(740d) Investigation of Coke Formation Mechanism on Platinum Surface during Ethane Dehydrogenation

Light alkane dehydrogenation receives much attention due to the recent increase of feedstocks from abundantly available shale gas in North America. Catalytic dehydrogenation over Pt alloys has been extensively studied as one promising processes for this reaction. The major challenge for this process is, however, coke deposition on the catalyst active sites leading to catalyst deactivation. Many studies have shown that alloying Pt or co-feeding hydrogen can enhance selectivity and reduce the coke amount. Although these studies have helped relieve this issue, it is still a significant problem and the mechanism of coke formation which could be essential to solving the problem remains unknown. In this work, the coke growth mechanism on Pt surface during ethane dehydrogenation was studied with density functional theory (DFT) calculations.

Coke formation paths including initiation of surface intermediates and the early propagation of them were explored on pristine Pt(111) surface by constructing Gibbs free energy diagrams. This started from chosen surface species (ethylidyne, vinyl, and acetylene) forming longer carbon chains. For ethylidyne (CH₃C), it was thermodynamically hard to further add surface radicals or detach hydrogens since it was very stably adsorbed on Pt. C-C bond formation with methylene was facile for vinyl (CH₂CH) or acetylene (CHCH) whereas dehydrogenation was thermoneutral (CH₂CH) or required much energy penalty (CHCH). This preference for C-C formation over C-H breaking was also shown for C₃H_x surface intermediates, which may imply that the former could occur before the latter at the early stage of carbon chain growth reactions. Ultimately, the network for C-C formation and C-H cleavage was established in order to obtain insight for coke formation mechanisms on Pt surface. This will lead to providing ideas for the design of novel coke-resistant catalysts.