Construction Of Amine-modified Pd-based Catalysts For The Formic Acid Hydrogen Storage Research At Room Temperature

With the over-exploitation and use of traditional fossil sources,the concentration of CO2 in atmosphere is increasing year by year,and the problems of energy shortage and environmental pollution are becoming increasingly,leading to serious challenge to human’s survival and development.Hydrogen is an important clean energy source,however,its low bulk density,flammability and explosiveness make it difficult to store and transport,which greatly limits its large-scale application.Recently,CO2-mediated formic acid-based reversible hydrogen storage has attracted much attention.On the one hand,this process can achieve hydrogen storage and carbon reduction through CO2 hydrogenation;on the other hand,using formic acid for hydrogen storage has the advantages of stable liquid-type hydrogen storage,non-flammability and high hydrogen storage density(4.4 wt%).However,due to the high thermodynamic stability of the CO2 molecule(ΔG=-396 kJ/mol),CO2-mediated reversible formic acid storage processes,especially CO2 hydrogenation to formic acid,are currently carried out under harsh reaction conditions such as high temperature and pressure and alkaline media,resulting in high energy consumption in reaction and separation processes.In this thesis,a series of amine-modified Pd-based nanocatalysts were designed and synthesized for the study of CO2-mediated formic acid-based hydrogen storage processes under mild conditions.In terms of catalytic effect,we have achieved the capture and conversion of C02 into formic acid and fertilizer at room temperature and pressure,and the one-step synthesis of pure formic acid solution under room temperature,and firstly verified the reversible formic acid based hydrogen storage under ambient conditions;in terms of catalyst conformation relationship,we explored the synergistic catalytic mechanism between amine functional groups and active metals.The specific results of the study are as follows:1.By constructing amino-modified Metal-Organic Frameworks(MOFs)carbon material loaded PdAu nanocatalysts(PdAu/CN-NH2),a technical route for CO2 capture and conversion to formic acid(HCOOH)and fertilizer(NH4H2PO4)under ambient conditions was achieved.Specifically,the pathway captures CO2 and converts it to NH4HCO3 using 0.5 M(NH4)2CO3 solution,followed by hydrogenation of the captured CO2 to HCOONH4 in the presence of the prepared catalyst.Then simple phosphorylation and distillation separation processes were applied to obtain pure HCOOH solution and NH4H2PO4,where the HCOOH can be used as a hydrogen storage medium and NH4H2PO4 can be used as fertilizer.The experimental results showed that the route possessed excellent CO2 capture capacity of 0.56 mmol/g and CO2 conversion rate of 32.6%under ambient conditions.With the Pd:Au ratio optimized to be 1:1,PdAu/CN-NH2 catalyst produced pure HCOOH at a concentration of 378.5 mmol/L and the highest turnover frequency(TOF)of up to 87 h-1.The catalytic activity reached the highest level for mild CO2 hydrogenation,which was comparable to the harsh reaction systems.According to detailed X-ray photoelectron spectroscopy(XPS)and in-situ Diffuse reflectance infrared fourier transform spectroscopy(in-situ DRIFTs),the catalyst’s PdAu alloy effect and the adjacent amino groups promote the adsorption of HCO3-,both of which synergistically contribute to the improved catalytic activity.2.Under base-free conditions,the hydrogenation of CO2 to pure formic acid solutions at room temperature has been made possible by the synthesis of ultra-fine Pd nanoclusters modified with organic amines.Based on the different characteristics of the interaction between amine groups and CO2,ultrafine Pd nanocluster catalysts modified with primary amine(PA),secondary amine(SA)and tertiary amine(TA)were designed and synthesized in this work.Their catalytic mechanisms for the direct synthesis of pure formic acid from CO2 at room temperature were investigated from those perspectives including Pd electronic properties,the basic strength of amine groups and corresponding steric hindrance.Various characterisation techniques,including high-resolution transmission electron microscopy(HRTEM),XPS,and ultraviolet-visible absorption spectroscopy(UV-VIS),have demonstrated that all three amine groups interacted strongly with Pd2+,with the strength of the interaction being TA-Pd+>SA-Pd3+>PA-Pd2+.This kind of interaction was found to promote the formation of ultrafine Pd nanoclusters(<2 nm)as well as the electrons transfer from amine N to Pd,allowing the Pd to exhibit electron-rich properties and higher reactivity in CO2 nucleophilic reaction.Meanwhile,there is a competitive relationship between Pd electron properties and steric hindrance of amine groups.The secondary amine modified catalyst(Pd/AC-SA)showed the best catalytic activity due to its relatively lower steric hindrance and Pd-rich electron properties,obtaining a TOF of 29.1 h-1 at 298 K and 4 MPa.The catalytic activity reached the highest level for the synthesis of pure formic acid.3.By synthesizing a commercial activated carbon loaded PdAu bifunctional catalyst(PdAu/AC-LA)modified with L-arginine(LA).a pH-controlled reversible CO2-formic acid-based hydrogen storage process was carried out under ambient conditions.In the proposed hydrogen storage system,CO2 is hydrogenated under alkaline conditions,and formic acid is dehydrogenated under acidic conditions to release hydrogen.By optimizing the amount of L-arginine content and the Pd/Au ratio,CO2 hydrogenation activity with TOF of 137 h-1 and formic acid dehydrogenation activity at 1760 h-1 were obtained,both of which reached the highest levels reported so far.XPS,CO2-programmed temperature desorption(CO2-TPD)and HCOO-adsorption studies showed that L-arginine promotes the hydrophilic performance and adsorption to acidic reactant of CO2 and HCOOH,and the PdAu alloy effect promotes the formation of electron-rich active Pd,both of which synergistically contribute to the increased reactivity of the catalyst.