Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce(3+) species in a ceria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce(3+) formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce(3+) was present as spectators. This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.

In photocatalytic H 2 formation, tertiary amines are commonly used as sacrificial electron donors, thereby limiting the pH range for studies in water and the concentration of free protons. We found that ascorbate rapidly reductively quenches the excited state of [Re(CO) 3 (bipy)(py)] + (bpy = 2,2Ј-bipyridyl; py = pyridine). In combination with the water reduction catalyst (WRC) [Co{(DO)(DOH)pn}Br 2 ] [(DO)(DOH)pn =

X-ray photoelectron spectroscopy has been employed for the qualitative and quantitative characterization of both model and real catalytic surfaces. Recent progress in the detection of photoelectrons has enabled the acquisition of spectra at pressures up to a few tens of millibars. Although reducing the pressure gap represents a remarkable advantage for catalysis, active sites may be short-lived or hidden in the majority of spectator species. Time-resolved experiments, conducted under transient conditions, are a suitable strategy for discriminating between active sites and spectators. In the present work, we characterized the surface of a Pt/CeO powder catalyst at 1.0 mbar of a reacting mixture of carbon monoxide and oxygen and, by means of time resolution, identified short-lived active species. We replaced oxygen with nitrogen in the reaction mixture while fast-detecting the core level peaks of cerium. The results indicate that active Ce sites form transiently at the surface when the oxygen is switched off. Analysis of the depth profile shows that Ce ions are located at the ceria surface. The same experiment, performed on platinum-free ceria, reveals negligible reduction, indicating that platinum boosts the formation of Ce active sites at the interface.

Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials,w hich play an important role in catalysts lowering automotive emissions,a se lectrode materials for fuel cells,and as antioxidants in biomedicine.W e demonstrated that active Ce 3+ species in ac eria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce 3+ formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is akinetically relevant step in CO oxidation, whereas afraction of Ce 3+ was present as spectators.This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redoxa nd nonredoxcatalytic mechanisms.Oxygen-storage materials play an important role in automotive three-way catalysis, [1] carbon monoxide oxidation, and water-gas shift reaction, [2,3] electrode materials for fuel cells, [4,5] antioxidants in biomedicine, [6,7] solar thermochemical and photocatalytic water and carbon dioxide splitting. [8,9] However,t he nature of active species responsible for these outstanding properties typically remains unknown. This is mainly due to the low concentration of active species and the necessity to distinguish them from inactive spectators,w hich requires direct spectroscopic identification of intermediates and quantitative comparison of their kinetic behavior to the global reaction rate. [10,11] Theb est catalysts for low-temperature CO oxidation, which are important for lowering automotive emissions,c ontain metal nanoparticles such as platinum, gold, and palladium supported on or promoted by metal oxides that demonstrate reducibility and oxygenstorage capacity (OSC), such as ceria, titania, and iron oxides. [12][13][14][15][16][17] Catalytic oxidation on such catalysts is simple only at first glance.I to ften remains unclear how oxygen molecules are activated, whether lattice oxygen is directly involved in the catalytic cycle, and if the catalytic mechanism is of redox type.Atransient infrared spectroscopy study combined with density functional theory (DFT) calculations showed that during low-temperature CO oxidation on titaniasupported gold nanoparticles oxygen activation takes place at the metal-support interface,w hereas adsorbed CO is delivered by neighboring titania. [12] Moreover,r ecent kinetic and infrared spectroscopy experiments combined with DFT calculations demonstrated that oxygen from the titania support is not directly involved in the rate-determining step of CO oxidation on the Au/TiO 2 catalyst at room temperature.Instead, weakly adsorbed water is involved in the ratedetermining step. [16] These results raise questions whether this type of associative rather than redox mechanism of lowtemperature CO oxidation can al...

Copper-ceria finds applications in various energyrelated and environmental catalysts. However, the versatile structure and complex redox activity of this material entangle uncovering structure−activity relationships and distinguishing active species from spectators. In this work, we monitored the dynamic structure of the active sites in a catalyst containing highly dispersed copper-oxo species on ceria during low-temperature CO oxidation using time-resolved X-ray absorption spectroscopy. We quantitatively demonstrate that the CO oxidation mechanism below 90 °C involves an oxygen intermediate strongly bound to the active sites as well as the redox activity of Cu 2+ /Cu + and Ce 4+ / Ce 3+ couples. The redox activity of cerium is much lower than that of copper; however, both metals change their oxidation states in concert, indicating that oxygen activation involves copper−oxo species in close interaction with ceria. In addition to short-lived Cu + and Ce 3+ intermediates that are generated in the CO oxidation cycle, long-lived Cu + and Ce 3+ species appear in the catalyst under the working conditions. We demonstrate that they do not participate in the main low-temperature CO oxidation mechanism, which is mediated by a strongly bound oxygen intermediate. Finally, our results confirm the high potential of element-specific time-resolved X-ray spectroscopy methods combined with a non-steady-state experimental strategy to uncover the mechanisms of catalytic processes in complex multicomponent systems.