt4f 5/2 peak located at 74.9 eV correspond to metallic Pt(0), and the Pt 4f 7/2 peak located at 71.7 eV and Pt 4f 5/2 peak located at 75.5 eV correspond to Pt(II) in PtO or Pt(OH) 2 . The fractions of the Pt(0) and Pt(II) species in Pt 2 CeO 2 /CNTs-300 and Pt 2 CeO 2 /CNTs-500 were calculated as (43.1%, 56.9%) and (41.8%, 58.2%), respectively. Obviously, the content of Pt(0) in Pt 2 CeO 2 /CNTs-400 is higher than Pt 2 CeO 2 /CNTs-300 and Pt 2 CeO 2 /CNTs-500, which shows 400°C is the most favorable pyrolysis temperature for obtaining more Pt(0).    [38,48]. The electrochemical active surface area (ECSA) was calculated by measuring the hydrogen adsorption/desorption charges after a double-layer correction and assuming a value of 210 μC cm −2 for the adsorption of a hydrogen monolayer [49]. Therefore, the ECSA of the Pt2CeO2/CNTs-400 was calculated to be 63.2 m 2 g −1 , which is much higher than those of the Pt2CeO2/CNTs (41.7 m 2 g −1 ) and Pt/C (21.3m 2 g −1 ). The larger ECSA of Pt2CeO2/CNTs-400 is most likely due to the higher dispersion of Pt2CeO2 HJNS on the MWCNTs. In order to explore the effect of pyrolysis   [38,48]. The electrochemical active surface area (ECSA) was calculated by measuring the hydrogen adsorption/desorption charges after a double-layer correction and assuming a value of 210 µC cm −2 for the adsorption of a hydrogen monolayer [49]. Therefore, the ECSA of the Pt 2 CeO 2 /CNTs-400 was calculated to be 63.2 m 2 g −1 , which is much higher than those of the Pt 2 CeO 2 /CNTs (41.7 m 2 g −1 ) and Pt/C (21.3m 2 g −1 ). The larger ECSA of Pt 2 CeO 2 /CNTs-400 is most likely due to the higher dispersion of Pt 2 CeO 2 HJNS on the MWCNTs. In order to explore the effect of pyrolysis temperature on catalyst performance, we tested the catalytic performance of Pt 2 CeO 2 /CNTs-300, Pt 2 CeO 2 /CNTs-400 and Pt 2 CeO 2 /CNTs-500 for MOR (Figure 4b). For Pt 2 CeO 2 /CNTs-400, the peak current density in the forward scan is 839.1 mA mg Pt −1 , higher than Pt 2 CeO 2 /CNTs-500 (620.3 mA mg Pt −1 ) and Pt 2 CeO 2 /CNTs-300 (459.2 mA mg Pt −1 ). The results show that the heat treatment of 400°C is more beneficial to the improvement of catalyst performance. Figure 4c shows the CV curves for MOR on the Pt 2 CeO 2 /CNTs-400, Pt 2 CeO 2 /CNTs and Pt/C. For the Pt 2 CeO 2 /CNTs-400, the peak current density of MOR in the forward scans is much higher than those on the Pt 2 CeO 2 /CNTs (641.6 mAmg −1 ) and Pt/C (229.9 mAmg −1 ). These results indicate that the electrocatalytic activity of Pt 2 CeO 2 /CNTs-400 for MOR is higher than Pt 2 CeO 2 /CNTs and Pt/C. In order to further evaluate the long-term performance of Pt 2 CeO 2 /CNTs-400, Pt 2 CeO 2 /CNTs and Pt/C, the CA were performed in 0.5 M CH 3 OH + 0.5 M H 2 SO 4 solution at 0.5 V for 7200 s. As shown in Figure 4d, in the initial period, all of the curves with fast current decay indicate poisoning of the electrocatalysts due to the formation of intermediate species such as CO ads [50]. After 7200 s, the Pt 2 CeO 2 /CNTs-400 catalyst maintained a higher current density (23.2mAmg −1 Pt ), which is almost 2.0 and 4.3 times those of the Pt 2 CeO 2 /CNTs (11.5 mA mg −1 Pt ) and Pt/C (5.3 mAmg −1 Pt ), respectively. In order to further explore the multicycle CV stability of the catalysts, accelerated degradation tests (ADT) were conducted to check the durability of catalysts in 0.5 M H 2 SO 4 + 0.5 M CH 3 OH solution for 500 cycles ( Figure S7 from Supplementary Materials). Obviously, the activity of the Pt 2 CeO 2 /CNTs-400 catalyst decreased rapidly (39.6%) during the first 200th cycle and decreased to 37.1% at the 400th cycle. By the 500th cycle, the activity had dropped to 35.0%. We can see that there is not much change in activity between the 400th and 500th cycles. However, the activity of Pt 2 CeO 2 /CNTs and Pt/C catalysts still decreased significantly between the 400th and 500th cycles. After 500 cycles, the Pt 2 CeO 2 /CNTs-400 maintained a higher current density (294.2 mA mg Pt −1 ) that is almost 2.6 and 4.9 times of the Pt 2 CeO 2 /CNTs (113.1 mA mg Pt −1 ) and Pt/C (59.9 mA mg Pt −1 ), respectively. These results further illustrate that Pt 2 CeO 2 /CNTs-400 exhibits higher electrocatalytic stability for MOR. Besides, the Pt 2 CeO 2 /CNTs-400 catalyst presents the better MOR mass activity in comparison with the recent research works on Pt-based catalysts (Table S1 Supplementary Materials).

