Abstract
Identifying the ripening modes of supported metal nanoparticles used in heterogeneous catalysis can provide important insights into the mechanisms that lead to sintering. We report the observation of a crossover from Smoluchowski to Ostwald ripening, under realistic reaction conditions, for monomodal populations of precisely defined gold particles in the nanometer size range, as a function of decreasing particle size. We study the effects of the CO oxidation reaction on the size distributions and atomic structures of mass-selected Au(561±13), Au(923±20) and Au(2057±45) clusters supported on amorphous carbon films. Under the same conditions, Au(561±13) and Au(923±20) clusters are found to exhibit Ostwald ripening, whereas Au(2057±45) ripens through cluster diffusion and coalescence only (Smoluchowski ripening). The Ostwald ripening is not activated by thermal annealing or heating in O2 alone.
Highlights
Sintering leads to the coarsening of the particles, and the irreversible deactivation of their catalytic activity as the average particle size increases over time.[3]
Populations of mass-selected clusters exhibit very narrow particle size distributions that are approximately Gaussian (see as examples Figure 1(b,d) and Figure 2(a,e,i)). This is advantageous given that Granqvist and Buhrman posited that the ripening mode could be identified by the weighting of the particle size distribution.[7]
The challenge to verify the Granqvist−Buhrman phenomenon experimentally has been 2-fold: to track the change in the particle size distribution given that supported nanoparticles tend to exhibit an inherent broad, log-normal distribution from the outset, and to perform the analysis at the single particle level while amassing meaningful statistics
Summary
The inhibition of sintering remains a central challenge for heterogeneous catalysis using ultrafine metal particles.[1,2] Sintering leads to the coarsening of the particles, and the irreversible deactivation of their catalytic activity as the average particle size increases over time.[3]. The imaging of small clusters at the atomic level, even in vacuo, remains challenging.[4,12] It has long been known that supported metal clusters exhibit sizedependent catalytic properties,[13] it has only been recently that catalytically active, size-selected PtN (N = 22, 68) clusters on oxide supports have been used to provide evidence of Ostwald ripening suppression in truly monomodal populations.[14] Particle size has long been associated with catalyst sintering kinetics,[15] but detailed studies of truly monodisperse nanoparticles at atomic resolution have proved elusive. Ostwald ripening is activated by the combustion of CO, and not by thermal annealing (0.35 bar, 250 °C) nor the presence of O2 alone (0.14 bar (Au923±20) and 0.12 bar (Au2057±45) at 250 °C)
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