Controlled Pretreatment and Reconstruction of a Bimetallic Pt–Ir/Al2O3/ZSM-5 Catalyst for Increased Stability during Butane Hydrogenolysis

Abstract

The activity and stability of bimetallic Pt–Ir nanoparticles supported on an Al₂O₃/ZSM-5 mixture were investigated as a function of pretreatment and regeneration conditions for butane hydrogenolysis to ethane. Catalyst characterization by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy before and after aging under butane hydrogenolysis conditions for 12 weeks confirmed that the bimetallic nanoparticles were resistant to sintering, coking, and bulk metal segregation. However, for catalysts that were pretreated through an initial H₂ reduction, n-butane conversion decreased from 68 to 34% after 12 days on stream while maintaining ∼76% selectivity to ethane. A specific regeneration (or pretreatment) protocol was identified, involving the exposure of the oxidized catalyst to a butane and hydrogen mixture followed by post-reduction, which recovered the catalyst activity and enhanced catalyst stability such that n-butane conversion decreased <5% after 6 days on stream. The influence of various treatments on the structure and surface composition of the bimetallic nanoparticles was hypothesized based on analysis of in situ and cryogenic CO probe-molecule diffuse reflectance infrared Fourier transform spectroscopy measurements. Based on this analysis, it was inferred that high-temperature H₂ treatment of oxidized catalysts resulted in intraparticle segregation into a Pt shell and Ir core that was detrimental to long-term catalyst performance. The core–shell structure was reversible upon catalyst oxidation in O₂, forming an oxidized Ir (IrO_x) shell and Pt core. Treatment of the oxidized catalyst with a butane and H₂ mixture deposited CO and hydrocarbon adsorbates on the IrO_x shell, which stabilized Ir on the nanoparticle surface, even under reductive conditions. Post-reduction in H₂ restored the initial n-butane conversion with improved catalyst stability due to the adsorbate-stabilized, Ir-enriched surface. Therefore, carefully designed pretreatment protocols that deposit stable spectator adsorbates are presented as a valuable tool for controlling the surface composition of bimetallic nanoparticles under reaction conditions to improve their catalytic performance.