Spatial Confinement of Pt Nanoparticles in Carbon Nanotubes for Efficient and Selective H2 Evolution from Methanol

Abstract H2 generation from methanol‐water mixtures often requires high pressure and high temperature (200–300 °C). However, CO can be easily generated and poison the catalytic system under such high temperature. Therefore, it is highly desirable to develop the efficient catalytic systems for H2 production from methanol at room temperature, even at sub‐zero temperatures. Herein, carbon nanotube‐supported Pt nanocomposites are designed and synthesized as high‐performance nano‐catalysts, via stabilization of Pt nanoparticles onto carbon nanotube (CNT), for H2 production upon methanol dehydrogenation at sub‐zero temperatures. Therein, the optimal Pt/CNT nanocomposite presents the superior catalytic performance in H2 production upon methanol dehydrogenation at the expense of B2(OH)4, with the TOF of 299.51 min‐130 oC. Compared with other common carriers, Pt/CNT exhibited the highest catalytic performance in H2 production, emphasizing the critical role of CNT in methanol dehydrogenation. The confinement of Pt nanoparticles by CNTs is conducive to inhibiting the aggregation of Pt nanoparticles, thereby significantly increasing its catalytic performance and stability. The kinetic study, detailed mechanistic insights, and density functional theory (DFT) calculation confirm that the breaking of O─H bond of CH3OH is the rate‐controlling step for methanol dehydrogenation, and both H atoms of H2 are supplied by methanol. Interestingly, H2 is also successfully produced from methanol dehydrogenation at −10 °C, which absolutely solves the freezing problem in the H2 evolution upon water‐splitting reaction.


Introduction
8][9][10] Hence, it is highly desired to develop sustainable, green and carbon-free energy.[18][19][20][21] It is obvious that major H 2 generation methods rely heavily on conventional fossil fuels.This is a complete departure from the principles of the utilization of hydrogen energy. [22]Additionally, the large-scale industrial application of hydrogen is severely hampered by its safety issues as to storing and transportation, because of its extremely low density, super-high explosibility, and liquefaction Scheme 1.The synthesis of Pt/CNT.
[37] Homogeneous catalysts for H 2 production from methanol are developed for ages, but they exhibited poor activity and selectivity. [38]Indeed, some low molecular weight organic matter (such as acetic acid, [39] formaldehyed, [40] formate salts, [41] methyl formate [42] and dimethyl acetal [43] ) were obtained as by-products, greatly decreasing the efficiency of methanol steam reforming. [44]So this reaction, which is typically catalyzed by heterogeneous catalysts, often requires high pressure and high temperature (200-300 °C). [45]However, CO can be easily generated and poison the catalytic system of fuel cell, as well as contaminate the H 2 gas under such high temperature. [46,47][50] For example, Zhou's group first reported [Cp*IrCl(phen)]Cl catalyzed H 2 generation from methanol at near-room temperature. [51]Herein, we have first reported carbon nanotube-supported Pt, Pd, and Rh nanocomposites (Pt/CNT, Pd/CNT, and Rh/CNT) as high-performance nanocatalysts, via stabilization of Pt, Pd, and Rh nanoparticles onto carbon nanotube (CNT), [52][53][54] for H 2 production upon methanol dehydrogenation at 30 °C (Equation 1).Therein, the optimal Pt/CNT nanocomposite presented the superior catalytic activity in H 2 evolution upon methanol dehydrogenation at the expense of B 2 (OH) 4 , with a TOF value of 299.51 min −1 at 30 °C.Among them, B 2 (OH) 4 was frequently applied in borylation reaction and reduction reaction, [22] and recently used as the sacrificial agent for H 2 production. [55,56]In order to speculate and verify its mechanism, the carrier effect, kinetic study, kinetic isotope effect, tandem reaction, and density functional theory (DFT) of H 2 production upon methanol dehydrogenation had been scrutinized in detail.Interestingly, H 2 was also successfully produced from methanol dehydrogenation at −10 °C, which absolutely solved the freezing problem in the H 2 evolution upon water-splitting reaction. 2MeOH

