Deciphering the Active Phase Evolution of Co-W-Based Catalysts during Carbon Nanotube Growth by First-Principles...

Co-W-based catalysts represent one of the most promising systems for chirality-controlled carbon nanotube (CNT) growth. However, the identity of the active catalytic phase and its evolution under growth conditions remain under debate, leaving a critical gap in understanding how Co-W-based catalysts enable chirality-selective CNT synthesis. In this work, we investigated the thermodynamic stability and phase evolution of Co-W-C catalysts during CNT growth using first-principles calculations. By including vibrational contributions, we constructed phase diagrams of the Co-W-C system as a function of temperature, carbon chemical potential, and the W/Co ratio. The results show that Co7W6 is thermodynamically favored at low carbon chemical potential. Conversely, higher carbon chemical potential promotes the formation of carbide phases, such as Co6W6C, Co2W4C, and eventually Co3C, depending on temperature and catalyst composition. These diagrams consolidate previously reported active phases of Co-W-based catalysts (e.g., Co7W6, Co6W6C, Co-W-C η-carbide phases) into a single thermodynamic picture. Furthermore, the results also suggest that carbon supply may regulate the catalyst phase by changing the carbon chemical potential within the particle. This work clarifies the dynamic nature of Co-W-C catalyst evolution and establishes a thermodynamic framework for understanding phase behavior in Co-W-based catalysts for chirality-controlled CNT synthesis.