Pt metal supported and Pt4+ doped La1−xSrxCoO3: non-performance of Pt4+ and reactivity differences with Pt metal

In the present work, we correlate the CO-oxidation activity with the oxidation state of platinum with combined experimental and DFT calculations. XPS reveals that Pt supported La1−xSrxCoO3 (Pt/La1−xSrxCoO3) and Pt doped La1−xSrxCoO3 (La1−xSrxCo1−yPtyO3) consist of Pt in 0 and + 4 oxidation states respectively. Further, catalytic CO oxidation over Pt-doped and Pt-supported La1−xSrxCoO3 in the presence of oxygen demonstrates the lowest activity of the doped compound. Pt supported La1−xSrxCoO3 showed the highest activity with almost 100% conversion at 150 °C. La1−xSrxCo1−yPtyO3 was slightly inferior to the blank La1−xSrxCoO3 suggesting that Pt4+ is an inactive or non-performing entity in the doped compound. Temperature programmed desorption (TPD) demonstrates the low amount of CO desorption from La1−xSrxCoO3 and Pt-doped La1−xSrxCoO3 due to the very weak interaction. On the other hand, Pt-supported La1−xSrxCoO3 shows a substantial amount of CO desorption due to strong interaction and large number of metallic sites available for adsorption. This was supported by density functional theory (DFT) based calculations which showed that Pt-supported La1−xSrxCoO3 surface has higher binding energy of CO than the La1−xSrxCoO3 surface confirming the strong CO interaction. Transient responses using mass spectrometer suggest that the Pt supported perovskite utilizes the lattice oxygen for the reaction and vacancies are formed which gets filled with gaseous oxygen. No such phenomenon is observed in the doped compound demonstrating the mechanistic differences in the two catalysts. Often, during the synthesis of Pt-based compounds, it is common to get mixed phases of platinum including Pt4+. From this study, it can be established that one can discard the contribution from Pt4+ in the calculations of kinetic data such as rate or turnover number because this oxidation state is inactive/nonperforming.by Anuj Bisht, Amita Sihag, Akkireddy Satyaprasad,Sairam S. Mallajosyala and Sudhanshu Sharm