Effect Of RE Doping On The Hydrogen Storage Of Nanomaterials And Surface Catalytic Activity: A First-principles Study

With the fast development of the global economy, the deficiency of fossil fuel resourcesand the deterioration of ecological environment present a tremendous challenge to long-termdevelopment of human society. The research and development of new green energy is ofgreat significance. Hydrogen as a clean, efficient and safe source has attracted tremendousattention over the years. Hydrogen is considered as an ideal fuel for many energy convertersbecause of its low mass density and nonpolluting nature. Hydrogen can also be directly usedin fuel cells for transportation applications. However, hydrogen storage with a safe, effectiveand stable storage medium remains a main challenge before any automotive applications canbe realized. Hydrogen storage material with excellent performance is the key technology inthe development of hydrogen energy. Nanostructures doped with metals have recentlybecome one of the most promising hydrogen storage solid materials due to the advantages ofhigh capacity and low cost.In recent years, carbon nanotubes (CNT) and boron nitride nanotubes (BNNT) haveattracted much attention as candidates for a hydrogen storage media. CNT is a very potentialhydrogen storage material because of its unique structure. Considerable research has beendevoted to understand the hydrogen storage capacity of CNT. BNNT is an analogous to CNTin many respects. Surprisingly, the amount of successful research work performed on BNNTwas negligibly lower compared to that on CNT. Since pure nanostructures adsorb H₂molecules with weak physical adsorption, which can not meet the target of hydrogen storagefrom U.S. Department of Energy (DOE) for practical applications. Metal doping must becarried out to enhance the chemical reactivity of nanotubes surfaces. It is promising fornanotubes in the application as hydrogen storage medium because of the unique hydrogenstorage properties of nanotubes induced by surface modifications. As the rare earth (RE)element has the special atomic structure, a lot of inner4f orbital unpaired electron, beenextremely rich in electronic energy level. Therefore, we chose the rare earth elementseuropium and cerium to dope nanomaterials in order to improve hydrogen storage properties.With the development of computer simulation technology, the computational ability isenhancing, which have become an effective complementary tool to the traditionalexperiments. It plays an irreplaceable role in various fields. These are more favorable for researching the properties of nanostructures in atomic level by the first-principles calculationwhich can investigate the atomic and electric structures more profound. Based on the reviewof the research and development of hydrogen storage nanostructures, the hydrogenadsorption process of RE doping nanostructures was selected as the object of the study. Thefirst-principles calculations of the adsorption of H₂on the RE doped CNT and BNNTsurfaces, the local structure around RE before and after doping was systematicallyinvestigated by means of Mulliken and DOS analysis to understand the hydrogen adsorptionprocess and mechanisms. In addition, Platinum is often used as the catalyst at the electrodesof fuel cells. However, use of the Pt catalyst gives rise to a major problem. The Pt metalsurface is easily poisoned by trace amounts of CO, especially in fuel cells (H₂). Most ofefforts to overcome this problem have focused on alloying Pt with a secondary metal. In thisthesis, using the first-principles calculations based on density functional theory (DFT), wealso investigate the effect of RE doping at the surface on the CO oxidation activity and theCO-tolerance performance of platinum catalyst.The main results obtained in the thesis are divided into four parts as following:Firstly, we investigate the interaction between H₂and Eu-doped single-walled carbonnanotubes (SWCNT). For the Eu/CNT system, the hollow site on the outer wall of CNT isthe most favorable for the adsorption. The charge analysis results show that two6s electronsin Eu transfer to CNT while4f electrons remain in Eu, and Eu atom is thus divalent. Theresults indicate that five H₂per Eu atom can be adsorbed in the Eu/CNT system while4.44wt.%H₂can be stored in the Eu3/CNT system. The interaction between H₂and Eu/CNT isbalanced by the electronic hybridization and electrostatic interactions.Secondly, the adsorption of H₂on Ce doped CNT and gaphene are investigated by usingDFT. For the both systems, it is found that Ce preferentially occupies the hollow site on theoutside. The results indicate that Ce/CNT system is a good candidate for hydrogen storagewhere six H₂per Ce can be adsorbed and5.14wt.%H₂can be stored in Ce₃/CNT system.Among metals doped SWCNT, Ce exhibits the most favorable hydrogen adsorptioncharacteristics in terms of the adsorption energy and the uptake capacity. The hybridizationof the Ce-4f and Ce-5d orbitals with the H orbital contributes to the H₂binding where Ce-4felectrons participate in the hybridization due to the instability of the4f state. Curvature ofnanotubes changes the size of binding energy of Ce and C and that of adsorption energy ofH₂on Ce.Thirdly, the adsorption of H₂on Ce doped BNNT is investigated by using DFT. For the Ce/BNNT system, it is found that Ce preferentially occupies the hollow site over the BNhexagon. The results indicate that seven H₂per Ce can be adsorbed and5.68wt%H₂can bestored in Ce₃/BNNT system. Both hybridization of the Ce-5d orbital with the H-1s orbitaland the polarization of the H₂molecules contribute to the hydrogen adsorption. Ce clusteringcan be suppressed by preferential binding of Ce atoms on BNNT, which denotes that BNNTas a hydrogen storage substrate is better than CNT due to its heteropolar binding nature.Lastly, we studied the reaction of CO oxidation on Ce-decorated Pt(111) surface. Theoptimal adsorption position for CO was the top site of Pt, which on both Pt(111) andCe/Pt(111) surfaces. The adsorption energy of CO molecule on both surfaces is different, asfollows, Pt(111)> Ce/Pt(111). Comparing their activation energy, we can find CO onCe/Pt(111) surface oxidation easier. It was indicated that the electronic effect of RE loweredthe Pt-CO bond energy to improve the activity of RE doped catalysts for CO oxidation andthe CO-tolerance performance. CO on the O2/Pt(111) surface, by O2molecule on the catalystsurface can be decomposed into O atoms, the CO by the surface reaction mechanism of CO+O→CO2to provide the O atom, is the reaction rate control step. More specifically, O2dissociation on Ce/Pt(111) surface is also easier than on pure Pt surface. Ce dopingmanifestly increased the reaction rate of Pt catalyst for CO oxidation. Thus, Ce/Pt(111)system is a good candidate for CO oxidation reaction.In conclusion, this study not only contributes to our understanding of the hydrogenadsorption process and mechanisms in the RE-doped nanostructures, and will also providethe theoretical support for their potential applications. In addition, we also provide usefulinformation and positive guidance for the preparation of CO-tolerance Pt alloy catalysts.Thus, there are of importance in fundamental science and nanotechnology application.