Single-walled carbon nanotube supported Pt-Ru bimetallic superb nanocatalyst for the hydrogen generation from the methanolysis of methylamine-borane at mild conditions

Several metal nanoparticle based catalysts have been synthesized for catalyzing the hydrogen production process by hydrolysis of methylamine-borane (MeAB). However, there was only one study that catalyzes the producing of hydrogenvia the methanolysis of MeAB, and it was carried out by our research group. For this reason, in this work, a new catalyst system entitled by single-walled carbon nanotube (SWCNT) supported bimetallic platinum-ruthenium nanoparticles were developed and called as PtRu@SWCNT. These NPs were characterized by several techniques (XRD, XPS, Raman, and TEM), and they were performed for the methanolysis of MeAB with high catalytic activity. The prepared PtRu@SWCNT NPs were also tested in the methanolysis of MeAB at different parameters including different temperatures, catalyst and substrate concentrations, and reusability performance. Experimental results revealed that the new PtRu@SWCNT NPs had excellent catalytic activity and reusability for removing of hydrogen from the methanolysis of MeAB at ambient conditions. According to the obtained data, the turnover frequency is 136.25 mole H2/mole PtRu × min, and the activation energy (Ea) is 17.29 kJ/mole. More than 99% of conversion was observed at room temperature.

In the literature, there are several metal nanoparticles based catalysts for catalyzing the hydrogen production process by hydrolysis of MeAB. However, until the present study, there was only one study that catalyzes the producing of hydrogen via the methanolysis of MeAB, and it was carried out by our research group 1 .
In this study, Pt-Ru alloy nanoparticle decorated on SWCNT was synthesized and characterized by several techniques. The prepared new PtRu@SWCNT NPs nanocatalyst was tested effectively to complete dehydrogenation of MeAB by the methanolysis reaction. The methanolysis reaction with the use of the PtRu@SWCNT NPs began without any observing induction time at room conditions. The detailed kinetic study of synthesized nanoparticles for the methanolysis reaction of MeAB catalyzed by PtRu@SWCNT NPs were performed with the help of Arrhenius and Eyring equations.

Results and Discussion
the chemical and morphological structure of ptRu@SWcnt nps. In order to reveal the chemical and morphological structure of PtRu@SWCNT NPs, various advanced analytical analysis techniques were conducted, and the details of characterization studies were given in supporting information. Figure 1 shows TEM analysis for PtRu@SWCNT NPs to reveal the mean particle size and distribution of PtRu alloy nanometals on SWCNT. As seen in Fig. 1, the mean particle size of PtRu@SWCNT NPs were found to be 3.62 ± 0.5 nm and this figure also show monodisperse and homogeneous distribution of the Pt and Ru metals on the supporting material. There was no agglomeration of PtRu nanoparticles on SWCNT.
XRD analysis was used to determine the crystalline structure of the monodisperse PtRu@SWCNT NPs nanocatalyst (containing 3.34 ± 0.02 wt % PtRu as founded using ICP-OES). Figure 2 shows XRD patterns of Pt@ SWCNT NPs and PtRu@SWCNT NPs. As seen in Fig. 2, the similar XRD patterns were determined for Pt@ SWCNT NPs and PtRu@SWCNT NPs, however, there was a small shift to the higher 2θ values which shows the  alloy formation of PtRu@SWCNT compared to the Pt@SWCNT after 2 nd metal addition. Both of Pt@SWCNT and PtRu@SWCNT have showed face centered cubic (fcc) structure and the XRD analysis also revealed the crystalline structures of PtRu@SWCNT NPs after the stabilization of metal ions to metallic forms 31 .
The Raman spectroscopy was shown in Fig. 3. The peaks observed at 1349 and 1589 cm −1 , correspond to the D and G bands of carbon based materials, respectively. The intensity of graphite and the degree of graphitization of the carbonaceous materials represent the density ratio of the D-G band (I D /I G ). After the functionalization of SWCNT with PtRu nanoparticles, I D /I G value increased from 1.31 to 1.42. The change in this ratio means the increase in deficiency of SWCNT which supports their functionalization with nanoparticles 32 .   the methanolysis of MeAB catalyzed by ptRu@SWcnt nps nanocatalyst. For the catalytic performance experiments, PtRu@SWCNT NPs nanocatalyst (0.96 mM) was added to a vacuum Schlenk tube. 4 mL of pre-dried methanol added to Schlenk tube and closed with the septum. 50 mM MeAB (0.25 mmol, 11.25 mg) was dissolved in 1 mL of dry methanol. In the presence of dissolved MeAB and N 2 gas, it is placed in a jacketed Schlenk. Then the timer is started at t = 0. The released hydrogen gas amount was recorded using a cylinder burette. The experimental results obtained from different PtRu@SWCNT NPs nanocatalyst concentrations (0.48-1.20 mM) in the methanolysis of MeAB at mild conditions were given in Fig. 5. The hydrogen evolves began no observing any induction time as seen in Fig. 5(a). The complete hydrogen releasing from MeAB catalyzed by PtRu@SWCNT NPs nanocatalyst occurred in a very little time like 3.5 min at mild conditions. The plot obtained from the experiments carried out at different PtRu@SWCNT NPs nanocatalyst concentrations is given in Fig. 5(b) (lnk obs versus to ln [PtRu]) and the obtained plot is linear. The slope of the plot was found to be 0.92. According to experimental results, the rate of MeAB methanolysis, in the presence of PtRu@SWCNT NPs nanocatalyst was determined as 0.92 nd depending on the concentration of PtRu@SWCNT NPs superb nanocatalyst.  To set the optimal temperature for the methanolysis of MeAB catalyzed with PtRu@SWCNT NPs various experiments were carried out containing 50 mM MeAB and 0.96 mM PtRu@SWCNT NPs nanocatalyst at different temperatures (25-55 °C). The results obtained from the experiment conducted at different temperatures and Arrhenius -Eyring equations were used to calculate activation (activation energy (Ea) enthalpy (ΔH # ) and entropy (ΔS ≠ )) and the kinetic parameters of MeAB catalyzed with PtRu@SWCNT NPs nanocatalyst. The results of the experiments conducted at different temperatures are given in Fig. 7(a). As seen in this figure, when increased temperatures, the catalytic activity of PtRu@SWCNT NPs nanocatalyst were increased. The activation energy (Ea) for the methanolysis of MeAB catalyzed with PtRu@SWCNT NPs nanocatalyst was calculated to be 17.29 kJ/mole using Arrhenius plot (given in Fig. 7(b)). Additionally, the observed reaction constant given in Fig. 7(c) was used to calculate enthalpy and entropy values for the methanolysis of MeAB catalyzed with PtRu@SWCNT NPs nanocatalyst, and these values were found to be ΔH # = 15.46 kJ/mole and ΔS # = −171.68 J/ (mole × K), respectively. To test the stability and recyclability of PtRu@SWCNT NPs nanocatalyst in the methanolysis of MeAB at room temperatures, the same concentration of MeAB was subsequently added the completed www.nature.com/scientificreports www.nature.com/scientificreports/ experiment reaction after the previous run. Finally, the recyclability performance of PtRu@SWCNT NPs nanocatalyst has been shown to maintain its initial activity (87%) and provides high conversion (>99%) at the end of the 5th catalytic cycle (Fig. 8).
The initial turn-over frequency (TOF initial ) for PtRu@SWCNT NPs was found to be 8175 h −1 (136.25 min −1 ) at room temperatures and the calculated this TOF value were compared the TOF values present in literature as seen in Table 1. This TOF value is higher than the other study used for the methanolysis of MeAB at room conditions. As a result, the synthesized PtRu@SWCNT NPs nanocatalyst exhibited a superior catalytic activity compared the previous catalyst used for the methanolysis of MeAB. This unique catalytic activity can be ascribed to the large surface area of the catalysts, the synergic effects of alloy metals (Pt-Ru) with SWCNT and ultrafine structure.

