Understanding the Effect of Platinum Particle Size on Ethane Dehydrogenation and Hydrogenolysis: Particle-Based Microkinetic Modeling at Finite Conversion

The effect of platinum particle size on the kinetics and mechanism of ethane dehydrogenation (EDH) and hydrogenolysis (EH) is elucidated using first-principles and multiscale modeling. A particle-based microkinetic modeling (PB-MKM) approach is used to couple the MKMs for individual facets, i.e., Pt(111), Pt(100), and Pt(211), into a variable-size Pt nanoparticle catalyst model. The PB-MKM accounts for different types of cross-facet communication, and special attention is paid to modeling cross-facet adsorbate diffusion. In agreement with experimental observations, EDH activity and selectivity increase with particle size between 1.7 and 9.5 nm. However, EDH activity is highest in the limit of subnanometer catalysts and declines beyond 9.5 nm. Facet cooperativity is observed for the EDH mechanism, where surface reactions occur predominantly on Pt(211), while adsorption and desorption largely proceed on the terrace facets. Ethylene is produced via both dehydrogenation and hydrogenation pathways, methane originates from C–C cleavage on Pt(211), and acetylene is produced, in trace amounts, on Pt(111). For EH, Pt particle size effects depend on reactant pressure, where the EH rate decreases with increasing particle size at low PH2 and increases at high PH2. At typical experimental conditions (PH2=0.2⁡bar;PH2/PCH3CH3=6.67), the EH rate marginally decreases with particle size while methane selectivity remains constant, in agreement with experiment. Unlike EDH, EH rates are insensitive to adsorbate diffusion because all CxHy species equilibrate with their corresponding gas-phase alkanes, and dehydrogenation/hydrogenation steps are quasi-equilibrated. C–C cleavage reactions are rate-controlling during EH and proceed on all three facets via different elementary reactions. Particle size effects are also observed in the H2 reaction order for both EDH and EH, with positive and negative reaction orders at low and high PH2, respectively. This work demonstrates that PB-MKM at finite reactor conversion can help achieve better kinetic agreement with experiments.