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Adsorption and dissociation of diatomic molecules in monolayer $1H$-MoSe$_2$

Two dimensional transition metal dichalcogenides appear as good candidates for gas sensing and catalysis. Here, by means of density functional theory, we characterize the adsorption and dissociation of selected diatomic molecules (CO, H$_2$, O$_2$, and NO) on the MoSe$_2$ monolayer. We consider that these processes occur on the pristine $1H$-MoSe$_2$ monolayer and in the vicinity of an isolated Se vacancy. The presence of Se vacancies both enhances the molecular adsorption and reduces the energy needed for dissociation, making it energetically favorable for the case of O$_2$ and NO molecules. For each case we evaluate the effect that each adsorbate has on the electronic structure of the MoSe$_2$ monolayer and the charge transfer that takes place between the adsorbate and the surface. Remarkably, adsorption of either CO or NO at the Se vacancy induces a finite spin-magnetization in the system that is spatially well localized around the adsorbate and the three closest Mo atoms.