den Breejen J P, Radstake P B, Bezemer G L, Bitter J H, Frøseth V, Holmen A, de Jong K P
Inorganic Chemistry and Catalysis, Debye Institute for NanoMaterials Science, Utrecht University, NL-3508 TB Utrecht, P.O. Box 80003, The Netherlands.
J Am Chem Soc. 2009 May 27;131(20):7197-203. doi: 10.1021/ja901006x.
The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.
金属颗粒大小在催化过程中的影响具有至关重要的科学和工业意义，需要深入理解。本文研究了费托（FT）催化中钴颗粒大小效应的起源。采用稳态同位素瞬变动力学分析（SSITKA）来提供反应中间体的表面停留时间和覆盖度，作为钴颗粒大小（2.6 - 16纳米）的函数。对于在210摄氏度和H₂/CO = 10体积比条件下的碳纳米纤维负载钴催化剂，结果表明，对于小尺寸（<6纳米）的钴颗粒，可逆键合的CHₓ和OHₓ中间体的表面停留时间增加，而CO的表面停留时间减少。小尺寸钴颗粒的不可逆键合CO的覆盖度更高，这归因于低配位表面位点的比例更大。从SSITKA获得的覆盖度和停留时间被用于定量描述表面比活性（TOF），并定性描述费托反应（220摄氏度，H₂/CO = 2）中甲烷选择性随钴颗粒大小的变化。<6纳米的钴颗粒较低的TOF是由边缘/角落位点的阻塞和小平台上较低的本征活性共同导致的。小尺寸钴颗粒较高的甲烷选择性主要是由其较高的氢覆盖度引起的。