Characterization of Pt/CNT
As illustrated in Figure 2a, the BET result presented the BET surface area, total pore volume and mean pore diameter of Pt/CNT nanocomposite is 97.64 m 2 g −1 , 0.47 cm 3 g −1 , and 19.08 nm, respectively, indicating that Pt/CNT possessed a mesoporous structure. [57]A distinct characteristic peak at 26 o , which is corresponding to graphene (002) (JCPDS card No. 75-1621), was recorded in Figure 2b. [58]The distinct peaks of Pt ( 311 ). [59]Next, the Pt content of Pt/CNT was measured by ICP to be 4.46 wt.%, which is just slightly lower than the theoretical value (4.65 wt.%).The obvious peaks of D-band (1383.02cm −1 ), G-band (1584.86 cm −1 ) and 2D-band (2758.13cm −1 ) were observed in Figure 2c. [60]Among them, the G-band and D-band were corresponding to graphite carbon resp.disordered carbon.The small value of I D /I G (0.27) illustrated that there were only few crystal defects in Pt/CNT.The external nano-structure and nano-morphology Pt/CNT nanocomposite had also been measured by TEM and HRTEM.As illustrated in Figure 2d,e, Pt/CNT nanocomposite possessed a nanotube structure.Some Pt nanoparticles (3.79 nm, Figure S8, Supporting Information) were located at the surface of CNT, other Pt nanoparticles were encapsulated into CNT.C (002), where its crystal lattice spacing is 0.33 nm, was recorded in the exterior (Figure 2f).While Pt (111) of 0.23 nm was presented at the interior, further confirming some Pt nanoparticles were encapsulated into CNT. [61]64] In addition, the chemical valence states of surface elements of Pt/CNT had also been determined by XPS in Figure 4a.The high-resolution Pt 4f 7/2 spectrum of Pt/CNT was divided into two peaks at 71.63 and 72.71 eV, which were attributed to Pt (0) (74.38%) resp.Pt (II) (25.63%) species (Figure 4b). [65]This result had indicated that Pt (0) was partly oxidized to Pt (II) by O 2 .As illustrated in Figure 4c, the C 1s spectrum was decomposed into three characteristic peaks of C sp 2 at 284.78 eV and C sp 3 at 285.61 eV, respectively. [66]As shown in Figure 4d, the O 1s spectrum of Pt/CNT nanocomposite was divided into two typical peaks of 531.88 and 533.52 eV, assigned to C═O and C─O, respectively. [67]These O-containing functional group in CNTs could protect the Pt nanoparticles by enhancing its stability and decreasing the leaching of Pt in the confined space.In addition, the XPS of Pt/ZrO 2 was also measured in Figure S9 (Supporting Information), a slight shift was found in Pt 4f of Pt/CNT as compared to Pt 4f of Pt/ZrO 2 , suggesting the electron transfer from Pt atom into CNT surface.Thus, the superior catalytic performance of Pt/CNT in methanol dehydrogenation might be ascribed to its electronic interaction effect and synergistic effect. [68] order to further identify the coordination and valence states of Pt/CNT, X-ray absorption near edge structure (XANES) spectra of the Pt L3-edge over Pt/CNT catalyst, Pt foil, PtO, and PtO 2 had been performed in Figure 5a. the green peak of Pt/CNT was lower than that of PtO and slightly higher than that of Pt foil reference, suggesting the chemical charge of Pt atoms in Pt/CNT is between 0 and +2. [69]Then, the Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra of Pt/CNT uncovered that the presence of Pt-Pt, Pt-O-Pt, and Pt-C/O coordinations (Figure 5b,c), [70,71] demonstrating that Pt nanoparticles had been successfully stabilized by CNT.As shown in Figure 5d, the presence of Pt-Pt and Pt-C/O coordinations was also confirmed by wavelet transformed XAS analysis. [72]In Table S1 (Supporting Information), the coordination configurations of Pt atoms in Pt/CNT were measured by quantitative least-squares EXAFS best-fitting analysis.The coordination number of Pt-C/O, Pt-Pt, and Pt-O-Pt was 1.6, 4.8, and 2.0, respectively.In summary, these results further confirmed that chemical charge of Pt atoms in Pt/CNT is between 0 and +2, which was in line with XPS.

Kinetic Study
The kinetic study (such as B 2 (OH) 4 dosage, Pt/CNT amount, and reaction temperature) of H 2 production upon methanol dehydrogenation at the expense of B 2 (OH) 4 was further studied in detail.First, the H 2 production catalyzed by 0.2 mol.%Pt/CNT was conducted in the various B 2 (OH) 4 amounts from 1.0 to 2.5 mmol at MeOH (2 mL).In Figure 6a, the H 2 production rate was independent of B 2 (OH) 4 amounts, suggesting H 2 production was a zero-order reaction in B 2 (OH) 4 concentration.In general, 1 mol of H 2 was generated upon the expense of 1 mol of B 2 (OH) 4 .While the H 2 production rate boosted with the increment of Pt/CNT concentration (Figure 6b), with the slope was 1.67, indicating H 2 production was a first-order reaction in Pt/CNT concentration.In order to obtain E a of H 2 production upon methanol dehydrogenation over Pt/CNT, the methanol dehydrogenation was conducted from 273 to 303 K (Figure 6c).Based on the Arrhenius law, the E a was found to 19.59 kJ mol −1 .Then other common alcohols, including ethanol and propanol, had also been tested for H 2 production.As described in Figure 6d, H 2 was also successfully generated from ethanol (207.37 min −1 ) and propanol (123.84 min −1 ), but it needed 2.5 and 8 min induction time, respectively.Inter-estingly, H 2 was also successfully produced from methanol dehydrogenation at −10 °C (Figure 6e), which absolutely solved the freezing problem in the H 2 evolution upon water-splitting reaction.

