Supported AuPd Alloy Catalysts For Decomposition Of Formic Acid To Hydrogen At Low Temperature

The shortage of energy and environmental issues are two major challenges in current and future developments of human society. Sustainable and benign energy sources and an efficient way of energy distribution are prerequisites. Hydrogen is considered as a clean and efficient energy carrier and has attracted increasing attention due to its wide application in proton exchange membrane fuel cells (PEMFCs). However, controlled storage, transportation and release of hydrogen have been a major challenge that limits the development of hydrogen energy.Suitable and efficient chemical energy carrier is an important and challenging topic in recent years. As a main byproduct of biomass process, formic acid is considered promising as a hydrogen carrier because of its relatively high hydrogen content, non-toxicity and stability, where hydrogen is released by catalyzed decomposition of formic acid.Compared to homogeneous catalysts, heterogeneous catalysts could be easier to carry, separate and recycle, and have been extensively studied. The choice and modification of support and active center were critical to the synthesis of catalyst. By controlling synthesis conditions (synthetic method, solvent, temperature, the concentration of metal precursors and addition of surfactant polymer or foreign ions and so on), the process of nucleation and growth of nano-particle on support were under control. Thus carbon supported AuPd alloy catalysts with high activity and selectivity for hydrogen generation from formic acid were synthesized in our lab.The support chosen in our lab were carbon with high specific surface area. The active center was AuPd alloy nanoparticles. The synthetic method was co-reduction in aqueous phase at low temperature. By controlling of temperature, the ratio of Au and Pd and foreign ions (mainly magnesium ion), the process of nucleation and growth of nano-particle on support were under control. Catalysts synthesized in our lab were characterized by power X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectrometer (XPS), UV-Vis, et. The results show that well dispersed Au-Pd alloy nanoparticles with average particle size of around 2 nm supported on carbon black was successfully synthesized. There are two main parts in this paper.(1) By reduction of mixed noble metal precursors in aqueous phase under ice-water bath conditions, well dispersed Au-Pd alloy nanoparticles supported on carbon black were facilely prepared. The catalyst exhibited high activity and selectivity at nearly 0℃, with an initial turnover frequency (TOF) up to 635 h-1. At room temperature the initial TOF of the catalyst reached 1075 h-1, which is rather high compared with those reported in the literature. The activation energy is calculated to be 21.98 kJ/mol, lower than activation energy reported in some earlier research. The activity of supported AuPd alloy catalyst was far better than that of supported Au and supported Pd nanocatalyst. Gas volume generated with catalysts synthesized with different Au and Pd ratios demonstrated that the content of Pd was crucial to the activity. Without alloying with Au the activity of pure Pd decrease greatly. Meanwhile, pure Au/C catalyst showed no activity towards dehydrogenation of formic acid. The surface property of Pd was modification because of alloying with Au. The element mapping by high-angle annular dark-field scanning TEM (HAADF-STEM) indicate that Au and Pd are evenly distributed in the metal nanoparticles. The higher activity of L-Au6 Pd4 could be ascribed to smaller particle size and better alloy structure resulting from the low reduction temperature. Low temperature efficiently controls the nucleation and of noble metal nanoparticles, which leads to larger surface area available for reactant.(2) The rate of nucleation, growth and aggregation of nannoparticle can be well controlled by adding proper surfactants, polymers, foreign ions, or ligands and adjusting other reaction parameters. By modulating the parameter of the synthesis, highly-dispersed AuPd alloy naocatalyst deposited on carbon have been successfully prepared with Mg2+-assisted co-reduction approach. AuPd alloy catalyst assisted with magnesium ion with average particle size of around 2 nm show high show high activity and selectivity for the decomposition of formic acid. The initial turnover frequency (TOF) could reach 1530 h-1 at room temperature, which was a rather high value comparable to that ever reported in literature. This remarkable improvement of catalytic performance of catalyst may strongly encourage the practical application of formic acid as hydrogen storage material in H2 economy. Meanwhile, the Mg2+-assisted synthetic method provides a new route for the synthesis of supported metal nanoparticles catalyst.