Results and Discussion
We used CO stripping experiments to investigate the CO tolerance of the as-prepared catalysts. Figure 5 shows the CO stripping voltammograms for the Pt 2 CeO 2 /CNTs-400, Pt 2 CeO 2 /CNTs and Pt/C. Apparently, the onset potential of the adsorbed CO oxidation of the Pt 2 CeO 2 /CNTs-400 is negatively shifted to 0.47 V, and the corresponding potentials are 0.56 V and 0.58 V on the Pt 2 CeO 2 /CNTs and Pt/C, respectively, indicating that thermal treatment effectively improves the CO oxidation ability of the catalyst [51,52].
The greatly enhanced electrocatalytic performance of the Pt 2 CeO 2 /CNTs-400 for the MOR may be due to four reasons: (1) the surface of CeO 2 coating contains more active sites of Pt deposition, and the Pt was dispersed more evenly, which reduced the surface energy; (2) the increased Lewis alkalinity of CeO 2 led to the strong anchoring of Pt to CeO 2 ; (3) the increase of Pt(0) composition during the carbonization of DES [47]; (4) the addition of appropriate CeO 2 , which changed the electronic state around the Pt atom, affected the adsorption of toxic intermediates. The addition of CeO 2 contributed to the uniform distribution of Pt and inhibited the agglomeration of Pt nanoparticles, but too much CeO 2 hindered the structure between Pt and CNTs, thus inhibiting the interaction between Pt, CeO 2 and CNTs. On the other hand, appropriate calcination temperature is conducive to the formation of fluorite structure of CeO 2 and the interaction between CeO 2 and CNTs, while higher calcination temperature may lead to CeO 2 agglomeration, which is not conducive to the uniform distribution of Pt nanoparticles. In addition, higher calcination temperature may also lead to the collapse of CeO 2 -CNTs structure and the reduction of surface area [53]. The mechanism of the advantages brought by the addition of DES and the influence of appropriate heat treatment on the catalyst are also under study. ducted to check the durability of catalysts in 0.5 M H2SO4 + 0.5 M CH3OH solution for 500 cycles ( Figure S7 from Supplementary Materials). Obviously, the activity of the Pt2CeO2/CNTs-400 catalyst decreased rapidly (39.6%) during the first 200th cycle and decreased to 37.1% at the 400th cycle. By the 500th cycle, the activity had dropped to 35.0%. We can see that there is not much change in activity between the 400th and 500th cycles. However, the activity of Pt2CeO2/CNTs and Pt/C catalysts still decreased significantly between the 400th and 500th cycles. After 500 cycles, the Pt2CeO2/CNTs-400 maintained a higher current density (294.2 mA mgPt −1 ) that is almost 2.6 and 4.9 times of the Pt2CeO2/CNTs (113.1 mA mgPt −1 ) and Pt/C (59.9 mA mgPt −1 ), respectively. These results further illustrate that Pt2CeO2/CNTs-400 exhibits higher electrocatalytic stability for MOR. Besides, the Pt2CeO2/CNTs-400 catalyst presents the better MOR mass activity in comparison with the recent research works on Pt-based catalysts (Table S1 Supplemen  We used CO stripping experiments to investigate the CO tolerance of the as-prepared catalysts. Figure 5 shows the CO stripping voltammograms for the Pt2CeO2/CNTs-400, Pt2CeO2/CNTs and Pt/C. Apparently, the onset potential of the adsorbed CO oxidation of the Pt2CeO2/CNTs-400 is negatively shifted to 0.47 V, and the corresponding potentials are 0.56 V and 0.58 V on the Pt2CeO2/CNTs and Pt/C, respectively, indicating that thermal treatment effectively improves the CO oxidation ability of the catalyst [51,52]. The greatly enhanced electrocatalytic performance of the Pt2CeO2/CNTs-400 for the MOR may be due to four reasons: (1) the surface of CeO2 coating contains more active sites of Pt deposition, and the Pt was dispersed more evenly, which reduced the surface energy; (2) the increased Lewis alkalinity of CeO2 led to the strong anchoring of Pt to CeO2; (3) the increase of Pt(0) composition during the carbonization of DES [47]; (4) the addition of appropriate CeO2, which changed the electronic state around the Pt atom, affected the adsorption of toxic intermediates. The addition of CeO2 contributed to the uniform distribution of Pt and inhibited the agglomeration of Pt nanoparticles, but too much CeO2 hindered the structure between Pt and CNTs, thus inhibiting the interaction between Pt, CeO2 and CNTs. On the other hand, appropriate calcination temperature is

