Pd Catalysts Supported On Porous Carbon Nitrogen For H₂ Production From Formic Acid

The exploration and large-scale application of new energy is an indispensable part of achieving sustainable development.Hydrogen energy is considered to be an ideal secondary energy source because of its rich source,high calorific value and no pollution.However,the disadvantages of flammable and explosive,making it be limited in storage and transportation.Formic acid exists in liquid form at room temperature and can be safely stored and transported,making it a chemical hydrogen storage material which can be widely used.The decomposition of formic acid to produce hydrogen usually requires the catalysis of the catalyst.Homogeneous catalysts are generally difficult to recycle,and heterogeneous catalysts are widely used.But some heterogeneous catalysts still have some shortcomings,such as lower hydrogen production efficiency,the by-product CO leading to catalyst poisoning and the reaction requiring high temperature conditions.In recent years,some scholars have found that carbon-based materials doped with nitrogen can not only effectively prevent the production of by-product CO but also increase the activity of the catalyst.However,there are many kinds of nitrogen species in the nitrogen doped carbon materials due to the different coordination environment of nitrogen.It is important to understand that the contribution of specific nitrogen species to the dehydrogenation of formic acid,and it is also important for the preparation of high-performance catalyst.But there are few systematic studies related to this.Therefore,in this paper,we prepared two kinds of nitrogen-doped carbon materials with different nitrogen species through controlled synthesis methods,and further prepared Pd-based catalysts for hydrogen production.The catalysts were systematically characterized by X-ray diffraction(XRD),N2 adsorption and desorption,elemental analysis,transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS)to investigate the contribution of specific nitrogen species to the dehydrogenation of formic acid combined with activity test result.The main findings are as follows:1.The covalent triazine polymeric porous materials(CTF)of different calcination temperatures(350,450,550,650?)were prepared by ion-heat copolymerization,and Pd was loaded on them as a carrier by precipitation deposition method to synthesize four kinds of Pd/CTF catalyst,activated carbon supported Pd(Pd/AC)as a comparative catalyst.With the increase of the calcination temperature of the support,the catalytic activity of Pd/CTF first increased and then decreased.Among them,the catalytic activity of Pd/CTF-450 was the highest,the activity of Pd/CTF-350 was the worst,and the catalytic activity of Pd/CTF was generally higher than that of Pd/AC(except Pd/CTF-350).XPS results showed that with the increase of calcination temperature of the support CTF,the content of pyridine nitrogen and Pd(II)content in the catalyst increased first and then decreased.The catalytic activity of Pd/CTF synthesized with different calcination temperatures showed the same pattern.TEM results showed that the particle sizes of Pd/AC and Pd/CTF-350 were much larger than other Pd/CTFs.After analysis,it was found that pyridine nitrogen in the support material plays a crucial role in enhancing the activity of the catalyst.Its main performance is as follows:First,pyridine nitrogen promotes the deprotonation of formic acid to accelerate the reaction rate;Secondly,the higher the content of Pd(?),the higher the content of pyridine nitrogen,and the Pd(?)can promote the cleavage CH bond in HCOO-to accelerates the reaction rate.Third,pyridine nitrogen can effectively promote the dispersion of Pd particles.The formation of small particles of Pd increases the active sites and accelerates the reaction rate.2.In order to obtain a more active catalyst,ethylenediamine with higher nitrogen content was selected as the nitrogen source,and carbon tetrachloride was used as the carbon source to prepare the mesoporous nitride of different calcination temperatures(400,600,700,800?).MCN was loaded with Pd by precipitation deposition and Pd was supported by mesoporous carbon MC-600 as a comparative catalyst.The activity results showed that the Pd/MCN-600 had the highest TOF value(597.8 h-1).Under ambient condition and without additives,the catalytic activity of Pd/MCN-600 was higher than that reported in most literatures,and no by-product CO was detected in the product.Elemental analysis and XPS results showed that the total nitrogen content,surface nitrogen content,pyridine nitrogen content,and Pd(II)content all showed a trend of gradual decrease.The nitrogen content in Pd/MCN was significantly promoted than that of Pd/CTF.When the calcination temperature is higher than 600?,the rules of the catalytic activity is the same to pyridine nitrogen and Pd(II)in the catalyst.The catalytic activity of Pd/MC-600 is lower than all other Pd/MCN,it was again verified that the pyridine nitrogen in the support material plays an important role in enhancing the activity of the catalyst.After five cycles of use,the catalyst can still maintain a high catalytic activity,indicating that the catalyst has a high recycling potential.