Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
J Am Chem Soc. 2010 Jan 13;132(1):138-46. doi: 10.1021/ja904307n.
Nanoparticles are important catalysts for petroleum processing, energy conversion, and pollutant removal. As compared to their bulk counterparts, their often superior or new catalytic properties result from their nanometer size, which gives them increased surface-to-volume ratios and chemical potentials. The size of nanoparticles is thus pivotal for their catalytic properties. Here, we use single-molecule fluorescence microscopy to study the size-dependent catalytic activity and dynamics of spherical Au-nanoparticles under ambient solution conditions. By monitoring the catalysis of individual Au-nanoparticles of three different sizes in real time with single-turnover resolution, we observe clear size-dependent activities in both the catalytic product formation reaction and the product dissociation reaction. Within a model of classical thermodynamics, these size-dependent activities of Au-nanoparticles can be accounted for by the changes in the adsorption free energies of the substrate resazurin and the product resorufin because of the nanosize effect. We also observe size-dependent differential selectivity of the Au-nanoparticles between two parallel product dissociation pathways, with larger nanoparticles less selective between the two pathways. The particle size also strongly influences the surface-restructuring-coupled catalytic dynamics; both the catalysis-induced and the spontaneous dynamic surface restructuring occur more readily for smaller Au-nanoparticles due to their higher surface energies. Using a simple thermodynamic model, we analyze the catalysis- and size-dependent dynamic surface restructuring quantitatively; the results provide estimates on the activation energies and time scales of spontaneous dynamic surface restructuring that are fundamental to heterogeneous catalysis in both the nano- and the macro-scale. This study further exemplifies the power of the single-molecule approach in probing the intricate workings of nanoscale catalysts.
纳米粒子是石油加工、能量转换和污染物去除的重要催化剂。与它们的体相相比，由于其纳米尺寸，它们通常具有优越或新的催化性能，这增加了它们的比表面积和化学势。因此，纳米粒子的尺寸对于其催化性能至关重要。在这里，我们使用单分子荧光显微镜研究了在环境溶液条件下球形 Au 纳米粒子的尺寸依赖性催化活性和动力学。通过实时以单轮分辨率监测三种不同尺寸的单个 Au 纳米粒子的催化作用，我们观察到在催化产物形成反应和产物解离反应中都存在明显的尺寸依赖性活性。在经典热力学模型中，由于纳米尺寸效应，可以用底物 Resazurin 和产物 Resorufin 的吸附自由能变化来解释 Au 纳米粒子的这些尺寸依赖性活性。我们还观察到 Au 纳米粒子在两种平行产物解离途径之间的尺寸依赖性差异选择性，较大的纳米粒子在两种途径之间的选择性较小。颗粒尺寸也强烈影响表面重构耦合催化动力学；由于较小的 Au 纳米粒子具有更高的表面能，因此更容易发生与催化相关的和自发的动态表面重构。使用简单的热力学模型，我们对催化和尺寸依赖性的动态表面重构进行了定量分析；结果提供了对自发动态表面重构的激活能和时间尺度的估计，这对于纳米和宏观尺度的多相催化都是基本的。这项研究进一步例证了单分子方法在探测纳米尺度催化剂复杂工作原理方面的强大功能。
