Thermodynamics driving the strong metal–support interaction: Titanate encapsulation of supported Pd nanocrystals

Abstract

The strong metal-support interaction (SMSI) is the encapsulation of a supported metal particle by an oxide layer that diffuses from the substrate. This process is usually described as being driven by a reduction in the surface energy of the metal particle and has a significant influence on the catalytic activity of the metal. Here, epitaxial Pd nanocrystals grown in ultrahigh vacuum on SrTiO3(001) and anatase TiO2(001) substrates are studied by scanning tunneling microscopy. At annealing temperatures above ~600 ºC, the Pd crystals can become encapsulated by a TiOx monolayer originating from the substrates. For both bare and encapsulated Pd crystals, their height-to-width ratio increases with the crystal height as a mechanism to partially release their interfacial misfit strain with the substrate.However, the rate of this increase is lower for encapsulated crystals, indicating that during the SMSI process the interface between the particle and the oxide is modified to form a lower energy interfacial structure which also results in less strain in the encapsulated particle. The SMSI is found to preferentially occur on larger crystals, driven by the reduction in their elastic strain energy, which scales with the crystal volume. Compared with the traditional view of SMSI our results provide a more complete description of the encapsulation process.

Tuning Strong Metal–Support Interaction Kinetics on Pt-Loaded TiO2(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study

“…A possible interpretation for our observations may be given by considering the surface/interfacial energies of the resulting Pt species. The surface free energy of TiO 2 is lower than that of metallic Pt but higher than that of α-PtO 2 . , Covering Pt particles by a TiO x overlayer therefore reduces the overall surface energy and is considered one of the main driving forces for the “classical” SMSI. , Under UHV, the growth of the TiO x layer is self-limited, most likely due to its polarity . At 1 × 10 –5 Torr O 2 and lower partial pressures, Pt particles retain their metallic character with high interfacial energies.…”

Atomic-Scale Interface for Pt Nanoparticles on SrTiO3 (001)

“…Previous scanning probe microscopy work showed strong metal−support interaction on the oxide adlayer for Pd nanocrystals supported on SrTiO 3 (001). 37,38 The metal film growth kinetics confirmed Pt has high oxygen affinities due to interfacial bonding, which leads to Pt adsorption sites directly above O atoms. 39 This Pt−O interaction was also observed to affect the site-specific valenceband structure as reported in our previous XSW-XPS study of the AD Pt/STO interface.…”

Metal–support interactions for heterogeneous catalysis: mechanisms, characterization techniques and applications

“…For example, Chen et al prepared epitaxial Pd nanocrystals on SrTiO 3 (001) and anatase Ti(001) substrates by precipitation and oxidation under ultra-high vacuum conditions. 167 The packaged crystal increased at a lower rate, suggesting that in the SMSIs method, high-energy interface structures are created between the particle and compound, which additionally ends up with smaller strain on the encapsulated particles. There is a reduction in the elastic strain energy of larger crystals, which is proportional to their volume, leading to SMSI.…”