During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO₂ 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/CeO₂ 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.

Nanoporosity is clearly beneficial for the performance of heterogeneous catalysts. Although exsolution is a modern method to design innovative catalysts, thus far it is predominantly studied for sintered matrices. A quantitative description of the exsolution of Ni nanoparticles from nanoporous perovskite oxides and their effective application in the biogas dry reforming is here presented. The exsolution process is studied between 500 and 900 °C in nanoporous and sintered La_0.52Sr_0.28Ti_0.94Ni_0.06O_3±δ. Using temperature‐programmed reduction (TPR) and X‐ray absorption spectroscopy (XAS), it is shown that the faster and larger oxygen release in the nanoporous material is responsible for twice as high Ni reduction than in the sintered system. For the nanoporous material, the nanoparticle formation mechanism, studied by in situ TEM and small‐angle X‐ray scattering (SAXS), follows the classical nucleation theory, while on sintered systems also small endogenous nanoparticles form despite the low Ni concentration. Biogas dry reforming tests demonstrate that nanoporous exsolved catalysts are up to 18 times more active than sintered ones with 90% of CO₂ conversion at 800 °C. Time‐on‐stream tests exhibit superior long‐term stability (only 3% activity loss in 8 h) and full regenerability (over three cycles) of the nanoporous exsolved materials in comparison to a commercial Ni/Al₂O₃ catalyst.