Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Abstract

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

Oxidation State and Symmetry of Magnesia-Supported Pd13OxNanocatalysts Influence Activation Barriers of CO Oxidation

“…In the first (type I) experiment, the Pd 13 cluster samples (as prepared) were exposed to oxygen ( 16 O 2 ) and carbon monoxide ( 13 CO) at 120 K prior to the TPR experiment. In the second (type II) experiment, the Pd 13 clusters were first oxidized at 370 K in an oxygen, 16 O 2 , background of 5 × 10 −7 mbar, and after being cooled to cryogenic temperatures, the Pd 13 16 O x cluster samples were exposed to 16 O 2 and subsequently to 13 CO. In the type I experiment, a broad peak around 400 K is observed as shown in the upper spectrum of Figure 1, whereas for the oxidized Pd 13 clusters three different reaction channels can clearly be distinguished (see middle spectrum of Figure 1), in which carbon dioxide ( 13 C 16 O 2 ) is formed at around 200, 300, and 400 K and which are labeled with α, β, and γ, respectively.…”

“…14,15 For each cluster deposition experiment, the magnesium oxide films were prepared in situ by epitaxial growth onto a Mo(100) single crystal. 16 The thicknesses of the films were typically around 10 monolayers; 16 O 2 partial pressure, evaporation rate, and growth temperature were 5 × 10 −7 mbar, 0.1 ML min −1 , and 320 K, respectively. Auger electron spectroscopy (AES), metastable helium impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) were employed to verify the cleanliness and characterize the electronic states of the oxide layer.…”

“…After the sample was cooled to 120 K and the residual 16 O 2 was pumped out of the system, the catalyst was sequentially exposed to 18 Figure 1, the oxidized palladium clusters contribute to the formation of 13 C 16 O 16 O in a broad temperature window of above 200 K to about 500 K. Two maxima in the desorption spectra are observed at 340 and 400 K (labeled β and γ in Figure 1). Interestingly, the contribution of the cryogenic 18 O 2 is much smaller with two active reaction channels for the formation of 13 C 18 O 16 O at ∼210 K (α-mechanism) and ∼330 K (β-mechanism).…”

“…To identify atomistic mechanisms underlying the TPD spectra of the type II experiments, we consider the Pd 13 O 6 nanooxide in the bottom panel of Figure 2d resulting from the initial 16 O 2 treatment at 370 K. As the clusters in the type II experiment were exposed to 18 Figure 3d shows a top and side view of the final structure with full CO coverage (with orange color code for the six 16 O of the nanooxide).…”

“…After this reaction, all 16 O atoms in the second adlayer have been consumed, and only one 16 O atom remains on the cluster, located in the first adlayer on top of a Mg. Moving this 16 O to the second adlayer (resulting in structure XII* in Figure 5a) is kinetically hindered by a high activation energy of E a = 1.22 [1.17] eV. A direct reaction of the neighboring CO on the corner bridge with the 16 O atom on the first adlayer site (reaction F in Figure 6a) has a lower activation energy of 0.93 [0.91] eV (corresponding to a ∼400 K reaction temperature). The formation and desorption of the last CO 2 culminates in a fully reduced C 2v Pd 13 with 2 CO on corner bridges.…”