Size-Dependence of Catalytic Activity of Gold Nanoparticles Loaded on Titanium (IV) Dioxide for Hydrogen Peroxide Decomposition
Size-Dependence of Catalytic Activity of Gold Nanoparticles Loaded on Titanium (IV) Dioxide for Hydrogen Peroxide Decomposition
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DOI：
10.1002/cphc.200900596
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发表时间：
2009-12-07
期刊：
影响因子：
2.9
通讯作者：
Tada, Hiroaki
中科院分区：
文献类型：
--
作者：
Kiyonaga, Tomokazu;Jin, Qiliang;Tada, Hiroaki
Haruta [1]发现金属氧化物负载的Au纳米颗粒（NPs）对CO氧化具有极高的催化活性，引发了对Au NP催化的深入研究，以揭示其对许多化学反应的高催化活性。[2，3]在这些情况下，最有趣的一点是Au粒度对催化活性的影响。然而，据我们所知，对催化活性的Au颗粒尺寸依赖性的系统研究仅限于气相CO氧化，[4] C2 H2氢化，[5]和液相乙醇氧化。[6]另一方面，使用H2 O2作为氧化剂的有机合成[7]和水污染物分解[8]是理想的液相绿色氧化过程，因为其原子效率高且除H2O外没有副产物。到目前为止，H2 O2的重要氧化反应，包括由环己烯合成己二酸[9]和烯烃的环氧化[10]以及醇的羰基化[11]也已报道。这些方法中的一个共同的关键课题是开发用于H2 O2活化的催化剂。最近，发现平均尺寸低于10 nm的悬浮Au水溶胶颗粒对H2 O2分解具有催化活性，[12]而本体Au已知是无活性的。[13]此外，H2 O2-Au/载体系统已被证明在从水中去除包括苯酚、甲醛和丙酮的低水平有机化合物方面是有效的。[8]此外，H2 O2在金属上的分解对于燃料电池也是重要的，因为H2 O2，O2还原反应的中间体，主要负责组成材料的降解。[14]在此，我们报告的H2 O2分解或活化的催化活性的依赖于Au NPs负载在TiO 2（Au/TiO 2）的大小，和应用的H2 O2-Au/TiO 2系统的化学选择性醇氧化。Au NPs通过沉积-沉淀（DP）方法结合在TiO 2上，其中改变加热温度（Tc）和时间（tc）。[15]图1显示了在不同加热条件下制备的Au/TiO 2的TEM图像（左）和Au颗粒尺寸分布（右）：A）Tc= 673 K，tc= 1 h; B）Tc= 773 K，tc= 4 h; C）Tc= 873 K，tc= 4 h; D）Tc= 873 K，tc= 24 h。在每个样品中，Au纳米粒子高度分散在TiO 2表面，具有相当尖锐的分布，相对标准偏差（RSD）小于约10%。14%（另见支持性信息，图S1）。当负载量保持不变（0.245 × 0.245）时，由于Tc和tc分别从673 K增加到873 K和从1 h增加到24 h，Au NPs（d）的平均尺寸从2.1 nm增加到12.5 nm。005质量%）。图2A显示了在298 K下在TiO 2（a）和Au/TiO 2（b-e）存在下H2 O2分解的时间过程。在Au/TiO 2体系中，H2 O2浓度随反应时间（tr）线性降低，而TiO 2和本体Au（数据未显示）是无活性的。此外，Au/TiO 2的活性强烈地依赖于Au颗粒尺寸（d）。在tr= 1h时的转换数[TON=分解的H2 O2（1.6 × 10 ↑ [5] mol）/Au（2.5 × 10 ↑ [7] mol）]被计算为约。对于Au（d= 3.5 nm）/TiO 2为60
The discovery of the extremely high catalytic activity of metaloxide-supported Au nanoparticles (NPs) for the CO oxidation by Haruta [1] has triggered intensive research on Au NP catalysis to reveal its high catalytic activity for many chemical reactions.[2, 3] In these cases, the most intriguing point is the effect of Au particle size on the catalytic activity. However, to our knowledge, a systematic study of the Au particle-size dependence of the catalytic activity is limited to gas-phase CO oxidation,[4] C2H2 hydrogenation,[5] and liquid-phase glycohol oxidation.[6] On the other hand, organic synthesis [7] and water pollutant decomposition [8] using H2O2 as an oxidant are ideal liquidphase green oxidation processes because of the high atom efficiency and the lack of by-products except for H2O. So far, important oxidations by H2O2, including adipic acid synthesis from cyclohexene [9] and the epoxidation of alkene,[10] and carbonylation of alcohols [11] have been reported as well. A common key subject in these processes is to develop the catalyst for H2O2 activation. Recently, suspended Au hydrosol particles with a mean size below 10 nm have been found to exhibit catalytic activity for the H2O2 decomposition,[12] whereas bulk Au is known to be inactive.[13] Also, the H2O2–Au/support system has been proven to be effective in removing low-level organic compounds including phenol, formaldehyde, and acetone from water.[8] Further, H2O2 decomposition on metals is also of importance in connection with fuel cells because H2O2, an intermediate of the O2 reduction reaction, is mainly responsible for the degradation of the constituent materials.[14] Herein we report the dependence of the catalytic activity for H2O2 decomposition or activation on the size of Au NPs loaded on TiO2 (Au/TiO2), and the application of the H2O2–Au/TiO2 system to chemoselective alcohol oxidation. Au NPs were incorporated on TiO2 by the deposition–precipitation (DP) method where both heating temperature (Tc) and time (tc) were altered.[15] Figure 1 shows TEM images (left) and the Au particle-size distribution (right) of Au/TiO2 prepared under different heating conditions: A) Tc= 673 K, tc= 1 h; B) Tc= 773 K, tc= 4h; C) Tc= 873 K, tc= 4h; D) Tc= 873 K, tc= 24 h. In every sample, Au NPs highly dispersed on the TiO2 surface have a fairly sharp distribution, with the relative standard deviation (RSD) smaller than ca. 14%(also see the Supporting Information, Figure S1). The mean size of Au NPs (d) increases from 2.1 to 12.5 nm as a result of increases in Tc and tc from 673 to 873K and from 1h to 24h, respectively, while the loading amount remains constant (0.245 Æ0. 005 mass%). Time courses for the H2O2 decomposition in the presence of TiO2 (a) and Au/TiO2 (b–e) at 298 K are shown in Figure 2A. In the Au/TiO2 system, the H2O2 concentration decreases linearly with reaction time (tr), while TiO2 and bulk Au (data not shown) are inactive. Also, the activity of Au/TiO2 strongly depends on the Au particle size (d). The turnover number [TON= H2O2 decomposed (1.6 10À5 mol)/Au (2.5 10À7 mol)] at tr= 1h was calculated to be ca. 60 for the Au (d= 3.5 nm)/TiO2