【作者】 夏美荣；

【导师】 魏子栋；

【作者基本信息】 重庆大学 ， 物理化学， 2009， 硕士

【摘要】 燃料电池作为清洁能源转换装置,是人类解决目前面临环境污染和能源短缺问题有效手段之一。目前燃料电池阴极氧还原催化剂主要是Pt/C,但这类催化剂容易受空气中的杂质气体中毒,如SO2、H2S、NO、NO2的影响,使催化剂的活性和稳定性大为下降。为了增强催化剂的抗毒性并降低成本,本文通过理论研究比较Pt和PtMo合金上抗毒性能,分析毒性物种使催化剂失活的本质原因,探索催化剂抗毒性-催化剂构型间的制约关系。论文通过理论计算揭示PtMo催化剂具有良好的抗SO2、H2S、NO、NO2气体中毒的性质以及氧还原活性。采用密度泛函理论计算掺杂Mo原子对Pt催化剂抗杂质气体毒性的影响以及造成该影响的原因。首先,分别计算SO2、SO2离解物种S、SO3,以及H2S、NO、NO2在Pt(111)和Pt:Mo=8:1的PtMo(111)表面的吸附构型,并获得各物种的几何、电子构型。然后,通过比较各吸附物种在Pt(111)和PtMo(111)面的吸附能、键长、键角变化,分析吸附前后Pt(111)和PtMo(111)面净电荷数、分态密度、d带中心以及差分电子密度的变化。最后,计算不同掺杂比例的PtnMom (n=7,m=2; n=2,m=1; n=5,m=4; n=4,m=5; n=1,m=2)合金,比较各物种在表面的吸附强度和在表面反应活性的关系。主要结论如下:首先,Mo的掺杂明显减弱了Pt-S间的相互作用,消弱SO2、S、SO3在PtMo(111)表面的吸附强度;Mo减弱了SO2吸附对PtMo(111)体系电子构型的影响,使催化剂尽量保持原有的电子构型及活性;Mo原子使S的吸附有选择性在靠近Mo的hcp位成键,并增强了Mo-Pt间的相互作用。因此,Mo掺杂不仅有效提高了PtMo的抗SO2中毒性,还提高了PtMo催化剂的稳定性;其次,H2S分子无论在Pt或是在PtMo体系上作用主要以平铺位置方式吸附,而且不容易发生脱附和H-S离解。与SO2的PDOS相比,H2S对催化剂表面d带态密度的影响更大,但PtMo体系可显著减小对H2S的吸附作用;再次,由于Mo的作用,NO和NO2在催化剂表面的吸附作用减小,而且使得NO在表面的吸附强度受吸附位的影响较大,且N原子转移的电子数也与吸附位有关。最后,研究不同掺杂原子比的PtnMom合金上杂质气体的吸附发现:Mo掺杂改善了表层原子HOMO的空间尺寸,和与杂质气体LUMO的重叠程度,从而降低了杂质气体在PtMo(111)表面的吸附强度。根据气体吸附强度同掺杂比关系,Pt:Mo原子比在7:2和4:5和5:4时,显示了很好的抗中毒性。对O2离解还原研究,根据轨道的重叠程度和O2分子在PtMo表面吸附键长的变化分析,Mo的掺杂改善了PtMoHOMO与氧气LUMO轨道重叠程度,利于O2发生离解反应;其中以PtMo原子的比值4:5和5:4时活性提高最为显著。更多还原

【Abstract】 Fuel cell as a clean energy transform device is one of effective measurements for energy shortage and environment pollution caused by fossil fuel direct combustion. Currently, Pt/C catalysts are in the dominant position for oxygen reduction reaction (ORR). However, Pt/C catalysts are susceptible to poison of the impurity gases in the air, such as SO2, H2S, NO2, NO, and the activity and stability of Pt catalysts would be exacerbated markedly.In this work, the effect of contamination (NOX, SOX) on the performance of Pt and Mo-Doped Pt was studied. The results show that Mo-Doped Pt shows better catalysis for the ORR, anti-poisoning capability and the stability. Density function theory (DFT) was used to elucidate relationship between the anti-poisoning capability and structure of catalyst PtMo. At first, the adsorption geometry and electronic structure of contamination gases SO2, S, SO3, H2S, NO, and NO2 were calculated on the pure Pt (111) and Mo-Doped Pt(111) with the atomic ratio 8:1 of Pt to Mo (PtMo(111)). And then, the adsorption energy, bond length and bond angle of adsorb species, and the change of net charge, partial density of state, d-band center and deformation density of Pt(111) and PtMo(111) were calculated and analyzed. In the end, the adsorption geometry and electronic structure of contamination gases SO2, S, SO3, H2S, NO, and NO2 were calculated on PtnMom with the different atomic ratio of Pt to Mo (n=7, m=2; n=2,m=1; n=5, m=4; n=4, m=5; n=1, m=2).The following conclusions have been figured out. Firstly, doped Mo atom weakens the bonding strength between S and Pt catalysts, thus reduces the adsorption energy of SO2, S and SO3 on PtMo (111). Secondly, doped Mo atom weakens the effects of SO2 on electronic structure of PtMo (111), maintains the original electronic structure and activity of PtMo (111) after SO2 adsorption. Thirdly, doped Mo atom makes the S adsorbed on the hcp site near Mo atom, and enhances the interaction between Pt and Mo. Furthermore, different from the situation of SO2, H2S adsorption takes place mainly in the way of spreading on the metal plane regardless of on Pt or PtMo. H2S is not easily dissociation and dehydrogenated. H2S is a more serious poison to compare with SO2. Fortunately, the presence of Mo atom appears to have high H2S tolerance. The adsorption of NO, and NO2 on Pt can also be weakened by Mo introduction. The absorption energy and electron transfer number from p orbital of N atom depend deeply on the adsorption sites.At last, the relationship between the electronic structure of PtnMom (111) and adsorption of contamination gases shows that: Comparied with Pt(111), doped Mo atom decrease the spatial size and orbital energy of HOMO of PtMo(111), which inhibit the overlap between the electron donor (catalysts) and the electron acceptor (SOX, NOX molecule). The PtnMom (111) with ratio of Pt/Mo 7:2 and 4:5和5:4 has best anti-poisoning capability, and the catalystic activity of PtnMom (111) for ORR also increased as the ratio of Pt/Mo is 4:5和5:4. The addition of Mo to Pt improves the overlap degree between PtMo HOMO and the O2 LUMO. Therefore the addition of Mo to Pt not only increases the anti- poisoning capability but also enhances the catalysis to the ORR.更多还原