| As a reproducible resource, hydrogen has attracted compressive attention in the decades. The high-effective preparation and utilization has become an important research issue. Formic acid (FA, HCOOH), the most simple organic acid, has a component of hydrogen up to 4.4 wt.%. In addition, the advantage of formic acid, such as non-toxic, nonflammability, easy to transport, easily oxidized in low temperature, made it to be a promising hydrogen source.There are two kinds of catalysts for dehydrogenation of formic acid, the heterogeneous catalysts and the homogeneous ones. Pt and Pd noble metals are taken as the most effective catalysts for the decomposition of formic acid in direct formic acid fuel cells. These two transition metals are always used as anodic electrodes, and the electrode reaction equation is as follows,HCOOH ' CO2+2H++2e-However, Pt and Pd electrodes could be poisoned by by-products, CO species, in the decomposition reaction of formic acid,HCOOH ' COads+H2O'CO2+2H++2e-the decomposition reaction of formic acid would be slow down due to inactivation of Pt and Pd electrodes. The direct formic acid fuel cells (DFAFCs) are considered as one of the most promising energy source systems. The electrocatalytic oxidation effeciency of formic acid is the critical factor that affect the performance of fuel cells. It is desirable to find the high catalytic activity and anti-poisoning catalysts of DFAFCs. Although the decomposition mechanisms of formic acid have been reported in the previous reports, it is still not enough to rationalize the reason of Pt and Pd catalyst poisoning for the barrier of dehydration is much higher than dehydrogenation. People are hunting for a balanced point that considering both the higher efficiency and the low cost as Pt and Pd are expensive noble metals. In this situation, many modified bimetal catalysts that behave the expected anti-poisoning of CO species based on Pt and Pd have been prepared, i.e., PtAg bimetal.In addition, the homogeneous catalysts have attracted many concerns due to its high efficiency in recent years. The study on homogenous catalysts, such as the transition metal complex of Ru, Rh, W, Ir, Fe, have made much progress. However, the microscopic mechanisms of the reaction, for example, the elementary steps of dehydrogenation of formic acid catalyzed by [FeH(PP3)]BF4 (PP3=P(CH2CH2PPh2)3), and the spin states transformation of Fe, are still not clear. The development and application of the new catalysts have been restricted due to these unsolved issues to some extent.Based on these matters mentioned above, the quantum chemical calculation based on the density functional theory (DFT) was employed to examine the decomposition mechanism of formic acid catalyzed by Pt, Pd, PtAg and [FeH(PPj)]BF4(PP3=P(CH2CH2PPh2)3) catalysts. This dissertation is focusing on the microcosmic mechanism of formic acid oxidation catalyzed by Pt-, Pd-based heterogeneous catalysts and [FeH(PP3)]BF4 homogeneous catalyst. In order to find out the microscopic essence of high catalytic activity of Pt- and Pd-based catalysts, and understand the key factors that restrict the catalytic activity, the possible oxidation paths of formic acid and the properties of thermodynamics and dynamics were discussed, and the geometries of reactive intermediates and transition states were discerned. This work lay a foundation for the development and designation of anode catalysts of DFAFCs and lowcost homogenous catalysts. The major points and valuable results in this dissertation are summarized as follows:1. By performing density functional theory (DFT) theory calculations, we studied the adsorption behaviors of the monomer and dimer of formic acid (HCOOH, FA) on the Pt(111) surface with and without the presence of water molecules. The monomer prefers to stand on the surface of Pt(111), and in the most stable adsorption configuration the carbonyl O of HCOOH binds to the atop site of a Pt atom and the hydroxyl H points asymmetrically to two neighboring Pt atoms. The dimer of HCOOH not only exists in the gasphase but also on Pt(111) surface, and the eight-membered ring dimer is identified as the energetically most favorable dimeric structure of HCOOH both in gas-phase and on Pt(111) surface. With the presence of water molecules, both the monomer and dimer of HCOOH prefer to lie parallel to the surface so as to maximize the number of H-bonds to adjacent water molecules. These results indicate that water molecules significantly influence the initial adsorption manner of HCOOH and further its decomposition reactivity on Pt(111) surface. The present work shows the adsorption behavior of HCOOH dimer on Pt(111) for the first time and also gives several new adsorption configurations of the monomer that are not reported in literature. The theoretical results are expected to provide a valuable input to understand the reactivity of HCOOH on Pt(111).2. The calculated results in literatures for the decomposition of formic acid on Pt(111) into CO cannot rationalize the well-known easy CO poisoning of Pt-based catalysts. The present work reexamines the formic acid decomposition on Pt(111) by considering both the monomer and dimer pathways without and with the presence of water molecules. Upon a thorough search, we locate some new adsorption