Hydrogenation of Carbon to Methane on Flat and Stepped Platinum Surfaces

Dehydrogenation of hydrocarbons on metal surfaces often leads to coke formation which deactivates the catalyst, but in a hydrogen-rich environment the adsorbed carbon and coke can be hydrogenated to form CH₄. This product can then desorb from the surface to open adsorption sites for other surface reactions. Coke hydrogenation, C* + 2 H₂ → CH₄, also has many common elementary steps with decomposition or hydrogenation of hydrocarbon molecular fragments, making it of general interest in thermal catalysis. Using density functional theory (DFT), we study the reaction mechanism of coke to methane conversion on different platinum surfaces to determine the most favorable reaction mechanism and the role of surface structure. We compared three surfaces, Pt(111), Pt(211) (which has (111) terrace and (100) oriented step edge), and Pt(511) (which has (100) terrace and (111) oriented step edge), calculations on the (100) surface are in progress. The reaction step CH₃ + H → CH₄ has the highest barrier of all elementary steps on all the three surfaces with barrier heights of 0.90 eV, 1.00 eV and 1.13 eV respectively for Pt(511), Pt(111) and Pt(211). On Pt(111) carbon adsorbs in a fcc site, while on Pt(211) carbon adsorbs in a hcp site on the upper terrace right next to the step edge, and on Pt(511) carbon adsorbs in a fcc site on the step edge; in between the step and the lower terrace. These adsorption sites are most likely to be blocked on a coked surface. Based on the adsorption strength of carbon and the barrier for hydrogenation, Pt(111) will be most affected by the coke and Pt(511) will be the least affected. These findings can help us understand coke formation on Pt surfaces, as well as coke removal on different surface structures, and prevent deactivation of the catalyst.