Zhou Wei Ping, Lewera Adam, Larsen Robert, Masel Rich I, Bagus Paul S, Wieckowski Andrzej
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois, 61801, USA.
J Phys Chem B. 2006 Jul 13;110(27):13393-8. doi: 10.1021/jp061690h.
We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.
我们报告了一项结合X射线光电子能谱（XPS）、循环伏安法（CV）和计时电流法（CA）的研究，该研究针对粒径范围为9至40 nm的无载体钯纳米颗粒催化剂上的甲酸电氧化反应。CV和CA测量结果表明，活性最高的催化剂由最小（9和11 nm）的钯纳米颗粒制成。除了高反应活性外，XPS数据表明，相对于费米能级，此类纳米颗粒显示出最高的芯能级结合能（BE）位移和最高的价带（VB）中心下移。因此，我们认为我们发现了甲酸氧化电流与BE和VB中心位移之间的相关性，而这又可直接与不同粒径钯纳米颗粒的电子结构相关。显然，使用无载体催化剂的这种趋势从未被报道过。根据多相催化的密度泛函理论以及机理考虑，观察到的位移是由催化剂表面COOH中间体吸附的键强度减弱引起的。这反过来又导致甲酸氧化为CO2的速率增加（以及相关的氧化电流增加）。总体而言，我们的测量证明了粒径对钯电子性质的影响，这在HCOOH氧化反应中产生了不同催化活性。我们的工作突出了芯能级结合能和d带位移中心在电催化中的重要性，并强调了将d带位移中心与催化反应性联系起来的理论的价值。
