Key Descriptors of Single-Atom Catalysts Supported on MXenes (Mo₂C, Ti₂C) Determining CO₂ Activation

Abstract

CO₂ activation is crucial to its upgrade to fuels and chemicals. In this work, we systematically studied CO₂ cleavage on single-atom catalysts (SACs) based on metals M = (Fe, Ru, Co, Rh, Ni, Pd, Pt, Cu, Ag, Au) supported on Mo₂COₓ (6/9 O ML) and Ti₂COₓ (7/9 O ML) MXenes via Density Functional Theory (DFT) calculations and Bader charge analysis to provide insights into the charge redistribution among the metal, MXene, interface, and CO₂ during the process. CO₂ activation involves a two-step mechanism, adsorbing at the M-MXene interface where it bends and acquires a highly anionic character and then breaks, forming CO* and O*. The energy barriers analyzed for the CO₂ activation on M/Mo₂COₓ and M/Ti₂COₓ surfaces show that Cu, Ni, Rh, and Pt on Mo₂COₓ and Cu, Ru, and Rh on Ti₂COₓ presented the lowest energy barriers. Comparing the two MXenes, the electrophilic nature of Mo atoms facilitates CO₂ cleavage, while the Ti atoms distribute charge differently, hindering the CO₂ activation process. The energy barriers toward CO₂ activation on M/Mo2COₓ and M/Ti2COₓ surfaces show that Cu, Ni, Rh, and Pt on Mo₂COₓ and Cu, Ru, and Rh on Ti₂COₓ presented the lowest energy barriers. Mo2COₓ systems presented geometrical structures of the transition states that were more product-like aligning with the Hammond’s principle, implying exoenergetic processes and lower energy barriers in contrast to Ti2COₓ. Moreover, the CO₂ activation on M/2D-Mo₂C follows a Brønsted-Evans-Polanyi (BEP) relationship while M/2D-Ti₂C breaks it, a crucial factor to identify better catalytic materials. The ExtraTreesRegressor machine learning algorithm effectively predicts adsorption and transition-state energies using a small set of descriptors. The findings underscore the importance of transition metal electronic states, charge transfer, and support structure effects for SACs on MXenes, providing valuable insights for the design of catalytic materials. This detailed analysis provides a deeper understanding of the mechanistic aspects of CO₂ activation, highlighting the role of single-atom metals and their interaction with metal-carbide surfaces.