Wolf Moritz, Fischer Nico, Claeys Michael
Catalysis Institute, DST-NRF Centre of Excellence in Catalysis cchange, Department of Chemical Engineering, University of Cape Town Rondebosch 7701 South Africa
Nanoscale Adv. 2019 Jun 10;1(8):2910-2923. doi: 10.1039/c9na00291j. eCollection 2019 Aug 6.
The inert nature of graphitic samples allows for characterisation of rather isolated supported nanoparticles in model catalysts, as long as sufficiently large inter-particle distances are obtained. However, the low surface area of graphite and the little interaction with nanoparticles result in a challenging application of conventional preparation routes in practice. In the present study, a set of graphitic carbon materials was characterised in order to identify potential support materials for the preparation of model catalyst systems. Various sizes of well-defined CoO nanoparticles were synthesised separately and supported onto exfoliated graphite powder, that is graphite after solvent-assisted exfoliation ultrasonication resulting in thinner flakes with increased specific surface area. The stability of the supported nanoparticles during reduction to metallic cobalt in H was monitored by means of X-ray diffraction and smaller crystallite sizes were found to be harder to reduce than their larger counterparts. A low cobalt loading of 1 wt% was required to avoid aggregates in the parent catalyst, and this allowed for the preparation of supported cobalt nanoparticles which were resistant to sintering at reduction temperatures below 370 °C. The developed model catalysts are ideally suited for sintering studies of isolated nano-sized cobalt particles as the graphitic support material does not provide distinct metal-support interaction. Furthermore, the differently sized cobaltous particles in the various model systems render possible studies on structural dependencies of activity, selectivity, and deactivation in cobalt oxide or cobalt catalysed reactions.
只要能获得足够大的颗粒间距离，石墨样品的惰性就使得在模型催化剂中对相当孤立的负载型纳米颗粒进行表征成为可能。然而，石墨的低表面积以及与纳米颗粒的微弱相互作用导致传统制备路线在实际应用中具有挑战性。在本研究中，对一组石墨碳材料进行了表征，以确定用于制备模型催化剂体系的潜在载体材料。分别合成了各种尺寸明确的CoO纳米颗粒，并将其负载到剥离石墨粉上，即经过溶剂辅助剥离超声处理后的石墨，超声处理后石墨薄片变薄，比表面积增加。通过X射线衍射监测负载型纳米颗粒在氢气中还原为金属钴过程中的稳定性，发现较小的微晶尺寸比较大的微晶尺寸更难还原。需要1 wt%的低钴负载量以避免母体催化剂中出现团聚，这使得能够制备出在低于370°C的还原温度下抗烧结的负载型钴纳米颗粒。所开发的模型催化剂非常适合用于孤立纳米级钴颗粒的烧结研究，因为石墨载体材料不会提供明显的金属-载体相互作用。此外，各种模型体系中不同尺寸的钴颗粒使得研究氧化钴或钴催化反应中活性、选择性和失活的结构依赖性成为可能。
