Influence of support morphology on the bonding of molecules to nanoparticles

Abstract

Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the COmetal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.nanoparticle | carpet growth | surface strain | adsorption | scanning tunneling microscopy N anoparticles exhibit properties distinct from their bulk counterparts (1-3). For instance, semiconductor particles smaller than ∼10 nm act as quantum dots (1-4) and oxide-supported gold nanoparticles are active for a variety of reactions including CO oxidation (5), water-gas-shift reaction (6), and epoxidation (7), whereas gold itself is not. Nanoclusters composed of ∼10 atoms have been shown to be exceptionally catalytically active for some reactions on some metals (8, 9). When the particle size is reduced, the relative number of undercoordinated atoms at the edges and corners increases. The proportion of perimeter sites at the interface between the metal and the support also increases. 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).The role of the support in modifying nanoparticle properties has also been recognized. For instance, the strong metal support interaction has been known for some time (2) and charge transfer either to or from the nanoparticles can lead to enhanced reactivity (18). More recently, it has come to light that the particle size itself may be governed by the interaction with the support (19). However, one effect that has not been discussed and yet should be present in any nanoparticle-support system is the influence of the support morphology such as steps.Here, we investigate the role of the support morphology on the reactivity of metal nanoparticles using scanning tunneling microscopy (STM). As our test system we choose Pd nanoparticles (20, 21) supported on TiO 2 (110) (22) simply because much is known about bo...

Simulation of Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements of CO on Supported Pd Nanoparticles

“…The average diameter of the (CO-covered) nanoparticle employed for the CO p* NEXAFS measurements is *1.5 lm [28], ten times larger than the attenuation length of the X-rays (0.151 lm). As such, the incident photon beam cannot transmit through the nanoparticle, and as a result, CO molecules bound on the side facets not in lineof-sight to the photon beam do not contribute to the X-ray absorption signal at all.…”

“…[11]. With the assumption of CO bonding to only the top facet, this reveals a polar bond angle (h B ) of 21.4°± 7.0° [28]. Other experimental details regarding the acquisition and analysis of the NEXAFS data can be found in Ref.…”

“…In this section, we apply our model to a set of carbon K-edge NEXAFS data that were obtained on a Pd nanoparticle covered with CO [28]. The experiments were performed using the SPELEEM end-station of Beamline I06, Diamond Light Source (UK).…”

“…Regarding the nanoparticle shape used in the simulation, previous STM experiments have shown that Pd nanoparticles that are grown on TiO 2 (110) using a method similar to that employed in the NEXAFS measurements in Ref. [28] are pseudo-triangular in shape. They are therefore characterized by a 111 f g top facet with 111 f gand 100 f g side facets [29].…”

“…Other experimental details regarding the acquisition and analysis of the NEXAFS data can be found in Ref. [28]. Red markers are the simulated I versus b curves that are obtained from nanoparticles constructed using different sets of (M, m, s) parameters that give an average particle diameter of *1.5 lm (d).…”