酸洗Pt-Fe和Pt-Co催化剂在CO氧化反应中的对比研究

doi:10.1016/S1872-2067(17)62838-9

摘要/Abstract

摘要：

负载型纳米催化剂表面结构与其催化性能之间关系的研究一直受到广泛关注.由于其结构复杂使得人们在研究催化剂构效关系时遇到了很多困难.近年来，大量研究发现反转催化剂在众多反应中表现出优越的催化性能.反转催化剂是将过渡金属氧化物负载于其它金属表面.和传统金属/氧化物催化剂相比，反转催化剂更能突出氧化物在催化反应中的重要作用.众多研究表明，在氧化物-金属界面处存在特殊的作用，这种作用可以改变氧化物的电子特性和化学性质，进而产生较高的催化性能.傅强等人创建了金属氧化物负载于Pt表面的反转催化体系，其表现出了高的低温CO氧化反应性能.在氧化物和Pt之间的界面限域效应可以稳定氧化物中配位不饱和的金属阳离子.这种配位不饱和的氧化物提供了活化O₂的活性位.目前，反转催化剂的研究主要集中在单晶模型体系中，在负载型催化剂中的研究还较少.我们以炭黑（CB）为载体，将还原后的Pt-Fe和Pt-Co催化剂经过酸洗制备了一种表面富Pt核为合金的结构.考察了酸洗后的Pt-Fe和Pt-Co催化剂经过不同温度氧化后的结构变化，并讨论了其结构与CO完全氧化反应（COOX）和CO选择氧化反应（CO-PROX）性能的关系.
X射线粉末衍射（XRD），电感耦合等离子体发射光谱（ICP），透射电镜（TEM）和X射线光电子能谱（XPS）表征结果表明，还原后的Pt基催化剂经过酸洗可以选择性去除纳米粒子表面的3d过渡金属，形成表面富Pt体相为合金的结构.将酸洗后的Pt-Fe和Pt-Co催化剂在不同温度下空气中氧化，发现近表层的Fe（Co）会扩散到粒子表面上，形成过度氧化的Fe₂O₃（Co₃O₄）表面结构.氧化后的催化剂在COOX和CO-PROX反应中表现出截然不同的催化性能.酸洗后的Pt-Fe （Pt-Co）催化剂经过不同温度氧化后在COOX反应中活性都较差，室温下的CO转化率只有不到30%，CO完全转化的温度超过100 ℃，相当于纯Pt催化剂的活性.这说明Pt表面过度氧化的Fe₂O₃（Co₃O₄）对CO氧化反应的促进作用不明显.而氧化后的催化剂在CO-PROX反应中表现出较高的活性，250 ℃ （或350 ℃）氧化后的酸洗Pt-Fe催化剂室温下的CO转化率接近100%，250 ℃ （或350 ℃）氧化后的酸洗Pt-Co催化剂室温下的CO转化率也达到了70%.结合表征和反应结果，我们认为氧化处理形成的表面过度氧化的金属氧化物（Fe₂O₃，Co₃O₄）在COOX的催化性能较差.通入CO-PROX反应气后，气氛中大量H₂的存在和Pt表面的氢溢流效应可以使得表面Fe₂O₃，Co₃O₄在室温下被还原成配位不饱和的FeO，CoO.这种配位不饱和的氧化物在表面Pt的限域作用和大量H₂气氛下比较稳定，并且具有较强的活化解离O₂的能力，进而提高了CO-PROX反应的活性.
为了进一步证实催化剂表面氧化物与其催化性能的关系，我们在室温下进行了两种反应气的循环实验测试.测试结果表明，对于氧化后的酸洗Pt-Fe催化剂，COOX反应中的表面Fe₂O₃和CO-PROX反应中的表面FeO可以通过变换反应气氛实现两种氧化物的相互转变，并表现出完全不同的催化性能.对于氧化后的酸洗Pt-Co催化剂，CO-PROX反应中形成的CoO表面结构在COOX反应中也比较稳定，在两种反应气中表现出相似的催化性能.

关键词: 铂-铁, 铂-钴, 酸洗, CO氧化, 表面氧化物

Abstract:

Leached Pt-Fe and Pt-Co catalysts were prepared by acid leaching the reduced catalysts in acid solution. Oxidation treatments of leached catalysts produced the structure of metal oxides decorat-ing the surface of nanoparticles. The fully oxidized Fe₂O₃ and Co₃O₄ species on Pt nanoparticle sur-faces result in the low performance of the CO complete oxidation (COOX) reaction. In contrast, un-saturated FeO and CoO surface species can be formed during exposure to the CO preferential oxida-tion (CO-PROX) reaction with an excess of H₂, leading to a high O₂ activation ability and enhancing the CO-PROX activity. The FeO_x surface structures can be transformed between these two states by varying the reactive gas environments, exhibiting oscillating activity in these two reactions. Con-versely, the CoO surface structure formed in the H₂-rich atmosphere is stable when exposed to the COOX reaction and exhibits similar activity in these two reactions. It is hoped that this work may assist in understanding the important role of surface oxides in real reactions.

Key words: Pt-Fe, Pt-Co, Acid leaching, CO oxidation, Surface oxide

徐红, 倪可, 李小昆, 朱胜, 范果红. 酸洗Pt-Fe和Pt-Co催化剂在CO氧化反应中的对比研究[J]. 催化学报, 2017, 38(7): 1261-1269.

Hong Xu, Ke Ni, Xiaokun Li, Sheng Zhu, Guohong Fan. Comparative studies of leached Pt-Fe and Pt-Co catalysts for CO oxidation reactions[J]. Chinese Journal of Catalysis, 2017, 38(7): 1261-1269.

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