*2.3. CO Oxidation at LSCO: Effects of Doping at the Co Site*

The results presented in the preceding section encourage us to investigate in more detail how doping LSCO with heavy 3d dopants, i.e. Ni, Cu, and Zn, can influence the energetics of the CO oxidation reaction. To this end, we consider the mechanism reported in Ref. [3], consisting of the following elementary steps: i. Adsorption of CO; ii. Abstraction of lattice O atom and formation of CO2; iii. Desorption of CO2; iv. Adsorption of O2; v. Capture of a second CO molecule and formation of CO2; and vi. Desorption of CO2 and restoration of the stoichiometry of the starting surface. On the basis of the above presented findings, CO has been assumed to be adsorbed at a regular Co site, and to abstract a nearby O oxygen to form CO2. We optimized all the intermediate species and labelled them with consecutive numbers, starting from the clean surface (**1**). The optimized structures of these species turn out to be quite similar for the pure and for the doped systems, and are sketched in Figure 3 for the case of the Cu-doped surface. Actually, the structures are also similar to those of the analogous

intermediates computed for SrTiO3 [3] in the case of Cu doping. In particular, both O2 and CO2 lay parallel to the surface.

The analysis of the energy profiles relative to the investigated systems, reported in Figure 4, reveals that, similarly to what has been observed for the structures of the intermediates, the reaction energetics is also scarcely perturbed by doping, which is in striking contrast to the SrTiO3 case [3]. In fact, step ii, where a surface O atom is abstracted by CO, is always exothermic by 1.2–1.3 eV for all the systems, with the exception of the Cu-doped sytem, where it is even more exothermic (~1.5 eV). Overall, the energy profile of the reaction is qualitatively very similar to that computed for Cu-doped SrTiO3: all the steps are exothermic, with the exception of those corresponding to the desorption of weakly adsorbed molecules, which are slightly endothermic

**Figure 3.** Intermediate species in the CO oxidation process on the Cu-doped LSCO(100) surface. The color codes are red = O, blue = Co, green = La, gray = Sr, cyan = Cu, brown = C. Asterisks mark the position of O vacancies.

**Figure 4.** Energy profiles reporting all the intermediates (see Figure 3) in the CO oxidation reaction occurring on the pure as well as on the Ni-, Cu-, and Zn-doped LSCO(100) surfaces.

We now want to compare the potential energy (PE) curves of the O abstraction process (step ii, *viz*. 2 → **3**) for the undoped and for the Cu-doped LSCO surfaces as obtained from nudged elastic band (NEB) calculations. In tune with the analogies found both in the structure of the intermediates, and in the energy profiles (see Figure 4), the PE curves, shown in Figure 5, appear to be quite similar, the barrier being slightly lower for the Cu-doped case (0.35 vs. 0.45 eV). Because of the exothermicity of the step, we find "earlier" transition states, with respect to the case of s SrTiO3. In fact, transition state (TS) structures (see Figure 5) are similar to semi-bridging CO molecules, whereas CO2-like structures were computed in the latter case [3]. For analogous reasons, the C-Co distance of the TS becomes shorter (1.811 vs. 1.765 Å) on passing from the undoped to the Cu-doped surface, while a slight shortening is computed also for the C-O distance (1.159 vs. 1.156 Å).

**Figure 5.** Potential energy surfaces for the oxygen abstraction step (**2** → **3**) on the pure and on the Cu-doped LSCO(100) surfaces. Inset pictures show the structure of the transition states. The colors are the same as in Figure 3.

#### **3. Theoretical Methods**

Similarly to our previous investigations of SrTiO3, we used the PWSCF code of the QUANTUM-ESPRESSO package [7] to solve the spin-polarized Kohn-Sham equations with the generalized gradient approximation (GGA), adopting the PBE exchange-correlation functional [8]. We point out that our aim is not giving accurate descriptions of the electronic structure, nor quantitative predictions of any kind. Hence, we rely on standard GGA calculations, which have been successfully tested in similar cases [9,10], and are certainly affordable when comparing trends, instead of performing GGA+U calculations, which are time-consuming and often problematic to converge. The valence-core interaction were described by ultrasoft pseudopotentials taken from the Garrity–Bennett–Rabe–Vanderbilt library [11]. All the calculations have been initialized in the ferromagnetic state. Though most of the calculations have been carried out using the recommended 40 ryd plane wave kinetic energy cutoff, for transition state calculations (vide infra) a 30 ryd cutoff was used. We actually found that reducing the cutoff from 40 to 30 ryd has minor effects on the geometries, and produces a general 0.02–0.05 eV reduction of the adsorption energies. Lattice constants of LaCoO3 and of La0.75Sr0.25CoO3 have been optimized and kept fixed for all the subsequent calculations, as dopants have been treated as diluted impurities. Bulk structures have been studied with cubic supercells containing 40 atoms. For LSCO, Sr ions were assumed to occupy the lattice positions as in the structure proposed by Fuks et al. [6], i.e., they were placed at the farthest possible distance in a 2 × 2 × 2 supercell. We have considered the (100) surfaces, which have been modelled with a 2 × 2 slab consisting of seven atomic layers of CoO2 and LaO stacked alternately. Only the top (CoO2-terminated) surface of the slab was used to model adsorption and reactions. The top three atomic layers are relaxed, whereas the bottom four atomic layers were kept fixed to simulate bulk. Effects of impurities and oxygen vacancies were evaluated by modifying the composition of the top surface. Thanks to the symmetric termination, a 12 Å thick vacuum space is sufficient to decouple the surfaces. Increasing the vacuum thickness to 18 Å changes adsorption energies by less than 0.01 eV. The surface Brillouin zone was sampled using a 2 × 2 k-point mesh. Transition states (TSs) were located using the climbing image nudged elastic band (CI-NEB). Zero-point energy and entropic contributions were not included, as they cannot change the computed trends. In computing the formation energy of oxygen vacancies, a correction was applied to compensate the well-known DFT-GGA tendency to overestimate the O2 dissociation energy [12].
