Control and Impact of the Nanoscale Distribution of Supported Cobalt Particles Used in Fischer-Tropsch Catalysis
Control and Impact of the Nanoscale Distribution of Supported Cobalt Particles Used in Fischer-Tropsch Catalysis
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DOI：
10.1021/ja500436y
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发表时间：
2014-05-21
影响因子：
15
通讯作者：
de Jong, Krijn P.
中科院分区：
文献类型：
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作者：
Munnik, Peter;de Jongh, Petra E.;de Jong, Krijn P.
纳米粒子的接近程度可能会影响负载型金属催化剂的性能，特别是稳定性。在催化剂制备过程中，由于聚集体的形成，往往会出现较短的颗粒间距。在高负载二氧化硅负载催化剂的合成过程中，纳米钴颗粒聚集的原因被发现源于二氧化硅颗粒与硝酸钴前驱体浸渍后的干燥过程。Co3O4纳米颗粒的最大间距是在100℃的N-2气流中通过沸腾床干燥获得的。在此温度以下，在固相沉淀之前和过程中发生了液体的重新分配，导致钴颗粒聚集。在较高的温度下，Co3O4在干燥过程中发生形核和生长，也会引起团聚。在工业条件下对未助剂和铂助剂的钴催化剂进行的Fischer-Tropsch催化实验表明，Co粒子聚集体的大小(13-80 nm)(约9 nm)对活性影响不大。大的聚集体对长链烷烃表现出更高的选择性，这可能与较高的烯烃形成以及随后的再吸附和二次链增长有关。最重要的是，更大的钴颗粒聚集体导致钴(高达75%)广泛迁移到宏观催化剂颗粒的外表面(38-75 PM)。虽然二氧化硅载体颗粒内部的颗粒尺寸没有增加，但钴向外表面的迁移部分导致了颗粒的生长，从而导致活性的丧失。通过最大化载体上纳米颗粒之间的距离来抑制这种宏观长度尺度上的钴迁移。显然，颗粒的纳米尺度分布是负载型催化剂和一般功能纳米材料的重要设计参数。
The proximity of nanoparticles may affect the performance, in particular the stability, of supported metal catalysts. Short interparticle distances often arise during catalyst preparation by formation of aggregates. The cause of aggregation of cobalt nanoparticles during the synthesis of highly loaded silica-supported catalysts was found to originate from the drying process after impregnation of the silica grains with an aqueous cobalt nitrate precursor. Maximal spacing of the Co3O4 nanoparticles was obtained by fluid bed drying at 100 degrees C in a N-2 flow. Below this temperature, redistribution of liquid occurred before and during precipitation of a solid phase, leading to aggregation of the cobalt particles. At higher temperatures, nucleation and growth of Co3O4 occurred during the drying process also giving rise to aggregation. Fischer-Tropsch catalysis performed under industrially relevant conditions for unpromoted and Pt-promoted cobalt catalysts revealed that the size of aggregates (13-80 nm) of Co particles (size similar to 9 nm) had little effect on activity. Large aggregates exhibited higher selectivities to long chain alkanes, possibly related to higher olefin formation with subsequent readsorption and secondary chain growth. Most importantly, larger aggregates of Co particles gave rise to extensive migration of cobalt (up to 75%) to the external surface of the macroscopic catalyst grains (38-75 pm). Although particle size did not increase inside the silica support grains, migration of cobalt to the external surface partly led to particle growth, thus causing a loss of activity. This cobalt migration over macroscopic length scales was suppressed by maximizing the distance between nanoparticles over the support. Clearly, the nanoscale distribution of particles is an important design parameter of supported catalysts in particular and functional nanomaterials in general.