Structure-Sensitivity of CO Oxidation Over Supported Pt Catalysts in the Subnanometer Regime and Its Origin

Abstract

Metal-oxide-supported Pt-group metal catalysts are regularly used in many industrially relevant reactions, including CO oxidation for automotive emission control. Being able to activate O2, these catalysts are also promising candidates for oxidative decomposition of chemical warfare agents and their simulants, and CO oxidation serves as a simple model reaction for that. For Pt-group metals, it is important to maximize the metal utilization to design efficient catalysts for these reactions. To achieve that, it is crucial to understand how the electronic properties of these catalysts change with metal nuclearity, especially in the subnanometer regime. It is also important to understand how the catalytic performance can be tuned by conducting surface modification for a certain metal size. Here, we investigate how the reactivity of CO oxidation reaction changes with the size of Pt particles supported on anatase TiO2 and CeO2 and its origin. We varied the Pt nuclearity from single-atoms to subnanometer clusters (0.8-0.9 nm) to small nanoparticles (1.5-1.8 nm). By coupling density functional theory calculations and microkinetic modeling with kinetic measurements, in situ/operando infrared and X-ray absorption spectroscopies, we show that single-atom Pt/anatase TiO2 follows a complex reaction pathway consisting of initiation steps to re-organize the active site and a multi-branch reactive cycle with competing Eley-Rideal and Langmuir-Hinshelwood pathways. Both reaction pathways proceed with the involvement of oxygen adatoms directly provided by O2 dissociation, and not lattice oxygen, leading to very low turnover frequency. We then show that CO oxidation activity increases by 10-fold and 60-fold as average Pt size increases to 0.9 nm and 1.8 nm, respectively. Contrary to the single-atom sample, lattice oxygen was found to be involved in the reaction on these samples, with Mars–Van Krevelen (MvK) being the likely reaction mechanism, and involvement of the lattice oxygen being the reason for the higher activity. Kinetic measurements and microkinetic analyses show that both O2 activation at the Pt-TiO2 interface and oxygen vacancy creation are rate-limiting steps, with activation energy for oxygen vacancy formation decreasing by 19 kJ/mol as average Pt size increases from 0.9 nm to 1.8 nm. This was further corroborated by temperature-programmed reduction by CO, which demonstrated that larger Pt particles enhance oxygen supply from the support by increasing its reducibility, thereby lowering the onset temperature for CO2 production during CO oxidation. We therefore performed CO oxidation on Pt single-atoms supported on rutile TiO2- a more reducible phase of TiO2 compared to anatase- and demonstrated that the activity increases by ∼ 15 times compared to Pt single-atoms supported on anatase TiO2, mostly due to higher reducibility of rutile TiO2. With these insights, CO oxidation was performed over similar Pt sizes (single-atoms to 1.5 nm) supported on CeO2, which has a significantly lower theoretical oxygen vacancy formation energy compared to anatase TiO2. By combining infrared, Raman, and X-ray photoelectron spectroscopy, we demonstrate that oxygen vacancy formation and replenishment occur readily at room temperature in 0.8, 1.0, and 1.5 nm Pt/CeO2 catalysts, resulting in catalytic activities that are 10 to 60 times higher than those of similarly sized Pt particles supported on anatase TiO2. We observe that there is significant electron transfer from Pt to CeO2 due to the low reduction potential of CeO2, leading to lower Pt electron density and an unusual Pt-Pt bond elongation for smaller Pt particles. This electron transfer phenomenon between Pt and CeO2 can be significantly affected by varying the concentration of surface oxygen vacancy in CeO2. We also show that CO oxidation over all the Pt sizes of Pt/CeO2 catalysts proceeds via Mars-van Krevelen mechanism involving lattice oxygen from CeO2 lattice. The O2 activation and/or O extraction from CeO2 lattice becomes more facile with increasing Pt size, which leads to higher activity. In this work, we show through systematic experimentation that the key factor in obtaining superior low-temperature activity for oxidation reaction is facile extraction of oxygen from the support lattice, and a larger Pt size helps achieve that by inducing higher reducibility to the support through electronic metal-support interaction.