酸性Al<sub>2</sub>O<sub>3</sub>层包裹Pt纳米颗粒:增强的金属-酸性双功能协同效应提高其甘油氢解反应催化性能

引用本文: 杜洪仪, 陈飔, 王恒伟, 路军岭. 酸性Al₂O₃层包裹Pt纳米颗粒:增强的金属-酸性双功能协同效应提高其甘油氢解反应催化性能[J]. 催化学报, 2017, 38(7): 1237-1244. doi: 10.1016/S1872-2067(17)62859-6 shu

Citation: Hongyi Du, Si Chen, Hengwei Wang, Junling Lu. Acidic alumina overcoating on platinum nanoparticles:Close metal-acid proximity enhances bifunctionality for glycerol hydrogenolysis[J]. Chinese Journal of Catalysis, 2017, 38(7): 1237-1244. doi: 10.1016/S1872-2067(17)62859-6 shu

酸性Al₂O₃层包裹Pt纳米颗粒:增强的金属-酸性双功能协同效应提高其甘油氢解反应催化性能

中国科学技术大学化学物理系, 微尺度物质科学国家实验室(筹), 能量转换材料中国科学院重点实验室, 能源材料化学协同创新中心, 安徽合肥 230026
基金项目:
国家自然科学基金（51402283，21473169）；青年千人计划；中央高校基本科研业务费专项资金资助（WK2060030017）.

摘要: 同时含有金属和酸性位点的双功能催化剂已广泛用于石油加氢裂解和可再生生物质转化中.这两种位点之间的距离对双功能协同作用起着至关重要的作用，进而影响催化剂的活性与选择性.近年来，由生物质转化生产生物燃料和化学品得到了广泛的关注.相比于石油裂解工艺，金属-酸性位点临近效应在生物质转化反应中鲜有报道.甘油是来自生物柴油生产过程中的廉价副产物（约总产量的10%）.通过选择性氢解将其转化为具有高附加价值的化学品如1，2-丙二醇和1，3-丙二醇，这是提高其附加值的主要途径.甘油氢解包含脱水与加氢两个过程，分别发生于酸性位点与金属位点上.根据文献报道，Lewis酸位点倾向于进攻甘油端位的羟基，生成中间产物丙酮醇，而Brønsted酸则更易进攻甘油中间位的羟基产生3-羟基丙醛；随后两者进一步加氢分别生成1，2-丙二醇和1，3-丙二醇.
负载型金属催化剂广泛应用于甘油氢解反应中，在金属催化剂中添加酸性助剂能显著提高催化剂的活性.大量研究表明，无论是将酸性物种添加到金属颗粒表面或者是载体上甚至是简单的物理混合，均能有效提升催化剂的催化性能.然而据我们所知，金属-酸性位点之间的临近效应还未在甘油氢解反应中报道过.本文利用原子层沉积技术（ALD）在Pt/Al₂O₃催化剂表面精确沉积了一层酸性多孔的氧化铝包裹层，同时提高了Pt催化剂的活性与1，2-丙二醇选择性；我们进一步通过高分辨透射电镜（HRTEM）、一氧化碳吸附漫反射红外光谱（CO DRIFTS）、吡啶DRIFTS等手段研究了Al₂O₃包裹层造成催化活性提升的原因.
30个ALD周期氧化铝包裹后的催化剂具有最高的活性与选择性，HRTEM观测到催化剂中的Pt纳米颗粒的尺寸为7nm，氧化铝包裹层厚度为3.6 nm.与未包裹的Pt/Al₂O₃催化剂相比，沉积在Pt纳米颗粒上的酸性Al₂O₃与Pt颗粒形成更多的金属-酸性位点界面，从而提升了Pt与Al₂O₃酸性位点的亲密性.由于生长的氧化铝薄膜与载体氧化铝为相同物种，因此催化剂包裹前后总体的酸度并未发生明显改变，与吡啶化学吸附实验结果相一致.TEM测试发现，氧化铝包裹层在催化反应测试后会发生部分脱落.CO DRIFTS结果同样表明，随着反应时间的增加，Pt上CO的吸收峰逐渐增强，再次证实了Pt颗粒表面包裹层的脱落；但还发现一个位于1963 cm^-1的新CO吸附峰.该峰可归属于吸附于Pt与Al₂O₃包裹层界面的桥式CO.
此外，我们对其丙酮醇中间产物做了加氢反应的对比实验.结果表明Al₂O₃包裹层对Pt的加氢性能并未增加，说明甘油氢解反应的速控步骤是脱水.因此，我们初步认为，Al₂O₃包裹对甘油氢解反应活性的提高是通过其酸性而促进甘油脱水反应所致.我们还研究了Pt尺寸效应对甘油氢解反应的影响，发现小颗粒Pt对1，2-丙二醇的选择性比大颗粒更高，而活性更低，这表明甘油氢解是一个结构敏感反应.因此，Al₂O₃包裹层对1，2-丙二醇选择性的提高可能是由于几何效应造成的，Pt颗粒表面被Al₂O₃包裹层分割为许多Pt聚集体，类似于减小颗粒尺寸，从而提高了反应选择性.

关键词:

English

Acidic alumina overcoating on platinum nanoparticles:Close metal-acid proximity enhances bifunctionality for glycerol hydrogenolysis

Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborate Innovation Center of Chemistry for Energy Materials (iChEM), CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, Anhui, China
Received Date: 15 April 2017
Revised Date: 09 May 2017
Fund Project:
This work was supported by the National Natural Science Foundation of China (51402283, 21473169), One Thousand Young Talents Program under the Recruitment Program of Global Experts, and the Fundamental Research Funds for the Central Universities (WK2060030017).

Abstract: Bifunctional catalysts that contain both metal and acidic functions have been widely used in re-newable biomass conversions. The bifunctionality closely depends on the distance between the metal and acid sites. However, the metal-acid proximity effect has rarely investigated in biomass conversions. In this work, we precisely deposited a porous Al₂O₃ overcoat onto a Pt/Al₂O₃ catalyst using atomic layer deposition to improve the proximity between the Pt metal and the alumina acid sites by increasing the area of the metal-acid interface. Diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS) of pyridine chemisorption confirmed that the overall catalyst acidity did not change considerably after applying the alumina overcoat. In the aqueous-phase, hydrogen-olysis of glycerol was used to demonstrate that the alumina overcoat significantly improved the activity approximately 2.8-fold, as well as the selectivity to 1,2-propanediol (1,2-PD) at high conver-sions. DRIFTS measurements of CO chemisorption indicated that the Pt-alumina interface had greater area for alumina coated Pt/Al₂O₃ than for the uncoated analog. Moreover, we used the hy-drogenation of acetol, the key reaction intermediate in glycerol hydrogenolysis, as a control exper-iment to confirm that the observed activity improvement in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. In brief, our work provides solid evidence that close metal-acid proximity enhances bifunctionality, thus improving the catalytic activity.

Key words:

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酸性Al₂O₃层包裹Pt纳米颗粒:增强的金属-酸性双功能协同效应提高其甘油氢解反应催化性能

English

Acidic alumina overcoating on platinum nanoparticles:Close metal-acid proximity enhances bifunctionality for glycerol hydrogenolysis

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English

Acidic alumina overcoating on platinum nanoparticles:Close metal-acid proximity enhances bifunctionality for glycerol hydrogenolysis

CCS Chemistry：嵌段共聚物自组装成 “三明治”二维有机材料，实现纳米级压力传感功能

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