Surface Termination Matters: How H and O Coverage Govern Mo2C MXene in Non-Oxidative Ethane Dehydrogenation Revealed...

Mo2C MXene is a promising non-precious metal catalyst for non-oxidative ethane dehydrogenation (EDH), but the relationship between its surface termination, thermodynamic stability, and catalytic performance remains poorly understood. Here, we present an integrated first-principles framework combining density functional theory, thermodynamics, and microkinetic modeling (MKM) to interrogate four distinct Mo2C surface terminations: bare Mo2C, hydrogen-decorated (Mo2C/6H), single-oxygen-decorated (Mo2C/1O), and fully oxidized (Mo2CO2) surfaces. DFT reaction energy profiles establish a Sabatier hierarchy across the four terminations: bare Mo2C overbinds intermediates (ΔEr = -1.13 eV for the first C-H step) resulting in severe product inhibition (C2H4 desorption: 1.93 eV); Mo2CO2 underbinds the reaction intermediates, with the second C-H step being endothermic (ΔEr = +0.46 eV) and exhibiting prohibitively high activation barriers (1.09 and 1.33 eV); and Mo2C/6H exhibits moderate binding but remains product-inhibited (desorption: 1.09 eV). Mo2C/1O uniquely occupies the Sabatier optimum: the lowest first C-H barrier of all surfaces examined (0.51 eV), all elementary steps thermodynamically exothermic, and a balanced energy profile that avoids both overbinding and endothermicity. MKM confirms Mo2C/1O achieves the lowest apparent activation energy (1.14 eV) and the highest turnover frequency at 823 K (525 s-1), surpassing bare Mo2C and Mo2C/6H by orders of magnitude. The surface phase diagram reveals asymmetric entropic destabilization of the hydride and oxide phase boundaries, with log(pH2O/pH2) governing surface oxygen chemical potential. The Mo2C/1O phase is kinetically stabilized within log(pH2O/pH2) ≈ -4.1 to +10, below which the bare surface is restored and above which higher-coverage oxidation prevails. These findings converge on a precise design principle: single-site oxygen decoration represents the Sabatier optimum for Mo2C EDH catalysis, decoupling C-H activation efficiency from product inhibition and providing a quantitative target for surface engineering of transition metal carbide catalysts.