Wang Yixuan, Balbuena Perla B
Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, USA.
J Phys Chem B. 2005 Oct 13;109(40):18902-6. doi: 10.1021/jp0543779.
The Gibbs free energies of key elementary steps for the electrocatalytic oxygen reduction reaction (ORR) are calculated with B3LYP type of density functional theory: O2 + M + H+ + e- (0 eV) --> HOO-M (deltaG1), HOO-M + M --> HO-M + O-M (deltaG2), O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (deltaG3), and HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (deltaG4), where H+ is modeled as H3(+)O(H2O)3 and M stands for the adsorption site of a metal catalyst modeled by a single metal atom as well as by an M3 cluster. Taking Pt as a reference, deltaG4 is plotted against deltaG1 for 17 metals from groups V to XII. It is found that no single metal has both deltaG1 and deltaG4 more negative than Pt, although some of them have either more negative deltaG1 or more negative deltaG4. This enables us to explain thermodynamically why no other single metal catalyzes the ORR as effectively as Pt does. Moreover, a thermodynamic analysis reveals that the signs of delta deltaG (the difference between deltaG of other metals and deltaG of Pt) strongly correlate with the valence electronic structure of metals, i.e., delta deltaG1 < 0 and delta deltaG4 > 0 for metals M with vacant valence d orbitals, whereas delta deltaG1 > 0 and delta deltaG4 < 0 for metals M' with fully occupied valence d orbitals. Thus, a simple thermodynamic rule for the design of bimetallic catalysts for the ORR is proposed: couple a metal M (delta deltaG1 < 0) with a second metal M' (delta deltaG4 < 0) to form an alloy catalyst MM'3. The rationale behind this selection is based on M being more efficient for the rate-determining step, i.e., for the formation of the adsorbed species M-OOH, while M' can enhance the reductions of O and OH in the last three electron-transfer steps.
采用B3LYP类型的密度泛函理论计算了电催化氧还原反应（ORR）关键基元步骤的吉布斯自由能：O2 + M + H+ + e- (0 eV) --> HOO-M (ΔG1)，HOO-M + M --> HO-M + O-M (ΔG2)，O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (ΔG3)，以及HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (ΔG4)，其中H+ 建模为H3(+)O(H2O)3，M代表金属催化剂的吸附位点，用单个金属原子以及M3簇进行建模。以Pt为参考，针对第V族至第XII族的17种金属，绘制了ΔG4对ΔG1的曲线。结果发现，没有单一金属的ΔG1和ΔG4都比Pt更负，尽管其中一些金属的ΔG1或ΔG4更负。这使我们能够从热力学角度解释为什么没有其他单一金属能像Pt那样有效地催化ORR。此外，热力学分析表明，ΔΔG（其他金属的ΔG与Pt的ΔG之差）的符号与金属的价电子结构密切相关，即对于价d轨道为空的金属M，ΔΔG1 < 0且ΔΔG4 > 0，而对于价d轨道完全占据的金属M'，ΔΔG1 > 0且ΔΔG4 < 0。因此，提出了一种用于设计ORR双金属催化剂的简单热力学规则：将金属M（ΔΔG1 < 0）与第二种金属M'（ΔΔG4 < 0）耦合形成合金催化剂MM'3。这种选择背后的基本原理是基于M在速率决定步骤中更高效，即对于吸附物种M-OOH的形成更高效，而M'可以在最后三个电子转移步骤中增强O和OH的还原。
