(85a) Dehydrogenation, Hydrogenolysis and Oxidation of Ethane On Pt: Density Functional Theory Study and Microkinetic Analysis

Partial oxidation of fuels on noble metals is key to producing (1) syngas and hydrogen for a potential hydrogen economy from fossil fuels and biomass derivatives and (2) alkenes, such as ethylene which is the basic unit for polyethylene. The former process requires C-C bond cleavage, whereas the latter (oxidative) dehydrogenation. The best catalyst for the latter process is platinum1. The degree of syngas or alkene forming depends critically on the fraction of oxygen fed in a reactor. Preliminary calculations we conducted using the bond-order conservation method indicate that C-C bond scission cannot explain the speed of the reaction observed experimentally. The mechanisms for dehydrogenation, hydrogenolysis and oxidation of ethane on Pt surface are still under debate.

Using DFT, we calculated all the activation barriers of dehydrogenation and hydrogenation of C2 species, and all the C-C bond cleavage reactions and isomerization reactions on Pt(111) and Pt(211). It has been found that: (i) CCH3 is the most stable C2 species on Pt(111) and Pt(211); (ii) On Pt(211), ethane dissociation to CH2CH2 and CHCH3 is a rapid process at low surface temperatures; (iii) Isomerization reactions appear to be energetically unfavored on Pt; (iv) On Pt(111), the lowest barrier of C-C cleavage is 0.9 eV in CHC. Microkinetic modeling is in good agreement with literature data and indicates that the reaction pathway for C-C cleavage mainly takes place via CHCH3 and the dominant species on the surface is adsorbed H. We have found that first-principle results do not describe ethylene hydrogenation to ethane well. Using our hierarchical multiscale approach, we have refined the microkinetic model. Our results are in good agreement with data and indicate the importance of adsorbate-adsorbate interactions.

Under O-rich conditions, some other reactions like oxidative coupling reactions, oxidative dehydrogenation, and decomposition of C2HxO and C2HxOH have been studied via DFT and will be discussed. The full model is then used to describe partial oxidation of ethane to ethylene.

References

1. A.S. Bodke, D.A. Olschki, L.D. Schmidt, E. Ranzi, Science, 285, 712 (1999).