Determination of the Particle Size, Available Surface Area, and Nature of Exposed Sites for Silica−Alumina-Supported Pd Nanoparticles: A Multitechnical Approach

Abstract

In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H 2 ) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO 2 -Al 2 O 3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6-8 Å) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/V Chemi ). The S/V Chemi value drops significantly upon increasing the H 2 -reduction temperature. According to TEM, the sintering process can account only for a very small fraction of the S/V Chemi decrease, suggesting an important poisoning of the potentially available Pd surface. This hypothesis is supported by a parallel experiment of thermal decomposition at the same temperature (in absence of H 2 ), showing a S/V Chemi value almost unchanged. FTIR spectroscopy of adsorbed CO, probing the nature of the Pd surface available for adsorption, confirms the hypothesis.

A non-syn-gas catalytic route to methanol production

“…8.0 at Pd-Pd distance of 2.73 Å from the first shell analysis. The results are consistent with the detailed size estimations of supported Pd particles from a recent EXAFS study alongside with other multiple techniques including gas adsorption, TEM and XRD 16 . Similarly, the derived CN of ca.…”

Influence of Cluster Size Distribution on Cluster Size Dependent Catalytic Kinetics

“…Note that in the studies of supported Pd particles the cuboctahedral shape of the particles was fully supported by the high resolution TEM investigation [25].…”

“…As for example the distance between Pd-Pd atoms is equal to 0.275 nm [25], when the cluster order exceeds 3 or 4 (the cluster diameter close to 1.1-1.6 nm) the ratio between squares and triangles becomes rather insensitive on the size. Below this cluster order when the differences in activity of various faces are prominent, they should be taken into account in the kinetic equation.…”

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…Feature 2 grows as soon as feature 1 starts to change. The final intensity of feature 2 (0.02 eV −1 ) is much smaller with respect to that observed for palladium metal foil (0.04 eV −1 ) [21], as expected for nanoparticles, due to the relevant fraction of low-coordination surface sites compared to the total amount of metal sites [50][51][52]. Complementary information regarding the palladium particle size can be obtained by the analysis of SAXS data.…”