Stability of Pt/CNT Nanocomposite
It is also vital to demonstrate the stability of Pt/CNT in H 2 production upon methanol dehydrogenation at the expense of B 2 (OH) 4 for the further industrial and practical application.As described in Figure 6f, the result confirmed that Pt/CNT still kept excellent H 2 production rate after at least five runs in methanol dehydrogenation.Then, the 5 th recycled Pt/CNT nanocomposite was further measured by TEM and XPS.(Supporting Information) exhibited the size of 5 th recycled Pt/CNT nanocomposite (3.92 nm, Figure S11, Supporting Information) kept the same as fresh one (3.79nm).In Figure S12 (Supporting Information), the contents of Pt (II) and Pt (0) of 5 th recycled Pt/CNT also remained the same as the fresh one.Indeed, H 2 production rate remained unchanged after five times recycling of Pt/CNT, suggesting that Pt/CNT nanocomposite was an excellent stable, heterogeneous, and recyclable nanocatalyst for methanol dehydrogenation.Moreover, the commercial Pt/C was also tested for H 2 production upon methanol dehydrogenation.Although commercial Pt/C was as efficient as Pt/CNT in H 2 production upon methanol dehydrogenation at first, the catalytic activity of Pt/C greatly deceased after five runs in methanol dehydrogenation Figure S13 (Supporting Information).According to TEM pictures, we found the size of 5 th recycled Pt/C had increased from 3.77 (Figures S14,S15, Supporting Information) to 5.15 nm (Figures S16,S17, Supporting Information).More importantly, H 2 production rate still remained unchanged after ten times recycling of Pt/CNT (Figure S18, Supporting Information).In summary, the confinement of Pt nanoparticles by CNTs is conducive to inhibiting the aggregation of Pt nanoparticles, thereby significantly increasing its catalytic performance and stability.

The Mechanism of Methanol Dehydrogenation
H 2 production upon methanol dehydrogenation was performed in CH 3 OH resp.CD 3 OD was recorded in Figure 7a.A large KIE of 2.22 was obtained indicating that the breaking of O─H bond of MeOH is the rate-controlling step for methanol dehydrogenation. [73]In Figure 7b, the gas mixture obtained from methanol dehydrogenation was confirmed by gas chromatograms (GC) to be only H 2 , illustrating that H 2 generation upon methanol dehydrogenation over Pt/CNT has been successfully realized towards fuel cell power systems.
H 2 generation upon methanol dehydrogenation is not only applied in the safe and efficient generation, storage, and transportation of H 2 , but also in its in situ tandem reactions.As shown in Figure S19 (Supporting Information), tandem reaction was carried out in a dual-chamber reactor for 1,1-diphenylethylene hydrogenation with in situ produced H 2 from methanol dehydrogenation, and the target product of 1,1-diphenylethane was provided in > 99% yield, which was confirmed by 1 H-NMR (Figure S20, Supporting Information).In addition, the by-production of B(OH) 2 OMe was further identified by mass spectrum (Figure S21, Supporting Information) and 1 H-NMR (Figure S22, Supporting Information).As shown in Figure 8, D 2 was successfully generated from CD 3 OD dehydrogenation, then deuterated 1,1-diphenylethane was also obtained in > 99% yield, which was confirmed by 1 H-NMR (Figure S23, Supporting Information).The formation of Ph 2 CD-CH 2 D was also verified by mass spectrum (Figure S24, Supporting Information).These result suggested both H atoms of H 2 are supplied by CH 3 OH.

DFT Calculation
To further verify the mechanism of H 2 generation upon methanol dehydrogenation, the systematic DFT calculation was also performed.According to our previous work, [74] Pt 18 clusters and methanol were chosen as models for H 2 generation upon methanol dehydrogenation, the plausible mechanism pathway and energy change of H 2 generation upon methanol

Conclusion
In summary, a sequence of carbon nanotube-supported Pt, Pd, and Rh nanocomposites (Pt/CNT, Pd/CNT, and Rh/CNT) have been designed and synthesized as high-performance nanocatalysts, via stabilization of Pt, Pd, and Rh nanoparticles onto CNT, for H 2 production upon methanol dehydrogenation.Therein, the optimal Pt/CNT presented the superior catalytic  A drawback of H 2 production upon methanol dehydrogenation is the difficulty of regenerating B 2 (OH) 4 from MeOB(OH) 2 , but this challenge is also appropriate for the other hydroborons. [75]Further study about the regeneration of B 2 (OH) 4 from MeOB(OH) 2 is currently under investigation in our group.
This work not only develops an efficient catalytic system for H 2 production from methanol in sub-zero temperatures, but it also proposes an easy and simple method for pure D 2 production upon CD 3 OD dehydrogenation.

