Effects Of Catalyst-Support Interaction And Electronic Structure On Catalytic Behavior

Fuel cells have been recognized as clean energy-converting devices due to their high efficiency and low emission, which have taken great advances in recent decades. Despite these efforts, there are several scientific and technological difficulties hampering the widespread commercialization of fuel cells, such as (1) high Pt loading on the electrodes; (2) poor stability of the Pt-based electrocatalysts; (3) low dynamic reaction rate on anode and cathode fuel cells.Based above problems, in this dissertation, quantum chemistry and experiment method are combined to study Pt-free, highly stabile and active catalysts for oxygen reduction reaction and methanol electroxidaiton. On the one hand, based on the experiment results, theory calculation is used to establish the quantum chemistry explanation behind the experimental phenomena. On the other hand, theory calculation is performed to explore the reaction mechanisms and design the highly active catalysts. And then the results of theory calculation are validated by the experiment at last.Firstly, experimental phenomenon shows that catalytic activity of Pt and Pd for oxygen reduction reaction (ORR) changes with catalyst supports from C to TiO2. Based on the phenomenon, density function theory (DFT) was used to elucidate the cause behind the difference in catalysis caused by catalyst supports. At first, factors closely associated with the first electron transfer of the ORR were assessed in the light of quantum chemistry. Then intermediate (atomic oxygen, O) adsorption strength on the catalyst surface was calculated. The results show that, in terms of minimum energy difference, the best orbital symmetry match, and the maximum orbital overlap, TiO2 does bring about a very positive effect on catalysts Pd/TiO2 for the first electron transfer of the ORR. Especially, TiO2 remarkably expands the space size of Pd/TiO2 HOMO orbital and improves orbital overlap of Pd/TiO2 HOMO and O2 LUMO. The analysis of deformation density and partial density of state shows that the strong interaction between Pt and Ti leads to a strong adsorption of intermediate O on Pt/TiO2, but the strong interaction between Pd and surface O causes positive net charge of Pd and a weak adsorption of intermediate O on Pd/TiO2. Thus, the ORR can proceed more smoothly on Pd/TiO2 than Pt/TiO2 in every respect of maximum orbital overlap and rate delay by intermediate O. The research also discloses that several factors lead to less activity of TiO2 supported Pt and Pd catalysts than the C-supported ones for the ORR. These factors include the poor dispersion of Pt and Pd particles on TiO2, poor electric conduction of TiO2 carrier itself, and bigger energy difference between HOMO of TiO2 carried metallic catalysts and LUMO of O2 molecule due to electrons deeply embedded in the semiconductor TiO2 carrier.Secondly, carbon nanotubes (CNTs) have large surface area, excellent conductivity, and high level of chemical stability. However, the interaction between the CNTs and Pt catalysts is still one of the main problems which induce the short lifetime of catalysts. In present work, thiol (-SH) is introduced into multiwalled carbon nanotubes (MWCNTs) to synthesize the Pt/SH-MWCNTs catalysts. It shows high durability during 600 repeated potential cycles. The platinum ECSA of the Pt/SH-MWCNTs decreases about 15% even after 600 cycles, while the JM-Pt/C and Pt/FMWCNTs catalysts have lost about 35% of their platinum ECSA. The high stability of Pt/SH-MWCNTs still has high catalytic activity for ORR. To compare the effect of thiol and hydroxyl group on stability of catalysts, and explore the stability mechanisms of Pt/SH-MWCNTs caused by thiol, density functional theory (DFT) calculations are further performed. As revealed by migration and agglomerate active energy calculation for Pt particles on O-SWNTs and S-SWNTs, the interaction between the Pt and O-SWNTs or S-SWNTs has indistinctive difference. The adsorption configurations of Oad atom on Pt5/S-SWNTs and Pt5/O-SWNTs show that S-SWNTs have stronger corrosion-resistance than that of O-SWNTs, becuase the S atom keeps the perfect structure of SWNTs. The lower-lying d-band center of Pt cluster on S-SWNTs can be considered as an indication for higher oxidation-resistance of Pt. The underlying mechanism, indicated by DFT calculations, could be ascribed to that the S atom of thiol group keeps not only the corrosion-resistance of carbon nanotubes, but also enhances the oxidation-resistance of Pt particles.Thirdly, methanol electrooxidation has been proven to be a surface structure sensitive reaction on platinum (Pt). It depends on desorption of underpotential deposition (upd) of hydrogen (Hupd), methanol dehydrogenation, OHad formation, and CO oxidation. The structure sensitivity of catalysts for methanol electrooxidation has hitherto not been well studied or understood. In this work, full potential-dependent periodic DFT calculations were performed to elucidate structure sensitivity and potential-dependent difference in Hupd desorption, water dissociation and CO oxidation over low Miller index planes (111) and (110)-(1×1) of platinum single-crystal electrode surface. The results show that the Hupd desorption is easier on Pt (111) than that on Pt (110)-(1×1), the water dissociation and OHad formation occur at same electrode potentials on Pt (111) as that on Pt (110)-(1×1), and the Pt (111) favors the CO oxidation at low potentials. At anodic potentials, the difference in Hupd adsorption and desorption potential on Pt surface is due to the difference in the adsorption energy on Pt surface, and the water dissociation and OHad formation deeply depend on the surface excess electrons. The relatively weak adsorption energy of OHad and CO on platinum surface would support an easy oxidation of CO on platinum electrode.Finally, theory calculation was performed to select the doped metal M to enhance the activity of Pt for CO and methanol oxidation. Three metals Pb, Bi and In were picked out. And then the Pb doped Pt (110) surface was calculated carefully to acquire the function of Pb on the activity of Pt (110) for methanol oxidation. The results show that the Pb dope to Pt (110) surface makes no good for Hupd desorption. The adsorption of CO on Pb doped Pt (110) surface is weakened in comparison with that on Pt (110). However, the adsorption of O on Pb doped Pt (110) surface is strengthened in comparison with that on Pt (110). What's more, CO is hardly adsorbed on Pb but oxygen species have astrong adsorption on Pb. Thus, Pb doped Pt (110) surface would be more conducive for methanol oxidation because of oxygen species richening by Pb atoms. Underpotential deposition of Pb on Pt electrode was adopted to prepare upd-Pb/Pt electrode, on which methanol oxidation in acid and basic solution was investigated. The results were identical with the conclusions from theory calculation.