Temperature and Reaction Environment Influence the Nature of Platinum Species Supported on Ceria

Abstract

Catalytic systems based on supported noble metals are extensively studied because of their widespread application. Discussions remain about the nature of the active species, whether they are atomically dispersed or nanoparticles, and their reactivity. In this work, combining in situ/operando spectroscopy with theoretical modeling, we propose a phase diagram of atomically dispersed platinum on ceria, demonstrating that it reversibly changes from PtIVO2 to PtIIO as a function of temperature and oxygen partial pressure. The phase diagram helps identify the stability domain of each species, while spectroscopies provide a quantitative evaluation depending on the reaction conditions. Finally, our results show that high-temperature activation in the presence of steam of supported atomically dispersed platinum enhances the activity toward low-temperature carbon monoxide oxidation because it promotes aggregation into nanoparticles. This work highlights the structure–activity relationship in supported metal catalysts and proposes a suitable approach to determine the amount of each species before the investigation of the reaction mechanism.

Operando XAS Study of Pt-Doped CeO2 for the Nonoxidative Conversion of Methane

“…59 Linear combination fitting of the XANES data (Figure S17 and Table S2) and thermogravimetric analysis (TGA, Figure S18) evidenced two reduction regimes from about 50 to 250 °C and 400 to 530 °C, most likely corresponding to the reduction of atomically dispersed Pt IV O 2 to Pt II O and subsequent reduction to Pt 0 nanoparticles, respectively. 60 This could be the reason for a small amount of coke formation (Figure 3), as reported by Xie et al 9 The operando XANES spectra of 0.5 Pt (FSP) in Figure 6b during the reaction with CH 4 at 975 °C revealed near-edge features similar to a Ce−Pt alloy (compared to CePt 5 reference, cf. the later section), hinting toward an increased Pt−Ce interaction.…”

Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports

“…Figure 5 and Table S4.3, Supporting Information, show that fewer ion-exchanged Pd 2+ sites were formed, and the PdO content increased after the wet air treatment in comparison to the dry air treatment at temperatures < 900 K, which is qualitatively consistent with our thermodynamic predictions (Figure 4) that increasing H 2 O pressures should reduce the Pd 2+ cation content and increase the agglomerated PdO content. Similar results have been reported for atomically dispersed Pt on CeO 2 116 where steam exposure led to an increase in particle agglomeration. Additionally, a larger fraction of residual metallic Pd remained after wet air treatments than after dry air treatments for temperatures between 673 and 873 K, an effect that may be due to H 2 O-induced hydroxylation of particle surfaces inhibiting the complete oxidation of Pd.…”

Surface Noble Metal Concentration on Ceria as a Key Descriptor for Efficient Catalytic CO Oxidation

“…During the last decades, the role of the oxide support was reassessed from a broader perspective: the carrier also plays a key role for the catalytic activity, selectivity, and stability of catalysts. − For instance, redox active supports like CeO 2 − have been intensively investigated, and different descriptors are proposed to influence catalytic performance: morphology/exposed facets, − porosity, − reducibility of surface and bulk species, ,− or noble metal particle size. − Furthermore, the strong noble metal−support interaction − is reported to tremendously affect the activity profile. Recent in-depth in situ and operando characterization and theoretical modeling revealed the inherent dynamics of such catalytic systems under reaction conditions, which further increases their complexity. ,,− This includes the evolution of the platinum species themselves but also noble-metal induced reconstruction of the support. ,,,− The dynamics make the catalyst a strongly changing system that responds to the surrounding conditions. ,,− Central to the Pt/CeO 2 catalyst is the formation of isolated Pt species on the CeO 2 surface when the catalyst is exposed to oxygen at high temperatures. This phenomenon leads to the disintegration of the more active Pt nanoparticles. ,,− The reverse process occurs under reducing conditions, when monatomic Pt species reagglomerate to form Pt nanoparticles. ,, Both processes occur in emission control catalysis.…”

“…Recent in-depth in situ and operando characterization and theoretical modeling revealed the inherent dynamics of such catalytic systems under reaction conditions, which further increases their complexity. ,,− This includes the evolution of the platinum species themselves but also noble-metal induced reconstruction of the support. ,,,− The dynamics make the catalyst a strongly changing system that responds to the surrounding conditions. ,,− Central to the Pt/CeO 2 catalyst is the formation of isolated Pt species on the CeO 2 surface when the catalyst is exposed to oxygen at high temperatures. This phenomenon leads to the disintegration of the more active Pt nanoparticles. ,,− The reverse process occurs under reducing conditions, when monatomic Pt species reagglomerate to form Pt nanoparticles. ,, Both processes occur in emission control catalysis. Whereas pollutants like carbon monoxide and hydrocarbons act as reducing agents (especially at λ<1), molecular oxygen is a strong oxidant particularly under lean conditions (λ>1).…”