Preparation of DES
DESs (choline chloride/urea) were prepared by a simple method according to the procedure in the reported literature [54]. Choline chloride [HOC 2 H 4 N(CH 3 ) 3 Cl] (Shanghai Chemical Reagent Ltd., Shanghai, China 99%) was recrystallized from absolute ethanol, filtered and dried under vacuum. Urea (Shanghai Chemical Reagent Ltd., Shanghai, China >99%) was recrystallized from Millipore water (18.0 MΩ cm) provided by a Milli-Q Lab apparatus (Nihon Millipore Ltd., Tokyo, Japan), filtered and dried under vacuum prior to use. Briefly, urea and choline chloride with mole ratio of 2:1 were mixed and stirred at 80 • C until a homogeneous and colorless solution was formed. Then, the obtained DESs were preserved in a vacuum drying oven before use.

Preparation of Catalysts
Firstly, raw MWCNTs were treated with H 2 SO 4 and HNO 3 to introduce surface oxygen groups and the samples collected after centrifugation were labelled as MWCNTs-AO [55]. Typically, a mixture containing appropriate ratios of H 2 PtCl 6 ·6H 2 O, Ce(NO 3 ) 3 (atomic ratio: Pt/Ce = 1:0.5) and MWCNTs-AO was ultrasonicated until complete dispersion in DES (10 mL). Then, NaBH 4 (200 mg) and NH 4 OH (5 mL) were added to this suspension and stirred continuously for 5 h at 80 • C. After stirring, the suspension was centrifuged and washed repeatedly with C 2 H 5 OH and tri-distilled water. Later, it was dried at 60 • C for 24 h and the obtained product was labelled as Pt 2 CeO 2 /CNTs. Finally, Pt 2 CeO 2 /CNTs were thermal treated at 300 • C, 400 • C and 500 • C in N 2 atmosphere for 2 h (refer to Pt 2 CeO 2 /CNTs-300, Pt 2 CeO 2 /CNTs-400 and Pt 2 CeO 2 /CNTs-500). Scheme 1 shows the preparation of Pt 2 CeO 2 /CNTs-400.

