Electronic parameters in cobalt-based perovskite-type oxides as descriptors for chemocatalytic reactions

Perovskite-type transition metal (TM) oxides are effective catalysts in oxidation and decomposition reactions. Yet, the effect of compositional variation on catalytic efficacy is not well understood. The present analysis of electronic characteristics of B-site substituted LaCoO3 derivatives via in situ X-ray absorption spectroscopy (XAS) establishes correlations of electronic parameters with reaction rates: TM t2g and eg orbital occupancy yield volcano-type or non-linear correlations with NO oxidation, CO oxidation and N2O decomposition rates. Covalent O 2p-TM 3d interaction, in ultra-high vacuum, is a linear descriptor for reaction rates in NO oxidation and CO oxidation, and for N2O decomposition rates in O2 presence. Covalency crucially determines the ability of the catalytically active sites to interact with surface species during the kinetically relevant step of the reaction. The nature of the kinetically relevant step and of surface species involved lead to the vast effect of XAS measurement conditions on the validity of correlations.


Supplementary Note 2: Analysis of spin and oxidation states
XAS at first row transition metal (TM) L2,3-edges measures excitation of 2p electrons into unoccupied 3d states. It thus probes the energetic level and density of states of the latter, which depend on spin state, valence and coordination geometry. Supplementary Figure 2a shows the occupancy of d orbitals for the abundant Co spin and oxidation states in the octahedral geometry. The determination of Co oxidation states required to account for different spin state contributions in Co 3+ (low spin, LS, or high spin, HS) by appropriate choice of reference spectra 2 . Subsequently, present Co 2+ (stable HS) contribution was subtracted from the experimental data before the spin states of Co 3+ were assessed with suitable reference spectra 3 .
Quantification for all states was conducted via least squares fit of a linear combination of references (Supplementary Figure 2b) to experimental data followed by integration of the respective curve areas. Very low abundance of Co 3+ high spin states or Co 2+ was not accessible in the quantification and is designated as < 5 % in the respective measurements, reflecting an estimated determination threshold for low abundance.
Supplementary Fig. 2 Analysis of Co oxidation and spin states. a Co states present in the catalyst and the respective simulated spectra of LS Co 2+ and Co 3+ (LS) in octahedral coordination from ref. 2 as well as mixed Co 2+ spin states from ref. 3 . b Co 3+ Analysis of TM states, here for LCA-28 measured in UHV at 623 K. Gray area represents the total area of Co 2+ and Co 3+ , while the scatter plot shows experimental data.

Supplementary Note 5: Analysis of oxygen surface species
O 1s core electron spectra ( Supplementary Figures 7 and 8) were aligned and normalized to the bulk oxide species in terms of position (528.9 eV) and height, respectively. Peaks related to surface species of hydroxyl, carbonate groups or surface oxygen are observed in a range of 530-534 eV in line with common findings for perovskites 9,10 . Hydroxyl groups are attributed to the peak 530.5 eV 9 but the origin of the other two peaks related to surface species is ambiguous and cannot be discerned between surface oxygen and carbonate surface in absence of respective C 1s spectra. An additional species with a peak center at 534.7 -535.7 eV appears for the catalysts LCZ-82 and LCA-82. The latter was analyzed in detail for a set of different conditions The elevated binding energy of the peak implicates a surface species that is less negatively charged than the other surface species, possibly a superoxide-type species. It is important to note that peaks at this binding energy have been related to O in fluorinated hydrocarbons that are a potential surface impurity 11 . In some measurements respective peaks 12 in C 1s (291-293 eV) and a F 1s signal is present in the survey measurement in few instances. The C1s and F 1s peaks, however, do not occur consistently with the O 1s species at elevated binding energy.
Vice versa, catalysts that show the presence of fluorinated hydrocarbons in C1s and F 1s do not show the peak in O 1s that we ascribe to superoxide species. As a result, we ascribe the peak above 534 eV to a superoxide species. Additional experiments to confirm this designation are in line.
Species above 534 eV have also been attributed to liquid phase H2O 13,14 , which was generated through an experimental setup that was not present in our study and, thus, liquid phase H2O is very unlikely to be the origin of the peak in question. Original publications that were cited to attribute the species above 534 eV to adsorbed water in references 15,16 do not distinctly describe adsorbed water at these binding energies; instead adsorbed H2O is ascribed to peaks below 534 eV in other reports 9,10 . Fig. 7 Comparison of O 1s spectra of catalysts in UHV at 623 K a Full spectra and components. b Fit of the same spectra of LCA-82 in a more detailed illustration of the peaks related to oxygen surface species, where Oand CO3 2were not distinguishable in terms of peak attribution.

Supplementary Note 6: Partial Pressure Dependence in CO Oxidation
The trend of CO oxidation with partial pressures was tested to confirm previous mechanistic assumptions for our study: Figure S8 shows similar trends of reactant partial pressures as described previously 17 , where CO and O2 partial pressures have a favorable effect on the CO oxidation rate, while increasing CO2 partial pressure has the inverse effect. CO and CO2 adsorb on the same O surface sites 17 . Thus, CO2 partially inhibits CO adsorption and its subsequent reaction. The increase of CO partial pressure accordingly leads to an increase in CO oxidation rate as its concentration on the surface increases in the competitive adsorption regime. As a result, we assume the mechanism described previously 17 is valid for the discussions in our work as well. Supplementary Fig. 9 Effect of reactant partial pressure on CO oxidation rates. Reaction rates measured on LC at 519 K. Lines were added to guide the reader.