Study on the activation mechanism of O-enhanced methane adsorbed on Pd-Cu catalyst
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摘要: 甲烷催化燃烧相比较传统燃烧有燃烧温度低,清洁以及高效的优点,在天然气汽车、固体氧化物燃料电池等多个领域具有较好的应用前景。为了揭示甲烷在不同掺杂比的Pd-Cu团簇上的脱氢机理,本研究采用密度泛函理论(DFT)对CH4*在不同团簇上的直接脱氢和O辅助脱氢进行计算。计算结果表明,Pd原子的掺杂提高了Cu(111)表面的吸附能力,在直接脱氢过程中,Pd的掺杂不仅使能垒由2.56 eV降低到2.43 eV,而且使速率控制步骤由CH*+*→C* + H*变为CH4*+*→CH3* + H*。预吸附O能够显著降低甲烷脱氢的能垒,速率控制步骤均为CH4* + O*→CH3* + OH*,甲烷在团簇上O辅助脱氢的最高能垒的大小为Cu(111)(1.56 eV)>Pd6Cu(111)(1.44 eV)>Pd2Cu(111)(1.38 eV),Pd的添加对于直接脱氢和O辅助脱氢的性能都有所提升。Abstract: Compared with traditional combustion, methane catalytic combustion has the advantages of low combustion temperature, clean and high efficiency, and it has good application prospects in natural gas vehicles, solid oxide fuel cell and other fields. In order to reveal the mechanism of dehydrogenation of methane on Pd-Cu clusters with different doping ratios, the density functional theory (DFT) is used to calculate the direct dehydrogenation and O-assisted dehydrogenation of CH4* in different clusters. The calculation results show that the doping of Pd atoms increases the adsorption capacity of Cu(111) surface, and in the process of direct dehydrogenation, the doping of Pd not only reduces the energy barrier from 2.56 to 2.43 eV, but also changes the rate determining step from CH*+*→C* + H* to CH4*+*→CH3* + H*. Pre-adsorbed O can significantly reduce the energy barrier of methane dehydrogenation, and the rate determining steps are CH4* + O*→CH3* + OH*. The highest energy barrier of O-assisted dehydrogenation of CH4* is Cu(111)(1.56 eV)>Pd6Cu(111)(1.44 eV)>Pd2Cu(111)(1.38 eV) on three clusters, which indicates that the addition of Pd has improved the performance of direct dehydrogenation and O-assisted dehydrogenation.
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Key words:
- CH4 activation /
- Pd-Cu catalyst /
- direct dehydrogenation /
- O-assisted dehydrogenation
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表 1 CHx*(x=0–4)在三种催化剂表面的吸附位点和吸附能(eV)
Table 1 Adsorption sites and adsorption energy (eV) of CHx*(x=0–4) over the surface of three catalysts
Species Cu(111) Pd2Cu(111) Pd6Cu(111) site energy site energy site energy CH4* top −0.02 top −0.03 top −0.02 CH3* hollow −0.98 top −1.41 top −1.12 CH2* bridge −2.79 hollow −3.13 hollow −2.93 CH* hollow −4.08 hollow −4.65 hollow −4.51 C* hollow −4.54 hollow −5.11 hollow −5.17 表 2 甲烷在三种催化剂上直接脱氢的能垒和反应热(eV)
Table 2 Energy barrier and heat of methane direct dehydrogenation on three catalysts (eV)
Elementary step Cu(111) Pd2Cu(111) Pd6Cu(111) barrier heat barrier heat barrier heat CH4*+*→CH3* + H* 1.80 1.40 2.43 1.19 2.43 1.46 CH3*+*→CH2* + H* 2.07 1.53 1.77 1.15 2.00 1.31 CH2*+*→CH* + H* 1.88 1.24 1.67 0.88 1.82 0.94 CH*+*→C* + H* 2.56 1.88 1.98 1.15 2.27 1.37 表 3 O辅助甲烷脱氢在三种催化剂表面上的能垒和反应热(eV)
Table 3 Energy barrier and heat of reaction of O-assisted methane dehydrogenation over three catalyst surfaces (eV)
Elementary step Cu(111) Pd2Cu(111) Pd6Cu(111) barrier heat barrier heat barrier heat CH4* + O* → CH3* + OH* 1.56 0.45 1.38 0.25 1.44 0.32 CH3* + O*→ CH2* + OH* 0.64 −0.79 0.66 −0.09 0.80 −0.18 CH2* + O*→ CH* + OH* 0.71 −0.41 0.44 −0.13 0.66 −0.27 CH* + O*→ C* + OH* 0.83 −0.26 0.90 0.24 1.02 −0.09 -
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