Size-Dependence of Adsorption Properties of Metal Nanoparticles: A Density Functional Study on Palladium Nanoclusters

Abstract

Interatomic distances in metal nanoparticles are reduced from their values in the bulk. We studied computationally how this size-dependent geometry change (from the bulk) relates to the size-dependence of other properties of large metal clusters, including their reactivity. For this purpose, using an all-electron scalar-relativistic density-functional approach, we calculated structures and binding energies for the example of CO adsorption on 3-fold hollow sites at the center of (111) facets of cuboctahedral nanoscale clusters Pd n (n = 55−260). The average nearest-neighbor Pd−Pd distance of optimized structures is 4−7 pm (2−3%) shorter than the extrapolated limit of the lateral distance within an infinite (111) surface. In consequence, the energy of CO adsorption on a cluster of ∼100 atoms is ∼15 kJ mol−1 smaller than the extrapolated limit. On the basis of these results, we suggest a strategy for modeling particles of larger size, e.g. of 1000 atoms and more, with the help of smaller model particles of up to ∼300 atoms where one keeps the core of a model cluster fixed at the bulk structure and restricts the structure optimization to the outermost shell of cluster atoms.

Particle-size dependent heats of adsorption of CO on supported Pd nanoparticles as measured with a single-crystal microcalorimeter

“…25 This finding agrees with the principle of bond order conservation: 26 in the contracted clusters, one expects weaker adsorption bonds and stronger binding within the adsorbate as a result of better saturated valences of the substrate atoms. The decrease in the adsorbate binding energy with the lattice constant can be also rationalized in terms of the strain effect, which is associated with a downward shift of the valance d band with decreasing lattice parameter.…”

“…The decrease in the adsorbate binding energy with the lattice constant can be also rationalized in terms of the strain effect, which is associated with a downward shift of the valance d band with decreasing lattice parameter. 25,27 Such effects were found not to be restricted to threefold hollow sites on ͑111͒ facets but reflect a general trend that also holds for adsorption at other sites of the particles such as bridge sites and edges. 25 A second reason for the decrease in adsorption heat of a gas-phase molecule on the small metal clusters is a feasible weakening of the dispersion force ͑van der Waals interaction͒ that is induced by dynamic response of bulk electrons of the metal to charge-density fluctuations in an adsorbed molecule.…”

“…25,27 Such effects were found not to be restricted to threefold hollow sites on ͑111͒ facets but reflect a general trend that also holds for adsorption at other sites of the particles such as bridge sites and edges. 25 A second reason for the decrease in adsorption heat of a gas-phase molecule on the small metal clusters is a feasible weakening of the dispersion force ͑van der Waals interaction͒ that is induced by dynamic response of bulk electrons of the metal to charge-density fluctuations in an adsorbed molecule. Previously, it has been shown that the electron population at the Fermi edge, which is mainly relevant for this interaction, drastically changes with the cluster size in the range of a few nanometers.…”

Influence of support morphology on the bonding of molecules to nanoparticles

“…All these sites have been shown to play a crucial role in some reactions (10,11). Reducing the particle size can also lead to a decrease in the interatomic bond length in small metal clusters (12,13), which in the case of Pd nanoparticles results in lower adsorption energies for both CO (14,15) and O 2 (16), although such weakening of CO binding on the nanoparticle can also arise from other factors such as encapsulation of the nanoparticles by the support (17).…”

Trends in der Bindungsstärke von Oberflächenspezies auf Nanopartikeln: Wie verändert sich die Adsorptionsenergie mit der Partikelgröße?

“…Diese Beobachtung stimmt gut mit den Literaturdaten überein, in denen strukturelle Unterschiede in der Pd-Oberfläche nur einen geringen Einfluss auf die CO-Bindungsenergie haben. [29] Zwei mikroskopische Effekte kçnnen den beobachteten Abfall der An- fangsadsorptionsenergie auf Pd-Nanopartikeln sowohl für Sauerstoff als auch für CO erklären: die Schwächung der Chemisorption und die Reduktion der Van-der-Waals(VdW)-Wechselwirkung. Der Abfall der Chemisorptionsenergie auf Pd-Partikeln wurde kürzlich in einer theoretischen Studie vorhergesagt.…”