*2.2. Doping LSCO at the Co Site*

On the basis of the above presented results, it seems interesting to investigate the effects of doping LSCO at the Co site, once more taking the vacancy formation energy as a gauge of the catalytic activity. To this end, we replace one of the surface Co atoms of the supercell by another 3d atom, namely V, Cr, Mn, Fe, Ni, Cu, and Zn. Next, we compute the stability of oxygen vacancies, taking into consideration both the sites adjacent (NN) and those not-adjacent (NNN) to the 3d-impurity. The same slab models are used to compute the adsorption energy of CO, which is in turn evaluated both at the impurity site (CO@M) and at regular cobalt sites (CO@Co). We resume the results of these calculations in Figure 1, from which the difference in the behaviour of the investigated systems can be inferred. In particular, it appears that though the formation of vacancies is favored both by light and by heavy 3d metal impurities (with the exception of Fe), vacancies are attracted only by light dopants. Furthermore, CO adsorption is always favored at regular (unsubstituted) Co sites.

**Figure 1.** CO adsorption energies (top) and vacancy formation energies (bottom) computed for TM-doped LSCO(100) surfaces. The CO@M and CO@Co lines indicate adsorption at impurity and at regular Co sites, respectively. Vacancies created at the nearest site and at the next nearest site of the impurity are labelled NN and NNN, respectively.

We now consider the effect of Co-site doping on the formation of double vacancies. This is done starting from the structure of single vacancies, and exploring the stability of the possible configurations corresponding to the creation of a second vacancy. The results, shown in Figure 2, indicate that, in contrast to the case of single vacancies, the formation of double vacancies is favored only by heavy 3d impurities, namely, Ni, Cu, and Zn. In contrast to that, doping with light 3d metals is detrimental, as it increases the formation energy of double vacancies by ~0.5 eV wrt the undoped LSCO host (corresponding to the Co label in Figure 2) which in turn indicates a worse performance for NO reduction. Therefore, doping with light 3d impurities, such as V, Cr, Mn, and Fe, is not a wise choice for improving the properties of LSCO in TWC converters.

**Figure 2.** Formation energy for double vacancies (i.e., 2 VO/cell) for LSCO doped at the Co site.
