Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Abstract

Abstract. Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO 2 (111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O 2 , CO, and CO + O 2 at 300 K. The particles are found to be stable in O 2 ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above ~ 1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO 2 (111) support, is observed in CO + O 2 (1:1) mixture at much lower pressure (~10 -3 mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…4.36). 431 Whereas the Au exclusively nucleates along step edges in the stoichiometric film, 20% of the particles develop on the terraces after creation of oxygen vacancies via mild oxide reduction. In selected cases, the associated lattice defects had even been identified in the STM prior to metal deposition (Fig.…”

“…The inset in (B) shows two defects in the CeO 2 surface, being assigned to oxygen vacancies. 261,431 An unambiguous correlation between atomically-defined point defects and a distinct binding behavior of atomic or molecular species has been provided for vanadia thin-films grown on Au(111). 262,432,433 The V 2 O 3 (0001) surface exposes a closed-packed layer of vanadyl groups when prepared at intermediated O 2 chemical potentials, 26 but forms a bulk-truncated O or V termination at strongly oxidizing or reducing conditions, respectively.…”

“…4.40A). 27,48,95,431 Their increased binding potential with respect to terrace sites results from the interplay of different effects: (i) a reduced coordination of the step and corner atoms,…”

Probing the 4f states of ceria by tunneling spectroscopy

“…[13][14][15][16] The vacancies are mainly generated with two approaches, namely thermal treatment of the oxide to stimulate oxygen desorption 13,14 and electron bombardment. 17 In both cases, they were exclusively observed on plain oxide terraces, although DFT calculations found the defect-formation energy to be substantially lowered at low-coordinated step and corner sites. [18][19][20] The presence of O vacancies along the ceria steps is also suggested from the preferred nucleation of ad-metals at those sites, as revealed from growth studies performed with Au and Ag.…”

Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

“…In relation to gold catalysis, it has been argued that surface defects like cerium and oxygen vacancies play a role in the nucleation and stabilization of gold nanoparticles [31][32][33][34][35][36]. Another relevant issue is the stabilization of cationic gold in ceria, either at the surface or as a solid solution.…”