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

Abstract

During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO 2 catalysts, we found that the Pt surface concentration is a key descriptor for the reaction rate. By increasing the surface noble metal concentration (SNMC) of a Pt/CeO 2 catalyst by a factor of ∼4, while keeping the weight hourly space velocity constant, the ignition temperature of CO oxidation was decreased by ∼200 °C in the as-prepared state. Moreover, the stability was enhanced at higher SNMC. Complementary characterization and theoretical calculations unraveled that the origin of this improved reaction rate at higher Pt surface concentrations can be traced back to the formation of larger oxidized Pt-clusters and the SNMC-dependent aggregation rate of highly dispersed Pt species. The Pt diffusion barriers for cluster formation were found to decrease with increasing SNMC, promoting more facile agglomeration of active, metallic Pt particles. In contrast, when Pt particle formation was forced with a reductive pretreatment, the influence of the SNMC was temporarily diminished, and all catalysts showed a similar CO oxidation activity. The work shows the general relevance of the proximity influence in the formation and stabilization of active centers in heterogeneous catalysis.

Interface Reactions Dominate Low-Temperature CO Oxidation Activity over Pt/CeO2

“…Previous experiments have reported similar apparent activation energies for CO oxidation over Pt/CeO 2 . Taking the example of ignition in lean conditions and Pt particles smaller than 5 nm, refs and report 0.57 and 0.69 eV, respectively.…”

Detailed microkinetics for the oxidation of exhaust gas emissions through automated mechanism generation

“…noble metal's oxidation state. Therefore, this study underscores the importance of including morphological changes of the Pt crystals and the support, such as sintering, 73 cluster formation, 95 re-dispersion, and oxidation 76,77,94,97 and surface reconstruction 96 into the microkinetic model and the automated mechanism generation procedure. However, such an integration on a first-principles basis is beyond the scope of the present work.…”

“…93 Additionally, other facets can and do contribute to the conversion of the pollutants and the entire multifaceted nature of the crystals on the support need to be considered in the microkinetic model. 80,94,95 In principle, RMG can construct microkinetic models for all possible Pt facets if accurate thermochemical tables and BEP relations are provided. Through the usage of RMG within the framework described in the manuscript, the microkinetics for the complex mixture can be constructed in a fraction of the time required to build them manually, and RMG thus streamlines the entire process.…”

“…94,95 Avanesian et al96 observed a major but still reversible reconstruction of the Pt nanoparticles when they were exposed to a CO atmosphere at elevated pressure. Consequently, we think that the discrepancies between the model and the recorded concentrations for the experiments without H 2 and also for the small deviations for the experiments with H 2 are not caused by missing reaction pathways, but rather are due to the omission of the dynamic and multifaceted nature of the catalyst under reaction conditions, including the…”

Detailed Microkinetics for the Oxidation of Exhaust Gas Emissions through Automated Mechanism Generation

“…Especially the combination of microkinetic modeling with transient operando experiments to elucidate the nature and oxidation state of the catalyst nanoparticles is very promising. 77,78,95,96 However, such an integration on a first-principles basis is beyond the scope of the present work. Figure 7 displays the coverage profile for experiment 7, but this is similar for all cases.…”