Catalytic dehydrogenation of ethane (EDH) to ethylene over platinum-based catalysts has continued to gain scientific attention as a profitable alternative to ethane steam cracking. For improved catalyst design, it is important to accurately identify the active site(s), investigate the mechanism, and reconcile experimental and computational results. However, computational investigations of heterogeneously catalyzed reactions using density functional theory (DFT) are often inaccurate, largely due to uncertainties in the choice of DFT functional (enthalpic uncertainty) and approximations for modeling adsorbate movement along the catalyst surface (entropic uncertainty). This work illustrates that both uncertainties are significant in the investigation of the ethane dehydrogenation (EDH) and hydrogenolysis on Pt catalysts by considering the complete deconstruction of ethane on Pt(100), Pt(111), and Pt(211) using microkinetic modeling (MKM). Hence, this work uses both non-calibrated and calibrated Bayesian MKMs to quantify and correct inaccuracies in macroscopic properties due to both uncertainties. A Bayesian approach to the correction of entropic errors was introduced using a defined ‘Modified Fermi Function (MFF)’ to calibrate in-between the two bounds of entropy represented by the harmonic oscillator (HO) and free translator (FT) approximations. Regardless of enthalpic and entropic uncertainties, all three surfaces are capable of ethane activation, however, Pt(211) was found to be the most active and is largely responsible for methane production. Next, Pt(111) is largely responsible for acetylene production, and Pt(100) has the highest ethylene selectivity but is most susceptible to coking. By comparing different calibrated models, the FT entropy approximation was found to better describe EDH and hydrogenolysis at typical EDH experimental conditions. Statistical evidence was found supporting both Pt(111) and Pt(211) as active sites for EDH, assuming one single site is responsible for the chemistry. On the three surfaces, competing second dehydrogenations to CH₂CH₂ and CH₃CH were observed as well as isomerization of CH₃CH back to CH₂CH₂ and deeper dehydrogenation of CH₃CH. C-C cleavage was found to largely proceed via the CH₃C intermediate on Pt(100) and Pt(111), while on Pt(211), via CHC.

Mubarak Bello

Appearances