Pt Metal Supported and Pt4+ Doped La1−xSrxCoO3: Non-performance of Pt4+ and Reactivity Differences with Pt Metal

Abstract: 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 La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> (Pt/La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf>) and Pt doped La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> (La<inf>1−x</inf>Sr<inf>x</inf>Co<inf>1−y</inf>Pt<inf>y</inf>O<inf>3</inf>) consist of Pt in 0 and + 4 oxidation states respectively. Further, catalytic CO oxidation over Pt-doped and Pt-supported La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> in the presence of oxygen demonstrates the lowest activity of the doped compound. Pt supported La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> showed the highest activity with almost 100% conversion at 150 °C. La<inf>1−x</inf>Sr<inf>x</inf>Co<inf>1−y</inf>Pt<inf>y</inf>O<inf>3</inf> was slightly inferior to the blank La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> 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 La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> and Pt-doped La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> due to the very weak interaction. On the other hand, Pt-supported La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> 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 La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> surface has higher binding energy of CO than the La<inf>1−x</inf>Sr<inf>x</inf>CoO<inf>3</inf> 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. Graphical Abstract: Pt supported perovskite (Pt/LSCO) utilizes the lattice oxygen for the CO oxidation reaction and the vacancies formed get filled with gaseous oxygen. No such phenomenon is observed in Pt doped perovskite (LSPtCO). [Figure not available: see fulltext.]