*2.1. LaCoO3 vs. (La,Sr)CoO3*

As the stoichiometry of perovskites can be changed by replacing both the A-site and the B-site ions, the range of possible stoichiometries is quite large. Thus, we decided to adopt a simple approach where we first compare the properties of the LaCoO3 (LCO) and of (La,Sr)CoO3 (LSCO) hosts. For the latter, we assumed a La0.75Sr0.25CoO3 stoichiometry, which has been computed to be a stable phase by Fuks et al. [6]. We considered the formation of both single and double oxygen vacancies (VO), as well as the adsorption of CO and NO molecules at the CoO-terminated (100) surface, testing all the configurations allowed by a 2 × 2 supercell. The main results are reported in Table 1.

**Table 1.** Formation energy for surface vacancies and CO adsorption energies (eV) at the (100) surface of LCO and LSCO.


Interestingly, the partial substitution of La by Sr atoms enhances the stability of vacancies, while having minor (and contrasting) effects on the adsorption of CO and NO. Hence, this replacement is in principle suitable for tuning the catalytic properties of LaCoO3. Another interesting finding is that when two vacancies are introduced in the supercell, they prefer to cluster together.
