Construction Of Heterogeneous Catalytic System For Hydrogen Generation And Storage Based On Formic Acid/formates

Hydrogen is the most abundant element in the Earth. The product of hydrogen combustion is water showing no pollution for environment, which is good agreement with the so-called "hydrogen economy" concept. Hydrogen was also considered to be the high energy density, ideal and clean fuels, especially applied on proton exchange membrane fuel cells (PEMFCs) for electrons and devices. Currently, the bottleneck of PEMFCs applications is the storage and transportation of ultra-pure hydrogen. Compared to physical hydrogen storage, chemical hydrogen storage shows high efficiency, high safety as well as other advantages. Formic acid (HCOOH, FA) has been attracting more and more attentions in the process of searching for much ideal material for chemical hydrogen storage. Compared to methanol, hydrazine hydrate, ammonia borane and others, FA exhibits non-toxic, low volatility, inexpensive, non-flaming, safe storage and transportation, convenient, and so on. FA, with4.4wt%hydrogen content, is liquid phase on room temperature. FA was mainly attained in industry with the method of methanol carbonylation and consequent methyl formate hydrolysis. Recently, FA is largely by-produced in the processes of biomass catalytic transformation with the rapid developing of biorefineries all over the world. Compared to the homogeneous catalysts with the shortages of air-sensitive and poor stability, the development of heterogeneous catalysts for high efficient and selective dehydrogenation of liquid-phase formic acid shows much more practical importance. The present dissertation aims to explore the new methods and new procedures for high chemical energy storage using heterogeneous catalysts to selectively catalytically decompose liquid-phase formic acid and/or formates for ultra-pure hydrogen gas under mild conditions. The following work was carried out in the present dissertation: 1. Hydrogen generation via liquid-phase formic acid decomposition under ambient temperatureInterest in using supported gold nanoparticles (NPs) or nanoclusters (NCs) as a low-temperature catalyst for green and sustainable chemical synthesis has gained considerable attention over the past few years. Our research group recently reported that small Au NPs (1.8nm) deposited on acid-tolerant zirconia(Au/ZrO2) can promote a facile hydrogen-independent transformation of aqueous bioderived levulinic acid (LA) and FA streams into y-valerolactone (GVL). The essential role of supported gold is to facilitate rapid and selective decomposition of FA to produce a hydrogen stream at120-180℃, thereby enabling a highly efficient reduction of LA without requiring an external source of hydrogen.Following the previous work, we prepared and found a novel gold catalyst comprising TEM-invisible subnanometric Au NCs deposited on ZrO2. The Au/ZrO2-NCs was the robust heterogeneous Au catalyst, which can be obtained with facile preparation process using modified NH4OH deposition-precipitation (DP) method, and much milder preparation and thermal treatment conditions. Au/ZrO2-NCs catalyst can selectively catalyze liquid-phase FA-NEt3dehydrogenation with high efficiency on ambient conditions, the reactivity was the best in heterogeneous catalytic systems at that time. The reaction was accelerated at higher temperatures, but even at room temperature, a significant H2evolution with turnover frequencies (TOFs) up to252h-1can still be obtained. The dehydrogenation reaction can generate pressurized H2for convenient application in PEMFCs in small-scale devices on the continuous supply of material FA, the TOF up to1,593h-1and turnover numbers (TONs) up to118,400at50℃. The present study not only disclosed the intrinsic catalytic ability of Au, but also established a more practical and convenient hydrogen generation process for industry with great prospect. ZrO2support can substantially facilitate crucial FA deprotonation appears to be a key factor for achieving high activity in the catalytic FA dehydrogenation owing to the amphoteric feature. Preliminary mechanistic studies suggest that the reaction is unimolecular in nature and proceeds via a unique amine-assisted formate decomposition mechanism on Au-ZrO2interface.2. Hydrogen generation via FA selective decomposition in base-free aqueous solutionOn the basis of the first section, we