Atom-to-nm ScaleEngineering of PtCo Alloy Catalystsover Supported Co Nanoparticles for Advanced Toluene HydrogenationEfficiency in Hydrogen Storage Applications

Hydrogen storage and transportation are pivotal for a hydrogen- and carbon-neutral society, yet current approaches face considerable limitations in efficiency and cost, which are typically related to catalyst design. Alloying a trace amount of Pt on supported Co nanoparticles has emerged as an innovative catalyst design that leads to a substantial reduction of the amount of precious metal used in the catalytic hydrogenation of toluene (TOL) to methylcyclohexane (MCH), which is an important aspect of hydrogen storage and transportation. However, because of the difficulty in controlling the surface structure of PtCo alloy catalysts, the structure that maximizes the synergistic interaction between the Pt and Co sites has not yet been identified. In this study, we explore the structure–function relationship in TOL hydrogenation over PtCo alloy catalysts by designing three distinct structures: PtCo single-atom alloys, PtCo alloy nanoislands on Co nanoparticles, and PtCo alloy nanoparticles on a support, i.e., monoclinic-phase ZrO₂ (m-ZrO₂). In the catalyst designs, m-ZrO₂ was employed as a support to control the Co nanoparticle diameter to 5–7 nm, maximizing the fraction of Co plane sites required for efficient TOL fixation/hydrogenation and enhancing the synergy with Pt site-derived functionality. The highest turnover frequency (TOF) had the following magnitude relationship: PtCo single-atom alloy > PtCo alloy nanoislands on Co nanoparticles > PtCo alloy nanoparticles on m-ZrO₂. Remarkably, the PtCo single-atom alloy enhanced Pt utilization efficiency by up to 51-fold over supported Pt nanoparticle catalysts, resulting in record activity in the literature. Based on the kinetic experiments coupled with in-depth structural analyses using the series of well-defined PtCo alloy catalysts, it was clarified that the high activity is attributed to (1) optimal competitive adsorption of TOL and H₂ created by Pt single atoms-containing Co ensemble sites and (2) large fraction of such sites. Stability tests coupled with microscopy confirmed that PtCo single-atom alloy catalysis remained stable for at least 24 h, indicating that the supported Co nanoparticles also play a crucial role in preventing Pt aggregation. Our findings underscore the importance of optimal atom-to-nm scale design of PtCo alloy catalyst for achieving ultrafast TOL hydrogenation even with extremely low Pt usage, offering an innovative approach for saving precious metal in hydrogen storage and transportation.