Department of Chemistry and Biochemistry, Center for Electrochemistry, University of Texas at Austin, 1 University Station, A5300, Austin, TX 78712-0165, USA.
Proc Natl Acad Sci U S A. 2012 Jul 17;109(29):11493-7. doi: 10.1073/pnas.1201370109. Epub 2012 Jun 4.
Here we report on the effect of the mass transfer rate (k(t)) on the oxygen reduction reaction (ORR) catalyzed by Pt dendrimer-encapsulated nanoparticles (DENs) comprised of 147 and 55 atoms (Pt(147) and Pt(55)). The experiments were carried out using a dual-electrode microelectrochemical device, which enables the study of the ORR under high k(t) conditions with simultaneous detection of H(2)O(2). At low k(t) (0.02 to 0.12 cm s(-1)) the effective number of electrons involved in ORR, n(eff), is 3.7 for Pt(147) and 3.4 for Pt(55). As k(t) is increased, the mass-transfer-limited current for the ORR becomes significantly lower than the value predicted by the Levich equation for a 4-electron process regardless of catalyst size. However, the percentage of H(2)O(2) detected remains constant, such that n(eff) barely changes over the entire k(t) range explored (0.02 cm s(-1)). This suggests that mass transfer does not affect n(eff), which has implications for the mechanism of the ORR on Pt nanoparticles. Interestingly, there is a significant difference in n(eff) for the two sizes of Pt DENs (n(eff) = 3.7 and 3.5 for Pt(147) and Pt(55), respectively) that cannot be assigned to mass transfer effects and that we therefore attribute to a particle size effect.
在这里，我们报告了传质速率（k(t)）对由 147 个和 55 个原子组成的 Pt 树状大分子包裹的纳米粒子（DENs）催化的氧还原反应（ORR）的影响。实验是在双电极微电化学装置中进行的，该装置能够在高传质速率（k(t)）条件下研究 ORR，并同时检测 H(2)O(2)。在低传质速率（0.02 到 0.12 cm s(-1))下，Pt(147)的有效电子数（n(eff)）为 3.7，Pt(55)的 n(eff)为 3.4。随着 k(t)的增加，ORR 的传质限制电流显著低于 4 电子过程的 Levich 方程预测值，与催化剂尺寸无关。然而，检测到的 H(2)O(2)的百分比保持不变，因此在整个 k(t)范围内（0.02 cm s(-1))，n(eff)几乎没有变化。这表明传质不会影响 n(eff)，这对 Pt 纳米粒子上 ORR 的机制有影响。有趣的是，两种尺寸的 Pt DENs 的 n(eff)存在显著差异（Pt(147)和 Pt(55)的 n(eff)分别为 3.7 和 3.5），不能归因于传质效应，因此我们归因于颗粒尺寸效应。
