Study On The Controllable Synthesis Of Supported Palladium Based Catalysts And Their Mechanisms For The Dehydrogenation Of Formic Acid

Hydrogen energy is abundant and a kind of ideal clean energies.The effective utilization of hydrogen is a powerful tool to solve the problems of environmental pollution and the depletion of fossil fuels.Formic acid is a nontoxic chemical hydrogen production material with a gravimetrical hydrogen storage density of 4.4 wt.%and exists in a liquid form under atmospheric pressure which is easy to operate and transport.It can be regenerated through the hydrogenation reaction of carbon dioxide.Nevertheless,the dehydrogenation of formic acid can not occur spontaneously at room temperature but only in the presence of catalyst to improve its performance for dehydrogenation reaction.On the basis of the overview on the research and development of formic acid for hydrogen storage,the palladium based catalysts can be optimized to improve the catalytic performance from three aspects of support,composition and structure.The catalytic mechanism during the process of dehydrogenation is also investigated.First of all,the dissertation prepares titanium dioxide nanosheets as the support of the catalyst.AuPd/TiO2(L)-400 composite is prepared by changing the preparation conditions and demonstrates the best catalytic performance.The initial turnover frequency(TOF)value can reach 373 molH2 molcat-1 h-1 at room temperature and the apparent activation energy is 11.8 kJ mol-1.It is also found that the catalyst shows the better performance employing titanium dioxide nanosheets as the support than that of titanium dioxide nanotubes and titanium dioxide particles as the catalysts' supports.The analysis shows that titanium dioxide nanosheets can effectively inhibit the growth of metal nanoparticles and transfer large electron to gold and palladium atoms,thus improving the catalytic activity of the catalyst.Secondly,the prepared nitrogen-decorated carbon nanosheets(n-CNS)are applied as the support.The content and configuration of nitrogen atoms on the nitrogen-decorated carbon nanosheets can be altered by changing the reaction conditions.The prepared AuPd/NC-Th-160 composite exhibits the catalytic activity with an initial TOF value of 459 molH2 molcat-1 h-1 at room temperature.The apparent activation energy is determined to be 28.7 kJ mol-1.It is found that the nitrogen atoms on the nitrogen-decorated carbon nanosheets can significantly improve the catalytic performance of catalyst for formic acid dehydrogenation.The higher ratio of graphitic nitrogen to pyridinic nitrogen on n-CNS is,the better performance of catalyst will be.The nitrogen bonding configuration can modify the electron structure distribution of carbon and facilitate the interaction between AuPd metal NPs and support.In order to reduce the application cost of the catalyst,the catalytic performance of the catalyst is improved by adding non-noble metal element of lanthanum to form ternary alloy structure of AuPd-La2O3.The of catalyst carbon nanotubes supported Au0.3Pd0.7-(La2O3)0.6/CNTs exhibits a catalytic activity with an initial TOF value of 589 molH2 molcate-1 h-1 at low temperature.The hydrogen selectivity is 100%and apparent activation energy is 30.7 kJ mol-1.The addition of La metal can improve the electron cloud density of the active metal Pd,so as to effectively enhance the performance of the catalyst.Finally,the dissertation also employs graphene as support to form AgPd-Hs/G.AgPd-Hs/G is synthesized by galvanic replacement reaction.AgPd-Hs/G composite has an excellent catalytic activity and can make formic acid release 78%of overall gas with 100%hydrogen selectivity.AgPd-Hs/G composite has an initial TOF value of 333 molH2 molcat-1 h-1 and an apparent activation energy of 28 kJ mol-1.The initial TOF value of AgPd-Hs/G composite catalyst is 4.5 and 1.9 times of graphene supported palladium catalysts(Pd/G),carbon supported AgPd catalysts(AgPd/C)under the same reaction condition.The analysis results demonstrate that the highly catalytic performance of catalyst derives from alloy effect between Ag and Pd,good dispersion of graphene and hollow structure of the metal nanoparticles.