Preparation of Catalysts
Firstly, raw MWCNTs were treated with H2SO4 and HNO3 to introduce surface oxygen groups and the samples collected after centrifugation were labelled as MWCNTs-AO [55]. Typically, a mixture containing appropriate ratios of H2PtCl6·6H2O, Ce(NO3)3 (atomic ratio: Pt/Ce = 1:0.5) and MWCNTs-AO was ultrasonicated until complete dispersion in DES (10 mL). Then, NaBH4 (200 mg) and NH4OH (5 mL) were added to this suspension and stirred continuously for 5 h at 80 °C. After stirring, the suspension was centrifuged and washed repeatedly with C2H5OH and tri-distilled water. Later, it was dried at 60 °C for 24 h and the obtained product was labelled as Pt2CeO2/CNTs. Finally, Pt2CeO2/CNTs were thermal treated at 300 °C, 400 °C and 500 °C in N2 atmosphere for 2 h (refer to Pt2CeO2/CNTs-300, Pt2CeO2/CNTs-400 and Pt2CeO2/CNTs-500). Scheme 1 shows the preparation of Pt2CeO2/CNTs-400.

Physical Characterization
The sizes, morphology and structure of all as-prepared nanocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) Scheme 1. Schematic illustration showing the preparation of Pt 2 CeO 2 /CNTs-400.

Physical Characterization
The sizes, morphology and structure of all as-prepared nanocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and (ICP-OES, Thermo Electron IRIS Intrepid II XSP, Waltham, MA, USA). XRD patterns were collected from a Rigaku D/max 2500Pc X-ray powder diffractometer(Rigaku D/MAX 2500 v/pc, Japan). SEM images were recorded using JSM-7500F electron microscopy. TEM and high-resolution TEM images were obtained with Talos F200S field emission electron microscope. The Pt contents in Pt/C, Pt 2 CeO 2 /CNTs and Pt 2 CeO 2 /CNTs-400 catalysts measured by ICP-OES were found to be 20.0, 19.3 and 17.7%, respectively.

Electrochemical Measurements
The catalyst-modified glassy carbon electrode (GC, diameter = 5 mm) was prepared based on a previously reported procedure [56]. An electrochemical workstation (Chenhua, Shanghai) was used to survey the electrochemical performances of prepared catalysts in a three-electrode system, where Pt foil and saturated calomel electrode served as the counter and reference electrodes, respectively. Earlier, GC electrode was polished with 5.

Conclusions
A simple and effective chemical reduction approach has been developed for the fabrication of Pt 2 CeO 2 /CNTs-400 with the help of DES and thermal treatment. The catalyst exhibited an enhanced electrocatalytic performance (higher activity, long-term durability and excellent CO tolerance) compared to the Pt 2 CeO 2 /CNTs and Pt/C. This study demonstrates the DES medium and CeO 2 coating in favor of a uniform distribution for Pt nanoparticles on the carbon support. The improved performance of Pt 2 CeO 2 /CNTs-400 is attributed to the addition of appropriate CeO 2 , which changed the electronic state around the Pt atom, formed new Ce-O-Pt bond at the interface between Pt and CeO 2 acting as new active sites, affected the adsorption of toxic intermediates and weakened the dissolution of Pt; on the other hand, with the assistance of thermal treatment, the DES is beneficial to the increase of the effective component Pt(0) in the carbonization process to enhance the dehydrogenation process of MOR.