This study aims to elucidate the influence of cobalt (Co) diffusion on the chemical vapor deposition (CVD) process of hydrogen-terminated diamond (100) surfaces, with a particular focus on the effects on dehydrogenation reactions and the adsorption behaviors of critical carbon-hydrogen (C-H) groups.Currently, pretreatment methods are commonly employed to remove cobalt from the substrate in order to mitigate its effects during diamond deposition. However, these methods tend to reduce the substrate's toughness and increase preparation costs. Moreover, even when cobalt is partially removed, some of it within the substrate can still diffuse to the film-substrate interface and into the diamond film during the deposition process, thereby compromising the quality of the diamond film.The primary objective of this study is to investigate, at the atomic scale, how cobalt atoms diffusing into the diamond substrate affect the key reactions during diamond growth—specifically, dehydrogenation and C-H group adsorption. Understanding these effects is crucial for developing strategies to mitigate cobalt's adverse impact on diamond deposition.Using first-principles calculations based on density functional theory (DFT), we constructed geometric models of single-crystal diamond and its (100) surface. Co atoms were introduced at various diffusion depths (ranging from the 2nd to the 5th layer beneath the diamond surface), and the surfaces were hydrogen-terminated to mimic experimental conditions.The Dmol3 module in Materials Studio was employed to simulate and analyze the energy barriers for dehydrogenation reactions and the adsorption energies of key C-H groups, which include CH, CH₂, CH₃.Transition state searches were performed to determine reaction pathways and energy profiles, while adsorption energies were calculated to assess the stability of C-H group binding at active sites.The presence of Co significantly elevated the energy barriers for dehydrogenation reactions.The magnitude of this increase was positively correlated with the projected distance (D_Co-H) between surface H atoms and Co atoms.Additionally, while the number of layers separating Co from the surface also influenced the energy barrier, this effect was less pronounced compared to D_Co-H.Co diffusion altered the adsorption energies of C-H groups, particularly increasing the adsorption energy of CH₃—a pivotal group in diamond growth.This resulted in reduced adsorption efficiency of CH₃, thereby degrading the quality of diamond deposition. The impact varied with Co's diffusion depth: at the 2nd layer, all C-H groups exhibited increased adsorption energies, indicating thermodynamic instability; at deeper layers (3rd to 5th), CH₃ consistently showed higher adsorption energies compared to Co-free conditions, while CH and CH₂ exhibited more complex behaviors with some layers showing decreased adsorption energies.Our findings provide crucial insights into the atomic-scale mechanisms by which cobalt affects diamond CVD.The significant elevation of dehydrogenation energy barriers and the altered adsorption behaviors of C-H groups, especially CH₃, underscore the challenges in depositing high-quality diamond films on WC-Co substrates.These results guide the development of strategies to mitigate cobalt's adverse effects, such as through optimized substrate pretreatments or barrier layer insertions, ultimately enhancing diamond film quality on cobalt-containing substrates.