Mechanistic and kinetic insights into the Pt-Ru synergy during hydrogen generation from ammonia borane over PtRu/CNT nanocatalysts

Increasing the utilization efficiency of noble metals by the synergy with less expensive metals is an important yet challenging issue in heterogeneous catalysis. Herein, exemplified with Pt-catalyzed hydrolytic dehydrogenation of ammonia borane, partial substitution of the Pt catalysts by over 11 times less expensive Ru is studied to obtain highly efficient and cost-effective Pt-based catalysts. It is observed that the Pt-Ru bimetallic catalysts, especially Pt0.5Ru0.5/CNT, deliver higher hydrogen generation activity and durability than the Pt/CNT and Ru/CNT catalysts, indicating a remarkable Pt-Ru synergy. The underlying nature of this synergy is elucidated by combining kinetic and isotopic analyses with multiple techniques such as HAADF-STEM, EDX, XRD, H2-TPR, HRTEM, XPS and ICP-AES. The appropriate activation energy and entropy of activation for the reaction are mainly responsible for the highly active Pt0.5Ru0.5/CNT catalyst, and a volcano-shaped relationship for the reactivity of these five catalysts as a function of entropy of activation is proposed. The insights revealed here might guide the rational design of catalysts for this reaction, and the methodology could be extended to unravel the nature of other bi- or multi-metallic catalysts.