configurations of formic acid, which were ignored in published literatures, for the subsequent C-H or O-H cleavages. From the calculated minimum energy pathways (MEPs), we propose that formic acid decomposition on Pt(111) initiates from its C-H bond scission rather than the O-H bond scission. The monomeric formic acid preferentially decomposes to CO2 regardless of absence or presence of water, while the dimeric one favors the formation of CO in the gas phase but competitively reacts to CO and CO2 with the presence of water molecules. The surrounding water molecules and the second formic acid in the dimer play substantial roles in the process of formic acid decomposition, which can be regarded as promoters assisting formic acid decomposition on Pt(111). In all situations, that is, regardless of the monomer or dimer, and of the presence or absence of water, the calculated barrier differences in the rate-determining steps of CO2 and CO formations are much smaller than those reported in the published works. These results improve our understanding for the catalytic oxidation mechanism of formic acid oxidation catalyzed by Pt-based catalysts and rationalize the performance durability problem of Pt-based catalysts used in direct formic acid fuel cells.3. While the high performance for electrooxidation of formic acid (HCOOH) has been recognized, Pd-based catalysts still suffer from CO poisoning, even though they are much more tolerant than Pt-based catalysts. Existing theoretical studies on the decomposition of HCOOH on Pd(111) surface cannot rationalize the catalyst poisoning effect. By performing density functional theory calculations, the present work reexamined the decomposition of HCOOH on Pd(111) along with the dual-path mechanism consisting of indirect and direct pathways. Two new adsorption configurations of HCOOH on Pd(111) are presented, from which the formation of CO is found to be either the same or more favorable in comparison with the formation of CO2. The present results are in distinct contrast to previous calculations where the barrier for the formation of CO2 was much lower than that for the formation of CO. Furthermore, this work also discussed the formation of CO through the reduction of CO2 and the effects of coadsorbed HCOOH and H2O molecules on the reactivity. From calculated results, it seems that the newly formed CO2 on Pd(111) can return to the surface to interact with adsorbed H atoms, partly contributing to the formation of CO. Coadsorbed HCOOH and H2O molecules are found to importantly affect the initial adsorption configuration and the decomposition mechanism of HCOOH on Pd(111). These results provide new insight into the reactivity of HCOOH on the Pd(111) surface and rationalize CO poisoning of Pd-based catalysts.4. Pt-based catalysts are known as the excellent electrocatalysts for formic acid (HCOOH) oxidation. Maximizing their use efficiency and reducing the CO poising effect are highly desirable, however, very challenging. Aiming at these interesting issues, this work presents a theoretical study of the catalytic decomposition of HCOOH on an ideal single-atom model catalyst of PtAg nanostructures, which consists of isolated Pt atoms anchored to Ag(111) surface and is referred as PtAg(111). The barrier of the rate-determining step of HCOOH decomposition to CO2 on PtAg(111) is calculated to be 0.38 eV, which is not significantly different from that on pure Pt(111) surface,0.35 eV. On the other hand, the barrier of HCOOH decomposing to CO on PtAg(111) is found to be higher than that on pure Pt(111),0.83 vs.0.67 eV. These results indicate that the single-atom PtAg(111) (Pt-decorated Ag(111) surface) presents promising catalytic performance of HCOOH oxidation, which promotes HCOOH dehydrogenation to CO2 as good as pure Pt(111) and inhibits HCOOH dehydration to undesirable CO that poisons the catalysts. The present results rationalize the experimental observation that Pt-Ag alloy electrocatalysts show improved catalytic performance toward HCOOH oxidation, and provide a clue for the rational design of Pt-based single-atom catalysts.5. The homogeneous catalysts are the efficient catalysts in the field of hydrogen production and storage in recent decades. Compared with the heterogeneous catalysts, the homogeneous catalysts do not produce CO species that poison the catalyst substances. It has been experimentally proved as a non-noble metal complex catalysts, the catalytic efficiency of [FeH(PP3)]+ for formic acid decomposition and hydrogenation of carbon dioxide is very high. In this dissertation, we calculated the stable point in potential surface of singlet and triplet, discussed the reactivity performance of [FeH(PP3)]+-HCOOH systemand and the thermodynamic and kinetic properties of the reaction. It is found that the reaction presents two states properties, and results in crossing between the potential energy surface. The two states both contribute to the reaction. The crossing in the potential energy surface makes the barriers of reaction lower, and facilitates the reaction process. The currently conclusions can reasonably explain the experimental phenomena and make contribution to the design of more efficient non-precious metal catalyst for the experiments. |
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