Anchoring Pd-nanoparticles on dithiocarbamate- functionalized SBA-15 for hydrogen generation from formic acid

Hydrogen (H2) generation from natural biological metabolic products has remained a huge challenge for the energy arena. However, designing a catalytic system with complementary properties including high surface area, high loading, and easy separation offers a promising route for efficient utilization of nanoreactors for prospective H2 suppliers to a fuel cell. Herein, selective dehydrogenation of formic acid (FA) as a natural biological metabolic product to H2 and CO2 gas mixtures has been studied by supporting ultrafine palladium nanoparticles on organosulfur-functionalized SBA-15 nanoreactor under ultrasonic irradiation. The effects of the porous structure as a nanoreactor, and organosulfur groups, which presented around the Pd due to their prominent roles in anchoring and stabilizing of Pd NPs, studied as a superior catalyst for selective dehydrogenation of FA. Whole catalytic systems were utilized in ultrasonic irradiation in the absence of additives to provide excellent TOF/TON values. It was found that propose catalyst is a greener, recyclable, and more suitable option for the large-scale application and provide some new insights into stabilization of ultra-fine metal nanoparticle for a variety of applications.


Scientific Reports
| (2020) 10:18188 | https://doi.org/10.1038/s41598-020-75369-y www.nature.com/scientificreports/ effect in catalytic systems for energy arena such as selective dehydrogenation has been unexplored to date. However, despite the unique ultrasonic technology and their effect in catalytic systems for energy arena such as selective dehydrogenation has been unexplored to date. On the other hand, dithiocarbamate (DTC) ligands possess well coordination capability, versatile coordination modes, and unique chemical properties of their metal complexes and nanoparticles [45][46][47][48] . The best affinity between the DTC ligand and the template walls is basically important for grafting and stabilizing the Pd NPs, and inhibition of the "Ostwald Ripening" process. Based on the above considerations, we envisioned an economical method for the bifunctional fabrication catalyst with features such as benign environmental, stable, and affordable for selective dehydrogenation of aqueous solutions of HCOOH We exploited both metal and organosites for the sustainability for H 2 generation from FA. The resultant of Pd NPs @SBA-15/DTC with uniform pores exhibit excellent catalytic activity in the presence of ultrasonic wave as a synergistic agent with the turnover frequency (TOF) value of 1952 h −1 , turnover number (TON) value of 3904 and 9.86 mL of gas (H 2 and CO 2 ) was obtained in 120 min. Characterization. The FT-IR spectra were performed using a PerkinElmer-Spectrum Two with ATR probe. Catalytic dehydrogenation of FA. Catalytic dehydrogenation of FA was accomplished in a two-necked round bottom flask (volume, 80 mL). One neck of the flask is connected to a gas burette and the other was reserved for adding 50 mg of Pd-catalyst mixed with 5.0 ml of FA-SF ( FA: 5% v/v with a mole ratio of 1:1) under ultrasonic irradiation. To examine whether or not CO gas was formed in the dehydrogenation process, the generation gases analyzed by GC and NaOH trap experiment. Using 6.64 mmol of FA under ultrasonic irradiation, the complete conversion is expected to develop 9.86 mL of the gases mixture with TOF value of 1952 h -1 , as calculated from the following equation.

Experimental
TOF was obtained from decomposition of FA/SF (1:1), which Patm is atmospheric pressure, Vgas is volume of produced gas, R is universal gas constant, T is room temperature and n Pd is mole number of Pd in the catalyst.

