Distinct Intercalation Mechanisms Across the MoS2, MoSe2 and MoTe2: From Weak Physisorption to Defect-Driven Chemistry

Intercalation within two-dimensional transition metal dichalcogenides (TMDs) presents an innovative frontier for catalysis, a strategy largely unexplored outside of energy storage applications. While these materials are promising, the fundamental impact of chalcogen identity (S, Se, Te) on interlayer chemistry is poorly understood, and systematic studies probing these effects are particularly rare. This leaves a critical gap in the rational design of 2D sensors and catalysts. This work addresses this gap through a first-principles investigation into the intercalation of CO and CO2 within pristine and vacant MoS2, MoSe2, and MoTe2 bilayers. While pristine materials exhibit weak interactions governed by the energetic cost of layer expansion, the introduction of a single chalcogen vacancy unlocks dramatically different and material-specific catalytic functions. These behaviors are dictated by an interplay between vacancy size, charge availability at exposed metal sites, and intermolecular interactions within the confined interlayer space. The compact S-vacancy in MoS2 is found to uniquely catalyze CO oxidation (2CO → CO2 + C) due to geometric constraints, while remaining inert to CO2. In stark contrast, the larger, more electron-rich vacancies in MoSe2 and MoTe2 are inactive for CO oxidation but instead promote the crucial CO2 reduction reaction (CO2 → CO + O). These findings establish that tuning chalcogen chemistry in concert with defect engineering provides a powerful and highly selective pathway for designing atomically precise catalysts for carbon oxide conversion.