Figure 3 .
Figure 3. a) STEM, b) combined Pt, O, N, and C, c) Pt, d) O, e) N, and f) C EDX mapping of Pt/CNT.

Figure 5 .
Figure 5. a) XANES spectra of the Pt L3-edge over the Pt/CNT catalyst, Pt foil, PtO, and PtO 2 ; b) FT-EXAFS spectra of Pt/CNT catalyst, Pt foil, PtO, and PtO 2 ; c) EXAFS k space fitting curves of Pt/CNT catalyst, Pt foil, PtO, and PtO 2 ; d) Wavelet transformed XAS signal of Pt/CNT.Moreover, the precise localization of C, N, O, and Pt elements in Pt/CNT nanocomposite had been further characterized by EDX.As shown in Figure3, the surface of Pt/CNT was consisted of Pt, O, N, and C elements.It is obvious that some Pt nanoparticles were confined into CNT.This confinement of Pt nanoparticles by CNTs is conducive to inhibiting the aggregation of Pt nanoparticles, thereby significantly increasing its catalytic performance and stability.[62][63][64]In addition, the chemical valence states of surface elements of Pt/CNT had also been determined by XPS in Figure4a.The high-resolution Pt 4f 7/2 spectrum of Pt/CNT was divided into two peaks at 71.63 and 72.71 eV, which were attributed to Pt (0) (74.38%) resp.Pt (II) (25.63%) species (Figure4b).[65]This result had indicated that Pt (0) was partly oxidized to Pt (II) by O 2 .As illustrated in Figure4c, the C 1s spectrum was decomposed into three characteristic peaks of C sp 2 at 284.78 eV and C sp 3 at 285.61 eV, respectively.[66]As shown in Figure4d, the O 1s spectrum of Pt/CNT nanocomposite was divided into two typical peaks of 531.88 and 533.52 eV, assigned to C═O and C─O, respectively.[67]These O-containing functional group in CNTs could protect the Pt nanoparticles by enhancing its stability and decreasing the leaching of Pt in the confined space.In addition, the XPS of Pt/ZrO 2 was also measured in FigureS9(Supporting Information), a slight shift was found in Pt 4f of Pt/CNT as compared to Pt 4f of Pt/ZrO 2 , suggesting the electron transfer from Pt atom into CNT surface.Thus, the superior catalytic performance of Pt/CNT in methanol dehydrogenation might be ascribed to its electronic interaction effect and synergistic effect.[68]

Figure 6 .
Figure 6.Plots of obtained H 2 volume versus time for H 2 evolution from MeOH with a) different concentrations of B 2 (OH) 4 , b) various amounts of Pt/CNT, c) various reaction temperatures; d) CH 3 OH, C 2 H 5 OH and C 3 H 7 OH, e) at 263 K, f) Stability test on the Pt/CNT catalyst in H 2 evolution.

Figure 7 .
Figure 7. a) H 2 evolution from MeOH catalyzed by Pt/CNT in CD 3 OD (red) or CH 3 OH (blue); b) GC spectra of evoluted gas mixture from MeOH over Pt/CNT at 30 °C.

Figure 9 .
Figure 9. Relative electronic energy (blue) diagram for H 2 generation over Pt/CNT.

Figure 10 .
Figure 10.The proposed mechanism of H 2 generation upon methanol dehydrogenation.
activity in H 2 production upon methanol dehydrogenation at the expense of B 2 (OH)4 , with a TOF value of 299.51 min −1 at 30 °C.Compared with other common carriers, including CeO 2 , ZrO 2 , NiO, ZnO, Fe 3 O 4 , and CoFe 2 O 4 , Pt/CNT exhibited the superior catalytic performance in H 2 evolution, emphasizing the critical role of CNT in methanol dehydrogenation.The confinement of Pt nanoparticles by CNTs is conducive to inhibiting the aggregation of Pt nanoparticles, thereby significantly increasing its catalytic performance and stability.The kinetic study (including Pt/CNT concentration, B 2 (OH) 4 amount, and dehydrogenation temperature) and detailed mechanistic insights, particularly KIE test, tandem reaction, GC result, and DFT calculation, confirmed that the breaking of O─H bond of MeOH was the rate-controlling step for methanol dehydrogenation, and both H atoms of H 2 were supplied by CH 3 OH.Interestingly, H 2 was also successfully produced from methanol dehydrogenation at −10 °C, which absolutely solved the freezing problem in the H 2 evolution upon water-splitting reaction.