Results and discussion
In this study, SBA-15 was prepared a supramolecular self-assembly strategy and then dithiocarbamate grafted on 3D ordered mesoporous silica. The resulted white powder was then utilized as the excellent support for immobilizing ultra-fine Pd NPs Fig. 1. The hydrophobic-hydrophilic properties of the mesoporous silica are controlled By embedding the DTC ligand into the pore walls. Plus, the DTC ligand and pores structure of nanoreactor SBA-15 has a vital role in the excellent control of Pd metal size and homogeneous dispersion, of that, then boosting the catalytic activity for selective dehydrogenation of FA. The chemical groups of the SBA-15, DTC/SBA-15, and Pd NPs @SBA-15/DTC were analyzed by FTIR spectra. As depicted in Fig. 2a, several absorption bands in 1400-1700 can be ascribed to ligand DTC in the SBA-15/DTC framework. The signal appears in 3241 cm −1 is attributed to N-H stretch mode, which suggesting the successful  . 2c). Also, all DTC ligand (Fig. 2e) and Pd particles (Fig. 2f) are well-anchored onto mesoporous silica (Fig. 2d). The existence of all constituent elements (i.e., Si, C, O, N, S, and Pd) in structures of Pd NPs @SBA-15/DTC has been proved by the EDS analysis.   Fig. 3a. After deposition of Pd NPs on SBA-15/DTC surface, the rod-like structures preserve, which demonstrated NPs were orderly anchored (Fig. 3b,c). The TEM images further reveal the spherical Pd NPs that are well-distributed on the surface of the SBA-15/DTC Fig. 3d. The surface of mesoporous silica with DTC ligand helps for the stabilization of Pd NPs and inhibition of agglomeration. The STEM-EDS Fig. 3e and elemental mapping images in Fig. 3f-k proved the existence of all constituent elements (i.e., Si, O, C, N, S, Pd) in the mesoporous silica structure. It demonstrates the uniform distribution of Pd NPs in SBA-15/DTC channels.
Ultimately, after the characterization of meso-material, the catalytic activity of Pd NPs @SBA-15/DTC, with its comparative component for the dehydrogenation of FA at room temperature, is stated in Fig. 5a. According to Fig. 5a, 9.83 mL of gases are released in 120 min catalyzed by Pd NPs @SBA-15/DTC, which corresponding to an initial TOF value of 1954 and TON 3904.
To evaluate the performance of Pd NPs @SBA-15/DTC (2.89 wt% Pd, determined by ICP analysis), for catalytic H 2 production, we utilize FA/SF (1: 1) as the reaction substrate. Very recently, a variety of noble metal NPs [54][55][56] has been used to catalyze the selective dehydrogenation of FA under mild conditions. Remarkably, Pd NPs @SBA-15/DTC had catalytic activity comparable to immobilized Pd complex (Pd (II) @SBA-15/ DTC), Pd (II) @SBA-15, SBA-15/DTC, and SBA-15 without the use of any additive in presence ultrasonic wave. This result demonstrates the DTC groups can apparently elevate the electron transfer from the host to Pd NPs and thus increases activity dehydrogenation FA. Even though the H 2 amount for Pd NPs @SBA-15/DTC is more than others, we continued our investigations with encapsulated Pd NPs. For instance, when we investigated the dehydrogenation progress with various solvents such as EtOH, MeOH, DMF, and H 2 O, we found that H 2 O with 9.86 mL H 2 gas and Tenover frequency (TOF) of 1952 is more efficient than the other solvent Fig. 4b. Using Pd NPs @SBA-15/ DTC, CO generation was insignificant (detected by GC and NaOH trap experiments), whereas H 2 generation as well associates with literature data 57,58 . When the amount of the catalyst increased from 10 to 50 mg, the FA dehydrogenation was also increased (Fig. 4c). On the other hand, the catalytic activity of Pd NPs @SBA-15/DTC is enhanced by the addition of SF, and we continued our studies in the best mole ratio of FA: SF is 1:1 (Fig. 4d).
Additionally, when the reaction was tested at various temperatures, it showed that ultrasonic condition in 335 K accelerates the decomposition of FA reaction Fig. 5a. Because of the enhanced rate of the dehydrogenation FA at room temperature and the significant importance of associated chemical processes and energy issues, we continued our studies under ambient conditions. According to this study, the activation energy (Ea) of the dehydrogenation FA is 37.48 ± 2.0 kJ mol -1 according to the Arrhenius plot, which is in agreement with previous studies Fig. 5b. The catalytic performance is related to the reaction temperature in a positive way, giving the TOF values of 1952, 1985, 2085, and 2154 h −1 at the corresponding reaction temperatures Fig. 5c, respectively. As represented in (Fig. 5d), the activity of all SBA-15 samples was enhanced under the wave of ultrasonic, and the Pd NPs @SBA-15/DTC produce a large amount of H 2 generation in comparison with that high-speed stirrer (HSS). Therefore the suggestion is that ultrasonic irradiation is the third function that acts as a synergistic agent in the catalytic decomposition of FA.
According to the promising results obtained above, NaOH trap experiments were performed to determine the volume ratio of H 2 to CO 2 in the dehydrogenation process. The produced gas was treated with/without NaOH trap (Fig. 6). The volume of produced gas was decreased to half after treating with the 10 M NaOH aqueous solution. The results demonstrate complete adsorption of CO 2 gas in NaOH solution (I), and the produced gas is H 2 and CO 2 with the volume ratio of 1:1. Finally, runs were repeated to check the activity and stability of the Pd@SBA-15, reusability tests were also performed by adding neat FA (5% v/v) to the reaction mixtures under the ultrasonic wave. The pre-catalysts was recovered and reactivated by common filtration after each reaction. As shown in Fig. 7a, the activity was approximately kept for the first eight-time suggesting that the Pd@SBA-15 was stable during the decomposition of FA for a long time. However, after each cycle of reaction, a small amount of decrease in yield was observed, which could be attributed to the reality that a small fraction of the catalyst is lost in each recovery. Stable mesoporous silica and Pd NPs aggregation after the reuse testified by TEM images (Fig. 7b). According to these results, the proposed catalyst after 8 th run affirming the overall structural integrity of the material without aggregation of Pd particles.

Conclusion
The purpose of the current study was to design DTC/SBA-15 as excellent support for anchoring and stabilizing Pd in nanoscale. The proposed catalyst shows high catalytic activity, good stability, and easy recyclability for the selective dehydrogenation of FA under ultrasonic irradiation. The results suggested that the mesoporous structure and synergistic effect of palladium metal, dithiocarbamate ligands in SBA-15 frameworks, and ultrasonic irradiation play serious roles in the catalytic performance of the H 2 generation from FA. Moreover, by modifying this mesoporous material with the various organic ligands for stabilizing other noble metal nanoparticles can be developed to prepare versatile heterogeneous catalysts for organic transformations and environmental application.