conclusions
In summary, even though several metal nanoparticles based catalysts have been synthesized for catalyzing the hydrogen production process by hydrolysis of MeAB, there was only one study related to the methanolysis of MeAB. For above reason, the superb PtRu@SWCNT NPs nanocatalyst was synthesized and tested as an effective catalyst in the methanolysis of MeAB with an easy and facile technique at mild conditions. With this report, a new and effective PtRu@SWCNT NPs nanocatalyst was developed for the methanolysis reaction of MeAB with complete hydrogen evolve at mild conditions. PtRu@SWCNT NPs superb nanocatalyst showed very high catalytic activity in the dehydrogenation of MeAB in dry methanol environment. The rate law of catalytic methanolysis of MeAB including PtRu@SWCNT NPs superb nanocatalyst was obtained as -d[CH 3 33 . For this aim, 0.1 mole (3.88 g) of NaBH 4 was weighed into a 250 mL two-necked flask, and 200 mL of anhydrous tetrahydrofurane (THF) was added. After stirring for 30 minutes at 25 °C, 0.1 mol (6.752 g) of methylamine hydrochloride was added through 24 hours at 25 °C under N 2 atmosphere. The mixture was filtered, and the liquid phase was evaporated, allowing the THF to leave the medium. THF was entirely removed from the medium, and 100 mL of dry ether was added. The mixture was stirred for 2 hours in a cryostat system at  www.nature.com/scientificreports www.nature.com/scientificreports/ a temperature of 0 °C. At the end of this period, the solid phase was re-filtered. The supernatant was left at room temperature for complete removal of water. When the supernatant was evaporated entirely, the white solids were formed as shown in following scheme (3 preparation of single-walled carbon nanotube supported platinum-ruthenium nanoparticles (ptRu@SWcnt nps). In the preparation of the new PtRu@SWCNT NPs nanocatalyst, an easy and facile one-step reduction technique was used at room conditions. Briefly, a solution containing 30 mg K 2 PtCl 4 , 30 mg RuCl 3 •xH 2 O , and 60 mg SWCNT were mixed in 20 mL of water. After that, a solution containing NaBH 4 was added to the mixture and waited until the bubble formation was finished. After that, black colored PtRu@SWCNT NPs superb nanocatalyst was obtained. The resulting mixture was filtered, and the obtained solid residue was washed with plenty of deionized water (3 × 10 mL), dried at inert medium at 80 °C.