Computational Insights into Hydrogen Interaction with the Ru (101 ̅1) and Ru (101 ̅0) Surfaces: Implications for Alkane and Polyolefin Hydrogenolysis

Hydrogen interaction with transition metal surfaces such as those exposed by ruthenium (Ru) nanoparticles is critical in applications like hydrogen storage and catalytic processes such as Fischer-Tropsch, Haber-Bosch, and plastic waste hydrogenolysis. While the Ru (0001) surface is well-studied, hydrogen interaction with the Ru (101 ̅1) and Ru (101 ̅0) facets remains mostly underexplored. In this contribution, we use density functional theory calculations to investigate hydrogen adsorption and dissociation and provide insights into the adsorbed hydrogen role in catalytic polyolefin plastic hydrogenolysis. We start our investigation by exploring all the unique surface and subsurface sites for hydrogen adsorption and dissociation and identify hcp and higher hollow as the most favorable atomic hydrogens adsorption sites on the Ru (101 ̅1) and Ru (101 ̅0) surfaces, respectively. We find that atomic hydrogen can easily migrate on these surfaces to achieve the most stable arrangement at different coverages. We then combine these findings with ab-initio thermodynamics and microkinetic modeling to build surface phase diagrams, which show that both surfaces are fully hydrogenated under typical catalytic conditions. We then study how the presence of the full hydrogen coverage affects the adsorption and dehydrogenation of butane as a proxy for polyethylene, as these are the initial steps in the catalytic hydrogenolysis of polyolefin plastic waste. We find that the adsorption energy of butane decreases when the two surfaces are fully hydrogenated but adsorption remains favorable. We then investigate two possible mechanisms for the dehydrogenation step. The most favorable dehydrogenation mechanism involves the reaction of a surface hydrogen with an alkane hydrogen to produce H2 gas and an adsorbed alkyl radical. However, both mechanisms have positive reaction free energies suggesting that polyolefin dehydrogenation will be slow on these surfaces.