further found that ZrO2support with different pure crystal phase supported Au catalyst exhibited the different and excellent activity for the dehydrogenation of FA in greener base-free aqueous system. ZrO2support with different crystal phases, including tetragonal (t-ZrO2) and monoclinic (m-ZrO2) polymorph, showing different acidic/basic properties and varied surface -OH concentrations owing to the different coordination environment exhibited by Zr and O. NH3-TPD results indicated that there is no significant distinction for the total amount of surface acidic sites on Au/t-ZrO2, Au/ZrO2and Au/m-ZrO2catalysts with the similar size of Au NPs (2.0nm). However, the CO2-TPD showed that there is a significant distinction for the total amount of surface basic sites on these three catalysts. The HCOOD-IR results demonstrated that Au/m-ZrO2catalyst with more surface basic sites can facilitate the H/D exchange between the OH group of support surface and HCOOD. Au/m-ZrO2catalyst can substantially facilitate crucial FA deprotonation appears to be a key factor for achieving higher activity in the catalytic dehydrogenation of FA in base-free aqueous solution owing to the Au/m-ZrO2catalyst possessing more basic centers than others. At80℃, Au/m-ZrO2catalyst displayed high efficiency for FA dehydrogenation reaction, the TOF up to730h-1, which was the best heterogeneous catalytic system under the similar conditions.3. Supported-nanoiridium catalyze reductive transformation of carbon dioxideAs an abundant, cheap, non-toxic, and renewable carbon source, CO2turns out to be an attractive C1building block for making valued-added organic chemicals. However, the conversion efficiency are always too low during the utilization of CO2owing to its strong stability. N,N-Dimethylformamide (DMF) is an important solvent generally used in the field of industry and lab, including plastics, pharmaceutical, paint, and many other chemical processes. TiO2with high surface area supported Ir catalyst was prepared with the NaOH deposition-precipitation (DP) method. Ir/TiO2catalyst exhibited excellent catalytic activity for the DMF synthesis from CO2, H2and dimethylamine under milder reaction conditions and without using extra solvents or additives. It demonstrated that Ir/TiO2-H and Ir/TiO2-A catalysts treated with different thermal conditions showed varied distinction on the aspect of morphology and catalytic activity. H2-TPR results showed that the IrOx species on the surface of Ir/TiO2-A catalyst can be partially in situ reduced to be metallic Ir NPs by pressurized H2gas during the reaction process. CO2-TPD results indicated the synergistic effect between IrOx and Ir NPs can selectively and chemically activate CO2and H2molecules, thus facilitated the high catalytic activity of Ir/TiO2-A catalyst. Furthermore, we also proposed a mechanism of DMF synthesis based on TiO2with high surface area supported bifunctional species of IrOx and Ir NPs, involving the synergistic effect on formate intermediates.4. Pd-catalyzed hydrogen generation-storage cycle based on formate-bicarbonate under mild conditionsFormate, such as potassium formate, with high hydrogen density and as the solid state on room temperature, can be easily stored and transported. Pd/r-GO catalysts with small and uniformly dispersed Pd NPs were prepared using the liquid chemical reduction method.1Pd/r-GO catalyst can efficiently catalyze dehydrogenation of potassium formate solution under mild conditions. The TOF of dehydrogenation up to11,299h-1at80℃, which was not only the best heterogeneous catalyst on the similar reaction conditions, but also was much higher than the best homogeneous catalytic system reported. There was no significant decline for the reactivity of1Pd/r-GO catalyst for potassium formate solution dehydrogenation after the5th reuse. Moreover,1Pd/r-GO catalyst can catalyze the transformation of potassium formate solution dehydrogenation to almost full conversion (97%). Meanwhile,1Pd/r-GO can catalyze hydrogenation of CO2.1Pd/r-GO exhibited the high efficiency (>92%) of catalytic hydrogenation of bicarbonate such as potassium bicarbonate solution on the condition of100℃and40bar H2. Therefore, the Pd-catalyzed cycle system for hydrogen generation and storage based on formate-bicarbonate can be constructed. This cycle system was CO2-neutral, and the efficiency of hydrogen generation and storage was up to95and90%, respectively.