(165g) Understanding Coking On Platinum Catalysts Under Ethane Dehydrogenation

Authors: 
Wu, J., University of California - Berkeley
Peng, Z., University of Rochester
Helveg, S., Haldor Topsoe Research Laboratories
Bell, A. T., University of California - Berkeley



Light alkenes are widely use as building blocks for the production of rubber, plastics, and other polymers, and are currently produced by steam cracking of alkanes or naphtha at high temperatures. However, these processes exhibit low alkene selectivity and produce significant amounts of methane and coke. Catalytic dehydrogenation of C2-C5 alkanes offers an attractive alternative because it utilizes low-cost reactants and can be carried out with high selectivity(1). Platinum is known to be the most effective transition metal for dehydrogenation of light alkanes. However, in the absence of a promoting element such as Sn, platinum catalysts deactivate rapidly due to coke formation, making it difficult to measure the true initial activity in comparison with bimetallic PtSn catalysts(2).  Furthermore, it is difficult to assess composition and nanoparticle size effects with catalysts synthesized by traditional impregnation methods.  We have developed a colloidal synthesis in which we form the metal particles ex-situ, enabling stricter control over the nucleation and growth of nanoparticles(3).  Following immobilization and reduction, model catalysts of fixed composition and size are used in this study.

This talk will present the effects of composition (PtxSny) and size (2-8 nm) on ethane dehydrogenation at low residence-time, a novel method that we show to significantly reduce coke formation and measure the true activity of these catalysts. Catalysts composition and size are characterized by XRD, TEM, and EDX. The results show that the addition of Sn favors higher ethene production and selectivity, even in the absence of coke formation.  Furthermore, the reaction is more favored on large particles that can be explained by poisoning of corner sites on small particles.  In conjunction with these studies, we have examined coke formation on Pt particles in-situ using a Titan 80-300 ETEM to better understand the location of carbon nucleation along with the formation of multiple graphene layers. If these issues are resolved, design of future catalysts to limit deactivation can be achieved(4). However, imaging challenges to avoid beam effects exist and obtaining an optimal imaging strategy to observe carbon formation on Pt will be discussed.

References:

1. V Galvita, G Siddiqi, P Sun, AT Bell, J Catal. 271 (2010) 209-219

2. DE Resasco, Encylopedia of Catalysis (2002)

3. F Somodi, Z Peng, A Getsoian, AT Bell, J Physical Chem C 115 (2011) 19084

4. Z Peng, F Somodi, S Helveg, C Kisielowski, P Specht, AT Bell, J Catal 286 (2012) 22-29