No abstract

Rational design of stable nanocatalysts Sintering of nanoparticles is one of the main causes of their catalytic deactivation. Rational design of nanocatalysts that are stable against sintering is a grand challenge in heterogenous catalysis. Hu et al . present kinetic theories for two competing sintering mechanisms, Ostwald ripening and particle migration, which relate the rates of both processes to fundamental interaction energies in metal nanoparticle-support combinations. Using kinetic simulations for hundreds of such pairs, the authors show a universal volcano dependence of the sintering kinetics on the metal-support binding energy that can serve as a single descriptor to predict nanoparticle growth rates. The revealed scaling relations are a good start in the development of high-throughput screening computational approaches to drive discovery of sintering-resistant nanocatalysts. —YS

Supported metal nanoparticles are of universal importance in many industrial catalytic processes. Unfortunately, deactivation of supported metal catalysts via thermally induced sintering is a major concern especially for high-temperature reactions. Here, we demonstrate that the particle distance as an inherent parameter plays a pivotal role in catalyst sintering. We employ carbon black supported platinum for the model study, in which the particle distance is well controlled by changing platinum loading and carbon black supports with varied surface areas. Accordingly, we quantify a critical particle distance of platinum nanoparticles on carbon supports, over which the sintering can be mitigated greatly up to 900 °C. Based on in-situ aberration-corrected high-angle annular dark-field scanning transmission electron and theoretical studies, we find that enlarging particle distance to over the critical distance suppress the particle coalescence, and the critical particle distance itself depends sensitively on the strength of metal-support interactions.

The stability of supported metal particles is one of the key issues for successful industrialization of catalysts. A theoretical study of Ostwald ripening of supported particles and its dependence on metal‐support interactions, sublimation energy, and surface energy is reported. Two distinct metal‐support interactions are differentiated: metal particle‐support and metal atom‐support. Although strong metal particle‐support interaction (small contact angle) stabilizes the supported particles and improve the ripening resistance, strong metal atom‐support interactions decrease the total activation energy and dramatically lower the onset temperature and half‐life time of ripening, a fact that should be prevented. Moreover, supported particles with low surface energy and/or high sublimation energy of supported particles would have a high onset temperature and a long half‐life time. Compared to the metal particle‐support interaction and surface energy, the metal atom‐support interaction and sublimation energy are most influential to the overall ripening resistance. The present work highlights the importance of interplay between two types of metal‐support interactions on the overall stability of supported particles.

Stability of dispersed particles on their supports is one of the central topics in heterogeneous catalysis and quantifying the influence of the particle size distribution on lifetime and thermal stability is highly valuable for rational design of efficient nanocatalysts. We report here a theoretical study of Ostwald ripening of supported particles, in particular, the kinetic evolution of particle number, average size, and dispersion with respect to time and thermal temperature in a wide range of size and monodispersity. Phase diagrams of half‐lifetime and onset temperature of ripening as functions of size and monodispersity were constructed. If decreasing the average particle size, though there is a modest gain in the dispersion, the stability declines dramatically; specifically, the half‐lifetime of ripening decreases exponentially and the corresponding onset temperature decreases by hundreds of Kelvin. Decline in stability owing to the decrease in size could, however, be systematically compensated by increasing the monodispersity of the size distribution. We find that the supported particles with the same half‐life time and onset temperature could originate from different particle size distributions, whereas the particle size distribution with the same apparent dispersion could have very different onset temperature and half‐lifetime. The result highlights the importance of both size and monodispersity in particle size distribution to the ripening resistance of supported particles, and the methodology developed for simulating ripening kinetics could be used to accelerate the aging protocols.

Supported transition metal (TM) particles on oxides severely deactivate because of sintering. Investigation of the dependence of Ostwald ripening kinetics on the composition and size of the metal particles is essential for understanding the sintering mechanism. On the basis of the first-principles kinetics simulation, we study here the ripening kinetics of TiO 2 (110)-supported late TMs (including Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) in a wide range of particle size. Density functional theory calculations show that the total activation energies of ripening are decided by the corresponding formation energy of the metal monomer on TiO 2 (110) and vary in the range of 3 eV following the order of Ag < Cu < Pd < Au < Ni < Rh < Pt < Ru < Ir. Isothermal and temperature ramping kinetic simulations are performed, and the corresponding half-life time and onset temperature of ripening are extracted, respectively. The results show that the half-life time of ripening exponentially increases with the total activation energy of the metal from Ag to Ir. The onset temperature of ripening increases more than hundreds of kelvin, which is consistent with variation in the melting points of the bulk counterpart. The ripening rate is found to dramatically increase with the decrease of the particle size, and the corresponding size effect increases pronouncedly with the total activation energy from Ag to Ir. This work provides valuable insights into the ripening kinetics of oxide-supported metal particles and is helpful in designing stable nanocatalysts.

No abstract

Metal oxide plays an important role on stability and catalytic performance of supported metal nanoparticles, but mechanistic understanding of structure sensitivity and optimization of the oxide supports remains elusive in heterogeneous catalysis. Taking Ostwald ripening of platinum nanoparticles supported on titanium dioxide (TiO2) as an example, we reveal here a great structure sensitivity of oxide facets and crystal phases on sintering of supported metal nanoparticles through first-principles kinetic simulation. Total activation energies of the Pt ripening on various pristine TiO2 surfaces of both anatase and rutile phases are calculated by density functional theory, and Ostwald ripening under isothermal condition and temperature programmed condition are simulated numerically. Calculated total activation energies are found inversely proportional to the corresponding oxide surface energies, and vary considerably from 1.76 to 3.56 eV. The Pt ripening rate on the pristine TiO2 surfaces follows the order of r(001) ≈ a(001) ≫ a(100) ≈ r(101) > r(100) > a(101) ≈ r(110). For TiO2 support exposing different facets, not only their intrinsic ripening rate but also their relative surface area determines the overall ripening kinetics and formation of transit bimodal particle size distribution. For pristine anatase TiO2 exposing a(001) and a(101) facets, ripening starts on a(101) facets only after ripening on a(001) facets finishes due to their order of magnitude difference in ripening rate, resulting a step-wise increase of average particle size with ripening time. For pristine rutile TiO2 exposing r(101) and r(110) facets, ripening could proceed simultaneously on both facets due to their modest difference in ripening rate, and the average particle size increases monotonically with ripening time. Compared to rutile TiO2, anatase TiO2 supports are less resistant to the metal nanoparticles ripening since a(001) facets with high ripening rate is likely to be exposed. The present work is compared to available experiments and the theoretical framework established could be expanded to various metal and oxide systems.