Supported rhenium (Re) catalysts are emerging as promising candidates for hydrogenation reactions, which are crucial in industrial processes such as biomass valorization, CO2 reduction, and petroleum refining. However, despite their broad application, the structural and mechanistic understanding of these systems remains limited. The strong oxophilic nature of Re, combined with its ability to adopt multiple oxidation states, complicates the characterization of the active species even with advanced experimental techniques. In this study, we employ density functional theory calculations, alongside ab initio thermodynamic analysis, to systematically explore the structural and electronic properties of single-atom Re catalysts on a TiO2 surface, providing insights that could inform the rational catalyst design. Our calculations reveal the formation of stable Re polyhydrides on the surface under hydrogen-rich conditions. Notably, even in highly reducing environments, Re species with low formal oxidation states are thermodynamically unfavorable. The stable Re species identified on TiO2 surfaces demonstrate high reactivity toward CO2 hydrogenation. The electronic properties and computed X-ray photoelectron spectroscopy (XPS) signatures of the feasible surface species are highly influenced by the ligand environment. This work highlights the limitation of routine interpretation of experimental XPS characterization data in terms of the formal oxidation state. ... Read more

Rational plastic recycling is critical for addressing the environmental challenges associated with plastic waste. Among the various recycling methods, chemical recycling, particularly via homogeneous catalysis, holds promise for converting plastic waste into valuable products. Post-consumer polymer wastes could present a challenge for catalytic upcycling due to the structural inhomogeneity and functionalization of the polyolefin chains. The impact of substrate aging on the performance of the upcycling catalyst can be viewed as an “inverse problem” of heterogeneous catalysis and has not received sufficient attention in mechanistic studies on this subject. Herein, we present a density functional theory study on the dehydrogenative upcycling of polyethylene (PE) with different in-chain impurities, representing the chemistry of post-consumption PE wastes. We selected the (^tBu4POCOP)-Ir pincer complex catalyzed dehydrogenation of PE as our model reaction. The calculations reveal that common in-chain impurities found in PE, such as carbonyl, hydroxyl, epoxides, and chlorine atoms, inhibit the overall catalyst performance. These impurities form stable molecular complexes with the catalyst, leading to a substantial increase in the energy barriers of the initial reaction step, the C-H bond addition. We also observe that the reaction on the ideal crystalline PE is also impeded. However, highly distorted PE chains exhibit greater susceptibility toward the (^tBu4POCOP)-Ir catalyst. Our mechanistic studies demonstrated that the reaction on the side alkane chains is kinetically favorable compared with the reaction on the PE backbone. The study highlights the critical role of in-chain heterogenieties in the catalytic activation of polymer chains and provides valuable insights into the development of effective technologies for upcycling plastic waste. ... Read more